Experimental design of Cu-based catalysts with different crystal facets for electrocatalytic CO₂ reduction

Electrocatalytic CO₂ reduction is an important approach to converting the greenhouse gas CO₂ into high-value chemicals and fuels under mild conditions, and this approach is highly compatible with renewable electricity. Cu-based catalysts are particularly important for CO₂ electroreduction because they can facilitate the deep reduction of CO₂ to hydrocarbons and oxygenates. However, product distributions over Cu catalysts are often complex and strongly influenced by surface structures. Therefore, crystal facet engineering is needed to regulate surface atomic arrangements, intermediate adsorption, and product selectivity. This experiment is designed around the controllable preparation of Cu₂O catalysts with different exposed facets and their application in electrocatalytic CO₂ reduction. It aims to guide students in mastering the complete research process, including material synthesis, structural characterization, electrochemical testing, product quantification, and structure–performance correlation analysis, thereby improving their scientific thinking, practical skills, and awareness of innovation.

Cu₂O nanoparticles with selectively exposed (100), (111), and mixed crystal facets (designated Cu₂O-A, Cu₂O-B, Cu₂O-C, and Cu₂O-D) were synthesized via a wet chemical reduction method by varying the amount of the reducing agent, hydroxylamine hydrochloride (NH₂OH·HCl). The physicochemical properties and surface structures of the synthesized catalysts were systematically characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT–IR), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS). Subsequently, the electrocatalytic CO₂ reduction performance of the catalysts was evaluated in a standard H-cell with a three-electrode configuration, while the gaseous and liquid products were quantitatively analyzed via gas chromatography and nuclear magnetic resonance spectroscopy, respectively.

XRD results confirmed that all Cu₂O samples exhibited characteristic diffraction peaks corresponding to Cu₂O without detectable impurity phases, indicating their high phase purity and crystallinity. FT–IR spectra showed a distinct Cu–O vibration band, further verifying the formation of Cu₂O. SEM images showed that Cu₂O-A presented a cubic morphology with a predominantly exposed (100) facet; Cu₂O-B and Cu₂O-C exhibited mixed-facet structures containing both (100) and (111) facets; and Cu₂O-D displayed an octahedral morphology with predominantly exposed (111) facets. As the amount of hydroxylamine hydrochloride increased, the proportion of exposed (100) facets gradually decreased, while that of the (111) facets increased. XPS analysis demonstrated that Cu⁺ was the main chemical state, accompanied by a small amount of Cu²⁺ resulting from partial surface oxidation. The O 1s spectra indicated the coexistence of lattice oxygen, oxygen vacancies, and adsorbed oxygen species. Electrochemical measurements showed that all Cu₂O catalysts exhibited higher current densities in CO₂-saturated electrolyte than in Ar-saturated electrolyte, confirming their CO₂ reduction activity. The Double layer capacitance (C_dl) values of Cu₂O-A, Cu₂O-B, Cu₂O-C, and Cu₂O-D were 60.0, 74.6, 55.0, and 65.0 μF cm⁻², respectively, indicating that Cu₂O-B possessed the largest electrochemically active surface area. Product analysis showed that different Cu₂O crystal facets resulted in different CO₂ reduction selectivity. At low overpotential, C₂H₄ was an important product, while H₂ evolution became increasingly competitive at more negative potentials. Among all samples, Cu₂O-D showed the highest CH₄ selectivity, reaching a maximum Faradaic efficiency of 49.2% at −1.6 V versus the reversible hydrogen electrode. Continuous electrolysis for 8 h showed that Cu₂O-D maintained a current density of ~20 mA cm⁻² and stable CH₄ production, demonstrating good catalytic stability.

This comprehensive experiment successfully transforms frontier research on crystal facet–regulated electrocatalytic CO₂ reduction into an undergraduate teaching experiment. The results reveal a clear structure–performance correlation between Cu₂O crystal facets and CO₂ reduction selectivity. Compared with the (100) facet, the (111) facet is more favorable for CH₄ formation. Through the full experimental workflow, including controllable synthesis, advanced characterization, electrochemical evaluation, and product analysis, students can gain an understanding of how surface structure regulates catalytic behavior and how experimental evidence supports mechanistic interpretation. This experiment integrates materials chemistry, physical chemistry, electrochemistry, and catalysis and is suitable for senior undergraduate comprehensive laboratory courses in chemistry, chemical engineering, materials science, and related disciplines. It provides an effective platform for cultivating students’ research-oriented thinking, practical skills, teamwork ability, and awareness of green and low-carbon development.