Solid Oxide Electrolyser Cell Testing up to the Above 30,000 h Time Range

Abstract

Steam (and co-) electrolysis demonstration projects with solid oxide cells (SOC) have reached a power range of several hundred kW (e.g., /1/). This generates an urgent demand for verification of the durability of the cells as core electrolyser elements. Cell lifetimes of several years are required, which largely exceeds the commonly reported testing periods. Apart from economic considerations, cell (or short stack) testing allows a more precise control of the cell temperature together with less temperature variation across the surface. This, in turn, facilitates quantification of degradation, in particular for the practically required low degradation rates of <10 mV/kh. Appropriate durability was confirmed with a 23 kh steam electrolysis test with an electrolyte supported cell with a Ni/GDC hydrogen electrode (20 kh at -0.9 Acm-2 with 7.3 mV/kh voltage degradation /2/). This result represents an example for SOFC/SOEC reversibility holding even for a current density magnitude exceeding the commonly applied values under SOFC operation (higher current densities are feasible in the electrolysis mode owing to the more favourable thermal conditions, but they will, in general, imply the development of appropriate cells). Comparably low degradation was confirmed in a one year long test coupling steady-state and ON/OFF switching operation (80,000 cycles) for a cell with a thin 3YSZ electrode instead of the 6Sc1CeSZ in the 23 kh test /3/. ON/OFF stack switching is part of an operation strategy for matching a fluctuating load (such as coupling with renewable energy sources or grid stabilisation).In this contribution, emphasis will be put on a cell test of even longer duration (>30 kh so far) with an electrolyte supported cell, maintaining the current density above the typical SOFC values (initial part of the test in ref. /4/). The operation conditions are chosen for a cell voltage close to the thermal neutral voltage (Uth ~1.3 V), in accord with the requirements for stack operation. The use of an electrolyte of high ionic conductivity, 10Sc1CeSZ, allows a moderate initial temperature below 800°C at the not degraded cell. Temperature is later increased for compensation of degradation. For an appropriate setting of the initial testing parameters together with a low cell degradation and a sufficiently wide temperature window, one should then obtain an operation close to the theoretical efficiency limit with zero voltage degradation during the entire envisaged lifetime. As in our previous work, the long-term cell behaviour is traced with in-situ impedance spectroscopy without interrupting the DC current flow. The main focus of the degradation analysis refers to the ohmic degradation, which typically dominates in the cells of concern /2-4/.