High temperature electrolysis based on solid oxide electrolysis cells (SOECs) is a very promising technology for energy storage and production of synthetic fuels. Compared with conventional low temperature electrolysis cells, SOEC has several advantages. The high operating temperature results in faster reaction kinetics and lower internal resistance. Besides, the heat demand required for electrolysis can be obtained from the Joule heat originating from the cell internal resistance or from renewable or waste heat sources, thus reducing the total electrical energy demand. A close to 100 % electricity-to-fuel efficiency can be reached if the SOEC is operated at the thermoneutral voltage (defined as the minimum thermodynamic voltage at which a perfectly insulated electrolysis unit would operate, if there is no net inflow or outflow of heat) [1].

In recent years extensive efforts have been devoted to improving performance and durability of SOEC cells and stacks. A number of long-term single cell or stack tests have been reported by various groups, with the longest testing period reaching 20,000 hours [2-4]. Due to historical reasons and the easiness of doing constant current tests, (almost) all the reported SOEC tests have been galvanostatic. The cell voltages were either below or above the thermoneutral voltage (~1.29 V for electrolysis of steam), resulting in an exothermic or endothermic electrolysis process, respectively, and lower efficiency. From a SOEC system point of view, it is highly desirable to operate the SOEC at constant thermoneutral voltage (or slightly above in order to account for potential heat losses), to ensure high efficiency and to ease the heat management of SOEC stacks and systems. Besides, from a testing point of view, the natural test method should be potentiostatic (constant voltage), because the electrochemical driving force of an electrode process is the overpotential, which is directly related to the cell voltage, and most probably some future applications will ask for constant voltage output.

In this work, we report test results on two types of SOEC cells operated for electrolysis of steam in potentiostatic mode at thermoneutral voltage. Both cells are Ni/YSZ fuel electrode supported type, one with LSCF/CGO (LSCF: (La,Sr)(Co,Fe)O₃; CGO: (Ce,Gd)O_2-d) and the other with LSC/CGO (LSC: (La,Sr)CoO₃) oxygen electrode. In our previous studies, the LSC/CGO cell showed lower degradation rate than the LSCF/CGO cell when tested at -1 A/cm² in galvanostatic mode [5]. When tested at 750 ^oC and 1.29 V in potentiostatic mode, with a mixture of 50 % H₂O + 50 % H₂ supplied to Ni/YSZ, the LSC/CGO cell again showed superior initial performance, demonstrating an initial current density of -1.1 A/cm² at 1.29 V. The LSCF/CGO cell showed a bit inferior performance, with an initial current density of -0.8 A/cm² at 1.29 V. The two cells differ also in the long-term degradation over the 2000 h testing period. The LSC/CGO cell showed large and irreversible degradation, while the LSCF/CGO cell degraded in the first 400 h, but afterwards activated and stabilized after 1500 h. Both cells reached a current density of about -0.6 A/cm² at the end of test period. Detailed impedance analysis indicates that degradation happened mainly at the Ni/YSZ electrode for both cells. Based on the experimental results and 2-D electrochemical modeling, the different degradation behavior of the two cells was further correlated to distribution of the over-potential across the cell and along the steam/hydrogen flow direction. Large over-potential on the Ni/YSZ fuel electrode was identified as the main cause of the degradation. Operation strategies were further proposed for fuel-electrode supported cells when operated at thermoneutral voltage.

[1] S. D. Ebbesen, S. H. Jensen, A. Hauch, and M. B. Mogensen, Chemical Reviews, 114 [21] 10697-10734 (2014).

[2] G. Corre and A. Brisse, ECS Transactions, 68 [1] 3481-3490 (2015).

[3] X. Sun, M. Chen, P. V. Hendriksen, and M. B. Mogensen, pp. B1305 in 11th European SOFC & SOE Forum, Lucerne, Switzerland, 2014.

[4] A. Brisse, J. Schefold, J. Dailly, pp. A0801 in 12th European SOFC & SOE Forum, Lucerne, Switzerland, 2016.

[5] P. Hjalmarsson, X. Sun, Y.-L. Liu, and M. Chen, Journal of Power Sources, 262 [0] 316-322 (2014).