Unleashing potential of novel 2D-Bi₂S₃/1D-SnO₂ heterostructure thin film anodes for light-fostered asymmetric electrochromic supercapacitors

Abstract

In this work, a novel 2D-Bi₂S₃/1D-SnO₂, n–n heterostructure thin film was employed as a pseudocapacitive photoanode for enhanced solar energy utilization, yielding a significant improvement in energy storage performance. The three-electrode system delivered an areal capacitance of 15.22 mF cm⁻² in 1 M Na₂SO₄ electrolyte at 0.2 mA cm⁻² under 1 sun illumination, achieving 33% enhancement compared to dark conditions. In addition, the fabricated Bi₂S₃/SnO₂‖PEDOT:PSS asymmetric photo-assisted electrochromic supercapacitor device exhibited a maximum areal capacitance of 1.78 mF cm⁻² at 0.06 mA cm⁻², which represents a 2.5-fold increase over its performance in the dark (0.70 mF cm⁻² at 0.06 mA cm⁻²). Under illumination, the device also showed an areal energy density (E_a) of 0.8 mWh cm⁻² and areal power density (P_a) of 356 mW cm⁻². The device retained excellent cycling stability, with capacitance retention of 82.2% and 77.2% at 0.2 mA cm⁻² after 1000 GCD cycles under dark and illumination, respectively. Mechanistic investigations revealed that the intercalation/de-intercalation of Na⁺ ions into 2D Bi₂S₃ (Bi₂S₃ + xNa⁺ + xe⁻ ↔ Na_xBi₂S₃) and SO₄²⁻ ions into the PEDOT:PSS chain during the charge–discharge process were facilitated by photon-induced redox activity and efficient charge separation by SnO₂ nanorods (NRs), thereby improving energy storage capability. This study underscores the potential of novel heterostructure design and material combinations for the development of next-generation photo-rechargeable supercapacitors, paving the way for self-powered electronic devices.

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Introduction

The development and deployment of renewable solar energy have garnered significant research attention in the pursuit of sustainable and robust energy alternatives, thereby ensuring environmental stability and long-term energy security.¹ Moreover, there is an urgent need for advanced techniques and compact energy harvesting and storage solutions to meet the growing power requirements of electric and hybrid vehicles (EHVs), public transportation, aerospace, and defence sectors, as well as wearable and portable electronics.² Therefore, photoassisted rechargeable supercapacitors (PSCs), an innovative solar-powered, self-charging system that integrates solar cells and electrochemical energy storage devices into a single compact unit, pave the way for high-power and efficient devices for the next-generation electronic industry. This dual functionality of the device is a fundamental aspect that enhances the viability and performance of solar electrochemical energy storage systems. Currently, various PSC architectures have been reported, wherein supercapacitors are integrated with dye-sensitized solar cells,^3,4 perovskite solar cells,^5,6 quantum-dot-sensitized solar cells,^7,8 and polymer solar cells.^9,10 In these integrated systems, photovoltaic and electrochemical storage components often share a common counter electrode to enable continuous and stable power output under solar irradiation.¹¹ Despite their promising potential, these complex device configurations are hindered by several intrinsic challenges, such as significant ohmic losses, low power conversion efficiency, Schottky barrier formation during the photo-charging process, energy dissipation, and electrolyte leakage.² In this context, the rational design and advancements of semiconducting materials with suitable band gaps and tailored electronic structures that exhibit both photoelectric and capacitive properties are highly desirable.¹² Such materials aid light-induced charge generation under illumination, which accelerates surface redox reactions, enhances specific capacitance and energy density, and improves the overall charge–discharge efficiency.¹³ However, the use of single-component photoelectrodes remains limited due to the rapid recombination of photogenerated electron–hole pairs, which significantly reduces their redox efficiency under illumination.

To date, various strategies have been employed to enhance charge carrier separation in photo-assisted supercapacitors, including elemental doping, interface engineering, nanostructuring, and heterostructure formation.^14,15 Among these, the construction of heterostructures has emerged particularly as an effective strategy, enabling spatial separation of photogenerated charge carriers through the alignment of energy bands between two semiconductors with different band gaps. This configuration facilitates efficient photo-charging and broadens the operating voltage window, a stringent requirement for integrated solar energy conversion and storage systems.¹³ Conventional fabrication of supercapacitor electrodes often involves the use of polymer binders and conductive additives, which can lead to poor electron conductivity due to the low utilization efficiency of active materials, ultimately hindering electrochemical performance. A more viable strategy involves the direct growth of active materials on conductive substrates such as indium-tin oxide (ITO), fluorine-doped tin oxide (FTO), nickel foam, and carbon cloth. This approach not only improves overall conductivity but also enhances the material adhesion over substrates, thereby establishing robust interfacial contact between the active layer and substrate, crucial for efficient charge transport and mechanical stability.¹⁶

Among various binary transition metal oxides, such as TiO₂,¹³ ZnO,^14,15 Fe₂O₃,¹⁷ BiVO₄,^18,19 V₂O₅,²⁰ MnO₂,²⁰etc., SnO₂ has emerged as a promising n-type semiconductor, due to its low cost, non-toxicity, facile synthesis, higher electron mobility (100–200 cm² V⁻¹ s⁻¹),²¹ and remarkable thermal and chemical stability. Additionally, its lower conduction band energy facilitates efficient electron injection from the electrolyte and promotes effective charge separation, thereby enhancing the overall electron transfer efficiency.²² One-dimensional (1D) SnO₂ nanorods (NRs) offer additional advantages such as high surface-to-volume ratio, reduced charge diffusion length, unidirectional electron transport, and improved light trapping capability, making them suitable for integration into self-chargeable supercapacitors for efficient surface charge storage.²³ However, the wide band gap of SnO₂ (3.6–4.0 eV) limits its photo-response to the UV region, resulting in short-lived photogenerated charge carriers, low photocurrent, and sub-optimal solar conversion efficiency, factors that significantly reduce its capacitive performance. To overcome these limitations, the formation of a heterostructure with a narrow band gap semiconductor is essential to extend light absorption into the visible region, enhance photogenerated charge carrier lifetimes, and improve charge separation and transfer dynamics.

Various transition metal sulfides and their heterojunctions have garnered significant attention for photo-enhanced capacitive applications owing to their excellent faradaic redox activity, low electronegativity, high electrical conductivity, and superior ionic diffusivity.²⁴ Among them, bismuth sulfide (Bi₂S₃) has emerged as an excellent choice for solar-driven photoanodic systems, primarily due to its strong light-harvesting capability and intrinsic n-type semiconducting behavior. Bi₂S₃ exhibits a two-dimensional (2D) layered lamellar structure formed by interconnected Bi–S bonds, and possesses several desirable optoelectronic and electrochemical properties, i.e., narrow direct band gap (1.3–1.7 eV) enabling efficient absorption across the visible to NIR region, moderate photoconversion efficiency (5%), high absorption coefficient (10⁵ cm⁻¹), high theoretical specific capacity (232 C g⁻¹), and better electron mobility (200 cm² V⁻¹ s⁻¹). Additionally, its favourable band edge positions promote efficient charge separation and facilitate charge carrier transport, making Bi₂S₃ a highly suitable material for integrated solar energy storage systems.²⁵

To enhance energy storage performance in terms of specific capacitance (C_S), Bi₂S₃ nanostructures have been synthesized via various techniques, for example, Bi₂S₃ nanorods via the solvothermal method (C_S = 270 F g⁻¹ @1 A g⁻¹),²⁶ Bi₂S₃ nanorods/graphene composite by the reflux method (C_S = 290 F g⁻¹ @1 A g⁻¹),²⁷ and Bi₂S₃ by the hydrothermal reduction of Bi₂O₃ through the ion exchange process (C_S = 565 F g⁻¹ @1 A g⁻¹).²⁸ The literature reports that Bi₂S₃ exhibits excellent photocurrent response and is widely acknowledged as an efficient photocatalyst.^25,29 However, the charge separation efficiency of photo-generated carriers in the pristine Bi₂S₃ interface is often limited by intrinsic photo-corrosion and rapid electron–hole recombination.²⁵ To address these drawbacks, the rational design of hetero-structured systems is essential, leveraging the synergistic interactions between the components to enhance the internal electric field, thereby promoting higher carrier concentration and more effective charge separation. In this regard, the construction of heterojunctions is a promising approach to improve the electrochemical energy storage performance. The staggered band alignment in systems facilitates the spatial separation of photo-generated electrons and holes, while the built-in electric field within the depletion region significantly enhances the charge carrier dynamics across the semiconductor interfaces.³⁰ Therefore, a heterostructure thin film comprising 2D Bi₂S₃ decorated on 1D SnO₂ NRs presents a promising approach for achieving enhanced solar-driven supercapacitor performance, while enabling deeper insights into the underlying mechanism of energy conversion and storage (Fig. S1). To the best of our knowledge, the application of Bi₂S₃@SnO₂ thin film photoanodes in photoassisted supercapacitors has not yet been explored in the existing literature.

Besides the photoanode, the counter electrode (CE) also plays critical role in influencing the overall photovoltaic response of the solar supercapacitor. An ideal CE should possess high electrical conductivity and large surface area to efficiently facilitate the electrocatalytic reduction of oxidized species in the electrolyte. Meanwhile, electrochromism, a phenomenon wherein materials exhibit a reversible colour change upon applying a small voltage, typically offers dual functionality by enabling both energy storage and visual colour modulation, and real-time indication of the charge–discharge state.³¹ Conducting polymers have been extensively investigated for electrochromic supercapacitors.³² The correlation between chromic behavior and electrochemical energy storage performance is significant for photo-assisted supercapacitors, especially when electrochromic materials are utilized as a counter electrode.

Considering the above facts, we have developed a 2D-Bi₂S₃/1D-SnO₂, n–n type-II heterostructure thin film photoanode for photoassisted supercapacitors. The staggered band offset between SnO₂ and Bi₂S₃ not only enhances incident light absorption but also promotes efficient separation of photoexcited carriers. The 2D-Bi₂S₃/1D-SnO₂ (BS16) heterostructure photoanode exhibited a higher areal capacitance of 15.22 mF cm⁻² at 0.2 mA cm⁻² in 1 M Na₂SO₄ with 33% photo-efficiency under illumination. A Bi₂S₃/SnO₂‖PEDOT:PSS asymmetric electrochromic supercapacitor achieved a C_s of 1.78 mF cm⁻² at 0.06 mA cm⁻² under light and a 2.5-fold increment in the dark with areal energy density (E_a) and areal power density (P_a) of 0.8 mWh cm⁻² and 356 mW cm⁻², respectively. The device retained 82.2% (with light) and 77.2% (without light) of its areal capacitance at 0.2 mA cm⁻² after 1000 charge–discharge cycles (GCD) cycles. The enhanced performance is attributed to light-assisted Na⁺ intercalation into Bi₂S₃ and efficient charge transport via SnO₂ nanorods, while SO₄²⁻ diffusion in PEDOT:PSS enables electrochromic switching during supercapacitive performance. These findings highlight a promising approach for multifunctional, solar-powered energy storage devices.

Experimental section

Preparation of the SnO₂ nanorod (NR) thin film

Initially, a colloidal seed solution of SnO₂ nanoparticles (NPs) was prepared by refluxing 0.1 M solution of tin(II) chloride dihydrate (SnCl₂·2H₂O, ≥98% purity, Sigma Aldrich) in 10 ml of isopropanol (IPA) at 70 °C for 1 h. The resulting mixture was subsequently aged at room temperature for 12 h to yield a transparent colloidal solution. Prior to coating, FTO-coated glass substrates (ρ = 7 Ω cm², Sigma Aldrich) with dimensions of 1.5 × 2.0 cm² were sequentially cleaned using soap solution, deionized (DI) water, ethanol, acetone, and IPA in an ultrasonic bath (15 min each), followed by drying using a hot air gun. The SnO₂ seed solution was then deposited on FTO substrates over 1.5 × 1.5 cm² area via spin coating in 3 successive cycles at 2500 rpm for 30 s each, which were dried at 90 °C for 10 min and subsequently annealed at 500 °C for 30 min (5 °C min⁻¹) to ensure the formation of a uniform and compact SnO₂ NPs seed layer. Alternatively, for the growth of SnO₂ NRs, a hydrothermal precursor solution was prepared by dissolving 30 mg of tin(IV) chloride pentahydrate (SnCl₄·5H₂O, ≥98% purity, Sigma Aldrich) in a mixture of 7 ml deionized water, 7 ml ethanol and 560 µl HCl (37%, reagent grade), which was stirred for 30 min under ambient conditions. The resulting solution, along with the seeded FTO substrate, was then transferred to a Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at 180 °C for 12 h. Post-synthesis, the as-grown SnO₂ NR thin films were annealed at 500 °C for 30 min (5 °C min⁻¹) to improve crystallinity. The synthesis process for SnO₂ NRs over FTO (SNO) is schematically shown in Fig. S2 (upper panel).

Preparation of the 2D Bi₂S₃@1D SnO₂ heterostructure thin film

Bi₂S₃ nanoflakes (NFs) were grown on an SnO₂ NR thin film through a two-step process involving spin coating followed by hydrothermal sulfurization, as illustrated in Fig. S1 (lower panel). Initially, 0.3 M solution of bismuth nitrate pentahydrate (Bi(NO₃)·5H₂O, 98% purity) was prepared in 10 ml of acetic acid (98%, SRL Chem) and stirred at room temperature for 10 h, followed by aging for 24 h to ensure homogeneity. Subsequently, 80 µl of the prepared Bi precursor solution was spin-coated on the SnO₂ NR thin films in two successive cycles at 2000 rpm for 30 s each and dried at 100 °C for 10 min. The resulting films were then annealed at 400 °C for 1 h with a ramp rate of 2 °C min⁻¹ to obtain Bi₂O₃ NPs decorated SnO₂ NR (BO-SNO) thin film. In the second step, a sulfurization solution was prepared by dissolving 0.1 M Thiourea (CH₄N₂S, 99% purity, SRL Chem) in 15 ml of 1 M ammonia solution (NH₄OH, extra pure, AR grade) under constant stirring at room temperature for 30 min. The previously prepared BO-SNO thin films were immersed in the sulfurization solution inside a Teflon-lined stainless-steel autoclave and subjected to hydrothermal treatment at different temperatures (140, 160, and 180 °C) for 4 h. This process led to the formation of a black-colored Bi₂S₃ layer on SnO₂ NRs, resulting in a 2D-Bi₂S₃@1D-SnO₂ heterostructure thin film, which was subsequently vacuum-dried at 80 °C for 2 h. The samples prepared at 140, 160, and 180 °C were designated as BS14, BS16, and BS18, respectively. For comparative analysis, pristine Bi₂O₃ (BO-FTO) and Bi₂S₃ (B16) thin films on FTO were also prepared under identical conditions at 160 °C.

Fabrication of the asymmetric (FTO/Bi₂S₃@SnO₂‖PEDOT:PSS/FTO) device

An asymmetric supercapacitor device (ASD) was fabricated using Bi₂S₃@SnO₂/FTO as the cathode and the PEDOT:PSS/FTO thin film as the anode. PEDOT:PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate), high conductivity grade, 1%, Sigma Aldrich) was dissolved in IPA at a volume ratio of 1 : 3 under constant stirring for 1 h, which was subsequently filtered through syringe filters (0.45 µm pore size). The filtered PEDOT:PSS solution was then 5 times spin-coated on FTO substrates (1.5 × 1.5 cm²) at 1500 rpm for 30 s per cycle. Subsequently, the film was annealed at 120 °C for 10 min to enhance film adhesion and conductivity. In the two-electrode cell configuration, the charge balance between the cathode and anode was optimized by adjusting the mass loading to ensure charge neutrality. Whatman glass fibre filter paper, pre-soaked overnight in 1 M Na₂SO₄ aqueous electrolyte, was used as a separator sandwiched between the two electrodes during ASD assembly. A thin layer of epoxy adhesive was carefully applied along the periphery of the FTO substrates to secure the cell assembly, ensuring that the adhesive did not interfere with the active electrode area or current collector terminals. Paper clamps were used to apply uniform pressure over the device.

Results and discussion

The XRD patterns and Raman spectra confirm the formation of tetragonal rutile SnO₂, tetragonal β-Bi₂O₃, and orthorhombic α-Bi₂S₃ phases, which are retained in Bi₂S₃@SnO₂. Furthermore, FESEM and EDS analysis reveal the uniform decoration of Bi₂S₃ over SnO₂ NRs, confirming the formation of the 2D-Bi₂S₃@1D-SnO₂ bilayer heterostructure. Additional details of XRD, Raman, FE-SEM, EDS, and elemental mapping analysis are provided in Fig. S3–S5 (SI). Based on these observations, the growth mechanism of the 2D Bi₂S₃@1D SnO₂ bilayer heterostructure is presented in the SI (Fig. S6). The micro-structure and interface of the bilayer heterostructure were investigated using TEM analysis (Fig. 1). A well-resolved, high magnification cross-sectional TEM image confirms the layered deposition of 2D Bi₂S₃ NSs on SnO₂ NRs (∼150 nm long and ∼50 nm diameter) grown on the FTO substrate (Fig. 1(a)), consistent with the cross-sectional FE-SEM images. The selected area electron diffraction (SAED) pattern (Fig. 1(b)), acquired from vertically aligned NRs (yellow marked circle in Fig. 1(a)), shows diffraction spots corresponding to (110), (200), and (211) planes of FTO substrates, whereas (101) and (202) planes are referred to the tetragonal rutile phase of SnO₂ NRs. Furthermore, a low-magnification HRTEM image (Fig. 1(c)) confirms the presence of entangled Bi₂S₃. The HRTEM image taken from the white marked region (Fig. 1(d)) shows a lattice spacing of 0.369, 0.285, and 0.179 nm, attributed to (101), (212), and (211) planes for three distinct NSs, with clearly visible interfacial textural boundaries. In contrast, the HRTEM image from the yellow marked region (Fig. 1(e)) reveals the lattice spacing of 0.334 nm, corresponding to the (301) plane of the orthorhombic Bi₂S₃ crystal structure, in agreement with XRD results. In addition, the Bi₂S₃@SnO₂ heterostructure is further distinguished from EDS spectra (Fig. 1(f)), the high-angle annular dark field image (HAADF) image, and the corresponding EDS elemental mapping images (Fig. 1(g)), which precisely show the uniform distribution of Sn, O, Bi, and S elements, indicating the existence of Bi₂S₃ and SnO₂. Additionally, XPS confirmed the phase purity and chemical states of the Bi₂S₃@SnO₂ heterostructure. Detailed XPS analysis is provided in the SI (Fig. S7).

Fig. 1 (a) TEM image of the Bi₂S₃@SnO₂ bilayer structure, (b) SAED pattern acquired from the yellow circular region marked in (a). (c) TEM image of Bi₂S₃ and HRTEM image acquired over the (d) white and (e) yellow marked region in (c). (f) EDS spectra of the Bi₂S₃@SnO₂ bilayer structure, and the corresponding (g) HAADF image along with elemental mapping images of Sn, O, Bi, and S elements.

The optical properties and band edge positions of SNO, BO-SNO, B16, BS14, BS16, and BS18 were determined by UV-visible spectroscopy (Fig. 2(a)). The bare SnO₂ (SNO) film exhibits an absorption onset near 300 nm in the UV region, with complete transmission in the visible range, where the observed indirect transitions are attributed to defect-induced mid-gap states in non-stoichiometric SnO₂.³³ The Bi₂O₃@SnO₂ (BO-SNO) film showed enhanced optical absorption with distinct onset at 300 nm (corresponding to SnO₂) and 500 nm in the visible region belonging to Bi₂O₃. The observed absorption range upon Bi₂O₃ loading indicates improved light harvesting and is attributed to charge transfer from O²⁻ to Bi³⁺.34 Furthermore, the Bi₂S₃@SnO₂ bilayer heterostructures prepared at different hydrothermal temperatures (140, 160, and 180 °C) exhibit a noticeable red shift in the absorption onset, extending from the visible to near infrared region after Bi₂S₃ incorporation. This red shift is ascribed to excitonic absorption associated with quantum confinement effects, which are pronounced at lower dimensions compared to the bulk counterparts. The optical E_g of all samples was evaluated using Tauc's plot relation:^12,15

(αhν)^ν = B (ην − E_γ) (1)

where, α is the absorption coefficient, hν is the incident photon energy, B is a constant, and n = 2 for indirect and n = ½ for direct transition bandgaps. The optical band gaps of 4.02, 2.54, 1.82, 1.78, 1.74, and 1.61 eV for SNO, BO-SNO, and Bi₂S₃ in B16, BS14, BS16, and BS18 films (Fig. 2(b and c)) are in good agreement with the existing literature. It should be noted here that the valence band (VB) of Bi₂S₃ is positioned higher (i.e., more negative) than its oxide counterparts, as it is primarily composed of S 3p orbitals, which have higher energy than the 2p orbital present in Bi₂O₃. Similarly, Bi₂S₃ exhibits an onset of conduction band at a lower energy position due to its narrow band gap relative to Bi₂O₃. Notably, the E_g of Bi₂S₃@SnO₂ films gradually decreased with increasing hydrothermal sulfurization temperature (140, 160, and 180 °C), enhancing their light absorption capabilities and making them more favourable for photoactive applications.

Fig. 2 (a) UV-visible spectra and corresponding Tauc plot of (b) SNO and (c) BO-SNO, B16, BS14, BS16, and BS18. (d) Valence band (VB) spectra recorded at the photon energy of 40 eV, (e) secondary cut-off edges, and (f) VB onsets of SnO₂ and Bi₃S₂.

The UPS measurements were performed at room temperature for both bare SnO₂ and Bi₂S₃ to investigate their electronic structure and the band edge positions. The full VB spectra of SnO₂ and Bi₂S₃ (Fig. 2(d)) were recorded to study their influence on photo-induced charge transfer and light-assisted electrochemical energy storage performance. The work functions (W_f) were determined from the difference between the incident photon energy (hν = 40 eV) and the secondary electron cut-off energies, measured to be 35.7 eV for SnO₂ and 35.8 eV for Bi₂S₃ (Fig. 2(e)). Accordingly, the calculated work functions relative to the vacuum level were 4.3 eV for SnO₂ and 4.47 eV for Bi₂S₃. Furthermore, the energy difference between the Fermi level and the valence band maxima (VBM) was estimated by extrapolating the linear tail of the VB onset in the lower binding energy region, yielding values of 3.87 eV for SnO₂ and 1.62 eV for Bi₂S₃ (Fig. 2(f)). The corresponding VBM (VBM = W_f + VB_onset) and conduction band minima (CBM = VBM − E_g) values were evaluated and are summarized in Table 1.

Table 1 Energy band levels, V_fb,N_D, and W for bare SnO₂ and Bi₂S₃ grown on FTO

Sample	W f (eV)	E g (eV)	VBM (eV)	CBM (eV)	V fb (V)	N D (cm^3)	W (nm)
SnO2	4.3	4.02	3.87	0.15	0.3	4.67 × 10^20	1.76
Bi2S3	4.2	1.82	1.62	0.2	−0.5	1.95 × 10^19	7.67

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Furthermore, Mott–Schottky (M–S) analysis was conducted from impedance spectroscopy in the −0.8 to 0.8 V range to investigate the electronic structure, charge carrier density (N_D), and space charge region width (W) in SnO₂ and Bi₂S₃ under applied potential. All M–S plots display positive slopes, confirming the n-type semiconducting nature for both materials and the n–n type heterostructure in the SnO₂@Bi₂S₃ photoanode, with electrons as the predominant majority charge carriers. Moreover, the CB edge (E_C) and VB edge (E_V) potentials for SnO₂ and Bi₂S₃ in the heterostructure were calculated on the NHE scale using Mulliken's electronegativity theory, as follows:³⁵

E_C(NHE) = χ − E_e − 0.5E_g (2)

E_V(NHE) = χ − E_e + 0.5E_g (3)

E_NHE = −4.44 eV − E_vac. (4)

Here, χ stands for absolute electronegativity (SnO₂ = 6.25 eV and Bi₂S₃ = 5.27 eV), E_e is the free-electron energy relative to the NHE scale (∼4.5 eV), and E_g is the optical band gap. After heterostructure formation, the E_g of SnO₂ and Bi₂S₃ was found to be 1.74 and 3.67 eV, respectively. Using eqn (3) and (4), on the NHE scale, the E_C for SnO₂ and Bi₂S₃ was found to be −0.085 and −0.1 eV, which further yields an E_V of 3.59 eV for SnO₂ and 1.64 eV for Bi₂S₃. These parameters were employed, along with the vacuum level work function and the NHE scale, to draw the band energy diagram of the Bi₂S₃@SnO₂ (BS16) photoanode (Fig. 3(a)), which confirms the formation of the n–n (staggered) hetero-structure. This configuration facilitates electrolyte ion diffusion while suppressing charge recombination, thereby enhancing the possibility of interfacial polarization and directional charge separation during the illuminated charge–discharge process in photo-assisted supercapacitor applications.

Fig. 3 (a) Schematic illustration of the working mechanism and charge separation route in the bilayer n–n type Bi₂S₃@SnO₂ (BS16) heterojunction thin film. Mott–Schottky plots of (b) SNO, (c) B16, and (d) BS16 films.

Furthermore, the M–S relationship is given by eqn (5):^14,35

Similarly, the N_D is evaluated from the slope (S) of the extrapolated M–S plot, and the relation between W and N_D is derived from eqn (6) and (7), as follows:¹⁴

where, ε₀ and ε_r represent the permittivity of free space and dielectric constant of the semiconductors, respectively. A is the active surface area of the photoanode, q is the electron charge, and K_BT is the Boltzmann constant. The flat band potentials (V_fb), N_D, and W for SnO₂ and Bi₂S₃ values are listed in Table 1. The V_fb for bare SnO₂ and Bi₂S₃ was extrapolated as +0.3 and −0.5 V vs. Ag/AgCl, respectively. Whereas, in the BS16 hetero-structure, the V_fb shifts to −0.06 V for Bi₂S₃, and 0.19 V for SnO₂, indicating the interfacial band bending that suppresses charge recombination (Fig. 3(b–d)), thus facilitating the charge transfer at the BS16 heterostructure/electrolyte interface. The larger donor density of SnO₂ confirms an increased concentration of free charge carriers available to participate in redox reactions, while its reduced depletion width facilitates more efficient separation and extraction of photo-generated charge carriers from Bi₂S₃. Together, these factors enhance the prospects of photocapacitive activity of the BS16 n–n type heterostructure for solar supercapacitor applications.

The electrochemical energy storage performance of all Bi₂S₃@SnO₂ heterostructure samples (BS14, BS16, and BS18) was examined in 1 M Na₂SO₄ electrolyte in a three-electrode configuration. CV curves, recorded at 10–50 mV s⁻¹ scan rates, illustrated the reversible electrochemical nature within the potential range of −0.85 to 0.05 V (V vs. Ag/AgCl) for BS14 (Fig. S8(a), SI), BS18 (Fig. S8(b)), and BS16 (Fig. S8(d)). Among these, BS16 exhibited a significantly higher peak current response at 10 mV s⁻¹ compared to BS14 and BS18 (Fig. S8(c)), indicating its enhanced charge storage capacity. Consequently, further electrochemical analyses were conducted with the BS16 sample. Notably, the CV curves of BS16 heterostructure demonstrated a distinct pair of reversible faradaic redox peaks, contributed by Bi³⁺/Bi⁰ redox transitions, i.e., (Bi₂S₃ + xNa⁺ + xe⁻ ↔ Na_xBi₂S₃) and the peak observed near 0.05 V is most likely associated with a conversion type mechanism involving the transformation of the Na_xBi₂S₃ intermediate into metallic Bi and Na₂S (Na_xBi₂S₃ + (6 − x)Na⁺ + (6 − x)e⁻ ↔ 2Bi + 3Na₂S). This indicates a transition from a layered intercalation phase (lamellar structure of Bi₂S₃) to a fully sodiated product, contributing to the overall capacitance through both intercalation and conversion pathways.^36,37

To investigate the photo-capacitive behaviour, the BS16 heterostructure photoanode was subjected to simulated solar illumination, and the CV curves were measured at different scan rates (Fig. S8(e)). Upon illumination, the CV curve area and the peak current at 10 mV s⁻¹ increased significantly (Fig. S8(f)), indicating enhanced photocapacitive activity and efficient transport of photogenerated charge carriers. Furthermore, a slight shift in the anodic and cathodic peak positions towards negative and positive potentials was observed after illumination. This shift represents that the Na⁺ ion transport is diffusion-limited due to the n-type semiconducting nature of the Bi₂S₃@SnO₂ heterostructure. The illumination-induced polarization results in an overpotential that suppresses the faradaic redox process, thereby increasing the peak current response and widening the peak separation.¹² The contribution of surface-controlled (k₁ν) and diffusion-controlled (k₂ν^1/2) processes to the total capacitance can be qualitatively estimated using the relationship between peak current (i_p) and the scan rate (ν), expressed as follows:^12,38

i_p(v) = k₁v + k₂v^1/2 (8)

log(i_p) = av^b = log(a) + b log(v) (9)

where a, k₁, and k₂ are constants, while b represents the power-law exponent obtained from the slope of the linear dependence of ‘i_p’ on ‘ν’. The b-value close to 0.5 indicates a diffusion-controlled process, whereas a value approaching 1 suggests surface-controlled (capacitive) kinetics.³⁸ The plots of log(i_p) vs. log(ν) under dark and illumination conditions (Fig. S9(a), SI) yield b-values of 0.6 and 0.5 in the cathodic region, respectively, evidenced by the intercalation pseudocapacitive nature of the BS16 electrode in the presence of Na⁺ ions.

In addition, the linear correlation coefficient (R²) values of the BS16 electrode under dark and light conditions were determined using the Randles–Sevcik plot (Fig. S9(b)). The R² values approaching unity reveal a diffusion-limited process with higher electrochemical reversibility.¹² The contributions of capacitive and diffusion-controlled processes at 10 mV s⁻¹ (Fig. S9(c)) and different scan rates (Fig. S9(d)) clearly demonstrate the strong influence of scan rates on Na⁺ ion migration and diffusion at the electrode/electrolyte interface. Importantly, diffusion-controlled contribution is significantly enhanced under illumination compared to dark conditions (Fig. S9(e and f)). This enhancement is primarily attributed to improved Na⁺ intercalation into the BS16 heterostructure photoanode and the more efficient separation and migration of photo-generated electron–hole pairs between Bi₂S₃ and SnO₂ semiconductors. This behavior is further facilitated by surface polarization induced by the built-in electric field at the heterostructure/electrolyte interface.³⁹ These synergistic effects collectively enhance the overall energy storage performance of the BS16 n–n type heterostructure under illuminated conditions.

Furthermore, GCD measurements of BS14 (Fig. S10(a), SI), BS16 (Fig. 4(a)), and BS18 (Fig. S10(b)) were performed at current densities from 0.2 to 0.6 mA cm⁻² within the potential window of −0.85 to 0.05 V. The areal capacitance (C_a) of BS14, BS16, and BS18 heterostructure electrodes was evaluated to be 4.16, 11.5, and 6.72 mF cm⁻², respectively, at 0.2 mA cm⁻². A notable increase in charge–discharge time and consequently in C_a was observed for the BS16 heterostructure compared to the other two electrodes (Fig. S10(c)). This enhancement is attributed to the uniform and dense coverage of interconnected 2D Bi₂S₃ on SnO₂ nanorods in BS16, which provides ample redox active sites and facilitates directional charge transport, thereby significantly improving the charge storage performance of photo-assisted supercapacitors. Subsequently, under illumination, a consistent increase in the charge–discharge time was observed across all current densities (Fig. 4(b)), indicating that exposure to solar light modifies the charge storage mechanism. Specifically, the total discharge time increased from 41 s under dark conditions to 58 s under illumination (Fig. 4(c)), confirming the crucial role of photo-excited charge carrier dynamics. Enhanced charge separation at the hetero-structure/electrolyte interface, driven by band bending and bandgap-driven acceleration of electrochemical redox kinetics, facilitates more efficient intercalation of Na⁺ ions into the lamellar structure of Bi₂S₃. This results in improved electron injection efficiency and overall enhancement in energy storage capability under solar irradiation.

Fig. 4 GCD recorded at various current densities (0.2–0.6 mA cm⁻²) under (a) dark and (b) illumination conditions, (c) discharge curves at 0.2 mA cm⁻² under dark and light conditions, and (d) areal capacitance (mF cm⁻²) of the BS16 electrode. Cycling stability of BS16 electrodes for continuous 2000 GCD cycles at 0.6 mA cm⁻² under (e) dark and (f) illumination conditions. (g) Post-stability XRD patterns, and (h) Nyquist plots at open circuit potential under (h) dark and (i) light conditions of the BS16 electrode.

The BS16 hetero-structure photo-anode exhibited a C_a of 15.22 mF cm⁻² at 0.2 mA cm⁻² under illumination, representing a 33% rise in photo-efficiency compared to dark conditions (Fig. 4(d)). The durability of the BS16 photo-anode was evaluated over 2000 GCD cycles at 0.6 mA cm⁻² under dark (Fig. 4(e)) and light (Fig. 4(f)) conditions. Interestingly, capacitance retention increased beyond 100% during the initial cycles under both conditions, which is attributed to the gradual activation of maximum redox active sites. After 2000 cycles in the dark, the BS16 electrode retained 94.36% (i.e., 6.7 mF cm⁻²) of its initial capacitance, confirming good long-term electrochemical cycling stability. In contrast, it is decreased to 76.61% (i.e., 9.6 mF cm⁻²) under illumination due to overheating issues caused by prolonged light exposure. Specifically, the increase in the thickness of the Helmholtz layer (charge separation distance) and the recombination rate of the charge carriers at the hetero-structure/electrolyte interface might have adversely affected the photoanode and resulted in a decrease in the capacitance. Despite this decline in retention, the overall capacitance under illumination remains higher than that in the dark, suggesting that photoexcitation still enhances the performance, even though thermal effects limit cycling stability.

Post-cycling XRD and surface morphological analyses under dark and illumination conditions (Fig. 4(g) and S11 (SI)) confirm the structural integrity of the BS16 hetero-structure photo-anodes, thereby supporting the conclusion that photo-assisted cycling enhances ion diffusion and energy storage capacity. EIS was performed on the BS16 heterostructure electrode in a three-electrode configuration over 0.01 Hz to 100 kHz to investigate the charge transfer kinetics at the heterostructure/electrolyte interface. The corresponding Nyquist plots before and after cycling stability tests under dark and illumination conditions, along with the fitted equivalent circuit (inset), are shown in Fig. 4(h and i). The fitted equivalent circuit comprises R_S, C_dl, R_ct, and Z_w components, representing the equivalent series or internal resistance, double layer capacitance, charge transfer resistance, and Warburg diffusion resistance, respectively, with the corresponding values summarized in Table S1 (SI). Prior to cycling stability tests, the BS16 electrode exhibited a lower R_S value under illumination (18.12 Ω) compared to the dark conditions (23.97 Ω), indicating an excellent photo-current response driven by the built-in electric field generated by exciton dissociation. A slight increase in R_S under prolonged illumination after cycling stability testing may be attributed to the limited generation of photo-induced charge carriers over time. A slight increase observed in C_dl under illumination is attributed to the enhanced electrostatic interactions facilitated by additional photoexcited electrons, indicative of intercalation-type pseudocapacitive behavior, consistent with the observed n values. Under open-circuit conditions, efficient surface polarization, improved charge separation, and accumulation of photoinduced charge carriers generate a barrier for interfacial faradaic charge transfer and surface ion diffusion,⁴⁰ leading to a significant increase in R_ct following initial light exposure. Furthermore, R_ct increased after prolonged cycling stability testing under both dark and illuminated conditions, suggesting an increasing kinetic barrier for electrolyte ion diffusion and interfacial redox reactions with the progression of charge–discharge cycles.¹² Initially, Z_w decreased significantly upon illumination, indicating enhanced charge transfer due to photoexcited electrons and reduced electron–hole recombination. This behavior supports a diffusion-controlled pseudocapacitive energy storage mechanism. However, Z_w increased after extended cycling under both conditions, indicating that ion diffusion becomes the rate-limiting process in the energy storage process over prolonged operation.

To investigate the practical applicability of the BS16 hetero-structure photoanode for real-time applications, it was integrated with the PEDOT:PSS conducting polymer (serving as anode) to fabricate an asymmetric photo-assisted supercapacitor device (APSD) with the configuration FTO/SnO₂@Bi₂S₃‖PEDOT:PSS/FTO as illustrated in Fig. 5(a). To evaluate the optimal operating potential window of APSD, both CV and GCD measurements were performed over a potential range of 0 to 1.8 V at a scan rate of 50 mV s⁻¹ (Fig. 5(b)) and current density of 0.06 mA cm⁻² (Fig. 5(c)), respectively. Interestingly, the PEDOT:PSS/FTO counter electrode exhibited simultaneous coloration and bleaching effects, indicative of a reversible ion intercalation and de-intercalation process. The detailed charge storage mechanism of the fabricated APSD under photo-assisted operation is elaborated in the subsequent section for greater clarity. Moreover, the device exhibited nearly ideal quasi-rectangular CV profiles and linear GCD curves with weak redox peaks and symmetric current distribution at high potential. This indicates that the operating potential window can be extended up to 1.8 V in 1 M Na₂SO₄ electrolyte. This behaviour reflects the synergistic contribution of electric double-layer capacitance and pseudocapacitive characteristics. Therefore, further electro-chemical studies were conducted at an extended potential window of 1.8 V. The CV profiles of the APSD were recorded at different scan rates (10 to 50 mV s⁻¹) under dark (Fig. S12(a)) and illumination (Fig. S12(b), SI) conditions. The retention of the quasi-rectangular shape and modulation of current response at higher scan rates suggest the excellent capacitive behaviour and efficient charge storage capabilities of the device.

Fig. 5 (a) Schematic illustration of the asymmetric electrochromic photoassisted supercapacitor device using BS16 heterojunction and PEDOT:PSS electrodes. Potential window dependent (b) CV profile @50 mV s⁻¹ and (c) GCD @0.06 mA cm⁻² in the dark, and comparison of (d) CV profiles @50 mV s⁻¹ and (e) GCD under dark and illumination conditions at a potential of 1.8 V. (f) Areal capacitance, (g) Ragone plots, and stability studies for 1000 GCD cycles @0.1 mA cm⁻² under (h) dark and (i) illumination conditions.

Further device performance is evaluated using GCD at various current densities (0.06 to 0.2 mA cm⁻²) under dark (Fig. S12(c)) and illumination (Fig. S12(d)) conditions. The observed non-linear charge–discharge profiles in both conditions suggest the presence of faradaic and reversible redox reactions, primarily occurring at the Bi₂S₃ component of the BS16 heterostructure in the presence of Na⁺ ions. Significantly, a noticeable increase in the CV curve area under illumination compared to the dark condition at a scan rate of 50 mV s⁻¹ (Fig. 5(d)) indicates enhanced charge transfer kinetics driven by strong photo-response. Moreover, the prolonged charge–discharge time under illumination (Fig. 5(e)) confirms the improvement in capacitive performance. Under illumination, APSD delivered a maximum areal capacitance of 1.78 mF cm⁻² at 0.06 mA cm⁻², which is ∼2.5-fold higher than the capacitance obtained under dark conditions (0.70 mF cm⁻²), as shown in (Fig. 5(f)). Ragone plot of the Bi₂S₃@SnO₂/FTO‖PEDOT:PSS/FTO APSD (Fig. 5(g)) demonstrates an areal energy density (E_a) of 0.8 mWh cm⁻² and areal power density (P_a) of 356 mW cm⁻² under illumination, which is either comparable or exceeds the performance of several state-of-the-art photo-rechargeable supercapacitors reported in the literature (Table S2, SI). The Nyquist plots obtained from EIS measurements under open circuit conditions in dark and illumination states are shown in Fig. S12(e and f). The resistance at the electrode/electrolyte interface (R_s) significantly reduced under illumination (52.29 Ω) compared to that in the dark (70.56 Ω), indicating improved photoelectrochemical reaction efficiency. A reduced charge transfer resistance (R_ct) of 13.1 kΩ under illumination compared to dark (16.5 kΩ) conditions signifies the improved photo-generated charge carriers facilitating enhanced electrolyte diffusion. Moreover, the relatively identical contribution of Z_w under illumination (12 kΩ) and dark (10 kΩ) conditions represents the stable diffusion response of the electrolyte ions in the inner core. Furthermore, cyclic GCD measurements of APSD over 1000 cycles at 0.2 mA cm⁻² under dark (Fig. 5(h)) and illumination (Fig. 5(i)) conditions demonstrated excellent cycling stability, retaining nearly 80% of its initial capacitance under both conditions.

Post-stability structural, morphological, and elemental analysis confirmed by XRD (Fig. S13(a)), FESEM (Fig. S13(b and c)), EDS (Fig. S13(d)), and EDS elemental mapping (Fig. S13(e and f), SI), revealed that the n–n type BS16 heterostructure remains structurally unaltered. However, cumulative effects of continuous Bi³⁺ ion extraction and rearrangement by Na⁺ intercalation resulted in the fragmentation of 2D Bi₂S₃, leading to the formation of minor secondary phases under both conditions. The originally well-separated 2D Bi₂S₃ NSs on SnO₂ nanorods appeared to undergo partial fusion, reducing the number of accessible surface-active sites. This transformation is possibly due to irreversible ion diffusion or redox reactions involving Na⁺ ions, which may have contributed to decreased capacitance retention. Nevertheless, the imperceptible amount of these structural alterations and ion diffusion highlight the promising stability of the BS16 heterostructure as stable electrode materials under both light and dark conditions.

To elucidate the plausible electrochemical energy storage mechanism, we systematically investigated the energy band alignment between SnO₂ and Bi₂S₃, along with the interfacial charge transfer pathways (Fig. 3(a)). Based on their respective band edge positions, the 2D Bi₂S₃@1D SnO₂ heterostructure exhibits an n–n (staggered) band alignment (Fig. 3(a-i)). Upon intimate interfacial contact between SnO₂ and Bi₂S₃, a charge rectification process occurs to equilibrate their Fermi levels, generating an inherent in-built electric field at the interface (Fig. 3(a-ii)). Due to the higher work function of Bi₂S₃, electrons flow from Bi₂S₃ to SnO₂, inducing upward band bending at the SnO₂ side and reinforcing the interfacial electric field. Under external bias, it counteracts the intrinsic electrochemical potential difference between the heterostructure and the electrolyte, inducing further band bending at the Bi₂S₃/electrolyte interface and establishing the space charge region (SCR).⁴¹ Under biasing, Na⁺ and SO₄²⁻ ions migrate toward their respective electrodes and are stored in Bi₂S₃ through a combination of electrostatic and faradaic processes. Subsequent unidirectional charge carrier transport from Bi₂S₃ to SnO₂ facilitates the anticipated surface redox reactions of Bi₂S₃ during the charge–discharge process under dark conditions (Fig. 3(a-iii)),³⁷ which the following equations can describe:

Bi₂S₃ + xNa⁺ + xe⁻ ↔ Na_xBi₂S₃ (10)

Upon illumination, Bi₂S₃ and SnO₂ both absorb photons to generate electron–hole pairs, causing the SCR at the Bi₂S₃/electrolyte interface to expand (Fig. 3(a-iv)). The resultant band bending and coulombic interactions drive photo-excited electrons to migrate from the CBM of Bi₂S₃ into the CBM of SnO₂, thereby participating in the redox reactions. During photo-charging, this enhanced charge separation further strengthens the built-in electric field, which contributes to improve the electron density and electron injection efficiency under illumination. The enhanced field then attracts a greater flux of Na⁺ ions towards the hetero-structure electrode, further boosting the redox activity.³⁷ Consequently, the synergistic effect of photo-generated excitons and electric-field-driven ion transport significantly enhances the overall charge storage capacity of the 2D-Bi₂S₃@1D-SnO₂ n–n hetero-structure photo-anode.

Conversely, under illumination, photo-generated electrons are extracted into the external circuit and participate in the oxidation of the PEDOT:PSS counter electrode during the charging process. PEDOT:PSS is a conducting polymer comprising a positively charged PEDOT polymer balanced by negatively charged PSS counterions, and it exhibits exceptional electrochromic behaviour when integrated as both the energy storage and electrochromic layer.⁴² The electrochromic mechanism arises from the reversible redox cycling of PEDOT under applied electric bias, where electrolyte ions intercalate (doping) and de-intercalate (de-doping) within the polymer matrix, modulating its optical transmission during bleached and colored states. In the present asymmetric device, SO₄²⁻ ions act as mobile counterions, while PSS⁻ provides complementary charge compensation (Fig. 6).⁴² During charging, SO₄²⁻ ions ingress into the PEDOT:PSS film, screening the coulombic attraction between PEDOT and PSS, and facilitating the oxidation of PEDOT⁰ to PEDOT⁺ (PEDOT⁰ → PEDOT⁺ + e⁻). The resultant PEDOT⁺ species, stabilized as polarons or bipolarons by intercalated SO₄²⁻, induce the color change from transparent to dark blue.⁴² Upon discharging, PEDOT⁺ follows the reverse reaction (PEDOT⁺ + e⁻ → PEDOT⁰). Here, de-intercalation (de-doping) of SO₄²⁻ leads to the collapse of the polaron/bipolaron structure, restoring the polymer matrix to its neutral, optically transparent (or pale blue) state as the anions exit the polymer matrix. The overall electrochromic coloration and bleaching mechanism at the PEDOT:PSS counter electrode in SO₄²⁻ ions can be summarized as follows:⁴²

2PEDOT⁰:PSS + 2SO₄²⁻ ↔ 2PEDOT⁺:PSS: SO₄²⁻ + 2e⁻ (11)

Fig. 6 Schematic illustration of the working mechanism of the asymmetric electrochromic photo-assisted supercapacitor device comprising the 2D Bi₂S₃ NFs@1D SnO₂ NRs bilayer n–n type heterojunction as the positive electrode and PEDOT:PSS/FTO as the negative electrode. The digital photographs in the inset represent the electrochromic (transparent to dark blue) properties of PEDOT:PSS at different applied potentials.

The optical properties of PEDOT:PSS offer fundamental insights into the underlying electronic structure. UV-visible spectra recorded in three electrode configurations under charging (doped) and discharging (de-doped) states (Fig. S14, SI) confirm π–π* conjugation. During discharging (de-doped) state, PEDOT:PSS appears transparent or pale blue throughout the visible region, reflecting the de-intercalation of SO₄²⁻ ion which exceeds the band gap beyond 3 eV.³² Upon charging, enhanced doping (intercalation) of SO₄²⁻ ions under positive potentials yields a dark blue film with strong absorption of the visible spectrum.^32,42 Electrochemical doping introduces new electronic states, polarons, bipolarons, or solitons, within the band gap, effectively transforming the polymer from an insulator to a quasi-metallic regime with localized mobile charge carriers propagating along its conjugated backbone.⁴³ Polaron forms via addition or removal of electrons, causing the localized distortion of the polymer backbone; bipolarons result from pairing like-charged polarons, and solitons arise in polymers with an odd number of carbon atoms, enabling unpaired electrons.⁴³ The energy-level diagram of inter-band transitions (T₁, T₂, T₃, and T₄) in the PEDOT:PSS film at increasing doping levels (Fig. S14(c)) highlights the emergence of bipolaron bands from pre-existing polaronic states and corresponding transitions to the valence band.⁴³ In the pristine PEDOT:PSS film, distinct absorption bands appear at 3.87, 3.43, 2.38, and 1.55 eV, corresponding to polaronic transitions. As the applied potential increases, doping progresses to intermediate levels, which results in the shifting of these bands, disappearance of the 2.38 eV polaron band, and emergence of broader bipolaron absorption bands.^43,44 Thus, the CV curve infers that polaron-dominated mid-gap states are formed during charging (oxidation) and bipolaron-dominated states predominate during discharging (reduction).⁴⁵

Conclusion

A 2D Bi₂S₃@1D SnO₂ (BS16) n–n heterostructure thin film photo-anode was successfully fabricated by using a two-step hydrothermal method for application in photo-assisted supercapacitors. The structural analysis confirmed the phase purity and preservation of the intrinsic lattice structure of the bilayer hetero-structure, while morphological and elemental analysis verified the uniform anchoring of 2D Bi₂S₃ nanostructures on SnO₂ nanorods, and the presence of Sn⁴⁺ and Bi³⁺ species. The UV-visible and UPS studies revealed a staggered band alignment, enabling efficient light harvesting and promoting the spatial separation of photo-generated charge carriers across the hetero-structure interface. The higher donor density of SnO₂ (4.67 × 10²⁰ cm⁻³) and narrower depletion width (1.76 nm) facilitated charge extraction from Bi₂S₃, enhancing photo-electrochemical activity. Under illumination, the three-electrode configuration exhibited an areal capacitance of 15.22 mF cm⁻² at 0.2 mA cm⁻², representing a 33% enhancement compared to dark conditions. Similarly, the asymmetric photo-assisted electrochromic device exhibited an areal capacitance of 1.78 mF cm⁻² at 0.06 mA cm⁻², 2.5 times higher than its dark performance, with excellent cycling stability. The enhanced performance is attributed to light-assisted Na⁺ ions intercalation into the Bi₂S₃ lattice (Bi₂S₃ + xNa⁺ + xe⁻ ↔ Na_xBi₂S₃). SnO₂ provides directional conducting charge transport pathways through its well-aligned conduction band with Bi₂S₃, while its wide band gap allows maximum incident light to pass through and get absorbed by Bi₂S₃, thereby indicating a strong synergistic effect during the charge–discharge process. Furthermore, SO₄²⁻ ion diffusion into the PEDOT:PSS chain induced reversible electrochromic transitions between doped (coloration, dark blue) and undoped (bleaching, transparent) states. Overall, this study demonstrates a multifunctional solar-powered energy storage system with integrated electrochromic capability, offering new design principles for next-generation optoelectronic and energy storage devices.

Conflicts of interest

There are no conflicts to declare.

Data availability

The data supporting this article have been included as part of the supplementary information (SI). Supplementary information: the details of experimental section and growth mechanism of 2D Bi₂S₃@1D SnO₂ bilayer heterostructure, materials characterization such as XRD, Raman, FESEM, XPS analysis and electrochemical analysis in 3-electrode configuration in 1M Na₂SO₄ electrolyte, and asymmetric electrochromic supercapacitor device (APSD). See DOI: https://doi.org/10.1039/d5ta09838f.

Acknowledgements

The authors acknowledge the Board of Research in Nuclear Sciences (BRNS), Science and Engineering Research Board (SERB), and DST-Ministry of Science and Technology (MST) under grant No. 58/14/08/2023/BRNS/11687, CRG/2022/000160, and DST/WOS-A/PM-118/2021, respectively. The authors are thankful to DST-FIST (SR/FIST/PSI-225/2016) for providing funding to procure the Raman spectrometer. MS acknowledges the Prime Minister Research Fellowship from the Government of India (PMRF-2101709) for the financial support during this research work. AM acknowledges the INSPIRE research fellowship (IF/210426) from DST India.

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