Types of electrolytes for carbon-based supercapacitors
- Autores: María Ángeles Moreno Fernández
- Directores de la Tesis: José María Rojo Martín (dir. tes.), Teresa Alvárez Centeno (codir. tes.)
- Lectura: En la Universidad Autónoma de Madrid ( España ) en 2016
- Idioma: español
- Tribunal Calificador de la Tesis: Marc A. Anderson (presid.), Vicente Torres Costa (secret.), José Manuel Amarilla Álvarez (voc.), Maria de Fátima Grilo Da Costa Montemor (voc.), Jesús Palma del Val (voc.)
- Materias:
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- Resumen
Supercapacitors are devices for energy storage showing high power density (up to 10 kW·kg-1), short response time (1-30 s) and long cycle life (1 million cycles of charge/discharge). However, they store low energy density (up to 5 W·h·kg-1), which is 10-20 times lower than the energy stored by the lithium ion batteries. Therefore, the main drawback of supercapacitors is their low energy density.
Many efforts have been focused on the development of materials for their application as electrodes in supercapacitors. Up to now, carbon materials are the most competitive candidates, due to their excellent physical and chemical properties and their low cost.
To enhance the energy density of carbon-based supercapacitors two approaches can be tried: (i) improving the specific capacitance by either increasing the extent of the electrochemical double-layer or by providing a pseudocapacitance in addition to the double-layer capacitance and (ii) by choosing an electrolyte with broad working voltage window. The first option leads to a linear increase of the energy. The second alternative leads to a quadratic increase of the energy.
This PhD thesis is focused on electrolytes showing different working voltage windows in presence of binder-free carbon electrodes with micropores or mesopores. The three types of electrolytes studied are: 2M H2SO4 aqueous electrolyte, the organic electrolytes 1M Et4NBF4, Pr4NBF4 and Bu4NBF4 solved in acetonitrile or propylene carbonate and the ionic liquid PYR14TFSI and its solution 1M in propylene carbonate. The carbon electrodes chosen are made of carbon monoliths and carbon cloths. With this regard this work is structured in seven chapters.
Chapter 1 provides a general introduction about energy storage devices focusing on supercapacitors. The history, the components and the energy storage mechanism in supercapacitors are described. A general overview about carbon materials used as electrodes (activated carbon powders, activated carbon monoliths and activated carbon cloths) and electrolytes (aqueous, organics and ionic liquids) is addressed. At the end, the objectives of this work are briefly presented.
Chapter 2 is the experimental section. The characterization techniques, the theories and methods used are reported. The characterization techniques involve textural, chemical, structural, morphological, electrical and electrochemical measurements.
Chapter 3 is focused on the aqueous electrolyte 2M H2SO4. The electrodes were four carbon monoliths, which were obtained by mould conforming of powder activated anthracite and a polymeric binder followed by carbonization of the binder. The monoliths displayed different activation degree and binder content. These monoliths showed high electrical conductivities (2-4 S·cm-1), medium densities (0.38-0.70 g·cm-3), large specific surface areas (1491-2117 m2·g-1) associated with the presence of micropores and high content of surface oxygen groups (685-1514 μmol·g-1 of CO). The specific capacitance reached values of ≈300 F·g-1. The working voltage window was ≈1 V. The specific capacitance and working voltage window due to the hydronium (H3O+) cation and bisulfate (HSO4 -) anion was measured separately. The pseudocapacitance for the H3O+ cation was estimated. From the comparative study of the double-layer capacitance due to the H3O+ and HSO4 - ion and the specific surface area determined for micropores with sizes above a certain value, the sizes of 0.4-0.5 nm and 0.5-0.6 nm were deduced for the H3O+ and HSO4 - ion, respectively, as they are electroadsorbed at the double-layer.
Chapter 4 presents the results obtained for three organic electrolytes having the salts Et4NBF4, Pr4NBF4 and Bu4NBF4 solved in acetonitrile (concentration 1M). The electrode was a commercial carbon monolith of cylindrical shape showing squared channels along the cylinder axis. The monolith displayed large surface area (1280 m2·g-1) associated with the presence of micropores, medium density (0.7 g·cm-3) and high electric conductivity (7 S·cm-1). The specific capacitance and the working voltage window due to the cation and the anion were measured separately. It was found a decrease of the two parameters as the ion size increases. The sizes of the ions BF4 -, Et4N+ and Pr4N+ as they are electroadsorbed at the double-layer were estimated from comparison of the specific capacitances and the specific surface areas associated with micropores having sizes above a certain value. The sizes of 0.52, 0.63 and 0.76 nm were deduced for the electroadsorbed BF4 -, Et4N+ and Pr4N+ ion, respectively.
Chapter 5 provides results about the bare ionic liquid PYR14TFSI and its solution in propylene carbonate (1M). The electrodes were a commercial carbon cloth and a carbon cloth obtained after heating at 950 ºC under N2 atmosphere for 2 hours. The former showed a high content of impurities (O, Al and Zn), being reduced by heat-treatment. The two carbon cloths had close electrical conductivities (∼ 10-2 S·cm-1) and microporous surface areas (∼ 1000 m2·g-1). The electrochemical measurements on the cell having the bare PYR14TFSI were carried out at 20 ºC and 60 ºC. At both temperatures the working voltage window was 3.5 V, but the specific capacitance was major at 60 ºC. At this temperature, specific interactions between the impurities and the electrolyte led to degradation of the electrolyte. The degradation was negligible for the PYR14TFSI solved in PC. Micropores with sizes in the range 1-2 nm (supermicropores) are compatible with the size of the cation PYR14 +.
Chapter 6 compares the aqueous 2M H2SO4 electrolyte, the organic 1M Et4NBF4 solved in PC one and the ionic liquid PYR14TFSI, all of them at room temperature. The electrode was a mesoporous carbon monolith. The mesoporous structure was chosen to make easier the access of the electrolyte to the carbon structure. The monolith showed moderate specific surface area (746 m2·g-1), high electrical conductivity (∼ 1 S·cm-1) and moderate content of surface oxygen groups. The specific capacitance for the organic electrolyte was close to that of ionic liquid but lower than the specific capacitance measured for the aqueous electrolyte. The specific capacitance for the aqueous electrolyte had a pseudocapacitive contribution in addition to the double-layer capacitance. The specific capacitance for the organic electrolyte and the ionic liquid showed a double-layer contribution only. The working voltage window increased from 1V for the aqueous electrolyte, to 2.7 V for the organic one and to 3.5 V for the ionic liquid. The energy density stored in the supercapacitor was higher for the ionic liquid than for the organic electrolyte and the aqueous electrolyte. The power density was greater for the organic electrolyte than for the aqueous electrolyte; the cell having the ionic liquid showed the poorest power density.
Chapter 7 collects the main conclusions of this work.
