Molecular Simulation Method Development and Implementation for Fuel Cell Catalyst Layers PDF Download

Author: Peter Vanya
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Author: Amin Nouri Khorasani
Publisher:
ISBN:
Category : Catalysts
Languages : en
Pages : 198

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Ionomer-free ultrathin catalyst layers have shown promise to enhance the performance and reduce the platinum loading of catalyst layers in polymer electrolyte fuel cell. The nanostructure of a catalyst layer affects the distribution and diffusion of reactants, and consequently its effectiveness factor. We employed classical molecular dynamics to simulate a catalyst layer pore as a water-filled channel with faceted walls, and investigated the effect of channel geometry and charging on hydronium ion and water distribution and diffusion in the channel. Equilibrium hydronium ion distribution profiles on the catalyst channel were obtained to calculate the effect of channel structure on the electrostatic effectiveness factor of the channel. Furthermore, we calculated the self-diffusion coefficient and interfacial water structure in the model channel. Results on proton concentration, diffusion and kinetics are discussed in view of catalyst layer performance.

Author:
Publisher:
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Category :
Languages : en
Pages : 19

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Dramatically improving the performance of fuel cell systems with their complex heterogeneous structures involving electrocatalysts, proton conducting membrane, reactant, and interfaces between them requires understanding the fundamental chemical, electrochemical, and physical phenomena at the heart of these complex materials and relating these fundamentals to the properties and performance of the membrane electrode assembly. Our goal is to develop a predictive model that can be used to estimate the changes in performance upon changes in the design and which can be used to monitor performance of working fuel cells. Our strategy is to start with first principles quantum mechanics (QM) and to develop overlapping simulation methodologies in which QM is used to train a reactive force field that can be applied for large-scale (millions of atom) molecular dynamics simulations while retaining the accuracy of QM. The results of molecular dynamics are used to extract a coarse grain or mesoscale description useful in modeling properties at much larger scales. This model would enable the conception, synthesis, fabrication, characterization, and development of advanced materials and structures for fuel cells and for the associated hydrocarbon fuel reformers in an overall fuel cell system. We illustrate here some of the progress toward this goal.

Author:
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Languages : en
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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.

Author: Partha P. Mukherjee
Publisher:
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Category :
Languages : en
Pages :

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The catalyst layer (CL) plays a crucial role in the overall performance of a polymer electrolyte fuel cell (PEFC) due to the sluggish oxygen reduction reaction as well as transport limitation in the presence of liquid water and ensuing flooding. Nevertheless, it is often treated either as a thin interface or a macrohomogeneous porous layer and the influence of underlying morphology and wetting characteristics on the catalyst layer performance and water transport is ignored. The macroscopic fuel cell models, therefore, employ effective transport properties for reactant and charge transport as well as arbitrary two-phase closure relations for capillary pressure and relative permeability, the experimental measurements of which are exceedingly difficult and might be impossible in the near future. The role of the catalyst layer flooding in the overall cell performance and the mechanisms of liquid water transport/removal from the CL remain unexplored. In order to reveal the underlying structure-performance relationship and to predict reliable closure relations, a pore-scale modeling framework comprising of a stochastic microstructure reconstruction model, an electrochemistry coupled direct numerical simulation (DNS) model and a two-phase lattice Boltzmann (LB) model is developed. The stochastic reconstruction model generates 3-D, statistically meaningful catalyst layer microstructure based on inputs from transmission electron microscope (TEM) images of an actual catalyst layer. Pore-level description of charge and species transport within the complex CL microstructure is achieved through the direct numerical simulation (DNS) model. The main purpose of the DNS model is to unravel the CL compositional influence on the performance and enable composition optimization for better performance. The mesoscopic lattice Boltzmann (LB) model simulates the liquid water transport through the CL microstructure in order to gain insight into the influence of structure on the pore-scale two-phase dynamics as well as to evaluate the two-phase constitutive relations in terms of capillary pressure and relative permeability as functions of liquid water saturation. A quantitative estimate of the detrimental effect of liquid water on the CL electrochemical performance in terms of the pore blockage and catalytic site coverage effects, which cannot be evaluated experimentally at present, is predicted from the combined LB and DNS models. These transport parameters can be used as reliable closure relations in macroscopic fuel cell models. Furthermore, a macroscopic model of CL flooding is devised based on a simplified structure-wettability representation and a physical description of water and heat balance. The role of evaporation in the CL liquid water saturation distribution and resulting flooding is elucidated. While the primary focus of the pore-scale modeling is to quantitatively estimate the transport parameters along with a detailed structure-transport-performance description, the macroscopic analysis reveals profound inter-relations of adjacent components and operating cell temperature with CL flooding.

Author: Tatsuhiro Okada
Publisher: Springer Science & Business Media
ISBN: 3540707581
Category : Science
Languages : en
Pages : 450

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Over the past decade the topic of energy and environment has been ackno- edged among many people as a critical issue to be solved in 21st century since the Kyoto Protocol came into e?ect in 1997. Its political recognition was put forward especially at Heiligendamm in 2007, when the e?ect of carbon dioxide emission and its hazard in global climate were discussed and shared univ- sallyascommonknowledge.Controllingtheglobalwarmingintheeconomical framework of massive development worldwide through this new century is a very challenging problem not only among political, economical, or social c- cles but also among technological or scienti?c communities. As long as the humans depend on the combustion of fossil for energy resources, the waste heat exhaustion and CO emission are inevitable. 2 In order to establish a new era of energy saving and environment benign society, which is supported by technologies and with social consensus, it is important to seek for a framework where new clean energy system is inc- porated as infrastructure for industry and human activities. Such a society strongly needs innovative technologies of least CO emission and e?cient en- 2 ergy conversion and utilization from remaining fossil energies on the Earth. Energy recycling system utilizing natural renewable energies and their c- version to hydrogen may be the most desirable option of future clean energy society. Thus the society should strive to change its energy basis, from foss- consuming energy to clean and recycling energy.

Author: Pavel Ruvinskiy
Publisher:
ISBN:
Category :
Languages : en
Pages : 232

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The aim of the present work is the development of novel 3D ordered catalytic layer architectures for fuel cell applications and their investigation in fuel cell related reactions such as oxygen reduction, hydrogen oxidation and CO oxidation reaction. This task is addressed via the preparation of catalytic layers based on Pt nanoparticles supported on vertically aligned carbon nano-filaments (Pt/VACNF). These catalytic layers were utilized both as a tool for gaining insights into the complex electrochemical reactions and as novel systems with promising activity for the use in PEM fuel cells. The growth of the arrays of the VACNF was performed using catalytic chemical vapor deposition from ferrocene/toluene mixture. Thus obtained systems were used as substrates for Pt deposition using various chemical and electrochemical methods. The obtained catalytic layers were investigated using a set of physical (TEM, SEM) and electrochemical (CV) tools. In order to get an insight into the complex interplay between the reaction kinetics and diffusion phenomena in these novel catalytic architectures we have currently chosen to study the hydrogen oxidation, the oxygen reduction and the CO oxidation reaction in liquid electrolyte using rotating disc and ring-disc methods coupled with mathematical modeling. These studies allowed understanding the influence of the key parameters such as the thickness of the catalytic layer and the Pt coverage on the performance of the catalytic layers, on the effectiveness of the Pt utilization as well as on bistable phenomena (as studied for dissolved CO oxidation). Finally, an approach has been developed for the incorporation of VACNF into MEA without loss of alignment. This work necessitated the scale-up of the growth procedure, Pt deposition, as well as required to establish the methods of a nondestructive membrane-electrode assembly production.

Author: Tatsuhiro Okada
Publisher: Springer
ISBN: 9783540866886
Category : Science
Languages : en
Pages : 434

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Book Description
Over the past decade the topic of energy and environment has been ackno- edged among many people as a critical issue to be solved in 21st century since the Kyoto Protocol came into e?ect in 1997. Its political recognition was put forward especially at Heiligendamm in 2007, when the e?ect of carbon dioxide emission and its hazard in global climate were discussed and shared univ- sallyascommonknowledge.Controllingtheglobalwarmingintheeconomical framework of massive development worldwide through this new century is a very challenging problem not only among political, economical, or social c- cles but also among technological or scienti?c communities. As long as the humans depend on the combustion of fossil for energy resources, the waste heat exhaustion and CO emission are inevitable. 2 In order to establish a new era of energy saving and environment benign society, which is supported by technologies and with social consensus, it is important to seek for a framework where new clean energy system is inc- porated as infrastructure for industry and human activities. Such a society strongly needs innovative technologies of least CO emission and e?cient en- 2 ergy conversion and utilization from remaining fossil energies on the Earth. Energy recycling system utilizing natural renewable energies and their c- version to hydrogen may be the most desirable option of future clean energy society. Thus the society should strive to change its energy basis, from foss- consuming energy to clean and recycling energy.

Author: Shankar Raman Dhanushkodi
Publisher:
ISBN:
Category :
Languages : en
Pages :

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Proton exchange membrane fuel cells (PEMFC) are attractive energy sources for power trains in vehicles because of their low operating temperature that enables fast start-up and high power densities. Cost reduction and durability are the key issues to be solved before PEMFCs can be successfully commercialized. The major portion of fuel cell cost is associated with the catalyst layer which is typically comprised of carbon-supported Pt and ionomer. The degradation of the catalyst layer is one of the major failure modes that can cause voltage degradation and limit the service life of the fuel cell stack during operation. To develop a highly durable and better performing catalyst layer, topics such as the causes for the degradation, modes of failure, different mechanisms and effect of degradation on fuel cell performance must be studied thoroughly. Key degradation modes of catalyst layer are carbon corrosion and Pt dissolution. These two modes change the electrode structure in the membrane electrode assembly (MEA) and result in catalyst layer thinning, CO2 evolution, Pt deposition in the membrane and Pt agglomeration. Alteration of the electrode morphology can lead to voltage degradation. Accelerated stress tests (ASTs) which simulate the conditions and environments to which fuel cells are subject, but which can be completed in a timely manner, are commonly used to investigate the degradation of the various components. One of the current challenges in employing these ASTs is to relate the performance loss under a given set of conditions to the various life-limiting factors and material changes. In this study, various degradation modes of the cathode catalyst layer are isolated to study their relative impact on performance loss _Fingerprints' of identifiable performance losses due to carbon corrosion are developed for MEAs with 0.4 mg cm−2 cathode platinum loadings. The fingerprint is used to determine the extent of performance loss due to carbon corrosion and Pt dissolution in cases where both mechanisms operate. This method of deconvoluting the contributions to performance loss is validated by comparison to the measured performance losses when the catalyst layer is subjected to an AST in which Pt dissolution is predominant. The limitations of this method iv are discussed in detail. The developed fingerprint suggests that carbon loss leading to CO2 evolution during carbon corrosion ASTs contributes to performance loss of the cell. A mechanistic model for carbon corrosion of the cathode catalyst layer based on one appearing in the literature is developed and validated by comparison of the predicted carbon losses to those measured during various carbon corrosion ASTs. Practical use of the model is verified by comparing the predicted and experimentally observed performance losses. Analysis of the model reveals that the reversible adsorption of water and subsequent oxidation of the carbon site onto which water is adsorbed is the main cause of the current decay during ASTs. Operation of PEM fuel cells at higher cell temperatures and lower relative humidities accelerates Pt dissolution in the catalyst layer during ASTs. In this study, the effects of temperature and relative humidity on MEA degradation are investigated by applying a newly developed AST protocol in which Pt dissolution is predominant and involves the application of a potentiostatic square-wave pulse with a repeating pattern of 3s at 0.6 V followed by 3s at 1.0 V. This protocol is applied at three different temperatures (40°C, 60°C and 80°C) to the same MEA. A diagnostic signature is developed to estimate kinetic losses by making use of the effective platinum surface area (EPSA) obtained from cyclic voltammograms. The analysis indicates that performance degradation occurs mainly due to the loss of Pt in electrical contact with the support and becomes particularly large at 80°C. This Pt dissolution AST protocol is also investigated at three different relative humidities (100%, 50% and 0%). Electrochemical impedance spectroscopy measurements of the MEAs show an increase in both the polarization and ohmic resistances during the course of the AST. Analysis by cyclic voltammetry shows a slight increase in EPSA when the humidity increases from 50% to 100%. The proton resistivity of the ionomer measured by carrying out impedance measurements on MEAs with H2 being fed on the anode side and N2 on the cathode side is found to increase by the time it reaches its end-of-life state when operated under 0 % RH conditions.

Author: Michael Eikerling
Publisher: CRC Press
ISBN: 1439854068
Category : Science
Languages : en
Pages : 567

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Book Description
The book provides a systematic and profound account of scientific challenges in fuel cell research. The introductory chapters bring readers up to date on the urgency and implications of the global energy challenge, the prospects of electrochemical energy conversion technologies, and the thermodynamic and electrochemical principles underlying the op