Activity Modulation of Two-dimensional MoS2 Based Catalysts for HER
Author:Luo Zhao Yan
The lack of a cost-effective replacement for Pt has plagued the scale-up of hydrogen electrochemical production for decades;the alternative catalytic materials are fundamentally limited by either a low catalytic efficiency or a short lifetime.Lamellar MoS2 has been regarded highly promising towards hydrogen evolution reaction(HER)since the activity of its metallic edges(△Gh=0.06 eV)was proved by theoretically and experimentally.The current guiding principles for advancing the MoS2 catalytic efficiency are as follows:First,increase the atomically undercoordinated active sites density in the trigonal prismatic phase(2H)MoS2,either through the preferentially exposing edge sites or through creating in-plane sulfur vacancies(SVs).However,unleashing the intrinsically high activity is retarded by the semiconductive feature of 2H-MoS2,where the charge transfer efficiency is limited by a deficiency of electrons at the reaction interface.Second,drive the 2H phase MoS2 into the conductive and therefore more catalytically active 1T phase.The basal-plane S atoms are regarded as active sites in 1T-MoS2;however,these S sites suffer from less favorable hydrogen adsorption features(△GH=0.17 eV)despite the greatly increased site density.Beyond the above mentioned problems of 2H-MoS2 and 1T-MoS2,one major issue that both these materials encounter is their reduced stability because defective 2H-MoS2 suffers from a high sulfur leaching rate and 1T-MoS2 is intrinsically metastable.Apparently,MoS2 only become truly applicable towards the HER when the electronic conductivity,site density,intrinsic activity,and stability issues are simultaneously solved.In view of the above problems,this paper carries out the following work:(1)We resolve these challenges concurrently through chemically activating the molybdenum disulfide(MoS2)surface basal plane by doping with a low content of atomic palladium using a spontaneous interfacial redox technique.Palladium substitution occurs at the molybdenum site,simultaneously introducing sulfur vacancy and converting the 2H into the stabilized 1T structure.Theoretical calculations demonstrate the sulfur atoms next to the palladium sites exhibit low hydrogen adsorption energy at-0.02 eV.The final MoS2 doped with only 1wt.%of palladium demonstrates exchange current density of 805 μA/cm2 and 78 mV overpotential at 10 mA/cm2,accompanied by a good stability.The combined advantages of our surface activating technique open the possibility of manipulating the catalytic performance of MoS2 to rival platinum.(2)Facilitating charge transfer through engineering better electrical contacts between the support and catalyst is an additional important variable,we arrived at resolving these issues by a cation and anion co-doping strategy;first,successfully stabilizing 1T phase MoS2 on reduced graphene oxides through anion(N)doping strategy;next,cation(Pd or Ru)doping induced energy level engineering and S-vacancies to further improve intrinsic activity and active site density.The high electrical conductivity,rich with catalytic active sites and high intrinsic activity as a result of the introduction of conductive substrate(rGO),stable 1T phase and heteroatoms,giving rise to up-to-date the lower over potential and highest intrinsic activity among MoS2-based catalysts,even comparable to or better than platinum-based catalysts.(3)We build a MoS2 di-anionic surface with controlled molecular substitution of S sites by-OH.We confirm the-OH group endows the interface with reactant dragging functionality,through forming strong non-covalent hydrogen bonding to the reactants(hydronium ions or water).This well-conditioned surface,in conjunction with activated sulfur atoms(by heteroatom metal doping)as active sites,giving rise to up-to-date the lowest over potential and highest intrinsic activity among all the MoS2 based catalysts,even comparable to(in acid)or better than(in base)platinum-based catalysts.The di-anion surface created in this study,with mixing of active sites and reactant dragging functionalities,represents a novel di-functional interface for boosted kinetic performance.(4)Heteroatom doping is recognized as an efficient strategy to tune and improve the hydrogen evolution reaction(HER)activity of molybdenum disulfide(MoS2).However,most heteroatom-doped MoS2 catalysts for HER in acid solution are either of large overpotential or lack of operating stability especially at high current densities.To tackle this challenge,we designed a dual-metal doping MoS2-based material via a facile partial cation exchange reaction method.The as-prepared Pd,Zn-MoS2 catalyst shows high HER performance,achieved a geometrical catalytic current density of 10 mA/cm2 at overpotentials as low as 86 mV and low Tafel slope of 76 mV/dec.Moreover,this catalyst also exhibits outstanding long-term operational stability.In addition,we also show that dual-doping can fine-tune the HER activity of MoS2-based material via energy level matching and increasing active density.This work proposes a new strategy to optimize HER electrocatalytic performance of MoS2 and also can be extended to other layered transition metal dichalcogenides.