Aqueous CO2 reduction on morphology controlled CuxO nanocatalysts at low overpotential

Abstract

Various Cu_xO catalysts with different special microstructures were synthesized using a simple one-step hydrothermal method by controlling the reaction time and temperature conditions. Scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM) were used to observe the morphologies of the received catalysts. The 3-dimensional (3D) hierarchical nanospheres (500 nm) comprised of secondary structured nanorods (50 nm) are formed at 180 °C for 2 hours. However, when increasing the hydrothermal reaction temperature to 220 °C, solid microspheres with a large size of 2.5 μm begin to appear instead of flabby hierarchical nanospheres. To further investigate the effect of morphologies on the activity and production selectivity of Cu_xO catalysts, cyclic voltammetry (CV) was used to evaluate the onset potential and current density of catalyzed CO₂ reduction combining linear sweep voltammetry (LSV) in 0.5 M KHCO₃ solution. The effect of catalyst loading was also tested by applying the gas diffusion layer (GDL) to make up a working electrode for CO₂ electroreduction. The results indicate that the synthesized temperature of 180 °C for 2 h is the optimal condition for Cu_xO nanospheres and the optimal loading is about 3 mg cm⁻², under which the onset potential for CO₂ electroreduction reaches −0.55 V vs. SHE. By ion chromatography measurement, the faradaic efficiency and production rate of produced formate was found to be 59%, which is much higher than most reported Cu-based catalysts at the same electrolysis conditions, indicating the high selectivity of the Cu_xO nanospheres due to their controlled special surface morphology.