Preparation And Electrochemical Performance Of Molybdenum Carbide-based Catalysts For Water Electrolysis

Hydrogen evolution reaction（HER）is a promising energy conversion method as it can effectively convert intermittent electrical energy into chemical energy.However,due to the slow kinetics,electrocatalyst are required to accelerate the reaction.The noble metal catalysts,such as Pt,possess high HER theoretical activity.However,the high cost,poor stability and scarcity of noble metal stand in the way of its widespread application.Among multitudinous non-noble metal catalysts,molybdenum carbide has become an ideal material to replace noble metal catalysts because of its Pt-like electronic structure,good corrosion resistance and strong mechanical stability.However,the performance of molybdenum carbide-based catalysts is still limited by the following aspects:First,the synthesis of molybdenum carbide requires extremely high temperature（>700℃）,which often lead to uncontrollable particles aggregation resulting in the decrease of active site concentration.Second,the strong adsorption of hydrogen also restricts the intrinsic catalytic activity of the molybdenum carbide HER catalyst.In addition,the inert oxide layer on the surface of molybdenum carbide also makes the actual activity of molybdenum carbide far below theoretical expectations.To solve above problems,in this study,we use a few strategies,such as loading with conductive substrate,heterojunction construction and surface activation to improve the HER activity of molybdenum carbide.Furthermore,a molybdenum-rich molybdenum carbide HER self-supporting electrode with high stability was constructed.The main research contents of this paper are as follows:I.The synthesis of molybdenum carbide requires a high temperature（>700℃）,which leading to agglomeration of active sites.To solve this problem,we entrapped molybdenum carbide into the pore of mesoporous carbon nanospheres.The mesoporous carbon can effectively limit the agglomeration of active sites of molybdenum carbide.The molybdenum carbide nanoparticles distribute uniformly with small size（～7 nm）.Compared with unentrapped molybdenum carbide,the electrochemical active surface area of the catalyst and the proportion of electrochemically accessible molybdenum carbide were significantly increased.Moreover,the inherent conductivity of carbon substrate also accelerates the electron transport.Thanks to the above advantages,this catalyst displayed excellent HER activity,including low overpotential of 134 m V and165 m V at current density of 10 m A cm^-2 in alkaline and acid media,respectively.Moreover,the catalyst can also maintain good activity and stability under the test of simulated industrial conditions.II.The strong Mo-H bonding molybdenum carbide prohibits the desorption of H_ads,thus affecting the catalytic performance.The inherent catalytic activity of molybdenum carbide can be increased while reducing hydrogen adsorption by improving the electronic structure.Therefore,in this paper,we took advantage of the electron-rich characteristics of Co to occupying molybdenum antibonding orbitals by transferring electrons to molybdenum carbide,thereby weakening Mo-H.Moreover,the easily oxidizable characteristic of cobalt metal under positive potential could be used to prevent Mo₂C from being oxidized and dissolved by a“self-sacrificing”effect,thereby improving the OER activity and stability of Mo₂C.Benefiting from the dual synergistic effects,the catalyst exhibited an excellent catalytic activity in alkaline electrolyte,only needing 98 m V and 254 m V to reached reached 10 m A cm^-2 for catalyzing the HER and OER,respectively.And its electrocatalytic performance for overall water splitting outperformed those based on commercial Pt/C and Ir O₂ catalysts,only requiring 1.59V to achieve a current density of 10 m A cm^-2.III.Because of the strong affinity with oxygen,Mo₂C is inevitably oxidized on the surface when exposed in air.The inert oxide layer prevents the contact between the active phase and the electrolyte,so that the activity of molybdenum carbide can not reach the theoretical expectation.To solve this problem,in this paper,we used sulfur as the“modifier”to modify the surface of molybdenum carbide by replacing a portion of the oxygen to achieve the purpose of surface activation.Moderate surface modification can optimize the central position of d-band of Mo,thus weakening the Mo-H bond and accelerating the desorption of hydrogen adsorption in HER process.The S-modified Mo₂C only needs 117 m V to reach the current density of 10 m A cm^-2,much lower than un-modified molybdenum carbide.The S-modified Mo₂C also maintain a good stability.IV.The reported molybdenum carbide catalysts are mainly in powder form.When in use,the powdery catalyst needs to adhere onto the conductive substrate with binder.The binder may block the active sites and reduce ionic conductivity.In addition,the mechanical force generated by the bubble bombardment at high current density often leads to the shedding of the active phase.In-situ constructing high activity and stable catalysts on substrates can solve this problem.In this paper,we designed and constructed a high current HER electrode with a molybdenum-rich molybdenum carbide/molybdenum substrate structure.The structure was constructed on the surface of molybdenum substrate by secondary carbonization.The charge transfer between the Mo and Mo₂C promote the Mo atom nearby the interface to be in a"molybdenum-rich"state.The optimized electronic structure weakened the strong adsorption of molybdenum to hydrogen and thus accelerated the hydrogen desorption process.In addition,the strong binding force between the active phase and the substrate ensure a high mechanical stability.The strong corrosion resistance of molybdenum carbide also guarantees its electrochemical stability.Thanks to the above advantages,the catalyst shows excellent activity and stability in alkaline conditions,especially under the high current density.In 1 M KOH electrolyte,the current density of 10,100,1000 m A cm^-2can be reached under the overpotential of 24.6 m V,126.6 m V,451.6 m V.Not only that,in 6 M KOH,1.5 A current,the prepared electrode can maintain stability for more than15 days.