Molecular-Scale Insights into Electrochemical Reduction of CO₂ on Hydrophobically Modified Cu Surfaces

Abstract

Depressing the competitive hydrogen evolution reaction (HER) to promote current efficiency toward carbon-based chemicals in the electrocatalytic CO 2 reduction reaction (CO 2 RR) is desirable. A strategy is to apply the hydrophobically molecular-modified electrodes. However, the molecular-scale catalytic process remains poorly understood. Using alkanethiol-modified hydrophobic Cu as an electrode and CO 2 -saturated KHCO 3 as an electrolyte, we reveal that H 2 O, rather than HCO 3 − , is the major H + source for the HER, determined by differential electrochemical mass spectrometry with isotopic labeling. As a result, using in situ Raman, we find that the hydrophobic molecules screen the cathodic electric field effect on the reorientation of interfacial H 2 O to a "H-down" configuration toward Cu surfaces that corresponds to the decreased content of H-bonding-free water, leading to unfavorable H 2 O dissociation and thus decreased H + source for the HER. Further, density functional theory calculations suggest that the absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of CO 2 RR intermediates, which consequently increases the CO 2 RR selectivity. This work provides a molecular-level understanding of improved CO 2 RR on hydrophobically molecule-modified catalysts and presents general references for catalytic systems having H 2 O-involved competitive HER.

Significance of density functional theory (DFT) calculations for electrocatalysis of N2and CO2reduction reactions

“…DFT calculations suggested that absorbed alkanethiol molecules alter the electronic structure of Cu sites, thus decreasing the formation energy barrier of key CO 2 RR intermediates (mainly COOH* and CO*), which consequently increases the CO 2 RR selectivity. 52 Similarly, we summarized the electrocatalytic performance and reaction mechanism of CO 2 RR electrocatalysts involved in this section in Table 2, including the current density, FE, electrolyte used, modulation strategy and PDS.…”

Selectivity and Intrinsic Activity of Functionalized Cu Surfaces: Can the CO2 Reduction Reaction be Improved on Cu?

“…Previous studies have posited that interfacial hydrophobicity and restricted water diffusion affects CO 2 RR product distribution. 21,42,53,[59][60][61] This includes showing that water accessibility affects selectivity; 60 that hydrophobic polymers increase C 2 H 4 selectivity while decreasing the diffusion coefficient D for H 2 O more than for CO 2 , while hydrophilic polymers do neither; 42,53 and finding a correlation between CO 2 RR product selectivity and the hydrophobicity of the interfacial layer. 21,61 As the Cu surface is functionalized, the water-Cu interface is replaced by a water-modifier-Cu interface.…”

Selectivity and Intrinsic Activity of Functionalized Cu Surfaces: Can the CO2 Reduction Reaction be Improved on Cu?

“…Previous studies have posited that interfacial hydrophobicity and restricted water diffusion affects CO 2 RR product distribution. 25,46,67,[75][76][77] This includes showing that water accessibility affects selectivity; 76 that hydrophobic polymers increase C 2 H 4 selectivity while decreasing the dif-fusion coefficient D for H 2 O more than for CO 2 , while hydrophilic polymers do neither; 46,67 and finding a correlation between CO 2 RR product selectivity and the hydrophobicity of the interfacial layer. 25,77 As the Cu surface is functionalized, the water-Cu interface is replaced by a water-modifier-Cu interface.…”