Adam Z. Weber

Nanostructured Au–CeO2 Catalysts for Low-Temperature Water–Gas Shift

The composite system of nanostructured gold and cerium oxide, with a gold loading 5–8 wt%, is reported in this work as a very good catalyst for low-temperature water–gas shift. Activity depends largely on the presence of nanosized ceria particles. Various techniques of preparation of an active catalyst are disscussed. The presence of gold is crucial for activity below 300°C. A dramatic effect of gold on the reducibility of the surface oxygen of ceria is found by H2-TPR, from 310–480°C to 25–110°C. All of the available surface oxygen was reduced, while there was no effect on the bulk oxygen of ceria. This correlates well with the shift activity of the Au–ceria system.

Effects of Microporous Layers in Polymer Electrolyte Fuel Cells

Our previous models and a catalyst-layer model are combined to study the effects of microporous layers in polymer electrolyte fuel cells. The combined sandwich model is used to fit data both with and without a microporouslayer with saturated feed conditions. In terms of water management, the simulations clearly show that the effect of a microporous layer is to keep water out of the cathode gas diffusion layer and move it through the anode. Additional effects of microporous layers include better ohmic behavior and perhaps better catalyst utilization, among other things. Optimizations for different structural parameters are investigated, as are the effects of microporous layers under different operating conditions. The discussion is geared toward how microporous layers increase performance and their effect on fuel cell water management.

Transport in Polymer-Electrolyte Membranes I. Physical Model

In this paper, a physical model is developed that is semiphenomenological and takes into account Schroeders paradox. Using the wealth of knowledge contained in the literature regarding polymer-electrolyte membranes as a basis, a novel approach is taken in tying together all of the data into a single coherent theory. This approach involves describing the structural changes of the membrane due to water content, and casting this in terms of capillary phenomena. By treating the membrane in this fashion, Schroeders paradox can be elucidated. Along with the structural changes, two different transport mechanisms are presented and discussed. These mechanisms, along with the membranes structural changes, comprise the complete physical model of the membrane. The model is shown to agree qualitatively with different membranes and different membrane forms, and is applicable to modeling perfluorinated sulfonic acid and similar membranes. It is also the first physically based comprehensive model of transport in a membrane that includes a physical description of Schroeders paradox, and it bridges the gap between the two types of macroscopic models currently in the literature.

Transport in Polymer-Electrolyte Membranes II. Mathematical Model

A mathematical model is developed that is based on our previous physical model. The governing equations are presented for both the vapor- and liquid-equilibrated transport modes as well as when they both occur. Thus, this model bridges the gap between the one-phase and two-phase macroscopic models currently used in the literature. In addition to being able to model such phenomena as Schroeders paradox, the model incorporates other relatively novel features including the effect of temperature on water uptake by the membrane from water vapor, and its associated effects on transport properties. Just as in the physical model, the mathematical model uses the wealth of knowledge contained in the literature to examine and determine values for the relevant transport and membrane parameters. This also helps in corroborating the physical model. The mathematical model developed is further validated and its results examined in a subsequent paper where it is placed in a simple fuel-cell model.

Modeling Two-Phase Behavior in PEFCs

UTC Fuel Cells, South Windsor, Connecticut 06074, USAA model is developed to examine quantitatively the effects of flooding on the operation of polymer-electrolyte fuel cells ~PEFCs!.Specifically, the change in the maximum power as a function of the structural properties of the diffusion media, including the bulkporosity, wettability, thickness, and pore-size distribution, is described. The porous-medium model developed includes analyticexpressions and a modeling methodology for handling both liquid and gas flow. The model is used in combination with ourprevious membrane model to simulate transport in typical gas diffusion layers and examine the effect of layer hydrophobicity onthe maximum power.© 2004 The Electrochemical Society. @DOI: 10.1149/1.1792891# All rights reserved.Manuscript submitted November 3, 2003; revised manuscript received March 24, 2004. Available electronically September 27,2004.

Toward an Ideal Polymer Binder Design for High-Capacity Battery Anodes

The dilemma of employing high-capacity battery materials and maintaining the electronic and mechanical integrity of electrodes demands novel designs of binder systems. Here, we developed a binder polymer with multifunctionality to maintain high electronic conductivity, mechanical adhesion, ductility, and electrolyte uptake. These critical properties are achieved by designing polymers with proper functional groups. Through synthesis, spectroscopy, and simulation, electronic conductivity is optimized by tailoring the key electronic state, which is not disturbed by further modifications of side chains. This fundamental allows separated optimization of the mechanical and swelling properties without detrimental effect on electronic property. Remaining electronically conductive, the enhanced polarity of the polymer greatly improves the adhesion, ductility, and more importantly, the electrolyte uptake to the levels of those available only in nonconductive binders before. We also demonstrate directly the performance of the developed conductive binder by achieving full-capacity cycling of silicon particles without using any conductive additive.

Coupled thermal and water management in polymer electrolyte fuel cells

Thermal and water management are intricately coupled in polymer-electrolyte fuel cells. In this paper, we simulate fuel-cell performance and account for nonisothermal phenomena. The transport of water due to a temperature gradient and its associated effects on performance are described, with the increase of reactant dilution by the water-vapor partial pressure being the most dominant. In addition, simulations are undergone to find the optimum operating temperature and maximum power density as a function of external heat-transfer coefficient. The shape of the optimization curves and the magnitudes of the nonisothermal phenomena are also detailed and explained.

Modeling, simulation, and design criteria for photoelectrochemical water-splitting systems

A validated multi-physics numerical model that accounts for charge and species conservation, fluid flow, and electrochemical processes has been used to analyze the performance of solar-driven photoelectrochemical water-splitting systems. The modeling has provided an in-depth analysis of conceptual designs, proof-of-concepts, feasibility investigations, and quantification of performance. The modeling has led to the formulation of design guidelines at the system and component levels, and has identified quantifiable gaps that warrant further research effort at the component level. The two characteristic generic types of photoelectrochemical systems that were analyzed utilized: (i) side-by-side photoelectrodes and (ii) back-to-back photoelectrodes. In these designs, small electrode dimensions (mm to cm range) and large electrolyte heights were required to produce small overall resistive losses in the system. Additionally, thick, non-permeable separators were required to achieve acceptably low rates of product crossover.

Transport in Polymer-Electrolyte Membranes III. Model Validation in a Simple Fuel-Cell Model

The previously developed mathematical model, based on our physical model, is validated by comparing simulations to experiments. The mathematical model is placed within a simple pseudo-two-dimensional fuel-cell (FC) model. The comparisons mainly involve the net flux of water per proton in the membrane. In the complete FC model, there is only one fitting parameter, the transport coefficient (effective permeability) of the diffusion media, and it is fit at only one simulation condition for each FC setup. The simulations agree very well with both values and trends taken from various literature sources under many different operating conditions, including pressure, temperature, current density, and inlet humidification and stoichiometry. Furthermore, the model allows for an explanation of those trends. The sensitivity and the details of the mathematical model are also examined. Overall, the mathematical model and the physical model are validated and shown to be generally applicable for describing water transport in a polymer-electrolyte membrane.

Modeling Low-Platinum-Loading Effects in Fuel-Cell Catalyst Layers

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.