Redox kinetics of co-doped Ceria ceramics for solar-thermochemical fuel production

Abstract

The utilization of clean and sustainable energy becomes important for solving global environmental problems and securing future energy supply. A possible method for producing CO for fuels is a two-step CO2 splitting reaction in a solar thermo-chemical cycle. Especially doped CeO2 are considered as promising redox materials for thermochemical purposes [1]. So, the oxidation kinetics of Zr doped ceria and co-doped ceria with Zr–La, Zr–Yb and Zr-Y were investigated by thermogravimetric analysis in synthetic air and CO2 to specify the influence of ionic radii and valence of dopants. Samples co-doped with Zr–La, Zr–Yb and Zr-Y exhibited high reduction states and oxidation kinetics that were much faster than 10mol% Zr-doped ceria. The extrinsic oxygen vacancy induced by the trivalent dopants improves the kinetics especially at oxidation temperatures below 700 ◦C, where the oxidation is more diffusion than surface exchange controlled. In the case of the oxidation with air the ionic radii of the dopant have a little influence. At lower temperature La doped ceria reoxidizes a little faster than Yb and Y doped ceria. However, the ionic radii become more important in the CO2 splitting reaction. Here La doped ceria shows significantly faster reoxidation both as a single doping and in combination with Zr. Since, in addition to the perfect doping, the sample shape is also crucial for fast oxidation also compacts consisting of fibers with diameters in the range of 8–10 µm have been successfully prepared by direct infiltration of commercial YSZ fibers with a cerium oxide matrix and subsequent sintering. The resulting chemically homogeneous fiber-compacts are sinter-resistant up to 1923 K and retain a high porosity of around 58 vol%. An evaluation of redox kinetics shows that the relaxation time of oxidation is five times faster than that of dense samples of the same composition. The fiber compacts show a high potential for the application in thermochemical redox cycling due its fast redox kinetics. [1] e.g. Jonathan R. Scheffe, Aldo Steinfeld, Oxygen exchange materials for solar thermochemical splitting of H2O and CO2: a review, Volume 17, Issue 7, September 2014, Pages 341-348