A review of ultrafast supercapacitors for AC-line filtering

Abstract

Filter capacitors play an important role in alternating current (AC)-line filtering for stabilizing voltage, suppressing harmonics, and improving power quality. However, traditional aluminum electrolytic capacitors (AECs) suffer from a large size, short lifespan, low power density, and poor reliability, which limits their use. In contrast, ultrafast supercapacitors (SCs) are ideal for replacing commercial AECs because of their extremely high power densities, fast charging and discharging, and excellent high-frequency response. We review the design principles and key parameters for ultrafast supercapacitors and summarize research progress in recent years from the aspects of electrode materials, electrolytes, and device configurations. The preparation, structures, and frequency response performance of electrode materials mainly consisting of carbon materials such as graphene and carbon nanotubes, conductive polymers, and transition metal compounds, are focused on. Finally, future research directions for ultrafast SCs are suggested.

1. Introduction

Filter capacitors are essential for maintaining circuit stability and efficiency by smoothing current and voltage fluctuations, thereby eliminating noise and high-frequency interference[1–4]. Currently, aluminum electrolytic capacitors (AECs) are commonly used due to their excellent frequency response and voltage withstand capability[5–7]. However, AECs are characterized by rigid cases, large size (～100 mm), low power density (< 0.1 W cm−3) and short lifespan (2–5 years), making them difficult to meet the growing demand for miniaturized and flexible electronic devices. Moreover, their low energy densities and unstable output voltage often fail to satisfy the stable operation of circuits[8–10]. With the rapid development of power electronic technology, there is a demand for faster frequency response, higher energy density and longer service life for filter capacitors. Therefore, exploring novel energy storage devices with high energy density and excellent stability has become the focus for replacing AECs.

Supercapacitors (SCs) have attracted significant attention as energy storage devices due to their excellent performance between traditional capacitors and batteries[11–12]. They are mainly divided into electric double-layer capacitors (EDLCs) and pseudocapacitors, based on different energy storage mechanisms[13–15]. EDLCs operate as physical energy storage devices, relying on ion adsorption and desorption at the electrode-electrolyte interface, which contributes to their long cycle life[16–18]. In contrast, the energy storage of pseudocapacitors is through rapid redox reactions at the electrode/electrolyte surface, which results in higher specific capacitances[19–20]. Despite these advantages, the intricate electrode structure in SCs leads to restricted ion diffusion and high ionic and electronic impedance. As a result, their charging and discharging capabilities are usually limited to frequencies below 1 Hz, restricting their application in high-frequency alternating current (AC)-line filtering[2,17,21]. Ultrafast SCs have been developed to address these limitations, offering superior power densities and rapid charging/discharging capabilities while maintaining stable operation under high-frequency conditions. This advancement enables effective noise filtration and enhances circuit stability. Nevertheless, ultrafast SCs still suffer from considerable challenges, particularly regarding their low energy density and limited operating voltage, necessitating further research and development to improve their performance.