Synthesis And Electrochemical Hydrogen Evolution Properties Of Molybdenum Phosphide Based Catalysts

Growing concern about the energy crisis and the issue of environmental pollution urgently demand the development of the new renewable energy sources to shorten the dosage of fossil fuels.Hydrogen is a potentially promising alternative energy carrier for traditional fossil fuel in virtue of its high energy density,high abundances and renewable features.Compared to the traditional hydrogen production technologies,the water electrolysis produces hydrogen cleanly and efficiently.At present,Pt-based catalysts are considered as the most efficient catalysts for the hydrogen evolution reaction（HER）,but their scarcity and high cost hinder the large-scale applications of electrochemical watersplitting electrolyzers.Therefore,it is highly desired to develop high active and stable non-precious metal electrocatalysts for HER.Molybdenum phosphide is one of the most widely reported HER catalysts,due to the platinum-like electronicstructure,low cost,efficient catalytic activity and remarkable durability over all pH range.However,molybdenum phosphide as HER catalyst suffers from two main limitations:insufficient active sites and poor electro-conductivity.In our work,the nitrogen-carbon codoped molybdenum phosphide（NC-MoP）,liked neural network of the N-doped carbon interconnected molybdenum phosphide（MoP@NC-MF）and3D self-supporting Ni₂P/Mo₄P₃ composite catalyst are successfully synthesized to improve the issues of insufficient active sites and poor electro-conductivity.Electrochemical hydrogen evolution performances testings demonstrate that the above MoP-based catalysts exhibited low overpotential,good conductivity and good stability.The primary study contents are as follows:（1）Nitrogen-carbon codoped molybdenum phosphide（NC-MoP）is synthesized through the one-step pyrolysis method by using phytic acid,ammonium molybdate tetrahydrate and melamine as reagents.NC-MoP exhibits high electrocatalytic activity and stability in a wide pH range for HER with a low overpotential of 123/207 mV required to achieve a current density of 10 mA cm^-2 in 0.5 mol·L^-1 H₂SO₄/1.0 mol·L^-1KOH solution.The onset overpotential of NC-MoP is only 57 mV,and the charge transfer resistance of NC-MoP is about 8Ωin acidic media.Compared with Bulk MoP,NC-MoP has higher electrocatalytic activity for HER,which can be attributed to the reason that the doping of nitrogen and carbon improves its conductivity.The simple synthetic method greatly simplifies the material preparation process without multi-step reaction,without introducing additional hybrid carbon sources,and without using strong reducing agents（such as NaH₂PO₂ or H₂/Ar,etc）or high boiling point organic solvents（such as triphenylphosphine）as phosphorus sources.（2）Inspired by human nerves which can rapidly transmit information relying on the interconnected network architecture consisting of neurons and axons,a novel neural network-like molybdenum phosphide composite（MoP@NC-MF）are constructed by using the low cost melamine resin foams as the initial scaffold.The neural network-like MoP@NC-MF composite consists of MoP nanoparticles with abundant active sites as the“neurons”and N-doped carbon skeleton as the“axons”.MoP@NC-MF exhibits high electrocatalytic activity and good stability with a low overpotential of 125 mV required to achieve a current density of 10 mA cm^-2 and a small Tafel slope of 53.0 mV·dec^-1under acidic condition for HER.Such a superior performance of MoP@NC-MF composite is attributed to the unique neural network structure with large specific surface area and the abundant MoP active sites,which enlarge the contact area between the electrode and electrolyte.Additionally,nitrogen doped carbon skeletons interconnect continuous electro-conductivity pass which promptly transfer electron.And the 3D networks with multilevel porous structures facilitate the liquid reactants/gaseous products transporting to/escaping from the reaction active centers.（3）The precursor oxides（NiMo₄/MoO₃）are in-situ grown on the surface of Ni Foam through hydrothermal method,and after the low temperature phosphating treatment the Ni₂P/Mo₄P₃ composite catalysts are prepared.Ni₂P/Mo₄P₃ composite catalysts with three-dimensional self-supporting structures,are directly used as working electrodes without any binder,which can expose more active sites and improve the mechanical stability of electrodes.Ni₂P/Mo₄P₃ composite catalysts are in situ grown on the three-dimensional porous Ni Foam as conductive substrates,which can effectively promote mass/charge transfer and timely transfer hydrogen gas away from the active sites.And there is synergistic effect between Ni₂P and Mo₄P₃.For these reasons,Ni₂P/Mo₄P₃ composite catalysts exhibit good hydrogen evolution activity and stability with a low overpotential of 122 mV required to achieve a current density of 10 mA cm^-2 and a small Tafel slope of 34.8 mV·dec^-11 in alkaline solution.