Design And Preparation Of POM-Mediated Noble-Metal-Free Electrocatalysts For Hydrogen Evolution Reaction

Clean hydrogen,as the most ideal alternative energy,has been widely recognized because of its high energy density,environment-friendly and renewable property to replace the excessive use of fossil fuels.Compared with hydrogen production from fossil fuels,water electrolysis is a theoretically attractive and potential method for hydrogen evolution reaction?HER?.However,a key limitation for this strategy is to explore highly efficient but cheap electrocatalysts.Pt-based catalysts perform the highest activity in HER,however,considering the high-cost and low-earth-abundance of Pt group metals,the development of various non-precious-metal-based electrocatalysts has become a hot research topic.Hitherto,much attention has been devoted to researching transition metals and their alloys,chalcogenides,carbides,nitrides,borides,phosphides and some carbon-based catalysts with heteroatomic doping and modification.These catalysts perform well in electrocatalytic HER activity,but most of them are still not comparable to the noble metal platinum.Thus,it is important that the development of new types of non-noble metal catalysts could reveal the external factors and internal mechanisms of improving the catalytic performance from molecular levels,which could continue to bring new breakthroughs in this researching field.Currently an important research strategy is to construct composite nanomaterials with multiple active components by using the synergistic effect of multiple components,reducing the free energy of hydrogenation of composite materials to enhance the catalytic activity and obtain novel catalysts.Nevertheless,the preparation of multi-component composite nanomaterials faced with the following problems:how to guarantee the composition consistency of the two types of components in nano-materials?How can composite nanomaterials prevent itself from overpolymerization and consolidation?How to maintain the uniformity?distribution of narrow size?of the particle size in the composite nanomaterial?So far these are still great challenges in the preparations of composite nanomaterials for electrcatalytic HER because of these problems.Polyoxometalates?POMs?,as one type of unique metal oxides clusters,have provided a class of excellent molecular pre-assemblies for constructing new composite nanomaterials due to their element-rich components,various structural types and nanometer size of natural uniform.Based on the above consideration,we intend to adopt POMs which contain abundant tungsten element as precursors,introduced in various catalytic transition metals and nonmetal elements by virtue of chemical synthesis,then annealed the so-prepared POM assemblies and obtained composite nanomaterials for HER in electrolysis of water.This strategy is expected to build a series of highly efficient and stable HER electrocatalysts.Based on above research strategies,two kinds of composite nano-catalysts were designed and prepared,tests in HER performance.The results are as follows:1?Wepre-assemblyDCAandthePOMprecursor Na₉?NH₄?₅[{(B-?-PW₉O₃₄)Co₃?OH??H₂O?₂?Ale?}₂Co]·35H₂O(Co₇P₆W₁₈)that is rich in Co,W and P elements,then the mixture experiences a one-step annealing in the tube furnace under N₂ flow.Finally the composite nanoheterojunction material of cobalt phosphide and tungsten carbide covered by N-doped carbon layers are obtained?abbr.Co₂P/WC@NC?.This material possesses the unique core of Co₂P/WC heterojunction,uniform particle size distribution and small particle size,confined to grow by the coating of thin layer of graphitized carbon.By means of PXRD,Raman spectrum,XPS,SEM and TEM,the heterojunction structure,component and morphology have been analyzed.The HER tests suggested that Co₂P/WC@NC exhibits extraordinary HER electrocatalytic activity and considerable durability in the whole pH range.By a series of comparison tests and DFT calculations,the results suggest that the Co₂P/WC@NC performed well owing to the synergy effect among Co₂P and WC in heterojunction structure,which could evidently decrease the hydrogen free energy of single component.Meanwhile,there is also a significant synergistic effect between the heterogeneous crystalline core and the N-doped carbon shells,which can further reduce the free energy of hydrogen generation and improve the overall catalytic activity of the composite catalytic materials.2?Meanwhile,we attempt to preassembly DCA and the POM precursor H₁₅Na₁₃[{Ni₄?OH?₃?PO₄?₄}₄(PW₉O₃₄)₄]·25H₂O(Ni₁₆P₂₀W₃₆)toobtainnickel phosphide/tungsten carbide coated by N-doped carbon shells through the same thermal conversion technique?Ni₂P/WC@NC?.By means of PXRD,Raman spectrum,XPS,SEM and TEM,the heterojunction structure,component and morphology have been analyzed.The HER tests suggested that Ni₂P/WC@NC also exhibits good electrocatalytic activity for HER and considerable durability.In summary,we developed a simple method of preparing heterojunction composite materials with multiple active components,and obtained two new types of HER electrocatalysts.By contrast experiments and the quantum calculations,we revealed the water electrolysis mechanism of these composite nanocatalysts and suggested a way to develop new non-noble-metal electrocatalysts for HER in the near future.