In recent decades, population growth and increasing energy demand have intensified CO2 emissions, destabilizing the carbon cycle and exacerbating global warming. In this context, CO2 capture and conversion emerge as promising alternatives, with CO2 electrochemical reduction (CO2ER) standing out as a strategy capable of converting CO2 into higher-value products, simultaneously enabling energy storage and climate mitigation [1].

The present work aimed to develop gas diffusion electrodes (GDEs) containing either CuO or Cu/Cu2O composites for application in CO2 electroreduction. The methodology was structured in three main steps: catalyst synthesis, electrode preparation, and electrochemical performance analysis.

For the synthesis, a solvothermal reactor was used for 20 h with an ethanolic solution (110 mL) of copper acetate (0.05 M), with and without the addition of monoethanolamine (ETA) at a Cu:ETA molar ratio of 2:3 [2]. Structural phases were characterized by X-ray diffraction. The sample without ETA showed a pattern corresponding to CuO (JCPDS 48-1548), whereas the sample with ETA resulted in a mixture of Cu2O (JCPDS 71-3645) and metallic Cu (JCPDS 04-0836), forming the Cu/Cu2O composite. The samples were named according to their identified phases.

For GDE fabrication, a suspension containing 18 mg of catalyst, 1.8 mg of carbon black, 45 µL of Nafion, 0.9 mL of isopropyl alcohol, and 1.8 mL of water was prepared and applied onto carbon paper via spray coating [3]. Electrochemical tests were conducted in a membrane electrode assembly (MEA) cell (2 cm²) under galvanostatic conditions at 150 mA·cm⁻2 for 2 h, with a CO2 flow of 30 mL·min⁻1. The counter electrode was nickel foam, separated from the cathode by a Sustainion X37-50 RT anion exchange membrane (~50 μm). Gas products were quantified by gas chromatography, while liquid products were analyzed by 1H NMR.

Results indicated that H₂ and CO were the main gaseous products, with H2 formation competing with CO2 reduction [1]. For CuO, CO decreased from 354 μmol (1 h) to 198 μmol (2 h) (–44.2%), while H2 increased from 118 μmol to 184 μmol (+56.1%). For Cu/Cu2O, CO dropped from 159 μmol to 88 μmol (–44.6%) and H2 rose from 367 μmol to 444 μmol (+21.0%).

Among the liquid products, formate was predominant. In CuO, it increased from 741 μmol to 1051 μmol (+41.8%), while ethanol grew from 37 μmol to 62 μmol (+66.0%) and propanol from 21 μmol to 46 μmol (+119.0%). In the Cu/Cu2O composite, formate increased from 1351 μmol to 2274 μmol (+68.3%), ethanol from 49 μmol to 87 μmol (+77.9%), and propanol from 23 μmol to 40 μmol (+74.0%).

The results show that, although both catalysts exhibited a decrease in CO conversion over time, the Cu/Cu2O composite stood out for its higher selectivity and energy efficiency, operating at a lower potential (2.5 V) compared to the CuO electrode (2.9 V). Among the products obtained, the formation of formate was particularly relevant, not only because it represents an accessible and selective two-electron reduction pathway, but also due to its strategic value as a chemical feedstock and energy carrier in sustainable technologies [4]. Therefore, the enhanced formate production achieved with Cu/Cu2O reinforces its potential as a promising catalyst for CO2 electroreduction in continuous and more energy-efficient processes.