Amine-Functionalized Copper Catalysts: Hydrogen Bonding Mediated Electrochemical CO<sub>2</sub> Reduction to C<sub>2</sub> Products and Superior Rechargeable Zn-CO<sub>2</sub> Battery Performance

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.