Photo- and Electrocatalytic Dual-Layer Cathode Facilitating Zn Peroxide Chemistry in Near-Neutral Zn–Air Batteries

Abstract

Rechargeable zinc-air batteries (ZABs) using near-neutral aqueous electrolytes are gaining significant attention due to their high energy density, low cost, high safety, and the excellent reversibility of the zinc (Zn) anode in mild electrolytes. However, the sluggish O₂/ZnO₂ conversion in the carbon-based cathodes of these batteries leads to a large voltage hysteresis (>600 mV) between charge and discharge. Metal- or metal oxide-based electrocatalysts are rarely used to reduce the overpotentials of this conversion because their presence may trigger undesirable H₂O-participated oxygen reduction/evolution reactions, disrupting the pH balance of the electrolyte. Here, we propose a dual-layer catalytic cathode comprising an outer photocatalyst layer (exposed to air) of gold (Au) nanoparticles (NPs) decorated tungsten oxide (Au@WO₃) loaded on carbon paper, and an inner electrocatalyst layer (exposed to the electrolyte) based on carbon nanotube (CNT). The hydrophobic inner CNT layer not only provides numerous active sites and ample accommodation for O₂/ZnO₂ conversion but also prevents the electrolyte from contacting the outer photocatalyst layer. Under light, the outer photocatalyst layer effectively separates photogenerated electron-hole pairs, which are then transferred to the inner CNT layer, reducing the overpotential of the O₂/ZnO₂ electrochemical conversion. As a result, the near-neutral ZAB demonstrates high stability at 0.1 mA cm^-2; with a very small voltage hysteresis (<150 mV), significantly improving energy efficiency.