Theoretical Study On Electrocatalytic Hydrogen Evolution And Carbon Dioxide Reduction Reaction

With the rapid increase of global energy demand and carbon emission,developing new and sustainable energy technologies has become an important task.Clean and efficient energy storage and conversion through fuel cells,water electrolyzers,carbon dioxide electrochemical reduction and ammonia electrosynthesis have attracted much attention.Electrocatalysts play a key role in these energy conversion technologies because they can improve the efficiency and selectivity of electrochemical conversion.However,cheap and high-performance electrocatalysts for these critical electrochemical reactions are still scarce.The development of new catalysts has been limited by trial and error method for a long time,which wastes many resources while the efficiency is very low.This is mainly due to the lack of in-depth understanding of the catalytic process and mechanism as well as the lack of rational and efficient catalyst design.At present,experimental characterization methods are often limited in their ability to explore the reaction mechanism under complex electrochemical environment,thus masking the further optimization of the catalytic performance of materials.In recent years,a catalyst development framework combining theoretical and experimental studies has gradually taken shape.For the study of reaction mechanism and the design of new catalyst,computational simulation is more economical and efficient than experiment,and experimental verification can further correct the calculation model,thus forming a positive feedback framework,which can provide reasonable guidance for the development of new catalyst.In this thesis,taking hydrogen evolution reaction and carbon dioxide reduction reaction as examples,the mechanism of reaction and the rational design of electrocatalyst are studied from the perspective of theoretical calculation by using the first principles method.It shows how to advance towards the ultimate goal of efficient,stable and cheap electrocatalysts based on the in-depth understanding of catalytic mechanism,promoting the development of new electrocatalysts and facilitating the widespread popularization of clean energy technologies.The main conclusions are summarized below:1)Activating inert basal planes of MoS₂ by defects for catalyzing hydrogen evolution.Nanoscale molybdenum disulphide(MoS₂)has attracted ever-growing interest as one of the most promising non-precious catalysts for hydrogen evolution reaction(HER).However,the active sites of pristine MoS₂ are located at the edges,leaving large area of basal planes useless.We systematically evaluate the capabilities of 16 kinds of structural defects including point defects(PDs)and grain boundaries(GBs)to activate the basal plane of MoS₂monolayer.Our first-principle calculations show that six types of defects(i.e.,V_s,V_{Mo S3},Mo _S2 PDs,4|8a,S bridge and Mo-Mo bond GBs)can greatly improve the HER performance of the in-plane domains of MoS₂.More importantly,Vs and Mo _S2PDs,S bridge and 4|8a GBs exhibit outstanding activity in both Heyrovsky and Tafel reactions as well.Moreover,the different HER activities of defects are well understood by an amendatory band-center model,which is applicable to a broad class of systems with localized defect states.Our study provides a comprehensive picture on the defect-engineered HER activities of MoS₂monolayer and opens a new window for optimizing the HER activity of two-dimensional dichalcogenides for future hydrogen utilization.In addition,we chemically activate the MoS₂surface basal plane by doping with a low content of atomic Pd using a spontaneous interfacial MoS₂/Pd redox technique.Due to the simultaneous increasing of the active site density and electrical conductivity,the final Pd-MoS₂ exhibits the highest HER performance ever achieved on heteroatom-doped MoS₂-based materials in an acidic solution.The surface activation approach proposed by us successfully turns the catalytic property of MoS₂ and pushes the process of replacing platinum-based material with MoS₂material.2)Hydrogen evolution catalyzed by molybdenum sulfide clusters and graphene composites.How to enhance electrical conductivity and maintain high intrinsic activity and active site density remains a challenge for molybdenum sulfide nanomaterials.We design a novel composite catalyst,which is composed of molybdenum sulfide clusters and defective graphene,holding excellent intrinsic activity,high-density active sites and high conductivity simultaneously.The strong S-C covalent bonds between clusters and graphene ensure the structural stability of the composite,avoiding the long-standing deactivation problem caused by cluster desorption.The clusters possess high-density active sites and the graphene acts as the conducting path to transport electrons from electrode to active sites efficiently.Moreover,the simulations of diffusion of clusters on defective graphene demonstrate that the composite catalyst is easy to synthesize by a simple drop-casting procedure.Our work provides new insights for the design of hydrogen evolution catalysts based on molybdenum sulfide clusters.3)Breaking scaling relations for efficient CO₂ electrochemical reduction through dual-atom catalysts.The electrochemical reduction of CO₂ offers an elegant solution to current energy crisis and carbon emission issues,but the catalytic efficiency for CO₂reduction is seriously restricted by the inherent scaling relations between the adsorption energies of intermediates.By combining the concept of single-atom catalysts and multiple active sites,we design heteronuclear dual-atom catalysts to break through the stubborn restriction of scaling relations on catalytic activity.Twenty-one kinds of heteronuclear transition-metal dimers are embedded in monolayer C₂N as potential dual-atom catalysts.First-principles calculations reveal that,by adjusting the components of dimers,the two metal atoms play the role as carbon adsorption site and oxygen adsorption site respectively,which results in the decoupling of key intermediates adsorption energies.Free energy profiles demonstrate that CO₂ can be efficiently reduced into CH₄ on Cu Cr/C₂N and Cu Mn/C₂N with low limiting potentials of-0.37 V and-0.32 V,respectively.This study provides a new strategy for the design of catalysts for complex electrochemical reactions containing multiple intermediates.4)Electrochemical CO₂ reduction:water/catalyst interface versus polymer/catalyst interface.Due to the low solubility and diffusion coefficients of carbon dioxide in aqueous solution,the carbon dioxide electrolytic cells with aqueous electrolytes has been difficult to achieve high conversion current density.The appearance of alkaline polymer electrolyte electrolytic cells(APEECs)brings hope to solve this problem.By utilizing advanced molecular dynamics methods,we compared the carbon dioxide reduction paths at the aqueous solution/copper electrode interface and polymer electrolyte/copper electrode interface from the perspective of reaction kinetics.Furthermore,our microkinetic model results show that by adjusting the hydroxyl ion concentration and water content of the polymer electrolyte,the carbon dioxide conversion current density of alkaline polymer electrolyte electrolytic cells can be hundreds of times higher than that of the aqueous-based electrolytic cells.Our study has laid a foundation for carbon dioxide electrolysis cell based on alkaline polymer electrolyte to step into the industrial age.