Low-platinum-content quaternary PtCuCoNi nanotubes with markedly enhanced oxygen reduction activity.

Polymer electrolyte membrane fuel cells (PEMFCs), among other clean energy sources, represent a highly efficient and potentially inexpensive solution for worldwide ever-growing energy demand and increasing environmental concern, and are expected to be able to find various applications ranging from automotive vehicles to stationary and portable devices. Presently, the widespread commercialization of PEMFCs is, however, substantially hampered by the slow rate of the oxygen reduction reaction (ORR) at the cathode and the relatively high costs resulting from the excessive use of precious metal platinum. Therefore, considerable research efforts have recently been dedicated to the development of low-cost and highly efficient electrocatalysts for the ORR. While great progress has been made towards non-platinum electrocatalysts, such as nitrogen-doped carbon nanotubes/ graphenes, conducting polymers, and other nonprecious metals/alloys, platinum-based catalysts are still more efficient for the ORR. To date, there are several major strategies being developed to reduce the cost of platinum or enhance platinum utilization in electrocatalysts. The most popular one is to alloy Pt with other nonprecious metals, for example, Fe, Co, Ni, Cu, Ti, Bi, Sc, and Y (PtM alloys). A variety of binary PtM alloy nanoparticle electrocatalysts have been investigated, and a 2–10-fold enhancement in ORR activity relative to pure Pt was repeatedly observed. It is generally accepted that this activity improvement can be ascribed to the lattice strain induced by the formation of Pt-skinned surfaces (i.e. PM alloy/Pt core/shell structure) through surface dealloying and the modified electronic structures, which can weaken the interaction between the Pt surface atoms and spectator species so as to increase the number of active Pt sites. The second frequently used approach to enhancing Pt utilization is to design core/shell nanostructures by covering the surface of nonprecious metal nanocatalysts with a Pt shell or monolayer, which can be accomplished either by solution-phase heterogeneous nucleation and growth on nonprecious metal nanoparticles or by Cu underpotential deposition and subsequent galvanic replacement reaction. The third strategy is to use hollow Pt nanostructures as electrocatalysts. In contrast to core–shell structures, the hollow geometry not only allows the reactants to access the external active Pt sites, but also renders the internal catalytically active sites accessible. Moreover, it was experimentally demonstrated very recently that the confinement effect occurring in the cavity of the nanocage catalysts can lead to a much higher frequency factor for the reduction of 4nitrophenol by NaBH4, a model reaction that can be used to evaluate the catalytic activity of metals. So far, a few reports have been dedicated to the development of hollow Pt electrocatalysts for both the ORR and methanol oxidation reaction (MOR). Nevertheless, in contrast with the other two approaches, Pt-based hollow electrocatalysts still remain insufficiently explored. Herein, we report the synthesis of low-platinum-content quaternary PtCuCoNi nanotubes (NTs) by means of template-assisted, one-step electrodeposition and testing of the suitability of these hollow multimetallic NTs as effective ORR electrocatalysts. We found that the as-prepared PtCuCoNi NTs exhibit markedly enhanced ORR activity over commercially available Pt black and Pt/C catalysts because of compositional (i.e. multicomponent alloys) and geometrical (i.e. hollow structure) properties of the material. The quaternary PtCuCoNi NTs were prepared by a onestep direct electrodeposition approach using a porous anodic aluminium oxide (AAO) membrane as the template. Onestep electrodeposition takes advantage of the sputtered annular base electrode and rapid consumption of metal ions at the deposition front as a result of a large deposition current density to grow tubular structures (see Figure S1 in the Supporting Information) without the need to modify the pore walls of the AAO template, and has recently been proved to be a simple and highly efficient way to fabricate metallic and alloy nanotubes. With this method, a growth rate of PtCuCoNi NTs of as high as 5 mmmin 1 was achieved. The electrodeposition was carried out under a condition that favors the reduction of less noble copper ions in the electrolyte (e.g. 0.8 V vs. Ag/AgCl). Afterwards, the AAO membrane was immersed into 10 wt% H3PO4 solution at 45 8C for 5 h. In this process, the porous alumina was completely removed. Meanwhile, mild dealloying also occurred in the as-deposited nanostructures. Figure 1a shows a representative scanning electron microscopy (SEM) image of the as-prepared nanostructures after removal of the AAO template. Unlike the nanoporous PtCo and PtNi nanowires reported previously, compact and robust fibers without discernable nanopores were observed after the acid treatment. Upon closer examination by transmission electron microscopy (TEM), it was found that most fibers actually possess hollow tubular morphology with an average diameter of 50 nm, as evidenced in Figure 1b,c. The [*] Dr. L. Liu, Dr. E. Pippel Max Planck Institute of Microstructure Physics Weinberg 2, 06120 Halle (Saale) (Germany) Fax: (+49)345-551-1223 E-mail: [email protected]