Review 1604685 DOI: 10.1002/adma.201604685 E-mail: [email protected] ON N2L3G1,Canada 200 UniversityAvenue West, Waterloo, University ofWaterloo Waterloo InstituteforSustainableEnergy Waterloo InstituteforNanotechnology Department ofChemicalEngineering Prof. M.Fowler, Prof.Z.Chen J. Fu, Z.P.Cano, M.G.Park,Prof.A.Yu, zinc–air batteriesisprovided. future researchdirectionstoprolongthelifespanofelectricallyrechargeable limitations, recentapplication-targeteddevelopments,andrecommended for bestpracticesareincluded.Finally, ageneralperspectiveonthecurrent dedicated tobattery-testingtechniquesandcorrespondingrecommendations alleviate theseissuestoenhancebatteryperformance.Additionally, asection affect individualcellularcomponents,alongwithrespectivestrategiesto compared, andanindepthdiscussionisofferedofthemajorissuesthat able zinc–airbatteriesisdiscussed,differentbatteryconfigurationsare safety, andlowcost.Here,thereactionmechanismofelectricallyrecharge- characteristics, suchasabundantrawmaterials,environmentalfriendliness, Besides theirhighenergydensity, theyalsodemonstrateotherdesirable a promisingcandidateforemergingmobileandelectronicapplications. research effortsrecentlyduetotheirhighenergydensity, whichmakesthem Zinc–air batterieshaveattractedmuchattentionandreceivedrevived energy mix. remains achallengetowidespreadadoptionintheglobal or headedbelowthatoffossilfuels,theirintermittentnature elized energycostforrenewablesourcesapproaching volatile energy economy in the near future. Even with the lev- fossil-fuel resources onEarth, it isreasonable to expect a more from fossilfuelstocleanrenewableenergy. Given thelimited change haspropelledandacceleratedtheinevitabletransition Ever-increasing demandsinenergyandawarenessofclimate 1. Introduction and ZhongweiChen* Jing Fu, ZacharyPaulCano, MoonGyu Park,AipingYu, MichaelFowler, Challenges, andPerspectives Electrically RechargeableZinc–AirBatteries:Progress, pensable forportableelectronicdevices.Electricvehicles(EVs), to bescaleddownsmallsizes,whichhasmadethemindis - advantages overtraditionalformsofenergystorageistheability in convertingandstoringchemicalenergy. Oneoftheirbiggest energy-storage systemsismoreurgentthanever. www.advmat.de For centuries, batterieshavebeenknownfortheirexcellence (1 of34) [1] With thisinmind,thetaskofdevelopingnew wileyonlinelibrary.com © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim Coulombic chargingefficiency. tion potentials typically lead to rapid self-discharge and poor densities comparabletolithium–air; however, theirlowreduc- are bothcompatiblewithaqueouselectrolytesandhaveenergy (1218 Whkg within ametal–airbatteryhas a relativelyhighspecificenergy abundant intheearth’s crust.Moreimportantly, zinc metal battery. Comparedwithlithium,zincisinexpensiveandmore its greaterenergyandcellvoltagewithinanaqueousmetal–air out ofthesetwo,zinchasreceivedfarmoreattentiondue to and canbechargedmoreefficiently inaqueouselectrolytes; aqueous electrolytes. plagued by its inherent instability when exposed to air and (nominally 2.96V).However, lithiuminthemetallicformis retical specific energy (5928 W h kg to beastronganodecandidatesinceithasthehighesttheo- For secondarymetal–airbatteries,lithiummetalisconsidered ious metalanodesinmetal–airbatteriesareshownFigure volumetric energy densities, and nominal cellvoltages of var theoretical specific energies(i.e.,gravimetric energydensities), sodium, andmagnesiumhaveattractedmuchattention. metals suchaszinc,aluminum,iron,lithium,potassium, charged. Primaryandsecondarymetal–airbatteries,with electrode andisstoredoutsideofthebatteryuntilitdis- ties, becauseoxygenisusedasthereactantatpositive density isparticularlydesirable formobileandportabledevices comparable tothatoflithium–air. Ahighvolumetricenergy Metal–air batteriesdisplayconsiderablyhighenergydensi- −1 ) andavolumetricenergydensity (6136WhL [5] of rangeanxietyandhighupfrontcost. could stillbedecadesawayduetotheissues that widespread consumer adoption of EVs of energystorage.However, manybelieve have thepotentialtobedominantform years, areanotherindustrywherebatteries combustion enginevehiclesinthecoming which areexpectedtoreplaceinternal native rechargeablebatterytechnologies. to intenseresearchactivityinvolvingalter electrode materials.Thesefactorshaveled sity isalsolimitedbythecapacityof in thepositiveelectrode).Theirenergyden- (the latterofwhichismostcommonlyused safety andthesupplyoflithiumcobalt cost and concerns regarding both their tages oflithium-ionbatteriesaretheirhigh 1990s. Goingforward,themaindisadvan- battery marketsincetheiradventinthelate which havedominatedtherechargeable Most EVs today uselithium-ionbatteries, Magnesium andaluminum–air batteries [3] Zinc and iron are more stable www.advancedsciencenews.com −1 ) and a high cell voltage Adv. Mater. 2017, 29, 1604685 [3,4] The −1 1. [2] ) - -

Review 1604685 (2 of 34) www.advmat.de received her Master’s received her Jing Fu under the degree (2011) of Prof. Jinli Qiao supervision Ma at East and Prof. Jianxin China University of Science Shanghai, and Technology, China, where she specialized After in alkaline fuel cells. with graduation, she worked as Corporation Cars Volvo a chassis design engineer pur- (2011–2014). She is now is Canada Zhongwei Chen is Canada Research Chair Professor in Advanced Materials for Clean Energy at University of Canada. Waterloo, Waterloo, He is also the Director of Graduate Collaborative Program in Nanotechnology He at University of Waterloo. received his Ph.D. (2008) in Chemical and Environmental Engineering from University received Zachary Paul Cano his bachelor’s degree (2012) and master’s degree (2015) in materials science and engineering from McMaster Hamilton, Canada, University, where he specialized in corro- sion science. He is now pur- suing his Ph.D. in chemical engineering (nanotechnology) under the supervision of Prof. Zhongwei Chen and wileyonlinelibrary.com 2. During discharge, the zinc–air battery suing her Ph.D. in chemical engineering under the supervi- suing her Ph.D. in chemical Chen at the University of Waterloo, sion of Prof. Zhongwei research interests include Her current Canada. Waterloo, electrode the development of advanced nanostructured recharge- materials and solid-state electrolytes for flexible able metal–air batteries. at the University of Waterloo, Prof. Michael Fowler His research is currently focused on the Canada. Waterloo, and cell designs development of novel cathode structures for long–lasting rechargeable metal–air batteries. Riverside. His expertise is advanced energy of California materials for zinc–air/lithium-ion/lithium–sulfur batteries and fuel cells. zinc electrode. A schematic of a rechargeable zinc–air battery zinc–air rechargeable of a schematic electrode. A zinc is shown in Figure electrochemical the through generator power a as functions

[6] [13] [8–10] Fluidic [12] However, However, . −1 A recent eco- [8,11] [15] price offered by offered price −1 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © until at least 2025. −1 Rechargeable zinc–air batteries for EVs batteries for EVs Rechargeable zinc–air The 160 US$ (kW h) (kW US$ 160 The [7] . [14] −1 29, 1604685 Therefore, the combination of high energy density, Therefore, the combination of high energy density, 2017, [16] Mater. Notwithstanding these problems, advances in materials sci- Notwithstanding these problems, advances Zinc, due to its low cost and high capacity, is the most is the to its low cost and high capacity, due Zinc, The zinc–air battery is typically composed of four main of four composed is typically battery zinc–air The Adv. www.advancedsciencenews.com nomic analysis also found that zinc–air batteries were the most economically feasible battery technology for smart-grid energy storage. their power performance has traditionally been a drawback their power performance has traditionally catalyzing reactions due mainly to fundamental challenges in corrosion Moreover, involving oxygen gas at the air electrode. another was reaction charging the during electrode air the of of lithium ion bat- critical issue, which, along with the advent at the end of teries, slowed down zinc–air battery development the 20th century. interest renewed about brought have nanotechnology and ence in this decade. in electrically rechargeable zinc–air batteries systems; the zinc–air Several companies have developed unique Fluidic Energy, most prominent ones are EOS Energy Storage, Aurora EOS Energy Storage’s Energy Solutions. and ZincNyx MW h zinc–air battery for utility scale product is a 1 MW/4 for as low as 160 US$ (kW h) grid storage, offered were heavily investigated between ca. 1975 and 2000. were heavily investigated ZincNyx Energy Solutions has developed a 5 kW/40 kW h zinc– Energy Solutions has developed a 5 kW/40 ZincNyx air backup system that is undergoing a field test at a subsidiary Resources. Teck of Moreover, the inherent safety of zinc means that zinc–air bat- of zinc means that zinc–air the inherent safety Moreover, where hood of an automobile, be placed in the front teries can in today’s well established for air access is already provision vehicles. in primary metal–air batteries. common anode material batteries are most notable for being Primary zinc–air source for hearing aids, where the predominant energy of volumetric energy density they provide an impressive 1300–1400 W h L forms rechargeable and electrically rechargeable Mechanically proposed. In mechanically recharged of the battery were both referred to as zinc–air fuel cells), the zinc–air batteries (also and re-supplying a battery was charged by removing spent zinc of poor zinc electrode fresh zinc anode. This avoided the issues air electrodes. However, bifunctional and unstable reversibility due to the high costs this concept was never widely adopted sta- supplying and recharging zinc of network a up setting of zinc–air rechargeable electrically The most successful tions. design, which greatly batteries employed a flowing electrolyte improved the durability of the zinc electrode. Energy has partnered with Caterpillar and Indonesia’s public public and Indonesia’s Energy has partnered with Caterpillar utility to provide over 250 MW h of its zinc–air batteries to store photovoltaic solar energy in 500 remote communities. (e.g., EVs and personal electronics) because there is a limited because there is a personal electronics) and (e.g., EVs applications. these in batteries the mounting for volume low cost, safe operation, and market-ready solutions leads us is batteries zinc–air for enthusiasm recent the believe that to particularly well-deserved among many other types of emerging batteries. components: an air electrode comprising a catalyst-painted gas and a an alkaline electrolyte, a separator, layer (GDL), diffusion EOS Energy Storage is remarkable given that manufacturing that given remarkable is Storage Energy EOS of rechargeable zinc–air batteries is far from mature; for com- parison, average lithium-ion battery prices are not expected to approach 160 US$ (kW h) Review 1604685 O2 electrode, formingzincate(Zn(OH) hydroxide ionsthenmigratefromthereactionsitetozinc electrolyte (liquid),andelectrocatalysts(solid).Thegenerated three-phase reactionsite,whichistheinterfaceofoxygen(gas), Reaction(1))ata oxygen reductionreaction(ORR)(forward electrode andisreadytobereducedhydroxideionsviathe same time,atmosphericoxygendiffuses intotheporousair while zinccationsareproducedattheelectrode.At the the zinctravelthroughanexternalloadtoairelectrode, tant (oxygen)fromtheatmosphere.Theelectronsliberatedat of analkalineelectrolytewithinexhaustiblecathodereac- coupling ofthezincmetaltoairelectrodeinpresence calculations andfurtherexplanations. density oftheanodeinfullydischargedstate.RefertoSupportingInformationfor charged andfullydischargedstates.Volumetric energydensitieswerecalculated usingthe values accountforoxygenuptakeinthebatterybynumericintegrationbetweenfully for variousmetalanodesinaqueousandnon-aqueousmetal–airbatteries.Specificenergy Figure 1. Zn(OH) ther decompose to insoluble zinc oxide (ZnO) at supersaturated can besimplyshownasZncombiningwithO face (backwardReaction(2)).Theoverallreaction(Reaction(3)) trolyte interface,whereaszincisdepositedatthecathodesur (OER) (backwardReaction(1)),occurringattheelectrode–elec - of storingelectricenergythroughtheoxygen-evolutionreaction redox reaction.Duringcharging,thezinc–airbatteryiscapable www.advmat.de 2Z Zn 22 The airelectrodereaction: The overallreaction: The zincelectrodereaction: nO ++ +⇔ +⇔ 2O (3 of34) HO 2 4 HZ Theoretical specificenergies,volumetricenergydensities,andnominalcellvoltages 2− −− concentrations.Reaction(2)showstheoverallzinc 2Z 4e −− nO, nO ⇔= 4O ++ E wileyonlinelibrary.com HO H, = 2 1.66 2e E Vv , 0.40 sS E HE =− Vv 4 2−

1.26 sS ) ions,whichthenfur HE Vv

2 sS toformZnO. © HE 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim

(2) (3) (1) - - divided intothreemainconfiguration types.Theconventional Electrically rechargeable zinc–air batteries can currently be 2. BatteryConfiguration research onelectricallyrechargeablezinc–airbatteries. will aidnewandexistingresearchersincarryingoutimpactful this reviewshouldserveasacomprehensivereferencethat spective ofrechargeablezinc–airbatteriesisprovided.Overall, the currentstatus,remainingtechnicalchallenges,andaper batteries and offer recommendations for best practices. Finally, include asectiondedicatedtoperformanceevaluationofzinc–air catalysts intoafree-standingairelectrodeassembly. We further of thezincelectrodeandnovelstrategiesforintegratingoxygen critical analysisofdifferent methodsofextendingthecyclelife previous articleshavenotextensivelyreviewed,including a of zinc–air batteries. We alsopay major attention to areas that atic issuesthatimpedetheefficiency, durability, andcycle-life address andoffer astate-of-art understandingoftheproblem- air electrode,electrolytes,andseparators.Particularly, wewill discussed, includingthereversiblezincelectrode,bifunctional formance ofrechargeablezinc–airbatterieswillbethoroughly disadvantages. Then,thekeycomponentsinfluencingper rations ofzinc–airbatteriesandidentifytheiradvantages begin bydifferentiating betweenthevariouspossibleconfigu- the typeofcatalystsforbifunctionalairelectrode.Here, we tion beingpaidtothecapacitydensityofzincelectrodeand improvements ofeachthecomponents,withmostatten- batteries. Theyalsoreviewedthemajordevelopmentsleadingto performance anddurability of electricallyrechargeablezinc–air overviews ofthecomponentsandmechanismsgoverning Recent reviewsonzinc–airbatteries Further, carbondioxide(CO iting thepowerdensityofzinc–airbatteries. facilitates bothredoxreactions,therebylim- electrode, itisdifficult tofindacatalystthat ponent faces its own challenges. For the air zinc–air battery, eachmainstructuralcom- the process.For theelectrically rechargeable electrocatalysts areoften usedtoaccelerate discharging cyclesarekineticallyslow; thus, tions of oxygen during the charging and voltage of1.66V. However, theredoxreac- spontaneous andproduceatheoretical mation andshapechange. zinc, whichisthemaincauseofdendritefor non-uniform dissolutionanddepositionof metal electrode,itisdifficult tocontrolthe while blockingthezincions.For thezinc- allows theflowofhydroxideionsexclusively that isrobustinabasicenvironment,yetalso separator, itischallengingtofindamaterial the GDL, which limits access to air. For the by-product canpotentiallyblocktheporesof environment insidethecell.Thecarbonate line electrolyte,thuschanging the reaction undergo acarbonationreactionwiththealka- Thermodynamically, bothreactionsare www.advancedsciencenews.com [17,18] Adv. have provided basic haveprovidedbasic Mater. 2 ) intheaircan 2017, 29, 1604685 - - - Review On 1604685 which [9,26] [27] with the Electrically Circulating [25] [9] (4 of 34) www.advmat.de [21,23,24] [20–23] . This configuration is similar to that - the shape, this configura 3. Besides wileyonlinelibrary.com 4 However, substantial loss of liquid elec- However, [22] Planar zinc–air batteries can either be posi- Planar zinc–air batteries operates identically to the air electrode of these zinc–air batteries in discharge mode. For reasons, rechargeable zinc–air flow batteries should provide higher operational and cycling lifetimes in comparison to conventional con- figurations with a static electrolyte. Figure by including from the button cell tion differs a plastic current collectors within conductive the tabs from to external in addition casing, The pris- negative electrodes. positive and the most common con- matic design is also electrically rechargeable figuration used in research research. Many zinc–air-battery of plastic plates groups use a combination together with bolts and and gaskets fastened de- and assembly quick allows which nuts, and electrolytes assembly of the electrodes being investigated. tioned horizontally (i.e., electrode surfaceselectrode (i.e., horizontally tioned the air-electrode side, precipitated carbonates the air-electrode (refer to Section 5.1) or other unwanted solids can be rinsed away by the flowing electrolyte and removed by an external filter. electrolytes are also commonly employed in alkaline fuel cells in order to combat carbonate precipitation within the air electrode, the large volume of circulating electrolyte avoids the problems of dendrite formation, Sec- to (refer passivation and change, shape tion 3.1) by improving current distribution and reducing concentration gradients. parallel to the ground, as depicted in Figure 2, 3) or vertically. 2, 3) or vertically. as depicted in Figure parallel to the ground, with the air electrodeHorizontally positioned zinc–air batteries, better current distribution inoffer facing upward, are claimed to from the air elec- the zinc electrode and easier oxygen removal trode during charging. rechargeable zinc–air batteries with a conventional planar con- rechargeable zinc–air batteries with a conventional market. How- figuration have not yet penetrated the commercial and other energy-storage they are good candidates for EVs ever, theirto volumes due and requiring low weights applications density. simple design, which prioritizes a high energy 2.2. Flow Batteries with a circulatingSeveral zinc–air batteries have been designed in a planar configura- electrolyte that flows past the electrodes tion, as shown in Figure batteries, of hybrid flow cells such as zinc–bromine main difference being that the zinc–air flow battery uses only being that the zinc–air flow battery uses only main difference a design helpssingle electrolyte channel. The flowing-electrolyte to alleviate performance and degradation issues concerning both the zinc electrode, the zinc electrode and the air electrode. For trolyte due to evaporation could lead to a complete loss of ionictrolyte due to evaporation could lead to a and air electrode in aconnectivity between the zinc electrode research groups employhorizontal configuration; therefore, most a vertical configuration in their investigations.

[7] as shown in shown as [7] 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © In small primary button cells button primary small In [19] 29, 1604685 2017, Schematic of an aqueous rechargeable zinc–air battery at charging status. Schematic of an aqueous Schematic representation of prismatic zinc–air battery configuration. Mater. Adv. www.advancedsciencenews.com Larger and multi-cell primary zinc–air batteries (historically Larger and multi-cell primary zinc–air used for railroad signaling, underwater navigation, and elec- prismatic configuration, a tric fencing) employ Figure 3. Figure 2. Conventional zinc–air batteries are constructed in a planar Conventional zinc–air batteries are constructed discussed in arrangement; in fact, most other configurations arrangement. This Section 2.2–2.4 also employ this general design in order configuration is favored over a spiral-wound the latter design has to maximize open air access, although consideration. some received 2.1. Conventional Planar Batteries 2.1. Conventional planar configuration was originally designed for primary zinc– planar configuration while zinc– density, air batteries and prioritizes a high energy and operational numbers cycle high prioritize flow batteries air emerging technology lifetimes. Flexible zinc–air batteries are an electronics promising for the advanced that is particularly due to the need for energy-dense power sources that industry, are compatible with flexible electronics. employed for hearing aids, the zinc-electrode compartment is compartment zinc-electrode the aids, hearing for employed with a gelled composed of atomized zinc powder intermixed from the air KOH electrolyte. This compartment is separated ionically conducting electrode by an electrically isolating and the density, energy the maximize to order In layer. separator casing and cap also act as the current collectors. button cell’s Review 1604685 Figure 4. a near-neutral-pH chloride-based electrolyte, arate compartments(eachwiththeirownairelectrodes). zinc particles, which allows discharging and charging in sep- Energy Solutionsutilizesaflowingelectrolytesuspensionof patents orpatentapplications); all chosentoemployflowingelectrolytes(accordingtheir developers ofelectricallyrechargeablezinc–airbatterieshave density. Nevertheless,the three most prominent commercial volume alsoresultsinreducedspecificandvolumetricenergy through thecells.Thetubing,pumps,andexcesselectrolyte to the requirement of pumping and circulating the electrolyte increased complexityandthedecreasedenergyefficiency, due past 15years. wide varietyofflexibleelectronicsoverthe of researchduetothedevelopmenta Flexible batterieshavebecomeamajorarea 2.3. FlexibleBatteries requirements arenotcritical. large-scale grid storage applications where weight and space However, thebulkynatureofthistechnologylikelylimitsitto cessful typeofelectricallyrechargeablezinc–airbatterytodate. Therefore, theflowconfigurationappearstobemostsuc- Energy usesasulfonate-containingionicliquid. design showninFigure batteries such asthe thin-film “wearable” and methodsforproducingflexible zinc–air groups havebeguninvestigating materials rigid protectivecasing.Therefore, research latter ofwhichnegatestherequirement fora high energydensity, andinherentsafety, the ible powersourcesduetotheirlowcost, excellent candidatetobedesignedintoflex- www.advmat.de Negative aspectsofzinc–air flow batteriesincludethe (5 of34) Schematic representationofzinc–airflowbatteryconfiguration. [32] Zinc–air batteriesarean wileyonlinelibrary.com 5. [33–35] [28,29] Acylindrical EOSEnergyStorageuses [29] Figure 5. while Fluidic © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [30] Zinc-Nyx Schematic representationofaflexible zinc–airbatteryconfiguration. [31]

cable-type flexiblebattery tery degree ofbending. must alsobeabletowithstandandmaintainoperationatahigh cient ionicconductivity. Theelectrodesandsupportmaterials is mechanicallyflexibleanddurablewhilemaintainingsuffi - battery researchisondevelopingasolid-stateelectrolytethat ment. Therefore,themajorfocusofcurrentflexiblezinc–air which canevaporateorleakontosensitiveelectronicequip- are opentotheair, itisnotdesirabletouseliquidelectrolytes, connection. plate withair-flow channelsratherthanthroughanexternal of anadjacentcellthroughelectricallyconductivebipolar tion ismadebetweentheairelectrodeandzinc single airelectrodeononlyoneofitssides.Aseriesconnec- arrangement (Figure 6b),eachzincelectrode ispairedwitha one cellandtheairelectrodeofadjacentcell.Inbipolar external connectionsaremadebetweenthezincelectrodeof is repeatedovermultiplecells.To connectthecellsinseries, two externally connected air electrodes and this basic unit (Figure to asmonopolarandbipolar. Inthemonopolararrangement cells can be stacked using two possible arrangements, referred the batteryvoltagetoarequiredlevelforitsapplication.The Several zinc–aircellscanbestackedinseriesordertoraise 2.4. Multi-Cell Configuration electrode areaslessthan400cm significant current-distributioneffects inalkalinefuelcellswith that edgecurrentcollectioncangenerallybeemployedwithout collect currentfromtheelectrodeedges.However, itisknown olar arrangement,sincethelatterusesexternalconnectionsto across theelectrodesofabipolararrangementversusmonop- wiring. Inaddition,thecurrentdistributionismoreeven can bepackagedmoreefficiently duetotheabsenceofexternal thickness. Thismeansthattheair-facing sideoftheairelectrode air electrodemustbeelectricallyconductiveacrossitsentire fuel cells.Adisadvantageofthebipolararrangementisthat batteries, whichtypicallyoperateatlowercurrentdensitiesthan for thebipolararrangement is likelytobeminimalinzinc–air A largeadvantageofthebipolararrangementisthatcells [37] havealsobeendemonstrated.Since zinc–airbatteries 6a), thezincelectrodeissandwichedinbetween [38] [33–36] [36] andabiocompatibleflexiblebat- 2 . [39] www.advancedsciencenews.com Therefore,thisadvantage Adv. Mater. 2017, 29, 1604685 Review

- 1604685 [41,44,45] ions is [45] 2− 4 Therefore, Upon con- [41] (6 of 34) www.advmat.de [44] ions are deposited pref ions are deposited 2− 4 However, it is important to However, dissolved ZnO. Electrodeposi- m identified a critical overpotential [44] [42,43,46] wileyonlinelibrary.com - Several studies of zinc electrodeposi established as a function of distance from established as a function Under this con- the zinc-electrode surface. dition, Zn(OH) erentially at raised surface heterogeneities, erentially at raised higher are which dislocations, screw as such gradient. up the concentration tion in alkaline solutions have identified tion in alkaline solutions key the zinc reduction overpotential as the batteries, zinc dendrites may form during zinc dendrites may batteries, fracture and process and can the charging (resulting in from the electrode disconnect can punc- or more critically, capacity losses), contact with and make ture the separator (resulting in a short the positive electrode arise as a circuit). Dendritic morphologies zinc elec- result of concentration-controlled a positively sloped trodeposition, whereby of Zn(OH) concentration gradient tinued deposition, these deposits grow past tinued deposition, these deposits grow region, the boundary of the diffusion-limited giving rise to dendrites rapidly growing under nearly pure activation control. Schematic representation of performance-limiting phenomena KOH containing 0.01–0.2 m parameter dictating the formation of zinc dendrites. parameter dictating the formation of tion at lower overpotentials tends to produce epitaxial, boulder tion at lower overpotentials tends to produce or sponge morphologies. For instance, Diggle et al. For even at relatively low overpotentials, multiple dissolution and and multiple dissolution even at relatively low overpotentials, can result in the deposition cycles under concentration control on zinc heterogeneities of surface amplification progressive growth. electrodes, eventually leading to dendritic 3.1.2. Shape Change is referred to as Another issue related to zinc-electrode failure shape change. This phenomenon is observed in zinc–air and note that, given a relatively long time for initiation, dendrites note that, given a relatively long time for can also form at lower deposition overpotentials. of 75–85 mV, above which zinc dendrites were initiated in above which zinc dendrites were initiated of 75–85 mV, 2 Figure 7. that may occur on the zinc electrode: a) dendrite growth, b) shape change, c) passivation, and d) hydrogen evolution. [39] 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © In secondary alkaline zinc-based a), ii) shape change (Figure 7b), iii) 7a), ii) shape change (Figure [40–43] 29, 1604685 Multi-cell zinc–air battery configuration with: a) monopolar arrangement, and Multi-cell zinc–air battery configuration with: 2017, Mater. Adv. www.advancedsciencenews.com b) bipolar arrangement. Air access channels are depicted as going into the page. b) bipolar arrangement. Air access channels are Figure 6. Zinc dendrites, which are defined as sharp, needle-like metallic Zinc protrusions, are well known to form under certain conditions during electrodeposition. The performance of a zinc electrode is limited by four major phenomena that occur during operation in a zinc–air battery: i) dendrite growth (Figure 3.1.1. Dendrite Growth 3.1. Performance-Limiting Phenomena 7c), and iv) hydrogen passivation and internal resistance (Figure in 7d). Each of these phenomena is discussed evolution (Figure the following four sections. 3. Reversible Zinc Electrodes supplied with an unlimited that zinc–air batteries are Given responsible for thesource of oxygen, the zinc electrode is solely should have a high A successful zinc electrode capacity. battery’s - capable of high-effi proportion of utilizable active material, be over long time periodsciency recharging, and sustain its capacity cycles. The followingand several hundred charge and discharge two sub-sections detail the scientific phenomena that constrain the achievement of these goals and the strategies that battery developers have used to battle and overcome these constraints. cannot be composed of a pure poly(tetrafluoroethylene) (PTFE) poly(tetrafluoroethylene) pure a of composed be cannot in order to maximize the hydro- preferred which is often layer, of the liquid elec- phobicity and minimize flooding or leakage that requires also arrangement bipolar The cell. the from trolyte to provide sufficient a certain pressure be maintained in order and bipolar plates. interfacial contact between the electrodes Review 1604685 ZnO solubilityasafunctionofKOHconcentration(datafromref.[52,53]). by theZn/Zn However, itcanalsobeseenthatzincredoxkinetics(indicated chosen duetothemaximumelectrolyteconductivityachieved. product and/orOH lating filmonitssurfacethatblocksmigrationofthedischarge cannot befurtherdischargedduetotheformationofaninsu - Passivation isthetermusedtodescribean electrodethat 3.1.3. PassivationandInternalResistance caused bythereasonsdescribedabove. dissolve, migrate,andre-depositundernon-uniformconditions during batteryoperationalargeamountofzincisexpectedto product increaseswithincreasingconcentration.Therefore, at thisconcentration,andthesolubilityofZnOdischarge discharged Zn(OH) up morevolumethanzinc)and finallyoccurswhenfreshly tion oftheporesizeduetoprecipitationZnO(whichtakes of aporouszincelectrode,passivationisprecededbythereduc - limit, ZnOisprecipitatedontheelectrodesurface.Incase and theZn(OH) charged form), trodes typicallyrequireaporosity of60–75%(inthemetallicor of theelectrodeandalossusablecapacity. Over manycharge–dischargecycles,thisleadstodensification a different locationsonthezincelectrodeduringcharging. electrolyte duringthedischargereactionandthendepositsat other alkaline–zinc batteries whenthezincis dissolved inthe Figure 8. volume. it toimmediatelyprecipitateand fullyplugtheremainingpore electro-osmotic forcesacrossthebattery. trode, unevenreactionzones,andconvectiveflowscausedby change tounevencurrentdistributionwithinthezincelec- modeling andmechanisticinvestigationshaveattributedshape tration ofapproximately6–7 is mosttypicallyusedinalkaline–zincbatteries.Ahighconcen- bated bythepropertiesofconventionalKOHelectrolytethat www.advmat.de It canbeobservedinFigure (7 of34) [51,54] Electrolyte conductivity, Zn/Zn Thishelpstoexplainwhyrechargeable zincelec- 2+ [48,55,56] exchangecurrentdensity)isnearitsmaximum 4 2− dischargeproducthasreacheditssolubility − 4 ions.Whenazincelectrodeisdischarged 2− wileyonlinelibrary.com whilethetheoreticalporosity required isfarabovethesolubilitylimit, causing m or25–30wt%KOHistypically 8 thatthisproblemisexacer 2+ exchangecurrentdensity, and [20,47,48,50,51] [47–49] © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim Ingeneral, - on the order of 1 potential, thehydrogen-evolutioncurrentonazincsurfaceis favors theformationofZnOoverZn(OH) fusive resistanceofhydroxideionsthroughtheelectrodepores known tocauseearlieronsetofpassivation,sinceincreaseddif to ZnOisonly37%.Increasingtheelectrodethicknessalso to physicallyaccommodatethevolumeexpansionfromzinc surface, evolution overpotentialissignificantlyreducedonaZnO range from 60–80%, The zincutilizationforconventionalpowder-based electrodescan ance becomestoohightomaintainasufficient operatingvoltage. zinc electrodebecomescompletelypassivatedoritsinternalresist- discharged. This percentage is limited by the point at which the of thezincmassthatisactuallyusedwhenelectrodefully electrodes. Itisdefinedasthepercentageoftheoreticalcapacity during dischargingandvoltageincreasescharging. of thezincelectrode,whichnaturallyleadstovoltagelosses ductive propertyofZnOalsoincreasestheinternalresistance the rateofself-discharge ofthezincelectrode. are thusneededtoimprovethechargingefficiency andreduce the overpotentialforsignificanthydrogenevolutiontooccur) egies todecrease the rate of hydrogen evolution (i.e.,increase increase asZnOformsonthedischarging zinc electrode.Strat- at 8.5×10 slope on a zinc electrode surface, which has been measured evolution isdefinedbyitsexchangecurrentdensityand Tafel trode duringcharging.However, theactualrateofhydrogen will consumesomeoftheelectronsprovidedtozincelec- be charged with 100% Coulombic efficiency, since theHER text) overtime.Thisalsomeansthatazincelectrodecannot (Reaction (5),referredtoasself-discharge inabatterycon- namically favoredandazincelectrodeatrestwillbecorroded SHE atpH14).Therefore,hydrogenevolutionisthermody- hydrogen-evolution reaction (HER)(Reaction(4),−0.83Vvs hydrogen electrode(SHE))atpH14)isbelowthatofthe The Zn/ZnOstandardreductionpotential(−1.26Vvs 3.1.4. HydrogenEvolution discussed belowcanpushthisvalueupto90%orabove. 6 the likelihoodof dendritic growth. Zinc passivation alsoreduces rent is kept constant) during charging, which in turn increases electrode. Thiscausesanincrease inoverpotential(whencur densification, whichreducesthe activesurfaceareaofthezinc action witheachother. For example,shapechangecanleadto The fourissuesdiscussedabove canoften havesignificantinter 3.1.5. InteractionofthesePhenomena Zn HO 22 m Zinc utilizationisacommonmetricthatreportedforzinc +→ KOH. +⇔ 2H 22 [60] e 22 −− OZ whichmeansthattheself-discharge rateshould [59] −7 mAcm Therefore,attheZn/ZnOstandardreduction OH nO × 10 + + [55,57,58] H H −2 −5 and0.124Vdecade

mA cm while novel developments that will be while novel developments that willbe −2 . www.advancedsciencenews.com [59] However, the hydrogen- Adv. 4 Mater. 2− −1 . [54] , respectivelyin Thenoncon- 2017, 29, 1604685 (5) (4) - - - Review - 1604685 [64] (8 of 34) www.advmat.de – – – – Open-cell metal Minimized [61] This is likely due to This is likely due to Hydrogen evolution evolution sites occurs) developed a novel casting Possibly minimized (If additive higher hydrogen evolution rate) [9,62,63] adsorption onto active hydrogen (Increases hydrogen overpotential) [61] Increased (Higher surface area causes 9) with a high specific and volumetric wileyonlinelibrary.com - reported a 3D hyperdendritic zinc elec – [65] Increased Minimized Minimized conductive than ZnO) precipitation is induced) discharge product is more Minimized (High surface area Possibly minimized (If trapped Possibly increased (If early ZnO (Increases electrode resistance) minimizes ZnO film thicknesses) (Improves electrode conductivity) (Improves electrode conductivity) Passivation and internal resistance Chamoun et al. Direct influence on: difficulties in achieving a thick, uniform metal electrodeposi- in achieving a thick, uniform difficulties foams. tion throughout the entire thickness of metal and heat-treatment procedure in order to produce a 3D zinc- and heat-treatment procedure in order to sponge electrode (Figure This electrode was capable of up to 89% zinc capacity density. cycles at high currents utilization and over 80 charge–discharge while this elec- without visible dendrite growth. However, hydrogen-evolution-suppressing and binder a included trode sup- rigid separate, a contain not did it that fact the additives, could only be demon- porting layer meant that a long cycle life the foam Presumably, strated when discharging to 23% DOD. foams (most commonly copper foam) are a common choice as choice common a are foam) copper commonly (most foams they possess a high surface area and since a current collector, addition to a high porosity (typically 95% mechanical rigidity in the active zinc species can be loaded. or higher) into which use electrodeposition to load the Most investigators typically the volumetric capacity foam; however, zinc into the metal the than lower times several is cases these in reported density zinc electrodes. theoretical value for performance. Increasing the surface area of the electrode and and electrode the area of surface the Increasing performance. overpotential, zinc-deposition a lower allows current collector formation of dendrite minimizing the likelihood therefore cur and electrode the designing Additionally, charging. upon well distrib- and electrolyte is such that current rent collector zinc minimizes the of three dimensions all throughout uted passivation. zinc’s the potential for the trode electrodeposited at a high overpotential (≈600 mV vs zinc trode electrodeposited at a high overpotential a metal-foam support. equilibrium potential) without the use of they could only cycle to 40% depth-of-discharge However, electrode structure. (DOD) due to the fragility of the 3D electrodes is Another issue with pure-zinc electrodeposited binders or hydrogen- that they contain no additives such as long-term for necessary often are that suppressors evolution investigation to lifetimes, and it would likely require extensive these additives in develop the methods necessary to co-deposit et al. the desired quantities. Parker solubility)

2− 4 [47] [47,48,51] Minimized Minimized Minimized Minimized Minimized Shape change (Discharge product migration is reduced) migration is reduced) Minimized (3D Structure improves current distribution) Minimized (Discharge product (Reduces Zn(OH) (Improves current distribution) (Improves current distribution) (Improves mechanical strength) can reduce the rate − 2

4 2− 4 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag concentra- © 2− 4 – – 1. It should be noted that these Dendritic growth Minimized (If additive Minimized (Decreases evolution sites occurs) Minimized (Zn(OH) charging overpotential) tion gradient is reduced) promotes denser deposits) Minimized (“Substrate effect” adsorption onto active hydrogen Minimized (Zn(OH) concentration gradient is reduced)

29, 1604685 2017, Strategies for improving zinc electrode performance. . Mater. Strategy (3) Carbon-based (3) Carbon-based electrode additives (4) Heavy-metal electrode additives (2) Polymeric binders (6) Electrolyte additives (7) Electrode coatings (1) High Surface Area/3D electrode structure (5) Discharge-trapping electrode additives Adv. www.advancedsciencenews.com A further example is the convective electrolyte flow caused by electrolyte flow example is the convective A further change. up the rate of shape which speeds hydrogen evolution, one of aimed at mitigating it can be seen that solutions Therefore, moreor one mitigate indirectly often can phenomena above the - there are also some cases where miti of the others. However, worse. For one make another of these problems can gating one solubility of Zn(OH) example, reducing the Table 1 the active surface area, leading to dendritic growth and increased dendritic growth and area, leading to the active surface of the current distribution. due to worsening shape change The geometry of the zinc electrode and the current col- The geometry of the zinc electrode to improve its efforts lector is an important consideration in 3.2.1. High Surface Area/3D Electrode Structure A multitude of strategies have been investigated to increase the A multitude of strategies have been investigated of cycle life (measured performance of zinc electrodes, in terms number), capacity by capacity retention as a function of cycle other factors), and (determined by zinc utilization, among - (determined by extent of hydrogen evolu Coulombic efficiency into seven methods tion). These strategies have been organized and are summarized in Table the performance of strategies are mostly aimed at improving As noted in zinc electrodes employed with a static electrolyte. in a zinc–air flow bat- Section 2.2, excess circulating electrolyte shape change, and tery greatly alleviates dendrite formation, passivation of the zinc electrode. 3.2. Strategies for Improving Performance - this can also lead to quicker onset of passiva of shape change, but of precipitated ZnO. The strategies fortion due the larger amount performance should be implementedimproving zinc-electrode properties, design in mind, as different with a specific battery and electrode thickness, and porosity, such as electrolyte volume, the most appropriate solutions. many others can dictate Review 1604685 powder-based electrode.Reproducedwithpermission. Figure 9. philic binderssuchascarboxymethyl cellulose(CMC), patibility withalkalineelectrolytes. relatively lowcost,easeofdispersion, andgoodchemicalcom- most common choice as a binder material, likely due to its ders toeachotherandthecurrentcollector. PTFEisthe to providemechanicalstabilitybybindingtheactivepow - electrodes composedofzincand/orZnOpowdersinorder As alludedtoabove, polymeric binders are usuallyadded to 3.2.2. PolymericBinders surface areas. as investigatorsdevelopzincelectrodeswithincreasinglylarger lution reductionmethodsthereforerequireincreasedattention Coulombic efficiency whenchargingthebattery. Hydrogen-evo- self-discharge whenthebatteryisnotoperating,andalsoworse increases withexposedsurfacearea.Thisleadstoincreased face-area zincelectrodeisthattherateofhydrogenevolution densified structureuponrecharging. structure couldcollapseatdeeperdischargedepths,leadingtoa prehensive studiesintheliterature ontheeffects ofbinderson been employed.Althoughthere doesnotappeartobeanycom- agar, www.advmat.de It shouldbenotedthatonedisadvantageofusingahigh-sur [67–69] (9 of34) Photograph and SEMimagesofa3Dzinc-sponge electrode, with schematic imageexhibiting its cycle-life advantage over a traditionalzinc- andpoly(vinylidene fluoride) (PVDF) wileyonlinelibrary.com [38,48,66] Other, morehydro- [61] [33,70] © Copyright 2014,TheRoyalSocietyofChemistry. 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim havealso [61,66] -

improves zincutilization.For example,Masri andMohamad improve the dispersion of Zn/ZnO powders in the electrode, trode helpstoavoidshapechange. the increasedmechanicalstrengthimpartedtozincelec- zinc-electrode performance,ithasgenerallybeenclaimedthat trical conductivitybyactingas conductivebridges,butalsoto conjunction with carbon black, not only to enhance the elec- from 68%to95%.Carbon nanofibershavealsobeenusedin improved thezincutilization ofazinc/agarpasteelectrode demonstrated thattheaddition of2wt%Super Pcarbonblack ments. ductivity andgoodchemicalresistancetoalkalineenviron - to lowertheresistanceofzincelectrodesduetheircon - based additives(mostoften carbonblack)aresometimesused electrode; thiscanleadtolowerzincutilization.Carbon- during discharge,increasetheinternalresistanceofazinc Polymeric binders, along with nonconductive ZnO that forms 3.2.3. Carbon-Based ElectrodeAdditives ance ofthezincelectrode. generally nonconductiveandthusincreasetheinternalresist- likelihood fordendritegrowth.However, polymericbindersare which increasestheeffective surfaceareaandthusreducesthe [33,67,69,72–76] Thishelpstoavoidpassivationandthus [47,71] www.advancedsciencenews.com Inaddition,binderscan Adv. Mater. 2017, 29, 1604685 [69] [68]

Review · 2 O. to to 2 1604685 −1 −1 as the ·nH − 0.25 with a 3:1 with a struc- www.advmat.de [111] (10 of 34) ](OH) [112] 2 Zinc utilization is utilization Zinc Therefore, capacity transfer better than (discharged form) or found it necessary to - perhaps because pre (OH) − −1 [109] [94] [94] 0.25 Al can occur slowly even in slowly even can occur since it forms a solid com- since it forms a solid 0.75 2− 4 , this material has a theoretical [58,75,78,94,97,98,107] [75,98,107] 1.125 This is intended to minimize to intended is This )O wileyonlinelibrary.com ions (usually by dissolving ZnO dissolving by (usually ions that can be written as Ca(OH) that can be written In a KOH electrolyte, the solubility In a KOH electrolyte, 2− 2− , which is significantly larger than that of , which is significantly larger than that of 0.25 4 4 layered double hydroxides −1 3 Al ). Assuming the layered double oxide has 10). Assuming the layered double oxide has demonstrated a zinc electrode using a Zn–Al demonstrated a zinc electrode using a Zn–Al [108,109] 0.75 ions during charging and discharging of the − O. /ZnO molar ratio of 0.25, which decreases the /ZnO molar ratio of [110] ions need to diffuse in order to be reduced is in order to diffuse ions need 2 2 in the charged form (neglecting other electrode in the charged form (neglecting other in the discharged form, or from 820 A h kg 820 A from form, or discharged in the 2− (charged form). 4 −1 −1 −1 ions that would normally be established during established during would normally be ions that ·2H 2 2− [56,63,66,87,88,94,98,110] [58,75,78,81,87,93,94,97,98,106,107] 4 Calcium (Ca) is a commonly used “discharge-trapping” “discharge-trapping” used is a commonly (Ca) Calcium Huang et al. Huang zinc electrode, thus delaying passivation and providing a max- imum zinc utilization of 87%. Minimal capacity fading of the electrode based on this material was also demonstrated over the an impressive 1000 charge–discharge cycles. Presumably, bonding of the charge-compensating layers helped to lock in the position of the active zinc during charging and discharging, shape change of the structure. preventing effectively 3.2.6. Electrolyte Additives investigators use a KOH electrolyte that is pre-sat- Many Zn(OH) with urated also increased with the addition of Ca, also increased with the use a Ca(OH) ture that can be written as [Zn cipitated calcium zincate facilitates OH charge-compensating anion in the interlayers, 505 A h kg A h 505 charging, which creates another benefit of reducing the likeli- benefit of reducing which creates another charging, formation. hood of dendrite additive, 523 A h kg precipitated ZnO on a Ca-free electrode, which would there- precipitated ZnO on a Ca-free substantial fore delay the onset of passivation. Unfortunately, required in order to meaningfully increase the addition is Ca et al. cycle life of a zinc electrode. Jain This structure has a high surface area and allows efficient This structure has a high surface area and allows efficient transfer of OH 2Zn(OH) powder). layered double-oxide powder, which was derived through cal- layered double-oxide powder, cination of ZnAl-CO - change are minimized through the addi losses due to shape reports have demonstrated to zinc electrodes. Several tion of Ca cycles with at least 80% capacity 100 to 550 charge–discharge strategy. retention using this theoretical capacity of the zinc electrode from 658 A h kg theoretical capacity of the zinc electrode shape change by inducing the formation of a solid ZnO discharge product at the zinc electrode at an earlier point the precipitation of ZnO from during discharge. However, discharged Zn(OH) anodically of this structure is lower than ZnO by a factor of 1.5 to 2.5, to 1.5 of factor a by ZnO than lower is structure this of concentration. depending on the KOH pound with Zn(OH) Zn:Al ratio (Figure the structure (Zn additives). Other authors used chemically synthesized cal- additives). Other authors used chemically which has an even cium zincate powders in their electrodes, lower theoretical capacity of 347 A h kg calcium zincate. Upon immersion in the electrolyte, it is trans- calcium zincate. Upon immersion in the with OH formed back into a double-layered hydroxide the Zn(OH) 595 A h kg capacity of 545 A h kg decreased. This minimizes the concentration gradients of concentration gradients This minimizes the decreased. Zn(OH)

3 O 2 [77,85] This [48] have also powders in ion. 3 [43,61,72,74,77–84] 2− Many of these Many O 4 2 Therefore, shape through coatings 2− For example, Zeng example, Zeng For 4 [72,85,102,103] [80,86,104] [48,105] discharge product is a 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag 2− appear to be the most appear to be the most © 4 and Sn [84,88,90,91,103] [56,66,77,79,85,92–98] In some cases, these additives have also In some cases, these additives have also [72,83,96,100,101] Ti, and Pb 29, 1604685 [33,34,76] [72,79,80,83] 2017, found that the addition of 10 wt% of 500 mesh bis- found that the addition of 10 wt% of showed that ZnO powder doped with 2.5 wt% In 2.5 with ZnO powder doped that showed [81] [88] Mater. [72,77–79,85,86,99] [61,77,79,85–91] Typically, investigators have physically mixed heavy-metal investigators have physically mixed Typically, Adv. www.advancedsciencenews.com change, dendritic growth, zinc passivation, and hydrogen evolu- hydrogen and passivation, zinc growth, dendritic change, heavy-metal additions. tion can all be reduced with appropriate have also been claimed for binary Synergistic beneficial effects heavy-metal additive combinations, mixtures of different Preventing migration of the Zn(OH) 3.2.5. Discharge-Trapping Electrode Additives 3.2.5. Discharge-Trapping the same proportions (84% zinc utilization). key to preventing shape change of zinc electrodes, as discussed While physical blocking of Zn(OH) earlier. although the mechanism behind this synergy has not been although the mechanism behind this synergy explained. powders together additives in the form of oxide or hydroxide electrode. However, with ZnO powders when fabricating the the of dispersion adequate regarding concern a up brings this are effects in the electrode, such that their beneficial additives of a dispersion can lead to the requirement fully realized. Poor mixture, which larger proportion of additives in the electrode zinc material and would then reduce the proportion of active Zhang instance, For capacity. overall of loss a to lead thus et al. zinc uti- better achieve method could the co-precipitation using charge–discharge lization (92%) and less capacity fading after cycling than a physical mixture of ZnO and In In, commonly used elements in this regard, while others, including in this regard, while others, including commonly used elements Cd, et al. et Another common additive in zinc electrodes is heavy metals is heavy electrodes in zinc additive common Another possessing more oxides/hydroxides/nitrides or heavy-metal compared to zinc. Bi, positive reduction potentials 3.2.4. Heavy-Metal Electrode Additives 3.2.4. Heavy-Metal provide mechanical stability to zinc electrodes in flexible zinc– zinc electrodes in flexible stability to provide mechanical air batteries. on the zinc electrode is one possible solution, another is to use Zn(OH) the to bond chemically that additives muth powder to a zinc electrode was required in order to reach muth powder to a zinc electrode was required a maximum in zinc utilization of 63%. Several investigations have addressed this problem by employing zinc electrodes with chemically doped heavy metals. leads to saturation and precipitation of the discharge products, close to or nearby their “trapped” and thus they are effectively original location during discharge. Upon charging, the distance been reported to impart a beneficial “substrate effect” to the effect” been reported to impart a beneficial “substrate upon charging, the additives promote zinc electrode, whereby, structures. zinc of compact formation the received attention in the literature. These additives improve the the literature. These additives improve the received attention in distribution within zinc electrodes, conductivity and current they stay in metallic form while the active owing to the fact that - and transformed to relatively insu zinc mass is discharged lating ZnO. additives are also reported to have a higher overpotential for additives are also reported to have a higher hydrogen evolution than zinc and ZnO. Review 1604685 Figure 10. permission. will reduce the tendency for zinc-electrode shape change. thus choosingelectrolyteswithlowerKOHconcentrations zinc thatwasplatedfromtheZn(OH) cycling revealedthattheporesbecameblockedwithadditional tron microscopy(SEM)analysisofaporouszincelectrodeafter problem withusingZnO-saturatedelectrolyte.Scanningelec- Zn(OH) concentration byasmuch possiblewithoutsubstantially gators haveapproachedthis problem byloweringtheKOH zinc–air batteriesduetoOhmic losses.Ni–Znbatteryinvesti- lower conductivities, which reduces the power performance of However, electrolyteswithlower KOHcompositionalsohave Also, recent work by Parker et al. ions toKOHsolutionsonlylowersitbyasmallamount. the electrolyte.Duringcharging,“extra” Zn(OH) KOH concentrationslowerthanapproximately6 lyte. AsshowninFigure 8,thesolubilityofZnOisreducedat be reducedbyalteringthecompositionofalkalineelectro - sition overpotentials. passivation, andeventuallydendritegrowthduetorisingdepo - to, uponcontinuedcycling,poorercurrentdistribution,earlier resulting densificationofthezincelectrodewouldlikelylead zinc electrode were both reduced to zinc atthe same time. The from theelectrolyteand“original” ZnOdischargedfromthe Zn/Zn www.advmat.de The solubility of the Zn(OH) (11 of34) 2+ 4 exchangecurrentdensity, theadditionofZn(OH) 2− Zn–Al layered double hydroxides with schematic of their formation and OH [110] -saturated KOHsolutions. Copyright 2015,TheRoyalSocietyofChemistry. wileyonlinelibrary.com 4 2− discharge product can also [115] [113,114] 4 2− revealed a potential originallypresentin Intermsofthe © m 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim KOH,and 4 2− ions 4 [93] [52] 2−

of 3.2–4.5 In addition,theminimumOH promise forNi–Znbatteries, additives, suchasperfluorosurfactants, therefore animportantresearch directionforzinc–airbatteries. aqueous electrolyte composition using the above approach is tivity. while maintainingabove75%oftheoriginalelectrolyteconduc- ronment. K ductivity. suspension of 0.5 KF, K and thenbyaddingotherioniccompoundsadditivessuchas have also been investigated in alkaline electrolytes for the in theair-electrode poresasCO cycle lifeof a Ni–Znbattery(the Li this willleadtoquickersaturationandprecipitationofK appropriate foraqueouselectrolytesofzinc–airbatteries,since reducing theperformanceofNiOOH/Ni(OH) Ni(OH) be substantially different from that required for the NiOOH/ isfactory performanceoftheOERatairelectrodemight be reducedtolessthanhalfofitsoriginalvalue(in7 NiOOH/Ni(OH) citric acid, Besides additivesthatreduceshape change,variousorganic [113] 2 CO 2 − electrodeduringcharging.Theoptimizationofthe Adler etal. [48] -migration property in analkalineelectrolyte. Reproduced with 3 m , K With this strategy, the Zn(OH) 3 [118] KOH,2 BO 3 BO and tetra-alkyl ammonium hydroxides 3 2 andK electrode 3 m , andK LiF resulted in optimal performance and [56,116] m KF,2 3 PO foundthatanelectrolytecomposed [48] 3 4 PO additives,whichhavealsoshown [92] ). However, CO m 4 thusseemtobeabetterchoice. − torestoretheelectrolytecon- concentrationrequiredforsat- 2 K entersfromtheoutsideenvi- 2 + www.advancedsciencenews.com CO additiveis beneficial tothe 3 , saturatedZnO,anda Adv. [117] Mater. 3 tartaric/succinic/ − 4 additivesarenot 2− solubility can 2017, 2 electrode, 29, 1604685 m KOH) 2 CO [119] 3

Review

- - 1604685 [123,124] ions. 2− www.advmat.de 4 (12 of 34) ions across the separator may have may separator the across ions wileyonlinelibrary.com 2− 4 The main function of the separator in Despite these features, their pore size is ions on the active sites of the catalysts. 2− 4 [122,123] [125,126] ions to pass while blocking soluble Zn(OH) − As seen in Table 2, none of the zinc electrodes developed in 2, none of the As seen in Table electrodes across reports from many different research groups. research many different across reports from electrodes by nor the specific capacities 2 attempts to “standardize” Table (including the charged electrode the entire mass of malizing to capacity den- and volumetric The specific capacities additives). voltage in practical zinc–air cell be multiplied by a sities can specific energy and volumetric energy order to calculate the into account the mass and thickness density (without taking it should be noted components). Finally, of additional battery 2 cannot be perfectly metrics in Table that the performance electrolytes, electrode due to different compared to one another and cycling conditions that were thicknesses, cell geometries, studies. used across the different - excellent performance across all the rel recent years displays specific high very a displays foam Cu Zinc-plated metrics. evant but its volumetric capacity density is too capacity and cycle life, 3D zinc-sponge elec- low for applications with space constraints. capacity densities, trodes display high specific and volumetric them to shallow but their lack of mechanical robustness limits life. Electrodes with depths of discharge and relatively low cycle cycle life at deep dis- discharge-trapping additives display good are relatively low due charge depths, but their capacity densities Electrodes with to the large proportion of non-active additives. appear to dis- chemically doped heavy metals and/or coatings performance metrics. play the most promising combination of advancements discussed of the zinc-electrode many However, thicknesses (>1 mm) above need to be demonstrated at larger are suitable for a wide and longer testing periods to prove they range of battery applications. The transfer of Zn(OH) of transfer The rechargeable zinc–air batteries is to prevent short circuits from zinc dendrite penetration upon cycling. In addition to the den- dritic inhibition, the separator should be electrochemically stable within a wide working potential window (≥2.5 V), remain intact in strong alkaline electrolytes (pH ≥ 13), and have a low the Ideally, electrical resistance and a high ionic conductivity. separator should also have a fine porous structure allowing OH Polypropylene membranes such as commercial Celgard films membranes such as commercial Celgard Polypropylene 5550 and with a laminated nonwoven structure (e.g., Celgard 4560) are typically used as separators in most studies Celgard of zinc–air batteries, due to their superior mechanical strength to retard dendritic penetration and broad electrochemical sta- bility window. 4. Separators is to act as a In batteries, the basic function of a separator allowing ionic flow while preventing physical physical barrier, an integral part of contact of the two electrodes. Despite being the separator has not received its deserved a zinc–air battery, So far, of the battery. attention in comparison to other parts specifically for most of the separators in use are not designed they are commonly taken from lithium- the zinc–air battery; based batteries. a detrimental effect on the polarization of the air electrode if a detrimental effect ZnO precipitates on its active surface or due to the interfer ence of Zn(OH)

[82] Yuan et al. Yuan ions to facilitate the ions to facilitate the − 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © [82,87,89,100,102] migration helps to mitigate migration helps to ions from migrating outside from migrating ions 2− 4 2− 4 migration outward during the discharge migration outward 2− studied the effect of a polyaniline coating of a polyaniline studied the effect 4 [121] ions to the inner Zn/ZnO cores while physically ions to the inner Zn/ZnO cores while physically 29, 1604685 − The coated zinc electrode showed improved discharge improved showed electrode zinc coated The 2017, 2 contains performance metrics of some zinc electrodes [120] Mater. O and OH Zhu and Zhao developed an ionomer film that was dip- Zhu and Zhao developed an ionomer ZnO powders coated with a thin layer of heavy-metal addi- ZnO powders coated with a thin layer of 2 Adv. www.advancedsciencenews.com charge and discharge processes, while blocking or reducing processes, while blocking or reducing charge and discharge the rate of Zn(OH) shape change and also reduces concentration gradients during reduces concentration gradients during shape change and also growth. dendritic for driving force the lowers which charging, a smaller proportion If these coatings are designed to take up additives discussed of the whole zinc electrode than other zinc pro- the maximizing by capacity larger a allow can they above, portion of active zinc material. zinc-powder elec- PTFE-bound conventional onto a coated trode. Table reported over the past five years, including capacity densi- ties, zinc utilization, and the number of cycles with over 80% It should be noted that many authors only retained capacity. reported the specific capacity of the zinc electrode being inves- tigated. Therefore, if it was not reported, a probable range of volumetric capacity densities was calculated from the available information in each reference. Also, it is important to note that some studies report the specific capacity relative to the mass of the charged electrode, while others report the mass of the discharged (i.e., oxidized) electrode. In addition, many authors the of mass the only to normalized capacity specific the report the mass than rather electrode, the zinc) in (i.e., material active to of the entire electrode including additives. It is thus difficult directly compare the capacity values of the large number of zinc 3.3. Evaluation the core. developed a ZnO powder with a Bi-based nanoparticle coating developed a ZnO powder with a Bi-based a maximum of 90% (5.1 wt% Bi overall), and demonstrated over 50 charge– zinc utilization and very little capacity fading the capacity retention discharge cycles. The authors attributed allowed passage of to the nanoporous Bi-based coating, which H on a zinc electrode, and found that shape change and dendritic on a zinc electrode, and found that shape capacity was achieved growth were minimized and stable cell over 100 charge–discharge cycles. tives have also been investigated. Coatings applied to the surface of zinc electrodes or to the the surface of zinc electrodes or to the Coatings applied to also been have electrode the zinc comprising powders zinc to improve cycle life. These coatings proposed as a strategy migration of OH should allow sufficient 3.2.7. Electrode Coatings suppression of dendrite growth and hydrogen evolution. It was and hydrogen evolution. of dendrite growth suppression through were reduced both of these phenomena claimed that onto the most electrochemically of the additives the adsorption process. during the charging of the zinc electrode active sites - 50 charge–dis after voltages and reduced dendritic growth was not reported. charge cycles, although the capacity retention et al. Vatsalarani blocking discharged Zn(OH) discharged blocking process. The reduced Zn(OH) process. The reduced Review 1604685 anion-exchange membranesremain toberesolved. the insufficient chemicalstability andhighproductioncostof avoided theprecipitationofZnO ontheairelectrode.However, of thecommercialCelgard separator, presumablybecauseit an increaseddischargecapacity almostsix-foldlargerthanthat lectivity. Thishighionicselectivityenabledthebatterytohave volumetric capacitydensitywasnotreported,arangeiscalculatedbasedontheelectrodematerialdensitiesandtypicalchargedporosityof60–75%; too largetolimitthecrossoverofZn(OH) c) a) 2 Table the batteries. to increasedpolarizationanddecreasedlong-termdurability of zinc cations,butsimultaneouslyallowedahighOH membrane washighlyeffective inpreventingthecrossoverof fonium membraneastheseparatorforazinc–airbattery. The zinc–air batteries. Dewi et al. applied inalkalinefuelcellscanbefeasibletoused in tance. For instance,thealkalineanion-exchangemembranes www.advmat.de [110] [82] [75] [88] [62] Theoretical [61] [65] Reference Depth ofdischargeisabbreviatedasDOD; Includes massofadditivesandbasedoncharged(non-oxidized)electrodewiththeexception100%solidZnO; Optimizing the ionic selectivity of the separators is of impor (13 of34) . Selected performancemetricsofrecentzincelectrodesdisclosedintheliterature. Strategies (Table 1) 1, 2,3, 1, 2,4, 1, 2,3, 1, 2,3, 1, 2,4 [17] 5, 6 6, 7 5, 6 4, 6 1, 6 1 5 wt%PTFE,6 5 wt%PTFE,6 powder +10wt%acetyleneblack powder +4.8wt%PTFE0.6 Bi-based nanoparticle-coatedZnO KOH +1.0 CMC, 4.5 In-doped ZnOpowder+8.3wt% Calcium zincatepowder+5wt% electrodeposited ontoNimesh, zinc +10wt%acetyleneblack wileyonlinelibrary.com PTFE +8.3wt%graphite,4.5 3D hyperdendriticzincsponge Zinc–electroplated Cufoam, Zn–Al-layered doubleoxide 3D zincsponge+300ppm In +300ppmBi,6 0.9 8 Electrode, electrolyte m KOH+0.5 m m composition + Sat’dZnO KF+0.1 m 100% ZnO KOH+1.6 8.9 100% Zinc NaOH+0.5 [127] m m m KOH+Sat’dZnO KOH+Sat’dZnO KOH developed a cationic polysul- d) m “+” indicatesthattheelectrodeprovided>80%ofitsinitialcapacitywhencyclingwasterminated; m LiOH m ZnO m KOH K m 2 BO LiOH 4 2− m 3 + ions,thusleading

© thickness Electrode 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [128] [mm] 0.28 1–4 0.2 0.2 0.3 e) 2 − permse- To avoid capacity [A hkg Specific - 728 469 385 586 656 658 820 719 754 −1 these issues,Kiros the membrane(3.52×10 The sulfonationtreatmentimproved theionicconductivityof based onacommercialmicroporous Celgard 2320membrane. Among them,theseparatorcoatedwithMn(OH) polarization loss.Wu etal. of theseparatorsmustbesufficiently hightoavoidlargeOhmic ticles. In addition to the ionic selectivity, the ionic conductivity plugging ofporestheseparatorbyveryfinecolloidalpar for the separation of Zn(OH) give the highest Zn(OH) a sulfonatedmicroporousmembrane exhibitedahigherbattery of theunsulfonatedone.The solid-state zinc–airbatteryusing including CaF porous separator(Celgard 3401).Various inorganiccompounds, an insolubleinorganicbarrierlayeronacommercialmicro ­ a) ]

Volumetric capacity 1025–1640 1282–2051 727–1164 density 552–883 338–540 [Ah L 3694 5846 928 39 −1 2 b) ] , Al(OH)

[129] utilization adoptedanotherapproachbycoating 3 , Mg(OH) Zinc [%] 4 89 87 98 92 90 88 92 −2 2− [130] Scm ion-separation capacity due to the 4 2− preparedasulfonatedseparator b) ions (Zn(OH) If thenecessaryinformationtocalculate 3 mAcm −1 2 100 mAcm www.advancedsciencenews.com ) by132%comparedtothat , andMn(OH) conditions 100% DOD 100% DOD 100% DOD 100% DOD 100% DOD (discharge) 5 mAcm 20% DOD 40% DOD ≈C/4 Rate C/5 Rate C/5 Rate 2C Rate 1C Rate Cycling −2 (charge) Adv. −2 c) −2 e)

Mater. Not reported. 4 2− # Cycles with>80% retained capacity permeability). 2 2017, wasfoundto 2 , weretested 1000+ 9000+ 250+ 100+ 29, 1604685 73+ ≈25 50+ [48,55–57] d) -

Review

- by 2 1604685 [140,141] [7] , resulting 2 For instance For www.advmat.de (14 of 34) [139] , electrolyte evapora- 2 In the solid configu- [36,138] The carbonates precipitate The carbonates precipitate [132] Although attempts to filter CO filter to attempts Although wileyonlinelibrary.com [135] In addition to the effect of CO In addition to the effect ), in aqueous systems the high-surface-area catalysts 11), in aqueous systems the high-surface-area [136,137] Another obstacle of batteries operated in air is that alka- operated in air is of batteries Another obstacle (Figure a facilitating electrolyte, the to access full have electrodes air at solid (catalyst)–liquid three-phase reaction zone, consisting of (electrolyte)–gas (oxygen), for oxygen electrochemistry. these issues is optimizing the composition of the electrolytes composition of the is optimizing the these issues 3.2) to (refer to Section the addition of additives together with - the Fara as well as increase zinc anodic polarization reduce the of the anode reaction. daic efficiency vulnerable to atmospheric CO line electrolytes are In contrast, in the solid configuration the three-phase interface reaction is severely restricted due to the poor wetting property interfacial resistance electrolyte; thereby, of the “immobilized” to hydroxide ions transporting over the catalyst surface is sub- stantially higher than that of aqueous systems. The high inter facial resistance will lead to undesirable low current rates and high overpotentials during battery operation. ration, however, improving the interfacial properties between ration, however, (especially the high- the solid electrolyte and a pair of electrodes surface-area air electrode) remains challenging. 5.2. Solid-State Electrolyte pos- for attention much drawn have electrolytes Solid-state and ion conduc- sessing both mechanical properties of solids batteries zinc–air characteristic, dual this to Due liquids. of tion operation substan- with these electrolytes would sustain cell electrolytes do; tially longer than common aqueous alkaline and flexible, include portable, applications their extended diversely shaped zinc–air batteries. tion and/or moisture invasion (depending on the atmosphere’s invasion (depending on the atmosphere’s tion and/or moisture temperature) is unavoidable when the relative humidity and Either situa- battery is exposed to a non-standard environment. - the viscosity and ionic conductivity of the electro tion can affect over time. At lytes and therefore decrease battery performance its level of hydra- room temperature, the 30 wt% KOH adjusts (60%). tion in accordance with the ambient humidity in the formation of carbonates. in the formation of inside the pores of the air electrode, which in turn reduces the air electrode, which in turn reduces the inside the pores of the air elec- path in the the air-diffusion and blocks conductivity an increased polarization and shortening trode, giving rise to battery. the of longevity - forma the carbonate can reduce adsorption physico-chemical of materials and management could be tion, the cost in terms high. - - For For Illustration of the three-phase reaction zone in different electrolytes. in different zone reaction the three-phase Illustration of [132] Figure 11. Figure / − 3 fast 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © [133] ), −1 /Zn(OH) 2 than that of the unsulfonated of the unsulfonated than that −2 ). −2 and relative humidity), causing undesir 2 Although the high [134] species are strongly asso- 2− 4 29, 1604685 An efficient strategy to address strategy An efficient Among these, concentrated KOH solutions dem- /ZnO). in Section as discussed species; [133] 2− 2− 2017, 4 4 [131] Mater. Adv. www.advancedsciencenews.com superior ionic conductivity and kinetic behavior, electrolytes superior ionic conductivity and kinetic behavior, with KOH concentrations of 20–40% are commonly used. At room temperature, a 30 wt% KOH solution has the maximum cm 640 mS (around conductivity zinc reaction kinetics in terms of the highest exchange current, and solubility of zinc discharge products (Zn(OH) 3.1.1, Zn(OH) solubility could suppress the passivation of the zinc surface and consequently increase yields supersaturated it often the capacity, Zn(OH) ciated with the formation of zinc dendrites zinc of formation the with ciated and are deemed detrimental to the battery cycle life. Despite the fact that neutral and acidic electrolytes can mini- Despite the fact that neutral and acidic carbonation, prevent and zinc dendrites the formation of mize for better zinc-corro- alkaline electrolytes are greatly favored kinetics, and wider sion resistance, higher oxygen reaction zinc–air batteries, For availability of electrocatalyst materials. and lithium potassium hydroxide (KOH), sodium hydroxide, alkaline elec- used commonly the most are solutions hydroxide trolytes. 5.1. Aqueous Electrolyte able electrolyte degradation and thereby inhibiting its use for able electrolyte degradation and thereby aqueous the to alternatives As operation. battery prolonged given to non-aqueous electrolyte, special attention has been conducting mediums electrolytes, including solid-state ionic up until now, and room-temperature ionic liquids. However, lower and energy far less exhibit systems non-aqueous the counterpart, due power performance than that of their aqueous provides insight to their lower ionic conductivities. This section electrolytes and non- into the current status of aqueous alkaline batteries. aqueous systems for rechargeable zinc–air mance in terms of ionic conductivity and interfacial properties. interfacial and conductivity of ionic in terms mance ambient the to is susceptible electrolyte alkaline the However, atmosphere (e.g., CO 5. Electrolytes - in zinc–air batteries is underesti The role of electrolytes pro- can Electrolytes materials. electrode to compared mated many aspects such as the battery performance in foundly affect With and cycling efficiency. capability, capacity retention, rate in high-performance, durable, flexible, the increasing interest elec- of development the batteries, zinc–air rechargeable and challenges as well as opportunities. Up trolytes is facing great electrolytes are still the most popular aqueous alkaline to now, owing mainly to their excellent perfor for zinc–air batteries, power density of 38 mW cm power density onstrate the highest ionic conductivity and the lowest viscosity, onstrate the highest ionic conductivity and the lowest viscosity, and thus are widely employed in zinc–air batteries for the sake of better electrochemical kinetics and mass transport. Zn(OH) membrane (20 mW cm membrane Review 1604685 lent cyclestabilityover120cyclesatarateof50Akg a flexibleandrechargeablezinc–airbattery, exhibitinganexcel- porous KOH-dopedpoly(vinylalcohol)gelledelectrolyteusedin facial resistance. unstable electrolyte–electrodecontactandleadstohighinter strength. ThepoormechanicalstrengthofAGEsresultsin there isatrade-off betweenionicconductivityandmechanical tical specificcapacityclosetothetheoreticalvalue.However, a highzincutilization(82–90%)and,accordingly, highprac- tation ofthisgelledelectrolyteinzinc–airbatteriesresults nitude as that of aqueous electrolytes. sity of5.7WhL glycol)-based alkalinegelelectrolyte,yieldinganenergyden- shaped zinc–airbatterywithapoly(vinylalcohol)/poly(ethylene either protic or aprotic in character. organic/inorganic anions;basically, RTILscanbeconsideredas liquid statebelow100°C,composedoflargeorganiccations and Room-temperature ionicliquids(RTILs)aremoltensaltsinthe 5.3. IonicLiquids rechargeable zinc–airbatteries. still necessaryforwide-scaleintegrationofhigh-performance ductivity, interfacialproperties,andmechanicalrobustnessare interest infullflexibility, furtherimprovementsinioniccon - the airelectrodeduringrepeatedcycles.Despiteincreasing ibility ofthezincelectrodeandacharge-transferprocess at ion availabilitytotheelectrodesfacilitatesasuperiorrevers- state ofthemembrane.Additionally, the uninhibitedhydroxide- while maintainingdimensionalstabilityinthewater-swollen retain a large degree of hydration for high ionic conductivity ture basedoncellulosenanofibersisofcriticalimportanceto conductivity of0.3Scm on poly(vinylalcohol)/poly(acrylicacid)exhibitedahighionic battery device(Figure a microporous structure, on a flexible rechargeable zinc–air ammonia-functionalized celluloseelectrolytemembrane,with zinc–air batteries.For instance,theydemonstratedaquaternary selectivity, as well as a strong mechanical property for flexible tured solidelectrolyteswithahighhydroxide-ioncapacityand co-workers tions), havebeendevelopedforprimaryzinc–airbatteries. weight polymersgelledwithalkalinesolutions(e.g.,KOHsolu- and co-workers Ohmic resistanceinbatteries). alkaline electrolytetoensurehighionicconductivity(thuslow matrices, andareswollenbyessentiallyalargeamountof mers andcellulosederivatives,areemployedasmechanical These low–molecular–weightpolymers,suchasvinylpoly- trode upon cycling). and internaldeformation(i.e.,volumechangeofthezincelec- when thebatteriesexperienceexternal(i.e.,flexibledevices) critical tothesuccessofAGEsinzinc–airbatteries,especially ously theionicconductivityandmechanicalpropertiesare teries. attracted researchattentionas electrolytesforlithium–airbat- trochemical windowandnonflammability, RTILshavelong www.advmat.de Alkaline gelelectrolytes(AGEs),namelylow–molecular– (15 of34) [150] However, theapplicationofRTILs aselectrolytesin [145,148] [147] [144] haveshedfurtherlightonmicroporous-struc- reportedaflexibleandstretchablefiber- [145] −1 Thus, solutions to improving simultane- wileyonlinelibrary.com . RecentadvancesmadebyChenand 12). Chen and co-workers −1 [145] , whichisofthesameordermag- Theinterconnectedporousstruc- [139] For instance,anAGEbased [149] [143] Due to their wide elec- Also, the implemen- [146] first reported a © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim −1 . Peng [142] -

in cost-competitivezinc–airbatteries. lytes, whichalsoposesachallengefortheirpracticalapplication RTILs arerelativelyexpensivecomparedtoaqueouselectro - can bedetrimentaltothecatalystsandairelectrode.Overall, pathway) andradicalperoxidespeciesintheelectrolyte,which a reducedenergydensity(comparedtothefour-electron ORR battery. However, thetwo-electronperoxidemechanismleadsto choosing anionsinsomeaproticRTILs. has beenproventocoordinatewithzinccationsbyselectively and betterreactionkinetics.Areversiblezincelectrochemistry high temperatures,enablingalowerthermodynamicbarrier using RTILshavethepotentialtooperateatmoderateand avoided. Duetotheirhighthermalstability, zinc–airbatteries lyte evaporation,zincself-corrosion, andcarbonation,canbe issues commonlyfacedbyaqueoussystems,suchaselectro- switching tolow-volatilityandwater-/metal-free RTILs,because ventional aqueouszinc–airbatteriesmaygreatlybenefitfrom by Wolfe and co-workers electron peroxideroute(proposedReaction(6))wasintroduced for zinc–air batteries, and the reaction mechanism with a two- containing sulfonateanionsastheelectrolytewasproposed helps topromoteafour-electron oxygen-reductionprocess. tages ofeachliquid,andconcludedthathighprotonavailability ethylene glycol) in RTILs as a means of combining the advan- investigated theeffect ofvariousproticadditives(e.g.,waterand the formationofhydrogenperoxide.Someresearchersfurther a two-electron-transferprocess(refertoSection6.2),leading more favorableinsupportingtheoxygenelectrochemistrywith parison, protic RTILs, which are able to dissociate protons, are either onthepresenceofhydroxideionsorprotons.Incom- well withthecurrentoxygenelectrochemistry, whichrelies to befeasible. RTILs, dendrite-freezincdepositionhasalsobeendemonstrated charge inalkalineelectrolytes. Thedevelopmentofahighly withstand theharshconditions duringrepetitivedischarge and trodes thatarebifunctionallyactive anddurable,suchthatthey able zinc–airbatteriesrelysignificantly onair(oxygen)elec- realization ofhighpower-performance, electrically recharge- discharge andreverselyevolves oxygenwhilecharging.For the A bifunctionalairelectrodeconsumesoxygenduringbattery 6. BifunctionalAirElectrodes zinc–air batteriesisstillarelativelynewareaofstudy. able zinc–air batteries have been conducted. systematic investigations of RTILs as electrolytes in recharge- a challengeforzinc–airbatteries.To ourknowledge,veryfew electrochemistry whileretaininghighreactionkineticsremains Thus thequesttofindanRTILthatfavorsreversibleoxygen and low conductivity, which substantially restricts current rates. are intrinsically slow because of their relatively high viscosity kinetics forboththezincandoxygenredoxreactionsinRTILs review article in ref. [151]. Nevertheless, the electrochemical trochemistry inproticRTILs,wereferreaderstoaspecific With regardtothemechanismforreversibleoxygenelec- Zn 2 +− ++ O2 22 e [153] ⇔ However, aproticRTILsgenerallydonotwork Zn O [30,155]

for Fluidic Energy’s zinc–air flow www.advancedsciencenews.com [152] Adv. With theseaprotic [155] Mater. An ionic liquid 2017, 29, 1604685 [151] Con- [154] (6)

Review

[160] 1604685 www.advmat.de (16 of 34) and fluorinated and [158] Copyright 2016, The Royal The 2016, Copyright a). The macroporous 13a). The macroporous [145] PVDF, [157] The degree of hydrophobicity in hydrophobicity of degree The [159] wileyonlinelibrary.com Two types of commercially available carbon-based GDLs types of commercially available carbon-based GDLs Two ethylene propylene (FEP). propylene ethylene Moreover, an ideal GDL should have properties such as fast Moreover, mechanical high repellency, electrolyte high with diffusion air in the case of flexible integrity (with optimum bending stiffness and reliable electro- designs), superior electrical conductivity, in strong and chemical durability stability chemical-oxidation alkaline electrolytes. that are commonly used in zinc–air batteries are nonwoven and and Sigracet) Freudenberg, carbon paper (e.g., Toray, and ELATTM). woven carbon cloth (e.g., Zoltek, GDL-CT, These carbon-based GDLs typically have a dual-layer design, backing and a thin consisting of a macroporous gas-diffusion microporous layer (MPL) (Figure the GDL influences the battery polarization behavior in terms of electrical conductivity (Ohmic resistance) and gas permea- bility (mass transfer); to achieve optimal performance, it should possess the capability of alleviating electrolyte evaporation and conditions. extreme under flooding electrolyte resisting 6.1. Gas-Diffusion Layer of bifunctional The GDL plays a vital role in the performance zinc–air batteries, the GDL has catalysts at the air electrode. For a physical support the following essential functions: i) provides ii) allows a uniformly distributed for the catalysts/catalyst layer, catalysts, iii) serves as transportation of air (oxygen) to/from the electrolyte from a wet-proofing electrode backing that prevents to provide leaking out, iv) possesses high electrical conductivity current collection. To an electrically conductive pathway for be the GDL should facilitate air transport in zinc–air batteries, nature. The hydropho- thin, highly porous, and hydrophobic in the GDL with hydro- bicity can be realized by impregnating PTFE, as such agents phobic ­ 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © Typically, the bifunctional air electrode Typically, [156] 29, 1604685 a) Schematic diagram of the solid-state rechargeable zinc–air battery using a functionalized nanocellulose membrane. (GDL: gas diffusion a) Schematic diagram of the solid-state rechargeable 2017, Mater. Adv. www.advancedsciencenews.com Society of Chemistry. bifunctionally active air electrode is quite challenging because challenging quite is electrode air active bifunctionally high over both the ORR and the OER show considerably Figure 12. layer; QA: quaternary ammonium). b) Schematic diagram of a flexible zinc–air battery device integrated with a sticking plaster. c) A demonstration diagram of a flexible zinc–air battery device integrated with a sticking plaster. layer; QA: quaternary ammonium). b) Schematic permission. with Reproduced bending. under LED a red power to finger index an around wrapped device flexible the of potentials, and thereby significantly relies on a truly bifunc- potentials, and thereby significantly relies promote both the ORR and tional electrocatalyst to efficiently must also be stable the OER processes. The electrode materials upon oxygen evo- with respect to highly oxidative conditions under oxygen lution, as well as strong reducing conditions range of poten- reduction at high current rates; a wide working elec- hydrogen tials spans from about 0.6 V (versus reversible trode (RHE), pH = 14) during discharge to about 2.0 V during for respective charging. The use of decoupled air electrodes cycling battery the improve to reported been has OER and ORR but this configuration unavoidably complicates the stability, cell design and incurs weight and volume penalties of power and energy density. consists of a hydrophobic GDL and a moderately hydrophilic The GDL provides a physical and conductive sup- catalyst layer. port for the catalysts, as well as a passage for oxygen diffusion The in and out during discharging and charging, respectively. oxygen–liquid (gaseous interface three-phase a at occurs ORR electrolyte–solid catalyst), while the OER takes place at a two- phase reaction zone (liquid electrolyte–solid catalyst). Thus, rational design of the air electrode in terms of an interfacial structure with optimal hydrophilicity is critical to optimize the catalytic activity and avoid flooding of the catalytic active sites. The search for bifunctional catalysts has mainly focused on nonprecious alternatives such as transition metal compounds (e.g., oxides, sulfides, nitrides, carbides, and macrocycles), former the of composed hybrids and materials, carbon-based two. In the following sections, the material and design aspects and bifunctional catalysts are dis- layers of the gas-diffusion cussed; advanced bifunctional air electrodes that uniquely inte- grate the former two are also reviewed. Review 1604685 Figure 13. mance tocarbon-basedGDLs inzinc–airbatteries. employed asdiffusion media, leading tocomparableperfor nium mesh,nickeland nickel/copperfoamhavebeen resistance betweenthemacroporouslayerandcatalystlayer. tively throughout the catalystlayer and minimizethe contact catalytic activematerials.TheMPLhelpstodistributeaireffec- carbon black)layerhavingfineporosityforsupportingthe while theMPLisathin,relativelyhydrophiliccarbon(e.g., graphitized carbonfiberstoprovidelargegas-diffusion pores, backing layer is madeup of a highly hydrophobic array of electrodes, andevensevereleakageofelectrolyte. reactions, uniformdistributionofcurrentthroughouttheair in lossesof active surface areafor electrochemical oxygen susceptible tocarboncorrosion,whichconsequentlyresults alkaline electrolytes, is still an issue. These GDLs are highly oxygen evolution(charging),aswellexposuretohighly fuel cells,theirstabilitytowardhighoxidationpotentialsduring successfully demonstratedinpolymer-electrolyte-membrane Although thesestate-of-the-art carbon-basedGDLshavebeen support forsmall-scale,flexiblezinc–air-battery applications. flexible, andtherebycanbeemployedasarobustcatalystlayer other hand,thickercarbonclothGDLsaremoremechanically mass transfer; however, they are quite brittle to handle. On the performance, intermsofmorefavorableOhmicresistanceand Thinner carbon-paperGDLsoften tendtohaveabetterbattery GDLs. layer) overawiderpotentialrange comparedtocarbon-based surface passivationviatheformation ofathinoxide/hydroxide dative stability (either througha higher reductionpotentialor higher electricalconductivityandabetterelectrochemicallyoxi - foam (Figure 13b).Ingeneral,metal-basedGDLsprovidea PTFE binderintoaporousmetallicsubstratesuchasmetal and pressingofamixturethecatalysts,metalpowders, strates. Typically, metal-based GDLs are fabricated bycasting graphitized carbonmaterialsorby the use ofmetallicsub- sion of the GDLs either by the use of highly corrosion-resistant, operation, itisthusofutmostimportancetopreventthecorro - corrosion oftheGDLs(refertoSection5.1).For long-term enced bydissolvedcarbonatespeciesresultingfromcarbon possible thatalkalineelectrolytesalsocanbenegativelyinflu- www.advmat.de (17 of34) [162] Metal substratesmadeofstainless-steelmesh,tita- Schematic diagramof:a)carbon-based(cross-sectionalview),andb)metal-basedGDLsintegratedwiththecatalysts. wileyonlinelibrary.com © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [137,140] [20,163,164] Itis [161] -

resistant, porousmetallicsubstrate.Brostetal. larly, theactivecatalystscanbedirectlysinteredonacorrosion- ties, madefromsinterednickelpowderasabaseandCo single-layer designwithdifferent porositiesandhydrophobici- the GDLandcatalystlayerwerecombinedinasinglelayer. The the mixtureofCo teries. Theelectrodewasfabricatedbycastingandpressingof as abifunctionalcatalyst,wasdemonstratedinzinc–airbat- negative influenceoftheporous geometrychange. tion ofthemicroporesinGDLs moreuniformcanalleviatethe With regard to the degradation of PTFE, making the distribu- but alsotoapparentreduction oflong-termbatteryoperation. only totheincreaseofmass-transfer resistanceatairelectrodes and mechanicalfailureofthe GDLs. Botheffects willleadnot operation times,resultingincollapseoftheporousstructure and oxidizing conditions and consequently degraded over long and metal-basedGDLs.PTFEisgraduallyattackedbycaustic binders (e.g., PTFE)is unavoidable in both the cases of carbon- conductivity. Itisworthmentioningthattheuseofplastic the batteriesduetoanincreaseofsurfaceareaandelectrical catalytic performanceoftheactivecatalystsandefficiency of lysts andnickel-foamscaffold. Thissignificantlyimprovesthe used toprovideadditionalcurrentpathwaysbetweenthecata - conductive additivessuchasmetalfibersorparticleswerealso facilitate the flow of air through theelectrode. In thiselectrode, ment. Theporosityofthescaffold wascontrolled(≈ the conductivescaffold byahigh-temperaturethermaltreat- Chen and Lee with perovskiteLa bifunctional airelectrodeconsistingofanickel-foamscaffold more than100cyclesatacurrentrateof17.6mAcm trode inzinc–airbatteriesledtoastablecyclingperformanceof mechanical failure and electrolyte leakage. Using this air elec- fusion avoidedtheissueofcarboncorrosionandsubsequent materials toformahierarchicalporousframeworkforairdif use ofnickelmacroparticlesandfoaminsteadcarbon efficient airdiffusion totheactivecatalysts.Inaddition, provide amacroporousstructureinthesinglelayer, allowing ness of150–250µ binder into a conductive nickel-foamsubstrate,with a thick- [162] reportedan integrative air electrode, wherein 3 O 0.6 m. Thenickelmacroparticleswereusedto 4 Ca catalyst,nickelmacroparticles,andPTFE 0.4 CoO 3 catalystschemicallybondedto www.advancedsciencenews.com Adv. Mater. [165] 2017, [137] prepareda 29, 1604685 − 80%) to 2 . Simi- 3 O 4 -

Review

- - - ) 2 [191] 1604685 orbital g State-of- /C (carbon- and RuO 2 2 [190] www.advmat.de (18 of 34) [185] This is particularly /C or IrO 2 [186] and lanthanum nickel and lanthanum nickel The relative stabilities of The relative stabilities [188] A thorough understanding thorough A [167] [167] Some transition-metal oxides, Some transition-metal A common approach to obtain (i.e., RuO 2 [185] - to replace these expen The drive [173] There are three main characteristics wileyonlinelibrary.com [184] These trends are complex due to many [173] or IrO 2 Noble-metal oxides (e.g., IrO Noble-metal [185] /C). Such an approach, however, is often is often an approach, however, /C). Such 2 nickel cobalt oxide, [177,183] [187] In these materials, surface metal cations (M) are In these materials, [189] Although recent advances in the computational analysis of Although recent advances in the computational : Catalytic activity should be considered to : Catalytic Activity Catalytic the case in spinel- and perovskite-type metal oxides such as and perovskite-type metal oxides such the case in spinel- cobalt oxide, exhibiting semiconducting properties, have demonstrated properties, have demonstrated exhibiting semiconducting oxygen evolution. good performance of high-performance bifunctional catalysts is to physically com- OER bine the above ORR catalysts (e.g., Pt/C) with efficient catalysts (e.g., IrO are highly OER active and display metallic-like conductivity, conductivity, metallic-like OER active and display are highly expensive. but are fairly Several volcano-like diagrams, which plot the catalyst activity reverse process of the ORR through four sequential electron- four sequential of the ORR through reverse process transfer steps. of the metal cations. catalysis enable us to predict and select potential candidates catalysis enable us to predict and select no exemplary for the ORR and the OER, there is currently catalysts to which benchmark performance for bifunctional all newly developed catalysts can be compared. these intermediates at metallic surfaces, namely the binding these intermediates at metallic surfaces, between and the activation barriers strength of the M–O bonds and step the rate-determining the intermediates, determine rates. OER overall the therefore considered as the active sites for the OER, where the reaction sites for the OER, where the reaction considered as the active of metal-stabilized intermediates (i.e., proceeds via a series and M–OO) and changes the oxidation M–OH, M–O, M–OOH, accordingly. state of metal cations oxide. factors, including electronic structure, electrochemically active factors, including electronic structure, electrochemically electrical surface area, surface composition and stoichiometry, and geometric structure (strain). conductivity, of the rate-determining steps could provide insight into how insight into provide could steps the rate-determining of electronic proper compositional, structural, geometric, and the reaction rates ties of catalysts could be tuned to enhance presented, been have trends activity of A number the OER. of in which the OER activi- computationally and experimentally, in the e ties are relative to the number of d-electrons sive noble-metal oxides has led to a class of catalysts consisting to a class of catalysts oxides has led sive noble-metal transition-metal (e.g., Co, Ni, and Mn) of earth-abundant oxides and (oxy)hydroxides. black-supported iridium oxide)) catalysts are currently the most black-supported iridium oxide)) catalysts for the ORR accepted benchmark electrocatalyst materials and the OER, respectively. that are important to an effective bifunctional catalyst: catalytic that are important to an effective and stability. selectivity, activity, increase the number of active sites, to obtain suitable adsorp- sur tion energy of the reaction intermediates on the catalysts’ face, and to reduce ionic and charge-transfer resistances. the-art carbon-supported Pt (i.e., Pt/C (carbon-black-supported Pt/C (i.e., Pt carbon-supported the-art and RuO platinum)) neither effective nor reliable, due to poor material compatibility nor neither effective of these composite catalysts. It is thus challenging, but desir able, to find a general principle to achieve rational designs of capability bifunctional yielding a strong catalysts, bifunctional battery zinc–air long-term of sake the for stability high and catalysts is a The development of bifunctional rechargeability. topic of a large body of research, for which readers can refer to a recent review on bifunctional electrocatalysts for recharge- able zinc–air batteries.

(7) (8) [180] [167,168] In con- In terms of However, it is However, [171,181,182] [174–176] - the dura In particular, 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag [24,125,169–172] © [167]

− Despite the progress, most catalyst H Noble-metal oxides such as ruthe- Noble-metal oxides

metal oxides (e.g., perovskites), [173] [166] H [179] 2 some transition-metal macrocycles (e.g., This is because the oxygen in this pathway this in oxygen the because is This OO 4O without producing peroxide in the solution − [175] [176] ⇔ ⇔+ −− [177,178] −− 29, 1604685 4e eH 2017, O2 HO Mater. ++ ++ Significant progress has been made in bifunctional-cata- progress has been Significant Precise reaction pathways during the OER are relatively are OER the during pathways reaction Precise For practical battery applications, catalysts that can promote For 22 22 OH O2 Adv. www.advancedsciencenews.com is reduced to OH trast, the two-electron reduction path involves peroxide species that are present in the electrolyte. The peroxide buildup not poi- but also of the ORR catalysis only reduces the efficiency sons the catalysts or carbon support materials because of its high oxidizability. still quite challenging to find a stable catalyst material without material without to find a stable catalyst still quite challenging due efficiency, catalytic significantly sacrificing bifunctional potential ORR/OER mainly to the wide operating range of the during the discharge and charge process. materials lyst development including transition-metal-based heteroatom- (oxides, chalcogenides, nitrides, and carbides), materials that con- doped carbon nanomaterials, and hybrid sist of the former two. iron phthalocyanine), and some nitrogen-doped carbon materials. To be commercially viable, while achieving catalytic efficiency efficiency catalytic achieving while viable, commercially be To air electrode, bifunc- for both the ORR and OER at the same have been extensively tional catalysts free of precious metals sought for practical zinc–air batteries. bility of existing noble-metal-based catalysts is still far from catalysts is still far from bility of existing noble-metal-based operation; zinc–air-battery rechargeable under satisfied being in the zinc–air system is also plagued their implementation noble metals. by the scarcity and resultant high cost of ambiguous and thus difficult to be determined. Generally to be determined. ambiguous and thus difficult accepted mechanisms for the metal-catalyzed OER involve the research is still generally based on the trial-and-error method. research is still generally based on the trial-and-error kinetics of the This is because the mechanism and reaction catalyst materials and ORR and OER vary between different electrolytes, and thus are hard to predict. nium (Ru) and iridium (Ir) oxides, though being excellent OER (Ir) oxides, though being excellent OER nium (Ru) and iridium for the ORR. catalysts, are less active the ORR through the direct four-electron reduction pathway are the ORR through the direct four-electron preferred. highly The kinetics for the electrochemical oxygen reactions are gen- are reactions oxygen electrochemical the kinetics for The are needed therefore, bifunctional catalysts erally rather slow; - to practi and OER kinetics the ORR to boost both in order zinc–air batteries. Platinum (Pt) exhibits cally usable levels in the ORR but rather poor performance for a superior activity for formation of a stable oxide layer with low the OER due to the electrical conductivity. 6.2.1. Oxygen Electrochemistry 6.2.1. Oxygen 6.2. Bifunctional Catalysts 6.2. Bifunctional phase. These catalysts are often noble-metal-based (e.g., Pt, Pd, phase. These catalysts are often Ag) materials, the ORR, the reduction of oxygen in aqueous electrolytes may may electrolytes aqueous in oxygen of reduction the ORR, the pathway four-electron proceed by two overall pathways: a direct (Reaction (8)). (Reaction (7)) and a two-electron pathway Review 1604685 well astheoxygenevolutionbyrulingoutCO ORR) andO their crystallographicstructures ( tron transferpathwaytoproducethedesiredOH selectivity cancatalyzetheoxygenreductionviafour-elec- et al. superior to thoseof the (111) planes. Meng and OERcatalyticactivitiesthat wereboth for ORR/OER catalysis,andexhibitedORR known asoneoftherate-determiningsteps oxygen species (e.g., OH (100) planesfacilitatedtheadsorptionof It wasfoundthatpreferentiallyexposed to carbonoxidationifthecatalystiscarbon-based). δ based oxides, reaction, havebeendevelopedfortheORR(e.g.,perovskite- against adsorptionenergyofreactionintermediatesforagiven study ofanisotropicMnOnanocrystals. ally catalyticactivitywashighlightedina the exposedlatticefacetsonbifunction- selectivity andactivity. Theimportanceof gies isvital,andcanleadtoahighcatalytic crystal planesandappropriatesurfaceener surface propertieswithpreferentiallyexposed cient bifunctionalcatalysts,precisecontrolof the catalystinquestion.For thedesign of effi- depends stronglyonthesurfacestructuresof alytic activitytowardtheORRandOER discussed below. chiometry, size,andmorphologyasfurther tallographic structure, composition/stoi- affected byavarietyoffactors,suchascrys- on theintrinsicpropertiesofcatalystsused,whichcanbe Activity andselectivityfortheORROERdependhighly 6.2.2. Catalytic ActivityandSelectivity respect tothesecharacteristics. rational designandimprovementofbifunctionalcatalystswith applied potentialsandpHconditions. thermodynamic stabilityofthevariousmetalsunderdifferent catalysts, Pourbaix diagramsareausefultooltoexaminethe corrosion/degradation, and phase change. For metal-based be duetomanyreasons,suchassurfacepassivation,material tion athighcurrentrates.Thestabilitylossofthecatalystscould tion, aswellstrongreducingconditionsunderoxygenreduc- with respecttohighlyoxidativeconditionsuponoxygenevolu- tion phaseandtoobtaindesiredend-productsofOH production of peroxide radicals as intermediates in the solu- there isanoptimalbindingenergy. These volcanotrendsshowedaclearactivitypeakatwhich of MnO OER (e.g.,metaloxides, www.advmat.de -MnO Crystallographic StructureControl:Thecat- The following sub-sections will discuss approaches to the Stability: is requiredtowithstandharshconditions Selectivity: referstotheabilityminimize [191] (19 of34) 2 , aswellamorphousMnO foundthatthecatalyticperformance 2 catalystswashighlydependent on 2 (for theOER).Abifunctionalcatalystwith a high (for [192] andheteroatom-dopedgraphene wileyonlinelibrary.com − [194] and O andmetal(oxy)hydroxides 2 α ), which is -, 2 β ), with -, and [196] -

2 evolution(due 2016, NaturePublishingGroup. cally active-vacancy-richO-terminated {111}facets.Reproducedwithpermission. textured nanopyramids. d) The dominant exposed facets of nanopyramids are electrochemi- pores present on the surface and across SC NRs. c) The surface of SC CoO NRs covered with Figure 14. © [195] 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim [193] − species,as ) andthe − (for the (for a) SCCoO NRsfabricateddirectly oncarbonfibersubstrate.b)Numerousnano- [185] ). lytic activityoftransition-metalcompounds(TM that tendtowardtwo-electrontransfer. Thisindicatedthat product (OH oxide-rich phase(e.g.,MnO α Mn ated withmanystoichiometries,fromametal-richphase(e.g., TM/A composition.For instance,manganeseoxidesaregener ORR/OER dependsstronglyonboththeTMactivecenterand both ORRandOERactivitiesfollowingtheorderof and Cuhavebeenreceivingincreasinginterest. Transition metals(referredto asTM)suchCo,Ni,Mn,Fe, mechanism of lizing Koutechy–Levichplotsrevealedthefour-electron transfer amorphous-MnO tutes fornoble-metal-basedcatalystsinzinc–airbatteries. referred toasA)areexpectedservecost-effective substi- phates, nitrides, andmacrocycles (these anions collectively metal-based compounds,includingoxides,sulfides,phos- electrocatalysts forboththeORRandOER,transition- the CoOandresultingsuperiorelectrocatalyticactivity. which wereresponsibleforthehighelectronicconductivityof by hybridizationofO-2p,Co-3d,andCo-3sinthebandgap, cies onthe{111}-Ofacetsinducedsomenewelectronicstates energies forORR/OER intermediates. In particular, thevacan- CoO topromotechargetransfer, aswelloptimaladsorption ture design,enabledgreatexposureoftheactivesites this surfacestructureengineering,intermsofelectronicstruc- By creatingoxygen-vacancy-richpyramidalnanofacetsof{111}, atomic structureof1Dsingle-crystalCoOnanorods(Figure mance bifunctionalelectrocatalystthroughtuningthesurface -MnO Composition Control:Aspromisingnonprecious-metal-based 3 O 4 ), whichfeaturesextensivemetal–metalbonding,toan 2 hadahigherselectivityforthefullyreducedoxygen − ). Recently, Lingetal. α -MnO 2

> β -MnO 2 in the ORR compared to other phases 2 2

> , MnO),whichexhibitsextensive δ - MnO [196] www.advancedsciencenews.com developedahigh-perfor 2 . Thekineticstudiesuti- Adv. Mater. x A [197] 2017, y ) towardthe [196] Thecata- α 29, 1604685 Copyright -MnO 14). [177] 2

> - -

Review

. 4 4 x x 4 O O O 3 3 3 1604685 exhib- 4 O 3 16a–c), for significantly www.advmat.de 15a–d). High- 4 Copyright 2016, Copyright (20 of 34) O 3 [202] (7.5)/CNTs (c), and (7.5)/CNTs x particle sizes. The OER particle x /CNTs, where OER activi- where /CNTs, x ; the microsized pores in the 4 O 3 (4.3)/CNTs were spinel-structured (4.3)/CNTs x (6.3)/CNTs (b), CoO (6.3)/CNTs x wileyonlinelibrary.com boosted the catalytic activity for both [24] and oxygen diffusion during the catalytic and oxygen diffusion − (Figure 6.3, 7.5, and 9.5 nm were 4.3, x /CNTs (Figure 15e–h) showed that the smallest CoO showed that the 15e–h) (Figure /CNTs (4.3)/CNTs (a), CoO (4.3)/CNTs x x whereas the other three samples consisted of the whereas the other were stably maintained after 1000 CV cycles in the range were stably maintained after 4 4, O O 3 3 : Morphology control can influence a Morphology Control: Morphology control of materials in the In addition to the varying morphology resolution transmission electron microscopy (HRTEM) images microscopy (HRTEM) transmission electron resolution of CoO reactions. Furthermore, cyclic voltammetric (CV) measure- reactions. Furthermore, of the 3DOM ments showed that the initial catalytic activities Co ties increased with decreasing CoO with decreasing ties increased with sizes of 3–10 nm, and investigated their size dependency their size of 3–10 nm, and investigated with sizes sizes of The average particle oxygen catalysis. for bifunctional the CoO instance, Park et al. instance, Park nanoparticles in the CoO nanoparticles in the nanoparticles. which, such as the specific surface area, properties, catalyst’s fashioning bulk Co By behavior. in turn, tune the catalytic (Figure into a 3D ordered mesoporous structure 3D 16d), the “honeycomb-like” Compared to bulk shape (Figure increased the morphology (referred to as “3DOM”) effectively active surface area of the Co nanoparticles from aggregating. in hybrid nano- same composition, morphological control OER-active materials) composites (e.g., hybridizing ORR- and the ORR and the OER, as well as the stability of spinel Co the ORR and the OER, as well as the stability 3DOM Co of the framework interconnected of 1.0–1.85 V (versus RHE), whereas the bulk Co of 1.0–1.85 V (versus RHE), cubic CoO phase. The particle-size-dependent catalytic activity particle-size-dependent catalytic activity cubic CoO phase. The series of CoO was observed in the enhancement was related to the increased oxidation state of the increased related to the enhancement was CoO specific activity in smaller-sized cobalt species and greater Co facilitated the OH ited larger polarization losses for both the ORR and the OER. ited larger polarization losses for both the to stable sufficiently be to proved 3DOM advanced the Thus, to highly oxidative withstand harsh conditions with respect as strong reducing conditions upon oxygen evolution, as well can stability enhanced The reduction. oxygen under conditions prevented Co be attributed to the 3DOM framework, which - - /CNTs) /CNTs) x synthesized microspheres 4 [199] O 2 synthesized carbon- 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag [202] © /CNTs. a–d) Bright-field TEM images of CoO a–d) Bright-field TEM images of /CNTs. x This size effect controls the This size effect and cubic MnCo Seo et al. [200] 4 O improve the ORR activity via the four- improve the ORR activity 2 [201] 4 O 2 29, 1604685 Characterizations of the CoO phase) favor the OER and the tetrahedral sites of phase) favor the OER Similar to manganese oxides, rational control of the oxides, rational control to manganese Similar 2017, 4 O 2 [198] (9.5)/CNTs (d). e–h) Corresponding atomic-resolution TEM images and FFT patterns (insets). Reproduced with permission. atomic-resolution TEM images and FFT patterns (insets). Reproduced (d). e–h) Corresponding (9.5)/CNTs x Mater. : For heterogeneous catalysis, the particle size Control: For Size Adv. www.advancedsciencenews.com sors. It was found that the octahedral sites of spinels (cubic the octahedral sites of spinels (cubic sors. It was found that MnCo by controlling the ratio of the respective carbonate precur by controlling the nanotube-supported cobalt oxide nanoparticles (CoO nanotube-supported cobalt oxide nanoparticles spinel tetragonal CoMn CoO Figure 15. oxide–oxide bonding. The bifunctional activity of manganese of activity bifunctional The bonding. oxide–oxide states of with the oxidation is closely associated oxide catalysts compo- Mn/O the by be controlled can which Mn cations, the sition. TM center and TM/A composition allows the ORR/OER catal- allows the ORR/OER composition and TM/A TM center spinel- and perovskite-type transition- ysis to be tuned on other control, for instance, was well demonstrated metal oxides. Such oxides. Spinel-type oxides (single, binary, in spinel-type metal of capable of hosting various species and ternary types) are et al. lattices. Menezes transition metals in their the tetragonal CoMn American Chemical Society. behavior and properties of catalysts such as electrochemically behavior and properties of catalysts such mag- surface area, electrical conductivity, catalytic reactivity, the nanoscale Unraveling and structures. netic permeability, catalysis can provide impor on ORR/OER particle-size effect tant clues for the rational design of efficient electrocatalysts. A electrocatalysts. design of efficient tant clues for the rational strain size-induced of absence the in that, is hypothesis simple the smaller the particle size, (geometry) and electronic effects, studies Some scattered activity. the greater the specific catalytic or OER have been on the size dependency for respective ORR into bifunctional been given has less insight however, reported; oxygen electrocatalysis. electron transfer path. Accordingly, engineering the composi- engineering Accordingly, electron transfer path. such properties, the of tuning fine spinel oxides allows of tion play vital roles in as crystalline and electronic structure, which and an optimal catalytic activity catalysis, affording ORR/OER selectivity. interaction with their of the catalysts usually has a significant is reduced to the catalytic activities. When the catalyst size of the catalysts nanoscale, the physical and chemical properties scales (bulk phase) change significantly from those at larger due to quantum effects. Review 1604685 CCBC and intertwined-CCBC, respectively. f) Reproducedwithpermission. CCBC andintertwined-CCBC,respectively. f) andTEM(g)imagesofthe for therechargeablezinc−airbattery. e)Schematicofthezinc–airbatteryusing CCBCattheairelectrode.f,g)SEM(f) Figure 16. Reproduced with permission. sion. (enhanced synergisticeffect) betweentheLaNiO morphology enablesanintimate interparticleinteraction (Figure 16g). Inthismaterial,thehighlytailoredcomposite intertwined NCNTs, knitting theclustersofnanoparticles Lee etal. of thecore–coronamorphologyinelectrocatalyticactivity, to batteryperformanceuponcycling.To takefulladvantage half-cell test results, while suffering much less degradation of state-of-the-art Pt/CandLaNiO ited excellentORRandOERcatalyticactivitysimilartothat through thehighlyporousframework.TheCCBCthusexhib- component), aswellenhancedcharge-transportproperties catalysts. could beanefficient strategytodevelopbifunctionalelectro- thesis, macrosizedLaNiO (NCNTs) Inthissyn- forzinc–airbatteries(Figure 16e,f). particles supportingnitrogen-dopedcarbonnanotubes LaNiO lighted asameansofcombiningadvantagestheOER-active phology onthebifunctionalelectrocatalyticactivitywashigh- (CVD) method. The importance of the core-and-corona mor seed forthegrowthofNCNTs viaachemicalvapordeposition compared tothatofPt/CandLaNiO ther demonstratedsimilardischargeandchargepolarization activity andselectivity. Azinc–airbatteryusingtheCCBC fur ration contributedsignificantlytothecatalyst’s bifunctional lyst structurearisingfromtheuniquecore–coronaconfigu- effect betweenthetwodifferent materialsandthestablecata- well assufficient stability. Itwasconcludedthatthesynergistic controlled LaNiO of theCCBC(referredtoasIT-CCBC) stemmingfromsize- structured bifunctional catalyst (CCBC)consistingof LaNiO cies duringtheelectrochemical oxygenreactions.Compared the diffusion ofreactants andchargetransferofactivespe- and theNCNTcoronamaterials, and,accordingly, improved www.advmat.de [125] (21 of34) 3 Copyright 2014,AmericanChemicalSociety. (corecomponent)andtheORR-activeNCNT(corona [173,203] [125] a–c) Schematic(a)andTEMimages(b,c)ofhoneycomb-like3DorderedmesoporousspinelCo presentedanoptimizedcore–coronamorphology Chenetal. 3 nanoparticlestopromoteprolificgrowthof wileyonlinelibrary.com [24] 3 particlesactedasthesupportand Copyright 2015, Wiley-VCH. e–g) Design and application of the core–corona-structured bifunctional catalyst (CCBC) [204] synthesizedacore–corona- 3 catalysts,respectively, as 3 , ingoodagreementwith © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim 3 nanocores 3 - -

[204] electron pathway; theORRcatalyzedbynitrogen- (N), (I) activity leadingtoafavorableformationofOH materials withheteroatomscouldsignificantlyboosttheORR have yettobereadilyemployed asbifunctionallyactivecatalysts Despite theORRenhancement, thesecarbonnanomaterials rials asmetal-freeelectrocatalysts hasbeenextensivelystudied. sulfur- (S), in alkalinesolutions. ally showacertainextentofcatalyticactivitytowardtheORR Aside fromtheirutilizationassupport,carbonmaterialsgener oxidation potentials. phitic degreeforenhancedelectrochemicalstabilityathigh electrical conductivityforrapidelectrontransfer, andhighgra- large surfaceareaforextensiveexposureofactivesites,high extensive attentioninheterogeneouscatalysis,owingtotheir graphenes aretwogroupsofcarbonnanomaterialsreceiving rosion incomparisontotheirlow-crystallinitycounterparts. owing to their relatively high anodic resistance to carbon cor carbon nanosheets,andgraphene,aremostcommonlyused such ascarbonnanotubes (CNTs), carbonnanofibers(CNFs), trode/electrolyte interface.Particularly, graphitizedcarbons, as supportstoincreasethenumberofactivesitesatelec- Carbon materials with high surfaceareas are typically employed 6.2.3. Carbon EffectsonCatalytic Activity, Selectivity, andStability electrocatalysts. tion of composite materials as highly efficient bifunctional show the importance of morphology control in theprepara- carefully engineered core–corona morphology. These results bifunctional activityandelectrochemicaldurability, duetothe half-cell andzinc–air-battery performance,intermsof the to theCCBC,IT-CCBC displayedenhancementof both Copyright 2012,AmericanChemicalSociety. g)Reproducedwithpermis- [210] anddual-atom-doped [207] phosphorous-(P), [205] [174] Significantly, dopingthesenanocarbon Amongthesenanocarbons,CNTs and [182,211] 3 O 4 . d)TEMimageofthebulkCo CNTs and/or graphene mate- [208] www.advancedsciencenews.com boron-(B), Adv. Mater. − 2017, viathefour- [209] 29, 1604685 iodine- 3 [206] [205] O 4 - - .

Review

nm. 1604685 prepared nm; e,f) 5 /C catalysts, [214] 2 www.advmat.de (22 of 34) µm; d) 500 µm; b,c) 1 . a–c) FESEM images. d) TEM image. 2 wileyonlinelibrary.com ). The ZIF-67 particles not only provided particles not only 17). The ZIF-67 This hybrid configuration not only inherits the This hybrid configuration not only inherits cm. Scale bars: a) 10 [214,215] Some studies have shown that covalently hybridizing gra- Some studies have Another type of carbon-based hybrid bifunctional catalyst Another type of carbon-based hybrid bifunctional a 3D hybrid catalyst composed of interconnected N-doped gra- a 3D hybrid catalyst composed of interconnected method. This catalyst demon- by the CVD phene and CNTs strongly comparable to strated a high ORR and OER activity, wt% IrO that of commercial 20 wt% Pt/C and 20 zeolitic imidazolate framework (ZIF-67) particles (referred to as (ZIF-67) particles (referred framework zeolitic imidazolate for zinc– active catalyst a promising bifunctionally as NCNTFs) (Figure air batteries CNTs for the growth of N-doped nitrogen source a carbon and but also cobalt nanoparticles, the in-situ-formed catalyzed by for the formation of the hollow structure. acted as the template activity provide a good catalytic CNTs It is known that N-doped unsatisfactory. their OER activity is often toward the ORR, but catalytic activity exhibited enhanced The resultant NCNTFs - strongly com as superior ORR activity, toward the OER, as well Pt/C catalysts. This enhancement parable to that of commercial to the modified electronic structure and was mainly related chemical which were induced by CNTs, carbon defects of the with nearby cobalt and carbon atoms. interaction of nitrogen be a prom- as well as heteroatom doping, can phene and CNTs, catalytic activity and ising strategy to enhance the bifunctional stability. advantages of both the CNTs and the graphene, such as high and advantages of both the CNTs but conductivity, specific surface area and superior electronic the extends the exposure of active sites through also effectively the intimate connection between gra- 3D framework. Moreover, C–C bonding enables fast through covalent phene and CNTs et al. electron and charge transfer as a whole. Tian respectively, as well as superior durability, thus establishing it as well as superior durability, respectively, catalyst. as a high-performance, metal-free bifunctional nanomaterials typically consists of strongly coupled inorganic by direct nuclea- and functionalized carbons, synthesized metals, oxides, tion and growth of inorganics (e.g., transition oxygen, nitrogen, or sulfides) on the functional groups (e.g., - - As , as −2 [195] Under [212] experimentally evolution). 2 [170] demonstrated a direct 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © [213] Copyright 2016, Macmillan Publishers Limited. 2016, Macmillan Copyright nm refers to the lattice spacing indicated by the white dashed lines, on the carbon (002) plane. The arrows in (e) and nm refers to the lattice spacing indicated by the white dashed lines, on the carbon (002) plane. [213] developed N,S-co-doped mesoporous carbon developed N,S-co-doped mesoporous carbon [182] 29, 1604685 . The identification of bifunctionally active sites . The identification of bifunctionally active −2 Morphology and structural characterization of NCNTFs obtained at 700 °C in the presence of H Morphology and structural characterization of NCNTFs 2017, Mater. Recently, for the first time, Yang et al. Yang first time, for the Recently, Recently, metal–organic frameworks (MOFs)-derived nano- metal–organic frameworks (MOFs)-derived Recently, Adv. www.advancedsciencenews.com synthesis of hollow-structured, N-doped CNTs derived from CNTs synthesis of hollow-structured, N-doped e,f) HRTEM images. In (f) 0.36 (f) indicate the direction of the graphitic layers. The inset of (a) is a digital photo, scale bar is 1 Reproduced with permission. associated with different nitrogen species provides important nitrogen species provides important associated with different metal-free implications for the development of carbon-based, applica- zinc–air-battery practical for catalysts bifunctional tions. Qu et al. Figure 17. due mainly to their fairly low OER catalytic activity and the OER catalytic activity to their fairly low due mainly high oxida- when exposed to the carbon corrosion unavoidable further carbon corrosion could of the OER; the tion potentials the active catalysts and block the aggregation of the aggravate (due to CO formed carbonates surface with high and graphene exhibit relatively mentioned above, CNTs conjugated extensively from their benefiting stability, oxidation could further be improved their stability graphitic structure; dopants. heteroatom introducing selectively through well as good cycling stability over 30 h at a current density of well as good cycling stability over 30 h at 20 mA cm standing the exact catalytic reaction mechanisms of the ORR of the reaction mechanisms exact catalytic the standing nanocarbon is thereby of impor and OER on heteroatom-doped as the bifunctional activity and selectivity, tance to finely tune of the overall stability. well as the improvement nanosheets using S-modified polydopamine as a precursor. as a precursor. nanosheets using S-modified polydopamine this material exhibited Benefiting from the multiple doping, even and durability, bifunctional activity excellent ORR/OER catalysts. better than that of transition-metal and noble-metal groups have carbon composites with abundant functional catalysts toward been investigated as promising bifunctional the ORR and the OER. Xia et al. identified the bifunctionally active sites in N-doped graphene- identified the bifunctionally N sites quaternary the electron-donating materials; nanoribbon ORR, whereas the electron- were responsible for a four-electron as OER active sites. The N species served withdrawing pyridinic graphene-nanoribbon zinc–air battery adopted with N-doped of 65 mW cm catalysts exhibited a peak power density Review 1604685 free bifunctionalairelectrodes. substrates, inordertomake carbon-support-free andbinder- growing nanostructuredcatalysts onconductiveandporous trodeposition andCVDhavebeen takenasmethodsofdirectly integration intotheairelectrode. practical performanceinzinc–airbatteries suffers frompoor Despite alltheprogressofbifunctionalelectrocatalysts,their 6.3. IntegrationofBifunctionalAirElectrodes coupling withheteroatom-dopedgrapheneandCNTs. to transitionmetals,oxides,hydroxides,sulfides,orphosphates the aforementioneddeficiencies. electrodes forrechargeablezinc–airbatteriesthatovercomes exists aneedfornovelintegrativedesignofbifunctionalair tentials fordischargingandchargingreactions. Thus, there and largeinterfacialimpedances,givingrisetohighoverpo - (i.e., CaCO as evidentbytheabsenceofprecipitationcarbonates good OERselectivityandstabilityagainstcarboncorrosion, ately highORRactivity, theN-MWCNTs-Co catalystexhibited on theelectrochemicalmeasurements,inadditiontomoder to asN-MWCNTs-Co) forrechargeablezinc–airbatteries.Based tional catalystcomprisingcobaltandN-dopedCNTs (referred or sulfur)ofananocarbonsupport. additional carbonsupportand binders,theseadvancedair to thatofCo3O4. activity ofthosehybridcatalysts,aswellstability, compared oxidized CNTs hasalsoledtodrasticallyimprovedbifunctional spinel-Co3O4 nanoparticlesandN-dopedgrapheneormildly trode surface.Therecognitionofthesynergisticeffect between their physicalmixtures.Thippanietal. the OER,incomparison to thoseofindividualcomponents or to the increased activity and durability for both the ORR and of asynergisticeffect betweenthetwocomponents,givingrise strong chemicalinteractionsandelectricalcouplingasameans environment. radation of polymeric binders in a strong caustic and oxidizing weakly boundcatalystsfromtheelectrodesurfaceduetodeg - formance. Anotherissueisassociatedwiththedetachment of leaching ofcatalysts,severelyreducingtheoverallbatteryper The carboncorrosionaggravatesaggregation/sinteringoreven highly oxidative electrochemical potentials during charging. lene blackisreadilycorroded,especially when encountering tive impactonthebatteryperformance.For instance,acety- black). However, theuseoftheseancillarymaterialshasanega- ionomers) andaconductivecatalystsupport(e.g.,acetylene binders (e.g.,Nafionperfluorinated, TokuyamaAS4alkaline as wellmassdiffusion. Theygenerallyincludepolymeric the airelectrode,forsakeofelectronandchargetransfer, continuous, porous,andionic/electronicconductivenetworkin lary materialsarerequiredintheseapproachestoconstructa as drop-casting,screen-printing,andspray-coating.Theancil- into theGDLframework,throughavarietyofmethods,such poration ofthebifunctionalcatalystsandancillaryadditives cation ofthebifunctionalairelectrodeinvolvesphysicalincor leading toalackofaccesstheelectrolyte.Typically, thefabri- sized catalystsareburiedduringtheair-electrode fabrication, www.advmat.de (23 of34) 3 forelectrolytescontainingCa(OH) [141] Thisissueresultsinlossofactivematerials [217] Such asynergisticeffect couldbegeneral wileyonlinelibrary.com [141] [218] [140,141,147,172,218–221] Thatis,anumberofnano- Approaches suchaselec- [216] [195] Thisapproachaffords synthesizedabifunc- 2 © ) ontheelec- 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim Without - - - 25 mAcm cycling stabilityover600hofoperationatacurrentdensity tubes (NCNTs); thesynergisticeffect betweentheCo a powerdensityof160.7mWcm was abletodeliverahighenergydensity of 847.6Whkg Using thisairelectrode,aflexiblesolid-statezinc–airbattery port, andsuperiormechanicalintegritywithrobustflexibility. against corrosionandaggregationofthecatalysts/catalystsup- properties andmasstransfer, enhancedair-electrode stability facile accessibilityofcatalyticactivesites,favorableinterfacial tages ofthiselectrodecanbeembodiedbyseveralaspects: rent collectorandtheairelectrode.Themorphologicaladvan- performance duetotheinterfacialresistancebetweencur completely eliminatesthevoltagedropthatreducesbattery a currentcollector at the air electrode is not necessary. This Given that the stainless-steel mesh is highly conductive, the NCNTs wasfoundto enhance bifunctional catalytic activity. Co catalyst ofthearraycontainedananoassemblymesoporous on astainless-steel mesh (Figure on anarrayofhair-like bifunctionalcatalystsverticallygrown electrode polarizationvaluescan aidintheunderstandingof methods. Inaddition,individual resistance,capacitance,and (Section 7.2),andgalvanostatic full-discharge(Section7.3) galvanodynamic polarization(Section 7.1),galvanostaticcycling zinc–air batteries.Theseparameters canberevealedwiththe eters thatshouldbeevaluated inthe investigation of novel density, powerdensity, andcyclicstabilityareessentialparam- operating voltage, energy efficiency, specific capacity, energy zinc–air batteries.Factors suchasopen-circuit voltage(OCV), techniques thatmaybeusedtoevaluatetheperformance of provides anoverviewofthevarietyexperimentaltesting be demonstratedwithinacompletezinc–aircell.Thissection operation. Therefore,itisimportantfornovelcomponents to have significantandcomplexinfluencesontheothersduring air battery, it must also be realized that each component can tics andperformanceoftheindividualcomponentsinazinc– While the previous sections have focused on the characteris- 7. Battery-PerformanceEvaluation are stronglydemanded. bendable substrates (e.g., carbonclothormetalmeshes/foams) realize flexibleintegrativeairelectrodes,physicallyrobustor which wouldbeofgreatbenefittolarge-scalefabrication. To and arefreeoftheconventionalcomplexpreparationprocesses, synthesized with thecurrent collector (conductive substrates), integrative bifunctionalairelectrodesasawholearedirectly sites andalowinterfacialcontactresistance.Additionally, such electrodes also can provideahighaccessibility of thecatalytic array. electrode throughmorphologicalemulationofahuman-hair example, werecentlydevelopedafree-standingbifunctionalair the powerdensity, andthecyclestabilityarecompared.Asan formance, thevoltagegapbetweendischargeandcharge, able zinc–airbatteries.To accesstheoverallair-electrode per integrative bifunctionalairelectrodesforelectricallyrecharge- Table 3 O 4 [141] nanopetalsinnitrogen-dopedmultiwalledcarbonnano- 3 Thisairelectrodeassemblywasconstructedbased presentsasummaryofsometherecentlyreported − 2 . 18). Each individual hair-like www.advancedsciencenews.com − 2 , aswellaremarkable Adv. Mater. 2017, 29, 1604685 3 O − 4 1 and and - - Review f) d)

1604685 ; 2 − ) to the ; NA for 200 s, − ; ≈50 h @ ; ≈83 h @ ; ≈55 h @ −2 −2 ; ≈136 h @ ; ≈600 h @ ; ≈125 h @ −2 −2 −2 ; ≈600 h @ −2 −2 −2 −2 for 10 min per −2 www.advmat.de (24 of 34) for 1 h per cycle for 200 s per cycle 2 − −1 @ 0.65 mA cm for 1 h per cycle cycle 2 − @ 58 mA cm −2 for 15 min per cycle for 20 min per cycle for 30 min per cycle −2 −2 −2 −2 @ 100 mA cm @ 300 mA cm @ 200 mA cm for 5 min–4 h per cycle for 20 min–10 h per cycle @ 168 mA cm @ 65 mA cm @ 260 mA cm @ 253 mA cm −2 Measurements at a discharge −2 −2 −2 −2 −2 −2 −2 −2 d) 50 mA for 6 h per cycle Power density and durability Power density 20 mA cm 0.55 mW cm 10 mA cm 20 mA cm 10 mA cm 27 mW cm 15 mA cm NA; ≈30 h @ 1 A g 25 mA cm 40 mW cm 166 min @ 5 mA cm 125 mW cm 185 mW cm 86.3 mW cm NA; ≈8.3 h @ 20 mA cm 174 mW cm 108 mW cm 167 mW cm ≈ Not reported; a) c) ] −2 wileyonlinelibrary.com d) f) e) c) @ 10 @ 25 @ 5 b) b) @ 17.4 b) NA Measurements at a charge current of 5 mA cm b) f) 0.84 @ 10 1.41 @ 20 0.85 @ 20 0.73 @ 10 0.82 @ 1 0.9 ; 1.04 0.72 0.82 @ 15 1.1 −1 E [V] @ j [mA cm ∆(charge–discharge)

m m m m m Unit of j is A g /air e) 2 KOH + KOH/air KOH/air KOH/air m m to as activation polarization). The oxygen reactions at the air to as activation polarization). The oxygen losses in zinc– electrode constitute the majority of activation increased, additional air batteries. When the current is further polarization, and thus voltage losses are dominated by Ohmic the current density. the voltage decreases almost linearly with from loss mostly comes region, the voltage In this Ohmic to ionic flow and the the internal resistance of the electrolyte and the electro- interfacial resistances between the electrodes lyte; this is the desirable operating regime for the zinc–air bat- Both the activation (influenced mainly by catalyst activity) tery. and Ohmic resistances (influenced mainly by the electrolyte bat- of zinc–air drivers performance are key power properties) teries. The sharp voltage drop at the end is associated with the limited mass transfer of reactants (i.e., oxygen or OH electrodes, referred to as mass-transfer polarization. The power density (blue solid line) of the zinc–air battery can be plotted as a function of current density by multiplying the discharge allowing determination voltage by the applied current density, of the peak power density (and corresponding current density). (red solid energy efficiency In addition, the zinc–air battery’s line) can also be determined at a certain current density by In voltage by the charge voltage. simply dividing the discharge - this example, at the peak power output, the battery energy effi ciency is 31.5%, indicating that almost 70% of the energy put into the battery is lost to the discharge and charge overpoten- the at discharged and is charged battery the assumes (this tials same current density). /air m m c) KOH + 0.1 KOH + 0.2 KOH + 0.2 KOH + 0.2 2 KOH + 0.2 ZnCl m m m m m NA −1 ZnCl zinc acetate/air zinc acetate/air zinc acetate/air for 10 min; Cell components Cell 20 g L zinc acetate/oxygen −2 Zinc plate/6 alkaline electrolyte/air Zinc plate/6 Zinc plate/6 gelled with 6 Zinc foil/3 Zinc powder/cellulose film Zinc powder/cellulose film Zinc plate/6 Zinc plate/6 Zinc plate/6 Zinc plate/6 Zinc wire/gelled PVA & PEO Zinc wire/gelled PVA a shows 19a Rigid Rigid Rigid Rigid Rigid Feature Flexible Flexible Flexible Flexible Flexible Flexible 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © for 2000 s. CVD/ −2 process pyrolysis pyrolysis deposition deposition Immersion Dip coating Pulsed-laser carbonization Chenical-bath Hydrothermal Hydrothermal Electrospinning/ Electrospinning/ electrodeposition Electrodeposition Synthesis method Electrodeposition/ The voltage gap was estimated based on the discharge–charge cycling measurements; The voltage gap was estimated based on the discharge–charge b) on carbon on CNT for 5 min, followed by a charge current of 7.5 mA cm 2 on carbon –NCNT on 2 paper 4 4 −2 fiber film carbon-fiber paper O O 4 3 on stainless steel 3 on stainless steel 29, 1604685 x 4 RuO stainless steel nanofiber mat N N-doped carbon 3 Co O Co N/intertwined carbon a-MnO 3 4 AgCu on nickel foam fibers on carbon cloth N-doped NiFe double- N-doped NiFe MnO Co Bifunctional air electrode Bifunctional air Co 2017, layered hydroxide/Ni foam Summary of recently reported binder-free, integrative bifunctional air electrodes. integrative bifunctional recently reported binder-free, Summary of P–g-C . Mater. Current density represented by j; Reference [224] [219] [141] [147] [223] [228] [222] [220] [140] [218] [221] Adv. www.advancedsciencenews.com a) current of 15 mA cm The galvanodynamic polarization technique is used to evaluate The galvanodynamic polarization technique voltage from operating the degree of departure of the battery’s the OCV in response to discharging and charging currents. The technique is generally carried out by applying negative (discharging) or positive (charging) currents with progres- sively higher magnitudes to the battery electrodes. The poten- of current, yielding are recorded as a function tial responses the discharge and charge polarization curves. Figure 7.1. Galvanodynamic Polarization various phenomena. Electrochemical impedance spectroscopy various phenomena. Electrochemical impedance (Section 7.5) may be (Section 7.4) and three-electrode testing employed to determine these values. a typical example of discharge and charge polarization plots (black and brown solid lines, respectively) of the rechargeable zinc–air battery obtained by the galvanodynamic polarization between the operating voltage and the method. The difference OCV (i.e., discharge or charge overpotential) directly represents the sum of the voltage losses associated with activation and mass-transfer polarization at each electrode, in addition to the Ohmic resistance of every component of the electrochemical Taking Information). S1, Supporting circuit (refer to Figure the discharge polarization curve as an example, the steep ini- tial voltage drop is mostly caused by the energy barrier for the electron-transfer reactions occurring at the electrodes (referred Table 3 followed by a discharge current of 0.5 mA cm Review 1604685 cally alignedNCNTframeworkdirectlygrownonthesurfaceofeveryindividualSSmeshfibers.c)SEMimage(Co d) DigitalphotooftheCo with permission. Figure 18. initial fewcycles. carbon-nanofiber catalyst(NCNF-1000)at10mAcm played ahigherdischargevoltagethanthatofnitrogen-doped The commercialPt/Ccatalystfromthisinvestigationdis - A typicalplotobtainedfromthistestispresentedinFigure 19b. the correspondingdischargeandchargevoltagesarerecorded. alternately applyingfixednegativeandpositivecurrentswhile The galvanostaticdischarge/chargecyclingtestisperformed by investigating the electrochemical durability of zinc–air batteries. Galvanostatic cyclingisanacceleratedtestingtechniquefor 7.2. GalvanostaticCycling a galvanostaticcyclingtestinvolves thechoiceofmoretest 1000 catalystwascomparatively moredurable. severe corrosion during the chargingprocess, while the NCNF- loss ofORRactivityforthePt/C catalystwaslikelycausedby only decreasedfromapproximately 1.2Vto1.05V. Thelarge hundreds ofcycles,whilethedischargevoltageNCNF-1000 was severelydecreasedfromapproximately1.3Vto0.8after www.advmat.de In comparisontogalvanodynamic polarizationtesting, (25 of34) a) Aschematicillustrationofhair-likearrayaCo [141] Copyright 2016,Wiley-VCH. [222] wileyonlinelibrary.com However, thedischargevoltageofPt/C 3 O 4 -NCNT/SS electrodewithtwistingtheto360°.Scalebars,10µm,1m(b),(c),2cm(d).Reproduced © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim −2 3 inthe O 4 -NCNT/SS airelectrode.b)Illustrationandcross-sectional SEM imagesofverti- because thetimelengthofeach cycleistypicallylongenough zinc–air batteryratherthanthe pulsecyclingmethod.Thisis able forevaluatingtherealrecharge capacityretentionofthe at 20hwithatotalof2cycles.This testingmethodismoresuit- Supporting Information showsthelengthofacycleisapplied cycles withlongertimelengths. For example,Figure S2binthe In contrast,thesecondcategoryinvolvesashorternumber of charged toasmallfractionofitstotalcapacityineachcycle. the airelectrode,sincezincelectrodeisusuallyonlydis - method, thecyclingdurabilityismostlikelytobelimited by densities withashortlengthofcycle(200spercycle).Inthis polarization performanceswerecomparedatdifferent current porting Information, the galvanostatic discharge and charge (i.e., pulsecyclingtest).For example,inFigure S2a intheSup- large numberofcycleswithshorttimelengthsforeachcycle divided intotwodifferent categories,thefirstofwhichusesa results. Cycling testsforzinc–airbatteriescanbebroadly for each cycle; each of these parameters will affect the test number ofcycles,andthetimelengthand/orcut-off voltage conditions, includingdischarge/chargecurrentdensity, the www.advancedsciencenews.com 3 O Adv. 4 -NCNT/SS airelectrode. Mater. 2017, 29, 1604685 Review 1604685 ) with an −1 nanowires 4 O 3 www.advmat.de (26 of 34) b) Copyright 2016, Copyright b) (50 A kg −1 [222] and 10 min are applied −1 wileyonlinelibrary.com Copyright 2013, Wiley-VCH. Copyright [140] different testing parameters (current density, length of a cycle, length of a cycle, (current density, testing parameters different to compare the and cycle number), which makes it difficult resultant performances. Therefore, it would be beneficial to on based parameters for testing standard a general determine smart grids, and including EVs, battery applications, different agreement on these standards is personal electronics. However, not likely to be reached in the short term. Therefore, zinc–air battery researchers should employ the same or similar testing parameters used by the prior investigations that are mostly closely aligned with their investigations (in terms of materials or applications). 7.3. Galvanostatic Full-Discharge The galvanostatic full-discharge test determines the capacity of the zinc–air battery by applying a fixed discharge current until nanowires sprayed on GDL (blue circles), and Pt/C sprayed on GDL (black triangles). Inset: high nanowires sprayed on GDL (blue circles), and Pt/C sprayed on GDL (black triangles). Inset: high 4 O 3 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © Copyright 2015, Wiley-VCH. d) Nyquist plots obtained by EIS using air under ambient conditions of Co 2015, Wiley-VCH. Copyright [146] this allows evaluation of the zinc-elec- 29, 1604685 [62,65,82,88,91] a) Typical discharge and charge polarization curves for the rechargeable zinc–air battery, as well as the power density and energy efficiency. as well as the power density and energy efficiency. zinc–air battery, discharge and charge polarization curves for the rechargeable a) Typical 2017, Mater. Unfortunately, no standard parameters have been estab- Unfortunately, Adv. www.advancedsciencenews.com inset view of the corresponding capacities. The gravimetric energy density and specific capacity calculation are based on the mass of zinc electrode. inset view of the corresponding capacities. The Reproduced with permission. lines. b) Typical galva- (M) polarization regions are identified on the discharge curve by black dashed lines. b) Typical The activation (A), Ohmic (O), and mass transfer (NCNF-1000) and precious catalysts (Pt/C) are tested. 10 mA cm nostatic discharge/charge battery cycling test. Nonprecious Figure 19. to utilize a significant portion of the zinc electrode’s capacity. capacity. to utilize a significant portion of the zinc electrode’s Investigations focused on the zinc electrode performance often instead of a defined time length for voltage employ a cut-off each cycle; frequency range of the Nyquist plot, and the equivalent circuit. Reproduced with permission. Wiley-VCH. c) Typical specific capacity curves of the battery with different thicknesses of zinc film at a current density of 250 A L specific capacity curves of the battery with different Wiley-VCH. c) Typical trode capacity with respect to cycle number. Another advan- trode capacity with respect to cycle number. tage of using longer cycling times is that more realistic voltage that times cycling to opposed as obtained, be to likely are values How- are too short to allow the voltages to reach a stable value. cycling methods provide uniquely since the two different ever, useful information, it is recommended to conduct both types of experiments in investigations of complete zinc–air batteries. S2 lished for the in-lab testing of zinc–air batteries. In Table zinc–air-battery rechargeable Information, Supporting the in categories of catalysts performances from three different evaluated by galvanostatic discharge/charge cycling tests are listed. It can be seen that the respective studies utilize widely as the current density and length of a cycle, respectively. The battery voltage is recorded versus zinc. Reproduced with permission. with Reproduced zinc. versus recorded is voltage battery The respectively. cycle, a of length and density current the as grown on a stainless-steel mesh (red squares), Co Review 1604685 entire thickness of the electrode. non-uniform wettingbythesolid-stateelectrolytethrough as afunctionofthickness,whichlikelyresultedfromthe However, thespecificcapacityofzincelectrodedecreased extremely flat,meaninglittleOhmicpolarizationofthebattery. are showninFigure 19c.Inthiscase,thedischargecurvesare discharge period.Dischargecurvesforaflexiblezinc–airbattery cific capacityandanaveragedischargevoltageovertheapplied zinc-electrode surface.ThisresultsinamoresevereZn(OH) of zinc(Whg tion, aspecificenergydensitynormalizedtotheconsumedmass equivalent circuit,areshownin Figure 19d.Thetwosemicircles battery usingdifferent catalysts, aswellthecorresponding overpotential. The typicalEISspectra (i.e., Nyquistplots)ofthe with respecttothedischargeand rechargeprocessesatafixed Typically, EISinrechargeablezinc–air batteriesisconducted enabling abetterunderstanding ofthemajorlimitingfactors. vidual resistancesandcapacitanceswithinazinc–airbattery, Electrochemical impedance spectroscopy (EIS) can reveal indi- 7.4. ElectrochemicalImpedanceSpectroscopy the theoreticalspecificzinccapacityof820mAhg within thezincelectrode;thisenablesarelativecomparisonto normalized bythemassofzincelectrodeor capacity isdictatedbythezincelectrode,often a definedcut-off voltageisreached. Since azinc–airbattery’s uted totheincreasedrateofZn(OH) results in a slightly lower attained capacity. This may be attrib- Typically, increasingthecurrentdensityinafulldischargetest example, refer to Figure S3, Supporting(for Information). two can vary depending on the discharge current employed full-discharge experiments, and therelationship between the the batteryasafunctionofdischargedepthisalsorevealedby how muchofthezinc’s capacitywaslosttoself-discharge. zinc–air batterybythemassofconsumedzinc,sinceitreveals this phenomenonbynormalizingthemeasuredcapacityof it tohavealowerspecificcapacity. Researcherscanelucidate trode reducetheexternalcapacitythatitsupplies,thuscausing Reaction (9).Self-discharge processesoccurringatthezincelec- the zincelectrode,causingittobeself-discharged accordingto electrode duringthedischargeprocessandreactdirectlywith travel alongsidefullyreducedhydroxideionstowardthezinc tion ofperoxideionsintheelectrolyte.Themay the two-electronpathway, andthusproduceagreaterconcentra- (Reaction (8))ORR.Apoorordegradedcatalystwilltendtoward pathway (Reaction(7))insteadofthetwo-electron-pathway of different catalyststoprovideandmaintainthefour-electron- posed explanationofthisbehaviorinvolvestherelativeability example, refertoFigure S4,Supporting Information).Thepro- (for affected bytheefficiency ofthecatalystinairelectrode extracted fromthezincelectrodeinazinc–airbatterycanbe the electrodesurface,andthuspassivationatanearlierstage. concentration gradient,causingearlierprecipitationofZnOat www.advmat.de Zn Interestingly, it has also been shown that the capacity +→ HO (27 of34) 2 −− Zn −1 ) canbeobtainedbysimplymultiplyingthespe- OO + wileyonlinelibrary.com H

[33] The working voltage of 4 2− ionproductionatthe © 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim −1 . Inaddi- [164,225] (9) 4 2−

line electrolytes. since thisisthemostwidelyused referenceelectrodeinalka- Hg/HgO referenceelectrode in three-electrodeexperiments, in zinc–airbatteries.Investigators maychoosetoemploya EIS experimentsaretypicallydominatedbytheairelectrode cuit including five elements: in theNyquistplotaremodeledbyprovidedequivalentcir ments, trocatalytic activityoftheairelectrode.Theconstantphaseele- electrochemical reaction,whichisdirectlyrelatedtotheelec- the charge-transferresistanceofairelectrodeduring the zincelectrode under investigation. wire orzincfoilcanbeemployed asthereferenceelectrodeto the zincelectrode, vide theopportunitytoperformequivalent-circuitmodeling of An EISexperimentusingathree-electrodesetupcanalsopro - discharge voltage. is appliedtoOERactivityoncharging),leadingahigher ance indicateshigherORRcatalyticactivities(thesametrend general analysisoftheresults,asmallercharge-transferresist- interface betweentheelectrodeandelectrolyte.Intermsof R electrolyte interfacialresistance,respectively. R in employing the zinc electrode as the WE is shown schematically electrode (RE).Anexampleofathree-electrodeapparatus while thepotentialofWEismeasuredagainstareference between a workingelectrode (WE) and counter electrode (CE), In athree-electrodeconfiguration,electricalcurrentflows two-electrode configuration(i.e.,theusualzinc–aircellsetup). perform theminathree-electrodeconfigurationratherthan For manyofthetestsdescribedabove,itcanbebeneficialto 7.5. Three-ElectrodeTesting Section 5.2). teristics comparedtoconventionalaqueouselectrolytes(refer solid-state electrolytes, due their relatively poor wetting charac- resistance valuesareparticularlyimportant to quantifyfor facial contacts,respectively, inthezinc–airbattery. Interfacial lower internalresistancesfromtheelectrolyteandatinter growth duringchargingorpassivationdischarging. be usedtoindicateprocessessuchasnucleationanddendritic accurate detectionofzinc-electrodeoverpotentials,whichcan utilize athirdelectrodeasreferenceelectrode.Thisallowsfor tigators researchingthezincelectrodeinazinc–airbattery cated bythelatter).Therefore,itisrecommendedthatinves - is typicallymuchlarger(resultingintheformerbeingobfus - two-electrode setup,sincetheoverpotentialatairelectrode is difficult todetectifitistestedagainstanairelectrodeina ying the zinc electrode. The overpotential at the zinc electrode complete confidence. whether theairelectrodeorzincisculprit,with cycling experiment,asitallowstheexperimentertodetermine when apolarizationincreaseisobservedduringgalvanostatic independently measured. This setup can be particularly useful trode orairelectrode(whicheverisemployedastheWE)tobe s andR The three-electrodetestsetupisespeciallyusefulforstud- Figure Q int int 20. This setup allows the potentialof the zincelec- signify the electrolyteresistance and theelectrode– andQ [56,72,92,116] [227] [229] dl , arecharacteristiccapacitancesfromthe Inaddition,smallerR sincetheimpedancesintwo-electrode Alternatively, asparepieceof zinc R s , R www.advancedsciencenews.com int , [20,230] R Adv. ct , Q Whilezincisnot a Mater. int s andR , and 2017, ct represents Q 29, 1604685 int dl imply . [226,227] [63] - -

Review - - - 1604685 www.advmat.de (28 of 34) poisoning, thereby poisoning, thereby 2 wileyonlinelibrary.com and particulate matter). The efficient stacking and particulate matter). The efficient 2 improving battery efficiency and cycle life. However, their life. However, and cycle improving battery efficiency as high-performance electrolytes immediate application several to due challenges, other faces batteries zinc–air in low reaction kinetics shortcomings, such as high viscosity, and high cost. For for zinc and oxygen electrochemistry, of the electrolytes is vital to flexible devices, “solidification” under var maintain the structural integrity of the batteries toward improving the electrolyte– ious deformations. Efforts emphasized when electrode interfacial properties must be versatile with electrolytes liquid incumbent the replacing solid-state alternatives. that exhibit electrocatalysts Novel, inexpensive bifunctional of com- i) bifunctionality (requiring in-depth understanding in electro- processes evolution reduction and plex oxygen in a wide range lytes), ii) versatility (requiring their function aqueous and non- of temperatures, voltages, and in both (ease of incorpo- aqueous electrolytes), and iii) scalability enabling structure, ration of catalysts into the air-electrode Moreover, commercialization and widespread adoption). results and to create a comparable link between laboratory must be studied commercial applications, the catalysts catalysts that show under practical test conditions. Often, cells do not per high activities in analytical three-electrode that demand form well in commercially relevant devices with long high current densities and operating temperatures lifetimes. design cur design. The air electrode air-electrode Advanced rently adopted in many zinc–air batteries in the research cells, fuel alkaline ordinary in used that to similar is phase carbon-based by mainly supported are catalysts the where carbon materials can However, electrodes. gas-diffusion be readily corroded (oxidized) under the harsh conditions imposed by repetitive discharging and charging in alkaline preparation is electrolytes. In addition, the air-electrode an inherent disadvantage owing to the tedious and has often use of ancillary materials. New strategies to use either highly corrosion-resistant carbon materials or metallic meshes/ foams, onto which the catalysts are directly grown, are sug- the con- free of are approaches by researchers. Such gested of be would which processes, preparation complex ventional great benefits to large-scale fabrication. Optimal cell and stack design. Although simple designs require supplemen- are preferred, zinc–air batteries often the air electrode needs “organs” tary components; namely, atmospheric of purification and supply the facilitate that air (e.g., CO of additives and/or chemical doping have been demon- doping have been and/or chemical of additives of these for meeting all three feasible solutions strated to be that additives are those The most promising requirements. of since a larger proportion in small quantities, are effective capacity. the overall zinc additives reduces opera- long-lasting allow that technologies electrolyte New aqueous alkaline elec- Up to now, tion of zinc–air batteries. most popular for zinc–air batteries, trolytes have been the with carbonate formation and electro- but issues associated to be addressed for long-term battery lyte evaporation need ionic liquids are attractive because operation. In this case, resist- exhibit and electrolyte evaporation they limit aqueous and CO ance to hydrogen evolution 3) 4) 5) 2)

) −1 −1 to [231] −1 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim KGaA, & Co. GmbH 2016 WILEY-VCH Verlag © Despite the intense research [38,232] ). −1 or volumetrically 800–1400 W h L −1 for lithium-ion batteries) and can be manu- be can and batteries) lithium-ion for −1 29, 1604685 Schematic representation (side-view) of a three-electrode test Schematic representation (side-view) of a three-electrode 2017, Mater. A reversible zinc electrode that i) has a high proportion of utilizable active material, ii) is capable of recharging with and iii) sustains its capacity over at least sev- high efficiency, eral hundred charge and discharge cycles. Structural modi- fication through electrodeposition and advanced casting techniques, as well compositional modification by means Adv. www.advancedsciencenews.com as low as 65 US$ (kW h) compared to other secondary batteries (e.g., 150–280 W h kg L h W 600–700 or factured at a very low cost (estimated from 135 US$ (kW h) setup employing the zinc electrode and air electrode as the WE and CE, setup employing the zinc electrode and air electrode plates and acrylic by using formed chamber is The electrolyte respectively. supplied filling channels the is filled using with circular holes, and gaskets at the tops of the plates. Figure 20. 8. Conclusion and Outlook 8. Conclusion urban environments, Energy storage in densely populated electric mobile applications, and flexible electronics require with low mass and space requirements. batteries affordable this in promising are batteries zinc–air rechargeable Electrically perspective; they can provide high energy density (gravimetri- cally 400–800 W h kg conventional reference electrode, its low polarizability and gen- conventional reference electrode, its low it an acceptable sub- eral stability in alkaline electrolytes make case. for this usage stitute for a Hg/HgO reference electrode 1) and improvements that were discussed, there is still much scope for further advancement of existing electrically recharge- able zinc–air batteries. Specific needs in materials science and engineering are as follows: Review 1604685 dies. the adoptionofEVs tobelargelyreliantongovernmentsubsi- and limitedenergydensityoflithium-ionbatterieshascaused most pressingofwhichisEVs. Thusfar, therelativelyhighcost teries suitableformanymoreenergy-storageapplications,the also be realized. Success in these areascan render zinc–air bat- and ultimatelymoreenergy-densewithstaticelectrolytescan cally rechargeablezinc–airbatteriesthataresmaller, lighter, progress intheresearchareasdiscussedthisreview, electri- Mazda, amongothers. baseload power, havealsobeenproposedforEVs byTesla and and regenerativebrakingwithahigh-energy-densitysourcefor systems, whichcombineahigh-powersourceforacceleration in vehicularacceleration.Inthisregard,dualenergy-storage capability ofbatteriesisneededforburstsenergy, suchas Additionally, ahighpowerperformance witharapiddischarge allowing themtobeemployedwithlessprotectiveequipment. higher energydensity, butalsobecauseoftheirinherentsafety, tion of zinc–air batteries, not only due to their lowercost and health, teries, whichpresenthazardstotheenvironmentandhuman batteries arealsoexcellentcandidatestoreplacelead–acidbat- charge capabilityforEVpropulsion. battery, providingremarkablyhighpowerdensityandfastdis - nickel–zinc andzinc–airreactions,ontherechargeable also demonstratedanovelhybridairelectrode,conductingboth power outputsatalmostanystateofcharge.Aresearcherhas component ofthesesystems,sincetheycanprovidestable by zinc–air batteriesmake them well-suited as the energy-dense curement of13MWzinc–air-battery storage. Californian investor-owned electricutilityannouncedthepro- rechargeable zinc–airbatterieswasdemonstratedwhena low environmentalimpact.Recently, confidenceingrid-scale years, withtheirmainadvantagebeinglowcostand trically rechargeablezinc–airflowbatteriesoverthepastfew didate to become a leading energy-storage solution. Existing tries, zinc–airbatterieshaveemerged asthemostlikelycan- attention in the academic literature on metal–air chemis- and spacerequirements. need forathick casing, thereby further reducingtheir weight are acrucial reason for this,theirsafe operation also lowersthe tions. Whiletheirhighspecificandvolumetricenergydensities ingly beingrecognizedasaviablesolutionfortheseapplica - batteries employingflexiblesolid-stateelectrolytesareincreas - current batterytechnology. Electricallyrechargeablezinc–air shapes iscurrentlylimitedbytherigidityandthickcasing of ment ofelectronicdeviceswithminiaturesizesandadaptable www.advmat.de Commercial developers have successfully introduced elec- While lithium–air batteries have thus far received the most escape duringrecharging. stack duringdischargeandallowevaluatedoxygentoeasily air channelsshouldfacilitateoxygen-gasflowthroughthe trode totheadjacentzincelectrodeinbatterypack.The channels (e.g.,bipolarplates),whichconnectstheairelec- success requiresasmartdesignofelectricallyconductiveair weight, volume,andstackimpedance.Thisperformance obtain high energy and power density by means of lowering of electricallyrechargeablezinc–aircellsinseriesiskeyto [235] (29 of34) [234] Thisproblemcanbealleviatedwiththeimplementa- intheresidentialstoragemarket.With significant wileyonlinelibrary.com [236] Theflatdischargecurvesexhibited [76] Meanwhile, thedevelop- © [233] 2016WILEY-VCHVerlag GmbH &Co. KGaA,Weinheim Zinc–air February 13,2017. Note: TheformattingoftheheaderrowTable 2wascorrectedon Lee attheUniversityofWaterloo forhisvaluablecomments. the Waterloo InstituteforNanotechnology. Theauthorsalsothank DongUn Research Council ofCanada (NSERC)andbytheUniversityofWaterloo and acknowledge thesupportprovidedbyNaturalSciencesandEngineering J.F. andZ.P.C.contributedequallytothiswork.Theauthorswish Acknowledgements from theauthor. Supporting InformationisavailablefromtheWileyOnlineLibraryor Supporting Information encouraged. technology intheacademicandindustrialsectorsishighly economy. Hence, acceleratedresearchanddevelopmentofthis needs ofanincreasinglyaffluent, digital,andlow-carbonglobal trically rechargeablezinc–airbatteriestosupporttheenergy of highenergydensity, safety, andlowcostshouldallowelec- able zinc–airbatteries. 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