Molybdenum Carbide And Nickel Sulfide Electrocatalysts For Hydrogen Evolution And Oxygen Evolution: In Situ Reconfiguration And Structural Design

Electrochemical water splitting is a promising technology for generating pure hydrogen energy.At present,platinum-group noble-metal catalysts are the most active electrocatalysts,but their earth-abundance is low and the cost is high.It is of great significance to exploit non-noble-metal catalysts?e.g.,metal carbides and sulfides?to replace Pt-based catalysts.Transition-metal?molybdenum,tungsten?carbides are extremely easy to be oxidized and then to afford heterostructures involving oxides naturally.Meanwhile,the surface of carbides would be restructured during electrocatalytic process.While the sulfides of transition-metals?iron,cabalt,nikel?are tend to form corresponding oxides/hydroxides/hydroxyloxides in alkaline electrocatalytic process,which participate in catalytic reactions as the actual active species.Thereby,investigation of the surface reconfiguration of noble-metal-free electrocatalysts under reaction conditions is greatly important for understanding structure-property relationships and developing efficient electrocatalysts.Here,molybdenum carbide and nickel sulfide are investigated as the typical representatives to form Mo₂C-Mo O_x and Ni₃S₂-Ni O_x heterostructures via oxygen plasma etching,which are investigated to reveal the functionalities of surface oxides.And their in situ reconfiguration process during electrocatalytic reaction is revealed by in-situ Raman.The main content of this dissertation is as follows:1.Heterostructured Mo₂C-Mo O_x on carbon cloth?Mo₂C-Mo O_x/CC?is designed and prepared through oxygen plasma.Raman spectroscopy combined with electrochemical tests identifies that surface Mo^VI oxides are in situ reduced to Mo^IV,along with the great promotion of the HER activity.As indicated by density functional theoretical?DFT?calculations,the in situ reduced surface with terminal Mo=O moieties brings the quite negative?35?G_H*on bare Mo₂C close to a thermodynamic neutral value,which addresses difficult H*desorption toward fast HER kinetics.The optimized Mo₂C-Mo O_x/CC gives a low overpotential(?₁₀)of 60 m V at-10 m A cm^-2in 1.0 M HCl O₄,outperforming most of the non-noble metal catalysts.And consistent in situ surface reconfiguration and promotion in HER are further evidenced on W₂C-WO_x.2.The Ni₃S₂-Ni O_x/NF heterostructures are fabricated from Ni₃S₂/NF via oxygen plasma treatment,which are used as bifunctional electrocatalysts for hydrogen and oxygen evolution reactions.Compared with Ni₃S₂/NF,the electrocatalytic activity of Ni₃S₂-Ni O_x/NF heterostructures has been greatly improved by optimizing the surface oxidation degree.The optimized Ni₃S₂-Ni O_x/NF affords a low overpotential of 105and 241 m V for HER and OER at 10 m A cm^-2 in 1.0 M KOH,outperforming most of recently reported nickel sulfide-based catalysts.Moreover,the promotion in HER is investigated by Raman spectroscopy combined with electrochemical tests,which identifies that the surface Ni O_x is in situ evolved into Ni?OH?₂ during hydrogen evolution reaction.This promotes the dissociation of water and the generation of H*,and thus accelerates the reaction kinetics of hydrogen evolution.In the OER process,the Ni O_x on the surface of Ni₃S₂-Ni O_x/NF in situ derives a large number of Ni OOH species,resulting in that electrons cannot be transferred from the catalyst surface to the internal matrix,which seriously hinders the mass transfer and charge transfer of the reaction.Thus,the activity of OER is rapidly declined.In summary,we proposed feasible strategies to construct the heterointerfaces of Mo₂C-Mo O_x and Ni₃S₂-Ni O_x and uncovered the effect of the surface oxides toward catalysts.This work provides insights into structure-property relationships and design principles for developing better catalysts in future research.