Theoretical Research And Design Of The Hydrogen Evolution Performance Of Transition Metal Sulfides

With the excellent electrochemical and optical properties,transition metal dichalcogenides(TMDCs)and related materials have been widely used in many electrochemical devices such as batteries,catalysts and supercapacitors.Theoretical research at atomic scale on the structural,electronic,and molecular adsorption of TMDCs provide deep understandings of the increasingly complex catalyst materials and prediction of HER catalytic performance in order to provide guidance and assistance to the design of materials and as well contribute to understand the mechanism of catalytic reaction from a microscopic perspective.In this paper,we performed systematic theoretical calculations to investigate how native point defects and dopants affect the structural,energetic,electronic and magnetic properties of bulk CoS2,and the stability and HER performance of the low-Miller-index surfaces(001),(110),(111),and(210)of CoS2.In addition,how the Sn doping affects the HER performance of Ni3S2,another transition metal sulfide in hot-topic research,was investigated through close collaborations with experiment.First-principles studies were performed to investigate the effects of various neutral defects and ion dopants on the structural,energetic,magnetic and electronic properties of the bulk CoS2.Our theoretical results show that the concentrations of single cobalt(VCo)and sulfur(VS)vacancies in CoS2 samples can be high under S-rich and S-poor conditions,respectively.Although the single vacancies induce defect states near the gap edge,they are still half-metallic.We find that the substitution of one S with O atom does not obviously change the structural,magnetic and electronic features near the Fermi level of the system.Most transition metal impurities(MnCo,FeCo,and MoCo)and Group Ⅳ and Ⅴ anion impurities(CS,SiS,NS,PS,and AsS)create impurity states that are deep and/or near the gap edge.However,NiCo and Group Ⅶ elements(FS,ClS,and BrS)causes very localized gap states close to the Fermi level in the minority spin channel,which may modify their electrochemical performances.Then we investigated the structural stability of the low-Miller-index surfaces of CoS2,including(001),(110),(210),and(111),and the results show that the stability of these surfaces follows the trend of(001)>(210)-2>(210)-1>(110)-R>(110)>(111),but the catalytic activities of HER is(110)-R>(210)-1>(110)>(111)>(001)>(210)-2.Moreover,we find that △GH of each catalytic site is closely related with its first-neighbor coordination number(CN).Whereas Co sites have few variety with the change of CN,the AGH of S sites are obviously affected by CN value and the existence of S-S dimer,following the tendency of SN4>SN21>SN11≈SN3>SN2.The analyses of projected density of states(PDOS)of each site show that the local states near the Fermi energy(EF)can make the △GH shift to the negative value.We then used the COS2(001)that is the most stable but the worst HER performance as an example to tune its HER performance through the modification of CN and electronic properties by inducing defects on surfaces.Interestingly,we find that the △GH of its SN21 site is as smaller as SN3 site of(110)-R surface.First-principles studies were also performed to investigate how Sn doping affects the HER performance of Ni3S2,with collaborations with experiments.Our theoretical results show that the most active site for HER is the Ni site on the Ni3Sn2S2(012)-NiSn.The possible reasons include(i)the steric effect of Ni3Sn2S2(012)-NiSn facet is almost negligible,(ii)the replacement of S atoms around the topmost Ni atoms in Ni3Sn2S2(012)with Sn atoms makes the Ni site more active,and(iii)the number of active sites on Ni3S2(101)(8.63 per nm2)is about two times than that on Ni3Sn2S2(012)(3.33 per nm2),therefore a moderate composition of Sn doping is required.