Impedance Spectroscopy Studies of the Behavior of MoNi-CeO2 Anode in SOFC Using H2S-Containing Hydrogen as Fuel

Development of anode materials that can be operated on sulphur-containing fuels is recognized as an important challenge for SOFC development. Doped or undoped ceria oxides have been investigated in metal cermet anodes due to good performance, low material cost and more resistance to sulphur poisoning. Ni-CeO₂/YSZ anodes showed excellent stability during CH₄ reforming and good tolerance to H₂S contamination [1]. In addition, molybdenum appeared in the composition of promising anode materials for running on hydrocarbons or H₂S-containing H₂ as fuels, such as double perovskites based on Sr₂MgMoO₆ [2] and mixed metal sulphides M-MoS₂ (M=Fe, Co, Ni) [3]. Based on these results, combined application of Mo and Ni with CeO₂ (MoNi-Ce) could increase the sulphur tolerance with an optimum performance in H₂ and CH₄. In a previously published manuscript [4], MoNi-Ce, was tested in a single cell as anode material under H₂ and CH₄, and the results revealed that this compound exhibited a high coke resistance and stability in pure CH₄.

In this work, the effect of H₂S on the cell performance for a single SOFC cell of MoNi-Ce/LDC/LSGM/LSCF has been investigated under different concentrations of H₂S at 750, 800 and 850 ºC using IV curves, impedance spectroscopy and load demands. Various levels of dry H₂S/H₂ (0-500 ppm) from a pressurized H₂S/H₂ bottle containing 500 ppm H₂S were mixed with humidified H₂ to keep a total constant fuel flow of 50 ml/min. As example, Fig. 1 displays IV curves and Nyquist plots measured at 850 °C. In general, it can be observed a small increase in the OCV values with an increasing H₂S concentration, being much more significant for 500 ppm H₂S that could be directly related to the different partial pressure of H₂O (pH₂O).  In the case of 500 ppm H₂S/H₂ composition, the gas is feeding without humidification. As expected, the maximum current density and the maximum power density (MPD) decrease with increasing H₂S concentration and the temperature reduction. At the 500 ppm H₂S, the MPD decreased by 44% (from 257 mW/cm² to 143 mW/cm²) at 750 ºC and by 26% (from 414 mW/cm² to 306 mW/cm²) at 850 ºC, compared with than that obtained in humidified H₂. This could be due to the adsorption of sulphur on the active sites. However, the cell performance remained stable under load demand, during 1h, in all H₂S/H₂ concentrations studied at three temperatures. To investigate the effect of H₂S on the electrocatalytic activity of MoNi-Ce anode for the H₂ oxidation reaction, the impedance data were fitted to an equivalent circuit, R_i(R₁Q₁)(R₂Q₂)(R₃Q₃) using Zview program. The R_i represents to the internal resistance of the cell and the (RQ) components correspond to the involved electrode processes. The polarization resistance (R_p) of the cell is defined as R₁+R₂+R₃. The impedance spectra revealed three distinct rate limiting processes at high, medium and low frequencies. At each temperature the internal resistance remained constant while the polarization resistance increased with the amount of H₂S. The analysis of impedance spectra shows that the high frequency arc is not affected by the H₂S concentrations and could be attributed to the charge transfer processes. The intermediate frequency arc is the most sensitive to the anode atmosphere and appears to be associated with the sulphur poisoning processes. While the low frequency arc became significantly larger when dry fuel was used, then it could be related to the change in pH₂O in the fuel. Upon the removal of H₂S from the fuel stream during 1 h, the anode performance can be partially recovered, being more significant at higher temperature.

Acknowledgement

This work was supported by Spanish Ministry of Economic and Competitiveness (MAT2013-45043-P).

 

References

 

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