To create truly effective electrocatalysts for the cathodic reaction governing proton exchange membrane fuel cells (PEMFC), namely the oxygen reduction reaction (ORR), necessitates an accurate and ...detailed structural understanding of these electrocatalysts, especially at the nanoscale, and to precisely correlate that structure with demonstrable performance enhancement. To address this key issue, we have combined and interwoven theoretical calculations with experimental, spectroscopic observations in order to acquire useful structural insights into the active site geometry with implications for designing optimized nanoscale electrocatalysts with rationally predicted properties. Specifically, we have probed ultrathin (∼2 nm) core–shell Pt∼Pd9Au nanowires, which have been previously shown to be excellent candidates for ORR in terms of both activity and long-term stability, from the complementary perspectives of both DFT calculations and X-ray absorption spectroscopy (XAS). The combination and correlation of data from both experimental and theoretical studies has revealed for the first time that the catalytically active structure of our ternary nanowires can actually be ascribed to a PtAu∼Pd configuration, comprising a PtAu binary shell and a pure inner Pd core. Moreover, we have plausibly attributed the resulting structure to a specific synthesis step, namely the Cu underpotential deposition (UPD) followed by galvanic replacement with Pt. Hence, the fundamental insights gained into the performance of our ultrathin nanowires from our demonstrated approach will likely guide future directed efforts aimed at broadly improving upon the durability and stability of nanoscale electrocatalysts in general.
We report a systematic computational search of molecular frameworks for intrinsic rectification of electron transport. The screening of molecular rectifiers includes 52 molecules and conformers ...spanning over 9 series of structural motifs. N-Phenylbenzamide is found to be a promising framework with both suitable conductance and rectification properties. A targeted screening performed on 30 additional derivatives and conformers of N-phenylbenzamide yielded enhanced rectification based on asymmetric functionalization. We demonstrate that electron-donating substituent groups that maintain an asymmetric distribution of charge in the dominant transport channel (e.g., HOMO) enhance rectification by raising the channel closer to the Fermi level. These findings are particularly valuable for the design of molecular assemblies that could ensure directionality of electron transport in a wide range of applications, from molecular electronics to catalytic reactions.
A template-directed, sol–gel synthesis is utilized to produce crystalline RuO2 nanowires. Crystalline nanowires with a diameter of 128 ± 15 nm were synthesized after treating the nanowires at 600 °C ...in air. Analysis of these nanowires by X-ray powder diffraction revealed the major crystalline phase to be tetragonal RuO2 with a small quantity of metallic ruthenium present. Further analysis of the nanowire structures by high-resolution transmission electron microscopy reveals that they are polycrystalline and are composed of interconnected, highly crystalline, nanoparticles having an average size of ∼25 nm. Uniform 3 nm Pt nanoparticles were dispersed on the surface of RuO2 nanowires using an ambient, solution-based technique yielding a hybrid catalyst for methanol oxidation. Linear sweep voltammograms (LSVs) and chronoamperometry performed in the presence of methanol in an acidic electrolyte revealed a significant enhancement in the onset potential, mass activity, and long-term stability compared with analogous Pt nanoparticles supported on commercially available Vulcan XC-72R carbon nanoparticles. Formic acid oxidation LSVs and CO stripping voltammetry revealed that the RuO2-supported Pt nanoparticles exhibit significantly higher CO tolerance, which leads to higher catalytic stability over a period of several hours. X-ray photoelectron spectroscopy results suggest that crystalline RuO2 leads to less-significant oxidation of the Pt surface relative to more widely studied hydrous RuO2 supports, thereby increasing catalytic performance.
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.
LiFePO4 materials have become increasingly popular as a cathode material due to the many benefits they possess including thermal stability, durability, low cost, and long life span. Nevertheless, to ...broaden the general appeal of this material for practical electrochemical applications, it would be useful to develop a relatively mild, reasonably simple synthesis method of this cathode material. Herein, we describe a generalizable, 2-step methodology of sustainably synthesizing LiFePO4 by incorporating a template-based, ambient, surfactantless, seedless, U-tube protocol in order to generate size and morphologically tailored, crystalline, phase-pure nanowires. The purity, composition, crystallinity, and intrinsic quality of these wires were systematically assessed using transmission electron microscopy （TEM）, high-resolution TEM （HRTEM）, scanning electron microscopy （SEM）, X-ray diffraction （XRD）, selected area electron diffraction （SAED）, energy dispersive analysis of X-rays （EDAX）, and high-resolution synchrotron XRD. From these techniques, we were able to determine that there is an absence of any obvious defects present in our wires, supporting the viability of our synthetic approach. Electrochemical analysis was also employed to assess their electrochemical activity. Although our nanowires do not contain any noticeable impurities, we attribute their less than optimal electrochemical rigor to differences in the chemical bonding between our LiFePO4 nanowires and their bulk-like counterparts. Specifically, we demonstrate for the first time experimentally that the Fe-O3 chemical bond plays an important role in determining the overall conductivity of the material, an assertion which is further supported by recent ＂first-principles＂ calculations. Nonetheless, our ambient, solution-based synthesis technique is capable of generating highly crystalline and phase-pure energy-storage-relevant nanowires that can be tailored so as to fabricate different sized materials of reproducible, reliable morphology.
Due to their unique size, surface area, and chemical characteristics, nanoparticles' use in consumer products has increased. However, the toxicity of nanoparticle (NP) exposure during the ...manufacturing process has not been fully assessed. Tungstate NP are used in numerous products, including but not limited to scintillator detectors and fluorescent lighting. As with many NP, no apparent toxicity studies have been completed with tungstate NP. The hypothesis that tungstate NP in vitro exposure results in reactive oxygen species (ROS) formation and cytotoxicity was examined. Differences in toxicity based on tungstate NP size, shape (sphere vs. wire), and chemical characteristics were determined. RAW 264.7 mouse monocyte macrophages were exposed to tungstate NP, and ROS formation was assessed via electron spin resonance (ESR), and several assays including hydrogen peroxide, intracellular ROS, and Comet. Results showed ROS production induced by tungstate nanowire exposure, but this exposure did not result in oxidative DNA damage. Nanospheres showed neither ROS nor DNA damage following cellular exposure. Cells were exposed over 72 h to assess cytotoxicity using an MTT (tetrazolium compound) assay. Results showed that differences in cell death between wires and spheres occurred at 24 h but were minimal at both 48 and 72 h. The present results indicate that tungstate nanowires are more reactive and produce cell death within 24 h of exposure, whereas nanospheres are less reactive and did not produce cell death. Results suggest that differences in shape may affect reactivity. However, regardless of the differences in reactivity, in general both shapes produced mild ROS and resulted in minimal cell death at 48 and 72 h in RAW 264.7 cells.
An ambient, surfactant-based synthetic means was used to prepare ultrathin binary (d ∼ 2 nm) Pd–Ni nanowires, which were subsequently purified using a novel butylamine-based surfactant-exchange ...process coupled with an electrochemical CO adsorption and stripping treatment to expose active surface sites. We were able to systematically vary the chemical composition of as-prepared Pd–Ni nanowires from pure elemental Pd to Pd0.50Ni0.50 (atomic ratio), as verified using EDS analysis. The overall morphology of samples possessing >60 atom % Pd consisted of individual, discrete one-dimensional nanowires. The electrocatalytic performances of elemental Pd, Pd0.90Ni0.10, Pd0.83Ni0.17, and Pd0.75Ni0.25 nanowires in particular were examined. Our results highlight a “volcano”-type relationship between chemical composition and corresponding ORR activities with Pd0.90Ni0.10, yielding the highest activity (i.e., 1.96 mA/cm2 at 0.8 V) among all nanowires tested. Moreover, the Pd0.90Ni0.10 sample exhibited outstanding methanol tolerance ability. In essence, there was only a relatively minimal 15% loss in the specific activity in the presence of 4 mM methanol, which was significantly better than analogous data on Pt nanoparticles and Pt nanowires. In addition, we also studied ultrathin, core–shell Pt∼Pd0.90Ni0.10 nanowires, which exhibited a specific activity of 0.62 mA/cm2 and a corresponding mass activity of 1.44 A/mgPt at 0.9 V. Moreover, our as-prepared core–shell electrocatalysts maintained excellent electrochemical durability. We postulate that one-dimensional Pd–Ni nanostructures represent a particularly promising platform for designing ORR catalysts with high performance.
In this article, we address two key challenges in the development of electrocatalysts for direct methanol fuel cells by rationally tailoring the morphology and chemical composition of Pd-based ...nanowires (NWs) for enhanced performance. First, we have examined the morphology and composition-dependent performance of Pt1–x Pd x NWs toward the methanol oxidation reaction (MOR). Elemental Pt NWs were found to possess a significant morphology-dependent enhancement of nearly 3-fold in terms of peak MOR-specific activity over that of commercial Pt NP/C. In addition, tailoring the chemical composition in Pt1–x Pd x NWs can lead to measurable increases in MOR kinetics, which can be attributed to improved oxidation of formic acid and, potentially, increased selectivity for a direct, CO-free pathway. Second, we have explored the stability of ORR performance in the presence of measurable concentrations of methanol as a function of chemical composition in Pt1–x Pd x NWs and Pt-free Pd9Au NWs. In the context of the Pt1–x Pd x NWs, a distinctive volcano-type dependence has been noted with respect to chemical composition, and on the basis of the MOR activities and methanol tolerant ORR behavior, Pt7Pd3 NWs have been highlighted as an optimal catalyst architecture. We have also analyzed the methanol tolerance in Pd9Au NWs, which represents a highly active, durable Pt-free alternative to traditional Pt-based nanostructured catalysts. Herein, we have demonstrated that Pd9Au NWs (0.42 mA/cm2) with no effective Pt content can outperform Pt-based nanostructures, such as Pt NWs (0.32 mA/cm2) and nanoparticulate Pt NP/C (0.24 mA/cm2) in the presence of 4 mM methanol/0.1 M HClO4.
The electrochemical oxidation of small organic molecules (SOMs) such as methanol and glucose is a critical process and has relevant applications in fuel cells and sensors. A key challenge in SOM ...oxidation is the poisoning of the surface by carbon monoxide (CO) and other partially oxidized intermediates, which is attributed to the presence of Pt–Pt pair sites. A promising pathway for overcoming this challenge is to develop catalysts that selectively oxidize SOMs via “direct” pathways that do not form CO as a primary intermediate. In this report, we utilize an ambient, template-based approach to prepare PtAu alloy nanowires with tunable compositions. X-ray photoelectron spectroscopy measurements reveal that the surface composition matches that of the bulk composition after synthesis. Monte Carlo method simulations of the surface structure of PtAu alloys with varying coverage of oxygen adsorbates and varying degrees of oxygen adsorption strength reveal that oxygen adsorption under electrochemical conditions enriches the surface with Pt and a large fraction of Pt–Pt sites remain on the surface even with the Au content of up to 50%. Electrochemical properties and the catalytic performance measurements of the PtAu nanowires for the oxidation of methanol and glucose reveal that the mechanistic pathways that produce CO are suppressed by the addition of relatively small quantities of Au (∼10%), and CO formation can be completely suppressed by 50% Au. The suppression of CO formation with small quantities of Au suggests that the presence of Pt–Au pair sites may be more important in determining the mechanism of SOM oxidation rather than Pt–Pt pair site density.
In this report, we examine the composition- and size-dependent performance in hierarchical Pd1–x Au x nanowires (NWs) encapsulated with a conformal Pt monolayer shell (Pt∼Pd1–x Au x ). The ultrathin ...Pd1–x Au x NWs are prepared by a solution-based method wherein the chemical composition can be readily and predictably controlled. Importantly, as-prepared Pd9Au NWs maintain significantly enhanced oxygen reduction reaction (ORR) activity (0.40 mA/cm2), as compared with elemental Pd NW/C (0.12 mA/cm2) and Pt nanoparticles (NP)/C (0.20 mA/cm2), respectively. After the deposition of a Pt monolayer, a volcano-type composition dependence is observed in the ORR activity of the Pt∼Pd1–x Au x NWs as the Au content is increased from 0 to 30% with the activity of the Pt∼Pd9Au NWs (0.98 mA/cm2, 2.54 A/mgPt), representing the optimum performance. We note that the platinum group metal activity of the ultrathin 2 nm NWs (0.64 A/mg) is significantly enhanced as compared with that of analogous 50 nm NWs (0.16 A/mg) and commercial Pt NP/C (0.1–0.2 A/mg), thereby highlighting a distinctive size-dependent enhancement in NW performance.
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