We have prepared gas diffusion electrodes for polymer electrolyte fuel cells (PEFC) using new organic/inorganic hybrid electrolytes. The catalyst layers were prepared by mixing 3-(trihydroxysilyl)-1-propanesulfonic acid [(THS) ProSO3H], 1,8-bis(triethoxysilyl) octane (TES-Oct), Pt loaded carbon black (Pt-CB) and water, followed by a sol-gel reaction. The polarization properties and the microstructure of the catalyst layer were investigated as a function of the composition. The catalyst layer exhibited higher catalyst utilization than that with conventional Nafion® ionomer. The maximum cathode performance was obtained at (THS)Pro-SO3H/CB = 1 (by weight). It was found by a mercury porosimetry that the volume of both primary and secondary pores decreased with increasing the acidic ionomer content. The high catalyst utilization with increasing the acid content is ascribed to an enhanced proton conduction, because the hybrid ionomer could penetrate both in the primary and secondary pores. However, an excess ionomer loading showed a detrimental effect due to disturbance of the gas diffusion. The novel organic/inorganic hybrid materials have proved to be a potential material as the ionomer in the electrodes for high temperature PEFCs.
For the wet cleaning of silicon surfaces, a few new reactants, such as ozone dissolved in UPW, have been proposed to replace the original RCA process using H2O2 solutions. In the present work we describe, for the first time, the mechanism of silicon surface oxidation by dilute solutions of elemental chlorine. Upon reaction with this highly oxidizing agent, the open circuit potential shifted immediately to positive values, the effect being identical for both n- and p-type Si substrates. The surface transformation was firstly investigated by electrochemical impedance spectroscopy which showed successive semicircles representing RC equivalent circuits, revealing a gradual growth of an insulating layer. XPS recordings demonstrated unequivocally the formation of a pure and uniform chemical oxide layer, the possible contamination by Cl element being negligible. The analysis of the charge transfer reaction by voltammetry led to the conclusion that the exchange between the semiconductor and the solution involved positive holes. The reduction current, at a negative bias potential, was extremely small with p-type Si as a consequence of a depletion layer appearance. On the contrary, in n-type substrates, an accumulation region was formed, so that the electric field, as high as 107 V cm-1, will promote a conduction mode through the insulating oxide layer. This novel technique of surface treatment seems promising with respects to the economy and environmental requirements, and also for the possible subsequent growth of multi-layer high-k dielectric structures.
For quantifying the vertical gravity effect on electron-transfer processes in an electrochemical reaction, a theoretical equation was derived, which depicted the reaction in the mixed-rate-controlling state of diffusion and reaction in a gravity field vertical to an electrode surface. Then, the vertical gravity effect on electron-transfer processes in a ferrocyanide-ferricyanaide system up to 650 g was studied. The Tafel lines obtained were compared with the results from the conventional chronopotentiometry in the natural gravity field. Consequently, it was concluded that there is no change in the reaction process in the gravity fields up to 650 g, and the validity of the equation was experimentally certified.
The fluctuations of corrosion potential, or potential noise, appeared during a polymer-coated iron electrode being immersed in a sodium chloride solution. Under this condition, corrosion blisters developed at an interface between a coating layer and an iron substrate. In order to investigate the electrochemical process of the potential noise, the fluctuations of applied current were measured when the electrode potentials were controlled at constant levels using a potentiostat. The current noise appeared at the potential of + 0.6 V vs. SSE. On the electrode used for this measurement, solely anodic blisters were formed. The places on the iron substrate where the anodic blister formed were covered with oxide films, and the traces of pitting were found on the films. The noise was also appeared on a short-circuit current through a pair of half cells that simulated the anodic and cathodic blister parts, respectively. From these results, it is estimated the potential noise was generated by the current noise resulting from a repetition of the breakdown and repassivation of the oxide films under the anodic blisters. The spectrum analysis of the potential noise was attempted. The intensity of the DC component of power spectrum decreased with immersion time. However, the slope of the frequency distribution of the spectrum increased with the time. This shows a possibility that corrosion activity under the coating of a coated iron might be monitored by this method.
Vanadium oxide films were deposited by r.f. magnetron sputtering technique on SUS 304 stainless steel substrates in Ar + O2 atmosphere using V2O5 target. The films obtained were characterized by X-ray diffractometry and scanning electron microscopy. The XRD and SEM observations show that the crystallographic orientation and surface morphology of the vanadium oxide films are changed with film thickness. For the thin film with thickness of 450 nm the V2O5 phase with the ab plane parallel to the substrate is formed, resulting in a highly smooth surface, while for thicker films with thickness of 1.0~4.0μm the V2O5 phase with the ab plane perpendicu1ar to the substrate is formed, giving a considerably rugged surface. The vanadium oxide films undergo a reversible lithium intercalation and deintercalation process, and the thicker film (4.0μm) showed more distinct stepwise discharge profile than the thin film (0.45 μm). The kinetics of intercalation process of lithium into the V2O5 film was studied using an electrochemical transient technique, deducing kinetic parameters such as chemical and lithium component diffusion coefficients and activation energy for lithium diffusion.
Flow cell technologies for electrochemical reactors were applied to the system for carbon dioxide absorption and its electrochemical reduction. Small flow-by type reactors with two chambers separated by a sheet of ion-exchange membrane were employed for both CO2 absorbtion and reduction. Absorbtion of CO2 into phosphate buffer solutions through the ion exchange membrane and succeeding electrochemical reduction of the absorbed CO2 were studied; and the opimum conditions to form formic acid with high current efficiency were discussed. Formic acid is expected to be available to chemiluminescent reagents for oxidant detections and chemical lamps. Almost 100% in current efficiency of CO2 reduction to form formic acid was realized by using In impregnated lead wire cathode. The applied cell voltage was reduced to under 1.5 V due to sulfur dioxide absorption into counter electrode solution.