Fabrication and characterization of activated carbon / graphene supercapacitor as an energy storage device and its equivalent circuit model

Abstract

This thesis presents the investigations of graphene materials’ contribution when used in combination with the traditional supercapacitor electrode material, the Activated Carbon (AC). The analyses were conducted via material characterization and electrochemical characterization methods. The study also proposed a new Equivalent Circuit Model (ECM) that can be used as another characterization approach besides producing a better supercapacitor model in a virtual electronic system environment. Literature works show that a small amount of graphene addition on the AC electrode enhanced the performance of the electrode in an improved specific capacitance and lowered the internal resistance. However, some researchers reported that further graphene addition would offset the improvement as graphene typically has a lower specific surface area compared to the AC. This work explores the performance of supercapacitor electrodes with pure AC, pure graphene, and several AC-graphene composites ratios. Two types of graphene were used, two-dimensional graphene using Graphene Nanoplatelets (GNP) and three-dimensional graphene using Graphene Aerogel (GA). It was found that increasing GNP wt% in the electrode would increase the prototype’s specific capacitance in a linear relationship, with an insignificant effect on the internal resistance. On the other hand, 20 wt% GA on the electrode performs the best capacitance among the GA-based prototypes, while further GA wt% increase decreases the capacitance. Higher internal resistances were also recorded with higher GA wt%. Besides the capacitance and internal resistance, the role of graphene addition was also observed in the prototypes’ self-discharge behavior, especially on the charge-redistribution effect under the Open Cell Voltage (OCV) procedure. 20 wt% addition of GNP retained the most charges among the prototypes after being left for 60 minutes in OCV. The self-discharge result was used for the ECM profile fitting. The proposed ECM produces the best circuit fitting on self-discharge among works of literature, especially on the short-term response. This was achieved by introducing an intermediate layer of RC circuit branch that represents the transitional charge location domain between the Helmholtz layer and the Diffuse layer. The ECM was successfully tested on commercial supercapacitors and the prototypes from this study with an average Root Mean Squared Error of 0.2 %. Applications with a short-term open circuit such as the stop/start features in micro-hybrid vehicles can benefit from this ECM by getting a more accurate State of Charge of the energy storage system used. New insights can be extracted from the ECM as the simulation shows that the graphene addition facilitated the ions’ movement into the Helmholtz layer, whereas for the prototype without graphene, most of the ions were restricted at the intermediate layer. The new ECM has the potential to be used as a new characterization method for understanding the supercapacitor’s electrode-electrolyte interface.