An electrochemical double-layer capacitor (EDLC) is a potential buffer for current power and energy supply. In this work, activated carbon derived from fluid coke as a brand new electrode material was studied due to its high specific surface area (SSA) and large portion of mesopores. A suitable electrode material formula, current collector, and cell configuration were investigated to fabricate a testable system and ensure the reproducibility of measurements. Cyclic voltammetry (CV) and constant current charge/discharge (CD) techniques were used to characterize the performance of the electrode material, as well as to study its fundamental behaviour. A new procedure was established for quantifying the capacitance (Cc) of EDLC from CV which isolates the effect of internal resistance on the measured capacitance (CM). The specific capacitance of single electrode made of activated carbon (~1900 m2/g) with approximately 80% mesopores and macropores was able to reach 180 F/g at scan rate of 0.5mV/s.

An electrochemical double-layer capacitor (EDLC) is a potential buffer for current power and energy supply. In this work, activated carbon derived from fluid coke as a brand new electrode material was studied due to its high specific surface area (SSA) and large portion of mesopores. A suitable electrode material formula, current collector, and cell configuration were investigated to fabricate a testable system and ensure the reproducibility of measurements. Cyclic voltammetry (CV) and constant current charge/discharge (CD) techniques were used to characterize the performance of the electrode material, as well as to study its fundamental behaviour. A new procedure was established for quantifying the capacitance (Cc) of EDLC from CV which isolates the effect of internal resistance on the measured capacitance (CM). The specific capacitance of single electrode made of activated carbon (~1900 m2/g) with approximately 80% mesopores and macropores was able to reach 180 F/g at scan rate of 0.5mV/s.

Hydrogen is considered one of the best alternatives to fossil-fuels for the transportation sector because hydrogen can be burnt cleanly and efficiently in a fuel cell to drive an electric motor. However, due to the low density of H2 at ambient conditions the conventional H2 storage technologies (cryogenic liquid and compressed gas) cannot achieve energy densities comparable to to gasoline and diesel. A second energy storage challenge onboard electric fuel cell vehicles is fuel cell power management at peak current loads, which requires an auxiliary power source like a battery or supercapacitor. The development of efficient onboard energy storage systems for H2 and auxiliary power is critical to realisation of a hydrogen economy. Mesoporous carbons were investigated as H2 storage materials in composites with magnesium hydride (MgH2),and as electrode materials for electrochemical double-later capacitors. The mesoporous carbons were prepared by two methods: (1) from porous silica and alumina templates, and (2) by chemical activation of a waste carbon source (waste coffee grounds). The experimental approach targeted reducing the cost of mesoporous carbon preparation by using a cheaper template, where the cost of alumina template was one-fifth the cost of the silica template (at the laboratory scale), or by using a waste material as a carbon source. The alumina template was found to be suitable to produce a mesoporous carbon with an average pore size of 4.8 nm. Chemical activation of coffee grounds with ZnCl2 produced activated carbons with BET surface areas up to 1280 m2/g. Mesopore volume increased with ZnCl2 impregnation ratio, with mesopore size distributions in the range 2 - 20 nm. The theoretical H2 capacity of MgH2 is 7.6 % but MgH2 application in fuel cell vehicles is limited by slow hydrogenation kinetics and high temperatures (> 573 K) for H2 release. Magnesium was impregnated on activated carbon fibres (ACF) and mesoporous carbon (prepared from silica and alumina templates) to improve H2 storage kinetics and thermodynamics by reducing the magnesium hydride particle size. Thermal gravimetric analysis (TGA) and temperature programmed desorption (TPD) studies showed that thermal decomposition of MgCl2 supported on ACF at 1173 K in N2 and H2 can produce a Mg-ACF composite. At 573 K and 2 MPa H2 pressure a Mg-ACF composite, containing 11.2 %wt Mg, showed improved H2 adsorption kinetics compared to bulk Mg powder, but the total capacity of the Mg-ACF composite was only 0.4 % wt H2. To achieve a target of 6 %wt for onboard H2 storage higher Mg loadings are required. Attempts to impregnate Mg in mesoporous carbon via the MgCl2 thermal decomposition process highlighted the difficulties of avoiding MgO formation, and show that MgH2 loaded carbon is unlikely to be a practical high density onboard H2 storage technology. Activated carbons from waste coffee grounds (CGCs) were used as electrode materials in electrochemical double-layer capacitors. The specific capacitance of CGCs was as high as 368 F/g in 1 mol/L H2SO4, with good capacitance retention at fast charge rates and stable cycling performance. The good electrochemical performance of CGCs is attributed to a porous structure featuring both micropores 0.5 - 1.0 nm wide, which are effective for double-layer formation, and small mesopores, which facilitate electrolyte transport at fast charge rates. The capacitance of CGCs is enhanced by pseudo-Faradaic reactions involving nitrogen and oxygen functional groups. At fast charge-discharge rates the CGCs had higher energy density and better stability than a commercial benchmark activated carbon (Maxsorb). The ZnCl2 activation process can be optimised to develop mesopores for improved capacitance at fast charge rates and capacitance in organic electrolytes. In 1 mol/L tetra ethyl ammonium tetrafluoroborate (TEABF4) / acetonitrile the CGC with the most mesopores, which was prepared with a ZnCl2 to coffee ratio of 5:1, has the highest capacitance at high power density. CGCs with greater mesopore content retained higher specific capacitance at fast charge-discharge rates as the mesopores acts as channels or reservoirs for electrolyte transport. An improved model for evaluation of contributions to capacitance from micropore surfaces and mesopore surfaces is proposed. From this model the double-layer capacitance of mesopores surface area was found to be about 14 μF/cm2 and did not change considerably with increasing current load. The contribution of micropores to capacitance is dependent on the accessibility of ions to the micropores, and this accessibility is proportional to the mesopore surface area. An exponential function was found to describe the contribution of mesopores and micropore surfaces to capacitance. The effective double-layer capacitance of the micropore surface area drops at fast charge-discharge rates as a result of restricted ion transport, and this result highlights the importance of mesopores to retain energy density for high power supercapacitor applications.

In this research we have investigated electrochemical and impedance characteristics of liquid phase exfoliated graphene electrodes. The exfoliated graphene electrodes were characterized in Electrochemical Double Layer Capacitors (EDLCs) geometry. Liquid phase exfoliation was performed on bulk graphite powder in order to produces few layer graphene flakes in large quantities. The exfoliation processes produced few layer graphene based materials with increased specific surface area and were found to have suitable electrochemical charge storage capacities. Electrochemical evaluation and performance of exfoliated graphene electrodes were tested with Cyclic Voltammetry, constant current charging discharging and Electrochemical Impedance Spectroscopy (EIS) at ambient conditions. We have used several electrolytes in order to evaluate the effect of electrolyte in charge storage capacities. Specific capacitance value of ~ 47F/g and ~ 262F/g was measured for aqueous and ionic electrolytes respectively. These values are at least an order of magnitude higher than those obtained by using EDLC's electrodes fabricated with the bulk graphite powder. In addition these EDLC electrodes give consistently good performance over a wide range of scan rates and voltage windows. These encouraging results illustrate the exciting potential for high performance electrical energy storage devices based on liquid phase exfoliated graphene electrodes.

No description available.

The focus of this thesis has been to determine the usable voltage range of carbon-based supercapacitors (SC). Supercapacitors are a relatively new type of capacitors with a vast increase in capacitance compared to capacitors which utilize a dielectric as charge separator. A SC consists of two electrodes and an electrolyte separating the electrodes. The charges are stored by electrostatic forces in the interface between the electrode and the electrolyte, forming the so called electrochemical double-layer (EDL). With porous electrodes the effective surface area of the interfacial zone can be made very large, giving SCs a large storage capacity. The limiting factors of a SC is the decomposition potential of the electrolyte and the decomposition of the electrodes. For commercially manufactured SCs the electrolyte is usually an organic solvent, which has a decomposition potential of up to 2.7-2.8 V. Compared to aqueous electrolytes with a thermodynamic limit of 1.23 V. The drawback of using non-aqueous electrolytes is that they are not environmentally friendly, and they increase the production cost. It is claimed that the voltage range can be up to 1.9 V using aqueous electrolytes. Some researchers have focused on aqueous electrolytes for these reasons. In this thesis two different electrolytes were tested to determine if the voltage range could be extended. The experiments were conducted using a three electrode cell and performing cyclic voltammogram measurements (CV). The carbon electrodes were made of  two different sources of grahite, battery graphite or exfoliated graphite, and nano fibrilated cellulose was added to increase the mechanical stability. The results show that the oxidation potential of the carbon electrode was the positive limit. A usable potential of about 1 V was shown. However, when cycling the electrodes to potentials below the decomposition limit, for hydrogen evolution, interesting effects were seen. A decrease in reaction kinetics, indicating a type of conditioning of the electrode was observed. An increase in charge storage capacitance was also observed when comparing the initial measurements with the final, probably corresponding to an increase in porosity. / KEPS projekt Sundsvall Mitt Universitet

Philosophiae Doctor - PhD / To meet a fast-growing market demand for next generation portable electronic devices with higher performance and increased device functionalities, efficient electrical energy devices with substantially higher energy, power densities and faster recharge times such as supercapacitors are needed. The overall aim of this thesis was to synthesize nanostructured sulphonated polyaniline and transition metal single, binary and ternary mixed oxide doped nanocomposites with electro-conductive properties. These nanocomposites were anchored on activated graphitic carbon and used in design of asymmetric supercapacitors. Tantalum(IV)oxide, tantalum(IV)oxide-nickel(II)oxide, tantalum(II)oxide-manganese(III)oxide, tantalum(II)oxide-nickel(II)oxide-manganese(II,III)oxide nanoparticles were synthesised using modified sol-gel methods. These were then dispersed, individually, in acidic media through sonication and incorporated in-situ into the polymeric matrix during the oxidative chemical polymerization of aniline doped with poly(4-styrene sulphonic acid). These novel polymeric nanocomposites were characterised with FTIR, UV-visible, TEM, SEM, EDS, XRD to ascertain successful polymerization, doping, morphology and entrapment of the metal oxide nanoparticles. SECM approach curves and interrogation of CV revealed that these nanocomposites are conductive and electro-active. The cells showed good supercapacitor characteristics with high specific capacitances of 170.5 Fg⁻¹ in TaO₂- PANi-PSSA, 166.1 Fg⁻¹ in TaO₂-NiO-PANi-PSSA, 248.4 Fg-1 in TaO-Mn₂O₃-PANi- PSSA and 119.6 Fg⁻¹ in TaO-NiO-Mn₃O₄-PANi-PSSA. Their corresponding energy densities were calculated as 245.5 Whg⁻¹, 179.4 Whg⁻¹, 357.7 Whg⁻¹ and 172.3 Whg⁻¹ respectively. They also gave respective power densities of 0.50 Whg⁻¹, 0.61 Whg⁻¹, 0.57 Whg⁻¹ and 0.65 Whg⁻¹ and showed good coulombic efficiencies ranging between 77.97% and 83.19%. These materials are found to have a long cycle life and therefore good electrode materials for constructing supercapacitor cells. / National Research Foundation (NRF)

Elektrochemische Doppelschichtkondensatoren (engl. Electrochemical Double-Layer Capacitors, EDLCs) stellen eine zunehmend wichtige Technologie auf dem Markt der elektrischen Energiespeicher dar. Sie zeichnen sich durch die Aufnahmefähigkeit großer Energiemengen, eine hohe Langzeitstabilität und ein schnelles Ansprechverhalten aus. Diese Eigenschaften sind Gründe, weshalb EDLCs als Speicherbausteine für Energierück-gewinnungssysteme oder zur Stabilisierung der Stromversorgung in diversen elektronischen Bauelementen eingesetzt werden. Die Aufnahme der Energie erfolgt über Ladungsseparation von Elektrolytionen an der Elektrodenoberfläche. Die Kapazität der Speicherfähigkeit wird dabei maßgeblich vom Betrag der Elektrodenoberfläche und dem Abstand der Elektrolytionen zur Oberfläche der Elektrode bestimmt (bei gleichbleibendem Elektrolyten). In der gegenwärtigen Forschung werden neue Elektrodenmaterialien entwickelt, um über deren Systemeigenschaften, wie Leitfähigkeit und Porosität, die Leistungsfähigkeit der Doppelschichtkondensatoren weiter zu optimieren. Gängige Komponenten für Elektroden in diesen Bauelementen stellen Kohlenstoffmaterialien dar, da diese chemisch inert und zumeist kostengünstig in der Produktion sind. In der vorliegenden Arbeit sollte die Eignung der Materialklasse der Siliziumoxykarbid-abgeleiteten Kohlenstoffe (engl. Silicon Oxycarbide-Derived Carbons, SiOCDCs) für die Anwendung in elektrochemischen Doppelschichtkondensatoren untersucht werden. Die SiOCDCs wurden über die Pyrolyse (700 – 1500 °C) und Chlorierung (700 – 1000 °C) eines kohlenstoffreichen Polysilsesquioxans mit der theoretischen Zusammensetzung C6H5SiO3/2 erzeugt. Dabei zeigte sich, dass sowohl die porösen Eigenschaften als auch die Leitfähigkeit innerhalb der erhaltenen Kohlenstoffmaterialien stark von der Synthesetemperatur abhängen. Somit konnten reine Kohlenstoffe mit spezifischen Oberflächen bis zu 2400 m2 g-1 und Porenvolumina von 1,9 cm3 g-1 synthetisiert werden. Im Verlauf der Arbeit wurde eine geeignete Methode zur Verarbeitung der erzeugten Oxykarbid-abgeleiteten Kohlenstoffe zu Elektroden evaluiert, um eine elektrochemische Charakterisierung vorzunehmen. Ein vielversprechender Ansatz stellt die vollkommen trockene Umsetzung der SiOCDCs zu freistehenden Elektrodenschichten dar. Dieses Verfahren nutzt die Verreibung der Aktivkomponente mit einem geringen Anteil (5 Gew.-%) eines Bindemittels (Polytetrafluorethylen, PTFE) aus, um flexible und selbsttragende Elektrodenfolien zu erzeugen. Die Vorteile dieses Prozesses gegenüber anderen Verarbeitungsarten liegen darin, dass aufwendige Trocknungsverfahren während der Elektrodenherstellung entfallen und die Schichtdicken der resultierenden Folien unmittelbar eingestellt werden können. Während der Untersuchung der unterschiedlichen Elektrodensysteme im organischen Elektrolyten (1 M Tetraethylammoniumtetrafluoroborat-Lösung in Acetonitril) konnten spezifische Kapazitäten von bis zu 120 F g-1 gemessen werden. Des Weiteren zeigte sich der Einfluss der Kohlenstoffstruktur innerhalb der Aktivmaterialien auf die elektrochemischen Resultate. So konnte festgestellt werden, dass eine zunehmende Graphitisierung im Kohlenstoff, welche mit einer steigenden Mesoporosität im SiOCDC einherging, zu einer verbesserten Leitfähigkeit innerhalb der EDLC-Elektroden führte, aber auch eine Verringerung der spezifischen Kapazität bedeutete. Die Verringerung der Widerstände im System weitete erheblich den Bereich der nutzbaren Arbeitsfrequenzen und die Strombelastbarkeit des Elektrodenmaterials aus. So bestand die Möglichkeit ein mesoporöses Kohlenstoffmaterial zu synthetisieren, welches mit einer maximalen Arbeitsfrequenz von 8 Hz einen Wert zeigte, der zwei Größenordnungen über der Arbeitsfrequenz eines kommerziell erhältlichen Standards (Aktivkohle YP-50F) lag. Dieses exzellente Ansprechverhalten bildet die Grundlage für den Einsatz in Hochleistungsspeichersystemen. Des Weiteren offenbarte sich, dass die trocken prozessierten Elektroden das Potential für eine hohe Langzeitstabilität besitzen, da je nach Elektrodensystem ein Erhalt von 94% der Ursprungskapazität über 10.000 Lade-/Entladezyklen beobachtet werden konnte. Die Modifikation der Elektrodenmaterialien mittels CO2-Aktivierung und eine damit verbundene Erhöhung der spezifischen Oberfläche führten zu einer Verbesserung der spezifischen Kapazität der Aktivkomponenten um bis zu 33%. Zusammenfassend bleibt zu erwähnen, dass poröse Oxykarbid-abgeleitete Kohlenstoffe erfolgreich über die Chlorierung von keramischen Vorläuferverbindungen synthetisiert werden konnten. Die Kohlenstoffmaterialien zeigten nach der Prozessierung zu freistehenden und flexiblen Elektrodenfilmen vielversprechende Eigenschaften bei der Nutzung in elektrochemischen Doppelschichtkondensatoren, wie hohe spezifische Kapazitäten, gute Langzeitstabilitäten und hohe Arbeitsfrequenzen bei Lade- und Entladevorgängen.

Nanoporous carbons have become a material of interest in many applications such as electrochemical double layer capacitors (supercapacitors). Supercapacitors are being studied for their potential in storing electrical energy storage from intermittent sources and in use as power sources that can be charged rapidly. However, a lack of understanding of the charge storage mechanism within a supercapacitor makes it difficult to optimize them. Two components of this challenge are the difficulties in experimentally characterizing the sub-nanoporous structure of carbon electrode materials and the electrical performance of the supercapacitors. This work provides a means to accurately characterize the porous structure of sub-nanoporus carbon materials and identifies the current limitations in characterizing the electrical performance of a supercapacitor cell. Future work may focus on the relationship between the sub-nano porous structure of the carbon electrode and the capacitance of supercapacitors, and on the elucidation of charge storage mechanisms.

Nanoporous carbons have become a material of interest in many applications such as electrochemical double layer capacitors (supercapacitors). Supercapacitors are being studied for their potential in storing electrical energy storage from intermittent sources and in use as power sources that can be charged rapidly. However, a lack of understanding of the charge storage mechanism within a supercapacitor makes it difficult to optimize them. Two components of this challenge are the difficulties in experimentally characterizing the sub-nanoporous structure of carbon electrode materials and the electrical performance of the supercapacitors. This work provides a means to accurately characterize the porous structure of sub-nanoporus carbon materials and identifies the current limitations in characterizing the electrical performance of a supercapacitor cell. Future work may focus on the relationship between the sub-nano porous structure of the carbon electrode and the capacitance of supercapacitors, and on the elucidation of charge storage mechanisms.