Theoretical Screening of 2D Materials as High-Efficiency Catalysts for Energy Conversion and Storage Applications

This Ph.D. thesis focuses on the exploration and design of new electrocatalysts; the theoretical screening and establishment of new high-efficiency catalysts for electrochemical reactions on 2D materials is based on extensive research, which provides a new route for designing heterogeneous catalysis and paves the way for the development of better electrochemical energy storage and conversion. Theoretical screening focuses on designing more selective, stable, and catalytically active electrocatalysts at a lower cost and finally replace commonplace catalysts. Achieving this requires sufficient theoretical knowledge of how a catalyst functions and works, including the active sites, reaction mechanisms, and expected products. The ultimate goal is to have a sufficient understanding of the significant influencing factors that determine the performance of a catalyst, which can be evaluated using the binding energy of the intermediates or D band center as descriptors. However, it is challenging to provide detailed information on reaction mechanisms using only experimental techniques. Extensive theoretical screening based on density functional theory (DFT) approaches has been employed to obtain basic guidelines for catalyst design. In this thesis, the electrocatalytic mechanisms of some representative electrochemical reactions are taken as examples to comprehensively analyze the present situation in terms of electrocatalyst technology that is to be used for application in energy storage and conversion devices, such as the oxygen reduction reaction (ORR) in fuel cells, carbon dioxide reduction (CO2 RR) in capturing CO2 , nitrogen reduction (NRR) in nitrogen fixation, and the nitric oxide (NO) reduction of ammonia (NORR) on two-dimensional (2D) materials. Theoretical screening is utilized to summarize the detailed design of the electrocatalyst. The understanding of 2D materials as catalysts for electrochemical catalysis can be broadened from various perspectives. The theoretical screening method guides the further development and discovery of highly efficient and inexpensive electrochemical catalysts, which can help confirm the preferable mechanism path to develop the control step of a reaction system. Achieving efficient catalytic activity for electrochemical reactions in promising future catalysts requires people to focus on highly flexible active sites, sufficient activity, selectivity, and a large species. Here, several great and novel 2D materials are considered as catalysts for electrochemical reduction, including the graphene family, 2D metal-organic framework M3 (2,3,6,7,10,11-hexaiminotriphenylene)2 [M3 (HITP)2], 2D transition metal carbides and carbonitrides (MXenes), and the novel 2D material MBenes that are based on boron analogs of MXenes. Our results suggest that these 2D materials can achieve high activity and selectivity in electrochemical reactions under extensive theoretical screening. The family of a single transition metal atom nitrogen co-coordination (MN4)-embedded graphene catalyst is known for its excellent activity, selectivity, and high atomic efficiency in the oxygen reduction reaction (ORR), and systematic theoretical research has proved that ORR works along a complete four-electron transfer pathway in acidic conditions, indicating that direct hydrogenation pathways are preferred over the O2 dissociative mechanism in ORR. Furthermore, 2D metal-organic frameworks M3 (HITP)2 and 2D transition metal carbides (MXenes) have been proven to possess high activity and low overpotentials when used for carbon dioxide electrochemical reduction reactions (CO2 RR). We performed density functional theory (DFT) calculations combined with the theoretical screening method and found that both 2D MOF and MXene materials are promising electrocatalysts for reducing CO2 to produce C1 hydrocarbons. Finally, 2D Metal Boride (MBenes) catalysts have the significant catalytic potential for the electroreduction of CO2 to CH4 ; MBenes are also great prospects for application in efficient electrocatalysts with high activity and high selectivity for the nitrogen reduction reaction (NRR) and nitric oxide reduction (NORR). 2D MBene catalysts have a low limiting potential, and their high selectivity is particularly desirable. Still, challenges surrounding the inadequate understanding of the catalytic mechanism need to be met if these materials are used commercially. To reduce nitric oxide to ammonia by electrochemical conversion as an efficient approach to solving air pollu tion, MBene materials are an attractive electroreduction 5 catalyst with which ammonia can be synthesized from nitric oxide (NO) in a process driven by renewable energy, such as wind and solar power. One promising strategy is the use of the Haber–Bosch process to synthesize ammonia under ambient conditions. Ammonia synthesis depends on the fixation of industrial nitrogen. It is important that the synthesis of chemicals and fertilizers while removing NO to solve air pollution is attractive and challenging in electrocatalysis. Current research generally focuses on these issues separately. However, the direct electrochemical reduction of N2 to NH3 is seldom mentioned. We propose a new electrocatalyst, metal boride (MBene), as a promising candidate catalyst for achieving the direct electroreduction of N2 to NH3. Therefore, our study evaluates a novel 2D material for use as a high-efficiency metal boride (MBene) electrocatalyst that can fix chemical nitrogen and remove NO, purifying exhaust gas. MBenes are also an effective alternative to the Haber–Bosch process currently used to synthesize ammonia