Aktiveeritud süsinike mikrostruktuuri ja poorsuse mõju elektrilise kaksikkihi kondensaatorite omadustele

Väitekirja elektrooniline versioon ei sisalda publikatsiooneKuna elektrijaamu (tuule-, päikese-, hüdro-, tuumaenergia jne) mugavalt kaasas kanda ei saa, on ikka vaja uuemaid ja paremaid energiasalvestusseadmeid, mida saab tasku panna. Energiat saab salvestada näiteks superkondensaatorisse. Energiasalvestusseadmeid iseloomustab nende energia ja võimsus. Mida rohkem energiat saab seadmesse, näiteks elektriauto akusse, salvestada, seda kaugemale saab see auto sõita enne, kui aku tühjaks saab. Kui sama aku on ka suure võimsusega, siis suudab see aku energiat kiiremini välja anda ehk auto kiirendus on suurem. Superkondensaatorid on väga hea võimsusega energiasalvestusseadmed (hind võimuse ühiku kohta 4500 EUR (kWh)−1). Seega kasutatakse superkondensaatoreid põhiliselt rakendustes, kus on oluline energiat salvestada/kätte saada väga kiiresti. Näiteks nutitelefoni kaamera välk saab oma energia superkondensaatorilt. Üks tähtsamaid superkondensaatori koostisosi on elektroodid. Tavaliselt tehakse elektroodid poorsest süsinikust, mis on olemuselt sarnane aktiivsöega, mida kasutatakse näiteks söetablettides. Poorseid süsinikke iseloomustab eripind ehk suur pindala väikese massi kohta. Selles töös sünteesiti mitmeid huvitava ehitusega suure eripinnaga süsinikmaterjale, kasutades lähteainetena glükoosi, sahharoosi ja turvast. Sünteesitud süsinikmaterjalidest tehti edasi superkondensaatori elektroodid ja neid kasutati superkondensaatori katserakus. Nähti, et mõned sünteesitud materjalid koosnesid 1 μm (umbes 100 korda väiksem kui juuksekarva läbimõõt) kerakujulistest osakestest. Nende väikeste sfääride sees on keeruline poorne võrgustik, mille eripind oli suurusjärgus 2000 m2 g−1 ja kuhu pääsevad ligi ioonid ja molekulid. Uurimuse käigus selgus, et igasuguse suurusega poorid pole võrdselt head selleks, et saada suure võimsusega superkondensaatorit. Selgus, et suure võimsusega superkondensaatorite valmistamiseks on äärmiselt vajalikud poorid, mille läbimõõt on ligi 1 nm või laiem (läbimõõt, mis on sarnane DNA molekuli läbimõõdule).Since the conventional power plants (wind, solar, hydro, nuclear, etc) currently do not fit in the pocket there is still an ever-increasing problem of needing newer and better energy storage devices. A part of the solution seems to come in the form of a device called supercapacitor. At first approximation, any energy storage system can be described by two main key parameters: energy and power. An electric car with a high energy battery system means it can drive further without needing to refill. If the same system had high power, then the car's acceleration from would be fast. That said, at equal basis, supercapacitors are considered to be cheap for obtaining high power values (4500 EUR (kWh)−1). Thus supercapacitors are used in applications that require high power intakes and outputs. For example, a smartphone camera uses supercapacitors for the flash. One of the most influential supercapacitor components is electrodes. Most commonly the electrodes are made of carbon materials that are in nature similar to the carbon materials used in charcoal tablet and carbon materials in water filters. They all function because of a high active surface area. In the current study, several promising high surface area carbon materials were synthesised using glucose, saccharose and peat as precursors. Resulting materials were meticulously evaluated in a supercapacitor test cell and interpreted using modern structural analysis methods. Obtained results revealed that some synthesised materials consisted of 1 m (about 100 times smaller than the diameter of a human hair) spherical particles. Inside of these extremely small particles lays a complex porous network where a lot of additional surfaces is located (>2000 m2 g−1). These pores are accessible to small particles like ions and molecules that can find a home at pores. However, not all pores are equally good. Data indicated that the best pores for making high power supercapacitors were approximately 1 nm in diameter or wider (close to the diameter of a DNA molecule).https://www.ester.ee/record=b536185

The role of conductive additives on the performance of hybrid carbon xerogels as electrodes in aqueous supercapacitors

Three different hybrid carbon xerogels containing Graphene Oxide (AXGO), Micronized Graphite (AXMG) and Carbon Black (AXCB) were synthesized using an easy, fast and affordable method. These three additives were initially selected to improve the electrical conductivity of the pristine activated carbon xerogel (AX) thus expecting to improve its performance in aqueous supercapacitors. Capacitances of the corresponding devices were measured as a function of current density and results of the high and low charge transfer regime of the supercapacitors were discussed separately. In both regimes, the differences observed between the hybrid electrodes were analyzed on the basis of the concurrent influence of the micro and mesoporosity, surface chemistry and electrical conductivity of the materials. Accordingly, even though all the hybrid carbon xerogels showed higher electrical conductivities, only AXGO rendered a better performance than AX, showing the highest capacitances in the whole interval of intensities studied. Consequently, at 16 A g−1, the energy and power densities of the AXGO supercapacitors increased up to 16% and 97%, respectively, with respect to AX, and of 143% and 409%, respectively, with respect to a commercial activated carbon used as reference. The performance of AXCB and, especially AXMG was worse than AX supercapacitors due to a combination of inadequate pore size distributions and/or a poor surface chemistry. Finally, TEM analysis helped to understand the different way the three additives were affecting the nanostructure (and final properties) of the hybrid carbon xerogels.Authors gratefully acknowledge the financial support from the Ministerio de Economía, Industria y Competitividad from Spain (Project CTQ2017-87820-R). MCR also acknowledges CSIC (Project I.E. 201880E010)

Study on the specific capacitance of an activated carbon cloth modified with reduced graphene oxide and polyaniline by cyclic voltammetry

This work describes a two-step process for the electrochemical coating of reduced graphene oxide (RGO) and polyaniline (PANI) onto an activated carbon cloth (ACC) by cyclic voltammetry (CV). The fact that the two syntheses are carried out independently of each other, makes it possible to select the experimental conditions for each one and to study the electrochemical response of RGO, PANI, and PANI onto RGO (RGOPANI), separately. Thus, by modifying the potential limits of the aniline-polymerization reaction, it was possible to observe the influence of RGO and the maximum amount of PANI that the carbon cloth can receive in terms of proper electrochemical response. Electrochemical properties were characterized by CV, galvanostatic charge-discharge curves (using three or two-electrodes symmetric cell configurations) and electrochemical impedance spectroscopy (EIS). A maximum improvement of 25%, 56% and 61% over the initial specific capacitance of ACC (about 129 F g−1) were obtained for RGO, PANI and RGOPANI coatings, respectively. Good cycling stability retaining 83% of the initial capacitance, after 1000 cycles stability test, was obtained for RGOPANI sample. Promising results of energy and power densities were also achieved. In the analyses by Fourier transform infrared spectroscopy (FTIR), the PANI-bands could be clearly identified which is indicative of a significant presence of PANI. Field emission scanning electron microscopy (FESEM) showed the morphology of RGO, PANI and RGOPANI onto the ACC fibers. These analyses helped to explain the electrochemical results.The authors wish to acknowledge to Chemviron Carbon who kindly donated the ZORFLEX (R) activated carbon fabric. The authors wish to thank the Spanish Agenda Estatal de Investigacion (AEI) and European Union (FEDER funds) for the financial support (contract MAT2016-77742-C2-1-P). Tim Vickers is gratefully acknowledged for help with the English revision.Fernández Sáez, J.; Bonastre Cano, JA.; Molina Puerto, J.; Del Río García, AI.; Cases Iborra, FJ. (2017). Study on the specific capacitance of an activated carbon cloth modified with reduced graphene oxide and polyaniline by cyclic voltammetry. European Polymer Journal. 92:194-203. doi:10.1016/j.eurpolymj.2017.04.044S1942039

Mikro- ja mesopoorsete hierarhilise struktuuriga süsinike sünteesimeetodite arendamine ja materjalide karakteriseerimine

Väitekirja elektrooniline versioon ei sisalda publikatsiooneLoodussäästlikule ning jätkusuutlikule energeetikale pööratakse kogu maailmas järjest enam tähelepanu võitlemaks globaalsete kliimaprobleemidega. Selleks, et taastuvaid energiaallikaid kasutada, on vajalikud ka efektiivsed energiasalvestussüsteemid. Üheks neist on superkondensaatorid, mis võimaldavad väga kiiret laadimist ja tühjendamist ning samas sobivad suurt võimsust nõudvate rakenduste korral. Üheks väga oluliseks superkondensaatorite töövõimet mõjutavaks elemendiks on kasutatav elektroodi materjal. Erinevad poorsed süsinikmaterjalid on selleks rakenduseks väga levinud. Poorsete süsinikmaterjalide alane teadustöö ning ka nende tootmine ja kasutamine maailmas aina kasvab. Lisaks energiasalvestusseadmetele kasutatakse neid ka näiteks veepuhastusseadmetes, vesiniku salvestamiseks jne. Sõltuvalt rakendusest peavad süsinikmaterjalid olema selleks vajalike omaduste ja struktuuriga, mis omakorda sõltuvad väga oluliselt antud süsinikmaterjalide sünteesi meetodist. Lähteaineks võivad olla näiteks suhkur, biomass, karbiidid, polümeerid jne. Selleks, et materjalid oleksid ka reaalselt erinevates rakendustes kasutatavad, on väga oluline hinna ja kvaliteedi suhe – otsitakse materjale, mida oleks võimalikult lihtne ja odav valmistada, kuid samas oma parameetritelt oleksid rakenduste jaoks sobivad. Käesoleva doktoritöö raames valmistati poorseid süsinikmaterjale ränikarbiidist ning uuriti erinevate järeltöötluse meetodite mõju saadud materjalide füüsikalistele omadustele (poorsus, korrapära ja struktuur). Seejärel valmistati uuritud materjalidest superkondensaatori elektroodid ja testiti mitmekülgselt ka nende sobivust kasutamiseks energiasalvestusseadmetes. Töö tulemused näitavad, et ränikarbiid, mis on teistest sarnastest kasutatavatest karbiididest 10 kuni 60 korda odavam, on väga lihtsa ning küllaltki odava sünteesi ja järeltöötluse meetodite abil võimalik töödelda efektiivseks materjaliks energiasalvestusseadmetes kasutamiseks.Sustainable and eco-friendly energy is increasingly being given worldwide attention to tackle the global climate challenges. The application of renewable energy sources needs effective energy storage systems. One of these are supercapacitors, which allow very fast charging and discharging and at the same time are suitable for high power applications. One very important component affecting the performance of the supercapacitors are the active materials used for preparing the electrodes. Different porous carbon materials are very common for this application. Research, production and use of porous carbons is growing worldwide and in addition to energy storage devices, they are also widely used for example in water purification equipment, hydrogen storage, air purification, etc. Depending on the application, the carbon material must have specific properties and structure. These in turn depend on precursors and methods used to synthesize the carbon materials. Different precursors can be used, for example biomass, sugars, carbides, polymers, etc. The price-quality ratio must also be kept in mind – the material should be cheap and easy to produce, but suitable for the specific application. In this thesis, porous carbon materials were synthesized from silicon carbide and various post-treatment methods were studied to analyze their effect on the resulting materials properties (porosity, structure). Subsequently, these carbon materials and their suitability for application in high energy and power energy storage devices were tested. The results showed, that silicon carbide, which is 10 to 60 times cheaper than other similar carbides used, can be processed by simple and relatively inexpensive synthesis methods into a suitable and efficient material for energy storage devices.https://www.ester.ee/record=b524280

Uudsete süsinikmaterjalide süntees ja karakteriseerimine suure võimusega superkondensaatorite rakendusteks

Väitekirja elektrooniline versioon ei sisalda publikatsiooneViimastel aastatel on üsna kiiresti kasvanud nõudlus mitmekülgsete energiasalvestussüsteemide järele. Mõnedes piirkondades on pidevalt suurenev energiatarbimine ja selle mõju keskkonnale on tekitanud vajaduse uute suure võimsuse- ja energiatihedusega energiasalvestite järele. Superkondensaatorid on pälvinud palju tähelepanu, kuna neil on suur erimahtuvus, pikk tööiga, suur võimsustihedus ja väga madalad hoolduskulud. Superkondensaatoreid saab kombineerida koos kõrge energiatihedusega patareide ja kütuseelementidega erinevates rakendustes, kus on oluline samaaegselt nii suur energia kui ka võimsustihedus. Superkondensaatorites salvestatava kui ka sealt vabaneva energia väärtused sõltuvad olulisel määral selle elektrilisest mahtuvusest, süsteemi takistusest ja maksimaalsest rakupotentsiaalist, mis kõik sõltuvad kasutatavate elektroodide materjalide poorsusest ja elektrolüüdi omadustest. Üks enimkasutatud elektroodimaterjale superkondensaatorites on erinevad poorsed süsinikud ja nende hulgas ka karbiidist saadud süsinikud, millede korral on võimalik pooride suurust väga kontrollitult varieerida ning läbi selle suurendada süsteemis salvestatava energia hulka. Teiseks oluliseks energia salvestamist piiravaks teguriks superkondensaatorites on kasutatavate elektrolüütide mõõdukas tööpotentsiaal. Superkondensaatori suurepärase jõudluse saavutamiseks on seega oluline optimeerida nii elektroodimaterjali mikro- ja mesopoorsust ja selle sobivust kasutatava elektrolüüdiga kui ka optimeerida kasutatava elektrolüüdi korral selle maksimaalset rakendatavat rakupotentsiaali. Antud doktoritöös kasutati mikro- ja mesopoorsete elektroodimaterjalide valmistamiseks sool-geel meetodit, mis annab esialgsele karbiidile täiendava mesopoorsuse, mis jääb alles ka karbiidset päritolu süsinikmaterjali ja mida ei eksisteeri kommertsiaalsest karbiidist süsteesitud süsinikus. Teiseks töötati välja ″operando″ aktiveerimise ja passiveerimise meetod elektroodide maksimaalse rakendatava rakupotsntsiaali suurendamiseks, et suurendada süsteemi energiatihedust.In recent years, the demand for versatile energy storage systems has risen quite fast. The environmental impact of energy consumption for some regions has additionally increased the necessity for new energy storage devices with high power and energy densities. Supercapacitors have gained much attention because they have high specific capacitance, long cycle life, high power density, and very low maintenance costs. Supercapacitors complement batteries and fuel cells in applications where high power is important. The energy storage and power delivery characteristics of supercapacitors are largely determined by the electrical capacitance, system resistance, and maximum cell potential which are all dependent on the electrode materials porosity and electrolyte properties used. One of the most used electrode materials in supercapacitors are different carbide derived carbons, which have the possibility to fine-tune the pore size. One limiting factor in achieving high power density is the moderate working cell potential of different electrolytes that are used in supercapacitors. To achieve excellent performance of a supercapacitor it is important to optimize the electrode’s micro-mesoporosity and for the used electrolyte, it is important that it has a high electrochemical window and that the electrolyte ions suit the selected electrode material. In this thesis, firstly, a sol-gel method was used for the preparation of well developed micro- and mesoporous electrodes, which gives additional mesoporosity to the initial carbide material. This results also in the derived carbon material, unlike when the commercially synthesized titanium carbide is used. Secondly, the operando activation and passivation method was developed for future enlargement of the ideal polarizability region of electrodes, to achieve higher energy densities.https://www.ester.ee/record=b5487650https://www.ester.ee/record=b548765

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

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

Süsinikmaterjalid energiasalvestus rakendustes

Väitekirja elektrooniline versioon ei sisalda publikatsiooneUudsete energia salvestuse süsteemide väljaarendamine ning olemasolevate energia salvestuse süsteemide optimeerimine on vajalik selleks et enamus kasutatavast energiast saaks pärineda taastuvatest energiaallikatest. Süsinikmaterjalid on tähtsaks komponendiks paljudes energiat salvestavates süsteemides, näiteks elektroodmaterjalidena patareides ning elektrilise kaksikkihi kondensaatorites või adsorbeerivate materjalidena energeetilise väärtusega gaasi hoiustamiseks. Süsinikmaterjalide struktuuri ning pinda saab muuta suurel määral lähtematerjali ning sünteesitingimuste valikuga. Paremate energiat salvestavate süsteemide, mis sisaldavat süsinikmaterjale, kavandamiseks on vaja hästi mõista süsinikmaterjalide struktuuri ja pinnaomaduste mõju antud süsteemide tööprotsessile. Süsiniku pinnaomaduste üheks tähtsamaks parameetriks on eripindala, ehk materjali pindala ühe massiühiku kohta. Suure eripinna tagavad eeskätt materjali pinnas esinevad väikesed avavused ning pinna ebaühtlused, ehk poorid. Poorid võivad omada erinevaid mõõtmeid, mille järgi klassifitseeritakse poorid suuruse järgi mikro-, meso- ning makropoorideks. Süsinikmaterjalide puhul omavad suurimat huvi mikropoorsed materjalid, kus pooride mõõtmed on alla paari nanomeetri. Erinevate pinnaomadustega ning struktuuri korrapäraga süsinikmaterjale uuriti elektrilise kaksikkihi kondensaatorite elektroodidena, metaani adsorbentidena ning kandematerjalina vesinikku salvestava materjali parendamiseks. Antud töö kinnitas, et energia mida on võimalik salvestada pinnaühiku kohta kasvab ühtlaselt pinna suurenemisega juhul kui toimub ilma laenguta gaasimolekuli adsorptsioon. Energia mida on võimalik salvestada pinnaühiku kohta omab piirilist väärtust pinna suurenemisega juhul kui toimub laetud osakeste adsorptsioon elektrilisse kaksikkihti. Lisaks eripinna suurele tähtsusele määrati metaani salvestamise suurem efektiivsus kui kasutati korrapäratumaid ning väiksemate pooridega materjalide. Süsinikmaterjali kasutamine komplekshüdriidi kandematerjalina alandas temperatuuri, mille juures algas vesiniku eraldumine, üle 100 °C võrreldes puhta kompleksmetallhüdriidiga.For standalone renewable energy production to be viable novel energy storage systems need to be developed or existent systems need to be improved. Carbon materials are an important component in many energy storage systems, from battery and electrical double layer capacitor electrodes to energetically valuable gas adsorbents. Depending on the carbon material synthesis conditions and used precursor material carbon materials surface properties and structure can be varied in a wide margin. To design better energy storage systems, which contain carbon materials, it is necessary to have a deep understanding of the influence of carbon materials structure and surface properties on the processes in energy storage systems. Specific surface area, surface per unit of mass, is one of the most important carbon surface parameters characterising the surface of carbon materials. A large specific surface area is possible in case of a porous structure. Based on the size pores are categorised as micro- meso- and macropores. In case of carbon materials micropores, pore dimensions under two nanometres, are of greatest interest. Carbon materials with different structural properties were investigated as electrode materials in electrical double layer capacitors, adsorbents for methane and as a supporting material to improve the hydrogen storage performance of a complex metal hydride. This work affirmed that the energy stored increases linearly with the increase of specific surface area in case of molecular gas adsorption. The energy stored has a limiting value with the increase of surface area in case of adsorption of ions in an electrical double layer. Efficiency of methane adsorption was improved when more disordered carbon materials with smaller pores were used. The use of microporous carbon as a supporting material for complex metal hydride, NaAlH4, lowered the temperature at which the release of hydrogen started by over 100 °C

Ioonsetel vedelikel põhinevate elektrolüütide elektrokeemilised omadused Bi(hkl) ja mikro-mesopoorsetel süsinik elektroodidel

Väitekirja elektrooniline versioon ei sisalda publikatsiooneTänapäeva tehnoloogias ja teaduses on olulisel kohal moodsad energia muundamise ja salvestamise seadmed. Ioonsed vedelikud omavad olulist tehnoloogilist potentsiaali antud seadmetes tänu nende kõrgele keemilisele stabiilsusele, laiale variatsioonile ning rakendatavusele nii elektrolüüdi kui solvendina. Selleks, et disainida effektiivsemaid ja kõrgema erienergiaga energiasalvesteid on oluline mõista mehhanisme, mis mõjutavad energia salvestamist ioonseid vedelikke rakendavates süsteemides, nagu näiteks super- ja hübriidkondensaatorites. Elektrilise kaksikkihi (EKK) tekke ja dünaamika ning lisandite mõju, nagu näiteks halogeniid ja leelismetalli ioonid, vesi ja orgaanilised solvendid, uurimine võimaldab meil teadlikult disainida paremaid elektrokeemilisi seadmeid. Antud töös uuriti nii mudelelektrood-süsteeme, et näidata lisandite mõju EKK tekkele vismuti monokristalli eri tahkudel, kui ka superkondensaatori test-rakke, et kontrollida, kuivõrd fundamentaaluuringutest saadavad teadmised on rakendatavad ka reaalsetes seadmetes. Antud töös kasutati elektrokeemilise impedantsspektroskoopia, skaneeriva tunnelmikroskoopia ja alalisvoolu elektrokeemilisi meetodeid, et iseloomustada erinevate ioonsete vedelike mahtuvuslikke, takistuslisi ja adsorptsioonilisi omadusi vismuti monokristallide ja mikro-mesopoorse süsinikmaterjali piirpinnal. Lisaks vaadeldi erinevate pindaktiivsete lisandite mõju vastavate süsteemide omadustele. Tulemustest nähtub, et kõige olulisemat mõju vastavate süsteemide elektrood│elektrolüüt piirpinnale omavad ioonse vedeliku anioonid ning lahustunud molekulid mis omavad tugevat vastastikmõju anioonidega. Näidati, et anioonide varieerimine muudab oluliselt piirpinna mahtuvust, eriti polariseeritavamate anioonide nagu näiteks halogeniid-ioonide puhul. Skaneeriva tunnelmikroskoopia tulemustest võib järeldada, et mahtuvuse muutus on tingitud kõrgelt struktureeritud tiheda adsorptisoonilise kihi tekkest vismuti monokristallide piirpinnale. Samuti uuriti vee kui lisandi mõju Bi│ioonne vedelik süsteemile. Kõrge hügroskoopsuse tõttu on väike vee lisand levinud paljudes uuritavates süsteemides, ning vastava mõju hindamine omab olulist väärtust rakendustele. Ka siin leiti, et kõige olulisem muutus puudutab ioonse vedeliku anioone, mis veemolekulide olemasolu korral on võimelised moodustama hüdrateeritud komplekse, mis omavad olulist mõju vismutelektroodi mahtuvuslikele omadustele, eriti positiivse pinnalaengu korral. Samuti on vee-lisandil arvestatav mõju takistuslikele parameetritele, kuna vesi on elektrokeemiliselt ioonsetest vedelikest vähem stabiilsem. Ioonsete lisandite mõju superkondensaatorites uurimiseks võrreldi puhast ioonset vedelikku kui elektrolüüti sisaldavat süsteemi ning lisati nendele nii halogeniid kui ka leelismetalli ioone. Vastavad lisandid olid kasutusel abrosbeerituna mikro-mesopoorses süsinikmaterjalis, milles oluline osa poorsusest on suurematele ioonse vedeliku anioonidele ja katioonidele suletud. Seega võis eeldada, et väikesemad lisandi-ioonid on peamiselt kontsentreerunud materjali mikropooridesse. Näidati, et kuigi leelis-metallide ioonid ei oma olulist mõju superkondensaatori mahtuvusele, siis halogeniidioonid aitavad parendada süsteemi elektrokeemilisi omadusi olulisel määral. Seega on ka antud süsteemides aniooni mõju elektrokeemilistele omadustele ja EKK tekkele määrava tähtsusega. Täpsem impedants-spektrite analüüs võimaldas lahtutada vastava mõju nii mahtuvuslikuks kui ka laengu-ülekande komponentideks, aidates mõista halogeniidioonide mõju hübriidkondensaatorile.Recently, the energy crisis has become more and more serious. Thus, it is important to accelerate energy upgrading and establish the growth of new energy economics. As one of the carriers of global energy transformation, the development of electric vehicles may contribute to establishing the electrification of society and the transformation of industrial structure. However, the rapid development of electric vehicles still represents several challenges and limitations, such as the charging capacity of power sources involving batteries and supercapacitors. With the continuous iteration in the technological progress of battery and supercapacitor materials, efforts have been made to develop high-performance electrochemical energy storage devices. The electrochemical energy storage process, in principle, occurs at the electrode-electrolyte interface. It is of great significance to study the electrochemical behavior at the interface. Therefore, the electrochemical behavior and energy storage characteristics of the electrode-electrolyte interfaces were studied in this work, aiming to achieve high capacitance performance. Ionic liquids (ILs) composed of anions and cations can show high thermal, chemical and electrochemical stability. This work focused on the electrochemical characteristics of the pure IL and IL salt mixtures. Regarding electrode materials, Bi(hkl) and micro- and mesoporous carbon electrodes were studied. As an excellent alternative electrode material for traditional mercury electrodes, bismuth electrode with low toxicity has been widely studied in the Department of Physical Chemistry, University of Tartu. The capacitance-potential curves of pure IL and IL salt mixtures at the Bi(111) electrode were discussed in this work, mainly conducted by the cyclic voltammetry and electrochemical impedance spectroscopy methods. The capacitance peaks of capacitance-potential curves suggest that the specifically adsorbed anions (i.e., I‾ and Br‾) can increase the capacitance. Additionally, electrochemical characteristics of water-contained ILs at Bi(hkl) electrodes were studied. The capacitance-potential curves have seen anomalous capacitance peaks due to the specific interaction between water and anions. Interestingly, small amounts of adsorbed water molecules do not affect the electrochemical stability potential range of the base electrolyte but contribute to the increase of capacitance. According to the equivalent circuit modeling fit of experimental impedance data, the kinetics of surface processes (mass transfer and faradaic characteristics) were shown in the resistance-potential curves. To characterize the surface structure of the interface in a nanoscale, in situ scanning tunneling microscopy measurement was applied to image the surface structure of electrodes in IL within the potential range applied. Results found that the highly ordered structures on Bi(111) and Bi(011 ̅) planes were recorded relating to the strongly adsorbed anions consistent with electrochemical results. Since the specifically adsorbed halide ions (Br‾, I‾) at the Bi(111) electrode can increase the capacitance at the interface, it is assumed that such halide ions could help improve the capacitance performance of supercapacitors. The pure IL, IL salts mixture containing halide ions and IL salts mixture containing alkali ions were applied to impregnate the micro- and mesoporous carbon electrodes of supercapacitors. Compared to the pure IL treated supercapacitor, supercapacitors treated with halide salt mixtures showed higher capacitance. In contrast, alkali salt mixtures did not improve the capacitance performance of the supercapacitor. Moreover, supercapacitors treated with halide (positive electrode) and alkali (negative electrode) salt mixtures showed consistently high capacitance, suggesting an effective doping methodology by symmetrically stabilizing both electrodes. According to the equivalent circuit modeling fit of experimental impedance data, the enhanced capacitances are attributed to the pseudocapacitive effect originating in the specifically adsorbed and redox-active halide ions within and near the carbon pores.https://www.ester.ee/record=b550050