Shigenori Onuma

Mass transport properties of Ce0.9Gd0.1O2−δ at the surface and in the bulk

Abstract The isothermal thermogravimetry was performed on the system of Ce 0.9 Gd 0.1 O 2− δ (CGO10) to determine the oxygen nonstoichiometry, δ , in the temperature range between 1073 and 1173 K and the oxygen partial pressure range of 10 −22 –1 bar. In addition, chemical diffusion coefficients, D chem , and surface reaction rate constants, k , were obtained in CO–CO 2 and H 2 –H 2 O atmospheres through the analysis of a weight relaxation process. The values of obtained chemical diffusion coefficients were consistent with those estimated from partial conductivity and oxygen nonstoichiometry through the P O 2 regions from predominant ionic conduction to predominant electronic conduction. Surface reaction rate constants are not directly related with oxygen partial pressure but depend on the gas species and the composition. The isotope exchange depth profiling with secondary ion mass spectrometry (SIMS) was done to examine the relationship between k and surface exchange coefficients, k s . It was shown that the hydrogen partial pressure dependence of k s converted from k was the same, although the absolute value was about 1 over 30 of those from SIMS analysis.

Electrochemical Oxidation in a CH 4 ‐ H 2 O System at the Interface of a Pt Electrode and Y 2 O 3‐Stabilized ZrO2 Electrolyte I. Determination of the Predominant Reaction Process

In order to elucidate the electrochemical reaction in a CH 4 -H 2 O system at the interface of a porous Pt electrode/Y 2 O 3 -stabilized ZrO 2 electrolyte, measurements were made on the gas composition before and after the reaction under directcurrent polarization and on electrochemical impedance spectra at 1073 K. The dominant electrochemical reaction was found to be the redox process of H 2 /H 2 O where H 2 and CO were generated by the steam reforming in a CH 4 -H 2 O system. The polarization curves showed good agreement with the model reported by Mizusaki et al. for a H 2 -H 2 O system.

Electrochemical Oxidation in a CH 4 ‐ H 2 O System at the Interface of a Pt Electrode and Y 2 O 3‐Stabilized ZrO2 Electrolyte II. The Rates of Electrochemical Reactions Taking Place in Parallel

The rate processes of electrochemical reactions were clarified in a CH 4 -H 2 O system at the interface of a porous Pt electrode/Y 2 O 3 -stabilized ZrO 2 electrolyte. Direct-current polarization measurements and ac impedance spectroscopy were made with gas analysis before and after the reaction at 1073 K. We proposed an analytical method to determine the rates of electrochemical reactions taking place in parallel. When the ratio p(H 2 O)/p(CH 4 ) of the inlet gas was close to zero, the observed relationship between the polarization current and electrode potential was interpreted by the electrochemical oxidation processes of H 2 , CO, C, and CH 4 in parallel using the proposed method. For example, the ratio of the oxidation rates for C/CO/CH 4 /H 2 is 1/1.3 x 10/1.9 x 10 2 /2.8 x 10 3 at E = -600 mV vs. air. This result was obtained under very low CH 4 concentration. The estimated oxidation rates of H 2 and CO as functions of the electrode potential were described by the model proposed by Mizusaki et al. for the reaction of H 2 -H 2 O and CO-CO 2 .