Evaluating the Electrochemical Properties of Supercapacitors using the Three-Electrode System

Supercapacitors have attracted enormous attention as suitable power sources for a variety of applications such as microelectronic devices, electric vehicles (EVs), and stationary energy storage systems. In EV applications, supercapacitors can be used for rapid acceleration and can enable the storage of regenerative energy during the deceleration and braking processes. In renewable energy fields, such as solar power generation¹ and wind power generation², supercapacitors can be used as stationary energy storage systems^3,4. Renewable energy generation is limited by the fluctuating and intermittent nature of these energy supplies; therefore, an energy storage system that can respond immediately during irregular power generation is required⁵. Supercapacitors, which store energy via mechanisms that differ from those of lithium-ion batteries, exhibit a high power density, stable cycle performance, and fast charging-discharging⁶. Depending on the storage mechanism, supercapacitors can be distinguished into double-layer capacitors (EDLCs) and pseudocapacitors⁷. EDLCs accumulate electrostatic charge at the electrode surface. Therefore, the capacitance is determined by the amount of charge, which is affected by the surface area and porous structure of the electrode materials. By contrast, pseudocapacitors, which consist of conducting polymers and metal oxide materials, store charge through a Faradaic reaction process. The various electrochemical properties of supercapacitors are related to the electrode materials, and developing new electrode materials is the main issue in improving the performance of supercapacitors⁸. Hence, evaluating the electrochemical properties of these new materials or systems is important in the progress of research and further applications in real life. In this regard, electrochemical evaluation using a three-electrode system is the most basic and widely utilized method in lab-scale research of energy storage systems^{9,10,11,12,13}.

The three-electrode system is a simple and reliable approach for evaluating the electrochemical properties, such as the specific capacitance, resistance, conductivity, and cycle life of supercapacitors¹⁴. The system offers the benefit of enabling analysis of the electrochemical characteristics of single materials¹⁵, which is in contrast to the two-electrode system, where the characteristics can be studied through the analysis of the given material. The two-electrode system just gives information about the reaction between two electrodes. It is suitable for analyzing the electrochemical properties of the entire energy storage system. The potential of the electrode is not fixed. Therefore, it is not known at what voltage the reaction takes place. However, three-electrode system analyzes only one electrode with fixing potential which can perform a detailed analysis of the single electrode. Therefore, the system is targeted toward analyzing the specific performance at the material level. The three-electrode system consists of a working electrode (WE), reference electrode (RE), and counter electrode (CE)^16,17. The WE is the target of research, assessment as it performs the electrochemical reaction of interest¹⁸ and is composed of a redox material that is of potential interest. In the case of EDLCs, utilizing high surface area materials is the main issue. Therefore, porous materials with a high surface area and micropores, such as porous carbon, graphene, and nanotubes, are preferred^19,20. Activated carbon is the most common material for EDLCs because of its high specific area (>1000 m²/g) and many micropores. Pseudocapacitors are fabricated with materials that can undergo a Faradaic reaction²¹. Metal oxides (RuO_x, MnO_x, etc.) and conducting polymers (PANI, PPy, etc.) are commonly used²². The RE and CE are used to analyze the electrochemical properties of the WE. The RE serves as a reference for measuring and controlling the potential of the system; the normal hydrogen electrode (NHE) and Ag/AgCl (saturated KCl) are generally chosen as the RE²³. The CE is paired with the WE and completes the electrical circuit to allow charge transfer. For the CE, electrochemically inert materials are used, such as platinum (Pt) and gold (Au)²⁴. All components of the three-electrode system are connected to a potentiostat device, which controls the potential of the entire circuit.

Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS) are typical analytical methods that use a three-electrode system. Various electrochemical characteristics of supercapacitors can be assessed using these methods. CV is the basic electrochemical method used to investigate the electrochemical behavior (electron transfer coefficient, reversible or irreversible, etc.) and capacitive properties of material during repeated redox processes^14,24. The CV plot shows redox peaks related to the reduction and oxidation of the material. Through this information, researchers can evaluate the electrode performance and determine the potential where the material is reduced and oxidized. Furthermore, through CV analysis, it is possible to determine the amount of charge that material or electrode can store. The total charge is a function of the potential, and the capacitance can be easily calculated^6,18. Capacitance is the main issue in supercapacitors. A higher capacitance represents the ability to store more charge. EDLCs give rise to rectangular CV patterns with linear lines so that the capacitance of the electrode can be calculated easily. Pseudocapacitors present redox peaks in rectangular plots. Based on this information, researchers can assess the electrochemical properties of materials using CV measurements¹⁸.

GCD is a commonly employed method for identifying the cycle stability of an electrode. For long-term use, the cycle stability should be verified at a constant current density. Each cycle consists of charge-discharge steps¹⁴. Researchers can determine the cycle stability through variations in the charge-discharge graph, specific capacitance retention, and Coulombic efficiency. EDLCs give rise to a linear pattern; thus, the specific capacitance of the electrode can be calculated easily using the slope of the discharge curve⁶. However, pseudocapacitors exhibit a nonlinear pattern. The discharge slope varies during the discharging process⁷. Furthermore, the internal resistance can be analyzed through the current-resistance (IR) drop, which is the potential drop owing to the resistance^6,25.

EIS is a useful method for identifying the impedance of energy storage systems without destruction of the sample²⁶. The impedance can be calculated by applying a sinusoidal voltage and determining the phase angle¹⁴. The impedance is also a function of the frequency. Therefore, the EIS spectrum is acquired over a range of frequencies. At high frequencies, kinetic factors such as the internal resistance and charge transfer are operative^24,27. At low frequencies, the diffusion factor and Warburg impedance can be detected, which are related to mass transfer and thermodynamics^24,27. EIS is a powerful tool for analyzing the kinetic and thermodynamic properties of a material at the same time²⁸. This study describes the analysis protocols for evaluating the electrochemical performance of supercapacitors using a three-electrode system.