Abstract:

The electrochemical carbon dioxide reduction reaction (eCO₂RR) can convert CO₂ into valuable chemicals, achieving a carbon cycle. Copper-based catalysts have demonstrated a unique ability to produce C₂₊ products in eCO₂RR, which is often limited by the scaling relationship of the reaction intermediates, complex reaction mechanism and competitive H₂ evolution. Organic functionalization is a promising strategy for regulating the activity and selectivity of eCO₂RR toward C₂₊ products. However, the mechanism behind such regulation of eCO₂RR, especially at the molecular level, remains elusive. In this study, Cu nanoparticles were prepared and functionalized with a set of amine derivatives, including hexadecylamine (HDA), N-methylhexadecylamine (N-MHDA), hexadecyldimethylamine (HDDMA), and palmitamide (PMM). The impact of the molecular structure of the amine surfactants on the selectivity and activity toward eCO₂RR was systematically explored through both experiments and theoretical calculations. X-ray photoelectron spectroscopy and density functional theory calculations revealed that HDA functionalization of the Cu catalyst surface resulted in negative charge transfer from amine molecules to Cu. ECO₂RR was examined in a 1.0 mol∙L⁻¹ KOH aqueous electrolyte. HDA functionalization of the Cu catalyst achieved the highest Faradaic efficiency (FE) of 73.5% for C₂ products and 46.4% for C₂H₄, respectively. It also provided the highest C₂ partial current density of 131.4 mA∙cm⁻² at −0.9 Ⅴ vs. reversible hydrogen electrode (RHE) among these amine derivatives functionalized Cu catalysts. In contrast, the highest FE and partial current density for C₂ products achieved with pristine Cu catalysts were only 27.0% and 50.5 mA·cm⁻², respectively. Theoretical studies demonstrated that hydrogen bonding interactions of HDA with CO₂ and eCO₂RR intermediates enriched CO₂, CO, and other intermediates, lowered the kinetic energy barrier of CO―CHO coupling and thereby promoted eCO₂RR to C₂ products. Replacing the H atoms of the amine group with methyl groups in N-MHDA and HDDMA resulted in dominant hydrogen evolution reaction (HER) in eCO₂RR. PMM bonding with the Cu surface through a Cu―O bond, instead of Cu―N bonding as in HDA, N-MHDA and HDDMA, resulted in preferred ethanol production. In situ Raman spectroscopy indicated CO adsorption on Cu at atop sites for HDA-capped Cu catalysts, instead of bridge site CO adsorption on clean Cu surfaces, possibly due to the enriched CO in the former case. HDA also increased the local pH relative to pristine Cu catalysts. The Cu-HDA-based rechargeable Zn-CO₂ battery exhibited a superior maximum power density of 6.48 mW∙cm^–2 at a discharge current density of 16 mA∙cm^–2 and remarkable rechargeable durability for 60 h, outperforming most of the reported catalysts in the literature. This work enhances CO₂-C₂ conversion by tuning the eCO₂RR activity and selectivity of Cu-based materials, unravels the reaction mechanism at the molecular level, and provides new insights for promoting C₂ products in eCO₂RR through surface functionalization with organic molecules.

Key words: CO₂ reduction, CO―CHO coupling, Organic functionalization, In situ Raman, C₂ product, Zn-CO₂ battery