Investigation On Preparation And Electrochemical Performance Of Titanium-supported Ni, Ni-Co And Ni-Sn Electrodes

As we all know that the electrocatalytic oxidation of small organic molecules has been receiving extensive investigation since the end of 20th century, which is of significant application both in fundamental research as model systems and in the fuel cells(FC). For example, study on electrocatalytic oxidation of small organic molecules has played an important role in recognizing the electrochemical phenomena, developing the kinetics of electrode processes, enriching the research content of electrooxidation model, and promoting the development of theory experimental methods concerned in physical chemistry. Electrocatalytic oxidation of small organic molecules on noble metals like Pt has been widely studied for about a half century. Nickel has been confirmed to present effective electrocatalytic activity towards some organic molecules such as methanol, ethanol, glucose and aspirin etc. Development of electrocatalysts with significantly electrocatalytic activities for electro-oxidation of small organic molecules has been considered as the most challenging problem in the study of fule cells(FC).Hydrothermal method, using metal salts, oxides or hydroxides as a solution or suspension, offers an alternative synthesis route for the nano materials in the elevated temperature and pressure. Because the reactions are carried out in the liquid phase, the chemical properties of many reactions will change. As a soft chemistry method, hydrothermal synthesis method has been applied to preparing the novel electrode materials in this paper. The properties of electrocatalysts made by hydrothermal synthesis are different from that by the traditional solid-state method. Nowadays, most of catalysts are loaded on carbon with electrodeposition techniques. In this thesis, we reported the preparation of nano-sized metal particales deposited on the surface of titanium by the hydrothermal process using hydrazine hydrate as the reduction agent, so as to effectively develop novel electrocatalysts with high electroactivity for the oxidation of small organic molecules. Novel titanium-supported nickel Ni/Ti, binary Ni-Co/Ti and Ni-Sn/Ti electrodes were synthesized by the hydrothermal method. Electro-catalytic activities of the prepared electrodes for methanol oxidation are investigated by conventional electrochemical techniques like voltammetric responses, chronoamperometric measurements and electrochemical impedance spectra, etc. In addition, the Ni-Co/Ti electrode was examined as an electrocatalyst for the electrooxidation of glucose in alkaline solutions. The main contents and conclusions in the thesis are as follows:1. Application of nickel hydroxide electrodes and electrocatalytic oxidation of small organic molecules are reviewed briefly. The electro-catalytic mechanism of the oxidation of methanol, ethanol and glucose is elaborated according to corresponding reports of literatures. The research progress of nickel hydroxide electrodes are introduced. Application of nickel hydroxide electrodes to organic electrosynthesis and other fields are summarized. The thesis points out that the main problem in the research of catalysts is the development of nano-catalysts with high electroactivity.2. Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes are synthesized by the hydrothermal process using hydrazine hydrate as the reduction agent. The electrocatalyst particles are directly deposited on the Ti surface under conditions of reaction temperature 120℃and reaction time 10 h.3. Scanning electron microscopy (SEM) and energy disperse spectroscopy (EDS) are employed to investigate the morphology and element compositions of Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes obtained by the hydrothermal process. The main results are as follows:(1) For the Ni/Ti electrode, SEM and EDS images show that the surface of Ti substrate is partly covered by nickel particles which were present as small balls with the almost uniform size of around 500nm. An interesting finding is that some nickel particles are connected with each other to form alveolate and cateniform structures. Ti substrate surface was not completely covered by nickel particles.(2) For the Ni-Co/Ti electrodes, SEM and EDS images show that the surface of Ti substrate is almost totally covered by nickel-cobalt catalyst particles. The samples exhibit similar SEM image to Ni/Ti electrode with the almost uniform size of around 500nm. But the bimetallic Ni-Co particles are tightly combined with each other and form three-dimensional porous network structures, resulting in a significant surface area.(3) For the Ni-Sn/Ti electrodes, SEM and EDS images show that the surface of Ti substrate is totally covered by nano-scale particles with the sizes of ca. 120～130nm. The particles are present as nano-scale flakes which thicknesses are around 40 nm. These flakes look like petals of flowers, resulting in a significant surface area and providing considerable numbers of active sites which are necessary for the adsorption and electro-oxidation of methanol on the catalyst.4. Electro-catalytic oxidation of methanol in sodium hydroxide solution on Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes, and electro-oxidation of glucose on Ni-Co/Ti electrode have been studied using cyclic voltammetry, pseudo-steady state polarization, chronoamperometry, and electrochemical impedance spectroscopy(EIS). The results are as follows:(1) It was shown from cyclic voltammograms in alkaline solutions that the oxidation current of methanol on the Ni/Ti electrode was much higher than that on a polycrystalline nickel electrode (Ni), and that the onset potential of methanol oxidation on Ni/Ti shifts to 0.36 V, which is less than that on the Ni, and that the oxidation peak current on the Ni/Ti was 5 times higher than on Ni. It was further observed from chronoamperometric measurements that the steady-state current (I_ss) on the Ni/Ti was also significantly higher than on Ni, and the I_ss is well linearly proportional to the methanol concentration. Electrochemical impedance spectra on the Ni/Ti reveal that the presence of methanol in 1.0mol/L NaOH enhances the charge transfer process of the oxidation of Ni(OH)₂ to NiOOH. In the activation range of methanol oxidation, the charge transfer resistance decreases with the increase of anodic potentials and methanol concentrations. This novel nickel electrode can be used repeatedly and exhibits stable electro-catalytic activity for the methanol oxidation.(2) Electrochemical measurements show that Co/Ti electrode doesn't present catalytic activity for methanol oxidation. However, the oxidation current of methanol on the Ni₈Co₂/Ti electrode was much higher than that on Ni/Ti electrode, and the onset potential on Ni₈Co₂/Ti shifts about 0.25V toward the negative potential in comparison with that on the Ni/Ti, The oxidation peak current on the Ni₈Co₂/Ti was 2.3 times higher than on Ni/Ti electrode. Addition of cobalt to the nickel enhances the methanol oxidation due to the formation of conducting oxide in low potential, especially to the co-precipitation of cobalt and nickel hydroxide which enhances the methanol electro-oxidation.(3) The Ni₈Co₂/Ti electrode was examined as an electrocatalyst for the electrooxidation of glucose in alkaline solutions. The Ni₈Co₂/Ti electrode exhibits significantly high current of glucose oxidation, and that oxidation reaction of glucose is controlled by the diffusing step. A high catalytic rate constant of 3.57×10⁴ cm³Â·mol^-1·s^-1 and the diffusion coefficient 6.50×10^-6 cm²Â·s^-1 of glucose was calculated from amperometric responses on the Ni₈Co₂/Ti electrode. Furthermore, amperometric datas show a linear dependence of the current density for glucose oxidation upon glucose concentration in the range of 0.05～0.5mmol/L with a sensitivity of 6.85×10^-4A·(mmol/L)^-1. A detection limit of 0.0012 mmol/L(1.2 μmol/L) glucose was found. Results show that the prepared Ni₈Co₂/Ti electrode is a promising biosensor for the construction of enzyme-free glucose detection.(4) Electrochemical measurements show that Ni-Sn/Ti electrode presents much higher anodic currents and lower onset potential for methanol oxidation than Ni/Ti electrode. The EIS datas indicate that under conditions of various anodic potentials and methanol concentrations, Ni-Sn/Ti displays significantly lower charge transfer resistances. Results show that the electrode of Ni:Sn=8:1 exhibits high electrocatalytic activity towards methanol oxidation.According to our review of literatures involved in the electrooxidation of some small organic molecules, it can be concluded that the electro-catalytic oxidation of methanol on Ni/Ti, Ni-Co/Ti and Ni-Sn/Ti electrodes, and electrooxidation of glucose on the Ni-Co/Ti electrode in sodsium hydroxide solution have not been reported. Our study will be of significance in the development of liquid fuel cells and in the electrochemical study on the electrooxidation of small organic molecules.