Pulsed plasma-polymerized alkaline anion-exchange membranes for potential application in direct alcohol fuel cells

Pulsed plasma polymerization is adopted to synthesize alkaline anion-exchange membranes (AAEMs) with high contents of functional groups. The attenuated total reflection Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy and thermo gravimetric analysis demonstrate that the benzyltrimethylammonium cationic groups can be successfully introduced into the polymer matrix. The content of the quaternary nitrogen in pulsed plasma-polymerized membrane is up to 1.93 atom%. The ultra-flat and undamaged morphology structure of the AAEMs indicates a low plasma ablation effect in the pulsed plasma polymerization. The excellent properties of the pulsed plasma-polymerized AAEMs, including good adhesion to the substrate, acceptable chemical stability and thermal stability, high ion-exchange capacity (1.42 mmol g⁻¹) and water uptake (59.73 wt%), interesting ionic conductivity (0.0205 S cm⁻¹ in deionized water at 20 °C) and ethanol permeability (3.37 × 10⁻¹¹ m² s⁻¹), suggest a great potential for application in direct alcohol fuel cells.     

Performance of air-breathing direct methanol fuel cell with anion-exchange membrane

This report details development of an air-breathing direct methanol alkaline fuel cell with an anion-exchange membrane. The commercially available anion-exchange membrane used in the fuel cell was first electrochemically characterized by measuring its ionic conductivity, and showed a promising result of 1.0 × 10⁻¹ S cm⁻¹ in a 5 M KOH solution. A laboratory-scale direct methanol fuel cell using the alkaline membrane was then assembled to demonstrate the feasibility of the system. A high open-circuit voltage of 700 mV was obtained for the air-breathing alkaline membrane direct methanol fuel cell (AMDMFC), a result about 100 mV higher than that obtained for the air-breathing DMFC using a proton exchange membrane. Polarization measurement revealed that the power densities for the AMDMFC are strongly dependent on the methanol concentration and reach a maximum value of 12.8 mW cm⁻² at 0.3 V with a 7 M methanol concentration. A durability test for the air-breathing AMDMFC was performed in chronoamperometry mode (0.3 V), and the decay rate was approximately 0.056 mA cm⁻² h⁻¹ over 160 h of operation. The cell area resistance for the air-breathing AMDMFC was around 1.3 Ω cm² in the open-circuit voltage (OCV) mode and then is stably supported around 0.8 Ω cm² in constant voltage (0.3 V) mode.     

Quaternized cardo polyetherketone anion exchange membrane for direct methanol alkaline fuel cells

Quaternized cardo polyetherketone (QPEK-C) membranes for alkaline fuel cells were prepared via chloromethylation, quaternization and alkalization of cardo polyetherketone (PEK-C). The chemical reaction for PEK-C modification was confirmed by nuclear magnetic resonance (¹H NMR) and energy-dispersive X-ray spectroscopy (EDAX). The QPEK-C membrane was characterized by X-ray photoelectron spectroscopy (XPS) and thermo gravimetric analysis (TG). The ion-exchange content (IEC), water and methanol uptakes, methanol permeability and conductivity of the QPEK-C membranes were measured to evaluate their applicability in alkaline methanol fuel cells. The ionic conductivity of the QPEK-C membrane varied from (1.6 to 5.1) × 10⁻³ S cm⁻² over the temperature range 20-60 °C. The QPEK-C membrane showed excellent methanol resistance. When the concentration of methanol was 4 M, the methanol permeability was less than 10⁻⁹ cm² s⁻¹ at 30 °C.     

Effect of hydroxide and carbonate alkaline media on anion exchange membranes

The effect of hydroxide and carbonate alkaline environments on the chemical stability and ionic conductivity of five commercially available anion exchange membranes was investigated. Exposure of the membranes to concentrated hydroxide environments (1 M) had a detrimental effect on ionic conductivity with time. Over a 30-day period, decreases in conductivity ranged from 27% to 6%, depending on the membrane. The decrease in ionic conductivity is attributed to the loss of stationary cationic sites due to the Hofmann elimination and nucleophilic displacement mechanisms. Exposure of the membranes to low concentration hydroxide (10⁻⁴ M) or carbonate/bicarbonate (0.5 M Na₂CO₃/0.5 M NaHCO₃) environments had no measurable effect on the ionic conductivity over a 30-day period. ATR-FTIR spectroscopy confirmed degradation of membranes soaked in 1 M KOH. Apparition of a doublet peak in the region between 1600 cm⁻¹ and 1675 cm⁻¹ confirms formation of carbon-carbon double bonds due to Hofmann elimination. Membranes soaked in mild alkaline environments did not show formation of carbon-carbon double bonds.     

Durability study of KOH doped polybenzimidazole membrane for air-breathing alkaline direct ethanol fuel cell

Recently, KOH doped polybenzimidazole (PBI/KOH) membrane has been reported as polymer electrolyte membrane for alkaline direct alcohol fuel cell (ADAFC), but little is known about its durability for ADAFC application. In this paper, the durability of PBI/KOH membrane for air-breathing alkaline direct ethanol fuel cell (ADEFC) is evaluated by means of ex situ and in situ tests. In the case of ex situ durability test, the ionic conductivity of PBI/KOH degrades from initial 0.023 S cm⁻¹ to 0.01 S cm⁻¹ after 100 h, and the degrading rate was 1.3 × 10⁻⁴ S cm⁻¹ h⁻¹. As for in situ test, Pt-free air-breathing ADEFC with PBI/KOH membrane can output a peak power density of 16 mW cm⁻² at 60 °C. Moreover, it can stably operate for 336 h above 0.3 V. In addition, the interaction between KOH and PBI matrix is also explored by density functional theory study.     

Measurements of water uptake and transport properties in anion-exchange membranes

This paper reports on the measured water uptake and water transport properties through an anion-exchange membrane, including the water diffusivity, the electro-osmotic drag (EOD) coefficient, and the mass-transfer coefficient of water at the cathode catalyst layer/membrane interface. The water uptake of the membrane in equilibrium with liquid water and water vapor at different values of relative humidity is measured at 30, 40 and 60 °C. The measured water uptake at each measured temperature is correlated in terms of the relative humidity. Like in Nafion^® membranes, the Schroeder’s paradox phenomenon is also found to exist in the anion-exchange membrane. The water diffusivity of the anion-exchange membrane shows the same order of 10⁻¹⁰ m² s⁻¹ as that of the Nafion^® membrane. The measured mass-transfer coefficient of water at the cathode catalyst layer/membrane interface falls in the range of 1.0 × 10⁻⁶ to 1.0 × 10⁻⁵ m s⁻¹. The EOD coefficients measured at 30 °C and 40 °C are, respectively, 2.3 and 3.6.     

A direct borohydride fuel cell based on poly(vinyl alcohol)/hydroxyapatite composite polymer electrolyte membrane

A new poly(vinyl alcohol)/hydroxyapatite (PVA/HAP) composite polymer membrane was synthesized using a solution casting method. Alkaline direct borohydride fuel cells (DBFCs), consisting of an air cathode based on MnO₂/C inks on Ni-foam, anodes based on PtRu black and Au catalysts on Ni-foam, and the PVA/HAP composite polymer membrane, were assembled and investigated for the first time. It was demonstrated that the alkaline direct borohydride fuel cell comprised of this low-cost PVA/HAP composite polymer membrane showed good electrochemical performance. As a result, the maximum power density of the alkaline DBFC based on the PtRu anode (45 mW cm⁻²) proved higher than that of the DBFC based on the Au anode (33 mW cm⁻²) in a 4 M KOH + 1 M KBH₄ solution at ambient conditions. This novel PVA/HAP composite polymer electrolyte membrane with high ionic conductivity at the order of 10⁻² S cm⁻¹ has great potential for alkaline DBFC applications.     

Anionic membrane and ionomer based on poly(2,6-dimethyl-1,4-phenylene oxide) for alkaline membrane fuel cells

Hydroxyl-ion conductive poly(2,6-dimethyl-1,4-phenylene oxide) (PPO) membranes with different characteristics were prepared via relatively simple bromination/amination serial reactions with reduced number of involved chemicals and shorter reaction time. The effects of reactants ratio, reaction atmosphere, polymer concentration, casting solvent, and hydroxylation treatment on reaction were investigated in details. The microstructure, water uptake, swelling ratio, ion-exchange capacity and ionic conductivity of the membranes were also studied. The obtained results demonstrate that, the ionic conductivity of the membrane is dependent on casting solvent. The N-methyl-2-pyrrolidonecast membrane exhibits the highest conductivity with the thinnest film. Although the membrane was prepared via a relatively simple preparation route with least toxic chemicals, a competitive ionic conductivity value of 1.64 × 10⁻² S cm⁻¹ was achieved at 60 °C. A power density of 19.5 mW cm⁻² has been demonstrated from the alkaline membrane fuel cell operated at 70 °C, assembled from the entirely homemade membrane electrode assembly without any hot-pressing.     

Performance of a direct ethylene glycol fuel cell with an anion-exchange membrane

This paper reports on the development and performance test of an alkaline direct ethylene glycol fuel cell. The fuel cell consists of an anion-exchange membrane with non-platinum electrocatalysts at both the anode and cathode. It is demonstrated that this type of fuel cell with relatively cheap membranes and catalysts can result in a maximum power density of 67 mW cm⁻² at 60 °C, which represents the highest performance that has so far been reported in the open literature. The high performance is mainly attributed to the increased kinetics of both the ethylene glycol oxidation reaction and oxygen reduction reaction rendered by the alkaline medium with the anion-exchange membrane.     

In and ex situ characterization of an anion-exchange membrane for alkaline direct methanol fuel cell (ADMFC)

Anion exchange membrane fumasep^® FAA-2 was characterized with ex and in situ methods in order to estimate the membranes’ suitability as an electrolyte for an alkaline direct methanol fuel cell (ADMFC). The interactions of this membrane with water, hydroxyl ions and methanol were studied with both calorimetry and NMR and compared with the widely used proton exchange membrane Nafion^® 115. The results indicate that FAA-2 has a tighter structure and more homogeneous distribution of ionic groups in contrast to the clustered structure of Nafion, moreover, the diffusion of OH⁻ ions through this membrane is clearly slower compared to water molecules. The permeability of methanol through the FAA-2 membrane was found to be an order of magnitude lower than for Nafion. Fuel cell experiments in 1 mol dm⁻³ methanol with FAA-2 resulted in OCV of 0.58 V and maximum power density of 0.32 mW cm⁻². However, even higher current densities were obtained with highly concentrated fuels. This implies that less water is needed for fuel dilution, thereby decreasing the mass of the fuel cell system. In addition, electrochemical impedance spectroscopy for the ADMFC was used to determine ohmic resistance of the cell facilitating the further membrane development.     

Study of poly(vinyl alcohol)/titanium oxide composite polymer membranes and their application on alkaline direct alcohol fuel cell

The novel poly(vinyl alcohol)/titanium oxide (PVA/TiO₂) composite polymer membrane was prepared using a solution casting method. The characteristic properties of the PVA/TiO₂ composite polymer membrane were investigated by thermal gravimetric analysis (TGA), a scanning electron microscopy (SEM), a micro-Raman spectroscopy, a methanol permeability measurement and the AC impedance method. An alkaline direct alcohol (methanol, ethanol and isopropanol) fuel cell (DAFC), consisting of an air cathode based on MnO₂/C inks, an anode based on PtRu (1:1) black and a PVA/TiO₂ composite polymer membrane, was assembled and examined for the first time. The results indicate that the alkaline DAFC comprised of a cheap, non-perfluorinated PVA/TiO₂ composite polymer membrane shows an improved electrochemical performances. The maximum power densities of alkaline DAFCs with 4 M KOH + 2 M CH₃OH, 2 M C₂H₅OH and 2 M isopropanol (IPA) solutions at room temperature and ambient air are 9.25, 8.00, and 5.45 mW cm⁻², respectively. As a result, methanol shows the highest maximum power density among three alcohols. The PVA/TiO₂ composite polymer membrane with the permeability values in the order of 10⁻⁷ to 10⁻⁸ cm² s⁻¹ is a potential candidate for use on alkaline DAFCs.     

Novel silica/poly(2,6-dimethyl-1,4-phenylene oxide) hybrid anion-exchange membranes for alkaline fuel cells: Effect of silica content and the single cell performance

Poly(2,6-dimethyl-1,4-phenylene oxide) (PPO)-based organic-inorganic hybrid alkaline membranes with enhanced hydroxyl (OH⁻) conductivity are prepared in response to the relatively low conductivity of previously reported PPO-based systems. The membranes also exhibit higher swelling-resistant properties and the hydroxyl (OH⁻) conductivity values are comparable to previously reported fluoropolymer-containing membranes: 0.012-0.035 S cm⁻¹ in the temperature range 30-90 °C. Other favorable properties for fuel cell application include high tensile strengths up to 25 MPa and large ion-exchange capacities in the range 2.01-2.27 mmol g⁻¹. Beginning-of-life fuel cell testing of a membrane with a thickness of 140 μm yielded an acceptable H₂/O₂ peak power density of 32 mW cm⁻² when incorporated into an alkaline membrane electrode assembly. Therefore, this class of hybrid membrane is suitable for application in alkaline membrane fuel cells.     

Direct methanol fuel cell (DMFC) based on PVA/MMT composite polymer membranes

A novel composite polymer electrolyte membrane composed of a PVA polymer host and montmorillonite (MMT) ceramic fillers (2–20 wt.%), was prepared by a solution casting method. The characteristic properties of the PVA/MMT composite polymer membrane were investigated using thermal gravimetric analysis (TGA), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), scanning electron microscopy (SEM), and micro-Raman spectroscopy, and the AC impedance method. The PVA/MMT composite polymer membrane showed good thermal and mechanical properties and high ionic conductivity. The highest ionic conductivity of the PVA/10 wt.%MMT composite polymer membrane was 0.0368 S cm⁻¹ at 30 °C. The methanol permeability (P) values were 3–4 × 10⁻⁶ cm² s⁻¹, which was lower than that of Nafion 117 membrane of 5.8 × 10⁻⁶ cm² s⁻¹. It was revealed that the addition of MMT fillers into the PVA matrix could markedly improve the electrochemical properties of the PVA/MMT composite membranes; which can be accomplished by a simple blend method. The maximum peak power density of the DMFC with the PtRu anode based on Ti-mesh in a 2 M H₂SO₄ + 2 M CH₃OH solution was 6.77 mW cm⁻² at ambient pressure and temperature. As a result, the PVA/MMT composite polymer appears to be a good candidate for the DMFC applications.     

Effect of morphology of mesoporous silica on characterization of protic ionic liquid-based composite membranes

Effects caused by the morphology of mesoporous silica on the characterization of protic ionic liquid-based composite membranes for anhydrous proton exchange membrane applications are investigated. Two types of SBA15 materials with platelet and fiberlike morphologies are synthesized and incorporated into a mixture of polymerizable monomers together with an ionic liquid (IL) [1-butyl-3-methylimidazolium bis(trifluoromethane sulfone)imide (BMIm-TFSI)] to form new conducting membranes using an in situ photo crosslinking process. Incorporation of a defined amount of fiber-shaped SBA 15 and platelet 15 significantly increases the ionic conductivity to between two and three times that of a plain poly(methyl methacrylate) (PMMA)/IL membrane (2.3 mS cm⁻¹) at 160 °C. The protic ionic liquid (PIL) retention ability of the membranes is increased by the capillary forces introduced by the mesoporous silica materials, while ionic conductivity loss after leaching test is retarded. The highest ionic conductivity (5.3 mS cm⁻¹) is obtained by incorporating 5 wt% of P-SBA 15 in the membrane to about six times that of plain PMMA/IL membrane (0.9 mS cm⁻¹) at 160 °C after leaching test.     

Alkaline direct oxidation fuel cell with non-platinum catalysts capable of converting glucose to electricity at high power output

Glucose is a potential fuel for fuel cells because it is renewable, abundant, non-toxic, and easy in handle and store. Conventional glucose fuel cells that use enzymes and micro-organisms as the catalyst are limited by their extremely low power output and rather short durability. In this work, a direct glucose fuel cell that uses an anion-exchange membrane and in-house non-platinum electrocatalysts is developed. It is shown that this type of direct glucose fuel cell with a relatively cheap membrane and catalysts can result in a maximum power density as high as 38 mW cm⁻² at 60 °C. The high performance is attributed mainly to the increased kinetics of both the glucose oxidation reaction and the oxygen reduction reaction rendered by the alkaline medium with the anion-exchange membrane.     

Proton conducting CS/P(AA-AMPS) membrane with reduced methanol permeability for DMFCs

A novel polyelectrolyte complex (PEC) membrane for direct methanol fuel cells (DMFCs) was prepared by blending a cationic polyelectrolyte, chitosan (CS), with an anionic polyelectrolyte, acrylic acid-2-acrylamido-2-methylpropane sulfonic acid copolymer (P(AA-AMPS)). The presence of –NH₃⁺ species detected by X-ray photoelectron spectroscopy (XPS) revealed that an ionic cross-linked interpenetrating polymer network (IPN) was formed between the two polyelectrolyte polymers. Methanol permeability and proton conductivity were measured and compared with the Nafion^®117 membrane. The dual function of P(AA-AMPS) as both an ionic crosslinker and a proton conductor led to not only a notable reduction in methanol permeability but also an increase in proton conductivity. The CS/P(AA-AMPS) membrane with a P(AA-AMPS) content of 41 wt.% exhibited a methanol permeability (P) of 2.41 × 10⁻⁷ cm² s⁻¹ which was fifteen times lower than that of the Nafion^®117 membrane, whereas its proton conductivity (σ) was comparatively high (3.59 × 10⁻² S cm⁻¹). In terms of the overall selectivity index (β = σ/P), the PEC membrane showed a remarkably higher selectivity than the Nafion^®117 membrane, and, furthermore, the overall selectivity index increased with the increase of P(AA-AMPS) content. The mechanism of proton transfer was tentatively discussed based on the activation energy of conductivity.     

Enhanced performance of a direct methanol alkaline fuel cell (DMAFC) using a polyvinyl alcohol/fumed silica/KOH electrolyte

A novel polymer-inorganic composite electrolyte for direct methanol alkaline fuel cells (DMAFCs) is prepared by physically blending fumed silica (FS) with polyvinyl alcohol (PVA) to suppress the methanol permeability of the resulting nano-composites. Methanol permeability is suppressed in the PVA/FS composite when comparing with the pristine PVA membrane. The PVA membrane and the PVA/FS composite are immersed in KOH solutions to prepare the hydroxide-conducting electrolytes. The ionic conductivity, cell voltage and power density are studied as a function of temperature, FS content, KOH concentration and methanol concentration. The PVA/FS/KOH electrolyte exhibits higher ionic conductivity and higher peak power density than the PVA/KOH electrolyte. In addition, the concentration of KOH in the PVA/FS/KOH electrolytes plays a major role in achieving higher ionic conductivity and improves fuel cell performance. An open-circuit voltage of 1.0 V and a maximum power density of 39 mW cm⁻² are achieved using the PVA/(20%)FS/KOH electrolyte at 60 °C with 2 M methanol and 6 M KOH as the anode fuel feed and with humidified oxygen at the cathode. The resulting maximum power density is higher than the literature data reported for DMAFCs prepared with hydroxide-conducting electrolytes and anion-exchange membranes. The long-term cell performance is sustained during a 100-h continuous operation.     

A preliminary study of the electro-oxidation of l-ascorbic acid on polycrystalline silver in alkaline solution

Electrochemical oxidation of l-ascorbic acid on polycrystalline silver in alkaline aqueous solutions is studied by cyclic voltammetry (CV), chronoamperometry (CA) and impedance spectroscopy (IS). The anodic electro-oxidation starts at −500 mV versus SCE and shows continued anodic oxidation in the cathodic half cycle in the CV regime signifying slowly oxidizing adsorbates. Diffusion coefficient of ascorbate ion measured under both voltammetric regimes is around 1.4 × 10⁻⁵ cm² s⁻¹. Impedance spectroscopy measures the capacitances associated with double layer and adsorption around 50 μF cm⁻² and 4 mF cm⁻² as well as the adsorption and decomposition resistances (rates).     

Tuning transport properties by manipulating the phase segregation of tetramethyldisiloxane segments in modified polyimide electrolytes

A series of copolymer electrolytes containing 4,4′-oxydianiline (ODA)-based sulfonated polyimide and siloxane segments, in various ratios, are prepared and characterized for direct methanol fuel cell applications. The chemical Structure of the sulfonated copolymers is confirmed by FT-IR and NMR. The prepared composite membranes are found to be flexible and show good thermal stability as well as good proton conductivity. A maximum proton conductivity of 5.78 × 10⁻² S cm⁻¹ (cf. Nafion117 = 8.31 × 10⁻² S cm⁻¹) is obtained for the sulfonated polyimide blended with sulfonated polyimide with a grafted tetramethyldisiloxane segment (cf. SPI_DSX75 membrane) at 90 °C. The membranes showed low methanol crossover below 10⁻⁷ cm² s⁻¹ (cf. Nafion117 = 10⁻⁶ cm² s⁻¹). The transport properties of the membranes are found to be strongly influenced by water uptake and by the number and nature of the ionic clusters in the hydrophilic domains. When the number of siloxane segments is increased, the relationship between the methanol self-diffusion coefficient (D_M) and water molecules per sulfonic acid group (λ) indicate that the water molecules are interacting with channels inside the membrane. In addition, the segregated nanophase also affects the ion transport and sometimes enhances the corresponding ionic conductivity. TEM and SAXS analyses shows evidence for phase segregation in the membranes and reveal the influence of flexible siloxane segments in ionic clustering.     

Modern generation of polymer electrolytes based on lithium conductive imidazole salts

In this paper the application of completely new generation imidazole-derived salts in a model polymer electrolyte is described. As a polymer matrix, two types of liquid low molecular weight PEO analogues e.g. dimethyl ether of poly(ethylene glycol) of 500 g mol⁻¹ average molar mass (PEGDME500) and methyl ether of poly(ethylene glycol) of 350 g mol⁻¹ average molar mass (PEGME350) were used. Room temperature conductivities measured by electrochemical impedance spectroscopy were found to be as high as 10⁻³-10⁻⁴ S cm⁻¹ in the 0.1-1 mol dm⁻³ range of salt concentrations. Li⁺ transference numbers higher than 0.5 were measured and calculated using the Bruce-Vincent method. For a complete electrochemical characterization the interphase resistance stability over time was carefully monitored for a period of 30 days. Structural analysis and interactions between electrolyte components were done by Raman spectroscopy. Fuoss-Kraus semiempirical method was applied for estimation of free ions and ionic agglomerates showing that fraction of ionic agglomerates for salt concentration of 0.1-1 mol dm⁻³ is much lower than in electrolytes containing LiClO₄ in corresponding concentrations.