Direct methanol fuel cells that would function with aerosol feed (instead of all-gas or all-liquid feed) have been proposed. As explained below, aerosol feed would afford the advantages of liquid feed, while reducing or eliminating some of the disadvantageous effects of liquid feed.
The invention of liquid-fed, polymer-electrolyte- membrane (PEM), direct methanol fuel cells (DMFCs) was something of a technological breakthrough; the performances of such fuel cells greatly exceed those of gas-fed methanol fuel cells because (1) the concentrations of methanol in the feed liquids (aqueous solutions of methanol) are much greater than those in the feed gases and (2) the increased concentrations result in increased transfers of mass to anodes.
In a liquid-feed PEM DMFC, methanol is supplied in excess to a catalytic anode on one side of a membrane/electrode assembly. Desirably, all or most of the methanol should diffuse into the anode and partially react. Undesirably, some of methanol diffuses through the polymer-electrolyte membrane from the anode to the cathode — a phenomenon called "methanol crossover." This phenomenon reduces cathode performance and fuel efficiency. It also becomes necessary to use a dilute methanol feed solution to minimize crossover. The depletion of methanol from the already dilute solution adjacent to the catalytic anode surface further reduces performance by increasing the resistance to transfer of mass into the cathode. The use of a dilute methanol solution also tends to result in flooding of the cathode with water; this flooding further reduces performance and leads to loss of water through evaporation.
Aerosol feed was conceived with the major objectives of reducing methanol crossover, increasing the concentration of fuel at the anode, and minimizing accumulation of water at the cathode. "Aerosol" as used here denotes a suspension of droplets of nearly pure methanol in a suitable gas, which could be the CO2 generated at the anode during normal operation. With a suitably controlled aerosol feed, very little methanol would be available at the membrane electrode for crossover, yet the concentration of the little methanol present at the catalytic anode surface would be high, as needed for high performance.
The sizes of the methanol droplets in an aerosol feed would lie between 0.1 and 10 µm. The aerosol feed for a given fuel cell could either be generated by one or more atomizers or equivalent devices located within the fuel cell near the anode, or else generated outside the fuel cell and blown in to the anode. Generating the aerosol outside the fuel cell would be advantageous in that the sizes of the methanol droplets could be controlled to optimize performance.
Control of droplet sizes would be important for the following reasons: During flow to the catalytic anode surface, some droplets of methanol would undesirably coalesce through the combined effects of diversion of the flow by some fuel-cell components, inertial deviation of droplets from deviated flow trajectories, and Brownian motion of the smallest droplets. To preserve the benefits of aerosol feed, it would be preferable to prevent the pores of the catalyst and the backing paper of the anode from becoming saturated with methanol, because if saturation were to occur, the anode would behave as in a liquid-feed cell, and crossover would rise substantially as a result. Minimization of droplet coalescence can prevent saturation. The quantitative measure of the tendency toward coalescence and saturation is called "collection efficiency," and it depends on droplet size and flow speed, as depicted in the upper part of the figure. One could minimize the collection efficiency by use of a droplet-size conditioner in the form of a porous diffuser, electrostatic separator, packed bed, or other device that would filter large droplets out of the flow, as indicated schematically in the lower part of the figure.
It would be necessary to vent some of the aerosol flow to limit the buildup of pressure caused by the generation of CO2. To limit the loss of methanol from such venting, the vent flow would be sent through a filter, membrane, or packed bed, which would cause methanol droplets to coalesce and would collect the coalesced droplets for recycling. However, unlike in a liquid-fed DMFC, there would be no need for a relatively bulky and heavy gas/liquid separator to separate the CO2 from the methanol in the flow downstream of the cathode; consequently, it should be possible to make an aerosol-fed fuel-cell system smaller and less massive than a liquid-fed system of comparable performance.
This work was done by Andrew Kindler, Sekharipuram Narayanan, and Thomas Valdez of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at ">www.nasatech.com/tsp under the Physical Sciences category.
In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to
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Refer to NPO-20745, volume and number of this NASA Tech Briefs issue, and the page number.
This Brief includes a Technical Support Package (TSP).
Direct Methanol Fuel Cells with Aerosol Feed
(reference NPO-20745) is currently available for download from the TSP library.
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