Synergistic enhancement on flexible solid-state supercapacitor based on redox-active Fe 3+ ions/natural spidroin modified vertically aligned carbon nanotube arrays.

The conductive skeleton and aligned carbon nanotube array (CNTA) structure can greatly shorten the ion transfer path and promote the charge transfer speed, which makes the CNTA an ideal electrode material for energy storage application. However, poor mechanical stability and low specific capacitance greatly impede its practical utilization. Here, we introduce a promising flexible electrode material based on the natural spider silk protein (SSP) modified CNTA(SSP/CNTA) with improved hydrophilicity and mechanical flexibility. The redox-active Fe 3+ doped SSP/CNTA flexible solid-state supercapacitor (FSSC) device with superior energy storage performance was assembled in a symmetric 'sandwich-type' structure. The synergetic interaction between Fe 3+ ions and the SSP are proved to greatly enhance the electrochemical performance especially the long-term cyclic stability. The Fe 3+ doped SSP/CNTA FSSCs device achieves an ultra-high volumetric capacitance of 4.92 F cm -3 at a sweep speed of 1 mV s -1 . Meanwhile it exhibited an excellent cycling stability with an increased capacitance by 10% after 10 000 charge-discharge cycles. As a control, a Fe 3+ doped CNTA composite device without SSP will lose over 74% of the capacitance after 10 000 cycles. The energy storage mechanism analysis confirms the dominated capacitive behavior of the device, which explained a considerable power density and rate performance. Our method thus provides a promising strategy to build up highly-efficient redox-enhanced FSSCs for next generation of wearable and implantable electronics.