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CAUSES OF DEGRADATION IN A SOLID OXIDE ELECTROLYSIS STACK
Introduction
Hydrogen is considered an alternative transportation fuel because its high gravimetric energy density
and its reaction product is water, a clean alternative to carbon oxides from fossil fuels. Although
hydrogen is the most abundant element, its diatomic gas cannot be mined or captured terrestrially.
Hydrogen gas is derived mainly from reforming fossil fuels or by the electrolysis of water. A High
Temperature Steam Electrolysis (HTSE) process based on Solid Oxide Fuel Cell (SOFC) technology
(Baukal, 1976; Doenitz, 1982; Salzano, 1985; Hino, 2004) and utilising electricity and waste heat
is presently being developed by the US Department of Energy’s Nuclear Hydrogen Initiative
(Herring, 2007).
In the HTSE process, multiple solid oxide cells are arranged in “stacks”, identical to stacks used in
SOFC systems, but unlike a SOFC, heat and a positive potential are applied to the stack to reduce the
steam to hydrogen; these being called Solid Oxide Electrolysis Cells (SOEC). Stacks of cells provide for
a manageable voltage/current input as well as a manufacturable cell size. However, stacks necessarily
have an increased cell area and a multiplicity of components that introduce a number of sources of
degradation that take place during operation. It has been observed that SOEC stacks show an
unusually large degradation rate compared to a SOFC stack (Mawdsley, 2009).
Argonne National Laboratory has analysed post-test, several cells and stack components run as
SOEC to determine the major causes of degradation. Resistance and elemental maps of stack component
surfaces where initially created using respectively, a four-point electrical probe and X-ray fluorescence
measurements at Argonne’s Advanced Photon Source. In addition, Raman micro-spectroscopy and
X-ray absorption spectroscopy were utilised to determine phase structure and oxidation state of the
main constituents and elements of stack components. These maps were used to identify regions of
probable degradation. From this information, stack components were cross-sectioned in the areas
of interest and analysed by scanning electron microscopy (SEM) and energy dispersive X-ray
spectroscopy (EDS). Results of the SEM/EDS work are presented here.
Experimental
Cells and stacks were fabricated by Ceramatec, Inc. (Salt Lake City), and tested at Ceramatec and INL.
After testing, the stacks were disassembled and several repeat units (cells, interconnects and gas flow
fields) were sent to Argonne for evaluation. Stack components from the following tests were
evaluated: i) a ten-cell stack operating for 200 h; ii) a 25-cell stack operating for 1 000 h, both stacks
described by O’Brien (2007); iii) a half-ILS module consisting of two 60-cell stacks, operating for 2 040 h
(Hartvigsen, 2007); iv) a ILS module containing four stacks of 60 cells each that operated for 420 h. This
article focuses on SEM and EDS analysis of components from the ½ILS module, including some
observations from other tests.
A typical stack repeat unit consists of a ferritic stainless steel interconnect plate including edge
rails, seals and corrugated gas flow fields attached to each side of the plate (O’Brien, 2007). Figure 1
shows a composite SEM image of the other essential components of a stack repeat unit. Beginning at
the top of the figure and moving down the cross-sections that were analysed are: i) the oxygen side of
the interconnect, including a native oxide layer at the interconnect interface; ii) the bond layer used to
electrically connect the interconnect plate to the corrugated flow field (not shown); iii) another bond
layer to attach the flow field to the oxygen electrode of the cell; iv) strontium-doped manganite
oxygen electrode; v) Scandia-stabilised zirconia electrolyte; vi) nickel zirconia cermet steam/hydrogen
electrode; vii) the bond layer on the steam/hydrogen electrode and the gas flow field, both which are
not shown; viii) nickel bond layer connecting the flow field to the interconnect; ix) the steam/hydrogen
side of the next interconnect plate. A stack repeat unit contains at least nine interfaces between
dissimilar materials.
Maps from electrical resistance and X-ray fluorescence (Mawdsley, 2009) measurements were
used to identify areas which showed probable signs of degradation. With resistance measurements,
we determined that the gas flow fields in the oxygen and steam-hydrogen compartments, as well as
the bond layers connecting the bipolar plate in the steam-hydrogen compartment, were functioning
well and maintained excellent electronic conductivity throughout all the tests. As a result, these were
not considered further. The other stack components were cross-sectioned in regions where
degradation was evident, then they were mounted, polished and examined using SEM and EDS.
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