Energy storage performance of binder-free ruthenium-oxide nano-needles based free-standing electrode in neutral pH electrolytes

Improving the energy density of symmetric flexible supercapacitors (FSSCs), while maintaining a high-power density under an environmentally safe system, is still a crucial challenge for researchers. The improvement can essentially be grasped by enlarging the voltage window through rational designing/selection of the electrode materials as well as utilizing such electrolytes that have extended stability range and non-corrosive/environmental properties. In this context, an articulate strategy is established, in the first step, fabricating the self-standing porous carbonaceous substrate (carbon nanotubes anchored at carbon fabric, denoted as CNTs-CF), in the second step, metal-oxide nano-needles (RuO2-NNs) decorated architecture was designed on the CNTs-CF substrate which served as a binder-free self-standing electrode (RuO2-NNs@CNTs-CF). The second strategy of regulating the harmless/non-toxic neutral aqueous electrolytes (NaCl, KCl & LiCl) is employed. The optimized performance of RuO2-NNs@CNTs-CF nanostructure holds a higher specific capacitance of 102.75 F g−1 in LiCl electrolyte and able to work at large voltage of 2.0 V, at the same time the retention capability of 98.6 % is achieved over 5000 cycles. Such an extended potential window is approximately twice higher as compared to the previously reported carbonaceous/metal-based supercapacitors using conventional electrolytes (acid/base in the aqueous state). In this regard, the physicochemical characteristics including mobility/diffusion and solvation of the ions are critically considered to find out their effect on the SSCs performance. Moreover, the FSSC device is also assembled using PVA/LiCl gel electrolyte; an excellent energy density of 62.94 W h kg−1 at 596.35 W kg−1 power density, is recorded with an extended voltage window of 2.35 V. The fabrication of RuO2-NNs@CNTs-CF FSSC with most safe and environmentally friendly components would unwrap new opportunities for the development of high-performance safe energy storage devices.