Miniaturized Electrochemical Supercapacitor with Patterned Horizontally Aligned Carbon Nanotube Sheets

Abstract

Introduction: Miniaturized energy storage systems have emerged as a promising approach to address the growing demands to power electronic devices on small footprint area. Microsupercapacitors have become a crucial part of this advancement and have drawn much attention in the research communities. Designing a microstructure and choosing the electroactive materials are two significant factors that greatly affect the performance of the device. Here, we report for the first time the successful fabrication and characterization of a microsupercapacitor with interdigitated structure based on carbon nanotube sheets (CNT sheet). Carbon nanotube sheet is a novel structure which is spun directly from the as-grown carbon nanotube forest. Horizontally aligned structure of CNT sheet offering excellent conductivity and large surface area enabled us to develop a light micro-device with higher electrochemical performance than other CNT-based counterparts in aqueous electrolyte. Materials and process: Spinnable carbon nanotube was grown via chemical vapor deposition (CVD) in a horizontal furnace at temperature of 780°C using acetylene as hydrocarbon and iron as metal catalyst. Multi-sheets of 5, 10, 20, 30 and 50 layers on two different substrates (glass slide and silicon wafer) and were patterned with reactive ion etch (RIE). CNT sheet was used as both active material and current collector. Interdigital structure (IDE) with gap size of 145 um, electrode width of 300um and the length of 5mm were fabricated. The effect of active material thickness on the device performance including specific capacitance and rate capability was studied. Furthermore, long-term stability of the devices was investigated using cyclic voltammetry technique at scan rate of 100mV/s for 10,000 cycles. Results: Thickness dependent specific capacitance of 5 mF/cm2 at the scan rate of 50 mV/s per footprint area of the device was obtained for multi-sheet of 50 layers. Stability and performance of the device was tested using an aqueous PVA-H3PO4 gel electrolyte that also offers desirable electrochemical capacitive properties. High Coulombic efficiency (around 95%), great rate capability (1.2 mF/cm2 at scan rate of 5V/s) and excellent capacitance retention (less than 12%) over 10,000 cycles were obtained. Conclusion: We have fabricated a long-term stable planar microsupercapacitor using horizontally aligned carbon nanotube sheets and achieved a noticeable specific capacitance and high rate capability which demonstrates the potential of CNT sheet for future thin film micro scale energy storage devices. Figure 1