Anke Kaltbeitzel

Phosphonated Hexaphenylbenzene: A Crystalline Proton Conductor

Proton conductivity has been widely studied because of its importance in biological and chemical processes. A fuel cell (FC) is a promising device that can provide electrical energy with high efficiency and low environmental impact. A critical issue that severely hampers FC performance is the synthesis of proton-exchange membranes (PEMs) that simultaneously provide high proton conductivity that is constant over temperature. In the case of automotive applications, a guideline of close to 1 10 1 Scm 1 for the proton conductivity of the membrane at 120 8C and 50 % relative humidity (RH) was established by the U.S. Department of Energy as target operating conditions. State-of-the-art polymeric electrolytes are sulfonic acid based perfluorinated polymers such as Nafion. These electrolytes present high but temperaturedependent proton conductivity, since proton transport is governed by the vehicle mechanism that is based on the diffusion of proton-containing groups. Alternatively, phosphonic acid has been suggested as a protogenic group for intrinsically conducting separator materials because of its amphoteric properties. It has been demonstrated that a high concentration of these acidic groups, which are able to aggregate, is required for a high intrinsic proton conductivity. Inorganic crystals (solid acid proton conductors) have been proposed as alternative materials to polymer electrolytes. However, despite their high intrinsic conductivities (10 –10 3 S cm ; Grotthuss-type mechanism), these crystals have certain disadvantages, such as poor mechanical properties, water solubility, and high-temperature operating conditions (above 230 8C under atmospheric pressure for CsH2PO4). [6] Although research has focused to date on increasing the flexibility of the protogenic groups, for example, by introducing spacers or by adding small molecules, we have followed a different approach, in which we proposed to increase proton mobility by using a self-assembly and preorganization concept. Herein, organic crystals of small molecules are suggested as an alternative to common polymeric electrolytes and inorganic crystals employed as PEM in FC systems. Although a lot of effort has been made to investigate inorganic crystals, very little information on entirely organic crystals has been reported to date. The crystal structure of hexaphenylbenzene (HPB) and its derivatives have been known for a long time. Most recently, crystallographic studies on acidic derivatives have been carried out. It was found that almost all the molecules present multiple hydrogen bonds in the molecular plane and additionally form hydrogen bonds between adjacent sheets in such a way that columnar supramolecular networks are formed. Inspired by the properties of phosphonic acids, as well as by the supramolecular self-assembly of HPB derivatives, we introduced phosphonic acid groups into the nonplanar structure of HPB. Hexakis(p-phosphonatophenyl)benzene (p-6PA-HPB) was synthesized in a three-step reaction (Scheme 1). Powder X-ray measurements at different temperatures and RH values were performed (see Figure 1 and Figures S4 and S5 in the Supporting Information). It was found that p6PA-HPB is crystalline and that the local order is only slightly affected by changing these parameters. These changes are attributed to small local packing variations probably caused by the evaporation of water (see Figure 1).

Facile Large-Scale Fabrication of Proton Conducting Channels

A new approach to the facile large-scale fabrication of robust silicon membranes with artificial proton conducting channels is presented. Ordered two-dimensional macroporous silicon was rendered proton conducting by growing a thick uniform polyelectrolyte brush using surface-initiated atom transfer radical polymerization throughout the porous matrix. The fabricated silicon-poly(sulfopropyl methacrylate) hybrid membranes were evaluated for their proton conductivity, ion exchange capacity, and water uptake. With proton conductivities in the range of 10(-2) S/cm, these proof-of-concept experiments highlight a promising alternative for producing tailorable proton conducting membranes. This approach constitutes a benchmark for the preparation and study of model systems and, in addition, for the large-scale fabrication of membranes suitable for a wide range of technological applications.

Highly Proton‐Conducting Self‐Humidifying Microchannels Generated by Copolymer Brushes on a Scaffold

Filling in the gaps: Macroporous silicon membranes modified with sulfonated polymer brushes have been synthesized by pore-filling surface polymerization (see picture) to give proton-conducting channels with tailor-made, finely tuned physicochemical characteristics. These membranes display high conductivity values (ca. 10(-2) S cm(-1)) regardless of the humidity, thus surpassing the performance of nafion.

Linear and non-linear optical properties of substituted polyphenylacetylene thin films

Optical third-harmonic generation (THG) at 1064 nm and wavelength dispersed degenerate four-wave mixing (DFWM) have been performed on substituted polyphenylacetylenes (PPAS). Strong changes in the linear and nonlinear optical properties can be achieved by varying the substituent. Resonant values of chi (3)(-3 omega ; omega , omega , omega ) up to 10-11 esu and chi (3)(- omega ; omega , - omega , omega ) of more than 10-9 esu with ultra-fast response times were observed. A linear relationship between chi (3)(- omega ; omega , - omega , omega ) and the absorption coefficient alpha was found. This is compatible with an inhomogeneous broadening of the absorption band or with phase-space filling by one-dimensional excitons. Model calculations are presented that relate the modulus and phase of chi (3)(-3 omega , omega , omega , omega ) to the linear optical properties of PPA and indicates a direct two-photon interaction with the absorption band. This may be due to a non-centrosymmetric electronic structure of the PPA backbone.

Resonant degenerate four wave mixing and scaling laws for saturable absorption in thin films of conjugated polymers and Rhodamine 6G

Abstract Wavelength dispersed degenerate four wave mixing (DFWM) experiments were performed with thin films of poly(3-decyl-thiophene), poly ( p -phenylene vinylene) and Rhodamine 6G. At the low energy side of the main absorption bands the resonant χ( 3 ) (-ω; ω, ω, -ω) values scale differently with the absorption coefficient α. In Rhodamine 6G, χ( 3 ) is proportional to α 2 due to saturable absorption in a two level system. In the case of polymers with a conjugated one-dimensional π-electron system a linear relationship between χ( 3 ) and α is found. This is attributed to phase-space filling effects by one-dimensional excitons.

Genotoxic effects of zinc oxide nanoparticles

The potential toxicity of nanoparticles has currently provoked public and scientific discussions, and attempts to develop generally accepted handling procedures for nanoparticles are under way. The investigation of the impact of nanoparticles on human health is overdue and reliable test systems accounting for the special properties of nanomaterials must be developed. Nanoparticular zinc oxide (ZnO) may be internalised through ambient air or the topical application of cosmetics, only to name a few, with unpredictable health effects. Therefore, we analysed the determinants of ZnO nanoparticle (NP) genotoxicity. ZnO NPs (15-18 nm in diameter) were investigated at concentrations of 0.1, 10 and 100 μg mL(-1) using the cell line A549. Internalised NPs were only infrequently detectable by TEM, but strongly increased Zn(2+) levels in the cytoplasm and even more in the nuclear fraction, as measured by atom absorption spectroscopy, indicative of an internalised zinc and nuclear accumulation. We observed a time and dosage dependent reduction of cellular viability after ZnO NP exposure. ZnCl2 exposure to cells induced similar impairments of cellular viability. Complexation of Zn(2+) with diethylene triamine pentaacetic acid (DTPA) resulted in the loss of toxicity of NPs, indicating the relevant role of Zn(2+) for ZnO NP toxicity. Foci analyses showed the induction of DNA double strand breaks (DSBs) by ZnO NPs and increased intracellular reactive oxygen species (ROS) levels. Treatment of the cells with the ROS scavenger N-acetyl-l-cysteine (NAC) resulted in strongly decreased intracellular ROS levels and reduced DNA damage. However, a slow increase of ROS after ZnO NP exposure and reduced but not quashed DSBs after NAC-treatment suggest that Zn(2+) may exert genotoxic activities without the necessity of preceding ROS-induction. Our data indicate that ZnO NP toxicity is a result of cellular Zn(2+) intake. Subsequently increased ROS-levels cause DNA damage. However, we found evidence for the assumption that DNA-DSBs could be caused by Zn(2+) without the involvement of ROS.

Optical third-harmonic generation in substituted poly(phenylacetylenes) and poly(3-decylthiophenes)

Abstract We have performed third-harmonic generation measurements at λ 0 = 1064 nm on ultrathin films of substituted poly(phenylacetylenes) and three forms of poly(3-decylthiophene). Both the absolute values and the phases of χ (3) have been estimated. The average values of the nonlinear susceptibility χ (3) (−3 ω ; ω , ω , ω ) were found to be between 1.3 ± 0.3 × 10 −12 and 1.3 ± 0.3 × 10 −11 e.s.u. for substituted poly(phenylacetylenes) and around 1.0 × 10 −11 e.s.u. for three neutral forms of poly(3-decylthiophene). Evaluation of the phases of χ (3) indicates that the two-photon resonance levels are located slightly differently with respect to the main absorption band in the two polymeric systems.

Polymer brush functionalized SiO2 nanoparticle based Nafion nanocomposites: a novel avenue to low-humidity proton conducting membranes

Polyelectrolyte membranes showing proton conductivity at moderate levels of relative humidity and temperatures are essential for the development of polyelectrolyte membrane fuel cells (PEMFCs). Herein, monomethoxy oligoethylene glycol methacrylate derived polymer brush functionalized silica nanoparticles (SiO2 NPs) are presented as humidifying-nanoadditives for the fabrication of Nafion nanocomposite membranes, exhibiting improved proton conductivities at moderate levels of relative humidity and temperatures. Polymer brush functionalized SiO2 NPs (SiO2-polymer-brush), fabricated via surface initiated atom transfer radical polymerization (SI-ATRP), are dispersed in the Nafion resin solution, and nanocomposite membranes (Nafion/SiO2-polymer-brush) are fabricated via solution casting. For comparison, composite membranes of Nafion are also prepared with bare SiO2 NPs. Spectroscopic measurements confirm the presence of polymer brushes in the final membranes and demonstrate increased water uptake in membranes with polymer brush-functionalized nanocomposite membranes. Electrochemical impedance analyses reveal that 1 wt% of functionalized SiO2 NPs is sufficient to achieve Nafion nanocomposite membranes with superior proton conductivities at ambient and moderately high temperatures over the entire range of relative humidity (RH). This study presents a facile avenue to membranes with superior proton conductivities under moderate levels of RH and temperature, and provides important insights into the scope of nanocomposite PEMs for fuel cell applications.

Proton Conductivity in Doped Aluminum Phosphonate Sponges

Proton-conducting networks (NETs) were prepared successfully by the insertion of phosphonated nanochannels into organic-inorganic hybrid materials that contain Al(3+) as the connector and hexakis(p-phosphonatophenyl)benzene (HPB) as the linker. Noncomplexed phosphonic acid groups remain in the framework, which depends on the ratio of both compounds, to yield a proton conductivity in the region of 10(-3) S cm(-1). This conductivity can be further improved and values as high as Nafion, a benchmark proton-exchange membrane for fuel cell applications, can be obtained by filling the network pores with intrinsic proton conductors. As a result of their sponge-like morphology, aluminum phosphonates adsorb conductive small molecules such as phosphonic acids, which results in a very high proton conductivity of approximately 5 × 10(-2) S cm(-1) at 120 °C and 50 % relative humidity (RH). Contrary to Nafion, the doped networks show a remarkably low temperature dependence of proton conductivity from external humidification. This effect indicates a transport mechanism that is different to the water vehicle mechanism. Furthermore, the materials exhibit an activation energy of 40 kJ mol(-1) at 15 % RH that starts to diminish to 10 kJ mol(-1) at 80 % RH, which is even smaller than the corresponding values obtained for Nafion 117.

Nonlinear Optical Properties of Poly(P-Phenylene Vinylene) Thin Films

Nonlinear optical investigations on conjugated polymeric systems have shown strong nonlinear effects and fast response times (1). For application of these phenomena it is necessary to fabricate thin films with high uniformity of thickness, low optical scattering, photostability and low absorption in the near IR. Recently the conjugated polymer poly(p-phenylene vinylene) (PPV) was described (2–7). Third harmonic generation (THG) measurements at 1.85 µm (8) and femtosecond degenerate four wave mixing (DFWM) experiments at 580 and 602 nm (9) on PPV thin films demonstrated high optical nonlinearity and subpicosecond response time. Therefore PPV is an interesting system for further nonlinear optical investigations at other wavelength regions.