Application of sodium conducting membranes in direct methanol alkaline fuel cells

A study of a direct methanol alkaline fuel cell (DMAFC) operating with sodium conducting membranes is reported. Evaluation of the fuel cell was performed using membrane electrode assemblies incorporating carbon supported platinum catalysts and Nafion^® 117 and 112 membranes. A membrane electrode assembly was also prepared by the direct chemical deposition of platinum into the surface region of the membrane. Evaluation of the chemically deposited assembly showed it to be less active than those based on carbon supported catalysts. SEM &; TEM analysis indicate that this behaviour is due to the low surface area of the chemically deposited catalyst layer. The fuel cell performance with Nafion membranes is reported and is not as good as achieved with hydroxide ion conducting membranes suggesting that Nafion may not be suitable for DMAFC operation.     

The effect of methanol and ethanol cross-over on the performance of PtRu/C-based anode DAFCs

In the present work, the cross-over rates of methanol and ethanol, respectively, through Nafion^®-115 membranes at different temperatures and different concentrations have been measured and compared. The changes of Nafion^®-115 membrane porosity in the presence of methanol or ethanol aqueous solutions were also determined by weighing vacuum-dried and alcohol solution-equilibrated membranes. The techniques of anode polarization and adsorption stripping voltammetry were applied to compare the electrochemical activity and adsorption ability, respectively. To investigate the consequences of methanol and ethanol permeation from the anode to the cathode on the performance of direct alcohol fuel cells (DAFCs), single DAFC tests, with methanol or ethanol as the fuel, have been carried out and the corresponding anode and cathode polarizations versus dynamic hydrogen electrode (DHE) were also performed. The effect of alcohol concentration on the performance of PtRu/C anode-based DAFCs was investigated.It was found that ethanol shows lower cross-over rates than methanol through the Nafion^® membrane in spite of the higher membrane porosity resulted in presence of ethanol aqueous solutions. Furthermore, it was found that ethanol presents less negative effect on the cathode performance due to both its smaller permeability through Nafion^® membrane and its slower electrochemical oxidation kinetics over Pt/C cathode.     

Preparation and characterization of nanocomposite polyelectrolyte membranes based on Nafion ionomer and nanocrystalline hydroxyapatite

In this study nanocrystalline hydroxyapatite (nHA) was synthesized and characterized by means of FT-IR, XRD and TEM techniques and a series of proton exchange membranes based on Nafion^® and nHA were fabricated via solvent casting method. Thermogravimetric analysis confirmed thermal stability enhancement of the Nafion^® nanocomposite due to the presence of nHA nanopowder. SAXS and TEM analyses confirmed the incorporation of nHA into ionic phase of Nafion^®. Furthermore, the incorporation of elliptical nHA into the Nafion^® matrix improved proton conductivity of the resultant polyelectrolyte membrane up to 0.173 S cm⁻¹ at 2.0 wt% of nHA loading compared to that of 0.086 S cm⁻¹ for Nafion^® 117. Also, the inclusion of nHA nanoparticles into nanocomposite membranes resulted in a significant reduction of methanol permeability and crossover in comparison with pristine Nafion^® membranes. Membrane selectivity parameter of the nanocomposites at 2.0 wt% nHA was calculated and found to be 106,800 S s cm⁻³, which is more than two times than that of Nafion^® 117. Direct methanol fuel cell tests revealed that Nafion^®/nHA nanocomposite membranes were able to provide higher fuel cell efficiency and also better electrochemical performance in both low and high concentrations of methanol feed. Thus, the current study shows that nHA enhances the functionality of Nafion^® as fuel cell membranes.     

Prediction of methanol and water fluxes through a direct methanol fuel cell polymer electrolyte membrane

Development of a direct methanol fuel cell (DMFC) mass flux model, using conventional transport theory, is presented and used to predict the fluid phase superficial velocity, methanol and water molar fluxes, and the chemical species (methanol and water) dimensionless concentration profiles in the polymer electrolyte membrane, Nafion^® 117, of a DMFC. Implementation of these equations is illustrated to generate the numerical data as functions of the variables such as the pressure difference across the membrane, methanol concentration at the cell anode, temperature, and position in the membrane.     

Proton-conducting membranes from phosphotungstic acid-doped sulfonated polyimide for direct methanol fuel cell applications

A series of novel hybrid proton conducting membranes based on sulfonated naphthalimides and phosphotungstic acid (PTA) were prepared from N-Methyl Pyrrolidone (NMP) solutions. These hybrid organic-inorganic materials, composed of two proton-conducting components, have high ionic conductivities (9.3 × 10^?2 S cm^?1 at 60 °C, 15% PTA), and show good performance in H₂|O₂ polymer electrolyte membrane fuel cells (PEMFC), previously reported by us. Moreover, they have low methanol permeability compared to Nafion^®112. In this paper we describe, for the first time, the behaviour of these hybrid membranes as electrolyte in a direct methanol fuel cell (DMFC). The maximum power densities achieved with PTA doped sulfonated naphthalimide membrane, operating with oxygen and air, were 34.0 and 12.2 mW cm^?2, respectively; about the double and triple higher than those showed by the non-doped membrane at 60 °C.     

Effect of operational parameters of mini-direct methanol fuel cells operating at ambient temperature

There is currently increased interest in small-size direct methanol fuel cells for portable applications. This work presents results of the influence of operational parameters on the performance of a mini-direct methanol fuel cell. The effects of methanol concentration, Pt load, membrane thickness and PTFE content in the cathode diffusion layer on the performance were studied. Two anodic materials were prepared, PtRu 75:25 at.% and PtRu 90:10 at.%, as nanoparticles supported on Vulcan XC-72 carbon, while for the cathodes Pt/C E-TEK catalysts were used. The materials were characterized physically by EDX and DRX and electrochemically in a half-cell. The results with single cells showed better performances with cells operating with 3 mg Pt cm^?2, 5 mol l^?1 methanol solution, Nafion^® 112 membrane and with 30 wt.% PTFE in the cathode diffusion layer deposited on only one face of the electrode support.     

Proton conducting hydrocarbon membranes: Performance evaluation for room temperature direct methanol fuel cells

The methanol permeability, proton conductivity, water uptake and power densities of direct methanol fuel cells (DMFCs) at room temperature are reported for sulfonated hydrocarbon (sHC) and perfluorinated (PFSA) membranes from Fumatech^®, and compared to Nafion^® membranes. The sHC membranes exhibit lower proton conductivity (25–40 mS cm⁻¹ vs. ∼95–40 mS cm⁻¹ for Nafion^®) as well as lower methanol permeability (1.8–3.9 × 10⁻⁷ cm² s⁻¹ vs. 2.4–3.4 × 10⁻⁶ cm² s⁻¹ for Nafion^®). Water uptake was similar for all membranes (18–25 wt%), except for the PFSA membrane (14 wt%). Methanol uptake varied from 67 wt% for Nafion^® to 17 wt% for PFSA. The power density of Nafion^® in DMFCs at room temperature decreases with membrane thickness from 26 mW cm⁻² for Nafion^® 117 to 12.5 mW cm⁻² for Nafion^® 112. The maximum power density of the Fumatech^® membranes ranges from 4 to 13 mW cm⁻¹. Conventional transport parameters such as membrane selectivity fail to predict membrane performance in DMFCs. Reliable and easily interpretable results are obtained when the power density is plotted as a function of the transport factor (TF), which is the product of proton concentration in the swollen membrane and the methanol flux. At low TF values, cell performance is limited by low proton conductivity, whereas at high TF values it decreases due to methanol crossover. The highest maximum power density corresponds to intermediate values of TF.     

Fuel cell performance of polymer electrolyte membrane based on hexafluorinated sulfonated poly(ether sulfone)

The potential-current fuel cell characteristics of membrane electrode assemblies (MEAs) using hexafluorinated sulfonated poly(ether sulfone) copolymer are compared to those of Nafion^® based MEAs in the case of proton exchange membrane fuel cell (PEMFC) and direct methanol fuel cell (DMFC). The hexafluorinated copolymer with 60 mol% of monosulfonated comonomer based acid form membrane is chosen for this study due to its high proton conductivity, high thermal stability, low methanol permeability, and its insolubility in boiling water. The catalyst powder is directly coated on the membrane and the catalyst coated membrane is used to fabricate MEAs for both fuel cells. A current density of 530 mA cm^?2 at 0.6 V is obtained at 70 °C with H₂/air as the fuel and oxidant. The peak power density of 110 mW cm^?2 is obtained at 80 °C under specific DMFC operating conditions. Other electrochemical characteristics such as electrochemical impedance spectroscopy, cyclic voltammetry, and linear sweep voltammetry are also studied.     

Zeolites in Microsystems for Chemical Synthesis and Energy Generation

Zeolites were incorporated as membrane and catalyst in chemical microsystems for portable energy generation and fine chemical synthesis. Microfabricated HZSM-5 micromembrane was used as a proton-exchange membrane in a miniature direct methanol fuel cell (μ-DMFC). The good proton conductivity of HZSM-5 micromembrane was attributed to a Grotthus-like diffusion of protons along the water molecules bridging neighboring aluminum sites in the hydrated HZSM-5. The 6-μm thick HZSM-5 micromembrane exhibited comparable proton flux as Nafion^® 117 and delivered a P _max of 2.9 mW cm^?2 (E = 0.33 V) at room temperature. This is smaller compared to 16.5 mW cm^?2 (E = 0.23 V) for a Nafion^®-based μ-DMFC and was believed to be caused by adsorbed methanol molecules interrupting the proton transport along the water bridge. A Cs-exchanged NaX on NaA bilayer catalyst-membrane incorporated in microreactor channels was used for the Knoevenagel condensation reactions between benzaldehyde and (1) ethyl cyanoacetate, (2) ethyl acetoacetate (EAA) and (3) diethyl malonate. Microreactor and membrane microreactor gave higher conversion compared to fixed-bed and batch reactors, but the reaction of benzaldehyde and EAA in the microreactor had poorer selectivity due to the slow diffusion of the product molecules in the microchannel that allowed their further reactions to form undesired byproducts.     

A novel heteropolyacid-doped carbon nanotubes/Nafion nanocomposite membrane for high performance proton-exchange methanol fuel cell applications

A novel proton-exchange polymer composite membrane was synthesized using Nafion^®, tetraethoxysilane-modified carbon nanotubes (CNTs) and phosphotungstic acid-modified carbon nanotubes with the aim of using direct methanol fuel cells (DMFCs). Physicochemical properties of the modified CNTs and fabricated composite membranes were investigated by Fourier transform infrared spectroscopy, field emission scanning electron microscopy, water uptake, thermogravimetric analysis, ion exchange capacity, proton conductivity and methanol permeability tests. It was demonstrated that chemical surface modification of CNTs and introduction of the phosphotungstic acid (PWA) groups effectively improved the performance of DMFC. It was found that the presence of PWA groups on the surface of CNTs led to the formation of strong electrostatic interactions between the PWA groups and clusters of sulfonic acid in Nafion^® macromolecules. Hence, the incorporation of inorganic phosphotungstic super-acid-doped silicon oxide-covered carbon nanotubes (CNT@SiO₂-PWA) into Nafion^® matrices enhanced the proton conductivity of the prepared membranes. Moreover, the methanol permeability was reduced to 2.63 × 10^?7 cm² s^?1 in comparison with the recast Nafion^® membrane (2.25 × 10^?6 cm² s^?1). Enhancing the proton conductivity and reducing the methanol permeability, the selectivity of the prepared nanocomposite membranes was enhanced to a greater value of 330,700 S s cm^?3 as compared to the value of 38,222 S s cm^?3 for recast Nafion^®.     

Effects of organically modified nanoclay on the transport properties and electrochemical performance of acid‐doped polybenzimidazole membranes

Nanocomposite polyelectrolyte membranes based on phosphoric acid (H₃PO₄) doped polybenzimidazoles (PBIs) with various loading weights of organically modified montmorillonite (OMMT) were prepared and characterized for direct methanol fuel cell (DMFC) applications. X‐ray diffraction analysis revealed the exfoliated structure of OMMT nanolayers in the polymeric matrices. An H₃PO₄–PBI/OMMT membrane composed of 500 mol % doped acid and 3.0 wt % OMMT showed a membrane selectivity of approximately 109,761 in comparison with 40,500 for Nafion 117 and also a higher power density (186 mW/cm²) than Nafion 117 (108 mW/cm²) for a single‐cell DMFC at a 5M methanol feed. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Transesterification of triacetin with methanol on Nafion® acid resins

Although homogeneous alkali catalysts (e.g., NaOH) are commonly used to produce biodiesel by transesterification of triglycerides (vegetable oils and animal fats) and methanol, solid acid catalysts, such as acidic resins, are attractive alternatives because they are easy to separate and recover from the product mixture and also show significant activity in the presence of fatty acid impurities, which are common in low-cost feedstocks. To better understand solid acid catalyst performance, a fundamental transesterification kinetic study was carried out using triacetin and methanol on Nafion^® (perfluorinated-based ion-exchange resin) catalysts. In particular, Nafion^® SAC-13 (silica-supported Nafion) and Nafion^® NR50 (unsupported Nafion) were investigated, because both show great promise for biodiesel-forming reactions. The reaction kinetics for a common homogeneous acid catalyst (H₂SO₄) were also determined for comparison. Liquid-phase reaction was performed at 60 °C using a stirred batch reactor. The swelling properties of the resin in solvents of diverse polarity that reflect solutions typically present in a biodiesel synthesis mixture were examined. The initial reaction rate was greatly affected by the extent of swelling of the resin, where, as expected, a greater effect was observed for Nafion^® NR50 than for the highly dispersed Nafion^® SAC-13. The reaction orders for triacetin and methanol on Nafion^® SAC-13 were 0.90 and 0.88, respectively, similar to the reaction orders determined for H₂SO₄ (1.02 and 1.00, respectively). The apparent activation energy for the conversion of triacetin to diacetin was 48.5 kJ/mol for Nafion^® SAC-13, comparable to that for H₂SO₄ (46.1 kJ/mol). Selective poisoning of the Brønsted acid sites on Nafion^® SAC-13 using pyridine before transesterification revealed that only one site was involved in the rate-limiting step. These results suggest that reaction catalyzed by the ion-exchange resin can be considered to follow a mechanism similar to that of the homogeneous catalyzed one, where protonated triglyceride (on the catalyst surface) reaction with methanol is the rate-limiting step.     

Investigation of direct methanol fuel cell performance of sulfonated polyimide membrane

Sulfonated polyimide (SPI) membranes have been evaluated as electrolyte membranes in direct methanol fuel cells (DMFCs). The membrane-electrode assembly (MEA) was made by hot-pressing the membrane, an anode and a cathode, catalyzed with PtRu/CB (PtRu dispersed on carbon black) and Pt/CB bound with Nafion^® ionomer, respectively. The performance of the cell based on SPI was compared with that of Nafion^® 112 in various operation conditions such as cell temperature (T_cell), cathode relative humidity (RH), and methanol concentration (C_MeOH). The methanol crossover at the cell based on SPI was a half of Nafion^® 112, resulting in the improved cell efficiency. Advantage of the use of SPI became much distinctive from the conventional Nafion^® 112 when the DMFC was operated at a higher T_cell or a higher C_MeOH.     

Research on methanol permeation of proton exchange membranes with incorporating ionic liquids

Methanol permeation and conductivity of membrane materials are important factors to evaluate the feasibility of application as proton exchange membranes (PEMs) in direct methanol fuel cell (DMFC). The methanol permeation values of these composite membranes based on ionic liquids of trifluoroacetic propylamine (TFAPA) and the disubstituted imidazolium cations with different anions were summarized, and the methanol permeation behaviors were investigated in this work. Although these polymer/ionic liquid composite membranes displayed satisfactory conductivities, the relative selectivity values of conductivity to methanol permeability were lower than the value of Nafion^® membrane. Moreover, polymerized ionic liquids (PILs) membranes showed the strong ability to hinder methanol permeation with a value around 10^?11 cm²/s at 10 M methanol solution. The maximum relative selectivity value reached (2.23–1.76) × 10⁶ S·s/cm³ for PVC-MIMCl membrane, which was near two orders of magnitude higher than the reported 2.47 × 10⁴ S·s/cm³ for Nafion-117 membrane at 2 M methanol solution.

Open image in new window

Graphical abstract ?

     

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.     

NiCoFe/C cathode electrocatalysts for direct ethanol fuel cells

A direct ethanol fuel cell (DEFC), which is less prone to ethanol crossover, is reported. The cell consists of PtRu/C catalyst as the anode, Nafion^® 117 membrane, and Ni–Co–Fe (NCF) composite catalyst as the cathode. The NCF catalyst was synthesized by mixing Ni, Co, and Fe complexes into a polymer matrix (melamine-formaldehyde resins), followed by heating the mixture at 800 °C under inert atmosphere. TEM and EDX experiments suggest that the NCF catalyst has alloy structures of Ni, Co and Fe. The catalytic activity of the NCF catalyst for the oxygen reduction reaction (ORR) was compared with that of commercially available Pt/C (CAP) catalyst at different ethanol concentrations. The decrease in open circuit voltage (V_oc) of the DEFC equipped with the NCF catalysts was less than that of CAP catalyst at higher ethanol concentrations. The NCF catalyst was less prone to ethanol oxidation at cathode even when ethanol crossover occurred through the Nafion^®117 film, which prevents voltage drop at the cathode. However, the CAP catalyst did oxidize ethanol at the cathode and caused a decrease in voltage at higher ethanol concentrations.     

The effect of catalyst support on the performance of PtRu in direct borohydride fuel cell anodes

A comparative investigation of direct borohydride fuel cell polarization behavior (DBFC) was carried out with respect to the effect of unsupported and supported PtRu anode catalysts using as supports both Vulcan XC-72R and graphite felt (GF). The Vulcan XC-72R-supported catalyst alleviated mass-transfer-related problems associated with hydrogen generation from borohydride hydrolysis taking place mainly on the Ru sites. However, the most significant improvement was obtained by using the three-dimensional GF support. Typically 1.0 mg cm^?2 PtRu was galvanostatically electrodeposited by a surfactant templated method on compressed graphite felt of 350 μm thickness. The PtRu/GF anode (Pt:Ru atomic ratio of 1.4:1) generated a DBFC peak power density of 130 mW cm^?2 at 333 K. The separator in the DBFC was a Nafion^® 117 membrane. The peak power density of the PtRu/GF was 270% and 60% higher compared with the catalyst-coated membrane configuration with unsupported PtRu and PtRu/Vulcan XC-72R, respectively.     

Utilization of Nafion<Superscript>®</Superscript>/conducting polymer composite in the PEM type fuel cells

The present study focuses on the problem of using conducting polymers (CPs) in proton exchange membrane fuel cell technology. It covers the electrocatalytic properties of the CP/Pt composite, permeability of the CP film for H₂, fixation of the compact CP film on the top of the Nafion^® membrane and first results of its utilization in a fuel cell. The present results did not confirm a previously reported increase in CP/Pt composite electrocatalytic activity when compared to the commercially available carbon supported catalysts. The main reason seems to be the very low permeability of the compact CP film for the fuel. This may be an advantage with respect to the minimization of fuel cross-over, which is a serious problem in the direct methanol fuel cell. On the other hand, it represents a serious danger in water management of the fuel cell. This fact has been recognized and alternative solutions are presented.     

Enhanced Fuel Cell Performance of Decalin Treated Nafion® Membranes

The interaction of Nafion^® 212 membrane with a carbocyclic fuel, decalin was studied. Membrane electrode assemblies (MEA) fabricated with decalin treated membranes exhibited significant increase in power density in a H₂/Air fuel cell at 60% relative humidity. Small angle X‐ray scattering experiments were used to understand the morphological changes in the membrane due to decalin treatment.     

Sulfonated derivatives of polyparaphenylene as proton conducting membranes for direct methanol fuel cell application

Proton conducting polymers derived from polybenzoyl-1,4-phenylene (PBP) and poly-p-phenoxybenzoyl-1,4-phenylene (PPBP) were synthesized by the Colon synthesis technique. The sulfonation of these proton conducting polymers was carried out using either sulphuric acid or tetramethylsiliylchlorosulfonate (TMSCl) as sulfonating agent, and their thermal properties were evaluated. Both sulfonated PBP and PPBP are thermally stable up to at least 215 °C. The sulfonated sPPBP exhibited good conductivity as proton conducting membranes at room temperature and were tested as electrolyte membranes for a single direct methanol fuel cell (DMFC) in terms of water absorption, methanol permeability and electrical performance. The water uptake of the sPPBP was found to be larger than that of the sPBP, i.e., 65 and 43 mol%, respectively. The permeability to methanol was found to be 10 times lower than sPPBP and sPBP compared to a Nafion^® membrane. In spite of this, performance in a single DMFC was found to be twice inferior to that with Nafion^® 117. Optimisation of the sulfonation level and of the electrode-membrane interfaces was lead to better results.