Monoethanolamine-assisted Cu/Cu₂O composites for efficient CO₂ electroreduction to formate

In recent periods, the increase in energy demand accompanied by population growth has intensified CO₂ emissions into the atmosphere, disrupting the carbon cycle and enhancing the greenhouse effect. In this context, CO₂ capture and conversion have emerged as promising strategies. Among them, CO₂ electrochemical reduction (CO₂ER) has gained particular attention. Accordingly, the present work aimed to develop gas diffusion electrodes (GDEs) with layers of CuO or Cu/Cu₂O composites for CO₂ER. The catalyst were synthesized by preparing an ethanolic solution of copper acetate (0.05 M), either with or without addition of monoethanolamine (Cu:ETA = 2:3). The solution was then subjected to 20 h in solvothermal reactor, resulting in CuO (JCPDS 48-1548) or Cu/Cu₂O composite (JCPDS 04-0836 and 71-3645), as confirmed by X-ray diffraction analysis. The samples were deposited onto carbon paper via spray coating to produce GDEs. CO₂ER tests were performed in membrane electrode assembly (MEA) cell (2 cm²) under galvanostatic conditions (150 mA·cm^-2, 2 h) with CO₂ flow of 30 mL·min^-1, using nickel foam as counter electrode and an anion exchange membrane. Gas and liquid products were quantified by gas chromatography (GC) and proton nuclear magnetic resonance (¹H NMR). After 2 h, formate (HCOO^‒) was the predominant liquid-phase product, reaching 1051 μmol in CuO and 2274 μmol in Cu/Cu₂O. The faradaic efficiency (FE) for formate formation was 14.1% for CuO and 30.5% for Cu/Cu₂O, indicating that the Cu/Cu₂O composite synthesized with ETA exhibited higher selectivity toward formate and greater energy efficiency, operating at 2.5 V compared to 2.9 V for CuO. Formate formation is particularly relevant, as it represents a selective two-electron reduction pathway and serves as a valuable chemical feedstock and energy carrier in sustainable technologies. ETA plays a key role in stabilizing Cu/Cu₂O phase, enhancing both selectivity and efficiency for formate production.