Rational Design of Engineered Porous Transition Metal-based Electrocatalysts for Rechargeable Zinc-air Batteries

The adoption of primary zinc-air batteries (ZABs) for telecommunication and medical applications underscores their commercial viability. However, the progress of ZABs have been hindered by challenges associated with air electrodes. The substantial electrode polarizations of the Oxygen Reduction Reaction (ORR) and the Oxygen Evolution Reaction (OER) pose significant energy barriers, impeding the efficiency of charge and discharge processes. Hence, there's an urgent need to develop bifunctional electrocatalysts with superior performance, energy efficiency, and long-term stability for ZABs. \nIn the first work, three-dimensional interconnected and ordered mesoporous Fe2Nx decorated on TiOy with the introduced nitrogen vacancies was constructed (Fe2Nx@TiOy). By creating defects in ordered porous materials, the increased surface area, pore volume, and active sites boost the kinetics of the ORR. Fe2Nx@TiOy with created nitrogen defects reveals a superior ORR performance, including a high half-wave potential (0.88 V vs reversible hydrogen electrode) and high current density (71 mA cm-2 at 0.8 V). The zinc-air battery assembled with Fe2Nx@TiOy catalysts presents a high specific capacity of 809 mAh g-1. Density functional theory (DFT) analysis and X-ray absorption spectroscopy further confirm that the engineering of nitrogen vacancies modulates the electronic environment of Fe and regulates the adsorption and desorption of intermediates to facilitate the ORR activity. The Fe d-band center moving toward the Fermi energy level strengthens the interaction between the adsorbate and substrate, allowing oxygen species to be favorably stabilized onto Fe2Nx@TiOy, while significantly reducing the kinetic barrier. This work serves as a guideline for developing effective defect engineering and ordered porous materials for efficient energy conversion and storage. \nIn the second work, among a series of ternary Cu-Ti-O electrocatalysts, a hierarchical macroporous Cu0.3Ti0.7O2 catalyst achieves a balance between structural stability and active sites exposure, showing an electron density reconfiguration in the Cu-Ti-O system. X-ray absorption fine structure analyses reveal the partial electron density reconfiguration presented among Cu, Ti, and O atoms can be the dominant reason for the peaks shift. It was demonstrated that Ti atoms tended to delocalize maximum charge by releasing it to the Cu atoms in the compositions of Cu0.3Ti0.7O2, which lower the thermodynamic barrier of the total reaction, and hence contributes to a remarkable enhancement in zinc-air battery. This work offers an attractive approach to developing the nonprecious transitional metal-based ORR/OER catalysts, and zinc-air battery for the design of performance-oriented electrocatalysts for wider electrochemical energy applications. \nIn the last work, a unique Mg-decorated three-dimensionally ordered mesoporous (3DOM) Co3O4 electrocatalyst is engineered and evaluated as cathodic material for zinc-air batteries. The modulation of electronic structure and bonding configuration of Co sites through coordination with substituted Mg atoms effectively enhance the interaction with oxygen species and, therefore, the ORR/OER activity. Meanwhile, the substitution of Co2+ with Mg2+ creates abundant, more catalytically active octahedral sites (Co3+) in 3DOM-MgxCo3-xO4. Moreover, the tailored 3D interpenetrating porous structure endows the electrocatalyst with large diffusion channels for oxygen species and highly accessible active sites. The as-prepared catalyst retains 99% and 98% of its initial ORR and OER current, respectively, after 16 h under chronoamperometric measurement. The zinc-air battery assembled with 3DOM-MgxCo3-xO4 exhibits a high power density of 253 mW cm-2 and long-term cyclability over 236 h, outperforming the commercial noble-metal-based catalysts in terms of performance and stability. This work offers a straightforward and promising design strategy for the development of robust bifunctional electrocatalysts toward practical applications of zinc-air batteries. \nIn summary, this thesis exhibits three types of transition metal-based materials with hierarchical three-dimensional porous structures applied in rechargeable zinc-air batteries. The main emphasis is focused on the synthesis and electrocatalytic activity as well as the underlying mechanisms for these materials in zinc-air batteries. It gives a prospect that is expected to engineer and synthesize porous transition metal-based materials for zinc-air batteries.

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