Electrochemical CO₂ reduction to formate on indium catalysts prepared by electrodeposition in deep eutectic solvents

The electrochemical conversion of CO₂ to high-value molecules is an elegant alternative for combining CO₂ utilization with renewable energy conversion and storage. Herein we report the preparation and characterization of indium catalysts for the electrochemical CO₂ reduction to formate. Indium coatings were prepared by electrodeposition from a deep eutectic solvent (DES) comprising 1:2 M choline chloride and ethylene glycol (12CE). The electrochemical behavior of indium chloride in this DES was investigated by cyclic voltammetry (CV) on copper, glassy carbon (GC) and platinum electrodes. The effect of InCl₃ concentration, electrolyte temperature and deposition method on the phase and morphology of the coatings were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Indium deposits on copper and carbon were deployed as catalysts for the CO₂ electrolysis in aqueous media. Chemical analysis by HPLC, GC, and NMR revealed an optimum efficiency toward formate at −1.9 V vs. Ag/AgCl. Indium coatings prepared by potentiostatic deposition showed faradaic efficiencies (FE) up to 72.5%. Gas diffusion electrodes (GDE) coated with indium led to formate concentrations up to 76 mM and formation rates of 0.183 mmol cm⁻² h⁻¹, which was considerably superior to indium coatings on planar electrodes.