Theoretical Investigations On The 2D Carbon Allotropes And Nonprecious Metal Phosphides Catalysts For Hydrogen Evolution Reaction And The Correlative Catalytic Mechanism

Along with the fast growing economy in the world,the contradiction between energy demand and environmental protection has become increasingly prominent,which has turned into a major challenge facing all mankind.It is very urgent to develop sustainable and environmentally friendly energy sources to replace depleted traditional fossil fuels.Hydrogen has been deemed as an ideal green energy carrier with virtues of the high energy density,recyclability and no pollution of reaction products etc.Electrochemical water splitting is deemed to be one of the most practical and feasible ways to commercially produce hydrogen,in view of the high hydrogen production efficiency,high purity of the product and almost no emission of greenhouse gases or other polluting gases.The electrocatalytic hydrogen evolution reaction,as one of the half reactions for water-splitting,is difficult unless the process is driven by the highly active catalysts to effectively minimize the overpotential.The noble metal Pt is regarded as the state-of-the art electrocatalyst for HER,but its high price and scarcity of resources hinder the practical and wide application.Thus,great effort has been made on seeking the nonprecious and highly efficient HER catalysts to replace the Pt-based materials.Based on the density functional theory?DFT?calculations,we have firstly investigated the hydrogen evolution reaction?HER?catalytic activities for three kinds of two-dimensional?2D?carbon allotropes consisted of four-,six-and eight-membered rings,including net C,net W and net Y,the cubic and tetragonal SnP systems as well as GeP₃ system.The findings are summarized below:?1?We investigate the hydrogen evolution reaction?HER?catalytic activities for three kinds of two-dimensional?2D?carbon allotropes consisted of four-,six-and eight-membered rings,including net C,net W and net Y.Different from the inert graphene only consisted of six-membered rings,all these three carbon allotropes can exhibit the HER catalytic activity,with the net C superior to net W and net Y.It is revealed that the strain in four-or eight-membered ring with the antiaromaticity can play a crucial role in dominating their HER activities.Furthermore,the HER catalytic activities for all of three carbon allotropes can be effectively enhanced by doping N,B,O,P and S atoms with the different electronegativity,and the achievement of their high HER activities can be mainly due to the cooperation of foreign-atom doping with the ring strain effect.The correlative carbon atoms,shared by four-,six-and eight-membered rings,can uniformly serve as the most active site for all these 2D pure or doped carbon systems.It is worth mentioning that the N-dopant concentration has an effect on the HER activity,indicating that employing the appropriate dopant concentration can be also considered as the effective approach to endow the doped carbon allotropes with the higher HER catalytic activity.It is obviously that introducing the correlative rings with the strain or antiaromaticity can be considered as the new and effective strategy to realize the highly efficient and nonprecious HER electrocatalysts,and all these intriguing findings obtained in this work can be advantageous for designing new kind of promising HER electrocatalysts.?2?We perform systematic investigations to explore the structures and HER catalytic activities for two kinds of bulk SnP systems with cubic and tetragonal symmetries,both of which can be considered as the stacking of SnP layers possessing structural features similar to the famous phosphorene.The BFDH results reveal that the?111?and?200?facets are the possible exposed surfaces of the cubic SnP system,and then the computed surface energies reveal that the P-terminated?111?and?200?can be the most stable surfaces.The computed?G_H*results show that the P-terminated?111?surface and the?200?surface can exhibit a considerably high HER catalytic activity over a wide range of hydrogen coverage,where the top sites over P atoms can serve as the most active sites on both the surfaces.The BFDH results show that the?101?,?10????,?110?,?002?and?00????facets are possible exposed surfaces for tetragonal SnP system.The P-terminated?101?and?10????surfaces as well as?110?surface can exhibit more favorable surface energy.The computed?G_H*results reveal that the P-terminated?101?and?10????surfaces as well as the?110?surface can possess the considerably high HER catalytic activity over a wide range of hydrogen coverage,even endowing the tetragonal SnP structure with higher HER activity than the cubic structure,and the top sites over P atoms can also serve as the most active sites on these surfaces.both the cubic and tetragonal SnP systems can be very promising candidates as the highly efficient and nonprecious HER electrocatalyst.Moreover,the correlative catalytic mechanisms have been analyzed in detail.All these intriguing findings at the atomic level can be advantageous for promoting the practical application of excellent SnP-based materials in catalyzing the water splitting process.?3?We conduct a series of investigations to study the structure and HER catalytic activity for the GeP₃ system as well as the defected?1????and?003?surfaces with P/Ge atom lack.The BFDH results reveal that the?1????,?003?and?1???2?surfaces are the possible exposed surfaces for the GeP₃ system and the computed surface energies reveal that the?1????-?and?003?are the most stable surfaces.The computed?G_H*results show that the five terminations can exhibit a considerably high HER catalytic activity.For the?1????,the Ge/P atoms out of plane can be mainly responsible for the high HER activity.Both Ge and P atoms can serve as the most active sites for?003?surface.We also investigate the P/Ge atom defect effect on HER activity for the most stable?1????-?and?003?surfaces.The computed results reveal that forming defect can improve the HER activity of original sites and produce some new active sites around the defect due to the decrease in the coordination number of correlative atoms.Overall,the formation of Ge/P vacancy can improve HER activity of?1????-?and?003?surfaces.All these striking theoretical insights at atomic level can open the door for the development of low-cost and high-performance HER electrocatalysts based on germanium phosphide and even other metal phosphides.