Novel anode materials for solid oxide fuel cells
Prof. Dr. Ir. D.N. Reinhoudt
Prof. Dr. J. Schoonman
Dr. B.A. Boukamp
This thesis deals with the solid oxide fuel cell (SOFC), which is currently at the forefront of environmental-friendly energy systems for this century. SOFCs directly convert chemical energy into electrical energy with high efficiency and low emission of pollutants. The emphasis in this work is on the SOFC anode, in particular its chemistry and development aspects. The hydrogen oxidation reaction at the state-of-the-art nickel/yttria-stabilised zirconia cermet anode was studied. Alternative anode materials – specifically perovskite and fluorite-type of materials - were tested in a hydrogen/water-vapour ambient. The overall aim was to obtain more insight into the properties of different anode systems, thus facilitating further improvement.
The electrodes were characterised electrochemically by polarisation measurements (I-h) and by electrochemical impedance spectroscopy as function of T, pH2 and pH2O. In certain cases the electrode impedance was also measured as function of bias (polarisation). The microstructure of the electrodes was investigated by scanning electron microscopy (SEM).
A general introduction on SOFC is presented in Chapter 1. The principles and materials employed are described. An overview is given on alternative anode materials that are currently being investigated.
Chapter 2 is a model study in which a porous nickel layer electrode is compared with a modified nickel layer electrode. The modification consists of the deposition of fine yttria-stabilised zirconia (YSZ) particles on the nickel surface. The marked similarity in the electrode properties of nickel and modified nickel in the hydrogen oxidation strongly suggests that essential steps in the electrode mechanism on both types of electrodes are similar. The total electrode conductivity increases with increasing triple phase boundary (TPB) length. However, at similar TPB length the electrode conductivity of the modified porous nickel electrode is 50% higher compared to that of the bare nickel electrode. This result indicates that the presence of fine YSZ particles on the nickel electrode surface creates additional active sites at which the electrode reaction can occur.
The kinetics of the hydrogen oxidation reaction at the Ni/YSZ anode is investigated in Chapter 3 using the approach originally used by Epelboin in 1970 to obtain an expression for the impedance. On the basis of the work reported, it is reasonable to conclude that charge transfer processes dominate the hydrogen oxidation kinetics occurring at the Ni/YSZ anode. The occurrence of a, so called, ‘inductive loop’ at low frequencies in the experimental impedance spectra originates from the potential dependence of the surface coverages of intermediate species. The characteristic inductive loop can be reproduced by assuming that two consecutive electron-transfer steps govern the surface coverage of intermediate OH- groups. However, more advanced reaction schemes must be invoked to fully explain the experimental observations.
Alternative anode materials have been tested in Chapters 4 and 5 for use as porous anodes in SOFC applications. In particular two types of lanthanum based chromite-titanates have been studied. They were stable under reducing conditions and measurements were reproducible with different samples of the same composition. The calcium- or strontium- substituted chromium-rich materials, with composition La0.7A0.3Cr0.8Ti0.2O3-d (A = Ca, Sr), showed p-type conductivity under reducing conditions. The titanium-rich composition, La0.7Ca0.3Cr0.2Ti0.8O3-d, showed n-type conductivity. Both type of materials were active electrodes in a H2/H2O gas mixture at 850°C. The electronic conductivity becomes an important limiting factor when the electrode thickness is reduced from 100 mm to 20 mm. The high frequency ‘cut-off’ resistance showed a significant dependence on pO2, similar to that found for the electronic conductivity. This effect could be ascribed to a limiting sheet resistance of the electrodes. The total electrode resistance of the electrodes was about a factor 10 higher than that of the state-of-the-art Ni/YSZ anode.
Hydrogen oxidation was studied for screen-printed cermet anodes composed of nickel metal and yttria-stabilised zirconia, with additions of 5 and 10 mol% titania. The impedance spectra at open circuit potential indicated the presence of three different processes dependent on the H2/H2O ratio and temperature. The measurements revealed a similar behaviour as functions of H2 and H2O partial pressures for the cermets with 5 and 10 mol% titania. The total electrode resistance increases with increasing the sintering temperature of the cermets from 1300°C to 1400°C. Contrary to earlier observations made for Ni/YSZ, the results showed that for the Ti-containing cermets the electrode conductivity increases with time. A remarkable result of this study is that these Ti-containing cermets, which have not been optimised yet, show a very good performance at 850°C.
Appendix A summarises the possibilities of percolation theory to describe and interpret transport properties in disordered composite media. Percolation properties play a key role in SOFC cermet electrodes. In these electrodes, ceramic and metal phases and pores have to form a completely percolative system.
Appendix B presents a model for electrophoretic deposition for a well-stirred suspension, a constant voltage difference across the deposition cell, and a nonionic solution. Based on first principles, the cast growth is described. The influence of the cast formation on the cast growth is implemented. Electrophoretic deposition is an interesting technique for depositing advanced ceramic materials for SOFC applications.
And last, the SOFC set-up is described in Appendix C. This set-up was utilised to carry out impedance and polarisation measurements on the different tested anode materials.
New mixed conducting anode materials studied in this thesis, by no means optimised, showed a good stability and were found to be reproducible. They were active in H2/H2O mixtures at 850°C. Nevertheless the electronic conductivity of these perovskites is too low, resulting in electrode conductivities a factor 10 lower that that of the state-of-the-art Ni/YSZ cermet. Ti-containing YSZ / Ni composites showed excellent anode performance in H2/H2O mixtures at 850°C, comparable to that of the state-of-the-art Ni/YSZ cermet, and an improved performance with time. There are strong indications that the ceramic phase is active in the electrode reaction. It can be concluded that composites present the best alternative for improved anodes, when compared to single-phase materials. It is essential to combine the best properties of different materials to achieve high performance.
Another conclusion from this research is that it is difficult to avoid Ni when excellent performance is wanted. When hydrocarbons are used as direct fuel, Ni must be replaced by another electronic conductor because of the severe carbon deposition problems. Hence further research should be focused on composites of mixed conducting ceramics and highly electronic conductors. Such research will necessarily include a study of the chemistry of layer formation, in order to optimise the microstructure of these composite anodes.