Redox-electrolytes for non-flow electrochemical energy storage: A critical review and best practice

Over recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte but also in the bulk electrolyte by redox activities of the electrolyte itself. Those redox electrolytes are promising for non-flow hybrid energy storage systems, or redox electrolyte-aided hybrid energy storage (REHES) systems; particularly, when they are combined with highly porous carbon electrodes. In this review paper, critical design considerations for the REHES systems are discussed as well as the effective electrochemical characterization techniques. Appropriate evaluation of the electrochemical performance is discussed thoroughly, including advanced analytical techniques for the determination of the electrochemical stability of the redox electrolytes and self-discharge rate. Additionally, critical summary tables for the recent progress on REHES systems are provided. Furthermore, the unique synergistic combination of porous carbon materials and redox electrolytes is introduced in terms of the diffusion, adsorption, and electrochemical kinetics modulating energy storage in REHES systems. © 2018 The Author(s

Performance improvement of electrochemical capacitors through the integration of advanced materials and the cell configuration assessment

209 p.The electrochemical capacitors or supercapacitors are envisioned as potential next-generation energy storage systems because of their excellent storage capacity, power density, and long-term durability. However, all these advantages are overshadowed by their poor energy density. Thus, this thesis aims to achieve a high-energy supercapacitor device without compromising its power performance to make them more commercially viable for many applications. The research work is associated with the improvement of the supercapacitors in different device configurations, such as EDL, asymmetric, and hybrid LIC systems by integration of advanced material and cell design. The results obtained from the studies of different supercapacitor systems demonstrate that the variation in electrode mass, cell voltage, and electrolyte has a huge impact on the overall electrochemical performance, stability, life expectancy, and safety of the device. Therefore, careful optimization of cell design and advancement in electrode materials retains the high importance driving factors of the supercapacitors for the development of future energy storage technology

Performance improvement of electrochemical capacitors through the integration of advanced materials and the cell configuration assessment

209 p.The electrochemical capacitors or supercapacitors are envisioned as potential next-generation energy storage systems because of their excellent storage capacity, power density, and long-term durability. However, all these advantages are overshadowed by their poor energy density. Thus, this thesis aims to achieve a high-energy supercapacitor device without compromising its power performance to make them more commercially viable for many applications. The research work is associated with the improvement of the supercapacitors in different device configurations, such as EDL, asymmetric, and hybrid LIC systems by integration of advanced material and cell design. The results obtained from the studies of different supercapacitor systems demonstrate that the variation in electrode mass, cell voltage, and electrolyte has a huge impact on the overall electrochemical performance, stability, life expectancy, and safety of the device. Therefore, careful optimization of cell design and advancement in electrode materials retains the high importance driving factors of the supercapacitors for the development of future energy storage technology

Modelling the ageing behaviour of supercapacitors using electrochemical impedance spectroscopy for dynamic applications

Diagnosis of ageing mechanisms in supercapacitors is made difficult by the enforcement of various ageing factors in the current ageing tests. The thesis presents the exact determination of the ageing mechanism by separating the impacts of high temperature, current cycling and constant voltage applications in accelerated ageing tests. The state of health (SOH) of the supercapacitors are monitored periodically with electrochemical impedance spectroscopy, cyclic voltammetry and constant current test to observe the evolution of ageing. The thesis identifies patterns of ageing from the changes at supercapacitor impedance. The thesis also presents the cause of the increase in ESR and the loss of capacitance in supercapacitors. High temperature application causes the appearance of high frequency semicircle which reflects the damage at the electrode-current collector interface. A tilt of the impedance line at low frequencies reflects modifications of electrodes and it is most sensitive to current cycling and constant voltage applications. The increase in ESR is observed to be caused by a single ageing mechanism while the capacitance loss is caused by multiple interactions of these ageing mechanisms at the same time. The thesis develops a supercapacitor model by means of electrical equivalent circuit. The model is divided into two parts based on the changes in its SOH: the baseline model represents the early stage of the supercapacitor life and the ageing model represents the phase of ageing. The models show dynamic interactions between ageing process and supercapacitor electrical performance. The supercapacitor model, in the form of fractional-order model, reduces the number of circuit components and shows excellent electrical behaviour particularly at the open circuit voltage decay and voltage recovery period. The parameterisation of model parameters shows that aged supercapacitors experience an increase of distributed resistance in the electrode pores and an increase of diffusion impedance at high temperature

Structure and properties of supercapacitor and lithium-ion battery electrodes: the role of material, electrolyte, binder and additives

Key parts of an electrochemical energy storage device are the active material, the electrolyte, the binder, and the conductive additives. This dissertation investigates the role of such individual components on the device’s overall performance and how they interact with each other to influence the device’s ability to store energy and longevity. Three aspects of the performance of electric double-layer capacitors are investigated: (1) The role of the conductive additives on performance and longevity, where 5 wt% admixture shows the best capability. (2) The role of the active material and the electrolyte with an increased capacitance when the pore width matches the ion size. (3) The volumetric expansion of carbon electrodes during charging is depending on the size ratio of the ions and the pore width. Further, an asymmetry in charging mechanism is found for two-dimensional metal carbides, MXenes, in ionic liquids. The charging mechanism is based on cation (de-)intercalation. The role of binder properties on the performance of battery electrodes was investigated with intercalation-induced volumetric changes of the active material. Moreover, the multi-length scale approach using different in situ measurement techniques reveals a promising way to understand mechanisms in electrochemical energy storage devices. The combination of dilatometry with quartz-crystal microbalance, X-ray diffraction or small-angle X-ray scattering shed light on potential-induced structural changes in the systems.Vier wichtige Bestandteile einer elektrochemischen Energiespeicherzelle sind aktives Material, Elektrolyt, Binder und Leitruß. In dieser Dissertation wird der Einfluss dieser Bestandteile untereinander und auf die elektrochemischen Eigenschaften untersucht. Drei Themenkomplexe werden in Bezug auf elektrische Doppelschichtkondensatoren untersucht: (1) Die Rolle von leitfähigen Additiven auf Leistung und Langlebigkeit, wobei eine 5 %-ige Beimischung die beste Leistung zeigt. (2) Die Rolle des aktiven Materials und des Elektrolyten mit einer erhöhten Kapazität, wenn die Porenbreite mit der Ionengröße übereinstimmt. (3) Die volumetrische Ausdehnung von Kohlenstoffelektroden während des Ladens hängt von dem Größenverhältnis der Ionen und der Porenweite ab. Es wurde eine Asymmetrie im Lademechanismus bei zweidimensionalen Metallkarbiden, MXenen, in ionischen Flüssigkeiten gemessen. Der Lademechanismus basiert auf Kationeninterkalation. Für ein Batteriesystem mit interkalationsbedingter Volumenänderung des aktiven Materials wurde der Einfluss vom Binder auf die Leistung untersucht. Darüber hinaus zeigt der Multi-Längenskalen-Ansatz mit verschiedenen in-situ-Messmethoden eine vielversprechende Möglichkeit um Mechanismen in elektrochemischen Energiespeichern zu verstehen. Die Kombination von Dilatometrie mit entweder Quarzkristall-Mikrowaage, Röntgenbeugung oder Kleinwinkel-Röntgenstreuung konnte ladungsinduzierte Strukturänderungen im System zeigen

Modelling the ageing behaviour of supercapacitors using electrochemical impedance spectroscopy for dynamic applications

Diagnosis of ageing mechanisms in supercapacitors is made difficult by the enforcement of various ageing factors in the current ageing tests. The thesis presents the exact determination of the ageing mechanism by separating the impacts of high temperature, current cycling and constant voltage applications in accelerated ageing tests. The state of health (SOH) of the supercapacitors are monitored periodically with electrochemical impedance spectroscopy, cyclic voltammetry and constant current test to observe the evolution of ageing. The thesis identifies patterns of ageing from the changes at supercapacitor impedance. The thesis also presents the cause of the increase in ESR and the loss of capacitance in supercapacitors. High temperature application causes the appearance of high frequency semicircle which reflects the damage at the electrode-current collector interface. A tilt of the impedance line at low frequencies reflects modifications of electrodes and it is most sensitive to current cycling and constant voltage applications. The increase in ESR is observed to be caused by a single ageing mechanism while the capacitance loss is caused by multiple interactions of these ageing mechanisms at the same time. The thesis develops a supercapacitor model by means of electrical equivalent circuit. The model is divided into two parts based on the changes in its SOH: the baseline model represents the early stage of the supercapacitor life and the ageing model represents the phase of ageing. The models show dynamic interactions between ageing process and supercapacitor electrical performance. The supercapacitor model, in the form of fractional-order model, reduces the number of circuit components and shows excellent electrical behaviour particularly at the open circuit voltage decay and voltage recovery period. The parameterisation of model parameters shows that aged supercapacitors experience an increase of distributed resistance in the electrode pores and an increase of diffusion impedance at high temperature

Redox electrolytes for non-flow electrochemical energy storage

In recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte, but also in the electrolyte by the redox activities of the bulk electrolyte itself. Such redox electrolytes are promising for non-flow energy storage (redox electrolyte aided hybrid energy storage systems, REHES) particularly when they are combined with electrodes made of nanoporous carbon. In this PhD work, I have established a fundamental understanding regarding ion diffusion, process kinetics, and adsorption of redox ions. For that, different REHES systems have been investigated including tetrapropylammonium iodide, zinc iodide, potassium iodide, potassium ferricyanide, vanadyl sulfate, tin sulfate, and tin fluoride. The basic understanding of REHES systems enabled the targeted improvement of the device performance throughout this PhD work. Compared to the energy storage capacity of a conventional (non-redox) electrical double layer capacitor of 4 Wh/kg (ca. 80 F/g), the use of the ZnI2 redox electrolyte yielded significantly higher performance of up to 226 Wh/kg. Furthermore, the specific power was also enhanced from 1.3 kW/kg to 20 kW/kg. As a key conclusion, this PhD work demonstrates the high attractiveness of REHES systems not only from a performance point of view, but also regarding low cost and simplicity of the system.Die Forschung der letzten Jahrzehnte hat eine neue Art der elektrochemischen Energiespeicherung hervorgebracht, bei der elektrische Ladung nicht nur an der Grenzfläche zwischen der Elektrode und dem Elektrolyten gespeichert wird, sondern auch im Elektrolyten selbst durch dessen Redoxaktivität. Diese Redox-Elektrolyte sind für hybride Energiespeichersysteme ohne extern-mechanische Bewegung des Elektrolyten (REHES) vielversprechend, insbesondere, wenn hochporöse Kohlenstoffmaterialien als Elektroden verwendet werden. In dieser Doktorarbeit wurden verschiedene REHES-Systeme hinsichtlich der Diffusion, der elektrochemischen Kinetik und der Adsorption von Redox-Ionen untersucht, um grundlegende Effekte und Prozesse aufzuklären. Die Kombination grundlegender Elektrochemie und Materialcharakterisierung ermöglichte es, die Leistungsparameter von REHES im Vergleich zum Stand der Technik deutlich zu verbessern. Die Energiespeicherkapazität des herkömmlichen wässrigen elektrischen Doppelschichtkondensators ohne Redoxelektrolyt von 4 Wh/kg (entspricht 80 F/g) wurde zum Beispiel im ZnI2 System auf bis zu 226 Wh/kg gesteigert, während die spezifische Leistung von 1.3 kW/kg auf 20 kW/kg verbessert werden konnte. Als Ergebnis zeigt sich, dass REHES ein besonders vielversprechender Ansatz zur Darstellung hochleistungsfähiger elektrochemischer Energiespeicher ist. Weitere Vorteile von REHES sind ein vereinfachtes Zellkonzept und die Verwendung von potentiell kostengünstige Einzelkomponenten

Redox electrolytes for non-flow electrochemical energy storage

In recent decades, a new type of electric energy storage system has emerged with the principle that the electric charge can be stored not only at the interface between the electrode and the electrolyte, but also in the electrolyte by the redox activities of the bulk electrolyte itself. Such redox electrolytes are promising for non-flow energy storage (redox electrolyte aided hybrid energy storage systems, REHES) particularly when they are combined with electrodes made of nanoporous carbon. In this PhD work, I have established a fundamental understanding regarding ion diffusion, process kinetics, and adsorption of redox ions. For that, different REHES systems have been investigated including tetrapropylammonium iodide, zinc iodide, potassium iodide, potassium ferricyanide, vanadyl sulfate, tin sulfate, and tin fluoride. The basic understanding of REHES systems enabled the targeted improvement of the device performance throughout this PhD work. Compared to the energy storage capacity of a conventional (non-redox) electrical double layer capacitor of 4 Wh/kg (ca. 80 F/g), the use of the ZnI2 redox electrolyte yielded significantly higher performance of up to 226 Wh/kg. Furthermore, the specific power was also enhanced from 1.3 kW/kg to 20 kW/kg. As a key conclusion, this PhD work demonstrates the high attractiveness of REHES systems not only from a performance point of view, but also regarding low cost and simplicity of the system.Die Forschung der letzten Jahrzehnte hat eine neue Art der elektrochemischen Energiespeicherung hervorgebracht, bei der elektrische Ladung nicht nur an der Grenzfläche zwischen der Elektrode und dem Elektrolyten gespeichert wird, sondern auch im Elektrolyten selbst durch dessen Redoxaktivität. Diese Redox-Elektrolyte sind für hybride Energiespeichersysteme ohne extern-mechanische Bewegung des Elektrolyten (REHES) vielversprechend, insbesondere, wenn hochporöse Kohlenstoffmaterialien als Elektroden verwendet werden. In dieser Doktorarbeit wurden verschiedene REHES-Systeme hinsichtlich der Diffusion, der elektrochemischen Kinetik und der Adsorption von Redox-Ionen untersucht, um grundlegende Effekte und Prozesse aufzuklären. Die Kombination grundlegender Elektrochemie und Materialcharakterisierung ermöglichte es, die Leistungsparameter von REHES im Vergleich zum Stand der Technik deutlich zu verbessern. Die Energiespeicherkapazität des herkömmlichen wässrigen elektrischen Doppelschichtkondensators ohne Redoxelektrolyt von 4 Wh/kg (entspricht 80 F/g) wurde zum Beispiel im ZnI2 System auf bis zu 226 Wh/kg gesteigert, während die spezifische Leistung von 1.3 kW/kg auf 20 kW/kg verbessert werden konnte. Als Ergebnis zeigt sich, dass REHES ein besonders vielversprechender Ansatz zur Darstellung hochleistungsfähiger elektrochemischer Energiespeicher ist. Weitere Vorteile von REHES sind ein vereinfachtes Zellkonzept und die Verwendung von potentiell kostengünstige Einzelkomponenten

Synthesis and functionality of boron-, nitrogen- and oxygen-doped shaped carbon-based nanomaterials and titania nanocomposites in electrochemical capacitors

Doctor of Philosophy in Higher Education. University of KwaZulu-Natal,Westville,2020Energy is a global fundamental sector and major concerns are inclusive of; making renewable power economical, reliable and accessible to all, maintain and improve power quality, voltage and frequency, amongst others. There is need for development of intelligent energy storage systems (ESS) that maximise and provides durable storage of electrical power generated. This is a suitable approach towards reducing gas emissions, lowering electricity bills, meet power needs at any time and for lowering excess power fluctuations. Much advancement is required on ESS to shift their optimum working regions towards preferred limits with both high justifiable power and energy. Advancement of ESS need to be sought through developing effective electrode materials. Shaped carbon nanomaterials (SCNMs) are suitable for ESS in the Smart Grids with potential better cost effective and scalable standards. The investigation of related physicochemical properties of SCNMs, modification of nano-structural parameters and development of appropriate strategies that would enhance their functionality in ESS is key in this regard. In this study, various ESS were reviewed with more focus on development of electrochemical capacitors (ECs) with a bias towards the use of SCNMs as electrodes. The work was aimed at understanding the influence of reagent ratio in the physicochemical properties of N-doped multiwalled carbon nanotubes (N-MWCNTs) and graphene oxide (GO). Also, it focused on modifying the functionality of MWCNTs, N-MWCNTs and reduced graphene oxide (RGO) in ECs via introduction and control of heteroatoms such as nitrogen and its functional moieties or introduction of oxygen-containing groups. Thirdly, the work investigated the effect of composite synthesis on the performances of individual components via control of wt.% ratios. Characterisation techniques used include transmission and scanning electron microscopies, atomic force microscopy, textural characteristics, thermogravimetric analysis, elemental analysis, cyclic voltammetry, electrochemical impedance spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction, ultraviolet-visible spectrophotometry, Raman and Fourier transform infra-red spectroscopies. N-MWCNTs were synthesized from N,N’-dimethyl formamide and acetonitrile as sp3 and sp hybridized nitrogen sources, respectively, as materials for ECs. The combination of ferrocene carboxaldehyde, N,N’-dimethyl formamide and acetonitrile in N-MWCNTs synthesis was a novel approach. Mixing the sp3 and sp sources in 1:3 ratio enhanced nitrogen content to 9.38% from that of both sp3 (5.87%) and sp (3.49%). The physical properties such as number of concentric shells were tailored by varying synthesis temperature. Pyrrolic N-doping was achieved as the main constituent of nitrogen moieties. Furthermore, GO was synthesized as a preliminary step for further N-doping. The effect of graphite: Na2NO4 reagent ratio in the synthesis of GO was studied to elucidate the influence of the initial step in GO synthesis, via modified Hummer’s method, and to develop novel strategies towards controllable products. The physicochemical properties such as content of oxygen-containing groups on GO and the surface areas were increased from 0% and 2 m2 g-1 to 30% and 188 m2 g-1, respectively, by increasing the proportion of Na2NO4 in reagents. The manipulation of the initial step was a novel means of tailoring the associated physicochemical properties of GO. Also, this study determined, for the first time, the most effective group one sulfate electrolyte at fixed concentrations. This aided the selection of the electrolyte used in the application of the SCNMs in this thesis. Oxygen moieties were introduced, by ultra-sonic waterbath treatment, onto MWCNT surfaces using various reagents namely; HCl, HNO3, H2O2 and HNO3/ HCl solutions. The study highlighted how the various reagents, commonly used to purify MWCNTs after synthesis, modify associated physicochemical properties and alter charge storage characteristics. Oxygen-containing groups increased capacitance of pristine MWCNTs and introduced pseudo charge storage mechanism via oxygen functionalities. HNO3 treated MWCNTs had a 77- and 2.5-fold upgrading from pristine using Li2SO4 and Na2SO4, respectively, whilst HNO3/ HCl was the best, 5 times better, in K2SO4. The oxygen-modified MWCNTs performance was highest and of best quality in Na2SO4. The effectiveness of common GO reductants, namely; ascorbic acid, hydrazine hydrate and sodium borohydride were practically investigated. This was done to select a reductant for the current work. This study also provided a viable novel chemical tuning approach for nitrogen moieties and content as well as to introduce boron, with sodium borohydride. Thirdly, under this particular study, the effect of heteroatoms, boron and nitrogen, as well as nitrogen moieties on physicochemical characteristics of RGO was also explored. Hydrazine hydrate was the most effective reductant and was associated with highest surface area and N-content of 390.55 m2 g-1 and 4.07 at.%, respectively. The nitrogen groups of RGO reduced by means of ascorbic acid, hydrazine hydrate and pristine were pyrrolic, pyridinic and sp3 N-C, respectively. N- doped RGO, particularly pyrrolic moieties, were 76-fold better than B-doped. A further iii iv thermal reduction, of RGO from hydrazine hydrate, increased surface area from c.a. 391 to c.a. 600 m2 g-1 at 750 ℃. The effect of oxygen-containing groups was then investigated in composites of titania with GO, RGO and cellulose reduced graphene oxide (CRG). The wt.% ratios of titania were varied; i.e., 5, 10, 20 and 40%. Based on earlier deductions in this thesis, reductant chosen was hydrazine hydrate. Titania enabled better exfoliation of GO but at higher wt.%, it culminated in larger agglomerates which in turn increased diffusion path-length. RGOTi at 5 wt.% titania increased surface area from 136.89 to 434.24 m2 g-1. The study generally showed that capacitance was better at lower wt.% titania in RGOTi and that cellulose surface area increase was outweighed by associated insulating effect. The present data infers that the impact of oxygen moieties on capacitance of SCNMs was subject to specific structures; MWCNTs, GO and RGO. Capacitance of titania and GO were improved by composite synthesis. Graphenated N-MWCNTs were targeted, as a means, to lessen agglomeration, without the use of surfactants, and to generate 3-D scaffolds for better electrical conductivity channels. Also, better physicochemical characteristics for higher capacitance were obtained via sol-gel than CVD method. The ratios of sp3- and sp-hybridized nitrogen in reagent mixtures, in this thesis, was effectively used to tune the composition of pyrrolic nitrogen moieties. Pyrrolic composition of N-MWCNTs was uniquely aimed because studies of typical moieties on RGO deduced pyrrolic to be better than pyridinic groups. The increase of pyrrolic nitrogen composition; 35, 45 and 60%, culminated in capacitance deterioration. Composite synthesis reduced Warbug length and amplified associated capacitance. The physicochemical properties of RGO, GO, MWCNTs and N-MWCNTs were positively tuned from reagent ratios, conditions and composite syntheses. The conjectured strategies could modulate their overall capacitance via manipulation of heteroatom content and functional groups, amongst others listed herein. Several traits that linked physicochemical properties and capacitance were successfully elucidated. This affirms the hypothesized potential of SCNMs in ESS through understanding and control of both nano-structural parameters and physicochemical properties

Investigating ageing behaviours in supercapacitor (cells and modules) using EEC (electrical equivalent circuit) models

This thesis contributes to the reliability and aging studies of supercapacitors for more efficient use in EV/HEV applications. This thesis demonstrates the effect of aging/failure in supercapacitor cells and module cells using accelerated tests employed to expedite the aging process. The tests, as explained below were categorized based on operational and environmental aging factors associated with supercapacitor failure in EV/HEV applications to; • Investigate supercapacitor cell performance at high temperature and constant voltage individual conditions, and also simultaneously (known as calendar test) • Investigate the effect of voltage balancing/equalization circuits on supercapacitor module cells’ performance during constant current cycling tests under certain environmental and electrical factors • Investigate supercapacitor module cells’ cycling performance in a lab-scale designed electrical DC programmable motor load system that emulates supercapacitor operational conditions in an EV/HEV application. The aging behaviors characterized by the three factors mentioned above are quantified in this thesis through the periodic monitoring of their electrical and electrochemical state of health with Electrochemical Impedance Spectroscopy, Cyclic Voltammetry, and Constant Current characterization tests. These tests help identity aging modes in supercapacitors, and it was observed that regardless of their aging factors; an increase in ESR and decrease of capacitance was determined. Although this information is required, the results from Electrochemical Impedance Spectroscopy (EIS) tests revealed more details distinctive to each aging factor. From this distinction, the aging mechanisms in relation to the aging factors, which causes the deterioration in the supercapacitor electrical performance, are identified and summarized as the following: 1. Loss of contact within supercapacitor electrode, given rise to the contact resistance due to the presence of high temperature as the main aging factor 2. Change of supercapacitor porous electrode emulating a charge transfer reaction thereby increasing its distributed resistance, caused by the effect of high voltage or cycling Mathematical models in the form of electrical equivalent circuits (EECs) distinctive of their aging factors are generated from EIS electrochemical behaviors to easily describe aging behaviors in supercapacitors. The EEC models developed using impedance modeling generated an initial model from dormant cells, which transitioned to aging models distinctive of their aging factors as soon as a 100% increase in ESR and/or an 80% decrease in capacitance is observed. The proposed EEC models were validated to show the dynamic interaction between aging of the supercapacitor cells on their electrical performance in both frequency and time domains. In summary, the EEC models encompass this thesis objective and as such considered the main contribution of this research work