Catalyst-Engineered Proton Transfer Pathways for Selective Hydrogen Peroxide Electrosynthesis in Solid-State Electrolytes

Polymer-based solid electrolyte (SE) cells promise electrochemical synthesis of pure hydrogen peroxide (H₂O₂), yet the protonation mechanisms governing the two-electron oxygen reduction reaction (2e⁻-ORR) remain unclear when using pure water as the proton source. Both Langmuir–Hinshelwood (LH, surface *H-mediated) and Eley–Rideal (ER, water-derived proton-coupled) pathways are theoretically plausible, but their practical dominance under SE conditions lacks experimental validation. Herein, we designed a hierarchical Ni─N₂─C─O single-atom/NiO nanocluster co-decorated porous carbon nanosheet catalyst (NiSA-NiO/pCNs) that achieved a Faradaic efficiency of 97% and a H₂O₂ partial current density of 356 mA cm⁻² (equivalent to 6.6 mmol cm⁻² h⁻¹ production rate) in a porous SE cell. Analysis of reaction intermediates and the local pH using in situ Raman spectroscopy, kinetic isotope effect, and density functional theory simulations showed the critical role of NiO nanoclusters in tuning the protonation pathway: NiO activates the ER mechanism via fast proton transfer from water dissociation, whereas NiSA/pCNs without NiO preferentially follow the LH mechanism through surface-adsorbed *H intermediates from interfacial transferred proton. These findings establish a catalyst design principle for proton transfer control in solid-state H₂O₂ electrosynthesis.