The Burstein–Moss (B–M) effect, which suggests that the optical band gap of degenerately doped semiconductors increases when all states close to the conduction band get populated due to shifting of an absorption edge to higher energy, is important, as it gives a chance to obtain different optical properties for the same material. Here, we report our observations of the similar shift in the optical band gap in Ni_xFe_2−xO₃ nanocomposites as a function of composition with the help of cyclic voltammetry (CV) and XPS valence band (VB) position measurements. The conduction band edge (CBE) position of the Ni_xFe_2−xO₃ nanocomposites as determined using CV was noted to move towards more negative potential with increasing Ni-concentration. A similar shift is also noted in the CBE estimated using XPS measurements (by subtracting the VB position from the optical band gap values). The observed shift in the optical band gap along with the CBE position gives the corresponding shift in the Fermi level, which is found to move closer to the CBE position, suggesting the observation of an effect similar to the B–M shift. Also, the extent of band bending estimated from the deviation of the CBE from the flat band potential (measured through Mott–Schottky plots) is found to increase with increasing Ni-concentration. Moreover, the Ni-composition has been observed to enhance the current density as well as to facilitate water splitting at a much lower onset potential compared to pure hematite. The Ni_xFe_2−xO₃ nanocomposite with an 11 mol% Ni-composition shows the highest photo-electrochemical response with an almost ten times enhancement in the current density at 1.9 V vs. RHE in alkaline medium, as compared to the dark current. This enhanced performance is attributed to the improved charge separation and higher charge carrier density as a result of the higher extent of band bending in the Ni_xFe_2−xO₃ nanocomposites.

The energy density of the present state-of-the-art materials remains the main challenge for commercial utilization of supercapacitor devices. Therefore, the development of highly active electrode materials with improved energy density, while preserving their exceptional power density besides introducing cost-effective and environmentally benign materials has recently become a major research interest. On this aspect, we present a cost-effective and highly efficient flexible supercapacitor based on SnO₂@carbon nanofibers synthesized via the electrospinning of SnO₂ precursor with PVP followed by calcination. The prepared highly porous, uniform, one-dimensional SnO₂@carbon nanofibers without externally added activated carbon mounted on a carbon cloth substrate were used to fabricate a flexible supercapacitor. The fabricated supercapacitor exhibited a very high specific capacitance of 646.15 F g⁻¹ at an energy density of 152 W h kg⁻¹ (0.08 W h cm⁻²) and power density of 39 kW kg⁻¹ (4.87 W h cm⁻²) at a very high current density of 60 A g⁻¹. An asymmetric supercapacitor also depicts high energy density of 4.7 W h kg⁻¹ (0.003 W h cm⁻²) at a power density of 3200 W kg⁻¹ (1.67 W cm⁻²) and a current density of 0.2 A g⁻¹. Furthermore, it displays an excellent cyclic stability even after 1000 continuous charge–discharge cycles at a constant but very high current density of 80 A g⁻¹. The presented material shows the highest specific capacitance ever reported for electro-spun SnO₂ nanofibers.