Authors: Thomas Erik Lyck Smitshuysen | Olivia Fjord Sloth | Marie Lund Traulsen | Martin N. Nielsen | F. Mattera | Dario Montinaro | Jan Pieter Ouweltjes | Mogens Bjerg Mogensen | Søren Højgaard Jensen
Source: ECS Meeting Abstracts (article, 2025-11-24)
Field: Materials Science — Materials Chemistry

The Necessity of AC:DC for Long-Term Operation: 20000 Hours of Operation on a 70-Cell SOEC Stack Solid oxide electrolysis cells (SOEC) offer the highest conversion efficiency, can electrolyze both CO 2 and H 2 O as well as being able to perform reversible operation; therefore it is the most industrially relevant and viable electrolysis option for the future, if the technical difficulties associated with SOEC can be solved[1]. One key challenge is to achieve sufficient operation lifetime. In this extended abstract, we present an unprecedented stable 3.5kW stack-test: 20000 hours (h) of operational time and performance loss of ~0.1%/kh on the most recent 17000 h. The unique parameter of this test is the AC:DC operation[2,3], which has been the sole electrolysis operation method of the stack throughout the last 18000 h of the test. Besides of the proven properties of AC:DC, such as its ability to decouple heat-loss and operational stack-voltage, and the elimination of Ni-migration[3], the hypothesis is that the reverse polarization pulses present in AC:DC increase the impurity tolerance. Methodology The main objective of the long-term SOE-stack test is to further establish the possibly substantial benefits of the AC:DC electrolysis operation[2,3] accompanied by the very important learnings resulting from running high temperature steam electrolysis equipment for such extended periods. SOE Stack - The operated stack of SolydEra brand includes 70 cells of 80 cm 2 active electrode area, the cells being grouped into 7 clusters of 9 cells and 1 cluster of 7 cells that are electrically connected in series. Voltage probes for individual cell-clusters are fitted. The cells are Ni/YSZ fuel electrode (FE) supported, YSZ electrolyte, GDC barrier layer and LSCF/GDC oxygen electrode (OE). The spring-loaded stack is fitted in a stack-box design with a furnace. Although it is relatively small, this stack is considered representative for the operation of SolydEra’s G8X platform[4], which allows for the realization of cost-efficient large scale electrolysis applications. Test rig - The stack-box was operated inside a test rig (EBZ GmbH). The rig has been post-fitted with a bidirectional G5.RSS power supply (Regatron), a DV4X direct steamer/evaporator (aDROP), and a custom-built fuel-gas recuperation system to allow H 2 recycling. Operation - Initially the stack was operated in AC:DC[3] at 0.5 A/cm 2 , inlet temperature of 745-750°C, 35 NLPM 10% H 2 in H 2 O(g) to the FE and 63% steam conversion, 372 NLPM air to the OE. The first 2700 h of the test saw periods of conventional DC load current, and AC:DC at 700°C. After 2060 h the H 2 content was lowered to 2%, and after 4300 h the air lowered to 200 NLPM. After 13700 h the average load current was reduced to 39.2 A, and increased to 40 A after 20900 h. Throughout the whole sequence, brief intermittent periods of 700°C at open-circuit voltage (OCV) were applied to perform electrochemical impedance spectroscopy (EIS). Figure A shows the average current and voltage during the test-period. The stack is still operating. Electrochemical Impedance Spectroscopy – EIS was obtained at 700°C under four different gas compositions: either 8% or 17% H 2 in steam at the FE and high flow of air at OE, and 17% H 2 at the FE and low flow of either 21% or 12% O 2 in N 2 at the OE with. Equivalent circuit-fitting and distribution of relaxation time (DRT) has been performed on the acquired data. Results The stack was heated up on the 28 th of August 2022, and operated the following day with a direct current load for the first ~250 h of 40 A (0.5 A/cm 2 ) at a resulting voltage of ~81.7 V. Across the 70 cells in the stack, this operation amounts to 2800 A of chemical reaction or 3.5 kW of H 2 gas (lower heating value, assuming 100% Faradaic efficiency) and a ~2.5 kg of H 2 /day out the tail pipe. Figure A shows the average current and voltage. At the time of writing, the test is over 21780 h old, with 20017 h of “hot” uptime out of which 18634 h or 93.1% has been under current load. The hourly-difference between uptime and load-time is corresponding with the time used on EIS-measurements. The down-time of 1763 h was mainly due to two events: an external interruption in power after 16600 h, enabling an exchange of a carbon-filter on the air gas-line, and leaving the stack cold for ~600 h; and a following incident after 17600 h, where the high-temperature heat-trace thread snapped due to wear and tear, and replacements requiring several weeks to arrive. Three other incidences of full shutdowns happened at 4700, 10700 and 18900 h into the test, but with a return to normal operation within a day. These points are visible in Figure A, as being the times when the stack-voltage drops to zero. Based on the average voltage of the stack during electrolysis, the first 2700 h features a “run-in” period with prominent degradation; where the voltage increases from 81.7 V to 85.2 V, which corresponds to a degradation rate of 37 mΩcm 2 /kh. From there on the voltage increases up to 86.8 V at 15000 h, after which the stack restores its capability until the resulting average voltage of 85.6, which gives a crude estimate of a degradation rate from point to point of just 0.7 mΩcm 2 /kh. The re-activation of the stack is coinciding with two occurrences: the exchange of a carbon-filter on our air gas-line, and a small tuning of the AC:DC parameters to avoid extreme min/max overpotential values, when shifting the polarization. The impedance measurements were conducted every 500-2000 h of test-time (every time, when the stack-voltage is at ~61 V on Figure A). Figure B shows the latest recorded data-point of EIS for cell-cluster #2 and the circuit-diagram used for fitting the EIS. Figure C shows an excerpt of the EIS analysis – the simulated resistances of the different elements annotated with the summit-frequencies of their respective RQ-element, for cell cluster #2 and from the EIS measured in 8% H2 in steam and high flow of air. Most notably on the EIS, it is seen that decreasing H 2 -content increases the low-frequency arc, resulting from a higher gas-conversion resistance[5]; and an increased ohmic resistance R s when decreasing the air-flow in OCV, arising from thermally deactivated lesser ionic conductivity when the outlet-temperature is 6-8 °C lower. The equivalent circuit used for modeling the impedance spectra is based on the results from DRT (plots not shown) modeling of the impedance spectra: five clear peaks with peak frequencies of 0.8, 5.4, 350, 6700 and 43000 Hz, suggests modeling the impedance spectra with a circuit of five RQ-elements, an ohmic resistance R s and an inductance element L a . The majority of the impedance is made up of a Z’-offset R s , a high-frequency arc R 2 and a low frequency arc R 4 , with the rest of R n fine-tuning the actual curve of these main-features, see Fig. B Nyquist plot. Within the first 1500 h, R s and R 2 Electrolysis ; Stack (abstract data type) ; High-temperature electrolysis ; Polarization (electrochemistry) ; Electrode ; Voltage ; Fuel cells Advancements in Solid Oxide Fuel Cells ; Hybrid Renewable Energy Systems ; Catalysts for Methane Reforming