Experimental and model analysis of IT-SOFC under syngas feed

This thesis is based on the experimental and model study of intermediate temperature solid oxide fuel cells (IT-SOFCs). The study began with the experimental and model investigation of Samaria-doped Ceria (SDC) based IT-SOFCs over a wide range of conditions. In particular, Pd-Cu-Pd-CZ80/SDC/LSCF electrolyte-supported cells (380 μm electrolyte, 150 μm anode and 40 μm cathode) were studied. The polarization and impedance experiments were carried out between 600 and 650 °C with the following mixtures (supplied with constant humidity): H2/N2, CO/CO2, syngas (changing the ratio H2/CO) and biogas, at different concentration of reactants and N2 dilution. A one dimensional, dynamic, isothermal and heterogeneous model was applied to simulate the polarization and impedance curves. The model allowed to extract global power-law rate equations for H2 electro-oxidation, CO electro-oxidation and O2 reduction reaction (ORR) independently. Having the kinetic expressions of anodic and cathodic semi-reactions, the experiments with syngas were examined with the model that eventually pointed out the simultaneous occurrence of H2 and CO electro-oxidation in parallel. The activation of a co-oxidative route at the presence of syngas feed was successfully verified at this point which is a most distinguishing feature of Ce-based cells compared to traditional cells. Dealing with MIEC electrolytes and composite electrodes inspired the necessity to develop a more sophisticated model which rigorously delves deeper into the electrochemical phenomena within the cell. In this regard, a dynamic, 1D, isothermal, distributed charge transfer model of an IT-SOFC with a MIEC electrolyte and composite electrodes was developed. This model constitutes a topic of originality of the second part of this thesis work. A physically-based description of the electronic leakage current in the electrolyte is included, together with mass and charge conservation equations. The reaction interface is extended along the whole length of the electrodes and a detailed, one-dimensional description of charge transfer phenomena is entailed. The profiles of the ionic and the electronic current density, as well as the profiles of the ionic and the electronic potential are described along the whole length of the cell. Distributed charge transfer IT-SOFC model was applied to analyze the experimental results of the cells based on SDC (Sm0.2Ce0.8O1.9) electrolytes, Cu-Pd-CZ80 anodes and LSCF-GDC composite cathodes. A significant increase of the ohmic resistance measured in the impedance spectra was evident at decreasing the H2 partial pressure (from 0.71 Ω cm2 at 100% H2 to 0.81 Ω cm2 at 30% H2) or increasing the voltage. Good agreement between the simulated and experimental polarization and EIS curves was achieved through fitting the exchange current density and the capacitance parameters of each electrode. Based on the results of the simulation, a detailed description is provided on how the electronic leakage current plays a key-role in the observed shift of ohmic resistance. Overall, the model allows to gain insight into relevant design parameters including electrochemical active thickness, current and potential distributions, mass diffusion gradients, etc., and therefore is able to assist for the optimal design of the IT-SOFC based on MIEC electrolytes. With reference to the SOFC application for distributed production of electricity, an important goal for improving the lifetime and thermos-mechanical resistance would be achieved by decreasing the operating temperature (e.g., ≤600 oC) and making the electrode nanostructure robust. Infiltration-derived composite LSF-YSZ electrodes were fabricated in a symmetric configuration by infiltration of LSF nanoparticles into a porous scaffold of YSZ electrolyte. The symmetric cells were then modified by atomic layer deposition (ALD) of ZrO2, La2O3 and Fe2O3 films of different thicknesses to determine the effect of surface composition on cathode performance. The growth rate of each oxide layer was determined gravimetrically using vacuum ALD procedure. For ZrO2 and Fe2O3, EIS tests on symmetric cells at 600 oC showed a regular progressive increase of polarization resistance with coverage in a manner implying simple blocking of oxygen adsorption sites. In case of La2O3 ALD, the polarization resistance first decreased with small numbers of ALD cycles and increased again at higher coverages similar to the previous individual trials. Finally, La2O3 and Fe2O3 were co-deposited aiming at achieving LaFeO3 perovskite phase. Impedance tests on symmetric cells treated with co-deposited films revealed low polarization resistances at high film coverages, indicating that O2 adsorption sites were formed. It is therefore demonstrated that an IT-SOFC electrode can be stabilized via ALD against sintering while maintaining and even slightly improving its performance.

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