Polythiophene-derived nickel cobaltite nanocomposites showing excellent photo-switching and photo-assisted enhanced supercapacitor properties

Abstract

Recently, the coupling of a photovoltaic cell with a supercapacitor device has attracted considerable attention in the field of photo-supercapacitors as it offers power generation and storage in a single device. Herein, we chose a substituted polythiophene (3-[1-ethyl-2-(2-bromoisobutyrate)]thiophene) (PT) system as an energy harvesting unit and spinel NiCo₂O₄ (NCO) as an energy storage unit to prepare a photo-assisted supercapacitor. PT-coated nano-octahedron-assembled NCO nanorods were synthesized via in situ oxidative polymerization by varying the ratios of PT and NCO (PT : NCO = 1 : 1, PTNCO-1 and PT : NCO = 2 : 1 (w/w), PTNCO-2) to obtain an optimum composition that showed excellent electrochemical performance under both dark and illuminated conditions. The porous PT matrix grown on the NCO nanorods was characterized using BET, SEM and TEM analyses, and the synergistic interaction between the components was confirmed using NMR, FTIR, Raman, UV-vis, fluorescence and XPS spectral analyses. PTNCO nanocomposites showed significant increase in dc-conductivity, dark current and photocurrent (∼10⁴ times) values compared to those of PT. A minimum photocurrent gain (I_on/I_off = 1.2) and extended stay of photoelectrons in PTNCO-1 revealed its higher photo-current storage capacity than those of NCO and PTNCO-2 samples. Electrochemical experiments using three-electrode devices indicated that PTNCO-1 showed a maximum specific capacitance (C_S) of 958 F g⁻¹ at a current density of 1 A g⁻¹. The PTNCO-1 solid-state device showed C_S values of 90 F g⁻¹ and 106 F g⁻¹ under dark and illuminated conditions, respectively, at a current density of 1 A g⁻¹, and at a current density of 5 A g⁻¹, the C_S values were 47.8 and 67.4 F g⁻¹, respectively. Alternatively, under UV light (λ = 365 nm), an increase in C_S values from 17.8% to 41% was observed between the current densities 1 to 5 A g⁻¹, revealing an increasing trend in C_S values with increasing current density values. The rate capability increased from 53% to 63.6%, i.e., an increase of 10.6% was observed, followed by a 5% increase in cycling stability at the illuminated state compared to the dark state. Impedance data supported the better storage capacity of photoelectrons in PTNCO-1, in which a decrease in charge transfer resistance and an increase in capacitance value of ∼44 mF were achieved under the illuminated condition compared to the dark condition. Thus, an increased photoconductivity facilitates an increase in specific capacitance, thereby establishing PTNCO-1 as a good photo-assisted enhanced supercapacitor.