Catalyst Layer Model Research Articles

A three-dimensional agglomerate model for the cathode catalyst layer of PEM fuel cells

In this work, a three-dimensional, steady-state, multi-agglomerate model of cathode catalyst layer in polymer electrolyte membrane (PEM) fuel cells has been developed to assess the activation polarization and the current densities in the cathode catalyst layer. A finite element technique is used for the numerical solution to the model developed. The cathode activation overpotentials, and the membrane and solid phase current densities are calculated for different operating conditions. Three different configurations of agglomerate arrangements are considered, an in-line and two staggered arrangements. All the three arrangements are simulated for typical operating conditions inside the PEM fuel cell in order to investigate the oxygen transport process through the cathode catalyst layer, and its impact on the activation polarization. A comprehensive validation with the well-established two-dimensional “axi-symmetric model” has been performed to validate the three-dimensional numerical model results. Present results show a lowest activation overpotential when the agglomerate arrangement is in-line. For more realistic scenarios, staggered arrangements, the activation overpotentials are higher due to the slower oxygen transport and lesser passage or void region available around the individual agglomerate. The present study elucidates that the cathode overpotential reduction is possible through the changing of agglomerate arrangements. Hence, the approaches to cathode overpotential reduction through the optimization of agglomerate arrangement will be helpful for the next generation fuel cell design.

Optimization of Polymer Electrolyte Fuel Cell Cathodes

We adopt an agglomerate catalyst layer (CL) model and recast it into a compact form for optimization of polymer electrolyte fuel cell cathodes. Our reformulation captures the transport processes within the gas diffusion layer and the CL and the transport/ reaction processes within an individual agglomerate particle. The governing equations are discretized using a finite-difference method, and the resulting optimization problem is linked to a state-of-the-art nonlinear programming algorithm. From the platinum (Pt) minimizations we obtain relationships between optimal Pt mass and current density at different operating voltages. For current density maximization we investigate the optimal distribution of material parameters along the CL width.

Multi-variable optimization of PEMFC cathodes using an agglomerate model

A comprehensive numerical framework for cathode electrode design is presented and applied to predict the catalyst layer and the gas diffusion layer parameters that lead to an optimal electrode performance at different operating conditions. The design and optimization framework couples an agglomerate cathode catalyst layer model to a numerical gradient-based optimization algorithm. The set of optimal parameters is obtained by solving a multi-variable optimization problem. The parameters are the catalyst layer platinum loading, platinum to carbon ratio, amount of electrolyte in the agglomerate and the gas diffusion layer porosity. The results show that the optimal catalyst layer composition and gas diffusion layer porosity depend on operating conditions. At low current densities, performance is mainly improved by increasing platinum loading to values above 1 mg cm −2, moderate values of electrolyte volume fraction, 0.5, and low porosity, 0.1. At higher current densities, performance is improved by reducing the platinum loading to values below 0.35 mg cm −2 and increasing both electrolyte volume fraction, 0.55, and porosity 0.32. The underlying improvements due to the optimized compositions are analyzed in terms of the spatial distribution of the various overpotentials, and the effect of the agglomerate structure parameters (radius and electrolyte film) are investigated. The paper closes with a discussion of the optimized composition obtained in this study in the context of available experimental data. The analysis suggests that reducing the solid phase volume fraction inside the catalyst layer might lead to improved electrode performance.

A Mathematical Model of Ultra-Thin Catalyst Layer in PEFC

A mathematical model for an ultra-thin catalyst layer in PEFCs is introduced. It utilizes Nernst-Planck and Poisson equations. Calculated polarization curves are shown to compare favourably with published experimental data for ultra-thin catalyst layers. Aspects of current conversion, reactant, current distribution, and catalyst utilization are explored. The effect of catalyst layers thickness on the Pt utilization is discussed. This study gives us a better understanding of transport and reaction at the mesoscopic scale and it furnishes the directions for optimization of this type of catalyst layer.

Steady and Unsteady Modeling of Single PEMFC with Detailed Thermoelectrochemical Model

A steady and unsteady water and thermal model was developed to consider the effects of local pressure on the cell performance, pressure drop, open-circuit voltage variation with stack temperature, and water-vapor effects on membrane conductivity. These considerations made the model physically more reasonable as well as more suitable for various operating conditions. Additionally, this model combined the along-flow-channel model and catalyst layer model, which represent a significant improvement to proton exchange membrane fuel cell (PEMFC) modeling. The model could predict the distributions of a series of important parameters along the flow channel and in the catalyst layer. Furthermore, the transient performance of the fuel cell can be simulated with this model. The modeling results agreed reasonably with the available experimental results from the literature. The results show that the humidification of both anode and cathode is very important for the performance of PEMFC which could also be improved by increasing the flow inlet temperatures within a reasonable range. Pressure loss is one of the important parameters that affect total system efficiency and optimization. This model could be used as part of a PEMFC stack or entire system modeling.

Analysis of a cathode catalyst layer model for a polymer electrolyte fuel cell

A macroscopic model for a non-isothermal cathode catalyst layer (CL) in a proton exchange membrane (PEM) fuel cell is presented, in which liquid water in the CL pores is neglected. The model couples three phases: an electrically conductive carbon/platinum phase, gas pores, and a proton-conducting Nafion phase. The reaction–diffusion dynamics are described by a nonlinear system of differential equations which, due to the wide range of physical parameters, is very stiff. To reduce the stiffness of the system, an appropriate scaled model is introduced. A numerical algorithm consisting of two embedded Newton loops, specifically developed to handle the stiffness, is presented. Several numerical results are used to investigate the dynamics of the layer and the validity of the interface reduction, or zero CL thickness limit, which is often used in large fuel cell computations. In the limit of infinite water adsorption rate between ionomer phase and pore, a boundary layer emerges at either side of the CL, in which the water sorption equilibrium [Zawodzinski, T., Derouin, C., Radzinski, S., Sherman, R., Smith, V., Springer T., Gottesfeld, S., 1993. A comparative study of water uptake by and transport through ionomeric fuel cell membranes. Journal of the Electrochemical Society 140, 1981–1985] between ionomer and pore is violated.

Effects of Microporous Layers in Polymer Electrolyte Fuel Cells

Our previous models and a catalyst-layer model are combined to study the effects of microporous layers in polymer electrolyte fuel cells. The combined sandwich model is used to fit data both with and without a microporous layer with saturated feed conditions. In terms of water management, the simulations clearly show that the effect of a microporous layer is to keep water out of the cathode gas diffusion layer and move it through the anode. Additional effects of microporous layers include better ohmic behavior and perhaps better catalyst utilization, among other things. Optimizations for different structural parameters are investigated, as are the effects of microporous layers under different operating conditions. The discussion is geared toward how microporous layers increase performance and their effect on fuel cell water management.

An improved two-dimensional agglomerate cathode model to study the influence of catalyst layer structural parameters

A two-dimensional model for PEM fuel cell cathode has been developed. The model treats the catalyst layer as agglomerates of polymer electrolyte coated catalyst particles. In this improved model, transport of the two charged species – electrons and ions – as well as that of the chemical species within the catalyst layer is considered. In addition, the transport of electrons through the gas diffusion layer is also considered. The model prediction shows that the cathode overpotential inside the catalyst layer is non-uniform influenced by the channel-land geometry, which implies that the implicit constant overpotential assumption in the commonly employed ultra-thin catalyst layer model is not an accurate representation of actual situation and may lead to overprediction of current densities. Unlike a majority of the multidimensional models that predict the current density to be higher either under the flow-channel area or under the land region, our model illustrates that the location of higher current (electrode reaction rate) changes from the region under the flow channel to that under the land area depending on the overpotential. Our simulation results suggest that charge transport, both electron conduction and the proton migration, is as important as oxygen diffusion in determining the overall electrode reaction rate. Using reaction effectiveness factor analysis, we found that catalyst utilization within the agglomerate is extremely poor at high current densities. The two-dimensional model was employed to investigate the effects of catalyst layer structural parameters including Nafion and platinum loadings and distributions on the cathode performance. The model predicts that there is an optimum level of Nafion loading.

Catalyst Layer Models for Proton Exchange Membrane Fuel Cells

This paper presents two simple catalyst layer models – the macro-homogeneous model and the agglomerate model for the proton exchange membrane (polymer) fuel cell using hydrogen gas as the feedstock. The former is solved analytically whereas the latter is solved numerically. The emphasis of this research is to parameterize factors affecting the performance of the fuel cell with simple approach without going into detailed catalyst layer modeling.