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 selfdiffusivity at small water loadings was observed to be diffusion timeindependent 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.
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...
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 CO ads . In this study, an all-queous colloidal synthesis method for producing highly dispersed Pt 2 Bi 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 μA cm Pt −2 ) is observed at the peak CO ads -stripping potential of 0.7 V vs RHE at 50 °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 Pt 2 Bi NPT. Resistance to poisoning coincides both with a reduction in the C ads specific activity onset potential by 25 mV relative to Pt-C and faster DME electro-oxidation kinetics. DDMEFC testing of the unsupported Pt 2 Bi NPT utilizing a phosphoric acid-doped polybenzimidazole (PBI) membrane operating at 240 °C yields a peak power of 56 W g PGM,anode −1. This represents a 30% increase relative to a commercial PtRu catalyst.
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.
In this work, evidence for ligand formation between Sn and Bi is provided utilizing time-resolved X-ray Absorption and 119Sn Mӧssbauer spectroscopy during the colloidal synthesis of Bi metal nanoparticles (NP)...
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