A highly durable platinum nanocatalyst for proton exchange membrane fuel cells: multiarmed starlike nanowire single crystal.

Despite significant recent advances, the long-term durability of Pt catalyst at the cathode is still being recognized as one of the key challenges that must be addressed before the commercialization of proton exchange membrane fuel cells (PEMFCs). 2] The loss of Pt electrochemical surface area (ECSA) over time, because of corrosion of the carbon support and Pt dissolution/aggregation/Oswald ripening, is considered one of the major contributors to the degradation of fuel cell performance. Up to now, highly dispersed Pt nanoparticles (NPs, 2–5 nm) on carbon supports are still being widely used as the state-of-the-art commercial catalysts, and most reported studies are focused on nanoparticles of Pt. However, Pt with nanosized particle morphologies has high surface energies, thereby inducing severe Oswald ripening and/or grain growth during fuel cell operation. One-dimensional (1D) nanostructures of Pt, such as nanowires (NWs) and nanotubes (NTs), have been demonstrated to overcome the drawbacks of NPs in fuel cells, owning to their unique 1D morphologies. Yan et al. reported that unsupported Pt nanotubes exhibit much enhanced catalytic activity and durability as fuel cell cathode catalyst. Sun et al. and Zhou et al. reported the improved oxygen reduction reaction (ORR) activities of Pt NWs at the cathode under fuel cell operating conditions. However, up to now, the durability of Pt NW-based electrocatalysts has never been reported in the literature. Here we describe a new approach to address, for the first time, both the activity and durability issues by using carbonsupported multiarmed starlike Pt nanowires (starlike PtNW/ C) as electrocatalysts. Interestingly, the durability can be further improved by eliminating the carbon support, that is, using unsupported Pt nanowires as the cathode catalyst. As a result of their unique 1D morphology, the starlike Pt nanowire electrocatalyst can provide various advantages. First, the interconnected network consists of multiarmed, star-shaped 1D NWs with arm lengths of tens of nanometers which makes the Pt less vulnerable to dissolution, Ostwald ripening, and aggregation during fuel cell operation compared to Pt nanoparticles. Second, this network structure reduces the number of embedded electrocatalyst sites in the micropores of the carbon supports relative to those in nanogrannular Pt. Third, the mass transfer within the electrode can be effectively facilitated by networking the anisotropic morphology. Carbon-supported multiarmed starlike platinum nanowires were synthesized by the chemical reduction of a Pt precursor with formic acid in aqueous solution at room temperature and under ambient atmosphere. No surfactant, which is usually harmful for catalytic activities, was used in the experiments. Figure 1A and B show the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images, respectively, of carbon-supported Pt nanowires at 40 wt % loading of Pt. It can be seen that the assynthesized Pt is nanostar-shaped, being composed of several short arms of Pt nanowires. The number of arms of each nanostar is found to vary ranging from several to over ten. Occasionally, single-armed nanowires standing on the carbon surface can also be observed. Diameter and length of the arms of starlike Pt nanowires are about 4 nm and 15 nm, respectively. More interestingly, from the connected atomic arrangement shown in the high-resolution TEM (HRTEM) images (see Figure S1 in the Supporting Information and the inset in Figure 1B), the nanostar is found to be a single crystal. The fast Fourier transform (FFT; see inset in Figure S1) of the original HRTEM image shows a dotted pattern, further proving that the nanostar is a single crystal. This indicates that the formation mechanism of the nanostar involves seeded growth rather than an aggregation of seeded particles or an assembly process of the nanowires. The X-ray diffraction (XRD) pattern (Figure S2) confirms that the carbon-supported Pt nanowires are crystallized in a face-centered-cubic (fcc) structure similar to bulk Pt, which is consistent with the HRTEM investigations. We believe that the growth of the multiarmed starlike PtNWs on carbon black supports follows a similar process to that for Pt NWs on other supports. Typically, Pt nuclei are first formed in solution through the reduction of H2PtCl6 by HCOOH, and they deposit on the surface of carbon spheres. The freshly formed nuclei act as the sites for further nucleation through the continual absorption and reduction of Pt(IV) ions leading to the formation of particle seeds. For fcc structures, the sequence of surface energies is g{111} < g{100} [*] Dr. S. Sun, Dr. G. Zhang, Dr. D. Geng, Y. Chen, R. Li, Prof. X. Sun Department of Mechanical and Materials Engineering The University of Western Ontario London, Ontario N6A 5B9 (Canada) Fax: (+ 1)519-661-3020 E-mail: xsun@eng.uwo. ca