Characteristics of water transport through the membrane in direct methanol fuel cells operating with neat methanol

The water required for the methanol oxidation reaction in a direct methanol fuel cell (DMFC) operating with neat methanol can be supplied by diffusion from the cathode to the anode through the membrane. In this work, we present a method that allows the water transport rate through the membrane to be in-situ determined. With this method, the effects of the design parameters of the membrane electrode assembly (MEA) and operating conditions on the water transport through the membrane are investigated. The experimental data show that the water flux by diffusion from the cathode to the anode is higher than the opposite flow flux of water due to electro-osmotic drag (EOD) at a given current density, resulting in a net water transport from the cathode to the anode. The results also show that thinning the anode gas diffusion layer (GDL) and the membrane as well as thickening the cathode GDL can enhance the water transport flux from the cathode to the anode. However, a too thin anode GDL or a too thick cathode GDL will lower the cell performance due to the increases in the water concentration loss at the anode catalyst layer (CL) and the oxygen concentration loss at the cathode CL, respectively.     

Study on the effect of humidity and stoichiometry on the water saturation of PEM fuel cells

This paper investigates the effects of relative humidity (RH) and stoichiometry of reactants on the water saturation and local transport process in proton exchange membrane fuel cells. A two‐dimensional model was developed, taking into account the effect of the formation of liquid water on the reactant transport. The results indicate that the reactant RH and stoichiometry significantly affect cell performance. At a constant anode RH = 100%, a lower cathode RH maintains membrane hydration to give better cell performance. At a constant cathode RH = 100%, a lower anode RH not only provides more hydrogen to the catalyst layer to participate in the electrochemical reaction but also increases the difference in the water concentrations between the anode and cathode. This enhances the back‐diffusion of water from the cathode to the anode, reducing possible flooding for better cell performance. Higher anodic stoichiometry results in the reduction of cathodic water saturation by increasing water back‐diffusion, thereby enhancing fuel cell performance. Higher cathodic stoichiometry also reduces water saturation by drying more liquid water to increase cathode local current density. Copyright © 2011 John Wiley & Sons, Ltd.     

Non-uniform agglomerate cathode catalyst layer model on the performance of PEMFC with consideration of water effect

We focus on the effect of cathode catalyst layer physical structure on the cell performance of proton exchange membrane fuel cell (PEMFC). At low polarization, high inlet humidification predicts better cell performance because of the more active surface in the CL. As polarization is extended near the mass transfer limited regime, high humidification only renders a flooded electrode and inferior cell performance. Catalyst layer with better capillary water transport parameters performs better than that with inferior water repulsion capability. Permeation in the gas diffusion layer (GDL) is important for efficient oxygen diffusion in mass transfer influenced regime. On the other hand, the permeability in catalyst layer only has secondary effect.The distribution of material properties in the CL is studied for the MEA fabrication strategy. The CL is divided into three sub-layers with changing material properties. With water effect considered, better performance is obtained for higher porosity near the GDL, higher electrolyte fraction in the agglomerate near the membrane. The effect of agglomerate particle size differs in the ohmic and mass transfer controlled regimes. Larger agglomerate size near GDL is preferred in the ohmic limited regime, while smaller size near GDL performs better if operated at mass transfer controlled regime.     

A theoretical model of the membrane electrode assembly of liquid feed direct methanol fuel cell with consideration of water and methanol crossover

An algebraic model of the membrane electrode assembly of the direct methanol fuel cell is developed, which considers the simultaneous liquid water and methanol crossover effects, and the associated electrochemical reactions. The respective anodic and cathodic polarization curves can be predicted using this model. Methanol concentration profile and flux are correlated explicitly with the operating conditions and water transport rate. The cathode mixed potential effect induced by the methanol crossover is included and the subsequent cell voltage loss is identified. Water crossover is influenced by the capillary pressure equilibrium and hydrophobic property within the cathode gas diffusion layer. The model can be used to evaluate the cell performance at various working parameters such as membrane thickness, methanol feed concentration, and hydrophobicity of the cathode gas diffuser.     

Parametric study of the porous cathode in the PEM fuel cell

The electrochemical behavior and the reactant transport in the porous gas diffusion layer (GDL) and catalyst layer (CL) are controlled by a large number of parameters such as porosity, permeability, conductivity, catalyst loading, and average pore size, etc. A three‐dimensional polymer electrolyte membrane fuel cell model is developed. The model accounts for the mass, fluid, and thermal transport processes as well as the electrochemical reaction. Using this model, the effects of the various porous electrode design parameters including porosity, solid electronic conductivity, and thermal conductivity of cathode GDL, and the catalyst loading, average pore size of cathode CL are investigated through parametric study. The model is shown to agree well with the experimental data of some porous electrode specifications. In addition, the model shows promise as a tool for optimizing the design of fuel cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Three-dimensional transient two-phase study of the cathode side of a PEM fuel cell

A three-dimensional transient two-phase isothermal model has been developed for the cathode side of a proton exchange membrane fuel cell (PEMFC). This has been done in order to fully investigate the effects and the time variation of liquid water formation as well as the gas phase transport under the start up condition. It is considered that the generated water in the cathode catalyst layer (CL) is liquid water and that the gas diffusion layer (GDL) is hydrophobic. A non-equilibrium water condensation-evaporation is also assumed. The time variations of liquid water distribution in along-channel and through-plane directions are investigated. This is to determine the liquid water accumulation at the start up time (above the channel under the CL), then the movement of the liquid water in the domain and the final accumulation at the steady state condition (above the rib and near the CL). It has also been found that it takes less time for a high average current density to attain the steady state condition which is due to the capillary pressure gradient inside the porous media. Validation of the numerical results has been implemented via a polarization curve comparison with the experimental data. Both sets show good agreement.     

A parametric study of cathode catalyst layer structural parameters on the performance of a PEM fuel cell

This paper is a computational study of the cathode catalyst layer (CL) of a proton exchange membrane fuel cell (PEMFC) and how changes in its structural parameters affect performance. The underlying mathematical model assumes homogeneous and steady-state conditions, and consists of equations that include the effects of oxygen diffusion, electrochemical reaction rates, and transport of protons and electrons through the Nafion ionomer (PEM) and solid phases. Simulations are concerned with the problem of minimizing activation overpotential for a given current density. The CL consists of four phases: ionomer, solid substrate, catalyst particles and void spaces. The void spaces are assumed to be fully flooded by liquid water so that oxygen within the CL can diffuse to reaction sites via two routes: within the flooded void spaces and dissolved within the ionomer phase. The net diffusive flux of oxygen through the cathode CL is obtained by incorporating these two diffusive fluxes via a parallel resistance type model. The effect of six structural parameters on the CL performance is considered: platinum and carbon mass loadings, ionomer volume fraction, the extent to which the gas diffusion layer (GDL) extends into the CL, the GDL porosity and CL thickness. Numerical simulations demonstrate that the cathode CL performance is most strongly affected by the ionomer volume fraction, CL thickness and carbon mass loading. These results give useful guidelines for manufactures of PEMFC catalyst layers.     

Numerical simulation of exchange membrane fuel cells in different operating conditions

A two-dimensional, steady state model for proton exchange membrane fuel cell (PEMFC) is presented. The model is used to describe the effect operation conditions (current density, pressure and water content) on the water transport, ohmic resistance and water distribution in the membrane and performance of PEMFC. This model considers the transport of species and water along the porous media: gas diffusion layers (GDL) anode and cathode, and the membrane of PEMFC fuel cell.     

Water management studies in PEM fuel cells,Part I: Fuel cell design and in situ water distributions

A proton exchange membrane fuel cell (PEMFC) must maintain a balance between the hydration level required for efficient proton transfer and excess liquid water that can impede the flow of gases to the electrodes where the reactions take place. Therefore, it is critically important to understand the two-phase flow of liquid water combined with either the hydrogen (anode) or air (cathode) streams. In this paper, we describe the design of an in situ test apparatus that enables investigation of two-phase channel flow within PEMFCs, including the flow of water from the porous gas diffusion layer (GDL) into the channel gas flows; the flow of water within the bipolar plate channels themselves; and the dynamics of flow through multiple channels connected to common manifolds which maintain a uniform pressure differential across all possible flow paths. These two-phase flow effects have been studied at relatively low operating temperatures under steady-state conditions and during transient air purging sequences.     

Effect of catalyst layer with zeolite on the performance of a proton exchange membrane fuel cell operated under low-humidity conditions

Proton exchange membrane fuel cells (PEMFCs) employ a proton conductive membrane as the separator to transport a hydrogen proton from the anode to the cathode. The membrane's proton conductivity depends on the water content in the membrane, which is affected by the operating conditions. A membrane electrode assembly (MEA) that can self-sustain water is the key component for developing a light-weight and compact PEMFC system without humidifiers. Hence, zeolite is employed to the anode catalyst layer in this study. The effect of the gas diffusion layer (GDL) materials, catalyst loading, binder loading, and zeolite loading on the MEA performance is investigated. The MEA durability is also investigated through the electrochemical impedance spectroscopy (EIS) method. The results suggest that the MEA with the SGL28BCE carbon paper, Pt loadings of 0.1 and 0.7 mg cm^?2 in the anode and cathode, respectively, Nafion-to-carbon weight ratio of 0.5, and zeolite-to-carbon weight ratio of 0.3 showed the best performance when the cell temperature is 60 °C and supplies with dry hydrogen and air from the environment. According to the impedance variation measured by EIS, the MEA with zeolite in the anode catalyst layer shows higher and more stable performance than those without zeolite.     

Simulation of species transport and water management in PEM fuel cells

A single phase computational fuel cells model is presented to elucidate three-dimensional interactions between mass transport and electrochemical kinetics in proton exchange membrane (PEM) fuel cells with straight gas channels. The governing differential equations are solved over a single computational domain, which consists of a gas channel, gas diffusion layer, and catalyst layer for both the anode and cathode sides of the cell as well as the solid polymer membrane. Emphasis is placed on obtaining a basic understanding of how three-dimensional flow and transport phenomena in the air cathode impact the electrochemical process in the flow field. The complete cell model has been validated against experimentally measured polarization curve, showing good accuracy in reproducing cell performance over moderate current density interval. Fully three-dimensional results of the flow structure and species profiles are presented for cathode flow field. The effects of pressure on oxygen transport and water removal are illustrated through main axis of the flow structure. The model results indicate that oxygen concentration in reaction sites is significantly affected by pressure increase which leads to rising fuel cells power.     

Effect of the cathode gas diffusion layer on the water transport behavior and the performance of passive direct methanol fuel cells operating with neat methanol

The passive operation of a direct methanol fuel cell with neat methanol requires the water that is produced at the cathode to diffuse through the membrane to the anode to compensate the methanol oxidation reaction (MOR). Hence, the anode performance of this type of fuel cell can be limited by the water transport rate from the cathode to the anode. In this work we theoretically show that the water transport from the cathode to the anode depends primarily on the design of the cathode gas diffusion layer (GDL). We investigate experimentally the effects of the design parameters of the cathode GDL, including the PTFE (polytetrafluoroethylene) content in the backing layer (BL), and the carbon loading and the PTFE content in the microporous layer (MPL) on the water transport and the performance of the passive DMFC with the help of a reference electrode. The results indicate that on one hand, these parameters can be adjusted to decrease the water concentration loss of the anode performance, but on the other hand, they can also cause an increase in the oxygen concentration loss of the cathode performance. Hence, an optimal balance in minimizing the both concentration losses is the key to maximize the cell performance.     

An impedance study for the anode micro-porous layer in an operating direct methanol fuel cell

This study presents the benefit to an operating direct methanol fuel cell (DMFC) by coating a micro-porous layer (MPL) on the surface of anode gas diffusion layer (GDL). Taking the membrane electrode assembly (MEA) with and without the anodic MPL structure into account, the performances of the two types of MEA are evaluated by measuring the polarization curves together with the specific power density at a constant current density. Regarding the cell performances, the comparisons between the average power performances of the two different MEAs at low and high current density, various methanol concentrations and air flow rates are carried out by using the electrochemical impedance spectroscopy (EIS) technique. In contrast to conventional half cell EIS measurements, both the anode and cathode impedance spectra are measured in real-time during the discharge regime of the DMFC. As comparing each anode and cathode EIS between the two different MEAs, the influences of the anodic MPL on the anode and cathode reactions are systematically discussed and analyzed. Furthermore, the results are used to infer complete and reasonable interpretations of the combined effects caused by the anodic MPL on the full cell impedance, which correspond with the practical cell performance.     

Parametric study of the cathode and the role of liquid saturation on the performance of a polymer electrolyte membrane fuel cell—A numerical approach

A two-dimensional two-phase steady state model of the cathode of a polymer electrolyte membrane fuel cell (PEMFC) is developed using unsaturated flow theory (UFT). A gas flow field, a gas diffusion layer (GDL), a microporous layers (MPL), a finite catalyst layer (CL), and a polymer membrane constitute the model domain. The flow of liquid water in the cathode flow channel is assumed to take place in the form of a mist. The CL is modeled using flooded spherical agglomerate characterization. Liquid water is considered in all the porous layers. For liquid water transport in the membrane, electro-osmotic drag and back diffusion are considered to be the dominating mechanisms. The void fraction in the CL is expressed in terms of practically achievable design parameters such as platinum loading, Nafion loading, CL thickness, and fraction of platinum on carbon. A number of sensitivity studies are conducted with the developed model. The optimum operating temperature of the cell is found to be 80-85 °C. The optimum porosity of the GDL for this cell is in the range of 0.7-0.8. A study by varying the design parameters of the CL shows that the cell performs better with 0.3-0.35 mg cm⁻² of platinum and 25-30 wt% of ionomer loading at high current densities. The sensitivity study shows that a multi-variable optimization study can significantly improve the cell performance. Numerical simulations are performed to study the dependence of capillary pressure on liquid saturation using various correlations. The impact of the interface saturation on the cell performance is studied. Under certain operating conditions and for certain combination of materials in the GDL and CL, it is found that the presence of a MPL can deteriorate the performance especially at high current density.     

An approach for determining the liquid water distribution in a liquid-feed direct methanol fuel cell

In determining the liquid water distribution in the anode (or the cathode) diffusion medium of a liquid-feed direct methanol fuel cell (DMFC) with a conventional two-phase mass transport model, a current-independent liquid saturation boundary condition at the interface between the anode flow channel and diffusion layer (DL) (or at the interface between the cathode flow channel and cathode DL) needs to be assumed. The numerical results resulting from such a boundary condition cannot realistically reveal the liquid distribution in the porous region, as the liquid saturation at the interface between the flow channel and DL varies with current density. In this work, we propose a simple theoretical approach that is combined with the in situ measured water-crossover flux in the DMFC to determine the liquid saturation in the anode catalyst layer (CL) and in the cathode CL. The determined liquid saturation in the anode CL (or in the cathode CL) can then be used as a known boundary condition to determine the water distribution in the anode DL (or in the cathode DL) with a two-phase mass transport model. The numerical results show that the water distribution becomes much more realistic than those predicted with the assumed boundary condition at the interface between the flow channel and DL.     

Three-dimensional two-phase flow model of proton exchange membrane fuel cell with parallel gas distributors

A non-isothermal, steady-state, three-dimensional (3D), two-phase, multicomponent transport model is developed for proton exchange membrane (PEM) fuel cell with parallel gas distributors. A key feature of this work is that a detailed membrane model is developed for the liquid water transport with a two-mode water transfer condition, accounting for the non-equilibrium humidification of membrane with the replacement of an equilibrium assumption. Another key feature is that water transport processes inside electrodes are coupled and the balance of water flux is insured between anode and cathode during the modeling. The model is validated by the comparison of predicted cell polarization curve with experimental data. The simulation is performed for water vapor concentration field of reactant gases, water content distribution in the membrane, liquid water velocity field and liquid water saturation distribution inside the cathode. The net water flux and net water transport coefficient values are obtained at different current densities in this work, which are seldom discussed in other modeling works. The temperature distribution inside the cell is also simulated by this model.     

Simulation on water transportation in gas diffusion layer of a PEM fuel cell: Influence of non-uniform PTFE distribution

Proton exchange membrane fuel cells (PEMFCs) are promising clean power sources with high energy conversion efficiency, fast startup, and no pollutant emission. The generated water in the cathode can cause water flooding of the catalyst layer (CL), which in turn can significantly decrease the fuel cell performance. To address this significant issue of PEMFC, a new gas diffusion layer (GDL) with non-uniform distribution of PTFE is proposed for water removal from the CL. The feasibility of this new GDL design is numerically evaluated by a Lattice-Boltzmann Method (LBM)-based two-phase flow model. The porous structure of the new GDL design is numerically reconstructed, followed by LBM simulations of the water transport in GDL. Three types of different wetting conditions are considered. It is found that liquid water transported 7.87% more with a single row of wetted solids and 13.36% more with two rows of wetted solids. The results clearly demonstrate that the liquid water can be effectively removed from the GDL by proper arrangement of hydrophilic solids in the GDL.     

Water management in alkaline anion exchange membrane fuel cell anode

Water management is an important issue for alkaline anion exchange membrane fuel cell (AAEMFC) due to its significant role in the energy conversion processes. In this study, a numerical model is developed to investigate the water transport in AAEMFC anode. The gas and liquid transport characteristics in the gas diffusion layer (GDL) and catalyst layer (CL) with different designs and under various operating conditions are discussed. The results show that the current density affects the liquid water distribution in anode most significantly, and the temperature is the second considerable factor. The stoichiometry ratio of the supplied reactant has insignificant effect on the liquid water transport in anode. The change of liquid water amount in anode with cathode relative humidity follows a similar trend with anode inlet relative humidity. Some numerical results are also explained with published experimental and modeling data with reasonable agreement.     

Three-dimensional multiphase modeling of alkaline anion exchange membrane fuel cell

Alkaline anion exchange membrane (AAEM) fuel cell is becoming more attractive because of its outstanding merits, such as fast electrochemical kinetics and low dependence on non-precious catalyst. In this study, a three-dimensional multiphase non-isothermal AAEM fuel cell model is developed. The modeling results show that the performance is improved with more anode humidification, but the improvement becomes less significant at higher humidification levels. The humidification level of anode can change the water removal mechanisms: at partial humidification, water is removed as vapor; and for full humidification, water is removed as liquid. Cathode humidification is even more critical than anode. Liquid water supply in cathode has a positive effect on performance, especially at high current densities. With more liquid water supply in cathode, liquid water starts moving from channel to CL, rather than being removed from CL. Liquid water supply in cathode is needed to balance the water amounts in anode and cathode. Decreasing the membrane thickness generally improves the cell performance, and the improvement is even enhanced with thinner membranes, due to the faster water diffusion between anode and cathode, which reduces the mass transport losses.