Electrochemical Impedance Study of Ag/Cu-Ca0.2Ce0.8O2+δ Anode for SOFCs Operating with Simulated Biogas

In recent years, researches on SOFC technology have been focused on lowering the operating temperature, primarily driven by the cost and durability of components. Unfortunately operation at lower temperature creates problems associated with the increase in the electrolyte resistance and the electrode polarisation as well as decrease in the rate of electrocatalytic reactions. Furthermore, the direct use of alternative fuels to hydrogen, such as natural gas or biogas, are still limited due to the catalyst deactivation by coking or fuel impurity poisoning. Therefore, it is necessary to continuously search for novel anode materials having superior electrocatalytic activity in the intermediate temperature range and less-prone to deactivation.

Biogas is mainly constituted by CH₄ and CO₂ while containing a few percent of H₂, N₂ and traces of NH₃, H₂S, halides and siloxanes. This composition fluctuates significantly during biogas production and it is highly dependent on the initial substrate used [1].

In previous works we have demonstrated the ability of nanocrystalline Cu-ceria based anodes to operate with CH₄ and H₂S-containing hydrogen [2-5]. The incorporation of a transition metal, such as silver, in an optimised anode formulation has improved its electrocatalytic properties for the complete CH₄ oxidation, whereas it shows excellent thermal and chemical compatibility with the common electrolyte materials [6]. Thus, this new material can be considered a promising anode for SOFC directly fuelled with biogas at intermediate temperature.

The composition of this anode material was Cu-Ag combined with Ca_0.2Ce_0.8O_2+δ (hereafter named AgCu-CaCe), metal load= 40 at. % (Cu/Ag 3:1). Under reducing atmosphere, it is mainly constitutes by a fluorite phase of the cerium mixed oxide and Cu⁰ and Ag⁰metal phases, as could be checked by X-ray diffraction.

In this work, electrochemical impedance spectroscopy (EIS), in symmetric cell configuration with LSGM-electrolyte, has been employed to measure the interface resistance of the deposited anode AgCu-CaCe in comparison with analogous Cu-doped ceria material (Cu-CaCe), at various temperatures (550-750 ºC) and different fuels (H₂ and CH₄) and three simulated biogas mixtures at 750 ºC, to quantitatively deduce the role of electrode microstructure on electrochemical activity.

The method for preparing the symmetric cell was as follow. A porous lanthanum strontium gallate magnesite (LSGM) layer was deposited onto an LSGM dense electrolyte by tape-casting method, using polymethyl methacrylate (PMMA) as pore former. The electrode materials were applied onto the LSGM wafers by screen-printing and after calcined at 750 ºC. Gold was used as current collector. The active area was 0.3 cm². Hydrogen, methane and simulated biogas mixtures were used as fuels and were supply to the anode at a flow rate of 50 ml/min, after passing through a saturator to control the gas humidity. Impedance data were fitted to equivalent circuits using Z-View software.

Fig. 1 shows the impedance spectra obtained at 750 ºC on hydrogen, methane and three simulated biogas mixtures (CH₄/CO₂/H₂; 50:45:5, 60:35:5 and 70:25:5 for 50bio, 60bio and 70bio, respectively). The generic form of the impedance response was composed of two partially resolved arcs under different fuels. The area specific resistance (ASR), cell resistance normalised by the cell area, is markedly lower in H₂ than in CH₄ or simulated biogas due to the lower pO₂ and other factors. When hydrocarbon fuels were used the low frequency arc increased significantly probably caused for the carbon deposition on catalytic active centres. Measurements on simulated biogas revealed that lower ASR values were obtained with higher CH₄content in the mixture (70bio). Note that a clearly reduction in ASR was observed with the incorporation of silver to the Cu-CaCe composition.

The electrochemical impedance responses were fitted to an equivalent circuit composed of a resistor in series with two (or three) parallel constant phase element (CPE)/resistor combinations. The analysis of these equivalent circuits revealed a strong dependence of the kinetics and mechanism involved in the electro-oxidation of the different fuels with the final anode formulation.

Acknowledgements

Authors thank the Ministerio de Educación y Ciencia (project MAT2005-02933 and MAT2013-45043-P) for financial support.

References

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