Theoretical Insights Into The Hydorgen Evolution Activity On Co/Fe Embedded Carbon Nanotubes

Hydrogen,one of the cleanest renewable fuels,is the best alternative to future fossil fuels.The best way of producing hydrogen is through water electrolysis or photoelectrolysis.So far,the best electrochemical hydrogen evolution catalyst is considered to be platinum.However,the high cost of platinum hinders commercial large-scale hydrogen production.Therefore,searching an efficient,stable and inexpensive catalyst for hydrogen evolution reaction（HER）is an urgent problem in the hydrogen economy.Transition metals have always been considered a good HER catalysts,yet its unstable property in acidic medium limits their further application in hydrogen production.In this thesis,we designed five modified Co embedded carbon nanotubes（CoCNT（3,3）,CoCNT（5,5）,CoCNT（7,7）,CoCNT（9,9）and CoNCNT（5,5））and four modified Fe embedded carbon nanotubes（FeCNT,FeBCNT,FeFeCNT and FeCoCNT）by replacing the carbon atom of metal embedded carbon nanotubes（MCNT）with different heteroatoms and inserting the second metal atom into the MCNT,and further demonstrated their stabilities by the formation energy,phonon dispersion spectra and cohesive energy.Next,the HER mechanism on the designed MCNT was investigated by first principles.Firstly,the hydrogen evolution reaction（HER）on modified cobalt embedded carbon nanotubes（CoCNTs）was investigated.The density functional theory（DFT）computations demonstrated that the HER on CoCNTs prefers to pass through the Volmer-Heyrovsky mechanism with the rate-determining step of the Heyrovsky reaction,and the most favorable active sites are the carbon atoms bonding to Co center.With the increasing curvatures of CNTs,the HER activities of CoCNTs exhibit a volcano-shaped trend,where CoCNT（5,5）has the best HER activity.In addition,after nitrogen atoms doped in CoNCNT（5,5）,the HER exchange current density of CoNCNT（5,5）is further enhanced,even higher than that of Pt（111）.In addition,the improvement of HER activity on CNT is primary on the basis of tuning the electronic structure rather than the geometric structure,mirrored by the linear dependence of both?G_H*and E_a with p-band center of active C atoms.Next,the HER on iron embedded carbon nanotubes（FeCNTs）was studied.The results suggested the most favorable active sites of FeCNT,FeBCNT,FeFeCNT and FeCoCNT are the active carbon atoms bonded to Fe atoms,and the HER on FeCNT is the Volmer-Heyrovsky mechanism with the rate-determining step of the Heyrovsky reaction.Moreover,when introducing the boron atoms into the FeCNT,the energy barrier of rate-determining step of Volmer-Heyrovsky on FeBCNT decreases by 0.30 eV compared with that of FeCNT.In addition,the hydrogen adsorption energy on FeCNT system would decrease with the incorporation of two metal atoms,and thus promoting the HER activity,especially,the exchange current density of FeFeCNT is approximate to that of Pt（111）.The results reavel that two metal atoms incorporation and boron doping for FeFeCNT could efficiently improve their HER catalytic performance via modifying the p-band center of active C atoms.