Preparation And Electrochemical Characteristics Of Titanium-supported NanoAg, Ag-Ni And Ag-Cu Electrodes

Hydrazine is an important chemical material and is widely used as a raw material in the manufacture of agricultural chemicals.Hydrazine is an ideal fuel for a direct fuel cell system because it does not exhaust environmentally loading materials such as CO₂ and the thermodynamic reversible potential for the direct hydrazine fuel cell(DHFC) is 1.56V.In addition,studies have shown that the hydrazine electrooxidation process did not suffer from any poisoning effect.Hydrazine is very important in pharmacology because it is recognized as a carcinogen and neurotoxin,which make against liver and brain.Electrochemical techniques offer the opportunity for portable,cheap and rapid methodologies of the hydrazine detection. Thus,development of electrocatalysts with significantly electrocatalytic activities for electrooxidation of hydrazine is considered as the most challenging problem.Sodium borohydride is a stable,nonflammable,easy-to-handle and non-toxic chemical material.Sodium borohydride is also an ideal fuel for direct borohydride fuel cell.The advantages of the DBFC include very high theoretical capacity(5.7Ahg^-1),very negative equilibrium potential and faster anodic kinetics at many metallic surfaces.It has been receiving extensive investigation.But,direct use of borohydride as an anodic fuel has a few problems that need to be overcome.The major problem is the spontaneous hydrolysis of BH₄^-ions during the discharging on a variety of electrocatalysts,causing decreasing coulombic efficiency and poor utilization of BH₄^-.It is,therefore,of primary importance to develop an anodic catalyst,which can only catalyze the electrochemical oxidation but not the hydrolysis of borohydride.In this thesis,we report the preparation of nano-sized metal particales deposited on the surface of titanium by the hydrothermal process using hydrazine and polyethylene glycol as the reduction agent,the novel titanium-supported silver Ag/Ti,binary AgNi/Ti and AgCu/Ti electrodes are synthesized by the hydrothermal method.Electro-catalytic activities of the prepared electrodes for hydrazine oxidation are investigated by conventional electrochemical techniques like voltammetric responses,chronoamperometric measurements,etc.The novel titanium-supported binary AgCu/Ti electrodes are fabricated by a electrochemical deposition technique using AgNO₃ and Cu(NO₃)₂ solutions as raw materials. Results show that dendritic metal particales are deposited on the surface of titanium.It can be used for amperometric detection of hydrazine in lower concentration range.The main contents and conclusions in the thesis are as follows:1.Application of silver electrodes and electrocatalytic oxidation of hydrazine and sodium borohydride are reviewed briefly.The electro-catalytic mechanisms of the oxidation of hydrazine and sodium borohydride are elaborated according to corresponding reports of literatures.The thesis points out that development of nano-sized catalysts would be the investigation trend of anodic materials used in fuel cells.2.Ag/Ti,AgNi/Ti and AgCu/Ti electrodes are synthesized by the hydrothermal process using hydrazine hydrate and PEG as the reduction agent.Dendritic metal particales are deposited on the surface of titanium by electrochemical deposition technique.The main features of the process are:nano-metal catalysts particles are directly deposited on the surface of titanium substrate,and the preparation of the electrodes is simple and have high stability.3.Scanning electron microscopy(SEM),energy disperse spectroscopy(EDS),are employed to investigate the morphology and element compositions of Ag/Ti,AgNi/Ti and AgCu/Ti.The main results are as follows:(1) For the Ag/Ti electrode,SEM and EDS images show that the surface of Ti substrate is partly covered by silver particles which were present as small balls with the almost uniform size of around 200-300nm.These particles connect with each other to form alveolate structures.(2) For the AgNi/Ti electrodes,SEM and EDS images show a good coverage of catalyst particles on the surface of Ti substrate.The particle sizes are 100～250mm.These nano-scale particles are connected with each other to form a three-dimensional texture.This porous structure provides stable immobilization of the AgNi particles on the Ti surface.(3) For the AgCu/Ti electrodes,SEM and EDS images show that the surface of Ti substrate is partially covered by particles with the sizes of 500～800nm.(4) For the electrodeposition fabrication of AgCu/Ti electrode,SEM and EDS images show that the surface of Ti substrate is totally covered by metal particales.These particles form a dendritic structure and the length of the branches are 100～110nm.These particles form structure-tipped stamens at the top of branches.4.Electro-catalytic oxidation of hydraine in sodium hydroxide solution on Ag/Ti, AgNi/Ti and AgCu/Ti electrodes,electro-oxidation of sodium borohydride on Ag/Ti electrode and amperometric detection of hydrazine in lower concentration range on electrodeposited AgCu/Ti electrodes are studied using cyclic voltammetry and chronoamperometry.The results are as follows:(1)It is show from cyclic voltammograms in alkaline solutions that the oxidation current of hydrazine and sodium borohydride on the Ag/Ti electrode is much higher than that on a polycrystalline silver electrode,and that the onset potentials of hydrazine and sodium borohydride oxidation on Ag/Ti shift to -0.60V and -0.70V respectively,which are less than those on the polycrystalline silver electrode.It is also observed that the oxidation peak current densities of hydrazine and sodium borohydride on the Ag/Ti are 212.8 mA/cm^-2 and 230.4mA/cm^-2 respectively,which are 8 and 12 times higher than those on polycrystalline silver electrode.It is further observed from chronoamperometric measurements that the steady-state current(I_ss) on the Ag/Ti is also significantly higher than on the polycrystalline silver electrode,and the I_ss is well linearly proportional to hydrazine and sodium borohydride concentration.(2) The electroactivity of the prepared titanium-supported nano-scale Ag/Ti and AgNi/Ti towards the hydrazine oxidation in alkaline solutions is evaluated by cyclic voltammetry and chronoamperometry.Results show that the electrodes present a low onset potential of ca. -0.6V and considerably high and stable anodic current density for the hydrazine oxidation. Among them,nanoAg₈₆Ni₁₄/Ti electrode exhibits highest anodic current density towards the hydrazine oxidation,showing an increment of electro-active sites on the nanoAg₈₆Ni₁₄/Ti due to the addition of Ni to the Ag particles.(3) The AgCu/Ti electrodes are examined as an electrocatalyst for the electro- oxidation of hydrazine in alkaline solutions.The Ag₇₉Cu₂₁/Ti electrode exhibits signi-ficantly high current density of hydrazine oxidation and low onset potential of ca.-0.70V.(4) AgCu/Ti electrodes are fabricated by a electrochemical deposition technique. Ultra-low concentration hydrazine is detected on the AgCu/Ti electrodes.Results show that anodic peak current density at -0.10V regularly declines with the concentration of hydrazine, and the plot of the reciprocal of the anodic peak current density I^-1 vs hydrazine concentration showes a good linear relationship.These electrodes can be used for the detection of ultralow concentration hydrazine.