Computational Study Of Graphyne Supported Single-atom Catalysts For Electrocatalytic Water Splitting

Water splitting by renewable clean energy such as solar energy,wind energy,and tidal energy is one of the most promising ways is to generate clean hydrogen energy.However,the Pt-based and Ir or Ru-based precious metal catalysts are the most efficient catalysts for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）in water splitting,respectively,which limits the large-scale industrial application of water splitting.Therefore,developing cheap and efficient catalysts that can replace precious metals has become the key to the practical application of water splitting.At present,there have been many studies on HER and OER catalysts both in experiments and in density functional theory（DFT）calculation,but there is still a lack of descriptors that can accurately describe and predict the HER/OER activity of catalysts.The good controllability of the new two-dimensional graphyne-supported single-atom catalysts（SACs）facilitates the construction of different types of catalysts and obtains models for accurately describing and predicting the HER and OER activity.Therefore,in this thesis,a series of two-dimensional graphyne-supported SACs with different active atoms were first cinstructed,and SACs with the same active atoms but different graphyne composition,lattice and number of layers through atomic doping,lattice strain and double-layer structure strategies were then constructed,respectively.The performance of these designed SACs in HER and OER was evaluated by DFT calculations.Finally,a model for describing and predicting HER/OER activity was established,which is expected to provide theoretical guidance for the further development of efficient HER/OER catalysts.The main research contents and results of this thesis are as follows:（1）The HER performance of the Co@GY and Mn@GY catalysts(adsorption free energy for H^*(ΔG_H^*)are 0.04 and-0.05 eV,respectively)and the OER performance of the Co@GY catalyst（the overpotential is0.55 V）are the best among the ten kinds of Graphyne supported SACs.Meanwhile,these TM@GY catalysts show metallic properties or reduced band gap,favoring electron transfer during the electrochemical processes.Besides,it is found that the d-band center of TM atoms in the TM@GY catalyst can refect the adsorption strength of the intermediate species.The scaling relation between HO^*and HOO^*was found,and according to this scaling relation,ΔG_HOO^*-ΔG_O^*can well describe the OER activity of the TM@GY catalysts,making the OER activity present a volcanic trend.（2）The HER activities of the Co@B₁-GY,Co@B₂-GY,and Co@N₁-GY catalysts(ΔG_H^*values are-0.03,0,and-0.02 eV,respectively)and the OER activity of the Co@N₁-GY catalyst（the overpotential is 0.42 V）are the best among the B or N atoms optimized Co@GY catalysts.The catalysts have strong structural stability after B or N atoms doping.These doped B atoms can make the charge density of the Co atom slightly decrease and upshift the d-band center of Co atoms,which strengthed the interaction between the H^*and Co atoms and thus improve the HER activity.The incorporation of N dopant in Co@GY can slightly increase the charge density of the surface of the catalysts.The increased charge density increases the number of active sites of Co@N₁-GY for HER and repels the adsorption of oxygen intermediates in different degrees,which rendersΔG₁,ΔG₂,andΔG₃ values close to each other and finally decreases the overpotential of OER.（3）A tensile strain of 0.5%on Co@N₁-GY gets an ideal HER performance,while the OER reaches the minimum overpotential of 0.33 V under the biaxial tensile strain of 3%.Compressive strain and tensile strain lead to less stability of Co@N₁-GY,and the distances between C and Co atoms increase linearly with the strain changing from compressive to tensile,thus linearly upshifting the p-band center of C atoms and d-band center of Co atoms.Besides,the biaxial strain has more remarkable effects on these properties than that of the uniaxial strain.From compressive to tensile strain,the chemisorption of electrochemically generated intermediates in both HER and OER is becoming weaker and weaker.The tensile strain reduced the overpotential of OER by making theΔG₁,ΔG₂,andΔG₃ values close to each other.（4）The HER performance of the Co@GY/Cu@GY and Co@GY/GY catalysts(ΔG_H^*values are 0.05 and-0.06 eV,respectively)and the OER performance of the Co@GY/Ni@GY catalyst（the overpotential is 0.38 V）are the best among the Co@GY/TM@GY（TM=Mn,Fe,Co,Ni,and Cu）catalysts.The TM atoms in the bottom TM@GY layer significantly modulate the electronic structure of the Co atom in the upper Co@GY layer,resulting in a strong linear relationship betweenε_d andΔG_H^*.ε_d can be used to describe the HER activity of the Co sites.There is also a linear relationship betweenΔG_HO^*andΔG_HOO^*,which makesΔG_HOO^*-ΔG_O^*a descriptor that can well descript the OER performance of the Co@GY/TM@GY catalysts,rending the OER performance presents a volcanic trend.The regression equations for predicting HER and OER activity areΔG_H^*=-1.518-1.242ε_d and ln(η^OER)=-2.163-1.128ΔG_O^*/eV+1.015ΔG_OOH^*/eV,respectively,which accurately describe and predict the HER and OER activities of catalysts.