Electrochemical Charge Storage Properties of Vertically Aligned Carbon Nanotube Films: The Activation-Enhanced Length Effect

Abstract

Improvement in the safety, efficacy, selectivity, and/or power consumption of devices used for neural stimulation requires a low impedance neural electrode interface that can rapidly store high charge densities without damaging the electrode or neural tissue. In this study, vertically aligned carbon nanotube (VACNT) films were synthesized and investigated in vitro for their potential use as a neural stimulation electrode. Materials and electrochemical (EC) characterization was performed prior and subsequent to EC activation, accomplished by slow potential cycling across the water window, to determine if activation improved the desired properties. EC characterization was performed at a wide range of frequencies using various techniques including cyclic voltammetry, electrochemical impedance spectroscopy, and high-rate potential transient measurements. The results indicated that EC activation, a technique often used to activate and greatly improve the performance of iridium oxide electrodes, was also favorable for VACNT electrodes. The effect of activation was more pronounced as the average length of the VACNTs was increased, especially at lower frequencies. The activation-enhanced length effect is believed to be a result of the VACNT film characteristics that are determined during the deposition process. A phenomenological model was developed to illustrate this concept and has important implications for various charge-storage applications.