Comparative study on various sponges as substrates for reduced graphene oxide-based supercapacitor

To solve environmental problems and address the exhaustion of fossil fuel resources, the development of environmentally friendly and alternative energy storage devices has attracted much interest.Supercapacitors are attractive devices for such a purpose because of their high power density, long cycle life, low maintenance, wide range of operating temperatures, and fast charging time compared to conventional capacitors and batteries [1][2][3][4][5].Supercapacitors can be divided into two general classes characterized by their unique mechanism for storing charge.The first class includes electrochemical double layer capacitors (EDLCs), which store a charge electrostatically or non-faradically, when the charge is distributed over surfaces through physical processes.EDLCs generally operate with stable capacitive performance for many charge-discharge cycles because there are no chemical or compositional changes.The second class is called pseudocapacitors.These store a charge faradically through redox reactions and electrosorption at the surface of a suitable electrode.Pseudocapacitors can achieve greater energy densities than EDLCs [5][6][7][8].Graphene materials, which consist of a few atomic layers of only graphite, have attracted great interest as novel electrode materials for energy storage devices because of their superior properties and advantages, such as high electrical and thermal conductivity, great mechanical strength, large specific surface area, and low manufacturing cost [9][10][11][12][13].In this work, reduced graphene oxide (RGO), obtained through the chemical reduction of graphene oxide (GO), was used as a capacitive material [14][15][16].Here, sponges similar to the ones used in ordinary life have been employed as supporting substrates for the fabricated supercapacitor electrodes.Sponges have suitable properties for improving capacitive performance, such as good wettability, highly porous structure, and substantial internal surface area.In demonstrations, they have provided smooth accessibility of ions to electrolytes.Among the various sponges easily available on the market, three types have been employed: polyvinyl alcohol (PVA), melamine, and polyurethane (PU) sponges.The capacitive performances of these types of sponge were compared.A facile dip and dry method was demonstrated, using an RGO solution to coat each sponge, to fabricate an RGO-coated sponge (RGO-sponge) electrode without additional binder and or conductive materials [17][18][19].All chemical reagents including graphite powder (<20 m) were purchased from Sigma Aldrich (USA).PVA sponge and Melamine sponge were purchased from CJ Olive Young, and PU sponge was purchased from Spongemart in the Republic of Korea.Elemental analysis was performed using a Flash 2000 (Thermo Scientific, UK).Atomic force microscopy (AFM) was performed using a Nano-R (Pacific Nanotechnology, USA).Attenuated total reflection Fourier transform infrared (ATR-FTIR) measurements were performed by a Vertex 80V (Bruker Optics, Germany).Raman spectroscopy was performed by an Alpha 300 (WI-Tec , Germany).The Brunauer-Emmett-Teller specific surface area (BET-SSA) was measured using a Tristar, ASAP2020 (Micromeritics, USA).Electrochemical measurements, including cyclic voltammetry (CV) and galvanostatic charge-discharge tests, were conducted with an Ivium Compactstat potentiostat (B0714).A GO suspension was synthesized from graphite powder using a modified Hummers method.The RGO was obtained by simple chemical reduction from GO.An RGO solution was obtained using ammonium hydroxide, with hydrazine hydrate as a reducing agent,