Synthesis Of C-doped H-BN Nanosheets As Advanced Catalysts Towards Electrochemical Reduction Of CO₂

Since industrial revolution,the coal and petroleum are the main sources of energy.Due to the overuse of these energy sources,the concentration of carbon dioxide(CO2)in the atmosphere and oceans has dramatically increased which is the ultimate cause of numerous ecological and environmental problems.Converting carbon dioxide to value-added products not only contributes to establishing a sustainable recycling society,but also mitigates its hazardous effect on environment.A vast variety of strategies have been developed by researchers for carbon dioxides conversion such as chemical reduction,photochemical reduction,electrochemical reduction,and biotransformation.Among them,electrochemical reduction of carbon dioxides is the most attractive and feasible,because the activity and selectivity of the catalyst can be controlled by adjusting the electrochemical parameters such as overpotential.Due to the controllable molecular structures,non-metallic catalysts have gained a lot of attention in recent years for the application of carbon dioxide electrochemical reduction.For most non-metallic catalysts,the carbon dioxide electroreduction process follows the proton-electron coupling conversion mechanism(PCET).Specifically,the COOH*or HCOO*intermediate forms a CO after accepting protons.Another typical mechanism is Langmuir-Hinshelwood(LH)mechansim.The intermediate are generated at the adjacent active sites and then combine to form the final products.The L-H reaction mechanism motivates us to search for atomic groups that can simultaneously activate carbon dioxides and protons.In this dissertation,carbon atoms were incorporated into hexagonal boron nitride(h-BN)by thermal decomposition method to prepare catalyst BCN-x.The carbon dioxide electroreduction performance of each sample was tested in KHCO3 electrolyte.The experimental results show that the catalyst exhibited excellent catalytic activity for the carbon dioxide electroreduction reaction and had high selectivity for HCOOH.Previous literatures reported that boron atoms in negatively charged h-BN were able to adsorb carbon dioxides molecules with high selectivity,while carbon atoms could efficiently activate protons to form H atoms.According to the L-H reaction mechanism,carbon was incorporated in the h-BN flakes to introduce the diatomic group C-B.Through the synergy of the C-B diatomic group,the relative concentrations of the intermediates H*and COOH*was adjusted in the solution.resulting in the tunable selectivity of the product.The first chapter mainly introduces the carbon dioxide electroreduction reaction and the recently reported catalysts.These catalysts can be divided into two major categories,metal-based and non-metal-based.In addition,how to design and fabricate the catalysts for superior activity is illustrated with specific examples.The reaction mechanism on the catalyst is also discussed.The second chapter is about synthetic route of catalysts,its detailed physiochemical characterization,and application for carbon dioxides electroreduction.The morphological and compositional details of BCN-x and h-BN are shown.Then,the eclectrocatalytic data for carbon dioxides reduction,theoretical calculation and the reaction mechanism are thoroughly explained.Finally,the whole research was summarized.The third chapter is about the challenges and future perspectives of electrochemical reduction of carbon dioxides.Other parameters that greatly influences the catalytic activity of the catalysts are discussed in details.