High temperature electrolysis cell (SOEC)
Electrolysis is a well-known process to split water (H2O) into hydrogen (H2) and oxygen (O2) by using electrical energy. If the water can be converted into steam by waste heat from other processes it is more efficient to perform a high temperature electrolysis (HTE) and convert the steam directly. The reasons are the more favourable thermodynamic and electrochemical kinetic conditions for the reaction. Thermodynamic conditions are more favourable in the sense that the molar Gibbs energy of the reaction (ΔG) drops from ~1.23 eV (237 kJ/mol) at ambient temperature to ~0.95 eV at 900°C (183 kJ/mol), while the molar enthalpy of the reaction (ΔH) remains essentially unchanged (ΔH ~1.3 eV or 249 kJ/mol at 900°C). A significant part of the energy required for an ideal (loss-free) HTE can thus be provided by heat (TΔS) coming from external sources or due to the unavoidable Joule effect in the electrolyser cell. As a consequence, less electricity is required per m3 of H2 generated compared with the other electrolyser technologies as shown in Figure 1. The transfer from water to steam electrolysis causes as significant drop in the electricity demand followed by a continuous decrease with increasing temperature. The theoretical SOEC electrical efficiency is close to 100 % for hydrogen production efficiency around 90 %.
Figure 1: Energy demand of hydrogen operation versus operation temperature
(Doenitz, W., et al., International Journal of Hydrogen Energy, 1980. 5: p. 55.).
Three operating modes can be distinguished for a HTE system: thermoneutral, endothermal, and exothermal as shown in Figure 2. The HTE operates at thermal equilibrium (1285 mV at 800 °C) when the electrical energy input equals the total energy demand and the electrical-to-hydrogen conversion efficiency is 100 %. In the thermoneutral mode, the heat demand Q=TΔS necessary for the water splitting equals the heat released by the joule heating (ohmic losses) within the cell.
Figure 2: Operation modes of high temperature steam electrolysis.
In the exothermal mode, the electric energy input exceeds the enthalpy of reaction, corresponding to an electrical efficiency below 100 %. In this mode, heat is generated from the cell and can be reused in the system to preheat the inlet gases. This mode has also the advantage to operate at higher current density allowing decreasing the size of the system. However, it could be a source of prematurely ageing of the system components. Finally, in the endothermal mode the electric energy input stays below the enthalpy of reaction which means a cell voltage below the thermoneutral one (< 1.286 V at 800°C). Therefore, heat must be supplied to the system to maintain the temperature. This mode means electrical-to-hydrogen conversion efficiencies of the SOEC above 100 %. This operation mode also allows minimal long-term degradation rates, since it is achieved at the lowest power densities.
As technique for HTE solid oxide cells (SOC) are used. So far they are mainly known and used as fuel cells (SOFC). SOC are usually made of different ceramics. They consist of a membrane for gas separation, a hydrogen electrode and an oxygen electrode. The denotation cathode and anode is skipped, because a single SOC can be operated as fuel cell and electrolyser cell, but as the current direction is inverted in the other mode, the denotation of the electrodes would have to be changed.
The SOC is easy scalable. So called Stacks are a series connection of multiple cells, and these stacks can as well be connected in series or in parallel, just as the specific application requires it.
Figure 3: SOC stack from sunfire GmbH.
The fact that the SOC can be operated as fuel cell and electrolyser cell as well shows the high potential of this technique. Development for one operation mode is also beneficial for the other, from the technical as well as from the economical point of view.