Vertically Oriented MXene Bridging the Frequency Response and Capacity Density Gap for AC‐Filtering Pseudocapacitors

Abstract

Electrochemical capacitors (ECs) with high-rate characteristics demonstrate great promise for replacing bulky aluminum electrolytic capacitors with alternate current (AC)-filtering functions. Currently, the performance of AC-line filtering ECs is limited by the vast gap between the ionic transport kinetics and capacity density of electrode materials. Herein, vertically oriented MXene and reduced graphene oxide composite (VMG) electrodes are prepared by electrochemical co-deposition to bridge the performance gap for AC-line filtering ECs. Benefiting from the unique pseudocapacitive characteristic and vertically interconnected microstructure, the VMG-based EC exhibits an outstanding areal capacitance of 1.14 mF cm −2 with a phase angle of −80° at 120 Hz. Moreover, by utilizing the advantages of MXene at negative potential, an asymmetric pseudocapacitor is constructed with an energy density up to 805 µF V 2 cm −2 at 120 Hz, which can be further incorporated in portable and wearable devices with excellent environmental ripple filtering capability in diverse scenarios.

MXene-Based Micro-Supercapacitors: Ink Rheology, Microelectrode Design and Integrated System

“…223,224 In filtering applications, especially in full wave rectification (120 Hz AC) waveforms, the RC time constant of the capacitor should be less than 8.3 ms, which means that the phase angle and frequency response of the capacitor should remain close to −90°up to 120 Hz frequency. 61,207,225 Due to the unique 2D morphology of MXene that forms ion channels between layers, its metallic conductivity (8,000−10,000 S cm −1 ), and rapid surface redox reactions, MXene MSCs often exhibit exceptional rate and power performance, which are critical for AC line circuit filtering. 27 In 2019, Jiang et al achieved wafer-scale micro-supercapacitors with superior frequency filtering performance by controlling the lateral size of MXene flakes (Figure 18a).…”

“…Subsequently, Zhang et al treated MXene/PEDOT:PSS coatings with tartaric acid, which exhibited similar AC filtering performance with a relaxation time constant of only 1.3 ms. 152 Additionally, Wen et al prepared vertical-oriented MXene and reduced graphene oxide composite (VMG) electrodes by electrochemical codeposition, which can maintain a phase angle of 80°at 120 Hz, an energy density of 805 μF V 2 cm −2 , and stable DC signals at different frequencies (Figure 18d). 61…”

“…Figure 15d shows an integrated system powered by wind. 61 The integrated system can convert AC wind signals into DC signals and store the collected energy in MSCs. The capacitor section is an asymmetric structure composed of VMG (vertically oriented MXene and reduced graphene oxide composite) and PEDOT, with a voltage of 1.4 V and areal energy density of 805 mF V 2 cm −2 , and exhibits excellent filtering performance.…”

Hierarchically 3D Fibrous Electrode for High‐Performance Flexible AC‐Line Filtering in Fluctuating Energy Harvesters

“…[19][20][21][22] The deep-seated issue is with electrode structures, which are hampered by the conflict between charge storage density and ion/electron transport kinetics. [23,24] Generally, high-capacity fibrous electrode materials must possess intricate microporous structures or abundant pseudocapacitance. However, neither tiny pores nor redox reactions can sustain quick ion transport, limiting frequency response capability (below 1 Hz).…”

“…[28][29][30] Accordingly, rationally designing the fibrous electrode structures to synergistically enhance the ion/electron transport properties and capacitance density is the key to realizing FSECs with good ripple filtering capability and further meeting compatible and wearable requirements of emerging self-charging devices. [24,31] Regarding the above concerns, a hierarchically 3D fibrous electrode with excellent line-filtering performances is developed. The primary framework is electrochemically reduced graphene oxide (ErGO) with a 3D interpenetrating structure, ensuring smooth ion transport channels.…”