Novel Nafion composite membranes with mesoporous silica nanospheres as inorganic fillers

Novel Nafion composite proton exchange membranes are prepared using mesoporous MCM-41 silica nanospheres as inorganic fillers. The novelty of this study lies in the structural design of inorganic silica fillers: the nanosized and monodisperse spherical morphology of fillers facilitates the preparation of homogenous composite membranes, whilst the superior water adsorption of the mesostructure in fillers consigns enhanced water retention properties to the polymer membranes. Scanning electron microscopy images of the composite membranes indicate that well-dispersed silica nanospheres are embedded in the Nafion matrix, but a large amount of added fillers (3 wt.%) causes some agglomeration of the nanospheres. Compared with the Nafion cast membrane, the composite membranes offer improved thermal stability, enhanced water retention properties, and reduced methanol crossover. Despite the enhancement of water retention, the composite membranes still exhibit a proton conductivity reduction of 10–40% compared with pristine Nafion. This is likely due to the incorporation of much less conductive silica fillers than Nafion. The composite membrane containing 1 wt.% of fillers displays the best cell performance in direct methanol fuel cell tests; it gives a maximum power density of 21.8 mW cm⁻², i.e., ∼20% higher than the Nafion cast membrane. This is attributed to its similar conductivity to Nafion, and its markedly reduced methanol crossover, namely, ∼1.2 times lower.     

Understanding of temperature-dependent performance of short-side-chain perfluorosulfonic acid electrolyte and reinforced composite membrane

The short-side-chain (SSC) perfluorosulfonic acid (PFSA) membranes are important candidates as membrane electrolytes applied for high temperature or low relative humidity (RH) proton exchange membrane fuel cells. In this paper, the fuel cell performance, proton conductivity, proton mobility, and water vapor absorption of SSC PFSA electrolytes and the reinforced SSC PFSA/PTFE composite membrane are investigated with respect to temperature. The pristine SSC PFSA membrane and reinforced SSC composite membrane show better fuel cell performance and proton conductivity, especially at high temperature and low relative humidity conditions, compared to the long-side-chain (LSC) Nafion membrane. Under the same condition, the proton mobility of SSC PFSA membranes is lower than that of the LSC PFSA membrane. The water vapor uptake values for Nafion 211 membrane, pristine SSC PFSA membrane and SSC PFSA/PTFE composite membrane are 9.62, 11.13, and 11.53 respectively at 40 °C and they increase to 9.89, 12.55 and 13.09 respectively at 120 °C. The high water content of SSC PFSA membrane makes it maintain high performance even at elevated temperatures.     

Nafion/polyaniline composite membranes specifically designed to allow proton exchange membrane fuel cells operation at low humidity

A Nafion and polyaniline composite membrane (designated Nafion/PANI) was fabricated using an in situ chemical polymerization method. The composite membrane showed a proton conductivity that was superior to that obtained with Nafion^® 112 at low humidity (e.g. RH = 60%). Water uptake measurements revealed similarities between the Nafion^® 112 and Nafion/PANI membranes at different humidities. The high conductivity of the Nafion/PANI membrane at low humidity is hypothesized to be due to the existence of the extended conjugated bonds in the polyaniline; proton transfer is facilitated via the conjugated bonds in lower humidity environments allowing retention of the relatively high conductivity. Correspondingly, the performance of a single cell fuel cell containing the Nafion/PANI composite membrane is improved compared to a Nafion^® 112-containing cell under low humidity conditions. This is important for portable fuel cells, which are required to operate without external humidification.     

Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs

A novel multilayer membrane for the proton exchange membrane fuel cell (PEMFC) was developed. Nafion was dispersed uniformly onto both sides of the sulfonated polyimide (SPI) membrane. The Nafion/SPI/Nafion composite membrane was prepared by immersing the SPI into the Nafion-containing casting solution. Through immersing both membranes into the Fenton solution at 80 °C for 0.5 h for an accelerated ex situ test, chromatographic analysis of the water evacuated from the cathode and the anode of the cells and a durability test of a single proton exchange membrane fuel cells, it was proved that the stability of the composite membrane has been greatly improved by adding the Nafion layer compared with the SPI membrane. The fuel cell performance with the SPI and Nafion/SPI/Nafion membranes was similar to the performance with the commercial product Nafion^® NRE-212 membrane at 80 °C.     

Balancing dimensional stability and performance of proton exchange membrane using hydrophilic nanofibers as the supports

In this work, we developed a novel composite membrane by anchoring perfluorosulfonic acid into the hydrophilic poly(lactic-co-glycolic acid) (PLGA) nanofibrous network which was synthesized by electrospinning method. It was clear that the PLGA/Nafion composite membranes possessed high Nafion loading, excellent dimensional stability and proton transport capacity. When the humidity of the membrane changed from soaking in water to 25 RH% at 90 °C, the PLGA fiber network effectively controlled the swelling of Nafion resin and reduced the humidity-generated shrinkage stress from 2.2 MPa (Nafion211 membranes) to 0.5 MPa (PLGA/Nafion composite membranes). The proportion of humidity-induced stress to the yield strength was also reduced to 4.4%, in comparison to 21.2% of that of Nafion211 membrane. The area proton conductivity of the PLGA/Nafion composite membrane achieved 48.2 S cm⁻², compared with 36.0 S cm⁻² of Nafion211 membranes in the same condition. The excellent proton transport capacity greatly improved the performance of fuel cell assembled with PLGA/Nafion composite membranes and effectively reduced the dynamic response time from 22 s (Nafion211 membranes) to 7 s (PLGA/Nafion composite membranes).     

Silica-related membranes in fuel cell applications: An overview

Silica is the most common inorganic filler used in fuel cells, especially for proton exchange membrane fuel cell and direct alcohol fuel cell applications. Silica has played an important role in improving the performance of fuel cells by enhancing their membrane properties. Recently, silica has been widely implemented in different types of membranes, such as fluorinated membranes (Nafion), sulfonated membranes (SPEEK, SPS, SPAES, SPI) and other organic polymer matrixes. The incorporation of silica into membrane matrices has improved the thermal stability, mechanical strength, water retention capacity and proton conductivity of the membrane. This review describes the interactions between silica and different types of polymer matrices in fuel cells and how they boost fuel cell performance. In addition, this review also discusses the current challenges of silica-related membrane-based fuel cells and predicts the future prospects of silica in membrane-based fuel cell applications.     

Physically stable proton exchange membrane with ordered electrolyte for elevated temperature PEM fuel cell

Dimensional change and humidity-induced stress of the proton exchange membrane were demonstrated to be main reasons for membrane physical failure during the long-term fuel cell operation. In this work, UV laser ablation was proposed to prepare physically stable polyimide supports to reduce the dimensional swelling and humidity-induced stress of the proton exchange membrane under variable humidities. Long-range ordered straight holes with definable open pattern and diameter of 50–200 μm were formed through the polyimide support. Composite proton exchange membrane prepared from the straight-hole polyimide support presented desirable performance and high durability in fuel cells. When Nafion fraction in the composite membrane increased to 48.67%, the proton conductivities of the composite membranes were equal to or greater than that of the conventional Nafion membrane with activation energies lower than that of the Nafion 211 membrane. The dimensions of the composite membranes are very stable in both low and elevated temperature conditions. The proportion of humidity-induced stress to the yield strength for the composite membrane is 0.20%–0.21%, much lower than that of the conventional Nafion membrane (24.77%). As a result, the composite proton exchange membrane prepared from the straight-hole polyimide presented high durability in the fuel cell operation. In the open circuit voltage accelerated test under in situ accelerating RH cyclic test, the irreversible OCV reduction rate of the composite membranes was 2.41–2.72 × 10⁻⁵ V/cycle, 37.1%–41.8% lower than that of the conventional Nafion 211 membrane.     

Operation of a proton exchange membrane fuel cell under non-humidified conditions using a membrane–electrode assemblies with composite membrane and electrode

Composite membranes with hydrophilic substances can retain water and allow the operation of proton exchange membrane fuel cells (PEMFCs) under non-humidified conditions. In this work, thin Nafion composite membranes with silica are prepared to operate a PEMFC with dry fuel and oxidant. In addition, the role of silica in the catalyst layer as a water retainer is studied. In particular, the anode and the cathode are modified separately to elucidate the effect of silica. The incorporation of silica in the membrane and the catalyst layer enhances single-cell performance under non-humidified operation. The cell performance of membrane–electrode assemblies using the composite membrane and electrode is higher than that of a MEA using commercial Nafion 111 membrane under non-humidified conditions.     

Fabrication and evaluation of Nafion nanocomposite membrane based on ZrO2–TiO2 binary nanoparticles as fuel cell MEA

Synthesis and characterization of nanocomposite membranes for proton exchange membrane fuel cell (PEMFC) operating at different temperatures and humidity were investigated in this study. Recast Nafion composite membrane with ZrO₂ and TiO₂ nanoparticles with 75 nm in mean size diameter, prepared for PEM fuel cells. Nafion/TiO₂ composite membranes have been also fabricated by in-situ sol–gel method. However, fine particles of the ZrO₂ were synthesized and Nafion/ZrO₂ composite membrane were produced by blending a 5% (w/w) Nafion-water dispersion with the inorganic compound. All nanocomposite membranes demonstrated higher water retention in comparison with unmodified membranes. Proton conductivity increased with increasing ZrO₂ content while TiO₂ additive (with mean size of 25 nm) enhanced water retention. Subsequently, structures of the membranes were investigated by Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) as well as X-Ray Diffraction (XRD). In addition, water uptake and proton conductivity of the modified membranes were also measured. The nanocomposite membrane was tested in a 25 cm² commercial single cell at the temperature range of 80–110 °C and in humidified H₂/O₂ under different relative humidity (RH) conditions. The membrane electrode assembly (MEA) prepared from Nafion/TiO₂, ZrO₂ presented highest PEM fuel cell performance in respect of I–V polarization under condition of 110 °C, 0.6 V and 30% RH and 1 atm.     

Nafion-impregnated electrospun polyvinylidene fluoride composite membranes for direct methanol fuel cells

Composite membranes consisting of polyvinylidene fluoride (PVdF) and Nafion have been prepared by impregnating various amounts of Nafion (0.3–0.5 g) into the pores of electrospun PVdF (5 cm × 5 cm) and characterized by scanning electron microscopy, differential scanning calorimetry, X-ray diffraction, and proton conductivity measurements. The characterization data suggest that the unique three-dimensional network structure of the electrospun PVdF membrane with fully interconnected fibers is maintained in the composite membranes, offering adequate mechanical properties. Although the composite membranes exhibit lower proton conductivity than Nafion 115, the composite membrane with 0.4 g Nafion exhibits better performance than Nafion 115 in direct methanol fuel cell (DMFC) due to smaller thickness and suppressed methanol crossover from the anode to the cathode through the membrane. With the composite membranes, the cell performance increases on going from 0.3 to 0.4 g Nafion and then decreases on going to 0.5 g Nafion due to the changes in proton conductivity.     

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.     

An enhanced proton conductivity and reduced methanol permeability composite membrane prepared by sulfonated covalent organic nanosheets/Nafion

Sulfonated covalent organic nanosheets (SCONs) with a functional group (−SO₃H) are effective at reducing ion channels length and facilitating proton diffusion, indicating the potential advantage of SCONs in application for proton exchange membranes (PEMs). In this study, Nafion-SCONs composite membranes were prepared by introducing SCONs into a Nafion membrane. The incorporation of SCONs not only improved proton conductivity, but also suppressed methanol permeability. This was due to the even distribution of ion channels, formed by strong electrostatic interaction between the well dispersed SCONs and Nafion polymer molecules. Notably, Nafion-SCONs-0.6 was the best choice of composite membranes. It exhibited enhanced performance, such as high conductivity and low methanol permeability. The direct methanol fuel cell (DMFC) with Nafion-SCONs-0.6 membrane also showed higher power density (118.2 mW cm⁻²), which was 44% higher than the cell comprised of Nafion membrane (81.9 mW cm⁻²) in 2 M methanol at 60 °C. These results enabled us to work on building composite membranes with enhanced properties, made from nanomaterials and polymer molecules.     

Multifunctional composite membrane based on a highly porous polyimide matrix for direct methanol fuel cells

A highly porous polyimide film with tunable pore size, porosity and thickness is synthesized and used as a matrix to construct a Nafion-infiltrated composite membrane. A very efficient way for an easy and complete infiltration of the proton-conducting polymer into this substrate is developed, which is usually a major problem for composite membranes. Due to the complete inertness to methanol and the very high mechanical strength of the polyimide matrix, the swelling of the composite membrane is greatly suppressed and the methanol crossover is also significantly reduced (80 times), where as while high proton conductivity (comparable with Nafion) and mechanical strength (4 times stronger than Nafion) is still maintained. This membrane demonstrates significantly improved cell performance compared with the Nafion membrane and is a promising candidate for use in direct methanol fuel cells.     

Methanol-blocking Nafion composite membranes fabricated by layer-by-layer self-assembly for direct methanol fuel cells

In this paper, a series of Nafion composite membranes with improved methanol barrier properties were fabricated from chitosan (CS) and silicotungstic acid (SiWA) by Layer-by-layer (LbL) self-assembly technique. The Fourier transform infrared attenuated total reflection (FTIR-ATR) spectra and scanning electron microscope (SEM) were employed to prove the successful deposition of CS/SiWA layers. The effect of the methanol-blocking layer on proton conductivity, methanol permeability and direct methanol fuel cell (DMFC) performance was studied. Both proton conductivity and methanol permeability of Nafion composite membranes decreased with the increase of CS/SiWA bi-layer number. The Nafion composite membrane with a reduced proton conductivity of 22% showed a reduced methanol permeability of 47%. Approximate 18% improvement of the selectivity was achieved as compared with plain Nafion membrane. But it did not result in better DMFC performance than that of plain Nafion membrane.     

Methanol crossover through PtRu/Nafion composite membrane for a direct methanol fuel cell

This study examined methanol crossover through PtRu/Nafion composite membranes for the direct methanol fuel cell. For this purpose, 0.03, 0.05 and 0.10 wt% PtRu/Nafion composite membranes were fabricated using a solution impregnation method. The composite membrane was characterized by inductively coupled plasma-mass spectroscopy and thermo-gravimetric analysis. The methanol permeability and proton conductivity of the composite membranes were measured by gas chromatography and impedance spectroscopy, respectively. In addition, the composite membrane performance was evaluated using a single cell test. The proton conductivity of the composite membrane decreased with increasing number of PtRu particles embedded in the pure Nafion membrane, while the level of methanol permeation was retarded. From the results of the single cell test, the maximum performance of the composite membrane was approximately 27% and 31% higher than that of the pure Nafion membrane at an operating temperature of 30 and 45 °C, respectively. The optimum loading of PtRu was determined to be 0.05 wt% PtRu/Nafion composite membrane.The PtRu particles embedded in the Nafion membrane act as a barrier against methanol crossover by the chemical oxidation of methanol on embedded PtRu particles and by reducing the proton conduction pathway.     

Sulfonated graphene oxide/Nafion composite membranes for high-performance direct methanol fuel cells

An easy and effective method for producing low methanol-crossover membranes is developed by dispersing sulfonated graphene oxide (SGO) into a Nafion matrix. A SGO/Nafion mixture with low SGO content exhibits unique viscosity behavior and allows for better SGO dispersion within the Nafion. After film casting, the composite membranes show lower methanol and water uptakes, a reduced swelling ratio, improved proton conductivity in low relative humidity, and extremely high methanol selectivity, which can be implemented in direct methanol fuel cells (DMFCs). The regular backbone of the composite membrane shows a higher storage modulus, increased α-relaxation (transition temperature), and improved tolerance to pressure during membrane electrode assembly (MEA). The small angle X-ray spectra indicate the shrinkage of the ionic clusters in the composite membranes, which thus reduce methanol crossover. The hybrid membranes applied to DMFCs demonstrate performances superior to that of the commercial Nafion 115 in 1 M and 5 M methanol solutions.     

Amelioration of PEMFC performance at high temperature by incorporation of nanofiller (sepiolite/layered double hydroxide) in Nafion membrane

Nanoheterostructured material composed of sepiolite clay mineral in which is assembled a MgAl layered double hydroxide (LDH) was used in the preparation of Nafion composite electrolyte membranes and their behavior compared to those of membranes filled with the LDH alone. Both, the neat MgAl LDH and the MgAl LDH-sepiolite hybrid materials were obtained via the co-precipitation method. Sepiolite fibers provide a large external surface area for bonding MgAl LDH particles while maintaining high microporosity and water molecules. The nanocomposite membranes were prepared incorporating different amount of LDH or LDH-sepiolite hybrid. Composite membranes present better water retention, good thermal properties and high proton conductivities at high temperatures than the pure Nafion membrane. The proton conductivity at 100 °C and 100% RH reaches a value of 0.13 S/cm for the LDH-sepiolite Nafion membrane whereas is only 0.010 S/cm in the case of the Nafion membrane. Fuel cell tests using Nafion membranes containing LDH or LDH-sepiolite hybrid as composite electrolytes show a good result for the operation of the PEMFC at 80 °C, 100 °C and 110 °C, with a clear favoring effect of the LDH-sepiolite filler for operation at the highest temperatures.     

Stabilizing phosphotungstic acid in Nafion membrane via targeted silica fixation for high-temperature fuel cell application

With PWA as proton transfer and silica as water retainer, stable phosphotungstic acid/silica/Nafion (PWA/Si–N) composite membrane is non-destructively fabricated and exhibits excellent stability and high temperature proton conductivity. Compared with pristine Nafion, high temperature proton conductivity is significantly enhanced due to the collaboration between –SO₃H ionic clusters and the in-situ filled silica embedded PWA nanoparticles. PWA is stabilized in the ionic clusters via in-situ catalyzing the hydrolysis silica precursor targeted filled into the –SO₃H ionic clusters. Stable proton conductivity of the PWA/Si–N membrane at 110 °C and 60% RH is high to 0.058 S/cm, which is 2.4 folds of that of Nafion. At the same time, the composite membrane still maintains good mechanical and thermal stability. As a result, high temperature fuel cell performance of the composite membrane is improved by 41% compared with the pristine Nafion membrane. The in-situ coating method proved to be an effective method to solve the stability of PWA in Nafion membrane, especially the inorganic oxide with good hygroscopicity as the modifier.     

Poly(1-vinylimidazole)/Pd-impregnated Nafion for direct methanol fuel cell applications

Nafion is modified by incorporating poly(1-vinylimidazole)/Pd composites into the ion cluster channels of the membrane. The poly(1-vinylimidazole)/Pd-impregnated (PVI/Pd-impregnated) membranes is characterized by means of X-ray photoelectron spectroscopy (XPS), proton conductivity and methanol permeability measurements and compared with those of the untreated Nafion. The dependence of the membrane proton conductivity and methanol permeability on the poly(1-vinylimidazole) (PVI) and palladium contents in the PVI/Pd-impregnated Nafion is studied. The performance of the cells employing the PVI/Pd-impregnated Nafion is evaluated using a direct methanol fuel cell (DMFC) unit cell. It is found that the best cell performance is obtained when Nafion is impregnated with the PVI/Pd composite solution for 20 h. This result suggests that there exists an optimum content of palladium and PVI in the modified membrane to obtain a high-proton conductivity and low-methanol permeability that result in a high-cell performance.