An improved design for a biplate in a methanol fuel cell provides increased efficiency in the removal of water, relative to older designs. For reasons explained below, this design both improves the performance of the fuel cell and increases the overall energy efficiency of the power-generating system of which the fuel cell is a part.
A typical methanol fuel cell includes a number of membrane/electrode assemblies (MEAs) stacked in alternation with biplates. Each biplate serves partly as an electrical contact between the cathode of the MEA on one side and the anode of the MEA on the other side. The biplate also contains channels for circulating air past the cathode and other channels for delivering fuel (methanol) to the anode.
During operation of the fuel cell, several parasitic chemical and physical effects cause water to accumulate in the air channels. If the water is not removed, then it impedes airflow in the channels, and consequently, the fuel-cell performance deteriorates. In the cases of most older biplate designs, considerable power is expended to supply pressurized air to blow the water out of the air channels; as a result, overall energy efficiency is reduced. The energy-efficiency issue becomes even more important if the fuel cell is operated at room temperature (about 25 °C, in contrast to a typical higher operating temperature of about 95 °C) because the fuel-cell output deceases with decreasing temperature in this range.
The improved biplate is designed to exploit gravitation and surface tension to remove water without need to supply pressurized air. It is still necessary to bring in air for the electrochemical reaction in the fuel cell, but the improved biplate offers little resistance to airflow, so that a low-power fan that supplies air at slightly more than atmospheric pressure is sufficient. Thus, a high level of performance can be maintained, and energy efficiency is enhanced.
To enable gravitation to drain water from the fuel cell, the airflow channels in the improved biplate are oriented vertically, with inlets at the top and outlets at the bottom (see figure). Most of the biplate is made of a graphite/epoxy composite material, which is hydrophobic. Unfortunately, hydrophobic surfaces tend to impede drainage. Therefore, the drainage surfaces of the biplate are treated to make them hydrophilic, so that immediately upon release at the cathode, a drop of water becomes part of a surface layer of water that drains to the bottom of the biplate. The treatment to make the surfaces hydrophilic includes coating with a commercial perfluorosulfonic acid-based ion-exchange polymer followed by coating with a carbon-supported platinum/ruthenium catalyst. The combination of coating materials was chosen because it is chemically compatible with other fuel-cell materials and is expected to be chemically stable in the long term.
If nothing were done to prevent it, surface-tension effects could cause an air-outlet port at the bottom of the biplate to become plugged with water. Two features prevent this from happening: (1) the port is made large enough that water cannot straddle it and (2) a metal insert extending from the edge of the biplate wicks the water away from the port. At the edge of the insert, water forms into drops, which then fall off.
To minimize resistance to airflow, air is supplied to the biplate via a large external manifold. The air passes from the manifold into the biplate through a large inlet port (equal in size to the outlet port). The air channels in the biplate are also made large to minimize resistance to flow.
This work was done by Andrew Kindler and Albany Lee of Caltech for NASA's Jet Propulsion Laboratory. 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-20308
This Brief includes a Technical Support Package (TSP).
Methanol-fuel-cell biplate improved for removal of water
(reference NPO20308) is currently available for download from the TSP library.
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