| Description |
The current status of solid oxide fuel cell (SOFC) development is reviewed. A parametric model based on real cell materials and components was developed. Current cathode material and structure are sufficient, and the limiting factors are gas diffusion through the supporting layer and ohmic loss through the whole cell. Once the latter two losses can be minimized, SOFC can offer the best performance. Most of the ohmic loss can be attributed to ionic resistance in the electrolyte, of which more than 50% is due to grain boundary resistance. A quantitative space charge theory is developed for YSZ by considering all the possible defect species and defect complex. Both oxygen vacancy depletion and yttrium segregation near the grain boundary contribute to the very high grain boundary resistivity. A nonequilibrium analysis suggests that quenching at higher temperature can improve both grain bulk and grain boundary conductivity. Nowadays, reversible solid oxide cells (SOC) can be used in both fuel cell mode to generate energy and in electrolyser mode (SOEC) to store energy. However, a SOC degrades much faster when working in the SOEC mode. Oxygen chemical potential in the solid electrolyte near the oxygen electrode could be very high and leads to crack growth. Introducing finite electronic conductivity by doping can prohibit such crack growth. A novel oxygen permeation technique has been developed to measure electronic partial conductivities in solid electrolyte materials. Oxygen can be pumped into the sample by applying DC bias. As excess oxygen is stored in the cavity, the generated Nernst potential drives permeation current. At steady state, oxygen pumped into the cavity equals that which permeates out. When DC bias is turn off, Nernst potential follows a slow decay with time. Analysis of both steady state data and transient process data gives electronic resistance. Proton exchange membrane fuel cell (PEMFC) is now used in electrical cars. The wide application of PEMFC is currently limited by its cost and degradation. Even with a trace amount of carbon monoxide, Pt catalyst will be poisoned. A novel technique is developed to study CO adsorption and desorption kinetics by monitoring the resistance variation. |
