Overcoming the limit of capacitance in Bi based supercapacitors by functionalizing their electrodes with NH4Bi3F10 cubes

In this research, flexible supercapacitors (SCs) were made with carbon nanotube fibers and NH4Bi3F10 (NBiF) particles. According to the analysis by microscopy, the NBiF consisted in a mixture of particles with irregular/cubic morphology. The cubic particles have lateral sides in the range of 170–1150 nm. SC devices made with and without NBiF particles were named CNBi-SC and CN-SC, respectively. The CNBi-SC device showed a capacitance of 818.2 F g-1 and energy density of 113.6 Wh kg-1. In contrast, the CN-SC device had a lower capacitance and energy density values of 203.7 F g-1 and 28.3 Wh kg-1, respectively. Thus, adding the NBiF particles on the SC electrodes increased the capacitance and energy density by ≈ 300%. In addition, the SCs made with and without NBiF were subjected to 500 bending or charging/discharging cycles and their capacitance retention was calculated. Despite the stressing conditions, the capacitance retention was in the range of 90–94% for the CNBi-SC device, but it was lower for the for the CN-SC device (82–85%). Raman, XPS and optical analyses demonstrated the presence of oxygen vacancy defects and Bi2+/Bi3+ species. Those ones were responsible of the redox reactions for the storage of charge. The analysis by impedance spectroscopy revealed that the series and charge transfer resistances decreased after the introduction of the NBiF particles in the SC electrodes, which facilitated the ion transfer from the electrolyte to the electrodes. Hence, the results presented here could pave the way for the improvement of SCs made with low-cost Bi compounds.