Preparation Of Molybdenum Carbide-based Composite For Water Splitting

Currently,the technology of water splitting offers a promising way for green hydrogen production,the key for its industrial application is to design and develop efficient and durable as well as low-priced electrocatalysts.Platinum-based materials have been recognized as the most efficient catalysts for water splitting so far,but the high cost and scarcity restrict their commercialization.Reducing the loading as well as improving the utilization and efficiency of precious metals is one important strategy to design electrocatalysts for water splitting.Meanwhile,the study of reducing the size of precious metal and combining it with other active component by interface engineering is the frontier.Furthermore,it is a sustainable strategy for water splitting to develop electrolytic water catalysts composed of cheap and earth-abundant elements.Herein,based on the interaction between metal salts and organic carbon sources,molybdenum carbide-based catalysts with special structures,components,and crystal phases were constructed by monophyletic precursor method.The optimization of the geometric structure and regulation of the electronic structure of molybdenum carbide-based catalyst were realized through morphological adjustment,element doping,interface engineering,and crystal phase regulation.The interaction between molybdenum carbide and loading metals was studied through various characterization methods.The relationship between the catalytic performance and the structure,and steps to control the reaction speed were investigated by electrochemical tests.Moreover,the active sites of catalysts for water splitting were revealed by studying the evolution of elements on the surface of the electrocatalyst.This paper mainly includes the following contents:（1）Using ammonium molybdate as the molybdenum source and the synthetic polyamine as the carbon source,the monophyletic precursor was synthesized by an organic-inorganic hybrid method which was followed by one-step pyrolysis,and the electrocatalyst composed of molybdenum carbide nanoparticles uniformly anchored on the surface of nitrogen doping porous molybdenum carbide microflower was acquired.The self-assembly mechanism of microflower was studied,the ratio of molybdenum source to carbon source,hydrothermal growth temperature and time,and the calcination temperature were optimized.The optimized electrocatalyst showed excellent activity and stability of HER under alkaline and acidic conditions,with initial overpotentials of41 m V and 69 m V,respectively.The current density of 10 m A cm^-2corresponded to 96m V and 118 m V,respectively,moreover,its activity is superior to that of commercial platinum carbon at a high current density.（2）Based on the synthesized porous molybdenum carbide microflowers,a microflower-like heterojunction catalyst（Ni-Mo2C/CF）composed of Mo2C nanoparticles,ultrasmall Ni particles,and nitrogen-doped carbon films was prepared in situ through optimizing the addition of organic carbon source and nickel salt.The prepared samples combined geometric structure design with electronic structure regulation.In addition,the synergistic effects between Mo2C and Ni are of great significance for the efficient activity of hydrogen evolution reaction.By regulating the contents of Ni,the performance of Ni-Mo2C/CF was optimized,and the hydrogen evolution current density of 10 m A cm^-2 was achieved in 1.0 M KOH electrolyte with only 81 m V overpotential.Compared with the catalyst prepared in（1）,the performance was further improved.（3）In combination with electronic structure and morphology regulation as well as interface engineering,the heterojunction catalysts with Fe Ni alloy,Fe Co alloy,Fe,Ni,and Co nanoparticles anchored on Mo2C/CF were constructed.Fe Ni-Mo2C/CF showed the best OER performance in the prepared samples.Through a variety of characterization methods,we studied the interaction between Fe and Ni and proved that the Fe Ni OOH which was in-situ generated on the surface of Fe Ni alloy nanoparticle to construct Fe Ni@Fe Ni OOH with core-shell structure was the real active site of oxygen evolution reaction,and the Fe Ni alloy in the core reduced the transmission resistance of electrons from the catalyst surface to the electrode.The ratio of Fe to Ni in the catalyst was optimized,and the current density of 10 m A cm^-2 could be obtained with only 228 m V for 0.7 Fe Ni-Mo2C/CF in 1.0 M KOH solution.The assembled alkaline electrolytic cell with Ni-Mo2C/CF as cathode and Fe Ni-Mo2C/CF as anode could achieve a current density of 10 m A cm^-2 of overall water splitting with a voltage of 1.53V,and showed excellent stability during the process of overall water splitting.（4）An ion-exchange method was used to combine Pt Cl6^2-with organic and inorganic hybrid of Mo-based composite to produce a monophyletic precursor.A heterojunction catalyst composed ofα-Mo C1-x andβ-Mo2C nanoparticles,nitrogen-modified carbon films,and Pt atoms with highly uniform dispersion was prepared by one-step calcination.The ratio ofβ-Mo2C toα-Mo C1-x in the final catalyst Mox C/CF@Pt was regulated by adjusting the content of Pt in the precursor.The ratio ofβ-Mo2C toα-Mo C1-x and the content of Pt was optimized to determine the composition of the catalyst with the best intrinsic activity.The catalyst Mox C/CF@Pt combined the advantages of the multi-pore sizes of Mox C carrier,the ability ofα-Mo C1-x to promote the cleavage of water,the synergistic regulation of hydrogen adsorption byα-Mo C1-x andβ-Mo2C,and the highly uniform dispersion of Pt atoms.When the HER overpotential is 100 m V,the mass current density of Mox C/CF@Pt is8.69 A mg^-1,which is eleven times higher than that of commercial 20%Pt/C(0.78 A mg^-1).