Theoretical Study And Structural Design Of Highly Efficient Electrocatalysts For Nitrogen Reduction And Oxygen Evolution Reactions

The development of clean energy and improvement of energy conversion efficiency involve numerous important chemical reactions,and the key issue is how to design and prepare efficient,inexpensive and environmentally friendly catalysts.In recent years,nitrogen reduction reactions（NRR）driven by electricity from renewable resources such as solar and wind energy has been extensively studied.However,limited by catalyst activity and selectivity,the traditional energy-intensive Haber-Bosch process is still irreplaceable.The oxygen evolution reaction（OER）can provide protons for the NRR process to hydrogenate nitrogen,but the best-performing OER catalysts,Ir O₂/Ru O₂,are costly and not very stable.In order to address the above problems,several catalysts including Fe dual-atom,Ir single atom,Pt₃In cluster and two-dimensional transition metal carbon/nitrides（MXene）loaded with gold clusters catalysts are designed,and their catalytic properties in some important reactions of clean energy field are investigated using first-principles calculations and reaction kinetics simulation.The low-valent Fe dual-atom catalyst（DAC）is designed to achieve highly efficient electrocatalytic ammonia synthesis.Based on the experimental configuration of graphene decorated with four neighboring pyrrolic nitrogen atoms,15 homonuclear DACs（TM₂N₄@graphene）are designed.Through the study of activity,selectivity and stability of catalysts,Fe₂N₄@graphene is screened out as the most efficient NRR catalyst with a limiting potential of only-0.32 V.Electronic structure analysis demonstrates that the low valence state of Fe（+1）remarkably activates the N₂molecular,which contributes to its excellent NRR catalytic activity.Moreover,the kinetic studies reveal that all barriers of the NRR elementary steps are smaller than that of the hydrogen evolution reaction（HER）,showing that HER is effectively suppressed.In addition,the integral crystal orbital Hamilton population（ICOHP）can be used as a descriptor to describe the Gibbs free energy of each step for the NRR performance.This work not only provides theoretical guidance for designing DACs for NRR but also promotes the understanding of DACs for N₂ fixation.The study of a single Ir atom doped Ni₂P performance for the oxygen evolution reaction.Via a computational standard hydrogen electrode（SHE）model coupled with the self-consistent theoretical overpotential method,a Ir single atom doped Ni₂P(Ir _SA-Ni₂P)catalyst is designed,and the OER activities of the stable Ni₂P（0001）surface are investigated before and after anchoring Ir atom.Considering the harsh conditions of OER,the surface Pourbaix diagrams of Ni₂P（0001）and Ir _SA-Ni₂P（0001）are constructed firstly,which are composed of the most stable surface phases at the given electrode potentials and p H values.The calculation results reveal that the exposed P,Ir,and Ni atoms on these two surfaces are chemically active towards water molecules,leading to the oxidation of catalyst surface under the OER operating conditions.This is also consistent with the experimental results that an oxide layer is formed on the sample surface after the reaction.In addition,both computational simulation and experimental results show that Ir single atom tends to occupy Ni sites on the surface.The reconstructed Ir-O-P/Ni-O-P bonding environment plays a crucial role in the adsorption and desorption of the OER intermediates,which leads to the remarkable increase of the OER catalytic activity.Meanwhile,the current density of Ir _SA-Ni₂P catalyst at 1.53V is 28 times that of commercial Ir O₂ catalyst.Our Ir _SA-Ni₂P catalyst designed by us provides a deeper understanding for further improving the OER catalytic performance.The study of intermetallic ordered Pt₃In clusters for oxygen reduction reaction（ORR）.Pt is the most active ORR catalyst,but its high cost and poor stability greatly limit the commercial application of fuel cells.Therefore,a series of Pt-based catalysts with ordered doping of In atoms are designed.Theoretical calculations show that the adsorption of ORR intermediates gradually weakens with the increase of the doping concentration of In atoms,and the overpotential of OER is gradually decreases.When the ratio of Pt:In is 3:1,the catalyst activity is closest to the peak of the volcano plot.The sub-3-nm ordered Pt₃In cluster catalyst is further synthesized and the test results show that the mass activity and specific area activity of the Pt₃In/C catalyst are 4.1 times and 2.7 times the industrial Pt/C catalyst,respectively.In addition,the Pt₃In catalyst is remarkably stable with negligible activity decay and structure corruption after 20,000 accelerated electrochemical durability cycles.MXene-based catalysts are rational design for water-gas-shift（WGS）reaction.Water molecules linked by hydrogen bonds are responsible for the high efficiency of bi-functional catalysts for the WGS reaction because water can act as a proton transfer medium.Hence,an associative pathway is proposed for WGS reaction assisted by water to realize hydrogen production.Based on this pathway,the oxygen-covered MXenes deposited by Au clusters are proved to be promising catalysts for WGS reaction by first-principles calculations.Furthermore,a comprehensive microkinetic model is constructed to describe the turnover frequencies（TOF）for the product at the steady-state conditions.The HER performances are evaluated for the oxygen-covered MXenes by a volcano curve,and the free energy of the adsorbed hydrogen can be significantly tuned by the deposited Au clusters.More importantly,there is a perfect linear scaling relation between the rate-determining barriers of WGS and the free energy of the adsorbed hydrogen.Our work not only opens a new avenue towards the WGS reaction but also provides many ideal catalysts for hydrogen production.