Unraveling the Role of Oxides in Electrochemical Performance of Activated Carbons for High Voltage Symmetric Electric Double‐Layer Capacitors

Abstract

Escalation of electricity demand, its heavy dependence on combustion of natural fossil fuels, and significant emission of greenhouse gases have urged industries and researchers to develop ecofriendly and high-performance techniques for the generation and storage of energy. A range of electrochemical storage systems such as batteries, fuel cells, and supercapacitors (SCs) have been extensively developed, among which SCs have gained tremendous attention in view of their long cycle life, high specific capacitance, and particularly, high power density. [1][2][3][4] SCs are classified into two categories: pseudocapacitors and electric double-layer capacitors (EDLCs). [5] In EDLCs, the energy originates from the reversible non-Faradic reactions at the interface of electrode/electrolyte. [5] Currently, SCs (filled with carbon materials) are widely used in a large variety of applications ranging from consumer portable electric devices, memory backup systems, industrial and energy management, to emergency doors on airplanes and low-emission hybrid electric vehicles. [1] Despite advances in controlling the characteristics of carbon materials and improvements in advanced nanomaterials, such as graphene or carbon nanotubes (CNTs), activated carbons (ACs) are still the primary choice in commercial SCs due to their low cost, abundance, ease of processability, and simple production procedures. [6] From the scientific point of view, ACs are of the most versatile carbon-based electrode materials applied in SCs [7,8] due to their outstanding properties of relatively inert electrochemistry, [9] large surface area (>1000 m 2 g À1 ), [6] controllable microporous structure, favorable pore size and pore volume (>0.5 cm 3 g À1 ), [6] variable surface chemical composition, [10] and great electrocatalytic active sites for redox reactions. [11] The electrochemical performance of ACs in SCs is greatly dependent on the carbon source and fabrication method. [12] ACs used to be derived dominantly from coal and petroleumbased materials [13] ; however, in recent years, to further diminish the carbon footprint in the environment, the approach has shifted toward the use of alternative carbon resources such as waste materials. Generally, there are two types of waste resources for ACs, 1) organic wastes such as biomasses and 2) synthetic wastes such as waste polymers and fabrics.