Uute mikro-mesopoorsete karbiididest sünteesitud süsinikmaterjalide valmistamine ning karakteriseerimine kõrge energia- ja võimsustih edusega elektrilise kaksikkihi kondensaatori elektroodimaterjalina

Väitekirja elektrooniline versioon ei sisalda publikatsioone.Viimaste aastate geopoliitiliste kriiside taga on tihti võitlus piiratud energiaressursside üle. See on oluline, kuid kindlasti mitte ainus põhjus, miks on järjest rohkem hakatud rääkima energiajulgeolekust ning moodsate tehnoloogiate kasutuselevõtust selle saavutamiseks. Need moodsad tehnoloogiad põhinevad suuresti päikesest ja tuulest elektrienergia tootmisele, mida omakorda on vaja kombineerida erinevate energiasalvestusseadmetega. Paraku on vajalike energiamuundurite ning suure erimahtuvuse ja erivõimsusega energiasalvestite konstrueerimine ilma konkurentsivõimeliste ja efektiivsete superkondensaatoriteta äärmiselt keeruline. Kommertsiaalsed superkondensaatorid on viimasel aastakümnel teinud suure arenguhüppe ning nende kasutamine kasvab kiiresti. Siiski ei ole superkondensaatorite hinna ja kvaliteedi suhe piisavalt hea, et rakendada neid massiliselt elektrivõrkudes taastuvenergeetika efektiivsemaks muutmise nimel. Kuigi väga suure soovi korral saaks kogu maailm juba täna minna üle taastuvale ning mittereostavale energeetikale, oleks selle hind kõrge ning inimesed pole valmis seda investeeringut tegema. Superkondensaatorite hinna ja kvaliteedi suhte parandamiseks on võimalik kas muuta tootmist odavamaks või oluliselt parandada seadmete elektrokeemilisi omadusi ning tõsta nende kvaliteeti. Üheks olulisimaks komponendiks superkondensaatoris on poorne kõrge eripinnaga elektrood, mille füüsikaline morfoloogia ja omadused määravad suuresti ka lõppprodukti karakteristikud nagu võimsus, mahtuvus ja eluiga. Selle tõttu on uute ning superkondensaatorites paremini käituvate elektroodimaterjalide väljatöötamine äärmiselt oluline. Tartu Ülikooli keemia instituudis on viimastel aastatel intensiivselt uuritud eriliste omadustega mikropoorse süsinikmaterjali sünteesi karbiididest ning selle kasutamist superkondensaatorite elektroodina. Me oleme võimelised üsna lihtsate meetoditega sünteesima väga unikaalsete füüsikaliste ja keemiliste omadustega süsinikmaterjale. Käesoleva doktoritöö raames valmistati uudseid süsinikmaterjale erinevatest karbiididest kõrgtemperatuurse halogeenimise meetodil. Esialgu teostati materjalidele erinevad füüsikalised mõõtmised mille käigus saadi parem arusaam nende morfoloogiast, kristallilisusest ja keerukast poorsest struktuurist. Seejärel valmistati süsinikmaterjalidest superkondensaatori elektroodid ning teostati põhjalikud elektrokeemilised mõõtmised ja analüüs. Töö käigus saadud tulemuste põhjalikumal analüüsil leiti mitmeid selgepiirilisi korrelatsioone, kuidas materjalide erinevad füüsikalised karakteristikud mõjutavad superkondensaatori elektrokeemilisi omadusi. Selle tulemusel leiti, millised sünteesitud materjalid sobivad superkondensaatori elektroodideks paremini ning milliseid füüsikalisi parameetreid peaks eriti silmas pidama, kui otsida uusi hea hinna ja kvaliteedi suhtega materjale superkondensaatorite elektroodide valmistamiseks.The geopolitical crises in recent years are often triggered by fights over limited energy resources. This is one of many reasons why it is becoming more and more important to invest in energy security and modern technologies. The latter are largely based on harvesting the energy of the sun and wind, which have to be backed up by various energy storage devices. The production of the necessary energy converter systems as well as high energy and power density storage systems is very difficult without competitive and efficient supercapacitors. In the last decade the commercial supercapacitors have made a great leap forward and their use is growing rapidly. However, their price-performance ratio is still not good enough to implement them on a large scale in electricity networks to make renewable energy more stable. If in great need, the world could meet its energy need from renewable resources even now. However, the price for this is still too high for the economically thinking mankind. To improve the price-performance ratio of supercapacitors there are two options: either lower the cost of production or significantly improve the electrochemical properties of supercapacitors. One of the key components in a supercapacitor is the porous electrode with a very high specific surface area. The morphology and physical characteristics of the electrode determine the supercapacitors electrochemical properties such as power density, capacity and lifetime. For these reasons, it is very important to develop new and better electrode materials for supercapacitors. In the Institute of Chemistry in University of Tartu a lot of work has been done in recent years to study unique carbon materials and their applicability to be used as supercapacitor electrodes. By fairly simple synthesis methods, we are capable of producing carbon materials with exceptional physical and electrochemical properties. In this work new carbon materials were produced from various carbides by means of high-temperature halogenation. Initially, these materials were physically characterized to better understand their morphology, crystallinity and complex porous structure. Thereafter the carbons were roll-pressed into supercapacitor electrodes and electrochemically analyzed. Clear correlations were established between the electrochemical and physical characteristics of the carbon materials synthesized. As a result, it was determined which materials were better suited for supercapacitor electrodes and which of the physical parameters should be especially kept in mind, when designing new materials for supercapacitors electrodes with a good price-performance ratio

Study of an off-grid wireless sensors with Li-Ion battery and Giant Magnetostrisctive Material

L'abstract è presente nell'allegato / the abstract is in the attachmen

Investigation of Bipolar Electrochemically Exfoliated Graphene for Supercapacitor Applications

Developing a reliable, simple, cost-efficient and eco-friendly method for scale-up production of high-quality graphene-based materials is essential for the broad applications of graphene. Up to now, various manufacturing methods have been employed for synthesizing high quality graphene, however aggregation and restacking has been a major issue and the majority of commercially available graphene products are actually graphite microplates. In this study, bipolar electrochemistry techniques have been used to exfoliate and deposit graphene nanosheets in a single-step process to enable high performance device application. In the first part of this study, bipolar electrochemistry concept is utilized to design a single-step and controllable process for simultaneously exfoliating a graphite source and depositing both graphene oxide (GO) and reduced graphene oxide (rGO) layers on conductive substrates. The electrochemical performance of the fabricated graphene-based materials as the electrode for supercapacitors has been investigated. Areal capacitance of 1.932 mF cm-2 for the rGO, and 0.404 mF cm-2 for GO at a scan rate of 2 mV s-1 were achieved. Moreover, a cut-off frequency of 1820 Hz was obtained, which is a promising characteristic for AC filtering applications. Although the physicochemical characteristics of produced graphene have been evaluated in the first part, the exfoliation and deposition mechanisms were still unclear. In the second part of this dissertation, a novel modified BPE system with an electrically connected graphite-platinum couple acting as the bipolar electrode has been designed in order to decouple and investigate the contribution of anodic/cathodic exfoliation and deposition of graphene in the BPE process. Electron microscopy and infrared spectroscopy results indicate that both anodic and cathodic exfoliation of graphene could take place regardless of the type of polarization; however, the morphology and deposition rate highly depend on the polarization. Furthermore, the graphene fabricated by anodic exfoliation was found to show higher levels of oxidation compared to the graphene produced by cathodic exfoliation. In the last part of this study, for the first time, a vertically aligned graphene layer was deposited on a micro-sized interdigitated gold current collector by a modified bipolar electrochemistry method. Both time domain and frequency domain electrochemical performance of on-chip micro-supercapacitors (MSCs) were evaluated. An areal capacity of 640.9 μF cm-2 at a scan rate of 2 mV s-1 and 239.31 μF cm-2 at discharge current density of 25 μA cm-2 was delivered with an excellent cyclability. Most importantly, the MSC exhibited a very fast response (cut-off frequency of 3486 Hz) and very close to ideal performance (phase angle reached -83.2°) at low frequencies. For the first time, this dissertation reported the modified BPE method as a novel approach for three in one exfoliation, deposition and reduction of high-quality graphene with vertically aligned and porous structure. The unique design of the BPE cell enabled the author to study the BPE mechanisms and measure the bipolar current for the first time. The method could successfully be employed to fabricate fast response microsupercapacitors based on vertically aligned graphene nanosheets

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

Electrochemical Capacitors for Miniaturized Self-powered Systems

Miniaturized self-powered systems with harvest-store-use architectures have been recognized as a key enabler to the internet-of-things (IoT), and further the internet-of-everything (IoE), 5G communication and tactile internet. Electrochemical capacitors (ECs), also known as supercapacitors, are promoted to be the energy storage component in such systems, because of their advantages such as an almost limitless cycle life that is ideal for the vision of “fit-and-forget” maintenance-free networks. Moreover, ECs are able to undertake tasks beyond energy storage. For example, high-frequency ECs can potentially replace the bulky electrolytic capacitors as AC line filters, with benefits in sizing down the circuitry boards and thus constructing compact systems which are pursued by the IoT technology.Bringing the IoT high-level requirements down to the device-level specifications, challenges to ECs are identified in different aspects, including device electrochemical performance, and device encapsulation/integration. Regarding the performance, challenges exist in (1) improving the energy density, (2) maximizing the operating voltage limit, (3) widening the working temperature range, (4) minimizing the self-discharge and leakage current, and (5) enhancing the frequency response property. Regarding the encapsulation and integration aspect, challenges exist in device design and fabrication. Novel encapsulation and integration EC concepts are thus appreciated to be compatible with the surface mount technology, allow for convenient adaption in the form factor and arbitrary choice of the EC materials (electrodes, electrolytes and separators). Moreover, the EC materials should be durable under the ambient conditions that occur during the encapsulation and integration processes, such as high-temperature exposure for the reflow soldering technique.The thesis research work addresses the device performance challenges. Specifically, the use of redox electrolytes is promoted for improving the energy density of ECs towards a battery-level, and at the same time keeping the capacitor-level power capability and cycling stability. With a redox-active electrolyte KBr, hybrid devices combining the features of both batteries and ECs are constructed, and a 1.9 V maximum operating voltage is achieved in the aqueous system. Furthermore, voltage- and history-dependent behaviors are revealed, reminding the complexity of hybrid systems. To explore the extreme high-temperature performance, a special measurement setup is customized and an EMImAc (1-Ethyl-3-methylimidazolium acetate) ionic liquid (IL) electrolyte is employed to enable an operation at a maximum of 150 \ub0C. It is observed that the energy and power densities at high temperatures may not be sacrificed when decreasing the operating voltage limit, therefore it is proposed that for neat IL-based ECs, a strategy of trading the voltage limit for gaining stability at extreme high-temperatures can be considered.With a graphite and carbon nanotubes hybrid material, it is demonstrated that the self-discharge and leakage current can be suppressed by employing a gel polymer electrolyte. Using the same electrode material, high-frequency ECs that are suitable for AC line filtering tasks are fabricated. The working frequency range is up to kHz with a state-of-art level areal (1.38 mF cm-2) and volumetric capacitances (345 mF cm-3), benefiting from a possible covalent bonding between graphite substrate and the CVD grown CNTs.Not limited to the above research findings, this thesis has critically reviewed and summarized the general strategies and methods to address all the identified challenges to ECs for their application in miniaturized self-powered systems

Rapid Fabrication of Miniaturized Electric Double-Layer Capacitors Based on Laser Patterned Graphene oxide-Poly (Furfuryl Alcohol) Nanocomposites

Miniaturization of various technologies has accelerated and drives new requirements for energy harvesting and storage solutions to power such devices. Reduction in size and designs requiring mechanical flexibility fuel the development of new materials and fabrication processes. The most advanced solutions to date have been to engineer thinner, more flexible rechargeable batteries. However, their short cycle-lifetime due to the intrinsic chemical nature of their energy storage mechanism, their low power density, high cost and safety concerns limit their application in variety of areas. For long-term electronic use in health-monitoring, smart-implants and the internet of things, a new form of power source which no longer requires replacement and offers consistent performance is needed. These concerns can be addressed if another energy storage technology is considered – the electrochemical supercapacitor. Such devices are generally reliable, offer long lifetimes, are safer and can be operated efficiently at high power. However, they typically store less energy per unit mass or volume than batteries. This thesis focuses on the development of miniaturized supercapacitors with improved performance as an energy storage solution for self-sufficient, micro-scale, and flexible electronics. Specifically, a type of supercapacitor called an electrical double-layer capacitor (EDLCs) is well known for high-power capabilities and a long-lasting cycle-life, typically above 100,000 charge and discharge cycles. The approach adopted in this study aims to improve the energy density using sustainable materials through a scalable and cost-effective fabrication technique. We will demonstrate that a polymer resin formed from an inexpensive monomer derived from waste biomass such as corn husks can be carbonized into a high surface area, electronically conductive material using a commercial CO2 laser. High resolution laser patterning of electrodes onto this material results in a micro-supercapacitor with much improved performance compared to state-of-the-art flexible and miniaturized supercapacitors. This thesis work initially focused on the understanding and development of laser irradiation as a fabrication technique using commercially available carbonaceous materials such as commercial Kapton® polyimide films as well as lab-made graphene oxide. A complete understanding of the variables involved in the fabrication process helped to optimize the laser irradiation technique. This achieved the highest reported micro-supercapacitor energy density compared to published research on similar materials. The main focus of this work was to develop an improved electrode material in terms of both cost and performance that could be laser activated in a similar way. To this end, we hypothesized that poly (furfuryl alcohol) (pFA), which is known to form a dense, microporous glassy carbon upon conventional heat treatment, might be a good candidate material for more effective laser activation. Several methods of polymer synthesis from the low cost, abundant, biomass waste derived monomer of pFA were explored to create a laser-inducible substrate including acid-catalyzed emulsion polymerization and bulk resinification. Surprisingly, despite a high yield conversion to carbon using conventional heating, the neat material could not be carbonized with the laser system. However, the addition of a small amount of graphene oxide was found to catalyze the formation of the carbonized structure which ultimately led to an improved supercapacitor. The optimum pFA synthesis conditions and the effect of graphene oxide incorporation were explored to maximize device performance. In each case, evolution of the carbonized structure was investigated by Raman and X-ray photoelectron spectroscopy. The resulting micro-supercapacitors boast a simple, inexpensive and rapid fabrication technique that are promising for future device applications. Notably, optimized electrodes achieved the highest specific areal capacitance presented in literature compared to other laser irradiated carbon precursors. The specific capacitance has been improved four times (4x) from literature’s highest measurement at 31 mF/cm2 to 147 mF/cm2 during this thesis work. pFA is heralded as a new renewable and environmental friendly energy storage source; recycled from common biomass waste into the solution to today’s most pressing micro-electronics need - power

Synthesis of electrolytic manganese dioxide (EMD) and biomass waste-derived carbon for hybrid capacitors

Renewable energy (RE) is expected to be the primary energy supplier in the future energy mix. This has created the necessity for low-cost, safe, and reliable energy storage to guarantee a continuous energy supply by the intermittent RE sources. Due to the inbuilt rich chemistry of manganese dioxide (MnO2) and the advantageous characteristics; of low cost, environmentally friendliness, and nontoxic, it can be adapted for a wide range of applications such as biosensors, humidity sensors, catalysts, and so on. Among the different forms of MnO2, electrolytic manganese dioxide (EMD) is well-demanded energy storage material. However, the limitations such as lower capacitance, irreversibility, and cyclability of EMD in comparison with other metal oxides such as cobalt and nickel oxides, have hindered its application in capacitor energy storage, which was one of the focuses of this thesis. Therefore, this Ph.D. research project aimed at synthesizing modified EMD materials as the positive electrode for hybrid capacitor applications. The modified EMD was coupled with the biomass-derived activated carbon (AC) which is synthesized as the negative electrode to fabricate hybrid capacitors. This Ph.D. research work has contributed to the existing knowledge through the following: 1) synthesizing pristine EMD using galvanostatic electrodeposition and studying its suitability for capacitor applications via experimental and theoretical analysis, 2) biopolymer alginate assisted EMD synthesis and optimization via experimental and computational modeling, 3) studying the effect of varying surfactants to improve the electrochemical characteristics of EMD, 4) synthesis of biomass waste-derived activated carbon and modeling their parameters for capacitance prediction. The results indicated the challenge and importance of the delicate tailoring of the EMD characteristics for capacitor application. Pristine EMD was synthesized under different electrodeposition experiment conditions by varying applied current density (100, 200, 300 A m-2) and deposition duration (4, 5, 6 h). The electrodeposition was carried out in a low acidic medium electrolytic bath where a lead (Pb) anode and stainless steel (SS) cathode were used. The EMD was deposited on the Pb anode via Mn2+ oxidation to form Mn4+ and its oxide MnO2. The physicochemical and electrochemical characterization of the obtained EMD powder concluded that the material deposited at 200 A m-2 for 5 hours, showing the spindle-like morphology was suitable over others for supercapacitor (SC) application. The pristine EMD at these experimental conditions delivered 98 F g-1 capacitance at 1 mA cm-2 applied current density tested in 2 M NaOH aqueous electrolyte and proved its potential development by modifying its characteristics. Therefore, the pristine EMD was modified by introducing the biopolymer alginic acid crosslinking to improve its electrochemical performance. The alginic acid was added to the electrolytic bath at varying concentrations; 0, 0.1, 0.25, 0.5, and 1 g l-1, to optimize the added bio-polymer amount to maximize the capacitance. At 0.5 g l-1, the pristine EMD morphology was rearranged to a cactus-shaped with flutes. The calculated specific capacitance of the modified EMD was ~5 times higher (487 F g-1) than the pristine EMD. The molecular dynamics simulation results determined the polymer-ion interactions in the electrolytic bath and provided evidence, showing that the alginic acid could act as a template for binding the Mn2+ ions in a relatively ordered manner for the growth of the EMD deposit. 0.42 of pyrolusite and 0.58 of ramsdellite fractions present in the modified material were quantitatively determined using the neutron powder diffraction (NPD) data. The slight increments of the lattice spacing observed in high-resolution transmission electron microscopy (HRTEM) images were well aligned with the NPD results of unit cell volume expansions of the EMD-polymer composite showing the polymer intercalation within the EMD structure influencing its characteristics. At 2 mA cm-2, the fabricated hybrid capacitor delivered 52 F g-1 specific capacitance, 14 Wh g-1 specific energy, 500 W g-1 specific power, and 94 % capacitance retention over 5000 cycles. The results highlighted the importance of the functional molecular structure of the biopolymer alginic acid to produce a binary composite of EMD-polymer as a capacitor material. Further, the pristine EMD was modified by electrodepositing the MnO2 using surfactant mediated electrolyte solutions. The electrochemical performance of the synthesized EMD in the presence of three novel cationic surfactants was compared with the pristine EMD and the EMD co-deposited with commonly used cetyltrimethylammonium ammonium bromide (C-AB) surfactant. The three surfactants with different molecular structures are Tetradecyltrimethylammonium bromide (T-AB), Didodecyldimethylammonium bromide (D-AB), Benzyldodecyldimethylammonium bromide (B-AB) used at varying concentrations (15, 30, 60 g l-1) in the electrolytic bath. Among the B-AB surfactant at 30 mg l-1, the EMD (EMD/B-AB30) showed the highest capacitance of 602 F g-1 tested at 1 mA cm-2 current density. The molecular dynamics simulation indicated that when the B-AB surfactant was attached to the Pb electrode via electrostatic, Van der Walls interactions, then the nucleation of MnO2 particles occurred surrounding the surfactant molecule. The unique molecular structure influenced the nucleation formation well-ordered, whereas, for pristine EMD, the nucleation was random. The hybrid capacitor comprises the best performed modified EMD (EMD/B-AB30), and biomass waste-derived AC exhibited 91 F g-1 specific capacitance, an outstanding energy density of 32.4 Wh kg-1 for a corresponding power density of 971 W kg-1. Valorization of the biomass waste, Mango seed husk (MS), and the Grape marc (GM) was carried out by converting the waste into AC for capacitor electrodes. The MS was carbonized, followed by chemical activation using KOH as the activating agent. Activation temperature was varied at 800, 900, 1000, and 1100 °C temperatures, among at 1100 °C highest surface area of 1943 m2 g-1, and the specific capacitance of 135 F g-1 was obtained for the MS-AC. The MS-AC experimental data were incorporated in four machine learning (ML) algorithms; linear regression (LR), decision tree (DT), support vector regression (SVR), and multi-layer perceptron (MLP) for capacitance prediction. Among, the MLP model showed the best correlation (R2 = 0.9868) between the experimental and predicted capacitance values and proved its potential application for computing the complex non-linear relationships between the input and output datasets. Further, the porous carbon materials were derived from GM using four synthesis routes by varying the parameters of activating agent (KOH and ZnCl2), dopant (Nitrogen), and carbonization (450, 600 °C) and activation (450, 800 °C) temperatures. Among the different GM-AC products, the GM carbon, doped with urea and activated by KOH (KACurea), exhibited better morphology, hierarchical pore structure, larger surface area (1356 m2 g-1), and the highest specific capacitance of 139 F g-1 in 2 M NaOH aqueous electrolyte. The miscellaneous collection of datasets based on AC experiments was used for specific capacitance and power prediction using the MLP ML model. Overall, this thesis showed that the EMD could be produced in bulk to be used for hybrid capacitor applications. Particularly, it provided insights about the specie interactions in the electrolyte solution that improved the material performance. This built the platform for further studies on altering the additive concentrations and combinations for developing high-performing EMD materials. This Ph.D. work also highlighted the opportunities to valorize the biomass waste to produce AC with desired characteristics of hierarchical pore structure, larger surface area, etc., to replace the conventional AC electrodes. Finally, the electrochemical performance of the hybrid capacitor fabricated using best performed EMD material (EMD/B-AB30) and biomass-waste derived AC (MS-AC 1100) surpassed the energy density values of the existing supercapacitors, proving its potential development in commercial applications

Novel Multiphysics Phenomena in a New Generation of Energy Storage and Conversion Devices

The swelling demand for storing and using energy at diverse scales has stimulated the exploration of novel materials and design strategies applicable to energy storage systems. The most popular electrochemical energy storage systems are batteries, fuel cells and capacitors. Supercapacitors, also known as ultracapacitors, or electrochemical capacitors have emerged to be particularly promising. Besides exhibiting high cycle life, they combine the best attributes of capacitors (high power density) and batteries (high energy storage density). Consequently, they are expected to be in high demand for applications requiring peak power such as hybrid electric vehicles and uninterruptible power supplies (UPS). This dissertation aims to make advancements on the following two topics in supercapacitor research with the aid of modeling and experimental tools: applying various thermophysical effects to design supercapacitor devices with novel functionalities and studying degradation mechanisms upon continuous cycling of conventional supercapacitors. The prime drawback of conventional supercapacitors is their low energy density. Most research in the last decade has focused on synthesizing novel electrode materials. Although such novel electrodes lead to high energy density, they often involve complicated synthesis process and result in high cost and low power density. A new concept of inducing pseudocapacitance developed in recent years is by introducing redox additives in the electrolyte that engage in redox reactions at the electrode/electrolyte interface during charge/discharge. The first section of this dissertation reports the performance of fabricated solid-state supercapacitors composed of redox-active gel electrolyte (PVA-K3Fe(CN)6-K4Fe(CN)6). The electrochemical performance has been studied extensively using cyclic voltammetry, constant current charge/discharge and impedance spectroscopy techniques, and then the results are compared with similar devices composed of conventional gel electrolytes such as PVA-H3PO4 and PVA-KOH on the basis of capacitance, internal resistance and stable voltage window. The second section explores the utility of the thermogalvanic property of the same redox-active gel electrolyte, PVA-K3Fe(CN)6-K4Fe(CN)6 in the construction of a thermoelectric supercapacitor. The integrated device is capable of being electrically charged by applying a temperature gradient across its two electrodes. In the absence of available temperature gradient, the device can be discharged electrically through an external circuit. Therefore, such a device can be used to harvest waste heat from intermittent heat sources. An equivalent circuit elucidating the mechanisms of energy conversion and storage applicable to thermally chargeable supercapacitors is developed. A fitting analysis aids in the evaluation of model circuit parameters providing good agreement with experimental voltage and current measurements. The latter part of the dissertation investigates the factors influencing aging in conventional supercapacitors. In the first part, a new imaging technique based on the electroreflectance property of gold has been developed and applied to characterize the aging characteristics of a microsupercapacitor device. Previous aging studies were performed through traditional electrical characterization techniques such as cyclic voltammetry, constant charge/discharge, and electrochemical impedance spectroscopy. These methods, although simple, measure an average of the structures’ internal performance, providing little or no information about microscopic details inside the device. The electroreflectance imaging method, developed in this work is demonstrated as a high-resolution imaging technique to investigate charge distribution, and thus to infer aging characteristics upon continuous cycling at high scan rates. The technique can be used for non-intrusive spatial analysis of other electrochemical systems in the future. In addition, we investigate heat generation mechanisms that are responsible for accelerated aging in supercapacitors. A modeling framework has been developed for heat generation rates and resulting temperature evolution in porous electrode supercapacitors upon continuous cycling. Past thermal models either neglected spatial variations of heat generation within the cell or considered electrodes as flat plates that led to inaccuracies. Here, expressions for spatiotemporal variation of heat generation rate are rigorously derived on the basis of porous electrode theory. Detailed numerical simulations of temperature evolution are performed for a real-world device, and the results resemble past measurements both qualitatively and quantitatively. In the last chapter of the thesis, a rare thermoelectric effect called the Nernst effect has been investigated in single-layer periodic graphene with the aid of a modified Boltzmann transport equation. Detailed formulations of the transport coefficients from the BTE solution are developed in order to relate the Nernst coefficient to the amount of impurity density, temperature, band gap and applied magnetic field. Detailed knowledge of the variation of the thermoelectric and thermomagnetic properties of graphene shown in this work will prove helpful for improving the performance of magnetothermoelectric coolers and sensors

Advanced Materials for Electrochemical Energy Conversion and Storage Devices

The book "Advanced Materials for Electrochemical Energy Conversion and Storage Devices" reports new, improved electrocatalytic materials for batteries, capacitors, and fuel cells. These advances are expected to significantly impact the performance of electrochemical energy conversion and storage devices and, consequently, their commercialisation. This book is intended to be a valuable tool for those from industry and academia interested in knowing more about the field