Nafion/Analcime and Nafion/Faujasite composite membranes for polymer electrolyte membrane fuel cells

The Nafion/zeolite composite membranes were synthesized for polymer electrolyte fuel cells (PEMFCs) by adding zeolite in the matrix of Nafion polymer. Two kinds of zeolites, Analcime and Faujasite, having different Si/Al ratio were used. The physico-chemical properties of the composite membranes such as water uptake, ion-exchange capacity, hydrogen permeability, and proton conductivity were determined. The fabricated composite membranes showed the significant improvement of all tested properties compared to that of pure Nafion membrane. The maximum proton conductivity of 0.4373 S cm⁻¹ was obtained from Nafion/Analcime (15%) at 80 °C which was 6.8 times of pure Nafion (0.0642 S cm⁻¹ at 80 °C). Conclusively, Analcime exhibited higher improvement than Faujasite.     

New solid polymer electrolyte membranes for alkaline fuel cells

New solid polymer electrolyte composite membranes have been prepared using chitosan as matrices and incorporating potassium hydroxide as the functional ionic source. These membranes were featured as a three‐layer structure having a porous intermediate layer while the two crosslinked surface layers are dense. Results from impedance spectroscopy analysis showed that the conductivity of some hydrated composite membranes, after hydration for 1 h at room temperature, reached about 10⁻² S cm⁻¹. Several composite membranes were then tested in alkaline fuel cells, using hydrogen as fuel, air as oxidant and platinum as the electrode catalyst. A current density of 35 mA cm⁻² has been achieved at 60 °C with a flow rate of hydrogen at 50 ml min⁻¹ and air at 200 ml min⁻¹. Copyright © 2004 Society of Chemical Industry     

Silicate-based polymer-nanocomposite membranes for polymer electrolyte membrane fuel cells

Proton-exchange membrane fuel cells have emerged as a promising emission free technology to fulfill the existing power requirements of the 21st century. Nafion^® is the most widely accepted and commercialized membrane to date and possesses excellent electrochemical properties below 80 °C, under highly humidified conditions. However, a decrease in the proton conductivity of Nafion^® above 80 °C and lower humidity along with high membrane cost has prompted the development of new membranes and techniques. Addition of inorganic fillers, especially silicate-based nanomaterials, to the polymer membrane was utilized to partially overcome the aforementioned limitations. This is because of the lower cost, easy availability, high hydrophilicity and higher thermal stability of the inorganic silicates. Addition of silicates to the polymer membrane has also improved the mechanical, thermal and barrier properties, along with water uptake of the composite membranes, resulting in superior performance at higher temperature compared to that of the virgin membrane. However, the degrees of dispersion and interaction between the organic polymer and inorganic silicates play vital roles in improving the key properties of the membranes. Hence, different techniques and solvent media were used to improve the degrees of nanofiller dispersion and the physico-chemical properties of the membranes. This review focuses mainly on the techniques of silicate-based nanocomposite fabrication and the resulting impact on the membrane properties.     

Sulfonated polysulfone as promising membranes for polymer electrolyte fuel cells

A new, milder sulfonation process was used to produce ion‐exchange polymers from a commercial polysulfone (PSU). Membranes obtained from the sulfonated polysulfone are potential substitutes for perfluorosulfonic acid membranes used now in polymer electrolyte fuel cells. Sulfonation levels from 20 to 50% were easily achieved by varying the content of the sulfonating agent and the reaction time. Ion‐exchange capacities from 0.5 to 1.2 mmol SO₃H/g polymer were found via elemental analysis and titration. Proton conductivities between 10⁻⁶ and 10⁻² S cm⁻¹ were measured at room temperature. An increase in intrinsic viscosity with increasing sulfonation degree confirms that the sulfonation process helps to preserve the polymer chain from degradation. Thermal analysis of the sulfonated polysulfone (SPSU) samples reveals higher glass transition temperatures and lower decomposition temperatures with respect to the unsulfonated sample (PSU). Amorphous structures for both PSU and SPSU membranes were detected by X‐ray diffraction analysis and differential scanning calorimetry. Preliminary tests in fuel cells have shown encouraging results in terms of cell performance. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1250–1257, 2000     

Recycling and regeneration of used perfluorosulfonic membranes for polymer electrolyte fuel cells

This paper describes a method for the recycling and regeneration of used perfluorosulfonic Nafion^® (Dupont) membranes by dissolution and recasting. The dissolution of the used Nafion^® membranes from polymer electrolyte fuel cells is realized using dimethyl sulfoxide as a solvent under atmospheric pressure and 190 °C. A mechanically robust membrane can be reproduced by a recast process of the dissolved Nafion^® solution at 170 °C. The recycled membrane has shown a good crystalline structure and high mechanical strength. Membrane properties, including water uptake, exchange capacity and resistance are similar to that of the as-received Nafion^® 115 membrane. Fuel cells prepared by the recycled membrane demonstrate a comparable performance to that of the fresh fuel cell.     

Degradation behaviors of perfluorosulfonic acid polymer electrolyte membranes for polymer electrolyte membrane fuel cells under varied acceleration conditions

The degradation of perfluorosulfonic acid (PFSA) membranes (e.g., Nafion membranes) in polymer electrolyte membrane fuel cells has caused wide widespread concern. However, their degradation behaviors, which lead to the damage of fuel cells, need to be investigated under alternative accelerating environments by the simulation of fuel‐cell operating conditions. Nafion membranes showed a homogeneous degradation behavior during hydrogen peroxide (H₂O₂) aging, whereas a nonhomogeneous (or crack‐type) degradation behavior occurs for Nafion membranes aged in an H₂O₂/Fe²⁺ system (Fenton's reagent), where plenty of the typical microcracks appeared. Interestingly, in the case of nonhomogeneous degradation, the membrane presented a lower fluoride emission rate than that with the homogeneous degradation; this indicates a possible selective attack model of free radicals to both CF₂ and the defect end groups in PFSA membranes. In addition, the effects of the different degradation behaviors on the thermal stability and water uptake of membranes were examined by thermogravimetric analyses. H₂ crossover and single‐fuel‐cell tests were carried out to evaluate the influence of the degradation behaviors on the fuel‐cell performance. These showed that the membrane with a nonhomogeneous degradation behavior had a higher hydrogen crossover and was more destructive than that with a homogeneous behavior. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013     

Silicone-containing polymer blend electrolyte membranes for fuel cell applications

Polymer electrolyte membranes are developed from blends of chemically durable silicone-containing epoxy (Si-Epoxy) and proton conducting sulfonic polyimide (SPI). A charge-transfer (CT) complex is formed between electron-donating dihydroxynaphthalene units in Si-Epoxy, and electron-accepting naphthalenediimide units in SPI, as confirmed via X-ray diffraction and visible spectroscopy. The blend membranes show comparable mechanical strength to Nafion 211, but the elongation to break is much lower, indicating better resistance to deformation under strain stress, attributed to CT complex formation. The chemical durability of the blend membranes was much higher than pure SPI according to Fenton's test, also attributed to CT complex formation. Meanwhile, the proton conductivity is dependent on the sulfonic acid content of the SPI, which in turn affects the fuel cell performance. The maximum proton conductivity was measured to be 23.1 mS cm⁻¹ at 80°C and 90 %RH for a 1:1 blend, and the membranes were successfully incorporated into PEFCs.     

Organic-inorganic nanocomposite polymer electrolyte membranes for fuel cell applications

Organic-inorganic nanocomposite polymer electrolyte membrane (PEM) contains nano-sized inorganic building blocks in organic polymer by molecular level of hybridization. This architecture has opened the possibility to combine in a single solid both the attractive properties of a mechanically and thermally stable inorganic backbone and the specific chemical reactivity, dielectric, ductility, flexibility, and processability of the organic polymer. The state-of-the-art of polymer electrolyte membrane fuel cell technology is based on perfluoro sulfonic acid membranes, which have some key issues and shortcomings such as: water management, CO poisoning, hydrogen reformate and fuel crossover. Organic-inorganic nanocomposite PEM show excellent potential for solving these problems and have attracted a lot of attention during the last ten years. Disparate characteristics (e.g., solubility and thermal stability) of the two components, provide potential barriers towards convenient membrane preparation strategies, but recent research demonstrates relatively simple processes for developing highly efficient nanocomposite PEMs. Objectives for the development of organic-inorganic nanocomposite PEM reported in the literature include several modifications: (1) improving the self-humidification of the membrane; (2) reducing the electro-osmotic drag and fuel crossover; (3) improving the mechanical and thermal strengths without deteriorating proton conductivity; (4) enhancing the proton conductivity by introducing solid inorganic proton conductors; and (5) achieving slow drying PEMs with high water retention capability. Research carried out during the last decade on this topic can be divided into four categories: (i) doping inorganic proton conductors in PEMs; (ii) nanocomposites by sol-gel method; (iii) covalently bonded inorganic segments with organic polymer chains; and (iv) acid-base PEM nanocomposites. The purpose here is to summarize the state-of-the-art in the development of organic-inorganic nanocomposite PEMs for fuel cell applications.     

直接聚合物电解质燃料电池的燃料

本文介绍了直接聚合物电解质燃料电池的几种甲氧基燃料包括二甲醚、二甲氧基甲烷、三甲氧基甲烷和甲醇的电化学性能及在电池中反应副产物情况，对其应用前景作了初步估计。     

Preparation of highly dispersed SiO2 and Pt particles in Nafion112 for self-humidifying electrolyte membranes in fuel cells

We have developed preparation protocol of practically large size self-humidifying polymer electrolyte membranes (PEMs) with highly dispersed nanometer-sized Pt and/or SiO₂ for fuel cells. The Pt particles were expected to catalyze the recombination of H₂ and O₂, leading to a suppression of the chemical short-circuit reaction at the electrodes, while the SiO₂ particles were expected to adsorb the water produced at the Pt particles together with that produced at the cathode reaction. Stable SiO₂ particles were formed in a commercial PEM (Nafion^®112) via in situ sol-gel reactions at 70 °C. It was found by SAXS that the hydrophilic cluster size increased by water adsorbed SiO₂, which may contribute to the increase in the proton conductivity once SiO₂ adsorbed water. Pt particles were uniformly dispersed in a Na⁺-form normal-PEM or SiO₂-PEM by an ion-exchange reaction with [Pt(NH₃)₄]Cl₂, followed by a reduction with 1-pentanol at 125 °C. The newly prepared Pt-SiO₂-PEM was found to perform a self-humidifying operation in a standard-size PEFC (25 cm² electrode area) with H₂ and O₂ humidified at 30 °C. The performance of the Pt-SiO₂-PEM cell operated with the low humidity reactant gases was as high as the normal-PEM cell fully humidified, because the ohmic resistance of the former cell was as low as the latter cell.     

Poly(vinyl alcohol)-based polymer electrolyte membranes for direct methanol fuel cells

We have prepared polymer electrolyte membranes (PEMs) from poly(vinyl alcohol) (PVA) and modified PVA polyanion containing 2 or 4 mol% of 2-methyl-1-propanesulfonic acid (AMPS) groups as a copolymer. The PEMs of various AMPS content and cross-linking conditions were prepared to determine the effect of AMPS content and cross-linking conditions on PEM properties. Proton conductivity and permeability of methanol through the PEMs increased with increasing AMPS content, C_AMPS, and with decreasing cross-linker concentration, C_GA, because of the increase in the water content. The permeability coefficient of methanol through the PEM prepared under the conditions of C_AMPS = 2.7 mol% and C_GA = 0.35 vol% was about 30 times lower than that of Nafion^®117 under the same measurement conditions. The proton permselectivity of the PEM, which is defined as the ratio of the proton conductivity to the permeability coefficient of methanol, gave a maximum value of 66 × 10³ S cm⁻³ s. The value is about three times higher than that of Nafion^®117.     

Pd-Co carbon-nitride electrocatalysts for polymer electrolyte fuel cells

In this paper is described the preparation of new platinum-free Pd-Co carbon-nitride electrocatalysts (Pd-Co-CNs) for application in low-temperature fuel cells. Two groups of materials with formula K_n[Pd_xCo_yC_zN_lH_m] were synthesized, which are grouped in two ensembles: the first is characterized by a molar ratio y/x > 1 (I), and the second by y/x < 1 (II). K_n[Pd_xCo_yC_zN_lH_m] materials were prepared through a two-step synthesis protocol. The effect of the Pd/Co molar ratio and of the temperature of the thermal treatments on the structure and properties of the products were studied extensively by thermogravimetry, scanning electron microscopy, and vibrational (FT-IR and micro-Raman) and XPS spectroscopy. Vibrational studies revealed that I and II systems consist of two polymorphs of α- and graphitic-like carbon-nitride nanomaterials. The electrochemical activity towards the oxygen reduction reaction (ORR) and the hydrogen oxidation reaction (HOR) was measured by cyclic voltammetry measurements with thin-film rotating disk electrode (CV-TF-RDE). The electrochemical performance of Pd-Co-CNs of group I obtained at t_f ≥ 700 °C resulted higher than that measured for a platinum-based commercial electrocatalyst in terms of both the activity towards the ORR and the HOR and of the resistance towards the poisoning effect of methanol towards the ORR.     

Transient analysis of polymer electrolyte fuel cells

A three-dimensional, transient model has been developed to study the transient dynamics of polymer electrolyte fuel cell (PEFC) operation. First, various time constants are estimated for important transient phenomena of electrochemical double-layer discharging, gas transport through the gas diffusion layer (GDL) and membrane hydration. It is found that membrane hydration occurs over a period of 10 s, the gas transport of 0.01-0.1 s, with the double-layer discharging being negligibly fast. Subsequently, extensive numerical simulations, with the transient processes of membrane hydration and gas transport taken into consideration, are carried out to characterize the dynamic response of a singe-channel PEFC with N112 membrane. The results show that the time for fuel cells to reach steady state is in the order of 10 s due to the effect of water accumulation in the membrane, consistent with theoretical estimation. In addition, overshoot or undershoot of the current densities is found during the step changes in some operating conditions, and detailed results are provided to reveal the dynamic physics of these phenomena.     

IrO2-deposited Pt electrocatalysts for unitized regenerative polymer electrolyte fuel cells

An IrO₂/Pt electrocatalyst for the polymer electrolyte-type unitized regenerative fuel cell (URFC) was prepared by deposition of iridium oxide (IrO₂) particles on Pt black via a colloidal iridium hydroxide hydrate precursor, and URFC performance was examined. After the iridium hydroxide hydrate deposited Pt was calcined at 400 °C in air for 1 h, rutile-structure IrO₂ particles (20–50 nm dia.) were formed on Pt particle clusters. TEM and pore volume distribution analysis revealed that the microstructure of the deposited IrO₂/Pt catalyst was different from the mixed IrO₂/Pt catalyst. The cell using the deposited IrO₂/Pt (20 at % Ir) catalyst showed similar fuel cell performance with the mixed IrO₂/Pt electrode of higher Pt content (10 at % Ir) while maintaining water electrolysis performance. Consequently, 51% round-trip energy conversion efficiency at a current density of 300 mA cm^–2 was attained.     

Plasma polymerized electrolyte membranes and electrodes for miniaturized fuel cells

Miniaturized fuel cells for portable systems like cellular phones, laptops, or other conventionally battery-driven devices, as well as long-term stationary monitoring electronics, have a potential market, especially for direct methanol fuel cells. However, design and fabrication technologies have to be adopted that allow the desired miniaturization of such a fuel cell. Thin film technologies like plasma polymerization and sputtering are suitable techniques for realizing membrane electrode assemblies only several microns in thickness that can be deposited on thin substrates (e.g., silicon wafers, porous foils, or others). Furthermore, plasma polymerized films exhibit a high degree of cross-linkage and are pinhole free even for films of only a few hundred nanometer in thickness, in contrast to conventionally polymerized films. In case of an electrolyte membrane these benefits yield a reduction of membrane resistance and a decreased methanol crossover. We have developed plasma polymerized electrolyte membranes using tetrafluoroethylene to generate the polymeric backbone of an ion-conductive membrane and vinylphosphonic acid to incorporate acid groups, which are responsible for the proton conductivity. Depending on the process parameters these films exhibit an ion conductivity in the range of 100 mS/cm to 200 mS/cm (at 80°C), determined by ac-impedance measurements. These films were optimized with respect to their use in direct methanol fuel cells to achieve a high ion conductivity and high thermal resistance. Porous graphite electrodes were fabricated using an acetylene plasma polymerization process. These films are combined with the plasma polymerized electrolyte membrane to form a thin film membrane electrode assembly.     

Proton-conducting electrolyte membranes based on hyperbranched polymer with a sulfonic acid group for high-temperature fuel cells

The hyperbranched polymers (HBP-SA-Acs) with both a sulfonic acid group as a functional group and an acryloyl group as a cross-linker at terminals in different ratios of sulfonic acid group/acryloyl group (SO₃H/Ac) were successfully synthesized as a new thermally stable proton-conducting electrolyte. The cross-linked hyperbranched polymer electrolyte membranes (CL-HBP-SAs) were prepared by thermal polymerizations of the HBP-SA-Acs using benzoyl peroxide, and their ionic conductivities under dry condition and thermal properties were investigated. The ionic conductivities of the CL-HBP-SAs were found to be in the range of 2.2 × 10⁻⁴ to 3.3 × 10⁻⁶ S/cm, depending upon the SO₃H unit contents, at 150 °C under dry condition, and showed the Vogel-Tamman-Fulcher (VTF) type temperature dependence, indicating that proton transfer is cooperated by local polymer chain motion. All CL-HBP-SAs were thermally stable up to 260 °C, and they had suitable thermal stability as electrolyte membranes for the high-temperature fuel cells under dry condition. Fuel cell measurement using a single membrane electrode assembly cell with a cross-linked electrolyte membrane was successfully performed under non-humidified condition. It was demonstrated that applying the concept of dry polymer system to proton conduction is one possible approach toward high-temperature fuel cells.     

Optimisation of polypyrrole/Nafion composite membranes for direct methanol fuel cells

Acidic and neutral Nafion^® 115 perfluorosulphonate membranes have been modified by in situ polymerization of pyrrole using Fe(III) and H₂O₂ as oxidizing agents, in order to decrease methanol crossover in direct methanol fuel cells. Improved selectivities for proton over methanol transport and improved fuel cell performances were only obtained with membranes that were modified while in the acid form. Use of Fe(III) as the oxidizing agent can produce a large decrease in methanol crossover, but causes polypyrrole deposition on the surface of the membrane. This increases the resistance of the membrane, and leads to poor fuel cell performances due to poor bonding with the electrodes. Surface polypyrrole deposition can be minimized, and surface polypyrrole can be removed, by using H₂O₂. The use of Nafion in its tetrabutylammonium form leads to very low methanol permeabilities, and appears to offer potential for manipulating the location of polypyrrole within the Nafion structure.     

Non-isothermal cold start of polymer electrolyte fuel cells

A multiphase, three-dimensional model has been developed to describe non-isothermal cold start of a polymer electrolyte fuel cell (PEFC) and to delineate intricate interactions between ice formation and heat generation during cold start. The effect of rising cell temperature is numerically explored by comparing a non-isothermal cold start with an isothermal one. It is found that more water is transported into the membrane and less ice formation occurs in the cathode catalyst layer (CL) in the presence of rising cell temperature. In addition, the more hydrated membrane and the rising cell temperature greatly lower the membrane resistance, thus giving rise to higher cell voltage. A lumped thermal analysis significantly over-estimates the overall thermal requirement of self-startup as a cell requires only a portion of its active area to reach the freezing point and be ice-free and operable. It is also found that pre-startup conditions have significant influence on cold start. Procedures to minimize residual water inside the cell prior to cold start, such as gas purge, are critically important. Finally, non-isothermal cold start becomes much easier from higher ambient temperatures.