Materials for IT-SOFC stacks: 35 years R&D: the inevitability of gradualness?

A personal view is presented of developments over the period 1964–1999 that have gradually made possible the fabrication of IT-SOFC stacks operating at 500°C. Although conceptual IT-SOFC designs were available in the early 1960s, their implementation had to await a much better understanding of various topics including: selection of optimal dopant type and concentration for solid electrolytes; the influence of microstructure on ionic conduction in ceramics; and the mechanism of oxygen ion formation and injection into solid electrolytes. The latter two topics have benefited immeasurably by the introduction of relevant instrumental techniques involving the use of frequency response analysers, and dynamic SIMS equipment to measure 18O/ 16O profiles in oxide electrolytes and mixed conductors. The parallel development of hydrocarbon reforming technology has also stimulated interest in IT-SOFC systems as syngas can now be effectively produced from natural gas at intermediate temperatures. With deregulation stimulating the introduction of distributed (embedded) electric power generation, the commercial future of micro-CHP units incorporating IT-SOFC stacks appears assured with entry into the market expected early in the next millennium.