A copper foam material for hydrogen generation from formaldehyde oxidation

The primary objective of this study is to elucidate the structure–activity correlation between copper valence evolution and formaldehyde oxidation reaction (FOR) performance, identify the intrinsic active sites for both thermocatalytic and electrocatalytic HCHO oxidation, and construct a high-performance mixed-valence copper-based catalyst system capable of enabling ultra-low-cell-voltage bipolar hydrogen production without CO₂ emissions.

A series of copper foam (CF)–based catalysts with precisely regulated valence states were fabricated via a facile three-step strategy involving chemical oxidation, inert atmosphere annealing, and controlled electrochemical reduction. The physicochemical properties of the as-prepared catalysts were systematically characterized using scanning electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy. Liquid-phase products (formate) were quantified using ¹H nuclear magnetic resonance spectroscopy with dimethyl sulfoxide as an internal standard, while gaseous H₂ production and generation rates were monitored in real time using gas chromatography. In situ differential electrochemical mass spectrometry (DEMS) with D₂O/NaOD electrolytes was employed to identify the hydrogen source. Linear sweep voltammetry and chronoamperometry measurements were conducted in a three-electrode H-type cell to evaluate electrocatalytic performance. Control experiments using pre-reduced Cu₂O/CF were further conducted to verify the synergistic effect of Cu⁰ and Cu⁺ species.

Structural characterization confirmed that the as-synthesized Cu(OH)₂/CF, Cu₂O/CF, and Cu₂O/Cu/CF catalysts exhibited distinct morphologies and valence-state distributions. Cu(OH)₂/CF consisted of vertically aligned nanorod arrays dominated by Cu²⁺ species, while Cu₂O/Cu/CF exhibited a partially collapsed nanostructure containing coexisting Cu⁺ and Cu⁰ phases. Thermocatalytic tests under open-circuit conditions revealed that Cu(OH)₂/CF achieved the highest H₂ generation rate of 0.433 mL·min^–1, surpassing those of Cu₂O/Cu/CF (0.388 mL·min^–1) and Cu₂O/CF (0.334 mL·min^–1). These results established the activity order of Cu²⁺ > Cu⁰ > Cu⁺ for the thermocatalytic FOR. Under an applied potential of 0.3 V vs. the reversible hydrogen electrode (RHE), the H₂ generation rate of Cu₂O/Cu/CF increased to 0.520 mL·min^–1, with a H₂ yield of 14.9 mL, representing a 34.2% improvement compared with that under open-circuit conditions. DEMS analysis demonstrated that all H atoms in the generated H₂ originated exclusively from the aldehyde group of HCHO rather than the electrolyte solvent. Electrochemical control experiments further proved that the Cu⁰–Cu⁺ synergistic structure served as the core active phase for the electrocatalytic FOR, while isolated Cu²⁺ or Cu⁺ species exhibited limited activity. Additionally, Cu₂O/Cu/CF achieved a Faradaic efficiency of 95.8% at 0.3 V vs. RHE, with a HCHO conversion rate exceeding 90% and a formate selectivity of over 93%. Post-reaction XRD indicated that high-valence Cu species were gradually reduced to metallic Cu during the reaction, confirming that the applied potential functions to dynamically regulate the surface valence state of the catalyst and accelerate reaction kinetics.

This study clarifies the distinct roles of copper valence states in thermocatalytic and electrocatalytic HCHO oxidation. Cu²⁺ serves as the dominant active site for spontaneous thermocatalytic H₂ production, whereas the Cu⁰–Cu⁺ interface acts as the intrinsic active center for the low-potential electrocatalytic FOR, with the applied potential regulating both reaction kinetics and valence-state evolution. The developed mixed-valence Cu₂O/Cu/CF catalyst achieves highly efficient, zero-carbon H₂ production under mild conditions, providing fundamental insights for the design of stable copper-based electrocatalysts and facilitating the development of ultra-low-energy bipolar hydrogen production systems as alternatives to traditional OER-coupled water electrolysis.