Monoethanolamine-Assisted Formation of Cu/Cu₂O Composites for Enhanced Electrochemical CO₂ Reduction to Formate

In recent years, increasing global energy demand driven by population growth has intensified CO₂ emissions, disrupting the carbon cycle and strengthening the greenhouse effect. In this context, CO₂ capture and utilization strategies have attracted significant attention, particularly electrochemical CO₂ reduction (CO₂ER). In this work, gas diffusion electrodes (GDEs) were developed using CuO and Cu/Cu₂O composite layers for CO₂ER. The catalysts were synthesized from an ethanolic solution of copper acetate (0.05 M), with or without monoethanolamine (MEA) addition at a Cu:MEA molar ratio of 2:3. The solutions were subjected to solvothermal treatment for 20 h, yielding CuO (JCPDS 48-1548) or Cu/Cu₂O composites (JCPDS 04-0836 and 71-3645), as confirmed by X-ray diffraction analysis. The resulting materials were deposited onto carbon paper via spray coating to fabricate GDEs. CO₂ER experiments were conducted in a membrane electrode assembly (MEA) cell (2 cm²) under galvanostatic conditions (150 mA·cm^-2 for 2 h), with a CO₂ flow rate of 30 mL·min⁻¹. Nickel foam and an anion exchange membrane were used as counter electrode and separator, respectively. Gas and liquid products were quantified by gas chromatography (GC) and ¹H nuclear magnetic resonance (¹H NMR). After 2 h of electrolysis, formate (HCOO⁻) was the main liquid product, reaching 1051 μmol for CuO and 2274 μmol for the Cu/Cu₂O composite. The corresponding Faradaic efficiencies for formate formation were 14.1% and 30.5%, respectively. The Cu/Cu₂O composite synthesized in the presence of MEA exhibited enhanced selectivity toward formate and improved energy efficiency, operating at 2.5 V compared to 2.9 V for CuO. These results demonstrate that MEA plays a crucial role in stabilizing the Cu/Cu₂O phase, promoting improved catalytic performance toward the selective two-electron reduction pathway to formate, a valuable chemical feedstock and energy carrier for sustainable technologies.