ECS H.H. Dow Student Achievement Award of the IEEE Division High Performance Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells

Introduction

      The hydrogen/air proton-exchange membrane fuel cell is a promising candidate for emission-free automotive power plants due to its high power output, energy conversion efficiency, and quick start-up.  Of critical importance for fuel cell commercialization is membrane-electrode-assembly (MEA) cost and durability. The supported Pt/C catalyst used in the anode and cathode is expensive and corrosion of the carbon support during fuel cell start-up is a continuing issue that has drawn considerable research attention.  Recently, R&D efforts have been directed at reducing Pt loading and fabricating catalysts with supports that can withstand the harsh automotive operating environment.  Work in this area includes the investigation of metal alloys, core-shell nanostructures, and metal oxide supports.^1,2  Researchers have also focused on creating oxygen reduction reaction catalysts that do not use platinum group metals.³

      Another strategy to lower cathode catalyst loading while maintaining high fuel cell power output is to improve the electrode morphology in order to maximize catalyst contact with reactant gases while having a sufficient number of pathways for effective proton and electron conduction.  Electrospinning offers the possibility of creating nanofiber structures from a rich variety of different catalytic materials.  Previously, we have shown that a cathode electrospun from  Johnson Matthey HiSpec 4000 catalyst with a Pt loading of 0.065 mg_Pt/cm² produced 29% more power than a painted decal cathode at a much higher Pt loading of 0.104 mg_Pt/cm² (360 mW/cm² vs 280 mW/cm² at 0.65 V).⁴ We have also shown that electrospinning is a robust electrode fabrication technique for creating both cathodes and anodes; fuel cell performance was largely insensitive to changes in binder content (28-45 wt%) and nanofiber diameter (250-475 nm).⁵  Additionally, the nanofiber cathodes exhibited less performance decay after accelerated stress tests that simulate automotive start-stop behavior.  Experiments at Nissan Technical Center North America showed that the end-of-life performance of MEAs with nanofiber anodes and cathodes was superior to MEAs with sprayed electrodes with the same catalyst and loading.⁶ The outstanding performance of the electrospun cathodes is attributed to the unique inter-fiber and intra-fiber porosity of the nanofiber mat, where the catalyst and ionomer are well dispersed, resulting in high electrochemical surface area and high catalyst mass activity.⁴

Experimental

      Electrospinning inks were prepared by mixing catalyst with a polymeric binder in an appropriate solvent.  For example⁴, a typical ink consists of: (a) Johnson Matthey  HiSpec 4000 (40% Pt on Vulcan carbon), (b) a solution of 20% Nafion ionomer  in a solvent of lower aliphatic alcohols + water (Aldrich) and (c) poly(acrylic acid) (PAA) (Mw=450,000, Aldrich).  The total polymer and powder content of the spinning solution was 14.0 wt%, where the Pt/C:Nafion^®:PAA weight ratio was 72:13:15. The ink was drawn into a 3 mL syringe and electrospun using a 22 gauge stainless steel needle spinneret, where the needle tip was polarized to a potential of +10.0 kV relative to a grounded stainless steel rotating drum collector. The spinneret-to-collector distance was fixed at 10 cm and the flow rate of the ink was 1.0 mL/h. The collector drum rotated at 100 rpm and oscillated horizontally to improve the uniformity of deposited nanofibers.  Figure 1 shows a nanofiber mat created with Johnson Matthey HiSpec 4000 catalyst, Nafion, and poly(acrylic acid).

      MEAs (5 or 25 cm²) were made using a Nafion 211 proton-exchange membrane with  electrospun anodes and cathodes.  Electrodes were hot-pressed to the membrane at 140^oC and 4 MPa for 1 minute after a 10 minute heating period at 140^oC and 0 MPa. Carbon paper gas diffusion electrodes (Sigracet GDL 25 BC from Ion Power, Inc) were physically pressed onto the MEA’s anode and cathode in the fuel cell test fixture prior to MEA evaluation.

      In this paper, recent experimental results will be presented on new cathode catalysts (e.g., non platinum group metal powders and Pt catalysts that do not employ a carbon support), ultra-low Pt–loaded anodes, and binders other than Nafion.  The goals of this work are to increase MEA power density, lower anode and cathode Pt loading, and further improve the durability of electrospun electrodes.

Acknowledgments

This work was funded by EMD Millipore and Nissan Technical Center North America.

References

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