Preparation Of Sulfur/Phosphorus Compound Self-Supporting Electrodes Based On Autogenous Growth And Their Electrocatalytic Performance

The water/seawater electrolysis technology powered by renewable energy has excellent development potential in industrial hydrogen production.Among them,in the context of scarcity of freshwater resources,overall seawater electrolysis can serve as a long-term goal for future hydrogen economic construction.The catalytic activity and stability of electrocatalysts for hydrogen evolution reaction（HER）and oxygen evolution reaction（OER）directly affect the conversion efficiency and production cost of water/seawater electrolysis.In addition,the complex seawater environment,especially the high concentration of chloride ions（Cl^-）,will result in the corrosion of the electrodes and other side reactions.Therefore,it is urgent to explore efficient,stable,inexpensive and corrosion-resistant non-precious metal catalysts for water/seawater electrolysis of hydrogen production.The electrocatalyst directly supported on the current collector is prone to problems such as poor wettability and easy detachment,and the large interface resistance between the catalyst and the substrate affects the rapid electron transport and reduces the catalytic performance.In this study,the corresponding transition metal sulfides and phosphorus oxides were synthesized in situ on the surface of nickel foam（NF）and nickel-iron foam（NFF）via hydrothermal vulcanization,etching oxidation and phosphorylation heat treatment to maximize the adhesion between the active components and the substrate,and explore the catalytic performance for water/seawater electrolysis.The self-supporting electrode prepared by self-growing strategy realizes the strong bond between the electrocatalyst and the substrate,and ensures the fast electron transport.It not only improves the catalytic efficiency of water/seawater electrolysis,but also helps to protect the substrate from seawater corrosion,killing two birds with one stoneUsing NF as the current collector and partial Ni source,an amorphous Mo S₂ coated Ni₃S₂ with nanosheets network composite architecture（a-Mo S₂-Ni₃S₂/NF）was prepared via hydrothermal vulcanization method for overall water splitting to hydrogen production.In 1M KOH,a-Mo S₂-Ni₃S₂/NF electrocatalyst exhibited overpotentials of 163 m V and 150 m V at 100 m A cm^-2 for HER and OER,respectively.In particular,when taken as the bifunctional electrocatalyst for the overall water electrolysis,the a-Mo S₂-Ni₃S₂/NF composite catalyst only required the battery voltage of 1.54 V at 100 m A cm^-2,which is superior to that of Pt/C‖Ir O₂.The synergistic effect between the a-Mo S₂ nanoflakes with highly disordered structure and Ni₃S₂ substrates with high conductivity significantly enhanced the electrocatalytic activity.To ensure the mechanical stability,efficient catalytic activity and corrosion resistance of the electrode,the self-supported a-Mo S₂-（Fe Ni）₉S₈ heterostructures epitaxially grown on NFF were（a-Mo S₂-（Fe Ni）₉S₈/NFF）in situ synthesized by a one-step hydrothermal sulfidation method for efficient and stable overall water/seawater splitting.In which NFF was the conductive substrate and the direct source of Ni and Fe elements.In 1 M KOH,a-Mo S₂-（Fe Ni）₉S₈/NFF electrocatalyst required 164.9 m V（HER）and 233 m V（OER）overpotentials to achieve 100 m A cm^-2,respectively.Benefiting from the strong interfacial interaction,efficient charge transfer,and synergistically enhanced corrosion resistance of a-Mo S₂-（Fe Ni）₉S₈,the self-supported a-Mo S₂-（Fe Ni）₉S₈/NFF electrocatalyst exhibits excellent catalytic performance in both freshwater/seawater electrolysis.In alkaline freshwater and natural seawater electrolytes,the a-Mo S₂-（Fe Ni）₉S₈/NFF electrolyzer required voltages of 1.58 and 1.62 V to reach 100 m A cm^-2,respectively,and exhibited a durability up to 72 h under alkaline seawater conditions.Transition metal phosphides（phosphorus oxides）would also significantly enhance their HER catalytic activity.In situ hydrothermal oxidation of NFF was carried ou in a mixed solution of H₂O₂,urea,and ammonium hydrogen fluoride without the addition of additional metal salts,and followed by the phosphorylation heat treatment to synthesize hollow nanospheres composed of amorphous Ni Fe-PO_x nanoflakes（Ni Fe-PO_x/NFF）for overall water splitting.In 1 M KOH electrolyte,the HER and OER catalytic processes required 110m V and 247 m V low overpotentials to reach 50 m A cm^-2,respectively.When taken as both anode and cathode for overall water splitting,the cell voltage as low as 1.52 V at 20 m A cm^-2.In addition,the current density was maintained at 88.6%after 40 h endurance testing,showing excellent catalytic stability.In addition to the enhanced density of active sites by the amorphous Ni Fe-PO_x surface,the high permeability hollow spherical nanostructures,efficient mass/charge transfer,and fully exposed active sites were beneficial to improve the electrocatalytic performance.To simultaneously improve the catalytic activity and corrosion resistance of the electrocatalyst for seawater electrolysis,we developed Mo-doped Ni Fe-phosphate hollow bird-nest architecture on NFF substrate（Mo-Ni Fe-PO₃/NFF）via hydrothermal in-situ chemical oxidation in a mixed solution containing Na₂Mo O₄ and subsequent phosphorylation annealing approach,and exhibited admired electrocatalytic activity and corrosion resistance for overall water/seawater electrolysis.As a bifunctional electrocatalyst for water electrolysis,the Mo-Ni Fe-PO₃/NFF electrolyzer only required 1.61 V voltage to achieve 100 m A cm^-2 in 1 M KOH.Under alkaline seawater conditions,the Mo-Ni Fe-PO₃/NFF electrolyzer exhibited 1.65 V and 1.78 V low voltages to obtain 100 and 500 m A cm^-2（industrial-level current density）,as well as excellent stability up to 100 h.The excellent catalytic performance of Ni Fe-PO₃/NFF in water/seawater electrolysis was attributed to the lattice distortion and the changes of electronic environment by Mo doping,which not only improved the electrocatalytic activity,but also cooperated with the polyanion phosphate group to resist Cl^-in seawater and improve the corrosion resistance.