Christodoulos Chatzichristodoulou

Defect Chemistry and Thermomechanical Properties of Ce0.8Pr x Tb0.2 − x O2 − δ

The oxygen nonstoichiometry (δ) of Ce 0.8 Pr x Tb 0.2-x O 2-δ (x = 0, 0.05, 0.10, 0.15, 0.20) was measured as a function of P o2 at temperatures between 600 and 900°C by coulometric titration and thermogravimetry. A nonideal solution model, allowing for a linear δ dependence of the partial molar enthalpy of reduction in the dopants, could successfully reproduce the experimentally determined oxygen nonstoichiometry. X-ray absorption near-edge spectroscopy measurements were performed at the Ce/Pr/Tb L3 and L2 edges. The valence state of each dopant was affected by the presence of the co-dopant. The redox properties strongly depended on the lattice strain energy and the mean metal-oxygen bond strength. The thermal and chemical expansion coefficients were determined by dilatometry. The strongly nonlinear behavior of the thermal expansion coefficient originated from the chemical strain due to increasing oxygen nonstoichiometry with increasing temperature.

Oxygen Permeation in Thin, Dense Ce0.9Gd0.1O1.95-δ Membranes II. Experimental Determination

Thin (~30 μm), dense Ce 0.9 Gd 0.1 O 1.95-δ (CGO10) membranes (5 × 5 cm 2 ) supported on a porous NiO/YSZ substrate were fabricated by tape casting, wet powder spraying and lamination. A La 0.58 Sr 0.4 Co 0.2 Fe 0.8 O 3-δ /Ce 0.9 Gd 0.1 O 1 . 95-δ (LSCF/CGO10) composite cathode was applied by screen printing. Oxygen permeation measurements and electrochemical characterisation of the cells were performed as a function of temperature with air and varying hydrogen/steam mixtures flowing in the feed and permeate compartments, respectively. The oxygen flux was found to reach 10 N mL min -1 cm -2 at ~1100 K and to exceed 16 N mL min -1 cm -2 at 1175 K. The measured oxygen flux was in good agreement with theoretical predictions from a model that takes into account the bulk transport properties of Ce 0.8 Gd 0.1 O 1.95-δ , the anode and cathode polarisation resistances, and the gas conversion and gas diffusion losses in the permeate compartment. The performance of the membrane was also investigated under varying CH 4 and H 2 O gas mixtures at 1106 K. The oxygen flux increased with decreasing steam to carbon ratio and was found to exceed 10 N mL min -1 cm -2 of O 2 for steam to carbon ratios below 4:3. Post-test analysis of the tested membrane did not reveal any significant microstructural degradation of the CGO10 membrane or the anode-support.

Oxygen Permeation in Thin, Dense Ce0.9Gd0.1O1.95-δ Membranes I. Model Study

A model of a supported planar Ce 0.9 Gd 0.1 O 1.95-δ oxygen membrane in a plug-flow setup was constructed and a sensitivity analysis of its performance under varying operating conditions and membrane parameters was performed. The model takes into account the driving force losses at the catalysts at the feed and permeate side of the membrane, related to the gaseous oxygen reduction . and fuel oxidation, respectively, as well as the gas conversion and gas diffusion resistances in the porous support structure at the permeate side. The temperature and oxygen activity dependence of the oxide ionic and electronic conductivity and the oxygen nonstoichiometry of Ce 0.8 Gd 0.1 O 1.95-δ were described based on literature data. The performance of the membrane was characterised by the delivered oxygen flux and the membrane voltage. The dependence of the performance on the various membrane and operating parameters was analyzed by a separation of the various losses. The chemical expansion of Ce 0.9 Gd 0.1 O 1.95-δ under operation was estimated from the calculated oxygen activity and nonstoichiometry profiles inside the membrane.

Electronic conductivity of Ce0.9Gd0.1O1.95−δ and Ce0.8Pr0.2O2−δ: Hebb–Wagner polarisation in the case of redox active dopants and interference

The electronic conductivity of Ce(0.9)Gd(0.1)O(1.95-δ) and Ce(0.8)Pr(0.2)O(2-δ) under suppressed ionic flow was measured as a function of pO(2) in the range from 10(3) atm to 10(-17) atm for temperatures between 600 °C and 900 °C by means of Hebb-Wagner polarisation. The steady state I-V curve of Ce(0.9)Gd(0.1)O(1.95-δ) could be well described by the standard Hebb-Wagner equation [M. H. Hebb, J. Chem. Phys., 1952, 20, 185; C. Wagner, Z. Elektrochem., 1956, 60, 4], yielding expressions for the n- and p-type conductivity as a function of pO(2). On the other hand, significant deviation of the steady state I-V curve from the standard Hebb-Wagner equation was observed for the case of Ce(0.8)Pr(0.2)O(2-δ). It is shown that the I-V curve can be successfully reproduced when the presence of the redox active dopant, Pr(3+)/Pr(4+), is taken into account, whereas even better agreement can be reached when further taking into account the interference between the ionic and electronic flows [C. Chatzichristodoulou, W.-S. Park, H.-S. Kim, P. V. Hendriksen and H.-I. Yoo, Phys. Chem. Chem. Phys., 2010, 12, 33]. Expressions are deduced for the small polaron mobilities in the Ce 4f and Pr 4f bands of Ce(0.8)Pr(0.2)O(2-δ).

High temperature and pressure electrochemical test station

An electrochemical test station capable of operating at pressures up to 100 bars and temperatures up to 400 °C has been established. It enables control of the partial pressures and mass flow of O2, N2, H2, CO2, and H2O in a single or dual environment arrangement, measurements with highly corrosive media, as well as localized sampling of gas evolved at the electrodes for gas analysis. A number of safety and engineering design challenges have been addressed. Furthermore, we present a series of electrochemical cell holders that have been constructed in order to accommodate different types of cells and facilitate different types of electrochemical measurements. Selected examples of materials and electrochemical cells examined in the test station are provided, ranging from the evaluation of the ionic conductivity of liquid electrolytic solutions immobilized in mesoporous ceramic structures, to the electrochemical characterization of high temperature and pressure alkaline electrolysis cells and the use of pseudo-reference electrodes for the separation of each electrode contribution. A future perspective of various electrochemical processes and devices that can be developed with the use of the established test station is provided.