Impact of PTFE distribution across the GDL on the water droplet removal from a PEM fuel cell electrode containing binder

By progress of the new generation of electrical powertrains and reducing of the fossil fuel resources, vehicle industry becomes more interested in utilizing proton exchange membrane (PEM) fuel cells. However, practical utilization of them is faced with some challenges including liquid water accumulation in the porous electrodes. The common belief for mitigating this issue is the treatment of electrodes' gas diffusion layers (such as carbon papers consisting of carbon fibers and binder for binding fibers) with a highly hydrophobic material such as poly-tetra-fluoro-ethylene (PTFE). In the current investigation, 3D stochastic reconstructions and 3D lattice Boltzmann simulations are employed to discover the impact of PTFE distribution as well as the role of binder content on the removal process of a water droplet from a PEM fuel cell electrode for the first time. Nine different simulations with three dissimilar PTFE distributions and three dissimilar binder contents are implemented. The results demonstrate that the PTFE distribution and the existence of binder can greatly affect the removal efficiency of water droplet from gas diffusion layer. Unexpectedly, for higher binder contents, the uniform distribution of PTFE is less effective. Besides, for a specific PTFE distribution adding binder can effectively hinder the removal process of droplet.     

Mechanism of action of polytetrafluoroethylene binder on the performance and durability of high-temperature polymer electrolyte fuel cells

In this work, new insights into impacts of the polytetrafluoroethylene (PTFE) binder on high temperature polymer electrolyte fuel cells (HT-PEFCs) are provided by means of various characterizations and accelerated stress tests. Cathodes with PTFE contents from 0 wt% to 60 wt% were fabricated and compared using electrochemical measurements. The results indicate that the cell with 10 wt% PTFE in the cathode catalyst layer (CCL) shows the best performance due to having the lowest mass transport resistance and cathode protonic resistance. Moreover, cyclic voltammograms show that Pt (100) edge and corner sites are significantly covered by PTFE and phosphate anions when the PTFE content is higher than 25 wt%. Open-circuit and low load-cycling conditions are applied to accelerate degradation processes of the HT-PEFCs. The PTFE binder shows a network structure in the pores of the catalyst layer, which reduces phosphoric acid leaching during the aging tests. In addition, the high binder HT-PEFCs more easily suffer from a mass transport problem, leading to more severe performance degradation.     

Electrode compositions for carbon power supercapacitors

The purpose of this work was to prepare economically-large electrodes in order to assemble mono-element supercapacitors with a capacity of more than 300 F and to reach powers in the range of 125 W (2 V/element) with the selected structure. In this paper, results are presented from different binder compositions—PTFE and a mixture of CMC/PTFE—that were tested in order to increase the volumetric capacitance of the electrode. The electrode composition was adapted to each binder composition used. For cost reasons, the amount of PTFE was reduced and the mixture of CMC/PTFE was examined as a cheaper alternative. We have obtained more than 25 F cm⁻³ per electrode, with a time constant close to 3 s, and power outputs compatible with automotive applications.     

Optimization of cathode microporous layer materials for proton exchange membrane fuel cell

The microporous layer (MPL) of diffusion medium has an important impact on the water management ability of proton exchange membrane fuel cells. In this study, six kinds of carbon black were used to prepare the cathode MPL. The thickness, conductivity, pore structure, hydrophobicity, and surface microstructure of MPL were characterized. The single cell was prepared and electrochemical tests were performed. The results showed that the single cell prepared by Acetylene black (ACET) and Vulcan XC-72R has a considerable power generation performance. In addition, polyvinylidene fluoride hexafluoropropylene copolymer P(VDF-HFP) was used to replace Polytetrafluoroethylene (PTFE) as hydrophobic binder. MPL with different P(VDF-HFP) contents were prepared, and the single cell performance was investigated. The results showed that all the single cells prepared by P(VDF-HFP) were worse than that of PTFE. This study provides an important reference for further improving the performance of fuel cells from the perspective of material optimization with MPL.     

Effect of polymer binders in anode catalyst layer on performance of alkaline direct ethanol fuel cells

In preparing low-temperature fuel cell electrodes, a polymer binder is essential to bind discrete catalyst particles to form a porous catalyst layer that simultaneously facilitates the transfer of ions, electrons, and reactants/products. For two types of polymer binder, namely, an A3-an anion conducting ionomer and a PTFE-a neutral polymer, an investigation is made of the effect of the content of each binder in the anode catalyst layer on the performance of an alkaline direct ethanol fuel cell (DEFC) with an anion-exchange membrane and non-platinum (non-Pt) catalysts. Experiments are performed by feeding either ethanol (C₂H₅OH) solution or ethanol–potassium hydroxide (C₂H₅OH–KOH) solution. The experimental results for the case of feeding C₂H₅OH solution without added KOH indicate that the cell performance varies with the A3 ionomer content in the anode catalyst layer, and a content of 10 wt.% exhibits the best performance. When feeding C₂H₅OH–KOH solution, the results show that: (i) in the region of low current density, the best performance is achieved for a membrane electrode assembly without any binder in the anode catalyst layer; (ii) in the region of high current density, the performance is improved with incorporation of PTFE binder in the anode catalyst layer; (iii) the PTFE binder yields better performance than does the A3 binder.     

Dilatometry studies of phosphotungstic acid pellets during hydration and dehydration processes and design of a room temperature fuel cell

Phosphotungstic acid (PWA) is still a potential solid electrolyte for fuel cells due to its high conductivity at room temperature. In this work, PWA was synthesised and characterised by X-ray, IR spectroscopy and thermal analysis. Proton number was determined by potentiometry. Characterization results agreed with those already published. Water of crystallization was determined by weight loss method on heating to anhydrous form. Changes in pellet thickness and diameter were measured as a function of water of crystallization and time for dehydration/hydration processes in controlled humid atmospheres. Geometric change parameter ΔL/L, where ΔL is the change in the volume (either thickness or diameter, and L is the initial dimension) was determined for each sample. After an initial hydration or dehydration process, pellets were subsequently dehydrated or hydrated in order to study the reversibility of the geometric changes. Effects of presence of polytetrafluoroethylene (PTFE) as a binder, pelletizing pressure and particle size of PWA powder are discussed. It was found that changes in pellet diameter were greater than the pellet thickness irrespective of the pelletizing pressure and particle size. Changes in pellet thickness were lower for pellets made with higher pelletizing pressure and greater particle size, whereas changes in the pellet diameter were almost identical. The presence of PTFE lowered both the dimensional changes and the rate at which water of crystallization was lost. Interestingly, steps in the changes in pellet thickness during hydration and dehydration were observed in line with the loss of water of crystallization. Possibility of pinhole generation in the dehydrated pellets was also observed. A new fuel cell was designed, which eliminated the need of sealing the electrolyte completely in the cell. The hydrogen and oxygen gases were prehumidified to 87% relative humidities, and there was no accumulation of water at the cathode side of the fuel cell. An open circuit potential of ∼0.7 V was observed for about 40 h during the testing of this cell. Further testing and use of this design incorporating PWA and other hydrated forms of proton conductors such as polymer membranes is suggested.     

SolarPV 3D在光电建筑中光伏发电系统设计时的应用研究

自主开发了一款基于SketchUp软件的三维数字化设计插件SolarPV 3D,并对该插件的主要算法进行了介绍,算法包括光伏阵列自动化布置算法和逐时阴影遮挡计算方法,解决了光伏发电系统数字化设计过程中的关键技术难题,并提高了其设计效率。以河海大学常州校区英才楼的屋面光伏发电系统布置为例,结合无人机拍照三维成像技术及SolarPV 3D,完成了屋面光伏发电系统的布置及系统性能的仿真。SolarPV 3D使光伏发电系统设计的三维可视化程度得到明显提升,此外其还可提供光伏发电系统的性能数据,从而提高了光伏发电系统设计的经济性和实用性,具有重要的工程应用价值。     

Stochastic 3D modeling of non-woven materials with wet-proofing agent

A novel, realistic 3D model is developed describing the microstructure of non-woven GDL in PEMFC which consists of strongly curved and non-overlapping fibers. The model is constructed by a two-stage procedure. First we introduce a system of random fibers, where the locations of their midpoints are modeled by a 3D Poisson point process and the fibers themselves by random 3D polygonal tracks which represent single fibers in terms of multivariate time series. Secondly, we transform the random fiber system into a system of non-overlapping fibers using an iterative method leaned on the so-called force-biased algorithm. The model is validated by comparing transport-relevant characteristics computed for experimental 3D synchrotron data, and for realizations sampled from the stochastic microstructure model. Finally, we suggest a model for the spatial distribution of PTFE, a wet-proofing agent often used in non-woven GDL, and combine this PTFE model with our new microstructure model for non-woven GDL.     

A novel and efficient water-based composite binder for LiCoO2 cathodes in lithium-ion batteries

The dispersion, adhesion strength, electrical, and electrochemical properties of LiCoO₂ cathodes in lithium-ion batteries with the addition of a new composite binder composed of two acrylic emulsions, poly(butyl acrylate)-based (PBA) and polyacrylonitrile-based (PA) latex in a ratio of 3:7, were evaluated. PBA binder has a low-glass transition temperature of 10 °C, which can improve the flexibility of the electrode. This new composite binder has a very good binding ability as same as the typical organic solvent-based binder, poly(vinylidene fluoride). The dispersions of the water-based cathode slurries with the composite binder were measured by analyzing the viscosity and sedimentation behaviors. The results show that the new composite binder can well disperse the LiCoO₂. Moreover, using the new composite binder could greatly improve the rate capabilities and the cycle stability of water-based LiCoO₂ cathodes.     

Effect of variation of hydrophobicity of anode diffusion media along the through-plane direction in direct methanol fuel cells

Reducing methanol crossover from the anode to cathode in direct methanol fuel cells (DMFCs) is critical for attaining high cell performance and fuel utilization, particularly when highly concentrated methanol fuel is fed into DMFCs. In this study, we present a novel design of anode diffusion media (DM) wherein spatial variation of hydrophobicity along the through-plane direction is realized by special polytetrafluoroethylene (PTFE) coating procedure. According to the capillary transport theory for porous media, the anode DM design can significantly affect both methanol and water transport processes in DMFCs. To examine its influence, three different membrane-electrode assemblies are fabricated and tested for various methanol feed concentrations. Polarization curves show that cell performance at high methanol feed concentration conditions is greatly improved with the anode DM design with increasing hydrophobicity toward the anode catalyst layer. In addition, we investigate the influence of the wettability of the anode microporous layer (MPL) on cell performance and show that for DMFC operation at high methanol feed concentration, the hydrophilic anode MPL fabricated with an ionomer binder is more beneficial than conventional hydrophobic MPLs fabricated with PTFE. This paper highlights that controlling wetting characteristics of the anode DM and MPL is of paramount importance for mitigating methanol crossover in DMFCs.     

Electrochemical characterisation of air electrodes based on La0.6Sr0.4CoO3 and carbon nanotubes

The efficiency of fuel cells suffers from the high activation polarisation at the cathode, where the oxygen reduction reaction takes place. In order to improve the performance, air electrodes composed of carbon nanotubes (CNTs) and the perovskite La_0.6Sr_0.4CoO₃ are produced by two different methods and investigated. In the first method CNTs are directly grown on the perovskite and in the second method CNTs and perovskite are combined by ultrasonic mixing. Their catalytic activity towards oxygen reduction in alkaline solution is evaluated by polarisation curves and electrochemical impedance spectroscopy. Best performance shows the electrode composed of 25 wt% CNTs, 55 wt% La_0.6Sr_0.4CoO₃ and 20 wt% PTFE as binder, produced by ultrasonic mixing. The Nyquist plot of this electrode displays two potential-dependent semi-circles, accounting for processes on the catalyst surface and for processes depending on the morphology of the electrode.     

Aging in chemically prepared divalent silver oxide electrodes for silver/zinc reserve batteries

The instability of silver(II) oxide electrodes used in silver/zinc reserve batteries is the well known cause of capacity loss and delayed activation in reserve batteries after they are stored in the dry, unactivated state for extended periods of time. Metal contaminants in sintered/electroformed electrodes destabilize the oxide and the solid state reaction between AgO and elemental silver results in the formation of the lower capacity monovalent oxide Ag₂O. Chemically prepared (CP) AgO can be used to avoid the metal contaminants and to minimize the interfacial contact area between AgO and Ag, thus minimizing the affects of aging on the electrodes.Electrodes were fabricated with CP AgO and polytetrafluoroethylene (PTFE) binder and expanded silver metal current collectors. Experimentally, both electrode active material compacts (AgO and binder only) and electrodes complete with AgO/binder and silver current collector were tested to evaluate the influence of the current collector on aging. The electrode samples were discharged at a constant rate of 50 mA cm⁻² before and after storage at 60°C for 21 days as well as after storage at room ambient temperature conditions for 91 months.The results indicate that the affects of aging upon the AgO/binder compacts are insignificant for long term storage at room temperature. However, thermally accelerated aging at high temperature (60°C) affects both transient and stabilized load voltage as well as capacity. In terms of capacity, the AgO/binder mix itself looses about 5% capacity after 21 days dry storage at 60°C while electrodes complete with current collector loose about 8%. The 60% increase in capacity loss is attributed to the solid state reaction between AgO and elemental silver.     

Phosphoric acid‐doped electrodes for a PBI polymer membrane fuel cell

A study of a phosphoric acid (PA)‐doped polybenzimidazole (PBI) membrane fuel cell is reported. The fuel cell used polytetrafluoroethylene (PTFE) in the catalyst layer of the membrane electrode assembly to act as a binder and did not use PBI. The PTFE provided an amorphous phase to hold the PA added to the catalyst layers. The study investigated several parameters of the fuel cell electrode, catalyst layer including: PA loading, PTFE content and catalyst loading and wt% of Pt in the carbon supported catalysts and doping of the PBI membrane. There was a minimum amount of acid doping that gave good cell performance for oxygen reduction in the cathode layer. Good performance of the fuel cell was achieved at 120°C with air of 0.27 W cm^?2 using a 0.51 mg_Pt cm^?2 loading of catalyst. Peak power of 0.4 W cm^?2 was achieved with air at 150°C using a membrane doping of PA of 5.6 PRU (doped acid molecules per repeat polymer unit). Heat treatment of the PTFE‐bonded electrodes to increase hydrophobicity did not improve the cell performance. The effect of a perfluorinated surfactant although reported to enhance oxygen solubility in the catalyst layer led to a poorer cell performance. Copyright © 2010 John Wiley & Sons, Ltd.     

Power generation using carbon mesh cathodes with different diffusion layers in microbial fuel cells

An inexpensive carbon material, carbon mesh, was examined to replace the more expensive carbon cloth usually used to make cathodes in air-cathode microbial fuel cells (MFCs). Three different diffusion layers were tested using carbon mesh: poly(dimethylsiloxane) (PDMS), polytetrafluoroethylene (PTFE), and Goretex cloth. Carbon mesh with a mixture of PDMS and carbon black as a diffusion layer produced a maximum power density of 1355 ± 62 mW m⁻² (normalized to the projected cathode area), which was similar to that obtained with a carbon cloth cathode (1390 ± 72 mW m⁻²). Carbon mesh with a PTFE diffusion layer produced only a slightly lower (6.6%) maximum power density (1303 ± 48 mW m⁻²). The Coulombic efficiencies were a function of current density, with the highest value for the carbon mesh and PDMS (79%) larger than that for carbon cloth (63%). The cost of the carbon mesh cathode with PDMS/Carbon or PTFE (excluding catalyst and binder costs) is only 2.5% of the cost of the carbon cloth cathode. These results show that low cost carbon materials such as carbon mesh can be used as the cathode in an MFC without reducing the performance compared to more expensive carbon cloth.     

Regulation of the hydrolysis reaction performance of aluminum composites by PTFE and investigation of the hydrolysis mechanism

Improving the hydrolysis reaction properties of aluminum to prepare renewable and environmentally friendly hydrogen is of great interest for the application of mobile hydrogen sources. Here, a series of poly(tetrafluoroethylene)-based activated aluminum composites were prepared, and their microstructures and hydrogen generation properties were investigated in detail.The results show that PTFE can effectively increase the hydrolysis reaction rate of activated aluminum composites. The sample Al–8Bi-2PTFE exhibits the fastest hydrolysis reaction rate. Through observing the microscopic morphology of the hydrolyzed products of Al–8Bi-2PTFE at different reaction times, the catalytic role of PTFE in the hydrolysis reaction of the composite was proved. PTFE can accelerate the rupture of the particles of Al–8Bi-2PTFE during the hydrolysis reaction, thereby accelerating the hydrolysis reaction rate of aluminum.     

Single walled carbon nanotubes as supports for metal hydride electrodes

The electrochemical generation and the storage of hydrogen employing metal hydrides has become a good alternative attending the requirement to search for new sources of clean energy. This work is devoted to study the hydrogen storage as hydrides in an AB₅-type metal alloy (MmNi_4.1Co_0.4Mn_0.4Al_0.5). The behaviour of the alloy containing electrode was evaluated employing several electrodes containing the alloy and diverse carbons. Carbons were prepared by using Single Walled Carbon Nanotubes (SWCNT) with different PTFE percentages (15%, 25% y 33%) and Carbon Vulcan XC72® with 33% of PTFE (VT). Several electrochemical techniques as Cyclic Voltammetry (CV), Charge–Discharge cycles and Electrochemical Impedance Spectroscopy (EIS) were used. Results demonstrate that at low discharge currents, electrodes containing SWCNT exhibit better hydrogen storage than Vulcan XC72® containing electrodes. Studies made with carbon supports only show that this little but not disregarded differences are related to different hydrogen sorption behaviour of SWCNT and Vulcan XC72®. From the kinetic point of view, Vulcan XC72® containing electrodes have a better behaviour than those prepared with SWCNT. On the other hand, the optimal percentage of PTFE for SWCNT was determined to be 25%.     

Modeling of composite fibrous porous diffusion media

To engineer the desired properties of fibrous porous media, a parametric modeling approach is needed to support the rational design of the materials before the fabrication. In this study, we propose a methodology that enables the accurate representation of three-dimensional (3D) microstructures of fibrous porous media and prediction of their transport properties. Toray TGP-H-060 gas diffusion layer (GDL) is selected as an example to demonstrate the feasibility of the suggested design methodology. The detailed microstructure of the GDL with the inclusion of locally distributed binder is constructed using an extended periodic surface (PS) modeling technique. A 3D morphological approach is taken to create the binder distribution within the fibrous microstructure. Transport properties including permeability, relative diffusivity, and tortuosity and local structure characteristics of the generated microstructure, under different binder loading are calculated. It is shown that the detailed model of the fiber-binder composite has a strong influence on the predicted properties.     

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).     

The cobalt content effect on the electrochemical behavior of nickel hydroxide electrodes

In this paper, the study of nickel hydroxide porous electrodes containing different concentrations of cobalt as additive (2–10%), polytetrafluoroethylene (PTFE) as binder material and prepared by chemical impregnation on nickel sintered substrate, are presented. The characterization of the different electrodes is performed using optical techniques such as scanning electron microscopy (SEM), energy dispersive X-ray analysis (EDAX) and electrochemical techniques as cyclic voltammetry, charge-discharge curves and electrochemical impedance spectroscopy (EIS). The results indicate that the concentration of 5% metallic Co improves the electrochemical behavior of the active material.     

Sulfonated poly(fluorinated arylene ether)s/poly(N-vinylimidazole) blend polymer and PTFE layered membrane for operating PEMFC at high temperature

We prepared a novel phosphoric acid-doped sulfonated poly(fluorinated arylene ether)s/poly(N-vinylimidazole) blend polymer and poly(tetrafluoroethylene) (PTFE) layered membrane. The thermal and electrochemical properties of the new layered membrane were investigated by thermogravimetric analysis (TGA), electrochemical impedance spectroscopy (EIS), as well as the performance of its membrane electrode assembly (MEA). The morphology of membrane was observed with field emission scanning electron microscopy (FE-SEM) and energy dispersive spectroscopy (EDS). The fabricated membrane shows good mechanical property comparable to PBI membrane. The conductivity of new layered membrane was similar to that of polybenzimidazole (PBI) at high temperature. The single cell test showed that new PTFE layered membrane had a good performance over 150 °C. The PTFE layered membrane can be an alternative approach for PEMFC applications at high temperature.