Catalyst development and air electrode structure engineering for rechargeable Zn-air batteries

Abstract

The fast-growing energy consuming of modern society requires the development of new energy conversion and storage technologies with low cost and high environmentally friendliness. Zn-air batteries (ZABs) have been an attractive candidate for the next generation rechargeable battery due to the abundant raw materials and the intrinsic safety of the aqueous electrolyte. Although the primary ZABs have been commercialized, the development of rechargeable ZABs is still challenging. Specifically, the large voltage gap and the consequent low energy efficiency is a critical problem, which mainly results from the sluggish oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) at the air electrodes. Meanwhile, the oxidative corrosion under the high charging potentials leads to the degradation of catalysts at the air electrodes, and thus insufficient cycle stability, which also impedes the practical application of Zn-air batteries. To address these challenges, efforts were paid in both the catalytic material development and electrode structure design, aiming at reducing the overpotential at the air electrodes, as well as enhancing the cycle life. For the catalytic material development, the hybrid strategy is utilized and thoroughly investigated, where the two components in a hybrid could combine the OER and ORR sites to obtain the bifunctional electrocatalysts. Besides, the strong interaction between the two components could regulate the electronic structure of the catalytic sites towards a higher intrinsic activity. Specifically, hybrid electrocatalysts of CeO2/LaFeO3 and La0.8Sr0.2Mn0.5Co0.5O3/RuOx were developed, which indicated that the interfacial elemental diffusion and the interfacial electron transfer play a critical role in enhancing the electrocatalytic activity. Additionally, the noble metal-nonnoble metal hybrid strategy was proposed, aiming at realizing a high atom utilization of noble metals, and thus highly active bifunctional electrocatalysts with low expense. Specifically, the Ir cluster anchored Co3O4 was synthesized, and mechanistic synergy between Ir and Co during the oxygen catalysis was scrutinized. It was demonstrated that the Ir sites assist the achieve the high reaction order due to the intrinsically fast kinetics, whereas Co plays the key role in accumulate sufficient charges due to the excellent pseudocapacitive characteristic. The in-depth understanding provides clear guidance for choosing suitable nonnoble metal supports for the noble metal-nonnoble metal hybrid catalysts. Furthermore, a smart design of electrode structure is carried out to achieve microscale decoupled OER and ORR sites at the air electrode. By using the decoupled-structure, the hydrophobicity of the air cathode could be optimized to realize abundant triple-phase interface and thus high discharge voltage. Moreover, the decoupled-structure could alleviate the oxidative degradation of ORR electrocatalyst and the carbon component during the charging process, and thus efficiently elongated the rechargeability and cycle life of the Zn-air batteries. This work displays a comprehensive study of the air electrode development from both chemistry and engineering aspects. The hybrid material design strategy, the in-depth mechanism of the oxygen catalytic reaction process, as well as the smart electrode structure design, would be valuable for the further improvement of the rechargeable ZABs.