The Study On The Cathode Materials For Nitrogen Fixation And The Underlying Mechanism Of The Energy Conversion

As one of the most important raw materials in industry and agriculture,ammonia is urgently required by human society.However,the traditional synthesis by Haber-Bosch method requires the huge energy consumption under the harsh condition and induces the prominent environmental pollution.Electrocatalytic nitrogen fixation,as a research direction that has emerged in recent years,possesses apparent advantages such as mild reaction and controllable rate,which has aroused the interest of numberous researchers.Of note,the catalytic efficiency is greatly decreased due to electrochemical corrosion and the decay of the cathode material itself which stimulates us to explore more promising electrode materials.This thesis aims to investigate several potential two-dimensional materials for nitrogen reduction such as 1T-Mo S₂,functionalized graphene and its nanoribbon.In the regard the conversion efficiency as well as the catalytic mechanism are uncovered by means of the density functional theory calculations.The specific focuses on the following:1.Firstly,the nitrogen reduction reaction（NRR）performance of the transition metal anchored Mo S₂ monolayer with 1T atomic structure（1T-Mo S₂）is systematically evaluated by density functional theory calculations.Our results reveal that the V decorated 1T-Mo S₂exhibits the outstanding catalytic activity toward NRR via distal mechanism where the corresponding onset potential is 0.66 e V,being superior to the commercial Ru material.Furthermore,the powerful binding energy between the V atom and the 1T-Mo S₂ provides the good resistance against clustering of the V dopant,indicating its stability.Overall,this work provides a potential alternative for the application of NH₃ synthesis.2.Secondly,the NRR performances of the VN₃ anchored graphene,graphane and fluorographene are systematically investigated via density functional theory calculations.Our results reveal that the VN₃ embedded graphene exhibits the superior activity through an enzymatic mechanism with an onset potential of 0.28 V.The graphene hydrogenation significantly depresses the competitive hydrogen evolution reaction（HER）and remarkably improves the NRR selectivity.Conversely,the graphene fluorination results into the worsening of the NRR selectivity,which rules out VN₃ decorated fluorographene as the NRR electrocatalyst.Moreover,the thermodynamic decomposition potentials of the N elements are 0.20 and 0.07 V for the supports of graphene and graphane,respectively,indicating that no stability deterioration is occurred by hydrogenation.The electronic structures analysis uncovers that the substrate is the electron reservoir for the charge transfers meanwhile the VN₃ moiety acts as a bridge to link the substrate and adsorbates,which provides the explanation for the influence of the substrate on the NRR performance.3.Finally,the NRR performances of the single transition metal atom anchored zigzag graphene nanoribbon with H/F termination,termed as TM-G/H and TM-G/F,are systematically investigated via density functional theory calculations wherein V,Fe and Mo are considered among the various TM elements.The N₂-to-NH₃ conversion is achieved on Mo-G/H and Mo-G/F under the onset potentials of 0.55 and 0.42 V via the distal mechanism,respectively.The electronic analysis reveals that the edge termination alters the interaction between the Mo atom and its coordination C atoms and subsequently modifies the affinity of the NRR intermediates.In this regard,the improvement of the NRR activity is originated from the edge fluorination in combination with Mo dopant.In addition,by means of transition state search,the kinetic barrier of Mo diffusion or aggregation at the edge and base plane is discussed,which proves that it possesses excellent stability in practical application.