Polytetrafluoroethylene Treatment Research Articles

Electrochemical performance and water management investigation of polymer electrolyte membrane fuel cell (PEMFC) using gas diffusion layer with polytetrafluoroethylene (PTFE) content gradients in through-plane direction

The water management capability and electrochemical performance of polymer electrolyte membrane fuel cell (PEMFC) using gas diffusion layers (GDLs) with polytetrafluoroethylene (PTFE) content variation in through-plane direction are investigated. Four PTFE content variation cases are considered by stacking GDLs with different amounts of PTFE content. Polarization curves and electrochemical impedance spectroscopy are experimentally obtained. The liquid water saturation and effective diffusion factor are numerically studied. In high humidity condition, a PEMFC containing a GDL without PTFE content variation and without PTFE treatment shows the best performance due to the highest effective diffusion factor and the lowest mass transport resistance. Despite the lowest liquid water saturation in a GDL with decreasing PTFE content toward channel, the effective diffusion factor of the GDL is not the highest because of the decreased porosity by PTFE treatment. In low humidity condition, a PEMFC containing a GDL with increasing PTFE content toward channel has the best performance because of the lowest ohmic resistance. The GDL has the largest liquid water saturation and the lowest effective diffusion factor, resulting in the better membrane hydration. In high current density, however, the performance of the PEMFC containing the GDL with increasing PTFE content toward channel drastically decreases due to large mass transport resistance.

Effects of the Anode Diffusion Layer on the Performance of a Nonenzymatic Electrochemical Glucose Fuel Cell with a Proton Exchange Membrane.

It is necessary to apply a nonenzymatic glucose fuel cell using a proton exchange membrane for an implantable biomedical device that operates at low power. The permeability of glucose with high viscosity and a large molecular weight in the porous medium of the diffusion layer was investigated for use in fuel cells. Carbon paper was prepared as an anode diffusion layer, and it was analyzed with a diffusion layer treated with polytetrafluoroethylene (PTFE) and a microporous layer (MPL). When untreated carbon paper was applied, the peak power density (PPD) and open-circuit voltage (OCV) increased as the glucose concentration and flow rate increased. On this occasion, the highest PPD of 17.81 μW cm–2 was achieved at 3 mM and a 2.0 mL min–1 glucose aqueous solution (at atmospheric pressure and 36.5 °C). The diffusion layer, which became more hydrophobic through PTFE treatment, adversely affected glucose permeability. In addition, the addition of an MPL decreased OCV and PPD with increasing glucose concentrations and flow rates. Compared with untreated carbon paper, the PPD was six times lower approximately. Consequently, it was confirmed that the properties of carbon paper, such as low hydrophobicity, high porosity, and thin thickness, would be advantageous for nonenzymatic glucose fuel cells.

Hydrophobicity of CO2 gas diffusion electrodes

In Nature Communications, Hu, Feng, and their colleagues present a scalable, facile method to tune the hydrophobicity of CO2 reduction gas diffusion electrodes by adding polytetrafluoroethylene (PTFE) nanopowder into the catalyst ink. For a copper catalyst CO2 reduction electrode, the additive significantly enhances CO2 reduction performance and imparts persistent hydrophobicity.

The Impact Evaluation of Factors on the Adhesion of Modified Polytetrafluoroethylene Films in a Glow Discharge Non-Thermal Plasma

Mechanical, chemical and plasma treatment are the main kind of treatment of polytetrafluoroethylene (PTFE) films. Each method is different from each other by the adhesive force: the value of the wetting angle. Mechanical treatment allows different particles to permeate into the structure of the polymer. Chemical treatment creates new functional groups on the polymer surface, but this method is toxic and dangerous. Plasma treatment, in a glow discharge non-thermal plasma, is a more ecological and practical method. The experiment showed that the plasma treatment successfully increases the adhesion, this has been proven by infrared spectroscopy and scanning electron microscopy. According to the obtained data of the wetting angle, the regression equation was derived. A graphical model is constructed by regression equations allows you to determine the main processing factor and choose the optimal values of treatment.

The hydrophilicity improvement of polytetrafluoroethylene by Ar plasma jet: The relationship of hydrophilicity, ambient humidity and plasma parameters

Currently, many kinds of plasma jets are used for enhancing the hydrophilicity of polytetrafluoroethylene (PTFE) attributed to the key radical OH by reaction with water. In order to increase the products of OH, the ambient humidity is regulated instead of working gas to avoid the instability of discharge in this paper. And the relationship of hydrophilicity, ambient humidity and OH density coupled with the Ar plasma jet characteristics is investigated. The results show that at water content of 6 mg/L, the electron excitation temperature and the argon metastable reach the peak value. Thus, the OH density reaches maximum of 4 × 1015 cm−3, whereas O atoms diminish gradually. Under this condition, more O-polar groups are injected into the surface because of the high production of OH, implying the dominant radical in the surface treatment of PTFE At the same time, the etching effect of the plasma is the strongest. Thus, the smallest contact angle is reduced to 37°. While OH product at water content of 2.3 mg/L is higher than that of 9.5 mg/L, the contact angle of the former is larger because of the etching effect.

PTFE treatment by remote atmospheric Ar/O2 plasmas: a simple reaction scheme model proposal

Polytetrafluoroethylene (PTFE) samples were treated by a remote atmospheric pressure microwave plasma torch and analyzed by water contact angle (WCA) and X-ray photoelectron spectroscopy (XPS). In the case of pure argon plasma a decrease of WCA is observed meanwhile an increase of hydrophobicity was observed when some oxygen was added to the discharge. The WCA results are correlated to XPS of reference samples and the change of WCA are attributed to changes in roughness of the samples. A simple kinetics scheme for the chemistry on the PTFE surface is proposed to explain the results.

PTFE Distributions in Diffusion Media of PEFCs and Related Effects on Water Transport

The diffusion media (DM) of polymer electrolyte fuel cells (PEFCs) typically consists of a macro-substrate (or gas diffusion layer, GDL) coated with a micro-porous layer (MPL). The GDL portion is manufactured from carbon fibers/binder and is often treated with polytetrafluoroethylene (PTFE) to increase the hydrophobicity. Similarly, the MPL is manufactured from a slurry containing carbon nanoparticles and PTFE. While the distribution of PTFE undoubtedly influences water transport within the DM, the actual 3-D distribution of PTFE is extremely difficult to capture using a single, standalone device. Therefore, the objective of this work is to examine the impact of PTFE distribution on water transport by generating various PTFE distributions within the measured 3-D structures of DM materials.A commercial DM sample was previously analyzed using X-ray computed tomography (XCT) and focused ion beam-scanning electron microscopy (FIB-SEM) to capture the 3-D structures of the GDL and MPL regions, respectively [1]. In this study, three different PTFE distribution cases are generated on the internal surface of the measured GDL structure: i) random PTFE surface pixels; ii) random PTFE surface clusters; and iii) wetting of PTFE into GDL crevices to mimic the PTFE treatment process used during manufacturing. Similarly, cases (i) and (ii) are generated in the measured MPL structure. In each case, PTFE is assigned to existing solid pixels so that the structure of the measured sample is preserved.The well-established pore morphology or full morphology (FM) model [1,2], used for determining capillary pressure-saturation behavior in uniformly wetting pore structures, was recently extended to account for locally different wetting surfaces in a structure [3,4]. The extended-FM approach is applied here in 3-D to capture the impact of the PTFE distributions on water transport in the DM, by assuming one contact angle for the carbon fiber/binder surface and a different contact angle for the hydrophobic PTFE surface. Two uniform surface conditions (i.e., traditional FM approach) are also considered for comparison: one in which an average contact angle for carbon fiber/binder and PTFE is assumed, and another in which PTFE is neglected (i.e., untreated GDL sample). The capillary pressure-saturation results are compared with experimental and modeling studies available in literature.

Effect of Polytetrafluoroethylene (PTFE) and micro porous layer (MPL) on thermal conductivity of fuel cell gas diffusion layers: Modeling and experiments

Through-plane thermal conductivity of 14 SIGRACET gas diffusion layers (GDLs), including series 24 & 34, as well as 25 & 35, and of micro porous layer (MPL) is accurately measured under different compression, ranging from 2 to 14 bar, at a nominal temperature of 60 °C. The effect of compression, PTFE loading, and of MPL on thermal conductivity and contact resistance is investigated experimentally, and measurements are presented for the first time for the contact resistance between an MPL and a GDL for an MPL-coated GDL substrate. A new and robust mechanistic model is presented for predicting the through-plane thermal conductivity of GDLs treated with PTFE and is successfully verified with the present experimental data. The model predicts the experimentally-observed reduction in thermal conductivity as a result of PTFE treatment, and provides detailed insights on the functional dependence of thermal conductivity on geometric parameters, compression, and PTFE. The model can be used in performance modeling and in design of polymer electrolyte membrane fuel cells.

Investigation on Effect of PTFE Treatment on GDL Micro-structure by High-resolution X-ray CT

To remove liquid water from a gas diffusion layer (GDL) effectively and minimize an increase of transport resistance, hydrophobic treatment with a polytetrafluoroethylene (PTFE) is widely used. Because the GDL micro-structure has strong impact on transport properties, detailed understanding of fiber and porous network and PTFE distribution is strongly requested. To investigate correlation between the GDL micro-structure, PTFE treatment, and transport properties, micro-structure of the GDL was visualized by high-resolution X-ray computed tomography (X-ray CT). Newly designed sample holder allowed to regulate compression pressure. Visualization results with spatial resolution of 1.0 um made it possible to distinguish carbon fibers, binder material, PTFE, and voids (air), and non-uniform distribution of PTFE was observed. Because non-uniform porosity and contact angle distribution in the direction of thickness change the liquid water morphology within the GDL, understanding and controlling the GDL micro-structure is important and manufacturing process of GDL should be scientifically developed.

Enhanced mechanical and electrochemical durability of multistage PTFE treated gas diffusion layers for proton exchange membrane fuel cells

Gas Diffusion Layers (GDLs) of Proton Exchange Membrane Fuel Cells (PEMFCs) are usually subject to polytetrafluoroethylene (PTFE) treatment in a single stage. The impact of multistage PTFE treatment on the mechanical and electrochemical durability of GDLs used in PEMFCs, is reported here. With the same total amount of PTFE in the GDL, substrates treated with PTFE in multiple stages are seen to possess distinctly improved mechanical and electrochemical durability compared to GDLs treated with PTFE in a single stage. The difference in structure and hydrophobicity of the GDLs, when they are subjected to the two different PTFE treatment routes, are examined to understand the reasons for the improvements. The results indicate that there is a change in surface morphology, pore size distribution, and hydrophobicity of the GDL samples depending on the treatment route adopted. It is observed that it is possible to establish a gradient in PTFE profile in the GDL by adopting multistage treatment. Such gradients counteract loss in hydrophobicity resulting from compression cycles during cell assembly and carbon corrosion due to electrochemical aging. The results reveal that GDLs subject to multistage PTFE treatment, can have increased lifetime as opposed to conventional single stage PTFE treated GDL.

Effect of spatial variation of gas diffusion layer wetting characteristics on through-plane water distribution in a polymer electrolyte fuel cell

Through-plane liquid water distributions recently visualized by Manahan et al. [1] and Turhan et al. [2] using the neutron radiography (NR) technique show that the peaks of the water distributions are located near the center of a gas diffusion layer (GDL). We suggest that the distinctive water profiles are caused by incomplete polytetrafluoroethylene (PTFE) treatment of the GDL and the resultant spatial variation of GDL wettability in the through-plane direction. Based on this hypothesis, we improve the macroscopic two-phase fuel cell model to describe two-phase transport through GDLs with variation of spatial wettability [3]. The proposed model successfully reproduces the shape of through-plane water profiles obtained from the NR experiments [1,2]. Therefore, the centrally located liquid saturation peak in the GDL can be attributed to incomplete PTFE treatment of the GDL. This occurs because liquid water is more easily accumulated in the relatively hydrophilic GDL pores encountered in the inner GDL region (rather than the outer GDL region) due to its incomplete PTFE treatment. Our results indicate that the overall characteristics of liquid water distribution in a GDL under an inhomogeneous wetting condition can be macroscopically predicted using the two-phase model presented here.

Monolithic Macroporous Carbon Cryogel Prepared from Natural Polymers

In this study, monolithic macroporous carbon cryogels with a surface area of 117.1 m/g, a sheet resistance of 23 Ω/ and a contact angle of 140◦ were prepared from a bacterial cellulose membrane. The microscopic morphology of the cryogels contained a macroporous networking structure with a large number of micropores. Before polytetrafluoroethylene treatment, the carbon cryogels were very hydrophobic because of the large number of micropores on the surfaces of the cryogels. Therefore, the carbon cryogels exhibited great potential for applications as a gas diffusion layer in fuel cells because of their high electrical conductivity, macroporous structure and hydrophobicity.

Surface Treatment of Hydrophobic Polymers by Atmospheric-Pressure Plasma

Surfaces of hydrophobic polymers were treated by atmospheric pressure plasma. The plasma system used was a newly developed damage-free dielectric barrier glow discharge. Ar was mainly used as a carrier gas. The treated polymer surfaces were mainly analyzed with contact angle measurement and X-ray photoelectron spectroscopy (XPS). In the treatment of high-density polyethylene (HDPE), atmospheric Ar plasma was found to alter the surface very hydrophilic, which is comparable to low-pressure O2 plasma. The addition of O2 in the plasma gas hardly improved the hydrophilicity. In the treatment of polytetrafluoroethylene (PTFE), atmospheric H2 (0.1%) plasma showed the greatest hydrophilicity but it was not as good as the treatment by low-pressure H2 plasma.

Effect of anode backing layer on the cell performance of a direct methanol fuel cell

We investigated experimentally the effect of the anode backing layers consisting of carbon papers with different thicknesses and different polytetrafluoroethylene (PTFE) contents on the cell performance of a direct methanol fuel cell (DMFC). The membrane electrode assemblies were prepared using the decal method such that the effect of different anode backing layers could be studied with the same anode catalyst layer, the same membrane and the same cathode. We found that a too thin anode backing layer resulted in lower cell voltages in the entire current density region, whereas a too thick backing layer led to a lower limiting current density. The reduced cell performance as a result of thinning the backing layer may be attributed mainly to the increased under-rib mass transport polarization as a result of weaker under-rib convection in a thinner backing layer. The experimental results also showed that the use of a PTFE-treated backing layer resulted in a lower limiting current density, attributing primarily to the increased mass transfer resistance as a result of the PTFE treatment.

Effects of DBD plasma operating parameters on the polymer surface modification

This paper, based on an orthogonal experimental design and analysis method, reports the effects of a dielectric barrier discharge (DBD) plasma surface treatment on polytetrafluoroethylene (PTFE), polyimide (PI) and poly (lactic acid) (PLA) films in terms of changes in surface wettability and surface chemistry. The purpose was to study the influence of the main operating parameters, i.e. plasma power, treatment period duration (treatment cycles) and electrode gap on the resultant surface properties. Statistical analysis was carried out to develop an equation which expresses surface properties (water contact angle and oxygen enrichment, as observed by XPS analysis) in terms of these operational parameters. It was observed that the plasma parameters have a selective effect on the changes observed for the polymers processed. In particular, plasma processing time (treatment cycles), plays an important role in the treatment of PTFE and PI in this study, whereas the size of the electrode gap plays the dominant role in the treatment of PLA. Fast surface activation can be achieved in all cases after only a few seconds of treatment duration. The wettability improvement observed in all cases was attributed to changes in both surface chemistry and surface micro-structure.

Preliminary studies of the influence of cryo‐treatment on the mechanical and tribological properties of ptfe and composites

AbstractCryogenic treatment of polytetrafluoroethylene (PTFE) has proved beneficial in improving the abrasive wear resistance of several polymers, and it was thus assessed in an adhesive wear mode, as well. Preliminary investigations on the effect of cryogenic treatment on the tribological properties, in adhesive wear mode, and mechanical properties of neat PTFE and it composites filled with bronze or short glass fibres (GF) were carried out. It was found that, although the improvement in the wear and friction performance of neat PTFE and a GF + PTFE composite was significant, no such positive effect was observed for the bronze + PTFE composite. On the contrary, this composite showed a deterioration in performance.The reason behind the improvement in the tribological behaviour of neat PTFE and the GF + PTFE composite could not be clearly understood. However, it was confirmed that, if the treatment adversely affected the mechanical properties, then the tribological performance also deteriorated. An examination of the worn surface of the material and the counterface disc using a scanning electron microscope revealed changes in the microstructure due to the treatment. It was also confirmed from these SEM studies that the compatibility of bronze and PTFE was very poor, which led to poor performance of the composite both in the untreated and the cryo‐treated form. Further detailed investigation and analysis of various materials and composites, however, are necessary to establish the utility of this technique.

On the low-pressure plasma treatment of PTFE (polytetrafluoroethylene) with SO 2 as process gas

Low-pressure plasma treatments of PTFE films using sulfur dioxide (SO 2) as process gas were carried out in order to insert functional groups containing sulfur and oxygen into the inert polymer surface. By modifying the plasma parameters: gas flow rate, power and treatment time, the best plasma conditions for a surface modification could be found. An increase of the wettability, in terms of having the lowest static contact angle, could be detected after the plasma treatment. ESCA and FTIR-spectra (ATR and transmission) of selected treated samples completed the characterization methods.

Ammonia Plasma Treatment of PTFE Under Known Plasma Conditions

Ammonia plasmas produced in a helical resonator have been investigated using an energy-selective mass spectrometer and an r.f.-compensated Langmuir probe. The effect of processing parameters on the plasma composition and ion energy distributions have been examined. The dominant ion present in the plasma was found to be NH 4 + , which varied in concentration between 95% and 65%, dependent upon operating conditions. Similarly, by relatively small variations in processing parameters it was possible to vary the NH 3 + ion concentration between 1.7 and 31%. In addition, no radicals (NH 2 . , NH . or N . ) were detected using threshold ionization mass spectroscopy. Polytetrafluoroethylene (PTFE) samples were treated under a variety of different plasma conditions, and the chemical changes induced were studied by in situ XPS. Defluorination was observed to be greater under conditions that yield low ion energies or high concentrations of NH 3 + ions. Conditions giving rise to significant concentrations of NH 3 + ions result in the production of NH 3 + F - groups. Evidence is provided suggesting that NH 3 + ions are much more reactive with PTFE surfaces than NH 4 + ions

Adhesion Studies of Fluoropolymers

Abstract A comparative study of the treatment of polytetrafluoroethylene (PTFE) and poly(vinyl fluoride) (PVF) with “Tetra-Etch” has been carried out. The treatment of PTFE resulted in extensive changes in surface chemistry and topography, whereas with PVF there was no significant change in topography and the chemical changes were much less marked. However, treatment of both polymers resulted in large increases in bond strength. Multiple bonding experiments in which samples are repeatedly fractured and re-bonded were carried out with untreated PTFE and PVF. These resulted in moderate increases in bond strength with PTFE and large increases with PVF. The results indicate that weak boundary layer (WBL) removal is a key element in adhesion improvement by “Tetra-Etch” on PVF. With PTFE, WBL removal also improves adhesion, but the chemical and/or topographical changes introduced by the “Tetra-Etch” are required for optimum performance.