The rising interest in fuel cell vehicle technology (FCV) has engendered a growing need and realization to develop rational chemical strategies to create highly efficient, durable, and cost-effective ...fuel cells. Specifically, technical limitations associated with the major constituent components of the basic proton exchange membrane fuel cell (PEMFC), namely the cathode catalyst and the proton exchange membrane (PEM), have proven to be particularly demanding to overcome. Therefore, research trends within the community in recent years have focused on (i) accelerating the sluggish kinetics of the catalyst at the cathode and (ii) minimizing overall Pt content, while simultaneously (a) maximizing activity and durability as well as (b) increasing membrane proton conductivity without causing any concomitant loss in either stability or as a result of damage due to flooding. In this light, as an example, high temperature PEMFCs offer a promising avenue to improve the overall efficiency and marketability of fuel cell technology. In this Critical Review, recent advances in optimizing both cathode materials and PEMs as well as the future and peculiar challenges associated with each of these systems will be discussed.
The rising interest in fuel cell vehicle (FCV) technology has created a growing and timely need and realization to develop rational chemical strategies to create highly efficient, durable, and cost-effective fuel cells.
In the search for alternatives to conventional Pt electrocatalysts, we have synthesized ultrathin, ternary PtRuFe nanowires (NW), possessing different chemical compositions in order to probe their CO ...tolerance as well as electrochemical activity as a function of composition for both (i) the methanol oxidation reaction (MOR) and (ii) the formic acid oxidation reaction (FAOR). As-prepared 'multifunctional' ternary NW catalysts exhibited both higher MOR and FAOR activity as compared with mono-metallic Pt NWs, binary Pt sub(7)Ru sub(3) and Pt sub(7)Fe sub(3) NWs, and commercial catalyst control samples. In terms of synthetic novelty, we utilized a sustainably mild, ambient wet-synthesis method never previously applied to the fabrication of crystalline, pure ternary systems in order to fabricate ultrathin, homogeneous alloy PtRuFe NWs with a range of controlled compositions. These NWs were subsequently characterized using a suite of techniques including XRD, TEM, SAED, and EDAX in order to verify not only the incorporation of Ru and Fe into the Pt lattice but also their chemical homogeneity, morphology, as well as physical structure and integrity. Lastly, these NWs were electrochemically tested in order to deduce the appropriateness of conventional explanations such as (i) the bi-functional mechanism as well as (ii) the ligand effect to account for our MOR and FAOR reaction data. Specifically, methanol oxidation appears to be predominantly influenced by the Ru content, whereas formic acid oxidation is primarily impacted by the corresponding Fe content within the ternary metal alloy catalyst itself.
In this Progress Report, we update covalent chemical strategies commonly used for the focused functionalization of single‐walled carbon nanotube (SWNT) surfaces. In recent years, SWNTs have been ...treated as legitimate nanoscale chemical reagents. Hence, herein we seek to understand, from a structural and mechanistic perspective, the breadth and types of controlled covalent reactions SWNTs can undergo in solution phase, not only at ends and defect sites but also along sidewalls. We explore advances in the formation of nanotube derivatives that essentially maintain and even enhance their performance metrics after precise chemical modification. We especially highlight molecular insights (and corresponding correlation with properties) into the binding of functional moieties onto carbon nanotube surfaces. Controllable chemical functionalization suggests that the unique optical, electronic, and mechanical properties of SWNTs can be much more readily tuned than ever before, with key implications for the generation of truly functional nanoscale working devices.
Rational covalent functionalization of single‐walled carbon nanotube (SWNT) surfaces can be used to carefully manipulate properties of SWNTs and enhance their performance in a wide range of diverse applications. In this review, we update on a number of diverse chemical strategies for productively functionalizing SWNTs at their ends, defect sites, and sidewalls. Potential applications in the fields of polymer composites and biological devices are also discussed.
As-prepared, single-crystalline bismuth ferrite nanoparticles show strong size-dependent magnetic properties that correlate with: (a) increased suppression of the known spiral spin structure (period ...length of ∼62 nm) with decreasing nanoparticle size and (b) uncompensated spins and strain anisotropies at the surface. Zero-field-cooled and field-cooled magnetization curves exhibit spin-glass freezing behavior due to a complex interplay between finite size effects, interparticle interactions, and a random distribution of anisotropy axes in our nanoparticle assemblies.
In this perspective, the catalytic shortfalls of contemporary DMFCs are discussed in the context of the materials that are currently being employed as electrocatalysts in both the anode and cathode. ...In light of these shortfalls, the inherent advantages of one-dimensional (1D) nanostructures are highlighted so as to demonstrate their potential as efficient, robust, and active replacements for contemporary nanoparticulate electrocatalysts. Finally, we review in detail the recent applications of 1D nanostructured electrocatalysts as both anodes and cathodes, and explore their potentially promising results towards improving DMFC efficiency and cost-effectiveness. In the case of cathode electrocatalysts, our group has recently prepared both 200 nm platinum nanotubes and ultrathin 2 nm platinum nanowires, which evinced two-fold and seven-fold enhancements in area specific ORR activity, respectively, as compared with contemporary commercial Pt nanoparticles. Similarly, the development of one-dimensional anodic electrocatalysts such as alloyed PtRu and PtCo nanowires, hierarchical Pt~Pd nanowires, and segmented PtRu systems have yielded promising enhancements towards methanol oxidation.
A simple and effective template-mediated protocol has been developed for the large-scale, room-temperature preparation of high-aspect-ratio, single-crystalline Tb-doped CePO4 nanowires, measuring ∼12 ...nm in diameter and over 10 μm in length. Moreover, we also isolated sheaf-like bundles of nanostructures. The synthesis mechanism likely involved a crystal splitting step. The resulting nanowires demonstrated an intense redox-sensitive green photoluminescence, which was exploited, in addition to their inherently high biocompatibility and low toxicity, for potential applications in biological imaging and labeling of cells.
Environmentally friendly synthetic methodologies have gradually been implemented as viable techniques in the synthesis of a range of nanostructures. In this work, we focus on the application of ...green‐chemistry principles to the synthesis of complex metal oxide and fluoride nanostructures. In particular, we describe advances in the use of the molten‐salt synthetic methods, hydrothermal protocols, and template‐directed techniques as environmentally sound, socially responsible, and cost‐effective methodologies that allow us to generate nanomaterials without the need to sacrifice sample quality, purity, and crystallinity, while allowing control over size, shape, and morphology.
What's small and green? The answer to this question is discussed in this Review, which highlights some of the techniques currently being used in groups worldwide to prepare novel and useful nanoscale materials in an environmentally friendly manner. Molten‐salt processes (used to prepare the orthorhombic Bi2Fe4O9 structures shown), hydrothermal syntheses, and templating techniques are but some of the methods designed to utilize safer precursors, produce less‐harmful byproducts, and generate less waste.
A size- and shape-dependent morphological transformation was demonstrated during the hydrothermal soft chemical transformation, in neutral solution, of titanate nanostructures into their anatase ...titania counterparts. Specifically, lepidocrocite hydrogen titanate nanotubes with diameters of ∼10 nm were transformed into anatase nanoparticles with an average size of 12 nm. Lepidocrocite hydrogen titanate nanowires with relatively small diameters (average diameter range of ≤ 200 nm) were converted into single-crystalline anatase nanowires with relatively smooth surfaces. Larger diameter (>200 nm) titanate wires were transformed into analogous anatase submicron wire motifs, resembling clusters of adjoining anatase nanocrystals with perfectly parallel, oriented fringes. Our results indicate that as-synthesized TiO2 nanostructures possessed higher photocatalytic activity than the commercial titania precursors from whence they were derived.
With the increased interest in the development of hydrogen fuel cells as a plausible alternative to internal combustion engines, recent work has focused on creating alkaline fuel cells (AFC), which ...employ an alkaline environment. Working in alkaline as opposed to acidic media yields a number of tangible benefits, including (i) the ability to use cheaper and plentiful precious-metal-free catalysts, due to their increased stability, (ii) a reduction in the amount of degradation and corrosion of Pt-based catalysts, and (iii) a longer operational lifetime for the overall fuel cell configuration. However, in the absence of Pt, no catalyst has achieved activities similar to those of Pt. Herein, we have synthesized a number of crystalline ultrathin PtM alloy nanowires (NWs) (M = Fe, Co, Ru, Cu, Au) in order to replace a portion of the costly Pt metal without compromising on activity while simultaneously adding in metals known to exhibit favorable synergistic ligand and strain effects with respect to the host lattice. In fact, our experiments confirm theoretical insights about a clear and correlative dependence between measured activity and chemical composition. We have conclusively demonstrated that our as-synthesized alloy NW catalysts yield improved hydrogen oxidation reaction (HOR) activities as compared with a commercial Pt standard as well as with our as-synthesized Pt NWs. The Pt7Ru3 NW system, in particular, quantitatively achieved an exchange current density of 0.493 mA/cm2, which is higher than the corresponding data for Pt NWs alone. Additionally, the HOR activities follow the same expected trend as their calculated hydrogen binding energy (HBE) values, thereby confirming the critical importance and correlation of HBE with the observed activities.
We describe the preparation and characterization of (a) discrete, individual motifs and (b) arrays of crystalline and pure semiconducting transition metal sulfide (CuS, PbS, and CdS) nanowires, ...synthesized via an inexpensive, generalizable, simplistic, and ambient modified template-directed technique. We have demonstrated control over the diameters and lengths of our one-dimensional (1-D) nanostructures through corresponding variations in the template membrane’s pore size and thickness. We have not only successfully generated cubic-phase 1-D CdS nanowires but also produced, at slightly elevated temperatures, unusual CdS cactus-like, hierarchical nanostructures, consisting of tiny nanoneedles projecting out from the outer surfaces of parent CdS nanotube motifs. Opto-vibrational properties of all of these metal sulfide nanomaterials have been extensively studied. In addition, our results indicate that our as-prepared hexagonal-phase CdS cactus-like nanotubes evinced a higher photocatalytic degradation activity than that of both cubic CdS nanowires and their commercial bulk counterparts.
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