Characterization of proton exchange materials for fuel cells by solid state nuclear magnetic resonance

Solid-state nuclear magnetic resonance (NMR) has been used to explore the nanometer-scale structure of Nafion, the widely used fuel cell membrane, and its composites. We have shown that solid-state NMR can characterize chemical structure and composition, domain size and morphology, internuclear distances, molecular dynamics, etc. The newly-developed water channel model of Nafion has been confirmed, and important characteristic length-scales established. Nafion-based organic and inorganic composites with special properties have also been characterized and their structures elucidated. The morphology of Nafion varies with hydration level, and is reflected in the changes in surface-to-volume (S/V) ratio of the polymer obtained by small-angle X-ray scattering (SAXS). The S/V ratios of different Nafion models have been evaluated numerically. It has been found that only the water channel model gives the measured S/V ratios in the normal hydration range of a working fuel cell, while dispersed water molecules and polymer ribbons account for the structures at low and high hydration levels, respectively. Although the cross-section morphology of Nafion has been derived from SAXS data, the structure in the third dimension, which is channel straightness, was not clear. With 2H NMR, D2O can be used as a probe to study channel straightness (persistence length). In drawn Nafion with straight channels, the exchange between bound and free D2O results in a residual quadrupolar splitting of 1-2 kHz; while in normal Nafion with coiled channels, the 2H quadrupolar splitting of D2O is ~ 10-fold smaller. It is explained by the motional averaging of the 2H coupling frequencies in the NMR timescale when D2O diffuses through differently-oriented segments. The simulations of line narrowing and T2 relaxation times of D2O revealed a persistence length within 30 to 80 nm for normal Nafion. The Nafion phosphatranium composite developed by Verkade and Wadhwa, which is a potential candidate for anion exchange membranes, has been characterized by solid-state NMR. The synthesized membrane has two major components, in which phosphatranium cations are bonded to Nafion side-groups via either P or N with a mole ratio of 2:1. Degradation of the phosphatranium cations has not been found in the composite membrane, which implies a good stability of the material. Nafion-silica (NafSil) and Nafion-zirconium phosphate (NafZrP) composites prepared by the in-situ growth of inorganic particles in the channels of Nafion membrane have been characterized. Under typical situations with an inorganic volume fraction of around 15%, elongated nanoparticles are formed inside the water channels. The inorganic particles have cylindrical shapes with a cross-section area of ~ 6 nm2 and surrounded by water layers with a thickness of ~ 0.8 nm. Zirconium phosphates (ZrP) synthesized in and outside Nafion have been characterized in detail by solid-state NMR and X-ray diffraction (XRD). It has been found that typical α-ZrP with water of crystallization transforms to anhydrous α-ZrP and condensed pyrophosphates after drying at 150oC. When grown in Nafion, ZrP favors a structure with two disordered layers and a majority of (HO)P(OZr)3 sites, different from regular α-ZrP, particularly after drying

Catalytic consequences of open and closed grafted Al(III)-calix[4]arene complexes for hydride and oxo transfer reactions

An approach for the control and understanding of supported molecular catalysts is demonstrated with the design and synthesis of open and closed variants of a grafted Lewis acid active site, consisting of Al(III)-calix[4]arene complexes on the surface of silica. The calixarene acts as a molecular template that enforces open and closed resting-state coordination geometries surrounding the metal active sites, due to its lower-rim substituents as well as site isolation by virtue of its steric bulk. These sites are characterized and used to elucidate mechanistic details and connectivity requirements for reactions involving hydride and oxo transfer. The consequence of controlling open versus closed configurations of the grafted Lewis acid site is demonstrated by the complete lack of observed activity of the closed site for Meerwein-Ponndorf-Verley (MPV) reduction; whereas, the open variant of this catalyst has an MPV reduction activity that is virtually identical to previously reported soluble molecular Al(III)-calix[4]arene catalysts. In contrast, for olefin epoxidation using tert-butyl-hydroperoxide as oxidant, the open and closed catalysts exhibit similar activity. This observation suggests that for olefin epoxidation catalysis using Lewis acids as catalyst and organic hydroperoxide as oxidant, covalent binding of the hydroperoxide is not required, and instead dative coordination to the Lewis acid center is sufficient for catalytic oxo transfer. This latter result is supported by density functional theory calculations of the transition state for olefin epoxidation catalysis, using molecular analogs of the open and closed catalysts

Understanding CO2 Dynamics in Metal–Organic Frameworks with Open Metal Sites

Hopping along: Metal-organic frameworks such as Mg-MOF-74 possess open metal sites that interact strongly with CO2. Molecular simulations reveal detailed CO2 dynamics (hops between metal sites and localized fluctuations), which can be used to accurately explain the experimentally measured 13C NMR chemical shift anisotropy pattern. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Robust estimation of bacterial cell count from optical density

Optical density (OD) is widely used to estimate the density of cells in liquid culture, but cannot be compared between instruments without a standardized calibration protocol and is challenging to relate to actual cell count. We address this with an interlaboratory study comparing three simple, low-cost, and highly accessible OD calibration protocols across 244 laboratories, applied to eight strains of constitutive GFP-expressing E. coli. Based on our results, we recommend calibrating OD to estimated cell count using serial dilution of silica microspheres, which produces highly precise calibration (95.5% of residuals <1.2-fold), is easily assessed for quality control, also assesses instrument effective linear range, and can be combined with fluorescence calibration to obtain units of Molecules of Equivalent Fluorescein (MEFL) per cell, allowing direct comparison and data fusion with flow cytometry measurements: in our study, fluorescence per cell measurements showed only a 1.07-fold mean difference between plate reader and flow cytometry data

Characterization of proton exchange materials for fuel cells by solid state nuclear magnetic resonance

Solid-state nuclear magnetic resonance (NMR) has been used to explore the nanometer-scale structure of Nafion, the widely used fuel cell membrane, and its composites. We have shown that solid-state NMR can characterize chemical structure and composition, domain size and morphology, internuclear distances, molecular dynamics, etc. The newly-developed water channel model of Nafion has been confirmed, and important characteristic length-scales established. Nafion-based organic and inorganic composites with special properties have also been characterized and their structures elucidated. The morphology of Nafion varies with hydration level, and is reflected in the changes in surface-to-volume (S/V) ratio of the polymer obtained by small-angle X-ray scattering (SAXS). The S/V ratios of different Nafion models have been evaluated numerically. It has been found that only the water channel model gives the measured S/V ratios in the normal hydration range of a working fuel cell, while dispersed water molecules and polymer ribbons account for the structures at low and high hydration levels, respectively. Although the cross-section morphology of Nafion has been derived from SAXS data, the structure in the third dimension, which is channel straightness, was not clear. With 2H NMR, D2O can be used as a probe to study channel straightness (persistence length). In drawn Nafion with straight channels, the exchange between bound and free D2O results in a residual quadrupolar splitting of 1-2 kHz; while in normal Nafion with coiled channels, the 2H quadrupolar splitting of D2O is ~ 10-fold smaller. It is explained by the motional averaging of the 2H coupling frequencies in the NMR timescale when D2O diffuses through differently-oriented segments. The simulations of line narrowing and T2 relaxation times of D2O revealed a persistence length within 30 to 80 nm for normal Nafion. The Nafion phosphatranium composite developed by Verkade and Wadhwa, which is a potential candidate for anion exchange membranes, has been characterized by solid-state NMR. The synthesized membrane has two major components, in which phosphatranium cations are bonded to Nafion side-groups via either P or N with a mole ratio of 2:1. Degradation of the phosphatranium cations has not been found in the composite membrane, which implies a good stability of the material. Nafion-silica (NafSil) and Nafion-zirconium phosphate (NafZrP) composites prepared by the in-situ growth of inorganic particles in the channels of Nafion membrane have been characterized. Under typical situations with an inorganic volume fraction of around 15%, elongated nanoparticles are formed inside the water channels. The inorganic particles have cylindrical shapes with a cross-section area of ~ 6 nm2 and surrounded by water layers with a thickness of ~ 0.8 nm. Zirconium phosphates (ZrP) synthesized in and outside Nafion have been characterized in detail by solid-state NMR and X-ray diffraction (XRD). It has been found that typical α-ZrP with water of crystallization transforms to anhydrous α-ZrP and condensed pyrophosphates after drying at 150oC. When grown in Nafion, ZrP favors a structure with two disordered layers and a majority of (HO)P(OZr)3 sites, different from regular α-ZrP, particularly after drying.</p

The Pros and Cons of Polydopamine-Sensitized Titanium Oxide for the Photoreduction of CO2

Photocatalytic reduction of CO2 into fuels is a promising route to reduce greenhouse gas emission, and it demands high-performance photocatalysts that can use visible light in the solar spectrum. Due to its broadband light adsorption, polydopamine (PDA) is considered as a promising photo-sensitization material for semiconductor photocatalysts. In this work, titanium oxides have been coated with PDA through an in-situ oxidation polymerization method to pursue CO2 reduction under visible light. We have shown that the surface coated PDA with a thickness of around 1 nm can enhance the photocatalytic performance of anatase under visible light to reduce CO2 into CO. Assisted with additional UV-vis adsorption and photoluminescence characterizations, we confirmed the sensitization effect of PDA on anatase. Furthermore, our study shows that thicker PDA coating might not be favorable, as PDA could decompose under both visible and UV-vis light irradiations. 13C solid-state nuclear magnetic resonance showed structural differences between thin and thick PDA coatings and revealed compositional changes of PDA after light irradiation

A unified description for polarization-transfer mechanisms in magnetic resonance in static solids: Cross polarization and DNP

International audiencePolarization transfers are crucial building blocks in magnetic resonance experiments, i.e., they can be used to polarize insensitive nuclei and correlate nuclear spins in multidimensional nuclear magnetic resonance (NMR) spectroscopy. The polarization can be transferred either across different nuclear spin species or from electron spins to the relatively low-polarized nuclear spins. The former route occurring in solid-state NMR can be performed via cross polarization (CP), while the latter route is known as dynamic nuclear polarization (DNP). Despite having different operating conditions, we opinionate that both mechanisms are theoretically similar processes in ideal conditions, i.e., the electron is merely another spin-1/2 particle with a much higher gyromagnetic ratio. Here, we show that the CP and DNP processes can be described using a unified theory based on average Hamiltonian theory combined with fictitious operators. The intuitive and unified approach has allowed new insights into the cross-effect DNP mechanism, leading to better design of DNP polarizing agents and extending the applications beyond just hyperpolarization. We explore the possibility of exploiting theoretically predicted DNP transients for electron–nucleus distance measurements—such as routine dipolar-recoupling experiments in solid-state NMR

Celery‐derived scaffolds with liver lobule‐mimicking structures for tissue engineering transplantation

Abstract Decellularized scaffolds have a demonstrated value in liver tissue engineering. Challenges in this area are focused on effectively eliminating the biological rejection of scaffolds and finding a suitable liver cell source. Here, inspired by the natural microstructure of hepatic lobules, we present a novel decellularized celery‐derived scaffold cultured with human‐induced pluripotent stem cell‐derived hepatocytes (hiPSC‐Heps) bioengineering liver tissue construction. Because of the natural hollow channels, interconnected porous structures, and excellent physicochemical characterization of the decellularized celery‐derived scaffold, the resultant bioengineering liver tissue can maintain the hiPSC‐Heps viability and the hepatic functions in the in vitro cultures. Based on this bioengineering liver tissue, we have demonstrated its good biocompatibility and the significantly higher expressions of albumin (ALB) and periodic acid‐schiff stain (PAS) when it was implanted in nude mice. These remarkable properties endow the hiPSC‐Heps integrated decellularized celery scaffolds system with promising prospects in the field of liver transplantation and other regeneration medicine