Environmental Engineering Reference
In-Depth Information
with the NiO cathode is that NiO particles grow as they creep into the molten
carbonate melt, which reduces the active surface area and can short-circuit
the cell. A solution to this problem is to add small amounts of Mg metal to
the cathode and the electrolyte for stability [34].
The electrolyte for MCFCs is a molten carbonate that is stabilized by an
alumina-based matrix [37]. Initially, Li
2
CO
3
/K
2
CO
3
(Li/K) carbonate materi-
als were used as electrolytes, but they tend to degrade [38]. Subsequently,
an alumina phase or ceria-based materials were added to stabilize the elec-
trolyte [4, 5].
9.2.5 Solid Oxide Fuel Cell (SOFC)
SOFCs employ a solid oxide material as electrolyte and are, thus, more stable
than the molten carbonate fuel cells as no leakage problems due to a liquid
electrolyte can occur [39, 40]. Due to the high operating temperature (700-
1000°C), a wide variety of fuels can be processed. Anodes for SOFC are also
based on Ni, and usually Ni cermet materials are used, which are more stable
than plain Ni metal. A NiO powder mixed with a YSZ powder together with
a resin binder produces an anode functional layer onto which the YSZ elec-
trolyte can be deposited and sintered [41, 42].
From the beginning of SOFC development, it was found that LaSrMnO
3
(LSM) electrodes had a high activity for oxygen reduction at high tempera-
tures and were stable under SOFC operation conditions [43]. These LSM
cathodes have been improved over time and particularly yttria-stabilization
of the cathodes improves the performance. Perovskite-type materials have
been investigated as cathodes for SOFCs as well [44]. Lanthanide-based
perovskites showed a high conductivity and a high catalytic activity for
oxygen reduction. For electrolytes, ZrO
2
- and CeO
2
-based electrolytes have
been found to be stable and afford reasonable conductivity.
9.3 CATALYSTS FOR OXYGEN REDUCTION REACTION
OF FUEL CELLS
As we mentioned above, oxygen reduction reaction is an important half reac-
tion in the fuel cells, and it decide the efficiency, power density durability
and cost of the fuel cells [45]. In a classic PEMFC, the catalyst phase is
contacted with a porous carbon layer that conducts the electronic current and
allows gas diffusion to the catalyst/membrane interface and liquid water to
be extracted from the catalyst layer, and the oxygen reduction reaction is
the rate-determining step. Noble metals such as Pt, Pd, and Au, are mainly
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