Fabrication of a composite anion exchange membrane with aligned ion channels for a high-performance non-aqueous vanadium redox flow battery.

Fabrication of high-conductivity ion exchange membranes (IEMs) is crucial to improve the performance of non-aqueous vanadium redox flow batteries (NAVRFBs). In the present work, anion exchange membranes with high-conductivity were fabricated by aligning ion channels of the polymer electrolyte impregnated in porous polytetrafluoroethylene (PTFE) under electric fields. It was observed that the ion channels of the polymer electrolyte were uniformly orientated in the atomic-force microscopy image. Its morphological change could minimize detouring of the transport of BF4 - ions. The results showed through-plane conductivity was improved from 12.7 to 33.1 mS cm-1. The dimensional properties of the fabricated membranes were also enhanced compared with its cast membrane owing to the reinforcing effect of the substrate. Especially, the NAVRFB assembled with the optimized membrane showed increased capacities, with a 97% coulombic efficiency and 70% energy efficiency at 80 mA cm-2. Furthermore, the optimized membrane made it possible to operate the NAVRFB at 120 mA cm-2. Its operating current density was 120 times higher than that of a frequently used AHA membrane for RFBs.

Arsenic removal from groundwater using low-cost carbon composite electrodes for capacitive deionization.

Affordable carbon composite electrodes were developed to treat low-concentrated groundwater using capacitive deionization (CDI). A carbon slurry prepared using activated carbon powder (ACP), poly(vinylidene fluoride), and N-methyl-2-pyrrolidone was employed as a casting solution to soak in a low-cost porous substrate. The surface morphology of the carbon composite electrodes was investigated using a video microscope and scanning electron microscopy. The capacitance and electrical conductivity of the carbon composite electrodes were then examined using cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS), respectively. According to the CV and EIS measurements, the capacitances and electrical conductivities of the carbon composite electrodes were in the range of 8.35-63.41 F g(-1) and 0.298-0.401 S cm(-1), respectively, depending on ACP contents. A CDI cell was assembled with the carbon composite electrodes instead of with electrodes and current collectors. The arsenate removal test included an investigation of the optimization of several important operating parameters, such as applied voltage and solution pH, and it achieved 98.8% removal efficiency using a 1 mg L(-1) arsenate solution at a voltage of 2 V and under a pH 9 condition.

Morphologically Aligned Cation-Exchange Membranes by a Pulsed Electric Field for Reverse Electrodialysis.

A low-resistance ion-exchange membrane is essential to achieve the high-performance energy conversion or storage systems. The formation methods for low-resistance membranes are various; one of the methods is the ion channel alignment of an ion-exchange membrane under a direct current (DC) electric field. In this study, we suggest a more effective alignment method than the process with the DC electric field. First, an ion-exchange membrane was prepared under a pulsed electric field [alternating current (AC) mode] to enhance the effectiveness of the alignment. The membrane properties and the performance in reverse electrodialysis (RED) were then examined to assess the membrane resistance and ion selectivity. The results show that the membrane electrical resistance (MER) had a lower value of 0.86 Ω cm(2) for the AC membrane than 2.13 Ω cm(2) observed for the DC membrane and 4.30 Ω cm(2) observed for the pristine membrane. Furthermore, RED achieved 1.34 W/m(2) of maximum power density for the AC membrane, whereas that for the DC membrane was found to be 1.14 W/m(2) [a RED stack assembled with CMX, used as a commercial cation-exchange membrane (CEM), showed 1.07 W/m(2)]. Thereby, the novel preparation process for a remarkable low-resistance membrane with high ion selectivity was demonstrated.

Designing a Highly Active Metal-Free Oxygen Reduction Catalyst in Membrane Electrode Assemblies for Alkaline Fuel Cells: Effects of Pore Size and Doping-Site Position.

To promote the oxygen reduction reaction of metal-free catalysts, the introduction of porous structure is considered as a desirable approach because the structure can enhance mass transport and host many catalytic active sites. However, most of the previous studies reported only half-cell characterization; therefore, studies on membrane electrode assembly (MEA) are still insufficient. Furthermore, the effect of doping-site position in the structure has not been investigated. Here, we report the synthesis of highly active metal-free catalysts in MEAs by controlling pore size and doping-site position. Both influence the accessibility of reactants to doping sites, which affects utilization of doping sites and mass-transport properties. Finally, an N,P-codoped ordered mesoporous carbon with a large pore size and precisely controlled doping-site position showed a remarkable on-set potential and produced 70% of the maximum power density obtained using Pt/C.

Surface modification and use of polymer complex agents to mitigate metal crossover of anion-exchange membranes.

The facile surface modification of a commercial anion-exchange membrane (i.e., Neosepta-AFX, Astom Corp., Japan) was investigated with brominated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO), which had non-charged polar groups, and its quaternized form (QPPO) to improve the acid recovery efficiency in diffusion dialysis (DD). By coating a thin layer of BPPO on the membrane surface, the significant changes in the surface compactness and charge density were observed while the electrochemical properties were mostly maintained. From the DD experiments, it was revealed that the membrane modified with 1 wt% BPPO exhibited the moderate acid permeability as well as the highest acid selectivity (KAcid/KFe3+=48.81), which is more than double compared with that of the pristine membrane (KAcid/KFe3+=22.48) among the tested membranes. The electron-rich polar groups contained in BPPO are believed to provide moderate proton transport while the reduced swelling property of the membrane surface can effectively mitigate the crossover of metal cations. In addition, the acid selectivity (KAcid/KFe3+=30.69) was largely improved by using small molecular weight poly(ethyleneimine) (PEI, Mn=1800) with a small content (in the range of 1-5 wt%) as a solution additive for the growing size of metal species by the formation of polymer-metal complexes.

Facile surface modification of anion-exchange membranes for improvement of diffusion dialysis performance.

In this study, a facile membrane modification method by spin-coating of pyrrole (Py) monomers dissolved in a volatile solvent followed by an interfacial polymerization is proposed. The surface of a commercial anion-exchange membrane (i.e., Neosepta-AFX, Astom Corp., Japan) was successfully modified with polypyrrole (Ppy) to improve the acid recovery performance in diffusion dialysis (DD). The result of DD experiments revealed that both the acid and metal ion transports are significantly influenced by the surface modification. The metal crossover through the membranes was largely reduced while mostly maintaining the acid permeability by introducing a thin Ppy layer with excellent repelling property to cations on the membrane surface. As a result, the anion-exchange membrane modified with the optimum content of Py monomer (5 vol.%) exhibited excellent acid dialysis coefficient (KAcid) and selectivity (KAcid/KMetal) which is approximately twice as high as that of the pristine membrane.

Isoindigo-based small molecules for high-performance solution-processed organic photovoltaic devices: the electron donating effect of the donor group on photo-physical properties and device performance.

Five solution processable isoindigo-based donor-acceptor-donor (D-A-D) small molecules with different electron donating strengths have been designed and synthesized. The variation in the electron donating strength of the donor group strongly affected the optical, thermal, electrochemical and photovoltaic device performances of the isoindigo organic materials. The highest power conversion efficiency of ~3.2% was achieved in the bulk heterojunction photovoltaic device consisting of ID3T as the donor and PC70BM as the acceptor. This work demonstrates the potential of isoindigo moieties as electron-deficient units and presents guidelines for the synthesis of D-A-D small molecules for producing highly efficient, solution-processed organic photovoltaic devices.

The effects of aspect ratio of inorganic fillers on the structure and property of composite ion-exchange membranes.

A new type of nanocomposite ion-exchange membranes containing sulfonated polyethersulfone (sPES) polymer matrix and sulfonated surface-functionalized mesoporous silica (SS) inorganic fillers was prepared. Various characterizations revealed that the addition of inorganic fillers with different shapes had a significant influence on the membrane structure. The mesoporous inorganic fillers not only created extra pore and water channels, assisting the ionic migration and improving conductivity of the composites, but also provided additional fixed charge groups upon surface modification. This allows the Donnan exclusion to work effectively and thus improve the selectivity of membranes. It was proved that the incorporation of appropriate amount of SS additive could significantly improve the conductivity (up to 20 folds) and permselectivity (about 14%) of the sPES membranes. The performance of these newly developed membranes in desalination by electrodialysis was comparable with that of a commercial membrane (FKE).

Preparation of ion exchanger layered electrodes for advanced membrane capacitive deionization (MCDI).

A noble electrode for capacitive deionization (CDI) was prepared by embedding ion exchanger onto the surface of a carbon electrode to practice membrane capacitive deionization (MCDI). Bromomethylated poly (2, 6-dimethyl-1, 4-phenylene oxide) (BPPO) was sprayed on carbon cloth followed by sulfonation and amination to form cation exchange and anion exchange layers, respectively. The ion exchange layers were examined by Scanning electron microscopy (SEM) and Fourier transform infrared spectrometer (FT-IR). The SEM image showed that the woven carbon cloth was well coated and connected with BPPO. The FT-IR spectrum revealed that sulfonic and amine functional groups were attached on the cationexchange and anionexchange electrodes, respectively. The advantages of the developed carbon electrodes have been successively demonstrated in a batch and a continuous mode CDI operations without ion exchange membranes for salt removal using 100 mg/L NaCl solution.

Interface resistances of anion exchange membranes in microbial fuel cells with low ionic strength.

The interface resistances between an anion exchange membrane (AEM) and the solution electrolyte were measured for low buffer (or ionic strength) of electrolytes typical of microbial fuel cells (MFCs). Three AEMs (AFN, AM-1, and ACS) having different properties were tested in a flat-plate MFC to which 5-mM acetate was fed to the anode and an air-saturated phosphate buffer (PB) solution was fed to the cathode. Current density achieved in the MFCs was correlated inversely with independently measured membrane-only resistances. However, the total interfacial resistances measured by current-voltage plots were approximately two orders higher than those of the membrane-only resistances, although membranes had the same order as with the membrane-only resistance. EIS spectra showed that the resistances from electric-double layer and diffusion boundary layer were the main resistances not the membrane's resistance. The electric-double layer and diffusion boundary layer resistances of the AEMs were much larger in the 10 mM PB electrolyte, compared to 100 mM PB. EIS study also showed that the resistance of diffusion boundary layer decreased due to mechanical stirring. Therefore, the interface resistance that originates from the interaction between the membrane and the catholyte solution should be considered when designing and operating MFC processes with an AEM. The AEMs allowed transport of uncharged O(2) and acetate, but the current losses for both were low during normal MFC operation.

Analyses of interfacial resistances in a membrane-electrode assembly for a proton exchange membrane fuel cell using symmetrical impedance spectroscopy.

Interfacial resistances between the polymer electrolyte membrane (PEM) and catalyst layer (CL) in membrane-electrode assemblies (MEAs) have yet to be systematically examined in spite of its great importance on the fuel cell performance. In order to investigate ionic transport through the PEM/CL interface, the symmetrical impedance mode (SIM) was employed in which the same type of gas was injected (H(2)/H(2)). In this study, the ionic transport resistance at the interface was controlled by the additionally sprayed outer ionomer on the surface of each CL. Effectiveness of the outer ionomer on ionic transport at the interface was quantitatively explained by the reduced contact, proton hydration, and charge transport resistances in the SIM. To characterize the ionic transport resistance, the concept of total resistance (R(tot)) in the SIM was introduced, representing the overall ohmic loss due to proton transport in an MEA. This concept was successfully supported via an agreement of the interpretation and the linear correlation that was obtained between the admittance (1/R(tot)) and the performance of a fuel cell in the ohmic loss region. This correlation will enable researchers to predict the performance of a fuel cell under the influence of proton transport by examining the R(tot) in the SIM.

Modifying a proton conductive membrane by embedding a "barrier".

For development of proton conductive membranes, it is a difficult dilemma to balance proton conductivity and methanol permeability; however, this research proposes a simple strategy to solve this problem, i.e., embedding a proton conductive "barrier" into the perflorosulfonated matrix. The strategy is exemplified by embedding the amphoteric sulfonated poly(phthalazinone ether sulfone kentone) (SPPESK) into a semicrystalline perflorosulfonic acid polymer matrix (FSP). After being annealed, the domain of SPPESK is converted to the barrier. Two acid-base interactions constitute the barrier for both the transfer of protons and the blockage of methanol, respectively. On one hand, poorly hydrophilic ionic acid-base interactions (-SO(3)(-)...NH(+)-) are formed between sulfonic acid group and phthalazinone group through annealing and are useful for methanol blocking. On the other hand, more hydrophilic hydrogen-bonded acid-base interaction (-SO(3)H...(H(2)O)(n)...N-, n ≤ 3) can also be formed under hydrated condition and facilitate proton transport according to the Grotthuss-type mechanism. As a result, the final membrane exhibits an extremely low methanol permeability (30% of that of Nafion-112) and an excellent fuel cell performance (as compared with Nafion-112 at 80 °C).

Kinetics determination of electrogenerated hydrogen peroxide (H2O2) using carbon fiber microelectrode in electroenzymatic degradation of phenolic compounds.

The kinetics of electrogenerated hydrogen peroxide (H(2)O(2)), which can activate peroxidases in an electroenzymatic process, was examined by an amperometric technique using a carbon fiber microelectrode that was modified by polyaniline (PAn) film and platinum particles. The electrogeneration of H(2)O(2) was found to be dependent on the pH and applied potential, and resulting in a variable current response of the carbon fiber microelectrode. The highest amount of H(2)O(2) was electrogenerated when 2.3 V was applied between the Pt/Ti anode and a reticulated vitreous carbon (RVC) cathode at pH 6.0, with a current response of 0.0190 microA min(-1). Phenol was completely degraded by the electroenzymatic reaction of the immobilized horseradish peroxidase (HRP), and the time required for the electrogeneration of H(2)O(2) increased according to the initial concentration of phenol. The degradation stoichiometric ratio between the electrogenerated H(2)O(2) and the aqueous phenol under HRP immobilized on RVC was found to be 1:1.

Investigation on removal of hardness ions by capacitive deionization (CDI) for water softening applications.

Capacitive deionization (CDI) for removal of water hardness was investigated for water softening applications. In order to examine the wettability and pore structure of the activated carbon cloth and composites electrodes, surface morphological and electrochemical characteristics were observed. The highly wettable electrode surface exhibited faster adsorption/desorption of ions in a continuous treatment system. In addition, the stack as well as unit cell operations were performed to investigate preferential removal of the hardness ions, showing higher selectivity of divalent ions rather than that of the monovalent ion. Interestingly, competitive substitution was observed in which the adsorbed Na ions were replaced by more strongly adsorptive Ca and Mg ions. The preferential removal of divalent ions was explained in terms of ion selectivity and pore characteristics in electrodes. Finally, optimal pore size and structure of carbon electrodes for efficient removal of divalent ions were extensively discussed.

Hydrogen bonding: a channel for protons to transfer through acid-base pairs.

Different from H(3)O(+) transport as in the vehicle mechanism, protons find another channel to transfer through the poorly hydrophilic interlayers in a hydrated multiphase membrane. This membrane was prepared from poly(phthalazinone ether sulfone kentone) (SPPESK) and H(+)-form perfluorosulfonic resin (FSP), and poorly hydrophilic electrostatically interacted acid-base pairs constitute the interlayer between two hydrophilic phases (FSP and SPPESK). By hydrogen bonds forming and breaking between acid-base pairs and water molecules, protons transport directly through these poorly hydrophilic zones. The multiphase membrane, due to this unique transfer mechanism, exhibits better electrochemical performances during fuel cell tests than those of pure FSP and Nafion-112 membranes: 0.09-0.12 S cm(-1) of proton conductivity at 25 degrees C and 990 mW cm(-2) of the maximum power density at a current density of 2600 mA cm(-2) and a cell voltage of 0.38 V.

Detoxification of simulated textile wastewater using a membraneless electrochemical reactor with immobilized peroxidase.

Simulated textile wastewater was degraded using a membraneless electrochemical reactor with immobilized peroxidase on the porous Celite. The optimal current density was 10 A m(-2), at which the highest amount of hydrogen (H(2)O(2)) could be generated. The decolorization efficiencies of the simulated wastewater using the electrochemical and electroenzymatic methods were 35% and 92%, respectively. Biodegradability, the ratio of 5-day biochemical oxygen demand to chemical oxygen demand (BOD(5)/COD), was enhanced about 1.88 times when using the electroenzymatic treatment rather than raw wastewater, which could not be achieved by the electrochemical treatment. The toxic unit (TU), calculated using the lethal concentration (LC(50)) of Daphnia magna (D. Magna), of effluent treated by electroenzymatic method was below 1, whereas those of simulated textile wastewater and effluent treated by electrochemical method were 11.4 and 3.9, respectively.

Preparation of a highly sensitive enzyme electrode using gold nanoparticles for measurement of pesticides at the ppt level.

A highly sensitive enzyme electrode was prepared based on gold nanoparticles for measurement of pesticides. Gold nanoparticles of 25-30 nm were synthesized on a glassy carbon electrode by double-pulse technique while the coverage was controlled by applied potential and time. The gold nanoparticles were modified to form a self-assembled monolayer, followed by covalent binding of tyrosinase. The TYR-AuNP-GC electrode was compared with bare GC, AuNP-GC, and modified AuNP-GC and TYR-Au (plate type) electrodes in terms of cyclic voltammetry. The voltammograms well represent the sensitivity of enzymatic oxidation of catechol, substrates for the enzyme activity. The prepared electrode integrated into a continuous flow system and was tested to detect pesticides, such as 2,4-D, atrazine, and ziram. Under the optimized conditions of the flow system, the electrode performed reasonably according to the inhibition mechanism in the concentration range of 0.001-0.5 ng mL(-1). The enhanced performance was attributed to the favored microenvironment for the enzyme activity provided by SAM on gold nanoparticles.

Optimized coverage of gold nanoparticles at tyrosinase electrode for measurement of a pesticide in various water samples.

Gold nanoparticles (AuNPs) were electrodeposited onto a glassy carbon (GC) electrode to increase the sensitivity of the tyrosinase (TYR) electrode. By controlling the applied potential and time, the coverage of AuNPs at the TYR electrode was optimized with respect to the current response. The voltammetric measurements revealed a sensitive enzymatic oxidation and electrochemical reduction of substrate (phenol and catechol). The quantitative relationships between the inhibition percentage and the pesticide concentration in various water samples were measured at the TYR-AuNP-GC electrode, showing an enhanced performance attributed by the use of AuNPs.

Control of the fixed charge distribution in an ion-exchange membrane via diffusion and the reaction rate of the monomer.

The fixed charge distribution of the ion-exchange membranes was controlled by introducing ion-exchangeable groups onto the glycidyl methacrylate (GMA)-g-polypropylene (PP) membranes. The membranes were prepared by plasma-induced graft polymerization with uniform or nonuniform graft distributions over the cross section. The effects of reaction conditions on the graft distribution in plasma-induced graft polymerization were investigated to obtain the GMA-g-PP membranes with different graft distributions. The examined reaction conditions were plasma power, gas pressure of the plasma, solvent, concentration of the monomer solution, and reaction temperature. The graft distribution of the membranes was directly observed by a microscopic Fourier transform infrared mapping method and field-emission scanning electron microscopy. Also, the graft distribution was correlated with the relative magnitude of the reaction rate to the diffusion rate, which may determine the grafting yield as a function of the distance from the surface. A high rate of diffusion compared to the reaction rate resulted in a more uniform graft distribution. Among the grafting conditions, control of the reaction temperature was found to be the most effective for selectively preparing both uniform and nonuniform graft distribution. Uniform graft distribution was achieved when the reaction was conducted at 1 degrees C because of the relatively rapid diffusion and the slow reaction of the monomer, while nonuniform graft distribution occurred at higher reaction temperatures. Consequently, uniformly and nonuniformly charged cation-exchange membranes were prepared through sulfonation of the corresponding GMA-g-PP membranes at temperatures of 1 and 40 degrees C, respectively.

Improvement of an enzyme electrode by poly(vinyl alcohol) coating for amperometric measurement of phenol.

A poly(vinyl alcohol) film cross-linked with glutaraldehyde (PVA-GA) was introduced to the surface of a tyrosinase-based carbon paste electrode. The coated PVA-GA film was beneficial in terms of increasing the stability and reproducibility of the enzyme electrode. The electrode showed a sensitive current response to the reduction of the o-quinone, which was the oxidation product of phenol, by the tyrosinase, in the presence of oxygen. The effects of the PVA and PVA-GA coating, the pH, and the GA:PVA ratio on the current response were investigated. The sensitivity of the PVA-GA-Tyr electrode was 130.56microA/mM (1.8microA/microM cm(2)) and the linear range of phenol was 0.5-100microM. At a higher concentration of phenol (>100microM), the current response showed the Michaelis-Menten behavior. Using the PVA-GA-Tyr electrode, a two-electrode system was tested as a prototype sensor for portable applications.