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The ongoing green energy transition increases the need for dynamic and efficient Power-to-X (PtX) systems to convert wind and solar power to green fuels and chemicals. Amongst the different electrochemical conversion technologies, solid oxide electrolysis (SOE) technology offers the highest energy conversion efficiency. However, operation below thermo-neutral potential, which is a likely event due to the fluctuations in power, creates thermal variations and thermomechanical stress. This, combined with degradation mechanisms such as Ni migration and electrode poisoning, limits the SOE lifetime and impedes commercialization of the technology. Here, we present a novel operating method that alleviates temperature variations under fluctuating power and reduces SOE degradation. The operation method relies on operating the SOE stack with a mix of alternating current (AC) and direct current (DC) and is therefore referred to as AC:DC operation. AC:DC operation establishes a flat thermal profile across the SOE, by switching several times per second between electrolysis mode and fuel cell mode, the thermal mass of the SOE preventing temperature fluctuations during the short switching period.Further, for steam electrolysis Si-based impurities can be desorbed from the fuel electrode via formation of Si(OH)4 during the pulses in fuel cell mode. Since Si-impurities are considered a precursor for Ni-migration, AC:DC operation helps to suppress Ni-migration. In this presentation we present experimental results and impedance measurements from a 5000+ hour 70-cell SOE stack test, with a remarkably low degradation rate.

The dataset contains all the data used for this paper as well as for the supplementary information. Data is labeled with the figure number-letter and sample type. All the figures related to the nano-XRF data (Fig4, FigS1-4) were obtained from the same dataset, shared for Fig4 and FigS1.

Solid Oxide Cells (SOCs) are under intensive development due to their great potential to meet the 2030 targets for decarbonization. One of their advantages is that they can work in reversible mode. However, in respect to durability, there are still some technical challenges. Although the quick development of experimental and modeling approaches gives insight into degradation mechanisms, an obligatory step that cannot be avoided is the performance of long-term tests. Taking into account the target for a commercial lifetime is 80,000 h, experiments lasting years are not acceptable for market needs. This work aims to develop accelerated stress tests (ASTs) for SOCs by the artificial aging of the fuel electrode via redox cycling, which follows the degradation processes of calendar aging (Ni coarsening and migration). However, it can cause irreversible damage by the formation of cracks at the interface anode/electrolyte. The advantages of the developed procedure are that it offers a mild level of oxidation, which can be governed and regulated by the direct impedance monitoring of the Ni network resistance changes during oxidation/reduction on a bare anode sample. Once the redox cycling conditions are fixed and the anode/electrolyte sample is checked for cracks, the procedure is introduced for the AST in full-cell configuration. The developed methodology is evaluated by a comparative analysis of current voltage and impedance measurements of pristine, artificially aged, and calendar-aged button cells, combined with microstructural characterization of their anodes. It can be applied in both fuel cell and electrolyzer mode. The results obtained in this study from the electrochemical tests show that the artificially aged experimental cell corresponds to at least 3500 h of nominal operation. The number of hours is much bigger in respect to the microstructural aging of the anode. Taking into consideration that the duration of the performed 20 redox cycles is about 50 to 60 working hours, the acceleration factor in respect to experimental timing is estimated to be higher than 60, without any damaging of the sample. This result shows that the selected approach is very promising for a large decrease in testing times for SOCs.

We investigate the influence of position, under large circular sputtering targets, on the final electrochemical performance of 35 mm diameter button solid oxide fuel cells with sputter-deposited Gadolinium doped Ceria barrier layers, positioned in order to almost cover the entirety of the area associated with a 120 × 80 mm2 industrial cell. We compare the results obtained via structural and morphological analysis to the Electrochemical Impedance Spectroscopy (EIS) measurements performed on the button cells, disentangling the role of different parameters. The Atomic Force Microscopy analysis makes it possible to observe a decrease in the roughness values from the peripheral to the central zones under the sputtering target, with peak-to-valley roughness values, respectively, decreasing from 380 nm to 300 nm, while Scanning Electron Microscopy and Energy Dispersive Spectroscopy show a dependence of the layer coverage from the position. The electrochemical performances of button cells with buffer layers of only 200 nm in thickness, and with negligible thickness gradients across them, show current density values of up to 478 mA/cm2 at 0.8 V and 650 °C, with an improvement of more than 67% with respect to button cells with standard (screen printed) buffer layers. These results point out the major influence exerted by parameters such as the thickness gradient and the coverage of the sputtered buffer layers in determining the final electrochemical performances.

Dynamische Messung an SOFC Stack