Numerical modeling and experimental study of the influence of GDL properties on performance in a PEMFC

This work is to study the effect of properties of gas diffusion layer (GDL) on performance in a polymer electrolyte membrane fuel cell (PEMFC) by both numerical simulation and experiments. The 1-dimension numerical simulation using the mixture-phase model is developed to calculate polarization curve. We are able to estimate optimum GDL properties for cell performance from numerical simulation results. Various GDLs which have different properties are prepared to verify accuracy of the simulation results. The contact angle and gas permeability of GDLs are controlled by polytetrafluoroethylene (PTFE) content in micro-porous layers (MPLs). MPL slurry is prepared by homogeneous blending of carbon powder, PTFE suspension, isopropyl alcohol and glycerol. Then the slurry is coated on gas diffusion mediums (GDMs) surface with controlled thickness by blade coating method. Non-woven carbon papers which have different thicknesses of 200 μm and 380 μm are used as GDMs. The prepared GDLs are measured by surface morphology, contact angle, gas permeability and through-plane electrical resistance. Moreover, the GDLs are tested in a 25 cm² single cell at 70 °C in humidified H₂/air condition. The contact angle of GDL increases with increasing PTFE content in MPL. However, the gas permeability and through-plane electrical conductivity decrease with increasing PTFE content and thickness of GDM. These changes in properties of GDL greatly influence the cell performance. As a result, the best performance is obtained by GDL consists of 200 μm thick non-woven carbon paper as GDM and MPL contained 20 wt.% PTFE content.     

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.     

Degradation of gas diffusion layers through repetitive freezing

This work investigates the degradation of an individual gas diffusion layer (GDL) by repeated freezing cycles. The pore size distribution, gas permeability, surface structure, and contact angle on the surface of the GDL were measured in four different types of GDL: SGL paper with a microporous layer (MPL); SGL paper with 5 wt% of polytetrafluoroethylene (PTFE) loading; Toray paper without PTFE loading; and Toray paper with 20 wt% of PTFE loading. After repeated freezing cycles, the porosity of the GDL without PTFE was reduced by 27.2% due to the volumetric expansion of the GDL. The peak of the log differential intrusion moved toward a smaller pore diameter slightly because of the repeated freezing process. The crack of the MPL increased in its width and length after repeated freezing cycles. The through-plane gas permeability of the GDL with the MPL doubled after repeated freezing cycles due to the growth of the crack in the MPL, but was very small for the GDLs with Toray paper. Besides, the GDLs with PTFE loading showed a relatively larger decrease in the contact angle on the surface than the GDL without PTFE loading due to the separation of PTFE from the carbon fiber during the repeated freezing process.     

Determination of effective water vapor diffusion coefficient in pemfc gas diffusion layers

The primary removal of product water in proton exchange membrane (PEM) fuel cells is through the cathode gas diffusion layer (GDL) which necessitates the understanding of vapor and liquid transport of water through porous media. In this investigation, the effect of microporous layer (MPL) coatings, GDL thickness, and polytetrafluorethylene (PTFE) loading on the effective water vapor diffusion coefficient is studied. MRC Grafil, SGL Sigracet, and Toray TGP-H GDL samples are tested experimentally with and without MPL coatings and varying PTFE loadings. A dynamic diffusion test cell is developed to produce a water vapor concentration gradient across the GDL and induce diffusion mass transfer. Tests are conducted at ambient temperature and flow rates of 500, 625, and 750 sccm. MPL coatings and increasing levels of PTFE content introduce significant resistance to diffusion while thickness has negligible effects.     

In situ characterization of metal hydride thermal transport properties

Thermal property characterization of a metal hydride was conducted in a high pressure hydrogen environment with a transient plane source apparatus. This apparatus was integrated with a pressure vessel to measure the effective thermal conductivity and the thermal diffusivity of non-reactive and activated metal hydrides in hydrogen pressures up to 275 bar and 253 bar, respectively. In a non-reactive oxidized condition, the effective thermal conductivity of Ti_1.1CrMn powder increased with hydrogen pressure from 0.8 to 1.6 W/m·K. For activated powder, effective thermal conductivity increased from 0.3 to 0.7 W/m·K with hydrogen pressure. It is postulated that the smaller particle size associated with the reactive condition caused the reduction in effective thermal conductivity. Also, the derived specific heat of the activated Ti_1.1CrMn increased with reaction progress by a factor of two, with a maximum value at the fully hydrided state.     

Effect of polytetrafluoroethylene-treatment and microporous layer-coating on the electrical conductivity of gas diffusion layers used in proton exchange membrane fuel cells

The purpose of this study is to investigate the effect of ploytetrafluoroethylene (PTFE)-treatment and microporous layer (MPL)-coating on the electrical conductivity of gas diffusion layers (GDLs), as used in proton exchange membrane fuel cells (PEMFCs). The results show that, for PTFE-treated GDLs, the electrical conductivity in orthogonal in-plane directions is almost invariant with the PTFE loading. On the other hand, the in-plane conductivity of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be higher than that of the counterpart SGL 10BC (25% PTFE) and this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Further, the conductivity of each GDL sample was measured in two perpendicular in-plane directions in order to investigate the in-plane anisotropy. The results show that the electrical conductivity of the GDL sample in one direction is different to that in the other direction by a factor of about two. The contact resistance, the main factor affecting the through-plane conductivity, of PTFE-treated GDLs shows a different trend to the corresponding in-plane conductivity, namely it increases as the PTFE loading increases. On the other hand, the contact resistance of the MPL-coated GDL SGL 10BE (50% PTFE) was found to be lower than that of the counterpart SGL 10BC (25% PTFE) and again this was explained by the presence of more conductive carbon particles in the MPL of SGL 10BE. Also, it was noted that the MPL coating appears to have a positive effect in reducing the contact resistance between the GDL and the bipolar plate. This is most likely due to the compressibility of the MPL layers that allows them to fill in the ‘gaps’ that exist in the surface of the bipolar plates and therefore establishes a good contact between the latter plates and the GDLs. Finally, good curve fitting of the contact resistance as a function of the clamping pressure has been achieved.     

Optimization of polytetrafluoroethylene content in cathode gas diffusion layer by the evaluation of compression effect on the performance of a proton exchange membrane fuel cell

This research investigates the optimal polytetrafluoroethylene (PTFE) content in the cathode gas diffusion layer (GDL) by evaluating the effect of compression on the performance of a proton exchange membrane (PEM) fuel cell. A special test fixture is designed to control the compression ratio, and thus the effect of compression on cell performance can be measured in situ. GDLs with and without a microporous layer (MPL) coating are considered. Electrochemical impedance spectroscopy (EIS) is used to diagnose the variations in ohmic resistance, charge transfer resistance and mass transport resistance with compression ratio. The results show that the optimal PTFE content, at which the maximum peak power density occurs, is about 5 wt% with a compression ratio of 30% for a GDL without an MPL coating. For a GDL with an MPL coating, the optimal PTFE content in the MPL is found to be 30% at a compression ratio of 30%.     

Freezing characteristics of supercooled water in gas diffusion layer of proton exchange membrane fuel cells

The freezing characteristics of supercooled water in a gas diffusion layer (GDL), which are the bases for the cold start-up of proton exchange membrane fuel cells (PEMFCs), were investigated. An experimental apparatus for noncontact temperature measurement and observation systems was developed. GDL and GDL with a microporous layer (MPL) were prepared, and freezing experiments using a water-containing GDL under various cooling rates were performed with variations in polytetrafluoroethylene (PTFE) content and water saturation. Furthermore, based on the experimental results, the freezing initiation probability was theoretically investigated to elucidate the freezing characteristics. Results showed that, with increasing supercooling of water in GDL, the freezing probability of water increased abruptly. The effect of saturation showed a different trend depending on PTFE addition. For the GDL without PTFE, the freezing initiations occurred at approximately 6 °C of supercooling degree, and the probability approached 1.0 at approximately 9.5–11.5 °C, with saturation dependency. In contrast, for both GDL and GDL + MPL containing PTFE, the initiation temperature characteristics were relatively similar, which were approximately 8–12 °C, regardless of the saturation and PTFE content. In these cases, the ice-nucleating activity of water in the GDL was possibly stronger than that in the MPL.     

Effects of basic gas diffusion layer components on PEMFC performance with capillary pressure gradient

The gas diffusion layer (GDL) is composed of a substrate and a micro-porous layer (MPL), and is treated with polytetrafluoroethylene (PTFE) to promote water discharge. Additionally, the MPL mainly consists of carbon black and PTFE. In other words, the optimal design of these elements has a dominant effect on the polymer electrolyte membrane fuel cell (PEMFC) performance. For the GDL, it is crucial to prevent water flooding, and the water flux within the GDL is strongly affected by the capillary pressure gradient. In this study, the PEMFC performance was systematically investigated by varying the substrate PTFE content, MPL PTFE content, and MPL carbon loading per unit area. The effects of each experimental variable on the PEMFC performance and especially on the capillary pressure gradient were quantitatively analyzed when the GDLs were manufactured by the doctor blade manufacturing method. The experimental results indicated that as the PTFE content of the anode and cathode GDL increased, the PEMFC performance deteriorated due to the deformation of the porosity and tortuosity of the GDL. Additionally, the PEMFC performance improved as the MPL PTFE content of the cathode GDL increased at low relative humidity (RH), but the PEMFC performance tendency was reversed at high RH. Further, the MPL carbon loading of 2 mg/${\text{cm}}^2$ demonstrated the best performance, and the advantages and disadvantages of the MPL carbon loading were identified. In addition, the effects of each experimental variable on liquid water, water vapor, and gas permeability were investigated.     

Thermal management in PEMFCs: The respective effects of porous media in the gas flow channel

This study aims to evaluate the convective heat transfer enhancement of the proton exchange membrane fuel cells (PEMFC) numerically. As the higher heat transfer surfaces lead to higher heat transfer rates, a flat plate porous layer is utilized in the gas flow channel (GFC). This enhancement in heat transfer stems from the corresponding modification in the temperature and velocity profiles. The influencing parameters on these profiles are the thickness, permeability, and porosity of the GFC porous layer. After performing the simulations, the results indicate that convective heat transfer has a direct relationship with GFC porous layer's thickness and permeability. However, lower values of porosity lead to the higher Nusselt numbers. Previous investigations have also mentioned the positive impact of the microporous layer (MPL) on the water management of these fuel cells. Therefore, six different sizes of MPL and the gas diffusion layer (GDL) are utilized to evaluate their impacts on the thermal management. Results indicate that although these sizes have negligible effects on the heat transfer, Nu increases by enhancing the total size of MPL and GDL. The results also show that thicker MPLs lead to higher heat transfer rates. The evaluation of the friction factor also indicates the adverse effect of the GFC porous layer, although this undesirable effect is negligible. Finally, all the simulated values are utilized to train an artificial neural network (ANN) model with high precision. This ANN model can produce more data for sensitivity analysis and presenting respective 3D diagrams of the influencing parameters on heat transfer.     

Optimization of GDLs for high-performance PEMFC employing stainless steel bipolar plates

The effects of polytetraflouroethylene (PTFE) content in the gas diffusion layer (GDL) on the performance of PEMFCs with stainless-steel bipolar plates are studied under various operation conditions, including relative humidity, cell temperature, and gas pressure. The optimal PTFE content in the GDL strongly depends on the cell temperature and gas pressure. Under unpressurized conditions, the best cell performance was obtained by the GDL without PTFE, at a cell temperature of 65 °C and relative humidity (RH) of 100%. However, under the conditions of high cell temperature (80 °C), low RH (25%) and no applied gas pressure, which is more desirable for fuel cell vehicle (FCV) applications, the GDL with 30 wt.% PTFE shows the best performance. The GDL with 30 wt.% PTFE impedes the removal of produced water and increases the actual humidity within the membrane electrode assembly (MEA). A gas pressure of 1 bar in the cell using the GDL with 30 wt.% PTFE greatly improves the performance, especially at low RH, resulting in performance that exceeds that of the cell under no gas pressure and high RH of 100%.     

Effects of ratio variation in substrate and micro porous layer penetration on polymer exchange membrane fuel cell performance

A gas diffusion layer (GDL) facilitates the diffusion of reactant gas and the discharge of the generated water. The GDL performs various functions, such as conducting heat and electrons generated by electrochemical reactions and providing mechanical support for the catalyst layer. In this study, the effects of ratio variation in the substrate and microporous layer (MPL) penetration region on the proton exchange membrane fuel cell (PEMFC) performance were investigated. Furthermore, the reasons for these performance tendencies are explained based on the thermogravimetric analysis, contact angle, scanning electron microscopy, mercury porosimetry, electrical resistance, electrochemical impedance spectroscopy, and capillary pressure gradient. The experimental results indicate that the MPL penetration ratio within 15–20% of the total GDL thickness and the combined ratio of the MPL and MPL penetration within 35–40% is the best for the overall PEMFC performance. In addition, when the substrate ratio is excessively low, water flooding substantially occurs in the substrate, and this accumulated water functions as a back pressure, causing severe capillary condensation in the MPL penetration region and thus depriving the supply of the reactant gas.     

Effective thermal conductivity and thermal contact resistance of gas diffusion layers in proton exchange membrane fuel cells. Part 2: Hysteresis effect under cyclic compressive load

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a PEM fuel cell. The analysis of this process requires the determination of the effective thermal conductivity as well as the thermal contact resistance between the GDL and adjacent surfaces/layers. The Part 1 companion paper describes an experimental procedure and a test bed devised to allow separation of the effective thermal conductivity and thermal contact resistance, and presents measurements under a range of static compressive loads. In practice, during operation of a fuel cell stack, the compressive load on the GDL changes.In the present study, experiments are performed on Toray carbon papers with 78% porosity and 5% PTFE under a cyclic compressive load. Results show a significant hysteresis in the loading and unloading cycle data for total thermal resistance, thermal contact resistance (TCR), effective thermal conductivity, thickness, and porosity. It is found that after 5 loading-unloading cycles, the geometrical, mechanical, and thermal parameters reach a “steady-state” condition and remain unchanged. A key finding of this study is that the TCR is the dominant component of the GDL total thermal resistance with a significant hysteresis resulting in up to a 34% difference between the loading and unloading cycle data. This work aims to clarify the impact of unsteady/cyclic compression on the thermal and structural properties of GDLs and provides new insights on the importance of TCR which is a critical interfacial transport phenomenon.     

Measurement of in-plane thermal conductivity of carbon paper diffusion media in the temperature range of −20 °C to +120 °C

Carbon paper is commonly used as the gas diffusion layer (GDL) in polymer electrolyte membrane (PEM) fuel cells as it exhibits high chemical and mechanical durability. This diffusion medium is also anisotropic, which directly affects its transport properties and specifically the thermal conductivity. In this study, the in-plane thermal conductivity of the carbon paper GDL was determined using thermal diffusivity measurements for a temperature range from −20 to +120 °C and four Teflon loadings (0, 5, 20 and 50 wt.%). It is important to understand the effect of temperature on the thermal conductivity since PEM fuel cells are designed to operate under various temperatures depending on the application of use. Further, Teflon is used to change the hydrophobic properties of the carbon paper GDL with 20 wt.% as the most widely used percentage. In this study, the Teflon loadings were chosen to gain a comprehensive understanding of the thermal resistance due to Teflon. In this study, a quasi-steady method was used to measure the thermal properties of the carbon paper; hence, the phase transformation in the presence of PTFE was investigated. The thermal conductivity decreases with an increase in temperature for all samples. The addition of as little as 5 wt.% Teflon resulted in high thermal resistance decreasing the overall thermal conductivity of the sample. Further addition of Teflon did not have major effects on the thermal conductivity. For all treated samples, the thermal conductivity lies in the range of 10.1–14.7 W/mK. Finally, empirical relations for the thermal diffusivity and conductivity with temperature were deduced.     

A novel approach to determine the in-plane thermal conductivity of gas diffusion layers in proton exchange membrane fuel cells

Heat transfer through the gas diffusion layer (GDL) is a key process in the design and operation of a proton exchange membrane (PEM) fuel cell. The analysis of this process requires determination of the effective thermal conductivity. This transport property differs significantly in the through-plane and in-plane directions due to the anisotropic micro-structure of the GDL.A novel test bed that allows separation of in-plane effective thermal conductivity and thermal contact resistance in GDLs is described in this paper. Measurements are performed using Toray carbon paper TGP-H-120 samples with varying polytetrafluoroethylene (PTFE) content at a mean temperature of 65-70 °C. The measurements are complemented by a compact analytical model that achieves good agreement with experimental data. The in-plane effective thermal conductivity is found to remain approximately constant, k ≈ 17.5 W m⁻¹ K⁻¹, over a wide range of PTFE content, and its value is about 12 times higher than that for through-plane conductivity.     

Ultra thin gas diffusion layer development for PEMFC

This research studies an ultra-thin carbon fiber paper fabrication process for proton exchange membrane fuel cells (PEMFCs). Polyacrylonitrile (PAN) based carbon fibers 6 mm long were dispersed and formed at aerial weights of 15 and 20 g/m² using a slurry molding machine. Polyscrylamide (PAM) and polyvinyl alcohol (PVA) dispersal agent solutions for fiber binding were added to evenly distribute the carbon fibers and increase the paper mechanical strength. The carbon fiber papers were dried after resin impregnation using a convective oven at 120 °C temperature for 10 min. The hot press machine was heated to 160 °C temperature and the workpieces were pressed for 5 min. Graphitization completed the gas diffusion substrate (GDS) process. GDL involves immersing the paper in a 5% polytetrafluoroethylene (PTFE) solution, coating the paper with a micro porous layer (MPL). This study shows the proposed ultra-thin GDL fabrication method is suitable for PEMFC applications and exhibits feasible functionality for fuel cells.     

Thermal conductivity and temperature profiles of the micro porous layers used for the polymer electrolyte membrane fuel cell

The thermal conductivity and the thickness change with pressure of several different micro porous layers (MPL) used for the polymer electrolyte membrane fuel cell (PEMFC) were measured. The MPL were made with different compositions of carbon and polytetrafluoroethylene (PTFE). A one-dimensional thermal PEMFC model was used to estimate the impact that the MPL has on the temperature profiles though the PEMFC.     

Thermal modeling of LmNi4.91Sn0.15 based solid state hydrogen storage device with embedded cooling tubes

A 2-D mathematical model is developed for predicting the minimum charging/discharging time of the metal hydride based hydrogen storage device by varying the number of cooling tubes embedded in it. This study is extended to 3-D mathematical model for predicting the hydriding and dehydriding characteristics of LmNi_4.91Sn_0.15 based hydrogen storage device with 60 embedded cooling tubes (ECT) using COMSOL Multiphysics 4.3. The performance of the hydrogen storage device during hydriding/dehydriding process is presented for different supply pressure (10–35 bar), hot fluid temperature (30–60 °C) and effective thermal conductivity of hydride bed (0.2–2.5 W/(m?K)). It is observed that the rate of heat transfer and the hydriding and dehydriding rates are enhanced when the number of ECT is increased from 24 to 70. For the reactor with 60 ECT, the rate of hydrogen absorption is rapid for the supply pressure of 35 bar and hydride bed effective thermal conductivity of 2.5 W/(m?K). The numerically predicted hydrogen storage capacity (wt%) and amount of hydrogen desorbed (wt%) are compared with experimental data and found a good accord between them.     

Effects of gas diffusion layer (GDL) and micro porous layer (MPL) on cathode performance in microbial fuel cells (MFCs)

This study focused on novel cathode structures to increase power generation and organic substrate removal in microbial fuel cells (MFCs). Three types of cathode structures, including two-layer (gas diffusion layer (GDL) and catalyst layer (CL)), three-layer (GDL, micro porous layer (MPL) and CL), and multi-layer (GDL, CL, carbon based layer (CBL) and hydrophobic layers) structures were examined and compared in single-chamber MFCs (SCMFCs). The results showed that the three-layer (3L) cathode structures had lower water loss than other cathodes and had a high power density (501 mW/m²). The MPL in the 3L cathode structure prevented biofilm penetration into the cathode structure, which facilitated the oxygen reduction reaction (ORR) at the cathode. The SCMFCs with the 3L cathodes had a low ohmic resistance (R_ohmic: 26-34 Ω) and a high cathode open circuit potential (OCP: 191 mV). The organic substrate removal efficiency (71-78%) in the SCMFCs with 3L cathodes was higher than the SCMFCs with two-layer and multi-layer cathodes (49-68%). This study demonstrated that inserting the MPL between CL and GDL substantially enhanced the overall electrical conduction, power generation and organic substrate removal in MFCs by reducing water loss and preventing biofilm infiltration into the cathode structure.     

Ex situ measurements of through-plane thermal conductivities in a polymer electrolyte fuel cell

In this paper thermal properties for materials typically used in the proton exchange membrane fuel cell (PEMFC) are reported. Thermal conductivities of Nafion membranes were measured ex situ at 20 °C to be 0.177 ± 0.008 and 0.254 ± 0.016 W K⁻¹ m⁻¹ for dry and maximally wetted membranes respectively. This paper also presents a methodology to determine the thermal conductivity of compressible materials as a function of applied load. This technique was used to measure the thermal conductivity of an uncoated SolviCore porous transport layer (PTL) at various compaction pressures. For the dry PTL at 4.6, 9.3 and 13.9 bar compaction pressures, the thermal conductivity was found to be 0.27, 0.36 and 0.40 W K⁻¹ m⁻¹ respectively and the thermal contact resistivity to the apparatus was determined to be 2.1, 1.8 and 1.1 × 10⁻⁴ m² K W⁻¹, respectively. It was shown that the thermal contact resistance between two PTLs is negligible compared to the apparatus’ thermal contact resistivity. For a humidified PTL, the thermal conductivity increases by up to 70% due to a residual liquid saturation of 25%.