Use of Carbon Additives Towards Rechargeable Zinc Slurry Air Flow Batteries

energies

Article Use of Carbon Additives towards Rechargeable Zinc Slurry Air Flow Batteries

Nak Heon Choi 1,2,*, Diego del Olmo 3 , Diego Milian 4, Nadia El Kissi 4, Peter Fischer 1, Karsten Pinkwart 1,5 and Jens Tübke 1,2 1 Applied Electrochemistry, Fraunhofer Institute for Chemical Technology ICT, Joseph-von-Fraunhofer, Straße 7, 76327 Pﬁnztal, Germany; peter.ﬁ[email protected] (P.F.); [email protected] (K.P.); [email protected] (J.T.) 2 Institute for Mechanical Process Engineering and Mechanics, Karlsruhe Institute of Technology KIT, Straße am Forum 8, 76131 Karlsruhe, Germany 3 Department of Chemical Engineering, University of Chemistry and Technology Prague, Technická 5, 16628 Prague 6, Czech Republic; [email protected] 4 CNRS, Grenoble INP, LRP, Institute of Engineering, Univ. Grenoble Alpes, LRP, 38000 Grenoble, France; [email protected] (D.M.); [email protected] (N.E.K.) 5 Faculty of Electrical Engineering and Information Technology, Karlsruhe University of Applied Sciences, Moltkestraße 30, 76133 Karlsruhe, Germany * Correspondence: [email protected]; Tel.: +49-721-4640-827

Received: 31 July 2020; Accepted: 20 August 2020; Published: 31 August 2020

Abstract: The performance of redox flow batteries is notably influenced by the electrolyte, especially in slurry-based flow batteries, as it serves as both an ionic conductive electrolyte and a flowing electrode. In this study, carbon additives were introduced to achieve a rechargeable zinc slurry flow battery by minimizing the zinc plating on the bipolar plate that occurs during charging. When no carbon additive was present in the zinc slurry, the discharge current density was 24 mA cm 2 at 0.6 V, · − while the use of carbon additives increased it to up to 38 mA cm 2. The maximum power density · − was also increased from 16 mW cm 2 to 23 mW cm 2. Moreover, the amount of zinc plated on the · − · − bipolar plate during charging decreased with increasing carbon content in the slurry. Rheological investigation revealed that the elastic modulus and yield stress are directly proportional to the carbon content in the slurry, which is beneficial for redox flow battery applications, but comes at the expense 1 of an increase in viscosity (two-fold increase at 100 s− ). These results show how the use of conductive additives can enhance the energy density of slurry-based flow batteries.

Keywords: zinc slurry air ﬂow battery; redox ﬂow battery; zinc-air battery; carbon additives; rheology

1. Introduction Lately, the use of energy storage systems (ESS) to support renewable energies, like solar energy and wind energy, has attracted remarkable attention [1]. To achieve high energy densities for renewable energy, redox flow batteries (RFBs) are a promising candidate owing to their ability to decouple the control of energy capacity and power [2,3]. The energy densities of RFBs can be tuned by the capacity of electrolyte and the concentration of active species. On the other hand, the power density is determined by the electrode size and material. Different types of RFB systems are under investigation, such as vanadium-based, hydrogen-bromine, and organic RFB couples [4–6]. Using zinc as an active metal is very attractive, due to its abundance and low cost, including the electrolyte component, such as potassium hydroxide and zinc oxide [7,8]. Furthermore, zinc itself has a relatively high theoretical 1 energy density (1350 Wh kg− ) compared to other active materials [3]. Conventional zinc-air batteries consist of a positive compartment where air can flow in and out and a static porous zinc electrode in

Energies 2020, 13, 4482; doi:10.3390/en13174482 www.mdpi.com/journal/energies Energies 2020, 13, x FOR PEER REVIEW 2 of 12 high theoretical energy density (1350 Wh kg−1) compared to other active materials [3]. Conventional zinc-airEnergies 2020 batteries, 13, 4482 consist of a positive compartment where air can flow in and out and a static porous2 of 12 zinc electrode in the negative compartment [9,10]. On the other hand, the zinc slurry air flow battery uses a negative electrode with zinc particles suspended in a highly alkaline electrolyte (such as potassiumthe negative hydroxide) compartment with [gelling9,10]. On agents the otherforming hand, a slurry, the zinc which slurry can air flow flow in battery and out uses of the a negative cell [5]. Theelectrode corresponding with zinc chemical particles suspendedreactions are in apresente highlyd alkaline below electrolyteand a schematic (such asdiagram potassium of this hydroxide) system configurationwith gelling agentsis shown forming in Figure a slurry, 1. During which discharge, can flow inthe and oxygen out of reduction the cell [5reaction]. The corresponding(ORR) occurs whilechemical zinc reactions particles are in presentedthe slurry below oxidize and to a zincate, schematic which diagram can offurther this system decompose configuration to zinc oxide is shown in KOH,in Figure and1 .the During reactions discharge, are reversed the oxygen when reduction charging reaction[11]. (ORR) occurs while zinc particles in the slurry oxidize to zincate, which can further decompose to zinc oxide in KOH, and the reactions are Negative Electrode: Zn + 4OH− ↔ Zn(OH)42− + 2e− (E0 = −1.26 V) reversed when charging [11].

Positive Electrode: O2 + 2H2O + 4e− ↔2 4OH− (E0 = +0.40 V) Negative Electrode: Zn + 4OH− Zn(OH) − + 2e− (E0 = 1.26 V) ↔ 4 − Electrolyte: Zn(OH)42− ↔ ZnO + H2O + 2OH− Positive Electrode: O + 2H O + 4e− 4OH− (E0 = +0.40 V) 2 2 ↔ 2 Electrolyte:Overall: Zn(OH) 2Zn + O− 2 ↔ZnO 2ZnO+ (H∆E0O =+ 1.662OH V)− 4 ↔ 2 Overall: 2Zn + O 2ZnO (∆E0 = 1.66 V) 2 ↔

Figure 1. Schematic diagram of a zinc slurry air flow battery. Figure 1. Schematic diagram of a zinc slurry air flow battery. The capacity of conventional zinc-air batteries is determined by the size of the porous zinc electrode embeddedThe capacity in the system.of conventional However, zinc-air for a zinc batteries slurry airis flowdetermined battery theby capacitythe size canof the be controlledporous zinc by electrodethe amount embedded of zinc in in the the slurry, system. independently However, for of thea zi cellnc size.slurry Thus, air flow the batterybattery capacity the capacity can be can tuned be controlledby varying by the the tank amount volume of or zinc the concentrationin the slurry, ofindependently zinc particles inof thethe slurry cell size. [12]. Thus, Another the potential battery capacityadvantage can of be zinc tuned slurry by air varying flow batteries the tank relates volume to theor the charging concentration process. of One zinc of theparticles main limitationsin the slurry of [12].zinc-air Another batteries potential is dendrite advantage growth of onzinc the slurry zinc air electrode, flow batteries which willrelates eventually to the charging pierce the process. membrane One ofand the result main in limitations a battery short-circuit. of zinc-air batteries In the zinc is slurrydendrite air flowgrowth battery, on the zinc zinc plates electrode, on the bipolarwhich platewill eventuallyor, preferably, pierce on the zincmembrane slurry, asand there result is noin statica battery porous short-circui electrode,t. suchIn the as zinc carbon slurry felts. air Hence, flow battery,this system zinc configurationplates on the bipolar provides plate a space or, preferably, between theon the bipolar zinc plateslurry, and as there membrane is no static which porous helps electrode,prevent membrane such as carbon damage felts. when Hence, charging. this syst Furthermore,em configuration it is provides reported a that space flowing between configurations the bipolar plateprevent and the membrane dendrites fromwhich growing helps prevent perpendicular membrane to the damage bipolar plate when [13 charging.–16]. Nonetheless, Furthermore, as slurries it is reportedare heterogeneous that flowing systems configurations composed ofprevent non-soluble the dendrites particles, therefrom isgrowing a need toperpendicular use gelling agents to the to bipolarprevent plate sedimentation [13–16]. Nonetheless, and ensure aas percolated slurries are network. heterogeneous systems composed of non-soluble particles,Gel polymerthere is a electrolytes need to use (GPEs) gelling are agents very interestingto prevent forsedimentation zinc slurryair and flow ensure batteries, a percolated because network.advantages from both aqueous electrolytes and solid electrolytes can be encountered [17]. Depending on the battery design, the rheological behavior can be tuned up to favor cell efficiency by using carefully chosen gelling agents [18], rendering storage systems more flexible and adaptable to different scenarios [19]. Carbopol 940 (polyacrylic acid) has been recently investigated in different configurations Energies 2020, 13, 4482 3 of 12 as conventional alkaline metal-air batteries like Al-air battery [20] and Zn-air batteries [21]. Moreover, polyacrylic acid (PAA) has been proven to show higher conductivity compared to other GPEs like PVA and PAM at 6 M KOH [22]. In this same study, rechargeability tests were performed with an 2 efficiency of 79% at 0.5 mA cm− , and it was proved that charge transfer resistance was reduced by using GPEs. As a result of its mechanical properties (see J. M. Piau [23]), and chemical stability [22], PAA has already been used to formulate Zn slurries [5], and further investigations are being made to assess its performance as a gelling agent. The use of electrolyte additives for improving the performance of non-flowing zinc-air batteries has also been widely investigated [24]. Furthermore, conductive additives (as graphite or carbon black) can be included in the formulation of Zn electrodes [25–28], leading to improved discharge capacities and power densities by increasing electronic conductivity, connectivity between Zn particles and providing nucleation sites for ZnO. Similarly, the use of carbon conductive additives for slurry redox flow batteries [29–33] and slurry capacitors [34,35] has also been investigated in the literature. Recently, Akuzum et al. [35] studied the use of three different carbon additives for a slurry capacitor—they concluded that the morphology of the additive plays an important role on the rheo-electric performance of the slurry electrode. Nonetheless, despite the use of carbon additives for zinc slurries has been proposed previously [36], its influence in the charging performance has not been investigated in detail. Herein, we introduce carbon additives into a zinc slurry and study how the carbon additive affects both the charge and discharge performance of the zinc slurry air flow battery. Furthermore, using slurries with different carbon contents, we study the cycling performance and energy efficiency of the system. Finally, we show how the use of carbon additives reduces the amount of zinc plating onto the bipolar plate as zinc preferentially deposits in the carbon present in the slurry.

2. Materials and Methods

2.1. Air Electrode The catalyst-coated electrode (CCE) approach was used to prepare the positive electrode. The catalyst ink consisted of a mixture of Pt black (Platinum black, Alfa Aesar, Kandel, Germany) and IrO2 (99% Iridium oxide, Alfa Aesar, Kandel, Germany) with a ratio of 1:1 with 10 wt.% Fumion FAA-3 Ionomer (Fumatech, Bietigheim-Bissingen, Germany), deionized water, and isopropanol. The ink was first homogenized in an ultrasonic water bath for 10 min, and then again, using an ultrasonicator for another 10 min, to ensure a well-dispersed mixture. Then, the ink was manually sprayed onto a 25 cm2 commercial gas diffusion layer (SGL Carbon, 29BC, Meitingen, Germany) with a spray gun. The platinum and iridium oxide loadings on the catalyst layer were fixed at 0.5 mg cm 2 each, and the · − ionomer content was set to 23 wt.%.

2.2. Zinc Slurry Preparation The base gel polymer electrolyte consists of 10 M KOH (Carl Roth, Karlsruhe, Germany) with Carbopol TM 940 (Acros Organics, Nidderau, Germany) as a gelling agent. To ensure a strong gel network, the GPE was mixed with a high shear homogenizer Yellow line, Ultra Turrax DI 25 basic (Ika, Staufen, Germany) at 9.500 rpm. Then, the zinc powder (GC 7-4/200 Bi/200ln, Grillo, Duisburg, Germany) with D50 = 55 µm, ZnO (VWR Chemicals, Dietikon, Switzerland), and carbon black (CB, carbon black, acetylene, 50% compressed, Alfa Aesar, Kandel, Germany) with D50 = 0.045 µm were added and the mixture was again homogenized for a total of 9 min. The slurry compositions are shown below in Table1; note that the amount of the carbon black and 10 M KOH added up to 61.5 wt.%. The total volume of zinc slurry used for each experiment was approximately 70 mL. Energies 2020, 13, 4482 4 of 12

Table 1. Composition of the zinc slurries.

Mass Fraction (wt.%) Zinc ZnO Carbopol Carbon Black KOH + Water 0 Carbon (0%CB) 0 61.5 0.2 Carbon (0.2%CB) 0.2 61.3 33.8 4 0.7 0.6 Carbon (0.6%CB) 0.6 60.9 1 Carbon (1%CB) 1 60.5

2.3. Cell Design and Electrochemical Characterization The electrochemical characterization of the slurries was conducted using an in-house designed single cell presented in our previous study [5]. It comprises bipolar plates with a geometric active area of 25 cm2, a separator, gaskets, and current collectors enclosed by two end plates. This is a single cell configuration, however, as it is focused on upscaling to a stacked design, the graphite plate at the negative electrode and metallic plate at the positive electrode are referred to as bipolar plates in this paper. As the charging voltage was higher than 2 V, the positive side bipolar plate was made of CuNi alloy, whereas the negative electrode bipolar plate was made of graphite to minimize zinc plating on it. The flow fields for both bipolar plates were serpentine as described in our previous study [5]. The CCE was placed between the positive-side bipolar plate and the porous membrane separator (Cellophane™ PØØ purchased from FUTAMURA, Hamburg, Germany). The electrochemical performance of each slurry was studied by current-voltage characteristic curves (polarization curves) and galvanostatic cycling using a BaSyTec GSM Battery Test System (BaSyTec GmbH, Asselfingen, Germany). The galvanostatic cycling had a time stop-condition of 10 min per half-cycle and the voltage cut-offs were 0.3 V for discharge and 2.3 V for charge. The flow rates of active materials were 160 mL min 1 for the zinc slurry in the negative compartment and 100 mL min 1 · − · − for the synthetic air in the positive compartment. The voltage reported in the polarization curves was averaged over 30 s at each current density, due to the fluctuations caused by using a slurry electrode.

2.4. Rheometry The rheology of the slurries was investigated by means of steady shear and oscillatory rheometry. A stress-controlled rheometer AR-G2 (TA instruments) equipped with a four-blade vane-in-cup geometry (L = 45.5 mm, D = 30 mm) was used to reduce flow-related heterogeneity issues [37,38]. The cup was equipped with a grill to reduce slippage effects [39]. This tool was calibrated following the procedure described by Baravian for large finite gaps [40]. All experiments were performed at room temperature. The steady-state measurements were obtained by applying a constant shear rate from 100 to 1 0.01 s− . It was chosen to start from high shear rates to low shear rates to ensure viscosity steady-state measurements [41]. To quantify the rheological parameters of the carbon slurries, shear stress vs. shear rates curves were fitted to a Herschel-Bulkley model:

. n τ = τy + Kγ

. where τ is the shear stress, τy is the yield stress, K is the consistency index, γ is the yield stress, and n is the ﬂow index. Oscillatory strain amplitude sweep tests were carried from 0.1 to 1000% at 1 Hz to determine the linear viscoelastic region and both storage and loss moduli.

3. Results Initial testing of the cycling behavior of our prototype zinc slurry air ﬂow battery revealed that, during charging, part of the regenerated zinc was plated on the bipolar plate (see Figure2a). The plating of metal on the bipolar plate is the working principle of ﬂow-assisted batteries, such as zinc-nickel [42]. Energies 2020, 13, 4482 5 of 12

Energies 2020, 13, x FOR PEER REVIEW 5 of 12 However, it is an undesired process for zinc slurry air ﬂow batteries, as it limits the capacity of the system.Furthermore, In this the situation, plated thezinc maximum can grow amountto form ofdendrites zinc that which can be could recharged block would the slurry be given flow by and, the Energies 2020, 13, x FOR PEER REVIEW 5 of 12 sizeeventually, of the cell break and through depth of the the membrane channel rather producing than the a totalshort volume circuit. of slurry. Furthermore, the plated zincFurthermore, can grow tothe form plated dendrites zinc can which grow could to form block dendrites the slurry which ﬂow and,could eventually, block the break slurry through flow and, the membraneeventually, producingbreak through a short the circuit. membrane producing a short circuit.

Figure 2. Zinc-side bipolar plate after charging: (a) 0% carbon black (CB), (b) 0.2% CB, (c) 0.6% CB, (d) 1% CB. Figure 2. Zinc-side bipolar plate after charging: (a) 0% carbon black (CB), (b) 0.2% CB, (c) 0.6% CB, (d) FigureTo try 2.toZinc-side mitigate bipolarthis effect, plate we after introduced charging: ( acarbon) 0% carbon additives black (CB),in the (b slurry) 0.2% CB,aiming (c) 0.6% to shift CB, the 1% CB. zinc (platingd) 1% CB. from the bipolar to the carbon additive in the slurry. This new formulation would allow the recharged zinc to flow in and out of the system leading to a slurry flow battery configuration ToTo trytry to to mitigate mitigate this this e ffeffect,ect, we we introduced introduced carbon carbon additives additives in thein the slurry slurry aiming aiming to shift to shift the zinc the rather than a flow-assisted configuration. The suitability of the concept was evidenced by visual platingzinc plating from from the bipolar the bipolar to the to carbon the carbon additive additi inve the in slurry. the slurry. This This new new formulation formulation would would allow allow the inspection of the bipolar plate after cycling slurries with different carbon contents. It can be seen in rechargedthe recharged zinc zinc to flow to flow in and in outand of out the of system the syst leadingem leading to a slurry to a slurry flow battery flow battery configuration configuration rather Figure 2 how the use of an increasing amount of carbon in the slurry led to lower zinc plating on the thanrather a flow-assistedthan a flow-assisted configuration. configuration. The suitability The suitability of the concept of the was concept evidenced was evidenced by visual inspection by visual bipolar plate. ofinspection the bipolar of platethe bipolar after cycling plate after slurries cycling with slurries different with carbon different contents. carbon It can contents. be seen It in can Figure be seen2 how in theFigure use 2 of how an increasing the use of amountan increasing of carbon amount in the of slurrycarbon led in tothe lower slurry zinc led plating to lower on zinc the plating bipolar on plate. the 3.1. Oscillatory and Shear Rheometry bipolar plate. 3.1. OscillatoryFirst, we anddetermined Shear Rheometry the rheological behavior of the slurries under study, to assess their suitability3.1. OscillatoryFirst, wefor determined slurryand Shear RFBs. Rheometry the Oscillatory rheological rheometry behavior of (Figure the slurries 3) revealed under study, a linear to assess viscoelastic their suitability region (LVR) where G’>>G’’, indicating that the total stress was given mostly by the elastic modulus G’. The for slurryFirst, RFBs.we determined Oscillatory the rheometry rheological (Figure behavior3) revealed of the a linearslurries viscoelastic under study, region to (LVR) assess where their LVR is characterized by a moduli plateau where viscoelastic properties do not depend on strain. This Gsuitability0>>G”, indicating for slurry that RFBs. the Oscillatory total stress rheometry was given (Figure mostly by3) revealed the elastic a moduluslinear viscoelastic G0. The LVRregion is behavior was found in all samples indicating that a percolated network exists within the linear region characterized(LVR) where byG’>>G’’, a moduli indicating plateau that where the viscoelastic total stress propertieswas givendo mostly not depend by the onelastic strain. modulus This behavior G’. The [43]. wasLVR found is characterized in all samples by a indicating moduli plateau that a where percolated viscoe networklastic properties exists within do not the depend linear region on strain. [43]. This behavior was found in all samples indicating that a percolated network exists within the linear region [43]. 1000

1000 100

100 10 G', G'' (Pa) G'' G',

10 1 0 Carbon

G', G'' (Pa) G'' G', 0.2 Carbon 0.6 Carbon 1 10 CarbonCarbon 0.1 0.2 Carbon 0.1 0.6 1 Carbon 10 100 1000 1 CarbonOscillation strain (%) 0.1 0.1 1 10 100 1000 Figure 3. OscillatoryOscillatory strain strain amplitude amplitude sweep sweep test testperformed performed at f = at 1 fHz= for1 Hzcarbon for carbonslurries. slurries.Closed Closed symbols represent storage moduli (GOscillation0) and open strain symbols (%) loss moduli (G”). symbols represent storage moduli (G’) and open symbols loss moduli (G’’). AsFigure shown 3. Oscillatory in Figure 3strain, higher amplitude carbon sweep concentrations test performed yielded at f higher= 1 Hz for moduli carbon values, slurries. representing Closed As shown in Figure 3, higher carbon concentrations yielded higher moduli values, representing networkssymbols that represent tend to behavestorage moduli more as (G’) elastic and solids,open symbols meaning loss that moduli an optimal(G’’). tradeoﬀ between sample networks that tend to behave more as elastic solids, meaning that an optimal tradeoff between sample structure and viscosity needs to be found. Another important characteristic of these curves is that, As shown in Figure 3, higher carbon concentrations yielded higher moduli values, representing after a given strain, the moduli trend was reversed (G’’ was greater than G’), indicating an inversion networks that tend to behave more as elastic solids, meaning that an optimal tradeoff between sample structure and viscosity needs to be found. Another important characteristic of these curves is that, after a given strain, the moduli trend was reversed (G’’ was greater than G’), indicating an inversion

Energies 2020, 13, 4482 6 of 12

Energies 2020, 13, x FOR PEER REVIEW 6 of 12 structure and viscosity needs to be found. Another important characteristic of these curves is that, afterin the a behavior given strain, as the the sample moduli breaks trendwas andreversed starts to (G”flow, was indicating greater thana yield G0), stress indicating [44]. As an discussed inversion inin thethe behaviorlatter reference, as the sample there breaksis no andstandardized starts to flow, calc indicatingulation of ayield yield stress stress [by44 ].oscillatory As discussed methods. in the latterTherefore, reference, the yield there stress is no of standardized the slurries calculationwas determined of yield using stress shear by oscillatoryrheometry. methods. Therefore, the yieldThe stresssteady-state of the slurriesmeasurements was determined (Figure using4) show sheared that rheometry. the slurries follow a non-Newtonian behavior,The steady-state more specifically measurements a shear thinning (Figure4 )behavior. showed thatSuch the behavior slurries proves follow beneficial a non-Newtonian to slurry- behavior,based RFBs more as it specifically reduces particle a shear sedimentation thinning behavior. and segregation Such behavior at rest proves and beneficialviscosity decreases to slurry-based as the RFBsslurry as is it pumped. reduces particleIn general, sedimentation the addition and of CB segregation led to a decrease at rest and in viscositythe flow decreasesindex (see asinset the Figure slurry is4b). pumped. Newtonian In general, fluids have the additiona flow index of CB n = led 1, while to a decrease carbon slurries in the flow analyzed index in (see this inset study Figure follow4b). a Newtonianshear-thinning fluids behavior have a(n flow < 1). index The flown = 1,index while increased carbon slurrieswhen carbon analyzed was inpresent, this study but at follow higher a shear-thinningcarbon concentrations, behavior it (ndecreased< 1). The at flow values index lowe increasedr than the when slurry carbon without was CB present, and shear but atthinning higher carbonbehavior concentrations, is well established. it decreased at values lower than the slurry without CB and shear thinning behavior is well established.

Figure 4. Steady-state rheometry. (a) Viscosity values vs. shear rate for carbon slurries. (b) Shear stress vs.Figure shear 4. Steady-state rate for carbon rheometry. slurries. (a Lines) Viscosity represents values Herschel-Bulkley vs. shear rate for carbon fitting. slurries. The inset (b in) Shear the graph stress showsvs. shear the rate values for ofcarbon yield stressslurries and. Lines flow represents index for all Herschel-Bulkley carbon slurries obtained fitting. The from inset the fit.in the graph shows the values of yield stress and flow index for all carbon slurries obtained from the fit. In Figure4a, we observe that the viscosity values decreased by a factor of 10 4 for 1%CB (highest 3 CB concentration)In Figure 4a, we and observe a factor that of 10the forvisc 0CBosity (lowest values decreased CB concentration) by a factor in theof 10 range4 for 1%CB of shear (highest rates 1 analyzed.CB concentration) The 0CB and and a 0.2%CBfactor of slurries103 for 0CB showed (lowest similar CB concentration) values at high in shear the ratesrange(from of shear 0.1 rates s− ), indicatinganalyzed. thatThe the0CB addition and 0.2%CB of 0.2 slurries wt.% of showed CB did notsimilar influence values the at viscosity,high shear even rates if it(from modified 0.1 s its−1), viscoelasticindicating that properties the addition (Figure of3). 0.2 For wt.% the slurries of CB di withd not higher influence CB contents, the viscosity, the addition even if of it carbon modified black its resultedviscoelastic in a properties five-fold viscosity (Figure 3). increase For the for slurries 1%CB atwith low higher shear ratesCB contents, and a two-fold the addition increase of atcarbon high shearblack rates.resulted This in isa infive-fold agreement viscosity with theincrease investigation for 1%CB performed at low shear by Gallierrates and et al. a two-fold [45], indicating increase that at contacthigh shear forces rates. increase This is the in agreement shear stress with because the investigation of the friction performed and roughness by Gallier of particles et al. [45], close indicating to each other,that contact before forces entering increase a lubricational the shear regimestress because at higher of shearthe friction rates where and roughness these contacts of particles are reduced. close to eachFigure other,4 bbefore shows entering the shear a stresslubricational as a function regime of at shear higher rate. shear These rates slurries where showed these a contacts yield stress are behavior;reduced. thus, the data were fitted to a Herschel-Bulkley model. As discussed by Smith et al. [13], severalFigure rheological 4b shows parameters the shear canstress be as tuned a function to improve of shear the rate. energy These effi ciencyslurries of showed the system, a yield as stress yield stress.behavior; As seenthus, in the the data inset were of Figure fitted4 b,to carbona Herschel-B black concentrationulkley model. wasAs discussed directly proportional by Smith et to al. yield [13], stress.several Wei rheological et al. [43] parameters have demonstrated can be tuned that higher to impr yieldove stressesthe energy lead efficiency to higher electronicof the system, conductivity, as yield whichstress. isAs beneficial seen in the for inset thebattery of Figure tested 4b, carbon in this study.black concentration On the other hand,was directly higher proportional pressure drops to mustyield bestress. attained Wei toet overcome al. [43] yieldhave stressdemonstrated which can that severely higher aff ectyield the totalstresses effi ciencylead to of thehigher system electronic [46]. conductivity, which is beneficial for the battery tested in this study. On the other hand, higher 3.2. Polarization Curve Analysis pressure drops must be attained to overcome yield stress which can severely affect the total efficiency of theWe system then [46]. studied the influence of the carbon additive in the electrochemical performance of the system. The resulting polarization curves are shown in Figure5. For discharge (Figure5a), 3.2. Polarization Curve Analysis We then studied the influence of the carbon additive in the electrochemical performance of the system. The resulting polarization curves are shown in Figure 5. For discharge (Figure 5a), it can be

Energies 2020, 13, x FOR PEER REVIEW 7 of 12 observed that, at low current densities, corresponding with the kinetic or activation region, the zinc slurries with carbon exhibited higher current densities than the one without. At 0.6 V, the current density of each slurry was 24 mA∙cm−2 for 0CB, 38 mA∙cm−2 for 0.2%CB, 36 mA∙cm−2 for 0.6%CB, and 28 mA∙cm−2 for 1%CB. The better electrochemical performance is attributed to stronger particle connectivity in the zinc slurry promoted by the carbon additive, which leads to a higher electrochemically active area (i.e., surface area of percolating zinc particles). However, at higher current densities, corresponding with the mass transfer limitation region, an increasing carbon content led to a decrease in electrochemical performance. The presence of carbon can difficult the Energiesaccess 2020of OH, 13,- 4482to the zinc particles by blocking them (as they are smaller) which results in the 7mass of 12 transfer being hindered by the excess of carbon in the slurry. Nonetheless, the slurry with 0.2%CB still outperformed the one without additive over the whole range of current densities tested. it can be observed that, at low current densities, corresponding with the kinetic or activation region, Moreover, the maximum power density of the samples with CB was also higher. The measured the zinc slurries with carbon exhibited higher current densities than the one without. At 0.6 V, values were 23 mW∙cm−2 for 0.2%CB, followed by 22 mW∙cm−2 for 0.6%CB, and 17 mW∙cm−2 for 1%CB, the current density of each slurry was 24 mA cm 2 for 0CB, 38 mA cm 2 for 0.2%CB, 36 mA cm 2 for whereas the zinc slurry with no carbon additive· − showed the lowest· maximum− power density· −of 16 0.6%CB, and 28 mA cm 2 for 1%CB. The better electrochemical performance is attributed to stronger mW cm−2. · − particle connectivity in the zinc slurry promoted by the carbon additive, which leads to a higher From the slopes of the polarization curves, we can observe that the ohmic resistance seems to electrochemically active area (i.e., surface area of percolating zinc particles). However, at higher current increase with increasing carbon content. It should be noted that the ohmic resistance covers both the densities, corresponding with the mass transfer limitation region, an increasing carbon content led to a ionic and electronic resistance, which in slurry electrodes are comparable [47]. Therefore, the stronger decrease in electrochemical performance. The presence of carbon can diﬃcult the access of OH to the network of particles in the slurry promoted by the carbon leads to better connectivity leading to− lower zinc particles by blocking them (as they are smaller) which results in the mass transfer being hindered electronic resistance. However, as the carbon content increases, the ionic resistance also increases as by the excess of carbon in the slurry. Nonetheless, the slurry with 0.2%CB still outperformed the one the volume fraction of electrolyte (equivalent to electrode porosity) decreases. Similarly, the intrinsic without additive over the whole range of current densities tested. Moreover, the maximum power porosity of the carbon particles (i.e., pores inside the CB particles that can be filled with2 electrolyte) density of the samples with CB was also higher. The measured values were 23 mW cm− for 0.2%CB, could further difficult the2 access of hydroxyl ions to the2 zinc particles. It can be also· observed that followed by 22 mW cm− for 0.6%CB, and 17 mW cm− for 1%CB, whereas the zinc slurry with no more viscous slurries· tend to increase the mass transfer· resistance of the system.2 carbon additive showed the lowest maximum power density of 16 mW cm− .

(a) (b)

Figure 5. (a) Discharge polarization curves with their power densities and ( b) charge polarization of the zinc slurries with diﬀfferenterent carboncarbon contents.contents.

Furthermore,From the slopes the of charging the polarization polarization curves, curves we can(Figure observe 5b) thatshow the that ohmic an resistanceincrease in seems carbon to contentincrease leads with increasingto a lower carboncharging content. performance. It should This be notedcan be, that again, the ohmicrelated resistance to the increasing covers both ohmic the resistanceionic and electronicgiven by the resistance, lower ionic which conductivity. in slurry electrodes In this case, are comparablethe charge transfer [47]. Therefore, resistance the of strongerthe zinc electrodenetwork of does particles not seem in the to slurry play a promoted big role as by in the di carbonscharge, leads therefore, to better the connectivity higher electroactive leading to area lower of theelectronic slurries resistance. with zinc However,particles does as the not carbon influenc contente the charge increases, transfer the ionic polarization resistance significantly. also increases as the volume fraction of electrolyte (equivalent to electrode porosity) decreases. Similarly, the intrinsic porosity3.3. Cycling of thePerformance carbon particles (i.e., pores inside the CB particles that can be filled with electrolyte) couldNext, further we distudiedfficult the the galvanostatic access of hydroxyl cycling ions perfor to themance zinc of particles. the slurries, It can the be results also observedare presented that inmore Figure viscous 6 below. slurries All the tend slurries to increase were thesuccessfully mass transfer cycled resistance at 10 mA of∙cm the−2, system.but at 20 mA∙cm−2 the slurry Furthermore, the charging polarization curves (Figure5b) show that an increase in carbon content leads to a lower charging performance. This can be, again, related to the increasing ohmic resistance given by the lower ionic conductivity. In this case, the charge transfer resistance of the zinc electrode does not seem to play a big role as in discharge, therefore, the higher electroactive area of the slurries with zinc particles does not influence the charge transfer polarization significantly.

3.3. Cycling Performance Next, we studied the galvanostatic cycling performance of the slurries, the results are presented in Figure6 below. All the slurries were successfully cycled at 10 mA cm 2, but at 20 mA cm 2 the slurry · − · − EnergiesEnergiesEnergies 2020 20202020,,, 13 1313,,, 4482 xx FORFOR PEERPEER REVIEWREVIEW 888 ofofof 121212 withwith 1%CB1%CB failedfailed toto completecomplete thethe 10-min10-min galvanosgalvanostatictatic charge-dischargecharge-discharge cycle,cycle, asas itit surpassedsurpassed thethe with 1%CB failed to complete the 10-min galvanostatic charge-discharge−2 cycle, as it surpassed the chargecharge cutoffcutoff voltage.voltage. Similarly,Similarly, itit cancan bebe notednoted thatthat eveneven atat 1010 mAmA∙∙cmcm−2 thethe voltagevoltage ofof thethe 1%CB1%CB charge cutoff voltage. Similarly, it can be noted that even at 10 mA cm 2 the voltage of the 1%CB slurry slurryslurry duringduring chargecharge waswas higherhigher thanthan thethe rest,rest, whicwhichh agreesagrees withwith· thethe− polarizationpolarization results.results. TheThe mainmain during charge was higher than the rest, which agrees with the polarization results. The main reason reasonreason forfor thisthis waswas thethe decreasedecrease inin ionicionic conductivity,conductivity, asas discusseddiscussed previously.previously. Moreover,Moreover, itit waswas for this was the decrease in ionic conductivity, as discussed previously. Moreover, it was observed observedobserved duringduring cyclingcycling thatthat thethe slurriesslurries withwith highhigh carboncarbon contentscontents (particularly(particularly 1%CB)1%CB) werewere notnot ableable during cycling that the slurries with high carbon contents (particularly 1%CB) were not able to flow toto flowflow smoothlysmoothly intointo thethe cellcell leadingleading toto thethe presencepresence ofof airair bubblesbubbles inin thethe channel.channel. SuchSuch bubblesbubbles smoothly into the cell leading to the presence of air bubbles in the channel. Such bubbles would reduce wouldwould reducereduce thethe effectiveeffective activeactive massmass inin thethe cellcell and,and, therefore,therefore, leadlead toto thethe lowerlower cyclingcycling the effective active mass in the cell and, therefore, lead to the lower cycling performance. performance.performance.

FigureFigure 6.6. Charge-dischargeCharge-discharge curvescurves ofof thethe didifferentdifferentﬀerent slurries.slurries.slurries.

OnOn thethe other other hand, hand, thethe restrest rest ofof of thethe the slurriesslurries slurries presenpresen presentedtedted aa asimilarsimilar similar cyclingcycling cycling behaviorbehavior behavior atat atbothboth both 1010 10 andand and 2020 −2 2 20mA mA∙cmcm−2. The− . energy The energy efficiency efficiency of theof 1st the and 1st 4th and cycles 4th at cycles each atcurrent each currentdensity is density shown is in shown Figure in7. mA∙cm· . The energy efficiency of the 1st and 4th cycles at each current density is shown in Figure 7. −2 2 FigureAt 10 7mA. At∙cm 10−2 mA, the cmenergy− , the efficiency energy e ffiin ciencythe 1st in cycle the 1stwas cycle around was around55% for55% all the for samples, all the samples, and it At 10 mA∙cm , the· energy efficiency in the 1st cycle was around 55% for all the samples, and it −2 2 anddropped it dropped to around to around 50% in 50%the 4th in thecycle. 4th At cycle. 20 mA At∙cm 20− mA2, thecm slurry− , the with slurry no carbon with no showed carbon a showed slightly dropped to around 50% in the 4th cycle. At 20 mA∙cm , the· slurry with no carbon showed a slightly ahigherhigher slightly energyenergy higher efficiencyefficiency energy e(46%)(46%)fficiency comparedcompared (46%) comparedtoto thethe slurrislurri toeses the withwith slurries carboncarbon with (43–44%)(43–44%) carbon atat (43–44%) thethe firstfirst cycle.cycle. at the AfterAfter first cycle.fourfour cycles,cycles, After four thethe cycles,energyenergy the efficiencyefficiency energy e offfiof ciency thethe slurryslurry of the withwith slurry nono with carboncarbon no carbon remainedremained remained thethe same,same, the same, whilewhile while forfor the forthe theslurriesslurries slurries withwith with carboncarbon carbon itit showedshowed it showed differentdifferent different trends:trends: trends: 0.2%CB0.2%CB 0.2%CB droppeddropped dropped toto to 40%40% 40% andand and 0.6%CB0.6%CB showed showed anan increaseincreaseincrease tototo 47%.47%.47%. NoteNote thatthat thethe energyenergy eefficiencyefficiencyfficiency waswas limitedlimited byby thethe highhigh voltagevoltagevoltage didifferencedifferencefference betweenbetweenbetween chargechargecharge andandand discharge,discharge,discharge, ratherratherrather thanthanthan by byby the thethe current currentcurrent e efficiency.efficiency.fficiency.

FigureFigure 7.7. EnergyEnergy efficiency eefficiencyﬃciency ofof thethe didifferentdifferentﬀerent slurries.slurries.slurries.

Energies 2020, 13, x FOR PEER REVIEW 9 of 12

3.4. Estimation of Charging Efficiency To demonstrate that zinc plates preferentially on the carbon additive present in the slurry, further experiments were performed. First, each zinc slurry was charged for 20 min at 20 mA∙cm−2. EnergiesThen, after2020, 13the, 4482 charge was completed, the zinc slurries were replaced by 10 KOH. This experiment9 of 12 aimed to estimate the discharge capacity of the zinc plated on the bipolar plate. It can be noted in Figure 8 that the discharge times were longer than the charge, leading to coulombic efficiencies over 3.4. Estimation of Charging Efficiency 100%. This is associated with two factors: (a) the existence of some dead volumes in the corners of the flow Tofield demonstrate could lead that to remaining zinc plates slurry preferentially present in on the the cell carbon when additive discharging; present (b) in the the zinc slurry, plated further on 2 experimentsthe bipolar plate were grows performed. in uneven First, shapes, each zinc which slurry coul wasd lead charged to zinc for particles 20 min atfrom 20 mAthe cmslurry− . Then,being · aftertrapped the into charge the was plated completed, zinc. the zinc slurries were replaced by 10 KOH. This experiment aimed to estimateFrom the the discharge curves in capacity Figure of8 we the can zinc estimate plated on th thee coulombic bipolar plate. efficiency It can of be the noted experiment, in Figure 80%CB that theshowed discharge the highest times werewith longer159%, then than 148% the charge, for 0.2%CB, leading followed to coulombic by 136% efficiencies for 0.6%CB, over 100%.and 132% This for is associated1%CB. Unfortunately, with two factors: due to (a) the the above-mentioned existence of some lim deaditations, volumes the experiment in the corners can only of the be flow taken field as coulda qualitative lead to measurement. remaining slurry Nonetheless, present in thehigher cell carb whenon discharging; contents led (b) to theshorter zinc discharge plated on thetimes bipolar with plateKOH growscirculating in uneven through shapes, the cell, which which could can lead be to associ zincated particles with from less zinc the slurry plating being on the trapped bipolar into plate the plated(as shown zinc. in Figure 2).

Figure 8. Charge-discharge curves to estimateestimate zinczinc platingplating onon thethe bipolarbipolar plate:plate: charge with each zinc slurryslurry andand dischargedischarge afterafter replacingreplacing thethe slurryslurry withwith 1010 MM KOH.KOH.

FromTo avoid the the curves effects in leading Figure 8to we “over-efficient” can estimate ch thearges, coulombic two slurries e ffi withoutciency ofzinc the particles experiment, were 0%CBprepared showed and tested. the highest The new with samples 159%, then consist 148% of for the 0.2%CB, base electrolyte followed (10 by M 136% KOH, for Carbopol 0.6%CB, and and 132% zinc foroxide) 1%CB. with Unfortunately, 1 wt.% and 2 due wt.% to of the carbon, above-mentioned note that the limitations, volume fraction the experiment of solids can in these only besamples taken asis amuch qualitative lower measurement.than in the zinc Nonetheless, slurries. As higher opposed carbon to the contents previous led tocycling shorter experiments, discharge times the withnew KOHslurries circulating must be throughcharged thefirst: cell, they which cannot can be be directly associated discharged with less as zinc they plating contain on no the zinc bipolar particles. plate (asThe shown results in of Figure the cycling2). tests are shown in Figure 9. To avoid the effects leading to “over-efficient” charges, two slurries without zinc particles were prepared and tested. The new samples consist of the base electrolyte (10 M KOH, Carbopol and zinc oxide) with 1 wt.% and 2 wt.% of carbon, note that the volume fraction of solids in these samples is much lower than in the zinc slurries. As opposed to the previous cycling experiments, the new slurries must be charged first: they cannot be directly discharged as they contain no zinc particles. The results of the cycling tests are shown in Figure9. As shown in Figure9, both slurries with 1 wt.% and 2 wt.% carbon could be cycled at 10 mA cm 2. · − However, at 20 mA cm 2, only the slurry with 2 wt.% of carbon was able to charge and discharge as · − 1 wt.% surpassed the experimental charging cut-off voltage (which leads to the premature termination 2 of the experiment). With 2 wt.% of carbon, at 10 mA cm− , charging voltage is lower than 1 wt.% · 2 carbon, which leads to stable charge-discharge performance at 20 mA cm− . The coulombic efficiency 2 · at 20 mA cm− of the slurry with 2 wt.% carbon was 89%, which suggests that hydrogen evolution was · occurring as a side reaction (due to the low voltage in the negative electrode). To prove that the zinc was charging on the carbon particles in the slurry not on the bipolar plate, with 2 wt.% carbon, after the last charge the slurry was removed and 10 M KOH was circulated and discharged. The coulombic Energies 2020, 13, 4482 10 of 12 efficiency, in this case, was only 4%, which confirms that there was almost no zinc plating on the bipolar whenEnergies charging. 2020, 13, x FOR PEER REVIEW 10 of 12

FigureFigure 9.9.Charge-discharge Charge-discharge curvescurves ofof slurriesslurrieswithout without zinc zinc particles. particles.

4. Conclusions As shown in Figure 9, both slurries with 1 wt.% and 2 wt.% carbon could be cycled at 10 mA∙cm−2. However,In conclusion, at 20 mA slurries∙cm−2, only with the di ffslurryerent ratioswith 2of wt.% carbon of carbon additives was were able introducedto charge and to enhancedischarge the as performance1 wt.% surpassed of zinc slurrythe experimental air flow batteries charging aiming cut-off to prevent voltage zinc plating(which onleads the bipolarto the platepremature when charging.termination The of rheology the experiment). of zinc-carbon With 2 slurrieswt.% of wascarbon, analyzed at 10 mA revealing∙cm−2, charging that, despite voltage higher is lower viscosity than values,1 wt.% highercarbon, carbon which contents leads to also stable led charge-discharge to stronger networks performance characterized at 20 by mA higher∙cm−2.storage The coulombic moduli valuesefficiency and at yield 20 stressesmA∙cm− that2 of the are beneficialslurry with to RFBs.2 wt.% Although carbon was the zinc89%, slurry which without suggests carbon that additiveshydrogen showsevolution relatively was occurring stable performances as a side reaction for both (due charging to the low and voltage discharging, in the itnegative was found electrode). that zinc To plated prove onthat the the bipolar zinc was plate charging during on charge. the carbon On the particles otherhand, in the whenslurry carbon not on wasthe bipolar added toplate, the with zinc slurry,2 wt.% thecarbon, discharge after the power last densitycharge the of theslurry system was remo was enhancedved and 10 and M KOH the zinc was plating circulated during and chargedischarged. was shiftedThe coulombic from the efficiency, bipolar to in the this carbon case, additive was only in 4% the, which slurry. confirms This was that proved there by was charge-discharge almost no zinc experimentsplating on the using bipolar carbon when slurries charging. without zinc particles. When the zinc plating is shifted from the bipolar plate to the carbon additives in the slurry, a higher total battery capacity can be achieved. 4. Conclusions Author Contributions: Conceptualization, N.H.C., D.d.O. and P.F.; Methodology, N.H.C. and D.M.; Software, D.d.O.;In Writing—original conclusion, slurries draft with preparation, different N.H.C.; ratios Resources, of carbon N.H.C. additives and D.M.;were Writing—reviewintroduced to enhance and editing, the N.H.C.,performance D.d.O., of D.M., zinc N.E.K.slurry andair flow P.F.; Supervision,batteries aiming P.F., K.P. to prevent and J.T. zinc All authors plating have on the read bipolar and agreed plate towhen the publishedcharging. version The rheology of the manuscript. of zinc-carbon slurries was analyzed revealing that, despite higher viscosity Funding:values, higherThis project carbon has contents received also funding led fromto strong the Europeaner networks Union’s characterized Horizon 2020 by researchhigher storage and innovation moduli programme under the Marie Skłodowska-Curie Grant Agreement no. 765289. values and yield stresses that are beneficial to RFBs. Although the zinc slurry without carbon Conflictsadditives of shows Interest: relativelyThe authors stable declare performances no conflict offor interest. both charging and discharging, it was found that zinc plated on the bipolar plate during charge. On the other hand, when carbon was added to the Referenceszinc slurry, the discharge power density of the system was enhanced and the zinc plating during 1.chargeSperstad, was shifted I.B.; Korpås, from the M. bipolar Energy to Storage the carbon Scheduling additive in Distribution in the slurry. Systems This was Considering proved by Wind charge- and dischargePhotovoltaic experiments Generation using Uncertainties. carbon slurriesEnergies without2019, 12 zinc, 1231. particles. [CrossRef When] the zinc plating is shifted 2.fromDing, the bipolar Y.; Zhang, plate C.; Zhang,to the L.;carbon Zhou, additives Y.; Yu, G. in Pathways the slurry, to Widespread a higher total Applications: battery capacity Development can ofbe achieved.Redox Flow Batteries Based on New Chemistries. Chem 2019, 5, 1964–1987. [CrossRef] 3. Caramia, V.; Bozzini, B. Materials science aspects of zinc-air batteries: A review. Mater. Renew. Sustain. Energy Author2014 Contributions:, 3, 28. [CrossRef Conceptualization,] N.H.C., D.d.O. and P.F.; Methodology, N.H.C. and D.M.; Software, 4.D.d.O.;Gentil, Writing—original S.; Reynard, D.; draft Girault, preparation, H.H. Aqueous N.H.C.; organic Resources, and redox-mediatedN.H.C. and D.M.; redox Writing—review flow batteries: and A editing, review. N.H.C.,Curr. D.d.O., Opin. D.M., Electrochem. N.E.K.2020 and, P.F.;21, 7–13. Supervision, [CrossRef P.F.,] K.P. and J.T. All authors have read and agreed to the 5.publishedChoi, version N.H.; del of Olmo,the manuscript. D.; Fischer, P.; Karsten, P.; Tübke, J. Development of flow fields for Zinc Slurry Air Funding:Flow This Batteries. projectBatteries has received2020, 6 ,funding 15. [CrossRef from the] European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement no. 765289.

Conflicts of Interest: The authors declare no conflicts of interest.

Energies 2020, 13, 4482 11 of 12

6. Bockelmann, M.; Kunz, U.; Turek, T. Electrically rechargeable zinc-oxygen flow battery with high power density. Electrochem. Commun. 2016, 69, 24–27. [CrossRef] 7. Jiratchayamaethasakul, C.; Srijaroenpramong, N.; Bunyangyuen, T.; Arpavate, W.; Wongyao, N.; Therdthianwong, A.; Therdthianwong, S. Effects of anode orientation and flow channel design on performance of refuelable zinc-air fuel cells. J. Appl. Electrochem. 2014, 44, 1205–1218. [CrossRef] 8. Li, Y.; Gong, M.; Liang, Y.; Feng, J.; Kim, J.-E.; Wang, H.; Hong, G.; Zhang, B.; Dai, H. Advanced zinc-air batteries based on high-performance hybrid electrocatalysts. Nat. Commun. 2013, 4, 1–7. [CrossRef] [PubMed] 9. Zelger, C.; Süßenbacher, M.; Laskos, A.; Gollas, B. State of charge indicators for alkaline zinc-air redox flow batteries. J. Power Sources 2019, 424, 76–81. [CrossRef] 10. Wang, K.; Yu, J. Lifetime simulation of rechargeable zinc-air battery based on electrode aging. J. Energy Storage 2020, 28, 101191. [CrossRef] 11. Mainar, A.; Iruin, E.; Colmenares, L.; Blázquez, J.; Grande, H.-J. Systematic cycle life assessment of a secondary zinc–air battery as a function of the alkaline electrolyte composition. Energy Sci. Eng. 2018, 6, 174–186. [CrossRef] 12. Ma, H.; Wang, B.; Fan, Y.; Hong, W. Development and Characterization of an Electrically Rechargeable Zinc-Air Battery Stack. Energies 2014, 7, 6548–6557. [CrossRef] 13. Fu, J.; Cano, Z.P.; Park, M.G.; Yu, A.; Fowler, M.; Chen, Z. Electrically Rechargeable Zinc-Air Batteries: Progress Challenges and Perspectives. Adv. Mater. 2016, 29, 1604685. [CrossRef][PubMed] 14. Aremu, E.O.; Park, D.J.; Ryu, K.S. The effects of anode additives towards suppressing dendrite growth and hydrogen gas evolution reaction in Zn-air secondary batteries. Ionics 2019, 25, 4197–4207. [CrossRef] 15. Zhao, Z.; Fan, X.; Ding, J.; Hu, W.; Zhong, C.; Lu, J. Challengis in Zinc Electrodes for Alkaline Zinc-Air Batteries: Obstacles to commercialization. ACS Energy Lett. 2019, 4, 2259–2270. [CrossRef] 16. Wang, K.; Pei, P.; Ma, Z.; Chen, H.; Xu, H.; Chen, D.; Wang, X. Dendrite growth in the recharging process of zinc-air batteries. J. Mater. Chem. A 2015, 3, 22648–22655. [CrossRef] 17. Liang, S.; Yan, W.; Wu, X.; Zhang, Y.; Zhu, Y.; Wang, H.; Wu, Y. Gel polymer electrolytes for lithium ion batteries: Fabrication, characterization and performance. Solid State Ion. 2018, 318, 2–18. [CrossRef] 18. Smith, K.C.; Chiang, Y.-M.; Carter, W.C. Maximizing Energetic Efficiency in Flow Batteries Utilizing Non-Newtonian Fluids. J. Electrochem. Soc. 2014, 161, A486–A496. [CrossRef] 19. Cheng, X.; Pan, J.; Zhao, Y.; Liao, M.; Peng, H. Gel Polymer Electrolytes for Electrochemical Energy Storage. Adv. Energy Mater. 2018, 8, 1702184. [CrossRef] 20. Zhang, Z.; Zuo, C.; Liu, Z.; Yu, Y.; Zuo, Y.; Song, Y. All-solid-state Al-air batteries with polymer alkaline gel electrolyte. J. Power Sources 2014, 251, 470–475. [CrossRef] 21. Lee, S.M.; Kim, Y.J.; Eom, S.W.; Choi, N.S.; Kim, K.W.; Cho, S.B. Improvement in self-discharge of Zn anode by applying surface modification for Zn-air batteries with high energy density. J. Power Sources 2013, 227, 177–184. [CrossRef] 22. Tran, T.N.T.; Chung, H.J.; Ivey, D.G. A study of alkaline gel polymer electrolytes for rechargeable zinc–air batteries. Electrochim. Acta 2019, 327, 135021. [CrossRef] 23. Piau, J.M. Carbopol gels: Elastoviscoplastic and slippery glasses made of individual swollen sponges. Meso- and macroscopic properties, constitutive equations and scaling laws. J. Nonnewton. Fluid Mech. 2007, 144, 1–29. [CrossRef] 24. Mainar, A.R.; Leonet, O.; Bengoechea, M.; Boyano, I.; de Meatza, I.; Kvasha, A.; Alberto Blázquez, J. Alkaline aqueous electrolytes for secondary zinc-air batteries: An overview. Int. J. Energy Res. 2016, 40, 1032–1049. [CrossRef] 25. Othman, R.; Yahaya, A.H.; Arof, A.K. A zinc-air cell employing a porous zinc electrode fabricated from zinc-graphite-natural biodegradable polymer paste. J. Appl. Electrochem. 2002, 32, 1347–1353. [CrossRef] 26. Masri, M.N.; Mohamad, A.A. Effect of adding carbon black to a porous zinc anode in a zinc-air battery. J. Electrochem. Soc. 2013, 160, A715. [CrossRef] 27. Tao, H.; Tong, X.; Gan, L.; Zhang, S.; Zhang, X.; Liu, X. Effect of adding various carbon additives to porous zinc anode in rechargeable hybrid aqueous battery. J. Alloys Compd. 2016, 658, 119–124. [CrossRef] 28. Chotipanich, J.; Arpornwichanop, A.; Yonezawa, T.; Kheawhom, S. Electronic and ionic conductivities enhancement of zinc anode for flexible printed zinc-air battery. Eng. J. 2018, 22, 47–57. [CrossRef] 29. Duduta, M.; Ho, B.; Wood, V.C.; Limthongkul, P.; Brunini, V.E.; Carter, W.C.; Chiang, Y.-M. Semi-Solid Lithium Rechargeable Flow Battery. Adv. Energy Mater. 2011, 1, 511–516. [CrossRef] Energies 2020, 13, 4482 12 of 12

30. Youssry, M.; Madec, L.; Soudan, P.; Cerbelaud, M.; Guyomard, D.; Lestriez, B. Non-aqueous carbon black suspensions for lithium-based redox flow batteries: Rheology and simultaneous rheo-electrical behavior. Phys. Chem. Chem. Phys. 2013, 15, 14476–14486. [CrossRef] 31. Youssry, M.; Madec, L.; Soudan, P.; Cerbelaud, M.; Guyomard, D.; Lestriez, B. Formulation of flowable anolyte for redox flow batteries: Rheo-electrical study. J. Power Sources 2015, 274, 424–431. [CrossRef] 32. Shukla, G.; del Olmo Diaz, D.; Thangavel, V.; Franco, A.A. Self-Organization of Electroactive Suspensions in Discharging Slurry Batteries: A Mesoscale Modeling Investigation. ACS Appl. Mater. Interfaces 2017, 9, 17882–17889. [CrossRef][PubMed] 33. Shukla, G.; Franco, A.A. Handling Complexity of Semi-solid Redox Flow Battery Operation Principles Through Mechanistic Simulations. J. Phys. Chem. 2018, 122, 23867–23877. 34. Hoyt, N.C.; Wainright, J.S.; Savinell, R.F. Current Density Scaling in Electrochemical Flow Capacitors. J. Electrochem. Soc. 2015, 162, A1102–A1110. [CrossRef] 35. Akuzum, B.; Singh, P.; Eichfeld, D.A.; Agartan, L.; Uzun, S.; Gogotsi, Y.; Kumbur, E.C. Percolation Characteristics of Conductive Additives for Capacitive Flowable (Semi-Solid) Electrodes. ACS Appl. Mater. Interfaces 2020, 12, 5866–5875. [CrossRef] 36. Kisdarjono, H.; Lu, Y.; Lee, J.-J.; Evans, D.; Wang, L. Air Cathode Battery Using Zinc Slurry Anode with Carbon Additive. Available online: https://patents.google.com/patent/US20140370401A1/en (accessed on 1 July 2020). 37. Hreiz, R.; Adouani, N.; Fünfschilling, D.; Marchal, P.; Pons, M.N. Rheological characterization of raw and anaerobically digested cow slurry. Chem. Eng. Res. Des. 2017, 119, 47–57. [CrossRef] 38. Hermoso, J.; Jofore, B.D.; Martínez-Boza, F.J.; Gallegos, C. High pressure mixing rheology of drilling fluids. Ind. Eng. Chem. Res. 2012, 51, 14399–14407. [CrossRef] 39. Owens, C.E.; Hart, A.J.; McKinley, G.H. Improved rheometry of yield stress fluids using bespoke fractal 3D printed vanes. J. Rheol. 2020, 64, 643–662. [CrossRef] 40. Baravian, C.; Lalante, A.; Parker, A. Vane rheometry with a large, finite gap. Appl. Rheol. 2002, 12, 81–87. [CrossRef] 41. Helal, A.; Divoux, T.; McKinley, G.H. Simultaneous Rheoelectric Measurements of Strongly Conductive Complex Fluids. Phys. Rev. Appl. 2016, 6, 064004. [CrossRef] 42. Ito, Y.; Nyce, M.; Plivelich, R.; Klein, M.; Steingart, D.; Banerjee, S. Zinc morphology in zinc–nickel flow assisted batteries and impact on performance. J. Power Sources 2011, 196, 2340–2345. [CrossRef] 43. Wei, T.S.; Fan, F.Y.; Helal, A.; Smith, K.C.; McKinley, G.H.; Chiang, Y.M.; Lewis, J.A. Biphasic Electrode Suspensions for Li-Ion Semi-solid Flow Cells with High Energy Density, Fast Charge Transport, and Low-Dissipation Flow. Adv. Energy Mater. 2015, 5, 1500535. [CrossRef] 44. Dinkgreve, M.; Paredes, J.; Denn, M.M.; Bonn, D. On different ways of measuring ‘the’ yield stress. J. Nonnewton. Fluid Mech. 2016, 238, 233–241. [CrossRef] 45. Gallier, S.; Lemaire, E.; Peters, F.; Lobry, L. Rheology of sheared suspensions of rough frictional particles. J. Fluid Mech. 2014, 757, 514–549. [CrossRef] 46. Tang, A.; Bao, J.; Skyllas-Kazacos, M. Studies on pressure losses and flow rate optimization in vanadium redox flow battery. J. Power Sources 2014, 248, 154–162. [CrossRef] 47. Petek, T.J.; Hoyt, N.C.; Savinell, R.F.; Wainright, J.S. Characterizing Slurry Electrodes Using Electrochemical Impedance Spectroscopy. J. Electrochem. Soc. 2015, 163, A5001–A5009. [CrossRef]

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).