High‐Performance Nanofiber Fuel Cell Electrodes

Catalysts are key cost components in proton-exchange membrane (PEM) fuel cells. Consequently, there has been ongoing research to improve electrode catalyst activity and durability, particularly for platinum-based cathodes in hydrogen/air PEM fuel cells. Herein, we present results on a nanofiber-structured oxygen reduction fuel cell cathode that was prepared by electrospinning a solution containing catalyst powder and polymer binder. The electrospun nanofiber mat, with an average fiber diameter of 470 nm, was hot-pressed onto a Nafion 212 membrane with a decal-processed anode and tested in H2/air and H2/O2 fuel cells. The nanofiber cathode performed extraordinarily well ; high power densities (more than 500 mWcm 2 at 0.6 V and 80 8C) were achieved at a very low platinum loading of 0.1 mgcm , the platinum mass activity was exceptionally high at 0.23 AmgPt , and the nanofiber electrode exhibited outstanding stability in a voltage cycling accelerated degradation test. Prior work on fuel cell electrodes has focused on eliminating platinum-group metal (PGM) catalysts entirely from the cathode, or on decreasing the loading of PGMs in the cathode by use of alloys, core–shell materials, 7] star-like structures, or other supports. 10] Another way to lower the catalyst loading is to alter the morphology of the cathode in a fuel cell membrane–electrode assembly (MEA) in order to maximize catalyst contact with reactant gases while maintaining a sufficient number of pathways for proton and electron conduction. Surprisingly, over the past twenty years, there has been little advancement on new fuel cell electrode constructs with improved catalyst utilization. Thus, most fuel cell MEAs are fabricated today using either : (i) a decal transfer method, (ii) a catalyst coated carbon paper or carbon cloth gas diffusion layer (GDL), or (iii) a catalyst coated PEM. Recently, researchers have attempted to fabricate new fuel cell electrode structures, such as electrosprayed layers of micrometer-size catalyst droplets, platinum nanowires, oriented platinum-coated whiskers (developed by 3M Corporation), and electrospun micrometer-sized core-sheath fibers reported by Asahi Glass Co (where a fiber core of Pt/C catalysts is covered by polyethylene oxide). While such new approaches are promising, none has lead to a substantial reduction in platinum catalyst loading for the cathode while maintaining high fuel cell power output and providing long-term electrode durability. Electrospinning is a well-known technique for producing polymer fibers with nanometer-sized diameters. Recently, it has been used successfully to fabricate proton-conducting fuel cell membranes, where a phase-separated composite structure was created with sulfonated polysulfone or perfluorosulfonic acid polymer nanofibers embedded in an inert (uncharged) polymer matrix. For the case of perfluorosulfonic acid (PFSA) nanofiber electrospinning, a small amount (0.3– 2.0 wt%) of an inert carrier polymer such as poly(ethylene oxide) or poly(acrylic acid) (PAA) must be added to the electrospinning solution because there is an insufficient number of PFSA polymer chain entanglements in normal organic electrospinning solvents. For the case of nanofiber electrode electrospinning, the present study builds upon these successful PFSA electrospinning studies. Thus, Nafion was used as the ionomer component of the catalyst ink and PAA was selected as the carrier polymer. The experimental apparatus used to fabricate the nanofiber mat electrode is shown schematically in Scheme 1. The cathode ink was a dispersion of Pt/C catalyst powder (40 wt% platinum on carbon black) in a mixed isopropyl alcohol/water solvent containing dissolved Nafion and PAA polymers. The ink was pumped out of a needle spinneret (22 gauge needle) and deformed into a Taylor cone by the strong applied potential at the needle tip, +7.0 kV relative to a grounded stainless steel rotating drum nanofiber collector. The spinneret-to-collector distance was fixed at 9 cm, and the flow rate of ink was 1.5 mLh . Nanofibers were collected on a carbon paper sheet that was fixed to the collector drum (rotating at 100 rpm). The drum oscillated horizontally to improve the uniformity of deposited nanofibers. The resulting electrospun nanofiber mat