Papers by Rolf Hempelmann
RSC Advances, 2016
We synthesize a new vanadium oxyfluoride VO 2 F (rhombohedral, R 3c) through a simple one-step ba... more We synthesize a new vanadium oxyfluoride VO 2 F (rhombohedral, R 3c) through a simple one-step ballmilling route and demonstrate its promising lithium storage properties with a high theoretical capacity of 526 mA h g À1 . Similar to V 2 O 5 , VO 2 F transfers into an active disordered rock-salt (Fm 3m) phase after initial cycling against the lithium anode, as confirmed by diffraction and spectroscopic experiments. The newly formed nanosized Li x VO 2 F remains its crystal structure over further cycling between 4.1 and 1.3 V. A high capacity of 350 mA h g À1 at 2.5 V was observed at 25 C and 50 mA g À1 . Furthermore, superior performance was observed for VO 2 F in comparison with a commercial crystalline V 2 O 5 , in terms of discharge voltage, voltage hysteresis and reversible capacity.
Physical Chemistry Chemical Physics, 2023
Ionic liquids are attractive liquid materials for many advanced applications. For targeted design... more Ionic liquids are attractive liquid materials for many advanced applications. For targeted design, in-depth knowledge about their structure-property-relations is urgently needed. We prepared a set of novel protic ionic liquids (PILs) with a guanidinium cation with either an ether or alkyl side chain and different anions. While being a promising cation class, the available data is insufficient to guide design. We measured thermal and transport properties, nuclear magnetic resonance (NMR) spectra as well as liquid and crystalline structures supported by ab initio computations and were able to obtain a detailed insight into the influence of the anion and the ether substitution on the physical and spectroscopic properties. For the PILs, hydrogen bonding is the main interaction between cation and anion and the H-bond strength is inversely related to the proton affinity of the constituting acid and correlated to the increase of 1 H and 15 N chemical shifts. Using anions from acids with lower proton affinity leads to proton localization on the cation as evident from NMR spectra and self-diffusion coefficients. In contrast, proton exchange was evident in ionic liquids with triflate and trifluoroacetate anions. Using imide-type anions and ether side groups decreases glass transitions as well as fragility, and accelerated dynamics significantly. In case of the ether guanidinium ionic liquids, the conformation of the side chain adopts a curled structure as the result of dispersion interactions, while the alkyl chains prefer a linear arrangement.
Applied sciences, Jun 19, 2021
Ionic liquids are attractive and safe electrolytes for diverse electrochemical applications such ... more Ionic liquids are attractive and safe electrolytes for diverse electrochemical applications such as advanced rechargeable batteries with high energy densities. Their properties that are beneficial for energy storage and conversion include negligible vapor-pressure, intrinsic conductivity as well as high stability. To explore the suitability of a series of ionic liquids with small ammonium cations for potential battery applications, we investigated their thermal and transport properties. We studied the influence of the symmetrical imide-type anions bis(trifluoromethanesulfonyl)imide ([TFSI] -) and bis(fluorosulfonyl)imide ([FSI] -), side chain length and functionalization, as well as lithium salt content on the properties of the electrolytes. Many of the samples are liquid at ambient temperature, but their solidification temperatures show disparate behavior. The transport properties showed clear trends: the dynamics are accelerated for samples with the [FSI] -anion, shorter side chains, ether functionalization and lower amounts of lithium salts. Detailed insight was obtained from the diffusion coefficients of the different ions in the electrolytes, which revealed the formation of aggregates of lithium cations coordinated by anions. The ionic liquid electrolytes exhibit sufficient stability in NMC/Li half-cells at elevated temperatures with small current rates without the need of additional liquid electrolytes, although Li-plating was observed. Electrolytes containing [TFSI] - anions showed superior stability compared to those with [FSI] -anions in battery tests.
Journal of Physical Chemistry B, Mar 3, 2021
A detailed understanding of the local dynamics in ionic liquids remains an important aspect in th... more A detailed understanding of the local dynamics in ionic liquids remains an important aspect in the design of new ionic liquids as advanced functional fluids. Here, we use small-angle X-ray scattering and quasi-elastic neutron spectroscopy to investigate the local structure and dynamics in a model ionic liquid as a function of temperature and pressure, with a particular focus on state points (P,T) where the macroscopic dynamics, i.e., conductivity, is the same. Our results suggest that the initial step of ion transport is a confined diffusion process, on the nanosecond timescale, where the motion is restricted by a cage of nearest neighbors. This process is invariant considering timescale, geometry, and the participation ratio, at state points of constant conductivity, i.e., state points of isoconductivity. The connection to the nearest-neighbor structure is underlined by the invariance of the peak in the structure factor corresponding to nearest-neighbor correlations. At shorter timescales, picoseconds, two localized relaxation processes of the cation can be observed, which are not directly linked to ion transport. However, these processes also show invariance at isoconductivity. This points to that the overall energy landscape in ionic liquids responds in the same way to density changes and is mainly governed by the nearest-neighbor interactions.
Permeability and Diffusivity Measurements on Polymer Electrolyte Membranes
Fuel Cells, Dec 19, 2012
An essential material property directly influencing the long‐life capacity of hydrogen‐polymer el... more An essential material property directly influencing the long‐life capacity of hydrogen‐polymer electrolyte membrane fuel cells (H2‐PEMFCs) systems at elevated temperatures (>80 °C) is the permeability of the PEM material toward gases. The membrane permeability with respect to hydrogen and oxygen is responsible for a decrease of fuel cell efficiency and life‐time. Furthermore permeating oxygen is suspected to form membrane degrading O‐radical species. In view of the importance of gas permeability for further PEM R&D activities we present a test bench for the experimental determination of gas diffusivities and permeabilities for such polymeric materials. The advantages of a direct coupling of the permeation measurement cell with a gas chromatograph as analyzer are outlined. Hydrogen and oxygen diffusivities as well as permeabilities of prospective new materials such as cross‐linked SPEEK are determined at different temperatures and are compared to state‐of‐the‐art PFSA material. A significantly lower permeability regarding hydrogen and oxygen was determined for cross‐linked SPEEK materials. In addition the influence of the hydro‐thermal history of PFSA materials, postulated by G. Alberti, on membrane diffusivity was studied.
Physical Chemistry Chemical Physics, 2020
Ionic liquids are an attractive material class due to their wide liquid range, intrinsic ionic co... more Ionic liquids are an attractive material class due to their wide liquid range, intrinsic ionic conductivity, and high chemical as well as electrochemical stability. However, the widespread use of ionic liquids is hindered by significantly higher viscosities compared to conventional molecular solvents. In this work, we show how the transport properties of ionic liquids can be altered significantly, even for isostructural ions that have the same backbone. To this end, structure-property relationships have been determined for a set of 16 systematically varied representative ionic liquids. Variations in molecular structure include ammonium vs. phosphonium, ether vs. alkyl side chains, and rigid vs. flexible anions. Ab initio calculations are used to relate molecular structures to the thermal, structural and transport properties of the ionic liquids. We find that the differences in properties of ether and alkyl functionalised ionic liquids are primarily dependent on minimum energy geometries, with the conformational flexibility of ether side chains appearing to be of secondary importance. We also show unprecedented correlations between anion conformational flexibility and transport properties. Critically, increasing fluidity upon consecutive introduction of ether side chains and phosphonium centres into the cation is found to be dependent on whether the anion is flexible or rigid. We demonstrate that targeted design of functional groups based on structure-property relationships can yield ionic liquids of exceptionally high fluidity.
International Journal of Hydrogen Energy, Apr 1, 2019
• We have fabricated cathode based on TiO x nanotubes decorated by RuO x nanowhiskers • Surface m... more • We have fabricated cathode based on TiO x nanotubes decorated by RuO x nanowhiskers • Surface morphology of cathode is tailored to facilitate gas bubble detachment • A pre-dominant type of hydrogen bubble release is resolved using wavelet analysis • The best HER efficiency is up to 64% associated with small bubble detachment 4
Balance Between Contact and Solvent-Separated Ion Pairs in Mixtures of the Protic Ionic Liquid [Et<sub>3</sub>NH][MeSO<sub>3</sub>] with Water Controlled by Water Content and Temperature
Journal of Physical Chemistry B, Apr 24, 2021
The formation of aggregates of ionic species is a crucial process in liquids and solutions. Ion s... more The formation of aggregates of ionic species is a crucial process in liquids and solutions. Ion speciation is particularly interesting for the case of ionic liquids (ILs) since these Coulombic fluids consist solely of ions. Most of their unique properties, such as enthalpies of vaporization and conductivities, are strongly related to ion pair formation. Here, we show that the balance of hydrogen-bonded contact ion pairs (CIP) and solvent-separated (SIP) ion pairs in protic ionic liquids (PILs) and in their mixtures with water can be well understood by a combination of far-infrared (FIR) and mid-infrared (MIR) spectroscopy, density functional theory (DFT) calculations of PIL/water aggregates, and molecular dynamics (MD) simulations of PIL/water mixtures. This combined approach is applied to mixtures of triethylammonium methanesulfonate [Et3NH][MeSO3] with water. It is shown that ion speciation in this mixture depends on three parameters: the relative hydrogen bond acceptor strength of the counter ion and the molecular solvent, the solvent concentration, and the temperature. For selected PIL/water mixtures, the equilibrium constants for CIPs and SIPs were determined as a function of the solvent content and temperature. Finally, for the studied PIL/water mixtures, the transition from CIPs to SIPs could be understood on enthalpic and entropic grounds. A detailed picture of this interconversion process could be described at the molecular level by means of MD simulations. In addition, the concentration dependence of ion pair formation can be well understood with help of a simplified "cartoon-like" statistical model describing hydrogen bond redistribution.
Dynamics of 18-Crown-6 Ether in Aqueous Solution Studied by Quasielastic Neutron Scattering
Berichte der Bunsengesellschaft für physikalische Chemie, May 1, 1991
The dynamical behaviour of crown ether 18‐crown‐6 in water has been investigated over the whole c... more The dynamical behaviour of crown ether 18‐crown‐6 in water has been investigated over the whole concentration range by incoherent quasielastic neutron scattering using the high resolution backscattering spectrometer BSS1 and the time‐of‐flight instrument SV22 at the research reactor FRJ2 in Jülich. In addition we have determined the dynamic viscosities and the mass densities of this binary system. Evaluation of the neutron scattering data by use of appropriate diffusion models gives information about the temporal and spatial development of translational, rotational and conformational motion of 18‐crown‐6. The decrease of the measured dynamical quantities on addition of water up to a concentration of 50 mole‐% suggests the formation of a 1:1 crown ether‐water adduct. Addition of more water yields a rapid increase of the translational diffusion coefficient and a decrease of the viscosity.
Analytical and Bioanalytical Chemistry, May 14, 2016
Lignin is the second most abundant natural biopolymer, and lignin wastes are therefore potentiall... more Lignin is the second most abundant natural biopolymer, and lignin wastes are therefore potentially significant sources for renewable chemicals such as fuel compounds, as alternatives to fossil fuels. Waste valorisation of lignin is currently limited to a few applications such as in the pulp industry, however, because of the lack of effective extraction and characterisation methods for the chemically highly complex mixtures after decomposition. Here, we have implemented high resolution mass spectrometry and developed twodimensional mass defect matrix plots as a data visualisation tool, similar to the Kendrick mass defect plots implemented in fields such as petroleomics. These 2D matrix plots greatly simplified the highly convoluted lignin mass spectral data acquired from Fourier transform ion cyclotron resonance (FTICR)-mass spectrometry, and the derived metrics provided confident peak assignments and strongly improved structural mapping of lignin decomposition product series from the various linkages within the lignin polymer after electrochemical decomposition. Keywords Lignin . High resolution mass spectrometry . Kendrick mass . Mass defect . 2D matrix plot . FTICR-MS
Quasielastic neutron scattering study of H motion in the hydrogen-stabilized C15-type phases HfTi2Hxand ZrTi2Hx
Journal of Physics: Condensed Matter, Mar 21, 2000
In order to clarify the mechanism of H diffusion in the hydrogen-stabilized phases Hf Ti2 Hx and ... more In order to clarify the mechanism of H diffusion in the hydrogen-stabilized phases Hf Ti2 Hx and ZrTi2 Hx with xicons/Journals/Common/approx" ALT="approx" ALIGN="TOP"/> 4, we have performed high-resolution quasielastic neutron scattering measurements on Hf Ti2 H3.5 , Hf Ti2 H4.0 and ZrTi2 H3.9 over the temperature range 10 - 327 K. It is found that the diffusive motion of hydrogen in these systems can be described in terms of at least two frequency scales of H hopping. A small fraction of H atoms participates in the fast localized motion within the hexagons formed by interstitial g (Hf2 Ti2 or Zr2 Ti2 ) sites; this fraction increases with increasing temperature. The slower jump process corresponds to H motion on the nearly completely filled sublattice of interstitial e (Hf Ti3 or ZrTi3 ) sites; transitions from one e site to another imply a number of jumps over intermediate g sites. The parameters of H motion in Hf Ti2 Hx and ZrTi2 Hx are found to be close to each other. The temperature dependence of the characteristic rate icons/Journals/Common/tau" ALT="tau" ALIGN="TOP"/> d -1 of the slower jump process in Hf Ti2 H4 is well described by the Arrhenius law with the activation energy of 0.17 +/- 0.02 eV.
LoLiPEM: Long life proton exchange membrane fuel cells
International Journal of Hydrogen Energy, 2016
Abstract This paper presents the main results obtained during the European project (FCH-JU) “LoLi... more Abstract This paper presents the main results obtained during the European project (FCH-JU) “LoLiPEM – Long-life PEM-FCH & CHP systems”. The paper describes significant improvements in the polymer electrolyte by tailored heat treatments for cross-linking of Sulfonated Poly(ether ether ketone) (SPEEK), obtained without any addition of cross-linker species. The reported properties of the ionomers include mechanical properties, gas permeability and ionic conductivity. Innovative gas-diffusion electrodes are fabricated by the electrochemical deposition of Pt catalyst; the fuel cell current–voltage characteristics are reported with Nafion and SPEEK-based binder. The fuel cell performances at 80 °C of membrane-electrode-assemblies containing a SPEEK membrane with a cross-linking degree of 32% are among the best in the literature compared with the PEMFC using membrane alternative than Nafion.
Zeitschrift für Physikalische Chemie, Feb 1, 2010
This tutorial introduction has been written for people who are not specialized in neutron scatter... more This tutorial introduction has been written for people who are not specialized in neutron scattering or in other scattering methods but who are interested and would like to get an impression and learn about the method of Quasielastic Neutron Scattering (QENS). The theoretical (scattering process) as well as the experimental basics (neutron sources, neutron scattering instruments, experimental periphery) are explained in a generally understandable way, with only the most essential formulas. QENS addresses the stochastic dynamics in condensed matter, and it is pointed out for which problems and for which systems in condensed matter research QENS is a powerful method. Thus sufficient information is provided to enable non-experts to think about their own QENS experiment and to understand related literature in this area of research.
Journal of Nanoparticle Research, Jan 28, 2012
Multifunctional nanoparticles (NPs) combining the superparamagnetism of Mn-Zn ferrite and the flu... more Multifunctional nanoparticles (NPs) combining the superparamagnetism of Mn-Zn ferrite and the fluorescence property of gold nanoclusters (NCs) have been prepared by wet chemistry. Magnetic NPs synthesized by co-precipitation method were coated several times with oppositely charged polyelectrolytes (PEs) using the layer-by-layer technique. Common techniques (Fourier transform infrared spectroscopy, electron microscopy, zeta potential, etc.) indicated the monodispersity and the stability of the coated NPs providing a positive charged surface. Fluorescent gold NCs bound to a standard protein bovine serum albumin were adsorbed on the surface of the magnetic NPs. Structural investigations proved the presence of small gold clusters (*2 nm) in a shell surrounding the magnetic nanomaterial. The stable nanocomposite kept the original fluorescence property of the metal clusters with 211-fold increase of the red emission (k = 690 nm) compared to the uncoated NPs. These NPs can be moved with a permanent magnet despite a 72-wt% increase of the non-magnetic fraction due to the PE coating and the protein adsorption.
Effect of Moisture on Methane Permeation Through Fermenter Covering Films in Biogas Plants
Zeitschrift für Physikalische Chemie, Nov 18, 2017
Methane permeation through polyamide and polyethylene mono and multilayer films has been measured... more Methane permeation through polyamide and polyethylene mono and multilayer films has been measured via direct coupling of a gaschromatograph with a permeation measurement cell. Methane permeabilities [in cm3 (STP)·mm·d−1·bar−1·m−2] for single films were found to be 335 for polyethylene (PE) at 40°C, 0.64 for polyamide (PA) 6/6.6 with dry gas and 8.7 for PA 6/6.6 with water vapour saturated test gas. As a result, under operating conditions of biogas production facilities (40°C, water vapour saturated), the replacement of PE by PA, as biogas membrane on top of a fermenter, can reduce methane emission through permeation by a factor of about 40.
Angewandte Chemie, Feb 3, 2015
… der Eigenschaften ionischer Flüssigkeiten liegt in der feinen Energiebalance zwischen Coulomb-W... more … der Eigenschaften ionischer Flüssigkeiten liegt in der feinen Energiebalance zwischen Coulomb-Wechselwirkung, Wasserstoffbrücken und Dispersionskräften. In ihrer Zuschrift auf S. 2834 ff. untersuchen R. Ludwig et al. den "Wettstreit" zwischen H-Brücken und Dispersionswechselwirkungen in den Coulomb-dominierten Flüssigkeiten mit Ferninfrarot-Spektroskopie. Mithilfe charakteristischer spektraler Signaturen kann der Übergang zwischen den beiden Arten der nichtkovalent gebundenen "Ionenpaare" quantifiziert werden.
Physical Chemistry Chemical Physics, 2021
Ionic liquids are modern liquid materials with potential and actual implementation in many advanc... more Ionic liquids are modern liquid materials with potential and actual implementation in many advanced technologies. They combine many favourable and modifiable properties but have a major inherent drawback compared to molecular liquids -slower dynamics. In previous studies we found that the dynamics of ionic liquids are significantly accelerated by the introduction of multiple ether side chains into the cations. However, the origin of the improved transport properties, whether as a result of the altered cation conformation or due to the absence of nanostructuring within the liquid as a result of the higher polarity of the ether chains, remained to be clarified. Therefore, we prepared two novel sets of methylammonium based ionic liquids; one set with three ether substituents and another set with three butyl side chains, in order to compare their dynamic properties and liquid structures. Using a range of anions, we show that the dynamics of the ether-substituted cations are systematically and distinctly accelerated. Liquefaction temperatures are lowered and fragilities increased, while at the same time cation-anion distances are slightly larger for the alkylated samples. Furthermore, pronounced liquid nanostructures were not observed. Molecular dynamics simulations demonstrate that the origin of the altered properties of the ether substituted ionic liquids is primarily due to a curled ether chain conformation, in contrast to the alkylated cations where the alkyl chains retain a linear conformation. Thus, the observed structure-property relations can be explained by changes in the geometric shape of the cations, rather than by the absence of a liquid nanostructure. Application of quantum chemical calculations to a simplified model system revealed that intramolecular hydrogen-bonding is responsible for approximately half of the stabilisation of the curled ether-cations, whereas the other half stems from non-specific long-range interactions. These findings give more detailed insights into the structure-property relations of ionic liquids and will guide the development of ionic liquids that do not suffer from slow dynamics.
Kinetics of bulk polymerisation and Gompertz's law
Physical Chemistry Chemical Physics, 2011
The kinetics of bulk photo-polymerisation of multifunctional monomers, leading to highly cross-li... more The kinetics of bulk photo-polymerisation of multifunctional monomers, leading to highly cross-linked polymers, has been investigated by in situ real-time Raman spectroscopy. The decrease with time of the monomer content is described by a modified Gompertzian function originally developed for population dynamics. This function is flexible enough to take into account inhibitor effect, gel effect and glass effect which are characteristic for curing of technical polymer coatings. Layers of commercial acrylic resins with a thickness of 50 μm, with commercial initiators, are taken as model systems for the UV-curing. The proof-of-concept is presented that the combination of Raman hardware and Gompertzian software enables systematic laboratory tests of curable systems.
Chemistry: A European Journal, Feb 20, 2022
We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosp... more We report the complex phase behavior of the glass forming protic ionic liquid (PIL) d3-octylphosphonium bis(trifluoromethylsulfonyl)imide [C 8 H 17 PD 3 ][NTf 2 ] by means of solid-state NMR spectroscopy. Combined line shape and spin relaxation studies of the deuterons in the PD 3 group of the octylphosphonium cation allow to map and correlate the phase behavior for a broad temperature range from 71 K to 343 K. In the solid PIL at 71 K, we observed a static state, characterized by the first deuteron quadrupole coupling constant reported for PD 3 deuterons. A transition enthalpy of about 12 kJ mol À 1 from the static to the mobile state with increasing temperature suggests the breaking of a weak, charge-enhanced hydrogen bond between cation and anion. The highly mobile phase above 100 K exhibits an almost disappearing activation barrier, strongly indicating quantum tunneling. Thus, we provide first evidence of tunneling driven mobility of the hydrogen bonded PÀ D moieties in the glassy state of PILs, already at surprisingly high temperatures up to 200 K. Above 250 K, the mobile phase turns from anisotropic to isotropic motion, and indicates strong internal rotation of the PD 3 group. The analyzed line shapes and spin relaxation times allow us to link the structural and dynamical behavior at molecular level with the phase behavior beyond the DSC traces.
Crystallite size effect on the monoclinic deformation of the bcc crystal structure of chromium
Physica B-condensed Matter, Feb 1, 2018
Abstract The modulated spin density wave magnetic orderings observed in chromium suggests that th... more Abstract The modulated spin density wave magnetic orderings observed in chromium suggests that the crystal structure of chromium cannot be described by the cubic space group I m 3 ¯ m . Our experimental studies of polycrystalline and nanocrystalline chromium by synchrotron radiation (SR) and neutron powder diffraction show a hkl-dependent Bragg peak broadening which can be interpreted by the low-symmetry monoclinic space group P 2 1 / n instead of the high symmetry cubic space group I m 3 ¯ m . The monoclinic angle is β m = 90.05(1)° and 90.29(1)° for polycrystalline Cr and nanocrystalline Cr, respectively. The relative monoclinic distortion observed in chromium is 5 times larger than those reported for several oxides: BiFeO 3 , α - Fe 2 O 3 , Cr 2 O 3 and calcite. The symmetry of the magnetic transverse spin density wave (TSDW) and the longitudinal spin density wave (LSDW) observed in Cr are described by using the superspace groups P 2 1 / n ( 0 β 0 ) 00 and P 2 1 ′ / n ′ ( 0 β 0 ) 00 , respectively. These superspace groups describe both the magnetic modulations and the atomic position modulations reported in the literature. The monoclinic symmetry of chromium is a robust effect which is observed in the paramagnetic as well as in the TSDW and LSDW phases.
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Papers by Rolf Hempelmann