Development of Pt/CoWO4 Nanofiber Catalyst-Support Hybrid Material Using Ex-solution for Zinc-air batteries

Abstract

Development of Pt/CoWO4 nanofiber catalyst- support hybrid material using ex-solution for zinc- air batteries Changho Lee HYU-KITECH joint department The Graduate School Hanyang University Zinc-air batteries (ZABs) have attracted attention as next- generation energy storage devices due to their high energy density per weight and volume compared with lithium-ion batteries, cost-effectiveness, and the use of an aqueous electrolyte for safety. In the structure of ZABs, the catalyst for the oxygen redox reaction at the air cathode is a critical factor in determining battery performance. Conventionally, novel metals (e.g., Pt) and metal oxides (e.g., IrO2, RuO2) incorporating carbonaceous supports have been utilized as air electrode catalysts for facilitating these oxygen redox reactions. Nevertheless, severe activity degradation induced by agglomeration, dissolution, and oxidation of catalyst nanoparticles (NPs) as well as the corrosion of carbonaceous support materials has prompted to excavation of highly active and sustainable catalyst-support systems. Considering this aspect, heterogeneous catalysts incorporating metal NPs on oxide supports have been extensively researched. These hybrid materials exhibit high catalytic performance due to their high dispersion, durable properties, and metal-support interaction. However, fundamental limitations, such as catalyst NP agglomeration and low electrical conductivity, have not been overcome. To address this issue, we focused on the ex-solution phenomenon to achieve uniformly dispersed metal nanoparticles (NPs) on a metal oxide support through a single heat treatment. The metal NPs formed via ex-solution exhibit excellent thermal and chemical stability, as well as aggregation resistance. Additionally, to overcome the low electrical conductivity of the oxide, we selected reduced WO3 with a low bandgap as the host material, incorporating cobalt for oxygen-based reactions. In this study, we suggest CoWO4 nanofibers (NFs) in which Co- doped Pt NPs ex-solved onto the surface, serving as a bifunctional hybrid material for ZABs air-cathodes. The hybrid materials were easily produced through the electrospinning of a single polymer solution and thermal treatment. Two phenomena were observed in the morphology of synthesized NFs as the reduction heat treatment temperature increased: 1) the appearance of surface roughness and small pores, and 2) the growth of ex-solved particles. We confirmed the excellent bifunctional performance of the proposed Pt/CoWO4 NFs hybrid material through half-cell tests and provided experimental evidence attributing the results to the effects of ex- solution. The optimized hybrid NFs displayed a half-wave potential of 0.89 V for ORR and a low potential of 1.69 V at 10 mA cm-2 for OER in alkaline media, showing excellent electrochemical performance compared with commercial catalysts. Furthermore, ZABs with the optimized Pt/CoWO4 NFs exhibited a large discharge capacity (55 mW cm-2 at 105 mA cm-2), a lower voltage gap (0.9 V), a long-term cycling property over 240 h, and successfully operated electronic devices. Our results provide a design strategy for multi-component catalyst-support hybrid materials, contributing not only to the advancement of the practical implementation of ZABs but also to various developments of oxygen redox catalysts using ceramic materials.