Formaldehyde Electrolysis in a Membrane-Free Electrolyzer: Low-Energy Hydrogen and Formate Co-Production with Cu-Based Boride Electrocatalysts

Abstract

Formaldehyde oxidation reaction (FOR) offers a low-energy alternative to the oxygen evolution reaction (OER) for electrochemical hydrogen production, enabling simultaneous generation of value-added formate. Here, we report a boron-doped copper catalyst on copper foam (B/CuₓO/CF) that efficiently catalyzes FOR with an ultralow onset potential of 0.06 V at 100 mA/cm2. Compared to its phosphorus- and sulfur-doped counterparts (P/CuₓO/CF and S/CuₓO/CF), B/CuₓO/CF exhibits markedly superior activity with ~97% formate yield. Surface analysis confirms the critical role of coexisting Cu⁺/Cu2+ species in B/CuₓO for facilitating key FOR steps of adsorption and C-H cleavage, while the presence of boron improves charge transfer and active site availability. B/CuₓO/CF limited HER activity was effectively addressed by coupling it with a Ni-based phospho-boride catalyst on Ni foam (NiPB/NF) that delivers selective and high hydrogen evolution performance, even in formaldehyde-containing media. The asymmetric NiPB/NF ǀǀ B/CuₓO/CF configuration achieved 100 mA/cm2 at only 0.25 V and sustained long-term stability with continuous HCHO replenishment, maintaining 300 mA/cm2 for over 8 hours. Operated in a membrane-free flow cell, the system maintained ~90% formate yield with a H2 production Faradaic efficiency of ~190%, nearly doubling hydrogen output. This hybrid strategy not only lowers energy input by 1.64 V compared to water electrolysis but also demonstrates viability for decentralized hydrogen and chemical co-production with economic benefits.