Abstract. Oxygen-ion conducting solid electrolyte systems have been reviewed with specific emphasis on their use in solid oxide fuel cells. The relationships between phase assemblage, electrolyte stability and ionic conductivity have been discussed. The role of parameters such as sintering temperature and atmosphere which influence the segregation of impurities, present in the starting ceramic powders, at grain boundaries and at the external surface of the electrolyte compacts has been emphasised. The stability of various electrolyte materials in contact with other fuel cell components and in fuel environments has been discussed in detail. The ageing behaviour at fuel cell operating temperatures has been described. Data on ionic conductivity, mechanical and thermal properties have been presented for a number of electrolyte materials.

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Solid‐state electrochemical cells based on oxygen‐ion conduction (pure ionic or mixed ionic/electronic conductors) allow selective transport of oxygen (oxygen‐ion conducting materials) in the form of ionic flux at high temperatures. Thus these systems can act as filters for molecular oxygen and can be used for both generation or removal of oxygen selectively. The usage of such devices are numerous, including production of high purity oxygen for medical applications, aqua‐culture and combustion processes; control of oxygen partial pressure in industrial environments; production of power and chemicals; and removal of oxygen from enclosures and gas streams. In this paper, a brief overview of the technology has been given and various membrane materials for construction of such devices have been discussed.

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With dwindling liquid fuel resources, hydrogen offers a credible alternative. The use of hydrogen in a fuel cell offers the highest fuel conversion efficiency compared with all other technologies and it also has the potential to substantially reduce greenhouse gas and particulate emissions at least at the end-user sites. One of the major barriers to the introduction of the hydrogen economy and its wider acceptance is the lack of the rather costly hydrogen generation, transportation and distribution infrastructure to meet the local transport fuel demands. On-site or distributed hydrogen generation would remove the need for this up-front infrastructure requirements and assist with the early large-scale trials of the fuel cell technology for both transport and stationary applications and also introduction of the hydrogen economy. In this paper, the development of polymer electrolyte membrane electrolysis technology for on-site, on-demand hydrogen generation has been discussed. The major emphasis is given on reducing catalyst cost; interface design and modifications; interconnect materials, design and fabrication; and investigation of the sources of degradation. Stacks to 2 kW H2 capacity have been constructed and tested and show initial efficiencies of >87% at 1 A cm −2 .