Steady Cu+ species via magnesium and boron co-modification for enhanced CO2 electroreduction to C2+ products: an in situ Raman spectroscopic study†

Abstract

The electrochemical carbon dioxide reduction reaction (CO₂RR) to produce high-value multi-carbon (C₂₊) compounds holds significant practical importance in realizing carbon neutrality. Copper-based electrocatalysts are promising for CO₂-to-C₂₊ conversion. However, the labile Cu valence at high current densities impedes C₂₊ product generation. Here, we present an electrocatalyst derived from CuO, featuring a heterostructure of Cu/Cu₂O/CuO/Mg(OH)₂via the co-addition of Mg and B during the preparation (referred to as Cu₅(B_{0.02 M})Mg₁). The Cu₅(B_{0.02 M})Mg₁ shows an impressive C₂₊ yield, with a Faraday efficiency (FE_C2+) of 79.59% at −1.57 V vs. RHE (reversible hydrogen electrode). Additionally, the partial current density of C₂₊ on the Cu₅(B_{0.02 M})Mg₁ catalyst is −317.03 mA cm⁻², 2.7 and 3.5 times higher than those catalysts lacking B (Cu₅Mg₁ catalyst) or Mg (Cu(B_{0.02 M}) catalyst), respectively. Over a wide potential range of 600 mV between −1.17 and −1.77 V vs. RHE, the overall FE_C2+ surpasses 60% on the Cu₅(B_{0.02 M})Mg₁ catalyst. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM) characterization studies indicate the NaBH₄ reactant could promote the formation of crystalline Mg(OH)₂ in the catalyst structure, which is found to better stabilize Cu⁺ at negative potentials compared to the amorphous phase. Further in situ Raman spectroscopy reveals that at increasingly negative potentials, the higher copper species (Cu²⁺) is inevitably reduced to the lower copper species (Cu⁰/Cu⁺). However, the synergy of Mg and B prolongs the presence of Cu⁺ on the catalyst surface across a broad potential range, and *CO and *CO₂⁻ could still be recorded at quite negative reduction potentials. This suggests enhanced binding strength of *CO intermediates on the catalyst surface, promoting the C–C coupling process.