Showing papers by "Shimshon Gottesfeld published in 1999"
TL;DR: In this paper, the authors show that the presence of recast ionomer in DMFC cathodes based on unsupported Pt as the catalyst is essential for optimum methanol fuel cell performance.
Abstract: The ionomer content in catalyst layers has a marked influence on direct methanol fuel cell (DMFC) performance. In an anode which contains unsupported PtRu as the catalyst, the recast ionomer may not always be necessary because the protonic conductivity of hydrous RuOx, the presence of which is inferred from the X‐ray diffraction pattern, may be sufficient to allow effective utilization of catalyst sites. To examine interpenetration of catalyst and membrane material as a possible explanation for the lack of an apparent need of added ionomer, ultramicrotomed thin sections of the membrane‐electrode assembly (MEA) were examined by scanning electron microscopy. Microscopic examination of such MEA cross sections revealed significant porosity in layers made by mixing unsupported catalysts with recast ionomer. Images of such sections did not reveal significant interpenetration, supporting the interpretation that hydrous RuOx may by itself provide sufficient protonic conductivity in PtRu catalyst layers prepared with no added ionomer. In contrast we show that the presence of recast ionomer in DMFC cathodes based on unsupported Pt as the catalyst is essential for optimum DMFC performance, because the recast ionomer is the primary source of protonic conductivity in the latter case. Having shown its potential function as proton conductor, we stress that Ru oxide is apparently not the key for maximizing DMFC anodic activity. © 1999 The Electrochemical Society. All rights reserved.
139 citations
TL;DR: In this article, the authors used thermogravimetric and powder x-ray diffraction analysis on C60 · H18.7 to study the dehydrogenation of C60.7.
Abstract: The dehydrogenation of C60 · H18.7 was studied using thermogravimetric and powder x-ray diffraction analysis. C60 · H18.7 was found to be stable up to 430 °C in Ar at which point the release of hydrogen initiated the collapse of a fraction of fullerene molecules. X-ray diffraction analysis performed on C60 · H18.7 samples dehydrogenated at 454, 475, and 600 °C displayed an increasing volume fraction of amorphous material. The decomposition product comprises randomly oriented, single-layer graphite sheets. Evolved gas analysis using gas chromatograph (GC) mass spectroscopy confirmed the presence of both H2 and methane upon dehydrogenation. Attempts to improve reversibility or reduce hydrogenation/ dehydrogenation temperatures by addition of Ru and Pt catalysts were unsuccessful.
19 citations
TL;DR: Moore et al. as mentioned in this paper proposed a control strategy that simultaneously optimizes DMFC fuel conversion efficiency versus power level, leads into a system level optimization of efficiency vs. power, and provides an operational strategy for controlling a direct-methanol fuel cell for maximum fuel efficiency from minimum to maximum power demand.
Abstract: Author(s): Moore, Robert; Gottesfeld, Shimshon; Zelenay, P | Abstract: For automotive applications, it is necessary to maximize the fuel conversion efficiency of a PEM direct-methanol fuel cell (DMFC) over the broadest possible dynamic range of power. The research reported here critically examines the efficiency of the DMFC stack when operated over a broad power range. This research establishes a basis for a control strategy that simultaneously: optimizes DMFC fuel conversion efficiency versus power level, leads into a system level optimization of efficiency vs. power, and provides an operational strategy for controlling a direct-methanol fuel cell for maximum fuel efficiency from minimum to maximum power demand.First, there is an explanation of the experimental conditions used to obtain the DMFC experimental data that is reported and analyzed. Next the DMFC methanol crossover phenomenon is discussed and characterized. Then the conceptual framework for the optimization of fuel conversion efficiency is presented. Finally, the optimized fuel conversion efficiency is viewed in terms of the conventional voltage efficiency and fuel utilization parameters traditionally used for direct-hydrogen and reformate fuel cells.The primary conclusion of the research is that, at a given DMFC fuel consumption rate, the DMFC power density and fuel conversion efficiency is maximized by simultaneously controlling both the concentration and flow rate of the methanol fuel. This yields an optimized efficiency curve (vs. power level of DMFC operation). An additional optimization of the air flow and pressure conditions is clearly also possible, but is not explicitly developed as part of the research reported in this paper.A key feature of the optimized fuel efficiency curve is its relative flatness versus power density (e.g., greater than 30% efficiency over a range from about 70 to 230 mW cm2). The major operational result is that it is conceptually possible to optimize the conversion efficiency of a DMFC power system by manipulating the methanol fuel feed stream as a function of the system power demand. Practical application of such a strategy, of course requires a variable concentration, variable flow methanol fuel control technology.
6 citations
TL;DR: Moore et al. as mentioned in this paper compared the fuel conversion efficiency and power density of a direct-methanol fuel cell (DMFC) with the equivalent parameters of a state-of-the-art direct-hydrogen fuel cells (DHFC), and concluded that a DMFC-powered vehicle can become directly competitive with a DHFCpowered ZEV in any localities or market niches where ZEVs are "a condition of doing business" as a vehicle manufacturer.
Abstract: Author(s): Moore, Robert; Gottesfeld, Shimshon; Zelenay, P | Abstract: For an automotive application of a fuel cell power system, it is important to maximize the fuel conversion efficiency, while also providing the required peak power levels for vehicle performance. This paper first compares the fuel conversion efficiency and power density of a state-of-the-art direct-methanol fuel cell (DMFC) with the equivalent parameters of a state-of-the-art direct-hydrogen fuel cell (DHFC). The cell level comparison is then extended to the system level for a potential ZEV automotive application. It is concluded that a DMFC-powered vehicle can become directly competitive with a DHFC-powered ZEV (Zero Emission Vehicle) in any localities or market niches where ZEVs are "a condition of doing business" as a vehicle manufacturer.Following a brief outline of the experimental conditions used to generate the DMFC data reported and analyzed in this paper, a technique for optimizing the conversion efficiency of a DMFC is briefly reviewed. The technique is then applied to the DMFC state-of-the-art data, and compared with the efficiency and power density of the DHFC state-of-the-art. Next a "system" level comparison is introduced that captures the major differences between the DHFC and DMFC systems for automotive applications. This system level comparison is then used to evaluate the attributes of a DMFC-powered vehicle against the "benchmark" of a DHFC-powered vehicle.Overall, the conclusion is that a DMFC powered FCV could meet the requirements for a general-purpose ZEV, and could be an effective competitor to a DHFC-based FCV. It would have an range of 350 miles, and provide the same class of performance (acceleration) as a lightweight purpose-built hydrogen-fueled FCV, if two key criteria were met:- DMFC cell/stack power density of 0.35 kW/L,- DMFC stack conversion efficiency over the required dynamic power range of 50% of the equivalent DHFC stack efficiency.In fact, these criteria are marginally met for the present state-of-the-art in laboratory DMFC performance. However, for implementation of a DMFC power system in an FCV, the development of a commercial DMFC stack technology and fuel control system is also required, and a number of RaD advances are needed before the DMFC cell, stack and system can fully realize its great promise. Even so, because of this promise there are significant worldwide RaD efforts underway to address the technical and commercialization barriers for direct-methanol fuel cell vehicles.
4 citations