Electrocatalysts for Using Renewably-Sourced, Organic Electrolytes for Redox Flow Batteries

catalysts

Perspective Electrocatalysts for Using Renewably-Sourced, Organic Electrolytes for Redox Flow Batteries

Robert S. Weber

Paciﬁc Northwest National Laboratory, Institute for Integrated Catalysis, Richland, DC 99352, USA; [email protected]

Abstract: Biomass could be a source of the redox shuttles that have shown promise for operation as high potential, organic electrolytes for redox flow batteries. There is a sufficient quantity of biomass to satisfy the growing demand to buffer the episodic nature of renewably produced electricity. However, despite a century of effort, it is still not evident how to use existing information from organic electrochemistry to design the electrocatalysts or supporting electrolytes that will confer the required activity, selectivity and longevity. In this research, the use of a fiducial reaction to normalize reaction rates is shown to fail.

Keywords: biomass; redox ﬂow batteries; electrocatalysts

1. Introduction Catalysis is a key component of technologies that promise to satisfy the growing demand for clean energy. However, we do not yet have ready access to correlations for selecting or interpreting the performance of electrocatalysts for storing the quantities of Citation: Weber, R.S. Electrocatalysts electricity that will permit grid-scale use of renewable resources (e.g., wind power and solar for Using Renewably-Sourced, power). This article proposes the use of an infrequently employed approach to normalize Organic Electrolytes for Redox Flow electrocatalytic reaction rates, namely, a fiducial reaction. Here, fiducial means “faithful”, Batteries. Catalysts 2021, 11, 315. https://doi.org/10.3390/catal11030315 in the sense that the fiducial reaction faithfully tracks the number and rate of the active sites. While the discussion below was motivated by our study of electrocatalysts for redox

Academic Editors: Justo Lobato and flow batteries, this article is not intended to be a review of that technology, nor to bear on Kelsey Stoerzinger the choice of the electrolyte. Rather, it introduces the idea of a fiducial reaction as a way to systematize the rates of electrocatalytic reactions. The examples shown and discussed Received: 29 December 2020 below illustrate an instance in which a fiducial reaction works and one in which it does Accepted: 22 February 2021 not, for the particular cases of electrolytes derived from biomass. Published: 28 February 2021 Projections from the International Energy Agency (IEA) [1] indicate that the installed capacity of renewable energy may soon surpass the installed capacity of coal- and natural Publisher’s Note: MDPI stays neutral gas-fired powerplants. However, because the availability of the renewable resources fluctu- with regard to jurisdictional claims in ates seasonally, diurnally and on even shorter times scales [2] (e.g., momentarily becalmed published maps and institutional affil- turbines, clouds passing overhead), the installed availability averages only about 40–70% of iations. the world’s installed capacity [3]. In the U.S., after nearly two decades of steady growth [4], there is sufficient battery capacity to cover about 0.1% of the power demand but only about 3 × 10−5% of the energy demand (Figure1). Therefore, supplying smooth, always-on power that the modern world demands will require technologies that can buffer multiple tranches Copyright: © 2021 by the author. of both power and energy. Consider that the characteristic power of a modern wind turbine Licensee MDPI, Basel, Switzerland. is on the order of 2 MW per generator [5]. The amount of energy to be buffered for a short This article is an open access article interruption in its operation is on the order of 10–100 MJ (= 2 MW × 5–50 s). distributed under the terms and Despite recent acceleration in the rate of growth of both capacities, projected devel- conditions of the Creative Commons opments in rechargeable batteries may satisfy power demands but not energy demands Attribution (CC BY) license (https:// across days or seasons. creativecommons.org/licenses/by/ 4.0/).

Catalysts 2021, 11, 315. https://doi.org/10.3390/catal11030315 https://www.mdpi.com/journal/catalysts Catalysts 2021, 11, 315 2 of 8

2. Flow Batteries In flow batteries, an electrochemically reversibly oxidizable and reducible species shuttles between the anode and cathode. Because the volume of the storage vessel can be independent of the size of the electrolysis cell, a flow battery, in principle, can satisfy the need for both large amounts of stored energy and large rates of energy delivery (power). Their features and limitations have already been reviewed elsewhere [2,6–9]. To store energy, the electrolyte in its oxidized form is reduced to a lower oxidation state. Then, when the energy is needed, the reduced electrolyte is directed to the anode of the electrolysis cell, where it is reoxidized. However, the species (or mixtures of them) that offer the right balance of rate and thermodynamics (potential difference) have proved to be costly and environmentally burdensome [6]. Here, we consider the implications for catalysis of substituting inorganic shuttles (e.g., V3+/5+, Br) with renewable energy carriers (e.g., liquids or solutes derived from biomass). Biomass-derived fuels are abundant enough to fulfill only about 6% of the global demand for fuel [10]. However, as redox looping agents, their annual production could nearly buffer the increase in the global demand for renewable power. About 160 Mt of lignin is produced each year for making pulp and paper [11]. Mostly, it is now burned for process heat. Instead, the lignin could be a feedstock for an organic electrolyte. Suppose, as an example, that a redox shuttle had a molecular weight in the range of 150 g/mol and that it could shuttle 2e− at 3 V cell potential. That high a voltage is in the range of recent developments in organic electrolytes [12–15]. Lignin comprises about 25% of the mass of the wood that is used for making pulp [16]. Therefore, its annual production could source an amount of electrolyte sufficient to store about half of the energy (Figure1) that is Catalysts 2021, 11, x FOR PEER REVIEWprojected to be produced from renewable, episodic sources like wind and solar [17]. That2 of is 8

enough energy to approximately buffer diurnal cycling.

Figure 1. ConsumptionConsumption or or storage storage of of renewable renewable electricity. electricity. The The solid solid black black line line shows shows projected projected global consumption of renewable electricity [6]. The dashed line shows how much electricity global consumption of renewable electricity [18]. The dashed line shows how much electricity might might be stored in a redox flow battery whose electrolytes were derived from the 160 Mt of lignin be stored in a redox ﬂow battery whose electrolytes were derived from the 160 Mt of lignin produced produced globally each year by the pulp and paper industry [7] if lignin were the source of a re- globallydox shuttle, each had year a molecular by the pulp weight and paper of 150 industry g/mol, and [11 ]could if lignin store were and the release source 2e− of and a redox an aspira- shuttle, − hadtional a molecular[8] cell voltage weight of 3 of V. 150 The g/mol, lowest and curve could is a storerecent and projection release 2efor theand production an aspirational of storage [12–15 ] cellbatteries voltage for ofall 3 applications, V. The lowest increasing curve is a at recent a compound projection annual for the growth production rate (CAGR) of storage of 21%. batteries for all applications, increasing at a compound annual growth rate (CAGR) of 21%.

2.1. Renewable Organic Redox Shuttles FlowMolecules Batteries that have been considered and tested [6,13,15,19] as organic electrolytes consist of heteronuclear or substituted aromatics. Some of them occur naturally as fragments In flow batteries, an electrochemically reversibly oxidizable and reducible species of lignin (Table1). shuttles between the anode and cathode. Because the volume of the storage vessel can be independent of the size of the electrolysis cell, a flow battery, in principle, can satisfy the need for both large amounts of stored energy and large rates of energy delivery (power). Their features and limitations have already been reviewed elsewhere [2,9–12]. To store energy, the electrolyte in its oxidized form is reduced to a lower oxidation state. Then, when the energy is needed, the reduced electrolyte is directed to the anode of the electrolysis cell, where it is reoxidized. However, the species (or mixtures of them) that offer the right balance of rate and thermodynamics (potential difference) have proved to be costly and environmentally burdensome [9]. Here, we consider the implications for catalysis of substituting inorganic shuttles (e.g., V3+/5+, Br) with renewable energy carriers (e.g., liquids or solutes derived from biomass). Biomass-derived fuels are abundant enough to fulfill only about 6% of the global demand for fuel [13]. However, as redox looping agents, their annual production could nearly buffer the increase in the global demand for renewable power. About 160 Mt of lignin is produced each year for making pulp and paper [7]. Mostly, it is now burned for process heat. Instead, the lignin could be a feedstock for an organic electrolyte. Suppose, as an example, that a redox shuttle had a molecular weight in the range of 150 g/mol and that it could shuttle 2e− at 3 V cell potential. That high a voltage is in the range of recent developments in organic electrolytes [8,14–16]. Lignin comprises about 25% of the mass of the wood that is used for making pulp [17]. Therefore, its annual production could source an amount of electrolyte sufficient to store about half of the energy (Figure 1) that is projected to be produced from renewable, episodic sources like wind and solar [18]. That is enough energy to approximately buffer diurnal cycling.

Renewable Organic Redox Shuttles Molecules that have been considered and tested [9,14,16,19] as organic electrolytes consist of heteronuclear or substituted aromatics. Some of them occur naturally as fragments of lignin (Table 1).

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Table 1. Examples of organic electrolytes that operate at high potential that could be derived from TableTable 1. 1. ExamplesExamples of oforganic organic electrolytes electrolytes that that oper operateate at high at highpotential potential that could that couldbe derived be derived from Tablelignin .1. Examples of organic electrolytes that operate at high potential that could be derived from ligninfrom. lignin. lignin. Molecular Compound Molecular −1 Reduction Potential/V Reference CompoundCompound MolecularWeight/gMolecular Weight/g mol mol-1 Reduction Potential/V Potential/V ReferenceReference Compound Weight/g mol-1 Reduction Potential/V Reference Weight/g mol-1

~160~160 2.4 2.4–2.7–2.7 [20] [20 ] ~160 2.4–2.7 [20] ~160 2.4–2.7 [20]

~160 ~3 [15,16] ~160~160 ~3~3 [[15,1614,15] ] ~160 ~3 [15,16]

~150 ~3 [16] ~150 ~3 [16] ~150~150 ~3 ~3 [16] [15 ]

The characteristics of an electrocatalyst suitable for use with an organic electrolyte The characteristics of an electrocatalyst suitable for use with an organic electrolyte are: 1)The high characteristics electrochemical of an reaction electrocatalyst rates (i.e., suitable at a small for useoverpotential), with an organic 2) selectivity electrolyte (to are: 1) high electrochemical reaction rates (i.e., at a small overpotential), 2) selectivity (to areavoid: 1) undesiredhigh electrochemical side reactions) reaction and 3)rates long (i.e., times at ona small stream. overpotential), These three requirements2) selectivity (toare avoidThe undesired characteristics side reactions) of an electrocatalyst and 3) long times suitable on stream. for use These with three an organic requirements electrolyte are avoidequivalent undesired to the side usual reactions) triad of andcharacteristics 3) long times of anyon stream. industrial These catalyst: three requirementsactivity, selectiv- are equivalentare: (1) high to electrochemicalthe usual triad of reaction characteristics rates (i.e., of at any a small industrial overpotential), catalyst: activity, (2) selectivity selectiv- (to equivalentity and longevity. to the usual triad of characteristics of any industrial catalyst: activity, selectiv- ityavoid and undesired longevity. side reactions) and (3) long times on stream. These three requirements are ity andIn longevity.the case of electrocatalysis, it is impossible to ignore the effect of the electrolyte, equivalentIn the tocase the of usual electrocatalysis, triad of characteristics it is impossible of any to industrial ignore the catalyst: effect activity,of the electrolyte selectivity, whichIn servesthe case to transportof electrocatalysis, the redox itspecies is impossible and which, to ignore unlike thea gaseous effect ofreaction the electrolyte medium,, whichand longevity. serves to transport the redox species and which, unlike a gaseous reaction medium, whichlikely exhibitsserves to str transportong gradients the redox in composition species and and which, structure unlike over a gaseous nanom reactioneter length medium, scales likelyIn exhibits the case str ofong electrocatalysis, gradients in composition it is impossible and structure to ignore over the n effectanom ofeter the length electrolyte, scales likelyin the exhibits double layerstrong adjacent gradients to thein composition catalytic site. and Therefore, structure in over addition nano mtoeter first length architecting scales inwhich the double serves layer to transport adjacent the to redox the catalytic species site. and Therefore, which, unlike in addition a gaseous to reactionfirst architecting medium, inthe the electron double transfer layer adjacent site, the to designer the catalytic of an site. electrocatalyst Therefore, inmust addition also consider to first architecting what could thelikely electron exhibits transfer strong site, gradients the designer in composition of an electrocatalyst and structure must over also nanometer consider length what could scales thejocularly electron be transfercalled the site, “Mr. the Rogers’designer Support of an electrocatalyst Effect”. Such must a support also consider effect facilitates what could the jocularlyin the double be called layer the adjacent “Mr. Rogers’ to the catalytic Support site. Effect Therefore,”. Such ina support addition effect to ﬁrst facilitates architecting the jocularlyapproach be to calledthe neighborhood the “Mr. Rogers’ of an Supportactive site Effect of the”. Suchreaction a support intermediates effect facilitates and solution the approachthe electron to the transfer neighborhood site, the designer of an active of an site electrocatalyst of the reaction must intermediates also consider and what solution could approachphase species to the that neighborhood stabilizes the of antransition active site state. of theThe reaction structure intermediates of an organic and electrolyte solution phasejocularly species be called that stabilize the “Mr.s the Rogers’ transition Support state. Effect”. The structure Such a of support an organic effect electrolyte facilitates phasenear the species surface that of stabilizethe solids catalystthe transition has been state. probed The structurethrough modeling of an organic [21,22 electrolyte], spectro- nearthe approachthe surface to of the the neighborhood solid catalyst has of anbeen active probed site through of the reaction modeling intermediates [21,22], spectro- and nearscopically the surface [23,24 of], and the empirically,solid catalyst through has been the probed use of throughsurfactants modeling [25–28 ].[21,22 ], spectro- scopicallysolution phase [23,24] species, and empirically, that stabilizes through the transitionthe use of surfactants state. The structure[25–28]. of an organic scopicallyThe kinetics [23,24], ofand the empirically, electrocatalytic through reactions the use may of besurfactants complex [25funct–28ions]. of the compo- electrolyteThe kinetics near the of the surface electrocatalytic of the solid reactions catalyst has may been be complex probed through functions modeling of the compo- [21,22], sitionspectroscopicallyThe of thekinetics electrolyte, of the [23 electrocatalytic even,24], andfor substrates empirically, reactions that through are may infinitely be the complex use miscible of surfactants funct withions the of [25 thesupporting–28 compo-]. sition of the electrolyte, even for substrates that are infinitely miscible with the supporting sitionelectrolyte of Thethe. Forelectrolyte, kinetics example, of theeven we electrocatalytic forfound substrates [29] that reactions that the are kinetics infinitely may beof complexthe miscible oxidation functions with of the methanol ofsupporting the composi- and electrolyte. For example, we found [29] that the kinetics of the oxidation of methanol and electrolytewatertion followed of. the For electrolyte, example, Hill–Langmuir we even found forkinetics, substrates[29] that which the that kineticsprevent are inﬁnitely edof thehigh oxidation miscibleextents of withof conversionmethanol the supporting and be- water followed Hill–Langmuir kinetics, which prevented high extents of conversion be- watercauseelectrolyte. followedwater displaced ForHill– example,Langmuir the methanol we kinetics, found from which [29 the] that vicinityprevent the kinetics edof thehigh electroactive of extents the oxidation of conversion site when of methanol be-the cause water displaced the methanol from the vicinity of the electroactive site when the causemethanoland water water concentration displaced followed the Hill–Langmuir fell methanol below about from kinetics, 0.5 the M. vicinity Organics which preventedof such the electroactiveas those high in extents Table site of1when could conversion the be methanol concentration fell below about 0.5 M. Organics such as those in Table 1 could be methanolexpectedbecause toconcentration watersegregate displaced from fell belowthe polar methanol about media 0.5 from M. [30 Organics the] needed vicinity such to ofstabilize as the those electroactive the in Tablepolarized, 1 site could whenlikely be the expected to segregate from the polar media [30] needed to stabilize the polarized, likely expectedchargedmethanol, tocurrent segregate concentration carriers, from leading fellthe belowpolar to a media aboutcomplex 0.5[30 M.dependence] needed Organics to stabilize suchof the as redoxthose the polarized,reaction in Table rates1 couldlikely on be charged, current carriers, leading to a complex dependence of the redox reaction rates on chargedtheexpected concentration, current to segregate carriers, of the substrates. leading from the to polar a complex media dependence [30] needed of to the stabilize redox thereaction polarized, rates on likely the concentration of the substrates. the charged,concentrationThe rates current of electrochemicalof carriers,the substrates. leading oxidations to a complex and reductions dependence catalyzed of the redox by supported reaction rateselec- on The rates of electrochemical oxidations and reductions catalyzed by supported elec- trocatalyststheThe concentration rates are of also electrochemical sensitive of the substrates. to the oxidations composition and reductionsand domain catalyzed size of the by metal supported [31], possi- elec- trocatalysts are also sensitive to the composition and domain size of the metal [31], possi- trocatalystsbly becauseThe are ratesof alsoassociated of sensitive electrochemical changes to the compositionin oxidations surface site and and densities. reductions domain Moreover,size catalyzed of the bymetalthe supported scarcely [31], possi- con- electro- bly because of associated changes in surface site densities. Moreover, the scarcely con- blytrolled, catalystsbecause and of arepossibly associated also sensitivepotential changes to-dependent the in composition surface, interaction site anddensities.domain of the Moreover,metal size ofpartic the the metalles scarcelywith [31 the], possiblycon- sur- trolled, and possibly potential-dependent, interaction of the metal particles with the sur- trolled,facebecause moieties and ofpossibly ofassociated the potential supporting changes-dependent electrode in surface, interaction and site electrolyte densities. of the Moreover, further metal particcomplicate theles scarcely withs efforts the controlled, sur- to face moieties of the supporting electrode and electrolyte further complicates efforts to faceidentifyand moieties possibly correlations of potential-dependent,the supportingbetween the electrode structure interaction andof electrocatalysts electrolyte of the metal further and particles their complicate electrocatalytic with thes efforts surface ac- to moi- identify correlations between the structure of electrocatalysts and their electrocatalytic ac- identifytivity.eties D correlationsespite of the decades supporting between of research electrode the structure [32,33 and], electrolyte ofalong electrocatalysts with further many reviewand complicates their articles electrocatalytic efforts (for example to identify ac-, tivity. Despite decades of research [32,33], along with many review articles (for example, tivity.[31,33,34]correlations Despite), monographs decades between of the[35 research– structure38] and [32,33 handbooks of electrocatalysts], along on with organic ma andny electrochemistry theirreview electrocatalytic articles ( (forfor exampleexample, activity., De- [31,33,34]), monographs [35–38] and handbooks on organic electrochemistry (for example, [31,33,34][39]),spite these decades), monographs complications of research [35 continue– [3832], and33], to alonghandbooks contribute with manyon to organic a lack review of electrochemistry evident articles (forcorrelations example, (for example, to [31 guide,33,34 ]), [39]), these complications continue to contribute to a lack of evident correlations to guide [39]),themonographs choice these ofcomplications catalyst [35–38 and] and continueto handbookstransform to contribute an ony organicparticular to electrochemistrya lack functional of evident group. (forcorrelations example, to [39 guide]), these the choice of catalyst and to transform any particular functional group. the choice of catalyst and to transform any particular functional group.

Catalysts 2021, 11, 315 4 of 8

complications continue to contribute to a lack of evident correlations to guide the choice of catalyst and to transform any particular functional group.

2.2. Choosing/Designing Electrocatalysts for Organic Redox Flow Batteries

Catalysts 2021, 11, x FOR PEER REVIEW Cathode catalysts are usually chosen from platinum group metals,4 of as 8 these elements

are all active as hydrogenation catalysts [6,13–15,21,40–47]. Their utility is limited when the supporting electrolyte is aqueous because they are also active for reducing water (to make Choosing/H2). ThisDesigning limitation Electrocatalysts can be obviated for Organic by usingRedox metalsFlow Batteries with a high overpotential against reducingCathode watercatalysts while are usually still maintaining chosen from platinum the ability group to metals transfer, as these hydrogen elements [48 ]. Alternately, areit all appears active as that hydrogenation some metals catalysts can [9,14 bind–16,21,40 the organic–47]. Their strongly utility enoughis limited towhen lower the surface theconcentration supporting electrolyte of H, such is aqueous that the because H–H they combination are also active reaction for reducing is suppressed water (to [41]. make HAnode2). This limitation catalysts can are be also obviated usually by using chosen metals from with carbon-supported a high overpotential platinum group against reducing water while still maintaining the ability to transfer hydrogen [48]. Alter- metals or metal oxides [44,49,50] because such materials do not readily oxidize. Boron- nately, it appears that some metals can bind the organic strongly enough to lower the surfacedoped concentration diamond offersof H, such a high that overpotentialthe H–H combination for the reaction oxygen is suppressed evolution [41]. reaction [51–55], and isAnode therefore catalysts useful are also for usually oxidizing chosen organics from carbon at voltages-supported above platinum the group decomposition met- voltage alsof or water.metal oxides [44,49,50] because such materials do not readily oxidize. Boron-doped diamondBeyond offers a those high overpotential rules of thumb, for the there oxygen is little evolution guidance reaction available [51–55], for and the is choice of either thereforethe cathode useful for or anode.oxidizing There organics are at structure/activity voltages above the decomposition correlations voltage in electrochemistry, of for water.example, volcano plots [40,56,57], but they are more interpolative than predictive towards Beyond those rules of thumb, there is little guidance available for the choice of either thethe cathode detailed or anode. aspect There of are the structure/activity metal (alloy composition, correlations in particleelectrochemistry, size, particle for ex- shape, etc.) or ampletowards, volcano the plots substrate. [40,56,57], but they are more interpolative than predictive towards the detailedFor example,aspect of the volcano metal (alloy curves composition, have been particle compiled size, particle for the shape, reductions etc.) or of three organic towardsoxygenates, the substrate. benzaldehyde, furfural, and heptanal [40,57]. The original correlations plotted theFor adsorption example, volcano energies curves of the have substrates been compiled on the for indicated the reductions metals of three on the organic x axis. Here, for the oxygesakenates of comparison,, benzaldehyde, we furfural, renormalized and heptanal the [40,57 y values]. The by original the turnover correlations rate plot- of the most active ted the adsorption energies of the substrates on the indicated metals on the x axis. Here, catalyst for each reaction and the x axis to span, in each case, from the weakest bonding for the sake of comparison, we renormalized the y values by the turnover rate of the most active(0.0) catalyst to the for strongest each reaction (1.0). and While the x the axis peaked to span, shape in each that case, gives from thethe correlationweakest its name is bondingevident (0.0) (Figure to the strongest2), in this (1.0). case, While the the trend peaked lines shape are that heavily gives the inﬂuenced correlation only its by the most nameactive is evident catalyst. (Figure The 2), activity in this case, of all the the trend other lines catalysts are heavily lies influenced in a band only approximately by the half as mosthigh. active The catalyst. volcano-like The activity pattern of all the exhibits other catalysts two more lies in characteristics a band approximately thatattenuate half its value. as Thehigh. trend The volcano for a particular-like pattern substrateexhibits two or more across characteristics substrates that does attenuate not vary its value. systematically with Thethe trend position for a particular of the catalytic substrate metal or across in thesubstrates periodic does chart; not vary cobalt systematically is much morewith active than its the position of the catalytic metal in the periodic chart; cobalt is much more active than its neighborneighbor rhodium rhodium for the for electrochemical the electrochemical hydrogenation hydrogenation of heptanal, of heptanal, but the two but ele- the two elements mentsexhibit exhibit about about equal equal activity activity for for the the hydrogenation hydrogenation of benzaldehyde. of benzaldehyde.

Figure 2. Superposed volcano graphs for the electrochemical hydrogenation of benzaldehyde (•), furfural (N) and heptanal (). Data from [40,57].

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Figure 2. Superposed volcano graphs for the electrochemical hydrogenation of benzaldehyde (●), furfural (▲) and heptanal (■). Data from [40,57].

Electrochemical hydrogenation of benzaldehyde as a fiducial reaction Plotting the rates of hydrogenations against each other (Figure 3) provides a compel- Catalysts 2021, 11, 315 ling depiction of the lack of predictive power of the volcano correlation: the rate of hydro-5 of 8 genation of heptanal tracks with the rate of hydrogenation of benzaldehyde (positive slope), but the rate of hydrogenation of furfural tracks inversely with the rate of hydrogenation of benzaldehyde (negative slope). 2.3.The Electrochemical issue is not the Hydrogenation accuracy of the of Benzaldehydecalculations asof aheats Fiducial of adsorption Reaction that produced the x valuesPlotting in the the ori ratesginal ofversion hydrogenations of Figure 2 against, nor is eachit the otheraccuracy (Figure of the3) providesexperiments a com- thatpelling measured depiction the reaction of the rates. lack ofRather, predictive it points power to the of inadequacy the volcano of correlation: the rationale the that rate of underlieshydrogenation the Balandin of heptanal concept, tracks which with has been the rate noted of hydrogenationpreviously [58], of and benzaldehyde the complex (pos- interactiitiveons slope), that butcontribute the rate to ofelectrochemical hydrogenation kinetics of furfural [59]. tracks inversely with the rate of hydrogenation of benzaldehyde (negative slope).

Figure 3. Comparison of rates of hydrogenation of furfural ( ) and heptanal ( ) with the rate of Figure 3. Comparison of rates of hydrogenation of furfural (▲) andN heptanal (■) with the rate of hydrogenationhydrogenation of benzaldehyde. of benzaldehyde. The Thelines lines serve serve merely merely to illustrate to illustrate that that the thetwo two test testreactions reactions trend trendoppositely oppositely when when compared compared with with the the rate rate of of the the index index reaction. reaction.

DiscussionThe issue is not the accuracy of the calculations of heats of adsorption that produced the x values in the original version of Figure2, nor is it the accuracy of the experiments thatAn measuredexample of the a biomass reaction-derived rates. Rather, electrolyte it points is 2-methoxyhydroquinone to the inadequacy of the [45] rationale, which that can underliesbe synthesized the Balandin from vanillin, concept, which which can, has in beenturn, notedbe derived previously from lignin [58], and[60–63 the]. complexThat materialinteractions has been that successfully contribute employed to electrochemical as a redox kinetics shuttle. [59 However,]. those experiments involved an aqueous electrolyte, so the corresponding cell voltage was closer to 1 V than to the3. 3 Discussion V assumed in constructing Figure 1. Regardless,An example deconvoluting of a biomass-derived the complexities electrolyte noted is above 2-methoxyhydroquinone for the oxidation and [ 45reduc-], which tionscan of besubstrates synthesized, such from as those vanillin, in Table which 1, can,will inrequire turn, beaccess derived to intrinsic from lignin kinetics, [60– 63pref-]. That erablymaterial differential has been kinetics successfully over wide employed ranges asof asubstrate redox shuttle. concentration However, [29]. those It experimentswill also requireinvolved access an to aqueous well normaliz electrolyte,ed reaction so the correspondingrates, preferably cell using voltage a site was-counting closer to method 1 V than to thatthe can 3 be V assumedused under in constructingconditions close Figure to those1. of the relevant reactions. Titration of the sites by aRegardless, selective poison deconvoluting is one classical the complexitiesmethod [31]. Other noted selective above for site the-counting oxidation meth- and re- ods ductionshave been of described substrates, [64,65 such], asbut those they inare Table used1, too will infrequently require access in the to intrinsicdevelopment kinetics, preferably differential kinetics over wide ranges of substrate concentration [29]. It will also require access to well normalized reaction rates, preferably using a site-counting method that can be used under conditions close to those of the relevant reactions. Titration of the sites by a selective poison is one classical method [31]. Other selective site-counting methods have been described [64,65], but they are used too infrequently in the development and description of electrocatalysts for organic reactions [66]. Finally, we need more frequent use of in situ and in operando probes of the organic surrounding of the electrocatalytic site [23,24]. Normalizing electrocatalytic rates by the rate of a ﬁducial reaction is an approach that uses in operando information. Its partial success here suggests that the Catalysts 2021, 11, 315 6 of 8

reported reaction rates may not reﬂect the intrinsic kinetics, but rather, are confounded with ﬁnite rates of transport.

4. Conclusions The annual production of biomass should be capable of keeping up with the materials demand for organic electrolytes, provided that it could be converted into, and employed as, a high voltage redox shuttle. While the prospects for this are plausible, regrettably, it is not yet evident how to use the corpus of organic electrochemistry to design catalysts to promote the intrinsic kinetics of the redox couples. Therefore, at this point, organic shuttles, supporting electrolytes and catalysts will need to be developed in parallel, and likely empirically. The advantage of doing so would be the production of a sustainable, and potentially environmentally friendly, electrical grid.

Funding: The work was supported by Pacific Northwest National Laboratory’s Laboratory Directed Research and Development (LDRD) program through the Chemical Transformation Initiative. PNNL is operated by Battelle for the U.S. Department of Energy under Contract DE-AC05-76RL01830. Conflicts of Interest: The author declares no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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