Vertically aligned carbon nanotubes for supercapacitor and the effect of surface functionalization to its performance

As reliable mobile energy sources became more and more needed and alternative energy demands became more prevalent, the research in advanced energy storage technologies turned into a topic of utmost importance in today's society. Innovative electrode materials that are able to provide increased energy densities and long lifetime must be then be developed. Nanomaterials such as carbon nanotubes are of tremendous prospects in that context. This master thesis aims at realizing an electrochemical capacitor using a vertically grown carbon nanotube array, which is used as electrode in dierent sets of experiments to evaluate its electrical energy storage capability. Cyclic voltammetry and electrochemical impedance spectroscopy are used to determine the specic capacitance of the device, which is composed of two nanotubes electrodes separated by a polypropylene lter, in an adequate electrolyte. The surface chemistry was modied by adding hydroxyl groups onto the nanotubes surface, thus making them hydrophilic and providing an ecient way to increase their storage ability by a factor of two to three. Dierent electrolytes were compared, both aqueous and non aqueous, and the important parameter in choosing an appropriate electrolyte for a high storage ability was shown to be the polarity of the solution. The carried out performance studies gave encouraging results with a maximum energy density of 21 Wh/kg at a power density of 1.1 kW/kg for a hydrophilic electrode using tetraethylammonium tetrafluoroborate in propylene carbonate. Respectively, a maximum power density was found to be 22 kW/kg at energy density of 2 Wh/kg for a hydrophobic sample. This device can be claimed to be entirely carbon-based, with a relatively small ecological print compared to most lithium-based capacitors and inexpensive. The lifetime is also extremely long, indeed more than hundreds of thousands cycles without failure have been achieved.

Use of Vertically-Aligned Carbon Nanotube Array to Enhance the Performance of Electrochemical Capacitors

Mélanie Guittet

In the domain of energy storage, electrochemical capacitors have numerous applications ranging from hybrid vehicles to consumer electronics, with very high power density at the cost of relatively low energy storage. Here, we report an approach that uses vertically aligned carbon nanotube arrays as electrodes in electrochemical capacitors. Different electrolytes were used and multiple parameters of carbon nanotube array were compared: carbon nanotube arrays were shown to be two to three times better than graphite in term of specific capacitance, while the surface functionalization was demonstrated to be a critical factor in both aqueous and nonaqueous solutions to increase the specific capacitance. We found that a maximum energy density of 21 Wh/kg at a power density of 1.1 kW/kg for a hydrophilic electrode, could be easily achieved by using tetraethylammonium tetrafluoroborate in propylene carbonate. These are encouraging results in the path of energy-storage devices with both high energy density and power density, using only carbon-based materials for the electrodes with a very long lifetime, of tens of thousands of cycles.

IEEE2011

Aqueous electrolytes for high-voltage batteries and supercapacitors

David Reber

Lithium-ion batteries are widely implemented as energy storage devices due to their high energy density and low cost. They enabled modern portable electronics and electric ve-hicles, and are key to manage the integration of intermittent renewable electricity sources, such as solar and wind, into the power grid and decentralized microgrids. The use of flam-mable organic electrolytes however raises safety concerns, particularly for large-scale sta-tionary batteries. Furthermore, supply chain risks and the use of critical raw materials are a major challenge for the ongoing battery revolution. Aqueous electrolytes offer promising properties, as they are inherently non-flammable and potentially more cost effective. Combined with electrode materials based on abundant raw materials, aqueous batteries are among the most promising candidates to replace cur-rent battery chemistries relying on critical cobalt and flammable organic electrolytes. How-ever, due to the narrow electrochemical stability window of water (practically ca. 1.5 V), aqueous batteries have not yet met the energy density to be serious competitors to current lithium-ion battery technology. The goal of this thesis was to develop aqueous electrolytes with enhanced electrochem-ical stability, thus enabling new high-voltage cell chemistries. Exploring aqueous electrolytes with high salt concentrations, so-called water-in-salt electrolytes, we significantly increased the electrochemical stability and developed highly stable 2 V class aqueous sodium-ion bat-teries. Operating near the solubility limit of the electrolyte salt, crystallization turned out to be a major challenge for water-in-salt electrolytes. In this thesis, we demonstrated that add-ing highly asymmetric anions into the solution can effectively suppress crystallization, allow-ing stable operation of saturated electrolytes at temperatures as low as -10 °C. We further examined the impact of anion asymmetry on the local solution structure and dynamics using molecular dynamics simulations, relating the enhanced supercooling to sustained rotational motion of the asymmetric anion. Further, we demonstrate that the anion has a crucial impact on solution structure and cell performance. We establish that high salt concentration alone does not guarantee high-voltage stability and that due to the limited solubility of most sodium salts new approaches are needed. We provide guidelines for future work and the evaluation of experimental data in order to spur progress in the field of aqueous batteries.

EPFL2020

Aqueous electrolytes for high-voltage batteries and supercapacitors

David Reber

EPFL2020