Electrochemical capacitors (ECs) have great advantage over batteries in e.g. power density, cycle life and working temperature range. Comprehensive efforts have been undertaken to make ECs’ energy density comparable to batteries, so that ECs could be aimed for electrical vehicles, uninterruptable power supplies and similar high power consumption (W to kW) applications. However, with the advent of industry 4.0 and an increasing importance of miniaturized smart systems such as self-powered wireless sensors for internet of things (IoT), a new set of requirements on ECs applies for such low power consumption (µW to mW) applications, with the primary focus on not only energy density, but also other metrics. Among various critical performance requirements, the need for fast frequency response in smart systems is distinct from that for high power consumption applications. On one hand, fast frequency response (related to high power capability) is desired to efficiently store the energy that is from the energy harvester and in the form of pulse signal at a certain frequency. On the other hand, EC with fast frequency response could perform functions more than energy storage; it is possible to replace the bulky aluminum electrolytic capacitors with a high frequency EC for rectifying / line filtering, with great benefit in system size reduction. Additionally, ECs for miniaturized smart systems should also have high areal / volumetric capacitance, low leakage current, be capable of working at low current (µA to mA) with long cycle life and satisfy encapsulation requirements. Currently, many existing EC systems have poor frequency response (due to inappropriate porosity), and are incapable of working at a low current (side reactions lead to serious lifetime reduction). As we can see, the development of EC for miniaturized smart systems is still in its infancy.
In this work, high frequency (cut-off frequency over 1 kHz) ECs based on flexible carbon substrate with vertically aligned carbon nanotubes (VACNTs) as an active material have been developed. Covalent bonding between carbon substrate and VACNTs is created through the in-situ CNT growth on the substrate. The bonding is beneficial for high conductivity and thereby the high frequency response. Performance metrics as a function of CNT height is identified, a strategy of suppressing the Faradaic processes at low current by using solid-state electrolytes is proposed. A demonstrative EC with 10 µm long VACNTs has a device areal capacitance of 1.38 mF/cm2 at 120 Hz with -84.85° phase angle, a 0.64 µA leakage current (after 5 min, 2 mF device) and 99.8% capacitance retention over 30000 cycles, representing state-of-art carbon based high frequency ECs. The presented device bridges the performance gap between electrostatic capacitor and conventional ECs in terms of capacitance and frequency response. We believe the high frequency EC will find applications in various miniaturized smart systems.