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Yttria‐stabilized‐zirconia samples with fairly narrow pore size distributions give well‐defined microstructure impedance semicircles which can be characterized by a blocking factor αR, a capacitance ratio αC and a frequency ratio αF. On an αR vs. αC diagram, the regimes where either the pores or the grain boundaries are dominant are clearly separated. As expected from the reference model, the αRαF product was found proportional to porosity. The fairly continuous variations of αF from the densification to the grain growth regime revealed that voids remained present along the grain boundaries. Comparison of different results shows a remarkable constancy of the average thickness of the grain‐boundary blockers in the samples sintered at high temperature. The electrical bulk properties obey simple laws as functions of porosity, which allows us to correct the conductivity and dielectric‐constant data obtained with imperfectly densified materials.

Gradual internal reforming is based on local coupling between steam reforming of the fuel which occurs on a catalyst and hydrogen electrochemical oxidation which occurs at the electrode triple-phase perimeter. In order to demonstrate the feasibility of this strategy, the catalytic and electrochemical properties of lanthanum chromite, pure and impregnated with ruthenium, were investigated. Ruthenium supported on lanthanum chromite exhibits very good catalytic activity for the steam reforming of methane. Full conversion of steam is obtained for ratios H20/CH4 even lower than 1 at 700°C. No carbon deposition could be detected after 100 h of operation. Electrochemical measurements, carried out by impedance spectroscopy on cone-shaped microelectrodes of lanthanum chromite, show that the overpotential resistance under H2/H20 is lower than under CO/CO2 and much lower than under CH4/H20. In the presence of ruthenium, impedance diagrams under hydrogen and methane are fairly similar and gas analysis shows that some methane is reformed. This observation demonstrates that gradual internal reforming can be implemented. A detailed analysis of the electrode impedance diagrams shows that the so-called high-frequency semicircle is virtually independent of the nature of the atmosphere. This indicates that it is not directly related to any chemical or electrochemical step of the electrode reaction.

Analysis of the gas, Pt/yttria‐stabilized zirconia electrode shows that electrocatalyzed reactions can occur at both the anode and the cathode. These reactions are promoted by the onset of electronic conductivity in the electrolyte subsurface. Such conductivity can result from either native or added electronic defects, which become active under appropriate experimental conditions pertaining to oxygen partial pressure or electrode polarization. Transition from pure ionic to mixed (ionic + electronic) subsurface conductivity is reflected by a sharp decrease in the corresponding overvoltage and by a drastic modification of the electrode impedance spectra in terms of magnitude and frequency distribution. Inductive loops appear to be characteristic of this transition. The promoting role of electronic defects in the electrode process is clearly demonstrated. At both the cathode and the anode, the contribution of mobile electronic defects also results in a spreading of the reaction zone around the triple contact point.

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New evidence is put forward within the framework of a recently introduced model. The electrodes are viewed as measuring the oxygen chemical potential in a “microsystem” which exchanges oxygen at a finite rate with the surrounding gas being analyzed. In high temperature oxygen gauges, the oxygen flux resulting from the electrochemical oxygen semipermeability of the electrolyte can have two effects other than reducing the theoretical emf by the factor false(1−t¯normalefalse) which appears to induce error of second order only. The oxygen semipermeability can obviously modify the oxygen content of the gas analyzed and also can disturb the equilibrium between the electrode microsystem and the analyzed gas. A special experimental setup was assembled to study both phenomena. The results showed that the second is generally far from being negligible and may lead to noticeable measurement error. A new arrangement with a zirconia point electrode is proposed to eliminate this source of error in oxygen gauges working at very high temperatures. The new arrangement was utilized to measure accurately the oxygen semipermeability of a ZrO2‐Y2O3 [9 mole per cent (m/o)] electrolyte at temperatures up to 1650°C. The semipermeability was found to be proportional to false(P3′1/4−P2′1/4false) , where P3′ and P2′ are the equivalent oxygen pressures in the layers of oxygen adsorbed on both bases of the pellet. The activation energy of the electronic conductivity equaled 2.02 eV.

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