Applications based on Solid Oxide Fuel Cells (SOFC) for distributed production of energy require the cell to operate both with traditional hydrocarbons (CH4, LPG) and biomass-derived fuels (biogas). Several benefits in terms of lifetime and thermo-mechanical resistance are achieved by decreasing the operating temperature to 500-700°C (IT-SOFC). On the one hand, the use of hydrocarbons leads traditional Ni-based anodes to deactivate due to coke formation; on the other hand, lower temperatures ask for electrolytes different from YSZ, mostly based on Ce oxides. Novel cells and materials are then required. In this work, IT-SOFCs based on Samarium doped Ceria (Sm0.2Ce0.8O1.9, SDC) electrolytes and Cu/Pd/CZ80 anodes are investigated for applications with syngas mixtures and CO2/CH4 mixtures. A physically-based, numerical model of the IT-SOFC was applied to analyze the experimental results and kinetic equations were derived for the electro-chemical reactions: in the presence of syngas, the occurrence of a co-oxidation route is proved, wherein H2 and CO are electro-oxidized contemporarily.
Materials and Methods
SDC electrolyte-supported cells with 15 wt% Cu, 0.15 wt% Pd, 15 wt% CZ80 (CZ80 20 wt% CeO2, 80 wt% ZrO2) composite anodes and LSCF cathodes were prepared by die-pressing. SDC powders mixed with graphite were pressed with pure SDC. The bilayer was calcined in air (1400°C, 3 h) producing a porous SDC scaffold (150 μm) supported on a dense SDC electrolyte (380 μm). The porous scaffold was impregnated with Cu/Pd/CZ80-based solutions and then the cathode (40 μm) was deposited. Polarization and EIS measurements were collected between 600 and 700°C with syngas mixtures (2.3 – 0.4 H2/CO ratio), H2/N2 mixtures (from 97 to 30% H2 v/v), CO/CO2 mixtures (from 97 to 50% CO v/v) and synthetic biogas (75% CH4/25% CO2). The EIS tests were performed at the OCV. A model was applied to analyze both the EIS spectra and the polarization curves. The model is one-dimensional, charge-distributed, Dusty-Gas, dynamic and heterogeneous and allows to predict the cell voltage based on physically-sound conservation equations for mass, charge and energy. The model includes molecular kinetic schemes for the anodic catalytic reactions (CO2 reforming of CH4, WGS) and for the electrocatalytic reactions (H2 and CO electro-oxidation, O2 reduction). Appropriate correlations account for the presence of the electronic current leakage in the electrolyte.
Results and Discussion
The IT-SOFCs were first tested in syngas mixtures at varying the H2/CO ratio: no variation was found passing from 2.3 to 0.4 ratio (Fig. 1a). Analogous observations were done with the EIS spectra. These results suggested that both the H2 electro-oxidation and the CO electro-oxidation occurred simultaneously in the presence of syngas. A model analysis of the polarization and EIS curves was performed to prove the activation of the co-oxidative route for CO and H2. The IT-SOFCs were thus tested in H2/N2 mixtures at varying H2 concentration to derive a power-law kinetic rate equation for the electro-oxidation of H2. The polarization curves (Fig. 1b) showed a decrease of the extracted current at decreasing H2 partial pressure. As well, an increase of the polarization resistance at decreasing H2 content was observed in the EIS tests (Fig. 1c). Coherently with the use of SDC, which is a MIEC electrolyte and activates a leakage current, the measured OCVs were smaller than the Nernstian ones. Application of the charge distributed model revealed the axial variation of the leakage current in the electrolyte and in the electrodes. To complete the kinetic investigation, EIS and polarization tests with CO/CO2 mixtures were performed to derive a power-law rate for the CO electro-oxidation reaction. A picture similar to that of the H2/N2 tests was observed. The rate equations and their parameters (reaction orders, activation energies and rate constants) were derived based on the best simulation of the impedance spectra. Once these rates were extracted, the experiments with syngas mixtures were simulated on a fully predictive basis: the close match of the simulations confirmed that the co-oxidative scheme could rationalize the data. Simulations based on different reactive schemes (H2 oxidation + WGS, CO oxidation + RWGS) failed in achieving a satisfactory description, suggesting that the parallel oxidation of H2 and CO was the only adequate scheme.
Conclusions
By means of model analysis of experimental polarization and impedance curves, we show that, in the presence of syngas, the electro-oxidation of H2 and that of CO occur simultaneously in a novel, Ni-free, IT-SOFCs based on Samaria-doped Ceria. The activation of a co-oxidative route is a most distinguishing feature of Ce-based cells, compared to traditional SOFCs.