Speaker
Malgorzata Molin
(Technical University of Denmark)
Description
The Solid Oxide Fuel Cells (SOFCs) are used for a direct conversion of chemical energy into electrical energy and are composed of three layers: a porous anode, a solid electrolyte and a porous cathode. The greatest market entry barrier of the SOFCs is their durability, and it has been shown [1] that their degradation is caused mainly by processes taking place in anode. A common anode material is Ni – YSZ (yttria stabilized zirconia) cermet. During the initial operation of the SOFC, nickel oxide particles are reduced to pure nickel, which determine the porous microstructure of anode and anode support. This microstructure determine the mechanical and electrochemical properties of the fuel cell [1], [2], which have direct influence on the performance and durability of the SOFC.
Simultaneous exposure to reducing atmosphere and external stress leads to accelerated creep, which changes the stress field in the SOFC [3]. This phenomenon is not fully understood, and the best way to explain it is simultaneous in-situ observation of phase transition and stress field development in NiO-YSZ layers. The best method for such measurements is energy, i. e. wavelength resolved radiography.
We present the results of Bragg edge neutron imaging experiments on SOFC anode supports performed ex-situ at different facilities (ISIS in UK, HZB in Germany, J-PARC in Japan). The anode supports were reduced under different temperatures in different times and under different values of stress, which was applied in direction causing bending of the sample (thus one side of the sample was under compression, and the opposite side was under tensile stress). We observed different content of Ni and NiO phases in different areas of the samples. In particular, we could resolve areas of thickness 200 µm in layers of thickness 1 mm, which allow us to see the gradient of reduced phase amount within one layer. With such resolution we were able to compare phase amount at different sides, which was crucial for the samples reduced under nonsymmetrical stress. We have shown that compressive stress accelerates the reduction rate, since we observed higher amount of reduced phase on the compressed side than on the opposite side. Moreover, it was possible to resolve the edges characteristic for NiO and Ni already even after 2 min of exposure, while the reduction process takes about 1 hour. Therefore, we can conclude that the achievable spatial and time resolution is sufficient to conduct in-situ measurements, which will be the next step in this research project.
[1] a. Hauch, S. D. Ebbesen, S. H. Jensen, and M. Mogensen, “Solid Oxide Electrolysis Cells: Microstructure and Degradation of the Ni/Yttria-Stabilized Zirconia Electrode,” J. Electrochem. Soc., vol. 155, no. 11, p. B1184, 2008.
[2] P. T. Moseley, A. Hauch, P. S. Jørgensen, K. Brodersen, and M. Mogensen, “Ni/YSZ anode – Effect of pre-treatments on cell degradation and microstructures,” J. Power Sources, vol. 196, no. 21, pp. 8931–8941, 2011.
[3] P. V. H. Henrik Lund Frandsen*, Fabio Greco, Declan Curran, Peter Stanley Jørgensen, “Accelerated Creep of SOFC anode supports during reduction.” .
Primary author
Malgorzata Molin
(Technical University of Denmark)
Co-authors
Dr
Anton Tremsin
(University of California at Berkeley)
Dr
Erik M. Lauridsen
(Xnovo Technologies)
Dr
Henrik L. Frandsen
(Technical University of Technology)
Dr
Luise Theil Kuhn
(Technical University of Denmark)
Dr
Markus Strobl
(ESS AB)
Dr
Nikolay Kardjilov
(Helmholtz Zentrum Berlin)
