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Article Systematic Study of Quaternary Cations for Sequestering Application in High Energy Density Electrolytes for Hydrogen Bromine Redox Flow Batteries

Michael Küttinger 1,2 , Paulette A. Loichet Torres 1 , Emeline Meyer 1 , Peter Fischer 1,* and Jens Tübke 1,2

1 Applied Electrochemistry, Fraunhofer Institute for Chemical Technology, Joseph-von-Fraunhofer Straße 7, D-76327 Pfinztal, Germany; [email protected] (M.K.); [email protected] (P.A.L.T.); [email protected] (E.M.); [email protected] (J.T.) 2 Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology KIT, Straße am Forum 8, D-76131 Karlsruhe, Germany * Correspondence: peter.fi[email protected]

Abstract: Bromine complexing agents (BCAs) are used to reduce the vapor pressure of bromine in the aqueous electrolytes of bromine flow batteries. BCAs bind hazardous, volatile bromine by forming a second, heavy liquid fused . The properties of BCAs in a strongly acidic bromine electrolyte are largely unexplored. A total of 38 different quaternary ammonium halides are investigated ex situ regarding their properties and applicability in bromine electrolytes as BCAs. The focus is on

 the development of safe and performant HBr/Br2/H2O electrolytes with a theoretical capacity  −1 of 180 Ah L for hydrogen bromine redox flow batteries (H2/Br2-RFB). Stable liquid fused , Citation: Küttinger, M.; Loichet moderate bromine complexation, large conductivities and large redox potentials in the aqueous Torres, P.A.; Meyer, E.; Fischer, P.; phase of the electrolytes are investigated in order to determine the most applicable BCA for this kind Tübke, J. Systematic Study of of electrolyte. A detailed study on the properties of BCA cations in these parameters is provided Quaternary Ammonium Cations for for the first time, as well as for electrolyte mixtures at different states of charge of the electrolyte. Bromine Sequestering Application in 1-ethylpyridin-1-ium [C2Py]Br is selected from 38 BCAs based on its properties as a BCA High Energy Density Electrolytes for that should be focused on for application in electrolytes for H2/Br2-RFB in the future. Hydrogen Bromine Redox Flow Batteries. Molecules 2021, 26, 2721. Keywords: electrochemistry; bromine; quaternary ammonium salts; sequestration; hydrogen bromine https://doi.org/10.3390/ redox flow battery; electrolyte; liquid/liquid phase equilibrium molecules26092721

Academic Editor: Farid Chemat

Received: 26 February 2021 1. Introduction Accepted: 26 April 2021 Within the last ten years, numerous chemical combinations of negative and positive Published: 6 May 2021 half-cell chemistries for redox flow battery application have been published. The majority of these compositions have been investigated on laboratory scale [1–3]. One commercialized Publisher’s Note: MDPI stays neutral and noticeable redox flow battery system for stationary energy storage application is the with regard to jurisdictional claims in hydrogen bromine redox flow battery (H2/Br2-RFB) [4–6]. published maps and institutional affil- One of the challenges attributed to the H2/Br2-RFB application is the high vapor iations. pressure of bromine in the electrolyte solutions commonly used in this system [7]. In this study, 38 cost-effective additives based on quaternary ammonium halide salts are investigated for their applicability as bromine complexing additives (BCA) and their effects on electrolyte properties. The most promising additive is selected for further investigation Copyright: © 2021 by the authors. in a cell. This systematic investigation is carried out for the first time for electrolytes in Licensee MDPI, Basel, Switzerland. H2/Br2-RFBs. This article is an open access article distributed under the terms and 1.1. State of the Art conditions of the Creative Commons In this flow battery system, the negative half cell resembles the negative half cell Attribution (CC BY) license (https:// of a proton exchange membrane fuel cell anode, but is operated in a reversible mode. creativecommons.org/licenses/by/ Liquid posolytes based on (HBr), bromine (Br2) and water (H2O) are 4.0/).

Molecules 2021, 26, 2721. https://doi.org/10.3390/molecules26092721 https://www.mdpi.com/journal/molecules Molecules 2021, 26, 2721 2 of 26

− pumped through the positive half cell, while bromide (Br ) is oxidized to Br2 during charge operation (Equation (1)) and vice versa during discharge operation [4,5,8]. In some research articles, gaseous positive half cells operating with bromine vapor are also discussed [4]. The thermodynamic cell voltage is 1.09 V [9]. Br2 itself is hardly soluble in water [10–14]. Nevertheless, if bromide are present, Br2 forms polybromides − − − such as tribromide (Br3 ), pentabromide (Br5 ) or higher polybromides Br2n+1 [13,15,16] following Equation (2): − − 2 Br Br2 + 2 e (1) − − Br (aq) + n Br2 (aq) Br2n+1 (aq) (2) Bromine and bromide form addition bondings by complexation, resulting in the − previously mentioned polybromide cations Br2n+1 [17]. Complexation allows the main- taining of high concentrations of Br2 in aqueous bromide solutions. These electrolytes are − particularly interesting for the application of Br2/Br electrolytes in RFBs due to their high theoretical energy density of up to 254 Wh L−1 and a theoretical volumetric capacity of up to 233 Ah L−1 (HBr 48 wt%). Ideally, in order to achieve high power densities in H2/Br2-RFBs, high concentrations of HBr and Br2 are indispensable [18]. Nevertheless, Br2 itself and in polybromide form is already quite volatile at ambient temperature [19–21] and can react with different electrode and cell components of the H2/Br2-RFBs [4,22–26]. One major concern is the crossover of Br2 from the positive to the negative half cell during cell operation. When Br2 comes into contact with the platinum-based catalysts in the hydrogen half cell, it can induce corrosive dissolution of platinum resulting in the formation of bro- moplatinates [4,22–26]. The decay of the H2 electrode catalyst can be avoided by reducing the concentration of Br2 in the positive half cell electrolyte. Stabilization of polybromides by conversion into a less volatile form would reduce the bromine vapor pressure, thus leading to a safer battery electrolyte [7]. Additives used to lower bromine’s volatility have been applied in zinc bromine RFB (Zn/Br2-RFB) [8,27–39] and vanadium bromine RFB (V/Br2-RFB) [40,41] electrolytes for a long time. These additives are mostly based on organic quaternary ammonium cation compounds and are called bromine complexing agents (BCA). Their bromide or chloride salts are usually highly soluble in aqueous electrolytes. However, their polybromide salts are only slightly soluble or even insoluble in aqueous media [33,36,42]. A further advantage of the polybromide salts of quaternary ammonium BCAs is their low melting point, leading to an insoluble ionic liquid at room temperature. In the electrolytes, these liquids form emulsions that coalesce over time by forming a heavy second phase in the electrolyte [36], as shown in Equation (3). For illustration, Figure1 shows a photo of an electrolyte solution with a − + two-phase liquid Br /Br2/H2O/[BCA] electrolyte in the center.

+ − [BCA] (aq) + Br2n+1 (aq) [BCA]Br2n+1 (aq) [BCA]Br2n+1 (fs) (3)

This heavy phase is often referred as the fused salt phase (fs), which sometimes tends to crystallize [31,42]. In the past, many BCAs have been evaluated for their application on zinc bromine batteries, out of which BCAs such as 1-ethyl-1-methylpyrrolidin-1-ium bromide [MEP]Br and 1-ethyl-1-methylmorpholin-1-ium bromide [MEM]Br [8,27–34,38,39,42,43] have received a lot of attention and have been investigated thoroughly. Other BCA structures such as heteroaromatic BCAs based on alkylated pyridine, picolines or imidazole [27,29,30,34,44], symmetrical and unsymmetrical alkylated aliphatic BCAs [33,40,42] and alkylated cyclic cations [27,29,36,42] have also been reported in the literature. Different side chains in N-position such as n-alkyl [27,29,31,33,34,36] or iso-alkyl groups [31] have been used to investigate their steric influence on melting point, com- plexation and solubility of the fused salt in Zn/Br2-electrolytes. 1-carboxymethyl [30], hydroxyalkyl groups [27,29] and 1-chloromethyl groups [33,36,42] have been identified to increase Br2 concentration in the aqueous phase in order to reduce precipitation of [BCA]Br2n+1. Polar side chain groups of BCAs have been found to also increase the sol- ubility of the BCA in aqueous solutions. Mixtures of BCAs have been chosen to overlap Molecules 2021, 26, x FOR PEER REVIEW 3 of 28

Molecules 2021, 26, 2721 3 of 26

[BCA]Br2n+1. Polar side chain groups of BCAs have been found to also increase the solu- bility of the BCA in aqueous solutions. Mixtures of BCAs have been chosen to overlap the positive properties of different BCAs and overcome the drawbacks of single BCA proper- the positive properties of different BCAs and overcome the drawbacks of single BCA ties [34,38,40–43]. properties [34,38,40–43].

Figure 1. Scheme of the selection procedure of BCAs from 38 different compounds derived from six building blocks in

an aqueous HBr/BrFigure 1.2 electrolyte.Scheme of the The selection selecting procedure process includes of BCAs the from properties 38 different of primarily compounds the aqueousderived from electrolyte phase, such as safety,six performance,building blocks stability, in an aqueous bromine HBr/Br sequestration2 electrolyte. and electrolyticThe selecting conductivities. process includes The resultsthe properties are used to draw of primarily the aqueous electrolyte phase, such as safety, performance, stability, bromine seques- conclusions about the interaction of the [BCA]+ cations with the polybromides Br −, which are used to explain the tration and electrolytic conductivities. The results are used to draw conclusions2n+1 about the interac- phenomena. On the basis of these results, one BCA out of 38 BCAs was selected for later investigations in the cell. tion of the [BCA]+ cations with the polybromides Br2n+1−, which are used to explain the phenomena. On the basis of these results, one BCA out of 38 BCAs was selected for later investigations in the Comprehensive work on BCAs has been carried out for Zn/Br -RFB electrolyte mix- cell. 2 tures and the properties have been described by Cathro et al. [42], Eustace [36] and Lancry et al. [31]. While the work on BCAs for Zn/Br2-RFB electrolytes is extensive and only a Comprehensive work on BCAs has been carried out for Zn/Br2-RFB electrolyte mix- few studies applying BCAs in V/Br -RFB have been published so far, no systematic study tures and the properties have been described by Cathro2 et al. [42], Eustace [36] and Lancry is available on the application of BCAs in HBr/Br2/H2O electrolytes for H2/Br2-RFB. In et al. [31]. While the work on BCAs for Zn/Br2-RFB electrolytes is extensive and only a few addition, the influence of the different polybromides on the electrolyte properties has not studies applying BCAs in V/Br2-RFB have been published so far, no systematic study is been investigated and is hardly documented in the literature for Zn/Br2-RFB [17]. available on the application of BCAs in HBr/Br2/H2O electrolytes for H2/Br2-RFB. In addi- tion, the influence1.2. Workof the Plan different polybromides on the electrolyte properties has not been investigated and is hardly documented in the literature for Zn/Br2-RFB [17]. In this work, we investigate 38 quaternary ammonium halides ex situ as prospective BCAs for their suitability for application in Br−/Br electrolytes. The aim is to obtain 1.2. Work Plan 2 electrolytes for the positive half cell of a H2/Br2-RFB with a theoretical volumetric capacity In this workof 180, we Ah investigate L−1. All investigated38 quaternary BCAs ammonium are shown halides in Table ex situ1. They as prospective comprise of substances BCAs for theirfrom suitability six building for application blocks: (i) in pyridines, Br−/Br2 electrolytes. (ii) pyrrolidines, The aim (iii) is morpholines, to obtain elec- (iv) piperidines, trolytes for the(v) positive 3-methylimidazoles half cell of a H and2/Br (vi)2-RFB tetraalkylammonium with a theoretical volumetric compounds. capacity In this of research article, 180 Ah L−1. Allthe investigated abbreviations BCAs listed are shown in Table in 1Table are used1. They for comprise simplicity. of substances In this study, from the BCAs are six building blocks:examined (i) pyridines, for solubility (ii) pyrrolidines, in the electrolyte, (iii) morpholines, the stability of(iv) a piperidines, second liquid (v) phase at room 3-methylimidazolestemperature and (vi) and tetraalkylammonium their bromine binding compounds. strength In for this a rese definedarch electrolytearticle, the composition. abbreviations BCAslisted thatin Table do not 1 are form used a stable for simplicity. liquid second In this phase study are, eitherthe BCAs not solubleare exam- in the electrolyte ined for solubilityor show in the too electrolyte, low bromine the bindingstability strengthof a second and liquid are excluded phase at from room a detailedtemper- study as they ature and theirare bromine not suitable binding for strength a battery for application. a defined e BCAs,lectrolyte which composition. are still of BCAs interest, that are intensively do not form a investigatedstable liquid oversecond the phase entire are state either of charge not soluble (SoC) in range the electrolyte of the electrolyte or show with a focus on too low brominebattery binding performance-relevant strength and are excluded parameters: from stability,a detailed concentration study as they of are Br not2 in the aqueous suitable for a batteryelectrolyte, application. redox potential BCAs, w ashich well are as still the of electrolytic interest, are conductivity intensively ofinvesti- the aqueous phase. gated over theFor entire the state first time,of charge detailed (SoC) investigations range of the regarding electrolyte parameter with a focus development on battery are carried out − performance-relevanton the basis parameters of the measured: stability,distribution concentration of Brof 2Bron2 in the the polybromides aqueous electro- tribromide (Br3 ), − − lyte, redox potentialpentabromide as well as (Br the5 )electrolytic and heptabromide conductivity (Br7 of) the in theaqueous aqueous phase. phase. For Propertiesthe of the fused salt phase are not discussed in detail here in order to retain the clarity of the article. However, fused salt generally has low conductivity [36] and is therefore not the preferred option for cell applications. The criteria are defined to select a BCA for the application; the

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(Brapplications. salt5 are(Brsaltare 5(Br)generally saltgenerally5) and 5 )notgenerally and5not generally) and )generally and anddiscussed heptabromide discussedThe heptabromide heptabromide heptabromideThehas heptabromideThehas Thecriteria has has lowcriteria haslowcriteria criteria lowin lowin conductivity lowconductivitydetail aredetail conductivity (Brconductivity are (Braredefinedconductivity are(Br 7 (Brhere defined(Br7) heredefined 7)defined in 7) in7 ) in ) thein into[36] the in [36] theorder order selecttothe[36] aqueous theto[36] toaqueous and [36]selectand aqueous select aqueousselectand aqueousto and to a is and isretain BCA retain thereforeais thereforeais phase. aBCA therefore phase.isBCA thereforeBCA phase. for thereforephase.the phase.the for forthe Propertiesfor clarity notclarityProperties notthe Properties the application; notPropertiesthe not Propertiesthe application;thenot application; application;theof ofthepreferred preferred thethe thepreferred of preferred of preferredarticle. of thearticle. of the ofthe the the thefused theoption theobserved fusedoption thefused However,option However,fusedoptionobserved fusedobserved optionobserved salt forsalt forsalt saltfor saltfor for phasephasephasephase are are are not arenot not discussednot discussed discussed discussed in in detail indetail in detail detail here here here here in in order inorder in order order to to retain toretain to retain retain the the the clarity theclarity clarity clarity of of the ofthe of the article. thearticle. article. article. However,− −However,− − However, However, basisbasisphenomenaHHphenomenabasis2phenomenabasis/BrbasisH2/Br of 2of2/Br- RFB.2ofthe of-the RFB.of2 -the theRFB. measuredthemeasured Theare measured Themeasured are aremeasured The explainedprinciple principleexplainedexplained principle distribution distribution distribution distribution distribution of in of thisin inof detail,this detail,thisdetail, selection of ofselection ofBrselectionofBrand of 2andBr 2 andBron Bron 22lastly,2 on process2onthe lastly, theonlastly,process theprocessthe polybromidesthepolybromides one polybromides polybromides oneispolybromidesone isdepictedBCA isdepicted BCABCA depicted is is isselectedtribromide tribromidein selectedselected tribromidein tribromideFigure tribromidein Figure Figure for for1.for (Br (Brfurther1. (Brfurther 1.3(Brfurther 3−(Br −),),33 3pentabro- pentabro-),3), ),usepentabro- pentabro- usepentabro-use in in ina aa phasefusedphenomenacellfusedphasecellphasefusedcellfusedphasefusedcellphenomena phaseapplications.fusedphenomena cellapplications. aresalt applications. saltapplications.are aresalt applications. saltsaltare notgenerallysaltare generallynot notgenerallyare notgenerallygenerallydiscussed notgenerally discussed discussed are explained arediscussed The discussedThe Thehas explained Thehasexplained criteria Thehas criteria hashas inlowhas criteria lowcriteriain indetaillowcriteria lowlowin detail lowinconductivity detail conductivity are detail detail, aredetailinconductivity conductivityinconductivity are conductivity herearedefined detail, heredefinedaredetail, here defined here definedhere and indefined in inorder and inorder andto[36] lastly,in to[36]order order toselect[36] orderto[36]select[36] lastly, [36]and lastly, toselect and selectto and one toretainselectand and to retainandaisto aisretain BCA one retainthereforeaBCA isone retain BCAthereforeaisis BCA is athereforeBCA the thereforetherefore the BCAtherefore forBCA the for theisclarity the for clarity forthe selectedclarity the for isnotclarity isnottheclarity the application; selectednot application;notselectedthenot ofthenot theapplication;of application; ofthe application;thetheofthe preferred fortheof preferredthe thearticle.preferred the preferredpreferred article.forfurtherpreferred forarticle. article. article.the furtherthefurther theHowever, option theoptionobservedHowever, observedusethe However,option optionHowever,observed However,observedoption useobservedusein for for a infor forin for a a −− 55−−−5− −− 77−−−7− Molecules 2021, 26, 2721 midemideH2Hmide2Hmide/Brmide22/Br 22/Br 2(Br2(Br- RFB.2 (Br2-25(Br -RFB.25RFB.(Br)) 5 and and)The) ) and The and The and heptabromide heptabromideprinciple principle heptabromideprinciple heptabromide heptabromide of of ofthis this (Brthis(Br selection (Br7(Br 7 selection(Brselection)) 7 in in)) ) in the in the in theprocess the aqueous theprocessaqueousprocess aqueousaqueous aqueous is is depictedis phase. phase. depicted depicted phase. phase. phase. 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- methylpyrrolidinmethylpyrrolidin1methylpyrrolidinmethylpyrrolidinmethylpyrrolidin-1methylpyrrolidiniumbromide,1--1iumbromide,Ethyl-1-Ethyl1iumbromide,--Ethyl1Ethyl-Ethyl-1-1---1-1---1 --- -Quaternary- -- Ammonium Compounds Used as BCAs morpholin1morpholin1-Ethyl-morpholin1Ethylmorpholinmorpholin1-morpholin-Ethyl1bromide,Ethyl-Ethylbromide,-1-bromide,1--methyl---1methyl-1---1-methyl-1-ium-methyl---1-ium- 1-1methyl--1-ium-- ium--ium-ium- dinbromide,1din1-bromide,Methylpyrroli--din1Methylpyrroli-dinbromide,-din1-1din-Methylpyrroli--1-Methylpyrroli--ium---1-iumMethylpyrroli-1-1-1-ium- ium-ium[HMP]Br iumhydro- [HMP]Brhydro- [HMP]Br hydro- hydro-hydro- hydro- Quaternary Quaternary Quaternary Quaternary Quaternary methylpyrrolidinmethylpyrrolidin1methylpyrrolidinmethylpyrrolidin1methylpyrrolidin-1methylpyrrolidin-iumbromide, Ammoniumn -1Ammoniumiumbromide,n-1- Butyl-Ammonium -niumbromide,1ButylAmmoniumn --Ammonium-ButylnButyl-Butyl-1-1- -- -11- - -- 1Compounds- Compounds -methylpyrrolidin- -methylpyrrolidinCompounds -Compounds- 1Compoundsmethylpyrrolidin1methylpyrrolidinmethylpyrrolidin-1-1methylpyrrolidiniumbromide,n--1n1iumbromide,-1-Hexyl--n1Hexyliumbromide,n---HexylnHexyl- Hexylused used-1 -used 1-used --used-1 1 -as - as--1 - asBCAs- asBCAs- - as- BCAsBCAs BCAs metmetmet1met1met-hylmorpholin1met-iumbromide,hylmorpholinn-1iumbromide,nhylmorpholin-1hylmorpholin-Butylhylmorpholin--niumbromide,hylmorpholin1Butyln---ButylnButyl-Butyl-1-1----11- - - 1------methylpyrrolidinmethylpyrrolidin11methylpyrrolidin-methylpyrrolidiniumbromide,-1iumbromide,1[C2MP]Brmethylpyrrolidin1-1-Ethyliumbromide,-[C2MP]Briumbromide,1Ethyl-iumbromide,1iumbromide,-[C2MP]Br1-Ethyl-Ethyl-Ethyl-1- 1- ---1- 1-- 1------morpholin1 morpholin1 -Ethyl-morpholin1Ethylmorpholin1-1[C2MM]Br-bromide,Ethyl-morpholinEthylbromide,-[C2MM]BrEthylbromide,-bromide,[C2MM]Brbromide,1-bromide,1--methyl---1-methyl-1--1-methyl--methyl--ium-- 1-methyl-ium- 1-- ium-- 1ium- - ium- bromide,bromide,din1din1-bromide,Methylpyrroli--bromide,dinbromide,1Methylpyrroli-dinbromide,-1-1--Methylpyrroli--din1-Methylpyrroli---ium-Methylpyrroli-1-ium1-(S)- ium-[HMP]Br1 ium[HMP]Br(S)- hydro-ium [HMP]Br (S)hydro- [HMP]Br[HMP]Br [HMP]Brhydro- hydro- hydro- methylpyrrolidinmethylpyrrolidin11methylpyrrolidin-[C4MP]Br1methylpyrrolidiniumbromide,-11-iumbromide,[C4MP]Br1methylpyrrolidinn1--1n-iumbromide,-1[C4MP]Briumbromide,--iumbromide,Butyl1-n-iumbromide,Butyln--n--ButylButyl-Butyl-1- (C)1- --(C)-1-1- - (C)1 --- methylpyrrolidin- methylpyrrolidin- 11-methylpyrrolidin-1[C6MP]Brmethylpyrrolidiniumbromide,1--1iumbromide,[C6MP]Brn-1methylpyrrolidin11-n1-iumbromide,1-[C6MP]Br-iumbromide,Hexyl-1-iumbromide,n-Hexyliumbromide,n--n-Hexyl-Hexyl-Hexyl- 1-(S)1- --(S)-1-1 --(S) -1------metmet1met1[C4MM]Br-1metiumbromide,-hylmorpholin1[C4MM]Br1-iumbromide,hylmorpholinmet1n1--1n[C4MM]Br-iumbromide,-hylmorpholin1iumbromide,--iumbromide,hylmorpholinButyl1-n-iumbromide,Butyln-hylmorpholin-n-Butyl-Butyl-Butyl-1- 1(C)--- -1-(C)1- - 1 --(C)- - - - methylpyrrolidinmethylpyrrolidin1(=[MEP]Br)1methylpyrrolidin-methylpyrrolidiniumbromide,-1(=[MEP]Br)[C2MP]Brmethylpyrrolidin1iumbromide,[C2MP]Br1--(=[MEP]Br)Ethyliumbromide,-1[C2MP]Briumbromide,Ethyl[C2MP]Br1[C2MP]Br--1[C2MP]BrEthyliumbromide,-Ethyl-Ethyl-1--1- -1 (C)- 1- -(C)1- - - (C)- --- - 1morpholinmorpholin-1Ethyl(=[MEM]Br)-1morpholinEthylmorpholin1(=[MEM]Br)-[C2MM]Brmorpholin1Ethyl[C2MM]Br-bromide,(=[MEM]Br)Ethyl-bromide,Ethyl[C2MM]Br[C2MM]Br[C2MM]Brbromide,-[C2MM]Brbromide,1--1bromide,-methyl--1-methyl-1--1methyl---methyl-ium-- -1-methyl-ium- 1 -(C)- 1-ium-- (C)ium-ium-- ium- (C) bromide,1bromide,-1bromide,Methylpyrroli--1bromide,dinMethylpyrroli-din1-bromide,din1Methylpyrroli--Methylpyrroli---Methylpyrroli--1-1(S)--(S)1 -ium-[HMP]Brium ium-[HMP]Br(S)ium (S)(S) [HMP]Br(S) [HMP]Br hydro- [HMP]Brhydro-hydro- hydro- 1[C4MP]Br1methylpyrrolidin1-[C4MP]Brmethylpyrrolidiniumbromide,--1methylpyrrolidin1iumbromide,[C4MP]Brn1-[C4MP]Br1-[C4MP]Brn-iumbromide,1[C4MP]Br-Butyliumbromide,1-n--Butyln-iumbromide,-nButyl-Butyl-Butyl- 1(C) -(C)-1- -1(C) - (C) 1-(C)- 1 (C)- - - -- 1-[C6MP]Br1methylpyrrolidin-[C6MP]Brmethylpyrrolidiniumbromide,-1methylpyrrolidin1niumbromide,[C6MP]Br-1-[C6MP]Brn-[C6MP]Br1iumbromide,-Hexyl[C6MP]Br11iumbromide,-n--Hexyln--iumbromide,nHexyl-Hexyl-Hexyl- (S)1 -(S)-1- -1(S) - (S)(S) 1- -(S)1 ------1[C4MM]Brmet[C4MM]Br11-metiumbromide,--met1[C4MM]Br1iumbromide,n[C4MM]Br1[C4MM]Br-1-[C4MM]Brn-iumbromide,hylmorpholin1-Butylhylmorpholiniumbromide,1n---hylmorpholinButyln-iumbromide,-nButyl-Butyl-Butyl-1 -(C) -1(C)--1 -(C) 1--(C) (C) 1 -(C) - -- - - dindin-1-1-ium-ium hydro- hydro- methylpyrrolidinmethylpyrrolidin(=[MEP]Br)methylpyrrolidin(=[MEP]Br)methylpyrrolidinmethylpyrrolidin[C2MP]Br(=[MEP]Br)(=[MEP]Br)[C2MP]Br(=[MEP]Br)(=[MEP]Br)[C2MP]Br[C2MP]Br[C2MP]Br (C) (C) (C) (C) (C)- (C)- -methylpyrrolidin -methylpyrrolidin - - - methylpyrrolidinmethylpyrrolidin - - morpholin morpholin(=[MEM]Br) (=[MEM]Br)morpholinmorpholin[C2MM]Br(=[MEM]Br)morpholin(=[MEM]Br)[C2MM]Br(=[MEM]Br)(=[MEM]Br)[C2MM]Br[C2MM]Br[C2MM]Br-1--1-ium-- 1-(C)ium- 1- (C)1ium- - ium-- (C) (C) ium- (C) (C) met met hylmorpholinhylmorpholin-- 11-iumbromide,-1iumbromide,1-1-iumbromide,-iumbromide,iumbromide,-iumbromide,1-Ethyl-1- bromide,bromide,1-Ethyl-1-bromide,bromide,bromide, dindinbromide,dinbromide,-dinbromide,1din--1-ium-1-ium1-(S)-1ium(S)-ium- (S)ium hydro-(S) [HMP]Br hydro-[HMP]Br(S)[HMP]Br hydro- [HMP]Br hydro- hydro- 11--Ethyl1Ethyl1-1-EthylEthyl-Ethyl--11---11--1-- methylpyrrolidinmethylpyrrolidinmethylpyrrolidin[C4MP]Br[C4MP]Brmethylpyrrolidinmethylpyrrolidin1[C4MP]Br1-[C4MP]Br1-iumbromide,-iumbromide,iumbromide,-[C4MP]Briumbromide,1-n-Butyl-1- (C) (C) (C) (C) - (C)- methylpyrrolidin- -methylpyrrolidin -methylpyrrolidin[C6MP]Br[C6MP]Brmethylpyrrolidinmethylpyrrolidin11[C6MP]Br-[C6MP]Br1-iumbromide,-iumbromide,iumbromide,-[C6MP]Br1-iumbromide,n-Hexyl-1- (S) (S) (S) (S) - (S)- - 1-1 --Ethyl1Ethyl1-1-EthylEthyl-Ethyl--11--methyl--1methyl-1--1-methyl-methyl--methyl-metmet[C4MM]Brmet[C4MM]Brmethylmorpholinmet1[C4MM]Brhylmorpholin1[C4MM]Br1--1hylmorpholin-iumbromide,-n[C4MM]Briumbromide,iumbromide,hylmorpholin--Butyl-1-iumbromide,hylmorpholin (C) (C) (C) (C)- (C)- - - - bromide,bromide, [HMP]Br [HMP]Br 1-iumbromide,11-iumbromide,-methylpyrrolidin-1-iumbromide, 11-iumbromide,-iumbromide, 11-iumbromide,-iumbromide, methylmorpholin-1-bromide,bromide,bromide, 11-iumbromide,-iumbromide, 11-1-Methylpyrrolidin-1-ium-Methylpyrroli-1Methylpyrroli-1-1-Methylpyrroli-Methylpyrroli--Methylpyrroli- (=[MEP]Br)(=[MEP]Br)1-1[C2MP]Br(=[MEP]Br)iumbromide,[C2MP]Br(=[MEP]Br)-1iumbromide,(=[MEP]Br)[C2MP]Br-[C2MP]Br[C2MP]Briumbromide,[C2MP]Br (C) (C) (C) (C) (C) 11--nmethylpyrrolidin-1-1n1---Butyl1-nButyln--n-ButylButyl-Butyl--11--- 11-- 1-- 11--methylpyrrolidin-1-n1n1---Hexyl1-nHexyln--n-HexylHexyl-Hexyl--11-- -11-- 1-- (=[MEM]Br)(=[MEM]Br)[C2MM]Br(=[MEM]Br)[C2MM]Br(=[MEM]Br)bromide,(=[MEM]Br)[C2MM]Br[C2MM]Brbromide,[C2MM]Br[C2MM]Brbromide, (C) (C) (C) (C) (C) methylmorpholin-1- 11--n1n1---Butyl1-nButyln--n-ButylButyl-Butyl--11---11--1- - bromide,bromide,bromide,bromide,bromide, [HMP]Br [HMP]Br [HMP]Br [HMP]Br [HMP]Br methylpyrrolidin methylpyrrolidinmethylpyrrolidiniumbromide, --1--1iumbromide,-1iumbromide,1-1iumbromide,-iumbromide,-iumbromide, 1-1iumbromide,-1iumbromide,1-1iumbromide,-iumbromide,-iumbromide, morpholinmorpholinmorpholiniumbromide,--11--1-ium-ium-ium--ium-1-1iumbromide,-1iumbromide,1-1iumbromide,-iumbromide,-iumbromide, hydrobromide,PyridinPyridin(S)(S)(S)- (S)(S)1 (S)--[HMP]Br1ium - ium (S) methylpyrrolidin methylpyrrolidin 1 -1 Ethylpyridin -Ethylpyridin -1----1 - 1 - 1[C4MP]Brn [C4MP]Br--niumbromide,Butylpyridin[C4MP]Br-[C4MP]Br[C4MP]BrButylpyridin[C4MP]Br (C) [C4MP]Br(C) (C) (C) (C) (C)-1 -- 1 1 -- 1n[C6MP]Br-[C6MP]Br-nButylpyridin[C6MP]Br-[C6MP]Br[C6MP]BrButylpyridin[C6MP]Briumbromide, (S) (S) (S) (S)(S) -(S)1 -- 1 morpholin -morpholin1 -1n--nHexylpyridin-Hexylpyridin--11--ium-ium-- -1[C4MM]Br-1[C4MM]Brn--[C4MM]Brniumbromide,Hexylpyridin[C4MM]Br[C4MM]Br-[C4MM]BrHexylpyridin (C) (C) (C) (C) (C) (C)- - dindindindin--din11-Pyridin-ium-1ium1--1-iumiumium- iumhydro- hydro- hydro-hydro-- hydro-1 hydro- - ium [C2MP]Br (=[MEP]Br)(=[MEP]Br)1 (=[MEP]Br)[C2MP]Br(=[MEP]Br) -[C2MP]BrEthylpyridin [C2MP]Br[C2MP]Br[C2MP]Br (C) (C)(C) (C) - methylpyrrolidin 1methylpyrrolidin - methylpyrrolidinmethylpyrrolidin 1methylpyrrolidin -n-Butylpyridin - - methylpyrrolidin--methylpyrrolidin-1--methylpyrrolidinmethylpyrrolidin 1methylpyrrolidin-n-Butylpyridin - - ---1--[C2MM]Br (=[MEM]Br)(=[MEM]Br)1(=[MEM]Br)[C2MM]Br(=[MEM]Br)-[C2MM]Br[C2MM]Brn[C2MM]Br-[C2MM]BrHexylpyridin (C) (C)(C) (C) met met -metmethylmorpholinmet1hylmorpholin-nhylmorpholinhylmorpholin-hylmorpholinHexylpyridin - - - - - (S)(S)(S) (S) (S) (=[MEP]Br)(=[MEP]Br)11-1-iumbromide,iumbromide,-iumbromide, (C) (C) [C4MP]Br[C4MP]Br[C4MP]Br[C4MP]Br[C4MP]Br(C) (C) (C) (C) (C) (C) [C6MP]Br[C6MP]Br[C6MP]Br[C6MP]Br[C6MP]Br[C6MP]Br (S) (S) (S) (S) (S) (S) (=[MEM]Br)(=[MEM]Br)bromide,bromide,bromide, (C) (C) [C4MM]Br[C4MM]Br[C4MM]Br[C4MM]Br[C4MM]Br[C4MM]Br (C) (C) (C) (C) (C) (C) hydrobromide,hydrobromide,Pyridin-1-ium 11 -iumbromide,-1iumbromide, iumbromide,1 -iumbromide,Ethylpyridin- iumbromide,(=[MEP]Br) (C) - 1 - 1 iumbromide,-niumbromide,-Butylpyridin -1 - 1iumchloride,-niumchloride,-Butylpyridin - 1 - 11-1(=[MEM]Br)-iumbromide,bromide,n-bromide,iumbromide,-Hexylpyridinbromide, (C) - 11-1-iumchloride,n-iumchloride,-Hexylpyridin - bromide,bromide,Pyridinbromide,bromide,Pyridinbromide,hydrobromide,PyridinPyridinhydrobromide,Pyridin [HMP]Br -[HMP]Br1-1 -[HMP]Br -ium-[HMP]Br1-ium [HMP]Br-1 -1ium- -ium ium 1 (=[MEP]Br)1 - Ethylpyridin (=[MEP]Br)- 1 Ethylpyridin (=[MEP]Br) 1 -1(=[MEP]Br) Ethylpyridin - iumbromide,(=[MEP]Br) -Ethylpyridin Ethylpyridin iumbromide, (C) (C) -(C) 1- (C)1 -(C) - - 1- 1 -1- 1 - - n- 1n 1 - 1-- Butylpyridin- 1iumbromide,n-Butylpyridin1iumbromide, -n1-iumbromide,-n1Butylpyridin-iumbromide,iumbromide,iumbromide,-Butylpyridin-Butylpyridiniumbromide, - 1- 1- - -1 1- 1 -1- n-1 1-n 1- 1-- Butylpyridin-1iumbromide,n-Butylpyridin1iumbromide,-n1-iumchloride,-n1Butylpyridin-iumbromide,iumbromide,iumbromide,-Butylpyridiniumchloride,-Butylpyridiniumbromide, - 1- 1 - - - 1- 1 -1(=[MEM]Br)- - 1 -(=[MEM]Br)n -- 1n (=[MEM]Br)- 1-Hexylpyridin-1(=[MEM]Br)n-Hexylpyridin(=[MEM]Br)-n-1iumbromide,nHexylpyridin---Hexylpyridiniumbromide,Hexylpyridin (C) (C) (C) (C) - (C) - 1- 1 - 1-n 1n- -1-iumbromide,-Hexylpyridin-1iumbromide,n-Hexylpyridin1-n--1-niumbromide,iumchloride,iumbromide,Hexylpyridin-iumbromide,---Hexylpyridiniumbromide,iumchloride,Hexylpyridin - - - - - [C2MP]Br[C2MP]Br[C2MP]Br[C2MP]Br[C2MP]Br [C2MM]Br[C2MM]Br[C2MM]Br[C2MM]Br[C2MM]Br hydrobromide,Pyridinhydrobromide,Pyridin[HPy]Brhydrobromide,Pyridinhydrobromide,hydrobromide,Pyridin[HPy]Brhydrobromide,Pyridin[HPy]Br(S)(S)-(S)1- (S)1 -(S)-ium- -1(S)ium 1 - -(S)-ium 1ium - (S) ium 1 1 - Ethylpyridin- iumbromide, 1 Ethylpyridin iumbromide, [C2Py]Br1 - -Ethylpyridin iumbromide,[C2Py]Br1 Ethylpyridiniumbromide, iumbromide, -iumbromide,[C2Py]BrEthylpyridin (S) (S)- 1- (S)1- - - -1 1- -1 - 1 n - 1 -n[C4MP]Br -iumbromide, 1[C4MP]Br -Butylpyridin iumbromide,-[C4Py]Br-n 1Butylpyridinn[C4MP]Br -iumbromide,[C4Py]Br[C4MP]Br-iumbromide,-Butylpyridiniumbromide,n[C4MP]BrButylpyridiniumbromide,[C4Py]Br-Butylpyridin (C) (S)(C) (S) (C) (C) - (S) 1-(C) 1 - - - 1 1- 1 1-- -n -1 n[C6MP]Br -1iumchloride,[C6MP]Br-Butylpyridin-[C4Py]Cliumchloride,-n 1Butylpyridinn[C6MP]Br[C4Py]Cl-iumchloride,-[C6MP]Br-Butylpyridiniumchloride,niumchloride,[C6MP]BrButylpyridin[C4Py]Cliumchloride,-Butylpyridin (S) (C)(S) (C) (S) (S)- (C) 1-(S) 1 - - - 1- 1 1- 1- -1 n1- 1-n -1iumbromide,[C6Py]Br-- Hexylpyridin-1iumbromide,-n1Hexylpyridin1[C6Py]Brn--1-iumbromide,--Hexylpyridiniumbromide,n[C6Py]Br-iumbromide,Hexylpyridiniumbromide,-Hexylpyridin (S) (S) (S) - - 1- 1 -- [C4MM]Br1n - [C4MM]Br-1n- -1[C6Py]Cliumchloride,-Hexylpyridin-[C4MM]Br1-niumchloride,1Hexylpyridin[C4MM]Br[C6Py]Cln1--[C4MM]Br1-iumchloride,-Hexylpyridinniumchloride,[C6Py]Cl-iumchloride,Hexylpyridiniumchloride,-Hexylpyridin (S)(C) (C) (S) (C) (C) (S) -(C) - - - - (=[MEP]Br)(=[MEP]Br)(=[MEP]Br)(=[MEP]Br)(=[MEP]Br) (C) (C) (C) (C) (C) (=[MEM]Br)(=[MEM]Br)(=[MEM]Br)(=[MEM]Br)(=[MEM]Br) (C) (C) (C) (C) (C) hydrobromide,Pyridinhydrobromide,Pyridin[HPy]Br[HPy]Brhydrobromide,Pyridinhydrobromide,Pyridin[HPy]BrPyridin[HPy]Brhydrobromide,[HPy]Br[HPy]Br-1-1- -(S)ium- - 1-ium(S)1- - 1-ium-(S) (S)iumium-(S) ium(S) 1 1 - Ethylpyridin- iumbromide, [C2Py]Br1 Ethylpyridin iumbromide,1 [C2Py]Br -1 - Ethylpyridin- iumbromide,[C2Py]BrEthylpyridin - iumbromide,[C2Py]Br[C2Py]BrEthylpyridin [C2Py]Briumbromide, (S) (S) (S) - (S)1-(S) 1(S)- - - -1- 1 -- 1- - n- 1n 1- iumbromide, [C4Py]Br- Butylpyridin- 1iumbromide,[C4Py]Br-n-Butylpyridin n--iumbromide,[C4Py]Brn-Butylpyridin-iumbromide,[C4Py]Br[C4Py]BrButylpyridin-[C4Py]BrButylpyridiniumbromide, (S) (S) (S) (S)(S) - (S)1 - 1 - - - 1 -1- 1 1- - n1-- -1n -1[C4Py]Cl iumchloride,- Butylpyridin-1[C4Py]Cliumchloride,-n-Butylpyridinn--[C4Py]Cliumchloride,n-[C4Py]ClButylpyridin-iumchloride,[C4Py]ClButylpyridin-[C4Py]ClButylpyridiniumchloride, (C) (C) (C) (C) (C) - (C)1- 1- - - - 1- 1 1- 1- -1 - -1n- -1 n[C6Py]Br-1- iumbromide,[C6Py]Br -Hexylpyridin-1-n-iumbromide,Hexylpyridin1n-[C6Py]Br--[C6Py]Brniumbromide,-[C6Py]Br-Hexylpyridin-1iumbromide,[C6Py]BrHexylpyridin--Hexylpyridiniumbromide, (S) (S) (S) (S)(S) (S)- - 1 - 1-- -1n 1 n[C6Py]Cl-1-[C6Py]Cl- iumchloride,-Hexylpyridin1-1n-iumchloride,Hexylpyridinn1-[C6Py]Cl--[C6Py]Cln[C6Py]Cliumchloride,-Hexylpyridin-1[C6Py]Cliumchloride,Hexylpyridin--Hexylpyridiniumchloride, (S) (S) (S) (S)(S) (S) ------1-Ethylpyridin-1- 1-n-Butylpyridin-1- 1-n-Butylpyridin-1- 1-n-Hexylpyridin-1- 1-n-Hexylpyridin-1- Pyridin-1-iumPyridinPyridinPyridin-1- hydrobromide,-ium1-1-ium -ium 1 - Ethylpyridin 1 1 -Ethylpyridin -Ethylpyridin -1 -- 1- 11- - -n 1- 1-Butylpyridin n-n-Butylpyridin-Butylpyridin- 1 -- 11 --1-n 1-- 1-Butylpyridinn-n-Butylpyridin-Butylpyridin-1- - 1- 11-- -n1 -1-Hexylpyridinn-n-Hexylpyridin-Hexylpyridin - 1--n1-1-Hexylpyridinn-n-Hexylpyridin-Hexylpyridin - -- hydrobromide,hydrobromide,Pyridin[HPy]Brhydrobromide,[HPy]Brhydrobromide,hydrobromide,[HPy]BrPyridin[HPy]Br[HPy]Br-1 (S)- ium(S)- 1(S) -(S) ium (S) 1 -iumbromide,[C2Py]Br Ethylpyridiniumbromide, [C2Py]Br1 iumbromide,[C2Py]Br -iumbromide,iumbromide,[C2Py]BrEthylpyridin iumbromide,[C2Py]Briumbromide, (S) (S) (S) (S)- 1 (S) - - 1 - niumbromide,[C4Py]Br 1iumbromide,-[C4Py]Br Butylpyridin-iumbromide,iumbromide,n[C4Py]Briumbromide,iumbromide,[C4Py]Br-iumbromide,Butylpyridin[C4Py]Br (S) (S)[C4Py]Br (S) (S) -(S) 1 - 1- 1 - n-[C4Py]Cliumchloride, 1-iumchloride,[C4Py]ClButylpyridin-[C4Py]Clniumchloride,iumchloride,[C4Py]Cliumchloride,-iumchloride,Butylpyridiniumchloride,[C4Py]Cl (C) (C) (C) (C) -(C) 1 - - 1 1 1 - -1n-[C6Py]Br iumbromide,-[C6Py]Br1-iumbromide,1Hexylpyridin--[C6Py]Briumbromide,1n-iumbromide,[C6Py]Br-iumbromide,iumbromide,--[C6Py]Briumbromide,Hexylpyridin (S) (S) (S) (S) (S) - 1 iumchloride, -1 - 1n[C6Py]Cl- [C6Py]Cliumchloride,-1-1iumchloride,Hexylpyridin1-[C6Py]Cl-n1-[C6Py]Cliumchloride,-iumchloride,iumchloride,-[C6Py]Cliumchloride,Hexylpyridin[C6Py]Cl (S) (S) (S) (S) (S) - - [HPy]Br (S) 1-1Ethyl-1Ethyl[C2Py]Br-Ethyl-4 - - 4- (S)- 4 - 1-1n--1nButyl--nButyl-Butyl(S)-4- - 4 --4 - 1-1n--1[C4Py]ClnHexyl--nHexyl-Hexyl- (C)4 -- 4-- 4 - [C6Py]Br (S) 1-1Ethyl-(S)1Ethyl-Ethyl-3-- 3--3 - 4hydrobromide,-hydrobromide,4Methylpyridine[HPy]Brhydrobromide,-[HPy]Br4Methylpyridinehydrobromide,hydrobromide,-[HPy]Br[HPy]BrMethylpyridine[HPy]Br[HPy]Br (S) (S) (S) (S)(S) (S) iumbromide, [C2Py]Briumbromide, [C2Py]Br iumbromide,[C2Py]Br iumbromide,[C2Py]Br[C2Py]Briumbromide,[C2Py]Br (S) (S) (S) (S)(S) (S) iumbromide,[C4Py]Briumbromide,[C4Py]Briumbromide,[C4Py]Briumbromide,[C4Py]Br[C4Py]Briumbromide,[C4Py]Br (S) (S) (S) (S)(S) (S) iumchloride,[C4Py]Cl[C4Py]Cliumchloride,iumchloride,[C4Py]Cl[C4Py]Cliumchloride,[C4Py]Cliumchloride,[C4Py]Cl (C) (C) (C) (C) (C) (C) 31 -13 [C6Py]BrMethylpyridineiumbromide,[C6Py]Br-1-iumbromide,3Methylpyridine1-[C6Py]Br1-iumbromide,[C6Py]Br-[C6Py]BrMethylpyridineiumbromide,-[C6Py]Briumbromide, (S) (S) (S) (S)(S) (S) 1 -1 [C6Py]Cliumchloride, [C6Py]Cl -1 iumchloride,1-[C6Py]Cl 1iumchloride,[C6Py]Cl[C6Py]Cl-iumchloride,[C6Py]Cl-iumchloride, (S) (S) (S) (S)(S) (S) PyridinPyridinPyridinPyridinPyridin--11--ium-1ium1--1-iumiumium- ium methylpyridine 1 1 -methylpyridine -Ethylpyridin 1Ethylpyridin methylpyridine1 -1 1 -Ethylpyridin1 -Ethylpyridin -Ethyl-Ethylpyridin 1Ethyl11-1-Ethyl-Ethyl-EthylEthyl-4-4----4--4-4 -14hy--1-- -- hy--1 1 - -hy-1methylpyridine 1- -n- methylpyridine 1n 1- -1 -Butylpyridinmethylpyridine1-n1Butylpyridin -n-n-n-1Butylpyridinn1-Butylpyridin1-Butyl-1-Butylpyridinn-Butyln-n-n-Butyl-Butyl-ButylButyl-4-4----4 --4hy--4-14 -1-hy---- 1-1 hy-1methylpyridine1---n1- methylpyridine 1n 1-1-- Butylpyridinmethylpyridine1-n-Butylpyridinn-n-1n-Butylpyridin1-1Butylpyridin--Hexyl1-n-ButylpyridinHexyln-n-n-Hexyl-Hexyl-HexylHexyl-4-4--- -4-hy-4-41- 14-hy--- -1 hy-11-1--1- -n -1n1- --Hexylpyridin1-nHexylpyridinn--n-HexylpyridinHexylpyridin-Hexylpyridin-- 1-1---n1n1---methylpyri-Hexylpyridin1-nHexylpyridin1n-methylpyri--nEthyl-Hexylpyridin1EthylHexylpyridin1methylpyri--1-Hexylpyridin1-Ethyl-Ethyl-EthylEthyl-3-3----3-3-3-3------44-Methylpyridine-hydrobromide,4[HPy]BrMethylpyridine44hydrobromide,-[HPy]Br4-Methylpyridine-Methylpyridine[HPy]Br-Methylpyridinehydrobromide,[HPy]BrMethylpyridine[HPy]Br (S) (S) (S) (S) (S) [C2Py]Br [C2Py]Br [C2Py]Br [C2Py]Br[C2Py]Br (S) (S) (S) (S) (S) [C4Py]Br[C4Py]Br[C4Py]Br[C4Py]Br[C4Py]Br (S) (S) (S) (S) (S) [C4Py]Cl[C4Py]Cl[C4Py]Cl[C4Py]Cl[C4Py]Cl (C) (C) (C) (C) (C) 3 3- Methylpyridine-[C6Py]Brhydrobromide,3Methylpyridine3[C6Py]Br3hydrobromide,-3-Methylpyridine[C6Py]Br-Methylpyridine-Methylpyridinehydrobromide,[C6Py]BrMethylpyridine[C6Py]Br (S) (S) (S) (S) (S) [C6Py]Cl [C6Py]Cl [C6Py]Cl [C6Py]Cl[C6Py]Cl (S) (S) (S) (S) (S) hydrobromide,hydrobromide,hydrobromide,hydrobromide,hydrobromide, methylpyridine methylpyridine methylpyridine methylpyridineiumbromide,methylpyridine iumbromide,methylpyridinedrobromide,11iumbromide,drobromide,iumbromide,-iumbromide,Ethyl-1iumbromide,Ethyldrobromide,1--Ethyl1Ethyl-Ethyl-4-4---4- 4hy- -- hy-- 4 - hy- hy- hy-methylpyridine hy- methylpyridine methylpyridine methylpyridineiumbromide,methylpyridine1iumbromide,methylpyridinedrobromide,1-niumbromide,-drobromide,iumbromide,1iumbromide,n-1iumbromide,-drobromide,Butyl--n1Butyln---ButylnButyl-Butyl-4-4-- -hy--4 hy-4 -- - 4 hy- hy- hy-methylpyridine- hy- methylpyridine methylpyridine methylpyridinemethylpyridineiumchloride,1iumchloride,methylpyridinedrobromide,1-n-drobromide,iumchloride,1iumchloride,niumchloride,-1-Hexyl-iumchloride,drobromide,-n1Hexyln---HexylnHexyl-Hexyl-4-4- --hy- -4 hy- 4- - - hy- 4 hy-hy- - hy- 1 1- -iumbromide,1iumbromide,1-1-iumbromide,iumbromide,iumbromide,-iumbromide, 1 1- methylpyri-dinebromide,-iumchloride,methylpyri-1iumchloride,1dinebromide,1--methylpyri-1-Ethyl-iumchloride,methylpyri-iumchloride,dinebromide,methylpyri-1iumchloride,Ethyl-methylpyri-1iumchloride,--Ethyl1Ethyl-Ethyl-3-3---3-3-- -3 - 44-hydrobromide,Methylpyridine-[H4MPy]hydrobromide,4Methylpyridine4[H4MPy]-hydrobromide,hydrobromide,Methylpyridine-hydrobromide,4Methylpyridine[H4MPy]hydrobromide,-MethylpyridineBrBr (S)Br (S) (S) 3 3 -hydrobromide,Methylpyridine-[H3MPy]Brhydrobromide,3Methylpyridine3[H3MPy]Br-hydrobromide,hydrobromide,Methylpyridine-hydrobromide,3Methylpyridine[H3MPy]Brhydrobromide,-Methylpyridine (S) (S) (S) [HPy]Br[HPy]Br[HPy]Br[HPy]Br[HPy]Br (S) (S) (S) (S) (S) [C24MPy]Br[C2Py]Brdrobromide,[C24MPy]Br[C2Py]Brdrobromide,[C24MPy]Br[C2Py]Brdrobromide,[C2Py]Brdrobromide,drobromide,[C2Py]Br1-Ethyl-4- (S) (S) (S) (S) (S) (S) (S) [C44MPy]Br[C4Py]Brdrobromide,[C44MPy]Br[C4Py]Brdrobromide,[C44MPy]Br[C4Py]Brdrobromide,[C4Py]Brdrobromide,drobromide,[C4Py]Br1-n-Butyl-4- (S) (S) (S) (S) (S) (S) (S) [C64MPy]Br[C4Py]Cldrobromide,[C4Py]Cl[C64MPy]Brdrobromide,[C4Py]Cl[C64MPy]Brdrobromide,[C4Py]Cldrobromide,drobromide,[C4Py]Cl1-n-Hexyl-4- (C) (C) (C) (S) (C) (C)(S) (S) [C6Py]Br[C6Py]Br[C6Py]Br[C6Py]Br[C6Py]Br (S) (S) (S) (S) (S) [C23MPy]Brdinebromide,[C6Py]Cldinebromide,[C23MPy]Br[C6Py]Cldinebromide,[C23MPy]Br[C6Py]Cldinebromide,dinebromide,[C6Py]Cl[C6Py]Cl (S) (S) (S) (S) (S) (S) (S) methylpyridinemethylpyridinemethylpyridinemethylpyridinemethylpyridine11-Ethyl-1Ethyldrobromide,1-1-Ethyl-Ethyl-Ethyl-4-4 --- -4- 4-hy- - 4-hy-- - hy- hy-methylpyridine methylpyridinehy-methylpyridinemethylpyridine11methylpyridine-n-1n1--drobromide,Butyl-1-n-Butyln--n--ButylButyl-Butyl-4-4- - --hy-4- 4hy--- 4- -hy- -hy- methylpyridine methylpyridinehy-methylpyridinemethylpyridine11-methylpyridinen-1n1--Hexyl-1drobromide,-n-Hexyln--n-Hexyl-Hexyl-Hexyl-4-4- --hy-- 4-hy- 4-- 4- -hy- -hy- hy- 3-Methylpyridine methylpyri-methylpyri-11-methylpyri-Ethyl-1-Ethyl-3-methylpyri-dinebromide,1Ethyl1-methylpyri-1-Ethyl-Ethyl-Ethyl-3-3 --- -3-3- -3- - - [H4MPy]hydrobromide,[H4MPy]hydrobromide,[H4MPy]hydrobromide,[H4MPy]hydrobromide,[H4MPy]4-Methylpyridine[H4MPy]hydrobromide,BrBr Br(S)Br Br(S)Br (S) (S)(S) (S) methylpyridine methylpyridine methylpyridine [H3MPy]Brhydrobromide,[H3MPy]Brhydrobromide,[H3MPy]Brhydrobromide,[H3MPy]Brhydrobromide,[H3MPy]Br[H3MPy]Brhydrobromide, (S) (S) (S) (S)(S) (S) 44-Methylpyridine-4Methylpyridine4-4-Methylpyridine-Methylpyridine-Methylpyridine [C24MPy]Br [C24MPy]Br (S) (S) [C44MPy]Br[C44MPy]Br (S) (S) [C64MPy]Br[C64MPy]Br (S) (S) 3 3-Methylpyridine-3Methylpyridine3-3-Methylpyridine-hydrobromide,Methylpyridine-Methylpyridine methylpyridinebromide, [C23MPy]Br [C23MPy]Br (S) (S) hydrobromide, [H4MPy]Brmethylpyridinemethylpyridine (S)[C24MPy]Brmethylpyridinemethylpyridinedrobromide,methylpyridine[C24MPy]Brdrobromide,1[C24MPy]Br-1[C24MPy]Brdrobromide,Ethyldrobromide,-1Ethyl1-drobromide,1hydrobromide,Ethyl-Ethyl-Ethyl-4--4- -4(S)- 4- -hy- 4 hy--(S) - (S) hy- (S)hy- methylpyridinehy- methylpyridine [C44MPy]Br methylpyridinemethylpyridine1drobromide,methylpyridine[C44MPy]Brdrobromide,-1[C44MPy]Brn-[C44MPy]Br1drobromide,n-drobromide,1-Butyl1-nhydrobromide,-drobromide,Butyln--nButyl-Butyl-Butyl-4- -4(S)- -4 hy-- 4hy---(S)4 - (S) hy-- (S)hy- methylpyridinehy- methylpyridine [C64MPy]Br methylpyridinemethylpyridine1drobromide,methylpyridine[C64MPy]Br-1drobromide,n[C64MPy]Br-1[C64MPy]Brdrobromide,n-1-drobromide,Hexylhydrobromide,1-n-Hexyldrobromide,n--nHexyl-Hexyl-Hexyl-4- -(S)4- -hy-4 - hy-4--(S) 4 (S)- hy- (S)-hy- hy- [C23MPy]Brdinebromide,methylpyri-dinebromide,[C23MPy]Br1methylpyri-[C23MPy]Brdinebromide,-1[C23MPy]Brmethylpyri-dinebromide,Ethyl-methylpyri-1Ethyldinebromide,methylpyri-1-1Ethyl-Ethyl-Ethyl-3--3- -3(S)-3-- 3 - (S) -(S) (S) 4-4[H4MPy]Methylpyridine-4[H4MPy]Methylpyridine4-[H4MPy]hydrobromide,4Methylpyridine-hydrobromide,[H4MPy]Methylpyridine-hydrobromide,Methylpyridine[H4MPy]BrBr Br(S)Br (S)Br (S) (S) (S) 3-3[H3MPy]BrMethylpyridine-3[H3MPy]BrMethylpyridine3-[H3MPy]Brhydrobromide,3Methylpyridine-hydrobromide,[H3MPy]BrMethylpyridine-hydrobromide,Methylpyridine[H3MPy]Br (S)(S) (S) (S) (S) (S) [C23MPy]Br (S) hydrobromide,hydrobromide, methylpyridine methylpyridine[C24MPy]Brmethylpyridine[C24MPy]Brmethylpyridinemethylpyridine[C24MPy]Br[C24MPy]Br[C24MPy]Br[C24MPy]Br (S) (S)hy- hy- (S) (S)hy- hy-methylpyridine (S) hy- methylpyridine [C44MPy]Br methylpyridine[C44MPy]Br methylpyridinemethylpyridine[C44MPy]Br[C44MPy]Br[C44MPy]Br[C44MPy]Br (S) (S)hy- (S) hy- (S) (S)hy- hy-methylpyridine (S)hy- methylpyridine [C64MPy]Brmethylpyridine[C64MPy]Br methylpyridinemethylpyridine[C64MPy]Br[C64MPy]Br[C64MPy]Br (S) (S)hy- hy-(S) (S) hy- hy- (S) hy- hydrobromide, hydrobromide, [C23MPy]Br[C23MPy]Brmethylpyri-[C23MPy]Brmethylpyri-[C23MPy]Brmethylpyri-[C23MPy]Brmethylpyri-methylpyri- (S) (S) (S) (S) (S) hydrobromide,hydrobromide,hydrobromide, drobromide,drobromide,drobromide,drobromide,1drobromide,-Ethyl-4 - drobromide,drobromide,drobromide,1drobromide,-drobromide,n-Butyl- 4 - drobromide,drobromide,drobromide,1drobromide,-drobromide,n-Hexyl- 4 - hydrobromide,hydrobromide,hydrobromide, dinebromide,dinebromide,dinebromide,dinebromide,dinebromide,1-Ethyl-3 - [H4MPy]1[H4MPy]hydrobromide,-hydrobromide,1n[H4MPy][H4MPy][H4MPy]--1hydrobromide,[H4MPy]nButyl--nButyl-ButylBr-Br3-- Br3(S)me-Br (S)--Br3me- (S) - (S)(S) me- (S) 1-1n1-1-n-Hexyl-Ethyl--Ethyl1nHexyl1--1-EthylHexylEthyl-Ethyl---434---3me--4--4-3me--4---me- 3-3Methylimidazol1-13Methylimidazol--nnMethylimidazol11--Butyl1-nButyln--n-ButylButyl-Butyl--44- --44-- 4-- - -11--1-nEthyln1-1-1Ethyl-Hexyl1-nHexyl-n-Ethyl-n-HexylHexyl--Hexyl3--3-me--4--43me---- -4me-4--4- - [H3MPy]Br[H3MPy]Brhydrobromide,1hydrobromide,[H3MPy]Br-[H3MPy]Br1[H3MPy]BrEthylhydrobromide,[H3MPy]Br-1Ethyl-Ethyl-3--3me- -(S)- 3me-(S)- me-(S) (S)(S) (S) 1-1n--1n1Propyl----EthylnPropylEthyl11--1Propyl-EthylEthyl-Ethyl- -3-3 ----3me- 3---3-me- -me- 44--Methylpyridine4Methylpyridine4-4-MethylpyridineMethylpyridine-Methylpyridine [C24MPy]Br [C24MPy]Br drobromide, [C24MPy]Brdrobromide, [C24MPy]Br[C24MPy]Br drobromide,[C24MPy]Br drobromide,drobromide, (S) (S) (S) (S) (S) (S) [C44MPy]Br [C44MPy]Brdrobromide,[C44MPy]Brdrobromide,[C44MPy]Br[C44MPy]Brdrobromide,[C44MPy]Brdrobromide,drobromide, (S) (S) (S) (S) (S) (S) [C64MPy]Br [C64MPy]Brdrobromide,[C64MPy]Brdrobromide,[C64MPy]Br[C64MPy]Brdrobromide,[C64MPy]Brdrobromide,drobromide, (S) (S) (S) (S) (S) (S) 3 3- -Methylpyridine3Methylpyridine3-3-MethylpyridineMethylpyridine-Methylpyridine [C23MPy]Br [C23MPy]Brdinebromide, dinebromide, [C23MPy]Br [C23MPy]Br[C23MPy]Br dinebromide,[C23MPy]Br dinebromide,dinebromide, (S) (S) (S) (S)(S) (S) [H4MPy]thyl11[H4MPy]-thyln-[H4MPy]1n1-thyl[H4MPy]1-Butyl-[H4MPy]1-n-pyridinebro-Butyln-n--n-pyridinebro-Butyl-Butyl-Butylpyridinebro-Butyl-Br3-Br3- Br-me--(S)-3-me-Br 3(S)-3Br- 3-me--(S)me- -me-(S) me- (S) methylpyridinemethylpyridine 1thyl1methylpyridine-methylpyridinethyln-1methylpyridinen1-1thyl-Hexyl-1-n-Hexylnpyridinebro--n--n-Hexyl-pyridinebro-Hexyl-Hexyl-Hexylpyridinebro--3-3--me---3-me-3-3 -hy- 3-me--hy-me--me- me- hy-hy-methylpyridine 3hy-methylpyridine3-1Methylimidazol-methylpyridine3methylpyridineMethylimidazol-313-methylpyridineium3-Methylimidazol-Methylimidazol1ium-MethylimidazolMethylimidazol-ium hydrobro- hydrobro- hydrobro- hy- hy- - hy--hy-methylpyridine hy-methylpyridine---methylpyridine-thylimidazol1methylpyridine1-methylpyridinethylimidazolEthyl-1Ethyl1thylimidazol1-1-Ethyl-Ethyl-EthylEthyl-3-3--me---3-me-3-3-3-me--- me--me-hy- 1hy-me---1 hy---hy-1 hy--[H3MPy]Brthylimidazol1[H3MPy]Br1-[H3MPy]BrthylimidazolEthyl-[H3MPy]Br1Ethyl1[H3MPy]Brthylimidazol1-1-Ethyl-Ethyl-EthylEthyl-3-3--me---3- me-3(S)-3- 3-me-(S)--me- -me-1(S)me- --(S) 1 (S)-- 1 1- 1-nthylimidazol-1nmethylpyri-1-1thylimidazolmethylpyri--Propyl-1-n-Propylnthylimidazol-nmethylpyri--methylpyri-n-Propyl-Propylmethylpyri--PropylPropyl - 3-3- - me-- -3-me- 3-31-3-me---me-1-me-me---1- hydrobromide,hydrobromide,hydrobromide,hydrobromide,hydrobromide, [C24MPy]Br [C24MPy]Br[C24MPy]Br[C24MPy]Br[C24MPy]Br (S) (S) (S) (S) (S) [C44MPy]Br [C44MPy]Br [C44MPy]Br[C44MPy]Br[C44MPy]Br (S) (S) (S) (S) (S) [C64MPy]Br [C64MPy]Br[C64MPy]Br[C64MPy]Br[C64MPy]Br (S) (S) (S) (S) (S) hydrobromide,hydrobromide,hydrobromide,hydrobromide,hydrobromide, [C23MPy]Br [C23MPy]Br[C23MPy]Br[C23MPy]Br[C23MPy]Br (S) (S) (S) (S) (S) drobromide,drobromide,drobromide, drobromide,drobromide,drobromide, drobromide,drobromide,drobromide,1-Ethyl-3- 1-Ethyl-3- dinebromide,1-dinebromide,dinebromide,n-Propyl thylmide,thyl1mide,1-thyln-thyl1thylmide,n-1thyl1--pyridinebro-Butyl--npyridinebro-1Butyln [C43MPy]Br--Butyl-3-methyl----Butylpyridinebro- npyridinebro-[C43MPy]BrButyl-pyridinebro--pyridinebro- Butyl[C43MPy]Br-3-3--me---3me-3--me--3me--me- thylmide,1thyl 1mide,-ndrobromide,thyl- 1thyldrobromide,nthylmide,-1-thyl-Hexyl-pyridinebro--n1Hexylpyridinebro-n 1--[C63MPy]Br--Hexylnpyridinebro- -Hexylpyridinebro-[C63MPy]Br-Hexyl-3-methyl--pyridinebro--pyridinebro- Hexyl[C63MPy]Br-3-3--me---3me-3- -me-- 3me--me- 331- mide,1Methylimidazol-3Methylimidazolium-drobromide,mide,3 1-iumdrobromide,1Methylimidazol-1-3mide,Methylimidazol1-ium-3-Methylimidazol-1--ium-iumMethylimidazol ium hydrobro- [HMIm]Brhydrobro- [HMIm]Br hydrobro- hydrobro- hydrobro- [HMIm]Brhydrobro- - - --thylimidazol thylimidazol1-drobromide,1iumbromide,-drobromide,thylimidazolEthyl-thylimidazol1thylimidazoliumbromide,Ethyl1thylimidazol--Ethyliumbromide,1Ethyl-Ethyl-3-3--me--3-me-3---me-13-me- 1- --me--1- 1-1-1-- -thylimidazolthylimidazol11-iumchloride,thylimidazolEthyl-thylimidazol1thylimidazolEthyliumchloride,1thylimidazol--Ethyliumchloride,1Ethyl-Ethyl-3-3--me--3-me-3---me-13-me-1- --me--1- 1-1-1-- 1-1thylimidazol-nthylimidazol-dinebromide,1ndinebromide,iumbromide,-1-thylimidazolPropyl-thylimidazol-nthylimidazoliumbromide,1Propylnthylimidazol---Propyliumbromide,nPropyl-Propyl - 3-3- -me- -3-1me--3 1--- me---3-me-1-1 --1-me-1-- - [H4MPy][H4MPy][H4MPy][H4MPy][H4MPy]BrBr Br(S) Br(S)Br (S) (S) (S) methylimidazol-1- [H3MPy]Br[H3MPy]Brmethylimidazol-1-[H3MPy]Br[H3MPy]Br[H3MPy]Br (S) (S) (S) (S) (S) -3-methylimidazol-1- pyridinebromide, [C43MPy]Br pyridinebromide, ium hydrobromide, mide,mide,mide,mide,mide,mide, [C43MPy]Br [C43MPy]Br [C43MPy]Br (S) [C43MPy]Br[C43MPy]Br [C43MPy]Br(S) (S) mide, mide,[C24MPy]Br [C24MPy]Br mide, mide, mide,[C24MPy]Brmide,[C24MPy]Br[C24MPy]Br [C63MPy]Br [C63MPy]Br [C63MPy]Br (S) [C63MPy]Br[C63MPy]Br [C63MPy]Br(S) (S) (S) (S) (S) (S) (S) mide, [C44MPy]Brmide, [C44MPy]Br mide,[C44MPy]Brmide,mide,[C44MPy]Brmide,[C44MPy]Br [HMIm]Br [HMIm]Br(S) [HMIm]Br [HMIm]Br(S)[HMIm]Br [HMIm]Br (S) (S) (S) (S) (S) (S) [C64MPy]Br [C64MPy]Br [C2MIm]Briumbromide,iumbromide,[C64MPy]Br[C2MIm]Br[C64MPy]Briumbromide,[C64MPy]Br[C2MIm]Briumbromide,iumbromide,iumbromide,iumbromide, (S) (S)(S) (S) (S) (S) (S) (S) [C2MIm]Cliumchloride,iumchloride,[C2MIm]Cliumchloride,iumchloride,[C2MIm]Cliumchloride,iumchloride,iumchloride, (S) (S) (S) [C23MPy]Br [C23MPy]Br[C3MIm]Biumbromide,iumbromide,[C23MPy]Br[C3MIm]B[C23MPy]Briumbromide,iumbromide,[C23MPy]Br[C3MIm]Biumbromide,iumbromide,iumbromide,r (S) (S)r(S) (S) r(S) (S) (S) (S) thyl1thyl1-thyln-thyl1n1--Butyl-pyridinebro-thyl1-n-Butylpyridinebro-n--n-Butyl-pyridinebro-Butylpyridinebro---Butylpyridinebro--(S)3-3--me---3-me-3- -3-me-- me-- me- thyl1thyl1-n-thyl1nthyl1--Hexyl-1-pyridinebro-thyln-Hexylnpyridinebro---n--Hexyl-[C63MPy]Br-pyridinebro-Hexyl-pyridinebro-Hexyl-pyridinebro--3-3--me---3-me-3--3-me-- me-- (S)me- 3 31- Methylimidazol1--3Methylimidazolium-31-ium3-1Methylimidazol--Methylimidazol-ium-1Methylimidazolium- hydrobro-[HMIm]Brium hydrobro- hydrobro- hydrobro- hydrobro- (S) - - ---thylimidazol-1thylimidazol1-thylimidazolEthyl-thylimidazol1Ethyl1-thylimidazol1-Ethyl-Ethyl-Ethyl-3-3--me---3-me-3---3-me-- 1-me--1 -me--- 1- 1- --1-thylimidazol1thylimidazol1-thylimidazolEthyl-thylimidazol1Ethyl1-thylimidazol1--EthylEthyl-Ethyl-3-3--me---3-me-3---3-me-- 1-me--1 -me--- 1- 1- --111--thylimidazoln-thylimidazol1n1--thylimidazolPropyl-1-thylimidazoln-Propyln--thylimidazoln-Propyl-Propyl-Propyl - 3-3- - me---3-me-1 -3-1 --3-me---- me--1me-1 -- -1 - [C2MIm]Br (S) [C2MIm]Cl (S) [C3MIm]Br (S) mide,mide,1-1mide,nthylmide,-1thyln-1mide,-Butyl1-n -[C43MPy]BrButyl n--[C43MPy]Br-nButyl-pyridinebro-(S) pyridinebro-Butyl-[C43MPy]Br(S) Butyl[C43MPy]Br -(S) [C43MPy]Br(S)3(S)- (S)-3-me- -3- me-3-- 3me--me--me- mide, 1mide, -1 mide,nthyl-mide,1thyl n-1-mide,Hexyl1-n- Hexyln-[C63MPy]Br- n[C63MPy]Br-Hexyl--pyridinebro-(S) Hexyl-pyridinebro-[C63MPy]Br(S) Hexyl[C63MPy]Br (S)- [C63MPy]Br(S)(S)3 -(S)-3- me-- 3- me- 3--3me--me--me- 3 -mide,3Methylimidazol mide,-3 Methylimidazol3-mide,1 3Methylimidazolmide,1--Methylimidazol--iummide,Methylimidazolium [HMIm]Br (S)[HMIm]Br(S) [HMIm]Br hydrobro- [HMIm]Br(S)hydrobro- (S) (S) [HMIm]Br(S) - - - - [C2MIm]Br-1[C2MIm]Briumbromide, -1iumbromide,Ethylthylimidazol[C2MIm]Br-1thylimidazol[C2MIm]Br[C2MIm]BrEthyliumbromide,1-[C2MIm]Briumbromide,1Ethyl-Ethyl-iumbromide,Ethyl-3--3-me--3-me- 3--(S) 3me--(S)me-- me- (S) - (S)(S)-1 (S)1 - - [C2MIm]Cl1[C2MIm]Cliumchloride,-1iumchloride,Ethylthylimidazol[C2MIm]Cl-1thylimidazol[C2MIm]Cl[C2MIm]ClEthyliumchloride,1-[C2MIm]Cliumchloride,1Ethyl-Ethyl-iumchloride,Ethyl-3--3-me--3-me- 3--(S) 3me--(S)me-- me- (S)- (S)(S)-1 (S)1- - 1 -1 [C3MIm]Bn-[C3MIm]B1iumbromide,n-1iumbromide,-Propylthylimidazol[C3MIm]B1-nthylimidazol-[C3MIm]B[C3MIm]BPropyliumbromide,n--[C3MIm]Biumbromide,nPropyl-Propyl-iumbromide,Propyl - 3-r- 3r -me-(S) -3-(S) rme-3-r- r 3me- (S)- r- (S)me--(S)1 me-(S)1 - - thylthylthyl-thylpyridinebro--pyridinebro--pyridinebro--pyridinebro- thylthylthyl-thylpyridinebro--pyridinebro--pyridinebro--pyridinebro- 11-ium-ium11-ium- iumhydrobro- hydrobro- hydrobro- hydrobro- thylimidazol thylimidazolthylimidazolthylimidazol-1-1--- 1-1 -- thylimidazolthylimidazolthylimidazolthylimidazol- 1-1 --- 1-1--thylimidazolthylimidazolthylimidazolthylimidazol -1- 1-- -1-1-- thyl-pyridinebro- thyl-pyridinebro- 1 -ium hydrobro- thylimidazol-1 - thylimidazol-1 - thylimidazol-1- mide,mide,thyl1mide,thylmide,-thyl1mide,nthyl-- 1n[C43MPy]Brpyridinebro-Butyl -[C43MPy]Br-pyridinebro-n-Butyl(S) pyridinebro--[C43MPy]Br (S)-[C43MPy]Brpyridinebro-[C43MPy]BrButyl [C43MPy]Br(S) (S)- 3(S)- -3 me--- 3me--me- mide,mide,thyl 1 mide,thylmide, -thyl1mide,nthyl-- 1n[C63MPy]Brpyridinebro-Butyl -[C63MPy]Br-pyridinebro-n-Butyl(S) pyridinebro--[C63MPy]Br (S)-[C63MPy]Brpyridinebro-[C63MPy]BrButyl [C63MPy]Br(S) (S)- 3(S)- -3 me--- 3me--me- mide,mide, 1 -1 mide,1iummide,n1--mide,1ium-1nHexylium- --ium[HMIm]Brn Hexyl [HMIm]Br(S)hydrobro--(S) Hexyl[HMIm]Br hydrobro- [HMIm]Br[HMIm]Br(S) hydrobro- [HMIm]Br(S)hydrobro- -(S)3 -- 3me--- 3me--me- [C2MIm]Br1 thylimidazol[C2MIm]Briumbromide,-1iumbromide,thylimidazoln[C2MIm]Br-thylimidazol[C2MIm]Br-iumbromide,1nthylimidazoliumbromide,Hexyliumbromide,-[C2MIm]Br-iumbromide,nHexyl-Hexyl-3- -(S)3 me-(S)-- 1 3me--(S) -1(S)- me-1 --(S) 1 - - [C2MIm]Cl thylimidazol[C2MIm]Cliumchloride,iumchloride,thylimidazol[C2MIm]Clthylimidazol[C2MIm]Cliumchloride,thylimidazoliumchloride,iumchloride,[C2MIm]Cliumchloride, (S) (S)- 1 -(S) -1(S)- 1-- (S) 1- - [C3MIm]B thylimidazol[C3MIm]Biumbromide,iumbromide,thylimidazol[C3MIm]Bthylimidazol[C3MIm]Biumbromide,thylimidazoliumbromide,iumbromide,[C3MIm]Biumbromide,rr (S) (S)r- r1 -(S) -1(S)r- 1 --(S) 1 - - 11--n1n1---Butyl1-nButyln--n-ButylButyl-Butyl--33--me--3me-3--3-me- me- -me- 11--n1n1---Hexyl1-nHexyln--n-HexylHexyl-Hexyl--33--me--3me-3-- 3-me- me--me- 3 3--Methylimidazol3Methylimidazol3-3-MethylimidazolMethylimidazol-Methylimidazol -- - - -11--Ethyl1Ethyl1-1-EthylEthyl-Ethyl--33--me--3me-3--3-me-me--me- 1 -1Methylpiperidin1-1-MethylpiperidinEthyl1Ethyl-1Methylpiperidin-1-EthylEthyl-Ethyl--33--me--3me-3 -- 3-me-me-- me- 111 --1Ethyl-n -1n1-Ethyl--Propyl1-nPropylnEthyl--n-PropylPropyl--1Propyl--1methylpi--- 1methylpi-- -3-3 methylpi-- - me--3me- 3 -- 3 -me- me--me- mide,mide,mide,mide,mide, [C43MPy]Br [C43MPy]Br [C43MPy]Br [C43MPy]Br [C43MPy]Br(S)(S)(S)(S) mide, mide, mide, mide,mide, [C63MPy]Br 1-[C63MPy]Br n[C63MPy]Br -Butyl-3-methyl-[C63MPy]Br [C63MPy]Br(S)(S)(S)(S) mide, mide, mide, mide,mide, [HMIm]Br [HMIm]Br1- [HMIm]Br n[HMIm]Br (S)-Hexyl-3-[HMIm]Br(S)(S)(S) iumbromide,iumbromide,[C2MIm]Br[C2MIm]Briumbromide,[C2MIm]Br[C2MIm]Briumbromide,iumbromide,1-n-Hexyl-3- (S) (S)(S) (S) iumchloride,iumchloride,[C2MIm]Cl[C2MIm]Cl[C2MIm]Cliumchloride,[C2MIm]Cliumchloride,iumchloride, (S) (S)(S) (S) iumbromide,iumbromide,[C3MIm]B[C3MIm]Biumbromide,[C3MIm]B[C3MIm]Biumbromide,iumbromide,r r (S) r (S)(S) (S) 1-1thyl1n-thyl-Butyl-3-methyl-imidazol-1-n-thyl1n1-thyl1-Butylthyl-1-n-Butyln-nimidazol-n-(S)Butylpyridinebro--imidazolButyl(S)-Butyl-pyridinebro-Butylimidazol- 3 -3--me---3-me-3-3--3me--1me--me---me-1--1-1thyl1-thyln-thyl1n1-thyl1-Butylthyl-1-n-Butyln-nimidazol-n-(S)Butylpyridinebro--imidazolButyl(S)-Butyl-pyridinebro-Butylimidazol- 3 -3--me---3-me-3-3--3me--1me--me---me-1--11-1-thylimidazoln-1nthylimidazol1-1-Hexyl-1-iumn-thylimidazolHexyln-n-iumn-Hexyl-(S)Hexyl-Hexyl(S)Hexyl hydrobro- - hydrobro-3-3--me---3-me-3-3-31-me--me---me-1me---1-1[C2MIm]Br1-thylimidazol[C2MIm]Brn-1nthylimidazolthylimidazol1-1-Hexyl-1-n-thylimidazolHexyln-n-n-Hexyl-Hexyl-HexylHexyl-3-3- -me--(S)- 3-me-(S)3-3-31-me--me-- -me--1 me---1-[C2MIm]Cl[C2MIm]Cl1-Methylpiperidinthylimidazolthylimidazol (S) (S) - 1--1-[C3MIm]B[C3MIm]Bthylimidazolthylimidazol1-Ethyl-1-rr (S) (S) - 1--1- thylthylthylthyl--pyridinebro-pyridinebro---pyridinebro-pyridinebro- thyl thylthylthyl--pyridinebro-pyridinebro---pyridinebro-pyridinebro-imidazol-1- 11--iumium11--iumiummethylimidazol-1- hydrobro- hydrobro- hydrobro-hydrobro- thylimidazol thylimidazolthylimidazolthylimidazolmethylimidazol-1---11-- -1 1-- thylimidazol thylimidazolhydrobromide,1hydrobromide,thylimidazol-thylimidazolMethylpiperidin - -11 - -- 11 - - thylimidazol peridinbromide,thylimidazol1peridinbromide,-thylimidazolEthylthylimidazol-1-methylpi- --1 1- --1 1 -- iumbromide,(S)(S)(S) (S) (S)[C4MIm]Br (S)(S)(S) (S) (S) (S)(S)(S) (S) (S) [C2MIm]Br[C2MIm]Br[C2MIm]Br[C2MIm]Br[C2MIm]Br (S) (S) (S) (S) (S) 1 1 -[C2MIm]ClMethylpiperidin-[C2MIm]Cl1Methylpiperidin1-[C2MIm]Cl1hydrobromide,Methylpiperidin-[C2MIm]Clhydrobromide,-Methylpiperidin[C2MIm]Clhydrobromide,Methylpiperidin (S) (S) (S) (S) (S) 1 1 -methylpiperidinbromide, [C3MIm]B Ethyl-1Ethyl [C3MIm]B1 -peridinbromide,1[C3MIm]BEthyl--Ethyl[C3MIm]Bperidinbromide,Ethyl[C3MIm]B-1-1--methylpi--1-methylpi--1-1methylpi--r-methylpi- rmethylpi-(S) r(S) r(S)r (S) (S) thyl1thyl1mide,-mide,n-iumbromide,thyl1mide,nthyl-thyl1-iumbromide,Butylthyl--nimidazol-1Butylnimidazoliumbromide,---Butyln -imidazolButyl-[C43MPy]Br imidazol[C43MPy]Br-imidazol imidazolButyl[C43MPy]Br-3-3--me---3me-3---1me--31me-- ---1me-- 1-1-1- --thyl1 thyl 1mide,-mide, n-iumchloride,thyl1mide,nthyl-thyl1-iumchloride,thylButyl--nimidazol-1Butylnimidazoliumchloride,---Butyln -imidazol-Butyl[C63MPy]Br imidazoliumchloride,[C63MPy]Br-imidazol imidazolButyl[C63MPy]Br-3-3--me---3me-3---1me--31 me-----1 me--1-1-1- --1thylimidazol 1 -thylimidazoln- mide,iumbromide,1mide,nthylimidazol-1thylimidazol-mide,thylimidazolHexyl-iumbromide,-nthylimidazol1Hexyln-iumbromide,--Hexylniumbromide,Hexyl -[HMIm]Br Hexyl[HMIm]Br -[HMIm]Br3-3--me---3me-31---1me--3 -me---1- 1me--1-1-- - 1 thylimidazol1- thylimidazoln-1iumchloride,nthylimidazol-1thylimidazol-thylimidazolHexyl-iumchloride,iumbromide,-nthylimidazol1iumbromide,Hexyln-iumbromide,-iumchloride,iumchloride,-HexylnHexyl-Hexyl-3-3--me---3me-31---1me-- 3-me---1- 1 me--1- 1-- - iumchloride,iumchloride,iumchloride, iumbromide,iumbromide,iumbromide, mide,mide,mide, [C43MPy]Br [C43MPy]Br [C43MPy]Br(S) mide, mide, mide, [C63MPy]Br [C63MPy]Br [C63MPy]Br mide,mide, mide, [HMIm]Br [HMIm]Br [HMIm]Br iumbromide,iumbromide,iumbromide, hydrobromide,hydrobromide,[HMPip]Briumchloride,iumchloride,[HMPip]Brhydrobromide,hydrobromide,[HMPip]Brhydrobromide,iumchloride,hydrobromide,[HMPip]Br (S) (S) (S) (S) peridinbromide, peridinbromide,[C2MPip]Briumbromide,iumbromide,peridinbromide,[C2MPip]Brperidinbromide,peridinbromide,peridinbromide,iumbromide,[C2MPip]Br (S) (S) (S) (S) [C4MIm]Br (S) [C4MIm]Cl[C4MIm]Cl (C) (C) [C6MIm]Br[C6MIm]Br (S)(S) [C6MIm]Cl[C6MIm]Cl (S) (S) 11-Methylpiperidin-1Methylpiperidin1-Methylpiperidin-1Methylpiperidin-Methylpiperidin 1 1- Ethyl-1 Ethyl1- -Ethyl1Ethyl-Ethyl-1-1--methylpi--1-methylpi-1---methylpi-1methylpi--methylpi- thyl1thyl1[C4MIm]Br-iumbromide,n-iumbromide,thyl1n[C4MIm]Brthyl1---iumbromide,Butyl1-n-[C4MIm]Briumbromide,imidazolthyl-Butyliumbromide,n-imidazoliumbromide,-n-Butyl-(S)Butyl-imidazol(S)-imidazolButyl-(S)-imidazol (S)3 -(S)3-- me-- - 3-me- 3-(S) -3 1-me- - 1me-(S)- -me-- -1(S) 1 - - - 1thyl 1-thyl1[C4MIm]Cl-iumchloride,n-thyliumchloride,1[C4MIm]Clnthyl1---Butyliumchloride,1-[C4MIm]Cln-imidazolthyliumchloride,-Butyliumchloride,n-imidazoliumchloride,-n-Butyl-(S)Butyl-imidazol(S)-imidazolButyl-(S)-imidazol (S)3 -(S)3-- me-- - 3-me- 3(C)- -3 1- me-- 1(C)me- - -me-- -1 (C) 1 -- -11 thylimidazol-1-thylimidazol[C6MIm]Briumbromide,n-1iumbromide,nthylimidazol1[C6MIm]Br--thylimidazolHexyl-1iumbromide,-n-Hexyl[C6MIm]Briumbromide,niumbromide,-thylimidazol-iumbromide,n-Hexyl-(S)Hexyl-(S)Hexyl(S) (S)- 3-(S)3- -me-- -3- me-3(S) -1- 3 -1me--(S) me---1- me- (S) 1- - -1 1 - thylimidazol[C2MIm]Br1-[C2MIm]Brthylimidazol[C6MIm]Cliumchloride,n-1iumchloride,nthylimidazol[C6MIm]Cl1-[C2MIm]Br-[C2MIm]BrthylimidazolHexyl-1-iumchloride,n-Hexyl[C2MIm]Br[C6MIm]Cliumchloride,n-iumchloride,thylimidazol-niumchloride,-Hexyl-Hexyl-Hexyl-3-3-- me--(S)- 3- me-(S)3-1- 3 -1me-- (S) (S)me-- -(S) 1-me- (S) 1- - - 1 - [C2MIm]Cl[C2MIm]Cl[C2MIm]Cl[C2MIm]Cl[C2MIm]Cl (S) (S) (S) (S) (S) [C3MIm]B[C3MIm]B[C3MIm]B[C3MIm]B[C3MIm]Br r (S) (S)r r (S)r (S) (S) 11-hydrobromide,[HMPip]BrMethylpiperidin-1hydrobromide,[HMPip]BrMethylpiperidin1-1hydrobromide,-[HMPip]BrMethylpiperidin-[HMPip]Brhydrobromide,Methylpiperidin[HMPip]Br-[HMPip]BrMethylpiperidinhydrobromide, (S) (S) (S) (S)(S) (S) 1 1 -peridinbromide, Ethyl-[C2MPip]Br 1peridinbromide,Ethyl[C2MPip]Br1 -1peridinbromide,-Ethyl-[C2MPip]Brperidinbromide,Ethyl[C2MPip]Br-[C2MPip]BrEthyl[C2MPip]Brperidinbromide,-1-1--methylpi---1-methylpi-1--1-methylpi--methylpi--methylpi- (S) (S) (S) (S) (S) (S) 1[C4MIm]Br-1[C4MIm]Briumbromide,n-1iumbromide,thyln-[C4MIm]Brthyl1-[C4MIm]BrButyl[C4MIm]Briumbromide,1-n-[C4MIm]Briumbromide,Butyln--nButyliumbromide,--imidazolButyl-imidazolButyl-3--3-me-- 3-(S)me- 3--(S)3me-- me-- (S) (S)me--(S) -1(S) 1 - - 1 [C4MIm]Cl-1[C4MIm]Cliumchloride,n-1thyliumchloride,[C4MIm]Cln-thyl1-[C4MIm]ClButyl[C4MIm]Cl1iumchloride,-n-[C4MIm]Cliumchloride,Butyln--nButyl-iumchloride,-imidazolButyl-imidazolButyl-3--3-me- -3-(C)me- 3-(C)-3me-- me-(C) - (C) me--(C) - 1(C) 1 - - 1 [C6MIm]Br-1[C6MIm]Brniumbromide,-1iumbromide,n-thylimidazol1[C6MIm]Br-Hexylthylimidazol[C6MIm]Br1-n[C6MIm]Briumbromide,-Hexyl[C6MIm]Briumbromide,n--nHexyl-iumbromide,Hexyl-Hexyl-3--3-me- -3(S)- me-(S)3--3me- - (S) me-- (S)(S)-me-1 (S)1 - - 1 [C6MIm]Cl-1[C6MIm]Clniumchloride,-1iumchloride,n-thylimidazol1[C6MIm]Cl-Hexylthylimidazol[C6MIm]Cl1-n[C6MIm]Cl-iumchloride,[C6MIm]ClHexyln-iumchloride,-nHexyl-iumchloride,Hexyl-Hexyl-3--3-me- -3(S)- me-(S)3--3 me- -(S) me-- (S)(S)-me-1 (S) 1- - MoleculesthylMoleculesthylMoleculesMoleculesthyl-thylimidazol-imidazol 2021- imidazol- 20212021imidazol 2021, 26,, 26,26- ,x1- ,26 ,1 -FOR xx- ,- FOR FORx1-1 -FOR -PEERthylthyl PEERPEER thylPEER-thyl imidazol-REVIEWimidazol REVIEWREVIEW- imidazol-REVIEWimidazol - 1-1 -- -1- 1--thylimidazolthylimidazolthylimidazolthylimidazol-1-1---1-1--thylimidazolthylimidazolthylimidazolthylimidazol -1- 1---1-11---Methylpiperidin11-Methylpiperidin-Methylpiperidin 1- Ethyl1 1-Ethyl-Ethyl-1--methylpi-1-1-methylpi--methylpi-5 5of5 of 5of 28 of28 28 28 1 hydrobromide,[HMPip]Br-1hydrobromide,[HMPip]BrMethylpiperidin-1Methylpiperidinhydrobromide,[HMPip]Brhydrobromide,[HMPip]Br-hydrobromide,Methylpiperidin[HMPip]Br (S) (S) (S) (S) (S) 1 peridinbromide, [C2MPip]Br-1 peridinbromide,Ethyl[C2MPip]Br -1Ethylperidinbromide,[C2MPip]Brperidinbromide,-[C2MPip]Brperidinbromide,Ethyl[C2MPip]Br-1--1methylpi--1methylpi--methylpi- (S) (S) (S) (S) (S) thyl[C4MIm]Br thyl[C4MIm]Brthyl [C4MIm]Brthyl-[C4MIm]Brthyliumbromide,imidazoliumbromide,-[C4MIm]Brimidazol-1-imidazol-imidazol-nimidazol-Butyl-1- (S) -(S)1- -1(S) - (S)-1 - 1--(S) 1 - -thyl[C4MIm]Cl thyl[C4MIm]Clthyl[C4MIm]Clthyl-[C4MIm]Clthyliumchloride,imidazoliumchloride,-[C4MIm]Climidazol-imidazol-imidazol-1-imidazoln-Hexyl-1- (C) -(C)1- -1(C) - (C)-1- 1-(C) -1 - -thylimidazol[C6MIm]Br thylimidazol[C6MIm]Brthylimidazol[C6MIm]Brthylimidazol[C6MIm]Briumbromide,thylimidazoliumbromide,[C6MIm]Br -(S) 1(S)--1 -(S) -1(S)- 1 --(S) 1- - thylimidazol[C6MIm]Cl thylimidazol[C6MIm]Clthylimidazol[C6MIm]Clthylimidazol[C6MIm]Cliumchloride,thylimidazoliumchloride,[C6MIm]Cl -(S) 1(S)--1 -(S) -1(S)- 1-- (S)1- - iumbromide,iumbromide,1-1iumbromide,niumbromide,--1nButyl--nButyl-Butyl-1--1 --1- iumchloride,iumchloride,1-1niumchloride,-iumchloride,-1nHexyl--nHexyl-Hexyl-1- -1 --1 - iumbromide,iumbromide,1,1,1-Trimethyl-1-iumbromide,iumbromide, n -tetradecylammoniumbromide, iumchloride,iumchloride,iumchloride,iumchloride, hydrobromide,1,1,1-Trimethyl-1-n-hexadecylammoniumbromide, peridinbromide, 11-methylpiperidinbromide,-n1n1---Butyl1-nButyln--n-ButylButyl-Butyl--33--me--3me-3-- 3-me- me--me- 11--n1n1---Butyl1-methylpiperidinbromide,nButyln--n-ButylButyl-Butyl--33--me--3me-3--3-me-me-- me- 1 1--n1n1---Hexyl1-nHexyln--n-HexylHexyl-Hexyl--33--me--3me-3--3-me-me--me-11--n1n1---Hexyl1-nHexyln--n-HexylHexyl-Hexyl--33 --me- -3me-3-- 3- me- me-- me-[HMPip]Brhydrobromide,[HMPip]Brhydrobromide,[HMPip]Brhydrobromide,[HMPip]Br[HMPip]Brhydrobromide,[HMPip]Br (S) (S) (S) (S)(S) (S) [C2MPip]Brperidinbromide,[C2MPip]Brperidinbromide,[C2MPip]Brperidinbromide,[C2MPip]Br[C2MPip]Brperidinbromide,[C2MPip]Br (S) (S) (S) (S)(S) (S) iumbromide,iumbromide,[C4MIm]Br[C4MIm]Briumbromide,iumbromide,iumbromide, (S) (S) iumchloride,iumchloride,[C4MIm]Cl[C4MIm]Cliumchloride,iumchloride,iumchloride, (C) (C) iumbromide,iumbromide,[C6MIm]Br[C6MIm]Briumbromide,iumbromide,iumbromide, (S) (S)[MTA]Br iumchloride,iumchloride,[C6MIm]Cl (C)[C6MIm]Cliumchloride,iumchloride,iumchloride, (S) (S) 1 1--Methylpiperidin1Methylpiperidin1-1-MethylpiperidinMethylpiperidin-Methylpiperidin[CTA]Br 11-- Ethyl 1Ethyl (C)1 -1-EthylEthyl-Ethyl--11--methylpi--1methylpi-1--1-methylpi-methylpi--methylpi- [C4MIm]Br[C4MIm]Brmethylpiperi-1thyl1thyl-methylpiperi-[C4MIm]Brn-thyl[C4MIm]Br1n[C4MPip]Br1-methylpiperi-1-Butyl-1--nButyl-nimidazol-imidazoln-imidazol-n-Butyl-imidazolButyl-ButylButyl-1- (S)1 -(S)- (S)--1-1 -1 -(S)- 1- -(S)11---1- - [C4MIm]Cl[C4MIm]Cl1methylpiperi-thyl1-thyl[C4MIm]Clmethylpiperi-n-thyl1[C4MIm]Cln1-1methylpiperi--Hexyl-1-n--Hexyl[C6MPip]Brn-nimidazolimidazol-imidazoln-Hexyl-Hexylimidazol-HexylHexyl- 1-(C) 1(C)----1- 1 (S)-1(C)- 1-(C)1-1--1-1,1,1 - [C6MIm]Br1,1,1[C6MIm]Br1,1,1thylimidazolthylimidazol[C6MIm]Br-[C6MIm]BrthylimidazolTrimethyl-Trimethyl-Trimethyl (S) (S)-1 -(S)-- 1(S)1n---1--1n -tetradecylammoni- --[C6MIm]Clntetradecylammoni-[C6MIm]Cl-tetradecylammoni-thylimidazolthylimidazol[C6MIm]Cl[C6MIm]Clthylimidazol (S) (S) -(S)- 1(S)1--1- 1,1,1- [HMPip]Br1,1,1[HMPip]Br1,1,1[HMPip]Br[HMPip]Br-[HMPip]BrTrimethyl-Trimethyl-Trimethyl (S) (S) -(S) 1(S) -- (S)1n ---1 nhexadecylammoni- [C2MPip]Br--n[C2MPip]Brhexadecylammoni--[C2MPip]Brhexadecylammoni-[C2MPip]Br[C2MPip]Br (S) (S) (S) (S) (S) thyl[C4MIm]Brthyl[C4MIm]Br[C4MIm]Br--imidazolimidazol (S)- -1 (S)1 -(S)- [C4MIm]Clthylthyl[C4MIm]Cl[C4MIm]Cl--imidazolimidazol (C)-- 1(C)1 -(C) - [C6MIm]Brthylimidazol thylimidazol[C6MIm]Br[C6MIm]Br (S)--1 1(S)- -(S) [C6MIm]Clthylimidazolthylimidazol[C6MIm]Cl[C6MIm]Cl (S) --1 1(S) - -(S) hydrobromide,hydrobromide, peridinbromide,peridinbromide, methylpiperi-[C4MIm]Brmethylpiperi-[C4MIm]Brdinbromidmethylpiperi-dinbromidmethylpiperi-[C4MIm]Brmethylpiperi-dinbromid e(S) ,(S)e , (S)e , [C4MIm]Clmethylpiperi-methylpiperi-[C4MIm]Cldinbromidmethylpiperi-[C4MIm]Clmethylpiperi-dinbromidmethylpiperi-dinbromid e(C) ,(C)e ,(C) e, 1,1,1 1,1,1 1,1,1[C6MIm]Br1,1,11,1,1[C6MIm]Br-Trimethyl-[C6MIm]BrTrimethyl-umbromid-Trimethyl-TrimethylTrimethylumbromidumbromid (S)- 1-(S)1- -n-e (S)-1-n1,- 1e -tetradecylammoni--[MTA]Br-n-,tetradecylammoni-n ne-[MTA]Br-tetradecylammoni-,[C6MIm]Cl-tetradecylammoni- tetradecylammoni-[MTA]Br[C6MIm]Cl[C6MIm]Cl (C) (C) (C) (S) (S) (S) 1,1,11,1,1 hydrobromide,1,1,1hydrobromide,1,1,11,1,1-hydrobromide,hydrobromide,Trimethyl-Trimethyl--umbromidTrimethyl-TrimethylTrimethylumbromidumbromid-1-1- -n --e1-n1-1,-hexadecylammoni- e-- n-[CTA]Brhexadecylammoni-n,peridinbromide,n -eperidinbromide,[CTA]Br-hexadecylammoni--,hexadecylammoni-hexadecylammoni- peridinbromide,[CTA]Brperidinbromide, (C) (C) (C) iumbromide,1iumbromide,1-niumbromide,-iumbromide,1iumbromide,n-methylpiperi-1iumbromide,-Butyl--n1Butyln---ButylnButyl-Butyl-1-1----1 1-- -1 - iumchloride,1iumchloride,1-n-iumchloride,1iumchloride,niumchloride,-1methylpiperi--Hexyliumchloride,--n1Hexyln---HexylnHexyl-Hexyl-1-1--- -1 1--- 1 - iumbromide,iumbromide,1,1,1iumbromide,iumbromide,iumbromide,iumbromide,-Trimethyl -1 -niumchloride,-iumchloride,tetradecylammoni-iumchloride,iumchloride,iumchloride,iumchloride, 1,1,1-Trimethyl-1-n-hexadecylammoni- [HMPip]Br[HMPip]Br[HMPip]Br[HMPip]Br[HMPip]Br (S) (S) (S) (S) (S) [C2MPip]Br[C2MPip]Br[C2MPip]Br[C2MPip]Br[C2MPip]Br (S) (S) (S) (S) (S) [C4MIm]Br[C4MPip]Br[C4MIm]Brmethylpiperi-dinbromidmethylpiperi-1[C4MPip]Brdinbromid1[C4MIm]Br-[C4MIm]Brmethylpiperi-n-[C4MPip]Brdinbromidmethylpiperi-1[C4MIm]Brndinbromid1-dinbromid-methylpiperi-Butyl-dinbromid1-n-Butyln--n-Butyl-Butyl-Butyl-1- e(S)1 - e(S) ,-(S)- -,1- e1 (S)-(S)e- (S)e 1,- - e,(S) ,(S)- , [C4MIm]Cl [C6MPip]Br[C4MIm]Clmethylpiperi-1dinbromidmethylpiperi-[C6MPip]Br1dinbromid[C4MIm]Cl-[C4MIm]Clnmethylpiperi--1[C6MPip]Brdinbromidmethylpiperi-n[C4MIm]Cl1-dinbromiddinbromid-Hexyl-1methylpiperi--dinbromidn-Hexyln--n-Hexyl-Hexyl-Hexyl- 1-e(C) 1 (C)e-,(S)- -, - 1- e(C) (S)1e-(C) e-, 1 - e ,(C)-,(S) - , 1,1,1 1,1,1 [C6MIm]Br1,1,1[C6MIm]Br1,1,1-[C6MIm]Br1,1,1[C6MIm]BrTrimethyl-[C6MIm]BrTrimethylumbromid-umbromid-TrimethylTrimethylumbromid-umbromidumbromidTrimethylumbromid (S) (S)-1- 1 (S)-e (S)-n e-,1-(S)n ,-1[MTA]Br- e tetradecylammoni- -[MTA]Bre n-e1tetradecylammoni-,ne ,-,[C6MIm]Cl- [MTA]Br -tetradecylammoni-[C6MIm]Cl,[MTA]Brn[MTA]Br tetradecylammoni-[MTA]Br-[C6MIm]Cltetradecylammoni-[C6MIm]Cl[C6MIm]Cl (C) (C) (C) (C) (C) (C) (S) (S) (S) (S) (S)1,1,1 1,1,1 1,1,11,1,1-1,1,1Trimethyl-Trimethylumbromid-umbromid-TrimethylTrimethyl-umbromidumbromidTrimethylumbromidumbromid-1-1-e-n-e1-,n- 1,-[CTA]Brhexadecylammoni- -en-[CTA]Bre1hexadecylammoni-en,-e ,-, [CTA]Br-hexadecylammoni- n,[CTA]Brhexadecylammoni-[CTA]Br -[CTA]Brhexadecylammoni- (C) (C) (C) (C) (C) (C) [C4MPip]Br[C4MPip]Brmethylpiperi-1dinbromidmethylpiperi-[C4MPip]Brdinbromid-1[C4MPip]Br[C4MPip]Brnmethylpiperi--1methylpiperi-[C4MPip]Brdinbromidn-dinbromid1methylpiperi--Butyl1-n-dinbromidButyln--nButyl-Butyl-Butyl-1- e-1(S) -e,(S)-1 -, e1--(S) e 1(S),-(S) ,-(S)e , [C6MPip]Br [C6MPip]Brmethylpiperi-1dinbromidmethylpiperi--1[C6MPip]Brdinbromid[C6MPip]Brn[C6MPip]Brmethylpiperi--1methylpiperi-[C6MPip]Brdinbromidn-1-dinbromidmethylpiperi-Hexyl1-n-Hexyldinbromidn--nHexyl-Hexyl-Hexyl-1- e(S)-1 e,-(S) -1,- e1-(S) -e (S),1(S)- ,(S)e- 1,1,1, 1,1,1 1,1,11,1,11,1,1-Trimethyl-Trimethylumbromid-umbromid-Trimethyl-Trimethyl-umbromidTrimethylumbromidumbromid-1-1-e-n-e,-1-n 1-,-[MTA]Br-e tetradecylammoni--1-[MTA]Brn-etetradecylammoni-,n- ,-[MTA]Brne -tetradecylammoni--[MTA]Brtetradecylammoni-,tetradecylammoni-- tetradecylammoni-[MTA]Br (C) (C) (C) (C) (C) 1,1,11,1,11,1,11,1,11,1,1-Trimethyl-Trimethylumbromid--umbromid-TrimethylTrimethyl-umbromidTrimethylumbromidumbromid-1-1-e-n--e1-n,1- -,-[CTA]Brhexadecylammoni--1e -n-[CTA]Brhexadecylammoni-en,-- ,n-[CTA]Brehexadecylammoni-- hexadecylammoni-[CTA]Br-,hexadecylammoni- [CTA]Br (C) (C) (C) (C) (C)

[C4MPip]Brmethylpiperi-[C4MPip]Brdinbromidmethylpiperi-dinbromid[C4MPip]Brmethylpiperi-[C4MPip]Brmethylpiperi-dinbromid[C4MPip]Brdinbromidmethylpiperi-dinbromid e(S)e , (S) , e (S)e ,(S)e , , ,(S) [C6MPip]Brmethylpiperi-[C6MPip]Br dinbromidmethylpiperi-dinbromid[C6MPip]Brmethylpiperi-[C6MPip]Brmethylpiperi-dinbromid[C6MPip]Brdinbromidmethylpiperi-dinbromid e(S)e ,(S) , e (S)e ,(S)e , , ,1,1,1(S) 1,1,1 1,1,1 1,1,11,1,1-Trimethyl-Trimethylumbromid-umbromidTrimethyl-Trimethyl-umbromidTrimethylumbromidumbromid-1--1e-ne-1,-n- ,1--[MTA]Brtetradecylammoni- e1n-[MTA]Br-etetradecylammoni-,n--e ,,n[MTA]Br tetradecylammoni- -,[MTA]Br[MTA]Br tetradecylammoni--[MTA]Brtetradecylammoni- (C) (C) (C) (C) (C) (C) 1,1,11,1,11,1,11,1,11,1,1-Trimethyl-Trimethyl-umbromidTrimethylumbromid-Trimethyl-Trimethylumbromidumbromidumbromid-1--1-ne-1e-,n-1- -,hexadecylammoni-[CTA]Br1 n-e-[CTA]Brehexadecylammoni-n-,-e ,n,hexadecylammoni--[CTA]Br ,[CTA]Brhexadecylammoni-[CTA]Br- hexadecylammoni-[CTA]Br (C) (C) (C) (C) (C) (C) 11--n1n1---Butyl1-nButyln--n-ButylButyl-Butyl--11---11--1-- 11--n1n1---Hexyl1-nHexyln--n-HexylHexyl-Hexyl--11---11--1-- dinbromid[C4MPip]Brdinbromid[C4MPip]Br[C4MPip]Brdinbromid[C4MPip]Brdinbromiddinbromide,e ,e (S) e, (S) (S)e ,(S) , dinbromid[C6MPip]Brdinbromid[C6MPip]Br[C6MPip]Brdinbromid[C6MPip]Brdinbromiddinbromide,e ,e (S) e, (S) (S)e ,(S) , umbromidumbromidumbromidumbromidumbromide,e [MTA]Br,e [MTA]Bre, e,[MTA]Br ,[MTA]Br [MTA]Br (C) (C) (C) (C) (C) umbromidumbromidumbromidumbromidumbromide,e [CTA]Br,e [CTA]Bre, e,[CTA]Br ,[CTA]Br [CTA]Br (C) (C) (C) (C) (C) [C4MPip]Br[C4MPip]Brmethylpiperi-methylpiperi-methylpiperi- (S) (S) [C6MPip]Br[C6MPip]Brmethylpiperi-methylpiperi-methylpiperi- (S) (S) 1,1,11,1,11,1,1--TrimethylTrimethyl-Trimethyl--11--1-nn--n-tetradecylammoni-tetradecylammoni--tetradecylammoni- 1,1,11,1,11,1,1--TrimethylTrimethyl-Trimethyl--11--1-nn--n-hexadecylammoni-hexadecylammoni--hexadecylammoni- [C4MPip]Brmethylpiperi-methylpiperi-[C4MPip]Br (S) (S) [C6MPip]Brmethylpiperi-methylpiperi-[C6MPip]Br (S) (S) 1,1,11,1,1 --TrimethylTrimethyl--11--nn--tetradecylammoni-tetradecylammoni- 1,1,11,1,1--TrimethylTrimethyl--11--nn--hexadecylammoni-hexadecylammoni- [C4MPip]Br[C4MPip]Br[C4MPip]Br[C4MPip]Br (S) (S) (S) (S) [C6MPip]Br[C6MPip]Br[C6MPip]Br[C6MPip]BrTetra- n(S)- (S) (S) (S) Tetraethylammoni-Tetraethylammoniumbromide,Tetraethylammoni-Tetraethylammoni-Tetraethylammoni-dinbromiddinbromiddinbromidee,,e , , TetradinbromidTetradinbromidTetradinbromidTetra-n---nnbu---nbu-bu--bu-ee,,e , , umbromidumbromidumbromidee,,e , [MTA]Br,[MTA]Br [MTA]Br (C) (C) (C) umbromidumbromidumbromidee,,e , [CTA]Br,[CTA]Br [CTA]Br (C)(C) (C) dinbromiddinbromidee, , dinbromiddinbromidbutylammoniumbromide,ee, , Tetra-umbromidumbromidn-octylammoniumbromide,ee, ,[MTA]Br [MTA]Br [TOA]Br(C) (C) (C) Tetra-umbromidumbromidn-octylammoniumchloride,ee, ,[CTA]Br [CTA]Br[TOA]Cl (C) (C) (C) [TEA]Br (C) TetraTetraTetraTetra-n---nnoctylammoniumbromid--noctylammoniumbromidoctylammoniumbromid-octylammoniumbromide,e e,, e , TetraTetraTetraTetra-n---nnoctylammoniumchlorid--noctylammoniumchloridoctylammoniumchlorid-octylammoniumchloride,e e,, e , umbromid[C4MPip]Br[C4MPip]Brumbromid[C4MPip]Brumbromidumbromide,e e(S) ,(S), e (S), tylammoniumbro- tylammoniumbro-tylammoniumbro-[C6MPip]Brtylammoniumbro-[C6MPip]Br[C6MPip]Br[TBA]Br (C) (S) (S) (S) [C4MPip]Br[C4MPip]Br (S) (S) [C6MPip]Br[C6MPip]Br (S) (S) [TOA]Br[TOA]Br[TOA]Br[TOA]Br (C) (C)(C) (C) [TOA]Cl[TOA]Cl[TOA]Cl[TOA]Cl (C) (C)(C) (C) [TEA]Br[TEA]Br[TEA]Br[TEA]Br (C) (C)(C) (C) midmidmidmide,e e[TBA]Br,, e [TBA]Br[TBA]Br, [TBA]Br (C) (C)(C) (C)

2. Results and Discussion 2.1.2.2. 2.Results 2. StabiltiyResultsResults Results and ofandand and Two-Phase D DDiscussion iscussionDiscussioniscussion Electrolytes and Bromine Binding Strength of BCAs 2.1.2.1.2.1. 2.1.StabiltiyA StabiltiyStabiltiy totalStabiltiy of of ofT 38 of woTT wowoT BCAs-Pwo--hasePP-hasehasePhase areE lectrolytesEE lectrolyteslectrolytesE synthesizedlectrolytes and andand andB romineforBBromine romineBromine this B indingworkBB indingindingBinding (markedS trength SStrength trengthStrength of with of ofBCAs of BCAsBCAs BCAs “(S)” in Table1) or orderedAA Atotal A totaltotal total from of ofof 38 of commercial3838 BCAs38 BCAsBCAs BCAs are areare aresynthesized suppliers synthesizedsynthesized synthesized (marked for forfor forthis thisthis this withwork workwork work “(C)” (marked (marked(marked (marked in Table with withwith with1 ).“ (S)““ The(S) (S)“”(S) ””in component in”in Table in TableTable Table 1) 1)1) or 1) oror or structureorderedorderedorderedordered from of fromfrom thefrom commercial commercial synthesizedcommercial commercial suppliers supplierssuppliers BCAs suppliers is (marked detected (marked(marked (marked with andwithwith with “ confirmed (C) ““(C) (C)“”(C) ”in” in”in Table in TableTable byTable 1) means 1)1). The .1). TheThe. The component of componentcomponent 1componentH NMR struc- and struc-struc- struc- 13tureCtureture NMR.ture of ofof theof thethe NMR thesynthesized synthesizedsynthesized synthesized results ofBCAs BCAs allBCAs BCAs synthesized is is isdetected is detecteddetected detected componentsand andand and confirmed confirmedconfirmed confirmed are availableby byby meansby meansmeans means in of theofof 1ofH 11H SMH NMR1 H NMRNMR (SectionNMR and andand and 213 )C1313 C C13 C

andNMR.NMR.NMR.NMR. are NMR verified NMRNMR NMR results resultsresults onresults the of ofof basisall of allall synthesizedall synthesizedsynthesized of synthesized NMR results components componentscomponents components from the are areare literature areavailable availableavailable available [45 in in–in the51 in thethe]. theSM SMSM SM(Section ((SectionSection (Section 2) 2)2) and 2) andand and areareare areverified verifiedverified verified on onon theon thethe thebasis basisbasis basis of ofof NMR of NMRNMR NMR results resultsresults results from fromfrom from the thethe theliterature literatureliterature literature [45 [45[45 –[455––1]55–1]1].5 .1]. .

2.1.1.2.1.1.2.1.1.2.1.1. Solubility SolubilitySolubility Solubility of ofof [BCA] of [BCA][BCA] [BCA]-S--altsSSalts-altsSalts in inin A in AqueousA queousAqueousqueous HBr HBrHBr HBr S olutions SSolutions olutionsSolutions

ForForFor Fortheir theirtheir their application applicationapplication application on onon theon thethe theH HH2/Br 2H2/Br/Br22/Br-RFB,22--RFB,RFB,2-RFB, t he tthehe tbromidehe bromidebromide bromide or oror chlorideor chloridechloride chloride salts saltssalts salts of ofof theof thethe theBCAs BCAsBCAs BCAs (Table(Table(Table(Table 1) 1)1) need 1) needneed need to toto be to bebe solublebe solublesoluble soluble in inin aqueous in aqueousaqueous aqueous acidic acidicacidic acidic electrolyte electrolyteelectrolyte electrolyte at atat room at roomroom room temperature. temperature.temperature. temperature. This ThisThis This be- be-be- be- haviorhaviorhaviorhavior is is isstudied is studiedstudied studied by byby aby asa imple sasimpleimple simple solubility solubilitysolubility solubility test. test.test. test. For ForFor Forall allall investigatedall investigatedinvestigated investigated BCAs, BCAs,BCAs, BCAs, a asolubilitya solubilityasolubility solubility test testtest test in inin in − − − − electrolyteelectrolyteelectrolyteelectrolyte solutions solutionssolutions solutions with withwith with 1.11 1.111.11 1.11 M MM [BCA]X M [BCA]X[BCA]X [BCA]X (X (X(X = (X =Br= Br=Br Bror− oror −Cl or ClCl )Cl −and)) −andand) and 5.47 5.475.47 5.47 M MM HBr M HBrHBr HBr in inin H in HH2O 2H2O Ois2 O is iscarried is carriedcarried carried outoutout outat atat θ at θθ= θ==23 =2323 ±23 ±±1 ±11°C. °C.1°C. °C.Most MostMost Most of ofof theof thethe theinvestigated investigatedinvestigated investigated substances substancessubstances substances are areare aresoluble solublesoluble soluble in inin thein thethe theelectrolyte. electrolyte.electrolyte. electrolyte. ExceptionsExceptionsExceptionsExceptions are areare arethe thethe thethree threethree three BCAs BCAsBCAs BCAs [CTA]Br, [CTA]Br,[CTA]Br, [CTA]Br, [TOA]Br [TOA]Br[TOA]Br [TOA]Br and andand and [TOA]Cl. [TOA]Cl.[TOA]Cl. [TOA]Cl. These TheseThese These compounds compoundscompounds compounds are areare arenot notnot not completelycompletelycompletelycompletely soluble solublesoluble soluble in inin the in thethe themixture mixturemixture mixture of ofof HBr of HBrHBr HBr and andand and H HH2O 2H2O Odue2 O duedue due to toto their to theirtheir their long longlong long nonpolar nonpolarnonpolar nonpolar alkyl alkylalkyl alkyl side sideside side chains.chains.chains.chains. Therefore, Therefore,Therefore, Therefore, [CTA]Br, [CTA]Br,[CTA]Br, [CTA]Br, [TOA]Br [TOA]Br[TOA]Br [TOA]Br and andand and [TOA]Cl [TOA]Cl[TOA]Cl [TOA]Cl are areare arenot notnot notconsidered consideredconsidered considered as asas BCAs as BCAsBCAs BCAs for forfor foran anan ap-an ap-ap- ap- plicationpplicationlicationplication in inin a in abatterya batteryabattery battery and andand and are areare arenot notnot notinvestigated investigatedinvestigated investigated further furtherfurther further in inin this in thisthis this work. work.work. work. All AllAll Allthe thethe theother otherother other BCAs BCAsBCAs BCAs of ofof of TableTableTableTable 1 1are1 are1are aresoluble solublesoluble soluble bromide bromidebromide bromide or oror chloride or chloridechloride chloride salts saltssalts salts and andand and are areare areinvestigated investigatedinvestigated investigated on onon theiron theirtheir their bromine brominebromine bromine bind- bind-bind- bind- inginging ingstrength strengthstrength strength and andand and solubility solubilitysolubility solubility properties propertiesproperties properties related relatedrelated related to toto the to thethe theaddition additionaddition addition of ofof Br of BrBr2 .Br 22.. 2.

2.1.2.2.1.2.2.1.2.2.1.2. Stability StabilityStability Stability of ofof T of woTT wowoT-woP--hasePP-hasehasePhase E lectrolytesEElectrolytes lectrolytesElectrolytes and andand and B romineBB romineromineBromine B indingBB indingindingBinding S trength SStrength trengthStrength of ofof BCA of BCABCA BCAs ss s TheTheTheThe soluble solublesoluble soluble salts saltssalts salts should shouldshould should form formform form a aaliquid liquidaliquid liquid fused fusedfused fused salt saltsalt salt phase phasephase phase in inin ain aabromine bromineabromine bromine electrolyte electrolyteelectrolyte electrolyte at atat at roomroomroomroom temperature temperaturetemperature temperature, as,, asas , crystallizationas crystallizationcrystallization crystallization of ofof BCA of BCABCA BCA salts saltssalts salts in inin the in thethe thepump pumppump pump or oror cellor cellcell cell would wouldwould would lead leadlead lead to toto re- to re-re- re- ducedducedducedduced volume volumevolume volume rates ratesrates rates and andand and rising risingrising rising parasitic parasiticparasitic parasitic pressures pressurespressures pressures. For.. ForFor. Forthe thethe the35 3535 soluble35 solublesoluble soluble BCA BCABCA BCA salts saltssalts salts, a,, aacom-, com-acom- com- plexationplexationplexationplexation test testtest test in inin electrolyte in electrolyteelectrolyte electrolyte solutions solutionssolutions solutions by byby addingby addingadding adding 1.11 1.111.11 1.11 M MM Br M BrBr2 Brwa22 wawa2 swa sperformed.s performed.performed.s performed. The TheThe The total totaltotal total elec- elec-elec- elec- trolytetrolytetrolytetrolyte mixture mixturemixture mixture corresponds correspondscorresponds corresponds to toto a to amixturea mixtureamixture mixture at atat an at anan SoCan SoCSoC SoC of ofof 33% of 33%33% 33% (5.47 (5.47(5.47 (5.47 M MM HBr, M HBr,HBr, HBr, 1.11 1.111.11 1.11 M MM Br M BrBr2 ,Br 221.11.,, 1.11.21.11., 1.11. [BCA]X[BCA]X[BCA]X[BCA]X in inin H in HH2O) 2H2O)O) 2atO) atat θ at θθ= =θ23= 23=23 ± 23 ±1± 1°C.±1 °C.1°C. °C.The TheThe The SoC SoCSoC SoC value valuevalue value is is ischosen, is chosen,chosen, chosen, as asas it as ititrepresents representsitrepresents represents a a1:1a 1:1a1:1 molar1:1 molarmolar molar ratio ratioratio ratio ofofof Brof BrBr2 andBr22 andand2 and BCA BCABCA BCA in inin the in thethe theelectrolyte. electrolyte.electrolyte. electrolyte. (A (A(A defintion (A defintiondefintion defintion of ofof the of thethe theSoC SoCSoC SoC range rangerange range is is isprovided is providedprovided provided in inin the in thethe theMaterials MaterialsMaterials Materials andandandand M MMethods Methodsethodsethods section sectionsection section.) ..In)) In.In )the In thethe thepresence presencepresence presence of ofof aqueous of aqueousaqueous aqueous polybromides polybromidespolybromides polybromides, the,, thethe, theBC BCBC ABCA Acations A cationscations cations and andand and the thethe the polybromidespolybromidespolybromidespolybromides form formform form either eithereither either an anan orangean orangeorange orange-brown--brownbrown-brown liquid liquidliquid liquid fused fusedfused fused salt saltsalt salt phase phasephase phase or oror crystallize or crystallizecrystallize crystallize in ininto intoto red-to red-red- red- dishdishdishdish-brown--brownbrown-brown or oror orange or orangeorange orange crystals. crystals.crystals. crystals. At AtAt theAt thethe thesame samesame same time, time,time, time, a aparta partapart part of ofof the of thethe theBr BrBr2 Brremains22 remainsremains2 remains in inin the in thethe theaqueous aqueousaqueous aqueous electrolyte,electrolyte,electrolyte,electrolyte, resulting resultingresulting resulting in inin a in ayellowa yellowayellow yellow to toto brown to brownbrown brown-colored--coloredcolored-colored aqueous aqueousaqueous aqueous electrolyte electrolyteelectrolyte electrolyte phase phasephase phase, depending,, dependingdepending, depending ononon theon thethe theapplied appliedapplied applied BCA. BCA.BCA. BCA. The TheThe The c oncentrations cconcentrationsoncentrations concentrations of ofof Br of BrBr2 Brin22 inin 2the in thethe theaqueous aqueousaqueous aqueous solution solutionsolution solution and andand and state statestate state of ofof aggre- of aggre-aggre- aggre- gationgationgationgation of ofof the of thethe thesecond secondsecond second bromine brominebromine bromine-rich,--rich,rich,-rich, heavy heavyheavy heavy phase phasephase phase of ofof all of allall BCAsall BCAsBCAs BCAs are areare areshown shownshown shown in inin Figure in FigureFigure Figure 2. 2. 2.The 2. TheThe The resultsresultsresultsresults in inin the in thethe thefigure figurefigure figure are areare aresorted sortedsorted sorted according accordingaccording according to toto the to thethe thedecreasing decreasingdecreasing decreasing Br BrBr2 Brconcentration22 concentrationconcentration2 concentration in inin the in thethe theaque- aque-aque- aque- ousousousous electrolyte electrolyteelectrolyte electrolyte solution solutionsolution solution, which,, whichwhich, which reflects reflectsreflects reflects the thethe theincreasing increasingincreasing increasing strength strengthstrength strength of ofof the of thethe theBCA BCABCA BCA to toto bind to bindbind bind Br BrBr2 Brin22 inin 2 in thethethe thesecond secondsecond second phase. phase.phase. phase. The TheThe The bromine brominebromine bromine binding bindingbinding binding strength strengthstrength strength of ofof the of thethe theBCA BCABCA BCA is is isdefined is defineddefined defined by byby theby thethe themolar molarmolar molar frac- frac-frac- frac- tiontiontiontion of ofof Br of BrBr2 storedBr22 storedstored2 stored in inin the in thethe thefused fusedfused fused salt saltsalt salt ph phphase phasease asecompared comparedcompared compared to toto the to thethe thetotal totaltotal total amount amountamount amount of ofof Br of BrBr2 inBr22 inin 2the in thethe thesample. sample.sample. sample. FromFromFromFrom left leftleft leftto toto right to rightright right in inin Figure in FigureFigure Figure 2, 2, 2,the 2, thethe thestrength strengthstrength strength of ofof the of thethe theBCA BCABCA BCA in inin bromine in brominebromine bromine binding bindingbinding binding increases increasesincreases increases. A.. Ad-A. d-Ad-d- ditionallyditionallyditionallyditionally, the,, thethe, thesolubility solubilitysolubility solubility of ofof [BCA] of [BCA][BCA] [BCA]+ cations++ cationscations+ cations in inin the in thethe theaqueous aqueousaqueous aqueous phase phasephase phase in inin the in thethe thepresence presencepresence presence of ofof polybro- of polybro-polybro- polybro- − − midesmidesmidesmides Br BrBr2n+1 Br2n+12n+12n+1 decreas− decreasdecreas− decreaseseses fromes fromfrom from left leftleft leftto toto right to rightright right following followingfollowing following Equation EquationEquation Equation 3. 3. 3.Fused 3. FusedFused Fused salts saltssalts salts that thatthat that crystallize crystallizecrystallize crystallize

Molecules 2021, 26, 2721 5 of 26

2.1.1. Solubility of [BCA]-Salts in Aqueous HBr Solutions

For their application on the H2/Br2-RFB, the bromide or chloride salts of the BCAs (Table1) need to be soluble in aqueous acidic electrolyte at room temperature. This behavior is studied by a simple solubility test. For all investigated BCAs, a solubility test in − − electrolyte solutions with 1.11 M [BCA]X (X = Br or Cl ) and 5.47 M HBr in H2O is carried out at θ = 23 ± 1 ◦C. Most of the investigated substances are soluble in the electrolyte. Exceptions are the three BCAs [CTA]Br, [TOA]Br and [TOA]Cl. These compounds are not completely soluble in the mixture of HBr and H2O due to their long nonpolar alkyl side chains. Therefore, [CTA]Br, [TOA]Br and [TOA]Cl are not considered as BCAs for an application in a battery and are not investigated further in this work. All the other BCAs of Table1 are soluble bromide or chloride salts and are investigated on their bromine binding strength and solubility properties related to the addition of Br2.

2.1.2. Stability of Two-Phase Electrolytes and Bromine Binding Strength of BCAs The soluble salts should form a liquid fused salt phase in a bromine electrolyte at room temperature, as crystallization of BCA salts in the pump or cell would lead to reduced volume rates and rising parasitic pressures. For the 35 soluble BCA salts, a complexation test in electrolyte solutions by adding 1.11 M Br2 was performed. The total electrolyte mixture corresponds to a mixture at an SoC of 33% (5.47 M HBr, 1.11 M Br2, 1.11. [BCA]X ◦ in H2O) at θ = 23 ± 1 C. The SoC value is chosen, as it represents a 1:1 molar ratio of Br2 and BCA in the electrolyte. (A defintion of the SoC range is provided in the Materials and Methods section.) In the presence of aqueous polybromides, the BCA cations and the polybromides form either an orange-brown liquid fused salt phase or crystallize into reddish-brown or orange crystals. At the same time, a part of the Br2 remains in the aqueous electrolyte, resulting in a yellow to brown-colored aqueous electrolyte phase, depending on the applied BCA. The concentrations of Br2 in the aqueous solution and state of aggregation of the second bromine-rich, heavy phase of all BCAs are shown in Figure2. The results in the figure are sorted according to the decreasing Br 2 concentration in the aqueous electrolyte solution, which reflects the increasing strength of the BCA to bind Br2 in the second phase. The bromine binding strength of the BCA is defined by the molar fraction of Br2 stored in the fused salt phase compared to the total amount of Br2 in the sample. From left to right in Figure2, the strength of the BCA in bromine binding increases. Additionally, the solubility of [BCA]+ cations in the aqueous phase in − the presence of polybromides Br2n+1 decreases from left to right following Equation (3). Fused salts that crystallize are shown as red bars, labelled by “S“, while stable liquid phases at room temperature are shown as black bars. Figure2 allows an initial classification of the BCAs with regard to the stability of their liquid fused salt phase and their binding strength towards Br2. The length of the alkyl side chain in N-position of BCAs has a decisive influence on the solubility of the cation in the presence of polybromide anions in the aqueous phase and on the stability of the fused salt. As the length of the alkyl side chain increases, bromine concentrations in the aqueous electrolyte phase (c(Br2(aq))) decrease, resulting in a rising binding strength of the BCA versus Br2/polybromides in the fused salt. Already for n-propyl, n-butyl and some BCAs with ethyl side chains in N-position, a concentration of c(Br2(aq)) < 0.1 M is present in the aqueous phase, whereby more than 91 mol% of Br2 is transferred into the fused salt. This increased binding of Br2 may increase the safety of the electrolyte, but it also sustains less Br2 in the aqueous electrolyte. This can lead to mass transport limitations during cell operation. Liquid fused salts with BCAs which are protonated in N-position [HMP]+, [H4MPy]+, [H3MPy]+ and [HMPip]+ show a lower bromine binding strength with Br2 concentrations between 0.22 to 0.33 M, which corresponds to a binding strength of only 70 to 80 mol% versus Br2 in the fused salt. For the right choice of BCA, a compromise between the safety due to strong Br2 binding property and the availability of polybromides in the aqueous electrolyte at the electrode has to be made. The aqueous phase is essential as it transports bromide, polybromides and protons, all required for the electrochemical Molecules 2021, 26, 2721 6 of 26 Molecules 2021, 26, x FOR PEER REVIEW 6 of 28

are shown as redreaction. bars, labelled The concentration by “S“, while values stable of Brliquid2 in aqueousphases at phases room oftemperature the 35 samples are are shown in shown as blackdetail bars. in Figure S1 in the SM.

Figure 2. ConcentrationsFigure 2. Concentrations of Br2 in the of aqueous Br2 in the electrolyte aqueous phaseelectrolyte and aggregatephase and state aggregate of the state fused of salt the (black fused bars = liquid, red bars = solid)salt (black of all bars BCAs = shownliquid, red in Table bars 1= forsolid) a selected of all BCAs state shown of charge in Table (SoC) 1 offor 33% a selected at θ = 23state± of1 ◦ chargeC. The sorting is carried out for(SoC increasing) of 33% at bromine θ = 23 binding± 1 °C. The strengths sorting (BCA is carried complexation out for increasing strength) bromine of the fused binding salt. Non-solublestrengths BCAs are (BCA complexation strength) of the fused salt. Non-soluble BCAs are shown on the right side. The shown on the right side. The binding strength (text in grey color) of the BCA indicates the fraction of Br2 related to 1.11 M binding strength (text in grey color) of the BCA indicates the fraction of Br2 related to 1.11 M Br2 Br bound in the fused salt at an SoC of 33%. 2 bound in the fused salt at an SoC of 33%. On the basis of our studies, we assume that the electron-donating inductive effect Figure 2 allows(+I-effect) an initial influences classification the observed of the trend BCAs in with Figure regard2. The to +I-effect the stability increases of their with rising alkyl liquid fused saltside phase chain and length their towardsbinding strength the nitrogen towards atom Br of2. The the quaternarylength of the ammonium alkyl side and leads to chain in N-position of BCAs has a decisive influence on the solubility of the cation in the higher bromine binding strength with longer alkyl side chain BCAs and lower [BCA]Br2n+1 presence of polybromidesolubility in anions the aqueous in the aqueous electrolyte. phase In detail,and on due the tostability the +I-effect, of the fused the polarity of the salt. As the lengthN atom of the is alkyl reduced, side makingchain increases, the atom bromine increasingly concentrations nonpolar in and the less aqueous soluble in the polar electrolyte phase (c(Br2(aq))) decrease, resulting in a rising binding strength of the BCA HBr/H2O environment as the chain length is increased. In these cases, stronger interactions versus Br2/polybromideswith the polybromides in the fused might salt. Already be given, for while, n-propyl, due to n- reducedbutyl and polarity, some BCAs the interaction with with ethyl side chains− in− N-position,− a concentration of c(Br2(aq)) < 0.1 M− is present in the Br3 , Br5 and Br7 is preferred compared to polar Br ions. To further explain the aqueous phase,differences whereby more in bromine than 91 complexation mol% of Br2 is with transferred increased into chain the fused length, salt. in This particular on N- increased bindingheterocycles, of Br2 may former increase studies the safety on bromine of the interactionelectrolyte, withbut it quaternary also sustains ammonium less ions can 2 − Br in the aqueousbe considered. electrolyte Easton. This can et al.lead [17 to], formass example, transport have limitations calculated during the interaction cell op- of Br3 and − + eration. LiquidBr 5 fusedwith salts different with BCAs BCAs by which DFT calculation are protonated and simultaneously in N-position determined [HMP] , the influence [H4MPy]+, [H3MPyof polybromides]+ and [HMPip] on the+ show hydrogen a lower atoms bromine in the binding region aroundstrength the with nitrogen Br2 con- atom in the BCA centrations betweenby 1H NMR0.22 to spectroscopy. 0.33 M, which The correspond protons ins theto aα binding-positions strength of pyridinium, of only 70 piperidinium to and 80 mol% versuspyrrolidinium Br2 in the fused rings salt. interact For the most right intensively choice of withBCA the, a compromise polybromide between anion [17 ], which has the safety duealso to strong been shownBr2 binding by Lungwitz property et and al. forthe theavailability application of polybromides of polyiodines in [52 the]. aqueous electrolyte at the electrode has to be made. The aqueous phase is essential as it transports bromide,2.1.3. Influencepolybromides of Bromide and protons, or Chloride all required Counter for the of electrochemical the BCA on Bromine reac- Binding Strength tion. The concentration values of Br2 in aqueous phases of the 35 samples are shown in detail in Figure S1 inAqueous the SM. electrolytes containing BCAs with chloride counter ions have an increased + On the basisbromine of our bindingstudies, strengthwe assume compared that the electron to the same-donating [BCA] inductivecations effect with ( bromide+I- counter effect) influencesions. the They observed bind trend bromine in Figure 27.9 to 2. 64.3% The +I more-effect strongly increases in with the fusedrising saltalkyl in comparison + side chain lengthto the towards equivalent the nitrogen [BCA] atomcation of with the quaternary bromide anions. ammonium Next and to [C2MIm]Cl, leads to all further higher bromine[BCA]Cl binding have strength a bromine with lon bindingger alkyl strength side chain of 93 BCAs mol% and or higher, lower [BCA]Br leading to2n+1 a very low Br2 solubility in theconcentration aqueous electrolyte in the aqueous. In detail, electrolyte, due to whichthe +I-effect significantly, the polarity limits theirof the application N in the

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RFB. Based on these results, this study concludes that chloride-based BCAs bind Br2 too strongly, yet a detailed study on the reason for this observation is not within the scope of this research article. It is only known in the literature that bromine forms with chloride mixed − − polyhalides such as ClBr2 or ClBr4 [53–55], which may have different properties when in contact with [BCA]+ cations. An evaluation of the differences between the complexation of polybromides or polyhalides for these electrolytes requires a further study.

2.2. Crystallization Behavior of Fused Salt Phases 2.2.1. Influence of the Organic [BCA]+ Cation Structure on the Crystallization of Fused Salt In order to understand the crystallization behavior of the various BCAs in the form of their fused salts, the structure of the BCA main component and the length of the alkyl side chain in N-position are considered. Crystallization of the fused salts of BCAs with N-hexyl side chains does not occur for all tested BCAs, and in the case of n-butyl substituents it occurs only sporadically. BCAs with an ethyl substituent ([C2MM]+) and 4-methylpyridin- 1-ium compounds tend to crystallize more frequently due to less steric hindrance and a higher localized charge. Apart from [C2MIm]+, [C24MPy]+ and [C44MPy]+, the fused salt of all heteroaromatic BCAs alkylated in the N-position remains liquid at SoC = 33% and room temperature. The stability of their liquid phase at room temperature has also been shown in the literature for Zn/Br2-RFB electrolytes, but based on different electrolyte composition [27,29,30,34]. Reports on the precipitation of heteroaromatic BCAs could not be found previously in the literature. The previously mentioned [C2MIm]+, [C24MPy]+ and [C44MPy]+, however, offer a pronounced symmetry and predominantly short alkyl side chains. Protonated heteroaromatic BCAs ([HPy]Br, [HMIm]Br) without additional alkyl groups at the aromatic ring also tend to crystallize, while [H4MPy]Br and [H3MPy]Br form a liquid fused salt phase. A stable phase exists for [HMP]+ and [HMPip]+. Both of the latter remain liquid, and all stable BCAs protonated in the N-position have comparatively high Br2 concentrations between 0.2 M < c(Br2(aq)) < 0.35 M in the aqueous solution. Their bromine binding strength is between 70 and 80 mol% (Figure2). All examined tetraalkylammonium ([TEA]Br, [TBA]Br and [MTA]Br) are soluble in HBr/H2O but crystallize with polybromides under the given conditions. The symmetry of the BCA molecules leads to crystallization in H2/Br2-RFB electrolytes, which is also reported for this group in Zn/Br2 electrolytes with a different electrolyte composition [42]. In conclusion, organic BCA cations with a high symmetry in structure and short alkyl side chains tend to crystallize in contact with polybromides, while BCAs with long alkyl side chains and/or an unsymmetrical BCA structure stay liquid when forming a fused salt with polybromides.

2.2.2. Polybromide Species in [BCA]Br2n+1 Crystals Beside the structure of BCAs, polybromides are found in the crystals. It raises the question of whether individual polybromides have a decisive influence on the tendency to crystallize with BCA cations. To investigate the reason behind the crystallization, Raman spectra of dry crystals are studied and are shown in Figure S2 of the SM for most of the [BCA]Br2n+1 crystals (except for [TBA]Br and [CTA]Br) and in Figure3a using the example of the BCA [C2MIm]Br. The Raman spectra of all show the same distinct features: a very strong peak in the Raman spectrum at a Raman shift (v)˜ between v˜ = 158–167 cm−1 and a weak peak at v˜ = 180–200 cm−1, which is an adjacent shoulder of the strong peak. An example of the Raman spectrum of [C2MIm]Br3 is shown in Figure3a. Both peaks are characteris- − −1 tic for tribromide ions Br3 due to its symmetrical (v˜ = 163 cm ) and antisymmetrical (v˜ = 198 cm−1) stretching vibration known from [56–58]. The absence of water’s characteris- tic Raman bands, such as v˜ = 1600 cm−1 and between v˜ = 3000 cm−1 and 3800 cm−1 [59–62], ensures that dry crystals have been studied. Peaks of higher polybromides Raman shifts −1 − −1 normally appear at v˜ = 210 cm (Br5 , antisymmetric stretching vibration), v˜ = 253 cm − −1 − (Br5 , symmetric stretching vibration) and v˜ = 270 cm (Br7 , symmetric stretching vibra- Molecules 2021, 26, x FOR PEER REVIEW 8 of 28

molecules leads to crystallization in H2/Br2-RFB electrolytes, which is also reported for this group in Zn/Br2 electrolytes with a different electrolyte composition [42]. In conclusion, organic BCA cations with a high symmetry in structure and short alkyl side chains tend to crystallize in contact with polybromides, while BCAs with long alkyl side chains and/or an unsymmetrical BCA structure stay liquid when forming a fused salt with polybromides.

2.2.2. Polybromide Species in [BCA]Br2n+1 Crystals Beside the structure of BCAs, polybromides are found in the crystals. It raises the Molecules 2021, 26, 2721 question of whether individual polybromides have a decisive influence on the tendency8 of 26 to crystallize with BCA cations. To investigate the reason behind the crystallization, Ra- man spectra of dry crystals are studied and are shown in Figure S2 of the SM for most of the [BCA]Br2n+1 crystals (except for [TBA]Br and [CTA]Br) and in Figure 3a using the ex- tion) according to Chen et al. [56] but are not detected during the investigation of crystals. ample of the BCA [C2MIm]Br. − − Higher polybromides, e.g., Br5 or Br7 are not present in the crystals.

(a) (b)

−1 −1 FigureFigure 3. 3. RamanRaman spectra spectra of of (a (a) )dry dry crystals crystals of of [C2MIm [C2MIm]Br]Br3 including3 including peak peak fitting fitting for for Raman Raman shifts shifts at atṽ =v˜ 165 = 165 cm cm and andṽ = 180 cm−1 and−1 (b) liquid fused salts of [C3MIm]Br2n+1, [C4MIm]Br2n+1 and [C6MIm] Br2n+1 at θ = 23 ± 1 °C each ◦at an SoC of v˜ = 180 cm and (b) liquid fused salts of [C3MIm]Br2n+1, [C4MIm]Br2n+1 and [C6MIm] Br2n+1 at θ = 23 ± 1 C each at an 33% (c(HBr) = 5.47 M, c(Br2)total = c([BCA]Br = 1.11 M). SoC of 33% (c(HBr) = 5.47 M, c(Br2)total = c([BCA]Br = 1.11 M).

TheHowever, Raman higher spectra polybromides of all show seemthe same to be distinct bound infeatures: the liquid a very fused strong salts. peak The spectrain the −1 Ramanof [C3MIm]Br spectrum2n+1 at, [C4MIm]Br a Raman shift2n+1 (andṽ) between [C6MIm]Br ṽ = 2n+1158–are167 shown cm and in Figurea weak3b, peak where at theṽ = 180presence–200 cm of−1 pentabromide, which is an adjacent and probably shoulder heptabromide of the strong is evidencedpeak. An example by the appearance of the Raman of a − − spectrumpeak at v˜ of = 255[C2MIm]Br cm 1. Figure3 is shown3b also in shows Figure an 3a. increase Both peaks in peak are intensity characteristic at v˜ =for 160 tribro- cm 1 − − −1 −1 mide(Br3 ions) with Br increasing3 due to its alkyl symmetrical side chain (ṽ length= 163 cm of the) and BCA, antisymmetrical whereas the identified (ṽ = 198 cm peak) −1 − − stretchingat v˜ = 255 cmvibration(Br5 known/Br7 ) from remains [56–58] constant.. The absence In general, of water the Raman’s characteristic peak shifts Raman of the bandscrystal, such samples as ṽ = are 1600 similar cm−1 and to those between of the ṽ = fused 3000 saltscm−1 and [58]. 3800 With cm an−1 increasing[59–62], ensures side chain that − drylength crystals at an have SoC been of 33%, studied. bromine Peaks is preferentiallyof higher polybromides added as BrRaman3 ions. shift Thiss normally means thatap- −1 − −1 − pearBCAs at withṽ = 210 longer cm (Br side5 , chainsantisymmetric preferentially stretchin formg vibration), a fused salt ṽ = in253 which cm (Br Br35 , symmetricacts as the + stretchingcounter ion vibration) to the [BCA] and ṽ =cation, 270 cm but−1 (Br does7−, symmetric not crystallize stretching due to vibration) steric hindrance according from to Chenthe long et al. alkyl [56] sidebut are chain. not Ondetected the other during hand, the fusedinvestigation salts formed of crystals. from BCAsHigher with polybro- short − − − midesalkyl, side e.g., chains Br5− or are Br7 stable− are not liquid present phases in the when, crystals. due to a mixture of Br3 , Br5 and Br7 , − the fractionHowever, of higher higher polybromides polybromides seem increases to be bound in comparison in the liquid to Br fused3 . Insalts. this The case, spec- the trasteric of [C3MIm]Br hindrance2n+1 of, the [C4MIm]Br polybromides2n+1 and and [C6MIm]Br charge distribution2n+1 are shown prevents in Figure the 3 formationb, where the of presencecrystals. of The pentabromide distribution ofand Br 2probablyin the polybromides heptabromide depends is evidenced on the by BCA the and appearance the specific of ainteractions peak at ṽ = between255 cm−1. BCAFigure and 3b thealso polybromides. shows an increase Haller in peak et al. intensity [63] assume at ṽ that= 160 higher cm−1 (Brpolybromides3−) with increasing are less alkyl strongly side boundchain length by BCAs of the than BCA lower, whereas polybromides the identified or bromide peak ions.at ṽ In summary, symmetric and small BCAs tend to crystallize at room temperature with − Br3 ions because of their higher charge density and lower steric hindrance. Both long side chains of heteroaromatic BCAs and large polybromides do not crystallize at room temperature due to steric hindrance. An exception is [C2Py]Br, which also remains liquid at room temperature. For further consideration and application, therefore, N-alkylated pyridine-1-ium compounds and 1-alkylated 3-methylimidazol-1-ium compounds are of particular interest. With the alkyl side chains ethyl, n-propyl, n-butyl and n-hexyl, they all exhibit a bromine binding strength of more than 80 mol% at an SoC of 33% resulting in a reduced vapor pressure in the aqueous electrolyte solution. In order to compare their properties with well- investigated BCAs such as [C2MP]Br and [C2MM]Br, these two BCA groups are included in further experiments. In this first analysis, the focus has been set on the electrolyte mixture of 1:1 ratio of Br2 to BCA at an SoC of 33%. However, electrolytes at an SoC of 33% only Molecules 2021, 26, 2721 9 of 26

provide a selective insight into the electrolyte system of a flow battery. In the following, the properties of the electrolytes are examined over the entire capacity range as a function of the SoC.

2.2.3. Stability of Liquid Fused Salts within the Whole SoC Range In the first preselection process, mainly heteroaromatic 1-alkylpyridin-1-ium and 1-alkyl-3-methylimidazole-1-ium bromides showed stable liquid fused salt phases at room temperature. This has been investigated for a fixed electrolyte mixture of 1.11 M [BCA]Br, 5.47 M HBr and 1.11 M Br2. To operate the battery with electrolytes containing BCAs, electrolyte stability over the entire SoC range between an SoC of 0% (7.6 M HBr, 1.11 M [BCA]Br and H2O) and an SoC of 100% (1 M HBr; 1.11 M [BCA]Br, 3.35 M Br2 and H2O) is required. Table2 lists the investigated BCAs: [C2Py]Br, [C4Py]Br, [C6Py]Br, [C2MIm]Br, [C3MIm]Br, [C4MIm]Br and [C6MIm]Br, as well as [C2MM]Br (known as [MEM]Br) and [C2MP]Br (known as [MEP]Br), and indicates their usable SoC range for θ = 23 ± 1 ◦C. For different mixtures, it is shown that all BCAs form a second phase over the entire SoC range, except for an SoC of 0%. If Br2 is added to the aqueous solution, a separate fused salt phase is formed immediately. For all 1-alkylpyridin-1-ium compounds investigated for the entire SoC range, all fused salts remain liquid at room temperature over more than three years. With the exception of [C2MIm]Br, all investigated 3-methyl-imidazolium compounds form a liquid fused salt phase at room temperature within the entire SoC range. Electrolyte samples with the aqueous and fused salt phase in the whole SoC range for [C2MIm]Br, [C2Py]Br and [C6Py]Br are shown in Figure4. [C2MP]Br, [C2MM]Br and [C2MIm]Br form a solid crystalline phase with polybro- mides in the lower SoC range (Table2). With these three BCAs, a maximum of 40 to 70% of the capacity range (71.8 to 125.7 Ah L−1) can be used at room temperature, as a crystallization of the fused salt cannot be accepted in the battery during operation. For [C2MM]Br, [C2MP]Br and [C2MIm]Br, dry crystals of the second phase in the lower SoC range are again examined by Raman spectroscopy. In the SoC range in which the fused − salts tend to form crystals, only tribromide Br3 salts of the BCAs are identified in addition to the characteristic peaks for the respective organic cation (Table2). In previous studies, [C2MM]Br and [C2MP]Br have been applied in electrolytes for Zn/Br2-RFB individually or in mixtures [8,28,30,33,38,41–43]. While the liquid fused salts of the Zn/Br2 electrolytes are stable at room temperature, these BCA form crystals for the selected acidic concentration at different SoC of a H2/Br2-RFB electrolyte. [C2MIm]Br, [C2MM]Br and [C2MP]Br are therefore excluded for use at room temperature and lower temperatures in H2/Br2-RFB since the formation of crystals in the cell or during operation can lead to strong overpressure in the system. [C2MM]Br and [C2MP]Br as BCAs are not investigated further in this study. On the other hand, [C2MIm]Br is reintroduced in the next experiments in order to compare its properties to the other derivatives such as [C3MIm]Br, [C4MIm]Br and [C6MIm]Br.

2.3. Bromine Binding Strength of [BCA]+(aq) Cations as a Function of the State of Charge (SoC) The bromine binding strength of the individual BCAs has been determined based on the concentration c(Br2) in the aqueous electrolyte solution compared to the total Br2 concentration at an SoC of 33% for the 35 chosen BCAs in Figure5. Now, the change of the [BCA]+ concentration over the SoC range will be considered to investigate its effect towards the solubility and bromine binding properties for the seven BCAs based on 1-alkyl-1-pyridin-1-ium bromides and the 1-alkyl-3-methylimidazol-1-ium bromides. For the measurement of the BCA concentration in aqueous solution, the intensity of the characteristic peaks with corresponding Raman shifts is investigated. The Raman shifts of the [BCA]+ cations in aqueous solution are listed in Table2. The Raman peaks selected for the evaluation have certain Raman shifts, which are highlighted in Table2 in bold. The designated Raman shifts for the investigated BCAs are in agreement with the literature [64]. Concentrations of [BCA]+ cations in the aqueous phase are shown in Figure5. Molecules 2021, 26, x FOR PEER REVIEW 9 of 28

= 255 cm−1 (Br5−/Br7−) remains constant. In general, the Raman peak shifts of the crystal samples are similar to those of the fused salts [58]. With an increasing side chain length at an SoC of 33%, bromine is preferentially added as Br3− ions. This means that BCAs with longer side chains preferentially form a fused salt in which Br3− acts as the counter ion to the [BCA]+ cation, but does not crystallize due to steric hindrance from the long alkyl side chain. On the other hand, fused salts formed from BCAs with short alkyl side chains are stable liquid phases when, due to a mixture of Br3−, Br5− and Br7−, the fraction of higher polybromides increases in comparison to Br3-. In this case, the steric hindrance of the polybromides and charge distribution prevents the formation of crystals. The distribution of Br2 in the polybromides depends on the BCA and the specific interactions between BCA and the polybromides. Haller et al. [63] assume that higher polybromides are less strongly bound by BCAs than lower polybromides or bromide ions. In summary, symmetric and small BCAs tend to crystallize at room temperature with Br3- ions because of their higher charge density and lower steric hindrance. Both long side chains of heteroaromatic BCAs and large polybromides do not crystallize at room tem- perature due to steric hindrance. An exception is [C2Py]Br, which also remains liquid at room temperature.

For further consideration and application, therefore, N-alkylated pyridine-1-ium compounds and 1-alkylated 3-methylimidazol-1-ium compounds are of particular inter- est. With the alkyl side chains ethyl, n-propyl, n-butyl and n-hexyl, they all exhibit a bro- mine binding strength of more than 80 mol% at an SoC of 33% resulting in a reduced vapor pressure in the aqueous electrolyte solution. In order to compare their properties with well-investigated BCAs such as [C2MP]Br and [C2MM]Br, these two BCA groups are included in further experiments. In this first analysis, the focus has been set on the electrolyte mixture of 1:1 ratio of Br2 to BCA at an SoC of 33%. However, electrolytes at an SoC of 33% only provide a selective insight into the electrolyte system of a flow battery. In the following, the properties of the electrolytes are examined over the entire capacity range as a function of the SoC.

2.2.3. Stability of Liquid Fused Salts Within the Whole SoC Range In the first preselection process, mainly heteroaromatic 1-alkylpyridin-1-ium and 1- alkyl-3-methylimidazole-1-ium bromides showed stable liquid fused salt phases at room temperature. This has been investigated for a fixed electrolyte mixture of 1.11 M [BCA]Br, 5.47 M HBr and 1.11 M Br2. To operate the battery with electrolytes containing BCAs, elec- trolyte stability over the entire SoC range between an SoC of 0% (7.6 M HBr, 1.11 M [BCA]Br and H2O) and an SoC of 100% (1 M HBr; 1.11 M [BCA]Br, 3.35 M Br2 and H2O) is required. Table 2 lists the investigated BCAs: [C2Py]Br, [C4Py]Br, [C6Py]Br, [C2MIm]Br, Molecules 2021, 26, x FOR PEER REVIEW 10 of 28 [C3MIm]Br, [C4MIm]Br and [C6MIm]Br, as well as [C2MM]Br (known as [MEM]Br) and Molecules 2021, 26, 2721[C2MP]Br (known as [MEP]Br), and indicates their usable SoC range for θ = 23 ± 1 °C. 10 of 26

Table 2. Selected BCAs to be investigated over the entire SoC range including the zone in which the second phase is ṽS (Br3−) 165–167 present as a liquid fused salt. Furthermore, characteristic Raman shifts of the [BCA]+ cation are shown, whereby the Raman −

685, 701, 2888, 2946, ṽS (Br5 ) 254 peak with theTable Raman 2. Selected shift in BCAsbold is to used be investigated to determine overSoC the the [BCA]≥ entire 60%+ SoCconcentration. range including In addition, the zone the in whichRaman the shifts second of the phase is present 1sym-Ethylmetrical-1-methylmorpholin stretchingas a liquid vibration fused salt.-1- iumbromide,of Furthermore, the individual characteristic polybromides Raman are shown, shifts ofwhich the2985 [BCA] are used+ cation for the are evaluation shown, whereby in Section the Raman peak ṽS (Br7−) 269 2.5. [C2MM]Brwith the Raman (= [MEM]Br) shift in bold is used to determine the [BCA]+ concentration. In addition, the Raman shifts of the symmetrical stretching vibration of the individual polybromides are shown, which are used for the evaluationṽS (Br3−) in Section165–167 2.5 . SoC Range of 647,Characteristic 1029, 1177, Cat-2883, Polybromide Raman S 5− SoC 0–100% ṽ (Br ) 253–254 BCA StructureBCA / Name Struc- / AbbreviationSoC Range ofLiquid Liquid FusedCharacteristic 2919,ion Raman 2945, Cation 2988, Shifts 3098 / Polybromide Shifts in Aqueous Raman Shifts Solu- in Aqueous 1-Ethylpyridin-1-iumbromide, [C2Py]Br ṽS (Br7−) 269 ture/Name/Abbreviation Fused Salt Salt Raman Shifts/cmcm−1− 1 tionsSolutions/cm / cm−1 −1 ṽS (Br3−) 167–170 − 651, 1030, 1179, 2880, ṽv˜S (Br(Br3 ) )165–169 165–169 S 5− SoC 0–100%899, 1452, 2954, 2964, ṽ (Br −) 254–257 SoC ≥ 30% v˜ (Br ) 255 2914,899, 1452, 2943, 2954, 2976, 2964, 3097 ṽS (Br5−) 255 2989 − MoleculesMolecules1-n 20212021-Butylpyridin,, 26261-Ethyl-1-methylpyrrolidin-,, xx FORFOR PEERPEER-1-iumbromide, REVIEWREVIEW [C4Py]Br SoC ≥ 30% ṽS (Br7 ) 2691010 ofof 2828

1-Ethyl-1-methylpyrrolidin[C2MP]Br-1-iumbromide, (= 2989 − 1-iumbromide, ṽv˜S (Br(Br73−) )164269–169 269 Molecules 2021, 26, [C2MP]Brx FOR PEER[MEP]Br) REVIEW(= [MEP]Br) 649, 1029, 2869, 2910, 10 of 28 SoC 0–100% ṽS (Br5−) 255–256 2939, 2973, 3097 −− 1-n-Hexylpyridin-1-iumbromide, [C6Py]Br ṽv˜SS (Br(Br37− ) )165269–167 165–167 685, 701, 2888, 2946, ṽS (Br3 −) 165–167 SoC ≥ 60% v˜ S (Br5− ) 254 Molecules 2021, 26, x FOR PEER REVIEW 685,2985 701, 2888, 2946, ṽS (Br53− ) 165254–10168 of 28 685, 701, 2888, 2946, ṽS (Br5 ) 254 Molecules 2021, 261-Ethyl-1-methylmorpholin-, x FOR PEER REVIEW SoC ≥ 60% ṽS (Br3−) 165–10167 of 28 1-Ethyl-1-methylmorpholin-1-iumbromide, SoC ≥ 60% 598, 1024,2985 1339, 1419, ṽS (Br5−) 253–254 1-Ethyl-1-methylmorpholin- 1-iumbromide, 2985 − 1-iumbromide, [C2MM]Br SoC ≥ 50% ṽv˜SS (Br(Br7− ) )269 269 Molecules 2021, 26, x FOR PEER REVIEW 685, 701, 2888, 2946, ṽS (Br75 ) 26925410 of 28 Molecules1-Ethyl 2021, 26-3[C2MM]Br,- xmethylimidazol FOR(= PEER [MEM]Br) (=REVIEW [MEM]Br)-1 -iumbromide, SoC ≥ 60% 2961 10 of 28 [C2MM]Br (= [MEM]Br) ṽS (Br7−) 269 1-Ethyl-1-methylmorpholin-1-iumbromide, 2985 − [C2MIm]Br ṽṽSS (Br73−−) 165–167 ṽv˜SS (Br(Br33−) )165269–167 165–167 [C2MM]Br (= [MEM]Br) 647,647,1029 ,1029 1177,, 1177, 2883, 2883, ṽS (Br3− ) 165–167 647, 1029, 1177, 2883, ṽSS (Br353−−) 165–167 ṽ SoC 0–100%SoC 0–100% v˜SS (Br(Br55−−) )253167–254170 253–254 SoC 0–100% ṽ (Br ) 253–254 2919,2919,685, 2945, 701,2945, 2988, 2888, 2988, 3098 2946, 3098 ṽS (Br5− ) 254 ṽS (Br3−−) 165–167 2919, 2945, 2988, 3098 S 5 1-Ethylpyridin1-Ethylpyridin-1--1-iumbromide, [C2Py]Br SoC ≥ 60% 685, 701, 2888, 2946, ṽṽS (Br(Br735−)) 165254269254–167 1-Ethylpyridin-1-iumbromide, [C2Py]Br 647, 1029, 1177, 2883, ṽṽSS (Br(Br73−)) 165269–167 1-Ethyl-1-methylmorpholin-1-iumbromide, SoCSoC 0≥– 100%60% 1023, 1337,2985 1417 , 2961 S 5−− ṽ 1-Ethyl-1-methylmorpholiniumbromide,-1-iumbromide, SoC 0–100% 2985 v˜ S (Br(Br7−−) )253–254 269 685, 701, 2888, 2946, ṽS (Br35 ) 254269 1-n-Propyl -3-methylimidazol-1-iumbromide, 2919, 2945, 2988, 3098 S (Br3−−) 167–170 685, 701, 2888, 2946, ṽṽSS (Br(Br75−)) 167269254–170 1-Ethylpyridin[C2MM]Br-1[C2Py]Br-iumbromide, (= [MEM]Br) [C2Py]Br SoC ≥ 60% 651, 1030, 1179, 2880, ṽṽSS (Br(Br77−)) 269269 1-Ethyl-1[C2MM]Br-methylmorpholin (= [MEM]Br)-1-iumbromide, SoC ≥ 60% 651, 10302985, 1179, 2880, S 5− ṽ [C3MIm]Br SoC 0–100% S (Br5−−) 254–257 1-Ethyl-1-methylmorpholin -1-iumbromide, SoC 0–100% 2985 ṽ (Br ) 254–257 2914, 2943, 2976, 3097 ṽS (Br73−−) 165269–167 2914, 2943, 2976, 3097 ṽṽv˜SS (Br(Br(Br337−−)) )167165269–170167 167–170 1-n-Butylpyridin[C2MM]Br-1-iumbromide, (= [MEM]Br) [C4Py]Br 651,651,647,1030 1030,1029 1179,,, 1179,1177, 2880, 2880,2883, ṽS (Br73−−) 165269–170 1-n-Butylpyridin[C2MM]Br-1-iumbromide, (= [MEM]Br) [C4Py]Br 647, 1029, 1177, 2883, ṽSS (Br75−−) 269 SoC 0–100%SoC 0–100% ṽv˜SS (Br(Br55− ) )253–254 254–257 SoC 0–100% ṽS (Br5 −) 254–257 ṽ − SoC 0–100%2914, 2919, 2943, 2945, 2976, 2988, 3097 3098 ṽS (Br(Br35−)) 253164165254––254169167255 1-n-Butylpyridin-1- 2914,2919, 2943,2945, 2976,2988, 30973098 ṽṽSS (Br(Br33−)) 164165––169167 1-1n--EthylpyridinButylpyridin--11--iumbromide,iumbromide, [C2Py]Br [C4Py]Br SoC 0–100% 1024,649,647, 1029 1339,, 2869,1177, 1419 ,2910,2883, 2963 ṽṽSS (Br(Br77−)) 269269 1-Ethylpyridin-1-iumbromide, [C2Py]Br 649,647, 10291029,, 2869,1177, 2910,2883, ṽSS (Br75−−) 269 1-n-Butyl-3-methylimidazol -1-iumbromide, SoC 0–100% ṽS (Br5 ) 253–254 iumbromide, SoC 0–100% v˜SS (Br(Br575−−) )255–256 269 SoCSoC 00––100%100% 2919,2939, 2945, 2973, 2988, 3097 3098 ṽṽS (Br(Br73−)) 255253167269––256254170 [C4Py]Br 2919,2939, 2945, 2973, 2988, 3097 3098 ṽS (Br3−−) 164167–169170 1-1n--EthylpyridinHexylpyridin[C4MIm]Br-1--1iumbromide,-iumbromide, [C2Py]Br [C6Py]Br 649,651, 10291030,, 2869,1179, 2910,2880, ṽS (Br7−−) 269 1-1n--EthylpyridinHexylpyridin-1-1-iumbromide,-iumbromide, [C2Py]Br [C6Py]Br 651, 1030, 1179, 2880, ṽṽSS (Br(Br775−−)) 269269 SoC 0–100% ṽ (Br ) 254–257 SoC 0–100% ṽS (Br5−−) 255–256 ṽ − SoC 0–100% 2914, 2943, 2976, 3097 ṽv˜S (Br(Br(Br3−)) )254165167164––257168170167 164–169 2914,2939, 2943, 2973, 2976, 3097 3097 ṽṽSS (Br(Br33−)) 165167––168170 11--nn--HexylpyridinButylpyridin-1-iumbromide, [C6Py]Br[C4Py]Br 649,651,1029 ,1030 2869,, 1179, 2910, 2880, ṽṽSS (Br(Br77−−)) 269269 1-n-Butylpyridin-1-iumbromide, [C4Py]BrSoC 0–100% 651, 1030, 1179, 2880, ṽv˜SS (Br(Br75−) )269 255–256 SoC 0–100% ṽS (Br5 ) 254–257 2939,598,1025, 2973, 1024, 1122, 3097 1339, 1341, 1419 , SS (Br55−−) 253253–254 SoCSoC 0 ≥– 100%50% 598,2914, 1024, 2943, 1339, 2976, 1419 3097, ṽṽS (Br(Br3−)) 253254164––254257169 1-n-Hexylpyridin-1- SoC ≥ 50% 2914, 2943, 2976, 3097 ṽS (Br3−−) 165–168 11-1n--nEthyl-Hexyl-Butylpyridin-3--3methylimidazol-methylimidazol-1-iumbromide,-1--1iumbromide,-iumbromide, [C4Py]Br 649, 102914182961, ,2869, 2961 2910, ṽS (Br(Br7−)) 164269–169 1-nEthyl-Butylpyridin-3-methylimidazol-1-iumbromide,-1-iumbromide, [C4Py]Br 2961 ṽS (Br7−−) 269 iumbromide, SoC 0–100% 649, 1029, 2869, 2910, ṽv˜SS (Br(Br(Br75− )) )255269–256 269 SoC 0–100% 598, 1024, 1339, 1419, ṽS (Br75−−) 253255269–254256 [C2MIm]Br[C6MIm]Br SoC ≥ 50% 2939, 2973, 3097 ṽS (Br3−) 164–169 1-n-Hexylpyridin[C2MIm]Br[C6Py]Br-1-iumbromide, [C6Py]Br 649,2939, 1029 2973,, 2869, 3097 2910, ṽS (Br37−) 164269–169 11--nEthyl-Hexylpyridin-3-methylimidazol-1-iumbromide,-1-iumbromide, [C6Py]Br 649, 10292961, 2869, 2910, ṽS (Br7−−) 269 SoC 0–100% ṽṽSS (Br(Br735−−)) 167255269–170256 SoC 0–100% 2939, 2973, 3097 ṽṽv˜SS (Br(Br(Br335−)) )167255165––170256168 165–168 [C2MIm]Br S 3−− 1-n-Hexylpyridin-1-iumbromide,For [C6Py]Brdifferent mixtures, it is shown598, that 1024,2939, all 1339, BCAs 2973,1419 form 3097, a secondṽṽS (Br(Br phase57−−)) over165 the254269–168 entire 1-n-Hexylpyridin-1-iumbromide, [C6Py]BrSoC ≥ 50% ṽṽv˜SS (Br(Br(Br557−)) )254269 253–254 SoC 0–100% 1023,598,2961 1024, 1337, 1339, 1417 ,1419 2961, ṽṽSS (Br(Br35−)) 167253––170254 1-n-Propyl1-Ethyl-3-methylimidazol-1- -3-methylimidazolSoC - 1range,-iumbromide, except for anSoC SoCSoC 0 of–≥100% 50% 0%. If 1023,Br598,2 is 1024, 1337,added 1339, 1417 to ,the1419 2961 aqueous, ṽṽSS (Br(Br solution,53−)) 253165 a separate––254168 1-n-Propyl -3-methylimidazol-1-iumbromide, SoC ≥ 50% ṽS (Br3−) 165–168 1-Ethyl-3-methylimidazol-1-iumbromide, 2961 ṽṽSS (Br(Br57−−)) 254269 1-Ethyl-3-methylimidazoliumbromide,fused-1- iumbromide, salt phase is formed immediately. For all 1-2961alkylpyridin -1-ṽiumv˜SS (Br(Br compounds77−) )269 investi- 269 [C3MIm]Br SoC 0–100% 1023,598, 1024,1337, 1339,1417, 14192961, ṽS (Br5−−) 253269–254 [C3MIm]Br[C2MIm]Br SoC ≥ 50% 598, 1024, 1339, 1419, ṽṽSS (Br(Br75 )) 253269–254 1-n-Propyl -3-methylimidazol[C2MIm]Brgated-1- iumbromide,for the entire SoC range, all fused salts remain liquid at room −temperature over 1-Ethyl-3-methylimidazol[C2MIm]Br- 1-iumbromide, SoC ≥ 50% 2961 ṽS (Br3− ) 165–170 1-Ethyl-3-methylimidazol-1-iumbromide, 2961 ṽS (Br37 −) 165269–170 ṽS (Br73−) 167269–170 [C3MIm]Brmore than three years. With the exception of [C2MIm]Br, all investigatedṽv˜SS (Br37−− ) 3-methyl-imid- 167–170 [C2MIm]Br ṽSS (Br(Br53−)) 167254269–170255 [C2MIm]Br ṽS (Br5 −) 254–255 azolium compoundsSoC form 0–100%SoC a liquid 0–100% fused1023, 1024,salt 1337, phase 1339,1417 , at1419 2961 room, 2963 temperature ṽṽv˜SS (Br(Br(Br35−)) )within165 the254–170 entire 254 1-n-Butyl-3-methylimidazol-1 -iumbromide, SoCSoC 00––100%100% 1024,1023, 1339,1337, 14191417,, 29632961 ṽṽSS (Br(Br53−)) 167254–170 1-n-Butyl-3-methylimidazol-1-iumbromide, ṽS 3− 1-n-Propyl -3-methylimidazol1-n-Propyl -1-iumbromide, SoC 0–100% 1023, 1337, 1417, 2961 ṽS (Br7− ) 167269–170 1-n-Propyl -3-methylimidazolSoC-1 range.-iumbromide, Electrolyte samples with the aqueous and fused saltṽ phaseS (Br75 − )in the 254whole269–255 SoC [C4MIm]Br SoC 0–100% 1024, 1339, 1419, 2963 ṽS (Br57−−) 254269 -3-methylimidazol-1-[C4MIm]Br[C3MIm]Brrange for [C2MIm]Br, [C2Py]BrSoC 0– and100% [C6Py]Br 1023, are1337, shown 1417, in2961 Figure ṽṽSS 4.(Br(Br 75 −)) 269254 1-n-Butyl-3-methylimidazol-1-iumbromide, v˜ S (Br7− ) 269 1-n-Propyl -3-methylimidazol[C3MIm]Briumbromide, -1-iumbromide, SoC 0–100% 1023, 1337, 1417, 2961 ṽS (Br3− ) 164–167 1-n-Propyl -3-methylimidazol-1-iumbromide, ṽS (Br37 −) 164269–167 ṽS (Br73−) 165269–170 [C4MIm]Br[C3MIm]Br ṽṽSS (Br(Br37−−)) 165269–170 [C3MIm]Br 1025, 1122, 1341, ṽS (Br5− ) 253 [C3MIm]Br SoC 0–100% 1025, 1122, 1341, ṽṽSS (Br(Br55−−)) 254253–255 SoC 0–100% ṽS (Br35−−) 164254–167255 1-n-Hexyl-3-methylimidazol-1-iumbromide, SoC 0–100% 1024, 14181339,, 29611419 , 2963 ṽv˜S (Br(Br3−) )165–170 165–170 11-n-n-Hexyl-Butyl-3-methylimidazol-1-iumbromide,iumbromide, SoC 0–100% 1024, 14181339,, 29611419 , 2963 ṽS (Br3−−−) 165–170 1025, 1122, 1341, ṽṽSS (Br(Br57−−)) 253269 1-n-Butyl-3-methylimidazol-1 -iumbromide,SoC 0–100% 1024, 1339, 1419, 2963 ṽṽv˜SS (Br(Br(Br757−)) )254269269–255 254–255 [C6MIm]Br SoC 0–100% ṽṽSS (Br(Br75−−)) 254269–255 1-n-Hexyl1--3n-methylimidazol-Butyl-3-methylimidazol-[C6MIm]Br[C4MIm]Br -1-iumbromide, SoC 0–100% 1024, 14181339,, 29611419 , 2963 1-n-Butyl-3-methylimidazol[C4MIm]Br -1-iumbromide, SoC 0–100% 1024, 1339, 1419, 2963 −− 1-n-Butyl-3-methylimidazol-1-iumbromide, ṽS (Br7 −) 269 1-iumbromide, ṽv˜S (Br(Br73−) )164269–167 269 [C6MIm]Br[C4MIm]Br For different mixtures, it is shown that all BCAs form a secondṽṽSS (Br(Br phase37−)) over164 the269–167 entire [C4MIm]Br[C4MIm]Br For different mixtures, it is shown that all BCAs form a second phase− over the entire 1025, 1122, 1341, ṽS (Br5− ) 253 21025, 1122, 1341, ṽS (Br5 −) 253 SoCSoC range,range, exceptexcept forfor anan SoCSoC 0ofof– 100% 0%.0%. If If BrBr2 isis addedadded toto thethe aqueousaqueousṽS (Br solution,solution,3−) 164aa separateseparate–167 1-n-Hexyl-3-methylimidazol-1-iumbromide, SoC 0–100% 1418, 2961 ṽv˜S (Br(Br3 ) )164–167 164–167 1-n-Hexyl-3-methylimidazolfused-1 For-iumbromide, salt different phase is mixtures formed ,immediately. it is shown1025, that For 1122, all all1418 BCAs1341, 1-alkylpyridin, 29611418 form, a second-1-ium phase compounds− over the investi- entire fused salt phase isSoC formed 0–100% immediately. For1025, all 1 -1122,alkylpyridin 1341, -1-iumṽv˜S (Br(Br compounds57−−) )253269 investi- 253 [C6MIm]BrSoC range, except for an SoC 0of–100% 0%. If Br2 1025,is2961 added 1122, to 1341, the aqueousṽṽSS (Br(Br solution,75 )) a 269253separate 1-n-Hexyl-3-methylimidazol[C6MIm]Br1-n-Hexyl-3-gatedgated -1- iumbromide,forfor thethe entireentire SoCSoC rangeSoCrange 0,–, all100%all fusedfused saltssalts1418 remainremain, 2961 liquidliquid atat roomroom temperaturetemperature overover 1-n-Hexyl-3-methylimidazol-1-iumbromide, 1418, 2961 − methylimidazol-1-fusedmore thansalt phase three isyears. formed With immediately. the exception For of all [C2MIm]Br, 1-alkylpyridin all investigated-1-iumṽS (Br compounds7−) 3-methyl269 investi--imid- [C6MIm]Brmore than three years. With the exception of [C2MIm]Br, all investigatedṽv˜S (Br(Br7 −) ) 3-methyl269-imid- 269 [C6MIm]Briumbromide,gated For for different the entire mixtures SoC range, it is, shownall fused that salts all BCAsremain form liquid a second at roomS phase7 temperature over the entireover azoliumazoliumFor compoundsdifferentcompounds mixtures formform aa, itliquidliquid is shown fusedfused that saltsalt all phasephase BCAs atat formroomroom a temperaturetemperature second phase withinwithin over thethe entireentire [C6MIm]BrmoreSoC range,than three except years. for Withan SoC the of exception 0%. If Br of2 is [C2MIm]Br, added to the all aqueousinvestigated solution, 3-methyl a separate-imid- SoCSoC range.range,range.For different ElectrolyteElectrolyteexcept mixtures for ansamplessamples SoC, it isof with withshown 0%. thetheIf thatBr aqueousaqueous2 isall added BCAs andand toform fusedfused the a aqueous secondsaltsalt phasephase phase solution, inin thetheover wholewholea the separate entire SoCSoC azoliumrangefusedFor forsalt compoundsdifferent [C2MIm]Br, phase is mixtures formed form [C2Py]Br a immediately., liquidit is andshown fused [C6Py]Br that saltFor allphaseall are BCAs1- alkylpyridinshown at roomform in a temperatureFigure second-1-ium 4. phase compounds within over thethe investi- entireentire rangefusedSoC range, saltfor [C2MIm]Br, phase except is formedfor [C2Py]Br an SoCimmediately. of and 0%. [C6Py]Br If BrFor2 isall areadded 1- alkylpyridinshown to the in Figure aqueous-1-ium 4. compoundssolution, a separate investi- SoCSoCgated range.range, for the Electrolyteexcept entire for SoC ansamples rangeSoC of ,with all0%. fused the If Braqueous 2salts is added remain and to fused liquidthe aqueoussalt at phaseroom solution, intemperature the whole a separate overSoC gatedfused saltfor thephase entire is formed SoC range immediately., all fused For salts all remain1-alkylpyridin liquid -at1- iumroom compounds temperature investi- over rangefusedmore forthansalt [C2MIm]Br, phase three isyears. formed [C2Py]Br With immediately. the and exception [C6Py]Br For of all [C2MIm]Br,are 1- alkylpyridinshown in all Figure investigated-1-ium 4. compounds 3-methyl investi--imid- moregated than for thethree entire years. SoC With range the, exceptionall fused saltsof [C2MIm]Br, remain liquid all investigated at room temperature 3-methyl-imid- over gatedazolium for compounds the entire SoCform range a liquid, all fused fused salt salts phase remain at room liquid temperature at room temperature within the entire over azolium compounds form a liquid fused salt phase at room temperature within the entire moreSoC range. than three Electrolyte years. Withsamples the withexception the aqueous of [C2MIm]Br, and fused all investigatedsalt phase in 3the-methyl whole-imid- SoC SoC range. Electrolyte samples with the aqueous and fused salt phase in the whole SoC azoliumrange for compounds [C2MIm]Br, form [C2Py]Br a liquid and fused [C6Py]Br salt phase are shown at room in temperatureFigure 4. within the entire range for [C2MIm]Br, [C2Py]Br and [C6Py]Br are shown in Figure 4. SoC range. Electrolyte samples with the aqueous and fused salt phase in the whole SoC range for [C2MIm]Br, [C2Py]Br and [C6Py]Br are shown in Figure 4.

Molecules 2021, 26Molecules, x FOR PEER2021 ,REVIEW26, 2721 11 of 28 11 of 26

Molecules 2021, 26, x FOR PEER REVIEW 12 of 28 Figure 4. Photos of electrolyte samples with different BCAs [C2MIm]Br, [C2Py]Br and [C6Py]Br as a function of SoC for an Figure 4. Photos of electrolyte samples with different BCAs [C2MIm]Br, [C2Py]Br and [C6Py]Br as SoC of 0, 10, 20, 30, 33, 40, 50, 60, 66, 70, 80, 90 and 100%. For electrolytes with [C2MIm]Br, between 0 < SoC < 60%, the a function of SoC for an SoC of 0, 10, 20, 30, 33, 40, 50, 60, 66, 70, 80, 90 and 100%. For electrolytes second electrolyte phase is shown to precipitate and to form crystals. For [C2MIm]Br at SoC ≥ 60%, [C2Py]Br, as well as with [C2MIm]Br, between 0 < SoC < 60%, the second electrolyte phase is shown to precipitate and + [C6Py]Br into the form entire crystals. SoC range,[BCA] For [C2MIm]Br the cations formation in at aqueous SoC of a ≥ liquid 60% solution, [C2Py]Br, fused salt are as phase listed well occurs asin [C6Py]BrTable at room 2. Thein temperature.the Raman entire SoCpeaks With selected increasing for the SoC, the volumesrange, of the the formation fusedevaluation salt of phase a liquid have increase. fused certain In salt particular Ramanphase occurs forshift the sat, application whichroom temperatu are of highlighted [C6Py]Br,re. With the increasingin coloration Table 2 SoC ofin the,bold. aqueous The des- phase from yellowthe volumes to brown of ignatedthe shows fused that Raman salt the phase bromine shifts increase. for concentrations the In investigatedparticular increase for theBCAs with application increasing are in agreement of SoC. [C6Py]Br, In addition, with the col-the the literature SoC range [64]. is specified byoration total concentrationsof the aqueousConcentrations forphase an SoCfrom of of yellow[BCA] 0% and to+ cations 100%.brown showsin the thataqueous the bromine phase concentrationsare shown in Figurein- 5. crease with increasing SoC. In addition, the SoC range is specified by total concentrations for an SoC of 0% and 100%.

[C2MP]Br, [C2MM]Br and [C2MIm]Br form a solid crystalline phase with polybro- mides in the lower SoC range (Table 2). With these three BCAs, a maximum of 40 to 70% of the capacity range (71.8 to 125.7 Ah L−1) can be used at room temperature, as a crystal- lization of the fused salt cannot be accepted in the battery during operation. For [C2MM]Br, [C2MP]Br and [C2MIm]Br, dry crystals of the second phase in the lower SoC range are again examined by Raman spectroscopy. In the SoC range in which the fused salts tend to form crystals, only tribromide Br3- salts of the BCAs are identified in addition to the characteristic peaks for the respective organic cation (Table 2). In previous studies, [C2MM]Br and [C2MP]Br have been applied in electrolytes for Zn/Br2-RFB individually or in mixtures [8,28,30,33,38,41–43]. While the liquid fused salts of the Zn/Br2 electrolytes are stable at room temperature, these BCA form crystals for the selected acidic concentration at different SoC of a H2/Br2-RFB electrolyte. [C2MIm]Br, [C2MM]Br and [C2MP]Br are therefore excluded for use at room temperature and lower

temperatures in H2/Br2-RFB since the formation of crystals in the cell or during operation can lead to strong(a) overpressure in the system. [C2MM]Br and [C2MP]Br(b )as BCAs are not investigated further in this study. On the other hand, [C2MIm]Br is reintroduced in the Figure 5. Concentration of [BCA]++ cations in the aqueous electrolyte phase containing (a) 1-alkylpyridine-1-ium bromides Figure 5. Concentrationnext experiments of [BCA] incations order in to the compare aqueous electrolyteits properties phase to containing the other (a) derivatives 1-alkylpyridine-1-ium such as bromides and (b) 1-alkyl-3-methylimidazole-1-ium bromides used as BCAs with different alkyl radicals in N-position as a function and (b) 1-alkyl-3-methylimidazole-1-ium[C3MIm]Br, [C4MIm]Br and bromides [C6MIm]Br. used as BCAs with different alkyl radicals in N-position as a function of ofthe the state state of of charge charge (SoC). (SoC All). All values values of of Figure Figu5re are 5 are printed printed in thein the SM SM in Tablein Table S1. S1. 2.3. Bromine Binding Strength of [BCA]+(aq) Cations as a Function of the State of Charge (SoC) WhenWhen the the BCA BCA is is added added to to the the polybromide polybromide solution solution the the solubility solubility equilibrium equilibrium leads leads The brominetoto a a separationbinding separation strength of of [BCA]Br [BCA]Br of the2n+12n+1 individual saltssalts by by forming BCAs forming has a fused abeen fused saltdetermined salt phase phase according based according on to toEquation Equation 3, the concentrationor(3),, as or,c(Br in asthe2) in case the ofaqueous case [C2MIm]Br of [C2MIm]Br, electrolyte, to the solution toformation the formation compared of crystals. of to crystals. the The total phase TheBr 2separation con- phase separation leads to centration at analeads decrease SoC to of a 33% decrease in thefor concentrationthe in 35 the chosen concentration BCAs of [BCA] in ofFigure+(aq) [BCA] regardless 5.+ Now,(aq) regardless the of change the [BCA] of of the the+ cation [BCA] + typecation as [BCA]+ concentrationshowntype as in shownover Figure the in 5. SoC FigureThe rangeconcentration5. The will concentration be considereddependence dependence to of investigate the [BCA] of the+ itscations [BCA]effect on to-+ thecations SoC shows on the + wards the solubilityaSoC characteristic shows and bromine a characteristic curve. binding At an curve. SpropertiesoC of At 0% an andfor SoC the thus of seven 0%c(Br and BCAs2) = thus0 M based, c(Bra concentration 2on) = 1 0-alkyl M, a- concentration of c([BCA] ) 1-pyridin-1-ium=of 1.11 c([BCA] bromides M is present,+) = and 1.11 thewhich M is1-alkyl present,decreases-3-methylimidazol which for all decreases BCAs -at1- ium forSoC all > bromides 0%. BCAs Especially at. SoC For >the for 0%. 3-methylim- Especially measurementidazol forof the 3-methylimidazol-1-ium- 1BCA-ium concentration derivatives, a inlinear derivatives,aqueous and BCAsolution, a- linearindependent the and intensity BCA-independent decrease of th ein charac- the [BCA] decrease+ concen- in the teristic peaks tration[BCA]with corresponding +betweenconcentration 0 ≤ SoC Raman between ≤ 20% shifts can 0 ≤be isSoC observedinvestigated.≤ 20% (Figure can The be 5b). observedRaman For p shiftsyridin (Figure of-1 -5theiumb). Forderivatives pyridin-, the same trend is observed between 0 ≤ SoC ≤ 10% in Figure 5a. Between 20 < SoC < 66%, the concentration of BCA continues to decrease for both groups of substances, although these changes become smaller as the SoC increases. In this range, the different bromine

binding strength and solubility of the [BCA]+ cation with respect to the polybromide spe- cies become visible. [C6Py]Br and [C6MIm]Br show a considerably lower concentration in aqueous solution and therefore solubility as well as their property to bind bromine more strongly, respectively, than that of BCAs with shorter alkyl side chains. Organic cat- ions of [C6Py]Br and [C6MIm]Br are no longer detectable in the solution even at SoCs of 50% and 40%, respectively. [C4Py]Br, [C4MIm]Br and [C3MIm]Br follow this trend at higher SoCs. [C2Py]Br and [C2MIm]Br show the lowest bromine binding strength versus polybromides. Thus, higher concentrations of [C2Py]+ and [C2MIm]+ remain in aqueous electrolytes at a higher SoC. From SoC ≥ 70% on, [C2Py]+ and [C2MIm]+ are not detectable in the Raman spectrum. Experimentally determined detection limits for [BCA]+ cations in aqueous solution are listed in Table 3/line 1. Due to an increasing total Br2 concentration with an increasing SoC, the [BCA]+ cations are gradually transferred into the fused salt phase. In principle, the SoC range can be divided into two sections: Depending on the [BCA]+ cation, there is a section for lower SoC values where [BCA]+ cations are present in the aqueous solution, and a section for higher SoC values, where [BCA]+ cations are not present in the aqueous solution but are completely present in the fused salt phase.

Molecules 2021, 26, 2721 12 of 26

1-ium derivatives, the same trend is observed between 0 ≤ SoC ≤ 10% in Figure5a. Between 20 < SoC < 66%, the concentration of BCA continues to decrease for both groups of substances, although these changes become smaller as the SoC increases. In this range, the different bromine binding strength and solubility of the [BCA]+ cation with respect to the polybromide species become visible. [C6Py]Br and [C6MIm]Br show a considerably lower concentration in aqueous solution and therefore solubility as well as their property to bind bromine more strongly, respectively, than that of BCAs with shorter alkyl side chains. Organic cations of [C6Py]Br and [C6MIm]Br are no longer detectable in the solution even at SoCs of 50% and 40%, respectively. [C4Py]Br, [C4MIm]Br and [C3MIm]Br follow this trend at higher SoCs. [C2Py]Br and [C2MIm]Br show the lowest bromine binding strength versus polybromides. Thus, higher concentrations of [C2Py]+ and [C2MIm]+ remain in aqueous electrolytes at a higher SoC. From SoC ≥ 70% on, [C2Py]+ and [C2MIm]+ are not detectable in the Raman spectrum. Experimentally determined detection limits for [BCA]+ cations in aqueous solution are listed in Table3/line 1. Due to an increasing total Br 2 concentration with an increasing SoC, the [BCA]+ cations are gradually transferred into the fused salt phase. In principle, the SoC range can be divided into two sections: Depending on the [BCA]+ cation, there is a section for lower SoC values where [BCA]+ cations are present in the aqueous solution, and a section for higher SoC values, where [BCA]+ cations are not present in the aqueous solution but are completely present in the fused salt phase.

+ Table 3. Detection limits and detection range for [BCA] in the aqueous electrolyte phase, maximum fractions of Br2 in the aqueous phase compared to the global concentration and c(Br2)aq for an SoC of 33%.

[BCA]+ [C2Py]+ [C4Py]+ [C6Py]+ [C2MIm]+ [C3MIm]+ [C4MIm]+ [C6MIm]+ SoC limit for detectable ≤70 ≤66 ≤40 ≤60 ≤50 ≤40 ≤33 [BCA]+/% [a] [b] c(Br2)(aq)/c(Br2)total/% ≤10.1 ≤10.3 ≤8.5 ≤10.4 ≤10.0 ≤10.6 ≤9.5 C(Br )(aq) at an SoC of 2 121 51 9 105 63 34 8 30%/mM [a] The SoC value corresponds to the highest SoC at which [BCA]+ cations can still be detected in the aqueous phase. [b] Highest ratio of Br2 concentration in the aqueous phase versus the total Br2 concentration in the sample detected at an SoC of 90 or 100%.

The individual, BCA-dependent bromine binding strength is determined from the [BCA]+ concentrations in the range of 20 ≤ SoC ≤ 60% and is applicable to investi- gate the solubility behavior of the BCA. The BCAs can be sorted according to their concentration in the aqueous phase. Low concentrations of [BCA]+(aq) in the aqueous phase describe a larger bromine binding strength and vice versa. The bromine binding strength is obtained in the following order: [C6MIm]+(aq) ≥ [C6Py]+(aq) > [C4MIm]+(aq) > [C4Py]+(aq) > [C3MIm]+(aq) > [C2MIm]+(aq) > [C2Py]+(aq). Correspondingly, the solubil- ity of [BCA]Br2n+1 in the aqueous electrolyte for the 1-alkylpyridine-1-ium polybromides and for the 1-alkyl-3-methylimidazole-1-ium polybromides decreases with an increasing length of the alkyl side chain. The detection limits of [BCA]+ cations in Table3 show that 1-alkyl-3-methylimidazol-1-ium is a slightly stronger bromine binding BCA than 1-alkylpyridin-1-ium if both have the same alkyl group in the N-position. As mentioned earlier, the bromine binding strength cannot be the only criteria to choose a BCA. Moderate Br2 concentrations in the aqueous phase along the whole SoC range are a prerequisite for the operability of the cell and will be investigated in the next section.

2.4. Effect of the Solubility Equilibrium on the Br2(aq) Concentration for the Whole SoC Range

Br2 is transported from the tank into the RFB half cell as an ingredient of the aqueous solution. Moderate Br2 concentrations dissolved as polybromide ions in the aqueous electrolyte phase are a prerequisite for stable cell operation. The bromine concentrations in the aqueous phase of the electrolytes for the investigated BCAs are plotted as a function of the SoC in Figure6. Molecules 2021, 26, x FOR PEER REVIEW 13 of 28

Table 3. Detection limits and detection range for [BCA]+ in the aqueous electrolyte phase, maximum fractions of Br2 in the aqueous phase compared to the global concentration and c(Br2)aq for an SoC of 33%.

+ + + + + + + [BCA]+ [C2Py] [C4Py] [C6Py] [C2MIm] [C3MIm] [C4MIm] [C6MIm]

SoC limit for detectable [BCA]+ ≤ 70 ≤ 66 ≤ 40 ≤ 60 ≤ 50 ≤ 40 ≤ 33 / % [a] c(Br2)(aq)/c(Br2)total / % [b] ≤ 10.1 ≤ 10.3 ≤ 8.5 ≤ 10.4 ≤ 10.0 ≤ 10.6 ≤ 9.5 C(Br2)(aq) at an SoC of 30% / 121 51 9 105 63 34 8 mM [a] The SoC value corresponds to the highest SoC at which [BCA]+ cations can still be detected in the aqueous phase. [b] Highest ratio of Br2 concentration in the aqueous phase versus the total Br2 concentration in the sample detected at an SoC of 90 or 100%.

The individual, BCA-dependent bromine binding strength is determined from the [BCA]+ concentrations in the range of 20 ≤ SoC ≤ 60% and is applicable to investigate the solubility behavior of the BCA. The BCAs can be sorted according to their concentration in the aqueous phase. Low concentrations of [BCA]+(aq) in the aqueous phase describe a larger bromine binding strength and vice versa. The bromine binding strength is obtained in the following order: [C6MIm]+(aq) ≥ [C6Py]+(aq) > [C4MIm]+(aq) > [C4Py]+(aq) > [C3MIm]+(aq) > [C2MIm]+(aq) > [C2Py]+(aq). Correspondingly, the solubility of [BCA]Br2n+1 in the aqueous electrolyte for the 1-alkylpyridine-1-ium polybromides and for the 1-alkyl-3-methylimidazole-1-ium polybromides decreases with an increasing length of the alkyl side chain. The detection limits of [BCA]+ cations in Table 3 show that 1-alkyl- 3-methylimidazol-1-ium is a slightly stronger bromine binding BCA than 1-alkylpyridin- 1-ium if both have the same alkyl group in the N-position. As mentioned earlier, the bro- mine binding strength cannot be the only criteria to choose a BCA. Moderate Br2 concen- trations in the aqueous phase along the whole SoC range are a prerequisite for the opera- bility of the cell and will be investigated in the next section.

2.4. Effect of the Solubility Equilibrium on the Br2(aq) Concentration for the Whole SoC Range

Br2 is transported from the tank into the RFB half cell as an ingredient of the aqueous solution. Moderate Br2 concentrations dissolved as polybromide ions in the aqueous elec- trolyte phase are a prerequisite for stable cell operation. The bromine concentrations in Molecules 2021, 26, 2721 the aqueous phase of the electrolytes for the investigated BCAs are plotted as a function13 of 26 of the SoC in Figure 6.

(a) (b)

Figure 6. Concentrations of Br2 solved in the form of polybromide ions in the aqueous electrolyte phase depending on the state of charge (SoC) for the investigated BCAs: (a) 1-alkylpyridine-1-ium bromides and (b) 1-alkyl-3-methylimidazole-1- ium bromides as BCAs with different alkyl side chains in the N-position at θ = 23 ± 1 ◦C. All values of Figure6 are printed in the SM in Table S2. Section I shows a strong dependence of the Br2 concentration in the aqueous phase on the alkyl side chain length of the BCA, while in section II the Br2 concentration is independent of the alkyl side chain length. The total Br2 concentration in the samples allows a direct comparison of the Br2 concentration in the sample and the aqueous phase for 10 ≤ SoC ≤ 100%.

2.4.1. General Influence of BCA Application on Br2 Concentration in Aqueous Solution

Due to the relatively low solubility of the BCA-polybromide salt, the Br2 concentration in the aqueous solution in equilibrium is low compared to the global concentration of Br2 indicated by the absolute concentration in Figure6. While the total concentration of Br2 varies linearly along the SoC range, between c(Br2(total)) = 0.35 M at an SoC of 10% and c(Br2(total)) = 3.35 M at an SoC of 100% (Figure6), the solubility equilibrium of the BCAs with the polybromides results in a significantly lower concentration of bromine (c(Br2(aq)) < 0.35 M) in the entire SoC range for all investigated BCAs. In addition, no linear relationship between c(Br2(aq)) and the SoC is observed. The maximum ratio of Br2 in the aqueous phase compared to the total Br2 concentration is present for SoC > 80% and is shown in Table3/line 2. A maximum fraction of 11 mol% Br 2 is present in the aqueous solution, while a fraction of more than 89 mol% Br2 is present in the fused salt phase. Although vapor pressures are not measured in this work, the safety of the electrolytes increases tremendously due to the low Br2 concentrations and the resulting reduced outgassing of Br2 from the aqueous electrolyte. Additionally, low concentrations of c(Br2(aq)) would decrease the available bromine that can crossover across the membrane to the H2 electrode inside the battery.

2.4.2. Individual Concentration Trends of Different [BCA]+ Cations For BCAs based on 1-alkylpyridin-1-ium and 1-alkyl-3-methylimidazol-ium, charac- teristic concentration curves for c(Br2(aq)) are obtained, whereby both diagrams in Figure6 can be divided into two sections: In the first section (section I), between 0 < SoC ≤ 40% the Br2 concentrations in the aqueous solution are different for each BCA but are approximately independent of SoC and are also listed for an SoC of 30% in Table3, on line 3. One exception is the trend of [C2MIm]Br, which will be explained later. There is a clear trend in Br2 con- centrations in the aqueous phase at an SoC of 30% for the applied BCAs, which corresponds to the trend of BCA-concentrations in Section 2.3: c(Br2; [C6MIm]Br) < c(Br2; [C6Py]Br) < c(Br2; [C4MIm]Br) < c(Br2; [C4Py]Br) < c(Br2; [C3MIm]Br) << c(Br2; [C2MIm]Br) < c(Br2; [C2Py]Br). In this range, the bromine binding strength of the [BCA]+ cation is clearly visible Molecules 2021, 26, 2721 14 of 26

again and corresponds exactly to the mentioned trends in Section 2.3: [C6MIm]+(aq) > [C6Py]+(aq) > [C4MIm]+(aq) > [C4Py]+(aq) > [C3MIm]+(aq) > [C2MIm]+(aq) > [C2Py]+(aq). The alkyl side chain length in the N-position in all BCAs is responsible for the differences in complexing strength, which is just a handy indicator for the solubility of the [BCA]Br2n+1. The longer the side chain, the lower the solubility of the [BCA]Br2n+1 in the aqueous solu- tion. Another finding is that in general, the 1-alkyl-3-methylimidazol-1-ium compounds are slightly stronger BCAs compared to the 1-alkylpyridin-1-ium, when the same alkyl + side chains are introduced. The concentrations of Br2 and [BCA] cations in the aqueous solution both show the same trend. For the Br2 concentrations of [C2MIm]Br between 10 ≤ SoC ≤ 50%, there is a con- centration plateau in Figure6b which differs from the c(Br 2(aq)) curves for [C3MIm]Br, [C4MIm]Br and [C6MIm]Br. As mentioned above, the second phase of [C2MIm]Br is crystalline and consists solely of the tribromide compound ([C2MIm]Br3). Consequently, the ratio of bromine to BCA cation is fixed to 1:1, and no higher polybromides can be stored in the second heavy phase. This leads to rising Br2 concentrations in the aqueous phase until the second phase liquefies again at SoC ≥ 50%, as presented in Figure4. The second section in Figure6a,b starts from SoC > 40% (section II). Here the Br 2 concentration increases noticeably for all BCAs, whereby the dependence of the Br2 con- centration on the choice of the [BCA]+ cation decreases with increasing SoC. For both 1-alkylpyridin-1-ium and 1-alkyl-3-methylimidazolium cations as BCAs, the Br2 concentra- tion for SoC ≥ 60% increases, whereby for each SoC value examined, approximately equal concentrations c(Br2(aq)) are present in the aqueous solution. In this section, the length of the alkyl side chain at the N-position of the BCA seems to have no influence on the binding strength of Br2 of the second phase in solution. However, 1-alkylpyridin-1-ium bromides bind bromine in section II slightly more strongly and tend to have a lower solubility in the presence of polybromides at SoC > 60% as slightly lower Br2 concentrations in the aqueous phase are present (Figure6). In general, for SoC ≥ 80% the Br2 concentrations rise only slowly or form a plateau. In that range, the solubility of Br2 in the aqueous solution is limited by a lack of bromide ions to form polybromides, known from BCA-free electrolytes [65]. It is noticeable over the whole range that for small amounts of Br2 at SoC ≤ 40%, the complexation strength of the individual BCA types has a major influence on bromine concentration in the aqueous phase, but at SoC ≥ 60% neither the type of BCA base nor the length of the alkyl side chain seem to play a noticeable role, where all BCAs are considered to complex polybromides to a similar extent. At the same time, there is a transfer of Br2 from the aqueous phase into the fused salt over the entire SoC range, but since it has been shown above that no BCA cations are present in the aqueous solution at higher SoCs, the transfer no longer occurs according to Equation (3).

2.4.3. Theoretical Comparison of Br2 Concentrations and Current Densities in the H2/Br2-RFB Cell

Overall, the concentrations of c(Br2(aq)) are concentrations of Br2 in chemical equi- librium for both phases and are only present during operation if the mass transfer of Br2 from the fused salt to the aqueous electrolyte phase takes place without any mass transfer limitation, i.e., the equilibrium state is maintained during the discharge process. Applying −2 this theoretical assumption, the current densities of i = 723 mA cm for c(Br2) = 0.3 M, −2 −2 i = 482 mA cm for c(Br2) = 0.2 M and i = 241 mA cm for c(Br2) = 0.1 M would be possi- ble in a cell with a geometrically active electrode/membrane area of 40 cm2 and a volume flow rate of 30 mL min−1. Data for this cell are described in [65]. For electrolytes with lower concentrations, the possible current densities would decrease further. Only [C2Py]Br forms a liquid fused salt over the entire SoC range and has concentrations c(Br2) ≥ 0.121 M in equilibrium between the two phases. In reality, lower concentrations can be expected in the aqueous solution. Therefore, a current density i < 250 mA cm−2 for galvanostatic charg- ing and discharging can also be expected for electrolytes with [C2Py]Br. As [H4MPy]Br, [H3MPy]Br and [HMPip]Br upon first glance have aqueous Br2 concentrations between Molecules 2021, 26, x FOR PEER REVIEW 16 of 28

Molecules 2021, 26, 2721 15 of 26 ing the distribution of bromine in the different polybromides. The method has been de- scribed in our earlier work [65]. The distribution of Br2 on Br3-, Br5- and Br7- is shown in Figure 7. The Raman spectra of all investigated samples are shown in the SM in Figure S3 to0.20 S9. M and 0.35 M at an SoC of 33% in Figure2, they seem to be more applicable for effectiveAn analysis cell operation. of the distribution From Figure of6 Brit2 is among noticed the that polybromides in the range Br of3−, anBr5 SoC− and of Br 33%,7− in theBr2 aqueousconcentrations phase shows in the basically aqueous similar solutions results are for lowest. both BCA For thegroups protonated as shown forms in Figure it is 7atherefore,b. For all expected investigated that for BCAs, higher the SoCs,storage much of Br larger2 in the Br aqueous2 concentrations electrolyte are in present the form and of Brelectrolytes5- predominates, cannot followed still be classified by storage as “safe”in Br3- and electrolytes. finally storage From thein Br results7−. This in is this in accord- section, anceit is expectedwith earlier that studies concentrations on pure mightBCA-free rise forHBr/Br a higher2/H2O SoC-electrolytes and lead [65] to greater, leading volatility to the resultof Br2 . that [C2Py]Br [BCA] tends+ has to only be the a minor [BCA] influence of choice in on order the fractions to reach performance of Br2 in the and different safety − polybromideof the Br2/Br specieselectrolytes. in the aqueous solution. For SoC < 40%, the n-hexyl side chain BCAs [C6Py]Br and [C6MIm]Br have a slightly increased Br2 content in Br3−, which means that 2.5. Distribution of Br on Polybromides in the Aqueous Phase less Br2 is distributed2 to the Br5− and Br7− anions. For SoC < 40%, n-butyl, n-propyl and ethyl AsBCAs presented have a inconstant the previous Br2 distribution section, the, which aqueous is independent Br2 concentration of the defines BCA. theFor powerBCAs alkylatedcapability with of the an cell.n-hexyl For group a deeper, more understanding Br3− is available, of thewhile complexation less Br2 is stored behavior in Br of5- and the Brdifferent7−. In this BCAs, range the, cations Raman of spectra [C6Py] of+ and the aqueous[C6MIm] phase+ are availabl have beene in analyzed the aqueous regarding solution the anddistribution already prefer of bromine to interact in the with different Br3- in polybromides. comparison toThe Br5- methodand Br7−. has This been is in described accordance in − − − withour earlier the composition work [65]. of The their distribution fused salt of phases, Br2 on shown Br3 , Br in5 Figureand Br 3b.7 is shown in Figure7. The Raman spectra of all investigated samples are shown in the SM in Figures S3–S9.

(a) (b)

− - −- -− Figure 7. Distribution ofof BrBr22 inin thethe aqueous aqueous electrolyte electrolyte phase phase on on the the polybromides polybromides Br 3Br3, Br, Br5 5 and BrBr77 , ,which which are are present, present, depending on the SoC with different BCAs with different alkyl side chains in the N N-position-position for ( a) 1 1-alkylpyridine-1-ium-alkylpyridine-1-ium bromides and (b) 1-alkyl-3-methylimidazole-1-ium bromides as BCAs at θ = 23 ± 1 °C.◦ All values of Br2 distribution on bromides and (b) 1-alkyl-3-methylimidazole-1-ium bromides as BCAs at θ = 23 ± 1 C. All values of Br2 distribution on polybromides are printed in the SM in Tables S3, S4 and S5. polybromides are printed in the SM in Tables S3–S5.

2 − − − ForAn analysisan SoC of of ≥ the 40% distribution, there is no of dependence Br2 among the of the polybromides Br distribution Br3 ,in Br the5 andaqueous Br7 phasein the on aqueous the length phase of showsthe alkyl basically radical similarpresent results in the forN-position both BCA of the groups BCA. as The shown distri- in butionFigure 7isa,b independent. For all investigated of the choice BCAs, of the the BCA storage basic of compound Br2 in the aqueous pyridine electrolyte or 3-methylim- in the − − - − idazole.form of BrWithin5 predominates, this SoC range, followed the fraction by storage of Br in2 st Brored3 and as Br finally3 increases storage from in Br 30.7%7 . This to − 41.6is in−47.3%, accordance while with proportionally earlier studies less onBr2pure is stored BCA-free in Br5 HBr/Br and the2/H curve2O-electrolytes decreases from [65], + − 60%leading to 46 to–50%. the result Br2 remains that [BCA] in Br7has within only the a minor whole influence SoC range on relatively the fractions constant of Br 2upin to the a maximumdifferent polybromide of 13.5%. Presumably species in, thea better aqueous solvation solution. of the For more SoC stable < 40%, Br the3− comparedn-hexyl side to - - − Brchain5 is present. BCAs [C6Py]Br This effect and is [C6MIm]Br obtained also have for a BCA slightly-free increased HBr/Br2/Br Br2solutionscontent in [65] Br.3 Since, which the − − Brmeans2 concentration that less Br of2 isc(Br distributed2(aq)) < 0.35 to M the and Br 5theand distribution Br7 anions. of Br For2 to the SoC polybromides < 40%, n-butyl, is n-propyl and ethyl BCAs have a constant Br2 distribution, which is independent of the − BCA. For BCAs alkylated with an n-hexyl group, more Br3 is available, while less Br2

Molecules 2021, 26, 2721 16 of 26

− − + + is stored in Br5 and Br7 . In this range, cations of [C6Py] and [C6MIm] are available − − in the aqueous solution and already prefer to interact with Br3 in comparison to Br5 − and Br7 . This is in accordance with the composition of their fused salt phases, shown in Figure3b. For an SoC of ≥ 40%, there is no dependence of the Br2 distribution in the aque- ous phase on the length of the alkyl radical present in the N-position of the BCA. The distribution is independent of the choice of the BCA basic compound pyridine or 3- − methylimidazole. Within this SoC range, the fraction of Br2 stored as Br3 increases − from 30.7% to 41.6−47.3%, while proportionally less Br2 is stored in Br5 and the curve − decreases from 60% to 46–50%. Br2 remains in Br7 within the whole SoC range relatively Molecules 2021, 26, x FOR PEER REVIEW 17 of 28 constant up to a maximum of 13.5%. Presumably, a better solvation of the more stable − − − Br3 compared to Br5 is present. This effect is obtained also for BCA-free HBr/Br2/Br solutions [65]. Since the Br2 concentration of c(Br2(aq)) < 0.35 M and the distribution of Br2 relativelyto the polybromides constant in isthe relatively whole range, constant no specific in the whole influence range, of this no specificdistribution influence on the of per- this formancedistribution or individual on the performance parameters or is individual given andparameters therefore cannot is given be anddiscussed. therefore cannot be discussed. 2.6. Influence of Different BCAs on Electrolyte Conductivity of the Aqueous Phase 2.6. Influence of Different BCAs on Electrolyte Conductivity of the Aqueous Phase For both flow-by and flow-through electrodes in RFBs, a low ohmic cell resistance is requiredFor to both achieve flow-by high and cell flow-through or stack performance. electrodes In in RFB RFBs,, this a lowstrongly ohmic depends cell resistance on the electrolyteis required composition to achieve high at different cell or stackSoCs, performance.usually targeting In RFB, the maximum this strongly electrolyte depends con- on ductivitythe electrolyte of the composition aqueous electrolyte at different phase. SoCs, Electrolyte usually targeting conductivities the maximum of the electrolyte aqueous conductivity of the aqueous electrolyte phase. Electrolyte conductivities of the aqueous phases from the seven pre-selected [BCA]/Br2 electrolytes summarized in Table 2 are shownphases from in Figure the seven 8. pre-selected In addition [BCA]/Br, the reference2 electrolytes conductivity summarized curves in Table of a2 areBCA shown-free in Figure8. In addition, the reference conductivity curves of a BCA-free HBr/Br /H O HBr/Br2/H2O electrolyte (from [65]) and a pure HBr/H2O acid solution within the2 SoC2 rangeelectrolyte are shown (from in [65 Figure]) and a8. pureAll measured HBr/H2O values acid solution are listed within in Table the SoCS6 and range S7 in are the shown SM. in Figure8. All measured values are listed in Tables S6 and S7 in the SM. The values for The values for pure HBr/H2O acid at θ = 20 °C are calculated from molar conductivities pure HBr/H O acid at θ = 20 ◦C are calculated from molar conductivities from [66] and from [66] and2 are adapted to the SoC scale. are adapted to the SoC scale.

(a) (b)

FigureFigure 8. 8. SpecificSpecific electrolyte electrolyte conductivity conductivity κκ ofof the the aqueous aqueous electrolyte electrolyte phase phase containing containing ( (aa)) 1 1-alkylpyridin-1-ium-alkylpyridin-1-ium bromides bromides andand ( (bb)) 1 1-alkyl-3-methylimidazol-1-ium-alkyl-3-methylimidazol-1-ium bromides bromides as as BCAs BCAs with with different different alkyl alkyl side side chains chains in in the the N N-position-position as as a a function function of state of charge SoC at θ = 23 ± 1 ◦°C. In addition, electrolyte conductivities of BCA-free HBr/Br2/H2O electrolytes are of state of charge SoC at θ = 23 ± 1 C. In addition, electrolyte conductivities of BCA-free HBr/Br2/H2O electrolytes are shown (green dots) from [65] and of pure HBr/H2O solutions (orange line) from [66] at θ = 20 °C◦ with the same total HBr shown (green dots) from [65] and of pure HBr/H2O solutions (orange line) from [66] at θ = 20 C with the same total HBr and/or Br2 concentrations as reference curves. and/or Br2 concentrations as reference curves.

ElectrolyticElectrolytic conductivities conductivities in in the the aqueous aqueous phase phase depend depend mainly mainly on on the the high high proton −1 concentrationconcentration and and are are larger larger than 300 mS cmcm−1 throughoutthroughout the the SoC SoC range, range, whereby whereby the the GrotthusGrotthus mechanism mechanism [67,68] [67,68 ]is is responsible responsible for for ion ionicic transport transport through through the thesolution. solution. In this In chain-hopping mechanism, bonds are attached and released between water and protons in oxonium ions. Ionic transport by the vehicle mechanism [67] is subordinated to the Grotthus mechanism in aqueous electrolyte solutions. With conductivities between 324.8 ≤ κ ≤ 745.0 mS cm−1, all aqueous electrolytes containing BCAs have large electrolyte con- ductivities and are feasible for application in the H2/Br2-RFB. The [BCA]+ cations in the electrolyte mixture influence the electrolyte conductivity within the entire SoC range compared to BCA-free HBr/Br2/H2O electrolytes, as shown in Figure 8. For 0 ≤ SoC ≤ 60% for both 1-alkylpyridin-1-ium electrolytes and for 1-alkyl-3- methylimidazol-1-ium electrolytes, κ rises approximately from 368.90 to 478.00 mS cm−1 at an SoC of 0% to reach a peak at κ = 643.1−681.84 mS cm−1 between an SoC of 50 and 60%. Within this range, there is a difference from the two reference curves in accordance with the concentrations of the [BCA]+ cation in solution, shown in Figure 5. Initially, the organic character of [BCA]+ cation decreases the conductivity. Organic [BCA]+ cations with longer

Molecules 2021, 26, 2721 17 of 26

this chain-hopping mechanism, bonds are attached and released between water and pro- tons in oxonium ions. Ionic transport by the vehicle mechanism [67] is subordinated to the Grotthus mechanism in aqueous electrolyte solutions. With conductivities between 324.8 ≤ κ ≤ 745.0 mS cm−1, all aqueous electrolytes containing BCAs have large electrolyte conductivities and are feasible for application in the H2/Br2-RFB. The [BCA]+ cations in the electrolyte mixture influence the electrolyte conductivity within the entire SoC range compared to BCA-free HBr/Br2/H2O electrolytes, as shown in Figure8. For 0 ≤ SoC ≤ 60% for both 1-alkylpyridin-1-ium electrolytes and for 1-alkyl-3- methylimidazol-1-ium electrolytes, κ rises approximately from 368.90 to 478.00 mS cm−1 at an SoC of 0% to reach a peak at κ = 643.1−681.84 mS cm−1 between an SoC of 50 and 60%. Within this range, there is a difference from the two reference curves in accordance with the concentrations of the [BCA]+ cation in solution, shown in Figure5. Initially, the organic character of [BCA]+ cation decreases the conductivity. Organic [BCA]+ cations with longer alkyl side chains lead to lower conductivity in the aqueous phase due to their increasing size. On the one hand, their migration in the electric field is slowed down and the ionic transport by the Grotthus mechanism is restricted. Grotthus charge transfer on a direct path between the electrodes is hindered and is forced to bypass the large organic cations. However, as the SoC is increased, the conductivity of the aqueous electrolyte in Figure8 is shown to increase. This observation can be directly related to the results presented in Figure5, whereby the [BCA] + concentration of the investigated BCAs in the aqueous electrolyte is shown to decrease with the SoC as an indication that the cations are now bonded to the fused salt phase and leaving the aqueous electrolyte phase. Thus, based on these results, the decrease in the aqueous electrolyte [BCA]+ concentration translates into the increased electrolyte conductivity trend seen in Figure8 between 0 ≤ SoC ≤ 60%. For electrolytes with 1-alkylpyridin-1-ium bromide as a BCA, the complexation strength with respect to the side chain can be read in Figure8a. Since [C6Py] + with − + polybromides Br2n+1 is less soluble in the aqueous phase than [C2Py] , their concen- tration decreases more rapidly compared to [C2Py]+. The conductivity of the solution increases correspondingly more quickly in Figure8a compared to electrolytes containing [C4Py]+ or [C2Py]+. In the second range for 60 < SoC ≤ 100%, there are hardly any [BCA]+ cations avail- able in the aqueous phase, as presented in Section 2.3. In this range, no direct depen- dence of conductivity on the alkyl side chain in the N-position of the [BCA]+ cation is obtained. Electrolytes with BCAs show approximately same conductivities. With an in- creasing SoC, the proton concentration decreases, and the conductivity of the solution decreases subsequently. However, electrolytes containing BCAs reach higher conductivities compared to BCA- free HBr/Br2/H2O electrolytes for SoC ≥ 50% (green dots, in Figure8). Due to the transfer of Br2 by use of the BCA from the aqueous phase into the fused salt, only concentrations lower than 0.35 M Br2 compared to the maximum total concentration of 2.99 M Br2 in BCA-free HBr/Br2/H2O electrolyte (calculated from values in [65]) are present in aqueous electrolytes. Since the absolute Br2 concentration in the aqueous electrolytes with BCA is much lower, while the concentration of protons in both solutions is assumed to be the same, the conductivity is higher when using BCAs and is comparable to pure HBr/H2O solutions as shown in Figure8. The Grotthus mechanism takes place directly between the electrodes and does not need to bypass larger polybromides, as is the case in BCA-free + HBr/Br2/H2O electrolytes [65]. As for SoCs > 60%, the [BCA] cations are not available in the aqueous phase; the conductivity of the resulting electrolytes in that range is shown to be higher than in BCA-free HBr/Br2/H2O electrolytes. Overall, in the complete SoC range there are conductivities between a minimum of 324.8 mS cm−1 (SoC 100%) and a maximum of 712.0−745.0 mS cm−1 (an SoC of 50%, depend- + ing on the [BCA] ), which are large in comparison to aqueous electrolytes of Zn/Br2-RFB using the example of Eustace [36] with conductivities between 44.4 ≤ κ ≤ 128.2 mS cm−1 (θ = 23 ◦C). Pure aqueous solutions of comparable BCAs [C2MIm]Cl, [C4MIm]Cl and [C6MIm]Cl offer Molecules 2021, 26, 2721 18 of 26

conductivities of κ ≤ 82 mS cm−1 at θ = 25 ◦C [69], whereby a clear dependence on the alkyl radical in the N-position of the 3-methylimidazole-1-ium can be confirmed from [69] with κ([C2MIm]Cl(aq)) > κ([C4MIm]Cl(aq)) > κ([C6MIm]Cl(aq)). In this work, the achieved conductivities correspond to those of a pure HBr/H2O solution, which for an SoC of ≥ 60% means an increase in conductivity compared to HBr/Br2/H2O electrolytes and has a positive effect on cell performance. These high conduc- tivities are due to the presence of the high proton concentration of c(HBr) ≥ 1 M. The influ- ence of BCAs is directly attributable to the presence of BCAs for an SoC of < 60% and indirectly for an SoC of ≥ 60%. These high conductivities allow one to obtain high current densities at a low expected electrolyte overvoltage, depending on the half-cell structure.

2.7. Comparison of the Redox Potential of BCA-Free and BCA-Containing Electrolytes

To obtain a high-power density on an H2/Br2-RFB cell, a high thermodynamic cell voltage is beneficial. The thermodynamic cell voltage of the H2/Br2-RFB is composed of the difference of the half-cell redox potentials ϕ vs. NHE in open circuit state of the RFB. − The redox potential of the positive half-cell ϕ(Br2/Br ) depends, in the simplest scenario, − − − on the concentrations of the redox pair Br /Br2 or Br /Br2n+1 according to Nernst [70], while assuming activity coefficients (γ) of γ = 1 for all components of the redox reaction according to Equation (4):

− 0 − −1 − 2 −1 ϕ(Br2|Br ) = ϕ (Br2|Br ) - 0.5 R T F ln(c(Br ) c(Br2) ) (4)

In Equation (4), F is the Faraday constant (96485 As mol−1), R the molar gas constant −1 −1 0 − (8.3145 J mol K ) and T the temperature in K. ϕ (Br2|Br ) is the standard redox − potential of the Br2/Br electrolyte with 1.09 V [9]. The theoretical potential curve following the Nernst equation (Equation (4)) and global concentrations of HBr and Br2 at T = 298.15 K are applied. The curve is shown in Figure9 as a function of the SoC (orange line). It has much higher values at corresponding SoCs compared to the measured redox potential of BCA-free electrolytes in Figure9 (green dots). In the electrolyte system under investigation, there are no ideally diluted solutions, but strongly concentrated electrolytes. As a result, redox potentials according to the NERNST equation must be considered including activity coefficients as shown in Equation (5) [70]:

− 0 − −1 − 2 − 2 −1 −1 ϕ(Br2|Br ) = ϕ (Br2|Br ) − 0.5 R.T.F ln(c(Br ) γ(Br ) c(Br2) γ(Br2) ) (5)

For low SoCs with high concentrations c(HBr) > 1.2 M of HBr, the activity coefficients for γ(HBr) > 1 are available (calculated with data from [71]). Increasingly higher activities a(HBr) = c(HBr) γ(HBr) must be taken into account in the calculation of the redox potential of the positive half cell due to constantly increasing activity coefficients of HBr in the aqueous solution [71]. Wlodarczyk et al. [70] clearly show that the increasing activity coefficients of HBr for low SoCs leads to a strong decrease in the redox potential of the bromine electrolyte at an electrode according to Nernst compared to a pure concentration- based calculation. − The measured redox potential ϕ(Br2/Br ) of the pure HBr/Br2/H2O electrolytes (green curve in Figure9) increases with an increasing SoC from 0.76 V vs. NHE (at an SoC of 0%) and a maximum value at an SoC of 100% at 1.12 V vs. NHE. For SoC ≥ 89.4%, Br2 is not completely soluble in the aqueous solution as explained in [65]. The aqueous solutions contain less Br2, and the redox potential flattens following the Nernst equation (Equation (5)). Molecules 2021, 26, x FOR PEER REVIEW 19 of 28

To obtain a high-power density on an H2/Br2-RFB cell, a high thermodynamic cell voltage is beneficial. The thermodynamic cell voltage of the H2/Br2-RFB is composed of the difference of the half-cell redox potentials φ vs. NHE in open circuit state of the RFB. The redox potential of the positive half-cell φ(Br2/Br−) depends, in the simplest scenario, on the concentrations of the redox pair Br−/Br2 or Br−/Br2n+1− according to Nernst [70], while assuming activity coefficients (γ) of γ = 1 for all components of the redox reaction accord- ing to Equation 4:

φ(Br2|Br −) = φ0(Br2|Br −) – 0.5 R T F−1 ln(c(Br −)2 c(Br2) −1) (4) In Equation 4, F is the Faraday constant (96485 As mol−1), R the molar gas constant (8.3145 J mol−1 K−1) and T the temperature in K. φ0(Br2|Br −) is the standard redox potential of the Br2/Br- electrolyte with 1.09 V [9]. The theoretical potential curve following the Nernst equation (Equation 4) and global concentrations of HBr and Br2 at T = 298.15 K are applied. The curve is shown in Figure 9 as a function of the SoC (orange line). It has much higher values at corresponding SoCs compared to the measured redox potential of BCA- free electrolytes in Figure 9 (green dots). In the electrolyte system under investigation, there are no ideally diluted solutions, but strongly concentrated electrolytes. As a result, redox potentials according to the NERNST equation must be considered including activity coefficients as shown in Equation 5 [70]:

φ(Br2|Br −) = φ0(Br2|Br −) – 0.5 R T F−1 ln(c(Br −)2 훾(Br −)2 c(Br2) −1 훾(Br2) −1) (5) Molecules 2021, 26, 2721 19 of 26

(a) (b)

- − FigureFigure 9. 9. RedoxRedox potentials potentials of of the the redox redox couple couple Br Br/Br/Br2 in 2aqueousin aqueous posolyte posolyte vs. NHE vs. NHE as a function as a function of its of state its stateof charge of charge SoC measuredSoC measured on a g onlassy a glassy carbon carbon electrode electrode without without [BCA]Br [BCA]Br and with and (a with) 1-alkylpyridine (a) 1-alkylpyridine-1-ium-1-ium bromides bromides and (b) 1 and-alkyl (b)-3 1-- methylimidazolealkyl-3-methylimidazole-1-ium-1-ium bromides bromides as BCAs aswith BCAs different withdifferent alkyl side alkyl chain side groups chain in groups the N- inposition the N-position of the BCA of theat θ BCA = 23 at± 1 °C. The orange line shows the simulated redox potential following the Nernst equation and is based on global concen- θ = 23 ± 1 ◦C. The orange line shows the simulated redox potential following the Nernst equation and is based on global trations of HBr and Br2. The measured redox potential of BCA-free HBr/Br2 electrolytes is shown with green dots for concentrations of HBr and Br . The measured redox potential of BCA-free HBr/Br electrolytes is shown with green dots comparison and was first published2 in [70]. Values of redox potentials of the electrolytes2 containing BCAs are printed in thefor SM comparison of this study and wasin Table first publishedS8. in [70]. Values of redox potentials of the electrolytes containing BCAs are printed in the SM of this study in Table S8. For low SoCs with high concentrations c(HBr) > 1.2 M of HBr, the activity coefficients In principle, the redox potential of the aqueous electrolyte solution with the use for γ(HBr) > 1 are available (calculated with data from [71]). Increasingly higher activities of [BCA]Br in the presence of Br is lower within the entire SoC range compared to a(HBr) = c(HBr) γ(HBr) must be taken2 into account in the calculation of the redox potential BCA-free HBr/Br /H O electrolytes (Figure9). The transfer of Br into the fused salt of the positive half2 cell2 due to constantly increasing activity coefficients2 of HBr in the reduces the bromine concentration in the aqueous solution, while at the same time the HBr aqueous solution [71]. Wlodarczyk et al. [70] clearly show that the increasing activity co- concentrations stay nearly constant in comparison to BCA-free HBr/Br2/H2O electrolytes. Hence, according to Equation (4), the redox potential of [BCA]Br electrolytes is slightly lower when compared to the BCA-free HBr/Br2/H2O electrolytes. For SoCs < 50%, the solubility of the respective [BCA]+ cation is responsible for the trend observed in Figure9. In accordance with lower bromine concentrations in the electrolyte for increasing the n-alkyl side chain length (Figure6), lower redox potentials are observed with the BCA electrolytes with larger n-alkyl chain. For SoCs > 50%, the difference between the potentials of the pure HBr/Br2/H2O electrolyte and the electrolytes containing BCA becomes increasingly larger. In global terms, the Br2 concentration increases strongly, which can be seen in the potential of the BCA-free HBr/Br2/H2O electrolyte, but at the same time the majority of the Br2 in the solutions with BCAs is transferred to the fused salt. The redox potentials of the electrolytes with BCAs increase at a slower rate with an increasing SoC. The use of [BCA]Br leads to an average reduction in the redox potential of the positive half cell and the cell voltage ∆E by 32 ≤ ∆E ≤ 114 mV for all BCAs and over the entire SoC range compared to pure HBr/Br2/H2O electrolytes. The performance of the cell is reduced by this effect, while it is already lowered by high activity coefficients of HBr [70,71]. Fabjan et al. [8] described for Zn/Br2-RFB electrolytes that solutions of [C2MM]Br and [C2MP]Br reduce the concentration of Br2 in the aqueous phase to approximately 0.01 M, and the redox potential is characteristically reduced by 50 to 70 mV. This applies to our electrolytes and both BCA series in Figure9 for the range 30 ≤ SoC ≤ 70%. A lower cell voltage due to the use of BCAs must be accepted as a trade-off for limiting the Br2 concentration in the aqueous solution. Molecules 2021, 26, 2721 20 of 26

2.8. Interpretation of the Study and Selection of a Promising BCA for Application in Cell Tests In the scope of this research, 38 cost-effective BCAs have been investigated and evalu- ated on the basis of their solubility, Br2 binding strength, ionic conductivity and equilibrium potential for their possible application as electrolytes on H2/Br2-RFBs. In the following, an elimination procedure is applied to select the most suitable BCA for battery operation. The BCAs are excluded on the basis of the results obtained in the previous sections. To start, based on their solubility, [CTA]Br, [TOA]Cl and [TOA]Br do not dissolve in a polar HBr/H2O solution due to their long nonpolar alkyl groups. In addition, other BCAs such as [HPy]Br, [TEA]Br, [C2MPip]Br, [C2MP]Br, [C2MM]Br, [C4MM]Br, [HMIm]Br, [C2MIm]Br, [MTA]Br, [TBA]Br, [C24MPy]Br and [C44MPy]Br form solid crystals in the aqueous solu- tion instead of a liquid fused salt when they come into contact with polybromides. Hence, none of these BCAs can be used in the cell as the spontaneous formation of crystals in the cell, tubing and pumps can lead to operational problems. Since the H2/Br2 RFB is intended to be operated at high current densities due to the good reaction kinetics of both electrode reactions, a high mass flow of Br2 into the cell during the discharge reaction is necessary, making high concentrations of Br2 desirable. At the same time, it is the task of the BCAs to reduce the bromine concentration in the aqueous phase in order to increase safety. All BCAs that form a liquid fused salt phase and have long alkyl side chains such as n-butyl or n-hexyl have a bromine binding strength of >95 mol%. Additionally, all BCAs with a liquid fused salt phase and ethyl or n-propyl as a side chain, with the exception of [C2Py]Br, have a bromine binding strength of >90 mol%. These BCAs do not leave sufficient Br2 in the aqueous phase of the electrolyte to achieve acceptable current densities. [HMP]Br, [H4MPy]Br, [H3MPy]Br and [HMPip]Br show only a bromine binding strength of 70 to 80 mol% at an SoC of 33% with concentrations between 0.22 to 0.33 M, while it is expected that for higher SoC values, larger Br2 concentrations than 0.5 M are available. The essential safety might not be achieved at these concentrations. It is therefore proposed to investigate 1-ethylpyidin-1-ium bromide [C2Py]Br as BCA in further studies in the RFB cell. From the seven BCAs investigated over the entire SoC range, [C2Py]Br exhibits the most advantages. Even though there is a reduction in the electrolyte conductivity and the redox potential when compared to the BCA-free one, the concentrations of Br2 in the aqueous solution are higher compared to solutions with [C4Py]Br and [C6Py]Br. In the [C2Py]Br, electrolytes at least 89 mol% Br2 remain in the fused salt phase, yet the electrolytes still maintain large Br2 concentrations, between 0.12 M to 0.26 M, in the aqueous solution. The availability of Br2 in the liquid electrolyte phase turned out to be a decisive factor in our selection, making this compound clearly stand out over all the other investigated BCAs. The initial cycling tests and investigation of the [C2Py]+ cations on the cell performance during the charge and discharge operation are shown in parallel in [72].

3. Materials and Methods A detailed description of the testing methods is given in the SM (Section1) for this arti- cle to enable reproducibility of the measurements. An abbreviated form is reproduced here.

3.1. Reagents Reagents used for preparation of bromine electrolyte samples in this work are hydro- bromic acid, bromine and distilled water. Chemicals for the synthesis of BCAs are named in the Supplementary Materials (SM) including suppliers and purities.

3.2. Synthesis of Bromine Complexing Agents The investigated BCAs are listed in Table1, including chemical structure, name and ab- breviation. Non-commercially available N-alkylated quaternary ammonium bromide salts based on pyridine ([C2Py]Br, [C4Py]Br, [C6Py]Br, [C6Py]Cl), on 3-picoline ([C23MPy]Br, [C43MPy]Br, [C63MPy]Br), on 4-picoline ([C24MPy]Br, [C44MPy]Br, [C64MPy]Br), on 1-methylimidazole ([C2MIm]Br, [C3MIm]Br, [C4MIm]Br, [C6MIm]Br, [C6MIm]Cl), on 1- Molecules 2021, 26, 2721 21 of 26

methylpiperidine ([C2MPip]Br, [C4MPip]Br, [C6MPip]Br,) and on 1-methylpyrrolidine ([C6MP]Br) are synthesized by alkylation reaction [45,73] according to Dzyuba et al. [74]. Some BCAs are available as golden liquids at reaction temperatures between 40 ◦C and 100 ◦C, while some precipitate as golden or white solids. 1H NMR and 13C NMR spectra of the synthesized substances are presented in SM and are consistent with NMR shifts found in the literature [45–49]. Protonated forms of 4-picoline, 3-picoline, 1-methylpiperidine, pyridine, 1-methylimidazole and 1-methylpyrrolidine ([H4MPy]Br, [H3MPy]Br, [HMP]Br, [HMPip]Br, [HPy]Br, [HMIm]Br) are prepared by dissolving the heterocyclic ammonium compound with a calculated excess of HBr in the electrolyte solution.

3.3. Electrolyte Formulation For each BCA, a sample of 10 mL is mixed in aqueous solution containing 5.47 M HBr, 1.11 M [BCA]Br or [BCA]Cl. HBr 48 wt% and H2O are added volumetrically. Solubility of [BCA]Br or [BCA]Cl is evaluated. To solutions, in which the BCA salt is dissolved, Br2 with a mass of 1.77 g (1.11 M Br2) is added gravimetrically. All mixtures correspond to an electrolyte mixture for an SoC of 33%. For selected BCAs [C2MM]Br, [C2MP]Br, [C2Py]Br, [C4Py]Br, [C6Py]Br, [C2MIm]Br, [C3MIm]Br, [C4MIm]Br and [C6MIm]Br, electrolyte properties are investigated ex situ within the entire SoC range. The SoC range is defined for SoC 0% with 7.7 M HBr, 1.11 M [BCA]Br and 0 M Br2 and for an SoC of 100% with 1 M HBr, 1.11 M [BCA]Br and 3.35 M Br2. The SoC definition is derived from another study [65]. Samples with a volume of 30 mL in a test series with SoC steps of 10%, and additionally for an SoC of 33% and 66%, are mixed.

3.4. Stability of Fused Salt Electrolyte The stability of the fused salt is checked by visual inspection. The criteria for qualifica- tion of the liquid range of a fused salt are the absence of visible orange or red crystals in the fused salt.

3.5. Concentration of [BCA]+ Cations in Aqueous Electrolytes Concentrations of 1-alkylpyridin-1-ium cations and 1-alkyl-3-methylimidazol-1-ium cations in the aqueous electrolyte phase are determined by Raman spectroscopy over the entire SoC range. Both cations show characteristic strong peaks at distinctive Raman shifts especially due to their aromaticity [64,75,76]. Characteristic Raman shifts of peaks of the BCAs are shown in Table2. Peak areas at this Raman shifts are determined and compared with peak areas at known [BCA]+ concentrations (an SoC of 0% and 1.11 M [BCA]Br). [BCA]+(aq) concentrations as a function of the SoC are therefore calculated (ESI).

3.6. Br2 Concentration in Aqueous Electrolyte Phase

Br2 binding strength of the BCA or solubility of the [BCA]Br2n+1 complex is investi- gated by measurement of Br2 concentration in aqueous electrolyte phase. Concentrations of Br2 are investigated by linear chronoamperometry at a rotating disk electrode. At a rotation speed of ω = 1000 rpm on vitreous carbon electrode, linear sweep scans between 0.8 V and −0.5 V vs. an Ag/AgCl/KCl(sat.) reference electrode and a scan rate of –40 mV s−1 are per- formed for all samples. Constant reduction currents result for half-cell potentials < −0.1 V vs. Ag/AgCl/KCl(sat.), which represent mass transport limitation by Br2 from the bulk phase [77,78]. According to Levich [77–80], reduction currents are directly proportional to the c(Br2) of the bulk solution. Concentrations c(Br2(aq)) in M are calculated from the limiting currents Ilim (in mA) shown in Equation (6):

−1 −1 c(Br2(aq)) = 0.00342466 mol L mA Ilim (6)

A linear calibration curve with known bromine concentrations is investigated before at same conditions. Br2 concentrations are measured for the aqueous electrolyte phase of 35 samples with different BCAs chosen at an SoC of 33% and the series of electrolyte Molecules 2021, 26, 2721 22 of 26

samples with [C2Py]Br, [C4Py]Br, [C6Py]Br, [C2MIm]Br, [C3MIm]Br, [C4MIm]Br and [C6MIm]Br at 13 different SoCs.

3.7. Polybromide Determination, Polybromide Distribution and [BCA]+ Concentration Determination by Raman Analysis By means of Raman spectroscopy on the aqueous electrolyte samples, second phase crystals and fused salt samples, the occurrence of the different polybromides tribromide − − − Br3 , pentabromide Br5 and heptabromide Br7 in electrolyte series of 1-alkylpyridin-1- ium cations und 1-alkyl-3-methylimidazol-1-ium cations electrolytes at different SoCs were investigated. Raman shifts for polybromides and bromine were determined between v˜ = 150 and 320 cm−1 as described in the literature [17,56,81,82]. Peak areas of the following stretching − −1 oscillations of the polybromides were determined: v(Br˜ 3 , sym.) = 162–163 cm [17,53,56], − −1 − −1 − v(Br˜ 3 , antisym.) = 190–198 cm [17,56,83], v(Br˜ 5 , antisym.) = 210 cm [17,56], v(Br˜ 5 , −1 − −1 sym.) = 250–253 cm [17,56,81,83,84], v(Br˜ 7 , sym.) = 270 cm [56,81]. In the literature, −1 pure Br2 shows a strong, single peak at v(Br˜ 2, sym.) = 300–325 cm [81,83,84]. Since there are differences in Raman shifts between the aqueous solution and the corresponding fused salt phase, Raman shifts actually used for fitting the individual peaks are shown in Table2 for the investigation of the aqueous electrolyte phase. Investigation of Br2 distribution in polybromides is explained in detail in the SM and literature [65].

3.8. Electrolytic Conductivities of Aqueous Phase Electrolytic conductivities of the aqueous electrolytes of 1-alkylpyridin-1-ium BCAs and 1-alkyl-3-methylimidazol-1-ium BCAs electrolyte at various SoC are determined at θ = 23 ± 1 ◦C in a conductivity cell by potentiostatic impedance spectroscopy. The cell constant is determined in a 1 M KCl aqueous solution at θ = 23 ± 1 ◦C with κ (1 M KCl at θ = 23 ◦C) = 103.9 mS cm−1 (calculated from [85]). A detailed description can be found in the ESI.

3.9. Redox Potential of the Electrolytes The redox potentials of the aqueous electrolyte solution are investigated on a glassy carbon stick electrode vs. an Ag/AgCl/KCl(sat.) reference electrode at θ = 23 ± 1 ◦C and corrected to present them vs. a normal hydrogen electrode. A detailed description can be found in the ESI.

4. Conclusions In this study, tertiary ammonium compounds of pyridine, pyrrolidine, morpholine, 3-methylimidazole, tetraalkylammonium and picolines have been alkylated with n-alkyl side chains in the N-position to form organic bromide or chloride salts. In combination with some commercially available organic salts, this resulted in 38 organic quaternary ammonium substances from these six N-containing building blocks. The substances have been investigated ex situ on their properties as BCAs in HBr/Br2-electrolytes: BCA solubility, Br2 binding strength, the stability of the second heavy fused salt phase, Br2 concentration for application, electrolyte conductivity and redox potential. This study has been performed to define their applicability of BCAs to limit the vapor pressure of Br2 in aqueous highly concentrated electrolytes for a high-energy density H2/Br2-RFB with a theoretical capacity of 180 Ah L−1/196 Wh L−1 (7.7 M HBr). From the 38 substances evaluated, 15 are not soluble in the electrolyte or form a crystal phase in contact with polybromides. This behavior makes them unsuitable for their application in RFBs. We found that BCAs with short side chains and/or symmetric structure tend to form crystals instead of forming liquid fused salts. An investigation of the [BCA]-polybromide crystal composition showed that all BCA crystals are tribromide salts [BCA]Br3. All the other BCAs form fused salt phases, while most of them exhibit a strong bromine binding strength (14 substances > 95 mol% and four substances between 90 to 95 mol%); four of the N-position protonated BCAs show insufficient binding strengths (<80 mol%). Molecules 2021, 26, 2721 23 of 26

The binding strength increases with an increase in alkyl side chain length, yet the structure of the six basic components (pyridine, picoline, etc.) only has minor influence on the bromine binding strength in solutions. 1-alkyl-3-methylimidazol-1-ium and 1-alkylpyridin-1-ium bromides have been inves- tigated within the whole SoC range to compare the influence of side chain alkyl groups on the different parameters evaluated throughout this study: For a low SoC (<40%), the selected BCAs show different bromine binding strengths. A clear trend has been found as short alkyl side chains lead to lower Br2 binding strength compared to those with long alkyl side chains. For an SoC of ≥40%, the concentrations of Br2 in the electrolyte’s aqueous phase increase, while the binding strength becomes independent of the alkylated side chain. However, to obtain a clear insight on the reason behind this behavior, further investigation on the fused salt phase would be needed which is beyond the scope of this study. On a final note regarding this comparison, it is worth mentioning that the ability to form a fused salt phase differed from that of the [C2Py]Br and the [C2MIm]Br BCAs as we observed that while [C2Py]Br forms a liquid fused salt, the cation of [C2MIm]Br tends to crystallize over a wide SoC range. The ionic conductivity of the aqueous phase is high for the whole SoC range be- tween 325 and 745 mS cm−1 and depends primarily on the available proton concentration and its ionic transport by the Grotthus mechanism. The concentration of [BCA]+ in the aqueous phase can, to a lesser extent, influence the measured ionic conductivities of the electrolytes as their presence can slightly reduce the conductivity or increase it indirectly by sequestration of Br2 from the aqueous phase. The redox potential of the electrolyte is reduced by 32 to 114 mV due to the use of BCAs compared to BCA-free electrolytes in the whole SoC range. Following the Nernst equation, the decrease in Br2 concentration in the aqueous phase leads to reduced redox potentials of the half-cell reaction. This is certainly a trade-off that must be accepted as it is a direct consequence of the ambition to lower bromine vapor pressure by limiting Br2 concentration in aqueous solutions. Based on the results of fused salt stability over the SOC range, electrolyte conductivity, the trend of the redox potential and especially the concentrations of Br2 remaining in the aqueous electrolyte phase, one BCA was selected from 38 BCAs and needs to be investigated in detail and applied in the positive half cell of the H2/Br2-RFB cell: 1-ethylpyridin-1-ium bromide ([C2Py]Br).

Supplementary Materials: The following are available online: Detailed description of the mea- surement methods, synthesis of BCAs, manufacturers’ data and chemical purities (Chapter S1), as well as 1H NMR and 13C NMR interpretation for synthesized BCAs (Chapter S2), tabulation of measured values for Figures within the main manuscript (Table S1–S8), Raman spectra of solutions (Figures S3–S9), Raman spectra of [BCA]Br3 crystals (Figure S2) and concentrations of Br2 in aqueous solution for all 35 BCAs investigated (Figure S1). Author Contributions: Conceptualization, M.K.; methodology, M.K.; synthesis, E.M. and P.A.L.T.; validation, M.K.; formal analysis, M.K., P.A.L.T. and P.F.; investigation, M.K., P.A.L.T. and E.M.; resources, P.F., J.T. and M.K.; data curation, M.K. and P.A.L.T.; writing—original draft preparation, M.K.; writing—review and editing, M.K., P.A.L.T., P.F. and J.T.; visualization, M.K.; supervision, M.K. and P.F.; project administration, M.K.; funding acquisition, M.K., P.F. and J.T. All authors have read and agreed to the published version of the manuscript. Funding: The authors thank the Federal Ministry of Education and Research (BMBF) in the project “PEDUSA” (03XP0135) and the State of Baden-Wuerttemberg/Germany in the project “RedoxWind” for the funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article or supplementary material. Molecules 2021, 26, 2721 24 of 26

Acknowledgments: The authors would like to thank Michael Holzapfel (Fraunhofer ICT) for the preliminary discussion of experiments. Conflicts of Interest: The authors declare no conflict of interest.

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