Performance of Reversible Solid Oxide Cells: A review

It was shown about 25 years ago that solid oxide fuel cells (SOFC) could also work in the solid oxide electrolyser cell (SOEC) mode. Electrolysis of both water (steam) and CO2 was demonstrated. During the 1990'ies very little electrolysis work was reported in the open literature, whereas an enormous amount of results on the SOFC R&D appeared. Recently, the increasing interest in hydrogen production has revived the interest in the solid oxide cell (SOC) as electrolyser. During the time since the work of Isenberg the power density of the SOC has increased significantly, or in other words, the area specific internal cell resistance has been decreased substantially at least for fresh cells. This paper describes briefly the development and status of the SOC performance over the latest 25 years, and problems in obtaining high performance, reliability and durability are outlined. Examples of results obtained in the SOC research at Risø are given as illustrations of obtainable results for the SOFC technology together with a short review of the international state of the art for SOC. H2O electrolysis with a new SOEC resulted in a record breaking current density of -3.6 A/cm2 at a cell voltage of 1.48 V, which is the overall thermoneutral voltage. Assuming an electricity cost of 3.6 US$/GJ, the H2 production price is estimated to 4.8 US$/GJ equivalent to 30 US$/barrel crude oil. The technical requirements to the SOC technology imposed by the economic situation of today are discussed. It is indicated that the SOC holds a considerable potential as an energy converter in a society with a high fraction of renewable electricity supply. Production of synthesis gas by electrolysis of a mixture of steam and CO2 may be used as basis for production of CO2-neutral hydrocarbon fuels. The type of R&D that is necessary in order to obtain a commercially viable SOC converter is finally discussed.