Enhanced co-production of H2 and formic acid via Ni-facilitated Cu+/Cu2+ cycling in an industry-level hybrid water splitting system

Replacing the oxygen evolution reaction (OER) with the glucose oxidation reaction (GOR) to produce formic acid (FA) is a promising development in green hydrogen production. However, at high current densities, competition with the OER reduces the Faradaic efficiency (FE) of the GOR, limiting its practical applicability and compromising safety. In this study, NiO/CuO nanocomposites grown on Ni foam (NiCu-O/NF) were prepared by oxidizing NiCu-OH/NF to produce a GOR electrocatalyst. The incorporation of Cu effectively suppressed the OER. Additionally, Cu²⁺ in CuO can spontaneously oxidize glucose, generating Cu₂O. However, the resulting Cu₂O fails to sustain a high-rate GOR. Owing to the higher surface work function of NiO than that of Cu₂O, high-valence Ni species facilitated the reconversion of Cu²⁺, as evidenced by characterization before and after the GOR. The robust Cu⁺/Cu²⁺ cycling enhanced the GOR catalytic performance and ensured good catalytic stability over 40 h. A potential pathway was proposed for producing FA by the GOR. A two-electrode electrolyzer assembled with Ni1Cu2-O/NF (anode) and Ni1Cu2-OH/NF (cathode) afforded 100 and 600 mA cm⁻² at 1.46 and 1.82 V, respectively, without any OER interference. Furthermore, FA was confirmed to be the primary anodic product, achieving a maximum FE of 92.67 % at 1.3 V.