No abstract

No abstract

No abstract

No abstract

No abstract

Under some conditions, a solid oxide fuel cell (SOFC) stack can fail provided one (or more) cell(s) exhibit higher resistance than the rest of the cells. Such a cell can operate under a negative voltage prior to the onset of degradation. This phenomenon is not limited to SOFC. Recently, a model for SOFC stack degradation when a cell begins to operate under a negative voltage due to 'cell imbalance' was presented (1). Subsequently, experimental evidence for degradation was presented by investigating cell (stack) failure mechanism (2). When operated under a negative voltage of a sufficient magnitude, anode/electrolyte interface delaminated. The model shows that electronic conduction through the electrolyte plays a significant role. In the present work anodesupported cells were made of two different electrolyte compositions: 8YSZ and 92% 8YSZ + 8% CeO 2 (8CYSZ). It was observed that when operated under a negative voltage, the cell with 8YSZ exhibited considerable degradation attributed to electrolyte/anode interface delamination. By contrast, the cell with 8CYSZ exhibited no degradation under similar testing conditions. Results are explained on the basis of electronic conduction and the previously described degradation model.

No abstract

Thermoelectric power measurements have been reported on many oxides over the past several decades. In many of these studies, difficulties were encountered in achieving sample and thus electromotive force ͑emf͒ equilibration. Thermoelectric power in the present work was measured on dense and porous samples of two materials: silver, a noble metal, and Ba 3 Ca 1.18 Nb 1.82 O ͑9−␦͒ or BCN18, a high-temperature proton conductor. The objective of the work on porous BCN18 was to facilitate rapid sample equilibration by substantially increasing diffusion kinetics because BCN18 can exchange oxygen and/or water vapor with the atmosphere. Silver is not expected to exhibit any exchange of matter with atmosphere. Upon change of temperature, the time required for equilibration of emf was about the same for dense and porous silver samples, consistent with expectations. Correspondingly, the measured thermoelectric power was also about the same for both samples. In BCN18, however, kinetics of emf equilibration was much faster in porous samples compared to that in dense samples. Thus, it was not possible to accurately measure thermoelectric power of BCN18 using dense samples, especially at low temperatures ͑below 700°C͒. The vast difference in equilibration kinetics between dense and porous BCN18 samples is attributed to differences in solid-state diffusion distances between porous samples ͑on the order of a few micrometers͒ and dense samples ͑on the order of a millimeter͒. Above ϳ700°C, thermoelectric power measured using dense and porous BCN18 was nearly the same, since experiments could be conducted for a long enough time to ensure equilibration even when using dense samples. Thermoelectric power on porous BCN18 was measured between 500 and 800°C over a range of partial pressures of water vapor between ϳ0.023 and ϳ0.31 atm. Issues relating to kinetics of emf equilibration in dense and porous samples of silver and BCN18 are discussed.