Preparation Of Ni-based Catalysts And Investigation On Electrooxidation Behavior Of Urea

Direct urea fuel cell（DUFC）has the advantages of easy transportation and storage of fuel,and no pollution of products.Increasing the catalyst activity of the urea electrooxidation reaction is essential to improving the performance of the direct urea fuel cell and accelerating its practical process.Recent studies have shown that nickel is an ideal transition metal catalyst with high activity and stability for urea electrooxidation.The investigates in this thesis mainly focuses on two aspects:（1）Ni-based catalysts were prepared on a novel catalyst carrier by different preparation methods,and the dispersion of N i particles on the carrier was improved.（2）Other elements are doped in the nickel-based catalyst,and the synergistic effect of doping element and the N i element is utilized to increase the activity of the catalyst.In this paper,three kinds of different reduction methods were used to prepare supported Ni-based catalysts.The crystal structure,element content and surface chemical state of the catalyst were characterized by XRD,SEM,TEM,XPS.The catalytic performance and reaction principle of supported nickel catalysts were investigated by cylic voltammetry,chronoamperometry and electrochemical impedance spectroscopy.The effects of doping elements on their structure and electrochemical properties were investigated.Ni-M/CB catalyst was prepared by impregnation-freeze-drying-reduction under H₂.The effect of N i-M/CB catalyst on the performance of urea electrooxidation was studied.The results showed that N i-Ce/CB catalyst prepared at 800℃had the highest activity for the electrooxidation of urea,and the oxidation current density was107.28 m A.cm^-2,and the reason was that the doping of Ce decreases the distribution of N i on the catalyst surface,increases the amount of N i²⁺and N i³⁺particles,decreases the grain size of Ni and increases the specific surface area,thus which lead to increasing the rate of urea electrooxidation.Graphite oxide was obtained by improved Hummers method.Then 3DNG aerogels were prepared by hydrothermally self-assembly of ethylenediamine after freeze drying,and electrocatalyst using 3DNG as support was prepared by immersion-freeze-drying-H₂ reduction method.The results showed that Ni-Ce/3DNG catalyst had the highest activity for the electrooxidation of urea,and the oxidation current density was 28.76 m A.cm^-2,and the reason was that the doping Ce increased the distribution of Ni O,Ni（OH）₂ and Ni OOH on the surface of the catalyst and accelerated the rate of Electrooxidation of urea.Ni-based catalysts supported on functional MWCNTs were prepared b y liquid phase reduction of polyols.The results showed that N i-Zn/OH-MWCNTs had the highest activity for urea electrooxidation and the oxidation current density was 96.31m A.cm^-2,and the reason was that the introduced Zn element formed alloys with N i,which reduced the size of N i particles and increased the active sites on the surface of the catalysts.The prepared B-doped amorphous N i-B/OH-MWCNTs catalysts by water phase reduction method also had high activity for urea electrooxidation.The oxidation current density was 109.09 m A.cm^-2,and the reason for the increase of catalytic activity was that the doping of B changed the crystal structure of N i,reduced the distribution of N i on the catalyst surface,and exposed more active sites,thus enhancing the activity of urea electrooxidation.