Silver single atom in polymeric carbon nitride as a stable and selective oxygen reduction electrocatalyst towards hydrogen peroxide synthesis

Abstract

Electrochemical hydrogen peroxide (H₂O₂) synthesis via the two-electron oxygen reduction reaction (2e⁻ ORR) offers a promising alternative to the traditional anthraquinone process. In this study, we report a silver single-atom catalyst Ag(I) coordinated within a polymeric carbon nitride (PCN) framework (Ag–PCN), as a highly selective and durable electrocatalyst for H₂O₂ generation. For the first time, particular attention was given to evaluating catalyst stability under harsh oxidative conditions, specifically 3% H₂O₂ solution for one week. Ag–PCN exhibited superior H₂O₂ selectivity in 0.1 M KHCO₃ compared to pristine PCN. Although pristine PCN initially showed higher activity, it suffered from poor oxidative stability, losing 9% of its mass, whereas Ag–PCN displayed only 1% of mass loss. Inductively coupled plasma (ICP) analysis further confirmed minimal Ag leaching (0.3 wt%) after one week, underscoring its superior chemical durability. Remarkably, Ag–PCN demonstrated that enhanced faradaic efficiency (FE) post oxidative stress, likely due to structural and chemical rearrangements occurring during the stability test. In H-type cell experiments, Ag–PCN-7 achieved an H₂O₂ concentration of 1.55 mg L⁻¹ within 2 hours, yielding a FE of 20% at 0.42 V vs. RHE. Additionally, Ag–PCN exhibited improved thermal stability compared to PCN. Density functional theory (DFT) calculations on a model heptazine Ag(I) complex revealed that Ag(I) serves as an active site, facilitating OOH* intermediate binding and mediating charge transfer from the PCN framework to the adsorbed species. Overall, these results establish Ag–PCN as a promising catalyst with high selectivity, remarkable chemical and thermal stability, and strong potential for electrochemical H₂O₂ production.