Printing of Graphene Supercapacitors with Enhanced Capacitances Induced By a Leavening Agent

Abstract

The rapid development of portable and wearable electronics has been driving a demand for lightweight and flexible energy storage systems. In this regard, flexible supercapacitors are considered as promising since they possess long cycle life and high power rates. Among available materials for supercapacitor electrodes, graphene has gained considerable attention in recent years, due to its high specific surface area, high thermal and electronic conductivity as well as other favourable mechanical features. A key challenge for the development of graphene supercapacitors is associated with the scalability and simplicity of the fabrication process. Although printing technologies offer a rapid, precise, scalable, and cost-effective fabrication method for energy storage systems, printed graphene-based supercapacitors usually deliver a lower capacitance compared to those fabricated with other methods, which is mainly related to the re-stacking of the graphene sheets after printing. This work presents a straightforward and scalable leavening agent-assisted approach in order to enhance the capacitance of printed flexible graphene-based supercapacitors. Our method consists of increasing the surface area of the reduced graphene oxide electrodes (rGO) due to the addition of the leavening agent ammonium carbonate to the GO ink. The inks are printed with a micro-dispensing technique. During reduction of GO to rGO, the leavening agent decomposes and gases are released which enlarge the inter-flake gaps and therefore suppresses the restacking of the GO flakes. The lateral size is a key parameter that controls the self-alignment of the GO flakes and, thus, has an impact on the capacitance of rGO electrodes. To assess the effect of the lateral flake size, three stock dispersions with different size ranges were prepared. In detail, the processed dispersions contained flakes with lateral sizes in the nano- (< 100 nm), submicro- (0.1 - 1.0 µm) or the micro-meter (> 1.0 µm) range. Experiments were performed to investigate the relationships between GO lateral size, the concentration of leavening agent and the capacitance of the printed rGO electrodes. In absence of the leavening agent, the specific capacitance slightly decreases with an increase of the flake size. In contrast, for inks with leavening agent concentrations of 3.0 and 6.0 wt. %, the specific capacitance increases with increasing flake size. The experiments reveal that there is a maximum specific capacitance of 112.1 ± 6.5 F g-1 at a leavening concentration of 3.7 wt. %. This specific capacitance is significantly higher compared to that of the electrode without the leavening agent approach (62.4 ± 5.2 F g-1) indicating a successful process for increasing the gaps between rGO flakes. With respect to cycle life, it is observed that the risen electrodes maintain better long-term capacitance stability compared to agent-free electrodes. The electrodes with the leavening agent approach feature an 85% capacitance retention after 2,000 charge-discharge cycles at a current density of 1.0 A g-1 while the leavening agent-free electrodes show a gradual capacitance decay to 80 %. Figure 1

Soft materials for wearable supercapacitors

“…The device has small capacitance decay upon bending conditions (from 8.7 mF cm -2 to 8.1 mF cm -2 at a curvature radius of 2.75 cm and a scan rate of 50 mV s -1 ) [82]. To further improve the restack tendency of graphene flakes, Barz et al [83] added a chemical leavening agent in rGO ink to suppress this restacking during the printing process (Figure 4d and e).…”