If you have direct methanol fuel cells
(DMFCs) in mind for your projects,
check out an article in the IEEE Power
Electronic Society's latest newsletter.
"Electrical Dynamic Behavior of a Direct
Methanol Fuel Cell" by M. Ordonez and
others of the Memorial University of Newfoundland reports on a series of empirical
evaluations carried out on a standard
experimental DMFC from Fuel Cell Technologies and relates their test results to a
theoretical fuel-cell equivalent circuit.
The good news is that the fuel cell's
intrinsic capacitance provides excellent
response to transient load changes. The
not so good news is that when the fuel
cell was operated with a switching supply
connected to it, any ripple in the dc supply's output showed a hysteresis effect in
the DMFC's voltage-current characteristic
that consumes power ().
The authors also note that "The theoretical voltage (E) developed by a fuel cell
is given as a function of the methanol
and oxygen feed concentrations by the
Nernst equation. The theoretically predicted value lies around 1.2 V for a single
cell. However, typically, the practical open
circuit output voltage stays below 0.8 V,
and the voltage drops further as current
is drawn from the cell."
In the DMFC model, various losses are
represented by resistances. One, designated Ra, represents activation and
mass-transport losses. The other resistance accounts for ohmic losses. A
capacitance parallels Ra.
"The double layer capacitance is a
characteristic of any interface between
an electron-conducting phase and an
ion-conducting phase. It arises from the
fact that (in the absence of a Faradaic process) charge cannot cross the interface when the potential across it is
changed," the paper says.
"In a PEM [proton exchange membrane]
fuel cell, this interface is the 3D, high surface area interface between the catalyst
particles in the electrode and the Nafion
solid electrolyte... The capacitance of a
fuel cell is thus typically 10-30 µF per cm2
of MEA [membrane electrode assembly],"
it adds. The capacitances of the anode
and cathode act in series, which is why it's
called a "double-layer" capacitance.
Go to http://ewh.ieee.org/soc/pels/pdf/pelsNL0107fnl.pdf to see the article
in full.
Memorial University of Newfoundland
www.mun.ca
Fuel Cell Technologies
www.fuelcelltechnologies.com