Unmodified Nafion Membrane Research Articles

Porous lanthanum titanium oxide nanostructure composite membrane to enhance the power output and chemical durability of low-humidifying polymer electrolyte fuel cells: impact of additive morphology

Tuning the morphologies for improving the properties of the additive materials receives immense attention during the fabrication of proton-exchange membranes for fuel cells. This study reports the synthesis of lanthanum titanium oxide (LTO) nanostructures with different morphologies (cube, spherical, and tubular) via the pyrolysis of nanofibers containing metal precursors. A mixed matrix membrane is fabricated by incorporating LTO cubes (LTO-C), LTO spheres (LTO-S), or LTO tubes (LTO-T) into a Nafion matrix, with improved physicochemical, thermomechanical, and electrochemical properties compared to those of the unmodified Nafion and Nafion-212 membranes. We investigate the effect of LTO morphology on the performance of the Nafion membranes in low-humidity polymer electrolyte fuel cells (LH-PEFCs). The water retention and diffusion behavior of LTO-T nanostructures improve the proton conductivity and LH-PEFC performance of the Nafion composite membranes. The coexistence of La3+ and Ti4+ affords efficient radical scavenging during long-term LH-PEFC operation, which realizes higher durability for the composite membranes compared to Nafion. Accordingly, this study proposes a novel strategy to resolve the major challenges associated with the operation of LH-PEFCs with Nafion membranes: dehumidification and oxidative degradation.

An innovative membrane-electrode assembly for efficient and durable polymer electrolyte membrane fuel cell operations

An innovative membrane-electrode assembly, based on a polyoxometalate (POM)-modified low-Pt loading cathode and a sulphated titania (S-TiO2)-doped Nafion membrane, is evaluated in a polymer electrolyte membrane fuel cell. The modification of fuel cell cathode with Cs3HPMo11VO40 polyoxometalate is performed to enhance particles dispersion and increase active area, allowing low Pt loading while maintaining performance. The POM's high surface acidity favors kinetics of oxygen reduction reaction. The mesoporous features of POM allow the embedding of Pt inside the micro-mesopores, avoiding the Pt aggregation during fuel cell operation and delaying the aging process, with consequent increase of lifetime. On the other hands, commercial Nafion is modified with superacidic sulphated titanium oxide nanoparticles, allowing operation at low relative humidity and controlled polarization of the MEA. Further MEAs, formed by unmodified Nafion membrane and the POM-based cathode, as well as sulphated titanium-added Nafion and commercial Pt-based electrodes, are used as terms of comparison. The cell performances are studied by polarization curves, electrochemical impedance spectroscopy, Tafel plot analysis and high frequency resistance measurements. The dependence of cell performances on relative humidity is also studied. The catalytic and transport properties are improved using the coupled system, despite the reduced Pt loading, thanks to rich proton environment provided by cathode and membrane.

Novel sulfonated poly(glycidyl methacrylate) grafted Nafion membranes for fuel cell applications

Nafion membranes have been modified by PGMA using persulphate initiation system, subsequent by sulfonation of PGMA graft branches, to convert the epoxy groups into sulfonic groups, which provide the modified membranes with more acidic sites to maintain its ionic conductivity. The grafting process and the sulfonation process have been confirmed by FT-IR, TGA and FT-IR, EDAX analysis and ion exchange capacity measurements, frequently. Moreover, the physicochemical properties of the modified membranes, such as water and methanol uptakes, and ion exchange capacity, have been studied. The results showed that the modified membranes have high water uptake. The ion exchange capacity measurements proved the conversion of the epoxy groups into sulfonic groups in the sulfonated grafted membranes since the sulfonated grafted membranes showed ion exchange capacity higher than that of grafted membranes. Also, the results showed that the modification process has no impact effect on the stability of the membranes dimensions in water and methanol. TGA analysis showed that the modified membranes exhibited high thermal stability than that of unmodified Nafion membranes. SEM analysis showed the homogeneity of the PGMA in the matrix of Nafion membrane. The methanol permeability of the modified membranes decreased with increasing the polymer content in the modified membranes until reached to minimum value 1.49 × 10−6 cm2/s of grafting percentage 18.76%, which helped to reduce the methanol permeability by 45.36% in addition to higher thermal stability and higher water uptake. The modified membranes showed high-performance factor (584 × 10−9) in comparison to unmodified Nafion membranes (309 × 10−9).

Nafion-sulfonated organosilica composite membrane for all vanadium redox flow battery

A novel Nafion-sulfonated diphenyldimethoxysilane (N-sDDS) composite membrane is prepared and employed in vanadium redox flow battery (VRB). Ion exchange capacity, proton conductivity, water transport behavior, and the cell performances are characterized. Fourier transform-infrared and X-ray diffraction analysis indicate that the sulfonated diphenyldimethoxysilane (sDDS) particles are successfully introduced into the Nafion matrix. In VRB single cell test, the VRB with N-sDDS membrane exhibits nearly the same coulombic efficiency as the unmodified Nafion membrane, but higher voltage efficiency than that of the VRB with unmodified Nafion membrane. The VRB with N-sDDS composite membrane keeps a stable performance after 60 times charge–discharge test. In the self-discharge test, the VRB with the N-sDDS membrane presented a lower self-discharge rate than that of the VRB with Nafion membrane. All results show that the addition of s-DDS is a simple and efficient way to improve the conductivity of Nafion, and the composite membrane shows good potential use for VRB.

Proton NMR Relaxometry Study of Nafion Membranes Modified with Ionic Liquid Cations

Proton nuclear magnetic resonance (NMR) relaxometry was used to study the ionic mobility and levels of confinement within Nafion membranes modified by incorporation of selected ionic liquid (IL) cations. These studies were performed aiming at understanding the effect of using different types of ionic liquid cations, and their degree of incorporation, in the values of the spin-lattice relaxation times (T(1)) obtained at different values of frequency and thus detect the influence of confinement level on the ions mobility. The frequency dependence of the proton spin-lattice relaxation rate, R(1) = 1/T(1), for the modified Nafion/IL cation membranes was compared with that obtained for an unmodified Nafion membrane, allowing for distinguishing different contributions of the motions of the molecules depending on the frequency tested. The experimental R(1) results were analyzed in terms of models that consider the sum of the most effective relaxation contributions, to estimate the translational self-diffusion coefficient of the moving molecular species in the modified membranes. The stability of these membranes with temperature in terms of the spin-lattice relaxation was compared with results obtained by thermogravimetric analysis.

Methanol and gas crossover through modified Nafion membranes by incorporation of ionic liquid cations

Nafion membranes were modified by incorporation of ionic liquid (IL) cations, at controllable degrees, in order to assess the influence of this modification in both the methanol and gas crossover. The effect of using different degrees of incorporated IL cations, as well as the type of IL cation incorporated, in the transport of gases and methanol was studied in detail. The results obtained were compared with those obtained with an unmodified Nafion membrane. Depending on the IL cation incorporated, a reduction in methanol crossover in the range of 60–600 times was obtained in this work, when compared with a Nafion-112 membrane. This reduction was related both with the type of cation incorporated and its incorporation degree which determine the amount of water retained by the membrane and its degree of structuring inside the membrane. Pure H 2, O 2, N 2 and CO 2 permeabilities were also determined, and a lower gas crossover through all of the modified Nafion/IL cation membranes was obtained when compared with those obtained through the unmodified membrane (Nafion/H +). It was concluded that the electric properties, methanol and gas crossover as well the stability at high temperatures of these membranes can be tuned by controlling the degree of incorporation as well as the type of cation incorporated. The best compromise between all these properties has to be found in order to considerer their use in direct methanol fuel cells (DMFCs).

Nafion/organic silica modified TiO 2 composite membrane for vanadium redox flow battery via in situ sol–gel reactions

To improve the selectivity of Nafion membrane, reduce the crossover of vanadium ions and decrease water transfer across the membranes used in VRB systems, the sol–gel method was employed to obtain a composite membrane of Nafion/organic silica modified TiO 2 (Nafion/Si/Ti hybrid membrane). The preparation technique and characteristics of the composite membrane were described. The water transfer and the permeability of vanadium ions were also measured. Results showed that crossover of vanadium ions and water transfer across the membrane for the composite membrane have a remarkable decrease comparing with the unmodified Nafion membrane. It has been confirmed by EDX and FT-IR analysis of the modified membrane that the Si and Ti elements distributed uniformly in the prepared membrane. As a result, the coulombic efficiency of the VRB single cell with modified Nafion membrane was higher than that of the VRB single cell with pristine Nafion membrane. The ion exchange capacity (IEC), the area resistance and the water uptake of the composite membrane were also evaluated. Furthermore, the open circuit voltage (OCV) of the VRB with modified and unmodified Nafion membrane has been carried out and cycle performance of the VRB with modified membrane proves that it has high stability in strong acid condition.

Temperature‐dependent Performances of a Fuel Cell Using a Superacid Zirconia‐doped Nafion Polymer Electrolyte

AbstractSulphated zirconia‐added Nafion membranes were prepared and used as polymer electrolyte separators in H2‐air fuel cells. The cell performances of the composite membrane were compared with those of an unmodified Nafion membrane having the same thickness. The cell response was studied by means of voltage–current dependence, in situ impedance spectroscopy and current interrupt‐based cell resistance. The effect of temperature on the cells behaviour was evaluated. When using the composite membrane as an electrolyte a general enhancement was found, both in terms of power density delivered and ohmic resistance. Moreover, the high‐temperature impedance response of the zirconia‐doped Nafion‐based cell was highly improved, showing a wellcontrolled charge transfer resistance.

Preparation and performance of nano silica/Nafion composite membrane for proton exchange membrane fuel cells

Composite membranes made from Nafion ionomer with nano phosphonic acid-functionalised silica and colloidal silica were prepared and evaluated for proton exchange membrane fuel cells (PEMFCs) operating at elevated temperature and low relative humidity (RH). The phosphonic acid-functionalised silica additive obtained from a sol–gel process was well incorporated into Nafion membrane. The particle size determined using transmission electron microscope (TEM) had a narrow distribution with an average value of approximately 11 nm and a standard deviation of ±4 nm. The phosphonic acid-functionalised silica additive enhanced proton conductivity and water retention by introducing both acidic groups and porous silica. The proton conductivity of the composite membrane with the acid-functionalised silica was 0.026 S cm −1, 24% higher than that of the unmodified Nafion membrane at 85 °C and 50% RH. Compared with the Nafion membrane, the phosphonic acid-functionalised silica (10% loading level) composite membrane exhibited 60 mV higher fuel cell performance at 1 A cm −2, 95 °C and 35% RH, and 80 mV higher at 0.8 A cm −2, 120 °C and 35% RH. The fuel cell performance of composite membrane made with 6% colloidal silica without acidic group was also higher than unmodified Nafion membrane, however, its performance was lower than the acid-functionalised silica additive composite membrane.

Modification of Nafion membrane using interfacial polymerization for vanadium redox flow battery applications

In order to reduce the permeation of vanadium ions across the ion exchange membrane during the operation of vanadium redox flow battery (VRB) based on Nafion membrane, the interfacial polymerization was applied to form a cationic charged layer on the surface of Nafion 117 membrane. The area resistance and the permeability of vanadium ions were measured. The results indicate that comparing with the unmodified Nafion membrane, the modification of Nafion membrane results in a dramatic reduction in crossover of vanadium ions across the membrane and a little higher area resistance of the membrane. As a result, the columbic efficiency for the VRB single cell based on the modified Nafion membrane(VRB-modified Nafion), which is related to the concentration of the incubation solution of polyethylenimine (PEI), was increased significantly. The value is 96.2–97.3%, which is higher than that obtained with the VRB single cell based on unmodified Nafion membrane(VRB-Nafion) (around 93.8%). Due to the little higher area resistance caused by the modification, the voltage efficiency of VRB-modified Nafion is lower than that of VRB-Nafion. Furthermore, the water transfer across the modified membrane was also reduced. The ion exchange capacity (IEC) of the modified Nafion membrane was also evaluated. The formation of the thin cationic charged layer on the membrane surface was confirmed by IR spectra analysis.

Nafion/Zirconium Phosphate Composite Membranes for PEMFC Operating at up to 120 ° C and down to 13 % RH

Nafion/zirconium phosphate (ZP) composite membranes of different structures containing in situ precipitated ZP, crystalline layer-structured α-zirconium hydrogen phosphate , and three-dimensional-network zirconium hydrogen phosphate were synthesized. The ZP materials used have different structure and surface properties (specific surface area, zeta potential, and particle morphology). The composite membranes were studied in an proton exchange membrane fuel cell (PEMFC) over a range of relative humidity (RH) from 13 to 50% at temperatures of 80 and . At the operating temperature of , all studied composite membranes demonstrated significant improvement in performance under dehydrating conditions compared to unmodified Nafion membranes. Among the tested membranes, the Nafion/α-ZPL composite membrane demonstrated the highest performance at at all RH values, especially at 13% RH. At 26 and 50% RH, the performance of Nafion/α-ZPL and Nafion/19% ZP in situ membranes were close, and the performance of the Nafion/ZPTD membrane was the lowest. The effects of filled polymer pore constriction, scaffold formation, membrane structure, and surface properties of the ZP particles on the membrane performance in PEMFCs are discussed in the paper.

Optimized DMFC Performance Comparison for Modified and Unmodified Nafion Membranes

The effects of methanol flow rate, methanol feed concentration, and membrane thickness on the performance of modified Nafion membranes (Nafion-FEP blends and stretched recast Nafion) and unmodified (commercial) Nafion 117 and 112 in a direct methanol fuel cell were examined. That flow rate which produced the highest power density at 0.4 V (60oC and 500 sccm air) for each membrane at a methanol feed concentration of 0.5, 1.0 and 3.0 M was identified. Experimental results indicate that flow rate adjustments, with or without cathode air backpressure, cannot be used to compensate for a high methanol crossover membrane (i.e., lower power densities were obtained for Nafion 112 and 117 due to their poor methanol barrier property). Under optimum flow rate conditions, stretched recast Nafion membranes worked best at all methanol feed concentrations, with a power density at 0.4 V ranging from 65-80 mW/cm2 (for ambient pressure air).

Optimized DMFC Performance Comparison for Modified and Unmodified Nafion Membranes

not Available.

Synthesis and characterization of PDDA-stabilized Pt nanoparticles for direct methanol fuel cells

Pt nanoparticles are synthesized by the alcoholic reduction of H 2PtCl 6 in the presence of a polycation, poly(diallyldimethylammonium chloride) (PDDA). The size of the PDDA–Pt nanoparticle colloids is in the range of 2–4 nm, depending on the PDDA to Pt ratio in the solution. The PDDA–Pt nonoparticles can be self-assembled to the sulfonic acid group, SO 3 −, at the Nafion membrane surface by the electrostatic interaction, forming a self-assembled monolayer (SAM). The study shows that such SAM reduced the methanol crossover and enhanced the power output of direct methanol fuel cells (DMFC) by as much as 34% as compared to the cell based on an un-modified Nafion membrane. In addition, PDDA–Pt nanoparticles synthesized with low PDDA/Pt ratios show considerable catalytic activity for the methanol oxidation reaction (MOR) in comparison to a commercial Pt/C catalyst. However, the electrocatalytic activity of PDDA–Pt nanoparticles decreased significantly with the increase in the PDDA/Pt molar ratio, indicating that the excess PDDA inhibits the MOR.

Nafion ∕ TiO2 Proton Conductive Composite Membranes for PEMFCs Operating at Elevated Temperature and Reduced Relative Humidity

composite membranes with different contents were studied in an proton exchange membrane fuel cell (PEMFC) over a wide range of relative humidity (RH) values from 26 to 100% at temperatures of 80 and 120°C. The composite membranes, which were prepared using a recast procedure, showed a pronounced improvement over unmodified Nafion membranes when operated at 120°C and reduced RH. For instance, at 50% RH, the Nafion/20% membrane demonstrated a performance identical to that of an unmodified Nafion membrane operated at 100% RH. This performance level was comparable to that of a bare Nafion membrane at 80°C. The high performance of the composite membranes at low RH was attributed to improved water retention due to the presence of absorbed water species in the electrical double layer on the surface. The zeta potential and thickness of the hydrodynamically immobile water layer at the interface were discussed as parameters influencing the water balance in the membranes. The obtained experimental PEMFC performance data were fitted using an analytical equation, and calculated parameters were analyzed as functions of RH and content in the composite membranes.

Synthesis and Characterization of Nafion/ 60SiO2 ­ 30 P 2 O 5 ­ 10ZrO2 Sol-Gel Composite Membranes for PEMFCs

A limitation of Nafion membranes in proton-exchange membrane fuel cells (PEMFCs) is their poor water retention above 100°C. The incorporation of gels into Nafion membranes can improve their thermal stability. Therefore, composite membranes were prepared either by infiltration of Nafion membranes with sol or by recasting a film using Nafion solution and sol. The membranes were characterized by thermal analysis, microstructural analysis, functional testing in a fuel cell, and electrochemical impedance spectroscopy. The incorporation of the gel resulted in improved electrochemical behavior at 130°C, with current density-cell potential values as good as those obtained at 80°C with unmodified Nafion membranes. © 2005 The Electrochemical Society. All rights reserved.

Nano-silica layered composite membranes prepared by PECVD for direct methanol fuel cells

Plasma enhanced chemical vapor deposition (PECVD) technique has been employed to deposit nano-scale films of silica (10, 32, 68 nm) on Nafion membrane. Ion conductivity, methanol permeability and single cell performance of the resultant nano-silica/Nafion composite membranes were measured to ascertain its suitability as a candidate membrane for direct methanol fuel cell (DMFC) applications. Experimental results revealed that ion conductivity of the composite membrane containing silica film with 10 nm thickness was similar to the unmodified Nafion membrane, but its methanol permeability was reduced to an extent of 40%. Cell performance of the composite membrane with 10 nm silica was higher than that of the bare Nafion ® membrane by about 20%. The open circuit voltage (OCV) was increased and the cell temperature at OCV was decreased with an increase in the thickness of the silica film. Physical and electrochemical analyses were conducted to investigate the properties of silica-layered membrane and the DMFC employing the membrane.

Evaluation of a Sol-Gel Derived Nafion/Silica Hybrid Membrane for Proton Electrolyte Membrane Fuel Cell Applications: I. Proton Conductivity and Water Content

Sol-gel derived Nafion®/silica hybrid membranes were investigated as a potential polymer electrolyte for fuel cell applications. Membrane proton conductivity and water content were measured as a function of temperature, water vapor activity, and silica content. The hybrid membranes have a higher water content at 25 and 120°C, but not at 150 and 170°C. Despite the higher water content, the proton conductivities in the hybrid membranes are lower than, or equal to, that in unmodified Nafion membranes under all conditions investigated. The proton conductivity of the hybrid membrane decreases with increasing silica content under all conditions. © 2001 The Electrochemical Society. All rights reserved.