The development of PTFE/Nafion/TEOS membranes for application in moderate and high temperature proton exchange membrane fuel cells

PTFE/Nafion (PN) and PTFE/Nafion/TEOS (PNS) membranes were fabricated for the application of moderate and high temperature proton exchange membrane fuel cells (PEMFCs), respectively. Membrane electrode assemblies (MEAs) were fabricated by PTFE/Nafion (and PTFE/Nafion/TEOS) membranes with commercially available low and high temperature gas diffusion electrodes (GDEs). The effects of relative humidity, operation temperature, and back pressure on the performance and durability test of the as-prepared MEAs were investigated. Incorporating TEOS into a PNS membrane and adding another layer of carbon onto a GDE would result in low membrane conductivity and low fuel cell performance respectively. However, in this work it is shown that HT-PNS MEAs demonstrate a higher performance than LT-PN MEAs in severe conditions - high temperature (118 °C) and low humidity (25% RH). The TEOS and additional carbon layer function as water retaining agents which are especially important for high temperature and low humidity conditions. The HT-PNS MEA showed good stability in a 50 h fuel cell test at high temperature, moderate relative humidity (50% RH) and back pressure of 14.7 psi.     

Optimum compositions of membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell

We investigated the effects of the compositions of catalyst layers and diffusion layers on performances of the membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell. The performances of the MEAs with different thicknesses of Nafion membranes were compared in this work. The optimal compositions in the anode are: 20 wt% Nafion content and 3.6 mg cm⁻² Pt loading in the catalyst layer, and 30 wt% PTFE content and 1 mg cm⁻² carbon black loading in the diffusion layer. In the cathode, MEA with 20 wt% Nafion content in the catalyst layer and 30 wt% PTFE content in the diffusion layer presented the optimal performance. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm⁻² among the three MEAs with different Nafion membranes. Copyright © 2009 John Wiley & Sons, Ltd.     

Investigation of degradation behavior of membrane electrode assembly with polytetrafluoroethylene/Nafion composite membrane

The main purpose of this study is to investigate the accelerated degradation of the PTFE/Nafion membrane electrode assembly through open circuit voltage and relative humidity cycling. The state of a PTFE/Nafion membrane electrode assembly is evaluated in a fuel cell by monitoring the polarization curve, AC impedance, cyclic voltammetry, and linear sweep voltammetry data over time. The experimental results are then fitted to equations of an equivalent circuit. The results of the first 160 experimental cycles show that catalyst degradation is the main cause for the decay of the membrane electrode assembly. During the 160-520th cycles, the membrane electrode assembly experiences creep deformation, which is due to relative humidity cycling. During the 640-840th cycles, the degradation causes a gradual transition from minor to major membrane cracking; after which the combustion reaction dramatically accelerates membrane electrode assembly degradation.     

Effects on the electrochemical performance of surface-modified mordenite in a PTFE Nafion composite membrane for direct methanol fuel cells

Mordenite (MOR)/PTFE Nafion composite membranes were produced by impregnating Nafion solutions in a PTFE porous support with a modified form of MOR. A 3-mercaptopropyl triethoxysilane (MPTES) - MOR mixture is used as a filler in the PTFE Nafion membrane to block methanol crossover and increase water uptake. An experiment was conducted with a membrane without M-MOR (PTFE Nafion membrane) and a membrane with M-MOR at 2, 4, and 10 wt% (2-, 4-, and 10-M-MOR/PTFE Nafion membranes, respectively). In order to improve the interfacial properties, surface modification of the zeolite was conducted with a silane coupling reaction. The MOR samples were analyzed by SEM, SEM-EDS, and BET surface area analysis. The Nafion membrane was analyzed by SEM, according to the water uptake, and by an electrochemical analysis. The 10-M-MOR/PTFE Nafion membrane showed greater water uptake of 75% compared to the PTFE Nafion membrane without M-MOR (28%). It was found that the addition of MOR had a positive effect on the reduction of the methanol permeability of the MOR/PTFE Nafion membrane. The DMFC (Direct Methanol Fuel Cell) power density of MEA with the 4-M-MOR/PTFE Nafion membrane (4-M-MOR MEA) was found to be 71% higher than that of the PTFE Nafion membrane (0-M-MOR MEA).     

Diagnosis of PTFE‐Nafion MEA degradation modes by an accelerated degradation technique

Cost and durability are central issues in the commercialization of proton‐exchange membrane fuel cells. This study used complex accelerated‐degradation protocols to diagnose the degradation modes of low‐cost polytetrafluoroethylene–Nafion membrane electrode assemblies (MEAs). Acceleration protocols included open‐circuit voltage, relative humidity cycling, and load cycling. Failure modes included measurements of cyclic voltammetry, linear sweep voltammetry, polarization curve, and AC impedance. The four modes (stages) of degradation were determined to be (i) catalyst aging, (ii) creep deformation, (iii) pinhole formation, and (iv) membrane failure. During catalyst aging, the maximum power density decreased by 0.117 mW cm^?2 cycle^?1. After the 280th cycle, creep deformation occurred, and the maximum power density decreased by 0.227 mW cm^?2 cycle^?1. Pinholes led to membrane failure and a final dramatic loss of performance (?0.453 mW cm^?2 cycle^?1). Therefore, membrane failure is the major factor in the failure of MEAs. Copyright © 2010 John Wiley & Sons, Ltd.     

High temperature operation of PEMFC: A novel approach using MEA with silica in catalyst layer

Composite membranes with inorganic substances can retain water and allow the operation of polymer electrolyte membrane fuel cells (PEMFCs) at high temperature under low humidity. In this work, the single cell was operated at high temperature using silica–Nafion composite membrane in addition with silica in catalyst layer. The cell was operated at various temperatures under different relative humidity conditions. We observed that the single cell performance decreased steeply as the cell temperature increased. The role of silica in the catalyst layer at high temperature operation was studied by varying the silica content in the catalyst layers. There was a gradual decrease in cell performance when the silica content increased in catalyst layer. The single cell performance of membrane electrode assemblies (MEAs) with composite membrane and electrode was higher than that of MEA with commercial Nafion 112 membrane for high temperature operation.     

Sulfonated polyimide/PTFE reinforced membrane for PEMFCs

A sulfonated polyimide (SPI)/PTFE reinforced membrane was synthesized by impregnating PTFE (porous polytetrafluoroethylene) membrane with a SPI/DMSO solution. The resulting composite membrane was mechanically durable and quite thin relative to traditional perfluorosulfonated ionomer membranes (PFSI). We expect the PTFE to restrict the swelling and dimensional change of the SPI when it is immersed into water. And the reinforcement with PTFE can increase the hydrolysis stability of the SPI due to the low swelling and low dimensional change. From ex-situ testing and a short term fuel cell “life” test, we conclude that the PTFE reinforced sulfonated polyimide had a higher hydrolytic stability than pure sulfonated polyimide. The thin SPI/PTFE membrane showed comparable fuel cell performance with the commercial NRE-212 membrane with H₂/O₂ at 80 °C under fully humidified conditions. The chemically modified membrane with Nafion layer showed good stability in a 120 h fuel cell test.     

Sulfonated poly(ether ether ketone) as an ionomer for direct methanol fuel cell electrodes

Sulfonated poly(ether ether ketone) has been investigated as an ionomer in the catalyst layer for direct methanol fuel cells (DMFC). The performance in DMFC, electrochemical active area (by cyclic voltammetry), and limiting capacitance (by impedance spectroscopy) have been evaluated as a function of the ion exchange capacity (IEC) and content (wt.%) of the SPEEK ionomer in the catalyst layer. The optimum IEC value and SPEEK ionomer content in the electrodes are found to be, respectively, 1.33 meq. g⁻¹ and 20 wt.%. The membrane-electrode assemblies (MEA) fabricated with SPEEK membrane and SPEEK ionomer in the electrodes are found to exhibit superior performance in DMFC compared to that fabricated with Nafion ionomer due to lower interfacial resistance in the MEA as well as larger electrochemical active area. The MEAs with SPEEK membrane and SPEEK ionomer also exhibit better performance than that with Nafion 115 membrane and Nafion ionomer due to lower methanol crossover and better electrode kinetics.     

Construction of low-cost and long-time proton exchange membranes by using slow-release radical scavenger

The bottlenecks of commercial application of proton exchange membranes (PEM) fuel cell are cost and oxidation stability of PEM. Hence, we encapsulate Resveratrol (Res, a kind of reductant) in hydroxypropyl-β-cyclodextrins (CDs) to prepare the inclusion complexes of Res and CDs (Res@CDs) under the guidance of theoretical arithmetic. Then the Res@CDs are evenly dispersed in Nafion emulsion, which is subsequently combined with porous polytetrafluoroethylene (PTFE) substrate by emulsion pouring method to form the antioxidative composite membrane (Res@CDs-Nafion/PTFE). The as-prepared Res@CDs-Nafion/PTFE shows the similar performance on proton conductivity (103.9 mS cm⁻¹) and hydrogen-air fuel cell (317.84 mW cm⁻²) compared to the Nafion/PTFE composite membrane. The content of Nafion in the Res@CDs-Nafion/PTFE is less than 30%, which dramatically reduces the production cost compared to pure Nafion membrane. The weight loss of Res@CDs-Nafion/PTFE and Nafion/PTFE immersed in Fenton's reagent after 36 h is 4.97% and 16.49%, respectively, which demonstrate that Res@CDs can enhance oxidation stability of composite membrane. The Res@CDs-Nafion/PTFE offer huge merits of low cost and enhanced oxidation stability, which greatly promotes the application process of long-lifetime PEM fuel cell.     

Effects of membranes thickness on performance of DMFCs under freeze-thaw cycles

The effect of membrane thickness on the performance degradation of DMFCs was investigated during freeze-thaw cycling across temperatures ranging from −32 °C to 60 °C. Three cells with Nafion membranes of varying thickness were prepared: Nafion 112, Nafion 115 and Nafion 117. Performance degradation was evaluated by comparing the changes to electrode polarization, electrochemical impedance and cyclic voltammetry over a range of freeze-thaw cycles. It was determined that freeze-thaw cycling affected the performance of the three membrane electrode assemblies (MEA). A cell with a Nafion 112 membrane showed a more significant increase in cathode overpotential than cells with either a Nafion 115 or Nafion 117 membrane. The charge transfer resistance of the cell with a thin membrane was more affected by freeze-thaw cycles than the cells with thicker membranes. All three cells showed a significant decrease in ECSA after freeze-thaw cycles. Freeze-thaw cycles damaged the triple phase boundary region and decreased the ECSA of cells with thinner membranes, which cause rapid performance degradation. The use of thick membranes in MEAs was determined to be the effective method for reducing performance degradation during freeze-thaw cycling.     

Performance enhancement of membrane electrode assemblies with plasma etched polymer electrolyte membrane in PEM fuel cell

In this work, a surface modified Nafion 212 membrane was fabricated by plasma etching in order to enhance the performance of a membrane electrode assembly (MEA) in a polymer electrolyte membrane fuel cell. Single-cell performance of MEA at 0.7 V was increased by about 19% with membrane that was etched for 10 min compared to that with untreated Nafion 212 membrane. The MEA with membrane etched for 20 min exhibited a current density of 1700 mA cm⁻² at 0.35 V, which was 8% higher than that of MEA with untreated membrane (1580 mA cm⁻²). The performances of MEAs containing etched membranes were affected by complex factors such as the thickness and surface morphology of the membrane related to etching time. The structural changes and electrochemical properties of the MEAs with etched membranes were characterized by field emission scanning electron microscopy, Fourier transform-infrared spectrometry, electrochemical impedance spectroscopy, and cyclic voltammetry.     

Performance improvement by a glue-functioned Nafion layer coating on gas diffusion electrodes in PEM fuel cells

To reduce the performance difference of membrane electrode assembles (MEAs) between catalyst coated membrane (CCM) and gas diffusion electrode (GDE) methods, this study presents a novel structure of a glue-functioned Nafion layer coating on the catalyst layer of GDEs to enhance performance. The process of hot pressing is omitted from the membrane electrode assembly (MEA) fabrication in this study. In exploring the effect of the glue-functioned Nafion layer, five MEAs are compared, including one made by the CCM method and four by the GDE method. Loadings of 0, 0.1, 0.3 and 0.5 mg cm⁻² of glue-functioned Nafion layer are coated on the surface of the catalyst layer at room temperature with a condensed Nafion solution (20 wt%). The performance of the 0.3 mg cm⁻² Nafion layer coating improves 55% compared with that without an extra Nafion layer for the peak power density, and the performance difference reduces from 61.2% to 39.4% when compared with the one using the CCM method. However, the performance of the 0.5 mg cm⁻² Nafion layer coating is almost the same as without the Nafion layer. This indicates that increased Nafion cannot guarantee higher performance.     

Degradation on a PTFE/Nafion membrane electrode assembly with accelerating degradation technique

Cost and durability are the main issues of Proton exchange membrane fuel cells (PEMFCs) commercializing. This study uses the accelerate degradation technique to analyze the durability of low cost PTFE/Nafion membrane electrode assembly (MEA). Before the MEA degradation experiment, the MEA must be activated at 65 °C until the performance is stable. Then increase the operation temperature to 80 °C. The experimental process for MEA degradation contains three steps in one cycle. The first step is to open circuit voltage (OCV) for 30 s under R.H. 100%. Then, set 0.6V for 150 s under R.H. 100%. The final step is to set 0.6V for 120 s under R.H. 0%. These three steps take around 5 min to complete. This MEA degradation experiment process includes the OCV, potential cycles, and R.H. cycles. This study uses the polarization curve, AC impedance, cyclic voltammetry (CV), linear sweep voltammetry (LSV), equations and equivalent circuit to analyze state of the MEA. At less than 160 experiment cycles, the result show that catalyst degradation is the main reason for the decay of MEA. After 280 cycles, the MEA begins to exhibit creep deformation due to the R.H. cycle. Electrochemical surface area and high frequency resistance can be used to estimate the degree of MEA degradation approximately.     

Enhancement of direct methanol fuel cell performance through the inclusion of zirconium phosphate

Nafion/zirconium hydrogen phosphate (ZrP) composite membranes containing 2.5 wt.% ZrP (NZ-2.5) or 5 wt.% ZrP (NZ-5) were prepared to improve the performance of a direct methanol fuel cell (DMFC). The influence of ZrP content on the Nafion matrix is assessed through characterization techniques, such as Thermogravimetric Analysis (TGA), X-ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Electrochemical Impedance Spectroscopy (EIS), and water uptake measurement. Performance testings of the DMFCs based on these composite membranes as well as commercial Nafion^® 115 membrane were performed using a computer aided fuel cell test station for different values of cell temperature (40 °C, 60 °C, 80 °C, and 100 °C) and methanol concentration (0.75 M, 1.00 M, and 1.50 M). Characterization studies indicated that incorporation of ZrP into polymer matrix enhanced the water uptake and proton conductivity values of Nafion membrane. The results of the performance tests showed that the Membrane Electrode Assembly (MEA) having NZ-2.5 provided the highest performance with the peak power density of 551.52 W/m² at 100 °C and 1.00 M. Then, the performances of the MEAs having the same NZ-2.5 membrane but different cathode catalysts were investigated by fabricating two different MEAs using cathode catalysts made of Pt/C–ZrP and Pt/C (HiSPEC^® 9100). According to the results of these experiments, the MEA having NZ-2.5 membrane and Pt/C (HiSPEC^® 9100) cathode catalyst containing 10 wt.% of ZrP exhibited the highest performance with the peak power density of 620.88 W/m² at 100 °C and 1.00 M. In addition, short-term stability tests were conducted for all the MEAs. The results of the stability tests revealed that introduction of ZrP to commercial (HiSPEC^® 9100) cathode catalyst improves its stability characteristics.     

Pt/CNTs-Nafion reinforced and self-humidifying composite membrane for PEMFC applications

A novel thin three-layer reinforced and self-humidifying composite membrane has been developed for PEMFCs. The membrane has two outer layers of plain Nafion and a middle layer of Pt/carbon nanotubes (Pt/CNTs) dispersed Nafion. The Pt/CNTs present in the membrane provides the sites for the catalytic recombination of H₂ and O₂ permeating through the membrane from the anode and cathode to produce water and improve the mechanical properties of the composite membrane at the same time. The water produced directly humidifies the membrane and allows the operation of PEMFCs with dry reactants. The electrochemical performance and mechanical properties of the composite membranes are compared with those of a commercial Nafion^® membrane. The self-humidifying composite membrane could minimize membrane conductivity loss under dry conditions and improve mechanical strength due to the presence of the Pt/CNTs.     

Novel multilayer Nafion/SPI/Nafion composite membrane for PEMFCs

A novel multilayer membrane for the proton exchange membrane fuel cell (PEMFC) was developed. Nafion was dispersed uniformly onto both sides of the sulfonated polyimide (SPI) membrane. The Nafion/SPI/Nafion composite membrane was prepared by immersing the SPI into the Nafion-containing casting solution. Through immersing both membranes into the Fenton solution at 80 °C for 0.5 h for an accelerated ex situ test, chromatographic analysis of the water evacuated from the cathode and the anode of the cells and a durability test of a single proton exchange membrane fuel cells, it was proved that the stability of the composite membrane has been greatly improved by adding the Nafion layer compared with the SPI membrane. The fuel cell performance with the SPI and Nafion/SPI/Nafion membranes was similar to the performance with the commercial product Nafion^® NRE-212 membrane at 80 °C.     

Fabrication of MEA with hydrocarbon based membranes using low temperature decal method for DMFC

The membrane electrode assembly (MEA) with hydrocarbon (HC) based membranes made by a low temperature decal method has been investigated for the direct methanol fuel cells (DMFCs). The conventional low temperature decal (LTD) transfer method (comprised of three layers; viz., carbon, Nafion bonded electrodes and outer ionomer layers over the decal Teflon substrates) meant for the MEAs made of Nafion type membranes is suitably modified to use with hydrocarbon (HC) based membranes. The modification of conventional LTD method is effected by means of modulating the three-layered structure and optimizing other parameters to facilitate complete transfer of catalyst layers onto the HC membranes. The MEAs prepared by the modified LTD method have yielded 21 % higher DMFC performance compared to that of the MEAs produced by conventional LTD method. The structure and electrochemical properties of the MEAs have been analyzed by the field-emission scanning electron microscopy (FE-SEM) and the electrochemical impedance spectroscopy (EIS).     

Development of a novel decal transfer process for fabrication of high-performance and reliable membrane electrode assemblies for PEMFCs

To improve the performance of a polymer electrolyte membrane fuel cell (PEMFC), various membrane electrode assemblies (MEAs) were fabricated by the decal process. When peeling the decal films away from a Nafion membrane, a novel liquid nitrogen (LN₂) freezing method was employed. The results of a Fourier Transform Infrared (FTIR) analysis of the Nafion membranes demonstrate that this proposed method has no impact on the molecular structure of the Nafion polymer. In addition, the method makes it possible to achieve complete decal transferring under a wide range of hot-pressing pressures and temperatures: 9.8-15.7 MPa and 100-140 °C, respectively. Another approach to optimize the decal technique is to dry catalyst layers under vacuum. Catalyst layers dried under vacuum show better cell performances than atmospherically dried ones. Vacuum drying significantly facilitates the formation of small pores within Pt/C agglomerates on catalyst layers. Third, the use of Additive-A as a commercial dispersant in the catalyst ink has been investigated. From rheological characterizations, including thixotropy and catalyst ink viscosity, it is obvious that the additive plays an important role in elevating the dispersion stability of the ink. In addition, surface images of the catalyst layers revealed that the dispersing agent reduces cracks or fractures within the layers. Although adding Additive-A did not have an effect on the single-cell performance, the MEAs with the dispersant are expected to have better results for a long-term performance test of a single cell.     

Correlations between the catalytic layer composition,the relative humidity and the performance for PEMFC carbon aerogel based membrane electrode assemblies

The parameters influencing the water management or the gas diffusion have a large impact on the performances of the PEMFC. In this work, we focused on the influence of the composition of the cathode catalytic layer on the performance of the MEA in relation to the relative humidity. A carbon aerogel was synthesized as catalyst support with a texture minimizing diffusive losses. We studied the influence of the Nafion content by preparing various cathode catalytic layers whose hydric properties were modified by adding PTFE as a hydrophobic agent. We evaluated the impact of the cathodic relative humidity by testing MEAs on a single cell test bench. We showed that modifying the composition of the catalytic layers playing on the content of both Nafion and PTFE, enables to improve the performance due to a better water management. The best performance was obtained with a Nafion/Carbon mass ratio (N/C) of 0.5 and a PTFE/Carbon (PTFE/C) mass ratio of 0.35 allowing 40% more power at 0.4 V and 75%RH. A first positive evaluation of the impact of PTFE is also done with TEC10E40E.     

New modified Nafion-bisphosphonic acid composite membranes for enhanced proton conductivity and PEMFC performance

Proton exchange membranes remain a crucial material and a key challenge to fuel cell science and technology. In this work, new Nafion membranes are prepared by a casting method using aryl- or azaheteroaromatic bisphosphonate compounds as dopants. The incorporation of the dopant, considered at 1 wt% loading after previous selection, produces enhanced proton conductivity properties in the new membranes, at different temperature and relative humidity conditions, in comparison with values obtained with commercial Nafion. Water uptake and ionic exchange capacity (IEC) are also assessed due to their associated impact on transport properties, resulting in superior values than Nafion when tested in the same experimental conditions. These improvements by doped membranes prompted the evaluation of their potential application in fuel cells, at different temperatures. The new membranes, in membrane-electrode assemblies (MEAs), show an increased fuel cell maximum power output with temperature until 60 °C or 70 °C, followed by a decrease above these temperatures, a Nafion-like behaviour when measured in the same conditions. The membrane doped with [1,4-phenylenebis(hydroxymethanetriyl)]tetrakis(phosphonic acid) (BP2) presents better results than Nafion N-115 membrane at all studied temperatures, with a maximum power output performance of ～383 mW cm^?2 at 70 °C. Open circuit potentials of the fuel cell were always higher than values obtained for Nafion MEAs in all studied conditions, indicating the possibility of advantageous restrain to gas crossover in the new doped membranes.