Page 285 - Biofuels Refining and Performance

264 Chapter Nine

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density in the range of 100–120 mA/cm . Studies have shown that DMFC
efficiency decreases with increasing methanol concentration. Therefore,
operating a fuel cell to maintain the maximum efficiency needs close con-
trol of methanol concentration and temperature. An online concentra-
tion sensor is used in the feedback loop for this purpose. Some of the
advantages of this system, relative to the hydrogen systems, are that
the liquid feed (methanol) helps in attaining the uniform stack tem-
perature and maintenance of membrane humidity; it is also easy to
refill since the fuel (methanol) is in liquid form.
As compared to the PEMFC, the DMFC has a very sluggish electro-
chemical reaction (significant activation over voltage) at the anode. It
therefore requires a high surface area of 50:50% Pt-Ru (a more expen-
sive bimetal) alloy as the anode catalyst to overcome the sluggish reac-
tion and an increase in catalyst loading of more than 10 times that for
the PEMFC. Even then, the output voltage on the load is only 0.2–0.4 V
with an efficiency of about 40% at operating temperatures between 60 C
and 90 C. This is relatively low, and therefore, the DMFC is attractive
only for tiny to small-sized applications (cellular phones, laptops, etc.)
[17]. Another potential application for the DMFC is in transport vehi-
cles; as it operates on liquid fuels, it would greatly simplify the onboard
system as well as the infrastructure needed to supply fuel to passenger
cars and commercial fleets and can create a large potential market for
commercialization of fuel cell technology in vehicle applications.

9.3.3 Alkaline-electrolyte fuel cells (AFCs)
Alkaline-electrolyte fuel cells (see Fig. 9.7) are one of the most developed
fuel cell technologies. They have been in use since the mid-1960s for
Apollo and space shuttle programs [3, 6, 18, 19]. The AFCs onboard
these spacecraft provide electrical power as well as drinking water.
AFCs are among the most efficient electricity-generating fuel cells with
an efficiency of nearly 70%. The electrolyte used in the AFC is an alka-

line solution in which an OH ion can move freely across the electrolyte.
Electrochemistry of AFCs. The electrolyte used in the AFC is an aqueous
(water-based) solution of potassium hydroxide (KOH) retained in a
porous stabilized matrix. The concentration of KOH can be varied with
the fuel cell operating temperature, which ranges from 65 to 220 C.

The charge carrier for an AFC is the hydroxyl ion (OH ) that migrates
from the cathode to the anode, where they react with hydrogen to pro-
duce water and electrons. Water formed at the anode migrates back to
the cathode to regenerate hydroxyl ions.

Anode reaction: 2H 4OH → 4H O 4e
2
2

Cathode reaction: O 2H O 4e → 4OH
2
2