Publication date: 10th April 2024
Solid oxide electrolysis is part of a potential solution to decarbonise the chemical, aviation and shipping industries via the production of green fuels. However, excellent electrochemical performance of the cells is a critical factor for the commercial success of the technology. In this work, industrially-relevant fuel electrode-supported solid oxide cells have been tested to investigate the effect of the reduction profile on initial electrochemical performance. The reduction profile refers to the operating conditions at which NiO is reduced to Ni in the fuel electrode of the cell. The reduction profile is believed to play a role in determining the initial microstructure of the cell, which in turn is known to be an important predictor of electrochemical performance. Electrochemical characterisation was performed on cells reduced at three different reduction temperatures (700˚C, 800˚C, 900˚C) and under a range of different gas atmospheres and flow rates. Microstructural analysis of the tested cells was conducted by scanning electron microscopy (SEM).
For each test, quantitative analysis of electrode resistance contributions was carried out using complex non-linear least-squares (CNLS) fitting of electrochemical impedance spectra (EIS) to an equivalent circuit model (ECM). Distribution of Relaxation Time (DRT) plots were used to estimate the electrode process frequency ranges which were provided as inputs to the CNLS fitting. It was concluded that a reduction temperature of 800˚C gives best initial performance for the cells studied, but that an acceptable range of reduction temperature exists between 700˚C and 800˚C. Increasing the steam partial pressure (from 0% to 25% steam) in the reducing gas flow was determined to be detrimental to initial performance. In each of these studies, performance differences were relatively small – on the order of 10% between the highest and lowest performing cells. This leads to the overarching conclusion that while the reduction profile does have a measurable effect on the initial electrochemical performance of the cell, it is not the most crucial process for determining cell performance – as long as the reduction occurs within an acceptable window of operating conditions.
Special thanks go to Henrik Henriksen and Francesco Mondi, whose assistance and expertise in the laboratory made this work possible. A thanks is also due to Mikkel Agerbro Essendrop and Niels-Gabriel Gaillard for their diligent work on microstructural analysis. Lastly, thanks to all of my colleagues at both Topsoe A/S and DTU Energy who have made doing this research a truly enriching experience.
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