Impact of catalyst loading of atomically dispersed transition metal catalysts on H2O2 electrosynthesis selectivity

Electrosynthesis of hydrogen peroxide (H2O2) via a two-electron oxygen reduction reaction (2e− ORR) has emerged as a promising alternative to the current anthraquinone process. Atomically dispersed transition metal catalysts (M–N/C catalysts; M=transition metal) have received particular attention as H2O2 electrosynthesis catalysts. Among the factors affecting catalytic properties, the catalyst loading on the electrode has significant impacts on the ORR activity and selectivity of M–N/C catalysts in particular and electrocatalysts in general. However, the loading effect has been largely neglected and underexplored in literature. In this study, we investigated the impacts of the catalyst loading and the metal center in M–N/C catalysts on the 2e− ORR activity and selectivity. We prepared a series of mesoporous carbons comprising M (Fe, Co, Ni)-based atomically dispersed species, meso‑M–N/C catalysts. At a fixed low catalyst loading, the meso‑Co–N/C and meso‑Ni–N/C catalysts exhibited the best 2e−ORR performance under acidic and alkaline electrolytes, respectively. At high catalyst loadings, the H2O2 production activity of the meso‑Co–N/C catalyst dramatically declined to ∼20% compared to that at the low loading. In the thick catalyst layer, the generated H2O2 is accumulated due to poor mass transport, leading to reduction and/or decomposition. In contrast, the H2O2 synthesis activity meso‑Ni–N/C catalyst was nearly insensitive to the catalyst loading, which can be ascribed to the high porosity and well-developed mesopores of the meso‑Ni–N/C catalyst, promoting mass transport.