Fabrication of porous carbons and mesoporous silica materials for energy storage and environmental applications

In the context of limited availability of fossil fuel and the impact of fossil-based energy utilization to the environment, novel porous materials have been extensively investigated for applications in environmentally friendly energy generation and storage. This thesis describes work wherein porous carbons and mesoporous silica materials have been systematically studied to include new synthesis strategies, material characterization. Two main themes of this thesis are, firstly, to investigate how porosity affects the utilization of activated carbons in energy storage and gas adsorption, and secondly, explore the stabilization of mesoporous silica materials. Chapter 1 discusses structures and classifications of pores. Porous carbons and mesoporous silica materials are introduced including the fundamental properties, preparation and important applications of the materials. Chapter 2 gives the basics of techniques used for characterization of the porous materials fabricated in this work. Gas sorption techniques applied for hydrogen storage and carbon dioxide uptake are introduced. The chapter also presents the electrochemistry and electrochemical methods used in this work. Chapter 3 briefly describes the preparation of highly porous carbons from lignin via hydrothermal carbonisation followed by chemical activation using KOH as activating agent. The work evidences the influence of activation temperature and KOH/carbon weight ratio on the structure of activated carbon and the performance of the gas storage capacity. Activation at KOH/carbon ratio of 2 generates highly microporous carbons which exhibit excellent CO2 uptake capacity; up to 4.6 mmol/g at 1 bar and 25 oC. Activation at KOH/carbon ratio of 4 can, on the other hand, generate lignin-derived carbons with ultrahigh porosity. These higher surface area lignin-derived carbons exhibit attractive hydrogen uptake capacity of up to 6.2 wt% at -196 oC and 20 bar. Chapter 4 is devoted to the use of lignin-derived activated carbons (LAC) as electrode materials for supercapacitors in aqueous and ionic liquid electrolytes. The study shows several factors affecting the electrochemical performance of carbon electrodes. It is demonstrated that a high surface area carbon (designated as LAC4800) electrode in 2 M H2SO4 exhibits a high capacitance value of 223 F/g or surface capacitance of 11.49 µF/cm2 and good cycling stability over 1000 cycles. The LAC electrodes also showed attractive capacitive performance with 175 F/g (6.92 µF/cm2) and the energy density can be enhanced in ionic electrolytes to reach 97.2 Wh/kg and power density of 2.0 kW/kg at 0.5 A/g for sample LAC4800 in BMImBF4 electrolyte. Chapter 5, regarding non-carbon materials, new forms of MCM-41 type silica mesostructures have been prepared by increasing the applied crystallization temperature to between 150 and 190 oC. The high temperature crystallisation resulted in enlargement of pore size and generated thicker pore walls. The sample prepared at 190 oC shows exceptional hydrothermal and thermal stability, even retaining long-range mesostructural ordering after refluxing in boiling water for 24 h or heating at 1000 oC for 4 h, which is unprecedented for pure silica MCM-41 materials. Finally, the conclusions for the thesis including the suggestion for future work are proposed in Chapter 6

Valorization of lignin waste: high electrochemical capacitance of lignin-derived carbons in aqueous and ionic liquid electrolytes

This report describes the utilization of waste lignin-derived activated carbons (LACs) as high-energy/high-power electrode materials for electric double layer capacitors(EDLCs). The influence of carbon-pore structure on the capacitance of the LACs in two aqueous (H2SO4 and KCl) and two ionic liquid (1-ethyl-3-methylimidazolium ethylsulfate, [EMIm][EtSO4], and 1-butyl-3-methylimidazolium tetrafluoroborate, [BMIm][BF4]) electrolytes is evaluated. In EDLCs containing aqueous H2SO4 as electrolyte, the LACs exhibit specific capacitances of up to 223 F/g and good cycling stability, with energy density of 5.0 Wh/kg at a power density of 200 W/kg. EDLCs containing KCl achieved a specific capacitance of 203 F/g, and energy density of 7.1 Wh/kg at a power density of 510 W/kg. The specific capacitances of the LACs in [EMIm][EtSO4] and [BMIm][BF4] were up to 147 F/g and 175 F/g, respectively. The energy density in the IL electrolytes, is up to 25 Wh/kg at power density of 500 W/kg, and 16.4 Wh/kg at 15 kW/kg. We demonstrate that the electrochemical performance of the LACs depends not only the surface area and pore size, but also on the pore-wall thickness

Highly Porous Renewable Carbons for Enhanced Storage of Energy-Related Gases (H2 and CO2) at High Pressures

Hydrochar, i.e., hydrothermally carbonized biomass, is generating great interest as a precursor for the synthesis of advanced carbon materials owing to economical, sustainability, and availability issues. Hereby, its versatility to produce adsorbents with a porosity adjusted to the targeted application, i.e., low or high pressure gas adsorption applications, is shown. Such tailoring of the porosity is achieved through the addition of melamine to the mixture hydrochar/KOH used in the activation process. Thereby, high surface area carbons (>3200 m2 g–1) with a bimodal porosity in the micromesopore range are obtained, whereas conventional KOH chemical activation leads to microporous materials (surface area <3100 m2 g–1). The micromesoporous materials thus synthesized show enhanced ability to store both H2 and CO2 at high pressure (≥20 bar). Indeed, the uptake capacities recorded at 20 bar, ca. 7 wt % H2 (−196 °C) and 19–21 mmol CO2 g–1 (25 °C) are among the highest ever reported for porous materials. Furthermore, the micromesoporous sorbents are far from saturation at 20 bar and achieve much higher CO2 uptake at 40 bar (up to 31 mmol of CO2 g–1; 25 °C) compared to 23 mmol of CO2 g–1 for the microporous materials. In addition, the micromesoporous materials show enhanced working capacities since the abundant mesoporosity ensures higher capture at high uptake pressure and the retention of lower amounts of adsorbed gas at the regeneration pressure used in PSA systems.This research work was supported by Spanish Ministerio de Economía y Competitividad, MINECO (MAT2012-31651), and by Fondo Europeo de Desarrollo Regional (FEDER). M. S. thanks the Ministerio de Ciencia e Innovación for her Ramón y Cajal contract. We thank the Rajamangala University of Technology Srivijaya (RMUTSV), Thailand for funding and a studentship for WS, and the Kingdom of Saudi Arabia for funding a PhD studentship for NB.Peer reviewe

A hygrothermal modelling approach to water vapour sorption isotherm design for mesoporous humidity buffers

This paper describes the development of a design technique using hygrothermal numerical modelling for top-down predictive design and optimisation of water vapour sorption isotherms to match any humidity buffering application. This was used to inform the design and synthesis of two new mesoporous silica (MS) materials suitable for specific applications. To validate the technique, the new materials were experimentally assessed using gravimetric dynamic vapour sorption (DVS). The experimental isotherms closely matched the optimised isotherm predictions from the design stage, and a positive correlation was observed between the rate of change in adsorbed water content, Δw and the time taken to exceed the permissible upper limit of humidity, φi,U in a closed environment. A positive non-linear correlation was determined between the interior volumetric moisture load, ωml and the mass of adsorbent required to fully achieve humidity buffering between specified lower/ upper limits (φi,L and φi,U). The kinetics of water vapour sorption/ desorption were found to have general agreement when using the current hygrothermal numerical model. Current hygrothermal models appear to significantly underestimate the rate of adsorption/ desorption in rapid-response mesoporous silica type materials. This is perhaps largely due to the current lack of consideration for scanning curve prediction within hysteresis loops and so is a priority for future research

Fabrication of porous carbons and mesoporous silica materials for energy storage and environmental applications

In the context of limited availability of fossil fuel and the impact of fossil-based energy utilization to the environment, novel porous materials have been extensively investigated for applications in environmentally friendly energy generation and storage. This thesis describes work wherein porous carbons and mesoporous silica materials have been systematically studied to include new synthesis strategies, material characterization. Two main themes of this thesis are, firstly, to investigate how porosity affects the utilization of activated carbons in energy storage and gas adsorption, and secondly, explore the stabilization of mesoporous silica materials. Chapter 1 discusses structures and classifications of pores. Porous carbons and mesoporous silica materials are introduced including the fundamental properties, preparation and important applications of the materials. Chapter 2 gives the basics of techniques used for characterization of the porous materials fabricated in this work. Gas sorption techniques applied for hydrogen storage and carbon dioxide uptake are introduced. The chapter also presents the electrochemistry and electrochemical methods used in this work. Chapter 3 briefly describes the preparation of highly porous carbons from lignin via hydrothermal carbonisation followed by chemical activation using KOH as activating agent. The work evidences the influence of activation temperature and KOH/carbon weight ratio on the structure of activated carbon and the performance of the gas storage capacity. Activation at KOH/carbon ratio of 2 generates highly microporous carbons which exhibit excellent CO2 uptake capacity; up to 4.6 mmol/g at 1 bar and 25 oC. Activation at KOH/carbon ratio of 4 can, on the other hand, generate lignin-derived carbons with ultrahigh porosity. These higher surface area lignin-derived carbons exhibit attractive hydrogen uptake capacity of up to 6.2 wt% at -196 oC and 20 bar. Chapter 4 is devoted to the use of lignin-derived activated carbons (LAC) as electrode materials for supercapacitors in aqueous and ionic liquid electrolytes. The study shows several factors affecting the electrochemical performance of carbon electrodes. It is demonstrated that a high surface area carbon (designated as LAC4800) electrode in 2 M H2SO4 exhibits a high capacitance value of 223 F/g or surface capacitance of 11.49 µF/cm2 and good cycling stability over 1000 cycles. The LAC electrodes also showed attractive capacitive performance with 175 F/g (6.92 µF/cm2) and the energy density can be enhanced in ionic electrolytes to reach 97.2 Wh/kg and power density of 2.0 kW/kg at 0.5 A/g for sample LAC4800 in BMImBF4 electrolyte. Chapter 5, regarding non-carbon materials, new forms of MCM-41 type silica mesostructures have been prepared by increasing the applied crystallization temperature to between 150 and 190 oC. The high temperature crystallisation resulted in enlargement of pore size and generated thicker pore walls. The sample prepared at 190 oC shows exceptional hydrothermal and thermal stability, even retaining long-range mesostructural ordering after refluxing in boiling water for 24 h or heating at 1000 oC for 4 h, which is unprecedented for pure silica MCM-41 materials. Finally, the conclusions for the thesis including the suggestion for future work are proposed in Chapter 6

Valorization of Lignin Waste: Carbons from Hydrothermal Carbonization of Renewable Lignin as Superior Sorbents for CO₂ and Hydrogen Storage

This report presents the preparation of renewable carbons from hydrothermally carbonized lignin waste. The hydrothermally carbonized mineral-free lignin-derived hydrochar was activated with KOH to yield carbons with surface area of 1157–3235 m² g^–1 and pore volume of 0.59–1.77 cm³ g^–1. Activation at KOH/carbon = 2, generates highly microporous carbons (≥97% micropore surface area and 93% micropore volume), which exhibit excellent CO₂ uptake capacity; up to 4.6 mmol g^–1 at 1 bar and 25 °C, and 17.3 mmol g^–1 at 20 bar and 25 °C, whereas at 0 °C and 1 bar, they store up to 7.4 mmol g^–1. Activation at KOH/carbon = 4 can generate carbons with surface area and pore volume of up to 3235 m² g^–1 and 1.77 cm³ g^–1, respectively, which have hydrogen uptake of up to 6.2 wt % at −196 °C and 20 bar. The simplicity of hydrothermal carbonization in generating hydrochars suitable for activation from readily available lignin waste, without the need for a demineralization step, makes these carbons attractive as gas storage materials for energy related applications. Furthermore, the lignin-derived carbons offer advantages with respect to attainable porosity and gas storage capacity compared to other forms of biomass (e.g., cellulose)-derived carbons

Highly Porous Renewable Carbons for Enhanced Storage of Energy-Related Gases (H₂ and CO₂) at High Pressures

Hydrochar, i.e., hydrothermally carbonized biomass, is generating great interest as a precursor for the synthesis of advanced carbon materials owing to economical, sustainability, and availability issues. Hereby, its versatility to produce adsorbents with a porosity adjusted to the targeted application, i.e., low or high pressure gas adsorption applications, is shown. Such tailoring of the porosity is achieved through the addition of melamine to the mixture hydrochar/KOH used in the activation process. Thereby, high surface area carbons (>3200 m² g^–1) with a bimodal porosity in the micromesopore range are obtained, whereas conventional KOH chemical activation leads to microporous materials (surface area <3100 m² g^–1). The micromesoporous materials thus synthesized show enhanced ability to store both H₂ and CO₂ at high pressure (≥20 bar). Indeed, the uptake capacities recorded at 20 bar, ca. 7 wt % H₂ (−196 °C) and 19–21 mmol CO₂ g^–1 (25 °C) are among the highest ever reported for porous materials. Furthermore, the micromesoporous sorbents are far from saturation at 20 bar and achieve much higher CO₂ uptake at 40 bar (up to 31 mmol of CO₂ g^–1; 25 °C) compared to 23 mmol of CO₂ g^–1 for the microporous materials. In addition, the micromesoporous materials show enhanced working capacities since the abundant mesoporosity ensures higher capture at high uptake pressure and the retention of lower amounts of adsorbed gas at the regeneration pressure used in PSA systems