Novel Hybrid Supercapacitors Based on Nanoarray Electrodes

With the ongoing depletion of fossil fuels, the exploration of sustainable energy resources and advanced energy technologies is necessary and the development of clean and sustainable energy storage devices has become an important topic worldwide. In this regard, rechargeable batteries and supercapacitors (SCs) are currently considered to be promising electrochemical energy storage systems for widespread applications in electronic devices, electric vehicles, and smart-grid energy storage stations. Batteries typically exhibit high energy densities but are limited by their low power density and relatively poor cycling performance. In contrast, SCs exhibit high power density, stable cyclability, and good safety, but the energy densities of SCs are generally inferior to those of batteries, which hinders their widespread application. A reliable approach to addressing this issue is to fabricate hybrid supercapacitors (HSCs) composed of battery-type and capacitive electrodes. This device configuration enables the direct integration of the high energy densities of batteries and high power densities of SCs, making HSCs a promising class of energy storage devices. However, the mismatch of capacity and rate performance between the battery-type and capacitive electrodes hinders the widespread applications of HSCs. A key challenge for the development of high-performance HSCs is to optimize the balance between both electrodes. Recently, tremendous efforts have been focused on the search for suitable electrodes and considerable progress has been achieved. Nevertheless, in traditional electrodes, binders are commonly used to combine individual active materials with conductive additives. Unfortunately, these binders are generally electrochemically inactive and insulating, reducing the overall specific capacity/capacitance and deteriorating the charge/mass transport. Recently, binder-free nanoarray electrodes have provided a promising opportunity for designing effective HSCs owing to the merits of their direct electron transport pathway, short ion diffusion length, and ordered-structure-enabled abundant reaction sites. This review briefly addresses the energy storage mechanism of HSCs and the advantages of array electrodes, and subsequently reviews the recent advances in emerging HSCs developed by our group. The performance-electrode structure relationship is discussed from the perspective of devices featuring different electrolytes, including organic, aqueous neutral and aqueous alkaline electrolytes. Moreover, some solutions are put forward to solve the existing issues of HSCs, and the potential applications of array electrode-based HSCs in flexible/wearable electronics are envisioned. Finally, the challenges and future development trends of HSCs are proposed.