Symmetric supercapacitors based on graphene hydrogels

Cilj ovog rada je bio provesti hidrotermalnu redukciju grafenova oksida (GO) potpomognutu reducensom, vegetacijskom vodom masline (OMW), u kojoj nastaje hidrogel reduciranog grafenovog oksida, rGOH+OMW. Također je provedena hidrotermalna redukcija bez reducensa u kojoj nastaje reducirani grafenov oksid, rGOH. Hidrotermalna sinteza se vodila pri temperaturi od 120°C i pri pH 7. Superkondenzatori su izrađeni od dobivenih materijala, GO, rGOH i rGOH+OMW na podlogama od nikla i staklastog ugljika (GC). Ispitani su metodama cikličke voltametrije, elektrokemijske impedancijske spektroskopije (EIS) i kronopotenciometrije. Specifični kapaciteti dobiveni metodom cikličke voltametrije iznose od 33 F/g do 57 F/g, s izuzetkom superkondenzatora koji je sastavljen od GO koji ima specifični kapacitet manji od 1 F/g. Rezultati dobiveni metodom EIS pokazuju kako se radi o superkondenzatorima dobrih kapacitivnh svojstava koji pokazuju manja odstupanja od idealnog ponašanja. Ovom metodom su također određeni otpori karakteristični za pojedini superkondenzator. Specifični kapaciteti izračunati iz kronopotenciometrije se kreću od oko 40 do 49 F/g i nešto su manji od kapaciteta dobivenih metodom cikličke voltametrije jer je mjerenje provedeno pri užem rasponu napona. Izračunate specifične snage se kreću od 18-40 kW/kg te prema Ragonevom dijagramu svrstavaju priređene superkondenzatore u skupinu izvora visoke snage. Vrijednosti specifične energije iznose od 28-40 W h / kg što odgovara iznosima karakterističnim za superkondenzatore.The objective of this research was to carry out hydrothermal reduction of graphene oxide (GO) supported by olive mill wastewater (OMW) that resulted in formation of reduced graphene oxide hydrogel, rGOH+OMW. Also, hydrothermal reduction was carried out without OMW that resulted in reduced graphene oxide hydrogel, rGOH. Hydrothermal synthesis was performed at 120°C and at pH 7. In order to assemble supercapacitor, the synthesized materials, GO, rGOH and rGOH+OMW, were applied on nickel or glassy carbon support. Supercapacitors were tested by using cyclic voltammetry, electrochemical impedance spectroscopy (EIS) and chronopotentiometry methods. The specific capacitance values were calculated from cyclic voltammetry responses and it was in the range from 33 F/g to 57 F/g, except for supercapacitor made of GO hydrogel with specific capacitance below 1 F/g. EIS results has shown that the obtained supercapacitors had good capacitive behaviour with small deviations from ideal behaviour. Also, this method enabled estimation of the internal resistance of supercapacitor. The specific capacitance values calculated from chronopotentiometry were from 40 F/g to 49 F/g. These values are slightly different from the values obtained from cyclic voltammetry response due to the lower voltage values that were applied. The obtained specific power values were in the range from 18-40 kW/kg and according to the Ragone's diagram prepared supercapacitors belong to the group of high power supercapacitors. The obtained specific energy values were in the range from 28-40 W h / kg which corresponds to the values characteristic for supercapacitors