Download or read book Modeling of Ultrathin Catalyst Layers in Polymer Electrolyte Fuel Cells written by Karen Ka Wing Chan and published by . This book was released on 2013 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Ultrathin catalyst layers (UTCLs) are emerging as a promising alternative to conventional catalyst layers in polymer electrolyte fuel cells. In comparison, UTCLs have dramatically reduced Pt loading and thicknesses and are ionomer-free. We explore two open questions in the theory of UTCLs (1) the proton transport mechanism within the ionomer-free layer and (2) water management in membrane electrode assemblies (MEAs) with UTCLs. To investigate (1), we present a UTCL model, which assumes the protons are drawn into the UTCL via their interaction with the metal surface charge. We consider a continuum model of a water-filled, cylindrical nanopore with charged walls. We derive the relation between metal potential and surface charge density from a Stern double layer model. The model suggests the proton concentration and reaction current density to be highly dependent on the charging properties of the metal-solution interface, which are parameterized primarily by the potential of zero charge. Therefore, materials for UTCLs should be selected not only for their intrinsic mass activities and durability, but also for their charging properties. A systematic evaluation of the interplay of electrostatic, kinetic, and mass transport phenomena in UTCL demanded an impedance variant of the model. Based on the general set of transient equations, we have derived analytical impedance expressions and equivalent circuit representations in 4 limiting cases. While the UTCL model suggests the charging of the metal-solution interface to be crucial to performance, theoretical studies on the charging behaviour of platinum are limited. We present a generalised computational hydrogen electrode that enables the ab initio simulation of metal-solution interfaces as a function of pH and potential. To address (2), we present a water balance model to MEAs with UTCLs. The model relates the current densities, capillary pressure distributions, and fluxes of vapor and liquid water. Analysis of the model suggests that UTCLs require efficient liquid transport paths out of the MEA at low and moderate temperature. We discuss strategies for increasing the current density for the onset of GDL flooding, via enhanced liquid permeabilities, vaporization areas, and gas pressure differentials.
Download or read book Modeling and Diagnostics of Polymer Electrolyte Fuel Cells written by Ugur Pasaogullari and published by Springer Science & Business Media. This book was released on 2010-07-23 with total page 412 pages. Available in PDF, EPUB and Kindle. Book excerpt: This volume, presented by leading experts in the field, covers the latest advances in diagnostics and modeling of polymer electrolyte fuel cells, from understanding catalyst layer durability to start-up under freezing conditions.
Download or read book Nanoscale Phenomena in Ultrathin Catalyst Layers of PEM Fuel Cells written by Amin Nouri Khorasani and published by . This book was released on 2013 with total page 198 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Polymer Electrolyte Fuel Cells written by Michael Eikerling and published by CRC Press. This book was released on 2014-09-23 with total page 567 pages. Available in PDF, EPUB and Kindle. Book excerpt: 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
Download or read book Pore scale Modeling and Analysis of the Polymer Electrolyte Fuel Cell Catalyst Layer written by Partha P. Mukherjee and published by . This book was released on 2007 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: 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.
Download or read book Physical Modeling of Local Reaction Conditions Inside of Cathode Catalyst Layer of Polymer Electrolyte Fuel Cells written by Tasleem Muzaffar and published by . This book was released on 2018 with total page 165 pages. Available in PDF, EPUB and Kindle. Book excerpt: The foremost practical objective in research on polymer electrolyte fuel cells is to design catalyst layers with high performance at markedly reduced platinum loading. The overarching goal is thus to enhance the effectiveness factor of platinum utilization inside the cathode catalyst layer. This requires design modifications in fuel cell components, understanding of local reaction conditions inside the cathode catalyst layer, accounting for the impact of surface charging phenomena at pore walls on catalyst activity, as well as understanding water distribution and fluxes in porous electrode media and how the water balance affects all the aforementioned effects. As a contribution towards this objective, this thesis presents models to understand the local reaction conditions inside the cathode catalyst layer. This refers to rationalizing the oxygen and proton density in the cathode catalyst layer from the macroscopic level to the nanopore level. This work has been divided into three parts. The first part focuses on understanding of surface charging phenomena and catalytic activity in water-filled pores that are bounded on one side by an ionomer-skin layer. The model-based analysis reveals that the density of charged side chains at the ionomer shell exerts a pronounced impact on the surface charge density at the Pt surface and thereby on the activity of the pore for the oxygen reduction reaction. In the second part, we employed physical models of catalyst layer operation to analyze large sets of experimental performance data of fuel cells with gradually decreased Pt loading. The analysis reveals systematic variations in physical properties of cathode catalyst layers with Pt loading that can be consistently explained with a variation in the fuel cell water balance. A correlation exponent was introduced, which can be used to assess the design of a catalyst layer in terms of the propensity to flooding. The last part serves the need for a comprehensive water balance model as revealed by research described in the previous paragraph. We present a basic 1D +1D model to rationalize variations in water distribution and water fluxes in catalyst layers, diffusion media, and flow fields in response to changes in structure, composition and operating conditions. The model-based analysis consistently reproduces major trends in performance upon a systematic reduction in Pt loading. The tools and analyses provided in this thesis could thus inform strategies for minimizing the Pt loading without running into the water trap.
Download or read book Polymer Electrolyte Fuel Cells 11 written by H. A. Gasteiger and published by The Electrochemical Society. This book was released on 2011 with total page 2388 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Physical Modeling of Water Fluxes in the Catalyst Layers of Polymer Electrolyte Fuel Cells written by Jianfeng Liu and published by . This book was released on 2016 with total page 113 pages. Available in PDF, EPUB and Kindle. Book excerpt: The ability to predict the electrochemical performance of the catalyst layer (CL) in polymer electrolyte fuel cells (PEFCs) hinges on a precise knowledge of the water balance. The key effective properties of this layer, like gas diffusivity and vaporization exchange rate constant, control water distribution and fluxes in the complete cell. Unfortunately, the knowledge of relevant properties of CLs is rare and not available with sufficient accuracy. A physical model of water fluxes in CLs is proposed to develop a methodology for the determination of the effective properties of CLs. For the purpose of this work, the CL is considered exclusively as a medium for vapor diffusion, liquid water permeation, and vaporization exchange. The presented model exploits an analogy of the water transport problem to the processes involved in charge transfer in a porous electrode, which is represented by the famous transmission line model (TLM). The expectation is that this analogy could lead to a diagnostic tool with similar capabilities as electrochemical impedance spectroscopy (EIS) in rationalizing the response of CLs to varying conditions and in extracting parameters of water transport and vaporization exchange. An analytical solution under steady state and isothermal conditions is presented that rationalizes the relation between controlled environmental conditions and the net water flux under partial saturation. The analysis of water flux data using this solution provides a method for the extraction of the net vaporization exchange rate, the activation energy of vaporization, vapor diffusivity, and the temperature exponent of the vapor diffusivity, which allows the transport mechanism of vapor diffusion in the CL to be identified. Transient analysis with a periodic perturbation is then explored. The overall impedance of water transport and the response function of a voltage change to a vapor change are analyzed for a specific scenario, where no effluence of liquid water from the CL is permitted. The methodology based on the transient analysis provides not only a way to extracting the effective properties of the CL, but also a way to estimate the liquid saturation in the CL.
Download or read book Exploratory Fuel cell Research written by Michael Lee Perry and published by . This book was released on 1996 with total page 244 pages. Available in PDF, EPUB and Kindle. Book excerpt:
Download or read book Polymer Electrolyte Fuel Cells written by Alejandro A. Franco and published by CRC Press. This book was released on 2016-04-19 with total page 608 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book focuses on the recent research progress on the fundamental understanding of the materials degradation phenomena in PEFC, for automotive applications. On a multidisciplinary basis, through contributions of internationally recognized researchers in the field, this book provides a complete critical review on crucial scientific topics related
Download or read book Mathematical Modeling and Experimental Validation of Thin Low Platinum Content and Functionally Graded Cathode Catalyst Layers written by Kailyn Domican and published by . This book was released on 2014 with total page 139 pages. Available in PDF, EPUB and Kindle. Book excerpt: Low catalyst content remains a key requirement for the commercialization of polymer electrolyte fuel cells (PEFCs) into the energy, transportation, and material handling sectors. Understanding the fundamental phenomena reducing fuel cell performance at various stages of operation play a major role in PEFC optimization and reducing catalyst content. In this thesis, high performance PEFCs with low to moderate Pt loadings (27 - 112 [mu]gPt/cm2) have been fabricated using inkjet printing. To better understand thin low Pt content electrodes the PEFCs are tested at various back pressures, relative humidity, and oxygen partial pressures. The characterized PEFCs are then simulated using OpenFCST, an open-source FEM based fuel cell simulation framework, to aid in the investigation of key performance limiting phenomena. The simulations are obtained using a macro-homogeneous, non-isothermal MEA model where a multi-step reaction kinetics describes the oxygen reduction reaction. The model is then validated against the experimental Pt loading, ionomer loading, and oxygen partial pressure study. The successfully validated model highlighted key performance limitations between low Pt content and conventional (400 [mu]gPt/cm2) loading electrodes, while also highlighting the possible phenomena dictating ionomer and oxygen partial pressure performance. A mathematical model is also developed allowing the simulation of functionally graded electrodes. The model is then used to aid experimental design. To validate the new model two functionally graded ionomer electrodes are fabricated, characterized, and compared to the simulated results.
Download or read book PEM Fuel Cells written by Jasna Jankovic and published by Walter de Gruyter GmbH & Co KG. This book was released on 2023-05-22 with total page 555 pages. Available in PDF, EPUB and Kindle. Book excerpt: This book is a comprehensive introduction to the rapidly developing field of modeling and characterization of PEM fuel cells. It focuses on i) fuel cell performance modeling and performance characterization applicable from single cells to stacks, ii) fundamental and advanced techniques for structural and compositional characterization of fuel cell components and iii) electrocatalyst design. Written by experts in this field, this book is an invaluable tool for graduate students and professionals.
Download or read book Modelling Cathode Catalyst Degradation in Polymer Electrolyte Fuel Cells written by Steven Giordano Rinaldo and published by . This book was released on 2013 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: Nano-sized Pt particles in the cathode catalyst layer of a polymer electrolyte fuel cell afford a high initial electrochemically active surface-area. However, the gain in active surface area for desired surface reactions is offset in part by enhanced rates of degradation processes that cause losses in catalyst mass, catalyst surface-area, and electrocatalytic activity. The loss of electrochemically active surface-area of the catalyst causes severe performance degradation over relevant lifetimes of polymer electrolyte fuel cells yet a consistent theoretical approach, linking experimental observations of surface-area loss related phenomena to purported mechanisms of degradation was missing. Accordingly, a dynamic model of surface-area loss and Pt mass balance phenomena based on the theories of Lifshitz, Slyozov and Wagner, and Smoluchowski is developed. It relates kinetic rates of degradation processes to the evolution of the particle-size distribution and its moments. We pursue model validation and evaluation by analyzing an extensive set of electrochemical surface-area loss experiments probing the impact of accelerated stress test control levers. Our Pt mass balance model unifies degradation characterization approaches and accordingly discriminates the predominant degradation mechanisms. The evaluation and validation approaches established a firm link between surface-area loss, Pt dissolution and Pt oxidation. As a consequence of our evaluation results, a kinetic model for Pt(111) oxide formation and reduction is developed and validated against a wide range of electrochemical, spectroscopic and theoretical work found in the relevant literature. The model provides a comprehensive picture of surface electrochemical processes that occur at Pt(111). In closing we discuss future routes of research. Foremost is the extension of cyclic voltammetry work to polycrystalline Pt and Pt nanoparticle electrodes, we suggest that these are the logical steps towards linking dynamic Pt oxidation with surface tension, Pt dissolution, surface-area loss and the oxygen reduction reaction.
Download or read book A Structure based Model for Cathode Catalyst Layer of Polymer Electrolyte Membrane Fuel Cell written by Jianfeng Liu and published by . This book was released on 2007 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: A structure-based performance model for Cathode Catalyst Layers (CCLs) of Polymer Electrolyte Membrane Fuel Cells (PEMFCs) is presented. A CCL is the major competitive ground for mass transport, electrochemical reaction, and vaporization in a PEMFC. Analytical solutions for the case of fast proton transport have revealed that the CCL plays a vital role in the conversion of liquid water to vapor and in regulating water fluxes towards Polymer Electrolyte Membrane (PEM) and Gas Diffusion Layer (GDL). Critical values of proton conductivity O* and oxygen transport coefficient D* are introduced to distinguish different regimes of operation for these transport processes. Subsequently, the role of the porous structure and of liquid water accumulation for the performance of CCL in PEMFCs is explored. The non-linear spatial coupling between liquid water accumulation and oxygen depletion triggers critical effects and bistability in current-voltage response curves. Stability diagrams are proposed as novel tools for assessing CCL performance.
Download or read book PEM Fuel Cell Electrocatalysts and Catalyst Layers written by Jiujun Zhang and published by Springer Science & Business Media. This book was released on 2008-08-26 with total page 1147 pages. Available in PDF, EPUB and Kindle. Book excerpt: Proton exchange membrane (PEM) fuel cells are promising clean energy converting devices with high efficiency and low to zero emissions. Such power sources can be used in transportation, stationary, portable and micro power applications. The key components of these fuel cells are catalysts and catalyst layers. “PEM Fuel Cell Electrocatalysts and Catalyst Layers” provides a comprehensive, in-depth survey of the field, presented by internationally renowned fuel cell scientists. The opening chapters introduce the fundamentals of electrochemical theory and fuel cell catalysis. Later chapters investigate the synthesis, characterization, and activity validation of PEM fuel cell catalysts. Further chapters describe in detail the integration of the electrocatalyst/catalyst layers into the fuel cell, and their performance validation. Researchers and engineers in the fuel cell industry will find this book a valuable resource, as will students of electrochemical engineering and catalyst synthesis.
Download or read book Convective Heat Transfer in Porous Media written by Yasser Mahmoudi and published by CRC Press. This book was released on 2019-11-06 with total page 366 pages. Available in PDF, EPUB and Kindle. Book excerpt: Focusing on heat transfer in porous media, this book covers recent advances in nano and macro’ scales. Apart from introducing heat flux bifurcation and splitting within porous media, it highlights two-phase flow, nanofluids, wicking, and convection in bi-disperse porous media. New methods in modeling heat and transport in porous media, such as pore-scale analysis and Lattice–Boltzmann methods, are introduced. The book covers related engineering applications, such as enhanced geothermal systems, porous burners, solar systems, transpiration cooling in aerospace, heat transfer enhancement and electronic cooling, drying and soil evaporation, foam heat exchangers, and polymer-electrolyte fuel cells.
Download or read book Development of a Micro scale Cathode Catalyst Layer Model of Polymer Electrolyte Membrane Fuel Cell written by Mobin Khakbazbaboli and published by . This book was released on 2013 with total page 358 pages. Available in PDF, EPUB and Kindle. Book excerpt: In this work, a micro-model of the catalyst layer of polymer electrolyte membrane fuel cell (PEMFC) was developed. The micro-model includes the transport phenomena and the reaction kinetics within a three dimensional micro-structure representing a sample of PEMFC catalyst layer. Proper physical boundary conditions have been described on the surfaces of the sample as well as on the interfaces between the regions through which all constituents are solved in a coupled manner. A four-phase micro-structure of CL was reconstructed, the platinum particles were resolved in the computational grid generation and the governing equations were solved within platinum region. A body-fitted computational mesh was generated for the reconstructed micro-structure of CL. The number of computational cells were optimized based on how close to an analytical sphere the magnitude of the surface area of a sphere can be captured after generating the computational cells. The interfaces with important physical phenomena were more refined than the rest of the interfaces, specially the electrochemically active reaction surface. The computational mesh was checked for a grid independent numerical solution. The Knudsen effects was included by calculating the characteristic length in the pore region. Four different cases of including Knudsen effects were studied. Also, a comparison was made between solution with and without Knudsen effects. A physical model of oxygen dissolution was developed, the oxygen dissolution at the interface between pore and ionomer was treated as an superficial phenomenon. The performance curves were produced and provided for the reconstructed micro-structure along with the distribution of field variables. A length study of the reconstructed micro-structure was conducted such that the results from the micro-modeling can capture the trend in variable distributions observed in the macro-modeling of CL or experiments. A platinum loading study was preformed and the anomalous phenomena of dramatic increase in oxygen transport resistance observed in some experimental works was explained by isolating the ionomer region of the CL micro-structure and numerically calculating the shape factor for diffusive transport. It was found that the increase in oxygen transport resistance is due to the increase in diffusion pathway and decrease in the transport surface area.
