Pulsed field gradient (PFG) NMR at high magnetic field was used to study microscopic diffusion of dimethyl methyl phosphonate (DMMP), a common chemical warfare agent (CWA) simulant, and water in Nafion membranes. PFG NMR measurements were performed for a broad range of molecular displacements. The self-diffusivities were measured as a function of the DMMP concentration for several fixed water concentrations. The measured data suggest that DMMP and water diffuse in different regions of Nafion. While water mostly diffuses in hydrophilic regions of the membrane, viz. water channels, DMMP diffusion is mostly limited to interfacial perfluoroether regions between these water channels and the semi-crystalline matrix.

Pulsed field gradient (PFG) NMR in combination with quasielastic neutron scattering (QENS) was used to investigate self-diffusion of water and acetone in Nafion membranes with and without immobilized vanillic acid (VA). Complementary characterization of these membranes was performed by small angle X-ray scattering (SAXS) and NMR relaxometry. This study was motivated by the recent data showing that an organic acid, such as VA, in Nafion can preserve its catalytic activity in the presence of water even at high intra-polymer water concentrations corresponding up to 100% ambient relative humidity. However, there is currently no clear understanding of how immobilized organic acid molecules influence the microscopic transport properties and related structural properties of Nafion. Microscopic diffusion data measured by PFG NMR and QENS are compared for Nafion with and without VA. For displacements smaller than the micrometer-sized domains previously reported for Nafion, the VA addition was not observed to lead to any significant changes in the water and/or acetone self-diffusivity measured by each technique inside Nafion. However, the reported PFG NMR data present evidence of a different influence of acetone concentration in the membranes with and without VA on the water permeance of the interfaces between neighboring micrometer-sized domains. The reported diffusion data are correlated with the results of SAXS structural characterization and NMR relaxation data for water and acetone.

The high mass and volume-specific energy of dimethyl ether (DME) relative to hydrogen make it an attractive alternative electrochemical fuel source for portable applications such as powering drones and eVTOLs. A key stumbling block to the development of direct DME fuel cells (DDMEFCs) is the poisoning of the electrocatalyst surface by oxidation intermediates such as COads. In this study, an all-queous colloidal synthesis method for producing highly dispersed Pt2Bi alloy nanoplatelets (NPT) to mitigate such poisoning is presented. NPT synthesis entails the use of stannous chloride as an autocatalytic reducing and stabilizing agent for both Pt and Bi salts in aqueous solution. Sn and Bi stripping from the surface of these NPT is found to maximize activity for DME electrooxidation (DMEOR) relative to commercial Pt-C. A stable chronoamperometric current of 3.3 A g(Pt)(-1) (15.8 mu A cm(P)(t)(-2)) is observed at the peak COads-stripping potential of 0.7 V vs RHE at 50 degrees C over a time interval where Pt-C activity becomes negligible. The response of anodic peak positions to potential sweep rates is used to reveal the impact of alloying (electronic structure) on the electro-oxidation rates of various intermediate species on Pt2Bi NPT. Resistance to poisoning coincides both with a reduction in the C a d s specific activity onset potential by 25 mV relative to Pt-C and faster DME electro-oxidation kinetics. DDMEFC testing of the unsupported Pt2Bi NPT utilizing a phosphoric acid-doped polybenzimidazole (PBI) membrane operating at 240 degrees C yields a peak power of 56 W g(P)(GM,anode)(-1). This represents a 30% increase relative to a commercial PtRu catalyst.

Pulsed field gradient (PFG) NMR at high field was utilized to directly observe a transition between two different diffusion regimes in a Nafion 117 membrane loaded with water and acetone. Although water self-diffusivity at small water loadings was observed to be diffusion time-independent in the limit of small and large diffusion times, it showed a significant decrease with increasing diffusion time at intermediate times corresponding to root mean square displacements on the order of several microns. Under our experimental conditions, no self-diffusivity dependence on diffusion time was found for water at large water loadings and for acetone at all studied acetone loadings. The diffusion time-dependent self-diffusivity at small water concentration is explained by the existence of finite domains of interconnected water channels with sizes in the range of several microns that form in Nafion in the presence of acetone. The domain sizes and permeance of transport barriers separating adjacent domains are estimated based on the measured PFG NMR data. At large water concentrations, the water channels form a fully interconnected network, resulting in time-independent self-diffusivity. The absence of such a percolation-like transition with increasing molecular concentration for acetone is attributed to a difference in the regions available for water and acetone diffusion in Nafion. The diffusion data are correlated with and supported by structural data obtained using small-angle X-ray and neutron scattering techniques. These techniques reveal distinct water channels with radial dimensions in the nanometer range increasing upon water addition, while acetone appears to be in an interfacial perfluoroether region, reducing the size of the radial channel dimension.

In this work, evidence for ligand formation between Sn and Bi during the colloidal synthesis of Bi metal nanoparticles (NP) in an aqueous suspension is provided utilizing time-resolved X-ray absorption and 119Sn Mössbauer spectroscopy .

Rapid in-situ chemical analysis of flowing gas streams is of interest in a wide range of applications but requires deconvolution of the time-scales associated with the analyte source concentration, its accumulation within a sampling chamber, and its detection by a sensor. A mathematical analysis is presented on the use of a flow-through sample chamber for rapid, in-situ breath analysis utilizing analyte diffusion through a Nafion membrane optode. We show that this approach yields apparently non-Fickian (anomalous or Case II) transport that varies from t1/2 to t as t → 0 with constant inlet concentration. Such behavior arises due to the transition from membrane-limited to sample chamber-limited transport dynamics depending on test conditions. The model is validated utilizing experimental data obtained from the color response associated with the Friedel-Craft acylation of acetone vapor with resorcinol reagent immobilized in Nafion membrane solid-state catalyst. Reduction of optode membrane thickness and increase in membrane humidification yield an optical response limited only by sample chamber material accumulation. At this limit, the exhaled breath signal for acetone obtained from a healthy individual is found to vary as t2 (apparently Super Case II transport). Utilizing a simplified material balance on the human lung, this observation is ascribed to a constant acetone exhalation rate as opposed to a constant exhaled acetone concentration. This conclusion is shown to have broad implications on the use of exhaled breath biomarkers for medical diagnosis, in particular, lung physiology and permeability.

Endogenous volatile organic compounds (VOCs) such as acetone in exhaled human breath are associated with metabolic conditions in the bloodstream. Development of compact, rapid detectors of exhaled breath chemical composition in clinical settings is challenging due to the small sample size that can be collected during a single exhalation as well as spectroscopic interference by the abundance of water. In this paper, we show that the activity of a catalytic polymer membrane (Nafion 117) toward the heterogeneous condensation reaction of immobilized resorcinol reagent with gas-phase acetone can be preserved even at 100% ambient relative humidity through the incorporation of organic acids such as vanillic or tiglic. The reaction produces a colored flavan product that permits highly selective and sensitive correlation to acetone concentration in exhaled breath. Such behavior suggests solvent displacement, analogous to homogeneous liquid-phase systems. However, unlike classic acid-base equilibria, the extent of optode water resistance is shown to increase with the pK(a) of the imbibed organic acid while peak signal intensity of the imbibed acid undergoes a bathochromic shift to longer wavelengths. These observations are consistent with competition between organic acid deprotonation by water in a mixed solvent system on the one hand and immobilization on the other. Finally, we demonstrate how when applied to the direct chemical analysis of acetone in exhaled human breath, the approach yields excellent correlation to blood glucose in diabetics.

The importance of stabilizing ligands cannot be understated for the colloidal synthesis of nanoparticles as well as their dispersion and assembly into macro-structures such as electrodes. We have recently demonstrated a novel all-inorganic ligand approach to Pt nanoparticle synthesis where surface-adsorbed Sn serves as both reducing agent and stabilizing ligand, producing remarkably monodispersed nanoparticles. By eliminating the need to remove organic surfactants prior to nanoparticle incorporation into electrodes, we were able to use electrostatic assembly to achieve well-defined nanoparticle dispersions in contrast to the aggregated structures common in the field. The approach has allowed us to elucidate the nature of structure sensitivity for electrocatalytic oxygen reduction reaction in acidic and alkaline media. In this paper, we provide a detailed investigation of the evolution of surface-adsorbed Sn-Pt ligand interaction during Pt nanoparticle growth. We show that surface adsorbed SnCl3 exists as pyramidally coordinated to the Pt nanoparticle through Sn-Pt bond formation, resulting in a distorted tetrahedrally coordinated Sn-moiety. Furthermore, we show the shift in the bond charge distribution as the Pt is reduced from its initial Pt2+ state to its metallic state. Direct evidence for the distorted tetrahedral coordination of Pt-SnCl3 and the shift in Sn-Pt bond charge during nanoparticle growth emerges from the Sn-119 Mossbauer quadrupole splitting (QS) and Isomer-shift (IS) respectively. Evolution of the structure and chemistry of inorganic complexes during nanoparticle growth has never before been demonstrated and our work is the first to identify the strength of the Sn-Pt interaction in these systems. Such an understanding of nanoparticle surface chemistry will permit extension of this technique to other metals and macro-structures. (C) 2018 Elsevier B.V. All rights reserved.

The dissolution of base metal cores made of Ni, Fe, and Co in the hot, acidic environment of proton exchange membrane fuel cells (PEMFCs) has impeded the use of novel core-shell nanoparticles (NPs) where the precious metals Pt and Ru are confined to surface. Since electrocatalysis occurs on surfaces, a substantial reduction in precious metal use could be achieved with core-shell NPs relative to pure systems, as long as the core remains stable during use. Bi is an abundant earth metal with a low environmental and human toxicity and water stability across a wide pH range, particularly under the acidic conditions of a fuel cell environment. These characteristics make Bi an ideal candidate for use as a durable and inexpensive core material to minimize precious metal use in highly dispersed core-shell Bi-Pt nanoparticle (NP) electrocatalyst in PEMFCs. Bi has been found to substantially promote direct alcohol fuel cell (DAFC) efficiency relative to pure Pd by mitigating catalyst poisoning due to irreversibly bound reaction intermediates such as CO and acetaldehyde. Unfortunately, the formation of Bi NPs is difficult and currently based on unsustainable methods employing toxic organic solvents, expensive organic precursors, and dangerous reducing agents such as hydrazine and lithium that frequently yield highly non-uniform structures. In this work, we describe an inorganic ligand metal redox synthesis approach which is used to synthesize a variety of well-defined core-shell as well as Pt-Bi alloy nanoparticle structures under sustainable, all-aqueous conditions. The approach uses SnCl3- as both a reducing agent and inorganic stabilizing ligand to provide unprecedented control over the size and structure of these nanoparticles. Furthermore, the ligand itself exists in a variety of structures that can be used to control electrostatic assembly onto various substrates. The materials to be discussed in this presentation have never been previously reported and their synthesis is unexpected given both that pure Bi has well-known instability in high salt, low pH environment and that Bi-Sn ligand formation has not been previously recognized. The implications of such metal NP synthesis are transformative with respect to the practical realization and wide application of alternative energy technologies given its simplicity and sustainability. We will provide data showing that such structures can double the electrocatalytic oxygen reduction reaction mass activity and increase the specific activity by nearly an order of magnitude relative to pure systems (see graphic below). As important, we observe simultaneous mitigation of carbon monoxide poisoning due to the persistence of Bi and Sn on their surface. These observations are discussed relative to the surface distribution of active sites as characterized utilizing electrochemical adatom stripping. Figure 1

Computational and experimental investigations have previously revealed that the surface chemistry of metal nanoparticles used as fuel cell electrocatalysts can influence both wetting by water as well as the morphology of deposited solid polymer electrolytes. However, the impact of nanoparticle surface chemistry on practical fuel cell operation at high current load is unknown. Mass-transport and ohmic overpotentials are found to dominate performance at high load, hindering the impact of significant recent advances in electrocatalytic activity during practical operation. Prior investigations with operating fuel cells have typically focused on altering the distribution and content of ionomer in the electrode as well as the introduction of hydrophilic agents but often neglect the role of metal nanoparticle surface chemistry. In this investigation, we contrast the electrocatalytic and practical fuel cell performance of commercial carbon-supported Pt nanoparticles to carbon-supported and unsupported Pt and Pt-alloy nanoparticles synthesized via a metal-redox scheme that utilizes stabilizing inorganic ligands to alter the nanoparticle surface chemistry. 119Sn Mossbauer spectroscopy, X-ray Absorption, and X-ray Scattering are used to characterize ligand surface chemistry and structure. The catalysts are also characterized with respect to their Pt surface atom coordination using electrochemical stripping of Bi and Ge adatoms. In addition, the surface wettability of the various catalyst components (support, ionomer, metal nanoparticles) is differentiated using dynamic contact angle analysis. We determine that hydrogen/air fuel cell operation at high load is well-correlated to the surface chemistry of the nanoparticles. For a given surface atom coordination and ORR mass activity, catalysts produced via hydrophilic surface ligands permit the electrodes to operate under substantially drier conditions. Comparable single cell maximum power of 0.5 W/cm2 is realized at 40% inlet gas humidification for nanoparticles synthesized utilizing hydrophilic surface ligands versus 80% when an industrial precipitation approach is used for Pt nanoparticles ~3 nm, independent of ionomer loading. A remarkable swap in polarization behavior with humidification is observed. Correlations of fuel cell humidification requirements to alternate nanoparticle morphologies with varying surface atom coordination, inorganic ligand structures, and wetting behavior are described. Figure 1

Rotating Disk Electrode (RDE) measurements on model glassy carbon (GC) substrates and Cyclic Voltammetry on more practical commercial carbon supports are used to demonstrate that the kinetics of the positive VO2+/VO2+ redox reaction can be substantially enhanced by using electrostatic layer-by-layer assembly (LbL) to decorate their surface with graphene nanoplatelets (GNPs). An exchange current density, i(0), is obtained that is more than two orders of magnitude greater than that observed with standard carbon supported Pt nanocatalyst with the deposition of only 20 GNP layers. Tafel slope analysis is compared to electron microscopy imaging to conclude that while faster redox kinetics is associated with an increase in the available active area, the prevalence of smaller GNPs and associated edge sites the can attenuate activity gains with increasing number of layers. Practical implementation to existing Vanadium Redox Flow Battery (VRFB) configurations was demonstrated through the application of a 370 nm (20 layer) LbL GNP coating on carbon felt (CF). The GNP coating yielded a 5% increase relative in voltage and overall efficiency of charge discharge curves obtained under typical VRFB cell operating conditions at 40 mA cm(-2). Furthermore, a substantial increase in the discharge time is observed with this GNP coating on CF.

Early stage discovery of lung cancer can significantly improve patient prognosis. Unfortunately, the impact of a well-known technique such as computed tomography screening on mortality rates is uncertain. It has been postulated that tumor growth may lead to peroxidation of cell membranes and is responsible for the observed emission of volatile organic compounds (VOC’s) such as acetone and formaldehyde. This observation has stimulated significant recent effort in the measurement of these compounds in exhaled breath. While promising approaches have been developed for analyzing average VOC concentrations in exhaled breath, an unrecognized drawback to current efforts is the presumption that gas partitioning between the blood and gas phases occurs so rapidly that the time averaged concentration of collected VOCs is representative of their saturation concentration in the lungs. In fact, when one considers the mass transport resistances encountered, VOC lung concentration is likely to be time-dependent and any sensing approach must have sufficient time resolution to capture the dynamic evolution of this concentration during a typical breath exhalation period of not more than 40 seconds in a high humidity environment. In this presentation, we describe development of an optical exhaled gas sensing approach utilizing a nanostructured polymeric membrane that meets these requirements and demonstrates the dynamic evolution of exhaled breath biomarker concentration. We show that the activity of a solid acid catalyst toward the heterogeneous condensation reactions of immobilized resorcinol reagent with gas-phase acetone and formaldehyde can be preserved even at 100% ambient relative humidity through the incorporation of organic acids such as vanillic or tiglic. The reaction produces a colored flavan and poly-condensation products that permits highly selective and sensitive correlation to acetone and formaldehyde concentrations in exhaled breath. Such behavior relies on the complex heterophase morphology of the membrane as will be described. Figure 1

Sulfonated polymers such as sulfonated polystyrene (sPS) and perfluorosulfonic acid (PSA) based materials have been of long-standing interest as sustainable solid acid catalyst alternatives to toxic homogeneous acid catalysts in the production of high-volume specialty chemicals. Our interest in these materials arises from the potential use of their uniquely heterogeneous hydrophobic-hydrophilic morphology for wearable chemical sensing in personal protective equipment (PPE). Sulfonated polymers can be applied as durable coatings on fiber blends using coaxial electrospinning to produce non-woven fabrics or onto polymeric Gore ® woven textiles. Prior research with these materials did not consider their catalytic properties and determined that water as liquid or vapor from sweat can actually promote chemical warfare (CW) agent transport. We demonstrate that the interface between dispersed hydrophilic clusters and the hydrophobic matrix of sulfonated polymers can be exploited to introduce catalytically active sites that trap and convert CW agents to a non-toxic organophosphate ester already used commercially as a flame retardant in clothing. Such trapping and conversion would not interfere with water transport and the associated color change provides a simultaneous means for optical detection. Despite significant understanding of how PSA polymer morphology impacts the transport behavior of ions, uncertainty remains over why some organic molecules become immobilized as they diffuse and react in this important class of materials while others do not. These aspects will be discussed in this presentation. Figure 1

The crystalline silicalite membrane has been demonstrated as an effective ion separator for acidic solutions of vanadyl sulfate and for potential application as a proton-permselective electrolyte membrane in the all-vanadium redox-flow battery. Silicalite contains uniform channels with an effective diameter of 0.56 nm, which permits the small H3O+ ions to diffuse through but is impermeable to the large hydrated multivalent vanadium ions due to steric effects. Unlike conventional polymeric ion exchange materials, silicalite is nonionic and its proton conductivity relies on the electric field-driven H3O+ transport through the sub-nanometer pores. The silicalite membrane exhibits high proton selectivity relative to vanadium ions and a significantly reduced self-discharge rate compared to that of Nafion ion exchange membranes.

Measurement of acetone in human breath samples has been previously shown to provide significant non-invasive diagnostic insight into the control of a patient's diabetic condition. In patients with diabetes mellitus, the body produces excess amounts of ketones such as acetone, which are then exhaled during respiration. Using various breath analysis methods has allowed for the accurate determination of acetone concentrations in exhaled breath. However, many of these methods require instrumentation and pre-concentration steps not suitable for point-of-care use. We have found that by immobilizing resorcinol reagent into a perfluorosulfonic acid polymer membrane, a controlled organic synthesis reaction occurs with acetone in a dry carrier gas. The immobilized, highly selective product of this reaction (a flavan) is found to produce a visible spectrum color change which could measure acetone concentrations to less than ppm. We here demonstrate how this approach can be used to produce a portable optical sensing device for real-time, non-invasive acetone analysis.

Understanding and controlling nanoparticle formation mechanisms is important because of increasing reports of unusual material properties in this critical size region in fields ranging from magnetics, electrocatalysis, optics, and heterogeneous synthesis. Here we use real-time, in situ small-angle and wide-angle X-ray scattering to dynamically monitor the production of Pt critical nuclei from Sn-Pt complexes on a length scale approaching 0.6 nm. A time resolution of 20 s is achieved due to the slow reduction and growth processes of this unique system. The structure and evolution of inorganic Sn ligands on these Pt nanopartide cores is of particular interest because of the ability of Sn to mitigate poisoning of electrocatalyst surfaces by adsorbed CO intermediates (COads) on the anode of direct alcohol fuel cells. Coherent scattering from the stabilizing ligand shell is identified in the wide-angle scattering data and suggests segregation of Sn ligands at the surface of the nanoparticles synthesized using our approach. We correlate the autoreduction kinetics of precursor complexes with number density, polydispersity, and Pt volume fraction, thereby establishing the mechanism of particle size control at the onset of atomic coalescence. Particle growth is shown to terminate when a Sn shell of well-defined thickness (about 0.6 nm) is completed for a range of Pt core diameters in this critical size range.

The promotion of the electrocatalytic ethanol oxidation reaction (EOR) on extended single-crystal Pt surfaces and dispersed Pt nanoparticles by Sn under acidic conditions is well known. However, the correlation of Sn coverage on Pt nanoparticle electrocatalysts to their size has proven difficult. The reason is that previous investigations have typically relied on commercially difficult to reproduce electrochemical treatments of prepared macroscopic electrodes to adsorb Sn onto exposed Pt surfaces. We demonstrate here how independent control over both Sn coverage and particle size can yield a significant enhancement in EOR activity in an acidic electrolyte relative to previously reported electrocatalysts. Our novel approach uses electroless nanoparticle synthesis where surface-adsorbed Sn is intrinsic to Pt particle formation. Sn serves as both a reducing agent and stabilizing ligand, producing particles with a narrow particle size distribution in a size range where the mass-specific electrocatalytic activity can be maximized (ca. 1-4 nm) as a result of the formation of a fully developed Sn shell. The extent of fractional Sn surface coverage on carbon-supported Pt nanoparticles can be systematically varied through wet-chemical treatment subsequent to nanoparticle formation but prior to incorporation into macroscopic electrodes. EOR activity for Pt nanoparticles is found to be optimum at a fractional Sn surface coverage of ca. 0.6. Furthermore, the EOR activity is shown to increase with Pt particle size and correlate with the active area of available Pt (110) surface sites for the corresponding Sn-free nanoparticles. The maximum area- and mass-specific EOR activities for the most active catalyst investigated were 17.9 μA/cm(2)Pt and 12.5 A/gPt, respectively, after 1 h of use at 0.42 V versus RHE in an acidic electrolyte. Such activity is a substantial improvement over that of commercially available Pt, Pt-Sn, and Pt-Ru alloy catalysts under either acidic or alkaline conditions.

Highly dispersed Pt nanoparticles have been extensively studied for the electrocatalytic oxygen reduction reaction (ORR). Pt bulk and supported-nanoparticle electrodes have exhibited varying degrees of surface structure sensitivity toward the ORR for two main reasons: first, preferential adsorption of supporting electrolyte or water; and second, intrinsic variation of reaction kinetics on different Pt(hkl) surfaces or atomic scale imperfections on the Pt surface (e.g. steps, kinks, edges, and corners). The impact of surface atom coordination on ORR activity is seldom reported because there are few techniques that lend themselves to detailed, in situ assessment of catalyst surface site distribution. Surface active sites on ORR electrocatalysts have been inferred from application of bulk crystal structure data to specific nanoparticle geometries that account for electrocatalytically active surface area, ECA (cm2/gPt). This approach fails to capture the wide variety of active sites present on electrocatalyst surfaces under operating conditions, particularly at nanoparticle sizes that span the atomic cluster to nanocrystal transition. In this paper, we apply the techniques developed by Feliu et al. to determine surface site distribution in situ and, for the first time in the field, correlate these observations with ORR mass activity, MA (A/gPt), and surface activity, SA (μA/cm2Pt) on Pt nanoparticle catalysts. This approach indicates that the predominant active site available for ORR on nanoparticles in the size range of 1.8–6.9nm is (110) or (311). This observation is confirmed by using perchloric acid, sulfuric acid, and potassium hydroxide to demonstrate that the supporting electrolyte has little influence on ORR kinetics for these nanoparticles. Such behavior suggests that the Pt nanoparticle surfaces investigated consist of stepped adlayers on (111) or (100) facets that eliminate the (111) terraces historically associated with ORR activity. The predominance of such a stepped surface on Pt ORR electrocatalysts is unexpected and demonstrates the need for in situ characterization of active site distribution.