Publications

This paper presents further results and extensions of our previous point defect model for time-dependent growth of passive oxide films on metal surfaces. Specifically, by accounting for vacancies as material species rather than just holes in the oxide lattice, the model incorporates more plausible expressions for interfacial reactions and associated kinetic rate expressions. We use the model to explore the general effects of varying metal valence and electrolyte pH on passive film growth. Furthermore, we examine key assumptions concerning the thickness dependence of the electric field within the film. When the electric field inside the film remains constant and the rate constant for oxygen vacancy production varies with applied potential, the model predicts trends in thickness versus potential in reasonable agreement with experimental data for a variety of metal/metal oxide systems. This represents a considerable improvement upon the previous high-field form of the model which assumed rate constants independent of potential and electric field in the film varying with thickness. © 2002 Elsevier Science Ltd. All rights reserved.

We present a simplified point defect model to describe the growth of the primary passive oxide film on the surface of iron. The model postulates a reduced set of elementary interfacial reactions to describe the formation and dissolution of the oxide film. By casting the model in dimensionless form, we obtain a relatively small set of parameters that must be assigned values. Parameter values are set by matching the film thickness predicted by the model with one experimental data point. The model is then used to predict variations in film thickness with time, temperature, and applied potential, yielding reasonable agreement with experimental data. The model also gives the correct qualitative trend in the dependence of film thickness on electrolyte pH. Although the model parameters used in our comparisons are probably not unique, they suggest that physical picture embodied in the model provides a suitable starting point for modeling the growth of passive films on Fe. © 2002 Elsevier Science Ltd. All rights reserved.

Placing a layer of Ru atop a Pt anode increases the carbon monoxide tolerance of proton-exchange membrane fuel cells when oxygen is added to the fuel stream. Sputter-deposited Ru filter anodes composed of a single Ru layer and three Ru layers separated by Nation-carbon ink, respectively, were compared to Pt, Pt:Ru alloy, and an ink-based Ru filter anodes. The amount of Pt in each anode was 0.15 mg/cm 2 and the amount of Ru in each Ru-containing anode was 0.080 mg/cm 2 . For an anode feed consisting of hydrogen, 200 ppm CO, and 2% O 2 (in the form of an air bleed), all Ru filter anodes outperformed the Pt:Ru alloy. The performance of the Pt + single-layer sputtered Ru filter was double that of the Pt:Ru alloy. (0.205 vs. 0.103 A/cm 2 at 0.6 V). The performance was also significantly greater than that of the ink-based Ru filter (0.149 A/cm 2 at 0.6 V). Within the filter region of the anode, it is likely that the decreased hydrogen kinetics of the Ru (compared to Pt) allow for more of the OH ads formed from oxygen in the fuel stream to oxidize adsorbed CO to CO 2 . © 2002 The Electrochemical Society. All rights reserved.

Typically Pt is alloyed with metals such as Ru, Sn, or Mo to provide a more CO-tolerant, high-performance proton exchange membrane fuel cell (PEMFC) anode. In this work, a layer of carbon-supported Ru is placed between the Pt catalyst and the anode flow field to form a filter. When oxygen is added to the fuel stream, it was predicted that the slow kinetics of Ru in this filter would become an advantage compared to Pt and Pt:Ru alloy anodes, allowing a greater percentage of to oxidize adsorbed CO to With an anode feed of 2% and up to 100 ppm CO, the filter anode performed better at 70°C than the Pt:Ru alloy. The oxygen in the anode feed stream was found to form a hydroxyl species within the filter. The reaction of these hydroxyl groups with adsorbed CO was the primary means of CO oxidation within the filter. Because of the resulting proton formation, the Ru filter must be placed in front of and adjacent to the Pt anode and must contain Nafion in order to provide the ionic pathways for proton conduction, and hence achieve the maximum benefit of the filter. © 2002 The Electrochemical Society. All rights reserved.

A numerical technique is developed for solving coupled electrochemical reaction-diffusion equations. Through analyzing the nonlinearity of the problem, a trial and error iterating procedure is constructed. The coefficient matrix is arranged as a tridiagonal form with elements of block matrix and is decomposed to LU form. A compact forward and backward substitution algorithm based on the shift of inversing block matrix by Gauss-Jordan full pivoting is developed. A large number of node points is required to converge the calculation. Computation experiences show that the iteration converges very quickly. The effects of inner diffusion on the electrochemical reaction are analyzed by numerical solutions. © 2002 Published by Elsevier Science B.V.

Lithium-ion batteries, ultracapacitors, and parallel combinations of these devices were characterized with respect to their ability to meet the power demands of pulsed loads. Data are presented in the form of Ragone plots that relate the impact of current amplitude and pulse duty to the specific power and energy storage capacities. Adding a 50 F ultracapacitor in parallel with the battery exhibited up to a 20.3% increase in energy capacity as compared to a continuous discharge of the battery alone. The peak current capacity of the hybrid system was limited to 10 A, to prevent exceeding the maximum safe current of 2.4 A for the battery alone. The hybrid systems also suffered less voltage droop during the pulse on time when compared to the battery alone. However, when considered on a per mass basis, the energy and power densities were lower for the hybrids than for the battery alone. © 2002 Elsevier Science B.V. All rights reserved.

The performance of a battery-ultracapacitor hybrid power source under pulsed load conditions is analytically described using simplified models. We show that peak power can be greatly enhanced, internal losses can be considerably reduced, and that discharge life of the battery is extended. Greatest benefits are seen when the load pulse rate is higher than the system eigen-frequency and when the pulse duty is small. Actual benefits are substantial; adding a 23 F ultracapacitor bank (3 × 7 PC10 ultracapacitors) in parallel with a typical Li-ion battery of 7.2 V and 1.35 A hr capacity can boost the peak power capacity by 5 times and reduce the power loss by 74%, while minimally impacting system volume and weight, for pulsed loads of 5 A, 1 Hz repetition rate, and 10% duty.

The stability of the NiO cathodes in molten carbonate fuel cell (MCFC) has been improved through microencapsulation of the NiO cathode with nanostructured Co. Cobalt was deposited on the NiO cathode using an electroless deposition process. The electrochemical oxidation behavior of the Co-coated electrodes is similar to that of the bare NiO cathode. The cobalt-coated electrodes have a lower solubility in the molten carbonate melt when compared to bare nickel oxide electrodes in the presence of cathode gas. The solubility decreased more than 50% due to microencapsulation with cobalt. The thermal oxidation rate was also lower in case of the cobalt-encapsulated electrode. Impedance data from the modified electrode indicate that the oxygen reduction reaction depended inversely on the CO2 and directly on the oxygen partial pressures respectively suggesting a similar reaction mechanism to that of nickel oxide. The results indicated that cobalt-encapsulated NiO is a viable solution in the development of alternate cathodes for MCFC applications. © 2002 Elsevier Science B.V. All rights reserved.

Sputterdeposition has been investigated as a tool for manufacturing proton-exchangemembrane fuel cell (PEMFC) electrodes with improved performance and catalystutilization vs. ink-based electrodes. Sputter-depositing a single layer of Pton the gas diffusion layer provided better performance (0.28 A/cm2at 0.6 V) than sputtering the Pt directly onto aNafion membrane (0.065 A/cm2 at 0.6 V) and equaled theperformance of the baseline for an equivalent Pt loading. Sputter-depositingalternating layers of Pt and Nafion-carbon ink (NCI) onto themembrane did not increase the performance over the baseline asmeasured in amperes per centimeter squared due to the excessivethickness of the NCI (the NCI accounted for 99.9% ofthe electrode thickness). However, three and six layer Pt/NCI membraneelectrode assemblies (MEAs) resulted in Pt activities double that ofthe 905 A/g at 0.6 V achieved by the ink-basedbaseline. Decreasing the thickness of each NCI layer increased theperformance of the six-layered Pt/NCI MEA from 0.132 to 0.170A/cm2 at 0.6 V, providing an activity of 2650 A/gat 0.6 V. It is likely that by further decreasingthe ratio of NCI to Pt in these electrodes, Ptactivity, and PEMFC electrode performance can be increased. ©2002 TheElectrochemical Society. All rights reserved.

This paper focuses on studying the performance of Li-ion cells using LiMn 2O 4 as the positive electrode material. The capacity of the cell has been optimized based on varying the charging current and the end potential. The capacity fade of these batteries has been studied at different charge currents, namely 0.1 A, 0.25 A, 0.5 A, 0.75 A and 1 A. The discharge rate for all cells was kept constant at 1 A. The lowest capacity fade is seen for the cell charged at 0.5 A indicating that this was optimum charging current for these batteries. For all charge currents, the resistance of the LiMn 2O 4 cathode remains lower than that of the carbon anode with cycling. This result is in contrast with cells made with LiCoO 2 cathode where the increase in cathode resistance with cycling causes the fade in capacity. XRD studies of carbon and LiMn 2O 4 at different cycles, reveals no structural changes. However, the lattice constants vary with cycling indicating changes in the lithium intercalated at both electrodes. Further studies are being done to understand the cause for the capacity fade.