Unveiling charge dynamics in high-performance binder-free photo-rechargeable supercapacitors

Abstract

In this study, binder-free nickel cobalt oxide (NiCo₂O₄) nanowire arrays with a cubic spinel structure were directly grown on nickel foam (NF) via an in situ hydrothermal process. The resulting one-dimensional nanowires exhibited a uniform morphology and a favourable bandgap of approximately 1.67 eV, making them ideal candidates as electrode materials for photo-assisted supercapacitors. Electronic structure analysis revealed the coexistence of Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺ redox pairs, significantly enhancing electrochemical kinetics and facilitating efficient photo-assisted charge storage. Under illumination, the NiCo₂O₄@NF nanowires demonstrated a remarkable 54% increase in areal capacitance, from 570 to 880 mF cm⁻² at 15 mA cm⁻², attributed to the efficient separation and storage of photo-generated charges driven by surface polarization effects. An asymmetric supercapacitor device was fabricated with activated carbon (AC) as the anode and NiCo₂O₄@NF nanowires as the photoactive cathode, maintaining 88% capacitance retention after 1000 illumination cycles. Density functional theory with the on-site Hubbard U correction (DFT + U) calculations further confirmed that nickel substitution in the Co₃O₄ matrix significantly reduces the bandgap and enhances the magnetic moment, supported by asymmetric spin-resolved density of states and band structure analyses. This research provides valuable insights for developing next-generation photo-assisted energy storage solutions.

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1 Introduction

The increasing demand for renewable energy arises from the limited nature of fossil fuels.¹ Therefore, innovative methods are required to address global energy challenges. Various sustainable energy sources include solar,² wind,³ hydropower,⁴ biomass,⁵ ocean,⁶ and geothermal energy.⁷ Solar energy stands out due to its abundance, accessibility, and environmental friendliness. However, solar energy's intermittent nature and low energy density present significant challenges for its direct application.⁸

Sunlight has been utilized in various applications such as photocatalysis,⁹ photovoltaic cells,¹⁰ photothermal systems,¹¹ rechargeable batteries,^12,13 and supercapacitors.^14,15 Several attempts have been made to integrate solar energy harvesting and storage into a single, small device known as a “photo-supercapacitor”, which integrates solar cells with supercapacitor electrodes utilizing common counter electrodes. However, despite achieving an energy efficiency of 1–7%, these systems face challenges such as complex configurations, limited storage capacity, and low charge retention, making them inefficient and expensive.^16–19 To overcome these challenges, it is crucial to develop bifunctional electrode materials capable of simultaneously harvesting and storing solar energy within a single compact unit, presenting a novel paradigm for photo-assisted rechargeable systems.

Recent efforts have focused on integrating light-harvesting functionalities into energy storage systems. Semiconductors like Co₃O₄,^20,21 Cu@Cu₂O,²² V₂O₅,^23,24 VO₂,^25,26 MoS₂,²⁷ and g-C₃N₄ ^28,29 have been explored as prospective bifunctional electrode materials. For example, An et al.²² employed a hybrid array electrode of nanoporous Cu@Cu₂O for the photo-assisted charge capacitance showing 37.9% enhancement at 1 A g⁻¹ under light. They established a mechanism by which light-induced holes boost the number of active sites and encourage protons to migrate toward the electrode surface. Similarly, Wang et al.³⁰ and Boruah et al.^28,31 demonstrated photo-assisted enhancements in supercapacitors using various nanostructured materials. In addition, the latest research has also explored plasmonic-enhanced photo-assisted supercapacitors, where the integration of noble metal nanoparticles with metal oxide nanostructures further improves the light-harvesting efficiency.^32,33 Such developments highlight the dynamic nature of this field and suggest additional pathways to enhance device performance.

Despite these advances, many photo-responsive electrodes incorporate binders that impede electrical conductivity and hinder the efficient collection of photo-generated charges. In situ grown materials on electrode surfaces are required to overcome these limitations as they offer superior electrical conductivity, large surface area, high energy density, and unobstructed light–matter interaction. This study demonstrates a binder-free synthesis of nickel cobalt oxide nanowires directly on Ni foam as the photocathode electrode. Under light illumination, the areal capacitance reaches 1039 mF cm⁻² at 5 mA cm⁻², significantly higher than the 700 mF cm⁻² observed in the dark. Furthermore, the electrode exhibits excellent cycling stability, maintaining over 85% capacitance retention over 10 000 cycles even at a high current density of 50 mA cm⁻². For real-life demonstration, an asymmetric device was constructed with AC as the anode and NiCo₂O₄@NF NWs as the photoactive cathode, achieving a stable cell voltage of 1.2 V and excellent durability. DFT + U calculations were used to reveal the electronic band structure of the NiCo₂O₃ and Co₃O₄ crystal structures. The substitution of Ni atoms in Co₃O₄ resulted in a reduced band gap energy and an increased magnetic moment. This change reflects in photon absorption and electrical conductivity, thereby significantly enhances the capacitance of NiCo₂O₄. This development offers us various opportunities for storing large amounts of solar energy efficiently.

2. Experimentation

2.1. Chemicals

Cobalt nitrate hexahydrate (Co(NO₃)₂·6H₂O) and nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O) were supplied by Thermo Fisher Scientific. Urea, poly(vinyl) alcohol (PVA), acetone, isopropyl alcohol (IPA), N-methyl-2-pyrrolidone (NMP), and polyvinylidene fluoride (PVDF) were procured from Sigma-Aldrich. Potassium hydroxide (KOH) was provided by MERCK, ammonium fluoride (NH₄F) by Finar Limited, nickel foam (1 mm thickness) by Global Nanotech, and AC by Sainergy Fuel Cell India. All chemicals and reagents were of analytical grade and used without further purification. Double deionized (DI) water was obtained using a Sartorius-mini plus UV system and utilized throughout the experiments.

2.2. Preparation of NiCo₂O₄@NF NWs

The NiCo₂O₄@NF NWs were synthesized via an in situ hydrothermal growth reaction,³⁴ as presented in Scheme 1. Prior to synthesis, the Ni foam was pre-treated by sonicating in 2 M HCl for 15 min, rinsed with DI water, and dried at 60 °C for 3 h in a vacuum oven. In a typical synthesis, Co(NO₃)₂·6H₂O (270 mg), Ni(NO₃)₂·6H₂O (130 mg), urea (100 mg), and NH₄F (120 mg) were dissolved in 50 mL of DI water and stirred for 0.5 h until a clear pink solution was obtained. The solution, along with pre-treated Ni foam (3 × 2 cm²), was transferred to a 100 mL Teflon-lined stainless steel autoclave and maintained at 100 °C for 24 h. After natural cooling to room temperature, the pink-coloured Ni foam loaded with the metal precursors was washed thoroughly with DI water and ethanol, dried in a vacuum oven at 80 °C overnight, and then calcined in air at 500 °C for 2 h. For a comparative study, Co₃O₄@NF NWs were also prepared under the same conditions. The mass loading of NiCo₂O₄@NF NWs and Co₃O₄@NF NWs was 1.95 and 1.8 mg cm⁻², respectively.

Scheme 1 Schematic illustration for the synthesis of NiCo₂O₄@NF NWs.

2.3. Characterization

The synthesized NiCo₂O₄ NWs on Ni foam were characterized using X-ray diffraction (Rigaku Smart Lab X-ray diffractometer) to determine their crystalline structure and by X-ray photoelectron spectroscopy (Thermo Fisher K-Alpha) for chemical composition analysis. Field emission scanning electron microscopy (FESEM, TESCAN-MIRA 3), energy dispersive X-ray spectroscopy (EDS, Quantax 200) and transmission electron microscopy (TEM, Thermo Fisher Talos F200 S) were used to study morphology and elemental distribution. Optical behavior was examined using a UV-vis spectrophotometer (Shimadzu UV-3600 Plus).

2.4. Electrochemical and photo-electrochemical measurements

Electrochemical measurements were performed using a Gamry (1010E) workstation. The as-synthesized NiCo₂O₄@NF NWs served as the working electrode in a three-electrode system, with Hg/HgO, and platinum coil as the reference and counter electrodes respectively, all in 1 M KOH aqueous solution. Illumination was provided by an AM 1.5 G solar simulator (ABET Technologies 10 500) with a 150 W Xenon arc lamp (ozone-free), delivering 1 sun (intensity: 100 mW cm⁻²) over a 35 mm diameter field. A two-electrode asymmetric supercapacitor was assembled with AC@NF as the anode, NiCo₂O₄@NF NWs as the cathode, and Whatman glass microfiber (GF/D) as the separator, soaked in a PVA + 3 M KOH gel electrolyte (10 min immersion). The entire device was wrapped with transparent adhesive tape to minimize electrolyte evaporation.

The areal capacitance from the CV and GCD curves is calculated by utilizing the following equations:^35,36

where ∫i(V) dV is the area enclosed by the cyclic voltammetry curve, S (cm²) is the active surface area of the electrode immersed in the electrolyte, v (V s⁻¹) is the scan rate, ΔV(V) is the stable potential window, I(V) (A) is the constant current applied to the GCD curve, and Δt (s) is the discharge time in the GCD curve.

2.5. Computational details

Density functional theory calculations were performed utulizing the Quantum-Espresso code (Version 6.8)³⁷ employing a plane waves basis set in the pseudopotential framework by supplying ultrasoft pseudopotentials with the generalised gradient approximation (GGA)³⁸ in the parametrization of Perdew, Burke, and Ernzerhof (PBE)³⁹ for the exchange-correlation functional. Additionally, the Hubbard correction (U parameter) was applied to eliminate self-interaction effects in the localized Co and Ni electrons of the d-orbital in tricobalt tetraoxide Co₃O₄ with a cubic normal spinel structure and cobalt nickel oxide (Co₂NiO₄). The Hubbard parameters were set as U_eff Co = 4.4 ⁴⁰ and U_Ni = 6.2 eV.⁴¹ The ultrasoft pseudo potentials were employed, and the valence states of [Ar] 3d⁷,4s², [He] 2s², 2p⁴ and [Ar] 3d¹⁰,4s² were considered for Co, O and Ni, respectively. DFT + U⁴² has been implemented in Quantum ESPRESSO. The core concept of DFT + U is to use a Hubbard-like correction to take into consideration the strong on-site Coulomb interactions of localised electrons that are not well captured by normal LDA or GGA.⁴³ Therefore, the total energy of DFT + U is:where η_μ is the projection on the atomic shell, U_μ is the U value corresponding to the atomic shell, and the energy term is zero for fully occupied or unoccupied shells and positive for slightly occupied shells. This can be interpreted as driving the on-site occupancy matrix towards impotency, or either completely occupied or totally unoccupied levels, by adding a penalty functional to the DFT total energy expression.^37,42

In this study, we used an optimized energy cut-off of 952 eV and a 7 × 7 × 7 Monkhorst–Pack grid to sample the Brillouin zone for Co₃O₄ and NiCo₂O₄. The optimization process used convergence thresholds of 10⁻⁶ eV atom⁻¹ for total energy, 10⁻³ Å for maximum displacement, 0.03 eV Å⁻¹ for maximum force, and 0.05 GPa for stress in all calculations. It is important to note that this is not an externally applied stress but a convergence criterion for optimization. Crystallographic information of Co₃O₄ and NiCo₂O₄ structures and their experimental cell parameters are tabulated in Table S1 (SI).

3 Results and discussion

3.1. Material characterization

3.1.1. Structural and optical analyses. Fig. 1(a) displays the XRD patterns of the NiCo₂O₄@NF NWs and Co₃O₄@NF NWs, confirming their crystalline nature. They both show diffraction peaks with 2θ values of 18.8°, 30.9°, 36.4°, 44.3°, 58.7°, and 64.4°, which are assigned to the diffraction planes (111), (220), (311), (400), (511), and (440), respectively. The aforementioned peaks are characteristic of the cubic spinel structure in which the metallic cations (Ni and Co) occupy both octahedral and tetrahedral sites, while oxygen ions are located in the remaining tetrahedral positions.⁴⁴ A slight shift toward lower 2θ values is observed for the NiCo₂O₄ sample compared to Co₃O₄. This shift is attributed to the substitution of Ni²⁺ for Co²⁺, which results in an increased d-spacing due to differences in ionic radii. The crystallite size, calculated using eqn S1 (SI), is approximately 14 nm. This nanoscale size is advantageous as it yields a high surface-to-volume ratio and a greater number of electrochemically active sites, which are beneficial for energy storage applications.⁴⁵

Fig. 1 (a) XRD pattern and (b) UV-vis spectra (inset: Tauc plot) of the NiCo₂O₄@NF NWs and Co₃O₄@NF NWs, respectively. (c) XPS survey scan of NiCo₂O₄@NF NWs and (d–f) high-resolution fine-fitted spectra for Ni 2p, Co 2p, and O 1s.

Fig. 1(b) (with the inset showing the Tauc plot) presents the UV-vis absorption spectra of the samples. Using the Tauc relation (eqn S2, SI),^46,47 the direct band gap is determined to be 1.67 eV for NiCo₂O₄@NF NWs and 1.97 eV for Co₃O₄@NF NWs in the visible portion. The lower band gap of NiCo₂O₄@NF NWs suggests enhanced absorption of the solar spectrum. This is particularly significant for photo-assisted energy conversion as the photogenerated electron–hole pairs can promote additional redox reactions. Furthermore, the p-type semiconductor nature of NiCo₂O₄, combined with its reduced band gap, makes it a more suitable candidate for energy conversion compared to Co₃O₄.⁴⁸ Theoretical observations also support that substituting Ni²⁺ for Co²⁺ results in a reduced band gap.

3.1.2. Compositional and morphological analyses. To investigate the chemical compositions and valence states of NiCo₂O₄@NF NWs, X-ray photoelectron spectroscopy (XPS) has been employed, as shown in Fig. 1(c–e). The existence of elements Ni, Co, O, and C is confirmed by strong peaks with binding energies of 850, 775, 527, and 278 eV in the XPS survey scan. Of these, the C element mostly occurs from organics that are adsorbed on the sample surface. This is a common occurrence that is always associated with XPS measurements. The chemical states were further investigated in detail using high-revolution XPS. As shown in Fig. 1(d) and (e), a Gaussian fitting approach yields a well-fitting of Ni 2p and Co 2p spectra for NiCo₂O₄@NF NWs with two spin–orbit doublets and two shakeup satellite peaks. Ni³⁺ is attributed to the fine-fitted peaks at 855.1 and 872.7 eV, whereas Ni²⁺ is assigned to the ones at 853.4 and 870.9 eV. The binding energies corresponding to the two satellite peaks are 879.1 and 860.8 eV, respectively. Moreover, there are two spin–orbit doublets in the Co 2p spectrum, corresponding to Co²⁺ and Co³⁺ at 781.1, 779.1, 796.1, and 794.2 eV, as well as two shakeup satellite peaks at 788.5 and 804.8 eV.⁴⁹ The deconvolution of the O 1s spectrum reveals three distinct components: a peak at 527.5 eV that is assigned to metal–oxygen bonds (O1), a peak at 531.1 eV attributed to defect sites or regions with poor oxygen coordination (O2), and a peak at 531.9 eV corresponding to adsorbed water molecules or hydroxyl groups (O3). The Gaussian fitting of the C 1s spectrum is provided in Fig. S1 (SI).⁵⁰ These findings confirm the existence of mixed oxidation states, specifically Ni²⁺/Ni³⁺ and Co²⁺/Co³⁺, which are essential for pseudocapacitive behavior, as well as oxygen vacancies that facilitate enhanced ion adsorption. The above observations confirm the existence of the NiCo₂O₄@NF NWs phase and are well in agreement with the results of the XRD pattern illustrated in Fig. 1(a).

Furthermore, the as-prepared NiCo₂O₄@NF NWs and Co₃O₄@NF NWs were characterized by field emission scanning electron microscopy (FESEM) to confirm the morphology and structure of the nanostructures. Fig. 2(a–d) show low and high-resolution FESEM images of Co₃O₄@NF NWs and NiCo₂O₄@NF NWs. The images confirm that the Ni foam is uniformly covered with one-dimensional nanowire arrays. The uniformity and dense packing of these nanowires enhance the active surface area, thereby promoting efficient ion diffusion during electrochemical cycling. The cross-sectional image (Fig. S2, SI) indicates that the nanowires have an average length of ∼3 μm.

Fig. 2 Low and high-resolution FESEM images of (a and b) Co₃O₄@NF NWs and (c and d) NiCo₂O₄@NF NWs, respectively. (e) TEM image, (f) HRTEM image, (g) elemental mapping, and (h) SAED pattern of NiCo₂O₄@NF NWs.

TEM images (Fig. 2(e) and S3, SI) further validate the nanowire morphology. High-resolution TEM (Fig. 2(f)) shows clear lattice fringes with an interplanar spacing of 0.29 nm (calculated using eqn S3), which corresponds precisely to the (220) diffraction plane observed in the XRD pattern. Elemental mapping (Fig. 2(g)) confirms a homogeneous distribution of Ni, Co, and O elements. The scanning transmission electron microscopy energy-dispersive spectroscopy (STEM-EDS) spectrum and atomic percentage of the NiCo₂O₄@NF NWs in Fig. S4 (SI) confirm the presence of Co, Ni, and O elements. The selected area electron diffraction (SAED) spectrum shown in Fig. 2(h) reveals concentric ring-like multicircles, showing that NiCo₂O₄@NF NWs have a typical polycrystalline structure. The typical diffraction planes obtained from the SAED spectrum are (220), (311), (400), and (440) which are in good agreement with the XRD results.

3.2. Electrochemical study of NiCo₂O₄@NF NWs

The electrochemical performance of Co₃O₄@NF NWs and NiCo₂O₄@NF NWs was evaluated to assess their potential for energy storage applications. A three-electrode system consists of a working electrode (Co₃O₄@NF NWs or NiCo₂O₄@NF NWs), Pt coil, Hg/HgO as a counter electrode, and reference electrode. A schematic representation of this three-electrode setup, along with the light source used for photo-assisted measurements, is shown in Fig. 3(a). The CV curves of both electrodes were recorded at scan rates ranging from 1 to 100 mV s⁻¹ under dark and light conditions, with the corresponding CV curves provided in Fig. S5(a) and (b), S6(a) and (b) (SI), respectively. At a scan rate of 10 mV s⁻¹ under dark conditions, the CV curves for Co₃O₄@NF NWs and NiCo₂O₄@NF NWs (displayed as dotted curves in Fig. 3(b)) reveal that the NiCo₂O₄@NF NWs, represented in pink, exhibit a higher integrated area, indicating superior performance. This enhancement is attributed to the synergistic effect between Ni and Co ions in the spinel structure of NiCo₂O₄@NF NWs, which facilitates improved charge transport and electrochemical activity.^34,51 Upon illumination, the CV curves (solid lines) exhibit an increased area compared to those recorded in the dark, clearly indicating that the active material responds to incident light. The observed performance enhancement under illumination is solely due to the photogenerated electron-ion synergistic storage mechanism, rather than a conventional temperature effect.⁵² A control experiment using Ni foam at a scan rate of 50 mV s⁻¹ under both dark and light conditions shows negligible change, suggesting that the substrate plays a minimal role in performance enhancement upon irradiation.

Fig. 3 The electrochemical performance of the three-electrode system of the Co₃O₄@NF NWs and NiCo₂O₄@NF NWs in 1 M KOH under dark and light conditions. (a) Schematic illustration of the photo charging mechanism in a three-electrode system of NiCo₂O₄@NF NWs electrodes. (b) CV responses of the Co₃O₄@NF NWs (green) and NiCo₂O₄@NF NWs (pink) at a scan rate of 10 mV s⁻¹ under dark (dotted) and light (solid) conditions (inset: CV response of Ni foam at 50 mV s⁻¹). (c) GCD curves of the Co₃O₄@NF NWs and NiCo₂O₄@NF NWs (pink) at a constant current density of 5 mA cm⁻² under dark (dotted) and light (solid) conditions. The areal capacitance of the (d) Co₃O₄@NF NWs and (e) NiCo₂O₄@NF NWs at varying current densities under dark (black) and light (yellow) conditions. (f) Percentage increment in the capacitance of Co₃O₄@NF NWs (green) and NiCo₂O₄@NF NWs (pink) at different current densities. (g) Nyquist spectra at the OCP of NiCo₂O₄@NF NWs under dark (black) and light (yellow) conditions (inset: equivalent circuit). (h) Cycling stability of the NiCo₂O₄@NF NWs at a constant current density of 50 mA cm⁻² as a function of the number of cycles (inset: FESEM images before and after stability testing). (i) Ragone plot of Co₃O₄@NF NWs and NiCo₂O₄@NF NWs under dark (black) and light (yellow) conditions, respectively.

GCD curves recorded at current densities ranging from 5 to 50 mA cm⁻² for Co₃O₄@NF NWs and NiCo₂O₄@NF NWs are presented in Fig. S5(c) and (d), S6(c) and (d) (SI), respectively. For a direct comparison, the GCD curves at a constant current density of 5 mA cm⁻² are shown in Fig. 3(c), which confirms that NiCo₂O₄@NF NWs exhibit better performance than Co₃O₄@NF NWs. Both illuminated (solid) and dark (dotted) GCD curves reveal that the discharge time of the active material increases upon light illumination, which is consistent with the enhanced photon absorption observed in the UV-vis spectroscopic measurements. The areal capacitance at different current densities was calculated using eqn (2) and is depicted in the bar diagrams of Fig. 3(d) and (e) for Co₃O₄@NF NWs and NiCo₂O₄@NF NWs, respectively. Under dark conditions, the maximum areal capacitance values for Co₃O₄@NF NWs and NiCo₂O₄@NF NWs are 375 and 570 mF cm⁻², respectively, at a current density of 15 mA cm⁻². Fig. 3(f) shows that upon light illumination, the areal capacitance of NiCo₂O₄@NF NWs increases from 570 to 880 mF cm⁻² and that of Co₃O₄@NF NWs increases from 375 to 485 mF cm⁻², as summarized in Table S2 (SI). The percentage increment in areal capacitance, calculated using eqn S6 (SI), is 54% for NiCo₂O₄@NF NWs and 29% for Co₃O₄@NF NWs, with the percentage increment further increasing at higher current densities. This improvement is attributed to the lower optical band gap and higher electronic conductivity of NiCo₂O₄@NF NWs, which result in higher light absorption and lower charge recombination.⁵³ At a current density of 50 mA cm⁻², the percentage increment in capacitance reaches 66% for NiCo₂O₄@NF NWs, compared to 56% for Co₃O₄@NF NWs (Table S3, SI). These improvements in areal capacitance surpass those reported in previous studies.

To further elucidate the charge transfer mechanism at the interface between the active material and the electrolyte, EIS measurements were carried out at open-circuit potential (OCP). The Nyquist and Bode plots at different applied potentials for Co₃O₄@NF NWs and NiCo₂O₄@NF NWs are shown in Fig. S7(a–d) and S8(a–d) (SI), respectively, under both dark and illuminated conditions. A direct comparison of the Nyquist spectra under dark conditions is provided in Fig. S9(b) (SI), where the high-frequency region exhibits a smaller semicircular radius for NiCo₂O₄@NF NWs than for Co₃O₄@NF NWs. This observation corresponds to a lower charge transfer resistance (R_ct) and hence, superior charge transfer characteristics.⁵⁴ Similarly, the Nyquist plots of Co₃O₄@NF NWs and NiCo₂O₄@NF NWs under dark and illuminated conditions are presented in Fig. S9(a) (SI) and Fig. 3(g), respectively. The absence of an ideal double-layer behavior suggests that the supercapacitor can be modeled using a constant phase element (CPE), solution resistance (R_s), R_ct, surface resistance (R₂) and a Warburg diffusion behavior (W_s).^55–57 After fitting the EIS data with the equivalent circuit shown in the inset of Fig. 3(g) and S9(a) (SI), the fitted parameters are tabulated in Table S4 (SI). Upon light illumination, the R_s values remain nearly constant in Co₃O₄@NF NWs and NiCo₂O₄@NF NWs, however, the R_ct value decreases significantly, reflecting that the photogenerated electron–hole pairs enhance charge transfer at the interface. In addition, Fig. S9(c) and (d) (SI) shows the comparison of Bode plots of Co₃O₄@NF NWs and NiCo₂O₄@NF NWs under dark and illuminated conditions, where the lower total impedance and higher phase angles at lower frequencies under light indicate an increased surface-capacitive contribution.

The cycling stability of NiCo₂O₄@NF NWs was evaluated at a current density of 50 mA cm⁻² over 10 000 cycles, as shown in Fig. 3(h). The electrode exhibits excellent stability with a capacitance retention of 85%. FESEM images of the NiCo₂O₄@NF NWs before and after cycling (inset of Fig. 3(h) and S10(a–d) in the SI) show that the free-standing NW structure remains largely intact. The values for the energy and power densities at different current were calculated using eqn S7 and S8 (SI), and the Ragone plot in Fig. 3(i) illustrates that NiCo₂O₄@NF NWs possess higher energy and power densities than Co₃O₄@NF NWs. These values, which are also tabulated in Tables S5 and S6 (SI), reach 52 μWh cm⁻² and 1500 μW cm⁻², respectively, for NiCo₂O₄@NF NWs under light illumination.

For further details, the insights into the charge storage method employed by NiCo₂O₄@NF NWs, the Dunn power relationship is utilized. The total current I(V) is quantitatively deconvoluted into contributions from the diffusion-controlled faradaic current and the surface-capacitive current by utilizing the Dunn power empirical formula:⁵⁸

I(V) = av^b (4)

I(V) = k₁v + k₂v^0.5 (5)

where b offers qualitative insights into the functioning mechanism, and k₁v is capacitive (b = 1), and k₂v^0.5 is the diffusion-controlled (b = 0.5) contributions. The slope (k₁) and intercept (k₂) of the graph of I(V)/v^0.5vs. v^0.5 were used to ascertain the relative contributions from capacitive and diffusion-controlled processes.^35,59Fig. 4(a) and (b) display the CV curves of NiCo₂O₄@NF NWs, deconvoluted into capacitive and diffusion-controlled contributions at a scan rate of 10 mV s⁻¹ under dark (Fig. 4(a)) and illuminated (Fig. 4(b)) conditions. Under dark conditions, the surface-capacitive contribution is 36%, which increases to 43% upon light illumination. This enhancement indicates that the incident light facilitates the electron–hole pair generation, with the holes facilitating the adsorption of electrolyte anions at the electrode surface.⁶⁰Fig. 4(c) and (d) further illustrate the percentage contributions from capacitive and diffusive processes at various scan rates ranging from 1 to 100 mV s⁻¹. At lower scan rates (<50 mV s⁻¹), the diffusive processes dominate, whereas, at scan rates >50 mV s⁻¹, the capacitive processes become more significant, as evidenced by the increased capacitive current contribution (Table S7, SI).^61,62 Overall, the energy storage mechanism is primarily governed by surface-controlled faradaic (pseudocapacitive) processes, with only a minor contribution from diffusion-controlled phenomena, particularly under light illumination.

Fig. 4 Comparison of the capacitive contribution and diffusion-controlled contribution for NiCo₂O₄@NF NWs. CV curves with capacitive and diffusion-controlled contributions at a scan rate of 10 mV s⁻¹ under (a) dark (black) and (b) light (yellow) conditions. Capacitive and diffusive contributions at different scan rates under (c) dark and (d) light conditions.

3.3. Electrochemical study of the asymmetric device

To further explore the potential of NiCo₂O₄@NF NWs obtained by the in situ growth process for practical applications, an asymmetric supercapacitor (AC@NF//NiCo₂O₄@NF NWs) was developed using an AC electrode as the anode, NiCo₂O₄@NF NWs as the cathode, and PVA-KOH as the gel electrolyte. The optical image and the experimental setup for the asymmetric device are displayed in Fig. S11 (SI). Fig. 5(a–b) show the design of the asymmetric device along with their corresponding CV responses recorded at a scan rate of 10 mV s⁻¹. The CV of the AC electrode exhibits a conventional rectangular shape with no redox peaks, indicating typical EDLC characteristics,⁶³ whereas the CV of the NiCo₂O₄@NF NWs exhibits distinct redox peaks that suggest a pseudocapacitive nature. As observed in Fig. 5(b), the potential windows for the AC and NiCo₂O₄@NF NWs electrodes are −1 V⁶⁴ and 0.6 V, respectively, with the overall device performance being limited by the viable windows imposed by electrocatalytic processes. Consequently, the assembly of AC and NiCo₂O₄@NF NWs in this study achieves a stable cell voltage of up to 1.2 V, which is higher than the typical 0.8–1.0 V observed in traditional AC-based symmetric supercapacitors.⁵⁴

Fig. 5 Electrochemical performance of the AC@NF//NiCo₂O₄@NF NWs asymmetric device under dark and light conditions. (a) Schematic representation of the AC@NF//NiCo₂O₄@NF NWs asymmetric device. (b) CV curves of AC@NF (anode) and NiCo₂O₄@NF NWs (cathode) at a constant scan rate of 10 mV s⁻¹. (c) Variation in CV curves by changing the intensity of the light source at a scan rate of 100 mV s⁻¹. (d) CV curves at a scan rate of 50 mV s⁻¹ under dark and light conditions. (e) GCD curves at a constant current density of 2 mA cm⁻² under dark and light conditions. (f) Nyquist plot at the OCP under dark (black) and light (yellow) conditions (inset: equivalent circuit). The areal capacitance at varying (g) scan rates and (h) current densities under dark (black) and light (yellow) conditions. (i) Capacitance retention and coulombic efficiency under dark cycles at a constant current density of 20 mA cm⁻² as a function of the number of cycles (inset: cycling stability under light for 1k cycles).

Fig. S12(a) and (b) (SI) depict the CV responses of the optimized AC@NF//NiCo₂O₄@NF NWs asymmetric supercapacitor at different scan rates (10–150 mV s⁻¹) under both dark and illuminated conditions, revealing that both pseudocapacitive and EDLC contributions are present at all scan rates. When the light intensity is increased from dark conditions to 2 suns (intensity: 200 mW cm⁻²), the CV area at a scan rate of 100 mV s⁻¹ is notably enhanced, as shown in Fig. 5(c). The areal capacitance at different light intensities was calculated using eqn S4 (SI) and is illustrated as a bar graph in Fig. S13(b) (SI), which demonstrates a 57% enhancement in areal capacitance at an intensity of 2 suns (Table S8, SI). This result indicates that the material exhibits excellent light absorption capabilities across varying intensities. Furthermore, the asymmetric device (AC@NF//NiCo₂O₄@NF NWs) demonstrates a higher areal capacitance under persistent illumination, as evidenced by the CV curve recorded at 50 mV s⁻¹ in Fig. 5(d). Additionally, the CV responses measured at a fast scan rate of 150 mV s⁻¹ (Fig. S12(a) and (b), SI) under both dark and light conditions show negligible differences, which demonstrates the advantageous ion adsorption and transfer capabilities of the device.

Photo-assisted GCD experiments were performed on the AC@NF//NiCo₂O₄@NF NWs asymmetric device at various current densities, as depicted in Fig. S12(c) and (d) (SI). Fig. 5(e) presents the GCD plot of the device at a current density of 2 mA cm⁻² under dark and illuminated conditions, which clearly demonstrates that the charge–discharge periods are longer under photoirradiation. Furthermore, the bar graphs in Fig. 5(g) and (h) show the areal capacitance measured under dark (black) and light (yellow) conditions at scan rates ranging from 10 to 150 mV s⁻¹ and at current densities from 1 to 5 mA cm⁻². These observations demonstrate a notable increase; specifically, the areal capacitance increases from 98 to 114 mF cm⁻² at a scan rate of 10 mV s⁻¹ (Table S9, SI) and from 57 to 70 mF cm⁻² at a current density of 1 mA cm⁻² (Table S10, SI). This improvement is ascribed to the enhanced light absorption by the NiCo₂O₄@NF NWs photoelectrode. The rate capability of the asymmetric device under dark and light conditions was examined by performing GCD measurements at current densities ranging from 1 to 5 mA cm⁻², as shown in Fig. S14 (SI). The fabricated device initially delivers an areal capacitance of over 60.4 and 82.5 mF cm⁻² in the dark and under light illumination, respectively. As the current density increases, the capacitance values gradually decrease due to the limited time available for ion diffusion. However, when the current density is reduced back to 1 mA cm⁻², the areal capacitance recovers to 60 and 82 mF cm⁻², indicating the strong electrochemical reversibility of the NiCo₂O₄@NF NWs photoelectrode.

Additionally, EIS was performed to get insights into the ion diffusion and charge transport kinetics of the asymmetric device. Fig. 5(f) shows the Nyquist plot (with the inset illustrating the equivalent Randles circuit) of the AC//NiCo₂O₄@NF NWs device in the dark and under light illumination. It can be observed that both curves exhibit vertical lines and similar arcs in the low-frequency and high-frequency regions, respectively. The high-frequency intercept on the real axis represents the electrode's ohmic resistance (R_s), which is influenced by solution resistance and the contact resistance between the electrodes and the electrolyte.⁶⁵ The R_ct is computed from the semicircle's diameter in the high-frequency zone, while the low-frequency slope reflects the electrolyte's Warburg impedance or diffusion coefficient. The resistance to leakage currents is indicated by the parallel resistance (R_p), which remains high and increases with applied voltage.^66,67 The absence of an ideal double-layer behavior at the cathode suggests that the supercapacitor can be modeled using a CPE and R_p. After fitting the EIS data with the equivalent circuit shown in the inset of Fig. 5(f), the fitted parameters are tabulated in Table S11 (SI). It is evident that the ohmic resistance (R_s) remains nearly the same under both dark and light conditions; however, under illumination, the device exhibits a higher Q value and a lower R_ct, decreasing from 14.20 Ω to 12.15 Ω. This reduction in R_ct is attributed to the photo-generated electrons enhancing conductivity, thereby facilitating faster charge transfer.⁶⁸ Moreover, the Bode plots in Fig. S13(a) (SI) demonstrate a decrease in the impedance value from 42 Ω to 36 Ω under light illumination.

The long-term cycling stability of the asymmetric device is critical for its practical application. Fig. 5(i) displays the coulombic efficiency and capacitance retention under dark conditions as a function of the number of cycles. At a constant current density of 20 mA cm⁻², the device achieves robust charge–discharge stability with 94% capacitance retention and 100% coulombic efficiency (eqn S9) over 10 000 cycles. Under continuous light exposure, the device maintains 88% of its areal capacitance after 1000 cycles, which is comparable to the reported values in the literature (Table S14, SI). The slight decrease in capacitance retention under light illumination may be attributed to photo-induced interfacial stress and localized heating during prolonged light exposure. These results suggest that the active material is highly robust and holds the potential to revolutionize photo-rechargeable supercapacitors.

To further elucidate the charge storage mechanism of the photo-assisted supercapacitor, a schematic illustration of the charging and discharging states under dark and light conditions is provided in Fig. 6(a)–(d). Fig. 6(a) depicts the chemical equilibrium state of the fabricated asymmetric device, where the ions reside in the electrolyte. During charging under dark conditions, as illustrated in Fig. 6(b), positive and negative ions migrate toward their respective electrode surfaces and become adsorbed, as evidenced by the rectilinear CV profile shown in Fig. 5(d). When the device is illuminated, the generation of electron–hole pairs at the photo-electrode causes electrons to move through the external circuit while the holes accumulate at the electrode surface, promoting the adsorption of additional ions (Fig. 6(c)). This increases the CV area, as seen in Fig. 5(d), and leads to an overall enhancement in the capacitance of the asymmetric device.⁶⁹Fig. 6(d) then depicts the discharging state under light illumination, demonstrating that an equilibrium is reached.

Fig. 6 A schematic illustration of the working mechanism of a photo-assisted supercapacitor: (a) chemical equilibrium, (b) charged in the dark, (c) charged under light illumination, and (d) discharged under light illumination.

3.4. Theoretical insights

Fig. 7(a)–(b) show the optimized unit cell structure for Co₃O₄ and NiCo₂O₄. The crystalline structure of Co₃O₄ is that of a face-centered cubic (FCC) spinel oxide belonging to the space group Fd3̄m [227], with lattice parameters a = b = c = 8.071 Å. In this cubic normal spinel structure, Co₃O₄ exhibits antiferromagnetic properties, and the metal ions are distributed in two distinct coordination environments: Co³⁺ ions are coordinated octahedrally, and Co²⁺ ions are coordinated tetrahedrally (Td).^70,71

Fig. 7 Optimized structure of (a) Co₃O₄ and (b) NiCo₂O₄. (c–e) Optimized structure, calculated density of states, and electronic band diagram (spin up and spin down) for Co₃O₄. (f–h) Optimized structure, calculated density of states, and electronic band diagram (spin up and spin down) for NiCo₂O₄.

In the nickel-doped systems shown in Fig. 7(b), one cobalt atom in the Co₃O₄ lattice is substituted with a nickel atom, resulting in the formation of NiCo₂O₄. In this doped system, nickel adopts a 2+ oxidation state while the cobalt being replaced is in the 3+ state. To achieve overall charge neutrality, unusual local electronic and structural adjustments occur, which are the origin of several interesting properties of NiCo₂O₄, including its high catalytic activity⁷² and ferromagnetic behavior.⁴¹ NiCo₂O₄ is therefore considered a mixed-valence oxide, where nickel exclusively occupies octahedral sites, and the remaining cobalt ions are distributed between octahedral and tetrahedral sites. For comparison, the optimized lattice constant obtained via the DFT + U approach for Co₃O₄ is 8.086 Å, while that for NiCo₂O₄ is 8.191 Å, a result that is in good agreement with experimental observations. These lattice constants, as determined by different approaches, are presented in Table S13 (SI).

The density of states (DOS) and electronic band structure of both Co₃O₄ and NiCo₂O₄ were calculated at high-symmetry points in the Brillouin zone, and the results are shown in Fig. 7(c–e) for Co₃O₄ and in Fig. 7(f–h) for NiCo₂O₄. The Co₃O₄ crystal exhibits semiconducting behavior, characterized by an energy gap around the Fermi level, which is consistent with other studies.^73–75 The direct band gap of Co₃O₄ is calculated to be 1.65 eV. In contrast, the spin-up channel of NiCo₂O₄ shows a smaller energy gap of 1.45 eV, indicating that NiCo₂O₄ has a higher light absorption capability than Co₃O₄. This reduction in the band gap is a direct consequence of the substitution of Co with Ni in the Co₃O₄ lattice. Furthermore, while the spin-up channel of NiCo₂O₄ remains semiconducting, the spin-down channel becomes conducting. This observation indicates that Ni doping drives a transition in the electronic character of Co₃O₄, converting it from a semiconductor into a semi-metal for one spin channel. Specifically, only the spin-down electrons associated with the Ni atoms (located at the octahedral sites) and the Co atoms (occupying the tetrahedral sites) cross the Fermi level. This crossing suggests that the presence of Ni not only facilitates electron transfer through exchange interactions but also greatly enhances overall conductivity. The changes induced by Ni doping, which originate from the interplay between Ni and Co, also impart ferromagnetic properties to NiCo₂O₄, as further supported by the data presented in Table S13 (SI).⁷⁶

In summary, the first-principles DFT + U calculations reveal that the substitution of Ni into the Co₃O₄ lattice causes significant changes in lattice parameters and electronic properties. Our results demonstrate that the incorporation of Ni leads to a lower band gap and the emergence of half-metallic behavior in NiCo₂O₄, which in turn enhances its electrical conductivity. These theoretical insights provide a fundamental understanding of the improved photo-assisted performance observed experimentally in NiCo₂O₄-based supercapacitors.

4 Conclusions

In summary, our work demonstrates a highly effective photo-assisted supercapacitor based on in situ grown NiCo₂O₄ NWs on Ni foam. The integration of these binder-free NWs as the photocathode offers significant advantages in terms of both structural integrity and electrochemical performance. Theoretical and experimental studies revealed that the NiCo₂O₄ NWs possess a well-defined cubic spinel structure with a reduced band gap and offer efficient electroactive sites, making them highly accessible for photo-assisted electrochemical energy conversion and storage systems. Electrochemical investigations demonstrated that under light illumination, the NiCo₂O₄@NF NWs exhibit a remarkable 54% increase in areal capacitance, achieving superior charge storage performance compared to their Co₃O₄ counterparts (29%). Additionally, the electrode maintains a high capacitance retention of 85% over 10 000 cycles at a current density of 50 mA cm⁻², attesting to its long-term stability and reliability. To bridge the gap between laboratory-scale performance and practical applications, an asymmetric supercapacitor device was fabricated using AC@NF as the anode and NiCo₂O₄@NF NWs as the cathode. This device delivered robust cycling stability with 94% capacitance retention over 10 000 cycles under dark conditions and 88% retention under continuous light illumination for 1000 cycles. DFT + U analysis verifies that the substitution of Ni atoms into the Co₃O₄ lattice leads to a significant reduction in the band gap and alters the magnetic properties, resulting in increased light absorption and higher electrical conductivity. The emergence of half-metallic behavior, characterized by a conducting spin-down channel alongside a semiconducting spin-up channel, further facilitates efficient charge transfer. These atomic-level modifications are crucial for the superior photo-assisted behavior observed experimentally. The combination of advanced synthesis, thorough experimental characterization, and rigorous theoretical analysis presented in this work demonstrates a viable and promising approach for efficient solar energy storage. These findings not only highlight the potential of NiCo₂O₄@NF NWs in enhancing supercapacitor performance under illumination but also open new avenues for the development of next-generation photo-rechargeable energy storage devices.

Author contributions

A. R. L.: writing – original draft, methodology, data curation, formal analysis, investigation, writing – review & editing, and conceptualization. S. J. and M. S.: software, data curation, formal analysis, investigation, methodology, and writing – original draft. A. S.: writing – review & editing, visualization, validation, supervision, investigation. K. P.: project administration, writing – review & editing, supervision, resources, investigation, visualization, validation, funding acquisition, conceptualization, formal analysis, methodology.

Conflicts of interest

The authors declare that they have no financial interests.

Data availability

The data that support the findings of this study are available from the corresponding author upon reasonable request. The data supporting this article have been included as part of the SI.

Supplementary information is available. See DOI: https://doi.org/10.1039/d5se00700c.

Acknowledgements

A.R.L. acknowledges the research fellowship from the Centre for Nano and Soft Matter Sciences (CeNS), Bengaluru, and CRF-CeNS for providing research facilities. Kavita Pandey sincerely acknowledges funding from the Science and Engineering Research Board (SERB) under project numbers SCP/2022/000943 and CRG/2022/006798. A. S. acknowledges DST, SERB, MoE (MHRD), MeitY, QURP (Government of Karnataka) and the Pratiksha Trust for their generous funding and support.

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