Rechargeable Metal-air Batteries Research Articles

Recent Advances in Rechargeable Zn-Air Batteries

Rechargeable Zn-air batteries are considered to be an effective energy storage device due to their high energy density, environmental friendliness, and long operating life. Further progress on rechargeable Zn-air batteries with high energy density/power density is greatly needed to satisfy the increasing energy conversion and storage demands. This review summarizes the strategies proposed so far to pursue high-efficiency Zn-air batteries, including the aspects of the electrocatalysts (from noble metals to non-noble metals), the electrode chemistry (from the oxygen evolution reaction to the organic oxidation reaction), electrode engineering (from powdery to free-standing), aqueous electrolytes (from alkaline to non-alkaline) and the battery configuration (from liquid to flexible). An essential evaluation of electrochemistry is highlighted to solve the challenges in boosting the efficiency of rechargeable metal-air batteries. In the end, the perspective on current challenges and future research directions to promote the industrial application of rechargeable Zn-air batteries is provided.

High-Entropy Prussian Blue Analogue Derived Heterostructure Nanoparticles as Bifunctional Oxygen Conversion Electrocatalysts for the Rechargeable Zinc-Air Battery.

In response to energy challenges, rechargeable zinc-air batteries (RZABs) serve as an ideal platform for energy storage with a high energy density and safety. Nevertheless, addressing the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in RZAB requires highly active and robust electrocatalysts. High-entropy Prussian blue analogues (HEPBAs), formed by mixing diverse metals within a single lattice, exhibit enhanced stability due to their increased mixing entropy, which lowers the Gibbs free energy. HEPBAs innately enable sacrificial templating, an effective way to synthesize complex structures. Impressively, in this study, we successfully transform HEPBAs into exquisite multiphase (multimetallic alloy, metal carbide, and metal oxide) heterostructure nanoparticles through a controlled synthesis process. The elusive multiphase heterostructure nanoparticles manifested two active sites for selective ORR and OER. By integrating CNT into HEPBA-derived nanoparticles (HEPBA/CNT-800), the HEPBA/CNT-800 demonstrates superior activity toward both ORR (E1/2 = 0.77 V) in a 0.1 M KOH solution and the OER (η = 330 mV at 50 mA cm-2) in a 1 M KOH solution. The RZAB with a HEPBA/CNT-based air electrode demonstrated an open-circuit voltage of 1.39 V and provided a significant energy density of 71 mW cm-2. Moreover, the charge and discharge cycles lasting up to 40 h at a current density of 5 mA cm-2 demonstrate its excellent stability. This work provides an alternative avenue for the rational design of HEPBA's derivative for a sustainable rechargeable metal-air battery platform.

Effect of cobalt-doped iron oxide as an electrocatalyst for water splitting and photocatalytic hydrogen evolution

Developing cost-effective and efficient electrocatalysts for the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is a significant challenge for economically viable energy conversion devices, such as electrolyzers, rechargeable metal-air batteries, and regenerative fuel cells. Here, we have synthesized cobalt doped iron oxides (CoFe2O3) with different Co:Fe wt%. Among them, Co[1:3]Fe2O3 catalyst demonstrated outstanding performance and stability positioning it as a standard electrocatalyst for OER and HER, and as a photocatalyst for H2 production. The catalysts were characterized using various techniques, including field emission scanning electron microscopy (FESEM), X-ray diffraction spectroscopy (XRD), and energy dispersive X-ray spectroscopy (EDS) demonstrating successful application in water electrolysis. Specifically, the onset potentials for OER and HER were 1.63 V/RHE and −0.23 V/RHE, respectively. The overpotential (η) required to achieve 10 mA/cm2 for OER (in alkaline medium) and HER (in acidic medium) was 0.44 and −0.3 V/RHE, respectively. Additionally, the developed OER and HER catalyst exhibited high current and potential stability over a 13-hour period. This approach is considered a promising avenue for making water electrolysis for hydrogen (H2) energy viable by minimizing the energy requirements of the electrolytic process. Similarly, the performance of the fabricated catalyst was also examined for the photocatalytic generation of H2 gas in a photo-reactor. Where the highest generation rate of 97.77 mL. (g.min−1) was recorded for Co[1:3]Fe2O3 as a photocatalyst. As a result, the Co[1:3]Fe2O3 catalyst has the potential to function as both a photocatalyst and an electrocatalyst for water splitting.

In Situ Construction of Perovskite Pr0.5Ba0.5Mn0.8Co0.1Ru0.1O2.5+δ/CoRu Nanoparticles with Co-N-C Composite Enabling Efficient Bifunctional Electrocatalyst for Zinc-Air Batteries.

Bifunctional catalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) are essential components of rechargeable zinc-air batteries. In this study, we synthesized a Pr0.5Ba0.5Mn0.8Co0.1Ru0.1O2.5+δ (PBMCRO) perovskite composite with in situ exsolved CoRu nanoparticles and Co-N-C, functioning as an efficient bifunctional electrocatalyst for zinc-air batteries. The in situ exsolution of CoRu nanoparticles from the perovskite oxide was facilitated by the reducing action of 2-methylimidazole (2-MIM). Concurrently, Co-N-C was used to decorate PBMCRO, forming a novel bifunctional composite electrode of Co-N-C-PBMCRO. The incorporation of CoRu nanoparticles introduces a significant number of electrochemically active oxygen vacancies in the perovskite matrix, enhancing ORR and OER performance. Additionally, the Co-N-C synergistically improves electrochemical activity while preserving the structural stability of the perovskite oxide. The prepared Co-N-C-PBMCRO catalyst demonstrates significantly enhanced bifunctional performance compared to the undecorated pristine perovskite Pr0.5Ba0.5MnO3-δ (PBMO). The zinc-air battery with Co-N-C-PBMCRO catalyst achieve a peak power density of approximately 90 mW/cm2 and exhibit remarkable cycling stability for 788 h. This study presents a novel and effective strategy to enhance the catalytic performance of perovskite-based air electrodes for rechargeable metal-air batteries.

(Invited) Graphene-Supported Fe/Ni Single Atoms and Feni Alloy Nanoparticles as Bifunctional Oxygen Electrocatalysts for Rechargeable Zinc-Air Batteries

To alleviate the severe energy crisis and environmental problems caused by the massive and unsustainable consumption of fossil fuels, it is urgent to develop renewable energy conversion and energy storage technologies to reform the traditional energy structure. Batteries are considered as one of the most promising energy storage technologies due to their sustainability, high efficiency and less pollution. Among various battery storage devices, rechargeable zinc-air batteries (ZABs) have attracted widespread attention on account of their high theoretical energy density (1086 Wh kg– 1), low cost, abundant zinc resources, low operating temperature, and good stability and safety in aqueous environments. However, the kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) occurred on the air electrode during the discharging and charging processes are extremely sluggish, leading to low energy efficiency, which significantly limits the large-scale application of rechargeable ZABs. Pt-based electrocatalysts exhibit high ORR activities, while Ru- and Ir-based oxides are excellent OER electrocatalysts, but natural scarcity, high cost and poor durability seriously reduce the marketability and commercial competitiveness of rechargeable ZABs. Therefore, the development of robust, cost-effective and high-efficiency non-noble metal bifunctional ORR/OER electrocatalysts is of crucial significance for the practical application of rechargeable ZABs.As a promising alternative, earth-abundant transition metal (Fe, Co, Ni, etc.)-based materials have been extensively studied due to their relatively high electrocatalytic activity and durability. Thereinto, transition metal and nitrogen co-doped carbon (M-NC), especially Fe-NC, has been demonstrated to be the most effective ORR catalyst, and the active sites are generally considered as atomically dispersed metal coordinated to N atoms (M-Nx sites, x indicates the number of N atoms coordinating the metal). Nevertheless, M-NC catalysts only possess good ORR activity with poor OER performance. Bimetallic FeNi nanocomposites (metals, oxides, carbides, sulfides) are promising for OER. At high potentials, highly active FeNi oxyhydroxide species are generated on the surface of the FeNi-based electrocatalysts. Since ORR and OER depend on different catalytic active sites, an ideal bifunctional oxygen electrocatalysts should integrate M-NC complex with high ORR activity and bimetallic FeNi nanocomposite with a high OER activity to achieve bifunctionality. However, due to the intense competition between transition-metal single atoms and nanocomposites in the preparation process, it is challenging to combine them properly for superior bifunctional ORR/OER catalytic activity.Herein, we report a bifunctional oxygen electrocatalyst consisting of ZIF-derived carbon-anchored Fe/Ni single atoms and FeNi alloy nanoparticles; meanwhile, graphene is also introduced as carbon support to improve the uniformity of active sites (denoted as Fe/Ni-NC||FeNi@G). Benefiting from this unique composition and structure, Fe/Ni-NC||FeNi@G electrocatalyst displays excellent bifunctional ORR/OER activity and durability in rechargeable ZABs. This work provides an effective approach to designing highly efficient non-noble metal multifunctional catalysts for practical applications in rechargeable metal-air batteries. Acknowledgements Thanks for the financial support from the Shenzhen Science and Technology Innovation Committee (JCYJ20180507183818040).

Manganese Oxide Gas Diffusion Electrode Conditioning and Electrochemical Doping for Reversible Oxygen Reduction/Evolution Reactions

Developing durable and active bifunctional gas diffusion electrodes for reversible oxygen reduction/evolution (ORR/OER) reaction catalysis is crucial yet challenging in the development of numerous electrochemical energy conversion systems, including regenerative fuel cells and rechargeable metal-air batteries. This research is focused on developing manganese oxide-based gas diffusion electrodes for reversible oxygen electrodes.Core-porous shell Mn/Mn3O4 (abbreviated as MnOx) nanoparticles (QSI) demonstrated increased ORR/OER activities compared to γ-MnO2 (EMD) and b-MnO2.[1] The unique behaviour of core-shell MnOx is attributed to the charge transfer between the low-valent Mn core and the high-valent Mn3O4 shell, thereby regenerating Mn(III) active sites through comproportionation.[1]The stability of transition-metal-based reversible oxygen electrodes remains a challenge. As shown in Figure 1, the deterioration of Mn/Mn3O4 during ORR and OER cycling can be attributed to three aspects: Mn active sites oxidation (Figure 1a), less active MnOx phase formation (Figure 1b), and material loss (carbon corrosion and dissolved MnO4 - formation) (Figure 1c).[2] As a result, the QSI MnOx electrode transformed from pristine to inactive status within 15 galvanostatic ORR/OER cycles. To improve the stability of the electrode, a set of in-situ electrochemical conditioning and electric field-driven metallic cation incorporation into MnOx GDE was investigated. Among the explored metal cations, the Ni2+ incorporation showed the most significant improvement in terms of activity and cyclic stability. The Ni-MnOx GDE demonstrated excellent stability for over 120 galvanostatic cycles at ±10 mA cm-2 in O2-saturated KOH without losing OER activity, surpassing the Pt/C-IrO2 benchmark.[3]These findings position the MnOx electrodes as promising and sustainable alternatives to precious metal-based electrodes for ORR/OER catalysis and provide valuable insights into designing reversible oxygen electrodes for energy conversion and storage in alkaline systems, contributing to a green future.[1] Pei, Y., Wilkinson, D. P., & Gyenge, E. L. Insights into the electrochemical behavior of manganese oxides as catalysts for the oxygen reduction and evolution reactions: monometallic core‐shell Mn/Mn3O4. Small, 2023, 19, 2204585.[2] Pei, Y., Wilkinson, D. P., & Gyenge, E. L. Gas Diffusion Electrode Design and Conditioning with Manganese (III/IV) Oxide Catalyst for Reversible Oxygen Reduction/Evolution Reactions. ACS Energy & Fuels (2023), 37, 23, 19278–19291.[3] Pei, Y., Wu, W. Y., Wilkinson, D. P., & Gyenge, E. L. High-Performance Reversible Oxygen Reduction/Evolution Gas Diffusion Electrodes with Multivalent Cation Doped Core-Shell Mn/Mn3O4 Catalysts. ChemElectroChem, in press. Figure 1

Acidic water oxidation with ultralow 131 mV overpotential over surface-reconstructed RuIr nanoalloy: Chloride-driven high performance

The oxygen evolution reaction (OER) in kinetics is sluggish but a key electrode reaction for various energy storage and conversion devices, such as green hydrogen from water electrolysis, rechargeable metal-air battery, sustainable carbon dioxide electroreduction, synthetic ammonia, etc. However, it still remains a major and cutting-edge challenge to pioneering catalysts with simultaneously ultrahigh activity and stability for OER, even with diverse performance enhancement strategies, such as complex composition designs, surface chemical reconstructions, and multiphase engineering have been implemented to accelerate this key electrochemical process. Herein, we proposed a chloride-triggered activation and stabilization strategy for the branched RuIr alloy coated by thin IrO2 layer to achieve unparalleled high-performance toward OER in acidic water. The ultralow overpotential of the activated catalyst is reduced to about 131 mV at 10 mA cm−2, with 79-fold enhancement of mass-activity than commercial IrO2. Moreover, a modified proton exchange membrane water electrolysis (PEMWE) device was first constructed by introducing NaCl into recyclable electrolyte. It run a stably and low cell potential (1.514 V) for 170 h, having no obvious performance decay at 50 mA cm−2 in 0.5 M H2SO4 with 1.6 M NaCl, which far exceeding traditional IrO2||Pt/C PEMWE. The outstanding performance stems from that Cl- ions enable a faster lattice oxygen evolution mechanism. The novel catalyst activates asymmetric O-Ir-Cl structure induced by Cl- filled oxygen vacancies, which modify the electronic and geometric properties, improving OER activity. Moreover, these adsorbed ions can also efficiently patch and refill the oxygen vacancies that ensure the long-term structure stability. Therefore, tailoring the chloride-activated Ir-based catalysts with simultaneously improved activity and stability may be a valuable guide to achieve surprisingly high-performances for acidic water splitting.

Interfacial nanoparticles of Co2P/Co3Fe7 encapsulated in N-doped carbon nanotubes as bifunctional oxygen electrocatalysts for rechargeable zinc-air batteries

Heterostructures have been generally proven beneficial to the electrocatalysis performance due to tailorable electronic structures. Herein, we develop a facile one-step strategy to synthesize Co2P/Co3Fe7 heterointerfacial nanoparticles encapsulated within N-doped carbon nanotubes (Co2P/Co3Fe7@N–C). The electrocatalyst exhibits a bifunctional oxygen evolution/reduction reaction (OER/ORR) activity (ΔE = 0.669 V) that far exceeds that of commercial Pt/C + Ir/C catalysts (ΔE = 0.76 V). In this study, Co2P/Co3Fe7@N–C-based zinc air batteries (ZABs) exhibit a higher power density (151.6 mW/cm2) and longer stability (1600 cycles) than commercial Pt/C + Ir/C-based ZABs (80 mW/cm2, 450 cycles). Density functional theory calculations reveal that the interfacial electronic interaction between Co2P and Co3Fe7 play key roles in boosting bifunctional ORR/OER efficiency. This work may open a new pathway for the rational design of superior bifunctional oxygen electrocatalysts for rechargeable metal-air batteries through interfacial engineering.

Transition Metal-Based Polyoxometalates for Oxygen Electrode Bifunctional Electrocatalysis

Polyoxometalates (POMs) with transition metals (Co, Cu, Fe, Mn, Ni) of Keggin structure and lamellar-stacked multi-layer morphology were synthesized. They were subsequently explored as bifunctional electrocatalysts for oxygen electrodes, i.e., oxygen reduction (ORR) and evolution (OER) reaction, for aqueous rechargeable metal-air batteries in alkaline media. The lowest Tafel slope (85 mV dec−1) value and the highest OER current density of 93.8 mA cm−2 were obtained for the Fe-POM electrocatalyst. Similar OER electrochemical catalytic activity was noticed for the Co-POM electrocatalyst. This behavior was confirmed by electrochemical impedance spectroscopy, where Fe-POM gave the lowest charge transfer resistance of 3.35 Ω, followed by Co-POM with Rct of 15.04 Ω, during the OER. Additionally, Tafel slope values of 85 and 109 mV dec−1 were calculated for Fe-POM and Co-POM, respectively, during the ORR. The ORR at Fe-POM proceeded by mixed two- and four-electron pathways, while ORR at Co-POM proceeded exclusively by the four-electron pathway. Finally, capacitance studies were conducted on the synthesized POMs.

Cobalt@Ruthenium Core@Shell nanoparticles embedded within nitrogen-doped carbon nanosheets as reversible oxygen electrocatalysts

Rational design and engineering of cost-effective, high-performance reversible oxygen electrocatalysts for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) is imperative in advancing the progress of rechargeable metal-air batteries (r-MABs). Herein, nanocomposites based on Co@Ru core@shell nanoparticles embedded within N-doped carbon nanosheets (Co@Ru/CS) are prepared via facile galvanic exchange reactions of RuCl3 with Co/NC and used as an effective oxygen electrocatalyst for rechargeable zinc-air battery (r-ZAB). Electrochemical studies demonstrate a remarkable bifunctional catalytic performance of Co@Ru/CS towards both ORR and OER, featuring a low potential gap (ΔE) of only 0.69 V between the OER potential (E10,OER) at 10 mA cm−2 and half-wave potential (E1/2,ORR) of ORR, which is much lower than that of commercial Pt/C + RuO2 catalysts (0.76 V). Combined studies of experimental characterizations and density functional theory calculations show that the ORR activity arises primarily from the N-doped carbon and CoNx moieties in the composites, whereas RuO2/CoOOH produced at high electrode potentials is responsible for the OER activity. Co@Ru/CS based r-ZAB exhibits an open circuit voltage of 1.447 V, specific capacity of 781 mAh gZn−1, and maximum power density of 115 mW cm−2 at 0.83 V, a performance better than that with commercial Pt/C + RuO2 (1.412 V, 760.56 mAh gZn−1, and 91 mW cm−2). Results from this research underline the substantial impact of structural engineering on optimizing the electrocatalytic activity of nanocomposites for r-MABs.

Intercalated and Surface-Adsorbed Phosphate Anions in NiFe Layered Double-Hydroxide Catalysts Synergistically Enhancing Oxygen Evolution Reaction Activity.

The oxygen evolution reaction (OER), a crucial semireaction in water electrolysis and rechargeable metal-air batteries, is vital for carbon neutrality. Hindered by a slow proton-coupled electron transfer, an efficient catalyst activating the formation of an O-H bond is essential. Here, we proposed a straightforward one-step hydrothermal procedure for fabricating PO43--modified NiFe layered double-hydroxide (NiFe LDH) catalysts and investigated the role of PO43- anions in enhancing OER. Phosphate amounts can efficiently regulate LDH morphology, crystallinity, composition, and electronic configuration. The optimized sample showed a low overpotential of 267 mV at 10 mA cm-2. Density functional theory calculations revealed that intercalated and surface-adsorbed PO43- anions in NiFe LDH reduced the Gibbs free energy in the rate-determining step of *OOH formation, balancing oxygen-containing intermediate adsorption/dissociation and promoting the OER. Intercalated phosphate ions accelerated precatalyst dehydrogenation kinetics, leading to a rapid reconstruction into active NiFe oxyhydroxide species. Surface-adsorbed PO43- interacted favorably with adsorbed *OOH on the active Ni sites, stabilizing *OOH. Overall, the synergistic effects of intercalated and surface-adsorbed PO43- anions significantly contributed to enhanced OER activity. Achieving optimal catalytic activity requires a delicate equilibrium between thermodynamic and kinetic factors by meticulously regulating the quantity of introduced PO43- ions. This endeavor will facilitate a deeper comprehension of the influence of anions in electrocatalysis for OER.

First‐Principles Investigations on Effects of B‐Site Substitution (B═Mn, Fe, and Co) on La‐Based Perovskite Oxides As Bifunctional Electrocatalysts for Rechargeable Metal–Air Batteries

AbstractThe effects of B‐site substitution (B═Mn, Fe, and Co) in La‐based perovskite oxides (LPOs); LaMnO3, LaFeO3, LaCoO3, as bifunctional electrocatalysts during oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in metal–air batteries (MABs) under an alkaline electrolyte (pH = 13) are investigated using density functional theory (DFT). It is found that LaMnO3 exhibits higher ORR activity than others with ORR overpotential (ηORR) of 0.57 V, but its OER activity is poor with OER overpotential (ηOER) of 1.12 V. The ηORR (0.59 V) and ηOER (1.13 V) of LaMn0.75Fe0.25O3 closely resemble those of LaMnO3, suggesting that Fe substitution does not yield appreciable enhancements in activity. Fe substitution reduces the ORR and OER activity because the adsorption energies of intermediate species on Fe‐substituted LPOs surfaces are too strong to obtain a potential determining step for ORR and OER. According to Sabatier's principle, the LaMn0.25Co0.75O3 demonstrates superior OER activity compared to the other composition, while ORR activity approximates that of LaMnO3, evidenced by ηORR of 0.65 V and ηOER of 0.53 V. The Co‐terminated LaMn0.25Co0.75O3 shows bifunctional activity higher than Mn/Co termination, indicating that Co is an active site for OER and Mn is a promoter for improved ORR activity.

Theoretical regulating the M-site composition of Janus MXenes enables the tailoring design for highly active bifunctional ORR/OER catalysts

Rechargeable metal-air batteries generally require efficient, durable, and safe bifunctional electrocatalysts to simultaneously support oxygen reduction/evolution reactions (ORR/OER). Herein, we employed first-principles calculations to explore the structure-activity relationship between the composition control of metal atoms and the catalytic activity of Pt-doped Ti2–xMnxCO2 single-atom catalysts (SACs). The research found a clear linear relationship between the proportion of Mn and bifunctional performance, which can effectively modulate catalytic activity. Additionally, it shows excellent bifunctional catalytic activity at medium concentrations, among which the catalyst of Pt-VO-Ti0.89Mn1.11CO2 displays the lowest overpotential (ηORR/OER = 0.26/0.28 V). Attributed to the modulation of the average magnetism of Mn and the d-band center of Pt by different components, the bonding strength of the active center of Pt to adsorption intermediates is changed, resulting in the enhancement of the catalyst activity. Crucially, the molecular orbital-level bonding between the active site Pt and the adsorbed intermediate OH is clarified, shedding light on the involvement of the partially occupied antibonding state of Pt's d orbital in the activation process. The research extensively explores the control of catalyst activity through composition, offering strong support for designing and optimizing highly active Janus MXene-supported SACs.

Earthworm-Inspired Co/Co3O4/CoF2@NSC Nanofibrous Electrocatalyst with Confined Channels for Enhanced ORR/OER Performance.

The rational construction of highly active and durable oxygen-reactive electrocatalysts for oxygen reduction/evolution reaction (ORR/OER) plays a critical role in rechargeable metal-air batteries. It is pivotal to achieve optimal utilization of electrocatalytically active sites and valid control of the high specific internal surface area. Inspiration for designing electrocatalysts can come from nature, as it is full of precisely manipulated and highly efficient structures. Herein, inspired by earthworms fertilizing soil, a 3D carbon nanofibrous electrocatalyst with multiple interconnected nanoconfined channels, cobalt-based heterojunction active particles and enriched N, S heteroatoms (Co/Co3O4/CoF2@NSC with confined channels) is rationally designed, showing superior bifunctional electrocatalytic activity in alkaline electrolyte, even outperforming that of benchmark Pt/C-RuO2 catalyst. This work demonstrates a new method for porous structural regulation, in which the internal confined channels within the nanofibers are controllably formed by the spontaneous migration of cobalt-based nanoparticles under a CO2 atmosphere. Theoretical analysis reveals that constructing Co/Co3O4/CoF2@NSC electrocatalyst with confined channels can greatly adjust the electron distribution, effectively lower the reaction barrier of inter-mediate and reduce the OER/ORR overpotential. This work introduces a novel and nature-inspired strategy for designing efficient bifunctional electrocatalysts with well-designed architectures.

Optimizing Oxygen Electrode Bifunctionality with Platinum and Nickel Nanoparticle-Decorated Nitrogen-Doped Binary Metal Oxides

Developing bifunctional oxygen electrode materials with superior activity for oxygen reduction (ORR) and oxygen evolution (OER) reactions is essential for advancing regenerative fuel cell and rechargeable metal–air battery technologies. This present work deals with the synthesis and characterization of electrocatalysts containing Pt and Ni nanoparticles supported on nitrogen-doped mixed metal oxides (Mn2O3-NiO) and the systematic evaluation of their bifunctional ORR/OER performance in an alkaline medium. These electrocatalysts have been successfully synthesized by a simple and fast microwave method. PtNi/Mn2O3-NiO-N with a binary metal oxide-to-N ratio of 1:2 demonstrated the best performance among the studied materials regarding bifunctional electrocatalytic activity (∆E = 0.96 V) and robust stability.

Highly Porous Metal-Organic Framework Entrapped by Cobalt Telluride-Manganese Telluride as an Efficient Bifunctional Electrocatalyst.

The electrochemical conversion of oxygen holds great promise in the development of sustainable energy for various applications, such as water electrolysis, regenerative fuel cells, and rechargeable metal-air batteries. Oxygen electrocatalysts are needed that are both highly efficient and affordable, since they can serve as alternatives to costly precious-metal-based catalysts. This aspect is particularly significant for their practical implementation on a large scale in the future. Herein, highly porous polyhedron-entrapped metal-organic framework (MOF)-assisted CoTe2/MnTe2 heterostructure one-dimensional nanorods were initially synthesized using a simple hydrothermal strategy and then transformed into ZIF-67 followed by tellurization which was used as a bifunctional electrocatalyst for both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). The designed MOF CoTe2/MnTe2 nanorod electrocatalyst exhibited superior activity for both OER (η = 220 mV@ 10 mA cm-2) and ORR (E1/2 = 0.81 V vs RHE) and outstanding stability. The exceptional achievement could be primarily credited to the porous structure, interconnected designs, and deliberately created deficiencies that enhanced the electrocatalytic activity for the OER/ORR. This improvement was predominantly due to the enhanced electrochemical surface area and charge transfer inherent in the materials. Therefore, this simple and cost-effective method can be used to produce highly active bifunctional oxygen electrocatalysts.

Single Transition-Metal Atom Anchored on a Rhenium Disulfide Monolayer: An Efficient Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions.

Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER) of 0.44 V and overpotentials for ORR (ηORR) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.

Excellent Bifunctional Oxygen Evolution and Reduction Electrocatalysts (5A1/5)Co2O4 and Their Tunability.

Hastening the progress of rechargeable metal-air batteries and hydrogen fuel cells necessitates the advancement of economically feasible, earth-abundant, inexpensive, and efficient electrocatalysts facilitating both the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Herein, a recently reported family of nano (5A1/5)Co2O4 (A = combinations of transition metals, Mg, Mn, Fe, Ni, Cu, and Zn) compositionally complex oxides (CCOs) [Wang et al., Chemistry of Materials, 2023,35 (17), 7283-7291.] are studied as bifunctional OER and ORR electrocatalysts. Among the different low-temperature soft-templating samples, those subjected to 600 °C postannealing heat treatment exhibit superior performance in alkaline media. One specific composition (Mn0.2Fe0.2Ni0.2Cu0.2Zn0.2)Co2O4 exhibited an exceptional overpotential (260 mV at 10 mA cm-2) for the OER, a favorable Tafel slope of 68 mV dec-1, excellent onset potential (0.9 V) for the ORR, and lower than 6% H2O2 yields over a potential range of 0.2 to 0.8 V vs the reversible hydrogen electrode. Furthermore, this catalyst displayed stability over a 22 h chronoamperometry measurement, as confirmed by X-ray photoelectron spectroscopy analysis. Considering the outstanding performance, the low cost and scalability of the synthesis method, and the demonstrated tunability through chemical substitutions and processing variables, CCO ACo2O4 spinel oxides are highly promising candidates for future sustainable electrocatalytic applications.

Surface Tailoring-Modulated Bifunctional Oxygen Electrocatalysis with CoP for Rechargeable Zn-Air Battery and Water Splitting.

The transition metal phosphide (TMP)-based functional electrocatalysts are very promising for the development of electrochemical energy conversion and storage devices including rechargeable metal-air batteries and water electrolyzer. Tuning the electrocatalytic activity of TMPs is one of the vital steps to achieve the desired performance of these energy devices. Herein, we demonstrate the modulation of the bifunctional oxygen electrocatalytic activity of nitrogen-doped carbon-encapsulated CoP (CoP@NC) nanostructures by surface tailoring with ultralow amount (0.56 atomic %) of Ru nanoparticles (2.5 nm). The CoP at the core and the Ru nanoparticles on the shell have a facile charge transfer interaction with the encapsulating NC. The strong coupling of Ru with CoP@NC boosts the electrocatalytic performance toward oxygen reduction (ORR), oxygen evolution (OER), and hydrogen evolution (HER) reactions. The surface-tailored catalyst requires only 35 mV to deliver the benchmark current density of 10 mA·cm-2 for HER. A small potential gap of 620 mV between ORR and OER is achieved, making the catalyst highly suitable for the development of rechargeable zinc-air batteries (ZABs). The homemade ZAB delivers a specific capacity of 780 mA·hgZn-1 and peak power density of 175 mW·cm-2 with a very small voltaic efficiency loss (1.1%) after 300 cycles. The two-electrode water splitting cell (CoP@NC-Ru||CoP@NC-Ru) delivers remarkably low cell voltage of 1.47 V at the benchmark current density. Stable current density of 25 mA·cm-2 for 25 h without any significant change is achieved. Theoretical studies support the charge transfer interaction-induced enhanced electrocatalytic activity of the surface-tailored nanostructure.

Bifunctional 2D structured catalysts for air electrodes in rechargeable metal-air batteries

The inherent technical challenges of metal-air batteries (MABs), arising from the sluggish redox electrochemical reactions on the air electrode, significantly affect their efficiency and life cycle. Two-dimensional (2D) nanomaterials with near-atomic thickness have potential as bifunctional catalysts in MABs because of their distinct structures, exceptional physical properties, and tunable surface chemistries. In this study, the chemistry of representative 2D materials was elucidated, and the comprehensive analysis of the primary modification techniques, including geometric structure manipulation, defect engineering, crystal facet selection, heteroatom doping, single-atom catalyst construction, and composite material synthesis, was conducted. The correlation between material structure and activity is illustrated by examples, with the aim of leading the development of advanced catalysts in MABs. We also focus on the future of MABs from the perspective of bifunctional catalysts, definite mechanisms, and standard measurement. We expect this work to serve as a guide for the design of air electrode materials that can be used in MABs.