Graphene hydrogel/electrospun fibres composite for supercapacitor application

Cilj ovog rada bila je hidrotermalna sinteza kompozitnog materijala grafen/ispredena vlakna. Od ispredenih vlakana u ovom radu su korištena vlakna željezovog oksida (α-Fe2O3) i polimerna polivinilpirildona (PVP). Navedena vlakna koristila su se kako bi se poboljšala struktura hidrogela, tj. njegova mehanička stabilnost i elastičnost. Suspenzija grafenovog oksida (GO), masene koncentracije γ=4 mg mL^-1, priređena je modificiranom Hummersovom metodom. Redukcija GO provedena je hidrotermalnim postupkom potpomognutim ekstraktom lista masline (ELM) pri 120°C. Dobiveni hidrogelovi okaraterizirani su metodama rendgenske difrakcijske spektroskopije (XRD), infracrvene molekulske spektroskopije s Furierovim transformacijama (FTIR) i elektronskom pretražnom mikroskopijom (SEM). Ispiranje hidrogelova od organskih nečistoća i zaostalih fenola pratilo se metodom UV/Vis spektroskopije. Strujni kolektori korištene u ovom radu bile su Ni-pločice. Superkondenzatori su ispitani metodama cikličke voltametrije (CV), kronopotenciometrije (CP) i metodom elektrokemijske impedancijske spektroskopije (EIS). Ispitivanje metodom CV provedeno je u granici napona od 0 do 1,2 V, kod različitih brzina promjene potencijala. Metodom kronopotenciometrije je provedeno ispitivanje punjenja/pražnjenja superkondenzatora tijekom 1000 ciklusa uz struju u granicama od 1,2 do 2,2 A g^-1. Kompozitni hidrogel grafen/ispredena vlakna rGO-4-Fe pokazao je najbolja svojstva. Na SEM mikrografijama vidljiva su vlakna ugrađena u strukturu hidrogela, a elektrokemijskim metodama su dobivene najviše vrijednosti kapaciteta, snage i energije superkondenzatora rGO-4-Fe. Superkondenzator koji je sadržavao kompozit sa polimernim PVP vlaknima također je pokazao veće kapacitete u usporedbi sa čistim hidrogelom sintetiziranim pri istoj pH vrijednosti.The objective of this research was the hydrothermal synthesis of composite material graphene / spun fibres. The iron oxide (α-Fe2O3) and polymer polyvinylpyridyl (PVP) fibres were used in this work. The fibres were used in order to improve the structure of the hydrogel, i.e. its mechanical stability and elasticity. Graphene oxide (GO) suspension was prepared by modified Hummers method and mass concentration was γ = 4 mg mL^-1. Reduction of GO was carried out by a hydrothermal process assisted by olive leaf extract (ELM). at 120 °C. The obtained hydrogels are characterized by X-ray diffraction spectra (XRD), infrared molecular spectroscopy with Furier transformations (FTIR) and scanning electron microscopy (SEM). ELM was removed from hydrogels by a multiple water exchange. In order to determine the amount of ELM within the hydrogel the exchanged water was caracterisired by UV/Vis spectroscopy. The current collectors used in this work were Ni-sheets. Supercapacitors were characterized by using cyclic voltammetry (CV), chronopotensometry (CP) and electrochemical impedance spectroscopy (EIS). CV was carried out within the voltage range from 0 to 1.2 V, at different scan rates. Charging / discharging test of supercapacitors was carried out by 1000 cycles and current density within the range of 1.2 to 2.2 A g^-1. The composite hydrogel rGO-4-Fe showed the best properties. SEM micrographs indicated that fibres were embedded within the hydrogel structure. Electrochemical methods revealed that the supercapacitor rGO-4-Fe had the highest specific capacitance value, specific power and specific energy. The supercapacitor with composite containing PVP fibres also showed higher specific capacitance value compared to pure hydrogel synthesized at the same pH value