Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks

High temperature steam electrolysis using solid oxide electrolysis cell (SOEC) technology can provide hydrogen as fuel for transport or as base chemical for chemical or pharmaceutical industry. SOECs offer a great potential for high efficiencies due to low overpotentials and the possibility for waste heat use for water evaporation. For many industrial applications hydrogen has to be pressurized before being used or stored. Pressurized operation of SOECs can provide benefits on both cell and system level, due to enhanced electrode kinetics and downstream process requirements. Experimental results of water electrolysis in a pressurized SOEC stack consisting of 10 electrolyte supported cells are presented in this paper. The pressure ranges from 1.4 to 8 bar. Steady-state and dynamically recorded U(i)-curves as well as electrochemical impedance spectroscopy (EIS) were carried out to evaluate the performance of the stack under pressurized conditions. Furthermore a long-term test over 1000 h at 1.4 bar was performed to evaluate the degradation in exothermic steam electrolysis mode. It was observed that the open circuit voltage increases with higher pressure due to well-known thermodynamic relations. No increase of the limiting current density was observed with elevated pressure for the ESC-stacks (electrolyte supported cell) that were investigated in this study. The overall and the activation impedance were found to decrease slightly with higher pressure. Within the impedance studies, the ohmic resistance was found to be the most dominant part of the entire cell resistance of the studied electrolyte supported cells of the stack. A constant current degradation test over 1000 h at 1.4 bar with a second stack showed a voltage degradation rate of 0.56%/kh.