Solid oxide fuel cell (SOFC) applications require lifetimes of several years on the system level. A big challenge is to demonstrate such exceptionally long lifetimes in ongoing R&D projects. Accelerated or compressed testing are alternative methods to obtain this. Activities in this area have been carried out without arriving at a generally accepted methodology. This is mainly due to the complexity of degradation mechanisms on the single SOFC components as function of operating parameters. In this study, we present a detailed analysis of approx. 180 durability tests regarding degradation of single SOFC components as function of operating conditions. Electrochemical impedance data were collected on the fresh and long-term tested SOFCs and used to de-convolute the individual losses of single SOFC cell componentselectrolyte, cathode and anode. The main findings include a time-dependent effect on degradation rates and the domination of anode degradation for the evaluated cell types and operating conditions. Specifically, the steam content as determined by fuel inlet composition, current density and fuel utilization was identified as major parameter, more important than for example operating temperature. The obtained knowledge is adopted to identify optimal operation profiles in order to acquire accelerated testing for lifetime investigation of SOFCs.

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This work aims at developing an innovative renewable energy storage solution, based on reversible Solid Oxide Cell (rSOC) technology. That is to say, one system optimized to operate either in electrolysis mode (SOEC) to store excess electricity to produce H 2 , or in fuel cell mode (SOFC) when energy needs exceed local production, to produce electricity and heat again from H 2 or any other fuel locally available. Firstly, work focused on optimization of the different layers constituting the single SOC cell to reach high initial performance applying state-of-the-art materials as previously reported [1]. Secondly, the initially highest performing cells were selected for long-term reversible SOFC/SOEC single cell tests. Thirdly, these cells were integrated in a stack design optimized for reversible operation at high degrees of H 2 and H 2 O utilization. The long-term single cell tests showed significant degradation in galvanostatic test periods during electrolysis but not in fuel cell mode prior to starting the reversible test operation while the degradation diminished during the subsequent rSOC operation of the cells operating at 700 C, +0.6 and -1.2 A/cm 2 in SOFC and SOEC modes respectively, at fuel utilization (FU) up to 80% in both modes. Electrochemical impedance spectroscopy analyses and post-mortem SEM investigations of tested single cells reveal that the fuel electrodes degraded significantly during the longterm single cell tests. Furthermore, long-term stack tests were conducted on 5-cell stacks, integrating both reference cells and optimised cells. The long-term stack tests were conducted applying different switches between SOFC and SOEC modes. Initially long duration tests (100h each mode) were performed in a mixture of 50% H 2 O and 50% H 2 to see the effect of the polarisation only. The alternating cycle SOEC/SOFC was repeated over a 1800 h testing period. Then stack switched daily from SOEC mode

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Renewable energy sources such as solar and wind power are fluctuating by nature, and energy supply is therefore not fully controllable. Certain countries and communities have already incorporated significant share of fluctuating renewable energy in their energy system e.g. reaching a share of electricity supply from wind of 43% as reported for the Danish energy grid for 2017, which in turn also lead to periods where the supply of wind power exceeds the electricity consumption. In that perspective there is already at present a starting need for efficient, flexible and durable energy conversion and storage solutions, and the need for increased capacity of energy conversion and storage will increase steadily on the path to an energy supply aiming for substituting all fossil fuels and turning to an energy supply based 100% on renewable energy. Among a variety of energy conversion technologies reversible solid oxide cells (SOC) are a good candidate to tackle this challenge of efficient energy conversion and storage. Their capability to operate reversibly, i.e. in ‘electrolysis mode (SOEC)’ or ‘fuel cell mode (SOFC)’, allows using the surplus energy of wind and solar farms to produce H2 or syn-gas (which then can be stored) and re-using the gasses to generate electricity (and possibly heat) on demand. However for solid oxide cells to be a future viable and flexible energy conversion and storage solution it is required to develop, test and demonstrate cells and stacks optimized for reversible operation with the capability to operate at high current densities and 85% fuel utilization in both operating modes. In this study, we investigate performance and durability of optimized SOC at cell and stack level. The cells and stacks have been targeted to operate at high current densities and are operated in both constant SOFC and SOEC mode and in load cycling mode at a temperature 700°C. This in turn means reversible operation of SOC at high overpotentials and daily shift exothermic and endothermic operation. Figure 1 shows an example of cell voltage development during single cell testing of two types of cells and the analysis of cell degradation is supplemented by analysis of electrochemical impedance spectra showing that the dominating degradation of the cell originates from the degradation of the fuel electrode. Furthermore, the tested cells have been investigated by SEM and low-voltage in-lens SEM to investigate the microstructure after load cycling test. In line with the single cell test; stack test (25 cells) applying similar cells have been conducted and results will be reported. Figure 1: Long-term degradation testing of two types of single cells. SOFC operation: fuel side with dry H2, oxygen side with air, current density of 0.6 A/cm2 corresponding to a H2 utilization of 85%; SOEC mode: fuel side with gas composition of H2O/H2:90/10, oxygen side with air, current density of -1.2 A/cm2 corresponding to a H2O utilization of 85%. Cycling between modes was 16/8 hours at SOFC/SOEC. Temperature of 700 ˚C. Figure 1