Microstructure of Catalyst Layers in PEM Fuel Cells Redefined: A Computational Approach

We report fitting of the physics-based model for the cathode side impedance to the experimental spectra of low-Pt loaded (0.1/0.1 mg Pt cm −2 ) and high-Pt loaded (0.4/0.4 mg Pt cm −2 ) PEM fuel cells measured in the range of current densities from 50 to 400 mA cm −2 . Fitting allowed us to separate the oxygen diffusion coefficients in the catalyst layer D ox and in the gas-diffusion layer D b , and the respective mass transfer coefficients of the electrodes of both types. In the low-Pt electrode, D ox is an order of magnitude lower, than in the high-Pt electrode; however, due to 4-fold difference in the electrode thickness, the respective mass transfer coefficients are close to each other. In both the electrodes, the oxygen diffusion and the mass transfer coefficients in the GDL are nearly the same and they are much higher, than the respective coefficients in the CCLs. The ORR Tafel slope and D ox exhibit linear growth with the cell current density; both effects could be attributed to "cleaning" of Pt surface from oxides at lower cell potential.

Section: Resultsmentioning

confidence: 99%

Impedance Spectroscopy Characterization of Oxygen Transport in Low– and High–Pt Loaded PEM Fuel Cells

Reshetenko

Kulikovsky

2017

“…The site-site radial distribution function (RDF), obtained from CGMD simulations, matches very well to those from the atomistic MD simulations. 43 The RDF between the sidechain beads and the other components of the mixture show that side chains are surrounded with water and hydrated protons. The autocorrelation functions exhibit similar dependences on bead separation at all λ, indicating a strong clustering of sidechains due to the aggregation and folding of polymer backbones.…”

Section: Resultsmentioning

confidence: 99%

“…CGMD simulations.-The details of the computational approach based on CGMD simulations are explained elsewhere [43][44][45]50 and is developed in two major steps. In the first step, Nafion chains, water and hydronium molecules are replaced by corresponding spherical beads with predefined sub-nanoscopic length scale.…”

Section: Overall Methodologymentioning

confidence: 99%

“…These techniques include Radial Distribution Function (RDF), Pore Side Distribution, Cluster Size analysis, and Pore Network Analysis. 43,44,50 Performance model.-General framework.-The performance model is a multi-paradigm multiscale single-cell model implemented within our in house simulation package MS LIBER-T (see Figure 3). [56][57][58] This is a software coded in a modular framework on an independent C/Python language basis, highly flexible and portable with multiple application domains already demonstrated.…”

Section: Overall Methodologymentioning

confidence: 99%

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A Multiparadigm Modeling Investigation of Membrane Chemical Degradation in PEM Fuel Cells

Quiroga

Malek

Franco

2015

Self Cite

We report a multi-paradigm model of the membrane chemical degradation in Polymer Electrolyte Membrane Fuel Cells (PEMFCs), by combining Coarse-Grained Molecular Dynamics (CGMD) and a multiscale cell performance model. CGMD is used to generate structural databases that relate the amount of detached (degraded) ionomer sidechains with the water content and the resulting PEM meso-microporous structure. The multiscale cell performance model describes the electrochemical reactions and transport mechanisms occuring in the electrodes from an on-the-fly coupling between Kinetic Monte Carlo (KMC) sub-models parametrized with Density Functional Theory (DFT) data and (partial differential equations-based) continuum sub-models. Furthermore, the performance model includes a kinetic PEM degradation sub-model which integrates the CGMD database. The cell model also predicts the instantaneous PEM sidechain content and conductivity evolution at each time step. The coupling of these diverse modeling paradigms allows one to describe the feedback between the instantaneous cell performance and the intrinsic membrane degradation processes. This provides detailed insights on the membrane degradation (sidechain detachment as well as water reorganization within the PEM) during cell operation. This novel modeling approach opens interesting perspectives in engineering practice to predict materials degradation and durability as a function of the initial chemical composition and structural properties in electrochemical energy conversion and storage devices. From the second half of the twentieth century, Polymer Electrolyte Membrane Fuel Cells (PEMFCs) have attracted much attention due to their potential as a clean power source for vehicles traction. Market introduction of FC vehicles is being recognized of highest priority in many developed countries due to their impact on the reduction of energy consumption and greenhouse gas emissions. However, PEMFC technologies have not yet reached all the requirements to be competitive, in particular regarding their high production cost of Membrane Electrode Assemblies and their low durability.Indeed, meso/micro-structural degradation leading to the PEMFC components aging is attributed to several complex physicochemical mechanisms not yet completely understood. The associated components meso/micro-structural changes translate into irreversible longterm cell power degradation.1-3 For instance, dissolution and redistribution of the catalyst reduces the specific catalyst surface area and the electrochemical activity. The corrosion of the catalyst carbon-support and loss or decrease of the hydrophobicity caused by an alteration of the Polytetrafluoroethylene in Catalyst Layers (CLs), Microporous Layers and Gas Diffusion Layers also affect the water management in the cell and thus the electrochemical performance.Regarding the polymer electrolyte in PEMFCs, a large number of materials has been tested, including sulfonated hydrocarbon polymers, phosphoric acid doped polybenzimidazole, polymer-inorganic composit...

“…Approaches based on kinetic Monte-Carlo techniques are usually coupled with molecular dynamics simulations 9 and are able to describe systems up to a few thousand molecules (or quasi-particles). Computationally complex, such methods can sometimes predict the discharge characteristics of highly simplified battery structures, 10 but are still inappropriate to predict the behavior of realistic devices in networks or systems.…”

mentioning

confidence: 99%

Changing the Cathode Microstructure to Improve the Capacity of Li-Air Batteries: Theoretical Predictions

Bevara¹,

Andrei²

2014

The transport equations in Li-air batteries are revisited and modified to account for different pore microstructures, pore size distribution effects and electron transport through the discharge product. Different material microstructures are analyzed including structures made of spherical and cylindrical pores, nanoparticles, carbon nanotubes and nanofibers, and a new hybrid model is proposed to describe the deposition of discharge product in the cathode. It is shown that although the different microstructures result in different dynamics in which the pores are being filled, they lead to relatively similar values of the energy and power densities. Microstructures based on carbon nanotubes, nanofibers, and spherical particles tend to increase the effective size of the fibers and particles during the first part of the discharge process and result in a slight increase of the cell voltage at the beginning of the discharge. The power density of Li-air batteries decreases with the variance of the pore size distribution function, while the capacity shows a slight increase with the variance of this distribution. In general, the resistivity of the deposit layer (e.g. Li 2 O 2 , Li 2 O, etc.) has a negative effect on the performance of Li-air batteries. However, it is shown that, as long as this resistivity is not too large, the deposit layer tends to increase the capacity of the battery by approximately 10%. This rather unexpected phenomenon is attributed to the fact that the voltage drop across the deposit layer reduces the reaction rate at the air side of the cathode and, in this way, delays the formation of the deposit product in this region. Growing greenhouse gas emissions and degrading air quality due to electric power generation by fossil fuels have resulted in a shift in interest toward less-polluting and sustainable energy sources such as wind and solar energy.1 Heat-trapping greenhouse gasses resulting from human activity, such as burning fossil fuels, contribute to climate change around the world including rising sea levels, above average temperatures, heavy precipitations and droughts in certain regions, dwindling ecosystems, and others.2-4 There is, therefore, an increasing pressure on governments and industry worldwide to move to safer, environmentally friendly, renewable sources. The penetration of renewable energy sources in electric grids is however expected to produce large power fluctuations that are detrimental to the security and safety of the grid. To smooth out these power fluctuations it is important to utilize storage systems that are charging when there is less demand for power in the grid and discharging during high-demand periods. Among the existing energy storage systems, electrochemical systems based on metal-air batteries can potentially provide compact, high energy density, and environmentally friendly energy storage devices for the electric grid.5 In addition to grid applications, metal-air systems also have great potential to be used in the electric vehicle industry. The theoretical spec...