Formaldehyde electrolysis in a membrane-free electrolyzer: low-energy hydrogen and formate co-production with Cu-based boride electrocatalysts

Abstract

The 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/Cu_xO/CF) that efficiently catalyzes FOR with an ultralow onset potential of 0.06 V at 100 mA cm⁻². Compared to its phosphorus- and sulfur-doped counterparts (P/Cu_xO/CF and S/Cu_xO/CF), B/Cu_xO/CF exhibits markedly superior activity with ∼97% formate yield. Surface analysis confirms the critical role of coexisting Cu⁺/Cu²⁺ species in B/Cu_xO in facilitating the key FOR steps of adsorption and C–H cleavage, while the presence of boron improves charge transfer and active site availability. The limited HER activity of B/Cu_xO/CF 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‖B/Cu_xO/CF configuration achieved 100 mA cm⁻² at only 0.25 V and sustained long-term stability with continuous HCHO replenishment, maintaining 300 mA cm⁻² 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 the 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.