Design strategies for developing non-precious metal based bi-functional catalysts for alkaline electrolyte based zinc–air batteries

Chao Han , Weijie Li , Hua-Kun Liu , Shixue Dou and Jiazhao Wang *
Institute for Superconducting and Electronic Materials, AIIM Building, Innovation Campus, University of Wollongong, Squires Way, North Wollongong, NSW2500, Australia. E-mail: jiazhao@uow.edu.au

Received 3rd April 2019 , Accepted 17th May 2019

First published on 20th May 2019


Compared with the current dominant energy storage system (lithium-ion batteries (LIBs)), rechargeable zinc–air batteries (ZABs) with alkaline electrolyte are safer and less expensive, have much higher theoretical volumetric energy density, can be manufactured in ambient air rather than a dry room, and have much higher tolerance to moisture and air during operation. A mature aqueous alkaline electrolyte could also significantly improve safety while minimizing the fabrication cost. Hence, ZABs have great potential to challenge the dominant position of LIBs in the future. Nevertheless, the widespread application of this energy storage system is seriously hindered by the sluggish kinetics of the oxygen reduction (ORR) and evolution reactions (OER) at the liquid–gas–solid phase cathode interface. Therefore, to further promote the development of this technology, the development of low-cost, high-activity catalysts for the OER/ORR has long been recognized as a crucial measure. This paper summarizes the existing strategies that could be used to develop non-precious-metal based, high activity bifunctional OER/ORR catalysts for the alkaline electrolyte based zinc–air system.


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Chao Han

Dr Chao Han received his doctorate degree from the Institute of Superconducting and Electronic Materials (ISEM), University of Wollongong, Australia. His research focuses on controlled synthesis of different nanomaterials using different wet chemical methods, as well as their applications in energy storage and conversion, such as metal–air batteries, solar cells and thermoelectrics.

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Weijie Li

Dr Weijie Li received her master's degree (2012) from Central South University. She obtained her PhD degree from the University of Wollongong, Australia. She is currently a research fellow at the Institute for Superconducting and Electronic Materials (ISEM), University of Wollongong (UOW). Her current research is focused on nanostructured and nanocomposite materials for energy storage (alkaline ion batteries and aqueous batteries) and conversion (photocatalysts, electrocatalysts).

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Hua-Kun Liu

Prof. Hua-Kun Liu is a distinguished professor at the Institute for Superconducting and Electronic Materials, University of Wollongong, Australia, and a fellow of the Australian Academy of Technological Science and Engineering. Her research interests are in Li/Na/K ion batteries, supercapacitors, and catalysts.

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Shixue Dou

Prof. Shixue Dou is a Distinguished Professor and Director of the Institute for Superconducting and Electronic Materials at the Australian Institute of Innovative Materials, University of Wollongong. He received his PhD in chemistry in 1984 from Dalhousie University, Canada. He was elected as a Fellow of the Australian Academy of Technological Science and Engineering in 1994. He was awarded the New Vice-Chancellors Senior Excellence Award in 2008 and a Vice-Chancellor Outstanding Partnership Award in 2012. He is the program leader for Automotive CRC2020. His research interests include energy storage, superconductors and electronic materials. He has supervised 65 PhD students and more than 50 postdoctoral and visiting fellows who have been widely spread across different countries.

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Jiazhao Wang

Prof. Jiazhao Wang received her PhD degree from the University of Wollongong, Australia, in 2003. She is a professor at the Institute for Superconducting and Electronic Materials, Australian Institute of Innovative Materials, University of Wollongong. Her research focuses on the design, synthesis, and characterization of new materials for batteries, including lithium-ion batteries, sodium-ion batteries, lithium–sulfur batteries, and metal–air batteries.


1. Introduction

With the growing demands for electricity use in transportation, industry, and daily life, sources of fossil fuel energy such as coal, natural gas, and petroleum are being depleted more quickly. The development of zero-emission electrical vehicles, high-efficiency smart grids, and green renewable energy sources has been a commonsense approach for humanity to address the energy crisis and fossil-fuel pollution. As one of the core technologies, however, the ongoing research on advanced energy storage systems has always been inadequate for further development in these areas. The current dominant contender in the commercial energy storage market is still non-aqueous lithium-ion batteries (LIBs), which have high energy density and long cycle lives, but are expensive and dangerous because of limited lithium resources and the air sensitivity of electrolyte and lithium, respectively. Compared with LIBs, rechargeable aqueous zinc–air batteries (ZABs) are much safer and lower cost; they can be manufactured in ambient air rather than a dry room, and they have much higher tolerance to moisture and air during operation.1–4 Aqueous electrolytes could be used in ZABs, thereby significantly improving their safety while minimizing their fabrication cost. Moreover, the theoretical volumetric energy density (4400 W h L−1) and specific energy density (180–200 W h kg−1) of ZABs are three times and 1.1 times greater than those of conventional Li-ion batteries (1400 W h L−1 and 160 W h kg−1).5–17Fig. 1 provides a comparison of the specific and volumetric energy densities of ZABs with those of different families of batteries and electrical double layer capacitors (EDLCs).
image file: c9mh00502a-f1.tif
Fig. 1 Comparison of ZABs with different power systems: (a) theoretical/practical/volumetric energy density;14 Copyright 2018, Royal Society of Chemistry. (b) Specific energies versus power densities (Ragone plots), with EDLC representing supercapacitors.15 Copyright 2014, Elsevier Limited.

Table 1 briefly compares the costs and cycle lives of the different kinds of energy storage systems existing on the market, from which it is easy to conclude that ZABs have unmatched low cost compared with the other existing commercial energy-storage systems.

Table 1 Comparison of the technical characteristics of the Zn–air battery and other mature, developed battery energy storage systems.2,21,24,32–40
Zn–air Pb–acid Ni–Cd Li ion
Anode ZnO/Zn Pb Cd Graphite
Cathode Air PbO2 NiOOH LiCoO2
Electrolyte Alkaline Acid Alkaline Organic
Efficiency (%, average) 50 75–90 65–90 >95
W h kg−1 180–200 30–50 10–75 160
W kg−1 200 75–300 150–300 400
Cycle life (C-rate) 200 (0.067C) 200–2500 300–2500 >1000
Storage durability Hours–months Minutes–days Minutes–days Minutes–days
Cost ($ per kW h) 10–400 200–700 400–1500 500–2500
Cost/cycle (cents per kW h per cycle) 3–5 25–107 20–115 15–100


Aqueous alkaline electrolyte based ZABs are a relatively mature technology and hold the greatest promise for future energy applications. Their primary batteries have been known since the late nineteenth century, and commercial products started to emerge in the 1930s.13 With the increasing awareness of the need for environmental protection and the growing popularity of electric vehicles, attempts to use ZABs in electric vehicles began in 1999. Electric Fuel Ltd (EFL) developed a high-energy zinc–air battery system, designed to allow electric vehicles to compete with conventional vehicles in price, performance, convenience, and safety, while offering superior ranges, highway speeds, equivalent cargo capacity, and quick refueling.18–23 Nevertheless, some big obstacles to the widespread application of rechargeable ZABs are their limited energy efficiency and poor cycling stability,24–31 which originate from the irreversible consumption of the zinc anode and sluggish oxygen evolution/reduction reactions (OER/ORR) that take place on the gas–liquid–solid interface of the oxygen electrode in alkaline electrolyte. Hence, the development of high-activity bifunctional (OER/ORR) catalysts has long been recognized as an efficient and necessary measure to enable further flourishing of the rechargeable Zn–air battery market, which has high potential for replacing the lithium ion battery. Although there have been a huge number of review papers on bifunctional OER/ORR catalysts,9,14,17,32–35 we briefly summarize the state of the art strategies that were used to further boost the performance of bifunctional catalysts with alkaline electrolyte.

2. Working principles and mechanisms of ZABs

Generally, typical primary and rechargeable alkaline electrolyte based ZABs are both composed of a zinc anode, alkaline aqueous electrolyte (6 M KOH and 0.2 M Zn(CH3COO)2), and a catalyst containing air electrode. The merits of alkaline electrolyte and other possible electrolyte systems have been well reviewed by Blazquez et al.27 The basic structure and reaction mechanisms of alkaline electrolyte-based rechargeable ZABs are shown in Fig. 2(a).
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Fig. 2 (a) Model and reaction mechanism in ZABs;16 Copyright 2017, Elsevier Limited. (b) Schematic of the ORR (blue path) and OER (brown path) in alkaline solution. (c) ORR (left) and OER (right) volcano plots for metal oxides.17 Copyright 2017, The American Association for the Advancement of Science (AAAS) Publishing Group. (d) Schematic diagram of E10, E1/2, and ΔE during the OER and ORR. E10: potential needed to achieve a current density (J) of 10 mA cm−2 during the OER; E1/2: half-wave potential for the ORR; RHE: reversible hydrogen electrode.

The reactions on each side are as follows:41–44

Anode:

 
Zn(s) + 4OH(aq) → Zn(OH)4(aq)2− + 2e (E0 = −1.25 V vs. SHE)(1)
 
Zn(OH)4(aq)2− → ZnO(s) + 2OH(aq) + H2O(aq)(2)

Cathode:

 
O2(g) + 2H2O(aq) + 4e → 4OH(aq) (E0 = +0.401 V vs. SHE)(3)

Overall reaction:

 
2Zn(s) + O2(g) → 2ZnO(s) (cell potential: 1.65 V)(4)

Side reactions:

 
2KOH(aq) + CO2(g) → K2CO3(s) + H2O(aq)(5)
 
Zn(s) + 2H2O(aq) → Zn(OH)2(aq) + H2(g)(6)
Here, E0 is the potential of the electrochemical reaction, and SHE is the standard hydrogen electrode. The theoretical reversible potential for ZABs is 1.65 V [reaction (4)]. Depending on the applied current density, however, the practical voltage can decrease to below 1.4 V. The origin of this discharge potential drop is largely due to the high overpotential at the air electrode during the ORR (discharge). The charge (OER) potential is usually above 1.6 V. Thus, rechargeable ZABs usually have low round-trip energy efficiencies below 55–65%. The main function of the catalysts on the air electrode is, therefore, to facilitate the OER and ORR during the charge and discharge processes, and thus increase the energy efficiency. By nature, the corresponding discharge (ORR) and charge (OER) reactions at the cathode of alkaline ZABs are interfacial and include a series of complex electron transfer reactions, involving four electron reaction steps. For alkaline ZABs, the detailed ORR/OER procedure is schematically presented in Fig. 2(b), which includes reactions (7)–(10). Generally, the four-electron ORR proceeds through the formation of *OOH from adsorbed O2, followed by its further reduction to *O and *OH, in which * refers to active sites on the catalyst. The OER (charging) takes place just in the opposite sequence, e.g. from (10) to (7).
 
* + O2(g) + H2O(l) + e ↔ *OOH + OH(7)
 
*OOH + e ↔ *O + OH(8)
 
*O + H2O(l) + e ↔ *OH + OH(9)
 
*OH + e ↔ OH + *(10)

The potential for each step in reactions (7)–(10) is determined by the difference in the binding energies of the different intermediates (i.e., *OH, *O, and *OOH) before and after a charge transfer. As demonstrated in Fig. 2(c), currently, the best unifunctional catalysts for the OER and ORR are still the expensive noble metal catalysts Ru/IrO2 and Pt, respectively.4,45 Besides their high prices, the stability of these two noble metal catalysts is also fairly poor, and Pt shows low tolerance towards methanol. Compared with unifunctional ORR/OER electrocatalysts, bifunctional catalysts are more attractive due to their convenience and low cost. The main obstacle, however, to achieving bifunctional catalytic activity towards the OER and ORR in one material lies in the interdependence of the adsorption energies towards OH groups and O2 molecules [reactions (7) and (9), or the Gibbs free energies (ΔG1 and ΔG3) of the first and third reactions]. The scaling relations (interdependence) between the *OH and *OOH binding energies prevent any compound with a single site from being both ORR and OER active.46 Hence, intrinsically, as shown in Fig. 3, to achieve bifunctional activity in one material, it is necessary to achieve a good balance between the formation energies of *OH and *OOH intermediates via the material structure, composition, and design of electronic states.


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Fig. 3 Two pyramids depicting the best achievable potential for the ORR (blue) and the potential of the potential determining step for the OER (green) as functions of the first (ΔG1) and third reaction (ΔG3) free energies. The constraint set by a constant offset of 3.2 eV between *–OH and *–OOH is represented by the red plane. The red plane that cuts the two pyramids creates two separate volcanoes for the OER and the ORR, which are darkened.50 Copyright 2016, Elsevier Limited.

To characterize the bifunctional catalytic activity, the potential difference (ΔE) between the potential for achieving a current density of 10 mA cm−2 during the OER (E10) and the half-wave potential (E1/2) during the ORR has been calculated, which is schematically shown in Fig. 2(d). Table 2 briefly summarizes some typical, high-performance (ΔE < 1.0 V), catalysts that are bifunctional for both the ORR and the OER in alkaline solution, which could be generally divided into the following three groups: transition metals; transition metal compounds, including oxides, sulphides, hydroxides, nitrides and phosphides; and carbon-based materials.14,32,47 Transition metal compounds offer a good foundation for achieving bifunctional results for the following reasons. (1) Transition metals and the corresponding ions have unfilled 3d orbitals, leading to variable and adjustable cationic oxidation states. Since the ORR and OER procedures both involve a redox reaction, catalysts with variable cationic states are predicted to possess good stability. (2) Moreover, variable structures of transition metal compounds provide excellent scaffolds to achieve suitable affinity for oxygen absorption and electronic structures. Moreover, the detailed OER/ORR catalytic mechanisms in alkaline solution for transition metal sulphides, phosphides, and nitrides are still unclear, but some reports have proved that they are all related to the formation of an amorphous oxide layer outside the transition metal compound.48,49

Table 2 Some state-of-the-art bifunctional ORR/OER catalysts in 0.1 M KOH. CNT: carbon nanotube; rGO: reduced graphene oxide
Category Catalyst E 10(OER) (V) E 1/2(ORR) (V) ΔE (V) Ref.
Transition metals Co–N–C 1.54 0.80 0.74 51
(FeCoNi)@N-graphene tube 1.54 0.89 0.65 52
Transition metal oxides CoO 1.56 0.85 0.71 53
Co3O4/N doped graphene 1.60 0.83 0.77 54
NiCo2O4–CNT 1.66 0.81 0.85 55
MnO2/N doped CNT 1.88 1.05 0.83 56
CoV1.5Fe0.5O4 1.57 0.74 0.83 57
Lax(Ba0.5Sr0.5)1−xCo0.8Fe0.2O3−δ 1.60 0.66 0.94 58
Transition metal sulfides NiCo2S4@S-graphene 1.59 0.90 0.69 59
CuS/NiS2 1.62 0.71 0.91 60
Co9S8/N,S doped rGO 1.55 0.79 0.76 61
Transition metal LDH Co(OH)2/N-rGO 1.66 0.81 0.85 62
Transition metal nitrides Ni3FeN 1.50 0.50 1.00 63
Ni3FeN/N-doped rGO 1.63 0.90 0.73 64
Transition metal phosphides Fe0.33CoP@nickel foam 1.54 0.80 0.74 65
Co2P 1.50 0.84 0.66 66
Carbon based materials N doped carbon microtube 1.52 0.89 0.63 67
P,N co-doped graphene 1.55 0.85 0.70 68
Nanoporous carbon fibre 1.81 0.81 1.00 69


Unlike transition metal compounds, carbon based materials also constitute a group of bifunctional catalysts because of their high electrical conductivity, structural variety, and rich heteroatom doping or defect chemistry.

Compared with the benchmark Pt/C and RuO2 systems, whose ΔE is around 0.70 V, most bifunctional catalysts are still unsatisfactory. Generally, increasing the number of active sites while increasing the intrinsic activity of each active site are recognized as the two basic principles for further enhancing catalytic activity for all catalysts. In more detail, this paper focuses on summarizing the most recent progress in detailed strategies to achieve these two objectives for further enhancing the intrinsic OER/ORR activities of non-precious-metal based catalysts.

3. Strategies for further enhancing the OER/ORR activities of non-precious-metal based catalysts

3.1. Control of the ion field intensity of transition metals

Since the adsorption of O2 and OH on catalytically active sites is an electrostatic attraction, adjusting the intensity of the cationic ion field will definitely affect the ORR/OER processes. The ion field intensity is obtained from the ion radius divided by the valence and would be affected by neighboring atoms. Transition metals can lose electrons more readily than other elements because they have unstable electrons in their outer orbitals. Some oxidation states are more common for transition metals than for elements in the main groups. Moreover, the radius of transition metal ions also varies depending on their species and valence. Different anions and neighboring atoms could also have an impact on their oxidation states. Transition metal oxides containing highly oxidized redox couples such as Ir4+/6+, Ru4+/8+, Co3+/4+, Ni3+/4+, Mn3+/4+, and Fe3+/4+ are known as active centers for the OER. The electrochemical performances of oxides for the OER follow the order IrO2 > RuO2 > Co3O4 and Ni-containing cobalt oxides > Fe, Pb, and Mn containing oxides. Among the numerous low-cost oxides investigated, cobalt and manganese oxides are promising for both the OER and the ORR due to their moderate ion field intensity.70

Jaramillo et al. synthesized a novel Mn(III) oxide (Mn3O4) film using the electrodeposition method, which demonstrated excellent bifunctional activity, with its individual ORR and OER activity comparable to those of the best reported metal oxides and even some precious metal materials.71,72In situ X-ray absorption spectroscopy (XAS) confirmed that a disordered MnII,III,III3O4 phase contributes to the ORR, while a mixed MnIII,IV oxide is related to the OER. Hence, simply by tuning the MnIII/MnIV ratio, the OER/ORR catalytic activity is modified. Based on this concept, other similar analogues could also be bifunctional for the OER/ORR once the composition and valence state of the transition metal have been carefully designed. As seen from Fig. 4, Suib's work also proved that Mn in different oxidation states showed different catalytic activities towards the ORR and OER, while the activity was slightly enhanced as the percentage of Mn3+ increased when the calcination temperature was increased to 350 °C.73 Porous, bifunctional NiFeOx-based electrocatalysts were also synthesized using the hydrothermal method followed by calcination in air, while heat treatment at different temperatures resulted in well dispersed nanorod structures with high surface areas and mixed chemical states of Ni ions in the spinel structure. The sample annealed at 250 °C showed the best OER/ORR performance due to its suitable mixed oxidation state of Ni.74


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Fig. 4 Bulky structures of MnO, Mn2O3, and MnO2 (with the purple atoms being Mn and the red ones O); below are the calculated elementary reaction pathways for the dissociation of H2O on different kinds of manganese oxides.73 Copyright 2016, Royal Society of Chemistry.

In general, variation of ion field intensities leads to electrostatic affinity tuning towards intermediates produced during the OER/ORR. Precise control over the surface valence state of a ubiquitous electrocatalyst opens up new avenues in the field of alternative energy applications in terms of offering suitable low-cost and earth-abundant metal oxides. The most widely used strategy for achieving different oxidation states is by hydrothermal or chemical-based methods.

3.2. Morphological engineering

As aforementioned, the OER/ORR is actually an interfacial process, so morphological engineering could significantly affect the catalytic activity. Several strategies could be proposed based on this point.
3.2.1. Creating nanostructures with high surface area using core–shell or porous structures. To further enhance the catalytic activity of bifunctional catalysts, one of the most direct and effective strategies is to expose more active sites by creating nanostructures with high surface area using core–shell or porous structures.75–82 This strategy is universal for enhancing the catalytic activity of almost all catalysts.

As examples, α-MnO2 nanospheres and nanowires were reported to outperform their microparticle counterparts, due to their smaller size and higher specific surface areas.83 Also, Chen et al. reported the synthesis of hierarchical mesoporous Co3O4 nanowire arrays as a highly efficient bifunctional ORR/OER catalyst, unlike other morphologies.78 Under a current density of 50 mA cm−2, a ZAB based on Co3O4 nanowire arrays showed an open circuit voltage of 1 V and a charge potential of 2 V. Remarkable charge and discharge potential retentions (97% and 94%, respectively) were shown even after 100 cycles (nearly a month).78 Qiao et al. reported the synthesis of porous Co3O4 nanowire arrays directly grown on Cu foil, which exhibited 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.84Fig. 5(a and b) displays the morphology and OER/ORR curves of chestnut-like NiCo2O4 spinels (NCO) synthesized using the hydrothermal method. The products showed different morphologies and specific areas with different hydrothermal times, while their NCO-10 sample demonstrated the best OER/ORR performance in 0.1 M KOH.79 Compared with bulk Co3O4, three-dimensional (3D) ordered porous Co3O4 showed obviously improved OER/ORR performance, as presented in Fig. 5(c and d).85 Three-dimensional (3D) ordered porous Co3O4 based ZABs also exhibited higher discharge and lower charge potentials of 1.24 and 2.0 V at 10 mA cm−2, respectively, as well as good durability within 200 cycles. Besides their porous structure, hollow-structured transition metal oxides offer low overpotentials, fast reaction rates, and excellent stability in oxygen related reactions, including the OER/ORR in metal–air battery systems. Recent progress in the oxygen-related catalysis of hollow-structured transition metal oxides has been summarized by Feng et al.86 One of the typical examples is illustrated in Fig. 5(e and g). Bai et al. prepared MnCo2O4.5 nanoframes using carbonization of a Mn doped zeolitic imidazolate framework-67 (ZIF-67) template. Linear sweep voltammetry (LSV) results showed that the hollowed-out MnCo2O4.5 structures exhibited improved OER/ORR performance compared to the MnCo2O4.5 nanocages due to their higher specific surface area.75


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Fig. 5 (a) Morphologies and surface needles of NiCo2O4 formed at various hydrothermal reaction times of 2, 6, 10, and 12 h (NCO-2, NCO-6, NCO-10, and NCO-12, respectively); and (b) the corresponding OER and ORR curves in 0.1 M KOH solution.79 Copyright 2017, John Wiley & Sons Inc. (c) Transmission electron microscopy (TEM) images of 3D porous Co3O4 prepared using a polystyrene template (inset: corresponding selected area electron diffraction (SAED) pattern); and (d) comparison of the OER/ORR performances in 0.1 M KOH solution between the bulk and 3D ordered porous Co3O4.85 Copyright 2016, John Wiley & Sons Inc. (e) Field emission scanning electron microscopy (FE-SEM) image of hollowed-out dodecahedral MnCo2O4.5 nanocages; (f) TEM image of MnCo2O4.5 dodecahedrons; and (g) ORR and OER polarization curves of hollowed-out dodecahedral MnCo2O4.5 nanocages (I), MnCo2O4.5 dodecahedra (II), and ZIF-67 (III) at 1600 rpm in 0.1 M KOH solution.75 Copyright 2018, John Wiley & Sons Inc.
3.2.2. Morphological design to expose more active sites. Usually, different facets in crystals have different arrangements of atoms, leading to differences in ΔG1 ∼ ΔG4 and finally fluctuations in OER/ORR catalytic activity. This point has been proved by considerable efforts on shape-controlled synthesis to tailor the ORR activity of noble Pt metal.87–91 In non-adsorbing electrolytes such as perchloric acid, the ORR activity of low-index facets in single crystalline Pt is known to follow the order (110) > (111) > (100) facets;87 while in adsorbing electrolytes such as sulfuric acid, the (100) facets exhibit higher activity than their (111) counterparts instead, as sulfate anions strongly adsorb onto the (111) facets, blocking the sites.88 Hence, atomic-level-engineering of the surface structure can be used to precisely manipulate the exposure of active sites and subsequently enhance the corresponding electrocatalytic activity.92 He et al. highlighted the facet-dependent electrocatalytic activity of MnO nanocrystals for the OER/ORR in 0.1 M KOH solution. The MnO (100) facets with a higher adsorption energy of O species can greatly promote the electrocatalytic activity [Fig. 6(a–d)].93 Cho et al. developed a micrometer-sized polyhedral bismuth ruthenate pyrochlore (P-BRO) for ZABs, which achieved highly improved catalytic activity by the development of (100), (110), and (111) planes, regardless of particle size. The ZAB based on P-RBO also performed far better than the benchmark Pt/IrO2 system.94 Plausible active sites for the ORR and OER that occur on the polyhedral bismuth ruthenate pyrochlore (P-BRO) were located using a two-dimensional (2D) map of theoretical overpotentials based on two descriptors of the adsorption free energies, ΔGOH* and (ΔGO* − ΔGOH*) [Fig. 6(e)]. It was found that the Bi(3) and Ru(4) surfaces exhibited the best ORR and OER performances with the lowest overpotential, respectively.
image file: c9mh00502a-f6.tif
Fig. 6 (a) TEM images of a MnO single rod (upper) and octahedral nanoparticle (lower). (b) Schematic illustration indicating the growth direction and exposed planes of MnO nanorods and MnO octahedral nanoparticles, where O is red and Mn is yellow. (c) Current density vs. potential for different morphologies. (d) Representative OH* adsorbate species on different MnO lattice surfaces. H is in white, O is in red, and Mn is in yellow.93 Copyright 2015, Royal Society of Chemistry. (e) Density functional theory (DFT) calculation results of the low-index facets on the surface of the P-BRO including (100), (110), and (111) planes during the ORR and OER are, respectively, displayed in the form of 2D maps of theoretical overpotentials (η) as functions of the adsorption free energies (i.e., ΔGO* − ΔGOH* and ΔGOH*).94 Copyright 2018, American Chemical Society.

The traditional methodology for facet-control in nanoparticles is based on the interplay of crystal plane energy and surfactants.95–102 The principle behind this methodology is that certain crystal planes might be preferentially stabilized by strongly binding surfactants over other crystal planes. Thus, the shape of the nanocrystal might be fine-tuned by utilizing various possible combinations of surfactants and different crystal planes. Theoretical analysis of the surfactant–catalyst facet interaction is far behind the experimental work; however, the use of strongly binding surfactants is detrimental to the OER/ORR processes. Lee et al. therefore summarized strategies for achieving facet control in catalytic nanoparticles without using surfactants.103

3.3. Defect engineering

In actual OER/ORR bifunctional nanostructured catalysts, it is impossible to prepare a truly perfect crystal without any defects. Due to the lack of deep understanding of the details of the OER/ORR processes and the necessary atomic characterization techniques, the effects of surface defects (disruptions or imperfections), which could also significantly alter the surface catalytic activity by tuning the surface electronic states, have long been ignored. Recently, more and more researchers have realized that surface defects, mainly in the form of single atom catalysts, heteroatom doping, cationic/anionic vacancies, and interstitial atoms, play more pivotal roles in achieving high catalytic activity than an intrinsically perfect surface.108–113 As shown in Fig. 7(a and b), Yao et al. prepared defect-rich graphene (DG) with various types of defects (pentagons, heptagons, and octagons) by removing N atoms from N-doped graphene (NG) through heat treatment.104 In comparison with perfect graphene, the OER/ORR catalytic activity of their NG sample was significantly improved. Interestingly, unlike previous explanations related to the enhancement of activity by N doping, Yao et al. found that, even without N element, defect-rich graphene (pentagons, heptagons, and octagons) exhibited even higher OER/ORR performance [Fig. 7(c)].104 The ZAB test suggests that the DG sample has very stable charge and discharge voltages, high current and power density, which are comparable to a Pt based ZAB.104 These results gave direct evidence that, as one kind of point defect, although doped N atoms could modify the electronic state of C or act as active sites for the OER/ORR, intrinsic carbon defects, such pentagonal, heptagonal, and octagonal carbon rings, are more efficient at boosting the OER/ORR activity. Moreover, it was also found that the point defect active sites have excellent selectivity towards different reactions.104–106,114 For example, the Fe–N–C single atom catalyst, whose active site is the Fe–N4–C moiety, has been widely studied as an alternative oxygen reduction reaction (ORR) catalyst to Pt.115 Ni@SV (SV: single vacancy) and Ni@DV (DV: double vacancy) were found to be selective for the CO2 reduction reaction (CO2RR) over the hydrogen evolution reaction (HER) at neutral pH, and Ni@DV is also active towards the OER in alkaline media.106,107 Ni@D5775 was identified as the active site for the HER in acidic electrolyte [Fig. 7(d)].106 On account of their effectiveness and selectivity, research on the effect of defects in electrocatalysts has been a hot topic, and typical review papers are blossoming.33,34,104,114,116,117
image file: c9mh00502a-f7.tif
Fig. 7 (a) Schematic illustration of the formation of defect-rich graphene. (b) High-angle annular dark field (HAADF) image of DG with an acceleration voltage of 80 kV. The hexagons, pentagons, heptagons, and octagons are labeled in orange, green, blue, and red, respectively. (c) LSV curves of pristine graphene, NG, and DG for the ORR and OER in different alkaline solutions, respectively.104 Copyright 2016, John Wiley & Sons Inc. (d) Schematic structure of Ni atoms (green balls) coordinated in single-vacancy (SV), double-vacancy (DV), and 5775 (D5775) carbon (gray balls) defects. Ni@SV and Ni@DV are selective for the CO2RR over the HER at neutral pH, and the latter is also active for the OER in alkaline media. Ni@D5775 was identified as the active site for the HER in acidic electrolyte.105–107 Copyright 2018, Elsevier Limited.

Despite the limitations of characterization techniques, however, recent research on defects has been mainly focused on point defects, although the effects of other one- or two-dimensional defects in crystals, including grain boundaries, dislocations, and edges, have still not been clearly stated and investigated.33,34,104,116–119 And for a similar reason, the detailed effects of defects are complex and mostly unclear. However, defects would break the uniform electronic states of perfect crystals, leading to unpredictable electronic coupling. Finally, the electrostatic affinity at the defects towards different intermediates was adjusted.

3.4. Effect of strain

Local lattice distortions or elastic strain in nanocrystal catalysts, which would induce slight changes in atomic–atomic bond lengths and modification of electronic states or atom re-arrangements in the first or second subsurface strain layer, therefore could be seen as an important general strategy to adjust catalytic activity towards the OER/ORR.120 Moreover, the strain in the crystal would increase the energy level of the crystal and lead to instability in the strain-affected area, increasing the reaction activity towards O2 or OH. Generally, surface strain inherently and universally exists in catalysts for two main reasons. First, compared with the inner atoms, the characteristic lower coordination or dangling bonds of surface atoms typically results in the emergence of internal strain in an attempt to minimize the surface energy.121 Second, nanocrystals should not be simply considered as small pieces of a bulk material. Unusual forms of structural disorder may exist and thus induce surface strain.120 Hence, besides defects, external forces, edges, core–shell structures, and fast or controlled growth of nanocrystals could also induce elastic strain on the surfaces or interfaces of catalysts. Correlating induced strain with catalytic performance is of fundamental importance for the design and construction of highly efficient catalytic nanomaterials, and tremendous progress has been made in this area in the past decade.122–124

Previous research on the effects of strain has been mostly on noble metals. An interesting example where the surface strain of Pt nanoparticles was tuned from compressive to tensile was achieved using a common Li-ion battery electrode material (LiCoO2 as the support).124 LiCoO2 undergoes a large volume change when Li ions are repeatedly intercalated and extracted during electrochemical charge and discharge on its surface, leading to compressive and tensile strains for the 5 nm-thick layer of Pt deposited on LiCoO2, respectively.124 As early as 2015, Yang et al., for the first time, found that epitaxial strain can tune the OER/ORR activity of perovskite LaCoO3 in alkaline solutions. They found that moderate tensile strain can further induce changes in the electronic structure, leading to increased catalytic activity towards both the OER and the ORR.

The resultant decrease in charge transfer resistance for movement to the electrolyte, however, reduces the overpotential in the ORR more notably than in the OER. Later, Lee et al. reported strained LaNiO3 (LNO) for enhanced bifunctional ORR/OER catalysis. They used different lattice-mismatched substrates to control the degree of strain from −2.2% to 2.7% [Fig. 8(a)].125,126 To systematically introduce strain into (001) LNO, epitaxial films (10 nm in thickness) were deposited by pulsed laser epitaxy on a range of lattice-mismatched substrates. The authors found that when LaAlO3 (LAO) was used as a substrate, a small strain of −1.2% produced in LaNiO3 can lead to enhanced bifunctional catalytic performance towards the ORR/OER compared to other strained samples and pristine LaNiO3 [Fig. 8(b and c)]. When the ORR and OER activities under an overpotential of 0.4 V are compared [Fig. 8(d)], the bifunctional activity drastically increases with compressive strain. As a result, the LaNiO3 sample with a strain of −1.2% has a bifunctional catalytic activity exceeding that of Pt [Fig. 8(e)]. Interestingly, the authors also found that compressive strain could significantly and simultaneously enhance both the OER and the ORR in 0.1 M KOH solution. Based on density functional theory (DFT) calculations, they ascribed the simultaneous enhancement of the OER and the ORR to the compressive strain-induced splitting of the eg orbitals, which can create orbital asymmetry at the surface and lead to shifts in the d-band center relative to the Fermi level.


image file: c9mh00502a-f8.tif
Fig. 8 (a) Lattice parameters and associated biaxial strain for LNO on various substrates. Polarization curves for the (b) ORR and (c) OER on the strained LNO films. Strain-relaxed (ε ∼ 0%) films grown on LaSrAlO4 (LSAO) (10 nm in film thickness) and on LAO (100 nm in film thickness) as well as Pt films are used for comparison; LSAT, STO, and DSO represent (LaAlO3)0.3(SrAl0.5Ta0.5O3)0.7, SrTiO3, and DyScO3, respectively. (d) Current densities (J) of both reactions at overpotentials of η = 400 mV (ORR = 0.823 V and OER = 1.623 V) increase with compressive strain. (e) Bifunctional η to achieve 30 μA cm−2 current density for both reactions shows that compressed LaNiO3 surpasses Pt.125 Copyright 2016, American Chemical Society.

Generally, both Lee's and Yang's explanations are based on the d-band model and the scaling effect.125–127 In their simplest interpretation, when a d-band transition metal is put under tensile strain, the interatomic spacing of the surface atoms increases, leading to less overlap of the d orbitals and a narrower d-bandwidth. As the number of d electrons remains unchanged, the fractional filling of the d band remains constant, and the central moment of the d band (the d-band center) shifts upward, leading to a strengthening of the adsorbate–surface interaction. Hence the general conclusion is that tensile strain leads to stronger binding towards all reactive intermediates, while compressive strain leads to weaker binding. This explanation, however, sometimes contradicts experimental results. Just recently, as shown in Fig. 9, Peterson et al. built a mechanics-based eigenstress model to rationalize the effect of strain on adsorbate–catalyst bonding.128 This model suggests that the sign of the binding-energy response to strain depends on the coupling of the adsorbate-induced eigenstress with the applied strain. Taking adsorption of CH2 on a Cu(001) surface as an example, if CH2 is adsorbed at the bridge site, tension makes the binding stronger (consistent with conventional explanations motivated by the d-band model), whereas if CH2 is adsorbed at the four-fold hollow sites, tension unexpectedly weakens the binding. They projected the d band onto the two unique types of surface atom in the (110) surface and plotted the responses of these projected d-band centers to strain [Fig. 9(b)]; their deviation cannot be attributed simply to an opposite response of this electronic band to strain at different sites. Instead, strain can make the binding either stronger or weaker, depending on the eigenstress characteristics of the adsorbate on the surface. Briefly speaking, if the intrinsic strain of the catalyst lattice is the same as the strain induced by adsorption of the intermediates, the strain would promote the adsorption, but if otherwise, it hinders the reaction. Therefore the effect of strain is actually more complex than for the simple d band model.


image file: c9mh00502a-f9.tif
Fig. 9 (a) Strain susceptibility of the energy of CH2 binding on a Cu(110) surface on both bridge and four-fold hollow sites. (b) Variations of the center (mean energy of states) of the d orbitals projected onto the top- and second-layer atoms of the undistorted strained slab along different directions. (c) The in-plane tensile strain on surface atoms induced by the presence of the adsorbate at the bridge site. (d) The in-plane compressive strain on surface atoms induced by the presence of the adsorbate at the four-fold site.128 Copyright 2018, Springer Nature Publishing Group.

3.5. Formation of composites

The formation of composites has long been recognized as one important measure to achieve multifunctional performance or combine the merits of different materials. It is natural to entertain the concept of developing bifunctional catalysts from composites. The term “composite”, however, is different from simply mixing different materials together; instead, here the composites are formed by atomic interactions, which would partially alter their electronic structure and thus affect their OER/ORR catalytic activity.47,61,64,124,129–140 Zhang et al. reported the OER/ORR catalytic performance of composites of Co9S8 with different kinds of carbon, including N,S co-doped porous carbon (NSPC) obtained at different annealing temperatures and with different amounts of cobalt precursor, and N,S, co-doped carbon (NSC).61 As shown in Fig. 10(a and b), simply by mixing NSPC with Co9S8 (Co9S8/NSPC), the OER/ORR performance is improved, while the in situ anchored Co9S8/NSPC9-45 sample showed a much lower OER/ORR overpotential (ΔE), indicating that the interaction between carbon and Co9S8 plays a pivotal synergistic role in boosting the OER/ORR performance. More direct evidence is given by Kim et al. in Fig. 10(d–f),139 who calculated the adsorption energy (ΔEad) of O2 on a perovskite Sm0.5Sr0.5CoO3−δ (SSC) and SSC/N doped graphene composite (3DNG). The tendency of charge transfer from 3DNG to O2 leads to an increased bond length of the O2 molecule. The changes in the reaction free energy from single SSC [0.44 eV (endothermic)] to the SSC/3DNG composite [−0.02 eV (exothermic)] resulted in an enhanced OER/ORR performance.
image file: c9mh00502a-f10.tif
Fig. 10 (a) OER polarization curves; (b) ORR polarization curves; and (c) potential gaps (ΔE) of N,S co-doped porous C (NSPC), Co9S8, Co9S8 + NSPC, Co9S8/NSPC, Pt/C, RuO2, and Co9S8/NSPC9–45 (annealed at 900 °C with 45 mg cobalt precursor).61 Copyright 2016, Springer Nature Publishing Group. (d) The model and calculated adsorption energies of O2 on an SSC SSC/3DNG composite. The adsorption energy (ΔEad) and the bond length of O2 are shown at the top of each model. (e) Schematic band diagrams of SSC and the SSC/3DNG composite. The electron transfer from 3DNG enhances the orbital hybridization between Co 3d and O 2p, and enhances the OER activity. (f) The change in the free energy, ΔG, shows that each reaction from SSC to SSC/3DNG is endothermic or exothermic, respectively.139 Copyright 2018, John Wiley & Sons Inc.

Besides modification of the electronic structures, the formation of composites, especially composites with porous carbon materials, also offers excellent mass/electron transfer paths and provide good durability in alkaline solution for catalytic cycles.138,140–143 Moreover, the interface between the different components also has a great influence on the electrochemical activity and selectivity due to their intensive effects towards balancing the adsorption and desorption of the intermediates on the catalyst, as well as the transportation of intermediates, electrons, or adsorbents due to the strain effect originating from lattice mismatch.144–146 Hence, the formation of composites has great importance in boosting the OER/ORR catalytic activity.

Moreover, it is well known that the electronic structure of a material is highly related to its structure and composition. Composition adjustment in catalysts has long been one of the traditional strategies to modify their catalytic activity.

To replace noble metal catalysts, bimetallic nanoalloys of transition metals with carbon supports are one important group of ideal materials due to their modified adsorption energies towards OH and O2. NiCo-based electrocatalysts exhibit promising ORR/OER activity, as the carbon supports prevent the aggregation of nanoalloy particles, although their ORR performance is still unsatisfactory.142,147 NiFe-based nanoalloys, on the other hand, show good OER performance only in alkaline solution.148 Based on these results, Cho et al. synthesized ternary NiCoFe by pyrolyzation of Fe, Co, and Ni metallocene precursors at 400 °C and 100 bar.149 The in situ X-ray absorption (XAS) and DFT calculations have proved that Co and Fe atoms are the active sites of the ORR and the OER, respectively, while Ni element enhances the conductivity of the catalyst. Interestingly, single element Co and Fe have poor ORR and OER catalytic activity, indicating that alloying has changed their electronic structures. Another example was reported in 2015 by Johnston et al.150 A series of crystalline Ag–Cu nanoalloy particles with an average size of 2.58 nm and different compositions were deposited on nickel foam with the help of a laser. Compared with pure Ag or Cu metal, alloyed Ag50Cu50 and Ag25Cu75 nanoalloy catalysts exhibited the best ORR and OER catalytic activity in alkaline solution, respectively [Fig. 11(a and b)]. DFT calculations proved that the d-band center of Ag12Cu is much closer to the Fermi energy level than in the pure Ag13 clusters, while the d-band center of Ag50Cu50 is closer to the Fermi energy level than that of pure Ag, as demonstrated in Fig. 11(c and d). The O2 adsorption energy increased from −0.86 eV in pure Ag13 clusters to −1.36 eV in the Ag12Cu clusters (Cu-shell). Therefore, it can be inferred that alloying Cu into Ag–Cu nanoparticles has thermodynamic benefits for the O2 adsorption via electronic effects. The d-band center also has an influence on the adsorption of OH and the ORR performance.150 The corresponding ZAB cycling measurements show the Ag50Cu50 catalyst exhibited a maximum power density of approximately 86.3 mW cm−2 and an acceptable cell voltage of 0.863 V for current densities up to 100 mA cm−2. The round-trip efficiency reached 50% at a current density of 20 mA cm−2.150 Besides individual transition metal elements, doping a different element into other transition metal compounds also offers possibilities for further enhancing their bifunctional catalytic activity.65,81,123,151–153


image file: c9mh00502a-f11.tif
Fig. 11 (a) ORR and (b) OER polarization curves of Ag and Ag90Cu10, Ag75Cu25, Ag50Cu50, and Ag25Cu75 nanoalloys in 0.1 M KOH solution at 1600 rpm. (c) d-Projected densities of states for the Ag13, Ag12Cu (Cu-core), and Ag12Cu (Cu-shell) structures. (d) Valence band spectra (VBS) of Ag and the Ag50Cu50 alloy.150 Copyright 2015, American Chemical Society.

4. Conclusions and remarks

In general, it is much easier to deal with secondary zinc–air batteries (ZABs) relative to other metal–air batteries such as Li–air batteries.154 All the components of alkaline electrolyte based ZABs are moderately stable towards moisture, and all the reactions can be carried out under ambient air conditions. Therefore, the manufacturing process for ZABs is less stringent and cheaper than that for Li–air batteries. Therefore, as an important and relatively mature technology, ZABs hold the greatest promise for future energy storage applications. Despite their early start and great potential, however, the wide range of applications of alkaline electrolyte based ZABs has been impeded by problems such as the unavoidable corrosion of the zinc anode, the volatile and corrosive nature of the alkaline electrolyte, and expensive, poor performance OER/ORR catalysts. The former two problems, however, could be simply overcome by the concept of mechanically rechargeable batteries in real applications, where the zinc electrode and electrolyte are physically removed and replaced,13 and solid/quasi-solid electrolyte techniques.11,12,136 Hence, the ultimate hindrance to this excellent energy storage technique has become the development of efficient and low-cost bifunctional catalysts that could significantly decrease the overpotential between charge and discharge to increase the energy utilization efficiency. Although in recent years, many high-performance non-precious-metal based bifunctional (OER/ORR) catalysts have been developed through engineering their chemical compositions, their structures, or their interaction with carbon supports, their catalytic activity are still unsatisfactory. This paper has summarized the most recent progress in detailed strategies to significantly enhance the intrinsic OER/ORR activities of non-precious-metal based catalysts.

(1) Although the OER/ORR mechanism has long been investigated, several issues are still ambiguous. For example, what is the main factor that affects the O2 adsorption mode, which finally leads to the different two- and four-electron ORR modes? The effect of the amorphous hydroxide layer that is formed at the initial stage during the OER/ORR needs to be further confirmed, while the effects of the bulk lattice (including structure, composition) on the formation of the amorphous layer also need further investigations. Moreover, the catalytically active sites need to be identified in some bifunctional catalysts, such as heteroatom-doped carbon and the interfaces of composites. Is there any other possible way to break the scaling effect by decoupling the binding energies of different intermediates during the OER/ORR procedures – for instance, by stabilizing OOH* with respect to OH*?155 It is believed that with the development of operando characterization techniques, the OER/ORR procedures would be revealed more clearly, and this would definitely promote better catalytic activity towards the OER and ORR.

(2) There are generally two strategies to improve the activity of an electrocatalyst: (i) increasing the number of active sites on a given electrode and (ii) increasing the intrinsic activity of each active site. Among all the summarized strategies, the effects of defects and strain have been the two most active directions for further enhancing bifunctional catalytic activity. Further elucidation of the relationship between different defect structures and electronic structures would offer an excellent scaffold for further boosting their bifurcated catalytic activity towards the OER/ORR. Theoretical calculations can further guide researchers to design defect-rich nanocatalysts through prediction of the accurate electronic structures of various materials. By combining the rapid development of various advanced characterization methods, experimental data and theoretical calculations, the positive effects of defects can be studied more clearly, and novel types of defects other than point defects and their corresponding effects on the OER/ORR may be discovered. An establishment of a database that shows the corresponding relations between defect types and their effects on catalytic activity is of great importance for the development of catalysts.

(3) The d-band model could explain most of the effects of electronic structure tuning, but not fully explain the strain-induced changes in the adsorption properties. Instead, the new theory that the strain effect could break the scaling relationship between the OER and the ORR provides new explanations. Nevertheless, precise quantification of the induced strain requires knowledge of the 3D positions of atoms in nanocatalysts with high accuracy, which is not easy to achieve with currently used techniques. In addition, the stability of strain during practical electrocatalytic processes also needs further investigation.

(4) The utilization of each single strategy always has several effects; for example, heteroatom doping on the surface of a catalyst usually modifies the electronic structure due to the composition adjustment, while introducing point defects and local strains. Hence, the enhancement of catalytic activity towards the OER/ORR is actually a synergistic effect of several strategies in most cases. Besides the stated strategies, any method that could alter atom configurations to induce electronic structure fluctuations could possibly balance the catalytic activity between the OER and ORR to achieve promotion of bifunctional catalytic activity.

(5) Since the OER/ORR procedures in ZABs are tri-phase reactions in nature, effective access among the liquid electrolyte, the gas phase of oxygen and the solid catalysts is also important for improving catalytic activity. Therefore, the adjustment of hydrophobicity/hydrophilicity using different ligands could also significantly affect the catalytic activity. Moreover, surface ligands may also alter the surface electronic states and thus the intrinsic catalytic activity. Although there are no relevant reports in this field in the current stage, it is highly believed this will be a hot topic in the near future.

We believe that the combination of a series of theoretical and experimental studies, together with the use of various operando characterization techniques, will further advance the development of highly efficient non-precious-metal based bifunctional ORR/OER electrocatalysts, and pave the way for commercial application of alkaline based zinc–air batteries in the near future.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Australian Research Council (ARC) through a Discovery Project (DP180101453). Dr W. Li wants to acknowledge the support from the Discovery Early Career Researcher Award via DE180101478. We would like to thank Dr Tania Silver for polishing the manuscript. The authors also want to thank the Institute for Superconducting and Electronic Materials (ISEM) at the University of Wollongong (UOW) for its support.

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