Some changes have been made in the fabrication of PSSA/ PVDF-based membrane/electrode assemblies for direct methanol fuel cells. The effect of the changes is to improve the electrochemical performances of the cells.
Some rather detailed background information is prerequisite to a meaningful description of the changes. Each fuel cell contains a membrane/electrode assembly (MEA), which is a composite of a solid-electrolyte membrane sandwiched between catalyzed electrode layers. PSSA/PVDF is a composite material that has been recently found to be useful for making solid electrolyte membranes, as reported in “PSSA/PVDF Polymer Electrolyte Membranes for CH3OH Fuel Cells” (NPO-20378), NASA Tech Briefs, Vol. 23, No. 6 (June 1999), page 54. To recapitulate: a PSSA/PVDF membrane consists of cross-linked polystyrene sulfonic acid (PSSA) immobilized within an electrochemically inert matrix of poly(vinylidene fluoride).
Heretofore, the fabrication of an MEA has typically involved the following process: An ink for each electrode is prepared from a combination of (a) an electrocatalyst (Pt for the cathode, Pt/Ru for the anode) and (b) a solution of a perfluoro-sulfonated ion-exchange polymer dispersed in lower alcohols. Each ink is applied to either (a) a sheet of poly(tetrafluoroethylene)- impregnated porous carbon paper or (b) a surface of a solid-electrolyte membrane. The membrane is sandwiched between the carbon papers, which are destined to become the electrodes. The sandwich, in a hydrated condition, is then pressed at a temperature of 145 to 150°C and a pressure of 2 kpsi (14 MPa).
Prior to the development of PSSA/ PVDF membranes, the membranes in the MEAs of the most advanced direct methanol fuel cells were made from perfluoro-sulfonated ion-exchange polymers. The principal advantage of PSSA/PVDF membranes over membranes made from those and other polymer electrolyte materials is that the PSSA/PVDF membranes are less permeable by methanol; this translates to less methanol crossover and thus greater fuel-utilization efficiency. Relative to MEAs made from perfluoro-sulfonated ion-exchange membranes, MEAs made from PSSA/PVDF membranes exhibit comparable proton conductivity and less methanol crossover. However, until now, the electrical performances of the PSSA/PVDF-based MEAs have not been adequate for use in fuel cells. The inadequacy (in particular, high electrical resistance and poor utilization of catalyst) has been attributed primarily to poor interfacial bonding of the electrocatalytic layers with the proton-conducting moieties of the membrane. This completes the background information.
The changes in the fabrication process are intended to improve the interfacial bonding and the formation of proton-conducting channels at the membrane/electrocatalyst interfaces. One of the changes is the addition of PVDF powder to the ink. The PVDF improves the interfacial bonding by making the ink more chemically and thermodynamically similar to, and thus more miscible with, the membrane. In addition, PVDF has low intrinsic permeability by methanol and thus helps suppress methanol crossover through the electrodes.
Another change is roughening the membrane prior to application of the catalytic electrode layers. Roughening enhances bonding by providing additional sites for anchoring the catalytic and polymeric electrode materials.
A third change is the addition of water and N,N-dimethylacetamide to the ink (which, in this case, is painted directly onto the membrane). These additions enhance bonding by increasing the plasticity of the membrane during hot pressing. These additions also enhance bonding by preventing undesired dryout during hot pressing.
In an experiment, the electrical performance of a fuel cell containing an MEA made by a process that incorporates these changes was measured, along with that of a fuel cell containing an MEA made by an older process. The cell containing MEA made by the newer process was operated at a lower temperature, yet it exhibited better performance (see figure). Inasmuch as the performance of a given cell increases with temperature, the performance of the MEA made by the newer process could be expected to exceed that of the other MEA by an even greater margin if both were tested at the same temperature.
This work was done by Sekharipuram Narayanan and Marshall Smart of Caltech and Tony Atti, Surya Prakash, and George Olah of the University of Southern California 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-20644, volume and number of this NASA Tech Briefs issue, and the page number.