Electrocatalytic Hydrogen Evolution Properties Research Of Molybdenum-based Composite Materials Prepared By Polypyrrole

Rapid global energy consumption and the consequent problems of environmental pollution have greatly contributed to the development of sustainable energy conversion technologies.Hydrogen is an environmentally friendly new energy source with high energy density and an ideal alternative to fossil fuels.Electrolysis of water is recognized as a green hydrogen production method,but the shortcomings of precious metal Pt-based catalysts limit the large-scale industrial application.Therefore,the development of efficient and low-cost hydrogen evolution electrocatalysts is essential for industrial electrolysis of water to produce hydrogen.Conductive polymers（polypyrrole,polyacetylene,polyaniline,etc.）show good electrical conductivity,stable redox properties and low cost,and can be used as potential electrocatalysts for hydrogen evolution reaction.In addition,polyoxometalates（POMs）are anionic metal-oxygen clusters with three-dimensional structures.The abundant active sites and large active surface area make POMs exhibit excellent hydrogen evolution activity as electrocatalysts.Based on the above discussion,a series of transition metal phosphide electrocatalysts were obtained by combining polypyrrole with molybdenum-based POMs by electrostatic attraction for efficient hydrogen evolution.The main contents are as follows:（1）The precursor was synthesized by electrostatic attraction between polypyrrole,phosphomolybdenum acid(PMo₁₂)and nickel ions(Ni²⁺).Then,sodium hypophosphite was used as the phosphorus source,and the electrocatalyst Mo P/Mo Ni P@C was synthesized by one-step high-temperature calcining and phosphating process.The hydrogen evolution performance of Mo P/Mo Ni P@C was investigated in acidic and alkaline media.The test results show that Mo P/Mo Ni P@C exhibits good hydrogen evolution activity.When the current density reaches 10 m A cm^-2,the overpotentials are138 m V and 135 m V,and the Tafel slopes are 61 m V dec^-1 and 88 m V dec^-1 in 0.5 M H₂SO₄ and 1.0 M KOH,respectively.In addition,the catalyst shows good stability for24 h in acid-alkaline solutions.（2）Using sodium hypophosphite as phosphorus source and polypyrrole as carbon and nitrogen source,it was compounded with polyoxometalates Mo₄₀ and Ni²⁺to form a precursor,and multi-component nanostructured electrocatalyst Mo P/Mo Ni P@NC was obtained by calcination at high temperature of 900°C.Mo P/Mo Ni P@NC exhibits excellent catalytic activity in acid-base electrolytes.At the current density of 10 m A cm^-2,the overpotentials of Mo P/Mo Ni P@NC are 125 m V（0.5 M H₂SO₄）and 144 m V（1.0 M KOH）,respectively,and the Tafel slopes are relatively low of 115 m V dec^-1 and85 m V dec^-1,respectively.In addition,the stability of the catalyst was also tested,and from the time-current（i-t）curve that Mo P/Mo Ni P@NC displays good stability and corrosion resistance in acid-alkaline media.（3）Polypyrrole and green phosphorus-source ammonium polyphosphate（APP）with polyoxometalates Fe Mo₆ were used to synthesize precursors through electrostatic attraction and coordination,and then Fe-Mo P@NC was obtained by one-step high-temperature calcination.When the current density reaches 10 m A cm^-2,the overpotentials of Fe-Mo P@NC are 141 m V and 245 m V in 0.5 M H₂SO₄ and 1.0 M KOH solutions,respectively.And the Tafel slopes are 66 m V dec^-1 and 69 m V dec^-1,respectively.It is proved that the catalyst had a fast reaction kinetics.The stability of Fe-Mo P@NC in acid-alkaline medium was tested,and the results show that Fe-Mo P@NC has durability of up to 24 h.（4）Polypyrrole,polyoxometalates Mo₅₄ and phytic acid（PA）were used as raw materials to obtain precursors by simple solvent heating evaporation reaction.Finally,Mo P@C was successfully synthesized by high-temperature calcination and in-situ phosphating.The hydrogen evolution test results show that at 10 m A cm^-2,the overpotential of the Mo P@C is 154 m V in acidic solution,and the Tafel slope is 64 m V dec^-1.The overpotential is 228 m V and the Tafel slope is 95 m V dec^-1 in alkaline solution.Similarly,stability tests have found that Mo P@C have good durability.