Two new siloxanic proton-conducting membranes: Part I. Synthesis and structural characterization

The development of stable polymer electrolytes having good proton conductivity, low cost and operating at medium temperatures represent a crucial step in the evolution of polymer electrolyte fuel cells. We describe two new siloxanic proton-conducting membranes that were synthesized through a two-stage protocol. In the first stage, a poly(methyl hydrosiloxane) precursor (P) bearing siloxane side chains with sulfonic acid groups was prepared. In the second step, the hydrolysis of pristine precursor or its derivative obtained by grafting siloxane chains on P yielded two types of membranes with the formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃Si(CH₃)₂O)O]₁₄Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26[Si(CH₃)((CH₂)₃SO₃H)O]_1.8[Si(CH₃)((CH₂)₃(Si(CH₃)₂O)_w)O]_v[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)O]_14 − vSi(CH₃)₃}_n (B), with w = 20.31. Polymer membranes of A and B were prepared by means of a hot-pressing process at 80 °C and 10 t/cm². Scanning electron microscopy showed that A and B are rubbery materials with rough and transparent surfaces. Thermogravimetric investigations performed under air atmosphere disclosed that A and B are thermally stable up to at least 198 °C. DSC measurements yielded T_g(s) of −44 and −60 °C for A and B, respectively. The polymers exhibit ionic exchange capacities of 0.33 (A) and 0.15 meq/g (B). FT-IR and FT-Raman investigations revealed that the polymers consist of reticulated siloxane networks with pendant silicone chains having sulfonic acid groups.     

Two new siloxanic proton conducting membranes: Part II. Proton conductivity mechanism and NMR study

The synthesis and structural characterization of two types of membranes with formulas {Si(CH₃)₃O[Si(CH₃)HO]_21.26-[Si(CH₃)((CH₂)₃SO₃H)O]_1.8-[Si(CH₃)((CH₂)₃Si(CH₃)₂O-)-O]₁₄-Si(CH₃)₃}_n (A) and {Si(CH₃)₃O[Si(CH₃)HO]_21.26-[Si(CH₃)((CH₂)₃SO₃H)O]_1.8-[Si(CH₃)((CH₂)₃(Si(CH₃)₂O-w₎)-Ov_][Si(CH₃)((CH₂)₃Si(CH₃)₂O-)-O_]14−vSi(CH₃)₃}_n (B), (w=20.31), were previously proposed.The ac electrical response of A and B was fully characterized in the 40 Hz-2 MHz frequency region by studying the impedance spectra in the medium and low frequency regions by equivalent circuits and complex dielectric spectra at high frequency in terms of dielectric relaxation modes. Results demonstrated that A and B conduct ionically by means of a proton exchange event which occurs via a vehicular mechanism between neighboring water clusters formed by water molecules aggregated around each sulfonic acid group of the siloxane side chains. The proton conductivities at 115 °C of ca. 1.9 × 10⁻³ and 1.8 × 10⁻⁴ S cm⁻¹ of fully hydrated membranes A and B, respectively, classify these silicone networks as good proton conductors.Membrane B was chosen for a closer investigation using NMR spectroscopy. Solid state ²⁹Si MAS NMR experiments gave further insight about the three-dimensional structure. Proton diffusion measurements provided some encouraging results about proton dynamics of this membrane signaling the great potential of siloxanic based proton conductors.     

Gel polymer electrolytes based on PMMA : III. PMMA gels containing cadmium

Gel polymer electrolyte containing cadmium perchlorate was prepared by polymerisation of methylmethacrylate with a solution containing Cd perchlorate in propylene carbonate (PC). Electric conductivity, fundamental electrochemical properties and Cd electrodeposition were investigated. The deposition of Cd was fairly homogenous.     

Water-entrained cement-based materials : II. Experimental observations

This paper concerns a new concept for the prevention of self-desiccation in hardening cement-based materials. The concept is based on using fine, superabsorbent polymer (hydrogel, SAP) particles as a concrete admixture. This permits a controlled formation of water-filled macropore inclusions—water entrainment—in the fresh concrete. Consequently, the pore structure is actively designed to control self-desiccation. In the paper, experimental observations in relation to this technique are described and discussed. The observations show that self-desiccation can be controlled by water entrainment. The paper forms the second part of a series. In the first part, the theoretical background was presented [Cem. Concr. Res. 31(4) (2001) 647].     

Conducting polymer films as model biological membranes: Electrochemical and ion-exchange properties of poly(pyrrole) films doped with asparagine and glutamine

This paper shows the application of conducting polymers (CPs) for constructing model biological membranes in order to study potential formation mechanism. Two amino acids, asparagine and glutamine, were incorporated in the poly(pyrrole) matrix during electrochemical polymerization. The polymer film was characterized by infrared and X-ray photoelectron spectroscopy. The film morphology was studied by atomic force microscopy. The ion-exchange behavior of PPy-Asn and PPy-Gln membranes in dependence on the conditioning solution are characterized using an open circuit potentiometric measurements. Close-to-Nernstian sensitivity was observed for the films under equilibrium. During equilibration provoked by the change in concentration of magnesium and/or calcium ions, the differences in the shape and evolution of the potential response with time were observed. The varying potential-time behavior after a bulk concentration change has been explained by a different participation of the magnesium and calcium ions on the ground diffusion layer model (DLM).     

Water electrolysis under microgravity: Part 1. Experimental technique

Water electrolysis was conducted in both alkaline (25 wt.% KOH, 2 wt.% KOH) and acid (0.1N H₂SO₄) solutions for 8 s under microgravity environment realized in a drop shaft. The gas bubble formation of hydrogen and oxygen on platinum electrodes was observed by CCD camera. In alkaline solutions, a bubble froth layer grew on the electrode surface. Hydrogen bubble size was smaller than that of oxygen. The current density at constant potential decreased continually with time. In spite of the growth of a bubble froth layer on the electrode, the electrolysis never stopped, apparently because fresh electrolyte is supplied to the electrode surface by microconvection induced by the gas bubble evolution. In acid solution, hydrogen gas bubbles frequently coalesced on the cathode surface, yielding a larger average bubble than that of oxygen. The current density did not vary at constant potentials from -0.4 to −0.8 V versus reversible hydrogen electrode (RHE), because the effective electrode surface area was significantly reduced by the larger bubble size compared to alkaline electrolyte. The present experiments indicate that, especially in a microgravity environment, the bubble evolution behavior and the resultant current-potential curves are significantly influenced by the wettability of the electrode in contact with the electrolyte.     

Characterization of membrane electrode assembly with hydrogen-hydrogen cell and ac-impedance spectroscopy: Part I. Experimental

A measurement system based on a symmetrical hydrogen-hydrogen cell was developed and presented. The net water flux through and impedance of Gore™ Primea^® Series 58 MEA were measured under different humidity conditions. An equivalent circuit approach was used to analyze impedance measurement data and the dependences of membrane conductivity and reaction kinetics on the humidity were obtained. Modeling work was found necessary to estimate the diffusion coefficient of water in the membrane, because humidity profile inside the cell was uneven and unknown during the net water flux measurements. Gas permeabilities of different cell components and water uptake of the MEA were measured to get parameter values for the modeling work. The actual modeling work was not conducted in this contribution, but is explained in Part II of this paper.     

Characterization of membrane-electrode assembly with hydrogen-hydrogen cell and ac-impedance spectroscopy: Part II. Modeling

A two-dimensional, one-phase model for hydrogen and water vapor transport inside a hydrogen-hydrogen cell is presented. The model was used to analyze the results of water transport measurements. As a result, the concentration dependence of the diffusion coefficient of water in membrane of Gore^™ Primea^® Series 58 MEA was achieved. Temperature differences inside a hydrogen-hydrogen cell were studied with modeling approach and it was found that the cell can be treated as an isothermal system.     

Novel polymeric systems for lithium-ion batteries gel electrolytes: I. Cross-linked polyfluorosilicone

The investigation of chemically cross-linked, self-supporting gel-type electrolyte membranes, based on hybrid polyfluorosilicone polymers reinforced with nanosized silica, for lithium-ion battery systems is reported. The polyfluorosilicone materials were selected on the basis of their high chemical and thermal stabilities. The precursors were synthesized with functional groups capable to form inter-molecular cross-linking, thus obtaining three-dimensional polymer matrices. The latter were undergone to swelling processes in (non-aqueous, lithium salt containing) electrolytic solutions to obtain gel-type polymer electrolytes. Several kinds of membranes, based on different types of polyfluorosilicone precursor, were prepared and characterized in terms of swelling behavior, ionic conductivity and electrochemical stability. The properties of the swelled matrices were evaluated as a function of dipping time, temperature, kind of electrolytic solution and cross-linking initiator content.     

Technology update: Vacuum infusion

Vacuum, or resin, infusion techniques are becoming increasingly popular, especially in the marine industry where they can produce high quality boat hulls quickly and cleanly (see Reinforced Plastics' Marine Supplement, October 2003).     

Rapoport-Samoilenko test for cathode carbon materials: I. Experimental results and constitutive modelling

The sodium expansion of semigraphitic commercial cathode material used for aluminium production has been measured on solid cylinder samples. Experimental results have been obtained for three current density values using the Rapoport-Samoilenko-type apparatus. A constitutive model for cathode carbon materials which is able to reproduce the relationship between the sodium expansion and time during the Rapoport-Samoilenko-type test has been proposed. Parameters required in the proposed model such as the sodium expansion after infinite time and the diffusion coefficient have been calculated from experimental data by a least-square minimization process. The distribution of sodium concentration, radial stress, tangential stress, axial stress, first principal stress and von Mises equivalent stress in the solid semigraphitic cylinder with time have been obtained.     

Electrosorption of hydrogen into palladium-rhodium alloys: Part 2. Pd-rich electrodes of various thickness

Hydrogen electrosorption into Pd-rich (≥80% Pd) Pd-Rh alloy layers of various thickness has been studied in acidic solutions (0.5 M H₂SO₄) using cyclic voltammetry and chronoamperometry. Pd-Rh electrodes were prepared as limited volume electrodes (LVEs) by electrochemical deposition. The influence of electrode potential and alloy thickness on the amount of electrosorbed hydrogen was investigated. For the thickness range examined (0.11-1.55 μm) no distinct effect on alloy absorption properties was found. The influence of scan rate on hydrogen oxidation charge was also studied. The results obtained suggest that the layer of subsurface hydrogen in Pd-Rh alloys is much less developed than in pure Pd and direct hydrogen absorption/desorption path is more probable.     

Electrochemistry of fluoren-9-one based conjugated copolymers: Analysis of voltammograms and correlation with spectroscopic data

Voltammetric and spectroscopic data for novel copolymers of fluoren-9-one with benzene, thiophene, and furan have been used to explore the relationship between the formal potentials and optical band gaps of conjugated polymers. It is shown that the optical band gap measured from the onset of absorption corresponds to the difference in the formal potentials for n- and p-doping. Voltammograms have been analyzed to establish how formal potentials should be defined for these and other conjugated polymers.     

PBI-based polymer electrolyte membranes fuel cells: Temperature effects on cell performance and catalyst stability

In this work, it has been shown that the temperature (ranging from 100 to 175 °C) greatly influences the performance of H₃PO₄-doped polybenzimidazole-based high-temperature polymer electrolyte membrane fuel cells by several and complex processes. The temperature, by itself, increases H₃PO₄-doped PBI conductivity and enhances the electrodic reactions as it rises. Nevertheless, high temperatures reduce the level of hydration of the membrane, above 130-140 °C accelerate the self-dehydration of H₃PO₄, and they may boost the process of catalyst particle agglomeration that takes place in strongly acidic H₃PO₄ medium (as checked by multi-cycling sweep voltammetry), reducing the overall electrochemical active surface. The first process seems to have a rapid response to changes in the temperature and controls the cell performance immediately after them. The second process seems to develop slower, and influences the cell performance in the “long-term”. The predominant processes, at each moment and temperature, determine the effect of the temperature on the cell performance, as potentiostatic curves display. “Long-term” polarization curves grow up to 150 °C and decrease at 175 °C. “Short-term” ones continuously increase as the temperature does after “conditioning” the cell at 125 °C. On the contrary, when compared the polarization curves at 175 °C a continuous decrease is observed with the “conditioning” temperature. A discussion of the observed trends is proposed in this work.     

Film formation from monodisperse acrylic lattices: Part 3: Drying and ageing of acrylic latex films

The influence of drying/ageing on the structure and properties of acrylic latex films was investigated using turbidity measurements in combination with gravimetry, scanning electron microscopy (SEM), atomic force microscopy (AFM) and water vapour permeability. Ageing above the minimum film formation temperature (MFFT) leads to marked changes in a dried latex film, whereas, after ageing below the MFFT, changes are hardly found. Above the MFFT there is a continuous change in the film properties with time. This becomes obvious from the decreases in the regenerated interference minimum, water vapour permeability and corrugation height. The influence of ageing on the water absorption of the films is less straight forward. It was expected that films with a more compact structure would absorb less water. This is correct for short drying times only, from 0.5 to 3 h. Ageing of better-dried films, however, yields the opposite result: by increase of the ageing time from 3 to 150 h the water uptake increases. There are various reasons for this increase; they are discussed briefly.     

Water electrolysis under microgravity: Part II. Description of gas bubble evolution phenomena

Water electrolysis is carried out in both alkaline (25 and 2 wt.% KOH) and acidic (0.1N H₂SO₄) solutions for 8 s under a microgravity (μ-G) environment realized in a drop shaft. The effects of gravitational strength on gas bubble evolution behavior are analyzed in consideration of various factors (bubble movement, bubble assembly and single bubble). Under a μ-G environment, a collection of fine gas bubbles forms a froth layer in alkaline solutions, whereas bubbles frequently coalesce in acidic solution. Moreover, H₂ gas bubbles in alkaline jump from a cathode surface and O₂ bubbles often coalesce on an anode. In acidic solution both H₂ and O₂ bubbles frequently coalesce on electrode surfaces. Such gas bubble movements are reflected in the coalescence number and bubble residence time. A single bubble is characterized by the bubble size and the dynamic contact angle between a gas bubble and a Pt electrode, however, these factors are not essentially influenced by the gravitational strength.     

Show review: INCHEM TOKYO 2003

In the first week of November 2003, the Filtration and Separation Symposium was concurrently held with INCHEM TOKYO 2003 in Tokyo, Japan. A review of both events are presented below.