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Author: Elliot Padgett
Publisher:
ISBN:
Category :
Languages : en
Pages : 225
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Book Description
Hydrogen fuel cells in fuel cell electric vehicles (FCEVs) are a promising technology to reduce, and eventually eliminate, carbon dioxide emissions from transportation. The Pt nanoparticles used to catalyze the fuel cell's electrochemical reactions are an important limiting factor because at present levels, the cost of the Pt catalyst will prevent widespread adoption of FCEVs. Catalysts must be developed to reduce the amount of Pt while meeting vehicle power demands even after many years of use. Strategically improving catalysts requires detailed and statistically robust characterization of their microscopic structure to understand the connections between catalyst synthesis, structure, performance, and durability. This dissertation presents the development and application of (scanning) transmission electron microscopy ((S)TEM) techniques to guide advancement of catalysts through nanostructural characterization. We develop a robust strain mapping technique for complex catalyst specimens. We deploy a new exit wave power cepstrum (EWPC) transform to nanobeam electron diffraction (NBED) patterns to enable precise, high-throughput, dose-efficient strain measurement. This approach is suitable for statistically representative measurements of many particles without special requirements such as zone-axis orientation. We apply this strain mapping technique to core-shell Pt-Co nanoparticles in combination with a continuum elastic theory model and demonstrate two mechanisms contributing to the relaxation of strain at the catalyst surface: lattice dislocations and Poisson expansion due to the spherical geometry. Comparison with electrochemical measurements suggests that the geometrical Poisson relaxation accounts for the activity of catalysts with thin shells, but catalysts with thick shells experience additional activity loss from dislocation-driven relaxation. We then turn to the larger-scale catalyst structure, investigating the impact of porous carbon support morphology, local reactant transport, and catalyst durability. Using statistical analysis of STEM images, we compare Pt and Pt-Co catalysts on porous and solid carbon supports. Comparison of 3D tomographic images and electrochemical accessibility measurements indicated that carbon pores prevent ionomer adsorption for particles embedded within them, improving the catalyst activity, while allowing proton access through condensed water. By comparing images and composition maps before and after electrochemical stability tests, we find that porous carbon supports suppress Pt particle coalescence, accounting for improved overall durability.
Author: Elliot Padgett
Publisher:
ISBN:
Category :
Languages : en
Pages : 225
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Book Description
Hydrogen fuel cells in fuel cell electric vehicles (FCEVs) are a promising technology to reduce, and eventually eliminate, carbon dioxide emissions from transportation. The Pt nanoparticles used to catalyze the fuel cell's electrochemical reactions are an important limiting factor because at present levels, the cost of the Pt catalyst will prevent widespread adoption of FCEVs. Catalysts must be developed to reduce the amount of Pt while meeting vehicle power demands even after many years of use. Strategically improving catalysts requires detailed and statistically robust characterization of their microscopic structure to understand the connections between catalyst synthesis, structure, performance, and durability. This dissertation presents the development and application of (scanning) transmission electron microscopy ((S)TEM) techniques to guide advancement of catalysts through nanostructural characterization. We develop a robust strain mapping technique for complex catalyst specimens. We deploy a new exit wave power cepstrum (EWPC) transform to nanobeam electron diffraction (NBED) patterns to enable precise, high-throughput, dose-efficient strain measurement. This approach is suitable for statistically representative measurements of many particles without special requirements such as zone-axis orientation. We apply this strain mapping technique to core-shell Pt-Co nanoparticles in combination with a continuum elastic theory model and demonstrate two mechanisms contributing to the relaxation of strain at the catalyst surface: lattice dislocations and Poisson expansion due to the spherical geometry. Comparison with electrochemical measurements suggests that the geometrical Poisson relaxation accounts for the activity of catalysts with thin shells, but catalysts with thick shells experience additional activity loss from dislocation-driven relaxation. We then turn to the larger-scale catalyst structure, investigating the impact of porous carbon support morphology, local reactant transport, and catalyst durability. Using statistical analysis of STEM images, we compare Pt and Pt-Co catalysts on porous and solid carbon supports. Comparison of 3D tomographic images and electrochemical accessibility measurements indicated that carbon pores prevent ionomer adsorption for particles embedded within them, improving the catalyst activity, while allowing proton access through condensed water. By comparing images and composition maps before and after electrochemical stability tests, we find that porous carbon supports suppress Pt particle coalescence, accounting for improved overall durability.
Author: Martin Gocyla
Publisher:
ISBN:
Category : Fuel
Languages : en
Pages :
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Book Description
Author: Andrzej Wieckowski
Publisher: John Wiley & Sons
ISBN: 1118063112
Category : Technology & Engineering
Languages : en
Pages : 652
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Book Description
A comprehensive survey of theoretical andexperimental concepts in fuel cell chemistry Fuel cell science is undergoing significant development, thanks, in part, to a spectacular evolution of the electrocatalysis concepts, and both new theoretical and experimental methods. Responding to the need for a definitive guide to the field, Fuel Cell Science provides an up-to-date, comprehensive compendium of both theoretical and experimental aspects of the field. Designed to inspire scientists to think about the future of fuel cell technology, Fuel Cell Science addresses the emerging field of bio-electrocatalysis and the theory of heterogeneous reactions in fuel cell science and proposes potential applications for electrochemical energy production. The book is thorough in its coverage of the electron transfer process and structure of the electric double layer, as well as the development of operando measurements. Among other subjects, chapters describe: Recently developed strategies for the design, preparation, and characterization of catalytic materials for fuel cell electrodes, especially for new fuel cell cathodes A wide spectrum of theoretical and computational methods, with?the aim of?developing?new fuel cell catalysis concepts and improving existing designs to increase their performance.? Edited by two leading faculty, the book: Addresses the emerging fields of bio-electrocatalysis for fuel cells and theory of heterogeneous reactions for use in fuel cell catalysis Provides a survey of experimental and theoretical concepts in these new fields Shows the evolution of electrocatalysis concepts Describes the chemical physics of fuel cell reactions Forecasts future developments in electrochemical energy production and conversion Written for electrochemists and electrochemistry graduate students, electrocatalysis researchers, surface and physical chemists, chemical engineers, automotive engineers, and fuel cell and energy-related researchers, this modern compendium can help today's best minds meet the challenges in fuel science technology.
Author: Ariel Jackson
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
Reliance on fossil fuels as society's primary energy source has detrimental effects on climate, air quality and public health, economic competitiveness, and geo-political stability. A rapid transition to renewable energy is required and hydrogen fuel cells offer a promising pathway, particularly in the transportation sector. Despite significant progress over the past two decades, large scale commercialization of fuel cell automobiles has not been realized. Several companies have been leasing prototypes and claim that production models will go on sale for the first time in 2015, however the scarcity and cost of platinum--required to catalyze the electrochemical reactions in the fuel cell--remains the primary impediment to full implementation of fuel cell powered vehicles. Most of the Pt in a fuel cell is used on the oxygen electrode (cathode) to catalyze the sluggish oxygen reduction reaction (ORR). The primary pathway to reducing the Pt loading is to develop catalysts that are more active than Pt. In this dissertation, I will focus on the development of a new type of ORR electrocatalyst, ruthenium-core platinum-shell (Ru@Pt) nanoparticles. Theoretical understanding of the ORR mechanisms has improved dramatically in the last decade, demonstrating that the key parameter for catalytic activity is the binding strength of oxygen to the catalyst surface. In a theory-experiment collaboration, density functional theory (DFT) calculations showed that the oxygen binding strength to a Ru@Pt surface was more optimal (slightly weaker) than pure Pt. Using the DFT calculations to guide the catalyst design, we prepared Ru@Pt nanoparticles using a liquid phase synthesis. We confirmed that the nanoparticles have the intended Ru-core Pt-shell structure using a combination of transmission electron microscopy (TEM), scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS), and Z-contrast annular dark field-scanning transmission electron microscopy (ADF-STEM). The activity of the catalysts was tested using rotating disk electrochemistry, and a greater than two fold improvement was exhibited in the specific (per reaction-site) activity of Ru@Pt over state-of-the-art commercial Pt nanoparticles. We devised a new electrochemical conditioning treatment, tailored to the Ru@Pt catalyst, which involves cycling the nanoparticles between highly oxidizing and reducing potentials. The conditioning further improved the activity of Ru@Pt by a factor of two. While unprotected Ru nanoparticles are unstable at the oxidative potentials encountered in the conditioning treatment, analysis with STEM-EDS shows that the Pt-shell protects the Ru-core, mitigating Ru dissolution. Optimization of the Ru@Pt nanoparticle structure led to a seven fold enhancement in mass activity (activity per gram of Pt) over the first generation Ru@Pt catalysts. The effect of Pt content in the synthesis was investigated and the particle size, surface area, and activity were found to vary with Pt composition, with the mass activity maximized at a Pt:Ru ratio equal to one. Optimized Ru@Pt exhibited a mass activity of 0.497 A mg-1Pt at 0.9 V vs. RHE, exceeding the Department of Energy 2020 target. The Ru@Pt catalyst was tested for durability and retained 85% of its starting mass activity after 30,000 stability cycles, compared to commercial Pt nanoparticles which had a lower initial mass activity and only retained 62%. The newly developed Ru@Pt catalysts demonstrate impressive activity and stability and are a promising platform for reducing Pt use in fuel cells.
Author: Zishuai Zhang
Publisher:
ISBN:
Category :
Languages : en
Pages :
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Book Description
"Electrocatalysts play an important avenue in clean and efficient energy conversion. Of the many electrocatalytic processes, the oxygen reduction reaction (ORR) attracts increasing attention due to its widespread importance in electrochemical cells. One of the most important applications is in proton-exchange membrane fuel cells (PEMFCs), which are considered as a promising power generation system because of its low operating temperature (70-90 ̊C), sustainable energy sources (hydrogen) and high energy efficiency. ORR is a key half reaction that takes place at the cathode with sluggish kinetics and requires noble metal (e.g. platinum)-based electrocatalysts to increase the reaction rate to attain practically usable levels. High cost and poor durability are major drawbacks of the commercial platinum on carbon (Pt/C) catalyst, and the corrosion of the carbon supports is considered one of the main reasons for the loss of expensive Pt, resulting in loss of performance of PEMFC. Therefore, more corrosion resistant, electrochemically stable and low-cost supports are highly desired for PEMFCs’ improved performance.Methanol oxidation reaction (MOR) is an anodic half reaction occurs at anodes of methanol fuel cells. Pt/C catalyst is also commonly used for that reaction. This thesis attempts to rationally design and synthesize various different nanostructured alternatives to carbon black for ORR and MOR. Graphene, a high conductive and stable two-dimensional carbon support, was successfully exfoliated electrochemically with little defects. Platinum (Pt) nanoparticles were deposited on graphene via double-pulse deposition technique. The catalyst was demonstrated to be highly efficient for MOR with a 920 mA/mg forward current density.Durable carbon nanotube (CNT) microspheres were synthesized through a facile and scalable ultrasonic bonding method without any binder or surfactant. The CNT microspheres with electrodeposited Pt were showcased as efficient ORR catalyst supports which showed no degradation after 12, 000 cycles (26.6 h). Furthermore, a soluble acicular calcium carbonate (aragonite, diameter 100 nm; length 800 nm) was used to created connected porosity in the microspheres to improve the mass transfer as the thickness increases. As for the ORR catalysis performance, the Pt decorated microspheres with macropores was 3.4 times higher (specific activity at 0.9V vs RHE) than non-macroporous microspheres with the identical Pt loading.Besides carbon-based supports, TiC was investigated as a potential carbon alternative due to its metallic electrical conductivity and excellent corrosion resistance. A cobalt oxide shell with high ORR activity was deposited onto TiC to improve its stability at high potential. We demonstrated that the oxide anchored Pt on TiC catalysts exhibited excellent durability (~100% catalytic activity remained at 0.1M KOH, and ~92% catalytic activity remained at 0.1M HClO4 after 16.7 h) compared to the Pt/C (~50% remained in both alkaline and acidic solutions). As assessed by transmission electron microscope (TEM), no significant Pt detachment or agglomeration was observed in oxide anchored catalysts, while heavily agglomeration has occurred to Pt/C.Hematene, two-dimensional layer of hematite (Fe2O3), has recently been exfoliated by means of liquid exfoliation. As the biodegradable metal, Fe-based materials attracts lots of attentions due to their ability to be entirely dissolved and cleared from the body. Electronics comprised of biodegradable metals can be programmed to degrade after the implantation. Here, we fabricated potentially biodegradable electrodes by using Au and hematene for glucose oxidation. It showed 9.5 mA/mgAu oxidation current density at the potential of 0.6V (vs. RHE), and high stability during the continuous cell cycling. Additionally, the prepared catalyst exhibited short response time and linear calibration range"--
Author: Haoran Yu
Publisher:
ISBN:
Category : Electronic dissertations
Languages : en
Pages :
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Book Description
The optimization of low catalyst loading electrode fabricated with reactive spray deposition technology (RSDT) is conducted for proton exchange membrane fuel cell (PEMFC) and proton exchange membrane water electrolyzer (PEMWE) applications. For PEMFC, the key catalyst layer parameters studied in this work include: ionomer-to-carbon (I/C) ratio, platinum particle size, and platinum loading on carbon. The RSDT-derived catalyst layers were shown to exhibit better ionomer coverage on the carbon support than traditional fabrication method, reducing the amount of ionomer needed for optimal fuel cell performance. Analysis on the fuel cell polarization sources for RSDT-derived catalyst layer was performed to further elucidate the influence of I/C ratio on the oxygen reduction reaction (ORR) activity and oxygen transport. The effect of platinum particle size and platinum loading on carbon were studied by employing two types of gradient cathode to mitigate platinum dissolution and migration to the electrolyte membrane. One type of the gradient cathode was made with platinum particle size of 5 nm near the membrane and platinum particle size of 2 nm toward the GDL. The platinum loading on carbon was kept constant at 40 wt% throughout the cathode. The other type of gradient cathode kept the platinum particle size of 2 nm throughout the cathode but used higher platinum loading on carbon (60 wt%) near the membrane and 40 wt% platinum toward the GDL. Accelerated stress test was performed using triangular wave potential cycling from 0.6V to 1.0V at 50 mV s-1for 30,000 cycles. The degradation mechanism was investigated using cross-sectional transmission electron microscopy (TEM). Durable anode catalyst layer for PEMWE was developed with an IrOx/Nafion composite thin film with the key catalyst layer parameters being the surface oxidation state of iridium and the homogeneity of the catalyst layer. The oxide rich iridium showed superior oxygen evolution reaction (OER) activity and higher catalyst stability in electrolyzer test. In addition, the catalyst layer homogeneity played an important role in the catalyst stability. The optimized electrolyzer durability achieved ~1400 hours with ~0.08 mg cm-2 iridium loading, more than 90% reduction compare to the commercial catalyst.
Author: Jaroslava Lavková
Publisher:
ISBN:
Category :
Languages : en
Pages : 298
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Book Description
Present doctoral thesis is focused on investigation of novel metal-oxide anode catalyst for fuel cell application by electron microscopy and associated spectroscopies. Catalysts based on Pt-doped cerium oxide in form of thin layers prepared by simultaneous magnetron sputtering deposition on intermediate carbonaceous films grown on silicon substrate have been studied. The influence of the catalyst support composition (a-C and CNx films), deposition time of CeOx layer and other deposition parameters, as deposition rate, composition of working atmosphere and Pt concentration on the morphology of Pt-CeOx layers has been investigated mainly by Transmission Electron Microscopy (TEM). The obtained results have shown that by combination of suitable preparation conditions we are able to tune the final morphology and composition of the catalysts. The composition of carbonaceous films and Pt-CeOx layers was examined by complementary spectroscopy techniques - Energy Dispersive X-ray Spectroscopy (EDX), Electron Energy Loss Spectroscopy (EELS) and X-ray Photoelectron Spectroscopy (XPS). Such prepared porous structures of Pt-CeOx are promising as anode catalytic material for real fuel cell application.
Author: Paul H. Matter
Publisher:
ISBN:
Category : Catalysts
Languages : en
Pages : 333
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Book Description
Abstract: In the field of catalysis, the development of alternative catalysts for the oxygen reduction reaction (ORR) in Polymer Electrolyte Membrane Fuel Cell (PEMFC) cathodes has been an ongoing task for researchers over the past two decades. Currently, the high expense and low availability of platinum will hinder the large-scale commercialization of PEM fuel cells. The most hopeful advances being made in replacing platinum are related to pyrolyzed organic macrocycles with transition metal centers (such as Fe or Co porphyrins and phthalocyanines). Encouragingly, it has recently been discovered that active electrodes could be prepared by heat-treating metal and nitrogen precursors (not necessarily organic macrocycles) together in the presence of a carbon support. In the first study of this dissertation, catalysts for the Oxygen Reduction Reaction (ORR) were prepared by the pyrolysis of acetonitrile over various supports. The supports used included Vulcan Carbon, high purity alumina, silica, magnesia, and these same supports impregnated with Fe, Co, or Ni in the form of acetate salt. The catalysts were characterized by N2 physisorption, conductivity testing, Transmission Electron Microscopy (TEM), Temperature Programmed Oxidation (TPO), Thermo-Gravimetric Analysis (TGA), X-Ray Diffraction (XRD), X-ray Photo-electron Spectroscopy (XPS), Mössbauer Spectroscopy, Rotating Ring-Disk Electrode (RRDE) half cell testing, and full PEMFC testing. The most active catalysts were formed when Fe was added to the support before the pyrolysis; however, samples in which no metal was added still showed elevated activity for oxygen reduction. Within a support family, the more active catalysts had a higher amount of pyridinic nitrogen, as determined from XPS. A theory has been proposed to explain this trend based on the formation of different nanostructures depending on which support material is used for the acetonitrile decomposition. According to this theory, nitrogen-containing carbon samples with nanostructures that result in more edge planes being exposed (the plane in which pyridinic nitrogen is found) will be more active for the ORR. In volume II of this dissertation, Cu-based catalysts for hydrogen production from methanol and water were studied. These catalysts have applications for mobile fuel cells that rely on hydrogen production from easier to store liquid fuels, such as methanol.
Author: R. T. K. Baker
Publisher:
ISBN:
Category : Science
Languages : en
Pages : 258
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Book Description
Author: European Commission. Directorate General for Research
Publisher:
ISBN:
Category : Fuel cells
Languages : en
Pages : 38
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Book Description
