The Theoretical Study Of Surface Heterostructures For Electrocatalytic Hydrogen Evolution Reaction

Hydrogen has a very high energy density,and the product is only water,which will not cause environmental pollution.Green hydrogen is one of the most promising energy in the future,whereas the low efficiency of hydrogen-producing and huge energy consumption restrict the further development.An important reason for the low efficiency of hydrogen-producing is the lack of low-cost and high efficient HER catalysts to overcome the reaction barriers.As the most effective HER catalyst,the use of Pt is limited by the scarcity and high price.Among the non-noble metal catalysts,transition metal sulfides（TMDs）are promising HER catalysts and important supplement and substitution for Pt-based HER catalysts,but their over-all performances are still far away from the Pt-based catalysts.With a view to solving those above problems,we have performed density functional theory study combined with the ab initio molecular dynamics（AIMD）and climbing image nudge elastic band（CI-NEB）method on increasing active sites and prolonging the life of TMDs and Pt-based catalysts.The research online is as follows:（1）Mo S₂ is one of the most potential HER catalysts,but the number of active sites is limited.Almost all atoms of Mo_xS_y clusters occupy surface/edge sites,which can make them have an optimized number of active sites,whereas the Mo_xS_y clusters may be susceptible to desorption from the electrodes in working environment due to the weak adsorbate-electrode interactions.The Ce O₂ support could provide strong support interaction for various Mo_xS_y clusters,such as Mo₃S₄,Mo₄S₃,Mo₄S₄ and Mo₆S₈,which may be a good resolution for the desorption of Mo_xS_y clusters.The formed Mo₃S₄/Ce O₂and Mo₄S₄/Ce O₂hetero-structures have modest hydrogen adsorption free energy(ΔG_H*)for HER,and theirΔG_H*values are about-0.10 e V.From the PDOSs of Mo_xS_y/Ce O₂,we see that the induced S-3p or Mo-4d states through the interaction with the Ce O₂（111）supports lead to an appropriate interaction with H,being critical for a better HER activity.The Mo₄S₄/Ce O₂shows not only good HER performance in Volmer-Heyrovsky reaction but also on the Tafel reaction,which the barrier of Tafel reaction to break one Mo_b-H bond and to form the H-H bond is 0.39 e V.（2）Based on the study of Mo_xS_y/Ce O₂,the possibility of Nb_xS_y clusters（Nb₃S₄、Nb₄S₃and Nb₄S₄）used for HER reaction has been explored.The free-standing and absorbed Nb_xS_y clusters are stable at room temperature.The formed Nb₃S₄/Ce O₂ and Nb₄S₃/Ce O₂hetero-structures also have modest hydrogen adsorption free energy(ΔG_H*)for HER,and theirΔG_H*values are about 0.10 e V suitable for Volmer-Heyrovsky reaction.As for the Nb₃S₄/Ce O₂,the barrier of Tafel reaction to release H₂is 0.27 e V.The interaction of Nb_xS_y clusters with the Ce O₂（111）support would induce electronic rearrangement of S-3p or Mo-4d states,leading to an appropriate interaction with H.（3）The most straightforward method to improve the utilization of rare Pt-based catalysts is by reducing the size of catalysts from nano particles,nano clusters to single atoms.The oxidation states（OSs）of Pt single atoms play a key role in the HER reaction.Taking Pt/Ce O₂ model catalysts as examples,we have systematically explored the effect of Pt^{4+/2+/1+/0/2-}OSs on HER reaction.The H atom could not absorb on the Pt⁴⁺and Pt_4c²⁺sites,and they prefer to locate on the adjacent O atoms of Pt⁴⁺and Pt_4c²⁺sites.As for the other OSs,the adsorption ability of H atoms follows the order of Pt⁰>Pt_3c²⁺>Pt¹⁺>Pt^2-.（4）Prolonging the life of Pt-based catalysts is another important method to deal with the scarcity of Pt.The protective layer of graphene（Gr）could protect Pt from the harsh external reaction environment,but the HER activity of Gr/Pt is unsatisfactory due to the weak absorption of H atoms.The Ir doping of Gr/Pt could optimize the H absorption on Gr layers,and this can be ascribed to the enlarged electronic disturbance of C-p sates caused by the Ir doping.This paper has proposed some feasible strategy to problems limiting the large-scale application of TMDs and Pt based HER catalysts,and analysed the relationship between their atomic structures,electronic structures and catalytic characteristics,shedding light on the subsequent understanding and design of excellent HER materials.