Electrolysis Using Fuel Cell Technology
Present global hydrogen demand is met to a large extent by fossil fuels. Hydrogen is produced via alkaline electrolysis of water only, when the required electric energy is readily available. Alternatively, hydrogen can be produced by water electrolysis with reversibly operated fuel cells, PEMFCs or SOFCs. PEM electrolysis is technologically mature with an energy-conversion efficiency (electrical-to-chemical) somewhat above the one of alkaline electrolysis (70-80%).
Higher efficiency for electrolysis is expected with increasing temperature as consequence of the decrease of the molar Gibbs energy of the reaction. This was first confirmed with the reversible operation of tubular SOFCs as solid oxide electrolyser cells (SOECs) in pioneering works in the 1980ies. With present SOFCs operated as SOECs, notably the planar cells, the theoretically predicted efficiencies are more and more approached in the experiment. State-of-the-art SOECs may be operated with and without external heat supply; already without heat supply the electrical-to-chemical energy-conversion efficiency is above the one of low temperature electrolysers. Operation cell voltages are around 1.1 to 1.3 V at 800°C at a Faradaic efficiency of 100%. The high efficiency may render the SOEC technology attractive for the storage of (intermittent) renewable energy, such as wind or solar.
In this chapter, examples for the actual state of research on SOECs as well as on SOEC stacks and systems are given, notably their current-voltage response and behaviour under long-term operation. Moreover, some development options – and potential hurdles like cell degradation – are summarised for this potentially new branch of the solid oxide cell technology.