Simulation of nanostructured electrodes for polymer electrolyte membrane fuel cells

Aligned carbon nanotubes (CNTs) with Pt uniformly deposited on them are being considered in fabricating the catalyst layer of polymer electrolyte membrane (PEM) fuel cell electrodes. When coated with a proton conducting polymer (e.g., Nafion) on the Pt/CNTs, each Pt/CNT acts as a nanoelectrode and a collection of such nanoelectrodes constitutes the proposed nanostructured electrodes. Computer modeling was performed for the cathode side, in which both multicomponent and Knudsen diffusion were taken into account. The effect of the nanoelectrode lengths was also studied with catalyst layer thicknesses of 2, 4, 6, and 10 μm. It was observed that shorter lengths produce better electrode performance due to lower diffusion barriers and better catalyst utilization. The effect of spacing between the nanoelectrodes was studied. Simulation results showed the need to have sufficiently large gas pores, i.e., large spacing, for good oxygen transport. However, this is at the cost of obtaining large electrode currents due to reduction of the number of nanoelectrodes per unit geometrical area of the nanostructured electrode. An optimization of the nanostructured electrodes was obtained when the spacing was at about 400 nm that produced the best limiting current density.     

Pore network modeling of cathode catalyst layer of proton exchange membrane fuel cell

In this paper, a pore network model is developed to investigate the coupled transport and reaction processes in the cathode catalyst layer (CCL) of proton exchange membrane fuel cell (PEMFC). The developed model is validated by comparing the predicted polarization curve with the experimental data, and the parametric studies are carried out to elucidate the effects of CCL design parameters. With the decrease of the CCL thickness and the Nafion content, the cell voltage reduces at the low current density but increases when the current density is higher. The cell performance is also improved by increasing the proton conductivity of the Nafion film in the CCL. As compared to the CCL of uniformly distributed Nafion, the CCL with the Nafion volume decreasing along the thickness direction exhibits better performance at the high current density.     

Study of current distribution and oxygen diffusion in the fuel cell cathode catalyst layer through electrochemical impedance spectroscopy

In this study, an analysis of the current distribution and oxygen diffusion in the Polymer Electrolyte Fuel Cell (PEFC) Cathode Catalyst Layer (CCL) has been carried out using Electrochemical Impedance Spectroscopy (EIS) measurements. Cathode EIS measurements obtained through a three-electrode configuration in the measurement system are compared with simulated EIS data from a previously validated numerical model, which subsequently allows the diagnostics of spatio-temporal electrochemical performance of the PEFC cathode. The results show that low frequency EIS measurements commonly related to mass transport limitations are attributed to the low oxygen equilibrium concentration in the CCL–Gas Diffusion Layer (GDL) interface and the low diffusivity of oxygen through the CCL. Once the electrochemical and diffusion mechanisms of the CCL are calculated from the EIS measurements, a further analysis of the current density and oxygen concentration distributions through the CCL thickness is carried out. The results show that high ionic resistance within the CCL electrolyte skews the current distribution towards the membrane interface. Therefore the same average current density has to be provided by few catalyst sites near the membrane. The increase in ionic resistance results in a poor catalyst utilization through the CCL thickness. The results also show that non-steady oxygen diffusion in the CCL allows equilibrium to be established between the equilibrium oxygen concentration supplied at the GDL boundary and the surface concentration of the oxygen within the CCL. Overall, the study newly demonstrates that the developed technique can be applied to estimate the factors that influence the nature of polarization curves and to reveal the effect of kinetic, ohmic and mass transport mechanisms on current distribution through the thickness of the CCL from experimental EIS measurements.     

Parametric study of a novel cathode catalyst layer in proton exchange membrane fuel cells

A steady-state mathematical model for the ordered cathode of proton exchange membrane fuel cells is developed to investigate the dependence of the cathode performance on the structural parameters of the catalyst layer. The model is based on the governing equations for oxygen concentration and potentials of the membrane and the solid phase, coupled by Tafel relation for the oxygen reduction reaction kinetics. The cathode current density optimization at a given electrode potential is presented with respect to nano-thread radius, porosity, platinum mass percentage, thickness, Nafion volume fraction and platinum loading of the catalyst layer. The simulation results suggest that small nano-thread radius is preferred. Except for quite low values as well as thin catalyst layers, porosity and platinum mass percentage have minor effects on cathode optimization. The cathode performance depends strongly on the catalyst layer thickness and additional attention should be paid to a thinner catalyst layer. The cathode can be efficiently optimized by increasing the highly sensitive parameters, Nafion volume fraction and platinum loading, to a suitable value which must avoid significant loss of oxygen transport.     

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 investigation of the effect of cathode catalyst layer structure and composition on polymer electrolyte membrane fuel cell performance

The effect of the cathode catalyst layer's structure and composition on the overall performance of a polymer electrolyte membrane fuel cell (PEMFC) is investigated numerically. The starting point of the sub-grid scale catalyst layer model is the well-known flooded agglomerate concept. The proposed model addresses the effects of ionomer (Nafion) loading, catalyst (platinum) loading, platinum/carbon ratio, agglomerate size and cathode layer thickness. The sub-grid scale model is first validated against experimental data and previously published results, and then embedded within a two-dimensional validated computational fluid dynamics code that can predict the overall performance of the fuel cell. The integrated model is then used to explore a wide range of the compositional and structural parameter space, mentioned earlier. In each case, the model is able to correctly predict the trends observed by past experimental studies. It is found that the performance trends are often different at intermediate versus high current densities—the former being governed by agglomerate-scale (or local) losses, while the latter is governed by catalyst layer thickness-scale (or global) losses. The presence of an optimal performance with varying Nafion content in the cathode is more due to the local agglomerate-scale mass transport and conductivity losses in the polymer coating around the agglomerates than due to the amount of Nafion within the agglomerate. It is also found that platinum mass loading needs to be at a moderate level in order to optimize fuel cell performance, even if cost is to be disregarded.     

Exploration of the oxygen transport behavior in non-precious metal catalyst-based cathode catalyst layer for proton exchange membrane fuel cells

High cost has undoubtedly become the biggest obstacle to the commercialization of proton exchange membrane fuel cells (PEMFCs), in which Pt-based catalysts employed in the cathodic catalyst layer (CCL) account for the major portion of the cost. Although non-precious metal catalysts (NPMCs) show appreciable activity and stability in the oxygen reduction reaction (ORR), the performance of fuel cells based on NPMCs remains unsatisfactory compared to those using Pt-based CCL. Therefore, most studies on NPMC-based fuel cells focus on developing highly active catalysts rather than facilitating oxygen transport. In this work, the oxygen transport behavior in CCLs based on highly active Fe-N-C catalysts is comprehensively explored through the elaborate design of two types of membrane electrode structures, one containing low-Pt-based CCL and NPMC-based dummy catalyst layer (DCL) and the other containing only the NPMC-based CCL. Using Zn-N-C based DCLs of different thickness, the bulk oxygen transport resistance at the unit thickness in NPMC-based CCL was quantified via the limiting current method combined with linear fitting analysis. Then, the local and bulk resistances in NPMC-based CCLs were quantified via the limiting current method and scanning electron microscopy, respectively. Results show that the ratios of local and bulk oxygen transport resistances in NPMC-based CCL are 80% and 20%, respectively, and that an enhancement of local oxygen transport is critical to greatly improve the performance of NPMC-based PEMFCs. Furthermore, the activity of active sites per unit in NPMC-based CCLs was determined to be lower than that in the Pt-based CCL, thus explaining worse cell performance of NPMC-based membrane electrode assemblys (MEAs). It is believed that the development of NPMC-based PEMFCs should proceed not only through the design of catalysts with higher activity but also through the improvement of oxygen transport in the CCL.     

Effect of catalyst layer mesoscopic pore-morphology on cold start process of PEM fuel cells

Water transport is of paramount importance to the cold start of proton exchange membrane fuel cells (PEMFCs). Analysis of water transport in cathode catalyst layer (CCL) during cold start reveals the distinct characteristics from the normal temperature operation. This work studies the effect of CCL mesoscopic pore-morphology on PEMFC cold start. The CCL mesoscale morphology is characterized by two tortuosity factors of the ionomer network and pore structure, respectively. The simulation results demonstrate that the mesoscale morphology of CCL has a significant influence on the performance of PEMFC cold start. It was found that cold-starting of a cell with a CCL of less tortuous mesoscale morphology can succeed, whereas starting up a cell with a CCL of more tortuous mesoscale morphology may fail. The CCL of less tortuous pore structure reduces the water back diffusion resistance from the CCL to proton exchange membrane (PEM), thus enhancing the water storage in PEM, while reducing the tortuosity in ionomer network of CCL is found to enhance the water transport in and the water removal from CCL. For the sake of better cold start performance, novel preparation methods, which can create catalyst layers of larger size primary pores and less tortuous pore structure and ionomer network, are desirable.     

PEMFC modeling based on characterization of effective diffusivity in simulated cathode catalyst layer

Oxygen diffusion in the cathode catalyst layer (CCL) is crucial to the high performance of polymer electrolyte membrane fuel cells (PEMFCs), especially in high current density or concentration loss regions. Recently, PEMFC performance has been reported to be enhanced by increasing CCL pore size and pore volume due to the reduction of diffusion resistance by capillary water equilibrium [Yim et al., Electrochimica Acta 56 (2011) 9064–9073]. Herein, we simulate these experimental results utilizing a new one-dimensional PEMFC model considering the effects of accumulated water film in CCL on oxygen diffusion. Two CCL microstructures were numerically generated based on agglomerate models to examine the experimental results obtained for two membrane electrode assembly (MEA) samples with different CCL porosity. The effective diffusivity of oxygen in the CCL was estimated by performing auxiliary simulations of oxygen concentration in CCL microstructures covered by a film of liquid water, with exponential correlation obtained between effective diffusivity and the thickness of the above film. Polarization curves predicted by the present model were in good agreement with experimental results. In agreement with the results of Yim et al., the present model predicts that the MEA featuring a CCL with smaller pores (which are more easily filled by liquid water) should exhibit a larger concentration loss at high current densities.     

Numerical analysis of catalyst agglomerates and liquid water transport in proton exchange membrane fuel cells

A two-dimensional computational fluid dynamics model of a proton exchange membrane fuel cell (PEMFC) was formulated by taking into account the liquid water transport through the membrane electrode assembly (MEA) on the basis of the agglomerate catalyst framework. Various modes of water transport in the PEMFC, including electro-osmotic drag, back diffusion, water generation by oxygen reduction reaction, etc., were considered in the presence of three-phase water, i.e., gas-, liquid- and dissolved-phase water. The presented model was validated with cell polarization data taken from experimental data prepared by the authors, and the agreement was good. Various parameters were investigated to analyze the influence on cell polarization and are summarized as follows: i) various compositions of Pt/C, Nafion and void in the catalyst layer and ii) various channel-to-shoulder ratios. Consequently, detailed investigations on the degree of flooding, the potential field, and polarizations were fully discussed.     

Issues of using inorganic proton conductor in the electrodes of polymer electrolyte fuel cells

We clarify the issues to be addressed when inorganic materials are employed in the electrodes of low temperature fuel cells, which conventionally operate around 80 °C. The employed inorganic proton conductor is zirconium sulfophenyl phosphonate (ZrSPP), which is incorporated as an ionomer in the electrode catalyst layers by coating the Pt-supported carbon nanotubes with a ZrSPP layer (ZrSPP–Pt/CNTs). Compared with an MEA with an electrode comprising Nafion and Pt/CNT without a ZrSPP coating, a membrane–electrode assembly (MEA) with an electrode comprising ZrSPP–Pt/CNT exhibits an improved performance at elevated temperatures of 90 °C and 100 °C, illustrating an advantage of the inorganic proton conductors. However, a ZrSPP coating on the Pt/CNTs decreases the cell performance at 80 °C. A detailed in situ analysis using limiting-current measurements reveals that the oxygen transport resistance through the solid ionomers increases by approximately six times with the incorporation of the ZrSPP layer. These results indicate that the mass transport through the inorganic materials should be addressed when they are employed in electrodes.     

Numerical investigation of the optimal Nafion ionomer content in cathode catalyst layer: An agglomerate two-phase flow modelling

A two dimensional, across the channel, isothermal, two-phase flow model for a proton exchange membrane fuel cell is presented. Reactant transport in porous media, water phase transfer and water transport through the membrane are included. The catalyst layer is modelled as a spherical agglomerate structure. Liquid water occupies the secondary pores of the cathode catalyst layer to form a liquid water coating surrounding the agglomerate. The thickness is calculated by coupling the two-phase flow model with the agglomerate model. Ionomer swelling is associated with the non-uniform distribution of water in the ionomer determined from several processes occurring simultaneously, namely (1) water phase transfer between the vapour, dissolved and liquid water; (2) membrane/ionomer water content depending on the water vapour pressure; (3) a water film covering the catalyst agglomerate; (4) water transport through the membrane via electro-osmotic drag, back diffusion and hydraulic permeation. The model optimises the initial dry ionomer content in the cathode catalyst layer. The simulation results indicate that, to achieve the best fuel cell performance, the initial dry ionomer volume fraction should be controlled around 10%, corresponding to 0.3 mg cm⁻². By considering the effect of ionomer swelling on the reduction in CCL porosity and the increase in oxygen mass transport resistance, the accuracy of the model prediction is improved, especially at higher current densities.     

Pore network model of the cathode catalyst layer of proton exchange membrane fuel cells: Analysis of water management and electrical performance

A pore network modeling approach is developed to study multiphase transport phenomena inside a porous structure representative of the Cathode Catalyst Layer (CCL) of Proton Exchange Membrane Fuel Cell. A full coupling between two-phase transport, charge transport and heat transport is considered. The liquid water evaporation is also taken into account. The current density profile and the liquid water distribution and production are investigated to understand the liquid production mechanism inside the CCL. The results suggest that the wettability and the pore size distribution have an important impact on the water management inside the cathode catalyst layer and thus on the performances of the proton exchange membrane fuel cell. Simulations show also that Bruggemann correlation used in classical models does not predict correctly gas diffusion.     

The influences of multi-walled carbon nanotube addition to the anode on the performance of direct methanol fuel cells

Different amounts of multi-walled carbon nanotubes (MWCNTs) are added to anode catalyst layer in the membrane electrode assemblies (MEAs) of direct methanol fuel cells (DMFCs). The MEA with 0.5 wt.% carbon nanotubes (CNTs) shows the best performance in DMFC. In the protonic conductivity tests, a 0.5 wt.% amount of MWCNTs results in the highest protonic conductivity. SEM and TEM observations show that a continuous and uniform distribution of Nafion ionomer layer is formed on the MWCNT surface. Therefore, the dispersed MWCNTs in the catalyst layer are considered to be helpful for developing the pathways of protons transport.     

Prediction of the effective conductivity of Nafion in the catalyst layer of a proton exchange membrane fuel cell

In a previous study, a simple acid catalyzed reaction (esterification) was found to predict excellently conductivity of a membrane contaminated with NH₄⁺ or Na⁺. Since measurement of the conductivity of Nafion in a catalyst layer is problematic, being able to predict this conductivity for various formulations and fuel cell conditions would be advantageous. In this study, the same methodology as before was used to examine the proton availabilities of supported Nafion (Nafion on carbon and on Pt/C), as exists in the catalyst layer used in a PEMFC, during impurity exposure (e.g., NH₃) as a means for prediction of its conductivity. It was found that the effect of NH₃ exposure on the proton composition (yH+) of supported Nafion was similar to that of N-211 under the same conditions. Determined values of yH+ were then used to estimate the effective conductivity of an ammonium-poisoned cathode layer using the correlation developed and the agglomerate model. The predicted conductivities were matched with the results available in the literature. This technique would be useful for the optimization of catalyst design and for fuel cell simulation, since it provides many benefits over conventional performance test procedures.     

Performance improvement by a glue-functioned Nafion layer coating on gas diffusion electrodes in PEM fuel cells

To reduce the performance difference of membrane electrode assembles (MEAs) between catalyst coated membrane (CCM) and gas diffusion electrode (GDE) methods, this study presents a novel structure of a glue-functioned Nafion layer coating on the catalyst layer of GDEs to enhance performance. The process of hot pressing is omitted from the membrane electrode assembly (MEA) fabrication in this study. In exploring the effect of the glue-functioned Nafion layer, five MEAs are compared, including one made by the CCM method and four by the GDE method. Loadings of 0, 0.1, 0.3 and 0.5 mg cm⁻² of glue-functioned Nafion layer are coated on the surface of the catalyst layer at room temperature with a condensed Nafion solution (20 wt%). The performance of the 0.3 mg cm⁻² Nafion layer coating improves 55% compared with that without an extra Nafion layer for the peak power density, and the performance difference reduces from 61.2% to 39.4% when compared with the one using the CCM method. However, the performance of the 0.5 mg cm⁻² Nafion layer coating is almost the same as without the Nafion layer. This indicates that increased Nafion cannot guarantee higher performance.     

Optimization of Nafion content in Nafion–polyaniline nano-composite modified cathodes for PEMFC application

In this research several Nafion–Polyaniline nano-composite modified cathodes have been fabricated and evaluated in oxygen reduction reaction (ORR) in order to use in proton exchange membrane fuel cell (PEMFC). Modified cathodes made by the wide range of Nafion content (from 0 to 1.6 mg cm⁻²) and investigated in the acidic solution by different electrochemical techniques at 25 °C. The results indicate the activity of the modified electrodes is increased by employing of Nafion–Polyaniline nano-composite in the reaction layer, but there is an optimum value for Nafion content in the catalyst layer. The modified electrode impregnated by 0.4 mg cm⁻² of Nafion shows the highest activity. Analysis of the surface morphology of the Nafion–polyaniline modified electrodes by scanning electron microscopy and electrochemical data reveal that the existence of polyaniline (PANI) nanofibers in the catalyst layer before adding Nafion solution, improves the homogeneity distribution of the ionomer in catalyst layer, change the morphology of electrode and increase the performance of gas diffusion electrodes (GDEs) in oxygen reduction reaction.     

Catalyst layers for proton exchange membrane fuel cells prepared by electrospray deposition on Nafion membrane

The electrospray deposition method has been used for preparation of catalyst layers for proton exchange membrane fuel cells (PEMFC) on Nafion membrane. Deposition of Pt/C + ionomer suspensions on Nafion 212 gives rise to layers with a globular morphology, in contrast with the dendritic growth observed for the same layers when deposited on the gas diffusion layer, GDL (microporous carbon black layer on carbon cloth) or on metallic Al foils. Such a change is discussed in the light of the influence of the Nafion substrate on the electrospray deposition process. Nafion, which is a proton conductor and electronic insulator, gives rise to the discharge of particles through proton release and transport towards the counter electrode, compared with the direct electron transfer that takes place when depositing on an electronic conductor. There is also a change in the electric field distribution in the needle to counter-electrode gap due to the presence of Nafion, which may alter conditions for the electrospray effect. If discharging of particles is slow enough, for instances with a low membrane protonic conductivity, the Nafion substrate may be charged positively yielding a change in the electric field profile and, with it, in the properties of the film. Single cell characterization is carried out with Nafion 212 membranes catalyzed by electrospray on the cathode side. It is shown that the internal resistance of the cell decreases with on-membrane deposited cathodic catalyst layers, with respect to the same layers deposited on GDL, giving rise to a considerable improvement in cell performance. The lower internal resistance is due to higher proton conductivity at the catalyst layer-membrane interface resulting from on-membrane deposition. On the other hand, electroactive area and catalyst utilization appear little modified by on-membrane deposition, compared with on-GDL deposition.     

Optimum compositions of membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell

We investigated the effects of the compositions of catalyst layers and diffusion layers on performances of the membrane electrode assemblies (MEAs) for direct dimethyl ether fuel cell. The performances of the MEAs with different thicknesses of Nafion membranes were compared in this work. The optimal compositions in the anode are: 20 wt% Nafion content and 3.6 mg cm⁻² Pt loading in the catalyst layer, and 30 wt% PTFE content and 1 mg cm⁻² carbon black loading in the diffusion layer. In the cathode, MEA with 20 wt% Nafion content in the catalyst layer and 30 wt% PTFE content in the diffusion layer presented the optimal performance. The MEA with Nafion 115 membrane displayed the highest maximum power density of 46 mW cm⁻² among the three MEAs with different Nafion membranes. Copyright © 2009 John Wiley & Sons, Ltd.     

Sublayers for Pt catalyst electrodeposition electrodes in PEMFC

The effect of sublayers on the deposited platinum (Pt) catalyst layer fabricated by electrodeposition, and on the resulting fuel cell performance, was investigated. The substrate was prepared by applying a hydrophobic sublayer, composed of polytetrafluoroethylene (PTFE) and carbon black, and a hydrophilic sublayer, composed of Nafion and glycerol, onto an uncatalyzed gas diffusion layer prior to the electrodeposition of the Pt catalyst. The hydrophilic sublayer was found to play a substantial role in the Pt electrodeposition, since the structure of the resultant Pt catalyst significantly depended on the presence of the hydrophilic sublayer, the total loading amount and the Nafion to glycerol weight ratio, which in turn affected the fuel cell performance. The hydrophobic sublayer, which did not directly contact with the plating solution, did not show as marked an effect on the Pt deposit structure compared to that of the hydrophilic sublayer, except at high PTFE to carbon black weight ratios (≥70:30). However, the suitable PTFE to carbon black weight ratio in the hydrophobic sublayer was still important for the water management, mass transport of reactant gases and ohmic resistance of the membrane electrode assembly (MEA) during fuel cell operation. In this study, a total hydrophilic loading of 0.8 mg cm⁻² with a Nafion to glycerol weight ratio of 50:50, and a PTFE to carbon black weight ratio in the hydrophobic sublayer of 30:70 was found to yield the best Pt catalyst layer for PEMFC.