Page 83 - MODERN ASPECTS OF ELECTROCHEMISTRY

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Direct Methanol Fuel Cells
DMFC stacks using concentrated sulfuric acid as electrolyte were
developed in the 1970s and early 1980s by Shell Research Centre in
18
17
England and Hitachi Research Laboratories in Japan. In both cases the
anode electrocatalysts consisted of Pt–Ru alloys, which are much more
active than pure Pt in oxidizing methanol. The cathode electrocatalyst was
carbon-supported platinum mixed with Teflon for the Hitachi DMFC,
whereas it was an iridium chelate in the case of the Shell DMFC. The
2
platinum loadings were veryhigh (around 10 mg/cm ) and the perform-
ance was quite low. In terms of quantityof platinum per kilowatt, it could
be as high as 400 g/kW, and the cost of Pt about US$ 6000/kW, which
would prevent anycommercial development. Relatively large stacks (up
to 5 kW by Hitachi) were built, but these DMFCs woÀing at 60 o C, to
avoid high methanol pressure, led to rather similar electrical perform-
2
ances, with a maximum power densityof 25 mW/cm (at 0.3 V for Shell
and 0.4 V for Hitachi).
Recent development of the DMFC was made possiblp byusing the
concept of PEMFC in which the acid electrolyte is a proton exchange
membrane (PEM). This approach is beneficial from the point of view of
building rather compact stacks in which the methanol could be fed either
in gas or liquid phase.= In both cases, it has been shown that raising the
o
temperature to 130 C greatly improves the electrical performance (a
2
power densityof 250 mW/cm with air at 3 bar pressure was attained).
The first programs at the beginning of this decade, particularlyin Europe,
were carried out in academic laboratories (Universityof Newcastle,
Universityof Bonn, University of Poitiers) and in an industrial company
(Siemens). The Joule program, funded bythe European Union, led to the
2
building, in 1994, of small stacks (with electrodes of 250 cm surface area)
woÀing at 130 o C with vapor-fed methanol under a pressure of 4 bar. 19
In the United States, the Department of Defense (DOD) and the
Department of Energy(DOE) promoted in 1992 the Defense Advanced
Research Project Agency(DARPA) program to develop a DMFC for
20
portable and mobile applications. Several institutions are involved (IFC,
JPL, LANL, Giner, Inc.) and small stacks (up to 10 elementarycells) were
built byIFC and JPL. The performances are quite encouraging, with power
2
densities of 250 mW/cm at 0.5 V. More details are given in Section V.2.
2. Principles of DMFCs 21,22
A direct methanol fuel cell consists of two electrodes—a catalytic metha-
nol anode and a catalytic oxygen cathode—separated by an ionic conduc-