Investigation On Urea Electro-oxidation Reaction Catalyzed By Nickel-based Materials And The Performance Of Direct Urea-hydrogen Peroxide Fuel Cell

Fuel cell is a kind of power generation technology that meets both high efficiency and cleanness owing to the fact that fuel cell is not limited by Carnot cycle.Among those fuel cells,the direct urea fuel cell is a novel one that adopts industrial urea,human or animal urine and urea-containing sewage directly as the fuel.Via such a fuel cell device,the dual purposes of electricity generation and sewage purification can be achieved.To date,the electric-output performance of direct urea fuel cell is still significantly lower than other types of fuel cells,which is attributed to the higher onset oxidation potential,less stability,etc.during urea electro-oxidation reaction.In order to promote the performance of direct urea fuel cell,several aspects such as electro-catalyst designing and preparation,kinetics of electrode reaction,catalytic reaction mechanism of urea electro-oxidation are systematically studied in this dissertation.Three-dimensional nickel or nickel-cobalt nanowire arrays electrode and three-dimensional nickel foam coated porous nickel or nickel-cobalt electrode are prepared by track-etched polycarbonate template and dynamic hydrogen bubble template,resepctively.The catalytic performance of both electrodes on urea electro-oxidation is investigated and the reaction mechanism of urea electro-oxidation is explored.In the meantime,a novel direct urea-hydrogen peroxide fuel cell is assembled using hydrogen peroxide as the oxidant and the cell performance is investigated afterwards.The main contents are as follows:Three-dimensional nickel nanowire arrays electrode is prepared by galvanostatically electrodepositing nickel within the pores and over-plating on the surface of polycarbonate(PC)template.Low-melting point alloy is exploited as the conductive layer of PC template.According to the scanning electron microscope photographs,the as-prepared nickel electrode contains two parts:nickel support and nickel nanowire arrays.The thickness of nickel support is about 130 ?m.The length and diameter of nickel nanowire is?6 ?m and?70 nm,respectively.The integrated structure avoids the uses of carbon-material support and organic binder,and thus allows its long-term use in harsh alkaline medium.Furthermore,the open three-dimensional nanowire arrays structure allows rapid diffusion of reactants and products.The true surface areas and active areas of nickel nanowire arrays electrode and nickel plate electrode obtained from two electrochemical methods are respectively compared.Then urea and urea peroxide electro-oxidation on nickel nanowire arrays electrode and nickel plate electrode are investigated,respectively.When compared with nickel plate electrode,nickel nanowire arrays electrode shows superior catalytic performance.The apparent catalytic current density of urea and urea peroxide electro-oxidation on nickel nanowire arrays electrode is 4.89 and 4.38 times as large as that of nickel plate electrode,respectively.The onset oxidation potential of urea electro-oxidation on nickel nanowire arrays electrode reaches up to 0.25 V vs.Ag/AgCl,exactly the same as the onset oxidation of Ni(OH)2/NiOOH conversion reaction.Urea electro-oxidation on nickel nanowire arrays electrode is processed in a way of E-C coupling mechanism.Due to the structural difference between nickel nanowire arrays electrode and nickel plate electrode,the former has slightly lower onset oxidation potential and drastically higher oxidation current density.Three-dimensional nickel-cobalt nanowire arrays electrode with different contents of cobalt is prepared by varying the molar ratio of nickel sulfate and cobalt sulfate in the nickel plating bath.When the molar ratio of nickel sulfate to cobalt sulfate is 10:0,9:1,7:3 and 5:5,the corresponding Ni 10,Ni 9,Ni 7 and Ni 5 electrodes are tested in order to acquire the electro-catalytic performance in the single-electrode system and the electric-output performance in the whole-cell system,respectively.The optimal nickel-cobalt nanowire arrays electrode is obtained by comparing the onset oxidation potential and oxidation current density of four electrodes in the single-electrode system,as well as the open circuit voltage,discharge current density and power density of four electrodes in the whole-cell system.It is found that when adopting Ni 9 electrode,the highest oxidation current density of urea electro-oxidation is achieved,that is,380 mA cm-2,meanwhile,the onset oxidation potential gives a relatively lower value,i.e.,0.19 V.Then the anolyte proportion is optimized when the best nickel-cobalt nanowire arrays electrode is used as the anode of DUPFC.The stability of DUPFC is also evaluated under the optimized anolyte proportion.It is found that when the molar ratio of nickel sulfate to cobalt sulfate is 9:1,the nickel-cobalt nanowire arrays electrode has the most excellent electro-catalytic and electric-output performance.DUPFC possesses an open circuit voltage of 0.92 V,a discharge current density of 50 mA cm-2 and a peak power density of 7.4 mW cm-2,when the anolyte contains 9.0 mol L-1 KOH and 0.33 mol L-1 CO(NH2)2.A novel three-dimensional porous nickel-cobalt film on nickel foam(Ni-Co@Ni foam)is successfully prepared using hydrogen bubbles as a dynamic template.According to the materials characterization and cyclic voltammetric results,When the molar ratio of nickel chloride to cobalt chloride in the electrolyte solution is 10:0,8:2,5:5,2:8 and 0:10,the corresponding Ni 10,Ni 8,Ni 5,Ni 2 and Ni 0 electrodes own diverse morphology and structure,different Co molar percents in the Ni-Co deposits.As a result,the true surface areas of five electrodes and onset oxidation potentials of Ni(OH)2/NiOOH conversion reaction are different as well.Results show that the cobalt molar percentages of Ni 10,Ni 8,Ni 5,Ni 2 and Ni 0 electrodes are 0%,19.85%,49.17%,79.02%and 100%,respectively,as well as the true active surface areas are 71.50,234.50,461.50,621.67 and 44.50 cm2,respectively.Then the electric-output performance of DUPFC is also studied when adopting different Ni-Co/Ni foam anodes,different proportions of anolyte and cell flow rates.When the anode is Ni 2 electrode,the anolyte composition is 7.0 mol L'1 KOH and 0.5 mol L-1 CO(NH2)2 and the cell flow rate is 15 mL min-1,DUPFC gives an open circuit voltage of 0.83 V,a discharge current density of 90 mA cm-2 and peak power density of 17.4 mW cm-2.Furthermore,the electric-output performance of direct urine-hydrogen peroxide fuel cell employing urine as the fuel is investigated.Such a direct urine fuel cell has an open circuit voltage of 0.83 V and a peak power density of 7.5 mW cm-2,indicating urine is also a kind of promising fuel and can be turned from a waste to a treasure through fuel cell devices.The urea electro-oxidation reaction mechanism is investigated by AC impedance method.The AC impedance spectra show various characteristics when changing the polarization potentials.The indirect E-C process and direct electro-oxidation process in urea electro-oxidation are concluded by judging the number of Nyquist semi-circles and the value of characteristic frequency.Afterwards,a reaction model of urea electro-oxidation is established accompanying by the AC impedance,cyclic voltammetric and chronoamperometric results.The findings are as follows:when the Nyquist semi-circle locates in the first quadrant,the rate determining step is urea electro-oxidation reaction;when the Nyquist semi-circle locates in the second quadrant,the CO2 desorption reaction is the rate determining step;when the inductance appears in the Nyquist semi-circle,the rate of urea electro-oxidation reaction is comparable to that of CO2 desorption reaction.The product from urea electro-oxidation,CO2,would adsorb on the electrode surface and hence,the electrode shows poor stability.What's more,the effect of KOH concentration on the AC impedance spectra is also investigated,giving a clue that KOH concentration also affects the rate determining step of urea electro-oxidation reaction.