Carbon Paper Gas Diffusion Layer Research Articles

Simulation of liquid water breakthrough in a nanotomography reconstruction of a carbon paper gas-diffusion layer

This study reports the feasibility of newly simulating liquid water intrusion into the porous gas diffusion layer (GDL) of a polymer electrolyte fuel cell (PEFC) using X-ray nano-tomography and two-phase lattice Boltzmann (LB) simulation. A digital 3D model of the GDL is reconstructed using X-ray nanotomography while two-phase porous flow is simulated at two different levels of surface wettability by applying a newly-developed numerical LB model. The results show liquid infiltration in a hydrophobic GDL is comparatively lower (pore saturation of 0.11 to 0.90) than that for a hydrophilic GDL (pore saturation of 0.36 to 0.96) over the liquid intrusion range of 1 kPa . 100 kPa. Visualisation of simulated results in three dimensions reveal dissimilar liquid infiltration characteristics for the two levels of wettability considered, yet also reveal a general breakthrough of liquid water at a pressure of 10 kPa due to specific structural features of the GDL.

Electrochemical Synthesis of Oxygen Reduction Catalysts Based on Pt Coated Polypyrrole Nanowires Using Starch as Template Molecule

This study demonstrates a new method for the development of low loading Pt catalysts for the oxygen reduction reaction (ORR) based on the electrochemical synthesis of Pt coated polypyrrole (PPY) nanowires. PPY was deposited on different electrodes such as graphite, carbon paper gas diffusion layer (GDL), and glassy carbon by using chemical template molecules such as heparin and starch. Pt coated PPY/starch nanowires show ORR catalytic activity after the electrochemical degradation of PPY, which suggests that PPY/starch nanowires can be electrochemically converted to a stable high surface area support for ORR catalysts. Moreover, these low loading ORR Pt catalysts can be easily incorporated in the membrane electrode assembly (MEA) structure as a gas diffusion electrode (GDE) when directly prepared on a GDL.

Effect of liquid water on transport properties of the gas diffusion layer of polymer electrolyte membrane fuel cells

Water management in polymer electrolyte membrane (PEM) fuel cells is of importance due to its impact on the performance, durability and ultimately the cost of the cell. In the gas diffusion layer (GDL), liquid water has a direct effect on species and heat transport. The amount of liquid water in the GDL affects the relative permeability and capillary pressure, which govern the convective and diffusive transport of liquid water. Liquid water acts as a barrier to the diffusion of gases through the void region and facilitates in heat transfer. In this study, the full morphology model was used in order to investigate the effects of liquid water presence on the transport properties of the carbon paper GDL and examine the applicability of using various laws to estimate the transport properties in the presence of liquid water. The numerical results were compared against published experimental data. Further, the method of standard porosimetry was used to experimentally measure the effect of Teflon treatment on the capillary pressure of carbon paper. It was found that the addition of PTFE to the GDL results in the increase of capillary pressure; however, further increases to the PTFE loading did not result in additional changes to the capillary pressure.

Simultaneous Measurement of Oxygen Diffusivity and Visualization of Moisture Distribution in Gas Diffusion Layer with Wettability Distribution for Improvement of Polymer Electrolyte Fuel Cell Performance

The mass transfer characteristics of gas diffusion layer (GDL) are closely related to cell performance in polymer electrolyte fuel cell (PEFC). It is necessary to characterize the liquid water distribution, the microscopic conformation, and the oxygen diffusion coefficient in a GDL containing moisture. The configurations of a novel hybrid type GDL, in which two porous media with different wettabilities are arranged alternately (1),(2) and two porous media with different pore size distributions were alternately arranged (3), have been proposed with the aim of improving the oxygen diffusivity by controlling the moisture distribution in GDL porous media. However, the basic principles of hybrid type GDLs were examined in the former reports as a pilot study by using thicker media than those used in practice. It is thus necessary to elucidate the performance of the hybrid type GDLs with thinner sizes for the practical use. The present study examined a hybrid type GDL to control the liquid water behavior in the porous media. A carbon paper GDL used in practice for PEFCs with a wettability distribution due to adjacent hydrophilic and hydrophobic region was used as a test material. The measurement of oxygen diffusivity and visualization of liquid water in the GDL with wettability distribution were performed simultaneously using a galvanic battery type oxygen absorber and X-ray radiography, respectively.

The Electrochemical Behavior and Catalytic Activity for Oxygen Reduction of MnO2/C– Toray Gas Diffusion Electrodes

Gas diffusion electrodes (GDE) composed of: Toray carbon paper gas diffusion layer (unteflonated and teflonated with 30%wt PTFE, respectively) and MnO2 supported on Vulcan XC72R catalyst layer with 20%wt PTFE, were investigated in 6 M KOH without and with O2, respectively. Four sources of MnO2 (Sigma-Aldrich, Tronox, Riedel and Merck Inc.) were comparatively studied by electrochemical methods accompanied by XRD characterization. Cyclic voltammetry scans of GDE in N2-purged electrolyte were used to estimate the Tafel slopes for Mn(IV) reduction and to identify active Mn(IV) sites that play an important catalytic role. Two oxygen reduction reaction (ORR) mechanisms were identified by porous rotating disk electrode (PRDE) polarization as a function of electrode potential. At high potentials (above −300 mV vs. MOE) the O2/HO2− step is mainly catalyzed by the quasi-unreduced MnO2 surface and active sites on the carbon support, while at potentials more negative than −300 mV, the redox catalysis by Mn(III) prevails. The main catalytic sites for the second step HO2−/OH−, are Mn(III) sites. The hydrophobic property of the porous electrode is important due to the effect on peroxide desorption/readsorption on the electrode surface. PRDE and flow cell experiments revealed the Sigma-Aldrich γ-MnO2 was the most active for ORR.

Phase-contrast X-ray imaging of the gas diffusion layer of fuel cells

The understanding of and in situ observation of the transport and distribution of water in carbon-paper gas diffusion layers (GDLs) using non-destructive imaging techniques is critical for achieving high performance in polymer electrolyte fuel cells (PEFCs). To investigate the behavior of water in GDLs of PEFCs, phase-contrast X-ray imaging via X-ray interferometric imaging (XII) and diffraction-enhanced imaging (DEI) were performed using 35 keV X-rays. The XII technique is useful for the radiographic imaging of GDLs and in situ observations of water evolution processes in operating PEFCs. DEI provides a way for tomographic imaging of GDLs in PEFCs. Because high-energy X-rays are applicable to the imaging of both carbon papers and heavy materials, which make up PEFCs, phase-contrast X-ray imaging techniques have proven to be valuable for investigation of GDLs.

High-Resolution Visualization of Water Accumulation Behavior in an Operating PEMFC by Soft X-ray Radiography

A carbon-cloth gas diffusion layer (GDL) displays better performance than a carbon-paper GDL under humidified condition. To investigate the relationship between GDL micro-structure and liquid water transport phenomena, water accumulation behavior in an operating Proton Exchange Membrane Fuel Cell (PEMFC) was visualized by soft X-ray radiography. Utilization of the soft X-ray range made it possible to visualize the liquid water in a PEMFC with the spatial resolution of 0.5 um and temporal resolution of 1.0 sec/frame. The water accumulated in a carbon-cloth GDL was locally distributed, and a larger amount of water accumulated in it than in a carbon-paper GDL. These results suggested that the broader pore distribution creates oxygen transport paths even if water accumulates in the GDL, and the pore structure has great importance. Furthermore, to investigate liquid water transport in more detail, stereoscopy technique has developed to get three-dimensional information with high temporal and spatial resolution.

Characterization of Microporous Layer in Carbon Paper GDL for PEM Fuel Cell

Characterization of a custom series of Toray TGP-H-060 carbon paper gas diffusion layer (GDL) was performed by using scanning electron microscopy (SEM) images, pore size distribution (PSD), MacMullin number, and fuel cell performance. The GDLs were customized by the addition of a microporous layer (MPL) and the treatment of, either or both, the substrate and MPL with 10% and 40% hydrophobic agent. SEM images correlated very well with the data shown for PSD. Distinction between the substrate layer and the MPL were clearly shown as two different slopes in the integral distribution and two different peaks in the differential distribution. The MacMullin number increased with increase in wet proofing but decreased with the addition of the MPL. Results showed that the cell performance with the different GDLs is affected by a competitive effect of pore volume in the substrate region, wet proofing treatment, and the MacMullin number.

Digital Volume Imaging of the PEFC Gas Diffusion Layer

The gas diffusion layer (GDL) plays a key role in the overall performance/durability of a polymer electrolyte fuel cell (PEFC). Of profound importance, especially in the context of water management and flooding phenomena, is the influence of the underlying pore morphology and wetting characteristics of the GDL microstructure. In this article, we present the digital volumetric imaging (DVI) technique in order to generate the 3-D carbon paper GDL microstructure. The internal pore structure and the local microstructural variations in terms of fiber alignment and fiber/binder distributions are investigated using the several 3-D thin sections of the sample obtained from DVI.

Pore-Level Liquid Water Transport Through Composite Diffusion Media of PEMFC

Liquid water transport at the pore level in a composite microporous layer (MPL) and carbon paper gas diffusion layer (GDL), used in a proton exchange membrane fuel cell (PEMFC), has been simulated for the first time by a topologically equivalent pore network (TEPN). Extracted from stochastic generated three-dimensional porous media that replicate typical patterns of void space and fiber connectivity observed in high resolution two-dimensional images of carbon paper GDL and a regular MPL made of carbon black and poly(tetrafluoroethylene), the TEPN for the composite diffusion media is used as input to an invasion percolation flow simulator. Steady-state liquid water distribution and saturation profiles in the MPL–GDL composite media are determined. In the interfacial region between the MPL and the GDL, liquid water saturation increases sharply due to a dramatic change in pore size, confirming the saturation discontinuity modeled in the continuum approach. The effects of current density and MPL thickness on liquid water saturation profiles are also explored. The results suggest that, in contrast to the MPL, liquid water saturation in the GDL is less affected by the current density. A thicker MPL helps to achieve a lower saturation level in the GDL by reducing the number of liquid injection sites into the GDL.

Determination of oxygen effective diffusivity in porous gas diffusion layer using a three-dimensional pore network model

In proton exchange membrane fuel cell (PEMFC) models, oxygen effective diffusivity is the most important parameter to characterize the oxygen transport in the gas diffusion layer (GDL). However, its determination is a challenge due to its complex dependency on GDL structure. In the present study, a three-dimensional network consisting of spherical pores and cylindrical throats is developed and used to investigate the effects of GDL structural parameters on oxygen effective diffusivity under the condition with/without water invasion process. Oxygen transport in the throat is described by Fick's law and water invasion process in the network is simulated using the invasion percolation with trapping algorithm. The simulation results reveal that oxygen effective diffusivity is slightly affected by network size but increases with decreasing the network heterogeneity and with increasing the pore connectivity. Impacts of network anisotropy on oxygen transport are also investigated in this paper. The anisotropic network is constructed by constricting the throats in the through-plane direction with a constriction factor. It is found that water invasion has a more severe negative influence on oxygen transport in an anisotropic network. Finally, two new correlations are introduced to determine the oxygen effective diffusivity for the Toray carbon paper GDLs.

Numerical Modeling of Two-Phase Behavior in the PEFC Gas Diffusion Layer

A critical performance limitation in the polymer electrolyte fuel cell (PEFC) is attributed to the mass transport loss originating from suboptimal liquid water transport and flooding phenomena. Liquid water can block the porous pathways in the fibrous gas diffusion layer (GDL) and the catalyst layer (CL), thus hindering oxygen transport from the flow field to the electrochemically actives sites in the catalyst layer. The cathode GDL is the component primarily responsible for facilitating gas and liquid transport, therefore plays a major role in determining the water management of a PEFC and hence the mass transport loss. The underlying pore morphology and wetting characteristics have significant influence on the flooding dynamics in the GDL. In this paper, the study of the two-phase behavior and the durability implications due to the wetting characteristics in the carbon paper GDL are presented using a pore-scale modeling framework.

Multiscale Modeling of Single-Phase Multicomponent Transport in the Cathode Gas Diffusion Layer of a Polymer Electrolyte Fuel Cell

This research reports a feasibility study into multiscale polymer electrolyte fuel cell (PEFC) modeling through the simulation of macroscopic flow in the multilayered cell via one-dimensional (1D) electrochemical modeling, and the simulation of microscopic flow in the cathode gas diffusion layer (GDL) via three-dimensional (3D) single-phase multicomponent lattice Boltzmann (SPMC-LB) modeling. The heterogeneous porous geometry of the carbon-paper GDL is digitally reconstructed for the SPMC-LB model using X-ray computer microtomography. Boundary conditions at the channel and catalyst layer interfaces for the SPMC-LB simulations such as specie partial pressures and through-plane flowrates are determined using the validated 1D electrochemical model, which is based on the general transport equation (GTE) and volume-averaged structural properties of the GDL. The calculated pressure profiles from the two models are cross-validated to verify the SPMC-LB technique. The simulations reveal a maximum difference of 2.4% between the thickness-averaged pressures calculated by the two techniques, which is attributable to the actual heterogeneity of the porous GDL structure.

An X-Ray Tomography Based Lattice Boltzmann Simulation Study on Gas Diffusion Layers of Polymer Electrolyte Fuel Cells

This work reports a feasibility study into the combined full morphological reconstruction of fuel cell structures using X-ray computed micro- and nanotomography and lattice Boltzmann modeling to simulate fluid flow at pore scale in porous materials. This work provides a description of how the two techniques have been adapted to simulate gas movement through a carbon paper gas diffusion layer (GDL). The validation work demonstrates that the difference between the simulated and measured absolute permeability of air is 3%. The current study elucidates the potential to enable improvements in GDL design, material composition, and cell design to be realized through a greater understanding of the nano- and microscale transport processes occurring within the polymer electrolyte fuel cell.

Pore-scale simulations on relative permeabilities of porous media by lattice Boltzmann method

Pore-scale simulations of two phase flows in a packed-sphere bed and in a carbon paper gas diffusion layer (GDL) are carried out using the free energy multiphase lattice Boltzmann method (LBM). The simulations are performed based on the detailed microstructure of the porous media under periodic boundary conditions such that the average phase saturations in the porous medium remain constant. A comparison of the simulated and measured relative permeabilities for the packed sphere bed as a function of non-wetting phase saturation is performed, and effects of the wettability and the anisotropic characteristics of relative permeabilities of the GDL are investigated.

Experimental measurements of effective diffusion coefficient of oxygen–nitrogen mixture in PEM fuel cell diffusion media

PEM fuel cells are increasingly designed to operate at high current densities. At these densities, mass transport limitations become very significant, but they are not well understood, with many modeling studies but few experimental observations. The use of accurate transport coefficients to simulate the mass transport at high current densities is crucial. In this study, experimental measurements have been carried out to determine the effective diffusion coefficient in the carbon paper gas diffusion layer that is commonly used in PEM fuel cells. It was found that almost all the existing theoretical models significantly overpredict the effective diffusion coefficient by as much as 4–5 times; thus, underestimating the transport limitations considerably. Further, the effects of temperature, Teflon treatment for hydrophobicity and porosity on the effective diffusion coefficient were investigated. It was found that temperature does not affect the overall diffusibility of the gas. The diffusibility is decreased with the increase of Teflon treatment and decrease in porosity. Further work on better understanding the diffusion process in the gas diffusion layer is under way.

An analytical model and parametric study of electrical contact resistance in proton exchange membrane fuel cells

This paper presents an analytical model of the electrical contact resistance between the carbon paper gas diffusion layers (GDLs) and the graphite bipolar plates (BPPs) in a proton exchange membrane (PEM) fuel cell. The model is developed based on the classical statistical contact theory for a PEM fuel cell, using the same probability distributions of the GDL structure and BPP surface profile as previously described in Wu et al. [Z. Wu, Y. Zhou, G. Lin, S. Wang, S.J. Hu, J. Power Sources 182 (2008) 265–269] and Zhou et al. [Y. Zhou, G. Lin, A.J. Shih, S.J. Hu, J. Power Sources 163 (2007) 777–783]. Results show that estimates of the contact resistance compare favorably with experimental data by Zhou et al. [Y. Zhou, G. Lin, A.J. Shih, S.J. Hu, J. Power Sources 163 (2007) 777–783]. Factors affecting the contact behavior are systematically studied using the analytical model, including the material properties of the two contact bodies and factors arising from the manufacturing processes. The transverse Young's modulus of chopped carbon fibers in the GDL and the surface profile of the BPP are found to be significant to the contact resistance. The factor study also sheds light on the manufacturing requirements of carbon fiber GDLs for a better contact performance in PEM fuel cells.

EFFECT OF GAS DIFFUSION LAYER CHARACTERISTICS AND ADDITION OF PORE-FORMING AGENTS ON THE PERFORMANCE OF POLYMER ELECTROLYTE MEMBRANE FUEL CELLS

The performance of five layer membrane electrode assemblies having different characteristics of gas diffusion layers was determined at 70°C cell temperature and ambient pressure. The maximum power density at 0.6 V was 0.36 W/cm2 for the membrane electrode assembly prepared with the gas diffusion layer having minimum thickness (SGL BC 30 type). On the other hand, the maximum power density at 0.5 V was 0.44 W/cm2 for the membrane electrode assembly prepared with SGL BC 34 type gas diffusion layer. It was found that resistance of a membrane electrode assembly is strongly dependent on gas diffusion layer thickness. Moreover, membrane electrode assemblies prepared with carbon paper gas diffusion layers resulted in higher performance than the assembly prepared with carbon cloth gas diffusion layer. Addition of pore-forming agents, which were ammonium carbonate, ammonium bicarbonate, ammonium sulfonate, and ammonium oxalate, to the catalyst ink lowered the performance.

Nonisothermal Through-Plane Transport Model of PEMFC with Local VLE Assumption

A nonisothermal model of the proton exchange membrane (PEM) fuel cell is developed with taking the through-plane heat and water transport into consideration. Local vapor-liquid equilibrium of water in the gas diffusion layer (GDL) is assumed. The steam pressure is determined only by temperature in the presence of condensed water. Only 1 oC temperature rise in GDL creates a steam flux corresponding to water generation at the current density of 0.74 A/cm2. Using stacked four PEM sheets, between which 20 μm thick thermocouples were inserted, the PEM temperature was measured in a cell having carbon paper GDLs, operated at 80 oC with hydrogen and oxygen supplied. About 1 oC rise in the PEM temperature was observed at a current density of 0.6 A/cm2. The effective thermal conductivity of the GDL was estimated from the experimental results. The temperature rise calculated by the one-dimensional numerical simulation well represents the experimental results.

Lattice Boltzmann simulations of anisotropic permeabilities in carbon paper gas diffusion layers

The multiple-relaxation-time (MRT) lattice Boltzmann method (LBM) with multi-reflection solid boundary conditions is used to study anisotropic permeabilities of a carbon paper gas diffusion layer (GDL) in a fuel cell. The carbon paper is reconstructed using the stochastic method, in which various porosities and microstructures are achieved to simulate different samples. The simulated permeability and tortuosity show anisotropic characteristics of the reconstructed carbon papers with in-plane permeability higher than through-plane, and in-plane tortuosity lower than through-plane. The calculated permeabilities are in good agreement with existing measurements. The relationship between the permeability and the porosity is fitted with empirical relations and some fitting constants are determined. Furthermore, the obtained relationship of tortuosity and porosity is used in a fractal model for permeabilities. The results indicate that the fractal model and the Kozeny–Carman equation provide similar predictions on the through-plane permeability of the carbon paper GDL.