Double crosslinked polyetheretherketone-based polymer electrolyte membranes prepared by radiation and thermal crosslinking techniques

Novel one-step preparation of polymer electrolyte membranes without a membrane casting process is achieved by radiation crosslinking of a polyetheretherketone (PEEK) film to prevent dissolution and deformation of the original film in sulfonating solutions. The films crosslinked with doses more than 33 MGy can be effectively sulfonated in a chlorosulfonic solution, resulting in a crosslinked sulfonated PEEK (sPEEK) electrolyte membrane with high proton conductivity comparable to Nafion. Nevertheless, its water uptake was high for application in fuel cells. The thermal treatment was effective for further crosslinking of the membrane; as a result, the water uptake and methanol permeability of the double crosslinked sPEEK membranes drastically decreased, compensating for a slight decrease of proton conductivity. In addition, unlike the traditional cast sPEEK membrane showing the irreversible swelling in hot water, the double crosslinked sPEEK membranes exhibited excellent stability toward 100 °C hot water for more than 200 h without any decrease in proton conductivity, and had the mechanical and thermal properties superior to those of Nafion.     

Studies on electrochemical properties of MWNTs-Nafion composite films based on the redox behavior of incorporated Eu by voltammetry and electrochemical impedance spectroscopy

MWNTs can be conveniently dispersed in Nafion solution on the basis of the special interactions between the sidewall of MWNTs and the hydrophobic domains of Nafion. Casting of the resulting mixture on electrode surfaces produced uniform composite films having wide electroanalytical applications. In this work, the electrochemical properties of the MWNTs-Nafion composite film on a glassy carbon electrode were systematically investigated by various electrochemical methods using incorporated europium(III) ions (Eu³⁺) as the probe. The voltammetric studies showed that the increase of MWNTs concentration in the composite film could effectively improve the redox currents of Eu³⁺ and reduce the peak separation, whereas the increase of Nafion concentration generally increased both the redox currents and the peak separation. These results suggested the different roles of MWNTs and Nafion in the composite films. The electrochemical impedance spectroscopic (EIS) investigations showed that MWNTs mainly contributed to the charge transfer and mass transfer processes of the composite film through the increases of the electrode/electrolyte interfacial area and the film porosity while Nafion generally dominated the mass transport from the solution into the film via ion exchange. The potential application of the sensitive response of Eu³⁺ at the MWNTs-Nafion composite film in electroanalytical chemistry was evaluated. In the range of 0.04-100 μM, the concentration of Eu³⁺ showed excellent linear relationships with the differential pulse voltammetric response with a low detection limit of 10 nM (S/N = 3) for 60 s accumulation at −0.1 V.     

Modification of Nafion membranes with ternary composite materials for direct methanol fuel cells

Composite membranes for direct methanol fuel cells (DMFCs) were prepared by using Nafion115 membrane modification with polyvinyl alcohol (PVA), polyimide (PI) and 8-trimethoxysilylpropyl glycerin ether-1,3,6-pyrenetrisulfonic acid (TSPS). The performance of the composite membranes was evaluated in terms of water sorption, dimensional stability, thermal stability, proton conductivity, methanol permeability and cell performance. The proton conductivity was slightly decreased by 1-3% compared with Nafion115, which still kept the high proton conduction of Nafion115. The methanol permeability of Nafion/PI-PVA-TSPS composite membranes was remarkably reduced by 35-55% compared with Nafion115. The power density of DMFCs with Nafion/PI-PVA-TSPS composite membranes reached to 100 mW/cm², exceeding that with Nafion115 (68m W/cm²).     

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.     

Effect of operating conditions on energy efficiency for a small passive direct methanol fuel cell

Energy conversion efficiency was studied in a direct methanol fuel cell (DMFC) with an air-breathing cathode using Nafion 117 as electrolyte membrane. The effect of operating conditions, such as methanol concentration, discharge voltage and temperature, on Faradic and energy conversion efficiencies was analyzed under constant voltage discharge with quantitative amount of fuel. Both of Faradic and energy conversion efficiencies decrease significantly with increasing methanol concentration and environmental temperature. The Faradic conversion efficiency can be as high as 94.8%, and the energy conversion efficiency can be as high as 23.9% if the environmental temperature is low enough (10 °C) under constant voltage discharge at 0.6 V with 3 M methanol for a DMFC bi-cell. Although higher temperature and higher methanol concentration can achieve higher discharge power, it will result in considerable losses of Faradic and energy conversion efficiencies for using Nafion electrolyte membrane. Development of alternative highly conductive membranes with significantly lower methanol crossover is necessary to avoid loss of Faradic conversion efficiency with temperature and with fuel concentration.     

Preparation and characterization of multilayer-type electrolyte membranes with hydrocarbon polymers

Multilayer-type polymer electrolyte membranes composed of a sulfonated poly(4-phenoxybenzoyl-1,4-phenylene) (S-PPBP) layer and a mono[poly(propylene oxide)methacrylate]phosphate ester (PPHP) layer were fabricated by solution-casting procedure (Method 1) and hot-pressing procedure (Method 2) in order to suppress methanol permeability of electrolyte membranes. No delamination was observed by SEM measurements of S-PPBP/PPHP interfaces, indicating that PPHP had good adhesive properties to S-PPBP surfaces. The methanol permeability of S-PPBP/PPHP membranes was lower than that of S-PPBP membranes and decreased with increasing the thickness of PPHP layers. The bilayer membrane with 12 μm PPHP and 40 μm S-PPBP layers showed a methanol permeability of 2.97 × 10⁻⁷ cm² s⁻¹ in 1 mol dm⁻³ methanol aqueous solution at 25 °C, which was 13% less than that of the S-PPBP membranes. The conductivity of this membrane reached its optimum with values as high as 1.57 × 10⁻¹ S cm⁻¹ at 80 °C and 90%RH.     

Assembly of 8YSZ nanoparticles into gas-tight 1-2 μm thick 8YSZ electrolyte layers using wet coating methods

The application of a thin film electrolyte layer with a thickness in the micrometer range could greatly improve current solid oxide fuel cells (SOFCs) in terms of operating temperature and power output. Since the achievable minimal layer thickness with conventional powder coating methods is limited to ∼5 μm, a variety of thin film methods have been studied, but results on regular large-scale anode substrates are still lacking in the literature. In this paper, a wet coating process is presented for fabricating gas-tight 1-2 μm thick 8YSZ electrolyte layers on a regular NiO/8YSZ substrate, with a rough surface, a high porosity and a large pore size. These layers were deposited in a similar way as conventional suspension based layers, but the essential difference includes the use of coating liquids (nano-dispersion, sol) with a considerably smaller particle size (85 nm, 60 nm, 35 nm, 6 nm). Successful deposition of such layers was accomplished by means of an innovative coating process, which involves the preparation of a hybrid polyvinyl alcohol/8YSZ membrane by dip-coating or spin-coating and subsequently burning out the polymer part at 500 °C. Results from He leak tests confirmed that the sintered layers posses a very low number of defects and with values in the range 10⁻⁴-10⁻⁶ (hPa dm³)/(s cm²) the gas-tightness of the thin film layers is satisfactory for fuel cell operation. Moreover, preliminary results have also indicated a potential reduction of the sintering temperature from 1400 °C to the range 1200-1300 °C, using the presented coating process.     

Preparation and properties of PVDF-fabric composite membrane for membrane distillation

The flat sheet PVDF-fabric composite membrane used for membrane distillation was prepared by coating and wet phase inversion process. The composite membrane consisted of a PVDF porous membrane layer and a fabric layer. The thin polyester filament woven fabric with water-and-oil repellent finish was used as the support of composite membrane. The effects of fabric texture, PVDF concentration in casting solution and functional finishing of fabric on the preparation and properties of the composite membrane were investigated. The experimental results showed that fabric texture, PVDF concentration and functional finishing of fabric had great influence on the preparation and properties of the composite membrane. When the PVDF concentration in casting solution was 10-12% and the support fabric, with 435 warps/10 cm and 273 wefts/10 cm and the area weight of 79 g/m², was finished with 2 g/L water-and-oil repellent agent FK-501, the prepared composite membrane exhibited better performance in tensile strength, peeling strength and water vapor permeability, with mean pore size of 0.63 μm and overall porosity of 57.6%.     

High temperature operation of a composite membrane-based solid polymer electrolyte water electrolyser

The high temperature behaviour of a solid polymer electrolyte (SPE) water electrolyser based on a composite Nafion-SiO₂ membrane was investigated and compared to that of a commercial Nafion membrane. The SPE water electrolyser performance was studied from 80 to 120 °C with an operating pressure varying between 1 and 3 bar abs. IrO₂ and Pt were used as oxygen and hydrogen evolution catalysts, respectively. The assemblies were manufactured by using a catalyst-coated membrane (CCM) technique. The performance was significantly better for the composite Nafion-SiO₂ membrane than commercial Nafion 115. Furthermore, the composite membrane allowed suitable water electrolysis at high temperature under atmospheric pressure. The current densities were 2 and 1.2 A cm⁻² at a terminal voltage of 1.9 V for Nafion-SiO₂ and Nafion 115, respectively, at 100 °C and atmospheric pressure. By increasing the temperature up to 120 °C, the performance of Nafion 115 drastically decreased; whereas, the cell based on Nafion-SiO₂ membrane showed a further increase of performance, especially when the pressure was increased to 3 bar abs (2.1 A cm⁻² at 1.9 V).     

Performance improvement of Nafion based solid state electrochemical supercapacitor

An electrochemical supercapacitor in all solid configuration using perfluorosulfonate ionomer as polymer electrolyte has been successfully realized. Electrodes of supercapacitor have been prepared using activated carbon material and Nafion ionomer. This latter had the double function of binder and electrolyte. Nafion 115 membrane has been used as electrolyte separator in the preparation of small scale supercapacitors. The capacitance performance of these devices is comparable or better than traditional systems, which use sulfuric acid as electrolyte. The electrochemical evaluation of studied supercapacitor has been carried out by cyclic voltammetry, dc charge/discharge measurements and electrochemical impedance spectroscopy. A capacitance of 90 F/g (referred to the weight of active carbon material in the electrode) has been obtained with carbon having surface area (SA) of about 1000 m²/g and, a capacitance of 130 F/g with activated carbon having SA of 1500 m²/g. These interesting results have been tentatively explained with an optimal configuration of electrodes and with the concomitant beneficial effects on the carbon pores of adsorbed water and Nafion distribution, which produce low distribute resistance in the carbon composite electrodes.     

Effect of current collector corrosion made from printed circuit board (PCB) on the degradation of self-breathing direct methanol fuel cell stack

Adopting printed circuit board (PCB) as the current collector for direct methanol fuel cell (DMFC) is a promising task. This paper found that the combinational 10 μm Ni/2 μm Au coatings on PCB could not endure halide attacking in its anodic polarization. With this coated PCB, the self-breathing DMFC stack with passive methanol supply exhibited progressively degradation in its lifetime test. By using a series of measurements, it is shown that initial corrosion would not obviously influence stack degradation, but the subsequently inter-facilitation, between F⁻ originating from Nafion membrane and corrosion products of Ni²⁺ and Cu²⁺ from PCB, would result in serious degradation, the final stack invalidation would present accompanied with the loss of membrane conductivity. The defects on coated PCB are crucial to initiate localized corrosion and stack degradation. Although cyclic voltammetry (CV) technology is effective to recover cell temporarily, its damage by releasing F⁻ should be further evaluated.     

Nanometer-thick films of titanium oxide acting as electrolyte in the polymer electrolyte fuel cell

0-18 nm-thick titanium, zirconium and tantalum oxide films are thermally evaporated on Nafion 117 membranes, and used as thin spacer electrolyte layers between the Nafion and a 3 nm Pt catalyst film. Electrochemical characterisation of the films in terms of oxygen reduction activity, high frequency impedance and cyclic voltammetry in nitrogen is performed in a fuel cell at 80 °C and full humidification. Titanium oxide films with thicknesses up to 18 nm are shown to conduct protons, whereas zirconium oxide and tantalum oxide block proton transport already at a thickness of 1.5 nm. The performance for oxygen reduction is higher for a bi-layered film of 3 nm platinum on 1.5 or 18 nm titanium oxide, than for a pure 3 nm platinum film with no spacer layer. The improvement in oxygen reduction performance is ascribed to a higher active surface area of platinum, i.e. no beneficial effect of combining platinum with zirconium, tantalum or titanium oxides on the intrinsic oxygen reduction activity is seen. The results suggest that TiO₂ may be used as electrolyte in fuel cell electrodes, and that low-temperature proton exchange fuel cells could be possible using TiO₂ as electrolyte.     

Proton exchange membranes based on sulfonated crosslinked polystyrene micro particles dispersed in poly(dimethyl siloxane)

Proton exchange membranes of sulfonated crosslinked polystyrene (SXLPS) particles dispersed in crosslinked poly(dimethyl siloxane) matrix were investigated. Three different sizes of particles—25, 8 and 0.08 μm—were used at loadings from 0 to 50 wt% and the influence of these variables on the water and methanol uptake and proton conductivity were observed. With the reduction in particle size in the composite membrane, more water or methanol uptake was observed. Three different states of water were revealed in the composite membranes by differential scanning calorimetry (DSC). The number of bound water molecules per SO₃H group was 11-15 in membranes with 8- and 25-μm SXLPS. The ratio of bound to unbound water molecules was more than one in these membranes, whereas it was less than one in membranes with 0.08-μm SXLPS. The proton conductivities of the membranes increased with the increase in particle loading. At particle loadings above 35 wt%, membranes containing 8-μm SXLPS had higher conductivity compared to 25-μm SXLPS at room temperature. The conductivity of membranes containing 0.08-μm SXLPS was restricted to 10⁻³ S/cm because of the inherently low IEC of the particles. Increasing the temperature from 30 to 80 °C drastically enhanced the conductivity of the composite membranes compared to Nafion^® 112. At 80 °C, conductivities as high as 0.11±0.04 S/cm were observed for membranes containing more than 30 wt% of 25-μm SXLPS particles.     

Nafion-TiO2 composite DMFC membranes: physico-chemical properties of the filler versus electrochemical performance

TiO₂ nanometric powders were prepared via a sol-gel procedure and calcined at various temperatures to obtain different surface and bulk properties. The calcined powders were used as fillers in composite Nafion membranes for application in high temperature direct methanol fuel cells (DMFCs). The powder physico-chemical properties were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS) and pH measurements. The observed characteristics were correlated to the DMFC electrochemical behaviour. Analysis of the high temperature conductivity and DMFC performance reveals a significant influence of the surface characteristics of the ceramic oxide, such as oxygen functional groups and surface area, on the membrane electrochemical behaviour. A maximum DMFC power density of 350 mW cm⁻² was achieved under oxygen feed at 145 °C in a pressurized DMFC (2.5 bar, anode and cathode) equipped with TiO₂ nano-particles based composite membranes.     

The study of PTFE/Nafion/Silicate membranes operating at low relative humidity and elevated temperature

The performance and stability of PTFE/Nafion/Silicate composite membranes (PNS membrane) were studied at low and medium operating temperatures with different humidity, and compared with the Nafion112 membrane at the same conditions. The PNS membrane was prepared by impregnation of PTFE/Nafion composite membrane via sol-gel process with TEOS (tetraethoxysilane). When operated cell at low temperature of 60 °C with 100% R.H. humidified H₂/O₂ gases, the PNS membrane performs better than Nafion112, with 1.0, and 0.4 W/cm², respectively. When operated cell at 60 °C with 37% R.H. humidified gases, the discharge stability of PNS membrane is stable than that of Nafion112, this is due to that silicate could hold more water in the PNS membrane at low relative humidity. While the inlet of cell gases temperature keeps at 80 °C, the cell temperature varied 90, 100, and 110 °C, with 20 psig back pressure, their relative humidities were 67, 48 and 33%, respectively. The stability of discharge current remains constant except in the case of cell temperature being as high as 110 °C. It is believed that silicate could hold water except in the case of cell temperature at 110 °C, which is resulted as the membrane dehydration. On the other hand, the Nafion112 cannot operate at low humidity with cell temperature higher than 80 °C owing to membrane dehydration. The silica modified PTFE/Nafion membrane shows the improving cell performance at lower relative humidity due to adsorbed water inside the membrane and catalyst layer.     

Nafion 115/zirconium phosphate composite membranes for operation of PEMFCs above 100 °C

Composite Nafion/zirconium phosphate membranes were investigated for high temperature operation of proton exchange membrane fuel cells (PEMFCs). The composite membranes were prepared via impregnation of Nafion films (either commercial Nafion 115 or recast Nafion) with zirconyl chloride and 1 M phosphoric acid at 80 °C. An MEA employing a composite membrane prepared starting from commercial Nafion 115 gave a H₂/O₂ PEMFC performance of about 1000 mA/cm² at 0.45 V at a temperature of 130 °C and a pressure of 3 bar; this result compares very favorably with the performance of an MEA based on commercial unmodified Nafion, which gave only 250 mA/cm² at 0.45 V when operated under the same conditions of temperature and pressure. Similar experiments performed with recast Nafion and recast Nafion/zirconium phosphate composites confirmed an analogous improvement of performance of the composite membranes over the unimpregnated ones. In this case, the composite recast Nafion/zirconium phosphate gave about 1500 mA/cm² at 0.45 V at a temperature of 130 °C and a pressure of 3 bar. The composite membranes showed stable behavior during time when maintained at 130 °C, while irreversible degradation affected Nafion under the same conditions.     

Electrical and stability performance of anode-supported solid oxide fuel cells with strontium- and magnesium-doped lanthanum gallate thin electrolyte

Anode-supported solid oxide fuel cells (SOFCs) comprising NiO-samarium-doped ceria (SDC) (Sm_0.2Ce_0.8O_1.9) composite anode, thin tri-layer electrolyte, and La_0.6Sr_0.4Co_0.8Fe_0.2O₃ (LSCF)-La_0.9Sr_0.1Ga_0.8Mg_0.2O_3−δ (LSGM) composite cathode were fabricated. The thin tri-layer consisting of an 11-μm thick LSGM electrolyte layer and a 12-μm thick La_0.4Ce_0.6O_1.8 (LDC) layer on each side of the LSGM was prepared by centrifugal casting and co-firing technique. The performance of the cells operated with humidified H₂ as fuel and ambient air as oxidant showed a maximum power density of 1.23 W cm⁻² at 800 °C. A stability test of about 100 h was carried out and some deterioration of output power was observed, while the open circuit voltage (OCV) kept unchanged. Impedance measurements showed that both the electrolyte ohmic resistance and the electrode polarization increased with time and the latter dominated the degradation.     

Synthesis and characterization of Nafion-stabilized Pt nanoparticles for polymer electrolyte fuel cells

Platinum nanoparticles are synthesized by alcohol reduction method using Nafion as a stabilizer under various conditions such as the Nafion/Pt molar ratio and reflux temperature. Nafion-Pt nanoparticles are characterized by agglomeration and the particle size is typically in the range of 2-4 nm. The electrocatalytic activity of Nafion-Pt nanoparticles for polymer electrolyte and direct methanol fuel cells (PEFCs and DMFCs) is investigated in comparison to that of unsupported Pt black and carbon-supported Pt/C electrocatalysts. Nafion-Pt nanoparticles prepared with low Nafion/Pt ratios show higher and/or comparable activities towards O₂ reduction reaction in the absence and presence of methanol in comparison to that of Pt black and Pt/C electrocatalysts. In contrast, the electrocatalytic activity of the Nafion-Pt nanoparticles for the methanol oxidation reaction is very low. The results indicate that Pt nanoparticles embedded in Nafion polyelectrolyte are potential methanol tolerant electrocatalysts for the O₂ reduction reaction in DMFCs.     

Synthesis, proton conductivity and methanol permeability of a novel sulfonated polyimide from 3-(2′,4′-diaminophenoxy)propane sulfonic acid

A novel sulfonated diamine monomer, 3-(2′,4′-diaminophenoxy)propane sulfonic acid (DAPPS), was successfully synthesized and the sulfonated polyimide (SPI) was prepared from 1,4,5,8-naphthalenetetracarboxylic dianhydride (NTDA) and DAPPS. The resulting SPI, NTDA-DAPPS, was soluble in common organic solvents. The SPI membrane displayed proton conductivity σ values of 0.12-0.35 S/cm at temperatures ranging from 35 to 90 °C in liquid water, which were similar to or higher than those of Nafion 117 and sulfonated hydrocarbon polymers. The σ of the SPI membrane decreased significantly with decreasing relative humidity (RH) and became much lower than that of Nafion 117 at 30% RH. The SPI membrane displayed good water stability at 80 °C and was thermally stable up to 240 °C. It showed reasonable mechanical strength of a modulus of 1.3 GPa at 90 °C and 90% RH. Its methanol permeability P_M was 0.57×10⁻⁶ cm²/s at 30 °C and 8.6 wt% methanol in feed, which was a fourth of that of Nafion 117. As a result, its ratio of σ/P_M was 21×10⁴ S cm⁻³ s, which was about 4 times larger than that of Nafion 117, suggesting potential application of the SPI membrane for direct methanol fuel cell.     

Fabrication and characterization of a novel Nafion-PTFE composite hollow fiber membrane

Nafion has been widely used in electrochemistry, but there are only a few reports on its application in other fields, such as, gas separation, even though it exhibits good performance. The primary reason for that is the high cost of Nafion and making a composite membrane with a thin Nafion layer is a potential solution to solve this problem. In this study, a novel Nafion-PTFE composite hollow fiber membrane, which had a thin (~5 μm) and detect-free Nafion layer on PTFE surface, without Nafion filling substrate pores was developed, differing from the reported ones in which Nafion resin was required to impregnate into porous PTFE membrane as thorough as possible to ensure the ion conductivity and operation stability. The surface morphology, crystallite, solubility in ethanol/water mixture, and water uptake of membranes were systemically investigated. The gas permeance tests were also conducted. The permeances of different gasses of prepared composite membranes were significantly enhanced compared with the commercial membranes due to the decrease of Nafion thicknesses, while the selectivity remained the same, verifying the detect-free structure of Nafion layer on PTFE substrate. This study provided a good reference for the preparation and application of low-cost Nafion composite membranes.