In-Operando Raman Spectroscopy Study on Oxygen Chemical Potential Gradient in Ni-SDC Cermet Anode for SOFCs

Solid oxide fuel cells are promising energy conversion systems for the next generation. High-temperature operation of this electrochemical device offers high fuel flexibility as well as high energy conversion efficiency. Currently in Japan, methane is utilized as the main fuel source for the residential SOFC cogeneration system and the pre-reformed gas is supplied to the cell chamber. In this case, the anode is requested to be active for the reforming reaction of hydrocarbons as well as the electrochemical oxidation of fuels. The tolerance to carbon deposition is also an important function to provide against accidents of the reforming system because the hydrocarbon gas will be introduced directly to the anode chamber.

The cermet of Ni‒oxide ion conductor is widely used as an anode in SOFCs. However, the usage of nickel induces various degradation phenomena during discharge operation. In most cases, the agglomeration and/or oxidation of nickel catalyst are the major degrading factors under steady operation. For instance, such degradation phenomena can be observed in the downstream part of the fuel gas flow channel accompanied with the microstructural evolution because the anode is exposed to the severe condition due to lean fuel and high partial pressure of steam. The electrochemical oxidation of nickel also sometimes proceeds during discharge under the fuel shortage condition. These will lead to the microstructural change accompanied with the reduction in the length of triple phase boundary (TPB). As mentioned above, furthermore, the carbon deposition leads to the performance degradation due to the obstruction of gas diffusion path and the metal dusting of nickel. A series of degradation phenomena is triggered by the change in the oxygen chemical potential in the anode. In general, the partial pressure of oxygen in the vicinity of anode/electrolyte interface is higher than that in the anode surface region under anodic polarization because the steam is generated via the electrochemical reaction. Therefore, it is of importance to elucidate the oxygen chemical potential gradient in the anode layer under polarization. At this stage, however, appropriate measurement methods have not been developed.

Ceria-based oxides are widely used as oxide components in Ni-based cermet anodes for SOFCs. These oxides show the mixed conduction in reducing atmospheres because the partial reduction of Ce⁴⁺ to Ce³⁺ occurs accompanied with a change in oxygen nonstoichiometry. Mineshige et al. reported that this variation in oxygen vacancy concentration in quenched samples can be detected quantitatively by Raman spectroscopy since the band related to oxygen vacancy changes depending on the partial pressure of oxygen in the atmosphere exposed [1].

In this study, then, Ce_0.8Sm_0.2O_2-δ(SDC) was used as a probe for the detection of oxygen chemical potential. The oxygen chemical potential gradient in Ni−SDC cermet anode at 700ºC under polarization was investigated by applying in-operando Raman spectroscopy. The change in oxygen nonstoichiometry of doped ceria in the cermet anode was successfully quantified. Furthermore, the effective reaction zone was estimated from impedance spectra. 

Acknowledgements

This study was partially supported by Japan Science and Technology Agency (JST), CREST. 

References

1. A. Mineshige et al. Solid State Ionics., 152-153, 493 (2002).