Matabosch Coromina, Helena
(2017)
Advanced carbons for gas and energy storage.
PhD thesis, University of Nottingham.
Abstract
This thesis describes the synthesis and the characterization of novel porous carbon materials with properties tailored for energy storage or gas sorption applications. The study of carbon-based materials with different texture and porosity properties has focused on improving the performance of electrode materials for electric double-layer capacitors (EDLCs), and also the performance for CO2 and H2 uptakes.
The performance of high-power and high-energy EDLCs containing novel activated-carbon electrodes derived from carbon nanotubes (CNTs) with unprecedented high surface areas and specific capacitances is described. The CNTs were synthesised from CCl4 and ferrocene at 180°C, and chemically activated using KOH at a range of temperatures (600 - 900°C). The activated CNTs had surface areas of 1479-2925 m2 g−1 and Brauner-Emmett-Teller (BET) analysis showed that the samples activated at 900°C contained a mix of micropores and small mesopores, while samples activated at lower temperatures were microporous only. In aqueous H2SO4, the highest specific capacitance (172 F g−1) was achieved using a mix of pore sizes and not necessarily the CNTs with the highest surface areas. In EDLCs containing the ionic liquids 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), capacitances of up to 150 F g−1 and wide electrochemical windows (3.0 V) were achieved. The CNT-based EDLCs showed higher specific energies (15 Wh kg−1) and higher power densities (1.5 kW kg−1) than state-of-the-art carbon electrodes for EDLCs and batteries respectively, thus bridging a long-standing performance gap in the Ragone plot describing the relative power and energy densities of these energy storage devices.
This thesis is also concerned with CO2 storage, to reduce this greenhouse gas present in the atmosphere with a direct link to global climate change. To this end, a series of novel activated carbons (ACs) from jujun grass and Camellia japonica for CO2 storage were prepared by hydrothermal carbonization of the raw materials, which yielded hydrochars. These were then chemically activated using KOH as the activating agent. The samples were activated at KOH/hydrochar ratios of 2 and 4, and at activating temperatures ranging between 600 °C and 800 °C. The resulting ACs had high surface areas and were predominantly microporous, with moderate to high surface areas (1050 – 3537 m2 g-1). CO2 adsorption by these ACs was investigated in the pressure range 0 - 20 bar at room temperature. The ACs activated at a KOH/hydrochar ratio of 2 were predominantly microporous with surface areas of up to 2,750 m2 g-1. 95% of their surface area was attributed to micropores, while 84% of the pore volume was taken up by micropores, with pore sizes between 5 and 7 Å, and exhibited very promising CO2 uptake capacity of 5.0 mmol g-1 at 1 bar. This is amongst the highest reported so far for biomass-derived carbons. On the other hand, activation at KOH/hydrochar ratio of 4 generates carbons with surface area and pore volume of up to 3,537 m2 g-1 and 1.85 cm3 g-1, respectively, and which, depending on the level of activation, simultaneously exhibit high CO2 uptake at both 1 bar (4.1 mmol g-1) and 20 bar (21.1 mmol g-1), i.e. under conditions that mimic both post combustion and pre combustion CO2 capture from flue gas streams. These observations confirm that CO2 is predominantly adsorbed within the micropores of porous carbons at 1 bar, while the CO2 uptake at 20 bar is proportional to the total surface area. The samples activated at a KOH/carbon ratio of 4, exhibited a H2 uptake capacity up to 6.2 %wt at 20 bar and -196 °C. Considering that the carbon precursors are readily available, cheap and renewable, and that the synthesis of the ACs is simple, the results indicate that these activated carbons are very promising for CO2 and H2 uptake and storage.
Nitrogen/sulfur co-doped ACs prepared from polypyrrole and polythiophene as a nitrogen and sulfur precursor, followed by chemical activation using KOH as activating agent were prepared. The ACs have surfaces areas up to 3000 m2 g−1 and the pore size distribution of the ACs depends on the PPY/PTh ratio in the precursor mixture. The ACs are predominantly microporous, but those prepared from 1:2 PPy/PTh mixtures contain a significant proportion of large mesopores (up to 27 Å in diameter). These materials have been tested for their CO2 uptake capabilities, presenting attractive uptake capacities at higher pressures, up to 45 bar. In addition, the materials were tested as electrode materials for supercapacitors in aqueous and ionic liquid electrolytes. In the cell containing an ionic liquid electrolyte, this specific capacitance results in a specific energy, Es, of 107.4 W h kg−1 at a specific power, Ps, of 9.9 kW kg−1, which is unprecedented for ACs in contact with ionic liquids. The study of these nitrogen/sulfur co-doped materials showed promising performance as gas adsorbents and high specific capacitance as electrode materials in EDLCs.
Overall, this study showed how to tailor the properties of carbon-based materials for high capacitive performance as electrode materials for supercapacitors as well as for high adsorption behaviour for gas uptake applications. The work revealed in this thesis provides promising results for novel materials for both supercapacitors and gas uptake.
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