Direct Methanol Fuel Cell Tests Research Articles

SPEEK-based Composite Membranes for Direct Methanol Fuel Cells

AbstractSulfonated polyether ether ketone (SPEEK)-based composite membranes doped with 23 and 50 wt% of hydrated tin oxide (SnO2·nH2O) were prepared and characterized for Direct Methanol Fuel Cell (DMFC) applications . The composite membranes were characterized in terms of ion exchange capacity (IEC), tensile tests, water uptake measurements, electrochemical impedance spectroscopy (EIS), and Pulsed-field-gradient spin-echo nuclear magnetic resonance (PFGSE NMR) spectroscopy. The body of results indicated the involvement of hydrated tin oxide in the proton conduction mechanism, the inorganic phase providing additional paths between the water clusters for proton transport. NMR measurements evidenced that SnO2·nH2O particles not only facilitate the proton transport but also hinder water diffusion. Therefore, due to the selectivity of SnO2·nH2O for proton diffusion, the filler effect was found to be also beneficial for the reduction of methanol crossover through the SPEEK membrane and for improving the performance of the SPEEK membrane when used as electrolyte in a DMFC test station.

Low methanol crossover and high performance of DMFCs achieved with a pore-filling polymer electrolyte membrane

We compared the performance of the membrane electrode assembly for direct methanol fuel cells (DMFCs) composed of a pore-filling polymer electrolyte membrane (PF membrane) with that composed of a commercial Nafion-117 membrane. In DMFC tests, the methanol crossover flux was 23% lower in the PF membrane than in the Nafion-117 membrane even though the thickness of the PF membrane was 43% that of Nafion-117. This led to a higher DMFC performance and the lower overpotential of the cathode of the PF membrane. Feeding an aqueous 10 M methanol solution at 50 °C produced a low cathode overpotential, as low as 0.40 V at 0.2 A in the PF membrane, whereas the potential was 0.65 V at 0.2 A in the Nafion-117 membrane. In contrast, the ohmic loss and anode overpotential were almost the same in the two membranes. We confirmed that a reduction in methanol crossover using the PF membrane results in lower cathode overpotential and higher DMFC performance. In addition, the electro-osmotic coefficient was estimated as 1.3 in the PF membrane and 2.6 in Nafion-117, based on a water mass-balance model and values showing that the PF membrane prevents the flooding of the cathode at a low gas flow rate using. A highly concentrated methanol solution can be applied as a fuel without decreasing DMFC performance using PF membranes.

Nafion based organic/inorganic composite membrane for air-breathing direct methanol fuel cells

Membrane electrode assemblies (MEA) with organic/inorganic composite membranes were fabricated and tested in air-breathing direct methanol fuel cells (DMFCs). Various composite membranes based on Nafion ® ionomer with functionalized montmorillonite (MMT) were employed to reduce methanol cross-over without sacrificing proton conductivity by introducing sulfonic acid group at the MMT surface followed by blending with Nafion ionomer. Through a systematic investigation, the composite membrane containing 5 wt.% of MMT modified with 1,3-propane sultone was found to exhibit the best membrane selectivity among other membranes. Membrane–electrode interface of the composite membrane was improved by additional ionomer layer incorporated between membrane and catalyst layers. As a result, an improved DMFC performance was achieved compared to that with Nafion 112 or 115. Employing a self-humidifying diffusion layer, semi-passive DMFC tests were performed for various membranes, and it was confirmed that the composite membranes exhibit higher power density than the Nafion membranes. In particular, MEA with a thin composite membrane showed the highest power density and fuel efficiency among the membranes tested in semi-passive DMFCs.

Highly stable PtRuTiO x/C anode electrocatalyst for direct methanol fuel cells

In the present investigation, PtRuTiO x /C electrocatalyst was prepared by a modified polyol synthesis method and the as-prepared electrocatalyst was treated under the reductive atmosphere (30 vol% H 2 in Ar) at 500 °C for 2 h (denoted as PtRuTiO x /C-500) to enhance the interaction between the metal particles and the support. For comparison, the commercial PtRu/C electrocatalyst was also treated by the same procedure as PtRuTiO x /C (denoted as PtRu/C-500). Transmission electron microscopy results indicated that PtRuTiO x /C electrocatalyst exhibited not only a uniform dispersion and narrow size distribution with a smaller particle size, but also excellent stability during the thermal treatment. In contrast, the commercial PtRu/C electrocatalyst is not stable during the thermal treatment and the metal particles greatly agglomerated. The results of CO-stripping voltammetry, single direct methanol fuel cell tests and life-time test jointly showed that PtRuTiO x /C-500 had better durability than commercial PtRu/C while keeping a desirable activity toward methanol electro-oxidation, which may be attributed to the addition of titanium oxide that improved the interaction between noble metal particles and the support.

Development of materials for mini DMFC working at room temperature for portable applications

Methanol permeability measurements and direct methanol fuel cell tests were performed at room temperature with different commercially available or recast Nafion ® membranes and sulfonated polyimide (SPI) membranes. Power densities as high as 20 mW cm −2 could be obtained with Nafion ® 115. However, in order to meet the technological requirements for portable applications, thinner membranes have to be considered. As the MeOH crossover increases greatly (from (7 to 20) × 10 −8 mol s −1 cm −2) while Nafion ® membranes thickness decreases, non-perfluorinated polymers having high IEC are promising candidates for DMFC working at room temperature. The development catalysts tolerant to methanol is also relevant for this application. In spite of the low permeability to MeOH of SPI membranes, the obtained electrical performance with E-TEK electrodes based MEAs was lower than that obtained with Nafion ® membranes. No significant increase of performances was neither evidenced by using homemade PtCr(7:3)/C and PtRu(4:1)/C catalysts instead of E-TEK electrodes with recast Nafion ® based MEAs. However, MEAs composed with thin SPI membranes (50 μm) and homemade PtCr/C catalysts gave very promising results (18 mW cm −2). Based on experimental observations, a speculative explanation of this result is given.

Performance and efficiency of a DMFC using non-fluorinated composite membranes operating at low/medium temperatures

In order to increase the chemical/thermal stability of the sulfonated poly(ether ether ketone) (sPEEK) polymer for direct methanol fuel cell (DMFC) applications at medium temperatures (up to 130 °C), novel inorganic–organic composite membranes were prepared using sPEEK polymer as organic matrix (sulfonation degree, SD, of 42 and 68%) modified with zirconium phosphate (ZrPh) pretreated with n-propylamine and polybenzimidazole (PBI). The final compositions obtained were: 10.0 wt.% ZrPh and 5.6 wt.% PBI; 20.0 wt.% ZrPh and 11.2 wt.% PBI. These composite membranes were tested in DMFC at several temperatures by evaluating the current–voltage polarization curve, open circuit voltage (OCV) and constant voltage current (CV, 35 mV). The fuel cell ohmic resistance (null phase angle impedance, NPAI) and CO 2 concentration in the cathode outlet were also measured. A method is also proposed to evaluate the fuel cell Faraday and global efficiency considering the CH 3OH, CO 2, H 2O, O 2 and N 2 permeation through the proton exchange membrane (PEM) and parasitic oxidation of the crossover methanol in the cathode. In order to improve the analysis of the composite membrane properties, selected characterization results presented in [V.S. Silva, B. Ruffmann, S. Vetter, A. Mendes, L.M. Madeira, S.P. Nunes, Catal. Today, in press] were also used in the present study. The unmodified sPEEK membrane with SD = 42% (S42) was used as the reference material. In the present study, the composite membrane prepared with sPEEK SD = 68% and inorganic composition of 20.0 wt.% ZrPh and 11.2 wt.% PBI proved to have a good relationship between proton conductivity, aqueous methanol swelling and permeability. DMFC tests results for this membrane showed similar current density output and higher open circuit voltage compared to that of sPEEK with SD = 42%, but with much lower CO 2 concentration in the cathode outlet (thus higher global efficiency) and higher thermal/chemical stability. This membrane was also tested at 130 °C with pure oxygen (cathode inlet) and achieved a maximum power density of 50.1 mW cm −2 at 250 mA cm −2.

Characterization and application of composite membranes in DMFC

The present work focuses on the characterization of membranes for direct methanol fuel cells (DMFC), prepared using composites of sulfonated poly(ether ether ketone) (sPEEK, with sulfonation degree, SD, of 42 and 68%) as polymer matrix. This polymer was inorganically modified incorporating different amounts of zirconium phosphate (ZrPh) pretreated with n-propylamine and polybenzimidazole (PBI). The investigated properties were: proton conductivity, water and aqueous methanol swelling, permeability coefficients for DMFC species and morphology. DMFC tests were performed at 110 °C with relative humidity (r.h.) in the cathode feed of 100 and 138%. The results obtained show that the inorganic modification of the polymer decreases the proton conductivity, water and aqueous methanol swelling and permeability towards DMFC species. In terms of morphology, it was found that the applied procedure enabled the preparation of membranes with good compatibility between inorganic and organic components. In terms of the DMFC tests of the composite membranes, working with the cathode feed at 100% r.h., the unmodified sPEEK membrane with SD = 42% proved to have the best performance, although with higher methanol crossover. In contrast, for r.h. of 138%, the best performance was achieved by the sPEEK composite membrane with SD = 68 and 20.0 wt.% of ZrPh and 11.2 wt.% of PBI.

Preparation of supported PtRu/C electrocatalyst for direct methanol fuel cells

In this work, high-surface supported PtRu/C were prepared with Ru(NO)(NO 3) 3 and [Pt(H 2NCH 2CH 2NH 2) 2]Cl 2 as the precursors and hydrogen as a reducing agent. XRD and TEM analyses showed that the PtRu/C catalysts with different loadings possessed small and homogeneous metal particles. Even at high metal loading (40 wt.% Pt, 20 wt.% Ru) the mean metal particle size is less than 4 nm. Meanwhile, the calculated Pt crystalline lattice parameter and Pt (2 2 0) peak position indicated that the geometric structure of Pt was modified by Ru atoms. Among the prepared catalysts, the lattice parameter of 40–20 wt.% PtRu/C contract most. Cyclic voltammetry (CV), chronoamperometry (CA), CO stripping and single direct methanol fuel cell tests jointly suggested that the 40–20 wt.% PtRu/C catalyst has the highest electrochemical activity for methanol oxidation.

Direct methanol alkaline fuel cell with catalysed metal mesh anodes

Platinised Ti electrodes were produced by thermal decomposition of H 2PtCl 6 onto mesh substrates. Surface properties and morphology of the Pt deposit on Ti mesh were characterised using SEM, EDAX and XRD analysis. A highly dispersed Pt deposit layer was obtained. Direct methanol fuel cell tests with anion exchange membrane electrolyte and platinised Ti mesh anode were carried out and compared to conventional hot-pressed carbon supported Pt catalysts. The platinised Ti mesh anode showed higher catalytic activity than the conventional Pt/C electrode, and gave a stable fuel cell performance.

Novel Process for Improved Long-Term Stability of DMFC Membrane-Electrode Assemblies

In this work, we introduce a new process for fabrication of Nafion(reg. sign)-based membrane-electrode assemblies for direct methanol fuel cell (DMFC). The process relies on modified formulation of catalyst inks and high-temperature treatment of the MEA, both leading to improved long-term performance stability relative to standard MEAs used in DMFC testing at LANL. Unrecoverable performance loss of the novel MEA after 3000-hour life test at a cell voltage of 0.50 V has been estimated to be {approx}10 mV (at 100 mA cm{sup -2}). This number compares favorably with the performance loss of standard MEA, measured at {approx} 60 mV under the same conditions. Most of the gains are due to improved prope rties of the Pt-Ru anode. In particular, better stability of the anode helps to reduce loss in the catalyst surface area over time, providing net performance gain of 25-30 mV over the standard anode. Improved anode stability also reflects on long-term performance of the cathode thanks to reduced methanol crossover at long operating times of the cell ({approx} 15 mV gain).