Ultrafine CoO nanoparticles and Co-N-C lamellae supported on mesoporous carbon for efficient electrocatalysis of oxygen reduction in zinc-air batteries

FeNiCo|MnGaOx Heterostructure Nanoparticles as Bifunctional Electrocatalyst for Zn-Air Batteries

Driven by the pressing demand for stable energy systems, zinc-air batteries (ZABs) have emerged as crucial energy storage solutions. However, the quest for cost-effective catalysts to enhance vital oxygen evolution and reduction reactions remains challenging. FeNiCo|MnGaOx heterostructure nanoparticles on carbon nanotubes (CNTs) are synthesized using liquid-phase reduction and H2 calcination approach. Compared to its component, such FeNiCo|MnGaOx/CNT shows a high synergistic effect, low impedance, and modulated electronic structure, leading to a superior bifunctional catalytic performance with an overpotential of 255 mV at 10 mA cm-2 and half-wave potential of 0.824 V (ω = 1600 rpm and 0.1 m KOH electrolyte). Moreover, ZABs based on FeNiCo|MnGaOx/CNT demonstrate notable features, including a peak power density of 136.1 mW cm-2, a high specific capacity of 808.3 mAh gZn -1, and outstanding stability throughout >158 h of uninterrupted charge-discharge cycling. Theoretical calculations reveal that the non-homogeneous interface can introduce more carriers and altered electronic structures to refine intermediate adsorption reactions, especially promoting O* formation, thereby enhancing electrocatalytic performance. This work demonstrates the importance of heterostructure interfacial modulation of electronic structure and enhancement of adsorption capacity in promoting the implementation of OER/ORR, ZABs, and related applications.

Size-controlled Co/CoO heterogeneous nanoparticles confined in N-doped mesoporous carbon for efficient oxygen reduction in zinc-air batteries

The size of metal compound particles and interface electronic structure of heterojunctions in a matrix greatly affect oxygen reduction performance in zinc-air batteries. However, it is still a big challenge to precisely control or regulate the size of these metal compound particles and the heterojunction structure. Herein, cobalt complexes with different-sized ligands are chosen as cobalt resources and adsorbed onto a mesoporous carbon, after which they are coated with polydopamine and calcined. Under the confinement effect of mesopores and the isolation effect of ligands and dopamine, the as-obtained Co/CoO heterogeneous nanoparticles are restricted to nano-size and uniformly dispersed in N-doped carbon (NC). The sizes of Co/CoO are estimated to be 39.7, 24.9 and 15.6 nm with increased CoO contents, corresponding to the adopted cobalt precursors of Co(OAc)2, Co(acac)2 and Co(acac)3, respectively. The smallest Co/CoO/NC-S shows excellent catalytic activity for oxygen reduction reaction, with a half-wave potential of 0.82 V vs. RHE and a limiting current density of 4.59 mA cm-2. When applied to the cathode of zinc-air battery, a high peak power density of 131.9 mW cm-2 is achieved, which surpasses that of the battery powered by Pt/C. The excellent performance can be attributed to the formation of heterogeneous structures between Co and CoO, the smaller Co/CoO nanoparticles, and N-doped mesoporous carbon with effective charge/mass transport. This work provides an effective way to regulate the size and phase contents of heterogeneous particles in mesoporous carbon, which is highly valuable in electrocatalytic systems.

In situ space-confined growth of Co3O4 nanoparticles inside N-doped hollow porous carbon nanospheres as bifunctional oxygen electrocatalysts for high-performance rechargeable zinc-air batteries

Developing high-performance and low-cost bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) is of great significance for accelerating the commercialization of rechargeable zinc-air batteries (RZABs). Herein, in situ grown Co₃O₄ nanoparticle-embedded N-doped hollow porous carbon nanospheres (Co₃O₄@N-HPCNs) are synthesized via template-assisted pyrolysis as efficient bifunctional ORR/OER electrocatalysts. The N-HPCNs efficiently seize and confine Co₃O₄ nanoparticles to enhance electronic conductivity and structural stability, while the hollow porous architecture offers adequate mass diffusion pathways to improve the accessibility of reactants and electrolytes on active sites. Therefore, the as-obtained Co₃O₄-10%@N-HPCNs display outstanding activity and stability for the ORR and the OER, even outperforming commercial Pt/C and Ru/C catalysts. Liquid RZABs assembled with Co₃O₄-10%@N-HPCN cathodes exhibit a large specific capacity of 768.3 mA h g^-1_Zn, a high peak power density of 145.6 mW cm^-2 and a long-term cycling stability for over 1000 h, demonstrating much-enhanced battery performance in comparison with that of Pt/C + Ru/C based RZABs. Also, flexible quasi-solid-state RZABs assembled with Co₃O₄-10%@N-HPCN cathodes exhibit a considerable power density of 132.0 mW cm^-2 and a stable charge-discharge voltage for a long period even upon bending. This work provides a new approach for the development of catalysts with high activity, long-term stability and low cost.

Metal-Organic Framework-Derived Atomically Dispersed Co-N-C Electrocatalyst for Efficient Oxygen Reduction Reaction

In this work, an atomically dispersed cobalt-nitrogen-carbon (Co-N-C) catalyst is prepared for the oxygen reduction reaction (ORR) by using a metal-organic framework (MOF) as a self-sacrifice template under high-temperature pyrolysis. Spherical aberration-corrected electron microscopy is employed to confirm the atomic dispersion of high-density Co atoms on the nitrogen-doped carbon scaffold. The X-ray photoelectron spectroscopy results verify the existence of Co-N-C active sites and their content changes with the Co content. The electrochemical results show that the electrocatalytic activity shows a volcano-shaped relationship, which increases with the Co content from 0 to 0.99 wt.% and then decreases when the presence of Co nanoparticles at 1.61 wt.%. The atomically dispersed Co-N-C catalyst with Co content of 0.99 wt.% shows an onset potential of 0.96 V vs. reversible hydrogen electrode (RHE) and a half-wave potential of 0.89 V vs. RHE toward ORR. The excellent ORR activity is attributed to the high density of the Co-N-C sites with high intrinsic activity and high specific surface area to expose more active sites.

Controllable Solid-Phase Fabrication of an Fe2O3/Fe5C2/Fe-N-C Electrocatalyst toward Optimizing the Oxygen Reduction Reaction in Zinc-Air Batteries

Preparing advanced electrocatalysts via solid-phase reactions encounters the challenge of low controllability for multiconstituent hybridization and microstructure modulation. Herein, a hydrothermal-mimicking solid-phase system is established to fabricate novel Fe₂O₃/Fe₅C₂/Fe-N-C composites consisting of Fe₂O₃/Fe₅C₂ nanoparticles and Fe,N-doped carbon species with varying morphologies. The evolution mechanism featuring a competitive growth of different carbon sources in a closed hypoxic space is elucidated for a series of Fe₂O₃/Fe₅C₂/Fe-N-C composites. The size and dispersity of Fe₂O₃/Fe₅C₂ nanoparticles, the graphitization degree of the carbonaceous matrix, and their diverse hybridization states lead to disparate electrocatalytic behaviors for the oxygen reduction reaction (ORR). Among them, microspherical Fe₂O₃/Fe₅C₂/Fe-N-C-3 exhibits an optimal ORR performance and the as-assembled zinc-air battery shows all-round superiority to the Pt/C counterpart. This work presents a mild solid-phase fabrication technique for obtaining a variety of nanocomposites with effective control over composition hybridization and microstructural modulation, which is significantly important for the design and optimization of advanced electrocatalysts.