Electrospun Nanofiber Fuel Cell Membrane-Electrode-Assemblies with Pt-Alloy Cathode Catalyst

The hydrogen/air proton-exchange membrane (PEM) fuel cell is a promising candidate for automotive power plants but Pt/C catalyst electrode cost and durability are still issues that require further study. In a series of previous papers and presentations [1-4], Pintauro and coworkers have shown that an electrospun nanofiber cathode, composed of Pt/C particles and a binder of Nafion + poly(acrylic acid), performs remarkably well in a hydrogen/air proton exchange membrane fuel cell. For example, a nanofiber electrode MEA with a 0.055 mg_Pt/cm² cathode and 0.059 mg_Pt/cm² anode produced more than 900 mW/cm² at maximum power in a H₂/air fuel cell at 80°C, 100% RH and high feed gas flow rates at 2 atm backpressure. In another study, electrospun nanofiber cathodes exhibited excellent durability, as determined from end-of-life polarization curves after an accelerated start-stop voltage cycling (carbon corrosion) test.  For example, after 1,000 simulated start-stop cycles, a nanofiber MEA with Johnson Matthey Pt/C catalyst maintained 53% of its initial power at 0.65 V and 85% of its maximum power, as compared to a 28% power retention at 0.65 V and 58% maximum power for a sprayed electrode MEA. The excellent initial performance of nanofiber fuel cell electrodes was attributed to the unique nanofiber electrode morphology, with inter-fiber and intra-fiber porosity which results in better accessibility of oxygen to Pt catalyst sites and efficient removal of product water. The superior end-of-life performance of the nanofiber MEA after a carbon corrosion test was attributed to the combined effects of a high initial electrochemical cathode surface area, the retention of the nanofiber structure after testing (no collapse of the cathode, as confirmed by SEM imaging), and the rapid/effective expulsion of product water from the cathode which minimizes/eliminates flooding. 

In this presentation, the performance of Pt-alloy catalysts in electrospun nanofiber fuel cell cathodes will be discussed.  The method for creating nanofiber mat cathodes with PtCo and PtNi powders, using a mixture of Nafion and poly(acrylic acid) as the cathode binder, will be described. These cathodes, with a Pt loading of 0.10 mg/cm² were incorporated into a fuel cell membrane-electrode-assembly (MEA) with a Nafion 211 membrane and a Pt/C nanofiber anode (0.1 mg/cm²).  Short-term fuel cell performance was assessed at 80^oC with 100% RH using hydrogen and air at flow rates of 500 and 2,000 sccm, respectively.  Power output was compared with an MEA containing conventional painted electrode gas diffusion electrodes, with neat Nafion binder and with Nafion + poly(acrylic acid) binder (same as the nanofibers). The performance of nanofiber MEAs was also examined and contrasted with that of a conventional electrode MEA after an accelerated carbon corrosion voltage cycling experiment, with 1,000 voltage cycles between 1.0 and 1.5V.

Representative performance results for a nanofiber MEA with a PtCo cathode catalyst at a loading of 0.10 mg/cm² are shown in Table 1. Under ambient pressure, the nanofiber morphology consistently provides a 25% improvement in power density at both 0.65V and at maximum power. This effect is even greater under backpressure where the improvement is consistently around a 35% increase in power density at both 0.65 and maximum power. In this case at 0.65V a power density above 1W/cm² has been achieved. Additional results and analyses will be discussed in the presentation.

Acknowledgments

This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2.