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DEGRADATION MECHANISMS IN SOLID OXIDE ELECTROLYSIS ANODES: Cr POISONING AND CATION INTERDIFFUSION
Introduction
Nuclear fission energy is one of the promising sources for clean energy at a large scale. In addition to
electricity production, nuclear plants can also be used for the production of alternate transportation
fuels such as hydrogen gas. High temperature steam electrolysis is one of the most efficient
electrochemical processes for hydrogen generation from water with no CO 2 emissions using electricity
and heat from nuclear plants (Yildiz, 2006, 2006a). It is carried out in devices called Solid Oxide
Electrolytic Cells (SOEC) at high temperatures.
An electrolytic cell consists of the hydrogen electrode (cathode) and the oxygen electrode (anode),
with an oxygen ion-conducting electrolyte sandwiched between them. The electrodes are electrically
coupled to an electricity source by external lines outside the cell. Figure 1 shows a schematic of one
such SOEC. Oxygen passes over the anode, and hydrogen and steam over the cathode. At the cathode,
two water molecules accept four electrons to form hydrogen gas and oxide ions. The oxide ions then
move through the electrolyte and go to the positive electrode (anode) where they lose electrons and
form oxygen gas. Cells are stacked together, and metallic interconnects are used between cells. Cell
interconnects provide cell-to-cell electrical connection and separate fuel and oxidant gas atmospheres
in a stack.
Figure 1: Representation of a SOEC
The electrodes for SOEC have to possess certain properties important for their proper functioning,
including high electronic conductivity, high catalytic activity and compatibility with other cell
components. The doped ABO 3 perovskite family of compounds (A = lanthanide group, B = transition
metal group) is the most common set of materials that is used as oxygen electrodes in SOEC inspired
from SOFC. Sr-doped lanthanum manganites (LSM) are conventionally put to use as oxygen electrodes.
Like the oxygen electrode, the hydrogen electrode must combine catalytic activity with electrical and
ionic conductivity. Ionic conductivity allows the cathode to spread the oxide ions across a broader
region of electrode/electrolyte interface and electronic conductivity ensures the transport of electrons
required for the electrode reaction from the external circuit. The hydrogen electrode generally consists
of a nickel-zirconia cermet electrode. Besides, a perovskite bond layer is used to provide better contact
between the oxygen electrode and the metallic interconnects. Zirconia-based electrolytes, possessing
the fluorite structure, have been the most popular choice for the electrolyte material.
SOEC operate on the same basic principle as that of solid oxide fuel cells (SOFC), though the
current flow is in the opposite direction. Though SOFC have been a subject of detailed study for a long
time, a keen interest in SOEC has only been generated in the past few years. Hence SOEC are not yet
as well understood as SOFC, though the main challenges are expected to be similar for both since they
are based on similar mechanisms. It has been observed that over a long period of operation, the SOFC
show a loss in performance (Virkar, 2007). This loss is attributed to degradation of the cells. This
degradation is principally due to the chemical interaction between the electrolyte and the electrodes at
the electrolyte/electrode interface at the high temperatures of operation, which leads to the formation
140 NUCLEAR PRODUCTION OF HYDROGEN – © OECD/NEA 2010