Design of Catalysts for Electrochemical Reduction of Carbon Dioxide to Value-Added Chemicals

Abstract

CO₂, as the main greenhouse gas, has been emitted immoderately with the utilization of fossil fuels since the industrial revolution, causing adverse climate change. In order to reverse the situation, electrochemical methods are developed to achieve the conversion of carbon dioxide into valuable carbon-based chemicals by using renewable electricity, forming the environmentally-friendly carbon loop, further accelerating the process of carbon neutrality. During the conversion process, limited to the thermodynamic stability of CO₂, effective electrocatalysts are designed to shorten the required energy for breaking the C=O bond in relatively mild conditions. Meanwhile, the CO₂ electroreduction reaction (CO₂RR) is a multi-electron reduction reaction involving various surface-bound intermediates, leading to complex multi-products. Therefore, it is imperative to design suitable electrocatalysts targeting the specific product for CO₂RR, which exhibit high catalytic activity, good selectivity, and cycle stability. This dissertation describes efforts to optimize several electrocatalysts constituted from earth-abundant and low-cost elements. Furthermore, to clarify the mechanism of different reaction pathways, density functional theory calculations (DFT) are carried out to simulate the electronic structure and catalytic process of the catalysts. The details can be listed as follows:

In the first study, based on the metal-support interaction strategy, a 3D conformal hetero-assembly of Bi₂O₃/rGO (CHA-Bi₂O₃/rGO) nanosheets is synthesized/designed, which can be in situ converted to CHA-Bi₂O₂CO₃/rGO promptly in aqueous carbonate solution. During the CO₂ electrochemical reduction process, the abundant Bi-O bonds and structure of Bi₂O₂CO₃ are well maintained due to the introduction of the rGO, with the unavoidable formation of partial metallic Bi. The as-obtained nanocomposite electrocatalyst exhibits high Faradaic efficiencies (FEs) of above 90% for formate formation in a wide potential range ranging from -0.77 to -1.27 V. More significantly, a high partial current density of 47.1 mA cm^-2 is achieved at -1.27 V. The excellent selectivity and efficiency are benefited from the unique composition and structure features of reduced Bi₂O₂CO₃/rGO hybrid nanosheets, where the highly conductive rGO substrate offers fast electron transfer, and the thin Bi₂O₂CO₃ nanosheets provide abundant active sites and shortened electrolyte diffusion distance. Moreover, the conformal structure not only ensures the abundant exposure of active sites but also effectively enhances the stability of the active sites. The mechanistic studies indicate that the CHA-Bi₂O₂CO₃/rGO processes higher adsorption energy of oxygen species compared with Bi₂O₂CO₃, and its derived nanocomposite shows the optimized energy barrier for the formation of formate. It is believed that such a 3D conformal hybrid structure design will bring new insight to the development of high-performance electrocatalysts for CO₂RR and further promote its practical applications.

In the second study, since the CO₂RR is a surface process involving the interaction between the surface atoms and reactants, we further design the high specific surface area nickel electrocatalyst with densely populated, monodispersed Ni atoms on nitrogen-doped graphene-like carbon support based on metal-carbon interaction engineering. The atomically dispersed catalysts featured with uniform coordination structure surrounding the active metal center demonstrate high atomic utilization efficiency, leading to excellent activity and selectivity. At the same time, the pyridinic-N, as the higher catalytic active site, is heavily introduced by N₂ plasma to decrease the reaction energy barrier for the rate-determining step. The pyridinic-N-rich single-atom Ni catalyst exhibits great electrocatalytic performance for the CO₂-to-CO conversion with high CO Faradaic efficiency above 91% over a wide potential range (from -0.57 V to -0.97 V), a large CO current density of 30 mA cm^-2 at 870 mV, and outstanding stability. Moreover, the method for employing plasma-assisted regulation of defects has been implemented effectively.

In the third study, by employing plasma-assisted regulation method, the selectivity of a Cu oxide-derived electrocatalyst for the generation of high-energy-density multi-carbon products for CO₂RR was investigated. Here, the 3D flower-liked CuO pre-catalysts were synthesized by microwave-assisted hydrothermal method. The oxidation state was modified by H₂ plasma treatment, with abundant oxygen vacancies generation (CuO-P1). The highest C₂H₄ Faradaic efficiency from CuO-P1 derived reaches 54.6% at -1.5 V, much higher than that from the CuO pre-catalyst, with a partial current density of 17.2 mA cm^-2. The Raman results prove that the assistance of abundant O-vacancies promotes the adsorption and stability of CO intermediate on CuO-derived.

In summary, this thesis explores designs to optimize common electrocatalysts for high-performance CO₂RR. The strategies with high efficiency, selectivity, and low cost can further help the utilizing CO₂ to produce high value-added chemicals.

Date of Award	10 Nov 2023
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Wenjun ZHANG (Supervisor)