S-doped NiCo layered double hydroxides enable efficient and durable methanol oxidation and oxygen reduction reactions for Zn-methanol-air batteries

Abstract

The employment of alcohol oxidation reactions (AORs) is thermodynamically more favorable to replace the anodic oxygen evolution reaction (OER) in rechargeable Zn-air batteries (ZABs). As a representative AOR-coupled ZAB, Zn-methanol-air battery (ZMAB) gratifyingly delivers a lower charge voltage, higher energy efficiency, longer cycle life and more valuable charge product in comparison with the traditional ZAB. For such emerging ZMABs, it is imperative to explore efficient, stable and cost-effective bifunctional electrocatalysts for both methanol oxidation reaction (MOR) and oxygen reduction reaction (ORR). Herein, sulfur-doped NiCo layered double hydroxides (termed NiCo-LDH@SOH) are fabricated by a room-temperature surface sulfurization route. Remarkably, the NiCo-LDH@SOH heterostructure catalyst achieves a superb bifunctional activity for MOR and ORR, performing a low potential gap ΔE between the Ej10 for MOR and the half-wave potential E1/2 for ORR of only 0.596 V, far surpassing that (0.790 V) for Pt/C-IrO2 benchmark catalyst. The electronic interactions between S2− and transition metal ions significantly accelerate the charge transfer process and generate highly efficient Ni3+ and Co2+ active sites on NiCo-LDH@SOH, which account for the exceptional MOR and ORR performance, respectively. Thanks to the favorable replacement of OER with MOR and the excellent MOR/ORR bifunctional activity of NiCo-LDH@SOH, the ZMAB can output a comparable discharge power density (140 mW cm−2) to the traditional ZAB counterpart (149 mW cm−2), but a much lower charge voltage than the latter. In consequence, the ZMAB demonstrates an extraordinary long-term cyclability of 2880 cycles for 480 h (20 days) with almost no degradation, which is more than 5 times longer than that of the traditional ZAB.