An abnormal size effect enables ampere-level O2 electroreduction to hydrogen peroxide in neutral electrolytes

Abstract

The principle of size effect was noticed a century ago, and is now widely used to express the activity enhancement of a catalyst as a result of size decrease. However, there are exceptions. In this study we observe an abnormal size effect in oxygen electroreduction comprised of two- (to generate H₂O₂) and four-electron-transfer pathways (to generate H₂O), where hydrogen peroxide (H₂O₂) is the target product. We have synthesized three zinc oxide nanoplate catalysts with different thicknesses (i.e., 87.7, 26.3 and 1.70 nm). In contrast to common expectation, the large-thickness zinc oxide plate (i.e., 87.7 nm) showed superior oxygen electroreduction activities with nearly 100% selectivity to H₂O₂ in the current density range of 0.05–1.3 A cm⁻² in neutral electrolytes, while other size counterparts only demonstrated moderate activities. A mechanism study through theoretical calculations and in situ Raman spectra highlight the critical role of abnormal size effects in enabling oxygen electroreduction at elevated current densities, which can not only activate O₂ but also stabilize the key reaction intermediate (*OOH) by shifting the d-band center toward the Femi level. Furthermore, the feasibility of the present technology is evaluated by following a rigorous techno-economic assessment protocol, showing a threshold current density of 0.2 A cm⁻² for economically profitable manufacture, and the lowest H₂O₂ production cost (0.4 $ kg⁻¹) achieved at 1.0 A cm⁻².