First-principles Study On Cobalt Phosphide Materials

Cobalt phosphide materials are widely used in hydrogen evolution catalysis and lithium storage.In this paper,based on the first-principles calculation method of density functional theory,we have studied the hydrogen evolution catalytic activity and lithium storage mechanism of cobalt phosphide materials.Furthermore,further theoretical calculations have been carried out on the performance mechanism of hydrogen evolution after doping.The specific work content is as follows:（1）First,we analyze the Co P unit cell parameters and electronic structure,the calculated bulk crystal lattice parameters are close to those obtained in the experiment.The density of states has a distribution at the Fermi level,indicating that Co P conducts well,which is beneficial to the hydrogen evolution reaction and the electron transfer between layers.We calculated the different surface atomic distribution configurations of different crystal planes,and found that the Co P（111）surface which cobalt exposed has a lower surface energy.The thermodynamic analysis of the hydrogen atoms adsorbed on the surface was performed,and the Gibbs free energy of adsorption was calculated.The Co P（111）surface△GH*is close to zero under the coverage of 2/8-3/8 H,which indicates that Co P has for better HER catalytic activity,the Co2 site on the surface has the best catalytic activity,and the calculation result of the d-band center also proves this point.We doped the inactive sites on the surface with cations,and found that doping with Mn and Os atoms changed the electronic environment of the Co P（111）surface,and the Gibbs free energy of H atom adsorption decreased,making the Co1 site effective and active for hydrogen evolution,thereby the number of sites increases,and the optimal activity range of the hydrogen evolution reaction is increased to 2/8-5/8,which improves the HER performance.（2）Secondly,we conducted a theoretical study on the optimal catalytic mechanism of hydrogen evolution on the Co P surface,optimized the layout of surface water molecules through molecular dynamics simulation,and tested the barrier value of the hydrogen evolution transition state before and after Os doping using the Cl-NEB method,and extrapolate the barrier value under different H coverage to the situation when the potential is 0.After doping,the surface Volmer reaction barrier value is reduced from0.519 e V to 0.380 e V,the reaction energy is reduced from 0.240 e V to 0.186 e V,and the surface Volmer reaction is easier to proceed.By comparing the barrier values of the Heyrovsky reaction and the Tafel reaction,we found that the barrier value（0.483 e V）of the Tafel reaction on the Co P（111）surface is significantly lower than that of the Heyrovsky reaction（0.847 e V）,and the Volmer-Tafel reaction mechanism is the main mechanism for HER.After the surface is doped with Os,the number of active sites increases,and the Heyrovsky reaction barrier（0.470 e V）of the new site is significantly lower.It is inferred that the surface HER is dominated by the Volmer-Heyrovsky reaction mechanism at this time.In addition,the difficulty of the decomposition of surface water molecules to produce hydrogen was also tested under neutral conditions.The potential barrier of 0.685 e V indicates that the reaction is difficult to proceed,and the pre-adsorption of hydrogen atoms hinders the further progress of the water splitting reaction.（3）Finally,we studied the application of transition metal Co P in the field of anode materials for lithium-ion batteries.We have studied the lithium adsorption on Co P（111）crystal plane by calculation.The adsorption energy for lithium on the Co-terminated surface is-1.204 e V at the most stable S2 site,and other adsorption sites also have relatively negative adsorption energy,indicating that the Co P（111）crystal plane is easy for lithium adsorption.In addition,we tested Co P（111）adsorbed one,two and three layers of lithium ions,and found that the surface can only stably adsorb one layer of lithium ions.By comparing the binding energy of vacancies,we found that the conversion reaction trend after Co P adsorbs Li.Compared with Co atoms,P atoms are easier to escape from the surface,and the remaining Co atoms on the surface tend to aggregate and rebuild to form Co⁰ nanoparticles.At the same time,voids are created to facilitate the further extraction of P atoms from the bottom layer.Search the latest structure database to calculate the most stable Co-P and Li-P compounds,which are used to calculate the average Li intercalation formation energy,we found that after Li is embedded in Co P,Co3P and Co2P can be considered as the intermediate state where Co P transforms into Co nanoparticle nucleation.After the P element is removed,Li3P is mainly formed and a small amount of Li P is present,and it can be quickly transformed into Li₃P.The formation of Li P proves the existence of Co P+L i⁺+e^-→L i P+Co and Li P+2Li⁺+2 e^-→Li₃ Pelectrochemical reaction mechanisms in the discharge process considered by the cyclic voltammogram in the experiment.The conversion of negative electrode materials will help the rise of a new generation of lithium-ion batteries.