Zinc and Nickel Based Oxides Anchored Onto Nitrogen-Doped Carbon Nanotubes As Bifunctional Catalysts for Zinc-Air Batteries

Abstract

Rechargeable zinc-air batteries are a promising battery technology as they are safe, environmentally friendly, low cost, and have a high theoretical energy density. However, there are several challenges with zinc-air batteries which occur mainly at the air electrode. The main challenge is the slow reaction kinetics for the oxygen evolution and reduction reactions (OER/ORR). To improve the OER and ORR precious metal catalysts have typically been added to the air electrode. However, using precious metals as catalyst materials inhibits large scale applications due to the high material cost and material scarcity. Transition metal oxides are an effective lower cost alternative. Combining transition metal oxides with nanocarbon structures can further increase OER and ORR rates and improve electrochemical stability. [1], [2]In this study, a combination of nitrogen-doped nanotubes (N-CNTs) and transition metal oxides is impregnated into a gas diffusion layer (GDL) made from porous carbon paper. This synthesis process ensures that the catalyst is present throughout the GDL, improving the reaction kinetics [2]. Zinc- and nickel-based tri-metallic and tetra-metallic oxides are investigated. Nickel-based oxides have demonstrated potential for high electrochemical activity but have not been fully studied [3]. Zinc is chosen, in part, because it appears to be incorporated into the transition metal oxide catalysts during battery cycling, so its effect on battery behavior is worth further study. The metal oxide combinations are determined from previous bi-metallic and initial tri-metallic oxide results. For each combination, different ratios are tested, since the amount of each transition metal affects the OER and ORR kinetics. Each combination is examined through various electrochemical and microstructural characterization techniques; e.g., linear sweep voltammetry, electrochemical impedance spectroscopy, electron microscopy and energy-dispersive X-ray spectroscopy. A Design of Experiments approach is used to determine the best metal ratios for each combination. The overall goal of this work is to find a catalyst that increases the efficiency, lowers the voltage gap, and improves the cyclability of zinc-air batteries.[1] N. Xu et al., “Self-Assembly formation of Bi-functional Co 3 O 4 /MnO 2-CNTs hybrid catalysts for achieving both high energy/power density and cyclic ability of rechargeable zinc-Air battery,” Sci. Rep., vol. 6, no. September, pp. 1–10, 2016.[2] D. Aasen, M. Clark, and D. G. Ivey, “ A Gas Diffusion Layer Impregnated with Mn 3 O 4 ‐Decorated N‐Doped Carbon Nanotubes for the Oxygen Reduction Reaction in Zinc‐Air Batteries ,” Batter. Supercaps, vol. 2, no. 10, pp. 882–893, 2019.[3] J. Yi et al., “Non-noble Iron Group (Fe, Co, Ni)-Based Oxide Electrocatalysts for Aqueous Zinc-Air Batteries: Recent Progress, Challenges, and Perspectives,” Organometallics, vol. 38, no. 6, pp. 1186–1199, 2019.