Development of highly efficient solid oxide electrolyzer cell systems

Jakob Dragsbæk Duhn

Research output: Book/ReportPh.D. thesis

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Abstract

Solid oxide electrolyzer cells (SOEC) are electrochemical devices capable of converting H2O or CO2 to H2 and CO, respectively. In this thesis, the possibility of production of CO with SOECs are investigated, since it is currently of commercial interest, but could also play a major role in the future energy system.
The overall objective of the thesis was to investigate the limits for the allowed CO concentration during electrolysis of CO2 in SOECs and how the limit could be increased. A high CO concentration is desired because it lowers the strain on the separation unit and amount of recycle, when SOECs are used in systems like Haldor Topsoe A/S’s “eCOs”. In this way, the overall eÿciency of SOEC systems are increased. The CO concentration was limited by carbon formation via the Boudouard reaction, a non-uniform flow in the fuel channels over the fuel electrode, and the di˙usion in the fuel electrode. The thesis has focused on obtaining knowledge on the limiting causes, through both experimental work and modeling.
The formation of carbon leads to delamination of the cell layers and ultimately destroys the cells. It was found that the carbon formation was hindered, when the Boudouard reaction was thermodynamically unfavored. From thermo-gravimetry analyzer experiments, the thermodynamics for the equilibrium for the Boudouard reaction was obtained for Ni-YSZ material used in SOECs.
It is important that the flow distribution in the cell is as uniform as possible, since non-uniformities will lead to regions with flow rate and therefore high CO concentration. The flow distribution in the fuel channels over the fuel electrode was investigated with computational fluid dynamics (CFD) modeling. The flow was optimized by changing the cell inlet geometry and an increase in the flow uniformity was observed. The impact on the allowed average (exit) CO concentration was quite high (from 22 to 32 %).
The effective diffusion in the fuel electrode was investigated with a Wicke-Kallenbach set up. Combined with measurements of the thickness, porosity and pore size, the tortuosity of the material was calculated. This made it possible to implement the di˙usion limitation in an in-house 3d stack model and evaluate the impact of di˙usion limitations (only included in the confidential version of the thesis). The local CO concentration is limited by the thermodynamics for the Boudouard reaction. The diffusion cause an increase in the CO concentration at the reaction sites, compared to the channel, and diffusion therefore limits the allowed CO concentration in the fuel channel. The diffusion was found to be low, resulting in large concentration gradients between the channel and reaction sites. This mean that the allowed CO concentration in the fuel channel is very limited by diffusion.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherTechnical University of Denmark
Number of pages179
Publication statusPublished - 2017

Cite this

Duhn, J. D. (2017). Development of highly efficient solid oxide electrolyzer cell systems. Technical University of Denmark.