High temperature steam electrolysis at DLR –from stack to system level

Electrochemical hydrogen production can be crucial for the future energy system due to its many application benefits. Hydrogen is used in the chemical industry, is a promising energy carrier to store electric power from renewables and can play a large role in future mobility and steel industry. Currently, hydrogen is mainly produced by partial oxidation of natural gas. For defossilising the future energy supply new methods and processes for hydrogen production need to step in. The German Aerospace Center (DLR) investigates environmentally friendly hydrogen production paths which do not use fossils fuels. One of the research topics is the high temperature steam electrolysis using solid oxide electrolysis cells (SOEC). This technology offers a great potential for a highly efficient energy conversion due to the high operating temperature. Additionally, previous studies have shown that the performance of solid oxide cells can be significantly improved by operating at elevated pressure [1]. A further reason for pressurization is the use of pressurized hydrogen in downstream processes for storage or fuel synthesis [2]. One of the test environments at DLR allows to run experiments of solid oxide cell stacks at elevated pressures between 1.4 to 8 bar. Experimental results of two commercially available planar 10-layer stacks with electrolyte supported cells will be presented. Both stacks run in SOEC mode under pressurized conditions. The performance of one stack was evaluated via steady-state and dynamically recorded U(i)-curves as well as via electrochemical impedance spectroscopy (EIS). It showed an increase of the cell voltages due to the elevated pressure and a slight positive effect on the overall stack performance was observed. Whereas the ohmic resistance is the major part of the entire resistance of an electrolyte supported cell, the activation and diffusion resistance play a subordinate role. The second stack was analysed within a constant-current operation over 1000 hours and at a pressure of 1.4 bar. This stack showed a comparably low degradation rate of 0.56 %/kh [3]. To play a significant role in the future energy system hydrogen production with SOECs from renewable electricity will have to be scaled up into the multi-MW range. To support this transition the DLR built a test environment for large SOC modules of up to 150 kWel which can be building blocks for the envisaged systems. The current status of the test rig’s operational possibilities and the ongoing investigation of modules from two German SOC system manufacturers will be discussed. [1] M. Henke et al., Theoretical study on pressurized operation of solid oxide electrolysis cells, International Journal of Hydrogen Energy, http://dx.doi.org/10.1016/j.ijhydene.2014.05.185 [2] S. H. Jensen et al., Characterization of a Planar Solid Oxide Cell Stack Operated at Elevated Pressure, Journal of the Electrochemical Society 2016 163: F1596-F1604, https://doi.org/10.1149/2.1171614jes [3] M. Riedel et al., Analysis of pressurized operation of 10 layer solid oxide electrolysis stacks, International Journal of Hydrogen Energy, https://doi.org/10.1016/j.ijhydene.2018.12.168