# A Review of Fabrication Technologies for Carbon Electrode-Based Micro-Supercapacitors
## 7 Conclusions  
This review has presented an overview of the research progress, mainly of the last five years, on the fabrication of carbon electrode-based micro-supercapacitors. The fast evolution in wearable small electronics, enabled by huge advances in nanotechnology, drives a need for flexible, thin energy storage devices. Supercapacitors have advantages over batteries in terms of their ultrahigh areal power density, long cycle lifetime, and fast charge and discharge time. Supercapacitors are divided into three types based on their charge mechanism: (i) EDLC supercapacitors, (ii) pseudosupercapacitors, and (iii) hybrid supercapacitors. Carbon-based materials are the most common electrode materials for EDLC supercapacitors because of their high thermal and electrochemical stability, low cost, good electrical conductivity, controlled porosity, and appropriate active sites. Micro-supercapacitors can occur in two main configurations, namely the stacking configuration and the in-plane interdigitated configuration, and a third type, the fibre-shaped configuration has also been presented. The performance of supercapacitors is highly impacted by the structural design of the electrode, and to achieve high capacitance, the carbon electrode materials should have a controlled pore structure, large specific surface area, and adequate surface functional groups. Carbon materials commonly used in micro-supercapacitors include activated carbon, carbon nanotubes, graphene, onion-like carbon, and carbide-derived carbon. Recent research on these carbon electrode materials show efforts focused on increasing specific capacitance, making hybrid electrodes, doping of activated carbon, doping graphene with fluorine, laser irradiation of graphene, decoration of onion-like carbon with Pt, making composites of carbon nanotubes with MXene, and composites of graphene with carbide-derived carbon. Also the electrolytes have an influence on the micro-supercapacitor performance. The electrolytes can be divided into five types. Aqueous electrolytes have the advantages of high conductivity, low cost, non-flammable nature, and being environment friendly, but their main challenge is the low operating voltage. A potential solution to the latter challenge is a supersaturated water-in-salt electrolyte. The most common aqueous electrolytes, reported in studies on micro-supercapacitors, include NaCl, H\({}_{2}\)SO\({}_{4}\), Li\({}_{2}\)SO\({}_{4}\), and Na\({}_{2}\)SO\({}_{4}\). A second important type of electrolytes concern the organic electrolytes, which have a wider potential stability window. However, they have disadvantages in terms of their toxicity, flammability, sensitivity to moisture, and their inferior power performance. In recent micro-supercapacitor studies, propylene carbonate with sulfolane has been used. Ionic liquids are a third important type of electrolytes, which have a wide potential stability window, high thermal stability, and lower sensitivity to moisture and oxygen in comparison to organic electrolytes. The challenges with ionic liquid electrolytes are mainly the lower conductivity and lower power density. An example of an ionic liquid electrolyte, reported in recent micro-supercapacitor studies, is 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide. Next, solid-state electrolytes have the advantage of being leak proof, having a high performance, simple packaging, and a fabrication method. Still, their challenge is the limited ion mobility, which has a negative influence on the power density. Finally, in wearable micro-supercapacitors, which are flexible and elastic, quasi-solid-state electrolytes are used, such as gel polymer electrolytes. The main challenge with those electrolytes is the evaporation of the solvent, which can decrease the conductivity. The most common gel electrolytes, reported in recent micro-supercapacitors studies, involve \(\mathrm{PVA}/\mathrm{H}_{2}\mathrm{SO}_{4}\) and \(\mathrm{PVA}/\mathrm{H}_{3}\mathrm{PO}_{4}\).