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.

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Introduction

Substitution of fossil fuels with renewable energy has become an important area of current research efforts to meet the energy demands for the sustainable development of society.^1–3 Photovoltaic (PV) systems have drawn significant attention in obtaining carbon-free energy owing to their development in various technical domains, such as photo-electrochemical water splitting (PEC),⁴ photocatalysis,⁵ and photo-electrochemical redox flow batteries.⁶ However, the utilization of solar power for high-efficiency energy conversion and low-loss energy storage devices is still challenging. Supercapacitors (SCs) have attracted great attention in the fabrication of electrochemical energy storage devices owing to their fast charge/discharge rate, high power density, long cycling stability and safe operation.^7–9 Supercapacitors comprising mainly metal oxide/hydroxide composites show low energy density due to their limited electrical conductivity and poor cycling stability for morphological detoriation.¹⁰

Transition metal oxides (TMOs) are excellent candidates for the construction of supercapacitor/battery electrodes owing to the presence of various reversibly switchable redox states of their metal atoms. A very interesting example is Co₃O_4, where on the one hand, catalytically active octahedral centers in the spinel structure are useful for oxygen evolution reactions (OERs).¹¹ On the other hand, the availability of Co²⁺/Co³⁺ reversible redox states and the formation of a layered CoOOH intermediate with a large layer spacing offer room for ion intercalation, leading to excellent performance of supercapacitors.¹² Mixed metal oxides having a spinel structure are even more attractive in view of their unique morphology and availability of various oxidation states of different metal atoms such as NiCo₂O₄.^13,14

Photo-induced methodologies have gained increasing interest for various photocatalytic or energy conversion/storage processes such as hydrogen evolution reactions (HERs),¹⁵ oxygen evolution reactions (OERs),¹⁶ oxygen reduction reactions (ORRs),¹⁷ and rechargeable batteries¹⁸ in view of their environmental friendliness, low cost and easier accessibility. Photo-assisted supercapacitors or photoirradiation-enhanced capacitance is emerging as a promising energy storage solution.^19–21 In such devices, a photo active material integrated with a capacitive electrode material is used where photo-generated carriers expedite the charging process^22,23 or may improve photoconductivity or photo-thermal effects.²⁴ In this respect, the interface engineering of heterostructured materials showing great promise for the separation of photo-generated carriers across the interface is very much conducive in the enhancement of electrochemical deliverables. It is apparent that improved interfacial contact between the photoactive and electrode materials by means of nanostructuring should be a good strategy for realizing better performance. Furthermore, nano-structuring and improved interfacial contact are conducive to having optimum band gap matching to facilitate the transfer of photo-generated carriers. Thus, exploration with a novel combination of electrode materials and photo-active materials is absolutely necessary at this stage to gain a better understanding of the structure–property relationship of such heterostructured materials.

In this work, to construct an effective photo-assisted supercapacitor, we chose a PT derivative as the PT chain can generate photoelectrons upon irradiation with white light.^25,26 In general, polythiophene derivatives can act as a better photovoltaic materials as the electron-donating thiophene rings can narrow the optical band gap by extending their π-conjugations during the photo-irradiation.²⁷ In addition, PT chains may laminate over the NCO nanorods giving extended electrode life. Here, 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. NiCo₂O₄ was chosen over both NiO and Co₃O₄ due to the high storage capacity and better electrochemical activity of NCO.²⁸ Hence, a PT-coated nano-octahedron-assembled NiCo₂O₄(NCO) nanorod composite (PTNCO) was synthesized via ‘in situ’ oxidative polymerization with different ratios of both PT and NCO (PT : NCO = 1 : 1, PTNCO-1 and PT : NCO = 2 : 1 (w/w), PTNCO-2) to obtain an optimum composition showing excellent electrochemical performance under both dark and illuminated conditions. The porous PT matrix grown on NCO nanorods was well characterized using BET, SEM and TEM analyses and the interaction between the components was studied using NMR, FTIR and UV-vis spectral studies. Significant synergy in operation between PT and NCO was indicated by the increased dc-conductivity of PTNCO nanocomposites along with an increase in the values of both the dark current and the photocurrent (about 10⁴ times) compared to those of PT. A minimum photocurrent gain (I_on/I_off) and longer stay of photoelectrons in PTNCO-1 predicted a better photocurrent storage capacity. The increased photoconductivity may assist in the increase in specific capacitance, establishing PTNCO-1 to behave as a good photo-assisted enhanced supercapacitor. Electrochemical performances, e.g. cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), cycling stabilities, indicated that PTNCO-1 offers an enhanced specific capacitance of 958 F g⁻¹ at a current density of 1 A g⁻¹ measured by a three-electrode technique. In the solid-state (two-electrode) device under illuminated conditions, the C_S value increased to 106 F g⁻¹ from 90 F g⁻¹ measured under the dark condition with 1 A g⁻¹ current density. However, an increase in C_S from 17.8% to 41% at 1 A g⁻¹ to 5 A g⁻¹ current density was observed when it is exposed to UV light (365 nm), showing a higher charge storage capability at a higher current density. The rate capability was also increased by 10.6% along with the increase in cycling stability by 5% at the illuminated state than that at the dark state. Electrochemical impedance spectral (EIS) data showed a lower charge transfer resistance and an increase in capacitance value by ∼44 mF supporting 17.8% higher C_S (106 F g⁻¹ at 1 A g⁻¹ current density) value in PTNCO-1 under the illuminated condition than that under the dark condition.

Experimental section

Materials and purification

The monomer 3-thiophene ethanol, 2-bromoisobutyryl bromide (BIB, 98%) and poly(vinyl alcohol) (PVA) (Sigma-Aldrich) were used as received. Nickel chloride (NiCl₂·6H₂O), cobalt chloride (CoCl₂·6H₂O), urea, ferric chloride (FeCl₃) and potassium hydroxide (KOH) were obtained from Merck, India. Solvents such as dichloromethane (DCM), chloroform, methyl alcohol (GR, E-Merck, India) and tetrahydrofuran (THF) (HPLC grade, Spectrochem, India) were purified by distillation and HPLC grade water was used throughout the experiment.

Preparation of substituted thiophene: 3-[1-ethyl-2-(2-bromoisobutyrate)]thiophene (T)

First, 3-thiophene ethanol (20 mmol) was dissolved in dry DCM (30 mL) in a 100 mL round-bottom flask, where triethylamine (22 mmol) was added with continuous stirring at 0 °C in a nitrogen atmosphere. 2-Bromoisobutyryl bromide (BIB, 22 mmol), diluted with 10 mL of dry DCM, was added into the reaction mixture dropwise using a pressure-equalizer and was stirred for 24 h. The reaction mixture was first filtered and the filtrate was washed repeatedly and successively with 1% HCl, saturated NaHCO₃, brine solution, and finally with distilled water using a separating funnel. The organic layer was collected from the separating funnel and was passed through anhydrous Na₂SO₄ to remove any water. For further purification, silica column chromatography was performed in a solvent mixture of hexane/ethyl acetate (95/5 in volume ratio). After solvent evaporation, a brown-colored liquid of 3-[1-ethyl-2-(2-bromoisobutyrate)] thiophene (T) was obtained (yield, 77%w/w). ¹H NMR (CDCl₃): δ = 1.77 (6H), 3.05 (2H), 4.40 (2H), 7.03–7.29 ppm (aromatic protons) (Fig. 1a).

Fig. 1 ¹H-NMR spectra of (a) T, (b) PT, (c) PTNCO-1 and (d) PTNCO-2 along with their assignments, taken in CDCl₃.

Preparation of substituted polythiophene: 2,5-poly(3-[1-ethyl-2-(2-bromoisobutyrate)])thiophene (PT)

In a 250 mL round-bottom flask, purged with N₂, anhydrous FeCl₃ (15 mmol) was dispersed in 30 mL dry chloroform. Then, T (3.5 mmol in 30 mL dry chloroform) was added dropwise into the mixture and stirred overnight at 30 °C. It was then added into an excess amount of methanol with continuous stirring. The solid precipitate was separated, repeatedly washed with methanol, and finally Soxhlet extracted with methanol for 24 h. The extract was dried under vacuum at 40 °C for 3 days and the solid product was dissolved in 150 mL of CHCl₃. Then, it was refluxed with an additional 100 mL of concentrated ammonia solution to remove any trace amount of FeCl₃. The organic layer was separated, concentrated, and precipitated out into an excess amount of methanol. The precipitate was separated and dried under vacuum at 40 °C for 3 days. A red solid was obtained as a final product (yield, 75%w/w). The number-average molecular weight (M̄_n) of PT, determined by SEC analysis, was ∼40 000 Da (Fig. S1†). The number of thiophene monomer units present in PT was about 144 (MW of T unit = 277.9). ¹H NMR (CDCl₃): δ = 1.82 (6H), 3.26 (2H), 4.50 (2H), 7.16–7.23 ppm (aromatic protons) (Fig. 1b).

Preparation of nickel cobaltite (NCO)

A modified hydrothermal procedure²⁹ was applied for the synthesis of NCO, where 2.5 mmol of NiCl₂·6H₂O, 5 mmol of CoCl₂·6H₂O, and 9 mmol of urea were taken in 40 mL of HPLC-grade water. Then, the entire solution was stirred at room temperature (∼30 °C) for 1 h, and the resulting solution was transferred into a 40 mL Teflon-lined stainless-steel autoclave and kept at 130 °C for 6 h. The autoclave was cooled to room temperature, and the solution was filtered and washed several times with water and ethanol, successively. The residue was dried at 50 °C for 2 days. Finally, the solution was annealed in air at nearly 350 °C to obtain NCO.

Preparation of polythiophene-based NCO nanocomposite (PTNCO-1)

In a 250 mL round-bottom flask, purged with N₂, anhydrous FeCl₃ (15 mmol) was dispersed in 30 mL dry chloroform. Then, T (3.5 mmol in 20 mL of dry chloroform) and NCO (3.5 mmol in 20 mL of dry chloroform) were added dropwise into the mixture and stirred at 30 °C overnight. It was then added into an excess amount of methanol with continuous stirring. The solid precipitate was separated, washed repeatedly with methanol, and finally Soxhlet extracted with methanol for a whole day. The extract was dried under vacuum at 40 °C for 3 days, and the solid product was dissolved in 150 mL of CHCl₃ and refluxed with an additional 100 mL of concentrated ammonia solution to remove any trace amount of FeCl₃. The organic layer was separated, concentrated, and precipitated out into an excess amount of methanol. The precipitate was separated and dried under vacuum at 40 °C for 3 days. A reddish black solid was obtained as a final product. ¹H NMR (CDCl₃): δ = 1.80 (6H), 3.23 (2H), 3.50 (2H),4.45 (2H), 5.32 (2H), 7.28 ppm (aromatic protons) (Fig. 1c).

Preparation of polythiophene-based NCO nanocomposite (PTNCO-2)

A similar technique of oxidative polymerization was used for the synthesis of PTNCO-2 nanocomposites. Here, T (7 mmol in 30 mL of dry chloroform) and NCO (3.5 mmol in 20 mL of dry chloroform) were added into the dispersed FeCl₃ solution. ¹H NMR (CDCl₃): δ = 1.80 (6H), 3.23 (2H), 3.50 (2H),4.45 (2H), 5.32 (2H), 7.28 ppm (aromatic protons) (Fig. 1d).

Instrumentation

The different instrumental techniques used to characterize PT and PTNCO composites and determine the properties, e.g., current–voltage (I–V) curves, photocurrent, cyclic voltammetry (three and two electrode), impedance spectroscopy, UV-vis spectroscopy, fluorescence spectroscopy and TCSPC, are presented in the ESI.†

Result and discussion

Synthesis and characterization

The synthesis of PT, PTNCO-1 and PTNCO-2 (Scheme 1) nanocomposite polymers was performed via oxidative polymerization in the presence of FeCl₃ in chloroform. The broadening of the aromatic proton signal in the region of ∼7.2 ppm (Fig. 1) of thiophene unit in NMR for the PTNCO-1 and PTNCO-2 samples in comparison with that of PT clearly indicated the formation of nanocomposites.

Scheme 1 Schematic of the synthesis of PTNCO-1 and PTNCO-2 nanocomposites via in situ oxidative polymerization.

The structures of T along with PT before and after the formation of nanocomposites with their respective proton signals can be inferred from the ¹H-NMR study in CDCl₃. The ¹H-NMR spectra of these samples are in agreement with their respective proton signals: T (Fig. 1a) (CDCl₃): δ = 1.77 (6H), 3.05 (2H), 4.40 (2H), 7.03–7.29 ppm (aromatic protons); PT (Fig. 1b) (CDCl₃): δ = 1.82 (6H), 3.26 (2H), 4.50 (2H), 7.16–7.23 ppm (aromatic protons); PTNCO-1 (Fig. 1c) (CDCl₃): δ = 1.80 (6H), 3.23 (2H), 3.50 (2H),4.45 (2H), 5.32 (2H), 7.28 ppm (aromatic protons) and PTNCO-2 (Fig. 1d) (CDCl₃): δ = 1.80 (6H), 3.23 (2H), 3.50 (2H), 4.45 (2H), 5.32 (2H) and 7.28 ppm (aromatic protons). It is also evident from the ¹H-NMR spectra of PTNCO-1 (Fig. 1c) and PTNCO-2 (Fig. 1d) that two new de-shielded peaks are generated in the region of 3.50 ppm and 5.32 ppm after the nanocomposite formation in both the cases. This may be attributed to the interaction between conjugated PT chains with the vacant d orbitals of metals [Co(III) or Ni(II)] present in NCO (pπ–dπ) interaction, which makes ‘b^/’ and ‘c^/’ protons de-shielded to some extent. The broadening of proton signals of PT chains in the composite might be due to the restriction of the mobility of thiophene rings in the presence of NCO.

The number average molecular weight (M_n) of PT was found to be ∼40 000 (dispersity = 1.73) from the SEC analysis against the polystyrene standard in the THF medium (Fig. S1†), and it is nearly accurate on the basis of the previous reports.³⁰ The number average molecular weight (M_n) of PTNCO-1 and PTNCO-2 cannot be measured as the SEC analysis of these nanocomposite polymers was avoided due to the presence of NCO nanoparticles, which can clog the column. However, it may be approximated that the molecular weight of PT would be the same in both the nanocomposites as the polymerization condition is the same as in pure PT.

The FTIR spectra of NCO, PT, PTNCO-1 and PTNCO-2 nanocomposites are shown in Fig. S2.† Different peaks at 3390, 2345, 1640, 1370, 654, and 550 cm⁻¹ are characteristic peaks of NCO (Fig. S2a†). The characteristic peaks at 3390 and 1640 cm⁻¹ were assigned to the O–H stretching and bending, respectively vibrations, indicating the presence of water molecules in the samples.³¹ The typical characteristic bands in NCO ascribed to the vibration of the tetrahedral (Td) cation and the asymmetric stretching vibration of octahedral ions (Oh–O–Td) appeared at 2345 cm⁻¹ and 1370 cm⁻¹. Two sharp vibration peaks at 654 and 550 cm⁻¹ were attributed to the metal–oxygen vibrations in NCO.³² Similarly, the peaks at 534, 630, 762, 780, 1116, 1164, 1276, 1644, 1738, 2973, and 3400 were characterized as the C–S bending, C–S–C ring deformation, C–S stretching vibration, C–H bending out of plane vibration, C–S–C bond vibration, C=S stretching vibration, C–H bending in plane vibration, C=C stretching vibration, C=O stretching vibration, C–H stretching vibration and O–H stretching vibration bands for PT (Fig. S2b†), respectively.^33–35 The characteristic peaks of both NCO and PT units are also observed in PTNCO-1 and PTNCO-2 (Fig. S2c and d†), confirming the nanocomposite formation during the polymerization. However, a distinct shift of C=S vibration (1164 cm⁻¹) to a lower wave number by 8 and 4 cm⁻¹ and for S–C–S vibration (1116 cm⁻¹) to a lower wavenumber by 6 and 2 cm⁻¹ were observed for PTNCO-1 and PTNCO-2, respectively. This may be attributed to the interaction between the loan pair of sulphur in the thiophene ring of PT chains and the vacant d-orbitals of metals [Co(III) or Ni(II)] present in NCO, leading to pπ–dπ interaction shifting the vibrational bands to a lower energy.³⁶ A decrease in the stretching frequency of the >C=O bond of PT was also observed by 6 and 4 cm⁻¹ for PTNCO-1 and PTNCO-2 nanocomposites, respectively, which may arise from the delocalization with vacant d orbitals of cobalt ions of NCO.

The Raman spectra of PT, NCO, PTNCO-1 and PTNCO-2 are shown in Fig. S3.† The characteristic peaks at around 1033, 1160, 1255, 1370, 1484 and 1634 were characterized for PT (Fig. S3a†) as C_β–H, C_α–C_α anti-stretching, C_α–C_α stretching, C_β–C_β stretching, C_α=C_β stretching and C_α=C_β anti-stretching modes, respectively.³⁷ Similarly, characteristic peaks at around 324, 490, 560, and 630 cm⁻¹ observed for NCO (Fig. S3b†) were associated with E_2g, E_g, F_2g, and A_1g vibration modes, respectively.³⁸ Those characteristic peaks for both NCO and PT units were also present in PTNCO-1 and PTNCO-2 (Fig. S3c and d†), which confirmed the nanocomposite formation during the polymerization. However, distinctive shifts to a lower energy for most of the Raman bands were observed for PT and NCO vibration modes in both the nanocomposites due to the synergistic interaction between NCO and PT polymer matrixes.

The typical X-ray diffraction (XRD) patterns of NCO, PT, PTNCO-1 and PTNCO-2 nanocomposites are shown in Fig. S4.† Different diffraction peaks of NCO (Fig. S4a†) were found at 2θ values at 18.90° (111), 31.21° (220), 36.73° (311), 38.47° (222), 44.68° (400), 55.49° (422), 59.17° (511), and 65.04° (400), respectively. These assigned peaks represented NCO as a cubic spinal structure³⁹ and the average crystal size was found to be around 15.81 nm by the Scherrer method.⁴⁰ Similarly the broad diffraction peaks at around 23.67° (200), 34.36° (120) and 44.17° (004) were observed for the PT (Fig. S4b†) chains as well.⁴¹ The broadness of the diffraction patterns of PT chains was attributed to their amorphous nature. The characteristic peaks of both NCO and PT crystallites were also found in both nanocomposites (PTNCO-1 and PTNCO-2), indicating the presence of the component crystalline structure in the nanocomposites. The diffraction pattern of NCO along with PT in PTNCO-2 (Fig. S4d†) has come as a bit broader and has shifted in comparison with both NCO and PT probably due to the presence of PT moieties to a greater extent. However, the diffraction peaks of PT are of very small size in PTNCO-1 (Fig. S4c†) probably due to the presence of a lower amount of PT chains.

The surface morphologies of NCO, PT, PTNCO-1 and PTNCO-2 were extensively analyzed by FESEM (Fig. 2). The formation of rod-like hierarchical microstructures of self-assembled nano octahedrons was observed for pure NCO materials, where Ni(II) remains incorporated into the interstices of octahedral Co₃O₄ (Fig. 2a).⁴²Fig. 2b demonstrates the fibrillar network structure along with few tiny spheroids of modified PT polymers, which predicts its effectiveness of generating a good conducting matrix material. After nanocomposite formation via ‘in situ’ oxidative polymerization, the NCO surface coated with polythiophene was observed for both the PTNCO-1 and PTNCO-2 samples (Fig. 2c and d). Interestingly, the morphology of NCO was also retained in both nanocomposite samples where PT got coated over NCO nanorods. Hence, it can be argued that this type of porous morphology may lead to the enhancement of electrochemical properties for the increase in higher electrolyte contact area with active surfaces. Further, the PT-coated NCO matrix may enhance electron transfer to improve the stability of the electrodes during the electrochemical process.⁴³ Despite having greater extent of PT matrixes over NCO, PTNCO-2 may not be as effective as PTNCO-1, probably not having a closer proximity of NCO rods over the PT-matrix decreasing the interconnectivity among the NCO rods to some extent. The EDX analysis of NCO, PTNCO-1 and PTNCO-2 is provided in Fig. S5,† indicating the presence of Ni, Co, S, O and carbon.

Fig. 2 FESEM images of (a) NCO, (b) PT, (c) PTNCO-1 and (d) PTNCO-2.

To understand the morphology of composites in a better way, HRTEM analysis (Fig. 3) of NCO, PT, PTNCO-1 and PTNCO-2 samples was performed. Fig. 3a presents closely packed nano-octahedron-assembled micro-nano rod-like structures of NCO and Fig. 3b shows an enlarged image of NCO nano-rods having a particle size around 15–20 nm, close to the calculated value from the XRD analysis (Fig. S4a†). The nano-octahedrons are clearly visible where a large number of pores between them (marked by the red arrow) are clearly observed.⁴³ The fibrous spheroidal networking structure of the modified PT polymer is clearly evident from the TEM analysis (Fig. 3c). In the TEM images of PTNCO-1 and PTNCO-2 composites (Fig. 3d and S6†), the self-assembled nano-rods of NCO become completely wrapped with PT showing the fibrous morphology of PT as well. Therefore, the HRTEM images of all the samples were consistent with the FESEM observations; however, the formation of some isolated nano-octahedrons of NCO is observed beside the main matrix and it may be attributed to the ultrasonication of the samples before casting them on a TEM grid.⁴³

Fig. 3 HRTEM images of (a) NCO, (b) magnified HRTEM images of NCO, (c) PT and (d) PTNCO-1.

XPS studies were carried out (Fig. 4) to confirm the chemical state of the elements. The survey spectrum of NCO (Fig. 4a) shows the presence of O, Co and Ni elements with their respective signals. The survey spectra of PTNCO-1 (Fig. 4b) also indicate the presence of S, C, O, Co and Ni with their respective signals. Nearly marginal signals of both Co and Ni (indicated by red arrow) confirmed that the mesoporous NCO nanotubes are completely covered by the PT.⁴⁴ Furthermore, the S (2p) spectrum (Fig. 4c) is deconvoluted into two peaks at 163.1 (2p_3/2) and 164.2 (2p_1/2) using Gaussian fitting techniques.⁴⁵ Similarly, the O (1s) spectrum (Fig. 4d) can also be deconvoluted into two components at 531.3 and 532.7, where the first band is assigned to the metal–oxygen bond and the second one is for defects, contaminants, and a number of surface species including hydroxyls, chemisorbed oxygen and lattice oxygen of the spinel.⁴⁴ The XPS spectrum of Co (2p) (Fig. 4e) has been fitted with the same Gaussian fitting technique, which shows two spin–orbit doublets attributed to Co(II) and Co(III) states. The binding energies at 778.8, 780.1 eV and 793.8, 795.1 eV are assigned to Co(II) and Co(III) respectively.⁴⁶ Two sharp satellite peaks were also observed at 783.3 and 797 eV along with the previous two peaks. Furthermore, the Ni (2p) XPS spectrum (Fig. 4f) is also fitted with two spin–orbit doublets, which indicate the presence of Ni(II) and Ni(III) states, along with two satellite peaks at 860.5 eV and 873.4 eV. The binding energies at 853.2, 855.2 eV and 870.4, 872 eV are assigned to Ni(II), and Ni(III) respectively.⁴⁷

Fig. 4 XPS spectra (survey scan) of (a) NCO and (b) PTNCO-1. Deconvoluted peaks of (c) O(1s), (d) S(2p), (e) Co(2p) and (f) Ni(2p) of PTNCO-1.

UV-vis spectra were analysed (Fig. S7a†) in order to understand the band gap of PT in the PTNCO nanocomposites. It is evident from Fig. S7a† that absorption peaks of π–π* transition of the conjugated PT chains (414 nm) become red shifted with NCO and the maximum red shift of about 7 nm is observed for PTNCO-1 nanocomposites. In PTNCO-2, a red shift of about 3 nm was observed compared to that of the PT absorption peak (414 nm). These results indicate the increased stability of conjugated π-electrons of the PT chains in the composites for the pπ–dπ interaction between PT conjugated chains and NCO nanoparticles. Additionally, Tauc plot (Fig. S7b†) provides a profound estimation of the energy bandgap from HOMO to LUMO of semiconducting nanocomposites for the interaction between the components.⁴⁸ It is evident from Fig. S7b† that pure PT exhibits a band gap of 2.57 eV, whereas in the nanocomposites, the band gaps decrease to 2.47 and 2.26 eV for PTNCO-2 and PTNCO-1, respectively. The decrease in the band gap of nanocomposites may be ascribed to the synergistic interaction between polythiophene (donor) and the nickel cobaltite (acceptor) increasing the conjugation length of PT. This decrease in band gap may facilitate the flow of current in the I–V and photocurrent properties of the nanocomposites.

In the emission spectra (Fig. S8†), a similar red shift of emission peak in the composite from that of pure PT is observed. All the samples were excited at 400 nm, and the emission peaks were found at the ∼550 nm region. Here also, PTNCO-1 shows a maximum red shift of about 5 nm which may be attributed to the stabilization of PT-excitons, thus emitting light of higher wavelengths.⁴⁹ Interestingly, a decrease in PL intensity was observed with the increase in nanoparticle concentration in the nanocomposites, which may be attributed to the non-radiative decay of PT excitons on the NCO surface. The maximum decrease in PL intensity was observed for PTNCO-1 due to the well-coated NCO surface by PT chains to a maximum extent.

The specific surface area and pore size distribution of NCO, PTNCO-1 and PTNCO-2 nanocomposites were determined by N₂ adsorption/desorption analysis, which is shown in Fig. S9.† The adsorption isotherms show hysteresis (Fig. S9a, c and e†), characteristics of type-IV adsorption.⁵⁰ The BET specific surface areas (SSAs) of NCO, PTNCO-1 and PTNCO-2 were measured to be 28 m² g⁻¹, 40 m² g⁻¹, and 33 m² g⁻¹, respectively. The stronger interaction between PT and NCO in PTNCO-1 is the probable cause for the largest surface area in PTNCO-1. The mesopore size distribution of the respective samples is shown in Fig. S9b, d and f,† representing sharp distribution of pore sizes at 6.22, 8.92 and 8.15 nm for NCO, PTNCO-1 and PTNCO-2, respectively. This result implies that the mesoporous PTNCO-1 nanocomposite with maximum SSA is highly effective as the electrode material for supercapacitor application because the mesoporous structure facilitates the ion transportation to a greater extent.⁵¹

dc-Conductivity, photocurrent and photo-switching properties

The dc-conductivity values, measured by a two-probe method, of PT, PTNCO-1 and PTNCO-2 are 0.19 ± 003 μS cm⁻¹, 4.2 ± 002 and 9.1 ± 004 mS cm⁻¹, respectively (Table 1), indicating the samples to behave as semiconducting materials, and conductivity increases with the increase in PT concentration. The current (I)–voltage (V) measurement of thin films of the samples casted on ITO glass was performed by a applying bias voltage ranging from −5 V to +5 V and the dark current of pure PT was 2.9 μA at +5 V, whereas, in PTNCO-1 and PTNCO-2, the dark currents were significantly increased with values of 21.15 mA and 13.4 mA at +5 V, respectively, showing about 7293 and 4620 times increase in current for PTNCO-1 and PTNCO-2, respectively, from that of PT due to the decrease of band gap in the composites. This remarkable enhancement of current may be attributed to the increase in the active surface area due to the adherence of PT on the NCO nano rods, facilitating easy charge flow. The extensive π-conjugated PT chain may also get a better coplanar arrangement for getting adhered over the surface of NCO nano rods. The samples upon exposure of light of 1 sun intensity exhibit more enhanced current from that under dark condition due to the contribution of photoelectrons. The obtained photocurrents are 4.1 μA, 42.4 mA and 26.9 mA for pure PT, PTNCO-1 and PTNCO-2, respectively at +5 V (Table 1). Surprisingly, for both the nanocomposites, the semiconducting nature of I–V curve mostly changes to conducting nature despite the presence of semiconducting PT chains (Fig. 5a, c and e), and it is prominent under the illuminated condition. The photo-current gets enhanced by 10 341 times in PTNCO-1 and 6561 times in PTNCO-2 from that of pure PT under illuminated conditions. About two times higher current at each voltage upon irradiation of white light in comparison with dark is noticed for each PTNCO nanocomposites whereas it is only 1.4 times in pure PT at +5 V bias (Table 1).

Table 1 dc-Conductivity and current (μA) at +5 V from the I–V plots under dark and illuminated conditions and those in photo-switching experiment

Sl no.	Sample	dc-Conductivity (μS cm^−1)	I–V (μA)		Photo-switching (current) (μA)		Gain
			Dark	Light	Light	Dark
1	PT	0.19 ± 003	2.9	4.1	4.2	2.1	2
2	PTNCO-1	9100 ± 400	21 150	42 400	43 300	36 200	1.2
3	PTNCO-2	4200 ± 200	13 400	26 900	28 100	20 000	1.4

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Fig. 5 I–V plots under dark condition and upon light exposure of 1 sun intensity of (a) PT, (c) PTNCO-1 and (e) PTNCO-2 and photo response behavior when turning on/off white light on devices made of (b) PT, (d) PTNCO-1, and (f) PTNCO-2.

This implies an increased separation of photogenerated holes and electrons to the maximum extent followed by their flow on the PTNCO composite surface on photo-irradiation due to intimate adherence of PT over the NCO nanorods in both the nano composites. Nanocomposites consisting of π-conjugated systems and nanomaterials exhibit brilliant photo-responsive property.⁵² A crucial feature of these systems is the donor–acceptor interaction, which plays a pivotal role in their functionality. When light interacts with the surface of a semiconducting polymer, photons are absorbed. If the energy of the absorbed photons exceeds the band gap energy of the π-conjugated material, then the electrons become excited from the valence band to the conduction band, resulting in the formation of electron–hole pairs. The effectiveness of charge separation and charge transport in these systems is critical for the photocurrent behavior. A well-designed donor–acceptor framework facilitates efficient electron–hole separation, preventing the recombination and enabling the generation of higher current under illumination. This characteristic makes donor–acceptor nanocomposites promising materials for applications requiring light-induced switching or energy harvesting, such as photovoltaic cells, photodetectors, and other optoelectronic devices including photo-supercapacitors. In the present PTNCO composites, the conjugated chain of PT act as a donor and NCO act as an acceptor particularly for the vacant d-orbitals of metals [Co(III) or Ni(II)] of NCO as evident from the UV-vis spectroscopic data.

Here, the photo-switching properties of PT and nanocomposites (PTNCO-1 and PTNCO-2) were studied by conducting cyclic on–off tests on illumination of 1 sun intensity at +5 V bias and monitoring the photocurrent growth for 50 s and the decay for 50 s. It is observed from Fig. 5b, d and f and Table 1 that pure PT is capable of showing a photocurrent of 4.2 μA, while under dark conditions, the observed current is 2.1 μA with a photocurrent gain (I_on/I_off) of 2.0. Interestingly, in the nanocomposites (PTNCO-1 and PTNCO-2), the generated currents under both illuminated and dark conditions are significantly increased. The maximum photocurrent of PTNCO-1 and PTNCO-2 are 43.3 mA and 28.1 mA, respectively, whereas the minimum dark current values are 36.2 and 20 mA with the photocurrent gains (I_on/I_off) of 1.2 and 1.4, respectively. It is interesting to observe from Fig. 5d and f that the maximum photo-current gradually increases with the increase in photo-switching cycles, probably due to some increased storing of photoelectrons with the increase in photo-cycles for the nanocomposites compared to that of pure PT polymer. Moreover, the maximum holding time of photo-electrons with the increased number of cycles was found to be most prominent in PTNCO-1 than in PTNCO-2 due to the most favorable synergistic interaction between PT and NCO in the nanocomposite (Fig. 5d). This causes higher dark current, resulting in the lowest photocurrent gain (1.2) in PTNCO-1 compared to that (1.4) of PTNCO-2, implying that the former would be a promising material to act as a better photo-assisted supercapacitor.

Furthermore, the photo-responsivity (R) of the photo-current generation is an interesting term to understand the photocurrent property of the material and the R values were calculated for pure PT, PTNCO-1 and PTNCO-2 using the following equation:⁵³

where I_ph = I_on − I_off and A is the effective surface area of the photoelectrode, and P is the intensity of the light. The R value of PT, PTNCO-1 and PTNCO-2 was calculated under light of 1 sun intensity taking the surface area of the device equal to 1 cm². For PT, I_ph = I_on − I_off = 4.2 − 2.1 = 2.1 μA, A = 1 cm², P = 1000 W m⁻² = 0.1 W cm²; hence, R = 2.1 μA/(1 cm² × 0.1 W cm⁻²) = 21 μA W⁻¹. Similarly, for PTNCO-1 and PTNCO-2, the R value was found to be 71 000 and 81 000 μA W⁻¹ respectively. Thus, PTNCO nanocomposites exhibit 3380 and 3857 times higher photo-responsivity than that of pure PT and the higher photo-responsivity in the composites may be attributed to the synergistic donor–acceptor interaction between PT and NCO. The lower photo-responsivity of PTNCO-1 from that of PTNCO-2 may be probably for the lower photocurrent gain in PTNCO-1 than that of PTNCO-2, as in the latter system, increased PT chains make the NCO nanorods more separated than that of PTNCO-1 causing lesser photoelectrons to be stored on the NCO surface.

To gain a better understanding of the photo-switching property, time-resolved dynamic photocurrent of PT, PTNCO-1 and PTNCO-2 was analyzed using the following equations:

For growth: I(t)_growth = I_D + A₁ exp(t/τ_g1) +A₂ exp(t/τ_g2) (2)

For decay, I(t)_decay = I_D + A₁ exp(−t/τ_d1) + A₂ exp (−t/τ_d2) (3)

where I_D is the maximum current at a particular time, and τ_gi and τ_di are the growth and decay time constants, respectively.^54,55 After fitting growth and decay data with the above-mentioned equations (Fig. S10†), the growth and decay times were calculated and are presented in Table 2. The photocurrent growth and decay curves are found to follow two-stage processes in each case.³⁶ It is evident from Fig. S10a† that in the case of pure PT, the photocurrent rises fast initially at the ‘on’ state, followed by a slower increase and the current initially decreases fast at the ‘off’ state, followed by a slower decrease, till it reaches saturation (cf.Table 2). The overall growth and decay time constant for PT were found to be comparable due to its semiconducting nature. A stimulating photocurrent growth and decay behavior was noticed for both the nanocomposites (Fig. S10b and c†), where PTNCO-1 shows faster photocurrent growth (growth time = [2.43 + 2.71 = 5.14 s]) than PTNCO-2 ([5.45 + 5.59 = 11.04 s] and [2.33 + 3.06 = 5.39 s]), as the latter exhibits two-stage growth processes (Table 2). The two stages of growth times in the case of PTNCO-2 indicated that after the first rapid growth (11.04 s), a very small span of growth time (5.39 s) is also present as the photoelectrons may get stored for an instant to get re-excited. The overall photocurrent growth of PTNCO-1 was found to be faster as it can be expected from a better coplanar PT chain over the NCO nanorods (cf. morphology) with a higher active surface area (cf. N₂ adsorption data) that facilitates easier separation of photoelectrons and holes and their flow from those of PTNCO-2. The overall decay times were also found to be lowered with the two comparable decay stages (21.19 [7.33 + 13.86] and 22.23 [2.90 + 19.33] s) for PTNCO-1 than PTNCO-2 (55.65 [4.72 + 50.93] s) nanocomposites, indicating easier decay of photoelectrons too (Table 2). The higher charge storage ability of photoelectrons (about 15.8 [21.19–5.39] s) with a slight uniform decay was found in case of PTNCO-1 than that of PTNCO-2. The reason behind this decay nature may be attributed to the presence of a larger amount of NCO in the PTNCO-1 nanocomposite, which may result in higher interfacial resistance. By comparing the growth and decay time, it was found that the ratio turns out to be about 8.45 (43.42/5.14) for PTNCO-1 and 3.40 (55.65/16.43) for PTNCO-2 (Table 2), indicating that the photoelectrons are held for more time in the PTNCO-1 composite than in the PTNCO-2 composite. These results indicate the efficacy of PTNCO-1 towards the application of photo-storage devices in a better aspect.

Table 2 Growth and decay time constants of PT and the PTNCO composites obtained by fitting the photocurrent growth and decay data using biexponential eqn (1) and (2)

Sample	Growth (s)	Decay (s)	τ g1 (s)	τ g2 (s)	τ g3 (s)	τ g4 (s)	τ d1 (s)	τ d2 (s)	τ d3 (s)	τ d4 (s)
PT	38.48	40.72	17.32	21.16	—	—	12.50	28.22	—	—
PTNCO-1	5.14	43.42	2.43	2.71	—	—	7.33	13.86	2.90	19.33
PTNCO-2	16.43	55.65	5.45	5.59	2.33	3.06	4.72	50.93	—	—

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In order to comprehend the reason of photo-switching effect more vividly, we measured the lifetime values using the time-correlated single-photon counting (TCSPC) experiment. The solid-state emission decay plots of PTNCO-1 and PTNCO-2 under both the dark and illuminated conditions, deposited on quartz substrates, are shown in Fig. S11.† The fitted parameters of the decay profiles are displayed in Table S1† after fitting with biexponential functions. The average lifetime values were determined using the following equations:

I = B₁ exp(−t/τ₁) + B₂ exp(−t/τ₂) (4)

where I is the normalized emission intensity, τ₁ and τ₂ are the lifetime decay constants, and B₁ and B₂ are the amplitude coefficients for the individual decay components. It is evident from Table S1† that both the average lifetimes of PTNCO-1 under dark (0.441 ns) and illuminated (0.037 ns) conditions get decreased compared to that of PTNCO-2 (0.638 and 0.049 ns). This again implies the supreme fabrication of the PTNCO-1 electrode, where the charges store and flow through the NCO nanostructure in the composite state due to the greater extent of coplanarity of PT chains as noticed in the morphological section. Interestingly, the remarkable decrease of lifetime values under the illuminated state (1 sun intensity) for both the nanocomposites (12–13 times) may be attributed to the transfer of photogenerated carriers, resulting in maximum electron–hole separation.⁵⁶ The solid-state UV-vis (Fig. S12a†) spectra was also performed to measure the band gap using Tauc plot (Fig. S12b and c†), and the band gap of PTNCO-1 (2 eV) is lower than that of PTNCO-2 (2.19 eV). Thus, it can be surmised that the PTNCO-1 nanocomposite offers a better photo-responsive material than PTNCO-2 because, electron–hole separation in PTNCO-1 is easier upon irradiation of light than in PTNCO-2.

Supercapacitor performance

The electrochemical energy storage capacity of NCO and influence of PT on the specific capacitance on NCO were measured by cyclic voltammetry (CV) using both three-electrode and two-electrode configurations. The influence of light exposure on the specific capacitance, energy density and power density was also investigated on the two-electrode solid-state devices. The specific capacitance (C_S), energy density (E) and power density (P) of symmetrical supercapacitors were calculated from the GCD tests according to the following equations:

Three-electrode measurements

In this configuration, the supercapacitor performance of PT, PTNCO-1 and PTNCO-2 composites was measured by CV and galvanic charge–discharge (GCD) study in 1 M KOH solution by coating the sample onto a glassy carbon electrode (GCE), which acts as a working electrode, a Pt wire as the counter electrode and Ag/AgCl as the reference electrode. The CV traces clearly show well-defined redox peaks at 0.511 and 0.254 V at 100 mV s⁻¹, typical of NCO system, while having quasi rectangular shapes for the capacitive nature. The capacitance value of the NCO electrode is mostly based on redox mechanism related to redox reactions of M–O/M–O–OH (M = Ni or Co).⁵⁷ The increased area under the CV traces of the composite electrodes with the maximum for PTNCO-1 (Fig. 6b and S13a†) indicates their superior energy storage capability to NCO (Fig. 6a). The observed capacitive nature in the CV trace of the typical battery electrode NCO is attributed to its nano structure found in the TEM image. The CV traces of the NCO electrode were found to retain their shapes with increased peak current under increasing scan rates. This indicates improved rate performance arising from faster ion adsorption–desorption on the electrode surface, thanks to the nano-structuring of NCO generating increased available surface area and decreased diffusion path length, which allow faster electrolyte ion transport over the electrode surface at a higher rate of charge sweep.^46,58,59 This is further apparent from the linear relationship of anodic and cathodic peak currents with the square root of scan rate (Fig. 6a, inset), indicating that the diffusion of OH⁻ ions is the rate-determining step in the whole process.⁵⁷ Interestingly, the same features are observed in the composites (Fig. 6b and c), which indicate thin and uniform coating by the PT chains without disturbing the nanostructuring of the NCO. The potential of both the peaks shift to more positive (oxidation) and negative (reduction) values in the nanocomposites due to the increased level of electrical polarization.⁶⁰ The overlay of CV traces at 100 mV s⁻¹ for NCO, PTNCO-1 and PTNCO-2 is compared in Fig. S13a,† indicating supremacy of PTNCO-1 over the other electrodes. It is interesting to note that anodic peak position has shifted from 0.425 V in NCO to 0.448 V in PTNCO-1 at 10 mV s⁻¹ scan rate, where the cathodic peak also shifted from 0.301 to 0.247 V and almost a similar shift is also noticed in PTNCO-2. The observed shifts possibly arises from the electronic interactions between the thiophene rings with Ni(II)/Co(III) of NCO. Furthermore, it is interesting to note that the measured conductivities of the composites (PTNCO-1 → 9.1 mS cm⁻¹ and PTNCO-2 → 4.2 mS cm⁻¹) have shown manifold increase from any of PT (0.19 μS cm⁻¹) or NCO (0.5 mS cm⁻¹). The observed increase in conductivity in the composites might have occurred due to doping of the PT chains (p-type) by the metal atoms,⁶¹ which are at higher oxidation states in NCO. Thus, it may be affirmed here that a significant synergy between the operations of the composite components has played an important role in upholding the electrochemical property of the composite materials. The GCD curves of NCO and composite electrodes (Fig. 6d–f), show nearly triangular shape with some deviations from symmetrical nature, possibly arising from the contributions of both pseudo-capacitance and double layer-capacitance.^62,63 The specific capacitance (C_S) values were calculated using eqn (5) from the GCD curves, and the C_S values of NCO, PTNCO-1 and PTNCO-2 were found to be 606, 958, and 712 F g⁻¹, respectively, for a current density of 1 A g⁻¹ in a three-electrode set up. The observed C_S values of the composites, particularly for PTNCO-1, is appreciably high as compared with the various NiCo₂O₄-based nanocomposites, as presented in Table S2.† The appreciably high C_S value for PTNCO-1 indicates improved synergistic interactions, where conducting PT chains assist in the flow of electrons and the flow of ions is facilitated through the mesoporous morphology of the NCO-based nanocomposites.⁴³ The GCD curves at 1 A g⁻¹ current density of NCO, PTNCO-1 and PTNCO-2 were compared in Fig. S13b† for a clear understanding of a higher C_S value of PTNCO-1 over the others. The lower C_S value in PTNCO-2 from that of PTNCO-1 is attributed to lesser interaction between the components, as evident from FTIR and UV-vis spectral data and also for a higher degree of dispersion of NCO rods (cf. morphology) within the PT matrix decreasing the interconnectivity between the NCO rods necessary for the charge storage.

Fig. 6 Cyclic voltammograms at different scan rates of (a) NCO, (b) PTNCO-1 and (c) PTNCO-2 (inset: plot of peak current vs. square root of ν) and GCD plots at different current densities of (d) NCO, (e) PTNCO-1, and (f) PTNCO-2.

Fig. 7a shows the retention of a significantly high value of specific capacitance (C_S ∼700 F g⁻¹) for PTNCO-1 even under an appreciably high current density of 5 A g⁻¹, implying improved rate capability. The energy density and power density values calculated from eqn (6) and (7) of NCO, PTNCO-1 and PTNCO-2 are plotted in Fig. 7b and the curve of PTNCO-1 is at the upper most position and that of NCO is at the lowest, while PTNCO-2 is at the middle position. Due to the synergistic interaction between PT and NCO in PTNCO-1 in the composite, PT adheres on the NCO surface very well, facilitating energy storage at the NCO-PT interface, thus showing the highest energy density. However, a higher degree of dispersion of NCO rods into a larger amount of PT matrix in PTNCO-2 decreases the interconnectivity among the NCO rods, thus decreasing the energy density to some extent compared to that of PTNCO-1. In order to understand the reason of higher specific capacitance and energy density of PTNCO-1, electrochemical impedance spectral (EIS) studies were performed in three-electrode cells, and the Nyquist plots are presented in Fig. 7c. An enlarged portion of Nyquist plots is also shown at the inset of Fig. 7c for better understanding. A quasi-semicircle in a higher frequency region and a linear plot in a lower frequency region were found in the Nyquist plot. In general, the former arises for the interfacial charge transfer resistance (R2) during the Faradaic reaction, whereas the latter one arises from the Warburg impedance, which actually depicts the impedance for the diffusion of OH⁻ ions at the electrode surface.

Fig. 7 (a) Plot of specific capacitance vs. current densities, (b) plot of energy density vs. power density (Ragone plot), (c) Nyquist plots of the entire electrode materials (inset: magnified portion of the Nyquist plots), (d) log i vs. log ν plots and dependence of voltammetric charge on the scan rate in a three-electrode cell (e) q vs. ν^−1/2 and (f) q⁻¹vs. ν^1/2 for NCO, PTNCO-1 and PTNCO-2 electrodes.

The representative equivalent circuit of the three samples, drawn using the Z-view software, is also shown at the inset of Fig. 7c portraying the contact resistance (R1) in series with charge transfer resistance (R2), and Warburg impedance (W) which are in parallel with the cell capacitance (C1). NCO, PTNCO-1 and PTNCO-2 nanocomposites showed contact resistance (R1) values of 8.45, 6.03 and 7.14 Ω, whereas the charge transfer resistance (R2) values were found to be 12.9, 9.11 and 10.73 Ω, respectively, obtained using the Z-view software.

The lower R1 values in the composites imply good contact with the connecting electrodes for the presence of functionalized PT chain in the composites. From the R2 values, it may be inferred that the PTNCO-1 nanocomposite shows the lowest interfacial resistance facilitating enhanced electrical conductivity in the composite. The maximum capacitance (C1) value of about 99.8 mF is found in the PTNCO-1 composite among the others supporting PTNCO-1 to behave as a good super capacitor. This highest capacitance value of the PTNCO-1 nanocomposite may be attributed to better adherence of a thin layer of PT chain with NCO storing the charges at the interface better than that of PTNCO-2.⁴³ The Nyquist plot of PTNCO-1 shows the most steeper rise in a lower frequency region, indicating the lowest Warburg impedance facilitating better hydroxyl ion diffusion, thus increasing the specific capacitance value.⁶⁴

To obtain an insight into the charge storage mechanism analysis of the CV curves with the power law relationship of current ‘i’ at a scan rate of ‘ν’, eqn (8) ⁶⁵ was used:

i = a × ν^b (8)

where ‘a’ and ‘b’ are constant and the exponent ‘b’ value near 0.5 represents a fully diffusion-controlled process, whereas b = 1 represents a fully capacitive response. Here, we consider anodic oxidation peaks from the CV curves (Fig. 6a–c) to determine the ‘b’ values from the slope of log i vs. log v plots (Fig. 7d), and they are 0.741, 0.854 and 0.784 for NCO, PTNCO-1 and PTNCO-2, respectively. These results indicate that the capacitive response was found to be the highest for the PTNCO-1 electrode in comparison with others, whereas, the diffusive charge storage character is prominent for NCO. Hence it can be concluded that there is some contribution of diffusive charge storage along with the majority of capacitive nature in all cases where the PTNCO-1 electrode is showing the maximum capacitive storage over others.

The capacitance differentiation method (Dunn method)⁶⁶ was also applied (eqn (9)) to understand the capacitive response in detail for all three electrodes using the following equation:

i(ν) = k₁ν + k₂ν^1/2 (9)

where the first term indicates the capacitive storage character and the second term indicates the diffusive charge storage character. Interestingly, the capacitive response of NCO, PTNCO-1 and PTNCO-2 electrodes was found from the plot of I (ν)/ν^1/2vs. ν^1/2 (Fig. S14a†), showing capacitive responsivities of 74.4, 89.6 and 77.4%, respectively. For better understanding of capacitive nature, current vs. potential plots are also provided in Fig. S14b–d,† where the highlighted areas of the voltammograms represent the capacitive nature, and the diffusive charge storage characters are evident from the blank spaces of the voltammograms for each of the NCO, PTNCO-1 and PTNCO-2 electrodes.

Further, to find the contribution of diffusion-controlled charge storage over electrical double layer capacitance (EDLC) in the total capacitance, the Trasatti method (eqn (10i) and (ii))⁶⁷ was also employed with all the three systems using the following equations:

q⁻¹ = constant × ν^1/2 + q_T (10i)

q = constant × ν^−1/2 + q_EDLC (10ii)

The plots of voltammetric charge (q) against scan rates (ν^−1/2) are demonstrated in Fig. 7e and q⁻¹vs. ν^1/2 plot is shown in Fig. 7f. The total maximum specific capacitance (C_STM) and the double layer capacitance (C_SDL) were found after dividing the voltammetric charge obtained from ν = 0 to ν = ∞, with the window potential of 0.6 V. Then, the contribution of diffusion-controlled processes (C_dc) was obtained from the difference of C_STM and C_SDL. An increased degree of the diffusion-controlled charge storage contribution for PTNCO-1 (82%), in comparison with the NCO (72%) and PTNCO-2 (77%) electrodes is ascribed to the comparatively increased surface area with mesoporous morphology, providing rather shortened diffusion path for electrolyte ions among the NCO octahedrons in the composite.

Two-electrode measurements of PTNCO-1 under dark and illuminated conditions

In the three-electrode setup, PTNCO-1 showed the highest specific capacitance, energy density and rate capability, hence for fabricating a solid-state two-electrode device, we chose the PTNCO-1 composite to compare the above-mentioned results measured under both dark and illuminated conditions with UV light of λ = 365 nm. A two-electrode solid-state device was prepared by depositing a PTNCO-1 composite on a carbon cloth,¹⁹ and two such electrodes were sandwiched with a PVA-KOH gel electrolyte for measuring the CV curves (Fig. 8a) with different scan rates from 10 to 100 mV s⁻¹ to evaluate the electrochemical performances. The digital image of the solid-state device in the flat state is shown in Fig. 8b. Here we are interested in the influence of light irradiation on the super-capacitor performances, so we chose PTNCO-1 because this composite exhibits the best performance in photocurrent and photo-switching properties, as well as it exhibits the highest specific capacitance in the three-electrode set up among the present samples. To understand the passage of light through the carbon cloth-based sandwiched supercapacitor, we measured transmittance data from the sandwiched carbon cloth made by introducing a quartz plate and also that of a quartz plate (blank) using a UV-vis spectrometer. The transmittance data are presented in Fig. S15† which shows about 87% transmittance through the quartz plate and about 20% transmittance through the sandwiched carbon cloth. The CV curves of PTNCO-1 at different scan rates ranging from 10 to 100 mV s⁻¹ are presented in Fig. 8c for dark conditions, and upon illumination with 365 nm UV light, the CV curves are presented in Fig. 8d at similar scan rates from 10 to 100 mV s⁻¹. A comparison of Fig. 8c and d (also Fig. S16a†) indicates that the area under the CV curves appears to be larger in Fig. 8d than those in Fig. 8c at identical scan rates. To get a quantitative idea of specific capacitance under both dark and illuminated conditions, the GCD curves are presented in different scan rates under dark and illuminated conditions (Fig. 8e and f). A comparison of GCD curves at a current density of 1 A g⁻¹ was also measured under dark and illuminated conditions, as shown in Fig. S16b,† indicating that under illuminated conditions, PTNCO-1 exhibits larger charge–discharge curves, indicating that specific capacitance would be larger at the illuminated state. In the solid-state devices, the C_S values were calculated from the discharge curves using eqn (5), and the device under dark and illuminated conditions exhibit specific capacitance (C_S) values of 90 and 106 F g⁻¹ at a current density of 1 A g⁻¹, respectively. In comparison with the various NiCo₂O₄-based nanocomposites presented in Table S3,† these C_S values measured under dark conditions are comparable. The C_S values under dark and illuminated conditions are 90 and 106 F g⁻¹ at a current density of 1 A g⁻¹ and those at a current density of 5 A g⁻¹ are 47.8 and 67.4 F g⁻¹, indicating an increase in C_S from [(106 − 90) × 100/90] = 17.8% to [(67.4 − 47.8) × 100] = 41% from 1 A g⁻¹ to 5 A g⁻¹ current density (cf.Fig. 9a, inset). With the increase in current density, the C_S value gradually decreases for both dark and illuminated conditions, but the C_S value under illuminated conditions gradually increases from 17.8% to 41% for the increase in current density from 1 to 5 A g⁻¹ (Fig. 9a, inset). The rate capability for the dark condition is (47.8 × 100)/90 = 53% and that for the illuminated condition is 67.4/106 = 63.6% between 1 and 5 A g⁻¹ current density, thus there is an increase in rate capability (63.6 − 53) = 10.6% for illumination with the UV light (λ = 365 nm). It is now necessary to compare the electrochemical performances of the PTNCO-1 device under the illuminated condition with those of previous works. Harini et al. fabricated a NiO/TiO₂/rGO nanocomposite,⁶⁸ which provides around 18% and 10% increase in the C_S value at 1 and 2 A g⁻¹ current density under the illuminated condition (1 sun intensity), respectively. Similarly, Zhao et al. reported a NiO/FeCo₂O₄ electrode,⁶⁹ showing an increase of C_S around 42% at 1 A g⁻¹ current density under simulated sunlight; however, it decreases to 4.5% when the current density reaches 5 A g⁻¹. Prakash et al. synthesized a photo-assisted V₂O₅-based supercapacitor,⁷⁰ which provides 53% increase in C_S value upon irradiation of 1 sun intensity at 0.2 A g⁻¹ current density; however, they did not mention any photo-capacitive response of C_S at a higher current density. Wu and coworkers showed an increase in C_S value to 16.7% in TiO₂-assisted Co₃O₄ electrodes⁷¹ compared to that of pure Co₃O₄ electrodes at 1 A g⁻¹ current density under UV-light. The C_S value was also found to be increased to 27% at a higher current density of 4 A g⁻¹ under UV light, and this is very similar to the present PTNCO-1 system. Bai et al.¹⁹ also reported some fascinating photo-supercapacitors (Co₃O₄/g-C₃N₄ electrode), where 19.7% increase in C_S was observed under simulated solar light at 4.8 A g⁻¹ current density. Hence, it can be surmised from the above comparison that the PTNCO-1 solid-state electrode device exhibits a better result, particularly at a higher current density, indicating PTNCO-1 to behave as a promising photo-supercapacitor. The percent increment of C_S data plotted against current density is well fitted in a parabola-type curve (Fig. 9a, inset), and it is really an interesting observation. One possible explanation is that with the increase in current density, the photoelectrons do not get easily annihilated, rather they remain stored in the composite increasing the specific capacitance value to a greater extent. The rate capability graph of PTNCO-1 shows 53% retention in the darkness, while in the light the specific capacitance retention is 63.6%, indicating 10.6% increase in specific capacitance retention under illumination with UV light at λ = 365 nm. This increase in specific capacitance retention under illuminated conditions may be attributed to the storing of photoelectrons at the interface of PT and NCO, where the charges find difficulty to be annihilated with the increase in current density. Therefore, under illuminated conditions, the specific capacitance increases and with the increase in charge density, percent increase of specific capacitance was observed along with the increase in rate capability.

Fig. 8 (a) Fabrication of a solid-state two-electrode CV measurement system using carbon cloth, (b) digital image of the solid-state device (2 mg cm⁻²) in the flat state, CV curves of PTNCO-1 at different scan rates under (c) dark condition and (d) UV light (λ = 365 nm) exposure, GCD plots of the PTNCO-1 solid-state device at different current densities under (e) dark condition and (f) UV light (λ = 365 nm) exposure.

Fig. 9 (a) Plot of specific capacitance with different current densities under darkness and UV light (inset: increment (%) of C_S under UV light from that of darkness). (b) Ragone plots of the PTNCO-1 device under darkness and UV light, cycling stability analysis at a current density of 5 A g⁻¹ under (c) darkness and (d) UV light conditions, (e) log(energy density) vs. log(power density) plots under darkness and UV light and (f) Nyquist plot of the device under darkness and UV light.

The power density (P, W kg⁻¹) and energy density (E, W h kg⁻¹) values of the PTNCO-1 composite were calculated using eqn (5) and (6), under both dark and illuminated conditions. In Fig. 9b, the plots of energy density vs. power density (Ragone plots) of PTNCO-1 are shown for both dark and illuminated conditions, and from the plot, it is evident that the energy density is higher under illuminated conditions than that under dark conditions at identical power densities. Under dark conditions, the supercapacitor exhibits a maximum energy density of 12.5 W h kg⁻¹ at a power density of 500 W kg⁻¹ and it shows a maximum power density of 2500 W kg⁻¹ at an energy density of 6.6 W h kg⁻¹. However, under illuminated conditions, it shows a higher energy density of 14.7 W h kg⁻¹ at a power density of 500 W kg⁻¹, and it shows a maximum power density of 2500 W kg⁻¹ at an energy density of 9.4 W h kg⁻¹. Thus, at 500 W kg⁻¹ energy density values increased by 17.8% and at 2500 W kg⁻¹ energy density values increased by 41%. These higher energy density values under illuminated conditions indicate that some light energy may remain stored in the illuminated supercapacitor, thus acting as a photo-assisted supercapacitor. The cyclic stability of PTNCO-1 solid-state supercapacitors under dark and illuminated conditions is shown in Fig. 9c and d for 5000 cycles. Under dark conditions, PTNCO-1 shows a specific capacitance retention of 80%, whereas under illuminated conditions, the specific capacitance retention is 85%, indicating that under illuminated conditions, the PTNCO-1 system exhibit 5% higher cycling stability than that under darkness. The GCD plots of 12–15 consecutive cycles before and after 18 h are also shown at the inset of cyclic stability plot for both dark and illuminated conditions, showing a negligible change in C_S value. Boruah et al. focused on the cycling stability under UV-illuminated conditions, where only 1.5% increase in cyclic stability for the NiCo₂O₄/ZnO electrode⁷² was observed. Bai et al.¹⁹ reported in Co₃O₄/g-C₃N₄ photo-supercapacitors, 83.3% retention of C_S after 5000 cycles under illuminated condition. Therefore, it may be inferred that the present PTNCO-1 system behaves as a good photo-assisted supercapacitor even after running 5000 cycles. Although it is mostly of pseudo-capacitor nature, the PTNCO-1 electrode shows very high cycling stability under both dark and illuminated conditions. This is probably attributed to the nano-rod morphology of NCO formed from the assembly with nano-octahedron in a close proximity range, which decreases the diffusion length among themselves leading to the controlled ion diffusion rate during the redox reaction.⁴³ To correlate the energy density with the power density of PTNCO-1 with an empirical power law, log(energy density) vs. log(power density) was plotted according to the equation: log(energy density) = B log(power density) + A, both under the dark and illuminated conditions.⁵⁴ Under dark conditions, intercept A = 2.16 and slope B = −0.39 with 〈R²〉 value = 0.97 and for illuminated condition A = 1.93 and the slope B = −0.28 with 〈R²〉 value = 0.96. Therefore, a comparison of the slope values clearly indicates that the rate of decrease of energy density with the increase in power density is lower under illuminated conditions than that under dark conditions (Fig. 9e). This indicates that the photo-assisted supercapacitor preserves its energy density more than normal supercapacitors upon increasing the power density.

The EIS measurements were also performed for the PTNCO-1 solid-state device (Fig. 9f) under both dark and illuminated conditions to understand the influence of light on the values of contact resistance (R1), charge transfer resistance (R2), Warburg impedance and capacitance (Table 3). Here also at a high frequency, there is a semicircle followed by a sharp rise in the lower frequency region, indicating Warburg impedance arising from the diffusion of ions. As reported earlier, the data are fitted using the Z-view software, showing similar equivalent circuits as in the three-electrode system. The R1 and R2 values are 1.34, 1.12 Ω and 2.5, 1.99 Ω, respectively, for the dark and illuminated conditions, while the capacitance (C1) values are 122 and 166 mF at dark and illuminated states, respectively. Thus, the R1 and R2 values were found to be lowered when the device (PTNCO-1) is exposed to the UV light (365 nm), whereas the capacitance value gets increased. This is an interesting observation as a lower R2 value indicates easy movement of photoelectrons and a higher C1 value under photo-irradiated conditions, indicating that photoelectrons can be stored easily at the PT-NCO interfaces, thus showing a higher specific capacitance value than that under the dark condition. Hence, from the above observation, it can be concluded that the device is highly photo-active and behaves as an efficient photo-assisted supercapacitor with higher specific capacitance, rate capability, energy density and cyclic stability than those measured under dark conditions.

Table 3 R1, R2, C1 and W values of NCO and PTNCO nanocomposites obtained from the Nyquist plot of EIS spectra fitting using the Z-view software

Sample	R1 (Ω)	R2 (Ω)	C1 (F)	W (Ω s^−1/2)
NCO	8.45	12.93	0.049792	0.39
PTNCO-1	6.03	9.11	0.099792	0.36
PTNCO-2	7.14	10.73	0.069792	0.38
PTNCO-1, dark	1.34	2.5	0.121792	0.33
PTNCO-1, UV-light	1.12	1.99	0.165791	0.32

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Mechanism of photo-assisted enhanced supercapacitor property

The calculated energy gaps of the valence band and conduction band of PT⁷³ and NiCo₂O₄ ⁷⁴ are presented in Scheme 2. It is apparent from the scheme that the energy level matching of the VB and CB of PT (Fig. S12c†) and NCO (Fig. S12b†) is fairly good and thus favourable for electron and hole transfer as shown below.

Scheme 2 Mechanism of the PAEC behavior in the PTNCO-1 supercapacitor after forming nanocomposites.

The cathode reactions during charging and discharging in the absence and presence of light are presented as follows:

In darkness,

While charging:⁷⁵

NiCo₂O₄ + 3OH⁻ → NiOOH + 2CoO₂ + H₂O + 3e⁻

While discharging:⁷⁵

NiOOH + 2CoO₂ + H₂O + 3e⁻ → NiCo₂O₄ + 3OH⁻

In the presence of light,

While charging:

PT + nhν → nh⁺ + ne⁻

PT + NiCo₂O₄ + OH⁻ + nhν → PT + NiOOH + 2CoO₂ + H₂O + (n + 3)e⁻ + nh⁺

While discharging:

PT + NiOOH + 2CoO₂ + H₂O + nhν → PT + NiCo₂O₄ + OH⁻ + (n − 3)e⁻ + nh⁺

where h⁺ represents the photogenerated holes and e⁻ stands for the photogenerated electrons or free electrons in the redox reactions, n being the number of photogenerated carriers. As photogenerated carriers increase from 0 to n with photoirradiation, more electrons are separated from the holes by the built-in electric field thus, the GCD capacity increases, so the energy density (E) also increases.

Conclusion

We have successfully developed a photo-assisted supercapacitor by polymerizing substituted polythiophene (3-[1-ethyl-2-(2-bromoisobutyrate)]thiophene) (PT) into a spinel NCO dispersion. The ¹H-NMR spectra of PTNCO composites showed two new de-shielded peaks at 3.50 and 5.32 ppm characterizing nanocomposite formation. The FTIR spectra showed a distinct shift of C=S vibration (1164 cm⁻¹) and S–C–S vibration (1116 cm⁻¹) for PTNCO composites, and PTNCO-1 showed more shift to a lower energy, indicating the presence of pπ–dπ interaction between the components. In the Raman spectra, characteristic peaks for both NCO and PT units were observed and distinctive shifts to a lower energy for most of characteristic Raman bands occurred arising from the synergistic interaction between NCO and PT components. Interestingly, the nanorod morphology of NCO was retained in nanocomposite samples, where PT chains got adhered over the NCO nanorods, also showing porous morphology. The XPS data showed peaks for binding energies at 778.8, 780.1 eV and 793.8, 795.1 eV for Co(II) and Co(III) respectively, and those at 853.2, 855.2 eV and 870.4, 872 eV are for Ni(II), and Ni(III), respectively. The absorption peak of π–π* transition of conjugated PT chains (414 nm) showed 7 and 3 nm red shift for PTNCO-1 and PTNCO-2, respectively, due to the pπ–dπ interaction between PT conjugated chains and NCO nanoparticles. In Tauc plot in the solution state, PT exhibited a band gap of 2.57 eV, whereas the band gap decreased to 2.26 and 2.47 eV for PTNCO-1 and PTNCO-2 respectively. The PL peak of PT showed a red shift and the PL intensity decreased, showing maximum drop for PTNCO-1 due to the non-radiative decay of PT excitons in the well-coated NCO surface. The BET specific surface areas of NCO, PTNCO-1 and PTNCO-2 were found to be 28, 40, and 33 m² g⁻¹, having pore sizes of 6.2, 8.9, and 8.2 nm for NCO, PTNCO-1 and PTNCO-2, respectively, indicating the mesoporous character of the samples. The dc-conductivity value of PT was 0.19 μS cm⁻¹, whereas PTNCO-1 and PTNCO-2 exhibited dc-conductivity values of 4.2 and 9.1 mS cm⁻¹, respectively. From the I–V plot at +5 V, the dark current of pure PT was 2.9 μA; however, dark currents were significantly increased to 21.15 mA and 13.4 mA in PTNCO-1 and PTNCO-2, respectively, showing about 7293 and 4620 times increase of dark current from that of PT due to decrease in band gap in the composites. On illumination with light of 1 sun intensity, the photocurrent values are 4.1 μA, 42.4 mA and 26.9 mA for pure PT, PTNCO-1 and PTNCO-2, respectively at +5 V, indicating 10 341 and 6561 times increase in PTNCO-1 and PTNCO-2, respectively from that of PT due to increased separation of photogenerated holes and electrons followed by their flow on the PTNCO surface upon photo-irradiation. In the photo-switching experiment, photocurrent gain (I_on/I_off) values of 2.0, 1.2 and 1.4 were observed for PT, PTNCO-1 and PTNCO-2, respectively. Further, analysis of time-dependent growth and decay currents surmised that the ratio of overall growth and decay times were 8.45 and 3.40 for PTNCO-1 and PTNCO-2, respectively, indicating that the photoelectrons were held for more time in the PTNCO-1 composite. These results indicated the efficacy of PTNCO-1 towards the application of photo-storage devices to a better extent.

In the three-electrode configuration, the C_S values of NCO, PTNCO-1 and PTNCO-2 were found to be 606, 958, and 712 F g⁻¹, respectively, for a current density of 1 A g⁻¹. The ‘b’ values from the slope of log i vs. log v plots were 0.741, 0.854 and 0.784 for NCO, PTNCO-1 and PTNCO-2, respectively, indicating that the capacitive response was highest for the PTNCO-1 electrode in comparison with others. The Dunn method also indicated capacitive responsivities of 74.37%, 89.61% and 77.42% for NCO, PTNCO-1 and PTNCO-2, respectively. An analysis by the Trasatti method indicated an increase in the diffusion-controlled charge storage contribution for PTNCO-1 (82%) in comparison with the NCO (72%) and PTNCO-2 (77%) electrodes. A two-electrode solid-state device was fabricated by depositing a PTNCO-1 composite on a carbon cloth, and a PVA-KOH gel electrolyte shows C_S values of 90 and 106 F g⁻¹ at 1 A g⁻¹ current density, under dark and illuminated conditions (λ = 365 nm), respectively, showing 17.8% increased C_S values at irradiated state. Furthermore, an increase of C_S from 17.8% to 41% at 1 A g⁻¹ to 5 A g⁻¹ current density was observed when exposed to light (λ = 365 nm), showing a higher percent increase of C_S at a higher current density. The rate capability of PTNCO-1 shows 53% retention in the dark state, while under the light, the specific capacitance retention is 63.6%, indicating 10.6% increase under illumination of UV light at λ = 365 nm. After 5000 cycles, 5% increase of cyclic stability at the illuminated state compared to that at the dark state was also noticed. The EIS data supported better storage capacity of the photoelectrons in PTNCO-1 where a decrease in charge transfer resistance and an increase in capacitance value by ∼44 mF were achieved in the equivalent circuit under the illuminated condition compared to the dark condition. Thus, PTNCO-1 behaves as a good photo-assisted supercapacitor and may find applications in various electronic and optoelectronic devices.

Data availability

We have not used any specific software or code for analyzing the data. All the data of the study (polythiophene-derived nickel cobaltite nanocomposites showing excellent photo-switching and photo-assisted enhanced supercapacitor properties) are available in the original manuscript and ESI,† and they are available on the journal website at http://www.rsc.org free of cost.

Conflicts of interest

We declare that there is no conflict of Interest in the work.

Acknowledgements

We gratefully acknowledge CSIR, New Delhi (ES) grant (21(1055)/18-EMR-II) for financial support. U. B. acknowledges SERB, New Delhi (Sanction order No. PDF/2023/002040) for providing the National Post-Doctoral Fellowship. P. G. acknowledges IACS for financial support.

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Footnotes

† Electronic supplementary information (ESI) available: Different Instrumental technique, SEC trace of PT, FTIR spectra of NCO, PT, PTNCO-1 and PTNCO-2, Raman spectra of NCO, PT, PTNCO-1 and PTNCO-2, XRD pattern of NCO, PT, PTNCO-1 and PTNCO-2, EDX spectrum analysis of NCO, PTNCO-1 and PTNCO-2, HRTEM analysis of PTNCO-2, UV-vis spectra and Tauc plot of PT, PTNCO-1 and PTNCO-2, photo-luminescence spectrum of PT, PTNCO-1 and PTNCO-2, TCSPC analysis of PTNCO-1 and PTNCO-2 in solid state, N₂ adsorption/desorption isotherms and pore size distribution of NCO, PTNCO-1 and PTNCO-2, photo-switching fitted curves of PT, PTNCO-1 and PTNCO-2, comparison table of C_S of NiCo₂O₄ nanostructure-based supercapacitor using both three- and two-electrode systems and Dunn method analysis of capacitance contribution of NCO, PTNCO-1 and PTNCO-2. See DOI: https://doi.org/10.1039/d4ta09051a

‡ Contributes equally.

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