Nathan Harris Mack

Isolation and purification of single-walled carbon nanotubes (SWCNTs) are prerequisites for their implementation in various applications. In this work, we present a fast (similar to 5 min), low-cost, and easily scalable bench-top approach to the extraction of high-quality isolated SWCNTs from bundles and impurities in an aqueous dispersion. The extraction procedure, based on aqueous two-phase (ATP) separation, is widely applicable to any SWCNT source (tested on samples up to 1.7 nm in diameter) and independent of defect density, purity, diameter, and length. The extracted dispersions demonstrate that the removal of large aggregates, small bundles, and impurities is comparable to that by density gradient ultracentrifugation, but without the need for high-end instrumentation. Raman and fluorescence-excitation spectroscopy, single-nanotube fluorescence imaging, atomic force and transmission electron microscopy, and thermogravimetric analysis all confirm the high purity of the isolated SWCNTs. By predispersing the SWCNTs without sonication (only gentle stirring), full-length, pristine SWCNTs can be isolated (tested up to 20 mu m). Hence, this simple ATP method will find immediate application in the generation of SWCNT materials for all levels of nanotube research and applications, from fundamental studies to high-performance devices.

Increased oxygen reduction reaction (ORR) kinetics, improved CO tolerance, and more efficient water and heat management represent significant advantages that high-temperature polymer electrolyte fuel cells (HT-PEFCs) operating with a phosphoric acid-doped polybenzimidazole (PBI) membrane offer over traditional Nafion-based, low-temperature PEFCs. However, before such HT-PEFCs become viable, the detrimental effect of phosphate chemisorption on the performance of state-of-the-art wt-based cathode catalysts needs to be addressed. In this study, we propose a solution to the severe poisoning of Pt-based PEFC cathode catalysts with phosphates (H2PO4 and HPO42-) by replacing standard Pt/C catalysts with phosphate-tolerant, nonprecious metal catalyst (NPMC) formulations. Catalysts with a very high surface area (845 m(2) g (-1)) were synthesized in this work from polyaniline (PANI), iron, and carbon using a high-temperature approach. The effects of metal precursors and metal loading on the morphology, structure, and ORR activity of the NPMCs were systematically studied. Electrochemical measurements indicated that as-prepared Fe-based catalysts (PANI-Fe-C) can tolerate phosphate ions at high concentrations and deliver ORR performance in 5.0 M H3PO4 that is superior to that of Pt/C catalysts. A 30 wt 96 Fe-derived catalyst was found to have the most porous morphology and the highest surface area among studied Fe-based catalysts, which correlates with the highest ORR activity of that catalyst. These cost-effective and well-performing ORR catalysts can potentially replace Pt/C catalysts in phosphoric acid-based HT-PEFCs.

In this work, a novel systematic bottom-up approach in the investigation of the role of each component in non-precious metal ORR electrocatalysts is developed. Liquid-phase exfoliated graphite and graphene oxide were used as model two-dimensional carbon precursors. Nitrogen doping of these precursors was achieved via high-temperature ammonia treatment, as well as a novel technique of energetic neutral atom beam lithography/epitaxy (ENABLE), introduced for the first time. ENABLE offers the ability of working at high kinetic energies, overcoming high thermal limitation barriers, and allowing for direct chemical bond activation of graphitic systems at lower temperatures. The correlation of chemical-structure to ORR activity of these model nitrogen-doped systems was investigated using ultrathin film electrodes.

Structure property performance relationships of disulfonated poly(arylene ether sulfone) multiblock copolymer membranes were investigated for their use in direct methanol fuel cell (DMFC) applications. Multiple series of reactive polysulfone, polyketone, and polynitrile hydrophobic block segments having different block lengths and molecular composition were synthesized and reacted with a disulfonated poly(arylene ether sulfone) hydrophilic block segment by a coupling reaction. Large-scale morphological order of the multiblock copolymers evolved with the increase of block size that gave notable influence on mechanical toughness, water uptake, and proton/methanol transport. Chemical structural changes of the hydrophobic blocks through polar group, fluorination, and bisphenol type allowed further control of the specific properties. DMFC performance was analyzed to elicit the impact of structural variations of the multiblock copolymers. Finally, DMFC performances of selected multiblock copolymers were compared against that of the industrial standard Nafion in the DMFC system.

A composite anode consisting of hollow SnO2 microspheres covered by glass-like B2O3 layers was prepared via a combined hydrothermal-impregnation method, which results in much improved electrochemical performance in lithium ion batteries, relative to pristine SnO2 anodes. The cycling and rate capabilities of the SnO2-B2O3 composite anodes were investigated as a function of B2O3 content. The balance between increased electron-acceptor effect and compromised electronic conductivity due to addition of B2O3 is maximized around 20 wt% B203 loading. The best performing SnO2-B2O3 composite anode exhibits a specific capacity of 622.7 mAh g(-1) up to 160 cycles, and is able to maintain a capacity above 528.6 mAh g-1 at rate of 5C. These enhanced performance characteristics are attributed to the unique composite structures consisting of the hollow SnO2 cores and the B2O3 buffer layers, which likely are beneficial for reducing the overall volume changes. Importantly, the decreased charge transfer resistance and increased Li+ diffusion coefficient, resulting from B2O3 coating, lead to overall improvement of rate performance for the composite anodes. Such-fabricated composite structures are stable during the Li+ insertion/extraction, thereby promoting cycling stability. (C) 2013 Elsevier B.V. All rights reserved.

A major, unprecedented improvement in the durability of polymer electrolyte membrane fuel cells is obtained by tuning the properties of the interface between the catalyst and the ionomer by choosing the appropriate dispersing medium. While a fuel cell cathode prepared from aqueous dispersion showed 90 mV loss at 0.8 A cm(-2) after 30000 potential cycles (0.6-1.0 V), a fuel cell cathode prepared from glycerol dispersion exhibited only 20 mV loss after 70000 cycles. This minimum performance loss occurs even though there was an over 80% reduction of electrochemical surface area of the Pt catalyst. These findings indicate that a proper understanding and control of the catalyst-water-ionomer (three-phase) interfaces is even more important for maintaining fuel cell durability in typical electrodes than catalyst agglomeration, and this opens up a novel path for tailoring the functional properties of electrified interfaces.

The effect of cathode structures on the chemical stability of Nafion membranes is investigated. Membrane electrode assemblies (MEAs) were prepared by using Nafion 212 membrane and commercially available carbon supported Pt electro-catalysts. The cathode structures were controlled by the use of long side chain (LSC) and short side chain (SSC) perfluorosulfonic acids (PFSAs) as well as three dispersing solvents for the electrode fabrication (a water-isopropanol mixture, N-methyl-2-pyrrolidone, or glycerol). The membrane durability was evaluated by the H-2 crossover current density after a 200-hour open circuit voltage accelerated stress test. The MEA with a glycerol-processed SSC PFSA-bonded cathode exhibited a 200-fold less H-2 crossover current density than the MEA with a water-isopropanol-processed LSC PFSA-bonded cathodes. The analyzes by electrochemical impedance spectroscopy and microscopy suggest that the structural uniformity of cathodes play the most significant role in the chemical stability of the Nafion membranes. This study emphasizes the importance of cathode structures on the durability of Nafion membranes. (C) The Author(s) 2014. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.