| Abstract | Cilj ovog rada je bio provesti hidrotermalnu redukciju grafenovog oksida (GO) pri čemu nastaje grafenov hidrogel (rGO). U procesu se kao reducens koristi ekstrakt lista masline (ELM). Cilj je također bio ispitati kapacitivna svojstva rGO hidrogelova te ih primijeniti u superkondenzatorima. U prvom koraku ovog rada provedena je hidrotermalna redukcija GO u rGO pri različitim pH vrijednostima (3, 7, 10), uz različite mase reducensa (0,28 g i 2,8 g), pri različitoj temperaturi (120 °C i 180 °C) te uz prisustvo i bez prisustva punila. Dobiveni rGO hidrogelovi su je osušeni te su potom uzorci naneseni na elektrodu od staklastog ugljika. Elektrode s rGO uzorcima ispitane su metodom cikličke voltametrije u 0,5 mol dm^-3 otopini Na2SO4. Dobiveni specifični kapaciteti iznosili su od 30-185 F g^-1. Pokazano je da veće specifične kapacitete imaju uzorci koji su priređeni kod većih pH vrijednosti, s većom količinom ELM i koji su pripremljeni pri većoj temperaturi. Prisustvo punila smanjuje specifični kapacitet, ali ne značajno. Hidrogelovi rGO4, rGO5 i rGO7 su direktno nakon sinteze i ispiranja korišteni za izradu Superkondenzator1, Superkondenzator2 i Superkondenzator3. Pripravljeni superkondenzatori su ispitani metodom cikličke voltametrije te je provedeno 1000 uzastopnih ciklusa punjenja i pražnjenja. Iznosi specifičnih kapaciteta superkondenzatora su veći nego vrijednosti dobivene prilikom ispitivanja u troelektrodnom sustavu, što je posljedica porozne strukture hidrogelova koji nisu sušeni. Specifični kapaciteti dobiveni metodom cikličke voltametrije kreću se u rasponu od 111-202 F g^-1. Specifični kapaciteti dobiveni iz kronopotenciometrije nešto su niži, jer je ispitivanje provedeno u užim granicama napona, a kretali su se od 175-45 F g^-1. Za superkondenzatore je određena i specifična energija koja je iznosila od 32,60 kW s kg^-1 do 127,36 kW s kg^-1. Za sve ispitane superkondenzatore, tijekom punjenja i pražnjenja praćena je promjena kapaciteta, otpora samopražnjenja i unutarnjeg otpora. Iz dobivenih rezultata se može zaključiti da Superkondenzator3 (rGO7, pH=3, bez punila, m(ELM)=0,28 g) ima najveće specifične kapacitete i najmanji unutarnji otpor, dok Superkondenzator1 (rGO4, pH=10, bez punila, m(ELM)=0,28 g) ima najveći otpor samopražnjenja. |
| Abstract (english) | The objective of this research was to carry out the hydrothermal reduction of graphene oxide (GO) that results in graphene hydrogel (rGO) formation. In the process, olive leaf extract (ELM) was used as a reducer. The objective of this work was also to examine the capacitive properties of rGO hydrogels and to apply them in supercapacitors. In the first step, the hydrothermal reduction of GO to rGO was performed at different pH values (3, 7, 10), with different masses of reducing agent (0.28 g, 2.8 g), at different temperatures (120 °C and 180 °C) and with the presence and without the presence of fillers. The obtained rGO hydrogels were dried and then it was applied to the glassy carbon support. Electrodes with rGO samples were tested by cyclic voltammetry in 0.5 mol dm^-3 Na2SO4 solution. The obtained specific capacitances were 30-185 F g^-1. It has been shown that larger specific capacitance values were obtained by using samples prepared at higher pH values, with a higher amount of ELM and prepared at a higher temperatures. The presence of fillers reduces specific capacity, but not significantly. The hydrogels rGO4, rGO5 and rGO7 were used directly after the synthesis and rinsing in preparin Supercapacitor1, Supercapacitor2 and Supercapacitor3. Prepared supercapacitors were tested by cyclic voltammetry method and 1000 consecutive charge and discharge cycles were carried out. The specific capacities of the supercapacitors are higher compared to the values obtained by using three electrode system experiment, as a result of the porous hydrogel structure. Specific capacities obtained by the cyclic voltammetry range from 111 to 202 F g^-1. The specific capacities obtained from the chronopotentiometry were slightly lower due to the narrow voltage limits used during examination, ranging from 175 to 45 F g^-1. For the supercapacitor, a specific energy of 32.60 kW s kg^-1 to 127.36 kW s kg^-1 was determined. For all tested supercapacitors, capacity, self-discharge resistance and equivalent series resistance have been monitored during charging and discharging. From the obtained results it can be concluded that Superkondenzator3 (rGO7, pH = 3, without filler, m(ELM) = 0.28 g) has the highest specific capacities and the smallest equivalent series resistance resistance, while Superkondenzator1 (rGO4, pH = 10, without filler, m(ELM) =0.28 g) has the highest self-discharge resistance. |
