Biomass-derived activated carbon for energy storage applications

Activated carbon is a porous carbon material with a broad range of applications as adsorbents in liquid and gas treatment, as well as in catalytic applications. The need for activated carbons is continuing to expand, because environmental pollution is an increasingly serious issue. Activated carbons’ (ACs) specific properties are determined by the properties of the starting materials and the activation methods utilised. Practically, the principal sources of commercial ACs are coal, wood and coconut shells. Given their low cost, sustainability and ready availability, various agricultural and forest by-products have recently gained considerable attention as alternative feedstock for the production of ACs. Accordingly, a major goal of this thesis is to explore and discover the synthesis conditions for generating highly porous materials from starting materials with little to no value, for example waste biomass. We employed (sawdust, SD), date seed (Phoenix dactylifera) and CNL carbon (from accidental and uncontrolled burning of wood under fierce fire conditions of the first Carbon Neutral Laboratory, CNL, building at Nottingham) as feedstock, in order to prepare the activated carbon. The principal objectives are to identify and investigate synthesis conditions for producing highly porous activated carbon for sustainable energy applications. The first chapter presents an overview of the climate change issue, positing some strategies towards reducing the harmful effects of atmospheric pollution. Furthermore, it describes the several types of porous materials used for gas adsorption, for instance MOFs, zeolites and porous carbon. Description of porous materials is a main focus of this chapter, which also covers pore classifications according to size, shape and type of the pores, as well as methods used to prepare activated carbons, and the use of the carbons in gas storage. In the second chapter, the techniques adopted to analyse porous carbons are briefly discussed, with Brunauer, Emmett and Teller (BET) theory and CO2 adsorption using gravimetric analysis methods using a XEMIS instrument being notable. The chapter also described the techniques that may be used to probe the nature of porous materials and includes description of Thermal Gravimetric Analysis (TGA), powder X-Ray Diffraction (XRD), Scanning Electron Microscopy Analysis (SEM), Transmission Electron Microscopy (TEM) and Elemental Analysis. In the third chapter, date seed (Phoenix dactylifera) is used as an example of how biomass with a low O/C ratio can be used to prepare activated carbons in a targeted manner. The chapter describes how the choice of carbonisation mode may be adopted to produce activated carbons with optimised porosity for methane storage. The elemental composition of the biomass precursor, specifically, a low O/C atomic ratio, can be used as a universal predictor of the nature of porosity generated for activated carbon prepared via KOH activation. The carbons can be tailored to have a mix of microporosity/mesoporosity, with high surface area density, high volumetric surface area, in addition to a high packing density. The activated carbons produced are highly microporous with surface area of 995 – 2609 m2 g -1 , pore volume of 0.43 – 1.10 cm3 g -1 and high packing density. The resulting carbons had pores of size 8 – 12 Å, which are suitable for methane uptake. At 25 ˚C and 35 bar, the carbons have an excess and total methane uptake of up to 196 cm3 (STP) cm-3 and 222 cm3 (STP) cm-3 , respectively, which is superior to any previously reported carbon and comparable to the best MOFs. In the fourth chapter, potassium oxalate (PO) and KOH were employed as activating agents to prepare activated carbons from date seed derived carbonaceous matter designated as ACDS (air-carbonised date seed). Previously, the action of the two activating agents has been compared but on different starting materials. In this study, identical procedures were used to synthesise the carbons with PO or KOH. The design of this study allowed a fuller understanding of the workings of the two activating reagents. The activated carbons resulting from PO activation had surface area of up to 1747 m2 g -1 , with up to 94% of the surface area being attributed to micropores. Their porosity could be tailored towards 6–8 Å pore channels, which are excellent for CO2 storage at low pressure. At 25°C, the PO activated carbons can store up to 1.9 and 4.8 mmol g−1 of CO2 at 0.15 bar and 1 bar, respectively. Unlike what is observed for hydroxide (KOH) activation, changing the PO/ACDS ratio between 2 and 4 did not affect porosity. At any given activation temperature, carbons activated at PO/ACDS ratio of 2 or 4 have comparable porosity and therefore similar CO2 uptake. On the other hand, KOH activated carbons reach higher surface area of up to 2738 m2 g -1 . At 25°C, KOH activated carbons can store up to 4.5 mmol g−1 of CO2 compared to 4.8 mmol g−1 for PO activated carbons. The PO activated carbons CO2 uptake of 1.9 mmol g−1 at 0.15 bar and 25 °C is amongst the highest for any porous material under those conditions. In chapter five, highly microporous activated carbon materials are prepared via direct activation of biomass (sawdust, SD) with potassium oxalate (PO) as a non-corrosive and less toxic activating agent (compared to KOH). The direct activation negates the need for hydrothermal carbonisation or pyrolysis, and generates carbons that are similar to conventionally activated (via hydrothermal carbonisation) equivalents. Overall, the carbons display high microporosity with surface area ranging from 550 to 2100 m2 g -1 , and pore volume between 0.3 and 1.0 cm3 g -1 . Unlike hydroxide activation, the PO/SD ratio does not have a significant effect on porosity, however the activation temperature plays a critical role in determining the textural properties at any PO/SD ratio. Porosity could thus be precisely controlled by varying the activation temperature only. This is a significant finding because it confirms that a more environmentally friendly and direct route to activation, using a milder activating agent, does not compromise achievable porosity. The direct activation, with potassium oxalate as an activating agent, generated activated carbons with pore sizes of 6–8 Å, which is conducive for post-combustion (low pressure) CO2 storage; the carbons capture up to 1.6 and 4.3 mmol g-1 of CO2 at 0.15 and 1 bar, respectively, and 25 °C. In chapter six, we synthesized activated carbons from pre-mixtures of polypyrrole (PPY) with CNL carbon or ACDS carbon. The pre-mixtures, which combined precursors that would normally produce mesoporous carbons with high pore volume (PPY) or microporous carbons with moderate pore volume (CNL and ACDS carbons), produced activated carbons with ultra-high surface area (up to 3890 m2 g -1) and pore volume (up to 2.40 cm3 g -1). Use of pre-mixtures as precursors generates carbons with substantially greater surface area than single use of any one of the precursors. An improved understanding of the way in which the activation process is influenced by the carbonisation phase has been combined with knowledge of the effect of the oxygen to carbon (O/C) ratios of the utilised carbonaceous materials in order to provide activated carbons with a high packing density and porosity that are ideal for methane storage. The resulted activated carbons exhibit greater methane uptake capacity (gravimetric and volumetric) than those produced from single use of the precursors