vanced patent was awarded to R. A. Rightmire who was a chemist at Standard Oil Company of Ohio (SOHIO) [11]. In 1971, SOHIO licenced the double-layer capacitor technology to Nippon Electric Company (NEC) and by 1978, NEC's supercapacitor was commercialised. Rapid development of electric vehicles and mobile electronics called for more advancements in the battery and supercapacitor industry, and in 1990s more research funding was advocated by United States Department of Energy (DOE) due to an increasing awareness of the extensive applications of battery and supercapacitors [7]. Today, the application of supercapacitors in the fields of electronics and electrochemistry are diverse. They can be used to generate power and be a peak assist in industry or for automotive applications [4]. They can also replace or supplement batteries in consumer electronics. In addition, micro-sized supercapacitors are widely utilized in biosensors and microelectromechanical systems as an on-chip element [6]. Depending on the type of application, the requirements and thus, supercapacitor cell design may vary. Based on the mechanism of charge storage, supercapacitors are divided into three major types, namely electric double-layer capacitors, pseudocapacitors and hybrid cells. The following sections (2.2-2.4) provide an overview of their working principles. 2.2 Electric double-layer capacitors (EDLCs) 2.2.1 Electric double-layer models An EDLC consists of two electrodes separated by an ion permeable separator. The space between the electrodes is filled with a liquid or quasi-solid electrolyte and it usually consists of solvated positive and negative ions suspended in solvent(s), or moving inside the quasi-solid backbone. When an EDLC is charged, two layers with opposite charges form at each electrode-electrolyte interface; these layers are commonly described as electric double-layers and they resemble two capacitors in series (Figure 2.2).