Modified Cu-based catalysts for electrochemical reduction of CO2 to multicarbon products

Abstract

The electrochemical reduction of carbon dioxide to produce valuable chemicals is widely recognized as a promising strategy for mitigating the environmental impact of CO2. This research focuses on the development of Cu-based materials, particularly oxide-derived copper (OD-Cu), as efficient electrocatalysts for the conversion of CO2 to multicarbon products. Normally, the electrocatalytic reduction of CO2 involves a complex reaction pathway and the simultaneous occurrence of the hydrogen evolution reaction (HER), which subsequently results in poor selectivity of the carbonaceous products. Furthermore, the deactivation of catalysts has a significant impact on their long-term stability. Therefore, various electrode preparation methods and modification techniques have been used to improve the catalytic performance and deepen our understanding of the underlying mechanism.
The surface hydrophobicity of CuO electrodes was controlled by coating PVDC to suppress the hydrogen evolution reaction and promote the conversion of CO2 to ethylene, a value-added bulk chemical. The systematic investigation of different coating materials, amounts and sequences revealed that the CuO electrode with a PVDC coating of only 50 µg/cm2 optimizes hydrophobicity (WCA = 122°). It is found that the modified electrode led to highly efficient production of ethylene (|j|C2H4 = 6.8 mA/cm2, FEC2H4 = 41.4%) at -0.89 V vs. RHE while effectively suppressing hydrogen evolution (FEH2 = 22.7%). The catalytic activity of CO2RR to ethylene of the PVDC-modified CuO electrode is inherently a 50.8% increase in comparison to that of the CuO electrode. PVDC modification balances proton transfer and CO2 availability. In addition, PVDC affects CuO reduction by increasing the proportion of Cu+ species on the CuO-PVDC surface and facilitating C-C coupling. The PVDC-modified electrode retains durable hydrophobic properties without affecting conductivity, which facilitates efficient CO2 conversion.
Besides, CuO was prepared using precursors with different mass ratios, both with and without microwave heating. The microwave-assisted synthesized CuO (MW-CuO) showed similar morphology but smaller particle size than the conventionally prepared CuO. MW-CuO exhibited a higher defect site density and significantly more grain boundaries (GBs). At a reactant ratio of Cu2+ and CO32- of 1.1, MW-CuO exhibited superior catalytic performance and achieved a remarkable FE of C2+ products (71.9% at -1.04 V vs. RHE, corresponding to a partial current density of about 11.2 mA/cm2). This performance, which is among the highest reported for OD-Cu catalysts, is attributed to the significantly higher Cu+ to Cu0 ratio of MW-CuO on the surface. These factors, an appropriate ratio of Cu+ and Cu0 species, and defective surface features improved the CO2RR selectivity for multi-carbon products and emphasized the sustainable approach in catalyst preparation.
Another modification strategy involves electrochemical deposition on the Cu2O-C3N4 electrode, resulting in a captivating dendritic 3D Cu structure that significantly enhances the catalytic activity. Electrochemical analysis revealed a fourfold increase in current density. Electrochemical analysis revealed a fourfold increase in current density for multicarbon products after electrodeposition with a peak value of 12.7 mA/cm2 (with |j|ethylene = 7.2 mA/cm2) at -1.04 V vs. RHE. Furthermore, the reduction and restructuring of Cu species during the electrocatalytic CO2 reduction reaction led to a high Cu+/Cu0 ratio on the Cu@Cu2O-C3N4 electrode, which correlates with enhanced C-C coupling as evidenced by higher product ratios of C2H4/CH4. The electrodeposition increased the electrochemically active sites by 1.5 times and reduced the charge transfer resistance by 0.4-fold. In particular, the intrinsic electrocatalyst in the original electrode contributed significantly to the improvement of the electrochemical performance of various electrodes during the deposition process. Furthermore, the mechanism of electrode deactivation was investigated, revealing that carbon deposition was the primary cause.
This thesis covers several relevant works, including the preparation of Cu-based catalysts and the regulation of the gas-liquid-solid interface. It presents universal strategies to improve the performance of oxide-derived Cu electrodes for electrochemical CO2 reduction reactions to multi-carbon products. The research findings indicate that maintaining a moderate level of hydrophobicity facilitates proton transfer while preserving unhindered CO2 adsorption. In addition, the presence of porous 3D structures, an optimal ratio of Cu0 to Cu+ and abundant surface defects accelerate C-C coupling processes. It contributes to a better understanding of the reaction process.

Date of Award	Nov 2024
Original language	English
Awarding Institution	University of Nottingham
Supervisor	Tao Wu (Supervisor), Hongfeng Yin (Supervisor) & Edward Lester (Supervisor)