Ptfe Porous Hollow Nanocapsule Electrocatalysts with Remarkably Enhanced Oxygen Reduction Activity

Expansion of the commercial applications of polymer electrolyte fuel cells (PEFCs) strongly requires great improvements in cost, performance, and durability. At a cathode where oxygen reduction reaction (ORR) occurs, platinum (Pt) catalysts show sluggish ORR leading to high cost and poor performance of PEFCs. In addition, carbon-supports for Pt catalyst degrade due to carbon-corrosion during start-up/shut-down operations; it results in a severe deterioration of PEFC performance.

Here we develop platinum-iron (PtFe) porous hollow nanocapsules (Fig. 1) as carbon-free electrocatalysts for ORR. The nanocapsules are composed of fused PtFe nanoparticles with a crystallite size of 6-7 nm and a chemically-ordered (face-centered tetragonal; fct) structure. The fused PtFe network possesses an electron conductivity, thus any carbon-support is not required at the cathode made of PtFe nanocapsules. We demonstrated the MEA using the carbon-free cathode catalyst-layer of PtFe nanocapsules exhibited sufficiently high IV performance, and excellent durability against start-up/shut-down operations at 80°C, by intrinsically eliminating the problem of carbon-corrosion.^[1] The exclusion of carbon-supports from the cathode catalyst layer also enabled the formation of the ultrathin catalyst-layer that would provide great advantages to mass transport of oxygen. Moreover, it is noteworthy that the specific ORR activity of the PtFe nanocapsule is approximately 8.5 times higher than that of the commercial Pt catalyst supported on carbon black (Pt/C). Interestingly, the specific activity of the nanocapsule is also 2~3 times enhanced, compared with that of fct-PtFe nanoparticles (without any fused networks) supported on carbon black. Thus, it implies that the unique structures of the nanocapsule, such as the connected network structure of fused PtFe metal nanoparticles, and the support-free structure, would allow to exhibit higher ORR activities. We have also investigated the structure and electronic structure via in-situx-ray absorption spectroscopy combined with electron diffraction and Rietveld structure refinement with powder x-ray diffraction data.

In this presentation, the relationship of the surface structures on the PtFe nanocapsules to their ORR activities will be discussed in details. The above results indicate that the newly-developed nanocapsule is a quite promising catalyst-layer material because it potentially can solve all the current problems, regarding ORR activity, durability, and mass-transport of oxygen, in the conventional cathode catalyst layers using Pt-based catalysts on carbon supports.

[1] T. Tamaki, H. Kuroki, S. Ogura, T. Fuchigami, Y. Kitamoto, and T. Yamaguchi, “Pt-Fe Porous Hollow Nanocapsule as Carbon-free Electrocatalyst for Oxygen Reduction”, Submitted.

Fig. 1. Schematic illustration of PtFe porous hollow nanocapsules as carbon-free electrocatalysts.