Modeling Low-Platinum-Loading Effects in Fuel-Cell Catalyst Layers

Abstract

The cathode catalyst layer within a proton-exchange-membrane fuel cell is the most complex and critical, yet least understood, layer within the cell. The exact method and equations for modeling this layer are still being revised and will be discussed in this paper, including a 0.8 reaction order, existence of Pt oxides, possible non-isopotential agglomerates, and the impact of a film resistance towards oxygen transport. While the former assumptions are relatively straightforward to understand and implement, the latter film resistance is shown to be critically important in explaining increased mass-transport limitations with low Pt-loading catalyst layers. Model results demonstrate agreement with experimental data that the increased oxygen flux and/or diffusion pathway through the film can substantially decrease performance. Also, some scale-up concepts from the agglomerate scale to the more macroscopic porous-electrode scale are discussed and the resulting optimization scenarios investigated. IntroductionThe catalyst layer (CL) is a very complex chemical and geometric environment for electrochemical reactions in proton-exchange-membrane fuel cells (PEMFCs). It is composed of supported catalyst particles, ionomer, and gas pores. The reaction occurs at sites where various reacting species such as protons, electrons, and gases meet. Modeling the structure has been approached by various means 1 , and a rigorous mathematical model of the CL is required to capture transport within the different phases, electrochemical reaction, and heat and water generation. Among previous models, one of the most accepted models is an agglomerate particle composed of the ionomer, gas voids, liquid water, and catalyst that is covered by a thin film of ionomer . This idea is supported by various experimental observations such as scanningelectron-and transmission-electron-microscopy studies. In this model, oxygen is dissolved in the ionomer film surrounding the agglomerate, and the dissolved oxygen diffuses to the agglomerate where simultaneous transport and reaction occur. Typically, the agglomerate model is embedded (i.e., distributed uniformly across the CL) into a porous-electrode model to describe the CL fully 1 . _ENREF_29Optimization of the CL for enhancing PEMFC performance is of great interest for researchers and industry. Parametric studies of CLs were accomplished by Yin 14 using an agglomerate model, with the model predicting the general polarization-curve trend as a function of parameters such as gas void fraction 7,16 and ionomer 4,5,10,29 and catalyst loadings 5,10,[30][31][32] within the CL.Similarly, several optimization studies 16,22,30 using an agglomerate model were conducted to obtain optimum design parameters such as catalyst loading, CL thickness, and Pt/C ratio for best performance at a given potential. However, all the studies were based on several assumptions 4 and issues that are not correct validated or possibly correct including a lack of ionomer films and a first-order oxygen dependence for the oxygen redu...

Oxygen Reduction Reaction Measurements on Platinum Electrocatalysts Utilizing Rotating Disk Electrode Technique

“…Redistribution subject to ECS terms of use (see 54.245.55.244 Downloaded on 2018-05-11 to IP A critical and unresolved challenge in PEMFCs that has arisen with the attempts at reducing Pt loading and cost is the lower than projected peak power for cathodes incorporating ultra-low Pt loadings. The issue is being studied intensively and different groups 114,[121][122][123][124][125] have identified several possible routes including catalyst interaction with Nafion. In MEAs of PEMFCs, since it is not possible to carry out an investigation in the absence of ionomer (ionomer is indispensable for proton conduction), the contribution of the ionomer adsorption/blocking cannot be easily examined.…”

A Critical Review of Modeling Transport Phenomena in Polymer-Electrolyte Fuel Cells

“…The validity of such assumptions were examined based on the magnitude of thermal and ionic transport properties. 133 The impact of different kinetic and boundary conditions was also recently analysed along with the above assumptions with a full 2D(1D) model, showing that even when the ionomer conductivity is 1 × 10 −4 , the effects on cell performance are insignificant due to reaction re-distribution within the electrode. 124 The diffusion and reaction into a spherical agglomerate is given by the equation (see equation 10)…”

Effects of catalyst agglomerate shape in polymer electrolyte fuel cells investigated by a multi-scale modelling framework

“…Also, using realistic values for the ionic conductivity of the ionomer phase, the agglomerate was shown to be ionically iso-potential [44]. Further, the fuel cell was assumed to operate under low-humidity conditions in order to isolate the effects of saturation.…”

“…a is the specific surface area of the platinum catalyst, i.e. the ratio between the surface area of the platinum particle and its volume and is given by [44]:…”