Preparation And Electrocatalytic Performance Of Nickel-based/Carbon Composites

Fuel cell is an energy conversion device that can convert chemical energy into electric energy.Due to its high efficiency and low pollution emissions,fuel cell can effectively alleviate the energy crisis and environmental pollution caused by traditional fossil fuels.Direct methanol fuel cells(DMFC)and direct urea fuel cells(DUFC)have attracted extensive attention because of their abundant fuel sources,low price and convenient transportation and storage.The anodic reactions of these two fuel cells are a 6-electron transfer process,and the reaction kinetics is relatively slow,so the anodic catalysts are needed to accelerate the reaction rate.At present,noble metal-based catalysts are commonly used as anode catalysts.Due to scarcity and high cost,their commercial applications in DMFC and DUFC are hindered.Therefore,it is necessary to develop cheap,highly active and stable non-noble metal-based catalysts.Among them,nickel-based catalysts have high activity for methanol oxidation reaction(MOR)and urea oxidation reaction(UOR).However,due to the poor conductivity and low specific surface area of nickel-based catalysts,they are usually loaded on the substrate with high specific surface area and good conductivity to prepare composite materials.In this thesis,nickel chloride or nickel hydroxide were loaded on polyaniline-polyvinyl alcohol(PPH)or poly 3,4-ethylenedioxythiophene-polyvinyl alcohol(PPSH)conductive polymer hydrogels(containing N or S atoms)through impregnation method or hydrothermal method to prepare NiCl₂@PPH,Ni(OH)₂@PPH and NiCl₂@PPSH precursors,which were pyrolyzed at different temperatures under nitrogen atmosphere to obtain Ni/N-C,Ni/NiO-N-C and Ni/S-C composites,namely,the nickel nanocrystalline or nickel/nickel oxide nanocrystals uniformly distributed in the N or S doped carbon matrixes.The above composites were characterized by TGA,XRD,SEM,TEM,Raman and XPS.Meanwhile,the catalytic performances of the composites as electrocatalysts for MOR and UOR were investigated by a series of electrochemical tests,including cyclic voltammetry(CV),chronoamperometric(CA)and electrochemical impedance spectroscopy(EIS).The specific contents and results are as follows:(1)PPH was immersed in different concentrations of nickel chloride aqueous solutions for several hours,and then freeze-dried to obtain NiCl₂@PPH,which was pyrolyzed at different temperatures under nitrogen atmosphere to obtain Ni/N-C composites,that is,the nickel nanocrystalline embedded in nitrogen doped carbon matrixes.The effects of different preparation conditions on the electrochemical performance of the catalysts for the oxidation of methanol and urea were investigated,such as the concentration of nickel chloride,soaking time and pyrolysis temperature.The structure and morphology of the as-prepared catalysts were characterized,and the relationship between the structure and properties of the catalyst was analyzed.When the NiCl₂ concentration,soaking time and pyrolysis temperature were 5 M,18h and 500?respectively,the as-synthesized Ni/N-C@500 catalyst has the highest catalytic activity.The oxidation current densities are 146.7 m A·cm^-2 for methanol and 192.7 m A·cm^-2for urea at the voltage of 0.6 V(vs.SCE).After 500 and 1000 CV tests for MOR and UOR,the retention rates of current density were 87.1%and 79%respectively.In addition,in the CO poisoning experiment of Ni/N-C@500 catalyst,its retention rate of the current density for MOR is 85%of the initial value,which proves that Ni/N-C@500 catalyst had excellent catalytic oxidation activity for methanol and urea,stability and good resistance to CO poisoning,which is mainly due to the fact that the N-doped carbon matrix enhances the electrical conductivity of the composites and also protects the Ni nanocrystals active component.(2)PPH was immersed into Teflon-lined stainless-steel autoclave containing 1 m M nickel chloride and 6 m M urea,sealed and heated in an oven under at certain temperature and time.After naturally cooled to ambient temperature,the as-prepared Ni(OH)₂@PPH was freeze-dried,pyrolyzed at different temperatures under nitrogen atmosphere to obtain Ni/NiO-N-C composites,namely nickel/nickel oxide nanocrystals embedded in nitrogen doped carbon matrix.The effects of different preparation conditions on the electrochemical performance of the catalysts for oxidation of methanol and urea were systematically investigated,such as pyrolysis temperature.The structure and morphology of the as-prepared catalysts were characterized,and the relationship between the structure and properties of the catalysts was analyzed.Under the optimal conditions,the resulting Ni/NiO-N-C-500 catalysts had the highest catalytic activity.At 0.8 V(vs.SCE)voltage,the oxidation current densities are 178.1 m A·cm^-2 for methanol and 301 m A·cm^-2 for urea,respectively.After 12 h i-t test,the retention rates of current density were 74%and 76%,respectively.Before and after 1000 CV cycles,the contact resistances were increased from 1.51?cm² and 1.35?cm² to 1.69?cm² and 1.39?cm²,respectively.The above results indicated that the Ni/NiO-N-C-500 catalyst had excellent catalytic activity and stability for methanol and urea,which is mainly due to the synergistic effect between active substances Ni and NiO.Furthermore,the N-doped matrixes not only regulate the electronic structure of the catalyst,but also enhance the electron transfer.(3)PPSH was immersed in different concentrations of nickel chloride solution for several hours,and then freeze-dried to obtain NiCl₂@PPSH,which was pyrolyzed at different temperatures under nitrogen atmosphere to prepare the Ni/S-C composites,that is,the nickel nanocrystalline of catalyst embedded in sulfur doped carbon matrixes.The effects of different preparation conditions on the electrochemical performance of the catalysts for oxidation of methanol and urea were systematically investigated such as the concentration of nickel chloride,soaking time and pyrolysis temperature.The structure and morphology of the as-prepared catalysts were characterized,and the relationship between the structure and properties of the catalyst was analyzed.PPSH was immersed in 3 M NiCl₂ solution for 18 h to obtain NiCl₂@PPSH,which was pyrolyzed at 500?under nitrogen atmosphere to get the optimal Ni/S-C composite.At the voltage of 0.8V(vs.SCE),its oxidation current densities are 213.4m A·cm^-2 for methanol and 326 m A·cm^-2 for urea.After 12 h i-t test,the retention rates of current density were 67%for methanol and 80%for urea,respectively.The results shows that the Ni/S-C catalyst had excellent catalytic activity and stability for the oxidation of methanol and urea,which are mainly attributed to that S doped carbon matrixes fix the Ni(III)active component based on the XPS analysis,the porous layer structure of composites promotes the mass transfer,and the good electrical conductivity of Ni₃S₂ enhances charge transfer in the process of catalytic reaction.