Preface

For decades, science has been intensively researching electrochemical systems that exhibit extremely high, and previously unattainable, capacitance values (on the order of hundreds of Fg-1). Research has not only demonstrated the unsuspected possibilities of supercapacitors, but also marked out a new direction for the development of electrical energy storage systems [1]. With the recent development of new materials and technologies, very large developed surfaces and very small inter-electrode distances have been achieved. Enormous pseudo-capacitance of several orders of magnitude larger than standard capacitors has been achieved for many materials, and such systems are called supercapacitors or, less frequently, ultracapacitors) [2].

 There are two types of supercapacitors, depending on the energy storage mechanism. Electric double-layer capacitors use the electrostatic principle, and for pseudo-capacitors, [3] the charge storage is caused by fast redox reactions [4]. Electrode systems using both mechanisms are called hybrid supercapacitors. Supercapacitors are widely used due to their fast charge and discharge, and a huge number of charge/discharge cycles [5].

Chapter 1 focuses on the need for further development and improvement of supercapacitor properties to power isolated systems (sensor networks, IoT), covering peaks of electricity consumption, filtration and new, more efficient topologies in power electronics. New technologies and materials, and their fabrication, modeling, characterization and applications are also presented.

Carbon allotropes, including fullerenes, carbon nanotubes, and graphene-based supercapacitors, are discussed in Chapter 2, while graphene-based nanocomposites for supercapacitor electrodes are presented in Chapter 3.

Transition metal oxides (TMOs) are pseudo-capacitor electrode materials that show high specific capacitance and are a better redox active material for energy storage applications [6]. Chapter 4 examines developments in the fabrication of six TMObased electrode materials (NiO, ZnO, MnO2, SnO2, WO3, V2O5) for enhanced electrochemical performance.

In Chapter 5, the authors show how various technologies are used to fabricate electrodes and supercapacitors, and present several applications.

> **Zoran Stevic** Technical Faculty in Bor, School of Electrical Engineering, University of Belgrade, Belgrade, Serbia
