An improved method of fabricating electrodes for fuel cells includes the use of sputtering to deposit thin layers of catalytic electrode metals. Previously, catalytic electrode metals were deposited by means of inks and decals - means that are not amenable to mass production. The sputtering process used in the improved method is better suited to mass production. Sputtering also increases the efficiency of utilization and thereby decreases the needed amounts of the catalytic electrode metals, which are expensive noble metals; whereas the catalyst loadings needed in electrodes made by older methods ranged between 4 and 12 mg/cm2, the catalyst loadings needed in electrodes made by the improved method range from 0.5 to 1.0 mg/cm2.
The improved method has been demonstrated in the fabrication of membrane/electrode assemblies for direct methanol fuel cells. Such a membrane/electrode assembly includes a proton-conducting solid-electrolyte membrane sandwiched between two electrode layers. The membrane is made of the ionomer Nafion™ (or equivalent) - a perfluorosulfonic acid-based hydrophilic, proton-conducting polymer. The electrode layers are made of carbon paper coated with catalytic noble metals; Pt for the cathode and an alloy of Ru/Pt for the anode.
Fabrication according to the improved method begins with air drying of the ionomeric membrane for 24 hours, followed by vacuum drying for 30 to 60 minutes. The membrane is mounted in a sputtering chamber, with one side facing up or down toward a target made of one of the noble-metal electrode materials (see figure). The chamber is evacuated, then backfilled with argon to a pressure between 10 and 50 millitorr (between 1.3 and 6.7 Pa). The sputtering process is initiated by applying a high voltage between the target and the chamber wall. The sputtering process is continued for an amount of time (typically ranging up to a few hours) that depends on the desired catalyst loading. Upon completion of sputtering, air is readmitted to the chamber and the membrane is removed.
The foregoing process is repeated to coat the other side of the membrane with the other noble-metal electrode material. Then using sheets of carbon paper (instead of the ionomeric membrane) as the deposition substrate, the process is again repeated, except that Pt is sputtered into both sides of one sheet of carbon paper, and Pt/Ru is sputtered onto both sides of the other sheet of carbon paper. Both sputter-coated carbon papers are coated with a solution that contains the ionomer in a liquid form. Then the membrane is sandwiched between the coated carbon papers and the sandwich is pressed at a temperature between 140 and 150 °C to obtain a membrane/electrode assembly.
This work was done by Barbara Jeffries-Nakamura, William Chun, Sekharipuram Narayanan, Ronald Ruiz, and Thomas Valdez of Caltech for NASA's Jet Propulsion Laboratory. NPO-20250