Summary

Electric double-layer capacitors (EDLCs) have the potential to replace aluminum electrolytic capacitors for alternating current line-filtering, aligning with the trend of device miniaturization. However, it is challenging for EDLCs to simultaneously achieve rapid ion migration, electrical response, and high areal and volumetric capacitances. Here, we demonstrate that three-dimensional, structurally integrated multi-layer carbon tube (3D-MLCT) frameworks are used as electrodes for high-performance filtering EDLCs. By simply increasing the number of tube layers, a high specific areal capacitance of 3.08 mF cm−۲ at 120 Hz is achieved with a phase angle of −۸۰٫۱°, exhibiting excellent capacitance and fast frequency response. The outstanding performance of the 3D-MLCT is attributed to the high density, high orientation, and high integrality of carbon tube arrays that facilitate and accelerate ion distribution onto the electrode surface. The findings of this work provide a solution for developing next-generation miniaturized filter capacitors with high capacitance and fast frequency response.

Introduction

Converting alternating current (AC) into direct current (DC) is essential for powering electronics.1,2,3 In the process, filter capacitors play a crucial role in smoothing the ripples in the rectified DC signal.4,5,6,7 Aluminum electrolytic capacitors (AECs) dominate this field but occupy the most significant volume in circuits.8,9,10 Electric double-layer capacitors (EDLCs), storing energy physically through reversible ion adsorption at the electrode-electrolyte interface, have been anticipated for fast frequency response applications.11,12,13,14,15 EDLCs are considered promising alternative devices for AC line-filtering to meet the demands of miniaturized electronic devices owing to their several-orders-of-magnitude-higher specific capacitance than commercial AECs.16,17,18,19,20,21,22,23 However, conventional EDLCs behave more like resistors than capacitors and possess poor frequency response performance at an indicator frequency of 120 Hz because of their tortuous and complicated pore structure as well as the high resistances of the electrodes.24,25,26,27

The first filtering EDLC was reported by Miller et al.,1 with discrete vertically oriented graphene (VOG) nanosheets grown on nickel substrates as electrodes, demonstrating an outstanding AC line-filtering performance. Subsequently, various carbon-nanomaterial-based electrodes with high orientation, thin thickness, and macro-porous structures have been explored for filtering EDLCs.28,29,30,31,32,33,34,35 However, the specific areal (CA) and volumetric capacitances (CV) are still at a low level—the bottleneck of miniaturized filter devices.36,37,38,39

Results and discussion

The fabricating process of 3D-MLCT is schematically illustrated in Figure 1. A 3D-AAO template with interconnected vertical and lateral channels was synthesized by anodizing impure aluminum foil in phosphoric acid electrolyte at 0°C under a voltage of 195 V (Figure S1).40,41 The template was placed into a horizontal tube furnace and heated to 1,000°C at 10°C/min under an Ar atmosphere. The first CT layer was deposited on the pore walls of the 3D-AAO template with a flow of acetylene gas under vacuum conditions for 50 min. Subsequently, an AlOx layer was deposited via an ALD process at 250°C on the inner wall of the first CT layer using trimethyl aluminum (TMA) and ozone (O3) as the precursors.42 The second CT layer was deposited on the AlOx surface via a second-time CVD process. Similarly, repeating the ALD and subsequent CVD processes can prepare a third CT layer. Finally, the 3D-MLCT was obtained after removing the AlOx layer(s) and 3D-AAO template in hydrofluoric acid solution, and the spacing of adjacent CT layers can be precisely controlled by the thickness of the AlOx layers. Furthermore, the size of the 3D-CT films can be adjusted by varying the area of the 3D-AAO as needed, and we prepared a 3D-CT film with an area of approximately 24 cm2 (see Figure S2).