New Developments in Electrospun Nanofiber Electrodes for Hydrogen/Air Fuel Cells

The hydrogen/air proton-exchange membrane (PEM) fuel cell is a promising candidate for automotive power plants due to its high power output, moderate operating temperature, and quick start-up.  The commercialization of such devices is hindered by the high price and inadequate durability of the platinum catalyst electrodes.  Recent 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. Work on non-platinum group metal catalysts for the oxygen reduction cathode in a hydrogen/air fuel cell has also been reported. The creation of particle/polymer nanofiber mat electrodes via electrospinning represents an entirely different strategy for lowering cathode catalyst loading while maintaining high power output in a hydrogen/air fuel cell.  A nanofiber electrode morphology with intra and inter-fiber porosity is advantageous because: (i) it increases catalyst contact with reactant gases, (ii) it provides a sufficient number of pathways for effective proton and electron conduction, and (iii) there is sufficient inter-fiber porosity for release of cathode product water.

Previously, we have shown that more power was generated from nanofiber electrode MEAs than a conventional MEA with decal electrodes [1,2]. For example, the maximum power density for a H₂/air fuel cell with a nanofiber cathode and anode (at loadings of 0.065 mg_Pt/cm² and 0.10 mg_Pt/cm², respectively) was 437 mW/cm² vs. 400 mW/cm² for a decal MEA with a cathode containing 40% more Pt (a loading of 0.104 mg_Pt/cm²) and an anode loading of 0.10 mg/cm².  In these experiments, the nanofiber catalyst binder was a mixture of Nafion^® and poly(acrylic acid), the binder used in decal electrodes was neat Nafion, and the membrane was Nafion 212. The fuel cell operating conditions were: 80^oC, 1 atm (ambient) pressure, 125 sccm H₂, and 500 sccm air. As another example, an electrospun cathode with a Pt loading of 0.055 mg/cm² produced a maximum power density of 906 mW/cm² at 80°C and 3 atm pressure with 2000 sccm fully humidified air and 500 sccm H₂. Additionally, the mass activity (0.16 A/mg_Pt at 0.9 V) and electrochemical surface area (~41 m²/g) of nanofiber cathodes containing Johnson-Matthey HiSpec™ 4000 catalyst were found to be very high and electrospun cathodes exhibited less degradation than a conventional decal cathode in an accelerated voltage cycling durability test.

In this presentation, new catalyst/binder electrode formulations will be discussed, in terms of the conditions required to electrospin well-formed fibers and the performance of the fiber mats as cathodes in a hydrogen/air fuel cell operating at 80^oC. MEAs are assessed in terms of the initial power density at high and low humidity operation and the changes in power output during voltage cycling Pt dissolution and carbon corrosion durability experiments. All anode and cathodes were prepared with Johnson Matthey  HiSpec 4000 catalyst (40% Pt on Vulcan carbon), at a Pt loading of  0.10 mg/cm².    

Acknowledgments

This work was funded by EMD Millipore and the NSF-funded TN-SCORE program, NSF EPS-1004083, under Thrust 2.