Microstructure characterisation of solid oxide electrolysis cells operated at high current density

High temperature solid oxide cells can be operated either as fuel cells or electrolysis cells for efficient power generation or production of hydrogen from steam or synthesis gas (H2 + CO) from steam and CO2 respectively. When operated under harsh conditions, they often exhibit microstructural degradation of cell components in relation to the loss of electrochemical performance specific to the mode of operation.

Thus descriptive microstructure characterization methods are required in combination with electrochemical characterization methods to decipher degradation mechanisms. In the present work, microstructure evolution of the Ni-yttria stabilized zirconia (YSZ) is followed as a function of galvanostatic steam electrolysis testing at current densities between -0.5 and -1.0 A cm-2 for periods of up to 750 hours at 800 °C.

The volume fraction and size of the percolating Ni particles was statistically quantified using the mean linear intercept method as a function of current density and correlated to increases in serial resistance. The above structural changes are then compared in terms of electrode degradation observed during the co-electrolysis of steam and CO2 at current densities up to -1.5 A cm-2.

In this study, the formation of ZrO2 based nano particles at the Ni-pore interface is responsible for the loss of Ni-YSZ particle contact and thus loss of triple phase boundary. Formation of similar nano particles at Ni internal grain boundaries is also thought to be responsible for loss of Ni percolation.