In this review, the efforts done by different research groups to enhance the performance of the electric double-layer capacitors (EDLCs), regarding the effect of the modification of activated carbon structures on the electrochemical properties, are summarized. Activated carbon materials with various porous textures, surface chemistry, and microstructure have been synthesized using several different techniques by different researchers. Micro-, meso-, and macroporous textures can be obtained through the activation/carbonization process using various activating agents. The surface chemistry of activated carbon materials can be modified via: (i) the carbonization of heteroatom-enriched compounds, (ii) post-treatment of carbon materials with reactive heteroatom sources, and (iii) activated carbon combined both with metal oxide materials dan conducting polymers to obtain composites. Intending to improve the EDLCs performance, the introduction of heteroatoms into an activated carbon matrix and composited activated carbon with either metal oxide materials or conducting polymers introduced a pseudo-capacitance effect, which is an additional contribution to the dominant double-layer capacitance. Such tricks offer high capacitance due to the presence of both electrical double layer charge storage mechanism and faradic charge transfer. The surface modification by attaching suitable heteroatoms such as phosphorus species increases the cell operating voltage, thereby improving the cell performance. To establish a detailed understanding of how one can modify the activated carbon structure regarding its porous textures, the surface chemistry, the wettability, and microstructure enable to enhance the performance of the EDLCs is discussed here in detail. This review discusses the basic key parameters which are considered to evaluate the performance of EDLCs such as cell capacitance, operating voltage, equivalent series resistance, power density, and energy density, and how these are affected by the modification of the activated carbon framework.

CO gas sensing mechanism of SnO₂/CuO bilayer sensor.

The schematic view of the TiO ₂ /NiO bilayer sensor with the platinum as bottom and top electrodes.

Supercapacitors provide remarkable eco-friendly advancement in energy conversion and storage with a huge potential to control the future economy of the entire world. Currently, industries focus on the design and engineering aspects of supercapacitors with high performance (high energy), flexibility (by the use of composite polymer based electrolytes), high voltage (ionic liquid) and low cost. The paper reviews the modelling techniques like Empirical modelling, Dissipation transmission line models, Continuum models, Atomistic models, Quantum models, Simplified analytical models etc. proposed for the theoretical study of Supercapacitors and discusses their limitations in studying all the aspects of Supercapacitors. It also reviews the various software packages available for Supercapacitor (SC) modelling and discusses their advantages and disadvantages. The paper also reviews the Experimental advancements in the field of electric double layer capacitors (EDLCs), pseudo capacitors and hybrid/asymmetric supercapacitors and discusses the commercial progress of supercapacitors as well.

FESEM surface morphology and the schematic view of the Pd/WO₃–ZnO composite sensor with Ag paste contacts.