Plasma-enhanced atomic layer deposition of ruthenium metal on free-standing carbon nanotube forest for 3D flexible binder-less supercapacitor electrodes

Abstract

Plasma-enhanced atomic layer deposition of ruthenium on functionalized free-standing multi-walled carbon nanotube (MWCNT) forest directly grown on a stainless steel mesh as a flexible supercapacitor electrode is done by a zero-oxidation state precursor and nitrogen plasma. Using this method for the fabrication of 3D flexible supercapacitor electrodes eliminates the need for any binder component and solution-based synthesis step as well as the production of any toxic by-products. Moreover, it enables the utilization of the entire high-surface-area MWCNTs and it alleviates the Ru agglomeration issue typically found in solution-based synthesis techniques. Accordingly, ruthenium is successfully deposited on the entire length of the carbon nanotubes in various process temperatures and plasma exposure durations with a thickness range of 15–60 nm. Electrochemical characterization of the electrodes shows a significant improvement in the capacitance of Ru coated MWCNT compared to bare MWCNT due to the additional surface faradaic redox reactions. Cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) are carried out to evaluate the electrochemical performance of the fabricated electrodes. The capability of the 3D flexible electrode is evaluated in different bending angles of 60, 80, and 180° and twisting conditions. Capacitance as high as 1834.5 mF.cm−2 (393 F.g−1) with a very low charge transfer resistance of 5.49 Ω.cm−2 is achieved that shows >400 % improvement in capacitance compared to bare MWCNTs. A capacitance retention of 92 % after 1000 CV cycles is achieved.