IntroductionSupercapacitors, also called ultracapacitors, as potential electrochemical energy storage devices are applied in a variety of military and commercial applications because of their long cycling lifetime, low maintenance cost, and extraordinarily high power density [1, 2]. A hybrid system combining lithium-ion batteries and supercapacitors has also been attracting the attention of researchers. Supercapacitors can be primary energy devices for power assist during acceleration and hill climbing, as well as for recovery of braking energy. Lithium-ion batteries can provide the energy during cruising. The hybrid system combines the power performance of supercapacitors with the greater energy storage capability of lithium-ion batteries. The hybrid system can also extend the life, downsize the volume, and reduce the cost of batteries.There are two types of supercapacitors (SCs): electric double-layer capacitors (EDLCs) and pseudocapacitors. EDLCs are non-faradaic supercapacitors which store energy using the adsorption of both anions and cations [3,4]. Currently, most of state-of-the-art EDLCs devices are based on high-surface-area carbon [5][6][7][8][9][10][11], such as porous activated carbon (AC). A main issue with AC-based SC electrodes is that the entire surface area is not electrochemically accessible by the electrolyte because of their small micropore size (pore size <2 nm) and hydrophobic graphene-like surface [12,13]. Therefore, carbon has a low surface-area-normalized specific capacitance of 10-40 μF cm −1 [2, 14] The commercial SC based on AC and organic electrolyte has only an energy density of ∼7 Wh kg −1 [15], although its power density can be as high as 10 kW kg −1 as shown in Figure 4.1. Therefore, it is essential to develop novel electrode materials with large accessible specific surface area (SSA) and better electrolyte affinity, contributing to a higher energy density.Unlike EDLCs which store energy through physical adsorption, pseudocapacitive materials [17] store energy through a faradaic process, which involves fast and reversible redox reactions between the electrolyte and electroactive materials on the electrode surface [18]. In principle, the electroactive species, which possess Graphene-based Energy Devices, First Edition. Edited by A. Rashid bin Mohd Yusoff.