Advanced Architectures of Air Electrodes in Zinc-Air Batteries and Hydrogen Fuel Cells

Synergistic effect of Fe-Ru alloy and Fe-N-C sites on oxygen reduction reaction

In the pursuit of optimizing Fe-N-C catalysts for the oxygen reduction reaction (ORR), the incorporation of alloy nanoparticles has emerged as a prominent strategy. In this work, we effectively synthesized the FeRu-NC catalyst by anchoring Fe-Ru alloy nanoparticles and FeN4 single atom sites onto carbon nanotubes. The FeRu-NC catalyst exhibits significantly enhanced ORR activity and long-term stability, with a high half-wave potential of 0.89 V (vs. RHE) in alkaline conditions, and the half-wave potential remains nearly unchanged after 5000 cycles. The zinc-air battery (ZAB) assembled with FeRu-NC demonstrates a power density of 169.1 mW cm-2, surpassing that of commercial Pt/C. Density functional theory (DFT) calculations reveal that the synergistic interaction between the Fe-Ru alloy and FeN4 single atoms alters the electronic structure and facilitates charge transfer at the FeN4 sites, thereby modulating the adsorption and desorption of ORR intermediates. This enhancement in catalytic activity for the ORR process underscores the potential of this approach for refining M-N-C catalysts, providing novel insights into their optimization strategies.

Sustainable oxygen evolution catalysis: Water-based fabrication of FeNi-MIL-100 on recycled stainless steel substrates

The anodic oxygen evolution reaction (OER) process is essential in new technologies such as water electrolysis and metal-air batteries. However, it often exhibits suboptimal efficiency and delayed kinetics. This study presents a novel and new design for the fabrication of homogeneous FeNiBTC/SSM (SSM = stainless steel material) with tunable crystalline properties by a self-sacrificial and in situ synthesis from a recycled stainless steel substrate. The modified stainless steel template enhances the material's properties compared to the original mesh substrate and its structure can be attributed to the typical MIL-100 (Material of Institute Lavoisier) structure, which is a hierarchically structured, highly chemically stable material formed by FeO6 octahedral clusters around a single shared oxygen anion. The as-synthesized FeNiBTC/SSM4 catalyst exhibited excellent electrocatalytic performance for OER, as indicated by its small Tafel slope (74.5 mV dec-1), low overpotential (η10 223.7 mV), and high current retention (95.4 %) after a stability test lasting 45 h. The study demonstrates the development of water-based and self-sustaining MOF electrocatalysts for the oxygen evolution reaction in a simple process, along with a novel method for reusing renewable resources.

Large-Scale Atomic Strain Defects on Palladium Surfaces for Enhanced Oxygen Reduction and Zinc-Air Batteries

Designing nano-electrocatalysts rich in surface defects is critical to improve their catalytic performance. However, prevailing synthesis techniques rely heavily on complex procedures that compromise defect extensiveness and uniformity, casting a high demand for methods capable of synthesizing large-scale crystalline defects. An innovative design strategy is herein proposed that induces ample strain/dislocation defects during the growth of palladium (Pd), which is well-known as a good oxygen reduction reaction (ORR) catalyst. The controlled defect engineering on Pd core is achieved by the tensile stress exerted from an intentionally applied Fe3O4 skin layer during synthesis, which changes the surface free energy of Pd to stabilize the defect presence. With such large-scale crystalline defects, this Pd catalyst exhibits significantly higher ORR activity than commercial Pt/C, enabling its promising future in zinc-air battery catalysis. Additionally, the protective Fe3O4 skin covering the catalyst also enhances its catalytic stability. Theoretical calculations show that the superior catalytic property of such defect-engineered Pd is associated with the correspondingly modified adsorption energy of *O intermediates onto its surface, which further improves the reaction rate and thus boosts ORR kinetics. Findings here are expected to provide a paradigm for designing efficient and stable metal catalysts with plentiful large-scale strain defects.

Electron Reservoir Effect of Adjacent Fe Nanoclusters Boosts Atomic Fe Active Sites on Porous Carbon for the Both Electrocatalytic Oxygen Reduction and CO2 Reduction Reaction

Electrochemical oxygen reduction reaction (ORR) and carbon dioxide reduction reaction (CO2RR) are greatly significant in renewable energy-related devices and carbon-neutral closed cycle, while the development of robust and highly efficient electrocatalysts has remained challenges. Herein, a hybrid electrocatalyst, featuring axial N-coordinated Fe single atom sites on hierarchically N, P-codoped porous carbon support and Fe nanoclusters as electron reservoir (FeNCs/FeSAs-NPC), is fabricated via in situ thermal transformation of the precursor of a supramolecular polymer initiated by intermolecular hydrogen bonds co-assembly. The FeNCs/FeSAs-NPC catalyst manifests superior oxygen reduction activity with a half-wave potential of 0.91 V in alkaline solution, as well as high CO2 to CO Faraday efficiency (FE) of surpassing 90% in a wide potential window from -0.40 to -0.85 V, along with excellent electrochemical durability. Theoretical calculations indicate that the electron reservoir effect of Fe nanoclusters can trigger the electron redistribution of the atomic Fe moieties, facilitating the activation of O2 and CO2 molecules, lowering the energy barriers for rate-determining step, and thus contributing to the accelerated ORR and CO2RR kinetics. This work offers an effective design of electron coupling catalysts that have advanced single atoms coexisting with nanoclusters for efficient ORR and CO2RR.

Structure-Activity Relationships in Oxygen Electrocatalysis

Oxygen electrocatalysis, as the pivotal circle of many green energy technologies, sets off a worldwide research boom in full swing, while its large kinetic obstacles require remarkable catalysts to break through. Here, based on summarizing reaction mechanisms and in situ characterizations, the structure-activity relationships of oxygen electrocatalysts are emphatically overviewed, including the influence of geometric morphology and chemical structures on the electrocatalytic performances. Subsequently, experimental/theoretical research is combined with device applications to comprehensively summarize the cutting-edge oxygen electrocatalysts according to various material categories. Finally, future challenges are forecasted from the perspective of catalyst development and device applications, favoring researchers to promote the industrialization of oxygen electrocatalysis at an early date.

Designing Highly Efficient Electrocatalyst for ORR and OER Based on Nb2CO2 MXene: The Role of Transition Metals and N-Doping Content

Finding efficient and stable electrocatalysts for the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is imperative for advancing zinc-air batteries. Herein, the effect of transition metal anchored on Nb2CO2 with different N content to form TM-Nx-Nb2CO2 on the catalytic activity of ORR and OER is investigated by density functional theory. Among all the designed TM-Nx-Nb2CO2, Pt-N12.50%-Nb2CO2, Pt-N37.50%-Nb2CO2, Pt-N50.00%-Nb2CO2, Pd-N68.75%-Nb2CO2, and Pd-N100%-Nb2CO2 are excellent ORR electrocatalysts with ηORR values of 0.38, 0.36, 0.38, 0.38, and 0.34 V, respectively. Rh-Nb2CO2, Rh-N12.50%-Nb2CO2, Rh-N31.25%-Nb2CO2, Rh-N37.50%-Nb2CO2, Rh-N50.00%-Nb2CO2, Pt-N50.00%-Nb2CO2, Rh-N68.75%-Nb2CO2, and Rh-N81.25%-Nb2CO2 are excellent OER electrocatalysts with ηOER values of 0.33, 0.37, 0.34, 0.36, 0.37, 0.34, 0.38, and 0.33 V, respectively. Notably, Rh-Nb2CO2 and Pt-N50.00%-Nb2CO2 exhibit outstanding ORR and OER bifunctional catalytic activity with potential gap values of 0.80 and 0.72 V, respectively, which are higher than the activities of most reported bifunctional catalysts. Furthermore, electronic structure analysis indicates that the moderate adsorption strength of oxygen-containing intermediates on active centers is crucial for achieving highly active bifunctional catalysts for ORR and OER. This study provides a strategy for the design of novel ORR and OER catalysts using 2D MXene materials.

Promoting Electrocatalytic Oxygen Reactions Using Advanced Heterostructures for Rechargeable Zinc-Air Battery Applications

In order to facilitate electrochemical oxygen reactions in electrically rechargeable zinc-air batteries (ZABs), there is a need to develop innovative approaches for efficient oxygen electrocatalysts. Due to their reliability, high energy density, material abundance, and ecofriendliness, rechargeable ZABs hold promise as next-generation energy storage and conversion devices. However, the large-scale application of ZABs is currently hindered by the slow kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). However, the development of heterostructure-based electrocatalysts has the potential to surpass the limitations imposed by the intrinsic properties of a single material. This Account begins with an explanation of the configurations of ZABs and the fundamentals of the oxygen electrochemistry of the air electrode. Then, we summarize recent progress with respect to the variety of heterostructures that exploit bifunctional electrocatalytic reactions and overview their impact on ZAB performance. The range of heterointerfacial engineering strategies for improving the ORR/OER and ZAB performance includes tailoring the surface chemistry, dimensionality of catalysts, interfacial charge transfer, mass and charge transport, and morphology. We highlight the multicomponent design approaches that take these features into account to create advanced highly active bifunctional catalysts. Finally, we discuss the challenges and future perspectives on this important topic that aim to enhance the bifunctional activity and performance of zinc-air batteries.

One-Pot Etching Pyrolysis to Defect-Rich Carbon Nanosheets to Construct Multiheteroatom-Coordinated Iron Sites for Efficient Oxygen Reduction

Constructing multiheteroatom coordination structure in carbonaceous substrates demonstrates an effective method to accelerate the oxygen reduction reaction (ORR) of supported single-atom catalyst. Herein, the novel etching route assisted by potassium thiocyanate (KCNS) is developed to convert metal-organic framework to 2D defect-rich porous N,S-co-doped carbon nanosheets for anchoring atomically dispersed iron sites as the high-performance ORR catalysts (Fe-SACs). The well-designed KCNS-assisted etching route can generate spatial confinement template to direct the carbon nanosheet formation, etching condition to form defect-rich structure, and additional sulfur atoms to coordinate iron species. Spectral and microscopy analysis reveals that the iron element in Fe-SACs is highly isolated on carbon nanosheet and anchored by nitrogen and sulfur atoms in unsymmetrical Fe-S1N3 structure. The optimized Fe-SACs with large specific surface area could show remarkable alkaline ORR performances with a high half-wave potential of 0.920 V versus RHE and excellent durability. The rechargeable zinc-air battery assembled with Fe-SACs air electrodes delivers a large power density of 350 mW cm-2 and a stable voltage platform during charge and discharge over more than 1300 h. This work proposes a novel strategy for the preparation of single-atom catalysts with multiheteroatom coordination structure and highly exposed active sites for efficient ORR.

Topological Engineering Electrodes with Ultrafast Oxygen Transport for Super-Power Sodium-Oxygen Batteries

Sodium-oxygen battery has attracted tremendous interest due to its extraordinary theoretical specific energy (1605 Wh kg-1 NaO2) and appealing element abundance. However, definite mechanistic factors governing efficient oxygen diffusion and consumption inside electrolyte-flooded air cathodes remain elusive thus precluding a true gas diffusion electrode capable of high discharge current (i.e., several mA cm-2) and superior output power. Herein, 3D-printing technology is adopted to create gas channels with tailored channel size and structure to demystify the diffusion-limited oxygen delivery process. It is revealed that as the clogging discharging products increase, large channel size, and interconnected channel structure are essential to guaranteeing fast O2 diffusion. Moreover, to further encourage O2 diffusion, a bio-inspired breathable cathode with progressively branching channels that balances between O2 passage and reaction is 3D printed. This elaborated 3D electrode allows a sodium-oxygen cell to deliver an impressive discharging current density of up to 4 mA cm-2 and an output power of 8.4 mW cm-2, giving rise to an outstanding capacity of 18.4 mAh cm-2. The unraveled mystery of oxygen delivery enabled by 3D printing points to a valuable roadmap for the rational design of metal-air batteries toward practical applications.