Rational Synthesis of Activated Carbons for Enhanced CO2 Uptake and Methane Storage

Abstract

The atmospheric concentration of CO2 has reached alarming levels, primarily due to the combustion of fossil fuels. This surge in CO2 concentrations has led to rising global temperatures, increased frequency of extreme weather events, and threats to all forms of life on Earth. This scenario necessitates the urgent need to develop CO2 capture and removal technologies to ensure a more sustainable future. Moreover, the use of fossil fuels not only contributes significantly to CO2 emissions but reserves and supply are also limited. This Thesis covers themes on the rational preparation of porous carbons that are targeted at the capture of CO2 or the storage of gases (e.g. methane) that may be used as transitional energy sources towards a fossil-free economy. In this regard, ‎Chapter 1 explores the state-of-the art on potential solutions, including CO2 capture and storage via physisorption on porous materials, with a focus on readily synthesised activated carbons. Additionally, the chapter highlights the potential of methane-based natural gas as an alternative to conventional fossil fuels for vehicular transport, given its abundant reserves and lower carbon emissions. The storage of methane for vehicular use may be achieved using porous materials. However, the preparation of porous materials that meet set targets, such as those outlined by the US Department of Energy (DOE), remains a challenge. The considerations that must be carefully considered in seeking suitable materials for such gas storage applications include precise control over porosity parameters such as surface area, pore volume, pore size, and packing density. Therefore, both the ability to accurately modulate the textural properties and a comprehensive understanding of the relationship between gas (CO2 or CH4) uptake capacity and pore size are essential. This thesis is aimed to address these issues.

‎Chapter 2 concentrates on material characterisation to gain a comprehensive understanding of the targeted porous carbon nanostructures. To fully assess these materials, a combination of chemical, structural, and surface characterisation techniques have been employed. This chapter provides a concise overview of the equipment and techniques used for the characterisation of the materials prepared during this research programme.

Regarding preparation routes to activated carbons, this work investigates two primary synthesis methods. Firstly, ‎Chapter 3 delves into the effect of the air-carbonisation treatment. This study aimed to investigate the impact of air-carbonisation temperature followed by potassium hydroxide (KOH) activation on the structure and gas adsorption performance of activated carbons. It primarily focused on the effect of air-carbonisation treatment on the O/C ratio (a measure of a precursor’s susceptibility to activation) in carbonaceous matter. The study found a role of air-carbonisation temperature in tailoring the porosity, surface area, and packing density of the activated carbons. It revealed a very notable trend in the O/C ratio of the carbonaceous matter, which decreases as the temperature increases. This knowledge enables the optimal design of the pore structure of the activated carbons, resulting in porous carbons with high CO2 uptake, at 25 °C, of 2.0 and 5.2 mmol g-1 at 0.15 and 1 bar, respectively. As a result of the ability to adjust the O/C ratio of the precursor, activated carbons with a high packing density of up to 0.79 g cm-3 were obtained, resulting in high CH4 uptake, wherein the gravimetric and volumetric uptake reach of 0.38 g g-1 and 278 cm3 (STP) cm-3, respectively, at 25 °C and 100 bar.

The second approach centres on the effect of the activation technique when utilising N-rich precursors generated via varying pathways. Thus in ‎Chapter 4, a N source was introduced to biomass-derived carbonaceous matter through the use of additives, resulting in a incorporation of N in the precursor. This process involves adding melamine or urea as a nitrogen source to an activation mixture containing biomass-derived carbonaceous matter of low O/C ratio (air-carbonised date seed, Phoenix dactylifera, ACDS), and KOH as an activating agent. These carbons exhibit a broad range of surface areas and porosity characteristics, controlled by varying the amount of melamine or urea, the KOH/ACDS ratio, and the activation temperature. The N added to the activation mix serves as both an N-dopant and porogen, with the later effect enabling the formation of larger pores, extending the pore size distribution into the mesopore region, and increasing the surface area. This results in carbons with tunable porosity and variable packing density, suitable for enhanced CO2 and methane uptake. The carbons exhibited excellent low-pressure CO2 capture at 25 °C, of 1.7 mmol g-1 at 0.15 bar and 4.7 mmol g-1 at 1 bar. The porosity and packing density of the carbons also were directed and modulated towards methane storage, showing a gravimetric uptake of up to 0.42 g g-1 at 25 °C and 100 bar, and volumetric storage capacity of up to 266 cm3 (STP) cm-3 at 25 °C and 100 bar.

Expanding on the knowledge gained in ‎Chapter 4 from KOH activation of precursors with nonhomogeneous incorporation of N additives and low O/C ratio precursors, ‎Chapter 5 explores activated carbons derived from precursors with homogeneous incorporation of N. ‎Chapter 5 explores the potential of N-rich crosslinkable imidazolium-based ionic liquids as a new class of carbon precursors for activated carbons. Upon carbonisation, the ionic liquids yield carbonaceous matter (designated as IL-C) with the unusual combination of a high N content and low O/C atomic ratio. During activation, the resulting ionic liquid-derived carbonaceous matter (IL-C) generates activated carbons with a mix of micro- and mesoporosity, having ultra-high surface area of up to ~4000 m² g-1 and a pore volume of up to 3.3 cm3 g-1. These carbons, due to their porosity and packing density, exhibit outstanding gravimetric and volumetric methane uptake of up to 0.53 g g-1 and 289 cm3 (STP) cm-3, respectively, at 25 °C and 100 bar. This surpasses both gravimetric and volumetric methane storage targets, making these ionic liquid-derived activated carbons the first porous materials (carbon or MOF) to meet both gravimetric and volumetric storage targets as set by the US DoE.

To further enhance understanding of the role of N in shaping the porosity of activated carbons, ‎Chapter 6 goes beyond KOH activation to explore non-hydroxide activation of IL-C. Activated carbons were generated via chemical activation of IL-C using a non-hydroxide activating agent, potassium oxalate (PO). Due to the very low atomic ratio O/C of 0.116 of IL-C, and gentler activating nature of PO, the generated activated carbons display moderate to high surface area of 447 to 2202 m2 g-1, depending on the activation temperature. The porosity of the activated carbon could be tailored to exhibit a highly microporous structure with porosity suitable for post-combustion CO2 uptake, having CO2 capacities of 1.6 and 4.2 mmol g-1 At 25 °C and pressures of 0.15 and 1 bar, respectively. As a benefit of PO being a milder and greener activating agent, the activated carbons have excellent properties in terms of high packing density (0.98 cm3 g-1) combined with a high surface area density (2039 m2 cm-3), which translates to excellent volumetric methane gas uptake. At 100 bar and 25 °C, the carbons achieve total volumetric uptake of 282 cm3 (STP) cm-3 along with excellent deliverable capacity of 200 cm3 (STP) cm-3 (for a pressure swing of 100 bar → 5 bar).

‎Chapter 7 provides a summary and conclusions of this thesis, along with a future look at specific aspects. As an overview, this thesis delves into the rational and intentional synthesis of activated carbons for which various precursors and activation techniques are explored. The outcome has yielded exceptional activated carbons distinguished by the possibility of deliberately tailoring the porosity towards high surface area and pore volume. The motivation of the thesis is achieving porosity in the activated carbons that is suitable for addressing challenges associated with CO2 uptake and optimising methane storage. This research also opens up new possibilities in terms of applications for the synthesised activated carbons. A particularly promising application for future exploration lies in the utilisation of these activated carbons for hydrogen storage, presenting an exciting prospect for further investigation.