Study On The Synthesis Of Transition Metal Phosphides And Electrocatalytic Performance For Water Splitting

Water electrolysis is of paramount significance for solving the energy crisis and environmental problems.At present,only 4%of hydrogen is produced by electrochemical water splitting technology due to the high cost arising from the high overpotentials of hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）.Employment of high-performance HER and OER electrocatalysts can lower the reaction energy barriers,reduce the overpotentials and increase the energy efficiency.Nowadays,the state-of-the-art electrocatalysts for water splitting are noble-metal materials.Pt-based materials have been recognized as the benchmark catalysts for HER,while Ru O₂ and Ir O₂ exhibit remarkable performance towards OER.However,the large-scale utilization of noble-metal catalysts is limited owing to their low abundance and high cost.Therefore,highly active and stable non-noble metal catalysts are crucial for the development of large-scale electrochemical water splitting technology.Particularly,utilization of bifunctional electrocatalysts can further lower the cost.Among the non-noble metal bifunctional electrocatalysts,the transition metal phosphides have been paid intensive attention owing to their comparatively high activity,facile preparation and strong corrosion resistance.However,the activity of transition metal phosphides under alkaline conditions are quite low because of the high energy barrier of water dissociation on them.Meanwhile,the traditional transition metal phosphates are generally converted from metal oxides or hydroxides,which leads to difficulties in activity enhancement through morphology manipulating and heteroatomic doping.Based on this,this paper utilizes MOFs as the precursor to construct porous Fe-doped Ni CoP nanoarrays（denoted as Fe-Ni CoP）directly on nickel foam.The specific surface area of the electrode is remarkably increased and the transfer of mass and electron are well improved.The adsorption free energies of hydrogen and oxygen-containing species on the catalysts are optimized via heteroatomic doping to enhance the intrinsic HER and OER activity.The main tasks of this paper are as follows.Synthesis of Fe-Ni CoP/Ni from MOFs for overall water splittingIn this part,the transition metal phosphates were prepared as high-performance bifunctional catalysts for HER and OER using MOFs as the precursor.Firstly,the Ni Co metal organic framework compound was grown in situ on the nickel substrate by hydrothermal method using terephalic acid（BDC）as the ligand,Ni（NO₃）₂·6H₂O and Co（NO₃）₂·6H₂O as the nickel and cobalt sources.The obtained Ni Co-MOFs were subjected to ion exchange in potassium ferricyanide solution to form Ni Co-PBA/Ni.The final Fe-Ni CoP/Ni was achieved after phosphating under moderate conditions in a tubular furnace.It was demonstrated that the ion exchange reaction between BDC and Fe（CN）₄^3-from potassium ferricyanide initiated the gradual transformation of smooth-surfaced nanoarrays to porous rough-surfaced hierarchical nanobricks with cubic structures on the surface.The introduction of Fe resulted in lattice contract in Ni CoP and electron level shift in Ni,Co and P.Electrochemical tests showed that when the molar ratio of Ni and Co was 1:4 and the concentration of K₃Fe（CN）₄ was 5 m M,the obtained Fe-Ni CoP/Ni possessed the highest catalytic activity.To drive a current density of 10 m A cm^-2 and 800 m A cm^-2,Fe-Ni CoP/Ni needs an overpotential of 31m V,190 m V for HER and 187 m V,262 m V for OER.When used as bifunctional electrocatalysts for overall water splitting,the cell voltage is only 1.46 V at a current density of 10 m A cm^-2.Synthesis of CoP@N-C NAs/Ni for hydrogen evolution reactionCo CH/Ni nanowires were first prepared on commercial nickel foam substrate by hydrothermal method and then immersed in a certain concentration of dimethyl imidazole solution to grow ZIF-67 shell on Co CH/Ni（Co CH/ZIF-67/Ni）with aid of ultrasonication to accelerate the reaction between Co ion and dimethyl imidazole and remove the thermodynamically instable phases on the surface of Co CH/Ni nanowires.Finally,after phosphating at moderate temperature,CoP/Ni nanorod arrays warpped by nitrogen doped carbon（CoP@N-C NAs/Ni）were obtained.The influences of ultrasonic time on the morphology,crystalline structure,composition and HER properties of CoP@N-C NAs/Ni were studied.