Membrane electrode assemblies modeling

Figure 15.2 Schematic representation of different electrochemical cell types used in studies of electrocatalytic reactions (a) proton exchange membrane single cell, comprising a membrane electrode assembly (b) electrochemical cell with a gas diffusion electrode (c) electrochemical cell with a thin-layer working electrode (d) electrochemical cell with a model nonporous electrode. CE, counter-electrode RE, reference electrode WE, working electrode.

Cost targets exist for all parts of the fuel cell for bipolar plates, from 10/kW (2004) to 3/kW in 2015 for electrocatalysts, from 40/kW (2005) to 3/kW in 2015 and for membrane electrode assemblies (MEA), from 50/kW (2005) to 5/kW in 2015 (Freedom Car, 2005 these cost targets are somewhat different from those mentioned by the IEA (2005)). Since 2004, the number of fuel-cell cars has been growing and at the time of writing they numbered approximately 1000 worldwide there are also around 100 fuel-cell buses in use worldwide in several demonstration projects. But these cars are produced as individual (hand-built) models and are extremely expensive, with production costs per vehicle currently estimated at around one million large-scale production is not expected before 2015, see Section 13.1. 

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion... 

A.Z. Weber, M.A. Hickner, Modeling and high-resolution-imaging studies of water-content profiles in a polymer-electrolyte-fuel-cell membrane-electrode assembly. Electrochimica. Acta. 53, 7668—7674 (2008)... 

The membrane conductivity was measured in HCl(aq) solutions of different concentrations and in 2 M HC1 + 0.2 M CuCl solution to model the catholyte and anolyte solutions in the electrolyser. All membranes were equilibrated in the same solutions for 20 hours before starting the measurements. Detailed characterisation data for a number of commercial anion exchange membranes are published elsewhere (Gong, 2009). The AHA membrane, which demonstrated the highest conductivity in HC1 (12.61 mS/cm) compared to other membranes with similar IEC and water uptake, was selected to prepare a membrane electrode assembly (MEA) and carry out electrolysis tests with this MEA. The ACM membrane with lower conductivity values was also chosen for the electrolysis tests due to its proton blocking properties and high Cl- selectivity. 

The membrane electrode assembly (MEA), which consists of three components (two gas diffusion electrodes with a proton exchange membrane in between), is the most important component of the PEMFC. The MEA exerts the largest influence on the performance of a fuel cell, and the properties of each of its parts in turn play significant roles in that performance. Although all the components in the MEA are important, the gas diffusion electrode attracts more attention because of its complexity and functions. In AC impedance spectra, the proton exchange membrane usually exhibits resistance characteristics the features of these spectra reflect the properties of the gas diffusion electrode. In order to better understand the behaviour of a gas diffusion electrode, we introduce the thin-film/flooded agglomerate model, which has been successfully applied by many researchers to... 

Molecular-Level Modeling of the Structure and Proton Transport within the Membrane Electrode Assembly of Hydrogen Proton Exchange Membrane Fuel Cells... 

The fuel cell is basically a two-scale system. The small and large scales are determined, respectively, by membrane-electrode assembly (MEA) thickness and by the length of the feed channel. The Q3D model is designed to investigate the interplay of small- and large-scale processes in PEFC/DMFC, so that the fully 3D model of the cell is split into a model of a cell cross section (internal model) and a model of the flow in the channel (channel model). The two models are coupled via the local current density along the channel and the overall Q3D solution is obtained by iterations. 

Based on the Model 3, a 10-cell stack of microfuel cells was assembled. It was reported that an output of near 10 W was achieved, as shown in Fig. 8.23 [47]. Recently, through membrane electrode assembly (MEA) improvement, a power density 290 mW cm of the cell with an air cathode has been achieved. A 5-cell stack with effective area of 67 cm demonstrated that the power reached IlOW when the operating temperature reached 60 °C, though the stack started at room temperature without humidification. The performances of single ceU and 5-ceU stacks are shown in Fig. 8.24. 

Sone et al. [34] reported conductivity data for Nafion 117 in the E-form (no heat treatment), measured using a four-electrode AC impedance method. Membranes used in fuel cells are typically heated during the manufacture of the membrane electrode assembly, and it may thus be more appropriate to fit to the data for a membrane in the N or S form however, fitting to the E-form data allows direct comparison with Thampan et al. [22] fit to their model. 

In this chapter, we discuss more deeply two aspects of a numerical model for direct methanol fuel cells published in [1]. This model describes in detail the processes in the membrane electrode assembly (MEA) of a direct methanol fuel cell (DMFC). We assume that the MEA consists of the following parts ... 

The goal of detailed modeling of the membrane electrode assembly is to identify the essential parameters and processes to reach predictive simulation results within a parameter range of practical interest. The energetic losses due... 

The earliest PEMFC system models [1,2] were for single cells at steady state, assuming isothermal and isobar conditions. Performance is averaged over the cross-channel direction, and transport in gas channels is decoupled from transport through the Membrane Electrode Assembly (MEA). The power of... 

Schematic illustration of the membrane electrode assembly (MEA) of a PEM fuel cell (top) and details which have been subjected to modeling and simulation work described in the following chapters. Atomic level simulations have been performed for water and proton transport within the hydrophilic domaine of hydrated ionomers and for the electrochemical processes taking place at the electrocatalysts surfaces. The latter include the introduction of polarizable solvents and electrostatic potential variations. Mesoscale modeling is aiming at a better description and understanding of the development of ionomer microstructures.

This chapter presents the simulation of a button cell data reported by Liu et al. [38]. The model parameters derived here are further used in the performance analysis presented in the later chapters. A schematic representation of button cell is given in Fig. 6.1. The configuration is basically a concentric cylindrical assembly intercepted by the membrane electrode assembly (MEA). The fuel and air inlet are through the inner cylindrical pipe, which reaches above the anode and cathode. The product gas outlet is through the concentric space between the inner and outer cylinder. 

H. Zhu and R. J. Kee. A general mathematical model for analyzing the performance of fuel-cell membrane-electrode assemblies. J. Power Sources 117, (2003) 61-74. 

Mechanistic models can generally be characterized by the scope of the model. In many cases, modeling efforts focus on a specific part or parts of the fuel cell, like the cathode catalyst layer [39], the cathode electrode (gas diffusion layer plus catalyst layer) [40-42], or the membrane electrode assembly (MEA) [43, 44]. These models are very useful in that they... 

A computational model of an entire cell would require very large computing resources and excessively long simulation times. The computational domain in this chapter is therefore limited to one straight flow ehannel with the land areas. The full computational domain consists of cathode and anode gas flow channels, and the membrane electrode assembly as shown in Figure 3.1. 

Zhu, H. and Kee, R.J. (2008) Modeling distributed charge-transfer processes in SOFC membrane electrode assemblies. 

Usually, the starting point of model derivation is either a physical description along the channel or across the membrane electrode assembly (MEA). For HT-PEFCs, the interaction of product water and electrolyte deserves special attention. Water is produced on the cathode side of the fuel cell and will either be released to the gas phase or become adsorbed in the electrolyte. As can be derived from electrochemical impedance spectroscopy (EIS) measurements [14], water production and removal are not equally fast Water uptake of the membrane is very fast because the water production takes place inside the electrolyte, whereas the transport of water vapor to the gas channels is difiusion limited. It takes several minutes before a stationary state is reached for a single cell. The electrolyte, which consists of phosphoric add, water, and the membrane polymer, changes composition as a function of temperature and water content [15-18]. As a consequence, the proton conductivity changes as a function of current density [14, 19, 20). 

While most modeling efforts have focused on steady-state operation, the dynamic behavior is of paramount importance for fuel-cell transportation appHcations due to the inherent load variation involved. Transient phenomena in automotive fuel cells are not yet fully understood. In addition to the complex dynamic response involving various time scales, severe degradation of membrane electrode assemblies... 

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