Untreated Gas Diffusion Layers Research Articles

Towards Improved Cold-Start Capability of PEFCs: Patterned Gas Diffusion Layers

Polymer electrolyte fuel cells (PEFCs) are electrochemical energy converters with high efficiency and zero pollutant emissions. The unassisted cold-start of PEFCs at -30 °C is one of the major challenges for automotive applications due to the freezing of supercooled water generated by the cathodic oxygen reduction reaction (ORR). Once triggered by the nucleus in cathode gas diffusion layers (GDLs), ice propagates through the active area, leading to oxidant gas blockage and cold-start failure1.The cold-start capability of PEFCs can be improved by preventing ice propagation, especially in GDLs. In our previous work2,3, after the GDLs were segmented by polytetrafluoroethylene (PTFE) into two regions, the PEFC kept operating for another 30 minutes after the first freezing event in one region. Nevertheless, the PTFE barrier limited the active area as well as the heat and electric conductivity of PEFCs. In contrast, patterned GDLs allow water cluster separation without the abovementioned limitations. The improvement of water management and cell performance under technical operating conditions was demonstrated4. Yet, the cold-start capability with patterned GDL needs to be further investigated.This work studies the effect of patterned GDLs on the water-ice phase transition and cold-start capability of PEFCs. First, non-isothermal differential scanning calorimetry (DSC) measurements were conducted to determine the onset freezing temperature distribution of supercooled water in patterned GDLs, taking into account various properties such as coating load and pattern size. It was observed that independent freezing events occurred in patterned GDLs in all cycles, as several freezing/exothermic peaks can be seen in Figure 1b. In contrast, ice propagated immediately in untreated GDL, as only one freezing peak was observed in all cycles (Figure 1d). By computing the integral of the heat flux curve, we obtained the corresponding cumulative icing ratio over 60 cycles, as presented in Figures 1a and 1c. The effective prevention of ice propagation is only observed in patterned GDLs. Second, to assess the cold-start capability, statistical isothermal cold-start measurements were conducted using a 4.4 cm2 differential cell, with the cell temperature held at -7.5 °C, -10 °C, -15 °C, and 20 °C. In summary, nonconcurrent freezing events in patterned GDLs indicate that the use of patterned GDLs significantly improved the cold-start capability of PEFCs under isothermal subfreezing conditions.The findings of this work enhance the understanding of the cold-start dependence on the GDL and contribute to the development of unassisted freeze-start technology for PEFCs and the expansion of PEFCs applications. Figure 1

Water droplet detachment characteristics on surfaces of gas diffusion layers in PEMFCs

Water management is one of the key issues affecting the performance and stability of proton exchange membrane fuel cells (PEMFCs). Water detachment on the gas diffusion layer (GDL) surface is critically important to water management in PEMFCs. In this study, water droplet detachment characteristics under various GDL surface contact angles and channel heights are investigated, by using a customized transparent model cell for direct ex-situ water visualization. The droplet height, chord, height/chord ratio, and contact angle hysteresis at the instant of droplet detachment are quantitatively analyzed. The droplet detachment is easier for higher gas Reynolds number (Reg). The height and chord of the droplet both decrease with Reg for both GDLs with and without PTFE but their decrement rates become smaller in higher Reg regions for all the channel heights investigated. Compared with droplets on the untreated GDL, the droplet height/chord ratio on the PTFE-treated GDL with larger static contact angle is increased by 36.7%, 64.1% and 76.0% and the contact angle hysteresis is reduced by 17.1%, 16.3% and 12.6% for the channel height H of 1 mm, 2 mm and 3 mm, respectively, which indicates that the PTFE-treated GDL improves water detachment. It shows that the water detachment is improved by reducing the channel height due to the smaller contact angle hysteresis at the instant of droplet detachment.

Effects of Structure and Hydrophobic Treatment on Water Transport Behaviour in PEM Fuel Cell Gas Diffusion Layers

This paper examines the effects of structure and wettability on water transport behaviour in PEM fuel cell gas diffusion layers (GDLs). Different GDL materials, including carbon cloth and carbon paper with and without PTFE and MPL coatings, were used in this study. The breakthrough pressure of liquid water, water flow rate and water retention in the GDL were measured. For the GDL with and without MPL, the results indicate that the paper GDLs require higher breakthrough pressure than the cloth ones. Compared to the untreated GDL, applying PTFE on the GDL increases breakthrough pressure up to 1.5 times while it decreases water flow rate and water retention up to about 3.8 and 0.5 times respectively, over the tested conditions. The results also show that applying MPL on the GDL surface has greater impact on breakthrough pressure than applying PTFE. Liquid water requires up to about 4 times greater pressure to break through the other side of the GDL with the MPL. By applying MPL, the amount of water retention and flow through the GDL also reduced greatly, to almost zero. In addition, the results suggest that the GDLs are degraded after being exposed to water, which causes a significant decrease in breakthrough pressure and an increase in water retention.

Experimental study on the membrane electrode assembly of a proton exchange membrane fuel cell: effects of microporous layer, membrane thickness and gas diffusion layer hydrophobic treatment

In this study, electrochemical impedance spectroscopy (EIS) is employed to analyze the influence of microporous layer (MPL), membrane thickness and gas diffusion layer (GDL) hydrophobic treatment in the performance of a proton exchange membrane (PEM) fuel cell. Results show that adding a MPL increases cell performance at low to medium current densities. Because lower ohmic losses are observed when applying a MPL, such improvement is attributed to a better hydration state of the membrane. The MPL creates a pressure barrier for water produced at the cathode, forcing it to travel to the anode side, therefore increasing the water content in the membrane. However, at high currents, this same phenomenon seems to have intensified liquid water flooding in the anode gas channels, increasing mass transfer losses and reducing the cell performance. Decreasing membrane thickness results into considerably higher performances, due to a decrease in ohmic resistance. Moreover, at low air humidity operation, a rapid recovery from dehydration is observed when a thinner membrane is employed. The GDL hydrophobic treatment significantly improves the cell performance. Untreated GDLs appear to act as water-traps that not only hamper reactants transport to the reactive sites but also impede the proper humidification of the cell. From the different designs tested, the highest maximum power density is obtained from that containing a MPL, a thinner membrane and treated GDLs.

Experimental Study of Thermal Conductivity and Compression Measurements of the GDL-MPL Interfacial Composite Region

Through-plane thermal conductivity and thickness variation under different compaction pressures were measured for a composite region of commercial gas diffusion layer (GDL), namely Freudenberg H1410 GDL, as received as well as saturated with a custom-made carbon ink/microporous layer (MPL). 3D X-Ray pictures were taken of the materials. Thermal conductivity for the composite region is higher than for the GDL alone. The compressibility of the composite region is in the same order of magnitude as pure GDL material. The found results motivate further investigation of this composite region that has gotten very little attention in the literature. A PEMFC consists of several components, i.e. the membrane electrolyte assembly (MEA) sandwiched between a thin MPL and a somewhat thicker GDL on each side. The MEA consists of a membrane (PEM) coated with catalyst layers (CL) on each side. MPL and GDL are often treated as separate layers in the literature. However, there exists a considerable interfacial region where the two different materials are intertwined. The fine material of the MPL can intrude considerably into the fiber structure of the GDL material. It is this region we are interested in. The temperature difference across the PEMFC can reach several °C despite being less than a millimeter thick between the gas flow plates. Temperature differences arise mainly across the GDL. Several research efforts have led to a good understanding of the thermal conductivity of the GDL and how it changes with compression, temperature, PTFE content, different fabrics, and water content. This work presents an effort to determine the thermal conductivity as well as the compressibility of the aforementioned composite region. [2] At a compaction pressure of 9.2 bar the thermal conductivity of untreated Freudenberg H1410 (114 μm), was found to be 0.111±0.009 W K-1 m-1 and for the custom-MPL-drenched Freudenberg H1410 material it was 0.124±0.005 W K-1 m-1. The untreated Freudenberg H1410 material was compacted to 87% of its original thickness at 9.2 bar compaction pressure. The MPL-treated H1410 material was compacted to 77% of its original thickness at 9.2 bar compaction pressure. The MPL-treated Freudenberg H1410 has a larger compression from atmospheric pressure to first compaction pressure, but a less steep compression gradient with rising compaction pressure than the original H1410 material, see figure. The thermal conductivity at 9.2 bar compaction pressure of another untreated Freudenberg GDL, namely H2315 (182 μm), was reported earlier to be 0.15±0.02 W K-1 m-1, roughly 50% higher than for the H1410 we measured. This complies with a trend seen in Toray paper, where the thermal conductivity at 9.3 bar compaction increases from 0.53±0.03 W K-1 m-1 for Toray TGP-H-060 (165 μm) to 0.65±0.02 W K-1 m-1 for Toray TGP-H-090 (265 μm) and to 0.81±0.03 W K-1 m-1for Toray TGP-H-120 (333 μm). [1] SIGRACET GDL 10 AA, also an untreated GDL, was measured to have a thermal conductivity of 0.38±0.03 W K-1 m-1 at 9.2 bar compaction pressure, also much higher than the Freudenberg GDL. A SolviCore PTL had a very similar thermal conductivity of 0.36±0.08 W K-1 m-1 at 9.2 bar compaction pressure. [1] The way the GDL is produced, the Teflon content, and the binder content all have influence on the final value of the thermal conductivity for a GDL material. Toray paper has a higher thermal conductivity when the thickness increases. An increase in PTFE content will lower the thermal conductivity. In general, Freudenberg GDLs have a lower thermal conductivity than other commercially available GDLs. By depositing MPL material into the GDL, thermal contact between GDL fibers is enhanced, hence the overall thermal conductivity of the material increases. This could be a means to improve thermal conductivity of GDL materials with low thermal conductivity to achieve better temperature management in the fuel cell. Acknowledgment The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under Contract No. DE-AC02-05CH11231. The authors would like to acknowledge assistance of Dula Parkinson with the experimental set-up at the beamline.

Thermal conductivity of a graphite bipolar plate (BPP) and its thermal contact resistance with fuel cell gas diffusion layers: Effect of compression, PTFE, micro porous layer (MPL), BPP out-of-flatness and cyclic load

This paper reports on measurements of thermal conductivity of a graphite bipolar plate (BPP) as a function of temperature and its thermal contact resistance (TCR) with treated and untreated gas diffusion layers (GDLs). The thermal conductivity of the BPP decreases with temperature and its thermal contact resistance with GDLs, which has been overlooked in the literature, is found to be dominant over a relatively wide range of compression. The effects of PTFE loading, micro porous layer (MPL), compression, and BPP out-of-flatness are also investigated experimentally. It is found that high PTFE loadings, MPL and even small BPP out-of-flatness increase the BPP-GDL thermal contact resistance dramatically. The paper also presents the effect of cyclic load on the total resistance of a GDL-BPP assembly, which sheds light on the behavior of these materials under operating conditions in polymer electrolyte membrane fuel cells.

Pore-network modeling of liquid water flow in gas diffusion layers of proton exchange membrane fuel cells

Abstract Fluid flow through the gas diffusion layer (GDL) of fuel cells is numerically studied using a pore network modeling approach. The model is developed based on an experimental visualization technique (fluorescence microscopy). The images obtained from this technique are analyzed to find patterns of flow inside the GDL samples with different hydrophobicity. Three different flow patterns are observed: initial invasion, progression, and pore-filling. The observation shows that liquid water flows into the majority of available pores on the boundary of the untreated GDL and several branches are segregated from the initial pathways. For the treated GDL, however, a handful of boundary pores are invaded and the original pathways extend toward the other side of the medium with minimum branching. The numerical model, developed based on an invasion percolation algorithm, is used to study the effects of GDL hydrophobicity and thickness on the flow configuration and breakthrough time as well as to determine the flow rate and saturation in different GDL samples. During the injection of water into the samples, it is numerically shown that the flow rates are monotonically decreasing for both treated and untreated samples. For the treated sample, however, the injection flow rate is constantly lower than that of the untreated sample, resulting in a lower overall water saturation at breakthrough. The numerical results also suggest that hydrophobic treatment of thick samples has minor effects on water management and overall performance. The developed model can be used to optimize the GDL properties for designing porous medium with effective transport characteristics.

Liquid water breakthrough pressure through gas diffusion layer of proton exchange membrane fuel cell

The dynamic behavior of liquid water transport through the gas diffusion layer (GDL) of the proton exchange membrane fuel cell is studied with an ex-situ approach. The liquid water breakthrough pressure is measured in the region between the capillary fingering and the stable displacement on the drainage phase diagram. The variables studied are GDL thickness, PTFE/Nafion content within the GDL, GDL compression, the inclusion of a micro-porous layer (MPL), and different water flow rates through the GDL. The liquid water breakthrough pressure is observed to increase with GDL thickness, GDL compression, and inclusion of the MPL. Furthermore, it has been observed that applying some amount of PTFE to an untreated GDL increases the breakthrough pressure but increasing the amount of PTFE content within the GDL shows minimal impact on the breakthrough pressure. For instance, the mean breakthrough pressures that have been measured for TGP-060 and for untreated (0 wt.% PTFE), 10 wt.% PTFE, and 27 wt.% PTFE were 3589 Pa, 5108 Pa, and 5284 Pa, respectively.

Investigation of Macro-Thermal Contact Resistances in the Cathode of PEM Fuel Cells

Polymer Electrolyte Membrane (PEM) fuel cells are electrochemical energy conversion devices, and have been a main focus of study in the area of renewable energy systems in the past decade. With the potential to generate electricity with zero local greenhouse gas emissions, when fed hydrogen and oxygen from air, PEM fuel cells also have other promising advantages, such as supplying a high power to volume ratio, operating with little to no noise and being able to quickly reach steady state conditions [1]. Although the technology is promising, before PEM fuel cells can reach widespread adoption, effective thermal management must be achieved in order to reach the performance and durability levels required for commercialization.The cathode of the PEM fuel cell, which is composed of the Gas Diffusion Layer (GDL), Micro-Porous Layer (MPL), and Catalyst Layer (CL), houses various processes which affect the performance of the cell. While the inlet fuel and oxidant gases are typically heated, heat is also produced from electrochemical reactions and water phase change, resulting in temperature gradients that can affect the conditions of the cell. These exothermic reactions generate temperature gradients, which in turn affect the cell’s relative humidity levels, water saturation levels, and the cell’s reaction kinetics [2]. Pathways for effective heat conduction are vital for avoiding excessive condensation of water from cooling, or material dry-out from overheating. It is therefore crucial to have a way of measuring how effectively thermal energy can be transferred out of the CL, the region of generation, through the MPL, and finally out of the GDL, in order to have better control of parameters involved in future improvements to cell components.Present thermal conductivity models provide insight for analyzing thermal energy transfer in the through-plane and in-plane directions of the GDL, but do not consider interfacial mechanics at the micro-level caused by surface roughness, waviness, and irregularities. Therefore in order to provide a more realistic representation of the through-plane thermal conductivity of the GDL of a PEM fuel cell, a model will be developed which incorporates information regarding fibre surface morphology measured using atomic force microscopy [3], and will also incorporate polytetrafluoroethylene (PTFE), and binding materials into the thermo-mechanical analysis. The thermal resistance network will be constructed using stochastic relationships derived statistical expressions regarding the contacts in the GDL found in a previous study [2], and it will be informed with experimental data obtained via micro-computed tomography [4]. Secondly, the thermal contact resistance between the MPL and CL will be determined using a modification of Greenwood’s Rough Contact Model. The surface structures of the MPL and CL will be analyzed using a combination of scanning electron microscopy, atomic force microscopy, and micro-computed tomography, from which analysis will be conducted to determine the thermal contact resistance of the interface.A realistic thermal conductivity model representing surface features of carbon fibre at the nanoscale will be presented. The model will incorporate PTFE and binding materials in the thermal analysis, for various compressive loads and GDL sizes/types, from which preliminary results are shown in Figure 1. Also, the thermal contact resistance of the MPL|CL interface will be explored providing insight into a more realistic thermal model of the macro-thermal resistances within the cathode of the PEM fuel cell. Figure 1. Through-plane thermal conductivity with porosity for untreated GDL considering surface features of carbon fibre.

Experimental and numerical comparison of water transport in untreated and treated diffusion layers of proton exchange membrane (PEM) fuel cells

In this paper, a fluorescein solution is injected into the gas diffusion layer (GDL) of a PEM fuel cell to simulate water flow through the porous medium under flooding conditions. The effect of the GDL hydrophobic treatment is studied by analyzing the images obtained from fluorescence microscopy. The intensity of images is correlated to the liquid water content in the medium. As a result, three distinguishable patterns are recognized between the treated and untreated GDL samples. These patterns include: (1) initial invasion, (2) progression and (3) pore filling. Based on the flow characteristics, a pore-network model is developed and applied to unique network geometry with different hydrophobic fractions to study the effect of hydrophobicity on the flow configuration. It is shown that the model predicts the flow configurations which are qualitatively in agreement with the experimental observation. The model is then used to obtain saturation for samples with different hydrophobic fractions. The trend is in agreement with the results reported in the past.

Heterogeneous Through-Plane Porosity Distributions for Treated PEMFC GDLs I. PTFE Effect

To the authors’ best knowledge, this is the first parametric investigation of the effect of polytetrafluoroethylene (PTFE) treatments on the heterogeneous through-plane local porosity distributions of PTFE-treated gas diffusion layers (GDLs) for polymer electrolyte membrane fuel cells (PEMFCs). In this work, the through-plane porosity distributions of uncompressed PTFE-treated paper GDLs were measured from high resolution microscale computed tomography visualizations. We analyzed paper GDLs with varying thicknesses and PTFE treatments. When compared to untreated GDL porosity distributions, we observed that the application of PTFE resulted in decreased local porosities near the surface regions and non-uniform distributions of PTFE throughout the material. The rate of porosity decrease with increasing PTFE application is in agreement with the literature. The changes to the overall through-plane porosity distributions are more significant than the changes to the average bulk porosities, which are attributed to batch-specific manufacturing variations. This work provides new insight into the effects of PTFE treatments on GDL through-plane porosity distributions, which are expected to inform future PEMFC models.

Droplet pinning by PEM fuel cell GDL surfaces

In this work, side view images of liquid–gas–solid interfaces are observed during the evaporation of liquid water droplets on various commercially available untreated gas diffusion layers (GDLs). The change in contact diameter as a function of evaporative volume loss is measured to quantify the unpinning rates of micro-sized droplets. This contact diameter pinning behaviour during evaporation is correlated to the material topography, which is quantified through profilometry measurements. The carbon fibre paper with the smallest average roughness (15 μm) exhibits the strongest degree of pinning (unpinning at a rate of 0.13 mm/μL). Higher average surface roughnesses for felt (30 μm) and cloth yarn (32 μm) result in higher unpinning rates, 0.21 mm/μL and 0.19 mm/μL, respectively. These results indicate that common GDL materials exhibit Cassie–Baxter wetting behaviour, and reduced GDL roughness promotes droplet pinning. The material-specific droplet contact diameter progression should be considered during GDL selection for polymer electrolyte membrane (PEM) fuel cells. This work provides insight into the effect of GDL material properties on gas channel water management, as water droplets are expected to experience similar pinning to that observed in this work within the cathode gas channels of a PEM fuel cell.

Measurement of through-plane effective thermal conductivity and contact resistance in PEM fuel cell diffusion media

An experimental study was performed to determine the through-plane thermal conductivity of various gas diffusion layer materials and thermal contact resistance between the gas diffusion layer (GDL) materials and an electrolytic iron surface as a function of compression load and PTFE content at 70 ° C. The effective thermal conductivity of commercially available SpectraCarb untreated GDL was found to vary from 0.26 to 0.7 W/(m ° C) as the compression load was increased from 0.7 to 13.8 bar. The contact resistance was reduced from 2.4 × 1 0 − 4 m 2°C/W at 0.7 bar to 0.6 × 1 0 − 4 m 2°C/W at 13.8 bar. The PTFE coating seemed to enhance the effective thermal conductivity at low compression loads and degrade effective thermal conductivity at higher compression loads. The presence of microporous layer and PTFE on SolviCore diffusion material reduced the effective thermal conductivity and increased thermal contact resistance as compared with the pure carbon fibers. The effective thermal conductivity was measured to be 0.25 W/(m ° C) and 0.52 W/(m ° C) at 70 ° C, respectively at 0.7 and 13.8 bar for 30%-coated SolviCore GDL with microporous layer. The corresponding thermal contact resistance reduced from 3.6 × 1 0 − 4 m 2°C/W at 0.7 bar to 0.9 × 1 0 − 4 m 2°C/W at 13.8 bar. All GDL materials studied showed non-linear deformation under compression loads. The thermal properties characterized should be useful to help modelers accurately predict the temperature distribution in a fuel cell.

Effects of Design Parameters in Paper Type Gas Diffusion Layer on the Performance of Polymer Electrolyte Fuel Cells (Measures to Prevent Flooding and Drying-Up)

The influence of gas diffusion layer (GDL) design parameters on the performance of a polymer electrolyte fuel cell (PEFC) was evaluated using a test apparatus in which the clamp pressure on the cell could be accurately controlled. The PEFC performance varies significantly depending on the humidity of the supplied gas. Under moderate humidity, the PEFC performance is enhanced when using an untreated GDL with high air permeability. Under high humidity, improved performance is achieved using a GDL with PTFE hydrophobic treatment. A microporous layer (MPL) coated GDL significantly reduces flooding on the electrode, resulting in a decrease in the concentration overpotential. This enhances the PEFC performance at high current densities. A substrate with high air permeability for the MPL coated GDL enhances the ability of the MPL to prevent flooding. Under low humidity, the MPL is also effective to prevent drying-up of the membrane electrode assembly (MEA), resulting in enhanced performance. A substrate with low air permeability for the MPL coated GDL is effective for the prevention of drying-up.

Influences of Gas Diffusion Layer Design Parameters on the Performance of Polymer Electrolyte Fuel Cells

Experiments examining the influences of gas diffusion layer (GDL) design parameters on the performance of polymer electrolyte fuel cells (PEFCs) are carried out under controlled clamp pressure conditions. The PEFC performance is strongly affected by the humidity of the supplied gas. Under moderate-humidity conditions, improved performance is obtained using an untreated GDL with high air permeability, whereas under high-humidity conditions, improved performance is obtained using a GDL treated with PTFE. The addition of a microporous layer (MPL) under humid conditions significantly reduces flooding on the electrode, thereby decreasing the concentration overpotential. The MPL is also effective for preventing the membrane electrode assembly (MEA) from drying out under low-humidity conditions.

Influence of cathode opening size and wetting properties of diffusion layers on the performance of air-breathing PEMFCs

Air-breathing PEMFCs consist of an open cathodic side to allow an entirely passive supply of oxygen by diffusion. Furthermore, a large fraction of the produced water is removed by evaporation from the open cathode. Gas diffusion layers (GDLs) and the opening size of the cathode have a crucial influence on the performance of an air-breathing PEMFC. In order to assure an unobstructed supply of oxygen the water has to be removed efficiently and condensation in the GDL has to be avoided. On the other hand good humidification of the membrane has to be achieved to obtain high protonic conductivity. In this paper the influence of varying cathodic opening sizes (33%, 50% and 80% opening ratios) and of GDLs with different wetting properties are analysed. GDLs with hydrophobic and hydrophilic properties are prepared by coating of untreated GDLs (Toray ® carbon paper TGP-H-120, thickness of 350 μm). The air-breathing PEMFC test samples are realised using printed circuit board (PCB) technology. The cell samples were characterised over the entire potential range (0–0.95 V) by extensive measurements of the current density, the temperature and the cell impedance at 1 kHz. Additionally, measurements of the water balance were carried out at distinct operation points. The best cell performance was achieved with the largest opening ratio (80%) and an untreated GDL. At the maximum power point, this cell sample achieved a power density of 100 mW cm −2 at a moderate cell temperature of 43 °C. Furthermore, it could be shown that GDLs with hydrophilic or intense hydrophobic properties do not improve the performance of an air-breathing PEMFC. Based on the extensive characterisations, two design rules for air-breathing PEMFCs could be formulated. Firstly, it is crucial to maximise the cathode opening as far as an appropriate compression pressure of the cell assembly and therewith low contact resistance can be assured. Secondly, it is advantageous to use an untreated, slightly hydrophobic GDL.