MOF Derived Bimetallic Phosphates As Efficient Electrocatalysts For The Hydrogen Evolution Reaction

The energy crisis is one of the biggest problems we are facing today.Therefore,the development and application of new energy can reduce the consumption of fossil energy and environmental pollution.Hydrogen is a kind of clean energy,and the water produced by combustion of hydrogen and oxygen can be reused repeatedly,which is a truly zero-pollution renewable energy.At present,industrial hydrogen production is mainly through fossil fuels,but this method not only produce by-products,but also damage the environment,so the electrolysis of water to produce hydrogen is undoubtedly the most environmentally friendly method.Electrolysis of water includes cathodic hydrogen evolution reaction（HER）and anodic oxygen evolution reaction（OER）.The rapid progress of these two reactions depends on precious metal catalysts.Therefore,it is very important to develop non-precious metal catalysts with high catalytic activity to replace expensive precious metal materials for large-scale electrolysis of water to produce hydrogen.Transition metal phosphate（TMP）shows excellent electrocatalytic properties in electrohydrolysis due to its unique structure.Most of MOFs have special skeleton structure and large specific surface area,so they have great application potential in the field of electrocatalysis.In this paper,MOF-71,a Co-based organic skeleton,is taken as the research object,and the research content is mainly divided into the following two parts:（1）In this paper,the metal organic framework MOF-71 was used as the precursor,and the sodium hypophosphite decomposition method was used to phosphorize at low temperature in Ar atmosphere.Two kinds of CoP with different morphology were obtained by direct and indirect phosphorization of MOF-71.Its structure and composition were characterized,its catalytic performance of hydrogen evolution was investigated,the preparation process conditions were optimized,and the reaction mechanism of hydrogen evolution was discussed.After optimizing the reaction conditions,the results show that the optimal phosphating ratio is m（MOF-71）:m（NaH₂PO₂·H₂O）=1:12,and the optimal phosphating temperature is 350℃.By comparing the electrochemical properties of Di-CoP/C and In-CoP/C,it was found that the former showed better hydrogen evolution performance In 0.5 MH₂SO₄ electrolyte,and the overpotential was 136 mV at the current density of 10 mA·cm^-2,and the Taefir slope was 52.1mV·dec^-1.Since P has strong electronegativity,part of the Co atom charge will transfer to P,so the Co adsorption gibbs free energy of the hydrogen closer to 0.In the process of hydrogen evolution by electrolysis of water,P can play the role of proton adsorption,while Co as the atomic active center can reduce hydrogen protons to hydrogen,and the layered structure of Di-CoP/C has a large specific surface area,so it shows an excellent hydrogen evolution activity.（2）Similarly,the metal-organic framework MOF-71 is used as the precursor.In Chapter 4,manganese chloride is mixed in the process of synthesis of the precursor,Mn element is introduced,and loaded on the nickel foam substrate to obtain the Co Mn bimetallic MOF material.The peony flower-like double transition metal phosphide Co-Mn-P was obtained by phosphating at low temperature under Ar atmosphere by sodium hypophosphite decomposition method.The phosphating ratio was adopted as m（Co Mn-MOF-71）:m（NaH₂PO₂·H₂O）=1:12,and the phosphating temperature was 350℃.After optimizing the doping ratio,the optimal Mn doping ratio was obtained as 3%.The hydrogen evolution performance and oxygen evolution performance of the catalyst in different pH solution were studied,and the overall water splitting under alkaline conditions were studied,too.The special surface structure has a larger specific surface area,and exposed more active site Mn doping can optimize the electron configuration of the catalyst,reduce the free energy of hydrogen adsorption,and thus improve the intrinsic hydrogen evolution and oxygen evolution activity of the catalyst.In 0.5 MH₂SO₄ solution and 1.0 M KOH solution,the overpotential of the catalyst is 66.6 mV and 52.0 mV at the current density of 10 mA·cm^-2,respectively.Meanwhile,the Co-Mn-P 3%material shows small Tafel slope and good long-term stability in acidic and alkaline media.And it only needs a cell voltage of 1.70 V to reach a current density of 10 mA·cm^-2 and exhibits strong stability of 12 h in an alkaline environment.