Prussian Blue Derived Vertical Graphene Structure for High Frequency Electrochemical Capacitors

Abstract

High frequency electrochemical capacitors (HF-ECs), which have large capacitance density and therefore reduced device size in substitution of the bulky aluminum electrolytic capacitors, are of increasing interest. To produce HF-ECs that can respond at hundreds and even kilo hertz range, the tortuous mesoporous electrode of a conventional EC must be replaced with a relatively straightforward and large pore-based structure, and the whole electrode resistance must be small. Various electrode structures have been investigated but very few can deliver an adequate capacitance density at 120 Hz.In this study, Prussian blue (PB) cubes, as a metal organic framework, are used as building blocks of a 3D scaffold for the vertical graphene (VG) nanosheets growth. In a rapid plasma enhanced chemical evaporation deposition (PECVD) process, a 3D conductive porous network comprised of VG and PB-derived carbon cages was successfully fabricated, and this material structure has a seamless interface with the current collector. Benefiting from the unique electrode material structure, the fabricated HF-ECs exhibited an equivalent series resistance (ESR)as low as 40 mΩ cm-2, 120 Hz phase angle (F 120 ) of 85.9o and 120 Hz electrode areal capacitance (CA 120 ) of 1.02 mF cm-2, or F 120 = -80.6o and CA 120 = 2.34 mF cm-2 for a thicker electrode, which are among the best reported overall performances thus far for HF-ECs. Integrated cell was assembled to work at 2.7 V for line frequency filtering of different waveforms, demonstrating excellent performance. Figure 1. (a) SEM and (b) TEM images of PB derived vertical graphene structure. (c) CV curves recorded at 100 V s-1,(d) complex-plane-impedance plotswith inset displaying the high-frequency region, and (e) the derived specific capacitance versus frequency (red line: 3 mg, yellow line: 5 mg and purple line: 7 mg of PB precursor treated in PECVD for 5 min). Figure 1