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(viii). Nanomaterials for Fuel Cells
1. One-dimensional (1-D) metal (Ag, Au and Pt) nanowires and their corresponding arrays have been synthesized using an ambient, surfactantless synthesis technique. The potential applicability of such crystalline, highly purified 1-D samples for practical uses was specifically demonstrated in their manifestation as electrocatalysts for an oxygen reduction reaction (ORR). Specifically, Pt 1-D nanostructures possessed a two-fold higher ORR activity as compared with that of commercial Pt nanoparticles alone. Ag and Au nanowires also evinced reasonable ORR activity in alkaline solution. Ref.: J. Phys. Chem. C, v.113, 5460 (2009).
2. We report on the synthesis, characterization, and electrocatalytic performance of ultrathin Pt nanowires with a diameter of less than 2 nm. An acid-wash protocol was employed in order to yield highly exfoliated, crystalline nanowires with a diameter of 1.3 ± 0.4 nm. The electrocatalytic activity of these nanowires toward the oxygen reduction reaction was studied in relation to the activity of both supported and unsupported Pt nanoparticles as well as with previously synthesized Pt nanotubes. Our ultrathin, acid-treated, unsupported nanowires displayed an electrochemical surface area activity of 1.45 mA/cm2, which was nearly 4 times greater than that of analogous, unsupported platinum nanotubes and 7 times greater than that of commercial supported platinum nanoparticles. Ref.: Nano Letters, v.10, 2806 (2010).
3. We report on the synthesis, characterization, and electrochemical performance of novel, ultrathin Pt monolayer shell–Pd nanowire core catalysts. Initially, ultrathin Pd nanowires with diameters of 2.0 ± 0.5 nm were generated, and a method has been developed to achieve highly uniform distributions of these catalysts onto the Vulcan XC-72 carbon support. As-prepared wires are activated by the use of two distinctive treatment protocols followed by selective CO adsorption in order to selectively remove undesirable organic residues. Subsequently, the desired nanowire core–Pt monolayer shell motif was reliably achieved by Cu underpotential deposition followed by galvanic displacement of the Cu adatoms. The surface area and mass activity of the acid and ozone-treated nanowires were assessed, and the ozone-treated nanowires were found to maintain outstanding area and mass specific activities of 0.77 mA/cm2 and 1.83 A/mgPt, respectively, which were significantly enhanced as compared with conventional commercial Pt nanoparticles, core–shell nanoparticles, and acid-treated nanowires. The ozone-treated nanowires also maintained excellent electrochemical durability under accelerated half-cell testing, and it was found that the area-specific activity increased by 1.5 fold after a simulated catalyst lifetime. Ref.: J. Am. Chem. Soc., v.133, 9783 (2011)
4. In this report, we utilize the U-tube double diffusion device as a reliable, environmentally friendly method for the size-controlled synthesis of high-quality, single crystalline Pd nanowires. The nanowires grown in 200 and 15 nm polycarbonate template pores maintain diameters of 270 ± 45 nm and 45 ± 9 nm, respectively, and could be isolated either as individual nanowires or as ordered free-standing arrays. The growth mechanism of these nanowires has been extensively explored, and we have carried out characterization of the isolated nanowires, free-standing nanowire arrays, and cross sections of the filled template in order to determine that a unique two-step growth process predominates within the template pores. Moreover, as-prepared submicrometer and nanosized wires were studied by comparison with ultrathin 2 nm Pd nanowires in order to elucidate the size-dependent trend in oxygen reduction reaction (ORR) electrocatalysis. Subsequently, the desired platinum monolayer overcoating was reliably deposited onto the surface of the Pd nanowires by Cu underpotential deposition (UPD) followed by galvanic displacement of the Cu adatoms. The specific and platinum mass activity of the core–shell catalysts was found to increase from 0.40 mA/cm2 and 1.01 A/mg to 0.74 mA/cm2 and 1.74 A/mg as the diameter was decreased from the submicrometer size regime to the ultrathin nanometer range. Ref.: ACS Nano., 5(9), 7471-7487 (2011).
4. We have employed an ambient, template-based technique that is simple, efficient, and surfactantless to generate a series of bimetallic Pd1–xAux and Pd1–xPtx nanowires with control over composition and size. Our as-prepared nanowires maintain significantly enhanced activity toward oxygen reduction as compared with commercial Pt nanoparticles and other 1D nanostructures, as a result of their homogeneous alloyed structure. Specifically, Pd9Au and Pd4Pt nanowires possess oxygen reduction reaction (ORR) activities of 0.49 and 0.79 mA/cm2, respectively, which are larger than the analogous value for commercial Pt nanoparticles (0.21 mA/cm2). In addition, core–shell PtPd9Au nanowires have been prepared by electrodepositing a Pt monolayer shell and the corresponding specific, platinum mass, and platinum group metal mass activities were found to be 0.95 mA/cm2, 2.08 A/mgPt, and 0.16 A/mgPGM, respectively. The increased activity and catalytic performance is accompanied by improved durability toward ORR. Ref.: Nano Lett., 12(4), 2013-2020 (2012).
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