Relating Catalysis Between Fuel Cell and Metal-Air Batteries

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Perspective Relating Catalysis between Fuel Cell and Metal-Air Batteries

Matthew Li,1,2 Xuanxuan Bi,1 Rongyue Wang,3 Yingbo Li,4,6 Gaopeng Jiang,2 Liang Li,5 Cheng Zhong,6,* Zhongwei Chen,2,* and Jun Lu1,*

With the ever-increasing demand for higher-performing energy-storage sys- Progress and Potential tems, electrocatalysis has become a major topic of interest in an attempt to Catalyst research for fuel cells has enhance the electrochemical performance of many electrochemical technolo- led to much advancement in gies. Discoveries pertaining to the oxygen reduction reaction catalyst helped humanity’s understanding of the enable the commercialization of fuel-cell-based electric vehicles. However, a underlying physics of the process, closely related technology, the metal-air battery, has yet to find commercial significantly enhancing the application. Much like the Li-ion battery, metal-air batteries can potentially uti- performance of the technologies. lize the electrical grid network for charging, bypassing the need for establishing In contrast, metal-air batteries a hydrogen infrastructure. Among the metal-air batteries, Li-air and Zn-air bat- such as Li-air and Zn-air batteries teries have drawn much interest in the past decade. Unfortunately, state-of-the remain to be solved. Although the art metal-air batteries still produce performances that are well below practical metal anode used in this these levels. In this brief perspective, we hope to bridge some of the ideas from systems does play a large role in fuel cell to that of metal-air batteries with the aim of inspiring new ideas and di- limiting their commercial success, rections for future research. catalysis also remains quite challenging. In this perspective, a INTRODUCTION discussion is provided on the similarities and differences Increasing the energy density of the battery system on board electric vehicles (EVs) between metal-air catalysts and has become a major topic of research interest throughout the electrocatalysis com- fuel cells for aqueous (alkaline/ munity.1–3 The development of catalysts that efficiently facilitate the oxygen reduc- acidic) and aprotic electrolytes. By tion reaction (ORR) allowed for great commercial advances in the field. Recent appli- attempting to bridge the cation of fuel cell technologies in EVs has led to the commercialization of a variety of discussion between the fields and fuel cell vehicles such as the Toyota Mirai.4 This EV boasts an attractive cruising providing our own opinion on the range of 500 km, cold start capability, and a significantly shorter refueling time subject, we hope that this of 3 min when compared with battery-based EVs.5 Unfortunately, the price remains perspective will present itself as a relatively high (7 million Japanese yen or 57,500 USD in 20144) due to the large starting point in emulating the impact of the Pt-based catalyst on the cost of the vehicle and the low production vol- success of catalysis in fuel cells in umes.6,7 While this might be within the purchasing power of some consumers (as a the metal-air systems. luxury vehicle), it will unlikely become widespread at this price point. Beyond the development of vehicle cost, a country- or even a city-wide hydrogen infrastructure is still far from realization and is considered a major hurdle against the widespread adoption of hydrogen fuel cells in the transportation sector.

In contrast to fuel cell technologies, the closely related metal-air batteries (MABs) can be seen as a decent compromise between lithium-ion batteries (LIBs) and hydrogen fuel cell systems. Theoretically, MABs such as the Li-air battery (LAB) should have drastically higher energy densities when compared with a LIB. While calculations taking into account the volume of some non-active material have led to more modest volumetric energy density estimates (less than LiCoO2-based LIBs), the gravimetric energy density has beencalculatedtobetwicethatofLIBs.8 More- over, the electricity infrastructure needed to recharge MABs will likely be able

32 Matter 2, 32–49, January 8, 2020 ª 2019 Elsevier Inc. to bootstrap the already present and quickly growing EV charging infrastructure (be it charging stations or potential future battery-swapping stations9). Although this is perhaps one of the most important advantages of MABs, it will inevitably come at the cost of energy density when compared with the hydrogen fuel cell. It is, however, important to note that this range disadvantage has already been rather well toler- ated by current LIB-based EV consumers, indicating that the energy difference between MABs and hydrogen fuel cells might not be a polarizing deciding factor. The difference in recharge/refuel time between MABs and fuel cells will likely decrease with more standardization among battery-pack designs, thus promoting battery-swapping stations.9 With many candidate MAB systems for EVs such as LABs and Zn-air batteries (ZABs) among other MAB systems still suffering from unre- solved technical challenges,10–14 the future applications of MABs remain unclear.

Although the commercial application of fuel cells in EVs has already gained substan- tial progress in recent years, most MABs have remained in their conceptual/proto- type stages. The clear difference in maturity between the two systems suggests very different technical challenges. On the surface, the MABs appear to be quite similar to fuel cells and are sometimes referred to as semi-fuel cells.15,16 Like the hydrogen fuel cells, all MABs also require an external oxidizing agent (O2)thatis ideally accessed from the ambient air, separate from the inherent mass and volume of the MAB system. Although there are many technical challenges associated with using a metal anode,17,18 the seemingly similar cathode between these systems have subtle yet significant differences. Variations in the nature of the discharge reaction and product, such as the solid/poorly soluble and insulating Li2O2 formed during the discharge of LABs19 versus the liquid water produced in hydrogen fuel cells, have led to a great divergence in their research strategies. While some researchers have attempted to relate the different systems, it is difficult to foresee how they will intersect.20–22 If clear analogies can be drawn between these systems, new para- digms in research can be devised. Therefore, it is timely to compare and contrast the different meaning of catalyst between these technologies, proposing a situation whereby similar design concepts can be used. We will first elaborate on the differences and similarities between ZAB, LAB, and fuel cell catalysis in aqueous media. 1Chemical Sciences and Engineering Division, Argonne National Laboratory, 9700 Cass Avenue, We will then compare these technologies in aprotic media and propose operational Lemont, IL 60439, USA conditions whereby more similarities can be found and leveraged. 2Department of Chemical Engineering, Waterloo Institute of Nanotechnology, University of Waterloo, 200 University Avenue West, Waterloo, AQUEOUS SYSTEMS ON N2L 3G1, Canada 3 Acidic Media Applied Materials Division, Argonne National Laboratory, 9700 Cass Avenue, Lemont, IL 60439, Among all the contemporary technologies for electrocatalysis, the most commer- USA cially successful is the application in the acidic medium hydrogen fuel cell, more 4Materials Science Division, Argonne National commonly known as the proton exchange membrane (PEM) fuel cell. Although Laboratory, 9700 Cass Avenue, Lemont, IL 60439, USA acid medium is not preferable for MABs (due to corrosion of Zn), it is important to 5College of Physics, Optoelectronics and Energy, contrast the catalytic mechanism. In acidic electrolyte fuel cells, it is commonly Center for Energy Conversion Materials & Physics accepted that an adsorbed oxygen will reduce to an adsorbed OH* or OOH* before (CECMP), Soochow University, Suzhou 215006, P. 23 R. China further reduction to H2O. During this process, the breakage of the O–O bond is 6 typically the rate-limiting step that requires a catalyst. Only by lowering the overpo- Key Laboratory of Advanced Ceramics and Machining Technology (Ministry of Education) tential associated with ORR can PEM fuel cells operate at sufficient energy effi- and Tianjin Key Laboratory of Composite and Functional Materials, School of Materials Science ciencies and power densities. The ORR begins with the adsorption of O2 molecules and Engineering, Tianjin University, Tianjin onto the catalytically active sites on the surface of platinum. The next step is the 300072, China weakening of the oxygen bond and reduction of the molecule by protons and elec- *Correspondence: trons forming an adsorbed OOH* followed by a series of steps and intermediates [email protected] (C.Z.), (O*, OH*), finally forming water.23 The overall process is presented in Figure 1A. [email protected] (Z.C.), [email protected] (J.L.) The reaction kinetics is largely controlled by the degree to which the intermediates https://doi.org/10.1016/j.matt.2019.10.007

Matter 2, 32–49, January 8, 2020 33 Figure 1. Properties of Oxygen Reduction Reaction

(A) O2 is shown to reduce to H2O with the various reaction rate constants denoted with a numbered k. Reproduced with permission from MarkovicandRoss.23 Copyright 2002, Elsevier. (B) Volcano plot for ORR activity versus HO* adsorption energy for various catalysts. On the left axis, Pt a plot of activity enhancement relative to Pt at U = 0.9 V (RHE) versus DGHO* À DG HO*,andonthe right axis a plot of U required for all reaction steps to be negative in free energy versus DGHO* À Pt 24 DG HO* Reproduced with permission from Stephens et al. Copyright 2012, Royal Society of Chemistry. are adsorbed by the catalyst surface. One would want a catalyst surface with the optimal adsorption energy for every intermediate to achieve the best catalytic activity. However, the binding energies of all the intermediates in ORR are governed by what is known as a linear scaling relation (LSR).24 LSR refers to the phenomenon whereby moving toward a stronger adsorption of OOH* (one intermediate) to facilitate the dissociation of O2 molecule will actually result in a less favored subsequent reduction and desorption of OH* (another intermediate). On the other hand, a surface with weaker adsorption of OOH* that promotes OH* desorption will result in difﬁculty for O2 adsorption. Therefore, based on an LSR, an ideal catalyst should have a surface that adsorbs all intermediates neither too weakly nor too strongly. In fact this is the general governing principle across all acidic, alkaline, and aprotic electrocatalysts. This phenomenon yields the well-known volcano-shaped plot of catalytic activity as a function of different catalyst surfaces, as shown in Figure 1B. On the left side of the volcano, the reaction rate is determined by OH* removal.

The reaction is controlled by O2 activation for those catalyst surfaces on the right side of the volcano. Practical catalyst designs are usually aimed at the optimization of intermediate adsorption energy by ligand,25 strain,26 or geometry effect.27 At the

34 Matter 2, 32–49, January 8, 2020 top of this volcano plot (highest catalytic activity) is Pt3Ni(111)withPt-skinsurface. The presence of subsurface Ni lowers the binding energy of reaction intermediates on the surface of Pt3Ni(111) through both ligand and strain effects. Despite the slightly weaker OH* adsorption energy than the optimal value (on the right side of the volcano), the Pt-skin surface represents the best man-made catalyst surface and shows a 90-fold catalytic enhancement compared with a commercial Pt/C cata- 28 lyst. Pt3Ni(111) is able to lower the binding strength of all three intermediates. With three-dimensional accessible surface and Pt-skin structure, the recently developed

Pt3Ni nanoframes have already shown more than 20-fold activity enhancement over commercial Pt/C catalyst.29 Despitemuchprogressinnon-platinumgroup metal catalyst development, the current practical catalysts for PEM fuel cell cathode are still based on platinum or platinum-based alloys, which will naturally incur a cost penalty.30,31 While the aforementioned catalyst designs are promising, their commercial application remains elusive due to lack of economical synthesis methods and troubles in reproducing catalytic results in real cell operations (membrane electrode assembly).

In addition to the economics of the catalyst for PEM fuel cells, the hydrogen storage and transport infrastructure throughout society will be a major hurdle. In light of this, the oxygen evolution reaction (OER) in acidic medium has become an attractive field of research due to the possibility of an acidic regenerative fuel cell (RFC) in which O2 and H2 are evolved during the charging process. However, a large majority of estab- lished OER catalysts are chemically unstable in acidic media.32 As shown in Fig- ure 2A, Ir-based catalysts are one of the few remaining catalyst systems that can produce reasonable results in both acidic and alkaline media without exhibiting serious 33,34 corrosion. Recently, IrO6 octahedral dimers in 6H-SrIrO3 have been shown to be a promising catalyst with an overpotential of 248 mV (as shown in Figure 2B).35 This particular material uniquely has both Ir–Ir metallic bonds and weak Ir–O bonds. From theoretical calculations, the face-shared (Figure 2C) Ir atom was found to be more catalytically active over the corner-shared Ir dimers. The slowest step in acidic + À 35 OER is the formation of the O–O bond, i.e., O* + H2O / HOO* + H +e . The face-shared Ir dimers were found to bond more weakly with the O* than the corner-shared Ir dimers, benefiting the aforementioned reaction. Other techniques include Ti-doping of the surface of MnO2, but their catalytic activity is relatively low.36 Overall,theOER/ORRprocessesinacidicmediumarefarmoredifficultto achievethaninalkalinemedium,renderingmuchofthematerialchoicesandde- signs somewhat limited. Acidic media might not be a problem for systems for which only ORR is required (PEM fuel cells) and Pt can still be used as the catalyst. However, for rechargeable systems such as MABs, acidic media-based studies have been scarcely investigated. For ZABs, the high chemical reactivity of Zn in acid entails another completely different engineering challenge, namely the separation of the anode from the acidic electrolyte.37 When coupled with the challenge of making this OER catalyst also functional for ORR (bifunctional), an MAB or RFC based on acidic medium appears not to be a good choice.

Alkaline Media Perhaps the most obvious point of convergence between MABs and fuel cells is when the electrolyte is water based and alkaline. This is essentially true among all the aqueous MABs and RFCs. For example, the alkaline ZAB poses great resemblance with the fuel cell ORR and water-splitting OER process in alkaline conditions (known cumulatively as the RFC38). It is well known that many of the alkaline fuel cell ORR catalysts have been deemed appropriate for ZAB ORR.39 Both of these ORR À processes entail the initial adsorption of O2 followed by either a direct 4 e reduction

Matter 2, 32–49, January 8, 2020 35 Figure 2. Catalyst for Acidic Media À À (A) Metal dissolution rate in mg cm 2 s 1 and OER overpotential of various metal catalysts. Reproduced with permission from Reier et al.34 Copyright 2017, Wiley-VCH. 35 (B) Catalytic activity of 6H-SrIrO3 versus IrO2. Reproduced with permission from Yang et al. Copyright 2018, Springer Nature.

À À À À À to OH ,ora2e reduction to HO2 and then another 2 e reduction from HO2 to À OH .40 In fact, the OER/ORR of fuel cell and alkaline ZAB are nearly identical in na- 41 ture, as demonstrated in the use the effectiveness of a-MnO2 in alkaline ZAB and alkaline RFC.42 Similarly, aqueous Li-air batteries (A-LAB) also have the ability to 43 achieve 4-electron transfer, forming water-soluble LiOH from O2. This occurs through a series of reactions,44 which are nearly identical to the ones occurring in the ORR of both ZAB45 and RFC.46 The rate-limiting step includes the O–O bond for- À mation or deprotonation of OOH , depending on which side of the volcano the catalyst falls.47,48 The activity of candidate catalyst is also governed by an LSR and, as such, the volcano plots. However, this does not actually offer any means for researchers to rationally design new and better catalysts. In contrast, catalytic descriptors that correlate with the LSR and are intrinsic to the material have been a topic of great discussion for catalyst design. Among all the proposed descriptors (oxidation state of transition metal, O 2p band center, 3d electron number of B- site ions), the eg ﬁlling appears to be the most persistent. For perovskite-based cat- alysts48 and other peroxidase-like transition metal oxides (TMOs),49 the catalytic activity was found to be intrinsically based on the eg bond orbital—speciﬁcally, the 3d electrons that participate in the s bond. This is because the s-bonding eg orbitals

36 Matter 2, 32–49, January 8, 2020 Figure 3. Catalytic Activity of Various Material and Operating Conditions

(A) Band diagram of LaCoO3 and SrCoO3 showing the lowering of the M 3d orbital to well below the O 2p, enabling the lattice oxygen mechanism. Reproduced with permission from Grimaud et al.50 Copyright 2017, Springer Nature. (B) Double volcano for OER and ORR processes. Reproduced with permission from Calle-Vallejo et al.52 Copyright 2011, Royal Society of Chemistry. (C) Varying catalytic activity with same catalyst tested with different alkali hydroxide electrolyte salts. Reproduced with permission from Strmcnik et al.53 Copyright 2009. Springer Nature. (D) Output from calculations showing that a catalyst optimization process of moving up the volcano plot, or optimized within the constraints of the linear scaling relationship (d optimized) followed by moving beyond the volcano plot, or optimized without the constraints of the liner scaling relationship (ε optimized) is optimal. Reproduced with permission from Govindarajan et al.54 Copyright 2019, American Chemical Society. have a stronger overlap with the oxygen intermediates than with the other t2g orbitals. It was furthermore found that the catalytic activity stemmed from the degree of which the s*-antibondings (eg) are ﬁlled at the surface of the catalyst. An eg ﬁlling of1wasfoundtobeideal.48 Additionally, a complementing mechanism proposed that the OER activity is also governed by the lattice oxygen mechanism.50 By increasing the covalence of the metal-oxygen bond or lowering the Fermi level past the O 2p in the perovskite, lattice oxygen becomes electrochemically active (as illustrated in Figure 3A). The lattice oxygen mechanism can be taken advantage of and in turn decouples the need for a concerted electron-proton transfer reaction step. This creates unique opportunities in catalyst design such as the separation of the oxygen evolution sites (Sr0.8Co0.8Fe0.2O3-d) from the proton stripper (Sr3B2O6), which resulted in better catalytic performance.51

In terms of bifunctional catalysis (OER/ORR), the design and choice of catalysts are nearly identical between alkaline fuel cells and aqueous MABs.55–57 The performance of bifunctional catalysts takes into account both the ORR and OER direction of reaction. Instead of a single onset potential, the performance index of

Matter 2, 32–49, January 8, 2020 37 bifunctional catalyst typically includes a subtraction of the discharge voltage (ORR) from the charge potential (OER) at relevant current densities.38 Pt/C-based catalysts have been shown to be quite effective in aqueous LAB in terms of enhancing both the OER and ORR.58 However, because TMO-based catalysts59 arestableinbasic conditions and the ORR process is comparatively facile in alkaline conditions, the TMO class of materials and not the precious metals have dominated the ﬁeld for A-LABs,60 ZABs,45 and alkaline RFCs.61 These catalysts are quite ORR/OER active 62 63 and corrosion resistant in alkaline electrolytes. Particularly, the spinel Co3O4 and its variations64–66 have long been understood as key non-precious catalysts with desirable adsorption/desorption characteristics for the reaction intermediates.52 In terms of design, much of the contemporary bifunctional catalyst design has focused on the cycle stability of these TMO materials. OER/ORR bifunctional catalysts mostly suffer from degradation over the duration of long cycling and low catalytic activity. Degradation of these catalysts occurs through the oxidation of the carbon-based catalyst scaffold. Because of the poor electron conductivity of metal oxide-based bifunctional catalysts, conductive additives in the form of carbon black are often added.67 At higher voltages, the carbon scaffold for alkaline

MABs and RFCs oxidizes into CO2, destroying the conductive network for the metal oxide catalysts. Furthermore, carbonate precipitate (pore-clogging agent) will form upon subsequent ORR cycles if the OER-generated CO2 is not completely removed from the water-based electrolyte. Pore cloggage and subsequent impedance arising from the deposition of solid insulating discharge product throughout the battery is one of the main reasons for cell failure. In the case without carbon, performance degradation can be attributed to the aggregation of catalyst particles68 or degradation of the carefully designed high-surface-area morphologies of TMOs.69 Much work has focused on designing stable morphology of TMOs with large amounts of active sites and the ability to maintain these active sites over long cycling. Interestingly, not much effort has been focused on understanding what makes a catalyst bifunctional. For bifunctional activity, both the OER and ORR LSR-derived volcano plots must be considered. As shown in the double volcano plot (Figure 3B), bifunctional catalysts are inherently not the best selection for the separate OER and ORR reactions due to the separation between the vertices of the two volcano plots.3,47,52,70 Accordingly, bifunctional catalysts are usually not the ideal choice for either OER or ORR when compared with their exclusive ORR or OER counterparts. An obvious solution is to separately add dedicated ORR catalysts and OER catalysts, but this will contribute to increased catalyst weight. Another promising direction might be to tune the adsorption characteristics from the electrolyte side. It was shown early on that by changing the electrolyte cation and, as such, the nature of the water solvation shell around the cations, the adsorption characteristics of the reaction intermediates and the corresponding catalyst activity changed (as shown in the activity of the catalyst with varying alkali hydroxides, Figure 3C).53,71,72 Additionally, the breakage of the LSR and the subsequent benefits of superseding the limitation of the volcano plot have been envisioned as one of the most attractive and impactful directions for catalyst research.54 In a calculation- based optimization study, it was shown that it is also beneficial to first find the best catalyst restricted by the LSR/volcano plot and then perform another optimization that is not restricted by the LSR (Figure 3D).54 Onepossiblemethodtobreak LSR depends on the covalence in the TMO bonding, which strengthens only certain M–OOH bonds. However, this is only true in vacuum (theoretical settings) because the solvent and the nature of the electrolyte were found to compensate for any difference in adsorption energy with solvation of the particular intermediate.73 Taken together, there might be an opportunity to change the nature of the adsorption energies of intermediate over the surface of the catalysts71,74 that goes beyond the

38 Matter 2, 32–49, January 8, 2020 morphological and compositional design of the catalyst and moves into the realm of designing catalysts as a system.

While the catalyst design for A-LABs will likely be similar to that of RFCs and ZABs, a major problem overarching A-LABs lies in the protection of the Li metal. For this reason, a large majority of work into LABs has focused on the non-aqueous LAB systems. This is in fact the same problem for acidic media-based ZABs. If an ideal (in terms of performance and scalability) separating design can be reached, it will represent a significant step for- ward for these types of technologies. On the other hand, aprotic electrolytes react in a relatively milder manner with Li metal and will often in time form reasonably stable passivation films. Switching to aprotic electrolyte will unfortunately alter significantly the reaction pathway of an LAB and in some ways shift the functionality of the catalyst for both ORR and OER. In the next section, ORR/OER for aprotic-based LABs will be discussed and compared with aqueous ORR/OER.

APROTIC ELECTROLYTES Oxygen Reduction Reaction A few early studies have attempted to transfer traditional heterogeneous ORR catalysts for LABs.75,76 However, the use of this type of ORR catalyst is a rather small subfield within LABs, with only a few studies present due to its lack of importance in comparison with OER. An early study on ORR for LABs investigated noble metals as catalyst. A volcano plot analogous to that of fuel cell ORR was discovered, with Pd having the highest activity (higher discharge plateau) followed by Pt, Ru, Au, and pure carbon.77 Metal-oxide-based ORR catalysts have also been investigated.78 While the results are intriguing, the voltage gained on discharge is only 0.2 V, which is not nearly as attractive as decreasing OER overpotential by significantly higher values.79 Furthermore, sudden and drastic changes in usable capacity were experienced after short cycling, indicating at least some different requirements for the catalyst system design.78 Arguably the most impactful application of heterogeneous catalyst for LAB ORR was for the formation and stabilization of crystalline LiO2. Ir-reduced graphene oxide was demonstrated to be an effective nucleation center to 80 promote the formation and stabilization of crystalline LiO2. While not a traditional catalyst in the sense of fuel cell ORR, the stabilization of LiO2 can appropriately allow for a lowered overpotential on the subsequent charge due to the calculated half- metal nature of LiO2. Even with a lowered theoretical energy density limit, the increased round-trip efficiency is highly valued for practical applications.80

In an aprotic LAB, the reduction of O2 usually leads to the formation of LiO2 and then 81,82 solid Li2O2 in aprotic electrolytes. This process does not completely break the O–O bonds83 and typically does not require a catalyst to deliver appreciably high discharge voltages. However, the initial part of ORR is similar, that is, the adsorption À of O2 and reduction to O2 . The remainder of the reduction reaction entails quite foreign processes such as the reduction of solid/minutely soluble LiO2 into solid 84 Li2O2. This might in fact imply an opportunity for a simplified design of the initial À À O2 +e / O2 catalyst due to the decreased number of intermediate reactions relevant for the solid catalyst. This initial catalyst is only required to adsorb O2 and À desorb O2 . Any further reduction over the surface of this catalyst will in fact not be beneficial to performance as it will create undesired solid/passivating Li2O2,resulting in quick cell death. The functionality of catalysis for the subsequent reactions changes significantly for aprotic media LABs. The initial intermediate LiO2 is rela- 85 tively more soluble compared with Li2O2 in aprotic electrolytes, leading to its dissolution and further subsequent electrochemical reduction or chemical

Matter 2, 32–49, January 8, 2020 39 Figure 4. Illustration of the Two ORR Process of Aprotic LAB with Solvents with Different Donor Numbers Solvents with low donor number (DN) are expected to reduce primarily on the surface of the electrode surface while high-DN solvents are expected to follow a disproportionation pathway in the electrolyte. Reproduced with permission from Johnson et al.87 Copyright 2014, Springer Nature.

81,82,86–89 disproportionation to Li2O2. The catalyst for this process can be argued as an electrolyte component that has a high donor number,87 which can stabilize and prolong the lifetime of LiO2 allowing it to diffuse, reduce, and deposit in a non- passivating manner.

In contrast to RFC and ZAB catalyst design, for aprotic LABs much work has also been devoted to the morphology and distribution of the conductive substrate to prevent pore blockage.90 It is perhaps more efﬁcient to design systems whereby the deposition of insoluble insulating Li2O2 canbecarefullycontrolled. Interestingly, the composition of aprotic electrolyte was found to play a key role on the discharge pro- 91 duct’s ﬁnal morphology and, as such, the distribution of insoluble Li2O2. Early work found the formation of micrometer-sized toroid-shaped Li2O2 discharge products in 91 ether-based electrolyte with <30 ppm of water. The morphology of Li2O2 is highly 85 dependent on the solubility and stability of the LiO2 intermediate in the electrolyte.87 Depending on the lifetime of the superoxide in the electrolyte, it can directly reduce to Li2O2 on the surface (poorly soluble electrolytes) or disproportionate

(more soluble electrolytes) and precipitate solid Li2O2 through a more solution- based growth mechanism,87 as shown in Figure 4. Among the heterogeneous catalyst designs, MnO2 is one of the key catalysts for aprotic LABs. Recently, it was found that the crystal phase of MnO2 had a profound effect on the reduction reaction route

(solution or surface directed). The b-MnO2 bipyramid prism (100) face produced a more surface-based reaction whereby passivation is promoted, whereas the octahe- dron (111) face yielded a more solution-based reaction.92 Accordingly, the octahe- dron phase could be seen as a catalyst that only helps catalyze the generation of À O2 , allowing for its solvation, and does not directly form solid Li2O2. Electrolyte additives such as Fe phthalocyanine were found to be useful because of its ability to À stabilize the O2 anion, promoting the Li2O2 to precipitate elsewhere in a non- passivating manner.93 In fact, much of contemporary research into enhancing the LAB ORR process has not entailed signiﬁcant efforts in the traditional heterogeneous catalyst design (relative to fuel cells), but instead have focused on the formulation of a proper electrolyte composition to tune Li2O2 morphology through mediators and

40 Matter 2, 32–49, January 8, 2020 Figure 5. Thermodynamics of the LAB Discharge Products Change in Gibbs free energy (kJ/mol) on the left axis and equilibrium potential (V)ontherightaxis as a function of temperature (K and C) for the

solvation control of LiO2 and Li2O2 working more like a homogeneous catalyst.83,93,94 Redox mediator-added homogeneous catalysts are among the highest-performing catalyst systems,95 often outperforming heterogeneous designs.92,96 This overall superior performance could just be a result of the higher distribution and overall activity of homogeneous catalysts over heterogeneous particle- based catalysts. A rationally designed LAB ORR catalyst system might eventually include a catalyst that is efﬁcient in reducing the overpotential associated with the À reduction of O2 to O2 , a superoxide stabilizing agent, and a spatially separate

Li2O2 seeding site that also facilitates the rapid reduction of LiO2 to Li2O2.

As we have stated before, the O–O bond does not fully break for an aprotic LAB. In fact, no report has been made of Li2O formation in an aprotic room-temperature LAB. The 97 net DGLi2O is thermodynamically unfavorable compared with Li2O2 formation. Addi- tionally, because the Li2O formation process requires complete O–O bond breakage, this process is both thermodynamically and kinetically unfavorable. Interestingly, this trend is reversed (i.e., DGLi2O2 > DGLi2O)above150 Casdemonstratedrecentlyby 98 Xia et al. As shown in Figure 5,above150 C the formation of Li2O2 is no longer thermodynamically favorable compared with the formation of Li2O. This work suggests that not only is the 4-electron transfer of LAB possible, but it also leads to interesting implications. Firstly, it might be worthwhile to devise a method to kinetically limit the formation of Li2O2 at room temperature to bring the overpotential down to the voltage required for Li2O formation. This has been shown in fuel cell ORR where the direct À 99 4e reduction of O2 is possible depending on the catalyst. By reducing the proportion À of indirect 2 e reduction of O2, the formation of undesirable H2O2 (undesirable due to its reaction with the membrane and likely reduction in power density)100,101 can be avoided. Secondly, while 150C is indeed substantially higher than room temperature, it is not completely dismissible as an option. EVs using a high-temperature LAB as the main supplementary energy system might be possible if the system design of the vehicle were to be reworked. Furthermore, application of 150C LABs will be highly applicable for grid storage,102 and as a competing technology with other operating high-temperature systems such as solid oxide fuel cells. If the notion of operating at 150C were to be hypothetically acceptable (as a new class of LAB), many new cell conﬁgurations could be explored to realize the 4-electron transfer of O2 to Li2O. ORR catalyst design concepts for fuel cell ORR might now be more applicable for LAB ORR operated at 150C as the complete breakage of the O–O is enabled. As such, investigation of this prospective area might be facile with the large volume of supporting aqueous ORR/OER literature. It should also be noted that the electron conductivity issue of a hy- pothetical Li2O-forming LAB will likely be very similar to that of a Li2O2 or Li2S (in Li-sulfur battery) system. The exact conductivity values of Li2O at elevated temperatures are unclear, but we expect that the charge requirements will be similar to those of Li2O2.Spe- ciﬁcally, only through good contact between the discharge product and conductive

Matter 2, 32–49, January 8, 2020 41 scaffold in addition to the use of soluble redox mediator will a 150C LAB be able to function.

Oxygen Evolution Reaction In contrast to aqueous MABs and RFCs, the typical LAB discharge product is a solid. Furthermore, the evolution of oxygen from an LAB does not require a catalyst in the same manner as water-splitting OER. In water splitting, the O–O is reformed through a kinetically hindered process often using noble metals as catalyst.103 Alackofcom- plete O–O bond breakage during the conventional room-temperature reduction process of O2 to Li2O2 renders this process signiﬁcantly different. The direct use of popular fuel cell catalysts such as Pt for the LAB OER can actually promote degradation of the aprotic electrolyte. This can not only cause degradation problems in the performance but also convolute any probing of the OER process.104,105 The only applicable portion of the aqueous OER catalytic process includes the ﬁnal À oxidation of O2 to O2 and the subsequent desorption of O2, which is not likely the source of OER overpotential.

The main origin of the charge overpotential for Li2O2 stems from the poor ionic and electron conductivity of solid Li2O2.Accordingly,oneofthemajorinﬂuencesonthe

OER process is the previous ORR cycle (i.e., Li2O2 formation process) and the resulting Li2O2 morphology. Due to the insulating nature of the discharge product, the morphology of deposition can strongly dictate the OER overpotential. The contact and interface conductivity between the discharge product and the conductive surface/catalyst is highly important. Like ORR, much research has accordingly focused 106 on the deposition control of Li2O2, intimately linking the ORR and OER processes.

Interestingly, it was found that the initial OER of Li2O2 occurs primarily at its interface/contact point with the conductive substrate, after which the Li2O2 physically de- 107 taches. This likely suggests that only the initial charging process of Li2O2 is dependent on the contact with conductive substrate, and the subsequent continued charging process of detached Li2O2 is a more solution-based process. Specifically, the main controlling factors are likely the existence of a solvated redox mediator (homogeneous catalyst108). This is further demonstrated with the use of an electrolyte- soluble redox mediator and chemical redox reactions between the various oxygen species in the system. Drastic decreases in the charge overpotential were achieved relatively independent of the previous cycle’s ORR process,79 effectively decoupling the OER from the ORR process. In fact, the dissolution process (presumably from delithiation) of detached Li2O2 was directly observed under in situ transmission electron microscopy during the charge process.82 The comproportionation reaction between Li2O2 and O2 is said to form LiO2 (which could be soluble depending on the donicity of the solvent), which can freely diffuse to conductive surfaces and conduct the OER reaction. By using a redox mediator, this process is further expedited by increasing the concentration and consistency of the charge transfer, and has been a focus of much aprotic LAB OER research in the past years.79,95,109 If the oxidation of Li2O2 to LiO2 can be efficiently achieved, the next step will be to lower the desorp- + tion energy of Li and O2. At this stage, the use of a heterogeneous catalyst might become more relevant for LAB operation. A relationship between both the O2 and Li+ desorption energy and surface acidity has been revealed and is summarized in the volcano plot shown in Figure 6. In this sense, the final step of OER in LABs will be quite analogous to the OER of water splitting.110 However, this step is likely not to be the rate-limiting step, as the O–O was never broken from the previous ORR.

If the O–O bond was to be broken, interesting implications would follow. In general, research into the OER process of LABs has been largely dictated by the assumption

42 Matter 2, 32–49, January 8, 2020 Figure 6. Volcano Plot of Oxygen Evolution Catalyst for Li-Air Batteries

Dashed black line represents O2 desorption energy versus surface acidity. Dashed red line represents Li+ desorption energy versus surface acidity. Solid blue line shows the charge voltage versus surface acidity. Reproduced with permission from Zhu et al.111 Copyright 2015, American Chemical Society.

that solid Li2O2 is the predominant discharge product. If this were to change, and

Li2O is now somehow assumed to be the predominant final discharge product (as inthecaseof150C LABs), the subsequent OER process during charge might be more akin to water splitting. Specifically, the formation of Li2O requires the complete breakage of the O–O bond, which might not result in the reformation of the O–O bond, a key step in the OER of Li2O. This offers interesting implications for bridging designs from other technologies. However, the solid nature of both the oxide and peroxide renders any adsorption and surface reaction over a heterogeneous catalyst unlikely. It might therefore be more applicable to assume that Li2O charges in a 112,113 manner similar to bulk Li2S in the well-studied Li-S battery system. For bulk + Li2S, the Li ions at the surface are first stripped, forming higher-oxidation-state anion species (high-order polysulfides), followed by their chemical redox reactions 114 with fresh Li2S. Although the charging process of Li2S benefits from the addition of redox mediators,112 whether or not redox mediators can function in a similar manner for Li2O remains to be seen in the literature. It is likely that if Li2Ocanbe at least initially charged in a similar manner to that for Li2S, the subsequent final

O2 formation might adhere to some of the catalyst designs for water splitting.

Differing from Li2S, the ﬁnal step of a fully charged LAB must be the evolution of

O2 out of the battery. The charging of Li2O might entail a homogeneous catalyst (redox mediator) for initial activation followed by another homogeneous/heterogeneous catalyst for promoting the formation of the O–O bond, and the ﬁnal evolution + of O2 and desorption of Li . However, much work needs to be done to validate and propagate the discussion presented here. In fact, even the direct use of bulk Li2Oas a starting electrode material for LABs has only been recently attempted with mainly 115 CO2 evolution. The only work with a close resemblance was the recently reported oxygen anion redox of a nano Li2O composite with Co3O4. Cycling between Li2O and Li2O2/LiO2 was demonstrated, which might be an indication of a catalytic effect 116 promoting the delithiation of Li2O. While this is not an OER process, the formation of the O–O (present in LiO2)fromLi2O should have been achieved.

À Direct comparison between water/OH and Li2O2 OER might not appear to be a À good approach, as the interactions between the water/OH molecule on the surface of a heterogeneous catalyst are not transferable to interaction between

Matter 2, 32–49, January 8, 2020 43 85 111 solid/essentially insoluble Li2O2 and a heterogeneous catalyst. Interestingly, 2À 2 compared with LABs, ZABs produce solvated Zn(OH)4 . Subsequently the 2À 2,65 Zn(OH)4 precipitates as ZnO, resulting in a final solid discharge product. Since the discharge products for both systems are technically solid (Li2O2 or Li2OforLABs and ZnO for ZABs), it could be expected that some analogies could be drawn. The major difference is that ZnO can appreciably dissolve in the ZAB’s alkaline aqueous 2À 2,20,117 electrolyte as Zn(OH)4 with a significant saturation concentration, while 85 Li2O2 and Li2O are essentially insoluble in the LAB electrolyte. It is foreseeable that if an aprotic electrolyte formulation (such as the inverse crown ether complex118) canallowforanappreciablesolvationofLi2O2 or even Li2O, the charge process of aqueous and aprotic LABs might be similar. In this situation, a similar volcano plot derived from a similar LSR between the adsorption energy of intermediates might existinsuchassystem.However,ifaLi2O2/Li2O solvating entity is employed, the scaling relationship between the intermediate’s adsorption might become more complicated. This might in fact offer more opportunity and flexibility toward catalyst designs in terms of finding an optimal OER/ORR bifunctional catalyst, and greatly re- sembles the same opportunities present in aqueous OER/ORR where tuning electrolyte composition can be beneficial.

As LABs are supposed to be rechargeable, the catalyst system should be bifunctional in nature. Interestingly, there has been a surprising lack of fundamental study or even an explicit statement of the bifunctionality requirements based the various aspects of aprotic LAB OER/ORR processes. Accordingly, we propose that an ideal bifunctional catalyst system will have a solid bifunctional catalyst that is efﬁcient at À À O2 adsorption and O2 desorption. The desorbed O2 should be received and stabilized by an electrolyte component to facilitate the subsequent reduction/disproportionation and controlled precipitation of Li2O2. During OER, the precipitated

Li2O2 must be able to oxidize, implying that the controlled precipitation of Li2O2 must be on something conductive. A redox mediator (i.e., soluble catalyst) of some form must be present to facilitate the subsequent charge transfer between the conductive network and isolated Li2O2 particles. Subsequent to this will be the À À oxidation of O2 to O2, likely on the surface of the initial ORR O2 / O2 bifunctional catalyst. This proposed generic catalyst system is further depicted in Figure 7.

PERSPECTIVE As interest in electrochemical energy storage for EVs continues to grow, the development of both fuel cells and MABs has progressed significantly in the past years. The practical application of electrocatalysts for oxygen-based EVs mostly remains in the realm of fuel cells. With a longer and more in-depth research history, fuel cell catalysis holds many design concepts that could be valuable to MABs. Although using aqueous electrolyte might appear to automatically enable direct transfer of fuel cell and ZAB knowledge to LABs, an often noted problem of A-LABs is the dif- ficulty in limiting contact between the aqueous electrolyte and the Li metal.119 While some works have developed coatings on the Li metal,120,121 it is significantly more difficult to develop a stable protection layer against a pure water electrolyte rather than just oxygen or water crossover from using ambient air for aprotic LABs.20,122 Similarly, though not discussed in this perspective, dendrite formation in both Li123 and Zn124 metal-based air battery systems is an inevitable problem that must be addressed. Furthermore, although this perspective has mostly discussed LABs asthemaintechnologyinfocus,manyofthesearealsotransferabletootheranalo- gous systems. For example, other alkali MABs such as Na-air and K-air could benefit from the same ideas. Through specific operational circumstances (higher

44 Matter 2, 32–49, January 8, 2020 Figure 7. Schematic of a Proposed Generic Catalyst System Design for Aprotic Li-O2 Battery À On ORR, the solid catalyst should facilitate the reversible O2 adsorption, reduction to O2 ,andthe À À subsequent desorption of O2 into the electrolyte. Thereafter, the O2 should be received by an electrolyte-soluble chemical that stabilizes and facilitates the diffusion and disproportionation/ À reduction of O2 to Li2O2, which will preferentially nucleate on a conductive Li2O2 seeder to present passivation of conductive support. On OER, the Li2O2 in contact with the conductive Li2O2 nucleator will oxidize and likely form soluble intermediates, which will then be stabilized again by an electrolyte-soluble chemical from which the solid catalyst will adsorb, oxidize, and evolve O2. temperature, use of discharge product solvating entities such as reverse crown ether), the differences between fuel cell and LAB ORR can be reduced and design heuristicscanthenbemoredirectlyshared between the ﬁelds. Admittedly, the topics discussed in this perspective have been rather general; nevertheless, we hope that the ideas presented here can spark cross-technology innovations and reduce the hurdles of passage between the different battery systems.

Finally, although used rather interchangeably throughout this perspective, it is important to clarify that an LAB and a Li-oxygen battery are distinct from one another. An LAB usually implies operation with a natural air source while a Li-oxygen battery requires a pure oxygen feed. Most work in this field has remained within the realm of Li-oxygen battery due to reasons pertaining to N2,H2O, and CO2 contam- ination in the cell. These contaminants will inevitably cause corrosion of the Li anode, building up impedance. Overarching the problems with Li corrosion is the sensitivity of the discharge product Li2O2 to moisture, forming LiOH-based products that are substantially more difficult to delithiate. However, even if the moisture level can somehow be practically brought down to the beneficial levels of <30 ppm,91 there still exists the problem of the similar practical reduction potentials of CO2 (2.8 V + 125,126 + 127 versus Li /Li) and O2 (2.7 V versus Li /Li), and a possible combined reaction pathway at 2.85 V versus Li+/Li.128 This indicates that a discharge of a true LAB (with ideal Li protection and dried) will still likely create various carbon-based discharge products such as oxalates129 or carbonates.128 If these carbon-based products are not to become a hindrance, any catalyst system for a true LAB might need to be at least tetra-functional in nature such that it can reduce/oxidize both

O2 and CO2 at around the same voltages. One interesting material is Mo2C, which 130 131 has been demonstrated to be able to separately discharge both O2 and CO2 to Li2O2 and Li2C2O4 (lithium oxalate), respectively, and reoxidize the discharge products. Like the OER/ORR aqueous bifunctional catalyst, an obvious compromise

Matter 2, 32–49, January 8, 2020 45 would be to place CO2 reduction/oxidation and OER/ORR catalyst separately at the cost of increased catalyst weight.

ACKNOWLEDGMENTS This work is supported by the US Department of Energy (DOE), Office of Energy Ef- ficiency and Renewable Energy, Vehicle Technologies Office. Argonne National Laboratory is operated for DOE Office of Science by UChicago Argonne, LLC, under contract number DE-AC02-06CH11357. C.Z. acknowledges support from the Na- tional Science Foundation for Excellent Young Scholars (no. 51722403), National Natural Science Foundation of China (no. 51771134), the National Natural Science Foundation of China and Guangdong Province (no. U1601216), and the National Youth Talent Support Program. M.L. and Z.C. would like to acknowledge financial support from the Natural Sciences and Engineering Research Council of Canada, University of Waterloo, and the Waterloo Institute for Nanotechnology.

AUTHOR CONTRIBUTIONS M.L., X.B., and J.L. conceived the theme. M.L., X.B., R.W., Y.L., G.J., L.L., C.Z., Z.C., and J.L. wrote the manuscript and designed the ﬁgures.

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