Studies of a high temperature proton exchange membrane based on incorporating an ionic liquid cation 1-butyl-3-methylimidazolium into a Nafion matrix

High temperature proton exchange membranes based on Nafion were prepared by incorporating the polymer with ionic liquid cation 1-butyl-3-methylimidazolium (BMIm) and doping with phosphoric acid (PA). We found that using the hydroxide form rather than the chloride form of BMIm incorporated more readily the BMIm cation into Nafion film. A mole ratio of about 2 of BMIm cation to Nafion repeat unit, i.e., λ_BMIm/Nafion, was reached with the hydroxide form BMIm. The incorporated BMIm cation enhanced the doping of phosphoric acid into Nafion. A proton conductivity of 10.9 mS cm⁻¹ and a tensile stress at break of 5.3 MPa were achieved, respectively, with a composite membrane of Nafion/2.3BMIm/5.2PA in molar ratio at 160 °C without humidification.     

Electrochemical synthesis of poly(p-phenylene) and poly(p-phenylene)/TiO2 nanowires in an ionic liquid

This study shows for the first time that poly(p-phenylene) (PPP) nanowires can be easily obtained by electrochemical synthesis at room temperature. The method involves the template assisted electropolymerization of benzene in the air and water stable ionic liquid 1-hexyl-3-methylimidazolium tris(pentafluoroethyl)trifluorophosphate ([HMIm]FAP). Track-etched polycarbonate membranes (PC) with an average pore diameter of 90 nm were used as templates. Dense and highly flexible bundles of PPP nanowires with a high aspect ratio (>160) were easily obtained by this method. In addition, we present here our first results to obtain PPP/TiO₂ nanowires by the combination of a sol–gel technique with electropolymerization. HR-SEM, TEM and EDX were used for the structural characterization of the nanowires.     

Heterogeneously doped polyethylene glycol as nano-composite soft matter electrolyte

We have applied the concept of heterogeneous doping [1] to prepare and examine composite electrolytes, consisting of silica particles, low molecular weight polyethylene glycol solvents and lithium perchlorate salt. These “soggy sand” electrolytes combine high ionic conductivities (on the order of mS cm⁻¹) and high Li transference numbers (typically 60–80%) with improved mechanical properties. They were characterized using differential scanning calorimetry, dc-polarization and ac-impedance spectroscopy, zeta potential measurements and viscosimetry. Oxide, size and concentration as well as solvent molecular weight were varied to better understand the influence of ceramic oxide fillers on the ion conduction in these systems. As regarding the filler content, we observe that both conductivity and transference number of Li⁺ start increasing already at low volume fractions of oxide particles, reach a maximum and subsequently decrease to low values. The percolating network is – after initial partial coarsening – found to be stable within the time periods of the measurements.     

Effect of sintering temperature on microstructure and transport properties of Li3xLa2/3−xTiO3 with different lithium contents

Li_3xLa_2/3−xTiO₃ (LLTO) powder with different lithium contents (nominal 3x = 0.03–0.75) was synthesized via a simple sol–gel route and then calcination of gel-derived precursor at 900 °C which was much below the calcination temperature required for synthesizing the LLTO powder via solid state reaction route. The LLTO powder of sub-micron sized particles, derived from such sol–gel method, showed almost no aggregation. Starting from the sol–gel-derived powder, the LLTO ceramics with different lithium contents were prepared at different sintering temperatures of 1250 and 1350 °C. It demonstrated that our sol–gel route is quite simple and convenient compared to the previous sol–gel method and requires lower temperature for the LLTO. Our results also illustrated that lithium content significantly affects the structure and ionic conductivity of the LLTO ceramics. The dependence of the ionic conductivity on the lithium content, lattice structure, microstructure and sintering temperature was investigated systematically.     

Enhanced high-temperature polymer electrolyte membrane for fuel cells based on polybenzimidazole and ionic liquids

Polybenzimidazole (PBI)/ionic liquid (IL) composite membranes were prepared from an organosoluble, fluorine-containing PBI with ionic liquid, 1-hexyl-3-methylimidazolium tri?uoromethanesulfonate (HMI-Tf). PBI/HMI-Tf composite membranes with different HMI-Tf concentrations have been prepared. The ionic conductivity of the PBI/HMI-Tf composite membranes increased with both the temperature and the HMI-Tf content. The composite membranes achieve high ionic conductivity (1.6 × 10⁻² S/cm) at 250 °C under anhydrous conditions. Although the addition of HMI-Tf resulted in a slight decrease in the methanol barrier ability and mechanical properties of the PBI membranes, the PBI/HMI-Tf composite membranes have demonstrated high thermal stability up to 300 °C, which is attractive for high-temperature (>200 °C) polymer electrolyte membrane fuel cells.     

Electrochemical studies and self diffusion coefficients in cyclic ammonium based ionic liquids with allyl substituents

Several cyclic ammonium-based ionic liquids (ILs) with allyl substituent are synthesized, these allyl substituent ILs have high ionic conductivity (up to 4.72 mS cm⁻¹ at 30 °C) and wide electrochemical window of 5 V. The electrochemical behaviors of two organometallic redox couples Fc/Fc⁺ (ferrocene/ferrocenium) and Cc/Cc⁺ (cobaltocene/cobaltocenium) have been studied in these ILs, the calculated Stokes–Einstein product (Dη T⁻¹) of ferrocene in ILs is larger than that of cobaltocenium in ILs. The self-diffusion coefficients of cation and anion in these ILs are studied using pulsed gradient spin-echo NMR technique. There are very few reports where electrochemically derived diffusion coefficients and self diffusion coefficients are available for comparison, so a new key concept in electrochemistry could be developed in this paper.     

Effect of Nafion ionomer and catalyst in cathode layers for the direct formic acid fuel cell with complex capacitance analysis on the ionic resistance

Using platinum (Pt) black and carbon-supported Pt (Pt/C) as cathode catalysts, membrane-electrode assemblies (MEAs) were fabricated with various Nafion ionomer content, and their direct formic acid fuel cell (DFAFC) performances were investigated. In MEAs incorporating Pt black catalysts, the current density at 0.6 V was highest at ionomer/catalyst volume ratio of 1.0, which was consistent with the electrochemical active area (EAS) variation measured by cyclic voltammetry. However, the current density measured at 0.3 V, the cell performance increased with Nafion ionomer content, especially at low ionomer loading, indicating that proton transport rate played an important role. The variation in ionic resistance (R_ion) of cathode layers with Nafion ionomer content was experimentally confirmed by using the complex capacitance analysis of impedance data implemented with nitrogen (cathodes)/hydrogen (anodes) atmosphere. For Pt/C, the layer thickness and EAS of cathode were larger than those of MEA cathode using Pt black; and the current densities at 0.6 V were lower than those of Pt black, suggesting that smaller fraction of EAS was utilized.     

Development of a Ag/Ag micro-reference electrode for electrochemical measurements in ionic liquids

We report on a novel miniaturized Ag/Ag⁺ reference electrode (RE) design suitable for electrochemical measurements in room temperature ionic liquids (RTILs). The electrode is based on capillaries with an outer diameter of 365 μm and contains a 10 mmol/l solution of a silver salt in a RTIL. The silver salt bears the same type of anion as the RTIL. While potential shifts of several hundred millivolts have been observed for common platinum or silver pseudo-reference electrodes, our Ag/Ag⁺ micro electrode provides a stable and reliable reference potential over a period of more than two weeks, if protected from light and stored in a nitrogen atmosphere. Due to the small dimensions of the RE, it can be placed close to the working electrode (WE) and it is well-suited for application in electrochemical micro cells as well as for potential-controlled in situ AFM, STM or electrochemical impedance measurements. The electrode characteristics were determined by voltammetric measurements on ferrocene and cobaltocenium hexafluorophosphate dissolved in a RTIL. The highest expected contamination of the sample with Ag⁺ ions was calculated and found to be below 4 ppm.     

Potential of ionic liquids for VOC absorption and biodegradation in multiphase systems

This work aims to evaluate the feasibility of using imidazolium ionic liquids (ILs) in the design of multiphase bioreactors for the removal of volatile organic compounds (VOCs). The IL affinity for three model VOCs (dimethyl sulfide, dimethyl disulfide and toluene) was evaluated by means of the dimensionless partition coefficient (K). It was observed that ILs showed K values comparable to typical liquid solvents used in multiphase bioreactors for VOC biodegradation (K values ranged from 0.009 to 0.011, 0.0012 to 0.0013 and 0.00061 to 0.00096 for dimethyl sulfide, dimethyl disulfide and toluene, respectively). Toxicity tests showed that both ILs at concentrations of 5% and 10% (v/v) inhibited the glucose uptake of an activated sludge during approximately 24 h. After such lag period, the microorganisms were able to recover its metabolic activity. However, VOC biodegradation experiments showed that ILs at 5% (v/v) were toxic for the activated sludge and a toxic synergistic effect of the IL–VOC combination likely occurred. After acclimation to the target VOCs, only the toluene biodegradation capacity was significantly increased in the presence of ILs. These toxic effects represent a key drawback for the potential of IL-based multiphase systems devoted to VOC biodegradation. Therefore, this study suggests that microbial acclimation only to the VOCs is not enough to get an efficient biodegradation in multiphase systems including ILs as non-aqueous phases.     

Non-equilibrium molecular dynamics simulation of electrokinetic effects on heterogeneous ionic transport in nano-channel

The presence of the electric double layer (EDL) near the solid/liquid interface has a great impact on the liquid flowing through nano channels. In this paper, a non-equilibrium molecular dynamics method (NEMD) model is developed to investigate the transport characteristics of the heterogeneous ionic fluid flowing in a 4.672 nm-depth channel due to the electrokinetic effect induced by the EDL. An external perturbing velocity is introduced on the liquid instead of a constant force to study the electrokinetic effects on the flow. The effect of charges adsorbed on channel surfaces on flow resistance is especially considered. Two different charged surfaces with discrete or uniform charge distributions are compared, respectively. Besides Lennard-Jones (L-J) potential energy and Coulomb force, the ion–dipole interaction between the charged particle and the water molecule in the liquid is especially taken into account. According to the simulation results, the liquid density reaches the peak value in the EDL. However, unlike that predicted by the Poisson–Boltzmann theory, the liquid density profile is not exactly an anti-parabola curve, which has a significant fall before the density reaching the peak value in the EDL. It is shown that the charges absorbed on the channel surface have little influence on the liquid density distribution, but they play an important role in the fluid velocity distribution. Meanwhile, the liquid velocity slippage in the EDL emerges, which is well in agreement with the prior experimental data available. Moreover, though the non-dimensional velocities are nearly the same under different external velocity fields, the velocity slippage for the uniformly charged wall is much larger than that for the discretely charged wall. All of these results provide a probable reason why the flow behavior in a nano-channel is particularly different from that in a macro duct.