Electrospun Nanofiber Electrodes for Hydrogen/Air Proton Exchange Membrane Fuel Cells

Abstract

Nanofiber particle/polymer cathode mats, with an average fiber diameter in the 400-600 nm range, were fabricated by electrospinning mixtures of a proton exchange polymer and commercial Pt/C catalyst particles, incorporated into membrane-electrode-assembles (MEAs), and evaluated in a fuel cell test fixture. Nanofiber cathodes with a commercial Pt/C catalyst and a binder of Nafion and poly(acrylic acid) (PAA) were shown to work extremely well in hydrogen/air fuel cell MEAs. As compared to conventional painted cathode MEAs, they had a higher electrochemical surface area (39-45 m2/g for nanofibers vs. 30-36 m2/g for painted), higher mass activity (~0.16 A/mgPt vs. 0.11 A/mgPt), and higher power output (e.g., 396 mW/cm2 at 0.65 V with H2/air at ambient pressure and a cathode Pt loading of 0.10 mg/cm2 vs. 292 mW/cm2). MEA power output with nanofiber cathodes was insensitive to changes in fiber diameter and Nafion/PAA binder composition, indicating that precise control of these parameters is not required for commercial scale-up. The nanofiber electrode architecture did not significantly change the way fuel cell cathodes degraded during load cycling tests (Pt dissolution tests), but the nanofibers had a clear advantage in power retention after accelerated durability tests that simulate start-stop cycling (carbon corrosion tests). A second generation of nanofiber cathodes was fabricated with a binder of Nafion and poly(vinylidene fluoride) (PVDF). The addition of PVDF altered the hydrophilicity/hydrophobicity of the cathode and slowed the deleterious effects of carbon corrosion. Carbon corrosion rates were the same for both nanofiber and painted Nafion/PVDF cathodes, but the effect of corrosion on power output was much less severe for nanofiber cathodes. Cathodes with a low Nafion/PVDF ratio produced low power initially but the power density increased over the course of a carbon corrosion test. This unusual result was associated with the formation of hydrophilic carbon oxidation species at the catalyst support surface, which increased the hydrophilicity of the cathode.