Air Electrode Study for Rechargeable and Long-Life Zinc-Air Batteries

Abstract

Energy storage technology has become a vital element in the design of electrical networks, especially when renewable energy systems are included. Some other application conditions should be kept in mind that renewable energy system installations are often in remote locations and even subjected to harsh temperatures and punishing humidity. Therefore, reliable and efficient energy storage in both off-grid and on-grid schemes are ultimate developing objects. However, current energy storage technologies, including Ni-Cd, Ni-MH and Li-ion, can barely satisfy both issues of long cycle lifetime and low capital cost. Amongst most developing technologies, the zinc air battery not only provides high theoretical energy capacities, but also includes several advantages, like low cost, environmentally benign and naturally abundant. Some issues remain, hindering commercialization of this technology. One of them is the development of air cathodes possessing both oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) efficiently. Hence, much work on bifunctional catalysts development has been done and published by researchers over the world in the past few years. For example, metal oxides/conductive nanocarbon hybrids is proven as an efficient approach to enhancing catalytic activity, such as Co3O4/graphene, CoO/NCNT, and MnxOy/NC. Spinel Mn–Co oxides were also found to be highly active for both ORR and OER. Additionally, long cycle life is a must for us as well. Herein, the bifunctional air electrode composed of alpha phase of manganese oxides or silver particles, as ORR catalysts, grown on the surface of stainless steel mesh catalyzing OER, was fabricated via electro-deposition method. This as-synthesized oxygen electrode was used directly for zinc-air battery assembly without the need of polymer binders mixing for coating on a desired gas diffusion electrode such as carbon papers. Commonly used binders have low conductivity and suffer from detaching when immersion in alkaline solutions, leading to battery failure and short cycle life. In our system, the zinc-air battery was comprised of as-synthesized air cathode, 6M KOH and zinc foil anode. The battery was operated at the current density of 10 mA/cm2 and its charge/discharge potential was 1.97V/1.15V, respectively. Most importantly, it showed stable performance with >80% coulombic efficiency over 2000 recharge cycles (over 600 hours). Acknowledgement: Authors would like to thank the Bureau of Energy (BOE), Ministry of Economy Affair (MOEA), Taiwan for the financial support.