Designing Bifunctional Bilayer Air Electrode for Zinc−Air Batteries
Authors: Liu Rui , Ikezawa Atsunori , Okajima Takeyoshi , Arai Hajime 
Published: December 2024

Rechargeable zinc-air batteries, typically composed of a zinc metal negative and an air positive electrode are promising candidate for next-generation energy-storage devices owing to its high theoretical energy density, abundant raw materials and safe non-flammable electrolytes. Nevertheless, the primary challenges encountered by air electrode are the high overpotential and limited cycling durability due to the sluggish kinetics of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER), occurring in the discharging and charge processes. It has been demonstrated that ORR and OER take place at three-phase boundaries and effective reaction fields are different between ORR and OER, leading to varying rate-limiting factors for each reaction, thus they require different chemical environment [1-3]. Therefore, constructing a bifunctional air electrode with highly active catalysts and optimized configuration for both ORR and OER is crucial for addressing mentioned issues in air cathodes.
Herein, a strategy of bilayer air electrode fabrication is proposed for high-performance and long-term durability zinc−air batteries. Concretely, ORR catalyst layer and OER catalyst layer were combined with gas diffusion layer (carbon paper) to fabricate gas diffusion electrode, where ORR catalyst layer was concentrated on gas diffusion layer side for promoting oxygen diffusion in electrolyte, while OER catalyst layer was concentrated on electrolyte side for facilitating hydroxide ions transfer [3]. In this study, the LaMnO3 (LMO) synthesized by the traditional polymerized complex method and the partially Fe-substituted Ni oxyhydroxide (Ni0.7Fe0.3OOH) prepared by a soft chemical method [4] were applied as OER catalyst and ORR catalyst, respectively. Through optimization of bilayer fabrication process and parameters of catalyst layer, we’ve successfully fabricated a firmly bound bilayer electrode. It exhibits desirable high ORR and OER activities in alkaline electrolytes, coupled with long-term durability, rendering it highly promising for practical application in Zinc-air batteries. In addition, the EDS mapping characterization (Figure 1) of the cross-section of CP/LMO/Ni0.7Fe0.3OOH bilayer gas diffusion electrode reveals a desirable bilayer structure, which LMO and Ni0.7Fe0.3OOH catalyst layers exhibit even distribution throughout the lower and upper layer, respectively, as well as displaying uniform morphology and similar microstructures. This strategy of designing bilayer electrodes relying on different reactions provides a feasible avenue for bifunctional electrode design and further motivates exploration of electrode configuration involving multiphase electrochemical reactions.

Keywords: Zinc-air batteries, bifunctional air electrode, bilayer gas diffusion electrode

References:

[1] Liu, C. Zhao, D. Ren, Adv. Mater., 34, 2109407 (2022).
[2] Ikezawa, K. Miyazaki, T. Fukutsuka, T. Abe, Electrochem. Commun., 84, 53 (2017).
[3] Ikezawa, K. Seki, H. Arai, Electrochim. Acta, 394, 139128 (2021).
[4] Ikezawa, S. Koito, H. Arai, Presentaion at ACEPS-12 (2024)

Acknowledgement: JRP-LEAD with DFG (JPJSJRP20221602)

Figure 1