Coordination Nanosheet-Based Electrochromic Supercapacitor with High Energy Storage, Switching Durability, and Long Optical Memory Properties

All reagents in this study were reagent grade and were employed without further purification. The reaction solvents were extrapure dichloromethane (DCM) and ethanol (EtOH). Spectrochemically pure acetonitrile (ACN) was employed for cyclic voltammetry (CV), device fabrication, and spectroscopic analyses. S D Fine-Chem Limited supplied reagent grade ACN, DCM, EtOH, and acetic acid (AA). Specifically, 4′,4‴′-(1,4-phenylene)-bis­(2,2′:6′,2″-terpyridine), iron­(II)-acetate, 4′-chloro-2′,2:6′,2″-terpyridine, LiClO₄, and Fe­(BF₄)₂·6H₂O were acquired from Sigma-Aldrich. Tetra-n-butylammonium perchlorate (TBAP), propylene carbonate (PC), and poly­(methyl methacrylate) (PMMA) were sourced from TCI Chemicals (India) Pvt. Ltd. Furthermore, 2-(hydroxymethyl)-2-methylpropane-1,3-diol and powdered KOH were obtained from Sisco Research Laboratories Pvt. Ltd. (SRL)-India. ITO-coated glass slides with a resistivity of approximately 20 Ω and transmittance exceeding 90% were procured from Shilpa Enterprise, India. Millipore Milli-Q water with a resistivity of 18 MΩ cm was employed as needed. Water-based nickel hexacyanoferrate (w-NiHCF) was obtained from Cica-Reagent from Kanto-chemical Co., Inc. and used further for film preparation after dilution with DI water. Synthesis of (4′,4‴′-((2-(((2,2′:6′,2″-terpyridin)-4′-yloxy)-methyl)-2-methylpropane-1,3-diyl)­bis­(oxy))­di-2,2′:6′,2″-terpyridine) (3TPY), as shown in Scheme S1a↗ in Supporting Information (SI), was conducted following our previously reported procedure. The details of the synthesis of Fe-3TPY film grown through interfacial bilayer polymerization using 3TPY ligand in DCM solution and Fe­(BF₄)₂·6H₂O in water and the collection procedure of the film in ITO are provided in our previously reported procedure. A schematic of the synthesis and collection of the Fe-3TPY film is mentioned in Scheme S1b↗ in the Supporting Information.

Electrochemical measurements were performed by using an ALS/CHI electrochemical workstation (Model 612B, CH Instruments, Inc.) with a two-electrode system. An integrated Ocean Optics modular spectrometer was connected to the electrochemical analyzer for monitoring the optical spectral behavior of the EC device upon potential application. A FEI, Apreo SEM instrument with a 30 kV operating voltage was used for field emission scanning electron microscopy (FESEM) to determine the film morphology. The film surface was gold-coated by sputtering with gold for 30 s to reduce the surface potential. The X-ray photoelectron spectral (XPS) study was conducted using a Thermo Scientific Multilab 2000 with AlKα radiation (1486.6 eV) operated at 15 kV and 10 mA (150 W). All the binding energies are with reference to C 1s at 284.85 eV. To check the powder X-ray diffraction (PXRD) pattern, CONASH's flakes from EtOH were dropped on a Kapton holder, dried at 55 °C overnight, and subjected to the Rigaku MiniFlex XRD instrument.

In our present study, Fe-3TPY films were directly deposited on ITO glass substrates. Cracked or uneven regions were carefully removed to obtain a uniform film area. However, the NiHCF-coated electrode was obtained by spin-coating an aqueous-NiHCF solution at 1200 rpm on the conductive side of ITOs. NiHCF-coated ITOs were oven-dried at 70 °C overnight, whereas the Fe-3TPY-coated films were air-dried first and then oven-dried at 70 °C for 2 h. The quasi-solid Li-ion-based gel-electrolyte was prepared with 0.9 g LiClO₄ in 21 mL ACN and 6 mL PC, followed by adding 2.1 g PMMA. The mixture was stirred at room temperature for 8 h to become a transparent viscous state. The point to be mentioned is that the gel-electrolyte bottle was tightly sealed after each use and stored in a desiccator with low humidity. Finally, the dried film-coated ITOs were sandwiched with the gel electrolyte and kept at room temperature for drying before use.

The flexible nonconjugated ligand 3TPY was synthesized according to our previously reported protocol. This included the reaction between 2-(hydroxymethyl)-2-methylpropane-1,3-diol and 4′-chloro-2,2′:6′,2″-terpyridine in the KOH base in the presence of anhydrous DMSO, as represented in Scheme S1a↗ (SI). The synthesized 3TPY was thoroughly characterized by NMR and mass spectroscopy, with detailed results provided in our previous literature., Furthermore, as illustrated in Scheme S1b↗, the synthesis of Fe-3TPY nanosheets was carried out under static conditions at the interface of 15 mL DCM containing 0.1 mM of 3TPY and 15 mL water containing 50 mM of Fe­(BF₄)₂·6H₂O. The formation mechanism of the Fe-3TPY film relies on the complexation between the TPY units of adjacent ligands, facilitated by the introduction of suitable Fe²⁺ ions diffusing from the aqueous subphase., As previously discussed, to control the film's thickness and color intensity, the reaction parameters, such as the concentration of metal and the duration of layering in the reaction setup, can be adjusted. Although other in situ polymerization methods, such as electrochemical deposition, exist, only interfacial polymerization allows the formation of two-dimensional CONASHs. DCM and water were chosen as the solvent because they are immiscible. The organic ligand is soluble only in DCM, and the Fe­(II) salt is soluble only in water. Moreover, our previous report determined the stoichiometric complexation ratio between the 3TPY ligand and Fe²⁺ ions is to be 2:3 by using UV–vis titration by gradually adding Fe²⁺ ions to a solution containing 3TPY. The chemical structure of Fe-3TPY is represented in Figurea by considering the above-mentioned stoichiometric ratio. The comparative UV–Vis spectroscopy analysis (Figureb) of 3TPY and Fe-3TPY revealed a notable spectral shift of the peaks. Initially, for 3TPY, a distinct absorption peak at 272 nm was observed, attributed to the π–π* transition within its aromatic unit. Upon complexation with Fe²⁺, this transition underwent a significant red shift to 312 nm in Fe-3TPY. Additionally, a minor peak appeared at 360 nm, corresponding to the metal (Fe²⁺) d–d transition, along with an intense peak at 556 nm owing to the MLCT (d of Fe²⁺ to π* of 3TPY) transition, also responsible for the intense purple color of the CONASH. The FESEM image consistently revealed a homogeneously smooth, flat nanosheet structure throughout its surface (Figurec). The average film thickness of the Fe-3TPY film was maintained at 350 ± 10 nm through AFM imaging, as shown in Figured, by preparing an interfacial nanosheet over 48 h. The bonding environment and structural connectivity of Fe-3TPY were comprehensively examined through XPS analysis. In the core-level scan of N 1s, as shown in Figuree, a distinct peak corresponding to the nitrogen atoms of 3TPY was observed at 398.17 eV, which exhibited a slight shift to 399.92 eV in Fe-3TPY. Analysis of the Fe 2p core-level scan of Fe-3TPY (Figuref) revealed two distinct peaks at Fe 2p_3/2 (708.66 eV) and Fe 2p_1/2 (721.23 eV). This outcome explicitly confirmed the coordination of terpyridine moieties with Fe­(II) within the Fe-3TPY nanosheets. Additionally, the XPS analysis-based atomic ratios indicated N/Fe ratios of 6.63:1.17, which were very close to the theoretical N/Fe ratios of the ideal standard bis­(terpyridine)–Fe­(II) complex of 6:1. The Raman spectra (Figureg) of CONASHs exhibited a downward shift in the characteristic aromatic C=C or C=N stretching vibrations compared to the free ligand, confirming the successful coordination of Fe­(II) to 3TPY. Additionally, the PXRD pattern (Figureh) of CONASHs showed an amorphous nature with a broad peak centered at 2θ = ∼21.9, corresponding to a short-range interaction at 4.05 Å. The extensive optical, structural, and thermal characterizations of Fe-3TPY are provided in our earlier report.

The Fe-3TPY and NiHCF-based sandwich-shaped gel-based quasi-solid-state ECESD was made utilizing a Fe-3TPY-coated ITO as the active working electrode (WE) material and NiHCF-coated ITO as the CE with a LiClO₄ and PMMA-based semigel electrolyte in between electrodes. Here, the thickness of the Fe-3TPY layer was 350 ± 10 nm, as mentioned, and the average thickness of the NiHCF layer was maintained at 140 ± 10 nm (given in the inset of Figurea). Before quantitatively analyzing the energy storage properties, the energy storage behavior of the Fe-3TPY-based ECESD was assessed through a CV study by varying scan rates. At the scan rates between 25 and 200 mV s^–1, the ECESD exhibited a distinct reversible redox peak identified with the Fe²⁺/Fe³⁺ redox couple (Figurea), indicating the characteristic reversible faradic behavior of ECESD. Notably, increasing scan speeds led to higher oxidative and reductive current densities, indicating the ECESD's low internal resistance and rapid charge-transfer kinetics, with the charge storage mechanism described by eq:i=avb1where a and b are configurable parameters, v is the scan rate, and i is the peak current. We determined b-values of 0.680 for anodic and 0.708 for cathodic current maxima by plotting log­(i) vs log­(v) (Figureb). This demonstrated the predominant pseudocapacitive behavior of the Fe-3TPY-based ECESD and facilitated quantitative evaluation of diffusive and capacitive contributions to energy storage by using eq:i=k1v+k2v1/22where k₁ and k₂ are constants derived from the i/v^1/2 versus the square root of the scan-rate plot.

From Figurec, we determined k₁ as 0.00556 and k₂ as 0.00223. Consequently, we calculated the pseudocapacitive contribution of the Fe-3TPY/NiHCF ECESD to be over 80% at a scan rate of 25 mV s^–1. The pseudocapacitive contribution of EECSD decreased with increasing scan rates, reaching 55% at a 200 mV s^–1 scan rate (Figured). These results demonstrate Fe-3TPY's potential for high-power energy storage applications, making it a viable option for supercapacitor technology.

To gain insight into the electrochromism behavior of asymmetric ECESDs, we monitored the transmittance spectrum change of the ECESD when applying different voltages. It is worth noting that the transmittance was measured by subtracting the background of the ITO/glass substrate with an electrolyte gel. The transmittance spectra of ECESD revealed the disappearance of the MLCT peak at 556 nm (Figurea), with a reversible color change from pristine pink to a pale yellowish of the ECESD, as shown in Figureb. The CV plot revealed that ECESD exhibited an oxidation peak for Fe²⁺ to Fe³⁺ at +0.64 V and a corresponding reduction peak for Fe³⁺ to Fe²⁺ at +0.37 V, observed at a scan rate of 25 mV/s (Figurea). The color change of the ECESD is linked to the reversible Fe²⁺/Fe³⁺ redox transition, resulting in the complete disappearance of the 556 nm MLCT peak within a potential window of 0.01 to 1.0 V. The Fe-3TPY-based ECESD exhibited a distinct color change, resulting in a high optical contrast (ΔT) of 57.4% at 556 nm. It is worth noting that the incorporation of NiHCF as a CE material did not hamper the overall transmittance of the device, as the NiHCF electrode revealed a transparent pale yellowish color (Figure S1↗ in Supporting Information) originating from the d–d transition in the material.

The fundamental EC parameters, such as the switching speed, coloration efficiency, and device durability, were determined later for the practical applicability of EC materials. The EC parameters of Fe-3TPY were determined using double potential chronoamperometric studies, sweeping the potential between +1 and 0.01 V. The switching times, defined as the time taken for a 95% change in ΔT, were calculated from the transmittance vs time plot shown in Figurec. In general, ECDs with a fast-to-moderate switching time and high coloration efficiency (η) are desirable for commercial applications. In this study, the switching times of ECESD were determined to be 1.28 s for coloration (t_c) and 1.69 s for bleaching (t_b). "η" is a measure of the device's energy efficiency, which can be calculated by dividing the quantity of charge injected or ejected (Q_d) per unit area by the ratio of optical density change (ΔOD). The "η" of the CONASH-based hybrid ECESD, calculated from the slope of the ΔOD vs Q_d plot in Figured, was 619 cm² C^–1, which is higher than that of previously reported Fe­(II)-containing MSPs.,,, We also derived the power and energy consumption of the ECESD from the chronoamperometric charge–discharge plot given in Figuree. The power and energy consumed by the CONASH-based hybrid ECESD during electrochromism were 2.18 mW/cm² and 3.6 mJ/cm², respectively, calculated by using eq as mentioned in the SI↗ at an operating voltage of +1.0 and +0.01 V by taking the 1.4 × 1.3 cm² film size. We also compare the energy consumption data of ECESD with the conventional display technologies in Figuref, revealing even lower energy consumption in our ECESD compared to other conventional display technologies.− In the cycle stability study, we evaluated the long-term switching capacity of the ECESD within specified holding intervals of 5, 4, and 3 s, as illustrated in Figureg. As the ECESD showed faster switching and very low coloration and bleaching times, we anticipated swift and reversible switching in any holding times beyond 2 s. Remarkably, the ECESD exhibited exceptional cycle stability, enduring over 50,000 cycles of operation. By accurately analyzing the initial and final optical contrast (ΔT), we determined that the ECESD retained approximately 90% of its initial ΔT following 50,000 repeated switching events. This endurance provides durable performance of the ECESD equivalent to a remarkable 14-year lifespan based on a usage scenario of 10 cycles per day., This impressive durability is attributed to this device's extremely low operational voltage (0.01–1 V) and reversible charge-balancing redox reactions occurring between the WE and CE. This highlights the robustness and suitability of fabricated ECESD for sustained practical applications in various fields.

High optical memory within an ECESD is an essential factor in assessing the power efficiency of smart windows. Typically, in an ECESD, the ability of a specific redox state, characterized by its color or bleaching level, to maintain its originality over an extended period in open-circuit conditions signifies the potential for creating a power-efficient device.,, In this work, after attaining the fully bleached state at +1 V bias, we continually monitored transmittance at 556 nm and the change in optical color to examine the EC memory of the Fe-3TPY-based ECESD. Herein, Figurea,b depicts the regeneration of transmittance and optical color over time under open-circuit conditions, starting from a fully bleached state. The observed gradual increment in transmittance peak and development of a pink hue over time resulted from the self-reduction of colorless Fe³⁺ centers by residual electrons from the electrode under open-circuit conditions. Interestingly, the ECESD demonstrated only a 33.3% recovery of its original color state after 36 h (Figurec). The electron transport activities between nearby Fe centers via the ligand π cloud and from the electrode to Fe³⁺ centers within the MSP in the bleached state play crucial roles in determining EC memory. In the Fe-3TPY system, the nonconjugated nature of the 3TPY ligand restricted electron hopping through the ligand's π cloud, showcasing a highly extending EC memory of Fe-3TPY-NiHCF-based hybrid ECESD. It is noteworthy that the EC optical memory of our ITO/Fe-3TPY//NiHCF/ITO-based ECESD is not only one of the best in the MSPs or CONASH category but also enhanced a lot compared to our earlier report of ITO/Fe-3TPY/ITO-type devices, which showed 50% of coloration recovery only after 25 min in open-circuit conditions. Incorporating NiHCF as the redox-complementary CE in the devices can reduce the number of residual electrons during the oxidation of Fe-3TPY in the WE to increase the EC optical memory in the hybrid device.

Moreover, it is essential to understand the electrochemical reactions occurring between the WE and CE to comprehend the EC phenomenon of the ECESD. In the diagram given in Figured, in the colored mode, the Fe-3TPY layer is distinguished by its pink hue, which indicates the presence of Fe²⁺ ions. Conversely, the NiHCF layer is represented in a light yellow shade, indicating the presence of Fe³⁺ ions. The electrolyte LiClO₄ facilitates electrochemical processes between the two electrodes. We propose a mechanistic insight into the electrochemical redox reactions occurring within the device, defined by eqs–.

Electrochemical reaction during bleaching of ECESD:

At the WE: [ − . [ + → [ − [ [ + Fe 2 + COO − ClO 4 − Fe 3 + COO − ClO 4 − e − 3 3 TPY n TPY n ] n CH 3 ] 2 n ] n CH 3 ] 2 n ] n 3

At CE: n CN n n n CN Ni 1.5 2 + Fe 3 + e − Li + Li + Ni 1.5 2 + Fe 2 + [ ( ] + + → [ ( ] ) 6 ) 6 4

Electrochemical reaction during coloring of ECESD: [ − [ [ + → [ − · [ + Fe 3 + COO − ClO 4 − e − Fe 2 + COO − ClO 4 − 3 3 TPY n TPY n ] n CH 3 ] 2 n ] n ] n CH 3 ] 2 n 5 n CN n n CN n Li + Ni 1.5 2 + Fe 2 + Li + Ni 1.5 2 + Fe 3 + e − [ ( ] → + [ ( ] + ) 6 ) 6 6

As a result, the predicted redox potential of the ECESD can be defined as E voltage window E Ox WE , E Red CE , = − 7

When a positive voltage is applied, the oxidation reaction transpires on the Fe-3TPY film (resulting in the conversion of Fe²⁺ to Fe³⁺ at the WE), inducing a color transition from pristine pink to a pale yellowish hue (eq). Simultaneously, to maintain charge neutrality within the system, a reduction reaction takes place on the NiHCF film (involving the conversion of Fe³⁺ to Fe²⁺) at the CE (eq). In contrast, Li⁺ and ClO₄^– ions migrate toward opposite electrodes (Figured). Conversely, during the coloration phase, the reverse reactions occur at each electrode, with reduction transpiring at the WE and oxidation at the CE (eqs and ), as depicted in Figuref. In open-circuit conditions, after fully bleaching the ECESD, the generated Fe³⁺ ions undergo self-reduction by taking the residual electrons from electrodes (Figuree). The inherent nonconjugation ligand can slow down the electron hopping to reduce the self-reduction of Fe-3TPY. Again, the NiHCF component in CE is critical as a charge source/sink, facilitating the swift and facile redox reactions within the Fe-3TPY system. It can also consume the electrons for its reduction during bleaching, effectively reducing the number of residual electrons in WE in open-circuit conditions after bleaching to increase the optical memory. Additionally, the redox potential of the NiHCF (in Figure S1c↗ in Supporting Information) regulated the overall potential of Fe-3TPY-NiHCF-based hybrid ECESD to obtain an overall potential window of +0.01 to +1.0 V.

The energy storage property of ITO/Fe-3TPY/LiClO₄/NiHCF/ITO-based ECESD was examined by galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) techniques. As discussed earlier, the nonlinear CV curves of the ECESD showed excellent reversible redox behavior, which indicated that the Fe-3TPY-based ECESD disclosed an effective charge-storage pseudocapacitive system. The capacity to store charge in Fe-3TPY-based ECESD was examined by a GCD study at different current densities ranging from 1 to 15 A/cm³ in the +0.01 to +1.0 V potential window (Figurea). The nonlinear charge–discharge curves in the GCD study again confirmed the predominant pseudocapacitive charge-storage mechanism for the electrodes, which prevailed by surface faradic reaction at both electrodes. The ECESD showed a negligible iR drop of only 0.15 V in the GCD behavior. The minimal iR drop (Figure S2a↗ in Supporting Information) originated from internal resistance caused by the electrolyte and electrode–electrolyte contact. Additionally, including a NiHCF layer as a charge-storage layer would probably increase facile ion diffusion, resulting in a minimal iR drop for this ECESD. The volumetric capacitance was calculated as 248.1 F/cm³ at 1 A/cm³ and slightly decreased to 211.5 F/cm³ at 15 A/cm³ (Figureb), indicating the ECESD's rapid and high-rate charge-storage ability. To understand the charge-transport properties of the ECESD, we performed an EIS study in the 5 Hz to 1 kHz frequency range. The Nyquist plot (Figurec) revealed an interfacial charge-transfer resistance (R_CT) of 180.2 Ω, and the slope in the lower frequency region showcased the ECESD's diffusive behavior. After 40,000 consecutive charge–discharge cycles at a current density of 15 A/cm³, the constructed ECESD demonstrated ∼80% retention of the initial remarkable long-term charge–discharge stability, retaining approximately 80% of its initial volumetric capacitance (Figured). Additionally, Figure S2b↗ in Supporting Information illustrates this trend's first and last GCD cycles. Furthermore, the Coulombic efficiency of the ECESD remained at 97% after completing 40,000 cycles, revealing its remarkable performance for commercial applications. At a current density of 1 A/cm³, the ECESD exhibited a power density of 0.5 W/cm³ and an energy density of 34.45 mW h/cm³. These values improved to 7.5 W/cm³ for power density and 29.37 mW h/cm³ for energy density at a higher current density of 15 A/cm³, as shown in Figuree. The intentional selection of a redox-active CE within the ECESD promoted redox activities between the anode and cathode, enabling a reversible push–pull effect that enhances the energy storage performance of Fe-3TPY- and NIHCF-based ECESDs. The volumetric energy and power densities of the hybrid ECESD were compared to those of previously published high-performance ECESD in a Ragone plot, as shown in Figuref. Uncracked smooth surface of the Fe-3TPY film after prolonged ECESD operation, confirmed by SEM, indicates the high durability of the nanosheets (Figure S3↗, Supporting Information). The above data demonstrate the efficient energy storage performance of the ECESD, highlighting the active participation of both electrode materials in the redox mechanism. This synergistic interaction enhances the overall performance, as each material contributes to the charge-storage and transfer processes. The Fe-3TPY and NiHCF electrodes work in tandem to provide high capacity and stability, ultimately producing robust and effective energy. We also fabricated a device using a larger film (3 × 2.4 cm²) by the interfacial polymerization method (Figure S4↗, Supporting Information). The ECESD using the larger film exhibited similar EC behavior at an applied voltage of +1.5/+0.01 V. The calculated energy consumption and power consumption of the device were 3.8 mJ/cm² and 2.2 mW/cm², respectively, which are consistent with the smaller ECESD described above. However, fabricating much larger films remains challenging due to the need for precise control of the interface. Further research is needed to design vibration-free platforms, develop optimized vessels, and achieve controlled solvent removal to enable mass production of large, crack-free films.

Integrating high-performance energy storage and EC behavior is very important for the device to be used as an indicative supercapacitor for next-generation electronic devices. While measuring the energy storage performance, the ECESD changes its observable color depending on the device's charge state, potential, and redox state. We monitored the in situ transmittance change of the hybrid ECESD at 556 nm during the charging and discharging process through GCD at a current density of 1 A/cm³ (Figurea). The figure provided the correlation between the transmittance and stored energy state of the ECESD, which allows users to predict the amount of charge storage by observing the color variation of the ECESD. The Fe-3TPY-based ECESD changes from pristine pink in its fully discharged state to a pale yellowish color when fully charged, as shown in Figurea inset. This demonstrates the correlation between charge-storage level and observable transmittance change. Furthermore, we have measured the transmittance changes of the devices during the charging–discharging in different current densities starting from 1 to 15 A/cm³. As shown in Figureb, the results revealed that our ECESD could track the color change over this wide range of current densities.

The EC energy storage performance of the nonconjugated Fe-3TPY-based ECESD was compared with a conjugated ligand-based MSP Poly-Fe, maintaining a similar device architecture. A chemical structure and the synthesis protocol of Poly-Fe are mentioned in Figure S5a↗. In our previous article, we have shown the detailed durable EC performance of a poly-Fe-based EC device with NiHCF as a counter material. As shown in Figure S5b↗, the device showed a reversible purple to bleached EC transition in the operating voltage window of +1.2/+0.01 V with an optical contrast of 55% at 580 nm (Figure S5c↗ in Supporting Information). The Poly-Fe- and NiHCF-based fabricated asymmetric device revealed comparatively much lower optical memory, as it can regain ∼17.8% of the initial optical contrast after 75 min, where Fe-3TPY-/NiHCF-based ECD could gain only ∼10% of color after 75 min, as shown in Figurea. The comparatively low optical memory in the Poly-Fe system is basically due to the conjugated configuration at the ligand that helped in faster electron transfer or electron hopping inside the system under open-circuit conditions to the self-reduction of the Fe­(III) center. The GCD study of the Poly-Fe-/NiHCF-based device revealed similar nonlinear behavior with a comparatively high iR drop of 0.5 V (Figure S5d↗ in Supporting Information), due to a certain self-reduction tendency of the device in the presence of a fully conjugated system. The volumetric capacitance of the device was further calculated as 32 F/cm³ at 0.05 A/cm³, further decreasing to 23.5 F/cm³ at 1 A/cm³ (Figure S5e↗ in Supporting Information). These observations suggested that the intentional incorporation of a nonconjugated system provided high optical memory and high volumetric capacitance with a slightly higher voltage window.

Moreover, we compared the EC energy storage performance of the Fe-3TPY-based device in the absence of NiHCF by fabricating the device as ITO/Fe-3TPY/LiClO₄/ITO. Due to the absence of NiHCF, the operating voltage window of the device increased to +2.5 to −2 V, as shown in Figure S6a↗ in Supporting Information. The GCD study of the device in Figure S6b↗ revealed nonlinear behavior with a very high iR drop of approximately 2.5 V. This large iR drop resulted in decreased performance, with the volumetric capacitance calculated at only 32 F/cm³ at 0.05 A/cm³, further decreasing to 23.5 F/cm³ at 1 A/cm³ (Figure S6c↗ in Supporting Information). The GCD cyclic performance in Figure S6d↗ showed a significant capacitance loss within just 1000 cycles. The EC study also revealed that the optical contrast of the device slightly decreased to 53% (Figure S6e↗ in Supporting Information). The optical memory of the device was drastically affected, as it regained its complete optical contrast within 60 min in open-circuit conditions (Figure S6f↗), indicating a fast charge loss of the energy storage device. These observations suggest that incorporating suitable CE materials can enhance the dual-ion-based redox mechanism. Each electrode's push–pull effect improves energy storage and EC performance.

Moreover, we have compared the key performance parameters of the Fe-3TPY system with those of the previously reported MSP-based EC supercapacitor materials in Table S1↗ of SI. The comparison revealed a higher coloration efficiency and capacitance for the Fe-3TPY system with a comparable energy and power density to previous reported materials. Finally, we have demonstrated a proof-of-concept LED bulb illumination experiment using our lab-built ITO/Fe-3TPY-/LiClO₄/ITO-based ECESD (Figurec), with a diameter of approximately 1.5 × 1.5 cm². The prepared three ECESD in series connection effectively illuminated 10-blue LED bulbs (∼2.8 V each) panel connected by parallel connection, as shown in Figured, once the devices were fully charged. The emission brightness of the LED panel gradually reduced as the devices regained their color from pale yellowish (charge state) to pristine pink (discharge state) over time, as shown in Figured–g. Therefore, the EC color-indexed hybrid energy storage device has the potential to be used as a visually monitored energy storage management system. As the reference, the air-referenced transmittance of the devices is also provided in Figure S7↗, Supporting Information.