Load Cycling Tests of Reversible Solid Oxide Cells – Effects of Current Density, Steam Content, and Utilization
Authors: Hauch Anne , Pylypko Sergii , Cubizolles Geraud , Mouginn Julie 
Published: July 2021

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) for steam electrolysis to store excess electricity and to produce H2, or in fuel cell mode (SOFC) when energy needs exceed local production, to produce electricity and heat again from H2. In this work rSOC tests were conducted at single cell, in order respectively to finely understand performance and degradation mechanisms and to perform tests in representative conditions; and single cell rSOC test are finally compared to stack test results.

Single cell rSOC tests were conducted to investigate the effect of current density, inlet steam content and degree of steam utilization both upon galvanostatic SOFC and SOEC testing, and upon load cycling operation. Cells having high initial electrochemical performance from Elcogen were used for these studies (1). Via four different long-term tests, we have investigated degradation at 700°C during constant galvanostatic SOFC (250 h) and SOEC (250 h) conditions followed by load cycling operation (up to 1000 h) in cycles of 16 h SOFC and 8 h SOEC. The applied current densities were either severe (0.6 A/cm2 in SOFC and -1.2 A/cm2 in SOEC) or moderate (0.4 A/cm2 and -0.6 A/cm2); applying different inlet gas composition ie. p(H2O)/p(H2) of 90/10 in SOEC and dry H2 in SOFC, or applying p(H2O)/p(H2) of 50/50 for both modes. Furthermore, two different steam and H2 utilization degrees of 80% and 40% were used. Figure 1 depicts examples of recorded cell voltage curves over time for two examples of the long-term rSOC testing where the inlet gas compositions were varied between p(H2O)/p(H2) of 50/50 for one test and p(H2O)/p(H2) of 90/10 and dry H2 for the other test.

The electrochemical durability was assessed recording voltage over time and via electrochemical impedance spectroscopy (EIS). The characterization of the cells was supplemented by post-mortem scanning electron microscopy studies.

Key findings include: 1) The cells degraded to a significantly larger degree in SOEC mode than in SOFC mode during the initial galvanostatic testing sequences. 2) At the given conditions and applying a current density of -1.2 A/cm2, the cells operate at a cell voltage above thermoneutral potential during the constant SOEC operation while this is not the case for the low-current density tests applying an electrolysis current of -0.6 A/cm2. 3) Load cycling does not increase the rate of degradation; rather the cell voltage stabilized for parts of the load cycling test sequences. 4) The major contribution to the resistance increase during long-term operation can be attributed to the fuel electrode and for the test, having the highest fuel electrode resistance increase (high current density test) a ~60% increase of the ohmic resistance was also observed during SOEC testing. 5) For the harshest tested (high current density) cells, Ni migration have been observed in parts of the fuel electrodes. 6) Nano-scaled detachment between Ni and YSZ particles were observed in several locations in several cells together with non-percolating Ni particles ie. where Ni-Ni connection has been lost; and a general trend was that the higher the fuel electrode overpotential during testing, the more pronounced was the effect of Ni-network changes upon microscopy post-mortem analysis in line with previous studies (2). 7) Inclusions of silica-containing impurities were found in the Ni/YSZ fuel electrodes as also observed in previous work (3, 4).

Based on the single cell tests the recommendation for stack and system operation will, at 700 °C be, to operate in the current density regime of 0.4 A/cm2 and -0.6 A/cm2 rather than 0.6 A/cm2 and -1.2 A/cm2 to ensure long-term durability. Single cell test results were supplemented and trends confirmed by long-term rSOC test on 5-cell stacks at similar conditions.

Figure 1: Cell voltage curves over time for two long-term rSOC test at 700°C. Inlet gas compositions of p(H2O)/p(H2):50/50 (blue curve) for one test and p(H2O)/p(H2):90/10 and dry H2 for the other test (red curve). Current densities of 0.4 A/cm2 and -0.6 A/cm2, respectively and a H2O (SOEC) and H2 (SOFC), utilization of 40%.

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Figure 1