Electrocatalytic Performance Of MXene Based Catalysts By First Principles

Due to the profound understanding of the nature of the electrocatalytic processes accumulated over the past few decades and the technological advances in spectroscopy and high resolution imaging in recent years,great changes have been made in understanding the electrochemical properties of material surface properties.Nowadays,one can selectively design electrocatalysts based on the theoretical knowledge of electronic structures,computer-directed material surface and material?electro?chemical properties,and then synthesize high-performance actual materials for specific reactions.The successful synthesis of graphene and other layered materials for more than a decade has prompted researchers to study two-dimensional?2D?materials due to their extraordinary electrochemical and optoelectronic properties.Among them,graphene,MoS₂,phosphoenene and its derivatives have been extensively studied.Transition metal carbides and nitrides?MXenes?are 2D inorganic compounds that are composed of transition metal carbides,nitrides or carbonitrides of several atomic layers.Ti₃C₂ is the first 2D layered MXene that was stripped out in 2011.This material is a layered material similar to graphite from its3D Ti₃AlC₂ MAX phase.Since then,materials scientists have determined or predicted the stable phases of 4,200 different MXenes based on a combination of various transition metals such as Ti,Mo,V,Cr and their compounds with C and N.A large number of experimental and theoretical studies have shown that they have excellent energy conversion and electrochemical energy storage potential.The potential of MXenes can only be fully explored by fully understanding their fundamentals,from synthesis and performance research to complex applications.The purpose of this paper is to obtain high conductivity,high activity and high stability electrocatalyst for the surface modification and electronic properties analysis of MXene-based materials for electrocatalytic decomposition of water to produce hydrogen and electrocatalytic nitrogen reduction.The main research work of this thesis is as follows:?1?The structure and electronic properties of 2×2×1 supercells of Ti ₂CO₂?MXene?were analyzed by first-principles method.We found that Ti₂CO₂shows good HER Gibbs free energy??35?G_H?at low hydrogen coverage,while the large band gap of approximately 1.00 eV limits its conductivity capability.By doping phosphor?P?or sulfur?S?to partially substitute the surficial O,the average free energy??35?G_H^a?at various hydrogen coverages has been dragged to approach zero,and the conductivity is also significantly improved by narrowing the band gap to lower than 0.30 eV.The partial charge analysis indicates that doping P or S on the surface induces electron of oxygen diffusion to conductive band minimum?CBM?,resulting in O-2p_z reaction with H-1s.The facial overlapping with H-1s for O-2p_z will strengthen the binding strength,hence lowering?35?G_H.The energy shift toward Fermi level of Ti-3d after P or S doping contributes to the reduced band gap and high conductivity.Surficial O or P vacancies are created to evaluate the vacancy effect on HER performance,which not only improves?35?G_H^a and conductivity but also leads to a low reaction barrier of H₂O splitting dissociation?<0.20eV?.The armchair nanoribbon?ANR?displays improved HER activity by P-doping at 50%ratio.Our research shows that modification of end-group in MXenes can effectively improve HER catalytic activity and conductivity,which is expected to promote their potential applications in electrocatalysis and energy conversion.?2?Electrocatalytic ammonia synthesis in aqueous solutions under mild conditions is a quite promising way in the earth's nitrogen cycle,but the development of efficient and stable catalysts remains a great challenge.Current research efforts for nitrogen reduction of MXenes catalysts are now focused on doping and loading on the terminal functional groups using the electrochemical method,while the non-terminated MXenes catalysts have not been explored.Herein,the N₂ fixation and reduction overpotential of different MXenes transition metal?TM?catalysts\nitrides with the formula M₂C\M₂N?M=Sc,Ti,V,Cr and Mn?have been proposed for the first time by means of the density functional theory?DFT?.And the following two metal carbons\nitrides?M=Fe and Mn?have been predicted to find out the effect of3d period transition metal on NRR.Our computations revealed that Mn₂C and Fe₂C can effectively activate the N₂ molecule through the enzymatic mechanism with a rather low onset potential?0.28 and 0.23 eV?.Moreover,the ammonia synthesis through the enzymatic mechanism prefers to proceed via the three different paths?including the first?NH*NH ₃*+H⁺+e^-=*NH+NH₃?and second?*NH₂+H⁺+e^-=*NH₃?NH₃ released from two intermediates and both NH₃ released from the final step?,and the potential required was different for different paths and ranged from 0.41 to 0.60 eV.Importantly,the non-designed catalyst is further demonstrated to synthesis easily due to previous experimental studies.Thus,by carefully controlling the reaction pathway and reduce the hydrogenation energy barrier,the pure Mn ₂C and Fe₂C are quite promising catalysts for the electrocatalytic ammonia synthesis.