electrolyte in the solid oxide
fuel cell is a solid, non-porous metal oxide, usually zirconia (ZrO2)
stabilised with 8-10 mole% yttria (Y2O3). Ionic conduction in the electrolyte
is assured by oxygen ion (O2-)
transport. By using
a solid as the electrolyte, the system is more stable and safe than the
MCFC. No leakage occurs and the cell can be cast into multiple shapes, such as
or monolithic. A normal operating temperature is around 1000°C,
but it is desired to develop cells operating at a reduced
temperature as low as 650°C. This would of course decrease the
conductivity of the presently used solid electrolyte material.
in the MCFC, no precious metals are needed in the electrodes
due to the fast kinetics at these temperatures. Typically, the anode is of
ceramic materials based on cobalt or nickel (Co-ZrO2 or Ni-ZrO2
cermet), and the cathode is either yttria-stabilised
LaSrMnO3 or lanthanide-based perovskite. The electrode reactions are
oxidation of hydrogen:
H2O + 2e-
reduction of oxygen:
the present operating conditions, internal reforming of fossil fuels is possible. The higher temperature
compared to the MCFC reduces the sulphur problem from the
reforming process to a minimum. SOFC's can handle about 5 times the amount
which means that gases produced from coal can be used. Internal partial reforming of the incoming fuel is
also possible. This
leads to a more homogeneous temperature distribution in the cells.
presently three main cell designs:
tubular, developed by Siemens Westinghouse, a heat-exchange integrated system
by Sulzer Hexis and the ordinary planar concept. The advantages of the non-planar cells are
easier sealing techniques and a better heat
exchange concept. In addition to the general high-temperature problem of
material degradation, both aspects represent challenges for the planar development.
Heat-Exchange Intergrated System SOFC.
The high temperature of the SOFC has its drawbacks.
Thermal expansion mismatches among materials occurs, and sealing between cells is
difficult in the flat-plate configuration. It also places severe constraints on materials
selection and results in difficult
fabrication processes for interconnections (bipolar
plates) and seals. The interconnections must be electrically conductive, seal the anode and
cathode gas chambers and withstand the high temperature. Suitable sealants
are even more difficult to develop. Most common now are glass materials. Due to the high operating temperature, the start-up and shut-down procedures are very time-consuming. SOFC and MCFC are therefore not
suited for dynamic operation. Combined with heat turbines, the same overall efficiency as
in the MCFC can be reached.
Westinghouse 100 kW SOFC.
100 kW SOFC system components.
oxide fuel cells are most suited for stationary
applications. Siemens Westinghouse is, together with Sulzer-Hexis, one of
the leading actors. Even though the SOFC is the fuel cell with
the longest continuous development period, starting in
the late 1950's, several years before the AFC, there are still many
material issues to be solved. The
development of low-cost materials (especially interconnections) with high durability at high
temperatures is the key technological challenge.