Fuel cells show great promise as high power density energy conversion devices. Current research is focused on improving the durability and performance of membrane-electrode-assemblies (MEAs) while reducing their cost. One way to lower costs is to eliminate or minimize the amount of Pt catalyst in the cathode electrode of an MEA. Two strategies to accomplish this goal are the use platinum-group-metal-free (PGM-free) powders or high ORR activity Pt-alloys catalysts.

It has been shown previously that a nanofiber particle/binder fuel cell cathode with a conventional Pt/C catalyst works exceptionally well in a H₂/air fuel cell, with high initial power and better durability than a conventional sprayed/painted cathode.^1-4 The nanofiber morphology with inter and intra fiber porosity improves oxygen access to catalyst sites and facilitates the removal of product water. The methodology of fabricating fiber mat cathodes is robust and amenable to different binders and catalyst powders. Thus, in the present study, nanofiber cathode MEAs were fabricated and tested with cathode catalyst that have been attracting attention recently, namely a Pt-alloy powder (PtCo from TKK) and an Fe-MOF-based PGM-free powder.⁵

Nanofiber mat cathodes with the PGM-free catalyst (at a loading of 3.0 mg/cm²) were prepared with a binder of Nafion + poly(vinylidene fluoride) (PVFD) to control the binder hydrophilicity, where the fiber composition was 70 wt% catalyst and 30 wt% binder. A PtCo/C cathode fiber mat (55 wt% catalyst and 45 wt% binder, at a loading of 0.1 mg/cm²) was electrospun using a Nafion + poly(acrylic acid) (PAA) binder. The cathode fiber mats were incorporated into a MEA using Nafion 211 and a Pt/C painted anode at 0.1 mg/cm².

For the PGM-free fiber mat cathodes, the use of a Nafion/PVDF binder allowed for long-term (100 hour) stable power output of 95 mW/cm² at 0.5V, 80°C and 1 atm backpressure. The nanofiber cathode also exhibited excellent resistance to the deleterious effects of carbon corrosion. Thus, a maximum power density of 83 mW/cm² was observed after a carbon corrosion voltage cycling accelerated stress test (150 voltage cycles from 1.0 V to 1.5 V). The excellent performance of the nanofiber cathode was attributed to the combined effects of the somewhat hydrophobic Nafion/PVDF binder and the nanofiber morphology which minimized/eliminated catalyst degradation by electro-generated peroxides and hydroxyl radicals.

The initial performance of nanofiber cathode MEAs with PtCo/C was evaluated in H₂/air fuel cells for a range of operating conditions: temperatures from 80^oC to 99^oC, 40%-100% relative humidity, and operating pressures of 100, 150, and 200 kPa. In general, the nanofiber cathodes produced ~30% more power than a conventional sprayed cathode with the same catalyst at the same loading. For example, the power density at 0.65V was 868 mW/cm² for a cell temperature of 80°C and 150 kPa_a, where the gas flow rates were 500 sccm for H₂ and 2,000 sccm for air. When a load cycling accelerated stress test for platinum dissolution was performed, 81% of the initial power at 0.654 v was retained after 30,000 voltage cycles (0.6 V to 0.95 V) vs. a 42% power retention for a sprayed cathode MEA.

In this talk, the procedures for fabricating nanofiber cathodes will be discussed and MEA performance at begging-of-life and end-of-life (i.e., after an accelerated stress test) will be presented for both cathode catalyst types.

Acknowledgments

This work was funded in part by the National Science Foundation (NSF EPS-1004083) through the TN-SCORE program under Thrust 2. And by the Fuel Cell Consortium for Performance and Durability DOE-EERE FC-PAD Project DE-EE0007653.