Scanning Electron Microscopy

The performance of solid oxide cells is highly dependent on the triple phase boundaries. An increase in the density of blocked three phase boundaries in an electrode structure leads to an increase in
the resistance of the cell and therefore a decrease in the performance. There are several degradation mechanisms that can influence the three phase boundary. Some of the typical degradation
mechanisms for solid oxide cells are: 

  • Internal or external impurities
  • Sintering of the Ni particles
  • Mobilization of Ni particles
  • Oxygen bobbles in the interphase
  • Delamination of cells

The degradation mechanisms can be investigated by use of electron microscopy.

Percolation of Ni particles

By use of low-voltage scanning electron microscopy and charge contrast it is possible to visualize the fraction of electronically conducting phase that is interconnected and thereby contributing to the electronic conduction within the electrodes [1]. us, the percolating and non-percolating Ni particles can be distinguished as shown in Figure 1.

 

Figure 1: A SEM image of the active anode layer showing the percolating and non-percolating Ni particles.

Difference in the percolation of the Ni particles can lead to a dramatic difference in the performance of the solid oxide cells. Low-voltage SEM and charge contrast is a fast, useful and simple imaging technique to visualize of the electronic conduction paths in tested solid oxide fuel cell (SOFC) electrodes. It can be used as a complementary technique to impedance measurements as it is able to visually pinpoint where the charges are taking place. Insights to gain from the percolation investigation:

  • Visualization of the electronic
    conducting path in the cell
  • Development potential for the
    electrode
  • The impact on the percolation when
    the load pattern changes
  • Delamination of cells

Silica impurities

A rapid decrease in performance in accelerated test of SOFC has been observed. the impedance spectroscopy obtained during operation shows that the Ni/YSZ electrode is passivized. The post mortem analysis performed by SEM and EDS of the cells reveals the reason for the observed passivation [2]. SEM/EDS clearly showed that silica-containing impurities have segregated to the fuel electrode/electrolyte interface during the fuel cell testing. Thus, leading to a decrease in the Ni percolation. The results are shown in figure 2.

 

Figure 2: Boundary between a non-percolating Ni and a YSZ particle. The non-percolation Ni particle could be caused by an increase in the Si concentration between the particles. The increase in the Si concentration is detected with an EDS line scan.

O2 bobbles in the interphase

Increasing the current density (above -1.0 A/cm2) in a solid oxide electrolysis cell (SOEC) leads to an increase in the polarization losses over the electrode and the electrolyte, encouraging formation of O2
bobbles at the interphase between the electrolyte and the O2 electrode [3]. This phenomena is schematic illustrated in figure 3, can be elucidated by impedance spectroscopy and scanning electron microscopy.

  

Figure 3: Schematic drawing of electrolysis cell. The experiment conditions leads to O2 formation at the interphase.

References

[1] K. ydén, Y. L. Liu and J. B. Bilde-Sørensen, Solid state Ionics. (2008) 178, 1984-1989.
[2] A. Hauch, S. H. Jensen, J. B. Bilde-Sørensen, M. Morgensen J. of Electrochem. Soc. (2007) 154, A619-A626
[3] R. Knibbe, M. L. Traulsen, A. Hauch, S. D. Ebbesen, M. Morgensen J of Electrochem. Soc. (2010) 157, B1209-B1217.

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24 JUNE 2019