Electrocatalyst durability has been recently recognized as one of the most important issues that have to be addressed before the commercialization of the proton exchange membrane fuel cells (PEMFCs). The durability problem is particularly severe in the cathode where the oxygen reduction reaction (ORR) occurs. At present, the most widely used cathode catalyst system is platinum (Pt) in the form of small nanoparticles (2-5 nm) supported on amorphous carbon particle aggregates (Pt/C). The poor durability of the Pt/C catalyst is reflected by a fast and significant loss of Pt electrochemical surface area (ECSA) over time during fuel cell operation. The carbon corrosion problem can be alleviated by the use of a more corrosion-resistant catalyst support, e.g., graphitized carbons and carbon nanotubes. No effective solutions, however, exist for addressing the other three mechanisms. Our approach to a durable electrocatalyst is to develop supportless Pt nanotubes (PtNTs) and Pt alloy nanotubes (e.g., PtPdNTs) as the cathode catalyst. Due to their unique combination of dimensions at multiple length scales, PtNTs and PtPdNTs can provide high Pt surface area without the need of a high surface area support (e.g., carbon black) by their nanometer-sized wall thickness, and at the same time, have the potential to eliminate or significantly reduce all of the degradation pathways as a result of their micrometer-sized length. First, PtNTs do not require a support, and thus the support corrosion problem is eliminated. Second, the micrometer-sized length of the PtNTs (1-D nanostructure) makes the PtNTs less vulnerable, than the Pt nanoparticles (0-D nanostructure), to dissolution, the Ostwald ripening, and aggregation during the fuel cell operation. Additionally, the PtNTs and PtPdNTs, like carbon nanotubes (CNTs), have an anisotropic morphology that can improve the mass transport and catalyst utilization. And if properly assembled in a catalyst layer (e.g., cubic or hexagonal close-packing and vertically aligned on the Nafion membrane), they can also lead to a thin catalyst layer (e.g., 0.5 µm at a PtNT wall thickness of 2 nm and a Pt loading of 0.2 mg/cm2), further improving the mass transfer characteristics within the catalyst layer. We have synthesized platinum nanotubes (PtNTs) and platinum-palladium alloy nanotubes (PtPdNTs) (50 nm diameter, 5 to 20 µm long and 4 -7 nm wall thickness) and tested their suitability as catalysts for ORR in PEMFCs. The activity for oxygen reduction reaction (ORR) at PtNTs as well as commercial 20 wt % Pt/C and Pt black from E-TEK were investigated in 0.5 M H2SO4 using the rotating disk electrode (RDE) technique. The durability of the catalysts was evaluated by repeated potential cycling in the range of 0-1.3 V at 5 mV/s under Ar atmosphere at 60 oC. The Pt electrochemical surface area (ECSA) of PtNTs only decreases about 20% even after 1000 cycles while the PtB and the Pt/C catalyst has lost about 51% and 90% of their Pt ECSA, respectively. Typical ORR polarization curves of them are obtained at room temperature in O2 saturated 0.5 M H2SO4 at 1600 rpm at room temperature. The PtNTs asymptotically approaches a maximum of ~20% Pt surface area loss in durability test, while the commercial PtB and Pt/C loses about 51% and 90% of their initial surface area, respectively. Moreover, the PtNTs and PtPdNTs show much higher specific activity (e.g., 3.8 and 5.8 times) than commercial Pt/C.

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