Redox-enhanced solid-state supercapacitor based on hydroquinone-containing gel electrolyte/ carbon nanotube arrays

Abstract

By introducing redox active substances into electrolyte, the energy density can be effectively increased without reducing the power density. Considering the influence of ionic conductivity and environmental safety, we introduce the redox small molecule hydroquinone (HQ) into the PVA/H<sub>3</sub>PO<sub>4</sub> gel electrolyte, which then will recombine with the carbon nanotube arrays (ACNT) possessing high specific surface area and vertical orientation structure. The symmetrical “sandwich” type redox-enhanced solid-state super capacitor is then designed and prepared. We systematically study the effects of oriented structure and pore space on the electrochemical properties of the ACNT@PVA@HQ device and charge storage mechanism. With the addition of hydroquinone (0.1%, mol%), the specific capacitance of ACNT@PVA@HQ composite device increases 6.4 times compared with that of the ACNT@PVA, and maintains the extremely high rate performance and cyclic stability. When the current density increases 10 times, the specific capacitance of the device still possesses 85% of the original value. The energy storage mechanism is mainly ascribed to a diffusion-control behavior at a low scan rate while it will change into a capacitive behavior at a high scan speed. Furthermore, we prepare highly densified oriented carbon nanotube arrays (DACNT) by solvent evaporation, enhancing the mechanical stability of carbon nanotube arrays and improving the specific capacitance and energy density of the devices. Compared with the specific capacitance of ACNT and random carbon nanotube (CCNT), that of DACNT@PVA@HQ device under the current density of 11.1 mA·cm<sup>–2</sup> increases up to 385 mF·cm<sup>–2</sup> (1674 mF·cm<sup>–3</sup>), which is 6.6 times higher than that of the CCNT@PVA@HQ device and 18 times higher than that of the ACNT@PVA device. The maximum energy density can finally reach as high as 0.06 mW·h·cm<sup>–2</sup> (0.26 mW·h·cm<sup>–3</sup>), which is much better than those of many other reported CNTs-based devices. The oriented structure of the arrays effectively shortens the ion migration path of the device, achieving a good rate performance and lower internal resistance. This new type of redox-enhanced solid-state supercapacitor not only has excellent electrochemical energy storage properties, but also meets the requirements for environmental protection and safety. This design provides a new idea for developing the new energy devices in the future, which has a good prospect in practical applications.