Efficient and Durable Bifunctional Oxygen Catalysts Based on NiFeO@MnOx Core-Shell Structures for Rechargeable Zn-Air Batteries

Enhanced Oxygen Evolution and Zinc-Air Battery Performance via Electronic Spin Modulation in Heterostructured Catalysts

Beyond optimizing electronic energy levels, the modulation of the electronic spin configuration is an effective strategy, often overlooked, to boost activity and selectivity in a range of catalytic reactions, including the oxygen evolution reaction (OER). This electronic spin modulation is frequently accomplished using external magnetic fields, which makes it impractical for real applications. Herein, spin modulation is achieved by engineering Ni/MnFe2O4 heterojunctions, whose surface is reconstructed into NiOOH/MnFeOOH during the OER. NiOOH/MnFeOOH shows a high spin state of Ni, which regulates the OH- and O2 adsorption energy and enables spin alignment of oxygen intermediates. As a result, NiOOH/MnFeOOH electrocatalysts provide excellent OER performance with an overpotential of 261 mV at 10 mA cm-2. Besides, rechargeable zinc-air batteries based on Ni/MnFe2O4 show a high open circuit potential of 1.56 V and excellent stability for more than 1000 cycles. This outstanding performance is rationalized using density functional theory calculations, which show that the optimal spin state of both Ni active sites and oxygen intermediates facilitates spin-selected charge transport, optimizes the reaction kinetics, and decreases the energy barrier to the evolution of oxygen. This study provides valuable insight into spin polarization modulation by heterojunctions enabling the design of next-generation OER catalysts with boosted performance.

High-Pressure Induced Continuous Structural Evolution of Kagome Antiferromagnet MgMn3(OH)6Cl2: A Structural Analogue to Quantum Spin Liquid Herbertsmithite

MgMn3(OH)6Cl2 serves readily as the classical Heisenberg kagome antiferromagnet lattice spin frustration material, due to its similarity to herbertsmithite in composition and crystal structure. In this work, nanosheets of MgMn3(OH)6Cl2 are synthesized through a solid-phase reaction. Low-temperature magnetic measurements revealed two antiferromagnetic transitions, occurring at ∼8 and 55 K, respectively. Utilizing high-pressure synchrotron radiation X-ray diffraction techniques, the topological structural evolution of MgMn3(OH)6Cl2 under pressures up to 24.8 GPa was investigated. The sample undergoes a second-order structural phase transition from the rhombohedral phase to the monoclinic phase at pressures exceeding 7.8 GPa. Accompanying the disappearance of the Fano-like line shape in the high-pressure Raman spectra were the emergence of new Raman active modes and discontinuities in the variations of Raman shifts in the high-frequency region. The phase transition to a structure with lower symmetry was attributed to the pressure-induced enhancement of cooperative Jahn-Teller distortion, which is caused by the mutual substitution of Mn2+ ions from the kagome layer and Mg2+ ions from the triangular interlayer. High-pressure ultraviolet-visible absorption measurements support the structural evolution. This research provides a robust experimental approach and physical insights for further exploration of classical geometrical frustration materials with kagome lattice.

High-Performance Aluminum Fuels Induced by Monolayer Self-Assembly of Nano-Sized Energetic Fluoride Vesicles on the Surface

Surface modification is frequently used to solve the problems of low combustion properties and agglomeration for aluminum-based fuels. However, due to the intrinsic incompatibility between the aluminum powder and the organic modifiers, the surface coating is usually uneven and disordered, which significantly deteriorates the uniformity and performances of the Al-based fuels. Herein, a new approach of monolayer nano-vesicular self-assembly is proposed to prepare high-performance Al fuels. Triblock copolymer G-F-G is produced by glycidyl azide polymer (GAP) and 2,2'-(2,2,3,3,4,5,5-Octafluorohexane-1,6-diyl) bis (oxirane) (fluoride) ring-open addition reaction. By utilizing G-F-G vesicular self-assembly in a special solvent, the nano-sized vesicles are firmly adhered to the surface of Al powder through the long-range attraction between the fluorine segments and Al. Meanwhile, the electrostatic repulsion between vesicles ensures an extremely thin coating thickness (≈15 nm), maintaining the monolayer coating structure. Nice ignition, combustion, anti-agglomeration, and water-proof properties of Al@G-F-G(DMF) are achieved, which are superior among the existing Al-based fuels. The derived Al-based fuel has excellent comprehensive properties, which can not only inspire the development of new-generation energetic materials but also provide facile but exquisite strategies for exquisite surface nanostructure construction via ordered self-assembly for many other applications.

Insights into the Electrochemical Behavior of Manganese Oxides as Catalysts for the Oxygen Reduction and Evolution Reactions: Monometallic Core-Shell Mn/Mn3 O4

Overcoming the sluggish electrode kinetics of both oxygen reduction and evolution reactions (ORR/OER) with non-precious metal electrocatalysts will accelerate the development of rechargeable metal-air batteries and regenerative fuel cells. The authors investigated the electrochemical behavior and ORR/OER catalytic activity of core-porous shell Mn/Mn₃ O₄ nanoparticles in comparison with other manganese dioxides (β- and γ-MnO₂ ), and benchmarked against Pt/C and Pt/C-IrO₂ . Under reversible operation in O₂ -saturated 5 M KOH at 22 °C, the early stage activity of core-shell Mn/Mn₃ O₄ shows two times higher ORR and OER current density compared to the other MnO₂ structures at 0.32 and 1.62 V versus RHE, respectively. It is revealed that Mn(III) oxidation to Mn(IV) is the primary cause of Mn/Mn₃ O₄ activity loss during ORR/OER potential cycling. To address it, an electrochemical activation method using Co(II) is proposed. By incorporating Co(II) into MnO_x , new active sites are introduced and the content of Mn(II) is increased, which can stabilize the Mn(III) sites through comproportionation with Mn(IV). The Co-incorporated Mn/Mn₃ O₄ has superior activity and durability. Furthermore, it also surpassed the activity of Pt/C-IrO₂ with similar durability. This study demonstrates that cost-effective ORR/OER catalysis is possible.

Recent Progress of Non-Noble Metal Catalysts for Oxygen Electrode in Zn-Air Batteries: A Mini Review

Oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) play crucial roles in energy conversion and storage devices. Particularly, the bifunctional ORR/OER catalysts are core components in rechargeable metal–air batteries, which have shown great promise in achieving "carbon emissions peak and carbon neutrality" goals. However, the sluggish ORR and OER kinetics at the oxygen cathode significantly hinder the performance of metal–air batteries. Although noble metal-based catalysts have been widely employed in accelerating the kinetics and improving the bifunctionality, their scarcity and high cost have limited their deployment in the market. In this review, we will discuss the ORR and OER mechanisms, propose the principles for bifunctional electrocatalysts design, and present the recent progress of the state-of-the-art bifunctional catalysts, with the focus on non-noble metal-based materials to replace the noble metal catalysts in Zn–air batteries. The perspectives for the future R&D of bifunctional electrocatalysts will be provided toward high-performance Zn–air batteries at the end of this paper.

A Porphyrinic Zirconium Metal-Organic Framework for Oxygen Reduction Reaction: Tailoring the Spacing between Active-Sites through Chain-Based Inorganic Building Units

The oxygen reduction reaction (ORR) is central in carbon-neutral energy devices. While platinum group materials have shown high activities for ORR, their practical uses are hampered by concerns over deactivation, slow kinetics, exorbitant cost, and scarce nature reserve. The low cost yet high tunability of metal-organic frameworks (MOFs) provide a unique platform for tailoring their characteristic properties as new electrocatalysts. Herein, we report a new concept of design and present stable Zr-chain-based MOFs as efficient electrocatalysts for ORR. The strategy is based on using Zr-chains to promote high chemical and redox stability and, more importantly, tailor the immobilization and packing of redox active-sites at a density that is ideal to improve the reaction kinetics. The obtained new electrocatalyst, PCN-226, thereby shows high ORR activity. We further demonstrate PCN-226 as a promising electrode material for practical applications in rechargeable Zn-air batteries, with a high peak power density of 133 mW cm-2. Being one of the very few electrocatalytic MOFs for ORR, this work provides a new concept by designing chain-based structures to enrich the diversity of efficient electrocatalysts and MOFs.

Graphite Nanoarrays-Confined Fe and Co Single-Atoms within Graphene Sponges as Bifunctional Oxygen Electrocatalyst for Ultralong Lasting Zinc-Air Battery

The inferior stability of bifunctional oxygen electrocatalysts in the air cathode is one of the main obstacles that impedes the commercialization of zinc-air batteries (ZABs). This work describes a self-assembly technique combined with subsequent calcination to prepare a bifunctional oxygen electrocatalyst of graphite nanoarrays-confined Fe and Co single-atoms within graphene sponges (FeCo-NGS). Specifically, graphene sponges overspread with graphite nanoarrays as a structure regulation, which can prevent the metal single-atoms from aggregating and accelerate the mass/electron transfer, provides a guarantee for the long-term operation. Furthermore, M-N₄ (M = Fe/Co) as the intrinsic activity regulation can effectively drive the heterogeneous oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) catalytic processes. Thanks to the rationally designed regulations, FeCo-NGS shows both extraordinary electrocatalytic activity for ORR and OER, even outperforming commercial Pt/C and IrO₂. Remarkably, ZABs with FeCo-NGS air cathode demonstrate a record-breaking cycle lifetime of more than 1500 h (over 9000 cycles) at 10 mA cm^-2 with a small charge-discharge gap.

Deep-Breathing Honeycomb-like Co-Nx-C Nanopolyhedron Bifunctional Oxygen Electrocatalysts for Rechargeable Zn-Air Batteries

Metal organic framework (MOF) derivatives have been extensively used as bifunctional oxygen electrocatalysts. However, the utilization of active sites is still not satisfactory owing to the sluggish mass transport within their narrow pore channels. Herein, interconnected macroporous channels were constructed inside MOFs-derived Co-N_x-C electrocatalyst to unblock the mass transfer barrier. The as-synthesized electrocatalyst exhibits a honeycomb-like morphology with highly exposed Co-N_x-C active sites on carbon frame. Owing to the interconnected ordered macropores throughout the electrocatalyst, these active sites can smoothly “exhale/inhale” reactants and products, enhancing the accessibility of active sites and the reaction kinetics. As a result, the honeycomb-like Co-N_x-C displayed a potential difference of 0.773 V between the oxygen evolution reaction potential at 10 mA cm⁻² and the oxygen reduction reaction half-wave potential, much lower than that of bulk-Co-N_x-C (0.842 V). The rational modification on porosity makes such honeycomb-like MOF derivative an excellent bifunctional oxygen electrocatalyst in rechargeable Zn-air batteries.

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A deep-breathing oxygen electrocatalyst with highly dispersed active sites was built

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Sculpturing ordered macropores in MOF derivatives enables fast mass transport

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The honeycomb-like Co-N_x-C nanopolyhedron worked well in rechargeable Zn-air battery

Bifunctional Oxygen Electrocatalyst of Mesoporous Ni/NiO Nanosheets for Flexible Rechargeable Zn-Air Batteries

One approach to accelerate the stagnant kinetics of both the oxygen reduction and evolution reactions (ORR/OER) is to develop a rationally designed multiphase nanocomposite, where the functions arising from each of the constituent phases, their interfaces, and the overall structure are properly controlled. Herein, we successfully synthesized an oxygen electrocatalyst consisting of Ni nanoparticles purposely interpenetrated into mesoporous NiO nanosheets (porous Ni/NiO). Benefiting from the contributions of the Ni and NiO phases, the well-established pore channels for charge transport at the interface between the phases, and the enhanced conductivity due to oxygen-deficiency at the pore edges, the porous Ni/NiO nanosheets show a potential of 1.49 V (10 mA cm-2) for the OER and a half-wave potential of 0.76 V for the ORR, outperforming their noble metal counterparts. More significantly, a Zn-air battery employing the porous Ni/NiO nanosheets exhibits an initial charging-discharging voltage gap of 0.83 V (2 mA cm-2), specific capacity of 853 mAh g Zn -1 at 20 mA cm-2, and long-time cycling stability (120 h). In addition, the porous Ni/NiO-based solid-like Zn-air battery shows excellent electrochemical performance and flexibility, illustrating its great potential as a next-generation rechargeable power source for flexible electronics.

Bifunctional Catalysts for Reversible Oxygen Evolution Reaction and Oxygen Reduction Reaction

Metal-air batteries (MABs) and reversible fuel cells (RFCs) rely on the bifunctional oxygen catalysts for oxygen evolution reaction (OER) and oxygen reduction reaction (ORR). Finding efficient bifunctional oxygen catalysts is the ultimate goal and it has attracted a great deal of attention. The dilemma is that a good ORR catalyst is not necessarily efficient for OER, and vice versa. Thus, the development of a new type of bifunctional oxygen catalysts should ensure that the catalysts exhibit high activity for both OER and ORR. Composites with multicomponents for active centers supported on highly conductive matrices could be able to meet the challenges and offering new opportunities. In this Review, the evolution of bifunctional catalysts is summarized and discussed aiming to deliver high-performance bifunctional catalysts with low overpotentials.

Defect Engineering on Electrode Materials for Rechargeable Batteries

The reasonable design of electrode materials for rechargeable batteries plays an important role in promoting the development of renewable energy technology. With the in-depth understanding of the mechanisms underlying electrode reactions and the rapid development of advanced technology, the performance of batteries has significantly been optimized through the introduction of defect engineering on electrode materials. A large number of coordination unsaturated sites can be exposed by defect construction in electrode materials, which play a crucial role in electrochemical reactions. Herein, recent advances regarding defect engineering in electrode materials for rechargeable batteries are systematically summarized, with a special focus on the application of metal-ion batteries, lithium-sulfur batteries, and metal-air batteries. The defects can not only effectively promote ion diffusion and charge transfer but also provide more storage/adsorption/active sites for guest ions and intermediate species, thus improving the performance of batteries. Moreover, the existing challenges and future development prospects are forecast, and the electrode materials are further optimized through defect engineering to promote the development of the battery industry.

Recent Progress in Electrically Rechargeable Zinc-Air Batteries

Over the past decade, the surging interest for higher-energy-density, cheaper, and safer battery technology has spurred tremendous research efforts in the development of improved rechargeable zinc-air batteries. Current zinc-air batteries suffer from poor energy efficiency and cycle life, owing mainly to the poor rechargeability of zinc and air electrodes. To achieve high utilization and cyclability in the zinc anode, construction of conductive porous framework through elegant optimization strategies and adaptation of alternate active material are employed. Equally, there is a need to design new and improved bifunctional oxygen catalysts with high activity and stability to increase battery energy efficiency and lifetime. Efforts to engineer catalyst materials to increase the reactivity and/or number of bifunctional active sites are effective for improving air electrode performance. Here, recent key advances in material development for rechargeable zinc-air batteries are described. By improving fundamental understanding of materials properties relevant to the rechargeable zinc and air electrodes, zinc-air batteries will be able to make a significant impact on the future energy storage for electric vehicle application. To conclude, a brief discussion on noteworthy concepts of advanced electrode and electrolyte systems that are beyond the current state-of-the-art zinc-air battery chemistry, is presented.

Engineering the Surface Metal Active Sites of Nickel Cobalt Oxide Nanoplates toward Enhanced Oxygen Electrocatalysis for Zn-Air Battery

Clarifying and controlling the surface catalytic active sites is at the heart of developing low-cost effective bifunctional oxygen catalysts to replace precious metals for metal-air batteries. Herein, a shape-control of hexagon nickel cobalt oxide spinel nanosheets was reported to engineer the surface metal active sites for enhanced electrocatalysis of oxygen evolution and oxygen reduction reactions (OER/ORR). Specifically, through simply tuning annealing temperature, different Ni³⁺/Ni²⁺ and Co³⁺/Co²⁺ atomic configurations on the nickel cobalt oxide surface were controllably synthesized. Electrochemical results show that the oxide treated at 250 °C (NCO-250) with the highest value of Ni³⁺/Ni²⁺ sites and the lowest value of Co³⁺/Co²⁺ sites exhibits superior OER/ORR activity in alkaline electrolytes and better discharge/charge performance in Zn-air batteries among all the samples. The optimized surface active site configuration of the NCO-250 sample leads to the optimal energy of adsorption, activation, and desorption for water molecules and oxygen species, thus promoting a high electrocatalytic activity. This work provides a strategy to design cost-effective, highly active, and durable electrocatalysts through regulating active sites on transition-metal surface for Zn-air battery and other advanced energy devices.

Mesoporous Hollow Nitrogen-Doped Carbon Nanospheres with Embedded MnFe2O4/Fe Hybrid Nanoparticles as Efficient Bifunctional Oxygen Electrocatalysts in Alkaline Media

Exploring sustainable and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is necessary for the development of fuel cells and metal-air batteries. Herein, we report a bimetal Fe/Mn-N-C material composed of spinel MnFe₂O₄/metallic Fe hybrid nanoparticles encapsulated in N-doped mesoporous hollow carbon nanospheres as an excellent bifunctional ORR/OER electrocatalyst in alkaline electrolyte. The Fe/Mn-N-C catalyst is synthesized via pyrolysis of bimetal ion-incorporated polydopamine nanospheres and shows impressive ORR electrocatalytic activity superior to Pt/C and good OER activity close to RuO₂ catalyst in alkaline environment. When tested in Zn-air battery, the Fe/Mn-N-C catalyst demonstrates excellent ultimate performance including power density, durability, and cycling. This work reports the bimetal Fe/Mn-N-C as a highly efficient bifunctional electrocatalyst and may afford useful insights into the design of sustainable transition-metal-based high-performance electrocatalysts.

Advanced Architectures and Relatives of Air Electrodes in Zn-Air Batteries

Zn-air batteries are becoming the promising power sources for portable and wearable electronic devices and hybrid/electric vehicles because of their high specific energy density and the low cost for next-generation green and sustainable energy technologies. An air electrode integrated with an oxygen electrocatalyst is the most important component and inevitably determines the performance and cost of a Zn-air battery. This article presents exciting advances and challenges related to air electrodes and their relatives. After a brief introduction of the Zn-air battery, the architectures and oxygen electrocatalysts of air electrodes and relevant electrolytes are highlighted in primary and rechargeable types with different configurations, respectively. Moreover, the individual components and major issues of flexible Zn-air batteries are also highlighted, along with the strategies to enhance the battery performance. Finally, a perspective for design, preparation, and assembly of air electrodes is proposed for the future innovations of Zn-air batteries with high performance.

Electrochemical Oxidation of Chlorine-Doped Co(OH)2 Nanosheet Arrays on Carbon Cloth as a Bifunctional Oxygen Electrode

The primary challenge of developing clean energy conversion/storage systems is to exploit an efficient bifunctional electrocatalyst both for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) with low cost and good durability. Here, we synthesized chlorine-doped Co(OH)₂ in situ grown on carbon cloth (Cl-doped Co(OH)₂) as an integrated electrode by a facial electrodeposition method. The anodic potential was then applied to the Cl-doped Co(OH)₂ in an alkaline solution to remove chlorine atoms (electro-oxidation (EO)/Cl-doped Co(OH)₂), which can further enhance the electrocatalytic activity without any thermal treatment. EO/Cl-doped Co(OH)₂ exhibits a better performance both for ORR and OER in terms of activity and durability because of the formation of a defective structure with a larger electrochemically active surface area after the electrochemical oxidation. This approach provides a new idea for introducing defects and developing active electrocatalyst.

Activating the Bifunctionality of a Perovskite Oxide toward Oxygen Reduction and Oxygen Evolution Reactions

This article presents a facile and effective approach to activate the bifunctionality of calcium-manganese perovskites toward the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). We substituted Nb into the Mn site of CaMnO₃ (CMO) and treated the material with H₂. The as-obtained CaMn_0.75Nb_0.25O_3-δ (H₂-CMNO) displays the same structure as that of CMO, and compared to that of CMO, H₂-CMNO exhibits significantly improved OER performance, including a lower overpotential, a reduced Tafel slope, a higher mass activity, and enhanced stability. In addition, the ORR performance of H₂-CMNO is also greatly enhanced, relative to CMO, with a higher ORR activity and a more efficient electron-transfer pathway. H₂-CMNO shows an even higher activity-per-catalyst cost and superior stability than that of state-of-the-art materials, such as IrO₂ and Pt/C. This great enhancement in ORR and OER activity of H₂-CMNO is attributed to several factors, including phase stabilization, optimized e_g filling, better OH^- adsorption, and improved electrical conductivity.

Highly efficient electrocatalytic oxidation of urea on a Mn-incorporated Ni(OH)2/carbon fiber cloth for energy-saving rechargeable Zn–air batteries

The introduction of Mn can effectively regulate the electronic structure of Ni hydroxides on CFC, exhibiting superior electrocatalytic oxidation activity toward urea and potential applications in energy-saving rechargeable Zn–air batteries.