Scan-Mode Atmospheric-Pressure Plasma Jet Processed Reduced Graphene Oxides for Quasi-Solid-State Gel-Electrolyte Supercapacitors

Abstract

A scanning atmospheric-pressure plasma jet (APPJ) is essential for high-throughput large-area and roll-to-roll processes. In this study, we evaluate scan-mode APPJ for processing reduced graphene oxides (rGOs) that are used as the electrodes of quasi-solid-state gel-electrolyte supercapacitors. rGO nanoflakes are mixed with ethyl cellulose (EC) and terpineol to form pastes for screen-printing. After screen-printing the pastes on carbon cloth, a DC-pulse nitrogen APPJ is used to process the pastes in the scan mode. The maximal temperature attained is~550 • C with a thermal influence duration of 10 s per scan. The pastes are scanned by APPJ for 0, 1, 3 and 5 times. X-ray photoelectron spectroscopy (XPS) indicates the reduction of CO binding content as the number of scan increases, suggesting the oxidation/decomposition of EC. The areal capacitance increases and then decreases as the number of scan increases; the best achieved areal capacitance is 15.93 mF/cm 2 with one APPJ scan, in comparison to 4.38 mF/cm 2 without APPJ processing. The capacitance retention rate of the supercapacitor with the best performance is~93% after a 1000-cycle cyclic voltammetry (CV) test. The optimal number of APPJ scans should enable the proper removal of inactive EC and improved wettability while minimizing the damage caused to rGOs by nitrogen APPJ processing.

Flexible reduced graphene oxide supercapacitors processed using atmospheric-pressure plasma jet under various temperatures adjusted by flow rate and jet-substrate distance

“…These could lead to the improved wettability [21]. It is noted that APPJ treatment also burns out ethyl cellulose (which contains large amount of -OH) in the pastes, and therefore, the overall -OH content is reduced [21]. The improved wettability can facilitate contact between the gelelectrolyte and the rGO-coated carbon cloth, thereby improving the capacitance value, as shown later.…”

“…x-ray photoelectron spectroscopy shows that APPJ treatment can introduce -COOH function groups and nitrogen doping to rGO-coated carbon cloth. These could lead to the improved wettability [21]. It is noted that APPJ treatment also burns out ethyl cellulose (which contains large amount of -OH) in the pastes, and therefore, the overall -OH content is reduced [21].…”

“…The exposure of rGOs can have better contact with the gel-electrolyte, thereby increasing the capacitance value as described latter. APPJ treatment for an overly long duration may completely burn out the rGOs [21,24]. Therefore, in this study, the APPJ processing time is limited to 20 s. Abundant rGOs are still present after APPJ processing at all temperatures.…”

“…In these experiments, the plasma reactivity at the surface of the material being treated was found to be related to the input power and the decay process of plasma reactive species. A higher flow rate reduces the temperature but increases the plasma reactivity when reaching the material with a fixed jet-substrate distance [19][20][21]. By contrast, a shorter jet-substrate distance could lead to higher temperature and higher reactivity of the plasma species.…”

“…By contrast, a shorter jet-substrate distance could lead to higher temperature and higher reactivity of the plasma species. Previously, we have fabricated flexible reduced graphene oxide (rGO) supercapacitors by applying an APPJ activation process to rGO-coated carbon cloth [17,21,22]. In this study, we further adjust the processing temperature by the varying flow rate and jet-substrate distance; the input power is not varied to avoid plasma instability.…”