Hydrogen Utilization: Fuel Cells - Hydrogen Generation, Storage, and Utilization - page 159

it has improved significantly in stability, performance, and cost [29]. These
characteristics have made the PAFC a good candidate for early stationary
applications [30]. In the operating range of 150-200°C, the expelled water
can be converted to steam for air and water heating (combined heat and
power). This potentially allows an efficiency increase of up to 70% [31]. P
AFC is CO 2 -tolerant ,and even can tolerate a CO concentration of about
1.5%, which broadens the choice of fuels that can be used. If gasoline is
used, sulfur must be removed. At lower temperatures, phosphoric acid is a
poor ionic conductor, and CO poisoning of the Pt electro-catalyst in the
anode becomes severe. However, PAFC is much less sensitive to CO than
PEFCs and AFCs. The disadvantage of PAFC is its rather low power density
and aggressive electrolyte [30]. The electrodes used in PAFCs are generally
Pt-based catalysts dispersed on a carbon-based support. For the cathode, a
relatively high loading of Pt is necessary for the promotion of the O 2 reduc-
tion reaction. The hydrogen oxidation reaction at the anode occurs readily
over a Pt/C catalyst. The electrolyte of PAFC is highly concentrated or pure
liquid phosphoric acid (H 3 PO 4 ) saturated in a silicon carbide matrix (SiC)
[32].

9.2.4 Molten Carbonate Fuel Cell (MCFC)

The development of MCFCs started about mid-twentieth century [17]. An
advantage of the MCFC is the possibility to allow for internal reforming due
to the high operating temperatures (600-700°C) and to use the waste heat in
combined cycle power plants [33]. A molten alkali carbonate mixture is
retained in a porous lithium aluminate matrix [34]. At the cathode, oxygen
reacts with carbon dioxide and electrons to form carbonate ions:

− 2 −

O + CO + → 2 e CO .

(9.10)

1 2

2 2 3

The carbonate ions flow through the electrolyte matrix from cathode to
anode. At the anode, the carbonate ions are consumed by the oxidation of
hydrogen to form steam and carbon dioxide, releasing electrons to the exter-
nal circuit:

2 − −

H + CO → H O CO + + 2 e .

(9.11)

2 3 2 2

Because Ni metal anodes are not stable enough under the MCFC operating
conditions, Ni-Al or Ni-Cr metal alloys have been employed as MCFC
anodes [35, 36]. For cathodes, NiO is active enough for oxygen reduction at
high temperatures so that a Pt-based metal is not necessary. One problem