Preparation,characterization And Multi-functional Catalytic Performance Of Self-supported Transition Metal Boride Electrodes

With the rapid development of economy,the massive use of fossil fuels has caused energy crisis and environmental pollution,which threatens the survival and sustainable development of human beings.The development of clean,efficient and economic energy conversion or storage carriers is very important for the utilization of clean energy.The production of hydrogen from electrolyzing water or hydrolysis has attracted the attention of many researchers.Although precious metal-based catalysts have shown excellent catalytic performance,their high price and very low natural reserves have greatly restricted the wide application of noble metals.Therefore,it is urgent to develop non-noble metal catalysts with high efficiency,abundance and low cost.At the same time,compared with powder catalyst,the self-supported metal foam catalyst does not need to use polymer binder.Its porous structure can greatly enhance the specific surface area and promote the mass transfer and/or charge transfer.In view of this,this dissertataion mainly studies the construction,characterization and catalytic performance and mechanism of self-supported transition metal boride catalytic materials.(1)One-step high-temperature solid-state boronizing method was adopted by fully contacting the metal foam skeleton of Fe Ni with amorphous boron powder.Through controlling reaction temperatures(600,700,800,900?),a series of composite boride phases with different surface morphologies and different crystallinity were obtained.X-ray diffraction(XRD),scanning electron microscopy(SEM),transmission electron microscopy(TEM)and electrochemical measurements show that the porous Fe Ni foam skeleton was covered by loose and porous structures consisting of numerous Ni-B particles doped with or without Fe at 700?.Self-supported Fe Ni@Fe Ni B-700 electrode only requires an overpotential of 272 m V at the current density of 10 m A cm^-2.The density functional theory calculation(DFT)was used to simulate the intermediate products of oxygen evolution.It can be seen that the Gibbs free energy of*OOH on Ni site is significantly decreased after Fe doping,which is conducive to oxygen evolution reaction.(2)In order to further optimize the surface morphology,Co Ni foam and boron powder were reacted at high temperatures(600,700,800,900,1000?).XRD,SEM,TEM and electrochemical tests exhibit that high density and ultra-thin Co-Ni-B nanostructured films with Co B,Co₂B,Co₃B and Ni doped Co-B are grown on the surface of Co Ni foam at 700?.Meanwhile,it comprises both crystal and amorphous heterogeneous structures.This special morphology indicates that the solid-phase boronization method can prepare regular nanostructures.At the current density of 10 m A cm^-2,only 262 m V overpotential is needed for O₂ evolution,and it shows good stability for 12 hours.At the same time,the Gibbs free energy on metal sites of Co B,Co₂B,Co₃B and Ni doped Co₂B was calculated by density functional theory.The results show that the Ni doped system can reduce the energy barrier of borides,and has more active sites on Co₂B.Finally,the optimal Ni-P||Co Ni B-700 two electrode system is conducted to water electrolyzing,with a cell voltage of 1.64 V at 10 m A cm^-2.(3)As the structure of boride layer prepared by solid-state boronizing method has the problem of falling off,the precursor of Co₃O₄ nanowires,which has strong combination with Ni foam substrate,was prepared by hydrothermal method and afterwards annealing treatment,and then the precursor was processed by the alternative drop coating method of metal salt and Na BH₄reductant.Through XRD and XPS analysis,a large number of oxygen vacancies and B defects are detected in Co₃O₄ lattice.The generation of O vacancy can greatly promote the charge transfer rate,increase the electrochemical specific surface area and number of active sites,and greatly improve the performance of hydrogen evolution and oxygen evolution.Under the current density of 50 m A cm^-2,the overpotentials of V_OB-Co₃O₄/NF are 184 and 315 m V for HER and OER respectively.V_OB-Co₃O₄/NF as a bifunctional catalyst for electrolytic water can reach to 1.67 V_{@10 m A cm-2} voltage,and maintain 20 h at 10 m A cm^-2.Moreover,the strong negatively charged B-O defects can greatly enhance the H₂-evolve performance by hydrolysis of alkaline Na BH₄.The hydrogen production rate is 7055 ml min^-1 g^-1,and the activation energy is 29.7 k J mol^-1.(4)In order to improve the operability of the preparation process,we first developed the electrodeposition process of transition metal boride applied to the energy conversion field.Using DMAB as the boron source and adjusting the content of the second metal cation,the amorphous nanosheet films of Co Ni_0.09BO and Co Ni_0.27BO show excellent hydrogen evolution and oxygen evolution performance,respectively.The B signals before and after HER and OER durability test revealed by XPS is very strong,implying that the introduction of B element improved the catalytic performance.With the increase of ambient temperature or KOH concentration,the total electrolyzing water performance of Co Ni_0.09BO||Co Ni_0.27BO would be greatly enhanced.Under ambient temperature of 303 K and high concentration of 2 M KOH solution,the overall water splitting performance is the best,and the required voltage at 50m A cm^-2 is merely 1.64 V.(5)For further guiding the preparation and synthesis of transition metal boride catalysts for hydrogen evolution,the H*Gibbs free energies of active sites on M₃B,M₂B,MB and MB₂ borides were calculated based on the content of B element.The results show that M₂B(Ni₂B?Fe₂B?Mo₂B),M₃B(Co₃B?Ni₃B),MB(Co B,Fe B,Mo B,Ni B)and MB₂(VB₂,Fe B₂,Mo B₂,WB₂)have the?G_H*in the range of-0.3 to 0.3 e V,exhibiting approximate or evev higher catalytic activity relative to Pt(?-0.1 e V).