Hydrogen Peroxide Electrosynthesis in Natural Water: Dual Role of Calcium Scaling in Two-Electron Oxygen Reduction Reaction.

The two-electron oxygen reduction reaction (2e- ORR) represents an attractive strategy for hydrogen peroxide (H2O2) electrosynthesis, which has stimulated growing interest in decentralized water treatment. Conventionally, ubiquitous Ca2+ in natural water induces electrochemical scaling, which has been widely regarded as the predominant factor suppressing the 2e- ORR. However, this study reveals unprecedented protective effects of calcium scale on the 2e- ORR, challenging previous perspectives. Calcium scales as alkaline barriers reduce proton availability and prevent parasitic reduction of in situ generated H2O2, thereby enhancing 2e- ORR selectivity. In contrast, theoretical and experimental results prove that accumulated Ca2+ at the electrolyte-cathode interface promotes the reduction of H2O2 by enhancing water dissociation, serving as the main contributor to deterioration in H2O2 electroproduction. The carbon catalysts with a lower oxygen content exhibit superior Ca2+ tolerance. Furthermore, CaCO3 modification strategies are proposed to achieve a remarkable 2- to 6-fold improvement in the H2O2 yield. Testing with natural water confirms the excellent H2O2 electroproduction performance of CaCO3-coated cathodes over a 10-cycle period. These findings offer a novel perspective for understanding the effects of Ca2+ and calcium scale on H2O2 electrosynthesis with potential implications extending to other proton-coupled electrochemical systems.