A Low Temperature Unitized Regenerative Fuel Cell Realizing 60% Round Trip Efficiency and 10,000 Cycle" of Durability for Energy Storage Application" Yagya N. Regmi1, Xiong Peng1, Julie C. Fornaciari1,2, Max Wei3, Debora J. Myers", #dam %. Weber1, Nemanja Danilovic1,* 1 )nergy *+orage and Di!+ribute$ Re!ource! Divi!ion, ,a-rence Berkeley Na+ional ,abora+ory, Berkeley, CA, 94720. 2 Department of Chemical and Biomolecular )ngineering, 5niversi+y of California Berkeley, Berkeley, CA, 94720 3 )nergy #nalysi! and )nvironmental Impac+! Divi!ion, ,a-rence Berkeley Na+ional ,abora+ory, Berkeley, C#, 94720. " Chemical *cience! and )ngineering Divi!ion, #rgonne Na+ional ,abora+ory, ,emont, 6,, 60439 * ndanilovic8lbl.gov ' Broader context #n elec+rolyzer and fuel cell +oge+her form an elec+rochemical ba++ery -i+ a !+orage capaci+y limi+e$ by !+orage media, not +he conversion device. # proton-exchange membrane (P)M< !ys+em +ha+ i! use$ +o !+ore and relea!e chemical energy from hydrogen molecule al!o ha! +he adde$ advantage! of extende$ !+orage life and portabili+y. =owever, elec+roc emical energy conversion using +-o di!cre+e P)M device! (fuel cell and elec+rolyzer) a$$! !igni>cant capi+al cos+!. 5ni+i9e$ regenera+ive fuel cell! ;5RFC) can reduce +he price i4 a !ingle uni+ can achieve high roundtri3 e?ciencie! (RT)). @hus, exploring +he opera+ional RT) limi+! of 5RFCs i! e!!ential. We a!!e!! +he RT)! of P)M-ba!e$ 5RFCs in an unconventional configura+ion -here 1 hydrogen oxida+ion and oxygen evolution occur on one elec+rode, and oxygen reduc+ion and hydrogen evolution on the other. We al!o show tha+ thi! configura+ion ha! the potential to opera+e a+ higher current densi+ie! and RT)! +han +radi+ional 5RFCs, -here bot oxygen reac+ions +ake place on one elec+rode and bot hydrogen reac+ions on +he other. @he!e a!!e!!ment! -ill provide guideline! for future inve!+iga+ions +o further optimi9e +he !y!+em and RT)! via ma+erial! and !ys+em innova+ions, ul+ima+ely bringing do-n +he uni+ price of +he elec+rochemical genera+ion and utili9a+ion of hydrogen. *' A%"tract 5ni+i9e$ regenera+ive fuel cell! ;5RFC) convert elec+rical energy +o and from chemical bonds in hydrogen. 5RFCs have +he potential +o provide economical means for e?cient long-term, !ea!onal, energy !+orage and on-demand conversion bac/ +o elec+rical energy. We >rs+ optimi9e +he ca+alys+ layer for di!cre+e elec+roly9er and fuel cell and +hen configure +he 5RFC. @-o pos!ible configura+ions of 5RFCs are compared, -hic empha!i9e +he advantage! of +he unconventional constant-electrode (CE) 5RFC over +he +radi+ional constant-gas (CG) configura+ion. We al!o !+udy +he !+abili+y via accelera+e$ !+re!! +e!+! ;#*@!< and demons+ra+e !+eady !+a+e opera+ion in a daily cycle for day +o night energy !hi4+ing. From +he inve!+iga+ions, +he optimum composi+ion of +he 5RFC anode ca+alys+ layer i! 02 a+A Ir-blac/ balance$ by P+: blac/ for bot CE and CB configura+ions. #+ C2 °C and 1 #Ecm2, +he optimi9e$ CE 5RFC achieve! FC and 61% RT) -i+ air and G2 a! +he reduc+ant ga!e!, re!pec+ively. We +hen evalua+e$ +he diHerence! in durabili+y using an #*@ over 10k charge:di!charge cycle!I +he re!ul+! reveal +ha+ +he -ider potential -indow a+ +he anode in CE (0.05-1.55 J< ha! minimal eHec+ on ca+alys+ layer !+abili+y compare$ +o CB (0.55-1.55 J). Furthermore, +here -a! no 2 degrada+ion up +o +he range of 2k-5k cycle!I beyond +ha+ +he fuel cell (di!charge< performance degrade$ -hile +he elec+rolyzer (charge< performance -a! !+ill !+able. @he ob!erva+ions here indica+e !ub!+antial potential +o employ 5RFC! a! e?cient and cos+-eHec+ive bidirec+ional energy-conversion device! -i+hin energy !+orage and utili9a+ion !ys+em! a4+er appropria+e +echnological and opera+ional optimi9a+ions. + ,ntroduction )nergy !+orage i! increa!ingly important for elec+rical gri$ re!iliency a! intermi++ent and variable energy !ource! intermingle on +he elec+rical grid.1 @here are numerous energy and +ime domains +ha+ require +he appropria+e energy-!+orage +echnology a+ a low price point. @he!e !olutions include +echnologie! -ho!e deployment i! regionally dependent, !uc a! hydropower and air compre!!ion, +ha+ can depend on +he pre!ence of geological fea+ure!.2, 3 In contra!+, elec+roc emical-ba!e$ !+orage +echnologie! can be readily deploye$ -i+hout geological cons+raint!, including li+hium:ion ba++erie!, Lo- ba++erie!, and elec+rolyzers ;),)." @he capi+al and opera+ional expendi+ure! (capex and opex, re!pec+ively) of eac +echnology dic+a+e +he amount and +ime!cale over -hic +he !ys+em can !+ore energy.F For example, even +hough +he cos+ of li+hium ba++erie! ha! come down +remendously due +o +he emerging ba++ery-elec+ric vehicle indus+ry, +he cos+ +o !+ore more +han four hours of energy make! +hem prohibi+ive for longer dura+ion energy-s+orage applica+ions.7 For longer +ime domains, Lo- ba++erie! and hydrogen !+orage have been propose$ due +o +he abili+y +o decouple +he energy-conversion and +he energy-!+orage component! of +he !ys+em, !ince !+orage +anks can be use$ +o hol$ elec+roly+e or hydrogen ;=2), re!pec+ively. =ydrogen, produce$ from elec+rici+y and -a+er in an elec+rolyzer ;),< i! considered, in particular, a! a versa+ile long-+erm and !ea!onal energy !+orage option. @he 3 hydrogen can +hen be use$ for hea+ genera+ion, +ransporta+ion, chemical fee$!+oc/ or converte$ bac/ +o elec+rici+y u!ing a fuel cell (FC), a! outline$ in +he Department of )nergy’s =28*cale ini+ia+ive.1 @he ), and FC device! +oge+her cons+i+ute a =-ion ba++ery, charging +he =N ions into =2 using elec+rici+y, and di!charging +he !+ore$ =2. Conventionally, +hi! =-ion ba++ery i! calle$ a di!cre+e regenera+ive fuel cell (RFC). When +he ), and FC func+ionali+ie! are combine$ in one device, +he device i! +erme$ a uni+i9e$ regenera+ive fuel cell ;5RFC). While RFCs or 5RFCs have conventionally been use$ or promote$ for u!e in limi+e$ payloa$ applica+ions !uc a! unmanne$ vehicle!,C +he emerging energy landscape demands cos+-compe+i+ive !olutions for various energy-!+orage !cenarios.F @he advantage of 5RFC over RFCs i! +he reduc+ion in capex due +o +he minimi9a+ion of cell component! from +-o !+ac/! into one. @hi! paper aim! +o demons+ra+e +he upper limi+ of roundtri3 e?ciency, durabili+y, and +he re!ul+ing opex of +he 5RFC in order +o de+ermine +he energy/+ime domain +ha+ -oul$ make +he 5RFC cos+ compe+i+ive wi+h other energy-s+orage technologie!. #n 5RFC ;Figure 1<, consi!+! of a membrane elec+rode a!!embly (M)#< +ha+ contains an ion- conduc+ing membrane, +ypically a =N-conduc+ing perfluorosulfonic:aci$ membrane, -hic !epara+e! +he elec+rode! +ha+ perform +he elec+roca+alysi! (ca+alys+ layer) and ga! or liqui$ +ransport (ga! diHu!ion or porou! +ransport layers).0 During charge ;), mode), +he oxygen- evolution reac+ion ;G)R) occurs a+ +he anode and +he hydrogen-evolution reac+ion ;=)R) a+ +he ca+hode. During di!charge (FC mode), +he hydrogen-oxida+ion reac+ion ;=GR) occurs a+ +he anode and +he oxygen-reduc+ion reac+ion ;GRR) a+ +he ca+hode. #n 5RFC require! +ha+ eac elec+rode i! bifunc+ional in order +o !us+ain bot mode! of opera+ion. *uc bifunc+ionali+y can be accompli!he$ in +-o configura+ionsO constant-gas (CB< or constant-electrode (CE). In +he CB " configura+ion, +he 5RFC consi!+! of an oxygen elec+rode ;GRR/G)R) and a hydrogen elec+rode 10 ;=GR/=)R) ;Figure 2<. @he advantage! of +he CG configura+ion are +ha+ =2 and G2 mixing i! avoided, and fa!+ !-i+ching +ime! be+-een charge and di!charge are pos!ible. @he di!advantage! have hi!+orically been +ha+ +he +-o mos+ +hermodynamically, kine+ically, and +ransport-limi+e$ reac+ions, +he GRR and G)R, are combine$ on +he !ame !ide of +he cell re!ul+ing in a confluence of ine?ciencie!. In +he CE configura+ion, +he 5RFC consi!+! of an anode ;=GR/G)R) and a ca+hode ;GRR/=)R) and +he ga!e! pre!ent on +he anode and ca+hode are !-i+che$ be+-een ), 11 and FC mode!. @hi! configura+ion ha! di!+inc+ advantage! in improving +he +ransport of G2 and kine+ic! of +he GRR by optimi9ing +he ca+hode ca+alys+ and ga!-diHu!ion layers ;BD,!), -hile +aking advantage of +he facile =GR on +he anode !ide, allowing for a reduc+ion in ca+alys+ loading and improve$ G)R kine+ic! a! -ell. @he di!advantage! of +hi! configura+ion are +ha+ eac elec+rode experience! a -ider range of potential! -hen +ransi+ioning be+-een FC and ), mode!, -hic can cause higher ra+e! of ma+erial! degrada+ion, limi+ing device life+ime, and +ha+ +here i! a nee$ +o engineer !afe +ransi+ions be+-een charge and di!charge mode! +o avoi$ mixing of oxygen and hydrogen. F Figure 1. Schematic of a Unitized Regenerative Fuel Cell mem rane electrode assembl! operating in the constant electrode (CE) configuration.