(Invited) Durability of Electrolyte Supported Solid Oxide Cells for Steam Electrolysis: Results from Cell Testing in the 20,000 h to 50,000 Hours Range

Abstract

There is a current fast development of water/steam electrolysis for the production of hydrogen from renewable energy sources. (Multi) MW power demonstration units using the different (low and high temperature) technologies are, e.g., subject of the R&D project H2Giga funded by the German Federal Ministry of Education and Research /1/. Steam electrolysis with solid oxide cells (SOC) is characterised by a high energy-conversion efficiency, notably if steam is available, such as via a heat source of moderate temperature for steam generation. Approaches for a matching to a fluctuating load, important in the context of operation with renewable energy sources, are also under development with the SOC technology /2/.For the SOC electrolysers, durability analysis of the cells in a time scale appropriate for application remains an important development step. Cell (or short stack) testing allows a rather precise control of the cell temperature which facilitates quantification of degradation for the practically required degradation rates well below 10 mV/1000 h (cf. /3/). In this contribution, our testing results will be updated, with up to about 50,000 h operation using electrolyte supported cells (ESC) with Ni/GDC hydrogen electrodes and different types of electrolyte (10Sc1CeSZ, 6Sc1CeSZ, 3YSZ). As reference for cell durability serves a 23,000 h steam electrolysis test with a cell with 6Sc1CeSZ electrolyte /4/. However, cells with the known robust 3YSZ electrolyte seem to yield a comparably low long-term degradation, together with better initial stability /5/. The use of an electrolyte of high ionic conductivity, 10Sc1CeSZ, allows a relatively low initial temperature (<800°C) for a cell voltage close to the thermal neutral voltage for 0.6 Acm-2 current density. This, in turn, leaves a wide temperature margin for compensation of voltage degradation via a temperature increase (cf. initial period of the test in /3/). The results from the durability tests serve as examples for a large degree of SOFC/SOEC reversibility achievable with the ESC. Moreover, the relatively thick electrolyte layer of the ESC with its large ohmic resistance comes along with a high ohmic contribution in the overall degradation in most tests. The different degradation contributions were separated with in-situ impedance spectroscopy without interrupting the DC current flow.