SwRI IRD 2000--Pulsatile Fuel Cell Operation, 03-9049

Background - Proton Exchange Membrane Fuel Cells (PEMFCs) often exhibit unsteady output, and many attempts have been made in the past four decades to exploit surges in hopes of boosting output. Though none of these attempts has resulted in an overall increase in power output, investigations into methods of generating surges have resulted in an increased understanding of how a cell may be better matched to loads that require a pulsatile electrical supply rather than drawing power steadily.

Approach - This project underwent several transformations as new information surfaced from the project itself and from outside sources. The original concept was to vary the reactant delivery pressures and to use "load chopping" to produce and exploit power surges during transient operation. Later, an internal control electrode was devised to enable the reactant concentrations to be varied within the polymer electrolyte in a manner analogous to the pressure variation method. Observations of the side effects of the internal electrode then suggested additional experiments aimed at developing a PEMFC version that can deliver high momentary output far in excess of the steady-state capacity.

Accomplishments - Several new modes of PEMFC operation have been identified, and four patents are being pursued as a result of the interposed control electrode experiments. First, the use of an interposed control electrode allows a unique and unprecedented control of the catalytic action of the membrane electrode assemblies, to the extent that the catalyst can be deactived or purged of contaminants at will. This control may even extend to actually reversing the polarity of the cell, which may ultimately make the use of mixed fuel and oxidant on both electrodes possible, so that the cell may be operated in either direction. This capability approaches alternating current operation, at least for small cells.

Second, the metallic layer within the membrane used to create the control electrode has been shown to operate as a "getter" for hydrogen, thus creating an internal reactant plenum that allows momentary high output surges. This effect, termed "pseudo-capacitance," not only renders the cell capable of generating strong pulses on demand, but effectively makes the cell capable of operating in a regenerative mode, electrolyzing water to hydrogen and oxygen within the membrane. This capability allows the cell to receive electrical energy in applications where it may be stored for later use, as in regenerative braking schemes in electric vehicles.

Third, the interposed electrode has been shown to exhibit electrical characteristics that make it suitable for a membrane hydration sensor for closed-loop control of membrane hydration. This asset is valuable in circumstances in which high power densities are contemplated, as membrane hydration must be controlled more precisely as power densities increase.

Finally, the use of multiple interposed layers has been shown to be additive for pseudo-capacitance, and the possibility exists that the "getter" function may be exploited to allow "edgewise" fueling, wherein fuel is not presented to the anode, but to the interior of the membrane itself. This capability could result in fuel cells that are "trickle-charged" from hydrogen or proton sources such as biological ferments. The enhanced hydrogen plenum may also be used to fuel the cell intermittently, so that, for low-power applications, a separate hydrogen storage is not required.

An experimental apparatus and circuit block arrangement for electrochemical impedance characterization of the three-electrode membrane electrode assembly fuel cell are illustrated.