Énergie chimique | EPFL Graph Search

L'énergie chimique est le potentiel pour une substance chimique à subir une réaction chimique pour se transformer en d'autres substances. Les piles, les aliments et les combustibles fossiles sont des exemples de supports de stockage de l'énergie chimique. La rupture ou la création de liaisons chimiques implique de l'énergie, qui peut être absorbée ou dégagée par un système chimique (sous forme de chaleur ou d'énergie électrique par exemple). L'énergie chimique constitue le principal moyen de stockage d'énergie chez les mammifères grâce à l'ATP, cette substance étant ensuite transformée en énergie mécanique à l'intérieur des muscles. L'énergie chimique est aussi un des moyens connus pour stocker de l'énergie électrique, grâce notamment aux batteries. Il est à noter que la production d'énergie électrique française est pour une part non négligeable issue de l'énergie chimique (centrales thermiques au fuel, au charbon ou au biogaz). En effet elle constitue 7,5 % de l'énergie électrique pr

Analysis of Fluid-Fluid Reaction Systems with Reactions in Both Phases

Adrien Calabria

Developing mathematical models for chemical reaction systems is essential for analysis, development, design, optimization and control of chemical, or biochemical industrial processes. Nowadays, more and more companies across the world have to deal with innovative concepts such as the Sustainable Chemistry (or "Green Chemistry"), which particularly require good knowledge of chemical reactions systems. Well-defined chemical operations involved in a process lead to its efficiency, energy saving and an improvement of the product quality. Furthermore, plants adopting this new philosophy minimize their byproduct production and pollutant formation. This master thesis works on developing a methodology for analysis and kinetic modeling of chemical reaction systems. In this present study, the case where chemical reactions take place in both phases of a heterogeneous biphasic fluid-fluid (F-F) reactor under isothermal conditions is considered. In order to model homogeneous or heterogeneous (F-F) chemical systems, a new approach called “Extent-based Incremental Identification” has recently been developed by the Laboratoire d’Automatique (EPFL). In contrast to the commonly used simultaneous approach, this extent-based incremental approach can compute parameters corresponding to reaction and mass transfer rates individually for each reaction and mass transfer laws, and also does not require prior postulation or knowledge of rate laws. This particular case is an extension of the dissertation done by Nirav Bhatt [1], which considers a heterogeneous chemical reaction system with reactions in one of the phases with steady state mass-transfer between the phases, and work done by Michael Amrhein [2], who developed a linear transformation that computed the extents of reaction from the numbers of moles in homogeneous reaction systems, with inlet and outlet streams. [1] Nirav Bhatt, EPFL Diss. n° 5028 (2011) [2] Amrhein et al., AIChE J. 56 (2010), 2873-2886

2013

The design and synthesis of nanostructured hydrogen oxidation reaction electrocatalysts for hydroxide exchange membrane fuel cells

Weiyan Ni

Hydrogen fuel cell is a promising green technology that transforms the chemical energy in hydrogen into electricity. Currently, most of the efforts are focused on developing proton exchange membrane fuel cells (PEMFCs), however they are heavily dependent on rare and expensive platinum group metal (PGM) catalysts. In contrast, hydroxide exchange membrane fuel cells (HEMFCs) work in alkaline media, thus allow the use of cheaper and more abundant non-PGM catalysts. However, there is no materials found suitable to catalyze the anodic hydrogen oxidation reaction (HOR). Therefore, developing new, active and robust HOR catalysts in alkaline media is of paramount importance. Hereby, this dissertation presented my recent research works on preparation, characterization, and application of HOR catalysts for HEMFCs.Chapter 2 described the exploration of new earth-abundant HOR catalysts. A series of Ni-based compounds were synthesized and tested for HOR, among which Ni3N had the best activity. With further nano-engineering, the mass activity reached the highest at the time reported and the oxidation resistance of the supported version can be significantly improved. Spectroscopic evidence suggested that the downshift of the metal d band weakened the binding energies of the catalyst toward adsorbates, which accounted for the improved activity and stability of Ni3N/C.Chapter 3 demonstrated how to exploit strain-engineering for HOR catalysis. A series of Ni/C composites were prepared by pyrolyzing a Ni-based metal-organic framework (MOF). The optimal catalyst, Ni-H2-2%, has the highest mass activity (at the time reported) and abnormal high intrinsic activity. Characterizations using synchrotron XRD suggested that it was the distinct but relative optimal strain level of Ni-H2-2% that rendered it high activity.Chapter 4 is a complicated version of Chapter 3. The Ni catalysts were also prepared from the same Ni-based MOF with more complex synthesis conditions. The best catalyst, Ni-H2-NH3, possessed highest intrinsic activity among non-precious metals. Combining UPS, H2 chemisorption, isotopic studies, and electrochemical characterizations, the balanced hydrogen binding energy (HBE) and hydroxide binding energy (OHBE) is likely to be the origin of the high activity of Ni-H2-NH3. In the end, MEA measurements confirmed its excellent performance: for non-PGM MEA, the peak power density could reach 488 mW/cm2, 6.4 times higher than the previous record, which demonstrates the feasibility of efficient PGM-free HEMFCs.Chapter 5 exploited the interaction between PGMs and carbon support to improve their HOR activity. A porous N-doped carbon (pN-C) support was found to have prominent enhancement effect toward PGMs. Especially, the Pt0.25Ru0.75/pN-C had the up-to-date highest mass activity and intrinsic activity toward HOR. Using characterizations including XPS, UPS, H2 chemisorption, CO chemisorption, in situ surface-enhanced infrared adsorption spectroscopy (SEIRAS), and electrochemical characterizations, we concluded the superior HOR activity of Pt0.25Ru0.75/pN-C was a combination of interactions between Pt, Ru and the pN-C, with modulated the HBE and strengthened the H2O binding strength. The HEMFC using Pt0.25Ru0.75/pN-C also possessed great performance that far exceeded the U.S. Department of Energy (DOE) 2022 target for HEMFCs, even though the testing condition was harsher. This work demonstrated the feasibility of HEMFC replacing PEMFC.

EPFL2021

Molecular Engineering of Electrode Surfaces for Electrocatalysis

Patrick Eduard Alexa

Electrocatalysts play an important part on the route towards environmental friendly energy sources. They support modern technologies like fuel cells to move further away from the utilization of fossil fuels and the resulting CO2 emission into our atmosphere. The main principle of such energy conversion technologies is to store chemical energy within chemical bonds and to release it as electric energy to fulfill the energy demand of today's society. However energy conversion chemistry in such technologies requires catalysts in order to be effective by providing an alternative reaction pathway that lowers activation energies and maximizes the energy output. Providing suitable catalyst surfaces requires a fundamental knowledge of the interaction of reactants and reaction intermediates with the catalyst surfaces. This concern is addressed in this thesis by fabricating specifically tailored molecular components on metal surfaces, which act as organic/inorganic hybrid electrodes for various energy conversion reactions in electrocatalysis. Characterizing the electrode surface is crucial in order to identify active sites of the catalyst. The molecular structure is investigated by surface sensitive techniques, such as STM and XPS in combination with a home-build transfer system, which enables electrode fabrication in UHV and electrocatalytic experiments in a liquid environment. A detailed characterization of organic polymer components on electrode surfaces is obtained by combining experimental data with MD simulations. The morphology of polymer networks can be successfully mimicked and depending on the chemical composition and the structure, spatial correlations on short length scales are reported. Hybrid electrodes with organic polymer co-catalysts reveal an increased catalytic activity for the HER by providing suitable docking sites that act as catalytic active sites. Reactants and reaction intermediates are stabilized via hydrogen bonding, which results in a higher catalytic turnover. The specific designed structure of the organic co-catalyst helps to identify a fraction of the reaction mechanism and highlights the importance of molecular components in electrocatalysis. The utilization of organic units on electrocatalysts provides also an excellent opportunity in order to use single metal atoms as catalysts. Engineering large pi-systems has no significant impact on the ORR activity, while the distance between active sites and the underlying electrode are crucial for enhancing the catalytic activity. A significant factor in using molecular components in electrocatalysis is the stability of the organics. Using electrografting allows to design organic/inorganic hybrid electrodes with covalent bonds between the molecular component and the metal substrate. Attaching empty porphyrin molecules to substrates introduces a powerful technique in molecular engineering, since empty porphyrin cycles can be metallized with various metal centers that can act as electrocatalysts for different electrochemical reactions. In summary, this thesis provides multiple engineering aspects, which are useful in designing molecular components on metal electrodes for energy conversion. Such hybrid catalyst surfaces are capable of improving the catalytic activity and allow to study the mechanism of catalytic reactions. Implementing organic components provides new routes for tailoring novel materials, which deliver new insight into the complex field of electrocatalysis.

EPFL2020