Nafion含量与阴离子吸附对于铂单原子层核壳结构催化剂制备的影响

本实验利用铜的欠电位沉积技术,在旋转圆盘电极上以碳负载的钯纳米颗粒为核,制备铂单原子层核壳结构催化剂. 电化学测试用于表征不同Nafion含量的添加对于核壳结构催化剂制备的影响. 实验证明,Nafion的存在会影响铜的欠电位沉积,铂与铜的置换反应,并决定最终制备的核壳结构催化剂的氧还原催化反应的活性. 当催化剂薄层中Nafion的含量低于5%的时候,添加Nafion不但可以帮助催化剂附着在旋转圆盘电极表面,而且可以保证制备的催化剂具有较好的氧还原反应催化活性. 在H₂SO₄溶液中,钯纳米颗粒的表面存在特殊的阴离子吸/脱附电化学信号峰,这些信号峰可以用来监测Nafion含量对于铂单原子层核壳结构催化剂制备的影响.     

Langmuir--Schaefer films of Nafion with incorporated TiO(2) nanoparticles

An easy method of incorporating TiO(2) nanoparticles into Nafion perfluorinated ionomer is proposed. Ultrathin films of Nafion were prepared by employing the Langmuir-Schaefer (LS) technique. The pressure-area isotherm study of a Langmuir monolayer of Nafion at the air-water interface on different concentrations of NaCl as the subphase allowed us to find the best experimental conditions for the deposition of stable Langmuir-Schaefer films. Incorporation of TiO(2) nanoparticles was performed by dipping Nafion LS films in a solution of TiO(2) nanoparticles. The uniformity of the TiO(2) incorporation was detected by UV-visible spectroscopy. The morphology of the Nafion, Nafion/TiO(2) nanoparticles thin films, and the changes due to the annealing procedure were investigated by atomic force microscopy. Interestingly, the AFM investigation showed that Nafion and Nafion/TiO(2) LS films have thermal stability up to 600 degrees C.     

Wetting and absorption of water drops on Nafion films

Water drops on Nafion films caused the surface to switch from being hydrophobic to being hydrophilic. Contact angle hysteresis of >70 degrees between advancing and receding values were obtained by the Wilhelmy plate technique. Sessile drop measurements were consistent with the advancing contact angle; the sessile drop contact angle was 108 degrees . Water drop adhesion, as measured by the detachment angle on an inclined plane, showed much stronger water adhesion on Nafion than Teflon. Sessile water and methanol drops caused dry Nafion films to deflect. The flexure went through a maximum with time. Flexure increased with contact area of the drop, but was insensitive to the film thickness. Methanol drops spread more on Nafion and caused larger film flexure than water. The results suggest that the Nafion surface was initially hydrophobic but water and methanol drops caused hydrophilic sulfonic acid domains to be drawn to the Nafion surface. Local swelling of the film beneath the water drop caused the film to buckle. The maximum flexure is suggested to result from motion of a water swelling front through the Nafion film.     

Morphology study of Nafion membranes prepared by solutions casting

The structures of Nafion membranes prepared by solutions casting from low aliphatic alcohols/water mixture solvents and N,N′‐dimethyl formamide (DMF) solvent were investigated using differential scanning calorimeter and small angle X‐ray scattering. The aggregation behavior of Nafion molecules in the casting solutions was also investigated using dynamic light scattering. We show that the morphology of membranes was strongly influenced by the conformations of Nafion molecules in the solutions. In aliphatic alcohol/water mixture solvents, which have a worse compatibility with Nafion backbones, the Nafion molecules aggregate and form fringed rod‐like structures. These primary rod‐like structures then aggregate again through fringed side chains to form secondary ionic aggregations. In DMF solvent, owing to its better compatibility with Nafion backbones, less Nafion molecules aggregate. The high degree of Nafion molecular aggregations in aliphatic alcohol/water mixture solvents leads to a high degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/aliphatic alcohol/water solutions. However, the lower degree of molecular aggregations in DMF solvent results in a lower degree of hydrophobic and hydrophilic phase separation for membranes prepared by casting from Nafion/DMF solution. © 2005 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 43: 3044–3057, 2005     

耗散粒子动力学模拟Nafion膜和PVA/Nafion共混膜的介观结构

采用耗散粒子动力学模拟方法研究了水化Nafion膜和水化聚乙烯醇(PVA)/Nafion共混膜的微结构.模拟结果表明水化Nafion膜和水化PVA/Nafion共混膜均能形成相分离的微结构.在水化Nafion膜中,水与磺酸根混合形成管状的水团簇.随着膜内水含量增多,管状水团簇的尺寸逐渐变大并在膜内形成连续的水通道.在水化PVA/Nafion共混膜中,PVA、水、磺酸根混合形成亲水性区域.共混膜中PVA的质量分数和水含量共同影响膜的微结构.当膜中PVA质量分数较低时,PVA主要分布在Nafion的磺酸根基团周围;PVA质量分数升高后,PVA会在膜内单独成一相.当膜中的水含量相对较低时,水分子会溶解于PVA中,此时膜内不存在单独的水团簇;膜中的水含量增多后,膜内会形成接近于球形的水团簇.本文工作可为直接甲醇燃料电池用的PVA改性Nafion膜的开发提供参考.     

Influence of Analyte Flow Rate on Signal and Response Time of the Amperometric Gas Sensor with Nafion Membrane

This paper presents results of an investigation on influence of volumetric flow rate on the signal and response time of a prototype of sulfur dioxide gas sensor with Nafion membrane. The sensors differing in type of working electrode and composition of internal electrolyte were compared. We used Au and Pt working electrodes obtained via vacuum sublimation deposition on a Nafion membrane surface. The electrolytes were aqueous solutions of sulfuric acid of the summary concentration 5 mol dm^?3 (electrolyte A). The electrolyte B contained an addition of dimethylsulfoxide (DMSO); the water/DMSO molar ratio was 2 : 1. Based on a proposed equation, which takes diffusion resistance into account, the obtained sensor signals were analyzed for the flow rate within a range of 0–100 cm³ min^?1. The sensor response time was also determined for the above flow rate range.     

介质对Nafion膜孔腔中限域TiO2纳米晶体形成的影响

本文在前期工作基础上，利用高分辨电子显微镜(HRTEM)、X射线衍射仪(XRD)、扫描电镜(SEM)，能谱(EDAX)研究了在全氟磺酸离子交换膜(Nafion)模板中有机浸泡液溶剂种类、Nafion膜含水量对前驱体Ti(OBu)₄在Nafion模板中形成单分散的TiO₂纳米晶体的影响；研究了Nafion膜断面各元素含量的面分布、线分布、TiO₂的含量、粒子大小、纳米晶体微观形貌及其结构。研究结果表明，溶剂决定了Ti(OBu)₄在Nafion膜中的荷载量及相应TiO₂荷载量的大小；Nafion膜含水量不同，对膜中TiO₂荷载量影响很大，但对TiO₂晶形、晶粒大小和各元素在Nafion膜断面中的均匀分布没有显著影响。     

Acidity in Nafion

Hammett acidity functions have been measured for Nafion films equilibrated with a series of nitroaniline dyes in hydrochloric acid solutions. The effect of ionic strength of alkali metal and tetraalky-lammonium bromides on the ability of the Nafion film to donate a proton to Bronsted bases is the reverse of that expected for acid in solution. Absorption of neutral and charged dye is also influenced by the concentration and hydrophobicity of the salt in solution. An interpretation based upon structural spectroscopic studies is discussed.     

Transport properties of composite membranes containing silicon dioxide and Nafion

Silicon dioxide (SiO₂) nanoparticles were incorporated into Nafion 115 membranes using the sol–gel method in order to investigate their effect on water retention/transport, proton concentration, effective proton mobility, and proton conductivity. By adjusting the sol–gel reaction time, Nafion/SiO₂ membranes were fabricated with SiO₂ content ranging from 5.9 to 33.3 wt%. Because the density of the membranes decreased with increasing SiO₂ content and because dimensional changes with swelling in water of the composite membranes were less than that of unmodified Nafion 115 despite having increased water content, the theory that rigid scaffolding is formed inside the membrane is supported. Water content increases with increasing SiO₂ content due to void space formed inside the membrane. This increase in water content dilutes the protons in the membrane leading to lower proton concentration and therefore lower proton conductivity. A decreasing effective proton mobility with increasing SiO₂ content, likely due to an increase in the tortuosity of the proton-conducting pathway, also contributes to the decreasing conductivity. However, as evidenced by the similar water vapour permeance values, the SiO₂ nanoparticles do not increase the effective tortuosity of the water vapour transmission pathways.     

Hofmeister effects on electron-transfer reactions of 1-pyrenesulfonic acid radical cation with nucleophilic anions in Nafion membranes

The electron-transfer reaction from nucleophilic anions such as SCN(-), N(3)(-), I(-), and Br(-) to the 1-pyrenesulfonic acid radical cation (Py(*+)SA(-)) generated via a resonant two-photon ionization process in the Nafion membranes was investigated with transient absorption measurements. The apparent quenching rates observed in the Nafion membrane (k(q)(Nf)) were almost 2-4 orders smaller than those observed in the bulk solutions (k(q)(bulk)). The attenuation factor (AF), which is defined as log(k(q)(Nf)/k(q)(bulk)), decreased in the order SCN(-) > N(3)(-) > I(-) > Br(-). This interesting behavior was interpreted in terms of the anionic Hofmeister effects. The effects of hydrophobic organic cations such as tetrabutylammonium ion (Bu(4)N(+)) and tetraethylammonium ion (Et(4)N(+)) exchanged into the Nafion membranes were also examined.     

Mechanical properties of Nafion and titania/Nafion composite membranes for polymer electrolyte membrane fuel cells

Measurements of the mechanical and electrical properties of Nafion and Nafion/titania composite membranes in constrained environments are reported. The elastic and plastic deformation of Nafion‐based materials decreases with both the temperature and water content. Nafion/titania composites have slightly higher elastic moduli. Thecomposite membranes exhibit less strain hardening than Nafion. Composite membranes also show a reduction in the long‐time creep of ～40% in comparison with Nafion. Water uptake is faster in Nafion membranes recast from solution in comparison with extruded Nafion. The addition of 3–20 wt % titania particles has minimal effect on the rate of water uptake. Water sorption by Nafion membranes generates a swelling pressure of ～0.55 MPa in 125‐μm membranes. The resistivity of Nafion increases when the membrane is placed under a load. At 23 °C and 100% relative humidity, the resistivity of Nafion increases by ～15% under an applied stress of 7.5 MPa. There is a substantial hysteresis in the membrane resistivity as a function of the applied stress depending on whether the pressure is increasing or decreasing. The results demonstrate how the dynamics of water uptake and loss from membranes are dependent on physical constraints, and these constraints can impact fuel cell performance. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2327–2345, 2006     

Nafion Coated Silver Amalgam Electrode for Determination of Trace Metals by Anodic Stripping Voltammetry

This paper describes characterization and application of Nafion coated solid silver amalgam electrodes to prevent surface fouling of surfactants in determination of trace metals by differential pulse anodic stripping voltammetry (DPASV). Polymer films of different thickness were tested using Nafion solutions in the range 0.25%–1%. Optimum thickness was archived using a 0.5% Nafion solution, resulting in both increased response and stability over time compared to uncoated electrodes. The influence of model surface active macromolecules was studied using triton X‐100, sodium dodecyl sulfate, dodecyl pyridinium chloride and bovine serum albumin as representatives for surfactants. The resistance to surfactants makes the studied Nafion coated solid silver amalgam electrodes an interesting alternative for practical use in environmental monitoring.     

Epifluorescence imaging of electrochemically switchable Langmuir-Blodgett films of Nafion

A combination of electrochemistry and luminescence methods was exploited to obtain information on the electrochemical activity and homogeneity of Nafion Langmuir-Blodgett films. The redox behavior of the Ru(bpy)3(2+) probe incorporated in the Nafion film was monitored by epifluorescence microscopy. The photoluminescent images, recorded by a charge-coupled device (CCD) camera, reflect the distribution of the probe in the film, which resulted as very uniform, particularly in comparison with spin-coated films. Apparent diffusion coefficients (Dapp) determined by cyclic voltammetry for films of less than 10 layers are in the range of 1 x 10(-12) to 8 x 10(-12) cm(2) s(-1), that is, 2 orders of magnitude lower than values reported in the literature for spin-coated Nafion films. The application to the electrode of a potential able to oxidize the luminescent Ru(bpy)3(2+) to the nonluminescent Ru(bpy)3(3+) switched off the photoluminescence with a response time that for the LB films was much shorter than that for the spin-coated ones. Experimental evidence and calculations indicate that lowering of the film thickness down to the nanometric level is very effective in shortening the switching time, notwithstanding the lowering of the Dapp value in LB films.     

In situ formation of platinum nanoparticles in Nafion recast film for catalyst-incorporated ion-exchange membrane in fuel cell applications

Platinum (Pt) nanoparticles were synthesized inside a Nafion polyelectrolyte membrane for use as a catalyst membrane integrated layer in fuel cells. The integrated membrane was prepared by making use of the cation exchange between the tetraammineplatinum (II) cations ([Pt(NH₃)₄]²⁺) and sulfonic groups in the Nafion molecules, followed by film casting and chemical reduction. The synthesized Pt nanoparticles, which had a cubic shape with diameters of 11.5–14.5 nm, dispersed in the recast Nafion film, increased its proton conductivity and open circuit voltage compared with the pristine Nafion membrane. The Pt-incorporated membrane provided a 29% increment of the maximum power density, seemingly by oxidizing the crossover methanol passing through the proton-exchange membrane. At a high loading of Pt (over 3 wt.% in this study), the Nafion clusters were likely squeezed by the synthesized Pt nanoparticles so as to decrease the water uptake and proton conductivity. This hypothesis was also supported by the increased Ohmic resistance in the I–V polarization curve.     

Nafion聚离子修饰纳米Pt的合成及其影响因素

采用化学还原的方法成功地合成了Nafion聚离子修饰的纳米Pt颗粒，平均粒径为～4nm；由于表面缺电子的特性，纳米Pt粒子与Nafion高分子长链上的-SO3^-基团有较强的结合，使粒子随-SO3^的存在状态而分散，在水溶液中呈现与离子团簇相似的分散状态。FTIR显示Pt颗粒存在表面原子暴露在修饰离子之外，因此，这种粒子在具有功能结构的催化领域有着很好的应用前景；合成过程中反应速率随Nafion含量的降低、pH的增加而增大。     

小角X光散射表征分散液中Nafion的微观结构

利用小角X光散射研究了全氟磺酸离子聚合物Nafion膜在不同比例的氮甲基甲酰胺和正丁醇混合溶剂中分散形成分散液的微观结构。 研究表明,主链刚性和主-侧链亲疏水性的协同作用使分散液中的Nafion呈典型的棒状胶束结构。 胶束的等效回转半径(R_g)对Nafion质量浓度表现出-0.42的标度,与聚电解质在无盐溶剂中的理论标度一致;而胶束间相关长度对Nafion质量浓度表现出-0.13的标度,与典型的中性聚合物溶液理论标度一致。 极性低的正丁醇促进Nafion主链溶剂化并利于长胶束形成,而极性高的氮甲基甲酰胺则能促进Nafion分散。 该研究将为理解Nafion分散液的性质以及湿法制备Nafion膜的微结构形成提供清晰指导。     

Thermal stability of ion-exchange Nafion N117CS membranes

The effect of exchanged ions on the thermal stability of Nafion N117CS membranes was investigated by X-ray photoelectron spectra (XPS), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), thermomechanical analysis (TMA), and ion exchange capacity determinations. The ion exchange of alkaline metal ions was effective in improving the thermal stability of the Nafion N117CS membrane. Findings reveal that when Nafion was exchanged for cations with a larger ionic radius, the membrane attained superior thermal stability. On the other hand, we confirmed that the Na-exchange Nafion N117CS membrane possessed a distinctive degree of thermal stability among the alkaline ion-exchange Nafions, although the order of ionic radii is K > Na > Li. Thermal stability improved the most when the Nafion membrane was exchanged for alkaline ions, followed by divalent ions, then trivalent ions. As for the Nafion membrane when it was exchanged for divalent ions or trivalent ions, Nafion following the ion exchange had a thermal stability proportional to an increase in the ionic radius of the cation. This stability may be explained by the reduction of water content and a greater interaction between the sulfonate groups and the cations with larger ionic radii. Since the Al cations acted as a Lewis acid center, the decomposition of the ether bonds of the perfluoroalkylether pendant-chains of the Nafion membrane was observed for the Nafion N117CS membrane that had been exchanged for Al ions. The activation of molecular mobility in Nafion was observed between the decomposition stages of the loss of water and the loss of sulfonic groups. The temperature of activation of cation-exchange Nafion became much higher than that of Nafion in an acid form.     

Influence of polyfurfuryl alcohol (PFA) loading on the properties of Nafion composite membranes

Composite membranes based on Nafion (N115) loaded with furfuryl alcohol (FA) were prepared by in situ acid-catalyzed polymerization technique, with the aim to improve the ionic conductivity of Nafion membranes. The functionalization, thermal stability, electrical properties and mechanical strength of N115-PFA composites was analyzed by means of Fourier transform infrared (FT-IR) attenuated total reflection (ATR) spectroscopy, small- and wide-angle X-ray scattering (SAXS/WAXS), thermogravimetric analysis (TGA), electrical impedance spectroscopy, dynamic vapor sorption (DVS) and dynamic mechanical analyser (DMA). The FA loading in the resultant composites had a positive correlation with the water uptake (W_u), water vapor uptake (W_vu), ionic conductivity and thermo-mechanical stability. At low polyfurfuryl alcohol (PFA) loading, these membranes displayed higher W_u and improved ionic and electrical properties. Further, the thermo-mechanical stability also gradually increased with the PFA loading. All the composites showed a well-defined glass transition temperature in DMA, which shifted to higher temperature with repeated PFA loading. Overall, the results indicate that the developed composite membrane are promising for low temperature polymer electrolyte membrane (PEM) fuel cells.     

Exfoliated Pt-clay/Nafion nanocomposite membrane for self-humidifying polymer electrolyte fuel cells

Monolayers of Pt nanoparticles of diameters of 2-3 nm with a high crystallinity were successfully anchored onto exfoliated nanoclay surfaces using a novel chemical vapor deposition process. Chemical bonding of Pt to the oxygen on the clay surface ensured the stability of the Pt nanoparticles, and hence, no leaching of Pt particles was observed after a prolonged ultrasonication and a rigorous mechanical agitation of Pt-clay in the Nafion solution during the membrane casting process. Systematic analysis using WAXD and TEM showed that the recasting process produced a new self-humidifying exfoliated Pt-clay/Nafion nanocomposite membrane with a high crystallinity and proton conductivity. In situ water production for humidification of the dry membranes without any external humidification was characterized by a combined water uptake and FTIR analysis of the as-prepared membrane after a single cell testing without using electrodes. The power density at 0.5 V of a single cell made of a Pt-clay/Nafion nanocomposite membrane was 723 mW/cm2, which is 170% higher than that made of a commercial Nafion 112 membrane of similar thickness. No compromise in mechanical properties was observed.     

Photochemical reaction of azobenzene in solvent-swollen Nafion membranes

The photochemical reactions of azobenzene (AB) incorporated into different solvent-swollen Nafion membranes were investigated. The location of azobenzene in water-swollen Nafion membranes was different from that in methanol-swollen Nafion membranes, which was responsible for the different photochemical reactions of AB. In methanol-swollen Nafion membranes only trans-cis isomerization took place, while in water-swollen Nafion membranes photochemical cyclization of AB occurred after rapid trons-cis isomerization. The relation between the relative quantum yields and the light intensity showed that the cyclization was a two-photo process. The numbers of AB molecules per cluster (nAB /cluster) in Nafion membranes with different AB concentrations and different water contents were calculated. Under the experimental condition two competitive photocyclization mechanisms were suggested by nAB /cluster in Nafion membranes, relative quantum yields of the cyclization, and the change of the ratio of benzidine to benz