Synthesis Of Bimetallic Compounds Containing Composites For Hydrogen Evolution Reaction With Polyoxometalates As Precursors

The gradually serious problems of current energy crisis and environmental pollution make it urgent to find a clean energy to replace traditional fossil energy.Hydrogen has been expected as a clean,efficient and environmentally friendly energy source.Current methods to produce hydrogen include pyrolysis of fossil fuel,photocatalysis and electrolysis of water.For electrolysis of water,expensive and resource-limited platinum-based catalysts are required,which greatly limits its industrial application.Therefore,it is important to find new catalysts with good catalytic activity and long stability to replace noble metal-based catalysts.So far,scientists have been endeavoring to develop economical,earth-abundant and high-performance electrocatalysts,such as transition metal nitrides/carbides/phosphides etc.,but their catalytic activity is still far from that of platinum-based catalysts.In addition,most of the currently reported catalysts contain only single metal component.There have been many reports showing that catalysts having multiple metal species can enhance catalytic activity by playing synergistic roles via increasing catalytic active sites and/or adjusting the electronic structure of materials.However,traditional methods for preparing catalysts having multiple metal species are normally complicated,limiting the development and application of these catalysts.To solve aforementioned problems,polyoxometalates containing transition metal species are used as precursors since they have multiple atomically mixed metal and non-metal elements.Besides,organic amines are selected to form organic-inorganic hybrids with polyoxometalates containing transition metal species,which then transform to multi-metallic compound-based composites after calcination under mild conditions.Catalytic properties of the obtained multi-metallic compound-based composites for hydrogen evolution reaction（HER）are studied and optimized.The main research work is as follows:（1）Bimetallic nitrides-containing composite materials（BNC）have been prepared from Na₆K₄[Ni₄（H₂O）₂(PW₉O₃₄)₂]·32H₂O and dicyandiamide.Optimization of the experimental procedures leads to WN/Ni₂W₃N@NC composite.Benefited from good HER catalytic activity of Ni₂W₃N and improved conductivity by the outermost nitrogen-doped carbon layer（NC）,the composite requires overpotentials of 166 and157 m V when the current density reached 10 m A·cm^-2 under acidic and alkaline systems,respectively,and Tafel slope is 68.2 and 73.5 m V·dec^-1,respectively,demonstrating it is a good amphoteric catalyst for HER.（2）Bimetallic carbides-containing composite materials（BNC）was prepared from Na₁₀[Co₄（H₂O）₂(PW₉O₃₄)₂]·27H₂O and octadecyl trimethyl ammonium chloride.Optimization of the experimental procedures leads to W/Co6W6C@NC composite,of which the particle size is about 20 nm and the outermost NC layer is around 3-5 nm.Benefited from good HER catalytic activity of Co6W6C,the rich interface structure between W and Co6W6C to promote electron transport,and the contribution of W and NC to the electrical conductivity of the composite,the catalyst has excellent electrocatalytic HER performance in an alkaline system,with an initial overpotential of only 9 m V,which is close to that of 20%Pt/C.And it only needs an overpotential of 80m V to reach 10 m A·cm^-2 with a Tafel slope of 60.1 m V·dec^-1.In addition,the W/Co₆W₆C@NC catalyst also shows excellent stability due to protection from the outmost NC layer.In summary,by using transition metal-containing polyoxometalates and organic amine as reactants,we have prepared BNC and BCC under relatively mild condition.The as-prepared composite materials have good electrocatalytic performance for HER.Our strategy is simple and facile and can easily adjust the phase composition and micro-morphology of final products,which provides a good reference for preparing similar multi-metallic compounds materials and developing of multi-metallic compound-based composites catalysts.