Specific energy

Summary

Specific energy or massic energy is energy per unit mass. It is also sometimes called gravimetric energy density, which is not to be confused with energy density, which is defined as energy per unit volume. It is used to quantify, for example, stored heat and other thermodynamic properties of substances such as specific internal energy, specific enthalpy, specific Gibbs free energy, and specific Helmholtz free energy. It may also be used for the kinetic energy or potential energy of a body. Specific energy is an intensive property, whereas energy and mass are extensive properties. The SI unit for specific energy is the joule per kilogram (J/kg). Other units still in use in some contexts are the kilocalorie per gram (Cal/g or kcal/g), mostly in food-related topics, watt hours per kilogram in the field of batteries, and the Imperial unit BTU per pound (Btu/lb), in some engineering and applied technical fields. The concept of specific energy is related to but distinct from the notion

Official source

https://en.wikipedia.org/wiki/Specific_energy

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Ultracapacitors, also called electrochemical double layer capacitors (EDLC) or supercapacitors, may improve the performance of conventional electrolytic capacitors in terms of specific energy, or improve the performance of rechargeable batteries in terms of specific power when combined with respective device. However, the application of ultracapacitors faces the problem of ageing – the deterioration of capacity and increase of equivalent series resistance of the system. This thesis intends to elucidate ageing mechanisms of ultracapacitors based on activated carbon electrodes with tetrafluoroborate in acetonitrile as electrolyte. Multi-method experimental investigations are presented which allow to obtain a comprehensive picture of ageing. Ultracapacitors were electrochemically aged under various conditions by applying voltages and temperatures close to the actual operation conditions. The influences of ageing on the electrodes were investigated by structural and chemical characterization. Gas volumetric measurement (porosimetry) and Raman spectroscopy were applied to follow the structural changes; elemental analysis, electron spectroscopy for chemical analysis (ESCA, XPS) and attenuated total reflection infrared spectroscopy (ATR-IR) were applied for chemical composition analysis. It was found that the changes of both structure and chemical composition, which occur on aged anodes and cathodes, are asymmetric. Aged anodes showed more significant changes such as stronger decrease of pore volume and specific area, and more complex chemical species. Chemically bound nitrogen species were found only on aged anodes, but not on fresh electrodes and aged cathodes. Based on the experimental results, several mechanisms for ageing are suggested. The functionalities in activated carbons play an important role for ageing. Various oxygen-containing functional groups can take part in the reactions. A higher amount of functionalities means that more electroactive sites can be found on the graphene sheets in activated carbons to promote electrochemical reactions. Thus one can assume that less functional groups in activate carbons would be preferable for the stability of ultracapacitors. Ageing tests were carried out on individual components of the electrodes, mainly on activated carbon powders and conductive carbon additives. Observations are consistent with those on assembled electrodes. It turned out that the activated carbons from synthetic resin precursors, which have fewer oxygen functional groups, suffer less from ageing than those from natural precursors with more oxygen functional groups. A similar phenomenon was also observed on the conductive agents.

EPFL2006

Catalytic Upgrading of Biomass-Derived Carboxylic Acids to Fuels and Chemicals

Jher Hau Yeap

Our reliance on fossil fuels, which is unsustainable due to dwindling reserves, has led to various environmental issues. Lignocellulosic biomass could serve as a renewable source of carbon and energy for the production of fuels and chemicals. While several valorization routes of biomass-derived sugars (via furans or sugar alcohols) exist, the upgrading of the biological carboxylate platform is relatively unexplored, mainly due to historical focus on ethanol production via fermentation. Research on biomass-derived carboxylic acids could increase our knowledge on carboxylic acid upgrading and open up new avenues towards sustainable production of fuels and chemicals from lignocellulosic biomass. The production of olefins from carboxylic acids via tandem hydrogenation/dehydration was carried out, with olefin selectivities > 90% at close to 99% conversion of carboxylic acids using a Cu/SiO2-Al2O3 catalyst. Interestingly, a sudden selectivity switch from olefins to predominantly alkanes was observed at full conversion. Using various intermediate products and catalyst surface studies, this selectivity switch phenomenon was ascertained to originate from the adsorption of small amounts of carboxylic acid on the catalyst surface, which prevented the binding and subsequent overhydrogenation of olefins. This finding suggests that carboxylic acids has potential to be used in influencing product distributions of other catalytic reactions, especially if selective double bond preservation is required. This phenomena could also be used to explain the difference in catalyst performance observed when using commercial model compounds versus real biomass-derived feed, given the prevalence of carboxylic acid impurities in biomass-derived streams. This route represents a single-step upgrading of carboxylic acids to olefins with no loss in carbon efficiency and without use of expensive stoichiometric reagents, which is important for economical production of bulk chemicals from lignocellulosic biomass. In parallel, a study on carboxylic acid upgrading via single-step ketonization/cascade aldol condensation to an aviation fuel additive was performed over Cu/ZrO2. Biomass-derived acetic acid and butanoic acid was converted to an organic oil composed of aromatics and cycloalkenes with a carbon number range of C8 - C16 at mass yields of around 20 wt %. By-products include gaseous hydrocarbons (mainly propylene) and water resulting from condensation. This organic oil represents up to 96% deoxygenation of the starting carboxylic acids, and has been tested to be compatible as a 10 vol % blend with Jet A-1 fuel in terms of specific energy and distillation properties. While previous research on ketonization and aldol condensation have performed the reactions separately, this route combines both in a single step to upgrade short carboxylic acids to long chain hydrocarbons, while valorizing the often neglected acetate fraction of biomass. This study shows a combination of C-C coupling and extensive deoxygenation, both crucial towards successful upgrading of lignocellulosic biomass to fuels.

EPFL2019

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Designing solid electrolytes for all-solid-state-batteries that can withstand the extreme electrochemical conditions in contact with an alkali metal anode and a high-voltage cathode is challenging, especially when the battery is cycled beyond 4 V. Here we demonstrate that a hydroborate solid electrolyte Na-4(CB11H12)(2)(B12H12), built from two types of cage-like anions with different oxidative stability, can effectively passivate the interface to a 4 V-class cathode and prevent impedance growth during cycling. We show that B12H12 anions decompose below 4.2 V vs. Na+/Na to form a passivating interphase layer, while CB11H12 anions remain intact, providing sufficient ionic conductivity across the layer. Our interface engineering strategy enables the first demonstration of a 4 V-class hydroborate-based all-solid-state battery combining a sodium metal anode and a cobalt-free Na-3(VOPO4)(2)F cathode without any artificial protective coating. When cycled to 4.15 V vs. Na+/Na, the cells feature a discharge capacity of 104 mA h g(-1) at C/10 and 99 mA h g(-1) at C/5, and an excellent capacity and energy retention of 78% and 76%, respectively, after 800 cycles at C/5 at

ROYAL SOC CHEMISTRY2020

EPFL2006

Catalytic Upgrading of Biomass-Derived Carboxylic Acids to Fuels and Chemicals

Jher Hau Yeap

EPFL2019