Kourosh Malek

On the Micro-, Meso-, and Macroporous Structures of Polymer Electrolyte Membrane Fuel Cell Catalyst Layers

In this work, N(2) adsorption was employed to investigate the effects of carbon support, platinum, and ionomer loading on the microstructure of polymer electrolyte membrane fuel cell catalyst layers (CLs). Brunauer-Emmett-Teller and t-plot analyses of adsorption isotherms and pore-size distributions were used to study the microstructure of carbon supports, platinum/carbon catalyst powders, and three-component platinum/carbon/ionomer CLs. Two types of carbon supports were chosen for the investigation: Ketjen Black and Vulcan XC-72. CLs with a range of Nafion ionomer loadings were studied in order to evaluate the effect of an ionomer on the CL microstructure. Regions of adsorption were differentiated into micropores associated with the carbon primary particles (<2 nm), mesopores ascribed to the void space inside agglomerates (2-20 nm), and meso- to macroporous space inside aggregates of agglomerates (>50 nm). Ketjen Black was found to possess a significant fraction of micropores, 25% of the total pore volume, in contrast to Vulcan XC-72, for which the corresponding fraction of micropores was 15% of the total pore volume. The microstructure of the carbon support was found to be a significant factor in the formation of the microstructure in the three-component CLs, serving as a rigid porous framework for distribution of platinum and the ionomer. It was found that platinum particle deposition on Ketjen Black occurs in, or at the mouth of, the supports micropores, thus affecting its effective microporosity, whereas platinum deposition on Vulcan XC-72 did not significantly affect the supports microstructure. The codeposition of ionomer in the CL strongly influenced its porosity, covering pores < 20 nm, which are ascribed to the pores within the primary carbon particles (pore sizes < 2 nm) and to the pores within agglomerates of the particles (pore sizes of 2-20 nm).

Nanophase segregation and water dynamics in hydrated Nafion: Molecular modeling and experimental validation

Reported results of coarse-grained molecular dynamics simulations rationalize the effect of water on the phase-segregated morphology of Nafion ionomers. We analyzed density maps and radial distribution functions and correlated them with domain structures, distributions of protogenic side chains, and water transport properties. The mesoscopic structures exhibit spongelike morphologies. Hydrophilic domains of water, protons, and anionic side chains form a random three-dimensional network, which is embedded in a matrix of hydrophobic backbone aggregates. Sizes of hydrophilic domains increase from 1 to 3 nm upon water uptake. At low water content, hydrophilic domains are roughly spherical and poorly connected. At higher water content, they convert into elongated cylindrical shapes with high connectivity. Further structural analysis provides a reasonable estimate of the percolation threshold. Radial distribution functions from coarse-grained and atomistic molecular dynamics models exhibit a good agreement. Water cluster size distributions from coarse-grained molecular dynamics and dissipative particle dynamics are consistent with small angle x-ray scattering data. Moreover, we calculated the water diffusivity by molecular dynamics methods and corroborated the results by comparison with pulsed field gradient NMR.

Highly corrosion resistant platinum–niobium oxide–carbon nanotube electrodes for the oxygen reduction in PEM fuel cells

Nanocomposite materials consisting of platinum deposited on carbon nanotubes are emerging electrocatalysts for the oxygen reduction reaction in PEM fuel cells. However, these materials albeit showing promising electrocatalytic activities suffer from unacceptable rates of corrosion during service. This study demonstrates an effective strategy for creating highly corrosion-resistant electrocatalysts utilizing metal oxide coated carbon nanotubes as a support for Pt. The electrode geometry consisted of a three-dimensional array of multi-walled carbon nanotubes grown directly on Inconel and conformally covered by a bilayer of Pt/niobium oxide. The activities of these hybrid carbon-metal oxide materials are on par with commercially available carbon-supported Pt catalysts. We show that a sub-nanometre interlayer of NbO2 provides effective protection from electrode corrosion. After 10,000 cyclic voltammetry cycles from 0.5 V to 1.4 V, the loss of electrochemical surface area, reduction of the half-wave potential, and the loss of specific activity of the NbO2 supported Pt were 10.8%, 8 mV and 10.3%, respectively. Under the same conditions, the catalytic layers with Pt directly deposited onto carbon nanotubes had a loss of electrochemical area, reduction of half-wave potential and loss of specific activity of 47.3%, 65 mV and 65.8%, respectively. The improved corrosion resistance is supported by microstructural observations of both electrodes in their post-cycled state. First principles calculations at the density functional theory level were performed to gain further insight into changes in wetting properties, stability and electronic structure introduced by the insertion of the thin NbO2 film.

PEMFC Catalyst Layers: The Role of Micropores and Mesopores on Water Sorption and Fuel Cell Activity

The effects of carbon microstructure and ionomer loading on water vapor sorption and retention in catalyst layers (CLs) of PEM fuel cells are investigated using dynamic vapor sorption. Catalyst layers based on Ketjen Black and Vulcan XC-72 carbon blacks, which possess distinctly different surface areas, pore volumes, and microporosities, are studied. It is found that pores <20 nm diameter facilitate water uptake by capillary condensation in the intermediate range of relative humidities. A broad pore size distribution (PSD) is found to enhance water retention in Ketjen Black-based CLs whereas the narrower mesoporous PSD of Vulcan CLs is shown to have an enhanced water repelling action. Water vapor sorption and retention properties of CLs are correlated to electrochemical properties and fuel cell performance. Water sorption enhances electrochemical properties such as the electrochemically active surface area (ESA), double layer capacitance and proton conductivity, particularly when the ionomer content is very low. The hydrophilic properties of a CL on the anode and the cathode are adjusted by choosing the PSD of carbon and the ionomer content. It is shown that a reduction of ionomer content on either cathode or anode of an MEA does not necessarily have a significant detrimental effect on the MEA performance compared to the standard 30 wt % ionomer MEA. Under operation in air and high relative humidity, a cathode with a narrow pore size distribution and low ionomer content is shown to be beneficial due to its low water retention properties. In dry operating conditions, adequate ionomer content on the cathode is crucial, whereas it can be reduced on the anode without a significant impact on fuel cell performance.

Catalyst Layer Modeling: Structure, Properties and Performance

Polymer Electrolyte Fuel Cells (PEFCs) are promising electrochemical devices for the direct conversion of chemical energy of a fuel into electrical work [1–5]. Enormous research programs worldwide explore PEFCs as power sources that could replace internal combustion engines in vehicles and provide power to portable and stationary applications. Typically PEFCs operate below ~80 °C. Anodic oxidation of H2 produces protons that migrate through the polymer electrolyte membrane (PEM) to the cathode, where reduction of O2 produces water. Meanwhile, electrons, produced at the anode, perform work in external electrical appliances or engines. Unrivalled thermodynamic efficiencies, high energy densities, and ideal compatibility with hydrogen distinguish PEFCs as a primary solution to the global energy challenge.

Molecular Modeling of Hydronium Ion and Water Distribution in Water-Filled Pt Nanochannels with Corrugated Walls

Classical molecular dynamics was employed to study the impact of nanoconfinement and surface nanostructure on electrostatic phenomena and transport properties in catalyst layer pores of polymer electrolyte fuel cells. The porous structure of an ionomer-free ultrathin catalyst layer was represented by a water-filled nanochannel with corrugated and negatively charged Pt walls. The equilibrium hydronium ion distribution in the channel was used to assess local reaction conditions. We rationalized the effects of nanostructuring and metal surface charge on the electrostatic effectiveness factor of the channel. Furthermore, we investigated the water dynamics in the nanochannel, calculating the self-diffusion coefficients of surface and bulk-like water.

Electrochemical Materials for PEM Fuel Cells: Insights from Physical Theory and Simulation

This chapter focuses on the role of physical theory, molecular simulation, and computational electrochemistry for fundamental understanding, diagnostics, and design of Polymer Electrolyte Fuel Cells (PEFCs). Development of stable and inexpensive materials is the most important technological hurdle that PEFC developers are currently facing. A profound insight based on theory and modeling of the pertinent materials will advise us how fuel cell components with optimal specifications could be made and how they can be integrated into operating cells. This chapter highlights major challenges and perspectives in research on electrochemical materials for fuel cells, arising at scales from Angstrom to meters. Topics include proton conduction, nanoparticle electrocatalysis, self-organization in complex media, effective properties of random heterogeneous media, role of water in various components and at various scales and effectiveness of catalyst utilization.

Transport in Protein Crystals, Part II: Diffusion Simulation and Chiral Recognition

This second part of a two-part series focuses on diffusion simulation and explores the role of chiral recognition during solute transport in cross-linked protein crystals.

Transport in Protein Crystals, Part I: Insights from Molecular Simulations

In addition to simulations of transport processes in protein crystals, this article highlights the importance of protein-solvent interactions. It is important, for example, to know whether the catalytic properties of proteins are the same in crystal and in solution.

Cost Modeling and Valuation of Grid-Scale Electrochemical Energy Storage Technologies

Electrochemical Energy storage (ES) technologies are seen as valuable flexibility assets with their capabilities to control grid power intermittency or power quality services in generation, transmission & distribution, and end-user consumption side. Grid-scale storage technologies can contribute significantly to enhance asset utilization rate and reliability of the power systems. The latter is particularly critical for deployment of regional and national energy policies of implementing renewable sources. Once the suitable storage technology is chosen, modeling and simulation of electrochemical storage devices are utilized extensively for performance or life cycle prediction purposes. The main challenge of adopting electrochemical storage technologies among utilities is how to match the right energy storage technology for a site-specific grid configuration to an appropriate grid service. The majority of system-level modeling efforts do not provide information that can be used for valuation of storage technologies. Battery performance models generally suffer from lacking techno-economic predictions and accurate assessment of performance characteristics of the emerging ES technologies. This chapter introduces a valuation framework that is built upon high-level electrochemical storage models. This valuation model can characterize and quantify different grid applications and services for which electrochemical storage devices are used. Taking local differences in electricity markets and storage value for several grid applications and services, the modeling framework is employed in case studies to identify the value that storage systems can provide to the grid.