Younkee Paik

Investigation of the local structure of the LiNi0,5Mn0,5O2 cathode material during electrochemical cycling by X-ray absorption and NMR spectroscopy

In situ X-ray absorption spectroscopy (XAS) of the Mn and Ni K-edges and 6 Li magic angle spinning (MAS) nuclear magnetic resonance (NMR) spectroscopy have been carried out during the first charging and discharging process for the layered LiNi 0 . 5 Mn 0 . 5 O 2 cathode material. The Ni K-edge structure in the X-ray absorption near-edge structure (XANES) spectrum exhibits a rigid positive energy shift with increased Li deintercalation level, while the Mn XANES spectra do not show any substantial energy changes. The Ni edge shifts back reversibly during discharge. Further Li-ion intercalation at ∼1 V (vs. Li) could be accomplished by reduction of the Mn 4 + ions. The 6 Li MAS NMR results showed the presence of Li in the Ni 2 + /Mn 4 + layers, in addition to the expected sites for Li in the lithium layers. All the Li ions in the transition metal layers are removed on the first charge, leaving residual lithium ions in the lithium layers.

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

Structural chemistry and magnetic properties of La2LiRuO6

The synthesis and characterization of a polycrystalline sample of the n = 2 Ruddlesden−Popper phase Nd2BaLiRuO7 are reported. X-ray and neutron diffraction show that the 6-coordinate sites are occupied by a 1:1 ordered arrangement of Li and Ru, that the 12-coordinate sites within the perovskite-like blocks are occupied by Ba, and that the cation sites on the block edges are occupied by Nd. Transmission electron microscopy shows that this compound is less susceptible to stacking faults then many other n = 2 phases, and 7Li MAS NMR spectroscopy shows that the environment of the single Li site in the structure is well-defined. The compound is an antiferromagnet below 32 K, with ordered magnetic moments of 2.2(2) μB and 2.53(8) μB on the Ru and Nd cations respectively at 2 K.

Solid‐state 31P NMR investigation on the status of guanine nucleotides in paclitaxel‐stabilized microtubules

Microtubule dynamics is a target for many chemotherapeutic drugs. In order to understand the biochemical effects of paclitaxel on the GTPase activity of tubulin, the status of guanine nucleotides in microtubules was investigated by 31P cross‐polarization magic angle spinning (CPMAS) NMR. Microtubules were freshly prepared in vitro in the presence of paclitaxel and then lyophilized in sucrose buffer for solid‐state NMR experiments. A 31P CPMAS NMR spectrum with the SNR of 25 was successfully acquired from the lyophilized microtubule sample. The broadness of the 31P spectral lines in the spectrum indicates that the molecular environments around the guanine nucleotides inside tubulin may not be as crystalline as reported by many diffraction studies. Deconvolution of the spectrum into four spectral components was carried out in comparison with the 31P NMR spectra obtained from five control samples. The spectral analysis suggested that about 13% of the nucleotides were present as GTP and 37% as GDP in the β‐tubulin (E‐site) of the microtubules. It was found that most of the GDPs were present as GDP‐Pi complex in the microtubules, which seems to be one of the effects of paclitaxel binding. Copyright

Molecular Dynamics in Paraelectric Phase of KH 2 PO 4 Crystals Studied by Single Crystal NMR and MAS NMR

Abstract The temperature dependences of the NMR spectrum and the spin-lattice relaxation times in KH 2 PO 4 were investigated via single-crystal NMR and MAS NMR. The stretched-exponential relaxation that occurred because of the distribution of correlation times was indicative of the degree of the distribution of the double-well potential on the hydrogen bond. The behaviors responsible for the strong temperature dependences of the 1 H and 31 P spin-lattice relaxation times in the rotating frame T 1ρ in KH 2 PO 4 are likely related to the reorientational motion of the hydrogen-bond geometry and the PO 4 tetrahedral distortion. Keywords NMR, CP/MAS NMR, Crystal growth, Phase transition, Ferroelctrics Introduction Potassium dihydrogen phosphate (KH 2 PO 4 ) is among the most widely used crystals in nonlinear optics and the electro-optics industry owing to its unique properties such as a wide region of optical transparency, ferroelectric and piezoelectric stability against high-power laser light, and relatively high nonlinear efficiency

The Status of Guanine Nucleotides in Taxol-Stabilized Microtubules Probed by 31 P CPMAS NMR Spectroscopy

Rapid exchange and hydrolysis of the tubulin-bound guanine nucleotides have been known to govern the dynamics of microtubules. However, the instability and low concentration have made it difficult for the microtubule-bound GTP to be observed directly. In this study, we circumvent these problems by lyophilization and using cross-polarization techniques. 31 P NMR signals were detected from the tubulin-bound GTP in microtubules for the first time. Analysis of the 31 P CPMAS NMR spectrum indicates that GTP hydrolysis was delayed by the presence of taxol.

7 Li MAS NMR Studies of Lithiated Manganese Dioxide Tunnel Structures: Pyrolusite and Ramsdellite

The one-dimensional 1×1 and 1×2 tunnel structures of manganese dioxides, pyrolusit(β-MnO 2 ) and ramsdellite (R-MnO 2 ), respectively, were chemically intercalated with LiI. Two 7 Li resonances were observed in lithiated pyrolusite. One isotropic resonance arising at 110 ppm shows a short spin-lattice relaxation time (T 1 ∼ 3 ms) and was assigned to Li + ions in the 1×1 tunnel structure. The other isotropic resonance arising at 4 ppm shows a long spin-lattice relaxation time (T1 ∼ 100 ms) and was assigned to Li + ions in diamagnetic local environments in the form of impurities such as Li 2 O or on the surface of the MnO 2 particles. Three 7 Li resonances were observed in lithiated ramsdellite at very different frequencies (600, 110 and 0 ppm). The resonance at 600 ppm, which is observed at low lithium intercalation levels, is assigned toLi + ions coordinated to both Mn(III) and Mn(IV) ions in the 1×2 tunnels, while the resonanceat 110 ppm is due to Li + ions coordinated to Mn(III) ions and appears at higher Li levels. The resonance at 0 ppm is associated with a long spin-lattice relaxation time (T1 ∼ 100 ms) and is also assignedto Li + ions in diamagnetic impurities.

Quantification of Methanol Concentration in the Polymer Electrolyte Membrane of Direct Methanol Fuel Cell by Solid-state NMR

Direct quantification of methanol in polymer electrolyte membrane (PEM) by solid-state nuclear magnetic resonance (NMR) spectroscopy was studied and the methanol concentrations in PEM produced by crossover and diffusion were compared. The error range of the quantification was not smaller than and the amount of the methanol crossed over in our direct methanol fuel cells (DMFCs) was less than the methanol diffused to PEM. The methanol concentration in the PEM of the DMFC operated at different current densities were equivalent.

Vibrational Spectroscopy and MAS NMR Studies of A Cryptomelane Family of Mixed ( 7 Li + , 2 H + ) Form Manganic Acids

Cryptomelane-type manganic acids (CMAs) synthesized by a soft-chemical redox process of KMnO 4 with MnSO 4 in H 2 SO 4 and α-MnO 2 s by a decomposition-oxidation process of (CH 3 ) 3 COK and MnCO 3 at 530°C were studied by using the vibrational spectroscopy and 7 Li- and 2 H-MAS NMR. The infrared absorption bands of the α-MnO 2 s at 706cm -1 were not changed irrespective of the sort of alkali cations and their uptake amount, while large shifts of this band were observed on the CMA exchanged by alkali cations. Hence, cation-exchange sites of these solids will occupy crystallographically different positions. 7 Li-MAS NMR spectra of these manganic acids showed different two chemical shifts depending on their synthetic routes. These findings suggest that different ion-exchange sites are formed at a local level in the 2×2 type tunnel, strongly depending on their synthetic conditions. These are crucial for the control of the cation affinity. These details at a molecular level cannot be obtained by using the conventional XRD analyses.