Structure-Directed CNT Arrays As Micro-Supercapacitor Electrodes and the Effect of Geometry on Electrolyte Selection

The demand for power-dense and energy-dense storage devices is growing rapidly. While supercapacitors offer high power density, improving the energy density can be obtained by precisely tuning the electrode nanostructure and pairing it with the appropriate electrolyte.  Multi-walled carbon nanotubes (MWNTs) are a promising electrode material for EDLCs because of their high electrical conductivity, chemical stability and exceptionally high surface area. Until single walled carbon nanotubes can be produced as solely conductive structures, low-diameter MWNTs offer the best properties with regard to carbon-based EDLC electrode materials. When MWNTs are fabricated in anodic aluminum oxide templates, a MWNT is grown within each nanopore, resulting in an ultra-high density vertically-aligned array. Vertically-aligned MWNTs offer numerous benefits, such as directed charge transport and high density packing, leading to a higher active surface area and better rate capabilities than observed with randomly-dispersed networks. We have demonstrated the fabrication and performance of these electrodes in non-pseudocapacitive supercapacitors with both organic and ionic liquid electrolytes. While these benefits are beneficial for all capacities of supercapacitors, they are increasingly important in the production of micro-supercapacitors, when it is essential to use all device volume efficiently. Despite the benefits, this highly oriented electrode design combined with the remnants of the template can lead to unique current distributions and behaviors in a coin cell test set-up. We will discuss the differences in electrochemical capacitive behavior of traditional and our anisotropic electrodes. There exists unique stability criteria in nanostructured domains that must be considered when designing this electrochemical system - specifically the current and potential distribution due to the MWNT curvature and constant phase element behavior. Through thorough characterization of the electrode structure and electrolyte behavior, we have successfully demonstrated cells with a projected energy and power density of 108 Wh/kg-carbon and 768 kW/kg-carbon.