Environmental Engineering Reference
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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.
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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