First-principles Study Of Defective Cobalt Phosphide Electrocatalytic Hydrogen Evolution And Oxygen Production

With the continuous improvement of environmental protection requirements around the world,the development of clean energy technology has aroused great concern.It is well known that the hydrogen energy is considered to be an optimal substitute.Hydrogen can effectively improve the environmental problems caused by the burning of fossil fuels.Therefore,how to efficiently produce hydrogen is an important part.Hydrogen evolution from electrolyzed water is an efficient method.The best HER catalyst is the noble metal Pt catalyst.However,due to its low earth reserves and high prices,it has largely hindered the application of electrolyzed water.The other half of the reaction of electrolyzed water is the oxygen production reaction.Since the OER reaction is a four-electron reaction and the reaction kinetics are slow with a high overpotential,so it is the rate control step of the entire reaction.In order to increase the activity of electrocatalytic reactions,it is also critical to study efficient OER catalysts.At present,Ir O2 and Ru O2 are the most active OER catalysts,but they also limit their large-scale application in actual production because of the high price and low reserves.Therefore,the study of HER and OER catalysts with excellent catalytic performance has received more and more attention.In recent years,transition metal phosphides have been widely used in HER and OER reactions due to their good electrical conductivity.Co P is attracting the attention of many researchers because of its abundant reserves,easy synthesis and good stability.A large number of articles have reported that Co P has excellent HER performance,but there is still a certain gap in performance compared with precious metal Pt.However,there are few reports on the application of Co P in OER.Therefore,we want to modify Co P to improve the reactivity of HER and OER.One common treatment method today is to build defects.Carbon defects,oxygen defects,and sulfur defects have a significant improvement in catalytic performance.Therefore,we introduced P defects to investigate whether the Co P with P defects has improved HER and OER activity.Since there is no reported P defect in the experiment,we use theoretical calculations to investigate the existence of P defects and whether it has an impact on HER and OER performance.First,based on Density functional theory,we discuss the possibility of Co P exposure facet.By comparing the surface energies,we determined that(011)is the most likely surface to be exposed.Next,we built defects on the(011)surface.We constructed 1,2,and 3 P defects.Taking into account the periodicity of the P atom,the defect formation energy of all the possibilities of the defect is calculated.By comparing the calculation results of all possibilities,the minimum energy required for the formation of P defects is 1.9 e V.Compared with the O defect formation energy,the possibility of P defect formation exists,so we apply Co P with P defects to HER and OER to analysis whether defects have an effect on HER and OER activity.Second,based on Density functional theory,we discuss the impact of P defects on the HER.Phosphorus top position remains the active site of Co P in the HER,and the catalytic activity of HER is improved due to the construction of defects.Appropriate amount of P defects can enhance the activity of active sites,increase electron density,capture hydrogen ions,increase hydrogen adsorption and improve catalytic performance.However,if the number of defects increases,the electrons around the active site are too dense,the electronegativity is too strong,and the adsorption of hydrogen ions is too strong,so that the reaction product is finally prevented from leaving the surface,and finally the reactivity is lowered.Third,based on Density functional theory,we discuss the impact of P defects on the OER.The top position of the cobalt head remains the active site of Co P in the OER.According to the results of OER Gibbs free energy,OER catalytic activity is improved with the introduction of defects.It can be seen from the differential charge that the introduction of defects can enhance the density of electron clouds around the active sites.When there are too many defects,the electrons around the active site are too dense,and the adsorption of OH-is too strong,which is not helpful to the desorption of the final product,and also affects the reactivity of OER.LDOS also shows that after the introduction of defects,the active site is more strongly bound to the reactants,which is beneficial to the adsorption of the reactants and thus the reaction.