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 (see the figure).
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.