Cu-based catalysts for electrochemical CO2 reduction

Abstract

Electrochemical CO2 reduction (CRR) that converts CO2 into high‐value fuels and chemicals is one of critical approaches to achieving carbon‐emissions‐neutral processes. The first step in developing this technology is to design and synthesize high-performance CRR electrocatalysts. Because Cu binds to CRR intermediates *CO neither too weakly nor too strongly, it is the only metal catalyst to electrochemical transform CO2 into various products, hydrocarbons and oxygenates. They are highly sought because of their large market volume and commercial value. However, CRR is a gas/solid/liquid three-phase interface reaction near the electrode in aqueous solution. Therefore, in addition to the composition and structure of various Cu catalysts, their catalytic activity and selectivity depend significantly on the local environment, including catalyst surface structure and electrolyte composition. Understanding the structure-activity relationship between catalyst composition and local environment with CRR activity and selectivity is crucial, which inspires researchers to synthesize the desired CRR catalysts. This thesis aims to solve this key scientific question from catalyst design and tuning electrolyte composition to steer CRR activity and selectivity of Cu-based catalysts. First, we proposed a new method, molecular cleavage, to synthesize Cu-based MOFs. This strategy can precisely regulate the local coordination environment of a metal−organic framework (MOF) together with the derivation of a new topological structure. Importantly, this method is designed to circumvent limitations in hard and soft acids and bases (HSAB) theory to fabricate a new ordered Cu2BDC (BDC = 1,4- benzenedicarboxylate) MOF composed of a soft acid metal and a hard base ligand. Starting from a reported CuBDC MOF, we demonstrated L-ascorbic Acid (LA) acting as a molecular scalpel to finely regulate the chemical state and coordination number of Cu metal centres to cleave BDC linkers. A controlled phase transition from CuBDC to Cu2BDC with resulting different chemical composition and topological structure was achieved. In composition to the pristine CuBDC, the Cu2BDC-derived sample can display a higher Faradaic efficiency (FE) for C2 products (ethylene and ethanol). This improvement results from the formation of Cu2O (111) and Cu (111) mixed phases, which can promote CRR activity and C2 product selectivity in a synergistic way. Second, a method was proposed to stabilize Cu2+ during catalytic operation through fabrication of a Cu-Ce-Ox solid solution, in which Cu ions are incorporate into CeO2 matrix. In situ formed Ce3+ in solid solutions from electrochemical reduction can provide the rapid electron transport channel to suppress the accumulation of electrons around Cu2+ sites and protect them from reduction to Cu0. We also employed in situ attenuated total reflectance infrared spectroscopy (ATR-IR) to observe that the stable Cu2+ sites can significantly improve the initial *CO adsorption and facilitate the *CO hydrogenation to produce *OCH3, which is a crucial intermediate of CH4 generation. As a result, Cu-Ce-Ox delivered a high FE of CH4 with significant suppression of the competing C2 products (i.e., C2H4). Third, a class of bipyridyl molecules work as linkers with tunable electrophilicity to steer CRR selectivity of Cu-MOFs. Theory calculations and in situ experiments confirm that the electrophilicity of the linker can tune the catalyst’s proton availability, which can promote or inhibit the critical proton-coupled electron transfer (PCET) process in CRR. Catalyst with a low-electrophilicity linker exhibits fast proton transfer to *CO, accelerating its protonation to achieve a high FE of 58.2% for CH4. By contrast, a high-electrophilicity linker can stabilize *CO and favor its C-C coupling step, resulting in a high FE of 65.9% for C2H4. In the end, we engineered the solvated structure of metal cations to tune interfacial water structure, which can fast PCET process of CO2 activation and increase the absorption ability for *CO intermediate on Cu2O during CRR operation, thereby promoting C-C coupling reaction during CRR. Dimethyl sulfoxide (DMSO) was employed as an electrolyte additive to tune the solvated structure of K ions because it can preferentially solvate with K ions over water. In situ spectroscopic characterizations revealed that the introduction of DMSO can form a higher percentage of 4-coordinated hydrogen-bonded water on the surface of Cu2O catalyst, consequently building stronger hydrogen bonds to stabilize *CO and fast its dimerization.