Synthesis and characterization of sulfonated polyimide membranes for direct methanol fuel cell

The sulfonated polyimide (SPI) membranes for direct methanol fuel cell (DMFC) were synthesized with 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BTDA), 2,2′-benzidinedisulfonic acid (BDSA), 4,4′-oxydianiline (ODA) through classical two-step methods: (1) preparation of sulfonated poly(amic acid) (SPAA) precursors with different sulfonation levels by controlling the molar ratio of BDSA to ODA, and (2) thermal imidization of the SPAA films. The chemical structure and the imidization from SPAA membranes were characterized by FT-IR with temperature, and the sulfonation levels were determined by elemental analysis. The thermal stability of the membranes was also characterized by TGA. From water uptake and small angle X-ray scattering (SAXS) experiments for different sulfonation levels, it was found that the number of water clusters in SPI membranes increased as the water uptake of membranes increased, but the size of water cluster was not changed with the sulfonation levels. The proton conductivity and the methanol permeability of SPI membrane showed a sudden leap like a percolation phenomenon around 35 mol% of sulfonation level. The SPI membranes exhibited relatively high proton conductivity and extremely low methanol permeability, and showed the feasibility of suitable polymer electrolyte membranes (PEM) for DMFC.     

Transport properties of Nafion membranes modified with tetrapropylammonium ions for direct methanol fuel cell application

This work presents a study of transport properties (proton conductivity, methanol permeability, and water uptake) and acid-base properties of commercial Nafion-112, -115, and -117 membranes modified with tetrapropylammonium (TPA) cations. In the interaction between TPA hydroxide and protons of sulfonate groups in the Nafion matrix, some of the protons are shown to be bound to sulfonate groups and do not participate in transport processes. These findings are confirmed by IR spectroscopy, acid-base titration, and data on proton conductivity of the modified membranes. Proton conductivity of the modified membranes is shown to be effectively described by a percolation model with parameters that agree with published data for commercial Nafion membranes. Based on these results, a model is proposed for the interaction of TPA cations with the sulfonate groups in Nafion membranes. According to this model, TPA cations form hydrophobic clusters in hydrophilic regions of the polymer matrix, thus preventing some of the protonated sulfonate groups from participating in transport processes.     

Poly(3,4-ethylenedioxythiophene)-modified nafion membrane for direct methanol fuel cells

Membranes Nafion 117 are modified with poly(3,4-ethylenedioxythiophene) (PEDT) by chemical polymerization of EDT with H₂O₂ or FeCl₃ as the oxidants in a two-compartment cell. Depending on the oxidant and polymerization conditions, PEDT is deposited either as a thin film on the membrane surface or inside the Nafion membrane depending on whether FeCl₃ or H₂O₂ is used as the oxidant. The decrease in the ionic conductivity and methanol permeability is studied as a function of the polymerization time. A linear dependence is found with H₂O₂ and a t ^−1/2 dependence, with FeCl₃. The contributions of PEDT and Nafion to the overall conductivity of the composite membranes are separated by impedance measurements. The modified membranes (FeCl₃) are also tested in direct methanol fuel cells (DMFC). The methanol permeation through the membranes is measured by operating the fuel cell in an electrolysis mode. The influence of the modified membranes on the DMFC current-voltage characteristics is studied with 2 M CH₃OH and O₂ at 1.2 bar_abs and 80°C. Membrane electrode assemblies (MEAs) are prepared by hot pressing the modified membrane between two commercial gas diffusion electrodes with 1 mg cm⁻² of Pt loading. A decrease of the methanol permeation of 25% is observed at MEA with the modified membrane (1 h polymerization time), compared with that of MEA with a Nafion membrane. However, the overall DMFC performance decreases in the same relation: a maximal power density of 36 W cm⁻² is measured at MEA with a PEDT-modified membrane compared with 45 W cm⁻² for MEA with a Nafion membrane. Published in Russian in Elektrokhimiya, 2006, Vol. 42, No. 11, pp. 1330–1339. Based on the report delivered at the 8th International Frumkin Symposium “Kinetics of the Electrode Processes,” October 18–22, 2005, Moscow. The text was submitted by the authors in English.     

Proton exchange membrane using partially sulfonated polystyrene-b-poly(dimethylsiloxane) for direct methanol fuel cell

A diblock copolymer ionomer containing a rubbery poly(dimethylsiloxane) block has been developed as a proton exchange membrane for direct methanol fuel cell (DMFC). The partially sulfonated polystyrene-b-poly(dimethylsiloxane) (sPS-b-PDMS) membrane with 38% sulfonation degree exhibited 3 times lower methanol permeability and 2.6 times higher membrane selectivity (proton conductivity/methanol permeability) compared to Nafion^® 115 at 25 °C. Coexistence of microphase domains and ionic clusters was confirmed from the morphological studies by small-angle X-ray scattering and tapping-mode atomic force microscopy. Gas chromatographic analysis revealed that water/methanol selectivity of sPS-b-PDMS was 20 times higher than that of Nafion^® 115. Such a high water/methanol selectivity can be attributed to the existence of PDMS microdomains minimizing methanol permeation through hydrophilic ion channels. sPS-b-PDMS membranes were fabricated into membrane electrode assembly (MEA), and air-breathing DMFC test for these MEAs showed a better performance compared to the MEA composed of Nafion^® 115.     

Synthesis of PtRu nanoparticles from the hydrosilylation reaction and application as catalyst for direct methanol fuel cell

Nanosized Pt, PtRu, and Ru particles were prepared by a novel process, the hydrosilylation reaction. The hydrosilylation reaction is an effective method of preparation not only for Pt particles but also for other metal colloids, such as Ru. Vulcan XC-72 was selected as catalyst support for Pt, PtRu, and Ru colloids, and TEM investigations showed nanoscale particles and narrow size distribution for both supported and unsupported metals. All Pt and Pt-rich catalysts showed the X-ray diffraction pattern of a face-centered cubic (fcc) crystal structure, whereas the Ru and Ru-rich alloys were more typical of a hexagonal close-packed (hcp) structure. As evidenced by XPS, most Pt and Ru atoms in the nanoparticles were zerovalent, except a trace of oxidation-state metals. The electrooxidation of liquid methanol on these catalysts was investigated at room temperature by cyclic voltammetry and chronoamperometry. The results concluded that some alloy catalysts showed higher catalytic activities and better CO tolerance than the Pt-only catalyst; Pt56Ru44/C have displayed the best electrocatalytic performance among all carbon-supported catalysts.     

Preparation and characterization of CPPO/BPPO blend membranes for potential application in alkaline direct methanol fuel cell

A series of hydroxyl-conducting anion-exchange membranes were prepared by blending chloroacetylated poly(2,6-dimethyl-1,4-phenylene oxide) (CPPO) with bromomethylated poly(2,6-dimethyl-1,4-phenylene oxide) (BPPO), and their fuel cell-related performances were evaluated. The resulting membranes exhibited high hydroxyl conductivities (0.022–0.032 S cm⁻¹ at 25 °C) and low methanol permeability (1.35 × 10⁻⁷ to 1.46 × 10⁻⁷ cm² s⁻¹). All the blend membranes proved to be miscible or partially miscible under the investigations of scanning electron microscopy (SEM) and differential scanning calorimeters (DSC). By condition optimization, the blend membranes with 30–40 wt% CPPO are recommended for application in direct methanol alkaline fuel cells because they showed low methanol permeability, excellent mechanical properties and comparatively high hydroxyl conductivity.     

A Pd-impregnated nanocomposite Nafion membrane for use in high-concentration methanol fuel in DMFC

Pd nanophases in a Nafion polymer membrane electrolyte were used to enhance DMFC performance by preventing or reducing methanol crossover through the electrolyte. Interestingly, the Pd-impregnated nanocomposite membrane showed a lower permeability for methanol and maintained a proton conductivity comparable to that of a pure Nafion membrane. Because of the modification of the membrane by the Pd nanophases, a high molar concentration of methanol could be applied for use in practical DMFC without any power loss.     

Modified poly(phthalazinone ether ketone) membranes for direct methanol fuel cell

Sulfonated poly(phthalazinone ether ketone)s (SPPEK)s were synthesized by the modification of poly(phthalazinone ether ketone) (PPEK) with 98% concentrated sulfuric acid or 98% concentrated sulfuric acid and chlorosulfonic acid mixture at 80–100°C. The presence of sulfonic acid groups in SPPEKs was confirmed by Fourier transform infrared (FTIR) analysis and nuclear magnetic resonance (NMR), and the DSs were determined by Energy Dispersive X‐Ray (EDX). A blend membrane of No. 21 SPPEK and phosphotungstic acid (PWA) was prepared. The methanol permeabilities of SPPEK and blend membranes were about 20 times lower than that of Nafion117 at room temperature. The direct methanol fuel cell (DMFC) test of 2 M methanol solution and air breathing showed that the blend membrane had a better performance than that of the Nafion117. Copyright © 2007 John Wiley & Sons, Ltd.     

Polyaniline decorated graphene oxide on sulfonated poly(ether ether ketone) membrane for direct methanol fuel cells application

In direct methanol fuel cells (DMFC), methanol crossover is a major issue which has reduced the performance of polymer electrolyte membrane (PEM) for energy generation. In this study, graphene oxide (GO) and conductive polyaniline decorated GO (PANI-GO) were used as additives in fabrication of sulfonated poly(ether ether ketone) (SPEEK) nanocomposite PEM membrane to reduce methanol crossover. PANI-GO was synthesized by in situ polymerization method and the formation of PANI coated GO nanostructures was confirmed by surface morphology and crystallinity analysis. The membrane morphology and topography analysis confirmed that GO and PANI-GO were well dispersed on the surface of SPEEK membrane. 0.1 wt% PANI-GO modified SPEEK nanocomposite membrane exhibited the highest water uptake and ion exchange capacity of 40% and 1.74 meq g^?1, respectively. The oxidative stability of the nanocomposite membranes also improved. Lower methanol permeability of 4.33 × 10^?7 cm^?2S^?1 was noticed for 0.1 wt% PANI-GO modified SPEEK membrane. PANI-GO modified SPEEK membrane enhanced the proton conductivity, which was due to the existence of acidic and hydrophilic group present in PANI and GO. PANI-GO modified SPEEK membrane held higher selectivity of 1.94 × 10⁴ S cm^?3 s^?1. Overall, these studies revealed that PANI-GO modified SPEEK membrane is a potential material for DMFC applications.     

Recent development of methanol electrooxidation catalysts for direct methanol fuel cell

Direct methanol fuel cells(DMFCs) are very promising power source for stationary and portable miniature electric appliances due to its high efficiency and low emissions of pollutants. As the key material, catalysts for both cathode and anode face several problems which hinder the commercialization of DMFCs.In this review, we mainly focus on anode catalysts of DMFCs. The process and mechanism of methanol electrooxidation on Pt and Pt-based catalysts in acidic medium have been introduced. The influences of size effect and morphology on electrocatalytic activity are discussed though whether there is a size effect in MOR catalyst is under debate. Besides, the non Pt catalysts are also listed to emphasize though Pt is still deemed as the indispensable element in anode catalyst of DMFCs in acidic medium. Different catalyst systems are compared to illustrate the level of research at present. Some debates need to be verified with experimental evidences.     

Tolerant cathode catalysts for direct methanol fuel cell

Effect of methanol on the reduction kinetics of oxygen on highly dispersed catalysts 60Pt/C (HiSPEC 9100), 40Pt/carbon nanotubes, and CoFe/carbon nanotubes for the cathode of a direct methanol-oxygen fuel cell was studied. It was shown that the CoFe/carbon nanotubes catalyst surpasses the platinum systems in tolerance to the alcohol. It was found that the tolerance of the cathode catalyst strongly affects the current–voltage characteristics of the fuel cell, which is the principal result of the study and constitutes its scientific novelty. The maximum power density of an alkaline methanol-oxygen fuel cell with nonplatinum cathode (260 mW cm^–2) exceeds the characteristics of similar fuel cells with platinum cathode catalysts, both obtained in the present study and described in the literature, which points to the practical importance of the study.     

Synthesis and characterization of Pd-La(OH)3/C electrocatalyst for direct ethanol fuel cell

The composite nanomaterial of Pd-La(OH)₃/C was successfully synthesized via intermittent microwave heating–glycol reduction method and characterized with X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. The TEM photograph shows that Pd-La(OH)₃ is well polymerized and dispersed on the carbon support. The performance of the prepared material for ethanol oxidation was evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CA), and chronopotentiometry (CP) measurements in alkaline media. The results reveal that Pd-La(OH)₃/C has significantly higher activity and stability than that of Pd/C with the same Pd loading of 0.1 mg cm^?2. The stable potential reaches to ?0.38 V vs. Hg/HgO at 20 mA cm^?2 on the Pd-La(OH)₃/C electrode in CP curve. Single direct ethanol fuel cell (DEFC) was constructed using Pd-La(OH)₃/C electrode and MnO₂/C electrode as the ethanol anode and air cathode respectively, where the cell voltage can stay at 0.4 V under the current density of 20 mA cm^?2 by discharge test at room temperature.     

Characterization of PVdF-HFP/Nafion/AlO[OH]n composite membranes for direct methanol fuel cell (DMFC)

Poly-(vinylidene fluoride-hexafluoropropylene) (PVdF-HFP)/Nafion ionomer/aluminum oxy hydroxide nanocomposite membranes were prepared by phase inversion technique. The resultant membranes were subjected to protonic conductivity, methanol permeability, infra-red and thermogravimmetric analysis. The infra-red spectroscopic measurements revealed the presence of sulfonic acid groups in the composite membranes. The thermal stability and ionic conductivity of the polymer membranes have been greatly varied upon the addition of AlO[OH]_n. Although the PVDF-HFP/Nafion/AlO[OH]_n composite membranes have moderate protonic conductivity it has lower methanol permeability and may be considered as a candidate for DMFC applications.     

Composite Nafion membrane embedded with hybrid nanofillers for promoting direct methanol fuel cell performance

A special type of hybrid nano-particles was incorporated into the Nafion^® matrix to form a composite membrane. These nano-particles possessed a core–shell structure consisting of silica core (<10 nm) and a densely grafted oligomeric ionmer layer, which was synthesized via atom transfer radical polymerization (ATRP) on the particles’ surface. Besides considerable improvement in the proton conductivity of the membrane, the presence of these hybrid nano-particles in the Nafion^® matrix also repressed its methanol permeability by almost four times. The composite membrane also demonstrated superior performance when tested in a single cell membrane-electrolyte assembly (MEA) under direct methanol fuel cell (DMFC) operating condition. It was found that the composite membrane enabled a power density output that was 1.5 times greater than that of pristine Nafion^®.     

Sulfonated poly(fluorenyl ether ketone) membrane prepared via direct polymerization for PEM fuel cell application

Sulfonated poly(fluorenyl ether ketone)s with inherent viscosity ranging from 0.68 to 0.93 dL/g were directly synthesized by aromatic nucleophilic polycondensation of bisphenol fluorene with various ratios of sulfonated difluorobenzophenone to difluorobenzophenone. The synthesized polymers were characterized by ¹H NMR method and elemental analysis. The polymers were tested to be soluble in dipolar aprotic solvents such as DMAc, DMF and DMSO and can be readily cast into tough films. These films showed highly thermal degradation temperature, glass transition temperature and excellent water affinity as well as excellent proton conductivity. The polymers showed a promising proton membrane material for PEM fuel cell application.     

Composite anode catalyst layer for direct methanol fuel cell

A novel composite anode catalyst layer for direct methanol fuel cell is reported in this paper. The dual-layer anode, which is based on the catalyst coated membrane technique, characterizes a morphological variety of the catalyst layer. The inner sub-layer with a dense morphology can effectively suppress methanol crossover. On the other hand, the outer sub-layer modified by the pore-forming agent, NH₄HCO₃ and the carbon nanotubes can enhance the electrochemical surface area and increase the catalyst utilization. The structural improvement of anode catalyst layer results in a 40% increment in maximum power density during the single cell test at 30 °C.     

Sulfonated polyethersulfone Cardo membranes for direct methanol fuel cell

Novel proton conducting membranes, sulfonated polyethersulfone Cardo (SPES-C), were prepared with concentrated sulfonic acid at room temperature. The degree of sulfonation was controlled by reaction time. Their proton conductivity and methanol permeability as a function of temperature were investigated. The SPES-C membranes with 70% DS were still not water soluble and had low degree of swelling. With the level of 70% sulfonation, proton conductivity was 0.011 S/cm at 80 °C, 0.0338 S/cm at 110 °C, which approached that of Nafion^® 115 membrane at the same conditions. Methanol permeability of SPES-C membranes was considerably smaller than that of Nafion^® 115 membrane over the temperature 25–80 °C.     

Preparation and characterization of long-lived anode catalyst for direct methanol fuel cells

Entry of direct methanol fuel cells into the market requires anode catalyst with stable activity. This paper presents a novel method for stabilizing the activity by immobilizing silica on the catalytic PtRu nanoparticles. Characterization was performed by STEM-EDX, XRD, and ICP. The silica-immobilized PtRu nanoparticles showed high and stable activity toward methanol oxidation. The activity was maintained for 1000 h in sulfuric acidic solution, while the activity of the catalyst with "bare" PtRu nanoparticles decayed after 100 h, showing high durability of the silica-immobilized PtRu nanoparticles catalyst in quasi-anodic acidic environment.     

Polymer blend membranes of sulfonated poly(arylene ether ketone) for direct methanol fuel cell

The blend membranes of sulfonated poly(arylene ether ketone) (sPAEK) (IEC = 1.0 mequiv./g)/Nafion^® and the blend membranes of sPAEK (IEC = 1.0 mequiv./g)/sPAEK (IEC = 1.7 mequiv./g) were prepared. sPAEK with low IEC was introduced to reduce the methanol permeability through the membrane. Morphology, water uptake, proton conductivity and methanol permeability of the blend membranes were investigated by SEM, AFM, AC impedance spectroscopy and permeability measuring instrument. The cross-sections of blend membranes showed phase-separated morphologies. The effect of phase-separated morphology on the properties of blend membranes was investigated. The properties like water uptake, proton conductivity, and methanol permeability of sPAEK/Nafion^® blend membranes showed similar values with sPAEK and properties of sPAEK/sPAEK blend membranes showed intermediate values of two polymers due to the difference in morphology of the blend membranes. sPAEK/sPAEK blend membranes showed relatively high proton conductivity and lowered methanol permeability compared to Nafion^®. sPAEK/sPAEK blend membranes could be a competent substitution for Nafion^®.     

Effect of sorbed methanol, current, and temperature on multicomponent transport in nafion-based direct methanol fuel cells

The CO2 in the cathode exhaust of a liquid feed direct methanol fuel cell (DMFC) has two sources: methanol diffuses through the membrane electrode assembly (MEA) to the cathode where it is catalytically oxidized to CO2; additionally, a portion of the CO2 produced at the anode diffuses through the MEA to the cathode. The potential-dependent CO2 exhaust from the cathode was monitored by online electrochemical mass spectrometry (ECMS) with air and with H2 at the cathode. The precise determination of the crossover rates of methanol and CO2, enabled by the subtractive normalization of the methanol/air to the methanol/H2 ECMS data, shows that methanol decreases the membrane viscosity and thus increases the diffusion coefficients of sorbed membrane components. The crossover of CO2 initially increases linearly with the Faradaic oxidation of methanol, reaches a temperature-dependent maximum, and then decreases. The membrane viscosity progressively increases as methanol is electrochemically depleted from the anode/electrolyte interface. The crossover maximum occurs when the current dependence of the diffusion coefficients and membrane CO2 solubility dominate over the Faradaic production of CO2. The plasticizing effect of methanol is corroborated by measurements of the rotational diffusion of TEMPONE (2,2,6,6-tetramethyl-4-piperidone N-oxide) spin probe by electron spin resonance spectroscopy. A linear inverse relationship between the methanol crossover rate and current density confirms the absence of methanol electro-osmotic drag at concentrations relevant to operating DMFCs. The purely diffusive transport of methanol is explained in terms of current proton solvation and methanol-water incomplete mixing theories.