Solid oxide fuel and electrolysis cells

Solid oxide cells (SOCs) are electrochemical energy converters that can operate in two modes within one unit. In fuel cell mode (solid oxide fuel cell, SOFC), they directly convert chemical energy into electrical energy through a Knallgas reaction, which is the chemical reaction of oxygen and hydrogen forming water in a controlled manner. The opposite reaction, the splitting of water into hydrogen and oxygen, is the electrolysis reaction (solid oxide electrolysis cell, SOEC). The Knallgas reaction is exothermic, while the water splitting reaction is endothermic. SOCs today typically operate between 650°C and 900°C. Only three layers are required for the fundamental electrochemical reactions: an electrolyte and two electrodes-the anode (the positive pole) and the cathode (the negative pole). In this chapter, the fundamental reactions, the geometrical designs, the material requirements, and the status of SOFCs and SOECs are presented. In the following seven chapters, the most important components of an SOC are described and the chapter ends with a summary and outlook. The described components are: (i) oxygen-ion conductors (electrolyte), (ii) anode materials for SOFCs (cathode in SOEC mode), (iii) cathode materials for SOFCs (anode in SOEC mode), (iv) proton conductors and adjacent electrodes, (v) stack sealing materials, (vi) ceramic coatings for metal interconnects, and (vii) degradation effects limiting lifetime and performance.