Showing papers on "Nafion published in 2013"

Proton Conduction with Metal-Organic Frameworks

Abstract: Proton-exchange membrane fuel cells (PEMFCs) generate electricity because the electrons generated by the reaction of hydrogen and oxygen must travel through an external circuit; the membrane electrolyte only transfers protons. The membrane materials of choice have been ionomeric polymers, such as sulfonated fluoropolymers (Nafion), that achieve proton conductivities of up to 1 S cm−1, but the requirement to keep these materials hydrated limits their operating temperature and efficiency. Metal-organic frameworks (MOFs), in which inorganic assemblies are joined by organic linkers, have inherent porosity that could be exploited for the development of proton-conducting membranes. Among recent studies of experimental proton-conducting MOFs [e.g., ( 1 )], two general targets for PEMFC operation have emerged: developing better materials for operations under humid conditions (below 100°C), and developing efficient anhydrous proton conductors that could unlock the cost efficiencies enabled by humidity-independent operation above 100°C.

A Critical Revision of the Nano-Morphology of Proton Conducting Ionomers and Polyelectrolytes for Fuel Cell Applications

Abstract: A few aspects of the nano-morphology of hydrated Nafion and other ionomers and polyelectrolytes in their acid form are revisited by examining the evolution of small angle X-ray scattering (SAXS) data which are recorded for a wide range of water volume fractions (Φwater ≈ 7–56 vol%). A consistency check with the recent “parallel cylinder model” discloses that this is most likely biased by a large uncertainty of the experimentally determined water content. We rather find our data to be consistent with locally flat and narrow (around 1 nm) water domains. The formation of relatively thin water “films” is suggested to be a common feature of many ionomers and polyelectrolytes, and the underlying driving force is most likely electrostatics within these highly dissociated systems. The water films may act as a charged (e.g., with positive protonic charge carriers) “glue”, keeping together the oppositely charged polymer structures. While this interaction tends to produce flat morphologies, the formation process is suggested to be constraint by limited conformational degrees of freedom of the corresponding polymer and the interactions between polymer backbones. This may leave severe tortuosities on larger scales which depend on the sample history (including swelling, de-swelling, aging, stretching, and pressing).

PEMFCs and AEMFCs directly fed with ethanol: a current status comparative review

Abstract: The last decade’s research on the performance of proton-exchange membrane direct ethanol fuel cells (PEM-DEFCs) and anion exchange membrane direct ethanol fuel cells (AEM-DEFCs) is included in the present review. Future research challenges are identified along with potential strategies to overcome them. Pt-containing or Pt-free PEM-DEFCs that use acid proton-exchange membranes (typically Nafion type) exhibit relatively low performance (i.e., the state-of-the-art peak power density is 110 mW cm−2 at 145 °C over 4 mg of total Pt loading), while Pt-containing or Pt-free AEM-DEFCs that use low-cost anion-exchange membrane have recently exhibited better performance values (i.e., the state-of-the-art peak power density is about 185 mW cm−2 at 80 °C over Au-modified Pd catalysts supported on carbon nanotubes. The required faster kinetics of the ethanol oxidation and especially for the oxygen reduction reaction seem to be satisfied from one side by the AEM-DEFCs and from the other by PEM-DEFCs only if working at intermediate temperature values (>150 °C). Moreover, new possibilities of using less expensive metal catalysts (as silver, nickel, and palladium) are opening mainly for AEM-DEFCs and the last years for PEM-DEFCs too. Finally, it is worth to be noticed that the best value ever reported (peak power density is 360 mW cm−2 at 60 °C) has been obtained in a very promising alkaline-acid direct ethanol fuel cell (AA-DEFC).

Self-Assembly and Transport Limitations in Confined Nafion Films

Abstract: Ion-conducting polymers are important materials for a variety of electrochemical applications. Perfluorinated ionomers, such as Nafion, are the benchmark materials for proton conduction and are widely used in fuel cells and other electrochemical devices including solar-fuel generators, chlor-alkali cells, and redox flow batteries. While the behavior of Nafion in bulk membranes (10 to 100s μm thick) has been studied extensively, understanding its properties under thin-film confinement is limited. Elucidating the behavior of thin Nafion films is particularly important for the optimization of fuel-cell catalyst layers or vapor-operated solar-fuel generators, where a thin film of ionomer is responsible for the transport of ions to and from the active electrocatalytic centers. Using a combination of transport-property measurements and structural characterization, this work demonstrates that confinement of Nafion in thin films induced thickness-dependent proton conductivity and ionic-domain structure. Confining...

Nafion/Ni(OH)2 nanoparticles-carbon nanotube composite modified glassy carbon electrode as a sensor for simultaneous determination of dopamine and serotonin in the presence of ascorbic acid

Abstract: The electrochemical oxidation of dopamine (DA) and serotonin (ST) have been investigated by application of Nafion/Ni(OH) 2 -multiwalled carbon nanotubes modified glassy carbon electrode (Nafion/Ni(OH) 2 -MWNTs/GCE) using cyclic voltammetry (CV), differential pulse voltammetry (DPV) and chronoamperometry (CA) methods. The modified electrode worked as an efficient electron-mediator for DA and ST in the presence of ascorbic acid (AA). Voltammetric techniques separated the anodic peaks of DA and ST, and the interference from AA was effectively excluded from DA and ST determination. The DPV data showed that the obtained anodic peak currents were linearly proportional to concentration in the range of 0.05–25 μmol L −1 with a detection limit (S/N = 3.0) of 0.015 μmol L −1 for DA and in the range of 0.008–10 μmol L −1 and with a detection limit of 0.003 μmol L −1 for ST. The proposed sensor was used for determination of ST and DA in human blood serum with satisfactory results.

Capacity Decay and Remediation of Nafion‐based All‐Vanadium Redox Flow Batteries

Abstract: We will show two new methods to restore capacity during the long term charge-discharge cycling and operation

Characteristics of self-assembled ultrathin Nafion films

Abstract: Self-assembled Nafion films of varying thickness were generated on SiO2 terminated silicon wafer by immersion in Nafion dispersions of different concentrations. The impact of solvent/dispersion media was probed by preparing films from two different types of Nafion dispersions—IPA-diluted dispersion and Nafion-in-water dispersion. The thickness of films was ascertained by three different techniques: variable angle spectroscopic ellipsometry (VASE), atomic force microscopy (AFM), and X-ray photoelectron spectroscopy (XPS). The three techniques yielded consistent nominal thicknesses of 4, 10, 30, 55, 75, 110, 160, and 300 nm for films self-assembled from IPA-diluted Nafion dispersions of concentrations 0.1, 0.25, 0.5, 1.0, 1.5, 2.0, 3.0, and 5.0 wt %, respectively. Films generated from 0.25–5.0 wt % Nafion-in-water dispersions generated comparable thicknesses. An interesting finding of our work is the observation of bimodal surface wettability, investigated by water contact angle. The sub-55 nm films were fo...

Optimized anion exchange membranes for vanadium redox flow batteries.

Abstract: In order to understand the properties of low vanadium permeability anion exchange membranes for vanadium redox flow batteries (VRFBs), quaternary ammonium functionalized Radel (QA-Radel) membranes with three ion exchange capacities (IECs) from 1.7 to 2.4 mequiv g(-1) were synthesized and 55-60 μm thick membrane samples were evaluated for their transport properties and in-cell battery performance. The ionic conductivity and vanadium permeability of the membranes were investigated and correlated to the battery performance through measurements of Coulombic efficiency, voltage efficiency and energy efficiency in single cell tests, and capacity fade during cycling. Increasing the IEC of the QA-Radel membranes increased both the ionic conductivity and VO(2+) permeability. The 1.7 mequiv g(-1) IEC QA-Radel had the highest Coulombic efficiency and best cycling capacity maintenance in the VRFB, while the cell's voltage efficiency was limited by the membrane's low ionic conductivity. Increasing the IEC resulted in higher voltage efficiency for the 2.0 and 2.4 mequiv g(-1) samples, but the cells with these membranes displayed reduced Coulombic efficiency and faster capacity fade. The QA-Radel with an IEC of 2.0 mequiv g(-1) had the best balance of ionic conductivity and VO(2+) permeability, achieving a maximum power density of 218 mW cm(-2) which was higher than the maximum power density of a VRFB assembled with a Nafion N212 membrane in our system. While anion exchange membranes are under study for a variety of VRFB applications, this work demonstrates that the material parameters must be optimized to obtain the maximum cell performance.

Highly selective and sensitive glucose sensors based on organic electrochemical transistors with graphene-modified gate electrodes

Abstract: The sensitivity of glucose sensors based on organic electrochemical transistors (OECT) is increased by co-modifying graphene or reduced graphene oxide (rGO) and enzyme (glucose oxidase) on the gate electrodes for the first time. The optimized device shows linear responses to glucose in a broad concentration region from 10 nM to 1 μM and with a detection limit down to 10 nM, which is two orders of magnitude better than that for the device without the graphene modification. The selectivity of the device is systematically studied for the first time. The device selectivity is dramatically improved when the gate electrode is modified with biocompatible polymers (chitosan or Nafion). The interfering effect caused by uric acid and L-ascorbic acid is almost negligible for practical applications. Therefore, highly sensitive and selective OECT-based glucose sensors can be realized by functionalizing the gate electrodes. In addition, the devices are solution processable and low-cost, and are thus suitable for disposable sensing applications.

Water Oxidation Catalysis by Nanoparticulate Manganese Oxide Thin Films: Probing the Effect of the Manganese Precursors

Abstract: Nanoparticulate manganese oxides, formed in Nafion polymer from a series of molecular manganese complexes of varying nuclearity and metal oxidation state, are shown to effectively catalyze water oxidation under neutral pH conditions with the onset of electrocatalysis occurring at an overpotential of only 150 mV. Although XAS experiments indicate that each complex generates the same material in Nafion, the catalytic activity varied substantially with the manganese precursor and did not correlate with the amount of MnOx present in the films. The XAS and EPR studies indicated that the formation of the nanoparticulate oxide involves the dissociation of the complex into Mn(II) species followed by oxidation on application of an external bias. TEM studies of the most active films, derived from [Mn(Me3TACN)(OMe)3]+ and [(Me3TACN)2MnIII2(μ-O)(μ-CH3COO)2]2+ (Me3TACN = N,N′,N″-trimethyl-1,4,7-triazacyclononane), revealed that highly dispersed MnOx nanoparticles (10–20 nm and 6–10 nm, respectively) were generated in ...

Selective formation of hydrogen and hydroxyl radicals by electron beam irradiation and their reactivity with perfluorosulfonated acid ionomer.

Abstract: Selective formation and reactivity of hydrogen (H(•)) and hydroxyl (HO(•)) radicals with perfluorinated sulfonated ionomer membrane, Nafion 211, is described. Selective formation of radicals was achieved by electron beam irradiation of aqueous solutions of H2O2 or H2SO4 to form HO(•) and H(•), respectively, and confirmed by ESR spectroscopy using a spin trap. The structure of Nafion 211 after reaction with H(•) or HO(•) was determined using calibrated (19)F magic angle spinning NMR spectroscopy. Soluble residues of degradation were analyzed by liquid and solid-state NMR. NMR and ATR-FTIR spectroscopy, together with determination of ion exchange capacity, water uptake, proton conductivity, and fluoride ion release, strongly indicate that attack by H(•) occurs at the tertiary carbon C-F bond on both the main and side chain; whereas attack by HO(•) occurs solely on the side chain, specifically, the α-O-C bond.

Electrochemically reduced graphene oxide and Nafion nanocomposite for ultralow potential detection of organophosphate pesticide

Abstract: The development of rapid and field monitoring tools is highly desirable for the sensitive detection of organophosphate pesticide (OP) residues in water and agriculture products without interference. A novel disposable acetylcholinesterase (AChE) biosensor with low detecting potential (+50 mV), good selectivity, and high sensitivity was developed based on electrochemically reduced graphene oxide and Nafion hybrid nanocomposite (Er-GRO–Nafion) modified electrode. The Er-GRO–Nafion nanocomposite with conductive and three-dimensional interpenetrating network owns excellent characters as electrode modification material. It effectively promoted the electron transfer rate of electrode interface, could catalyze the anodic oxidation of thiocholine at ultra-low potential, and facilitated the access of substrates to the active centers. The modified electrode was used for the electrochemical detection of dichlorvos after adsorbing AChE on the biocompatible matrix. By dexterously controlling the electrochemical reduction variables and optimizing the electrode preparation parameters, the proposed electrochemical biosensor showed high sensitivity, wide linear range (from 5.0 to 100 ng mL−1 and from 1.0 to 20 μg mL−1), low detection limit (2.0 ng mL−1), good stability and reproducibility. It also had high precision for the real sample analysis, which indicated the good perspective for field application.

Durable Sulfonated Poly(arylene sulfide sulfone nitrile)s Containing Naphthalene Units for Direct Methanol Fuel Cells (DMFCs)

Abstract: Sulfonated poly(arylene sulfide sulfone nitrile)s (SN) were synthesized to investigate the effects of naphthalene units in the polymer backbone on membrane properties. The small and planar naphthalene in the main chain reduced interdomain distance, as confirmed by molecular simulations and small-angle X-ray scattering patterns. The SN polymer membranes exhibited excellent chemical and mechanical properties, better than those of their phenylene counterpart (SP). In particular, the water uptake and swelling ratio of the SN membranes were much lower than those of the SP membranes. Furthermore, the SN membranes exhibited a greatly reduced methanol permeability ((9–17) × 10–8 cm2 s–1) compared to Nafion 212 (330 × 10–8 cm2 s–1) at 30 °C in 10 M methanol. Moreover, sulfide- and naphthalene-based chemical structure of the SN membranes enhanced their DMFC single cell performance and long-term stability during fuel cell operation.

Electrochemical synthesis of ammonia directly from air and water using a Li+/H+/NH4+ mixed conducting electrolyte

Abstract: Ammonia has been successfully synthesised directly from air and water using an electrochemical cell based on an H+/Li+/NH4+ mixed conducting membrane and Pt/C electrodes. It is found that the Nafion 211 membrane exhibits mixed H+/Li+ conduction after exchanging in 0.1 M Li2SO4 solution. The ionic conductivity of the mixed conductor is slightly lower than that of H+-form Nafion 211. The introduction of Li+ ions to the cell did not improve the ammonia formation rates in our experiments. Reasonably higher temperature may favour ammonia formation and the highest ammonia formation rate (9.37 × 10−6 mol m−2 s−1) and Faraday efficiency (0.83%) was obtained at 80 °C when a voltage of 1.2 V was applied. The ammonia formation rate decreased when 0.1 M Li2SO4 solution instead of water was used in the cell. Under the applied potential, the presence of Li+ ions might have a blocking effect on the transfer of protons resulting in a lower current at higher applied voltage, particularly at lower temperatures.

Resonance Stabilized Perfluorinated Ionomers for Alkaline Membrane Fuel Cells

Abstract: Perfluorosulfonic acids such as Nafion are industrial standard cation exchange ionomers for polymer electrolyte membrane fuel cells because of their high gas permeability, hydrophobicity, and inertness to electro-chemical reaction. In this research, pentamethylguanidinium functionalized, perfluorinated hydroxide conducting ionomers for alkaline membrane fuel cells were prepared and characterized. The alkaline stability of the ionomers largely depended on the adjacent group that connected the cation; Sulfone guanidinium functionalized ionomer degraded almost completely after soaking in 0.5 M NaOH at 80 °C for 24 h, while phenylguanidinium functionalized ionomer did not degrade under the same conditions for 72 h. Spectroscopic data and density functional theory calculation suggested that the stability of the phenylguanidinium ionomer was greatly improved by charge delocalization of the formed resonance structure. Alkaline membrane fuel cells using the resonance stabilized perfluorinated ionomer in the catal...