Robert N. Carter

Artifacts in Measuring Electrode Catalyst Area of Fuel Cells through Cyclic Voltammetry

Cyclic Voltammetry (CV) is a well-established technique to measure the electrochemically active surface area (ECA) of a catalyst in an electrode through hydrogen adsorption/desorption (HAD)[1]. In the field of proton exchange memebrane fuel cells this method can be applied over a wide range of scales ranging from rotating disc electrodes (RDE) to single-cell and even multiplecell stacks where the working electrode is part of a membrane-electrode assembly (MEA). It is important, however, to recognize there are two convoluted processes occurring at low potential (ca. Evs RHE < 0.1 V): HAD and hydrogen evolution.

Modeling of Membrane-Electrode-Assembly Degradation in Proton-Exchange-Membrane Fuel Cells - Local H2 Starvation and Start-Stop Induced Carbon-Support Corrosion

Carbon-support corrosion causes electrode structure damage and thus electrode degradation. This chapter discusses fundamental models developed to predict cathode carbon-support corrosion induced by local H2 starvation and start–stop in a proton-exchange-membrane (PEM) fuel cell. Kinetic models based on the balance of current among the various electrode reactions are illustrative, yielding much insight on the origin of carbon corrosion and its implications for future materials developments. They are particularly useful in assessing carbon corrosion rates at a quasi-steady-state when an H2-rich region serves as a power source that drives an H2-free region as a load. Coupled kinetic and transport models are essential in predicting when local H2 starvation occurs and how it affects the carbon corrosion rate. They are specifically needed to estimate length scales at which H2 will be depleted and time scales that are valuable for developing mitigation strategies. To predict carbon-support loss distributions over an entire active area, incorporating the electrode pseudo-capacitance appears necessary for situations with shorter residence times such as start–stop events. As carbon-support corrosion is observed under normal transient operations, further model improvement shall be focused on finding the carbon corrosion kinetics associated with voltage cycling and incorporating mechanisms that can quantify voltage decay with carbon-support loss.

Measurements of Effective Oxygen Diffusivity, Pore Size Distribution, and Porosity in PEM Fuel Cell Electrodes

This paper presents the measurement results of effective oxygen diffusivity (DO2), pore size distribution, and porosity of proton exchange membrane fuel cell (PEMFC) electrodes, and the calculated tortuosities using these measurement results. A novel method has been developed to directly measure effective oxygen diffusivity in electrodes as a function of temperature and relative humidity (RH) on a conventional PEMFC platform with 50 cm 2 flow fields. The pore size distributions and porosities of catalyst powder and electrodes are measured by nitrogen adsorption with the BJH model and mercury intrusion porosimetry (MIP). The measured porosities from the MIP results are considerably less than those calculated from material densities, suggesting that ionomer blocks some pores in electrodes. The calculated tortuosities are larger than those predicted by Bruggeman correction, indicating the Bruggeman correction underestimates the tortuosity and thus overestimate effective DO2 in PEMFC electrodes.

Rapid Thermal Response Catalyst for Treatment of Automotive Exhaust

To meet the increasingly strict automobile emissions requirements for conventional vehicles, considerable improvements in catalyst technology are required. It is now recognized that a primary challenge in meeting the ultra-low emissions vehicle standards (ULEV) for automobiles is in reducing cold-start emissions, and this generally requires a catalytic converter that has a rapid temperature response. Conventional converters (typically made from ceramic or metal monoliths) have a relatively slow temperature response due to their high thermal mass and limited heat transfer characteristics. The authors present details of a novel catalyst technology (Microlith{trademark}) that offers considerable advantages for this application because of greatly enhanced heat and mass transfer characteristics. Results of performance tests are also presented that demonstrate this catalyst`s effectiveness. The effects of thermal aging on microstructure and catalyst performance are also discussed.

FUEL CELLS – PROTON-EXCHANGE MEMBRANE FUEL CELLS | Catalysts: Life-Limiting Considerations

Material degradation mechanisms of carbon-supported platinum-based electrocatalysts in proton-exchange membrane fuel cells (PEMFCs) are reviewed in the context of automotive applications. Transient operations such as idling, sitting at open-circuit voltage, load cycling, and startup/shutdown pose significant durability concerns over the state-of-the-art catalysts in the expected lifetime range. High cathode potentials and potential cycling can cause damage to both the catalyst and the carbon support, leading to the growth of platinum particles and the loss of platinum into the ionomer phase, as well as carbon-support corrosion. Startup and shutdown are the most damaging transient operations in PEMFC systems, which can cause significant cathode electrode carbon-support corrosion. PEMFC performance decays dramatically when the electrode structure is damaged due to carbon corrosion.

In situ vapor sorption apparatus for small-angle neutron scattering and its application

An in situ vapor sorption apparatus has been constructed for use in small-angle neutron scattering (SANS) measurements. The apparatus adapts two independent operating mechanisms, with and without a carrier gas, to control relative or absolute pressure, respectively, in the SANS sorption cell. By controlling the absolute pressure, a target relative vapor pressure between 0% and 90% can be reached within 1–2 min. The short response time makes it possible to correlate diffusion kinetics and/or sorption/desorption isotherms with structure evolution during wetting/drying, which is not possible in gravimetric methods. Also, one can extract diffusion coefficients and interaction parameters. Other uses include the enhancement of scattering contrast in the study of semicrystalline polymers by the preferential vapor sorption of deuterated vapor into the amorphous regions. Thus, one can obtain the same structural information as small-angle x-ray scattering measurements on dry samples. Also, the apparatus has the cap...

Ultrathin transparent membranes for cellular barrier and co-culture models

Typical in vitro barrier and co-culture models rely upon thick semi-permeable polymeric membranes that physically separate two compartments. Polymeric track-etched membranes, while permeable to small molecules, are far from physiological with respect to physical interactions with co-cultured cells and are not compatible with high-resolution imaging due to light scattering and autofluorescence. Here we report on an optically transparent ultrathin membrane with porosity exceeding 20%. We optimize deposition and annealing conditions to create a tensile and robust porous silicon dioxide membrane that is comparable in thickness to the vascular basement membrane (100-300 nm). We demonstrate that human umbilical vein endothelial cells (HUVECs) spread and proliferate on these membranes similarly to control substrates. Additionally, HUVECs are able to transfer cytoplasmic cargo to adipose-derived stem cells when they are co-cultured on opposite sides of the membrane, demonstrating its thickness supports physiologically relevant cellular interactions. Lastly, we confirm that these porous glass membranes are compatible with lift-off processes yielding membrane sheets with an active area of many square centimeters. We believe that these membranes will enable new in vitro barrier and co-culture models while offering dramatically improved visualization compared to conventional alternatives.

Catalytic Combustion Technology Development for Gas Turbine Engine Applications

Catalytic combustion is one means of meeting increasingly strict emissions requirements for ground-based gas turbine engines for power generation. In conventional homogeneous combustion, high flame temperatures and incomplete combustion lead to emissions of oxides of nitrogen (NOx) and carbon monoxide (CO), and in lean premixed systems unburned hydrocarbons (UHC). However, catalyst-assisted reaction upstream of a lean premixed homogeneous combustion zone can increase the fuel/air mixture reactivity sufficiently to provide low CO/UHC emissions. Additionally, catalytic combustion extends the lean limit of combustion, thereby minimizing NOx formation by lowering the adiabatic flame temperature. An overview of this technology is presented including discussion of the many materials science and catalyst challenges that catalytic combustion poses ranging from the need for high temperature materials to catalyst performance and endurance. Results of ongoing development efforts at Precision Combustion, Inc. (PCI) are presented including modeling studies and experimental results from both bench-scale and combustor-scale studies.