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Design of Efficient Bifunctional Oxygen Reduction/ Evolution Electrocatalyst: Recent Advances and Perspectives Zhen-Feng Huang, Jiong Wang, Yuecheng Peng, Chi-Young Jung, Adrian Fisher, and Xin Wang* technologies to meet the future energy requirements for various applications.[2–5] Typically, metal-air batteries are divided into two types according to the electrolyte: one is a cell system using an aqueous electrolyte such as representative Zn-air batteries and the other is a water-sensitive system using organic electrolyte such as representative Li-air batteries.[5] To date, metal-air batteries are believed potentially the most viable energy system to replace the mature Li-ion battery and even hydrogen fuel cell technologies that has almost reached theoretical performance limits and confronted with formidable challenges ranging from difficult hydrogen production/storage/transportation to complicated cell design, respectively.[4–7] Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the two most important reactions in electrochemically rechargeable metal-air battery (Figure 1): oxygen molecules are reduced by electrons during discharging, and the reverse process occurs during charging.[8] Considering the low cost, wide availability, high ionic conductivity and safety for metal-air battery in alkaline electrolyte, this review is limited to give an insight into bifunctional oxygen electrocatalysis in alkaline electrolyte.[5] One of the most important challenges for electrochemically rechargeable metal-air battery is to increase the efficiencies of both ORR and OER, which will require the development of efficient and stable electrocatalysts. However, ORR and OER require very different conditions, particularly the electric potentials present during the reaction, which narrows down the number of materials capable of catalyzing both reactions, and introduces more complicating factors that can inhibit catalytic activity. For example, Pt-based materials are the best ORR catalysts, but they are not very efficient towards OER, due to the formation of Pt oxides on the catalyst surface at high overpotentials. Also, the fabrication of working electrode using two different materials for individual ORR or OER, raises cost concerns and introduces manufacturing complexities. Alternatively, recent advances in catalyst have made possible the fabrication of air electrode using a bifunctional ORR/OER material. For example, Zhang et al. reported a MOFderived Co-N-C bifunctional catalyst for Zn-air battery with excellent activity and durability.[9]

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are the two most important reactions in rechargeable metal-air battery, a promising technology to meet the energy requirements for various applications. The development of low-cost, highly efficient and stable bifunctional ORR/ OER catalysts is critical for a large-scale application of this technology. In this review, the authors first introduce the fundamentals of bifunctional ORR/ OER electrocatalysis in alkaline electrolyte. Various types of nanostructured materials as bifunctional ORR/OER catalysts including metal oxide, hydroxide and sulfide, functional carbon material, metal, and their composites are then reviewed. The crucial factors that can be used to tune the activity of the catalyst towards ORR/OER are summarized, including (1) phase, morphology, crystal facet, defect, mixed-metal and strain engineering for metal oxide; (2) heteroatom doping, topological defects, and formation of metal-N-C structure for carbon material; (3) alloy effect for metal. These experiences lay the foundation for large scale application of metal-air battery and can also effectively guide the rational design of catalysts for other electrocatalytic reactions.

1. Introduction With increasing demands for clean and sustainable energy, the development of low cost and efficient energy technologies has received extensive research attention in recent years.[1] Metalair batteries, composed of metal electrode, oxygen electrode and electrolyte, have been considered as one of such promising Dr. Z.-F. Huang, Dr. J. Wang, Y. Peng, Prof. X. Wang School of Chemical and Biomedical Engineering Nanyang Technological University 62 Nanyang Drive, Singapore 637459, Singapore E-mail: [email protected] Dr. C.-Y. Jung Hydrogen and Fuel Center Korea Institute of Energy Research (KIER) 20-41, Sinjaesaengeneogi-ro, Haseo-myeon Jellabuk-do 56332, Republic of Korea Prof. A. Fisher Department of Chemical Engineering and Biotechnology University of Cambridge New Museums Site, Pembroke Street, Cambridge CB2 3RA, UK The ORCID identification number(s) for the author(s) of this article can be found under https://doi.org/10.1002/aenm.201700544.

DOI: 10.1002/aenm.201700544

Adv. Energy Mater. 2017, 1700544

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© 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

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Figure 1. Schematic illustration of metal-air battery based on bifunctional ORR/OER electrode.

The most common way of measuring a good bifunctional catalyst for ORR and OER is by measuring its overpotential at a given current density and based on same catalyst loading. A smaller overpotential at a given rate is a good indicator for a promising bifunctional catalyst. Also, it is necessary to create a bifunctional catalyst that is stable enough to withstand harsh environments encountered during ORR and OER, while still delivering significant activity. In the past decade, many significant and encouraging breakthroughs on nanostructured materials have been made for highly efficient bifunctional ORR/ OER. For one of the most important reasons, DFT calculations have provided the insight into predicting the ORR/OER activity of the materials.[3,10–16] This physical framework is used to estimate the free energy changes corresponding to elementary steps involving proton-coupled electron transfer on the catalyst surface. For the other, some analytical and characterization techniques including Fourier transform infrared spectroscopy (FTIR), Raman spectroscopy, X-ray absorption/diffraction, Mössbauer spectroscopy and so on, allow us to monitor the ORR/OER in situ and confirm the actual active phase/intermediates and electrochemical behavior during ORR/OER on the surface of the materials.[17–25] This review describes recent progress on the design and synthesis of heterogeneous catalysts and their emerging catalytic activities in bifunctional ORR/OER used in alkaline electrolyte. The fundamentals of bifunctional ORR/OER electrocatalysis including reaction mechanism are first introduced. We then discuss various types of nanostructured catalysts for bifunctional ORR/OER including metal oxide, hydroxide and sulfide, functional carbon material, metal, and their composites. Crucial factors determining the ORR/OER performance are summarized, including (1) phase, morphology, crystal facet, defect, mixed-metal and strain engineering for metal oxide; (2) heteroatom doping, topological defects, and formation of metal-N-C structure for carbon material; (3) alloy effect for metal.

2. Fundamentals of Bifunctional ORR/OER Electrocatalysis 2.1. Reaction Mechanism for ORR and OER 2.1.1. ORR Mechanism Generally, ORR mechanism can either be dissociative or associative in nature, depending on the oxygen adsorption mode


Zhen-Feng Huang is currently a research fellow under the supervision of Prof. Xin Wang in the School of Chemical and Biomedical Engineering, NTU. He received his Ph.D. and master degrees (2016) under the supervision of Prof. Ji-Jun Zou in the School of Chemical Engineering and Technology from Tianjin University. His research interests mainly focus on the design and synthesis of nanostructured materials for highly efficient photocatalytic and electrocatalytic water splitting and CO2 reduction. Xin Wang is currently a full professor in the School of Chemical and Biomedical Engineering, NTU. He received his bachelor’s (1994) and master’s (1997) degrees in chemical engineering from Zhejiang University and his Ph.D. degree (2002) in chemical engineering from Hong Kong University of Science and Technology. He has been working on electrocatalysis and electrochemical technologies for energy harvesting. His recent research focus includes electrocatalyst and electrode development for fuel cells, CO2 electroreduction, water electrosplitting, and electrochemical reactors with co-generation of electricity and valuable chemicals. and dissociation barrier on the catalyst surface.[14] There are two adsorption modes: bidentate O2 adsorption and end-on O2 adsorption, which lead to the direct four-electron pathway without peroxide formation and two-electron pathway with peroxide formation, respectively.[14,26] Typically, four-electron pathway predominates on noble metals, and two-electron pathway primarily occurs on carbonaceous materials.[4] Interestingly, various ORR pathways exist for metal oxide materials, relying on the specific crystal structure, molecular composition, or experimental parameters. For example, the reported bioinspired Cu catalyst shows dual-pathway ORR mechanisms with both direct four-electron and indirect four-electron pathways competing with each other.[27a] In high potential ranges, the direct pathway is dominating, while at low potential the peroxide-mediated indirect pathway is the main pathway. For ORR, multistep electron-transfer processes and complicated oxygen-containing species such as OOH*, OH*, O* are involved. Understanding how to control binding energies of reactive intermediates on a surface is the key to design materials with improved activity. The free energies of all the above intermediates have been calculated on a variety of close-packed metal surfaces, and a volcano plot was constructed relating the

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Figure 2. a) ORR volcano plot for metals. b) ORR theoretical limiting potential plot for fcc (111) and (100) facets of metals and alloys. c) OER volcano plot for metal oxides. Reproduced with permission.[3] Copyright 2017, American Association for the Advancement of Science. d) Relation between the OER catalytic activity and the occupancy of the eg-symmetry electron of the transition metal (B in ABO3). Reproduced with permission.[21] Copyright 2011, American Association for the Advancement of Science.

theoretical ORR activity to ΔEO, with Pt sitting near the top (Figure 2a).[3a,14] For metals that bind oxygen too strongly, the activity is limited by proton-electron transfer to O* or OH*. On the other hand, for metals that bind oxygen too weakly, the activity is limited by proton-electron transfer to O2* (associative mechanism) or splitting of the O-O bond in O2 (dissociative mechanism), depending on the applied potential. Note that Pt is not located at the “summit of volcano” (Figure 2a,b), and further improvement has been demonstrated through alloying effect by addition of 3d transition metals.[3] Different from HER/HOR with one reaction intermediate H*, the ORR involves multiple intermediates (OOH*, OH*, O*), the binding energies of which are strongly correlated and cannot be decoupled easily because of scaling relations. In fact, the scaling relation ΔGOOH = ΔGOH + 3.2 ± 0.2 eV was found to apply universally to both close-packed (111) and open-packed (100) facets of face-centered cubic (fcc) metals and their alloys.[3a] Due to this nonideal scaling between OOH* and OH*, even a catalyst calculated to be at the top of the ORR volcano plot with optimal ΔEO will have a nonzero theoretical overpotential of 0.3 to 0.4 V (Figure 2b). This is the origin of the observed overpotential among even the very best ORR catalysts, including the extensively studied Pt-based systems. Note there is a similar case about overpotential for best OER catalysts.


2.1.2. OER Mechanism According to the same reason, one might expect Pt, which is the best pure metal ORR catalyst, to perform as well for OER; however, this is not the case observed experimentally.[3a] One reason is that microscopic reversibility only holds for a process taking place close to equilibrium. When large overpotentials are needed to drive the reaction in the two directions, the requirements for catalysis in each direction could be substantially different. In addition, at the high positive potentials required for the OER, metals including Pt generally undergo oxidation, which presents a different type of surface than that pertaining under ORR conditions. Because catalyst materials for the OER are generally metal oxides, volcano plots for the OER have been constructed for a wide variety of metal oxide surfaces (including rutile, perovskite, spinel, rock salt, and bixbyite oxides) using ΔGO − ΔGOH as the descriptor. Experimental overpotentials at 1 mA cm−2 cat are seen to overlay well on the theoretical overpotential volcano when plotted against this simple descriptor (Figure 2c).[3a] Interestingly, Shao-Horn et al. further found a simple activity descriptor of eg electron for perovskite oxide (Figure 2d).[21] The intrinsic OER activity exhibits a volcano-shaped dependence on the occupancy of the 3d electron with an eg symmetry of surface transition metal cations, and the highest OER activity is predicted to be at an eg occupancy close to one.

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2.2. Ideal ORR/OER Catalyst

fundamental parameters e.g., onset potential, current density, Tafel slope, turnover frequency, electrochemical surface area and potential gap.

ORR/OER on metal surfaces, are made up of sequential proton-coupled electron transfer reactions. The overpotential results from the rate-determining step with the most positive free energy change ΔGmax (η = ΔGmax e−1).[12] Bifunctional ORR/OER materials involve four elementary reaction steps, in which ORR proceeds through the formation of OOH* from adsorbed O2, followed by its further reduction to O* and OH*, while OER proceeds in the reverse direction (see below). *

+ O2 ( g ) + H2O (l ) + e − ↔ HOO* + OH−

(1)

(2)

HOO* + e − ↔ O* + OH− O* + H2O (l ) + e − ↔ HO* + OH−

(3)

HO* + e − ↔ OH− + * Overall (alkaline electrolyte) : O2 + 2H2 O (l ) + 4e − ↔ 4OH−

(4)

Typically, the ORR activity is limited by the OH* reduction step (black line) and O2 reduction steps (blue line), and the OER activity is limited by the OOH* (green line) and O* formation step (red line), as shown in Figure 3.[28,29] In this sense, the binding energies of the intermediates (e.g., O*, OH*, OOH*, etc.) for the most active OER and ORR catalysts are not identical, and thus the best OER catalyst does not offer the best ORR catalysis, and vice versa.

2.3. Useful Notions in ORR/OER Test To evaluate and compare the performance of electrocatalysts for ORR/OER, it is necessary to clarify the definitions of some

2.3.1. Onset Potential Onset potential normally refers to the potential that the current deviates from the baseline. Experimental determination of onset potential can be quite arbitrary. A common approach is to get the intersection of the tangents between the baseline and the rising current in the voltammgram (Figure 4a). Note that the background should be properly corrected to avoid the overestimation of the onset potential.[30,31] For example, the current for ORR can be obtained by measuring the polarization curves under O2 and N2 using identical experimental parameters (scan rate, direction, rotation rate) and subsequently subtracting the curve under N2 from that under O2.[30] For OER, the accurate onset potential can be investigated by the Rotating Ring-Disk Electrodes (RRDE) method in N2-saturated 0.1 m KOH,[31a] where O2 generated at the disk moved to the ring by forced convection. O2 was then reduced at the ring held constantly at 0.40 V vs RHE under masslimiting conditions; i.e., the ring current only depends on the concentration of oxygen. Ring currents can be used for onset potential analysis shown in Figure 4b as they represent exclusively the catalytic current due to the OER, rather than other side reactions like substrate or catalyst oxidation.[31a] Also, this method allows the calculation of Faradaic efficiency (ε) of the system as follows: ε = jr (jdN)−1. Other techniques such as use of glassy carbon free disk as working electrode or fluorine-doped tin oxide (FTO) as a substrate to avoid substrate or catalytic material oxidation have also been reported.[31b] As the reduction of O2 and oxidation of water have to overcome the kinetic energy barrier, the onset potential is always lower (for ORR) and higher (for OER) than the standard redox

Figure 3. Theoretical ORR and OER activity volcano plot as a function of oxygen binding energy on metal a) and metal oxide b) surfaces, respectively. Reproduced with permission.[28] Copyright 2007, Elsevier.


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Figure 4. a) Polarization curves for ORR and OER. b) Onset potential analysis for OER using RRDE technique. Reproduced with permission.[31a] Copyright 2016, American Chemical Society.

potential of O2, respectively. The standard redox potential of O2/OH−(H2O) at 298 K is 1.23 V – 0.0591 pH vs normal hydrogen electrode (NHE).

2.3.2. Current Density The current density (j) is typically normalized by the surface area or the mass of the catalysts. The overall current density has an important relationship with the conversation or production rate of O2. This parameter is a crucial indicator on the practical performance of the catalyst at device level.

ring current and N stands for the collection efficiency.[27] HO2− percentage can be calculated by the following equation: % (HO2−) = 200(jr N−1) (jd + jr N−1)−1.[27] To investigate the reaction mechanism for OER, the RRDE technique can be employed with a Pt ring electrode potential of 1.50 V to oxidize the HO2− intermediates formed at the disk electrode (ranged from 1.0 V to 1.8 V vs RHE) during OER measurement.[27] A very low ring current suggests that there is only little HO2− formation and water oxidation undergoes a desirable four-electron pathway.

2.3.5. Turnover Frequency (TOF) 2.3.3. Tafel Slope The overpotential is generally logarithmically related to the current density (j) and its linear portion is given as Tafel equation: η = a + b log(j) where b is Tafel slope. The calculated value is an indicator of the reaction pathway and the rate-determining step. Also, when η = 0, the obtained current density from this equation is called exchange current density (j0). This represents the intrinsic activity of the catalysts under equilibrium states. Thus, a bifunctional ORR/OER catalytic material is desirable to possess a high j0 and a small b. Note that a plot of log Rct−1 vs overpotential from the EIS data also gives Tafel slope, which can reflect purely the charge transfer kinetics, in contrast to Tafel slopes obtained from voltammetry data which may include contributions from catalyst and electrode resistance. 2.3.4. Electron Transfer Number and HO2− Percentage ORR and OER can occur by two different mechanisms: direct four-electron pathway without peroxide formation or twoelectron pathway with peroxide formation. For ORR, electron transfer number (n) can be calculated by the following equation: n = 4jd (jd + jr/N)−1 based on RRDE technique, where jd stands for the faradic disk current, jr stands for the faradic Adv. Energy Mater. 2017, 1700544

The TOF value is calculated from the equation: TOF = (j × A) (n × F × m)−1, where j is the current density at a given overpotential, A is the surface area of the electrode, n is the electron transfer number at a given overpotential, F is the Faraday constant, and m is the number of moles of metal on the electrode. Active species such as metal content is typically detected by inductively coupled plasma techniques or voltammetry techniques. TOF also represents the intrinsic activity of the catalysts. Thus, a bifunctional ORR/OER catalytic material is desirable to possess a high TOF.

2.3.6. Potential Gap In addition to these general parameters for electrocatalytic reactions, there are some unique parameters to evaluate the performance for bifunctional ORR/OER catalyst. The potential gap (ΔE) between an ORR current density of −3 mA cm−2 and an OER current density of 10 mA cm−2 (E 10) is often used as the descriptor to assess the overall oxygen activity of the bifunctional electrocatalysts. Attention should be paid that the comparison of potential gap is made based on similar catalyst loading. Also, the potential gap between the ORR half-wave potential (E1/2) and OER potential (at 10 mA cm−2) is sometimes used as a common criterion for evaluating the overall bifunctional activity.

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3. Nanostructured Bifunctional ORR/OER Material The materials that have been explored so far for bifunctional oxygen electrocatalysis include metal oxide, hydroxide and sulfide, carbon material, and the composite materials such as metal/metal oxide, metal/carbon material, and so on. In the below sessions, we will elaborate the investigation of these materials for such purpose and practical approaches that can be used to tune the activities of the catalysts.

Rational design of active facets with favorable atomic arrangement and coordination is the most promising route.[38] For example, Liu et al. synthesized well-defined Co3O4 cubes with exposed (001) plane and octahedrons with exposed (111) plane and investigated the facet-dependent electrocatalytic performance.[39] They found that Co3O4 (111) has a lower activation barrier of O2 desorption than Co3O4 (001), which is of great significance to regenerate active sites of catalyst for excellent cyclic performance.

3.1.4. Defect Engineering 3.1. Transition Metal Oxide Transition metal oxides including Mn, Co, Ni, and Fe, have attracted great attention as bifunctional ORR/OER catalysts due to their intrinsic activity and sufficient stability in oxidative electrochemical environments.[32]

3.1.1. Phase Engineering The crystalline structure and chemical composition have great influence on the electrocatalytic performances. For example, Chen et al. found that MnO2 materials exhibit different catalytic activities depending on the crystal structures, following an order of α → β → γ-MnO2.[33] Also, Jaramillo et al. reported a novel Mn(III) oxide (Mn3O4), which demonstrates excellent bifunctional activity and its individual ORR and OER activity is comparable to the best reported metal oxide and even some precious metals materials.[34,35] In situ X-ray absorption spectroscopy confirmed that a disordered Mn3II,III,IIIO4 phase contributes to the ORR, while a mixed MnIII,IV oxide is related to the OER.

3.1.2. Morphology Engineering Among the same phase, varying morphologies possess different activities. For example, α-MnO2 nanospheres and nanowires were reported to outperform the counterpart of microparticles, due to their smaller size and higher specific surface areas.[36] Also, Chen et al. reported the synthesis of hierarchical mesoporous Co3O4 nanowire array as a highly efficient bifunctional ORR/OER catalyst, different from other morphologies.[37a] Similarly, Qiao et al. reported the synthesis of porous Co3O4 nanowire arrays directly grown on Cu foil, which exhibit higher OER activity (10.0 mA cm−2 at 1.52 V in 0.1 m KOH solution), more favorable kinetics, and stronger durability in comparison to those of IrO2/C.[37b] Furthermore, they can also efficiently catalyze ORR with a half-wave potential of 0.78 V, featuring a desirable four-electron pathway for reversible oxygen evolution and reduction.

Oxygen vacancies (OVs) typically serve as donors to narrow the bandgap, thus increase the electron density and electrical conductivity of materials. Moreover, surface OVs can enhance the electron transfer from O-vacancies to metal d band to further effectively tune the adsorption of surface species for catalysis.[40–42] For example, Chen et al. investigated the influence of OVs on the oxygen electrocatalysis of rutile-type β-MnO2 by introducing intrinsic OVs without modification by foreign additives.[42] They found that OVs-containing oxide demonstrates a more positive potential, larger current and lower peroxide production for ORR catalysis, and also promotes OER catalysis. DFT calculations further indicate that the presence of OVs strengthens the interaction between catalyst surface and oxygencontaining species to thus reduce the reaction kinetic barrier. Also, Wu et al. reported the influence of OVs on the oxygen electrocatalysis of BaTiO3. They found that OVs-containing BaTiO3−x exhibits higher activity and stability than the pristine one.[43] Precisely tailoring of OVs on the desired facets of metal oxide can further greatly influence the surface electronic structure metal oxide, which is beneficial to enhance the corresponding intrinsic activity for ORR/OER. For example, Qiao et al. recently reported the surface structure engineering of single-crystal (SC) CoO nanorods (NRs) by the introduction of desired facets and OVs (Figure 5a).[44,45] The formation energy of OVs on {111} facet is about 3 eV lower than the corresponding values of {100} and {110} facets (Figure 5c,d). Experiments, microscopic and spectroscopic characterization and DFT calculations demonstrate that the OVs located on the (111) facets can be beneficial to tailor the electronic structure of CoO NRs, leading to rapid charge transfer and favorable reaction kinetics for both ORR and OER (Figure 5b,e,f). Thus, CoO NRs show onset potential of 0.96 V (versus RHE), half-wave potential (E1/2) of 0.85 V and Tafel slope of 47 mV dec−1 for ORR catalysis, meanwhile they exhibit moderate potential (E10) of 1.56 V at a current density of 10 mA cm−2 and Tafel slope of 44 mV dec−1 for OER catalysis. When compared to benchmarked catalysts, the ORR activity is closed to that of Pt-based catalysts and the OER activity outperforms that of RuO2 catalysts. Moreover, the overall potential gap of 0.71 V (ΔE = E10 − E1/2) is competitive to that of the widely reported bifunctional ORR/OER catalysts.

3.1.3. Crystal Facet Engineering 3.1.5. Mixed-Metal Engineering Atomic-level-engineering of the surface structure can be used to precisely manipulate the exposure of active sites and subsequently enhance corresponding electrocatalytic activity.


Different surface doping at surface/interface and homogenous metal-mixing can manipulate the inherent electronic and/or

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Figure 5. a) Schematic synthesis of SC CoO NRs. b) SEM, TEM and HRTEM images of SC CoO NRs. c) Atomic model of SC CoO NRs. d) OV formation energies on {100}, {111} and {111}-O facets of CoO. e) Bifunctional ORR and OER performance of SC CoO NRs. f,g) Atomic models of O2 adsorption, ORR/OER free energy diagram at 1.23 V on different facets, respectively. Reproduced with permission.[44] Copyright 2016, Macmillan Publishers Limited.

surface structures of the host material toward better activity more effectively. Thus, spinel and perovskite structure with multi-metal components are largely used for ORR and OER catalysis.[32] Spinel oxides (AxB3−xO4) can be used as bifunctional ORR/OER catalyst in alkaline media.[46] Among spinel oxides, the electron transfer occur with quite low activation energy between the cations of different valences. For example, Chen et al. developed a facile and rapid room-temperature method to prepare highly active CoxMn3−xO4 for bifunctional ORR/OER.[47] The synthesis is based on the strategy of reduction-recrystallization of amorphous MnO2 precursors. As a result, tetragonal and cubic CoxMn3−xO4 with different morphologies, were obtained using NaH2PO2 and NaBH4 as the reductants, respectively. Due to the high surface areas and abundant vacancy sites, the as-prepared CoxMn3−xO4 exhibited higher ORR/OER activities than that traditionally synthesized at high temperature. Moreover, cubic CoxMn3−xO4 showed higher intrinsic ORR activity than that of tetragonal phase, but the opposite result is observed for OER, resulting from the dissimilar oxygen adsorption energies on Co and Mn defect sites. As an air electrode using CoxMn3−xO4 in Zn-air battery, it showed a stable galvanostatic discharge curve and considerable specific energy densities. Based on these, they recently reported the ultrasmall CoxMn3−xO4 by a simple solution-based oxidation/precipitation Adv. Energy Mater. 2017, 1700544

strategy (Figure 6a).[48] It is worth noting that this strategy fabricates ≈10 nm CoxMn3−xO4 with independently controllable Co/Mn ratio and crystallographic cubic or tetragonal phase. Furthermore, the cubic phase exhibited significantly improved ORR and OER activities as comparable to benchmark Pt/C catalyst (Figure 6b). Owing to the excellent bifunctional ORR/OER activity and stability, the cubic CoxMn3−xO4 can be assembled in Zn-air batteries with high energy density of about 650 W h kg−1 at 10 mA cm−2 normalized to consumed Zn anode (Figure 6c). Similarly, other spinel oxides including NiCo2O4 and LiCoO2 have also been reported as high-performance ORR/OER catalysts for metal-air batteries, and their active sites are originated from the Co4O4 cubane subunits.[49,50] Perovskite oxides (ABO3) are also widely investigated as bifunctional ORR/OER catalyst in alkaline electrolytes.[21,51–58] Their compositions and properties can be finely tuned by partially substituting A and B with other metal components. Typically, A-site mainly influences on the capability of oxygen adsorption, and B-site mainly influences on the activity of the adsorbed oxygen. Currently, the synthesized ABO3 with large sizes from traditional high-temperature calcination hinder the widely application due to the poor mass transfer and limited active sites. To address this problem, Shao-Horn et al. synthesized 50 nm perovskite nanoparticles with the composition of Lax(Ba0.5Sr0.5)1−xCo0.8Fe0.2O3−δ (BSCF), by the

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Figure 6. a) Schematic illustration of the formation of cubic (I) and tetragonal (II) phases of CoxMn3−xO4. b) Bifunctional ORR and OER performances of c-CoxMn3−xO4 and Pt/C. c) Electrochemical performance of assembled Zn-air battery. Reproduced with permission.[48] Copyright 2015, Macmillan Publishers Limited.

control of La concentration and calcination temperature.[56] The optimized BSCF exhibited excellent activity towards both ORR and OER: i.e., the ORR to water with 95% yield comparable to Pt/C, and 20 times higher gravimetric OER activity than IrO2. As expected, the charge/discharge performance of assembled Zn-air batteries from the optimized BSCF surpassed that of commercial Pt/C. Yamada et al. further reported quadruple Mn-based perovskites including CaMn7O12 and LaMn7O12 with the enhanced OER activities while intrinsically high ORR activities maintained.[57] A possible origin of the high OER activity is the unique surface structure through corner-shared planar MnO4 and octahedral MnO6 units to promote direct O−O bond formations.

3.1.6. Strain Engineering As aforementioned, eg electron number for perovskite oxide has a volcano relationship with OER activity.[21] Thus, strain


engineering can finely tune the degree of this eg orbital splitting and polarization in the octahedra, the absorption of oxygen-contained intermediates, and the overall ORR/OER activities. Recently, Lee et al. reported the strained LaNiO3 for the enhanced bifunctional ORR/OER catalysis using different lattice-mismatched substrates to control strain degree from −2.2% to 2.7% (Figure 7a).[59] They found that when LaAlO3 (LAO) is used as substrate, the produced small strain of −1.2% in LaNiO3 can lead to the enhanced bifunctional ORR/OER compared to other strained samples and pristine LaNiO3 (Figure 7b,c). When ORR and OER activities under the overpotentials of 0.4 V are compared (Figure 7d), the bifunctional activity drastically increases with compressive strain. As a result, the LaNiO3 with the strain of −1.2% has a bifunctionality outperforming that of Pt (Figure 7e). DFT calculations confirmed that eg-center decreases with the compressive strain, which leads to weaker strength of M−O bond and enhanced oxygen activity. (Figure 7f,g).

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Figure 7. a) Lattice parameters of strained LaNiO3. b,c) ORR and OER activities of strained LaNiO3. d) Relation between ORR/OER activities (overpotentials of 0.4 V) and compressive strain. e) Relation between bifunctionality (overpotential gap at 30 µA cm-2 for ORR and OER reactions) and compressive strain. f,g) DFT calculated relationship between electrocatalytic activity and eg center for compressive strain. Reproduced with permission.[59] Copyright 2016, American Chemical Society.

3.2. Transition Metal Hydroxide and Sulfide Other metal-based material such as metal hydroxide and sulfide are also investigated in ORR/OER, but the literatures are very limited.[60–63] For example, Sun et al. first found that layered double hydroxides (LDHs) can be used as bifunctional oxygen electrocatalysts.[60] NiCoFe-LDH shows a modest bifunctional


ORR/OER activities. The preoxidation treatment can further enhance its ORR/OER activities due to the partial conversion of Co2+ to Co3+ to accelerate charge transfer efficiency. The optimized material demonstrated a small potential gap (0.8 V for a reversible current density of 20 mA cm−2) as well as a high stability, superior to that of commercial Pt/C. Additionally, Zhang et al. demonstrated the synthesis of urchin-like NiCo2S4

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sub-micron spheres for bifunctional oxygen electrocatalysis, which was attributed to the unique d-electronic configuration of the surface Co3+ from sulfur vacancies.[63]

3.3. Carbon Materials Carbon materials have been widely reported as electrocatalysts due to their high surface area, high conductivity, tunable structure, facile preparation, and economic viability.[64–70] Rational design of carbon materials with multicomponent active centers can in principle lead to multifunctional catalysts for the ORR/OER. Typically, metal-free heteroatom (e.g., N)-doped carbon (C-N) and N- and transition metal“co-doped” carbon (M-N-C, M = Fe or/and Co) are widely investigated.

3.3.1. Heteroatom Doping Nonmetal atoms such as N, P, S and B can be doped into sever allocations within the carbon structure, thus resulting in multiple possible configurations. Being more electronegative than carbon, these heteroatoms make neighboring carbon atoms electron deficient, thereby promoting oxygen adsorption on the carbon nanostructure.[65,71] Recently, Wang et al. reported

the synthesis of hollow frameworks of N-doped carbon nanotubes (NCNTs) using ZIF-67 as precursor (Figure 8a).[72] Amazingly, during thermal treatment, the reductive H2 atmosphere plays a critical role in the formation of CNTs. As comparison, no CNTs were formed using other atmospheres such as N2. It is found that Co nanoparticles are initially formed in the presence of the H2 atmosphere for the subsequent catalytic growth of NCNTs to finally produce NCNT frameworks (NCNTFs). Further acid treatment can remove the accessible Co nanoparticles in NCNTFs. Due to the synergistic effect between the unique N-doping and hollow structure, the obtained metal-free NCNTFs exhibited higher electrocatalytic activity and stability for ORR and OER than commercial Pt/C (Figure 8b). Also, using MOF as precursor and template, an efficient bifunctional electrocatalyst with core-shell structure was obtained from ZIF-8@ZIF-67 through hydrothermal and carbonization treatment.[73] The resulted material, i.e., highly graphitic carbon (GC, carbonized from ZIF-67) on N-doped carbon (NC, carbonized from ZIF-8) (NC@GC), combines the distinguished advantages of NC, including high surface area, presence of Co doping and high N content, and those of GC including high crystallinity, good conductivity and stability of GC. This unique core-shell structure with potential synergistic interaction leads to high activities towards ORR and OER. Cheng et al. reported the synthesis of porous N-doped carbon microtube (NCMT) sponge via a simple and low-cost process

Figure 8. a) Morphology characterization of N-doped carbon nanotube framworks (NCNTFs). b) Bifunctional catalytic activities of NCNTFs and Pt/C toward both ORR and OER. Reproduced with permission.[72] Copyright 2016, Macmillan Publishers Limited. c) Synthesis and morphology characterization of N, S co-doped graphitic sheets with SHG. Reproduced with permission.[78] Copyright 2017, Wiley-VCH.


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of pyrolyzing facial cotton.[74] Due to its unique structure with a micron-scale hollow core and well-graphitized and interconnected porous walls, the NCMT sponge exhibited excellent electrocatalytic activity for ORR and OER with a small potential gap of 0.63 V between the OER current density at 10 mA cm−2 and the ORR current density at 3 mA cm−2. Suib et al. demonstrated that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs can be used for bifunctional ORR/OER with a small overpotential gap of 0.84 V between the OER current density at 10 mA cm−2 and the ORR current density at 3 mA cm−2.[75] Similarly, Liu et al. reported the synthesis of N-doped graphene nanoribbon networks (N-GRW) for bifunctional ORR/ OER with excellent activity and durability.[76a] In situ characterization including UPS and XANES demonstrated that the electron-donating quaternary N sites were responsible for ORR, whereas the electron-withdrawing pyridinic N moieties were responsible for OER. As a result, the as-prepared N-GRW can be assembled to an air cathode in Zn-air batteries with an opencircuit voltage of 1.46 V, a specific capacity of 873 mA h g−1, and a peak power density of 65 mW cm−2. Also, Guo et al. used both experimental and DFT computation techniques to identify the active sites for N, P co-doped carbon material. They found that the P-N sites are responsible for OER and the N-doped sites are responsible for for ORR.[76b] Dai et al. further reported N, P co-doped mesoporous carbon foam (NPMC) using polyaniline aerogel as precursor.[77] Due to the N, P co-doping and graphene edge effects and large surface area, the obtained NPMC exhibit excellent electrocatalytic properties for both ORR and OER as compared to Pt/C and RuO2, respectively. Furthermore, this optimized NPMC can be used as air electrode for Zn-air batteries with open-circuit potential of 1.48 V, specific capacity of 735 mA h gZn−1, a peak power density of 55 mW cm−2, and stable operation for 240 h after mechanical recharging. Similarly, they also reported N, S co-doped graphitic sheets with a unique hierarchical structure (Figure 8c).[78] The presence of stereoscopic holes over the graphitic surface (SHG) ensures a high surface area with abundant interfacial active sites for electrochemical reactions. The abundant accessible active sites coupled with efficient pathways for electron and electrolyte/reactant transports make the newly developed SHG an efficient metal-free ORR/OER/HER tri-functional catalyst with long-term stability in alkaline electrolytes (e.g., 0.1 m KOH). The SHG exhibited an ORR half-wave potential comparable to that of the commercial Pt/C, and showed a comparable OER activity to RuO2 nanoparticles. Intriguingly, they also reported N, P, and F tri-doped graphene for ORR/OER/HER and Zn-air battery with excellent performances. Other literatures also reported similar ORR/OER catalyst using heteroatom-doped carbon materials with a metric potential difference of 0.72 and 0.88 V, respectively.[79,80]

3.3.2. Topological Defects In addition to heteroatom dopants, the topological defects including carbon defects, edge sites, lattice reconstruction, nonhexagonal topologies, and dangling groups, also can redistribute the local electron density to produce a stronger


adsorption to the oxygen-containing intermediates.[81] Based on DFT computation, Hu et al. investigated the roles of intrinsic carbon defects in pristine carbon nanocages on decreasing the reaction free energy and facilitating the electron transfer for OOH*. As a result, this defective carbon nanocages exhibited a good ORR activity with a high onset potential about 0.88 V (vs RHE) in 0.10 m KOH, which is even superior to N-doped CNTs.[81b] Also, Wang et al. reported edge-rich and dopant-free graphene using Ar-plasma etching for ORR, and found that the reactive sites are edge carbon.[81c] Zhang et al. further reported a novel N-doped and edgerich graphene material (NGM) for bifunctional ORR/OER (Figure 9a).[81d] Different from other reported heteroatomdoped graphene, the in situ fabricated MgO templates are critical for effective graphitization of solid carbon/N sources and the generation of abundant in-plane holes. As a result, NGM exhibits one of the highest bifunctional activities with overpotential gap of 0.90 V between ORR and OER among all the reported metal-free catalysts (Figure 9b). DFT calculations confirmed that both edge effects and topological defects are mainly responsible for both ORR and OER, instead of the N-doped sites. The lowest overpotential for both ORR and OER is calculated for N-free configuration with adjacent pentagon and heptagon carbon rings (C5+7, as shown in Figure 9c). Furthermore, NGM can be constructed in Zn-air batteries with a current density of ≈6.0 mA cm−2 and a peak power density of ≈3.0 mW cm−2.

3.3.3. Metal-N-C Metal-N-C structure is widely studied for catalysis including ORR and OER.[82–84] Its synthesis can typically be classified into three categories. The direct carbonization of MOF or phthalocyanine compounds under inert atmosphere is the most typical method to produce highly porous metal-N-C structure.[82] The carbonization of mixtures of carbon materials, N-containing compounds such as urea, melamine or polyaniline, and metal salt under inert atmosphere can also produce metal-N-C structure.[83] The third method involves using NH3 as N source with the rest similar to the second method.[84] For example, Guo et al. reported an approach of obtaining highly effective ORR/OER electrocatalysts of Co-N-C frameworks using bimetallic CoZn-MOFs as precursor.[85a] By the optimization of different carbonization conditions, degree of graphitization, porosity and oxidation state of Co can be finely tuned. As a result, the highest ORR (with the Tafel slope, halfwave potential, electron transfer number and limiting current density comparable to Pt/C) and OER (with the potential of 1.67 V at 10 mA cm−2 better than lrO2/C of 1.72 V) activities can be obtained when the carbonization temperatures are set to 1000 and 1100 °C, respectively. Zhang et al. reported a novel Co-N-C structure from the carbonization of ZIF-67/polypyrrole nanofibers network rooted on carbon cloth (Figure 10).[9] Due to the unique composition and structure, it exhibits excellent bifunctional oxygen electrocatalytic activity and stability: i.e., a low overpotential of 0.31 V at 10 mA cm−2 for OER, a high halfwave potential of 0.8 V for ORR, and a good durability for about >20 h. Furthermore, the assembled Zn-air batteries from the

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Figure 9. a) Schematic of synthesis of N-doped graphene mesh (NGM). b) Bifunctional ORR/OER activities and assembled Zn-air battery performances. c) DFT calculated ORR/OER mechanism. Reproduced with permission.[81d] Copyright 2016, Wiley-VCH.

optimized Co-N-C exhibit a low discharge-charge voltage gap of 1.09 V at 50 mA cm−2 and long cycle life up to 408 cycles. Graphitic carbon nitride (g-C3N4), with a graphene like framework, contains periodic heptazine units connected via tertiary amines.[85b,c] The high level of pyridine-like nitrogen in heptazine heterorings provide rich electron lone pairs to capture metal ions in the ligands. Recently, Qiao et al. using g-C3N4 as a platform, investigated a series of g-C3N4 organometallic electrocatalysts as a new class of M-N/C materials for bifunctional oxygen electrocatalysis.[85b] As a preliminary trial, they theoretically predicted and experimentally measured the Co-g-C3N4 complex, with a single coordinated Co atom, as an efficient electrocatalyst for the ORR and OER in alkaline media. A combination of experimental and DFT computation confirmed that the high activity originates from the precise Co-N2 coordination moiety in the g-C3N4 matrix to provide the appropriate d-band position of the catalyst.

3.4. Composite materials Various materials have shown evident catalytic activity toward both ORR and OER. The combination of them as composites represent a new route for the preparation of highly attractive bifunctional ORR/OER catalysts. Composite materials composed of two individual components- one for ORR, and the other for ORR- are also expected in highly efficient bifunctional ORR/OER. For example, Strasser et al. reported the physical mixing of a OER active NiFe-LDH with a ORR active Fe-N-C can produce a highly efficient bifunctional oxygen catalyst with


overpotential gap of 0.74 V between ORR and OER.[86] Similarly, Shao-Horn et al. combined the high intrinsic ORR and OER activities of La0.8Sr0.2MnO3−δ and Ba0.5Sr0.5Co0.8Fe0.2O3−δ to develop a novel bifunctional oxygen catalyst with enhanced stability and activity.[87]

3.4.1. Metal Oxide/Metal Oxide It is frequently reported that MnO2 and RuO2 materials are good catalysts toward ORR and OER, respectively.[32] Thus, the formation of RuO2/MnO2 composite is promising for bifunctional ORR/OER. For example, Sun et al. reported the synthesis of RuO2 nanoparticles supported on MnO2 nanorods (np-RuO2/nr-MnO2) via a two-step hydrothermal reaction.[88] As expected, np-RuO2/nr-MnO2 exhibits excellent bifunctional electrocatalytic activities than MnO2 nanorods for both ORR and OER. Also, Kim et al. recently reported 1D hollow RuO2 and Mn2O3 composites by controlling ramping rate of electrospinning. As shown in Figure 11a, two different and novel composites including phase separated RuO2/Mn2O3 fiber-in-tube (i: RM-FIT) and RuO2/Mn2O3 tube-in-tube (ii: RM-TIT) are synthesized.[89] As comparable to 20 wt% Pt/C, both RM-FIT and RM-TIT demonstrated excellent bifunctional ORR/OER activities in 0.1 m KOH solution. Engineering high-energy interfacial structures can greatly facilitate adsorption of reactants on the surface of the catalyst, and charge transport, which results in enhancing catalytic performance.[90] For example, Qiao et al. reported the fabrication of high-energy interfacial structures by chemical coupling active

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Figure 10. a) Synthesis and charaterazation of Co-N-C structure from ZIF-67. b,c) Zn-air batteries performances. Reproduced with permission.[9] Copyright 2016, American Chemical Society.

of CoO nanoclusters and high-index Mn3O4 nano-octahedrons (hi-Mn3O4) from electrostatic assembly (Figure 11b).[90a] The strong interactions between both components generate interfacial Mn-O-Co bond and high oxidation state of CoO due to the transferred electrons from CoO to hi-Mn3O4, offering favorable physicochemical characteristics such as rich active sites, fast charge transfer, reversible oxidation capability towards oxygen-involving electrocatalysis. As a result, the assembled battery with the CoO/hi-Mn3O4 provided a discharge voltage of 1.22 V and a charge voltage of 1.97 V, comparable to those (1.21 and 1.98 V) of the battery with Pt/C+Ru/C, verifying high performance of CoO/hi-Mn3O4 towards both ORR and OER. 3.4.2. Metal Oxide/Carbon Transition metal oxides and functioned carbon material are widely reported as bifunctional oxygen electrocatalysts. Thus, the design and development of composite electrocatalysts based on them attracted great attentions.[91] For example, Dai et al. and other researchers reported the synthesis of Co3O4/NG for oxygen electrode in alkaline solutions. For ORR, it demonstrates comparable ORR activity and even higher durability than Pt/C.[92,93] For OER, the composite shows a small overpotential of 0.31 V at 10 mA cm−2 and Tafel slope of 67 mV dec−1. Similarly, Chen et al. further reported pomegranate-like Co3O4-based Adv. Energy Mater. 2017, 1700544

composite based on sphere-like Co-glycerate as precursor for bifunctional oxygen electrocatalysis. As a result, the composite shows a high halfwave potential of 0.842 V for ORR and a low overpotential of only 0.45 V at 10 mA cm−2 for OER, respectively.[94,95] Liu et al. reported the synthesis of ZnCo2O4 quantum dots (QDs) (about 3.0–3.5 nm,) supported on N-doped carbon nanotubes (N-CNTs) for bifunctional ORR/OER electrocatalysis.[96] For ORR, it shows a high halfwave potential of 0.87 V and a small Tafel slope of 52.9 mV dec−1, which is superior to individual ZnCo2O4 QDs (0.84 V, 81.6 mV dec−1) and the referred Co3O4/N-CNTs (0.86 V, 54.3 mV dec−1) and comparable to Pt/C (0.87 V, 70.1 mV dec−1). For OER, it also shows a low potential of 1.65 V at 10 mA cm−2 and a small Tafel slope of 70.6 mV dec−1, which is superior to individual ZnCo2O4 QDs (1.69 V, 90.4 mV dec−1) and the referred Co3O4/N-CNTs (1.67 V, 82.1 mV dec−1) and comparable to IrO2/N-CNTs (1.64 V, 74.5 mV dec−1). All these experimental results indicated that the critical role of Zn substituted in Co3O4 for ORR/OER. For example, ESCA (ZnCo2O4 QDs) confirmed that Zn2+ substitution can improve the active Co3+ site in Co3O4 by the occupation of tetrahedral site. Additionally, the morphology of QDs as well as the heteroatom N doping contribute to the overall ORR/OER activity. As expected, the assembled Zn-air batteries from the ZnCo2O4 QDs/N-CNTs showed a high open-circuit of 1.47 V, a peak power density of 82.3 mW cm−2 at 120 mA cm−2, an energy density of 595.57 W h kg−1, superior to Pt/C and IrO2/N-CNTs.

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Figure 11. a) Syntheis of hollow RuO2/Mn2O3 for bifuctional ORR/OER electrocatalysis. Reproduced with permission.[89] Copyright 2016, American Chemical Society. b) Syntheis of high-energy interfacial structures of CoO/hi-Mn3O4 for bifuctional ORR/OER electrocatalysis. Reproduced with permission.[90a] Copyright 2017, Wiley-VCH. c) Synthesis of NiCo/PFC aerogels for bifunctional oxygen electrocatalysis, and Zn-air battery. Reproduced with permission.[103] Copyright 2016, American Chemical Society.

Also, Muhler et al. and Wu et al. reported spinel CoxMn1−xO4 nanoparticles partially embedded in N-CNTs catalysts with excellent bifunctional ORR/OER activity.[97,98] This hybrid catalyst exhibits much higher OER activity than that of IrO2, and comparable ORR activity to Pt/C with identical onset potential (0.96 V) in alkaline media. Furthermore, the CoxMn1−xO4/ N-CNTs catalyst was studied as a cathode in both primary and rechargeable Zn-air batteries, demonstrating similar performance to commercial Pt/C or (Pt/C+IrO2), respectively. The ORR activities of perovskite oxides are much inferior to other ORR catalysts, such as N-doped carbons, whereas their OER activities are comparable to those of other OER catalysts. Therefore, the improvement in the ORR activity is required for ensuring the bifunctionality of perovskite oxides by reducing the potential gap between ORR and OER.[99] For example, Shanmugam et al. reported the synthesis of perovskite LaTi0.65Fe0.35O3−δ nanoparticles entangled both at the surface and within the N doped carbon nanorods (NCNR) as a bifunctional ORR and OER catalyst with excellent performance.[100]


Song et al. further demonstrated the simply mixed, composite catalysts of perovskite oxide catalysts and polypyrrole (pPy).[101] Without any strong interactions between the two components, the overpotentials for ORR and OER on perovskite oxide catalysts were significantly reduced simply by mixing the catalyst particles with polypyrrole/carbon composites (pPy/C). A mechanism based on the sequential task allocation between pPy and oxide catalysts for the ORR was proposed: (1) molecular oxygen is incorporated into pPy as a form of superoxide (pPy+O2−), (2) the superoxide is transferred to the active sites of perovskite catalysts, and (3) the superoxide is completely reduced along the four-electron ORR process.

3.4.3. (Alloyed)Metal/N-Doped Carbon Metal atoms can be stabilized by N-doped carbon supports due to the metal-support interaction from the intimate contact with N-induced defect.[82] To some extent, the formation of isolated

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metal species bound to N sites is associated with the ability of the metal to form complexes with N ligands. In this sense, N-doped carbon materials can be considered as “solid ligands”. Bao et al. reported the synthesis of uniform Fe nanoparticles encapsulated in N-doped carbon as shell (Fe@N-C) using dicyandiamide and ammonium ferric citrate as processors.[102] The resulting Fe@N-C material shows excellent bifunctionality for ORR/OER in alkaline solution compared to state-of-the-art Pt/C and IrO2. Also, it can be used in in Zn-air battery with high performance and cycling durability. More recently, one approach that has been suggested to enhance the catalytic activity for ORR/OER is alloying, to precisely tune the binding strength of intermediates on a catalyst surface.[103,104] Goodenough et al. demonstrate excellent performance of NiCo nanoparticles anchored on porous fibrous carbon aerogels (NiCo/PFC aerogels) as bifunctional ORR/ OER catalysts, as shown in Figure 11c.[103] This material is designed and synthesized using K2Ni(CN)4/K3Co-(CN)6-chitosan hydrogel as processors. At a discharge-charge current density of 10 mA cm−2, the NiCo/PFC aerogels enable a Zn-air battery to cycle steadily up to 300 cycles for 600 h with only a small increase in the round-trip overpotential, notably outperforming the more costly Pt/C+IrO2 mixture catalysts (60 cycles for 120 h). Wu et al. further reported ultralarge sized N doped graphene tubes (N-GTs) (>500 nm) decorated with FeCoNi alloy particles for bifunctional ORR/OER.[104] These tubes are prepared from an inexpensive precursor, dicyandiamide, via a template free graphitization process. The ORR/OER activity and the stability of these graphene tube catalysts depend strongly on the transition metal precursors. The best performing FeCoNi-derived N-GT catalyst exhibits excellent ORR and OER activity along with adequate electrochemical durability over a wide potential window (0–1.9 V) in alkaline media. Extensive electrochemical and physical characterization indicated that high graphitization degree, thicker tube walls, proper N doping, and presence of FeCoNi alloy particles are vital for high bifunctional activity and electrochemical durability of tubular carbon catalysts.

CNT, which exhibits higher activity than Cu@CNTs. In addition to metal, metal oxide also can mutually tune the electron structure of CNT. As a result, further annealing Cu@NCNT loaded by cobalt nitrate can produce Cu@NCNT/CoxOy composites, where CoxOy nanoparticles were selectively decorated on Cu@NCNT surfaces (Figure 12b–e). Note aqueous solution could not infuse into inner spaces of the CNTs because of the hydrophobic CNT walls, and thus cobalt nitrate aqueous solution only coated surfaces of the Cu@NCNTs. Because of these outstanding benefits, further N doping, and synergistic coupling between CoxOy nanoparticles and Cu@NCNTs, the Cu@NCNT/CoxOy composites demonstrated high ORR activity similar to commercial Pt/C and OER activity higher than IrO2. The overpotential gap between ORR (3 mA cm−2) and OER (10 mA cm−2) is 0.78 V. Furthermore, this approach was extended to the fabrication of different metal/metal oxide composites such as Ni@NCNT/CoxOy with similar hierarchical structures (Figure 12f). Similar to the reported hierarchical NCNTs frameworks in section 3.4, Muhler et al. reported a ZIF-67-derived bifunctional ORR/OER electrocatalysts: Co@Co3O4 core-shell nanoparticles embedded in NCNTs frameworks. In addition to the pyrolysis under H2 atmosphere, subsequent controlled oxidative calcination is necessary to obtain Co@Co3O4.[106] The catalysts show excellent ORR/OER activities in 0.1 m KOH with 0.85 V overpotential gap between ORR and OER, surpassing Pt/C (1.0 V), IrO2 (1.41 V), and RuO2 (1.27 V) and even the reported nonprecious-metal materials. Wu et al. reported the combination of NiCo alloys, their oxides and N-doped multiwall carbon nanotubes (NCNT/CoO-NiO-NiCo) as bifunctional ORR/OER catalyst with excellent activity and stability.[107] As expected, the corresponding assembled Zn-air batteries exhibits superior performance to state-of-the-art Pt/C or Pt/C+IrO2 couple. Also, Chen et al. reported the loading of Pt clusters on porous CaMnO3 nanoparticles with carbon as support as a bifunctional ORR/OER catalyst with excellent activity and durability.[108]

3.4.5. Metal Ion/Heteroatom-Doped Carbon Composites 3.4.4. Metal/Metal Oxide/Carbon Composites Introduction of metal into metal oxide/carbon represents another effort towards the design of efficient bifunctional composite electrocatalyst. Recently Baek et al. demonstrated the fabrication process of Cu@NCNT/CoxOy composites for highly efficient bifunctional oxygen electrocatalysis (Figure 12).[105] Cu@CNTs were fabricated by immersing AAO in copper nitrate aqueous solution and drying it, and following lowand high-temperature CVD processes at 450 and 800 °C, with acetylene as a carbon precursor. DFT calculations indicated that interaction of Cu nanoparticles with CNT walls induced decreased work function of CNT surfaces and improved adsorption of hydroxyl ions onto the CNT surfaces (Figure 12g), indicating it can function as a better bifunctional ORR/OER catalyst than pure CNTs. As expected, better ORR and OER activities of Cu@CNTs than pure CNTs were achieved, as shown in Figure 12c. They further performed ammonia posttreatment of Cu@CNTs at 900 °C to introduce N-doping into


Metal ions, as common active sites in metal-based catalytic materials, have a tunable ligand capability with oxygen-containing intermediates due to their d-electron difference.[27,109–111] N doped graphene (NG) is a modest bifunctional ORR/OER catalyst or an outstanding support with high surface area, good electrical conductivity and chemical stability to load active sites. As a result, the combination of suitable metal ion with NG may lead to different activities. For example, Xia et al. proposed the incorporation of heterogeneous metal ion (M = Fe2+, Co2+, Ni2+, Mn2+, Cu2+, Zn2+) on N doped graphene complexes (NG-M) under room conditions (Figure 13a).[109] They found that a volcano-like trend between OER activity (potential at 2 mAcm−2) for NG-M and corresponding d electron numbers of Mx+ (Figure 13b) is observed. Co2+ is the best metal ligand for NG, and the as-prepared NG-Co thus shows the best OER activity with an overpotential of 0.40 V and a TOF of 2.53 s−1 under 0.1 m KOH solution. They further investigated the activity of NG-Co-L that can be fine-tuned using different counter anions (L = Ac−, SCN−, SO42−, NO2−,

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Figure 12. a) Scheme of the fabrication process of Cu@CNTs. b-e) SEM, TEM and HRTEM images of Cu@NCNT/CoxOy composites. f) TEM image of Ni@NCNT/CoxOy composites. g) Electrostatic potential profiles averaged on the plane perpendicular to the X-axis as a function of the X-axis of the supercell of SWCNT and Cu4@SWCNT. h) Bifunctional ORR/OER activity of Cu@NCNT/CoxOy, Cu@CNT, Cu@NCNT composites, compared with CNT, IrO2 and Pt/C. Reproduced with permission.[105] Copyright 2017, Wiley-VCH.

acac−, NO3−, Br− and I− ions). Similarly, a volcano-like trend between OER activity (potential at 2 mA cm−2) for NG-M-L and corresponding DPV potentials of Co2+ ions on NG is observed (Figure 13c). Meanwhile, NG-Co also can be used as ORR catalyst with the enhanced activity than individual NG. Based on these, Wang et al. further used S/N/O-, S/O-, N/O-, and O-doped graphene as new ligand to immobilize Co2+ ions (denoted as SNG-Co2+, SG-Co2+, NG-Co2+ and OG-Co2+, respectively) from Co(acac)2 by replacing parts of the acac− groups, respectively (Figure 13d).[110] For OER catalysis, SNG-Co2+ and SG-Co2+ exhibited the best OER activities (0.266 s−1 vs 0.268 s−1 at η of 0.35 V, higher than that of many Co-based nanostructures and IrO2 catalysts) from the aspect of TOF than the other referred samples (Figure 13e, indicating that S-containing group is much more effective than the N-containing group. Coupled with other spectral characterization and electrochemical characterization, Co2+ ions combined with a sulfoxide configuration within the graphene are regarded as the most efficient active sites. Unlike conventional Co3O4, where dual Co sites are regarded as the active sites, the OER was found to be catalyzed by single Co ions with terminal oxo ligands formed


after a sequential conversion of Co2+ → Co3+ → Co4+ coupled with HO− (H+) transfer (Figure 13f). In addition to Co2+ ion, Xia et al. reported the optimization of the d-electron number of Cu2+ ion to design a highlyefficient bifunctional ORR/OER electrocatalyst. In this case, not only the synergistic electronic connection between Cu2+ and NG, but also that of Cu2+ and Cu0 were considered (Figure 13g).[27a] Using Cu2+-1,10-phenanthroline (Cu(phen)2) supported on graphene oxide (GO) as precursor, the one-pot thermal decomposition at different temperature can obtain the target NG-Cu2+/0 (donated as CPG-t, where t is pyrolysis the temperature). Raman spectra shows that the intensity ratio of D-band to G-band increases with the pyrolysis temperature, suggesting the gradual incorporation of Cu2+-N structure into the graphene forming active sites. The optimized CPG-900 showed the best ORR activity with an onset potential of 0.978V versus RHE that is even superior to commercial Pt/C catalyst. Furthermore, it can also be used as an excellent OER catalyst with negligible onset overpotential. In situ fluorescence spectroelectrochemistry measurements further established a brief correlation between ORR and OER in

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Figure 13. a) Schematic configurations of Mx+ ions located in NG. b) OER activity for NG-M as a function of d electron numbers of metal ions. c) OER activity for NG-Co-L (L: corresponding anion) as a function of the DPV potentials of Co2+ ions. Reproduced with permission.[109] Copyright 2016, Royal Society of Chemistry. d) Synthesis of Co2+ ions immobilized onto S-, N-, O-atom-doped graphene (SNG-Co2+). e) TOFs of SNG-Co2+, SG-Co2+, NG-Co2+ and OG-Co2+ for OER. f) Mechanism of SNG-Co2+ for OER in an alkaline solution. Reproduced with permission.[110] Copyright 2017, American Chemical Society. g) Proposed active sites of in CPG-900. h) Scheme of identifing oxygen containing intermediate (OCI) involved in ORR by in situ fluorescence spectroelectrochemistry. i) Schematic illustration of the bifunctional ORR/OER with a common transient state of HO·radical. Reproduced with permission.[27a] Copyright 2014, Macmillan Publishers Limited.

alkaline solutions using HO· radical as a shared intermediate (Figure 13h,i).

4. Conclusion and Perspectives Oxygen reduction reaction and oxygen evolution reaction are the two most important reactions of metal-air battery. In the past decade, great breakthroughs have been made on the design and synthesis of heterogeneous bifunctional electrocatalysts towards ORR/OER in alkaline electrolyte (Table 1) including metal oxide, hydroxide and sulfide, functional carbon material, metal, and their composites. All these reported materials have their advantages and disadvantages. For example, metal hydroxide, sulfide and carbon materials can be easily oxidized at high operating potential for OER, but their ORR activities and stabilities are relatively good. Transition metal oxide catalysts are inherently more stable than traditional carbon materials in oxidizing environments, while their activity is still not satisfactory. Also, the synthesis of perovskites needs very high temperature and the concomitant sintering and agglomeration


Table 1. Some state-of-the-art bifunctional ORR/OER catalyst in 0.1 m KOH. E10

E3(E1/2)

ΔE (E10 − E3(1/2))

Reference

Co3O4

1.52 V

0.78 V

0.74 V

[37b]

CoO

1.56 V

0.85 V

0.71 V

[44]

c-CoxMn3−xO4

1.78 V

0.68 V

1.10 V

[48]

BSCF

1.60 V

0.66 V

0.94 V

[56]

NCMT

1.52 V

0.89 V

0.63 V

[74]

NCNTs

1.60 V

0.84 V

0.76 V

[72]

P, N Co-doped graphene

1.55 V

0.845 V

0.705 V

[76b]

Co-N-C

1.54 V

0.80 V

0.74 V

[9]

Fe-N-C

1.60 V

0.84 V

0.76 V

[84] [90]

Catalyst

CoO/hi-Mn3O4

1.60 V

0.70 V

0.90 V

Co3O4/NG

1.60 V

0.83 V

0.77 V

[92]

Fe@N-C

1.70 V

0.84 V

0.86 V

[102]

NiCo/PFC aerogels

1.62 V

0.76 V

0.86 V

[103]

N-GT(FeCoNi)

1.54 V

0.89 V

0.65 V

[104]

Cu@NCNT/CoxOy

1.60 V

0.82 V

0.78 V

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is normally unavoidable, but they have a major advantage of tunable composition in ABO3 formulation. Overall, it is difficult for these materials as individual to possess excellent ORR and OER activities simultaneously. The composite materials are believed to be promising to complement one another, and finally function as excellent bifunctional ORR/OER catalysts with good stability. Additionally, in the review, some crucial factors determining the ORR/OER performance are summarized, which include: (1) phase, morphology, crystal facet, defect, mixed-metal and strain engineering for metal oxide, hydroxide and sulfide; (2) heteroatom doping, topological defects, and formation of metal-N-C structure for carbon material; (3) alloy effect for metal. Despite extensive advances to develop bifunctional ORR/OER catalysts, most state-of-the-art catalysts still require substantial overpotentials of 0.25 to 0.4 V to reach current densities of interest (i.e., 10 mAcm−2 for OER and −3 mA cm−2 for ORR). Therefore, more researches are still needed which can be directed along the following aspects: (1) Better understanding of mechanism for ORR and OER. The theoretical framework presented above shows that this is largely due to scaling relations among reactive intermediates involved in the ORR/OER. Overcoming this limitation requires decoupling the binding energies of different inter mediates-for instance, by stabilizing OOH* with respect to OH*.[11] Although the volcano framework has helped to elucidate this key concept, its implementation will require substantial effort. A deeper understanding of the electrodeelectrolyte interface and the associated kinetics would allow for targeted strategies to design efficient ORR/OER catalysts. (2) Identification of active sites of ORR/OER catalyst. There are generally two strategies to improve the activity of an electrocatalyst: (i) increasing the number of active sites on a given electrode (e.g., through nanostructuring to expose more active sites) or (ii) increasing the intrinsic activity of each active site. Thus, identification of the nature of the active site is important and should be the first step in the design of effi cient bifunctional electrocatalyst. For this reason, the development of in situ characterization techniques is crucial, since they can possibly disclose more key information regarding the intermediate and active sites, therefore provide guidance for subsequent catalyst design. Some techniques such as scanning probe microscopy, ambient pressure X-ray photoelectron spectroscopy and soft X-ray absorption spectroscopy may have the potential to realize the purpose of in situ characterization. (3) Extension from academic studies to industrial production. So far academic studies mainly focus on the performance and stability of metal-air batteries at the catalytic material level. Consideration for industrial production of such catalytic materials is rare. Note that most of the powder catalysts with extremely high performance are nanostructured catalysts, which have a high cost due to the expensive precursors and complicated process. In addition, many such processes can only produce small quantities of product. A scalable process suitable for large scale production of low-cost and efficient catalytic material is needed.


We believe a combination of theoretical and experimental work, together with the use of in situ/in operando characterization techniques will further push the development of highly efficient bifunctional ORR/OER electrocatalyst, and pave the way for commercial application of metal-air battery in the near future.

Acknowledgements The authors appreciate the supports from the National Research Foundation (NRF), Prime Minister’s Office, Singapore, under its Campus for Research Excellence and Technological Enterprise (CREATE) programme. We also acknowledge financial support from the Center for Programmable Materials, Nanyang Technological University, and the academic research fund AcRF tier 2 (M4020246, ARC10/15), Ministry of Education, Singapore.

Conflict of Interest The authors declare no conflict of interest.

Keywords bifunctional catalyst, electrocatalysis, nanostructured materials, oxygen reduction and evolution, rechargeable metal-air batteries Received: February 28, 2017 Revised: April 16, 2017 Published online:

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