# A Review of Fabrication Technologies for Carbon Electrode-Based Micro-Supercapacitors
## 1 Introduction  
The last decades have seen a tremendously fast evolution in technology of electronics [1]. Importantly, advances in nanotechnology have facilitated the ever further miniaturization of electronic devices [2, 3]. The progress in wearable electronics also drives a huge need for the development of wearable energy devices to power those electronics [4, 5, 6, 7] for applications, such as human health monitoring [8, 9], intelligent electronic skins [10, 11, 12, 13], foldable displays [14, 15, 16], and medical microdevices [17, 18]. Batteries and supercapacitors are commonly used as energy storage units, but they are generally bulky and rigid, which is not ideal for wearable devices [19, 20, 21, 22]. Therefore, efforts are focused on the development of flexible and very thin batteries and supercapacitors [23, 24, 25, 26, 27, 28, 29]. There are a few recent review papers on the advances in materials for supercapacitors [30, 31, 32]; however, a comprehensive review on the fabrication technologies for carbon electrode-based micro-supercapacitors is lacking. Batteries and supercapacitors have different performance properties (Figure 1). Batteries have the advantage of high energy density [33, 34], whereas supercapacitors have the advantages of ultrahigh areal power density (more than 10 kW per kg), long cycle lifetime (millions of cycles), and fast charge and discharge time (seconds) [35, 36]. The charge in supercapacitors is limited to the surface of the electrode materials, so the energy density of supercapacitors is still limited to about 5 to 20 Wh per kg [37, 38, 39]. Thus, supercapacitors fill the gap between batteries and aluminium electrolytic capacitors [40, 41]. There are three types of supercapacitors based on the charge storage mechanism (Figure 2). The first type is the electric double layer capacitor (EDLC), which does not involve a chemical oxidation-reduction reaction but rather the accumulation of electrostatic charges at the electrode/electrolyte interface upon an applied voltage [42]. Charges are stored by ion diffusion from the electrolyte into the pores of the electrode of opposite charge across the separator, based on the attraction force between opposite charges, resulting in physical adsorption [41; 43]. The concentration of the electrolyte remains constant, and there are no volumetric or morphological changes with charging or discharging, so a very long cycle lifetime can be achieved. An EDLC supercapacitor has high power density but low specific capacitance [44]. The second type is the pseudocapacitor, which involves fast reversible redox reactions (faradaic processes, providing high energy density) between the electrode active materials and electrolytes at the electrode surface and the near-surface [45; 46]. There are three main mechanisms of pseudocapacitors, namely: (i) reversible adsorption, (ii) redox reaction in metal oxides, and (iii) reversible doping/de-doping electrochemical processes in conductive polymer-based electrodes. Pseudocapacitors have high capacitance but low power density and cycle lifetime [47]. The third type is the hybrid supercapacitor, which uses both faradaic and non-faradaic processes to store charge [48]. Hybrid supercapacitors use EDLC type materials as the conductive backbone for pseudocapacitive materials with the aim of enhancing the electrochemical performance of the system [48]. Figure 1: Ragone plot, showing power density versus energy density, for different types of energy storage materials. Figure 2: Diagram of the different types of supercapacitors and their main electrode materials. This review paper on fabrication technologies for carbon electrode-based micro-supercapacitors is organized as follows. First, the micro-supercapacitor configurations are briefly described. Then, the different types of carbon materials used for electrodes in micro-supercapacitors are presented, followed by the different types of electrolytes.