Oc Hee Han

SAXS and NMR analysis for the cast solvent effect on sPEEK membrane properties.

The cast solvent effect on the structure and properties of sulfonated poly(ether ether ketone) (sPEEK) was studied. PEEK was sulfonated to have different sulfonation degrees of 65, 70, and 75%, and its membrane was prepared using the two types of solvents, N,N-dimethylacetamide (DMA) and 1-methyl-2-pyrrolidinone (NMP). Ionic cluster size was analyzed using small-angle X-ray scattering (SAXS), and it was correlated with a few essential membrane properties such as water uptake, methanol permeability, proton conductivity, and cell performance in direct methanol fuel cells (DMFCs). Synchrotron SAXS and solid state NMR data revealed the structural difference between the sPEEK membranes prepared using NMP and DMA, regarding the cluster dimensions of 3.22 and 2.70 nm, respectively. Although the water uptake, methanol permeability, and proton conductivity of the membranes prepared with NMP were higher than those with DMA, the overall cell performance was vice versa. The dimensional instability associated with high water swelling as well as high methanol permeability were the main causes for this inferior cell efficiency of NMP cast membranes. This report demonstrates the importance of selection of cast solvent in preparation of SPEEK electrolyte membranes for DMFC application.

Catalytic Reactions in Direct Ethanol Fuel Cells

For fuel-cell applications, ethanol is becoming a more attractive fuel than methanol or hydrogen because it has higher mass energy density and can be produced in great quantities from biomass. Additionally, ethanol is less toxic than methanol and easier to handle than hydrogen. 3] However, the C C bond in ethanol leads to more complicated reaction intermediates and products during oxidation, and catalysts must be able to activate C C bond scission for complete oxidation to CO2. Consequently, much effort has been made to investigate the reaction mechanisms of direct ethanol fuel cells (DEFCs) with various analytical methods. Especially the intermediates and products that are generated during the electrochemical reaction at different ethanol concentrations and potentials have been investigated and quantified by chromatographic techniques, infrared reflectance spectroscopy (IRS), and differential electrochemical mass spectrometry (DEMS). These studies revealed that most of the ethanol was oxidized to acetic acid (AA) or acetaldehyde (AAL) on Pt, but not much to CO2. Additionally, investigations of ethanol oxidation on various catalysts showed that alloying Pt with other transition elements improves the catalytic activity. 10, 12,13] However, DEMS is limited to the detection of volatile chemicals, and IRS requires smooth electrodes with sufficient reflectivity. On the other hand, liquid-state nuclear magnetic resonance (NMR) spectroscopy is a straightforward analytical method which can be applied to an operating fuel cell without any modification. In liquid-state NMR spectroscopy, peak areas are linearly proportional to the abundance of chemical species that are identifiable by their chemical shifts. The DEFC anode exhaust has been shown to give well-resolved C peaks that can unambiguously identify chemical species. We have used C liquid-state NMR spectroscopy to identify and quantify the reaction products present in the liquid anode exhaust of DEFCs that were operated with three different anode catalysts at various potentials. The results were used to explain the effect of elements such as Ru and Sn on the Pt/C anode catalyst and to propose reaction mechanisms of ethanol on Pt-based catalysts. The C liquid-state NMR experiments were performed on DEFCs containing 40 wt% Pt/C, PtRu/C, or Pt3Sn/C anode catalysts prepared by a polyol method. Full experimental details are described in the Supporting Information. Figure 1 shows the C NMR spectra of the anode exhaust from the DEFCs with Pt3Sn/C anode catalysts. The spectra were expanded in the y scale while maintaining the relative peak heights. The chemical species were assigned to the peaks in the spectrum according to literature data, and C atoms that are responsible for C NMR signals are underlined. In the exhaust, the dominant reaction products were AAL (d = 207 ppm), AA (d = 177 ppm), and ethane-1,1-diol (ED, d = 88 ppm) at various potentials. Ethyl acetate (d = 62, 175 ppm) and ethoxyhydroxyethane (d = 63, 95 ppm) also appeared, but only in trace amounts and hence were ignored. The coupling constants of 2.8 and 1.6 Hz between the C-labeled sites were used to distinguish CH2 groups in ethyl acetate and ethoxyhydroxyethane, respectively. For comparison purposes, the NMR spectra were also obtained for the DEFCs containing Pt/C and PtRu/C anode catalysts, and AA, AAL, and ED were major products detected for all three catalysts. Figure 2 shows the relative quantities of the major organic chemicals in the anode exhaust of the DEFCs with different anode catalysts at different potentials. For the DEFC with Pt/C anode catalyst, the NMR peak areas of the reaction products were monotonically depleted with increasing operating potential above 0.1 V versus the standard hydrogen electrode. Thus, more oxidation products were produced from the fuel when the DEFC was operated at a higher current and a lower potential. However, the addition of Ru or Sn to Pt caused variations in the NMR spectral patterns. Production of AA dramatically increased. Subtracting the product populations for Pt/C from those for PtRu/C and Pt3Sn/C (dotted lines in Figure 2) separates the contributions of Ru or Sn from those due to Pt/C. For example, the enhanced AAL and ED production on PtRu/C and Pt3Sn/C compared to on Pt/C was almost zero at 0.1 V and slightly increased above 0.2 V. In contrast, AA production was greatly enhanced and different production behaviors were observed depending on the anode catalysts. On the PtRu/C anode catalysts, AA production [*] Dr. I. Kim, Dr. O. H. Han, Dr. S. A. Chae, Dr. Y. Paik, S.-H. Kwon Analysis Research Division, Daegu Center Korea Basic Science Institute, Daegu, 702-701 (Korea) Fax: (+ 82)53-959-3405 E-mail: ohhan@kbsi.re.kr

Isolation and structural characterization of the elusive 1:1 adduct of hydrazine and carbon dioxide

A solid hydrazine was isolated as a crystalline powder by reacting aqueous hydrazine with supercritical CO(2). Its structure determined by single crystal X-ray diffraction shows a zwitterionic form of NH(3)(+)NHCO(2)(-). The solid hydrazine is remarkably stable but is as reactive as liquid hydrazine even in the absence of solvents.

Nanometer-Scale Water- and Proton-Diffusion Heterogeneities across Water Channels in Polymer Electrolyte Membranes†

Nafion, the most widely used polymer for electrolyte membranes (PEMs) in fuel cells, consists of a fluorocarbon backbone and acidic groups that, upon hydration, swell to form percolated channels through which water and ions diffuse. Although the effects of the channel structures and the acidic groups on water/ion transport have been studied before, the surface chemistry or the spatially heterogeneous diffusivity across water channels has never been shown to directly influence water/ion transport. By the use of molecular spin probes that are selectively partitioned into heterogeneous regions of the PEM and Overhauser dynamic nuclear polarization relaxometry, this study reveals that both water and proton diffusivity are significantly faster near the fluorocarbon and the acidic groups lining the water channels than within the water channels. The concept that surface chemistry at the (sub)nanometer scale dictates water and proton diffusivity invokes a new design principle for PEMs.

Non-sticky silicate replica mold by phase conversion approach for nanoimprint lithography applications

We have developed a transparent, non-sticky silicate nano mold with high mechanical strength and excellent releasing properties through a simple phase conversion process of polyvinylsilazane (PVSZ) replica mold. This inexpensive inorganic polymer derived silicate mold does not require extra surface modification and could be used as an ideal mold with low adhesion force for nanoimprint lithographic applications. The silicate hard nano mold allows fabrication of sub-100 nm patterns down to 30 nm. The mold can be used for both UV and thermal NIL duplication processes in a repeated manner. The economic efficiency of the mold fabrication as well as the high durability and excellent releasing properties could be quite valuable to physical contact nanolithography for high-throughput fabrication of nano-devices.

Influence of metal cleaning on the particle size and surface morphology of platinum black studied by NMR, TEM and CV techniques

Abstract Cyclic voltammetry (CV), transmission electron microscopy (TEM) and 13 C nuclear magnetic resonance spectroscopy (NMR) were employed to investigate the particle size and surface morphology of fuel cell grade platinum black samples as received and prepared under three different cleaning methods. The Pt particle growth was most markedly caused by extensive CV treatment, moderately by cleaning with chromic acid, and negligibly by holding the potential at 250 mV. Even single anodic sweep CV removed Pt adatoms; in contrast, cleaning with chromic acid and holding the potential at 250 mV did not. Our results demonstrate that a cleaning method should be chosen with the knowledge of its effect on the metal surface and particle size. The possibility of detecting the particle size change due to cleaning procedures by 13 C NMR is also discussed.

Blue-silica by Eu2+-activator occupied in interstitial sites

A blue-emitting SiO2:Eu2+ compound has been successfully synthesized and characterized. The PL intensity of SiO2:Eu0.0022+ compound is about 24 times higher than that of the O-defective SiO2 compound (without activators), which emits blue light. The valence state of the Eu ions responsible for the highly enhanced blue emission was determined to be Eu2+ using reference materials (EuCl2 and EuCl3) and XPS measurements. The Eu2+-activator ions occupied in the interstitial sites of the SiO2 matrix were confirmed by FT-IR, XPS, and 29Si MAS-NMR spectroscopy. Even though the void spaces formed structurally in both α-quartz and α-cristobalite can accommodate Eu2+ ions (ionic radius = 1.25 A at CN = 8), SiO2:Eu2+ compound fired at 1300 °C under a hydrogen atmosphere is destined to be deficient in O or Si atoms, indicating the formation of the wider void spaces in the SiO2 crystal lattice. A sputtered depth profile of SiO2-related compounds obtained by time-of-flight secondary ion mass spectrometry (TOF-SIMS) corroborates the O-defective SiO2 induced by hydrogen. In particular, the interatomic potentials, depending on the interstitial positions of Eu atoms in α-cristobalite and α-quartz, are calculated based on Lennard-Jones and coulomb potentials. For α-cristobalite, the minimum potential value is −51.47 eV and for α-quartz, the value is 221.8 eV, which reveals that the Eu2+-activator ions more preferably enter the interstitial sites of α-cristobalite than those of α-quartz. Thanks to the stable Eu2+-activator ions enclosed by Si–O linkages, the SiO2:Eu0.0022+ compound emits blue light and its PL emission intensity is about 24 times higher than that of the O-defective SiO2 compound. This phosphor material could be a platform for modeling a new phosphor and for application in the solid-state lighting field.

Temperature‐ and pressure‐dependent structural transformation of methane hydrates in salt environments

Understanding the stability of volatile species and their compounds under various surface and subsurface conditions is of great importance in gaining insights into the formation and evolution of planetary and satellite bodies. We report the experimental results of the temperature- and pressure-dependent structural transformation of methane hydrates in salt environments using in situ synchrotron X-ray powder diffraction, solid-state nuclear magnetic resonance, and Raman spectroscopy. We find that under pressurized and concentrated brine solutions methane hydrate forms a mixture of type I clathrate hydrate, ice, and hydrated salts. Under a low-pressure condition, however, the methane hydrates are decomposed through a rapid sublimation of water molecules from the surface of hydrate crystals, while NaClu2009·u20092H2O undergoes a phase transition into a crystal growth of NaCl via the migration of salt ions. In ambient pressure conditions, the methane hydrate is fully decomposed in brine solutions at temperatures above 252u2009K, the eutectic point of NaClu2009·u20092H2O.

Solid-state NMR evidence for the presence of two crystallographically distinct tetrahedral sites in zeolite merlinoite

Two crystallographically distinct tetrahedral sites in zeolite nmerlinoite are identified by 29Si MAS NMR and 2D 27Al nquadrupole nutation NMR spectroscopies.

Critical Role of the Chemical Environment of Interlayer Na Sites: An Effective Way To Improve the Na Ion Electrode Activity of Layered Titanate

The chemical environments of the interlayer Na sites of layered titanate are finely controlled by the intercalation of n-alkylamine with various alkyl chain lengths to explore an effective way to improve its electrode functionality for sodium-ion batteries (SIBs). The n-alkylamine intercalation via ion-exchange and exfoliation-restacking routes allows the modification of in-plane structures of layered titanate to be tuned. Among the present n-alkylamine-intercalates, the n-pentylamine-intercalated titanate shows the largest discharge capacity with the best rate characteristics, underscoring the critical role of optimized intracrystalline structure in improving the SIB electrode performance of layered titanate. The creation of turbostratic in-plane structure degrades the SIB electrode performance of layered titanate, indicating the detrimental effect of in-plane structural disorder on electrode activity. 23Na magic-angle spinning nuclear magnetic resonance spectroscopy demonstrates that the n-alkylamine-intercalated titanates possess two different interlayer Na+ sites near ammonium head groups/titanate layers and near alkyl chains. The intercalation of long-chain molecules increases the population of the latter site and the overall mobility of Na+ ions, which is responsible for the improvement of electrode activity upon n-alkylamine intercalation. The present study highlights that the increased population of interlayer metal sites remote from the host layers is effective in improving the electrode functionality of layered metal oxide for SIBs and multivalent ion batteries.