The selection of electrode materials and their fabrication play a critical role in improving the capacitive performance of the cells. In applications electrodes must provide thermal stability, high specific surface area, corrosion resistance, high electrical conductivity, proper chemical stability and appropriate surface wettability. They should also be low-cost and environmentally friendly. In addition, their ability to transfer Faraday charges is important to improve capacitive performance. The electrode materials are categorized based on three main classes, including carbonaceous materials, transition metal oxides (TMOs), and conducting polymers (CPs).
Primarily carbonaceous materials are used as electrode materials for devices because of their improved properties such as high surface area, low cost, eco-friendliness and easy synthesis methods. The physicochemical properties of carbon-based materials depend on a pore size of approximately less than 1 nm. Moreover, their thermal stability, chemical and electrochemical stability (in various solutions from strongly acidic to alkaline media), high electrical conductivity, and symmetrical constant current charge-discharge curves and good C-V curve rectangles indicate that carbon-based materials are useful electrode materials. Carbon-based materials can be used in various forms such as powders, fibers, monoliths and foils present in the devices. There are several common examples of carbon-based materials, such as carbon aerogels, graphene , activated carbon, activated carbon fibers , carbon nanotubes and carbon cloth, and various carbon-based composites.
Transition metal (TM)-based compounds are the most common redox-active materials are used as electrode materials for devices, especially in oxide (TMOs) and hydroxide forms (TM(OH)s). form. Generally, there are various TMs, such as Ni, Fe, Co, Ti, Mo, V and Nb. TMOs electrodes have high energy and power because of their higher specific capacitance and lower resistance compared to carbon materials, as well as their high electrical conductivity. Moreover, TM(OH)s can provide more charge storage and higher electronic conductivity of their corresponding oxides compared to their corresponding oxide forms. However, TM(OH)s exhibit lower structural stability and therefore have lower multiplicative performance or poor cyclic performance. There are several strategies to address this limitation and improve the cyclic stability of the materials, such as layering engineering of TM(OH) by anionic and cationic doping processes. For example, the cyclic lifetime of α-Ni(OH)2 can be obtained by partial substitution of other TM cations such as Co2+, Ni3+, etc. for bimetallic coordination.
The conducting Polymer-Based Material (CPs) are a widely studied electrode material because they can modulate redox activity through chemical modification, which have good conductivity in the doped state, a high voltage window, and are easy to produce with low environmental impact and low cost. CPs can store a large amount of charge because no structural changes such as phase transitions occur during the charge/discharge mechanism. Compared to other carbon-based electrode materials, CPs have higher conductivity, capacitance, and low equivalent series resistance. The high performance in CPs stems from the fast reversible redox reactions induced by π-conjugated polymer chains, in which ions move to the polymer backbone through an oxidation process (also called doping) and they are released back into the electrolyte solution during reduction or de-doping.
For their different properties, electrode materials have different applications. Carbonaceous materials are promising electrodes with high specific surface area, good chemical and thermal stability, and low resistance, but their low energy density (derived from their surface or quasi-surface energy storage) limits their large-scale application, so they are not yet officially in industrial production and are mainly used in scientific research. MOs such as RuO2, V2O5 and MnO2 show significant improvements as electrode materials (positive and negative) due to their wide variety, excellent specific capacitance and environmental friendliness, and dominate the battery material market. Polymer composite based or hybrid electrodes provide shorter diffusion paths for electrons and ions, adding new ways to improve battery conversion efficiency in the future.