Electrolyte Engineering in Redox-Enhanced Electrochemical Capacitors with Zn Anodes: The Role of Colorimetric Indicators
Ming Chen, Qinglong Luo, Xiaolei Wang, Kaiyuan Shi
TL;DR
Researchers developed color-changing dyes for use in zinc-ion capacitors, improving performance and allowing real-time diagnostics.
Contribution
The use of triphenylmethane dyes as colorimetric indicators in electrolytes for redox-enhanced zinc-ion capacitors is novel.
Findings
Colorimetric indicators enable proton-electron transfer, supporting health diagnostics and redox reactions.
Dye-based electrolytes extend Zn||Zn cell lifespan to 4,000 hours with 87.7% capacity retention after 20,000 cycles.
RZICs achieve 152.4 mAh g−1 capacity within a 0.2–1.6 V window using phenol–quinone transformation.
Abstract
Triphenylmethane dyes as colorimetric indicators were developed for fabricating functional electrolytes in redox-enhanced zinc-ion hybrid capacitors (RZICs), integrating pH buffering, electrochromic response, and redox activity.The colorimetric indicators exhibit proton-electron transfer behavior, where proton transfer enables health state diagnostics and electron transfer facilitates reversible redox reactions.The dye-containing electrolytes provide a wider voltage window, high capacity, and long cycling stability for high-performance RZIC devices.Incorporating colorimetric indicators extends the cycling lifespan of Zn||Zn cells to 4,000 h, allowing RZICs to deliver a capacity of 152.4 mAh g−1 within a 0.2-1.6 V voltage window, with 87.7% capacity retention after 20,000 cycles. Triphenylmethane dyes as colorimetric indicators were developed for fabricating functional electrolytes in…
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TopicsSupercapacitor Materials and Fabrication · Advanced battery technologies research · Advancements in Battery Materials
Introduction
Organic dye-based colorimetric indicators present a powerful approach for real-time monitoring and optimization of aqueous electrolytes [1]. These probes exhibit distinct chromatic transitions in response to electrochemical stimuli, including pH variations, metal-ion concentrations, and redox behavior, enabling real-time diagnostics while improving charge transfer and electrochemical stability [2, 3]. pH-sensitive dyes track proton activity [4, 5], whereas metal-ion indicators detect cations and interfacial byproducts to mitigate parasitic reactions [6, 7]. Meanwhile, the inclusion of redox-active molecules enables the identification of structural changes across various oxidation states, offering insights into charge transfer dynamics and electrolytic reaction mechanisms [8, 9]. The structural versatility of organic dyes enables tailored molecular design to enhance sensitivity and specificity for different electrochemical environments [10, 11]. Triphenylmethane dyes stand out due to their intense coloration, reversible redox behavior, and structural robustness [12]. These dyes possess solvatochromic characteristics that facilitate reversible color transition, allowing simultaneous operation as sensors and redox mediators [13, 14]. Furthermore, structural adaptability and proton-dependent colorimetric sensing render them promising candidates for advanced electrolyte design and continuous monitoring in electrochemical systems [15, 16].
Aqueous energy storage devices, including supercapacitors (SCs), zinc-ion batteries (ZIBs), zinc-ion hybrid capacitors (ZICs), and redox-enhanced zinc-ion hybrid capacitors (RZICs), operate through distinct mechanisms (Scheme 1a). The SCs store energy capacitively via surface charge accumulation (Fig. S1), while the ZIBs depend on bulk redox reactions for charge storage (Fig. S2). Electrochemical capacitors exhibit two primary behaviors [17, 18]: (i) non-Faradaic electrical double-layer capacitance and (ii) Faradaic pseudocapacitance involving charge transfer at the electrode–electrolyte interface. Symmetric SCs with activated carbon electrodes deliver high power density, although their energy density is limited due to the capacitive nature. In contrast, ZIBs/ZICs with metallic anodes offer higher energy densities but face anode degradation issues, such as dendritic growth and surface corrosion [19–21]. Traditional pseudocapacitive systems result in changes to the surface composition of electrodes through electron transfer. The redox-enhanced SCs, on the other hand, improve energy storage by facilitating ionic shuttling within aqueous electrolytes [22, 23], leading to enhanced energy density without compromising power performance (Fig. S3). The distinctive electrochemical behavior produces characteristic cyclic voltammetry (CV) profiles, as illustrated in Scheme 1b. The differences in interface kinetics can be identified through analysis using Dunn’s methods [24–26]. Furthermore, the combination of capacitive and redox-active behaviors in RZICs enables greater charge storage at a broadening voltage range [27, 28].Scheme 1. Illustrative representations and electrochemical characterization of aqueous energy storage devices. a Operational principles for supercapacitor, zinc-ion battery, hybrid capacitor, and redox-enhanced hybrid capacitor. b Corresponding CV profiles measured in-house for each system at 1 mV s^−1^, where quantitative analysis using Dunn’s method demonstrates the distinct kinetic behaviors of the different systems. c pH-dependent colorimetric response and UV–vis spectra of triphenylmethane dyes (PV, MCP, XO, and BCP, left to right) in aqueous solutions
Incorporating pH indicators into aqueous electrolytes offers a reliable tool for monitoring and regulating electrochemical conditions [29]. These indicators undergo distinct color transitions in response to variations in H^+^ concentration, enabling real-time visualization of proton transfer and electrolyte status [29, 30]. We herein explore four triphenylmethane dyes, including pyrocatechol violet (PV), m-cresol purple sodium (MCP), xylenol orange sodium (XO), and bromocresol purple sodium (BCP), which exhibit proton-dependent colorimetric responsiveness (Scheme 1c). Beyond pH sensitivity, these dyes exhibit reversible redox behavior due to proton–electron transfer (PET) behavior, enabling the fabrication of redox electrolytes [31]. The synergy between acid–base equilibria and redox behavior induces structural changes, leading to visible color shifts for dynamic electrolyte diagnostics. Our results demonstrated that the colorimetric indication, coupled with the electrochromic behavior of these dyes, presents a holistic approach to electrolyte engineering for redox-enhanced energy storage.
Experimental Section
Materials
Triphenylmethane dyes (PV, MCP, XO, and BCP), polyvinylidene fluoride (PVDF), and activated carbon (AC) were supplied by Aladdin. Zinc sulfate heptahydrate (ZnSO_4_·7H_2_O), oxalic acid, and Super-P carbon black were obtained from Alfa Aesar. Potassium hydroxide (KOH) was sourced from Thermo Fisher, while standard pH buffer solutions were supplied by SENHOPE Technology. Potassium permanganate (KMnO_4_) and sulfuric acid (H_2_SO_4_) were acquired from Guangzhou Chemical Reagent Ltd. All chemicals were used as received without further refinement. Before cell assembly, commercial zinc foil and titanium foil were polished using silicon carbide grinding paper.
Preparation of AC Electrodes for Electrochemical Capacitors
AC electrodes were prepared by mixing the AC powder, carbon black, and PVDF binders in N-methyl-2-pyrrolidone solvent at a weight ratio of 8:1:1. This mixture was then coated onto stainless steel substrates, which served as current collectors. The AC electrodes serve as both cathodes and anodes in SCs, and as cathodes in ZICs and RZICs.
Preparation of α-MnO2 Cathodes for Zn-ion Batteries
Cryptomelane-type α–MnO_2_ nanofibers were synthesized using a modified protocol based on previous studies [32]. In a typical method, a 100 mL solution of 0.1 M KMnO_4_ was mixed with 100 mL of 0.15 M oxalic acid at a rate of 1 mL min^−1^. The reaction was carried out at 60 °C for 1 h. The resulting brown material was filtered, washed with distilled water, and then annealed in a muffle furnace at 600 °C for 4 h to obtain α-MnO_2_ nanofibers. The cathodes of ZIBs were prepared by mixing α-MnO_2_ nanofibers, carbon black, and PVDF in a weight ratio of 8: 1: 1, followed by coating onto stainless steel substrates.
Fabrication of Electrolytes and Energy Devices
Baseline cells were fabricated using 2 M ZnSO_4_ (ZS) in deionized water. Redox electrolytes were prepared by adding 10 mM triphenylmethane dyes (PV, MCP, XO, and BCP) to 2 M ZnSO_4_ aqueous solutions, denoted as the xxx-containing electrolytes. The stability of metallic zinc was evaluated using Zn||Zn and Zn||Ti half-cells. Aqueous energy storage devices, including SCs, ZIBs, ZICs, and RZICs, were prepared using various electrolytes. Specifically, the SCs were constructed with symmetric AC electrodes in a 1 M H_2_SO_4_ electrolyte. The ZIBs consisted of an α-MnO₂ cathode and zinc anode in ZS electrolytes, while the ZICs were configured with an AC cathode and zinc anode in ZS electrolytes. The RZICs were prepared similarly to the ZICs but utilized redox electrolytes containing triphenylmethane dyes. The electrochemical performance of cells was evaluated by assembling 2032 coin-type cells, using a glass fiber (Whatman 1823–025) as the separator in 50 μL electrolyte.
Instrumental Characterization
The electrode morphology was analyzed using a Quanta 400FEG scanning electron microscope (SEM). Structural analysis was performed using X-ray diffraction (XRD) with a Rigaku D/max 2200 diffractometer using Cu-Kα incident radiation. Ultraviolet–visible (UV–vis) spectroscopy was conducted by a Shimadzu UV 1601 spectrophotometer with a 10 mm path-length quartz cuvette. The dynamic contact angles were measured on a KRUSS DSA100S analyzer by dropping 10 μL of the electrolytes on Zn foils. In situ electrochemical Raman spectroscopy was performed in a three-electrode system (Zn working, Ag/AgCl reference, Pt counter) at -50 and + 50 mV polarization for 15 min each, recorded by a Renishaw inVia instrument.
Electrochemical performance of half-cell and full-cell devices was evaluated using different electrolytes. CV measurements were recorded with a Chenhua CHI440c potentiostat at various scan rates. Additional measurements, including chronoamperometry (CA), chronopotentiometry (CP), linear sweep voltammetry (LSV), and electrochemical impedance spectra (EIS), were conducted using a GAMRY Interface 1010E workstation. CA measurements were performed at an overpotential of -150 mV, while CP analysis was carried out at a current density of 1 mA cm^−2^. EIS data were collected over a frequency range of 0.2 Hz to 100 kHz with an alternating voltage of 5 mV. Galvanostatic charge/discharge tests were performed using a LAND CT3001A system. The specific capacity was determined using the active mass of the AC materials.
Results and Discussion
Characterization of Colorimetric Indicators
Triphenylmethane dyes serve as effective titration indicators due to their vivid colorimetric responses and strong metal-ion binding properties [12]. These molecules feature a triphenylmethane frame with hydroxyl (-OH), sulfonate (-SO_3_^−^), carboxyl (-COOH), and halogen (-Br) groups that chelate metal cations through lone-pair electrons, modifying the electronic conjugation [6, 33]. The optical characteristics of dye-containing solutions are determined by their solvation behavior and physicochemical properties, such as polar functional groups, accessible surface areas, and solvent-excluded volume (Table S1). As shown in Fig. 1a, the PV solution shifts from violet in deionized water to orange in ZS electrolyte. The other three dyes exhibit similar color transitions attributed to the structural rearrangements upon metal-ion binding. It has been demonstrated that the –OH groups in triphenylmethane dyes facilitate metal-ion complexation and enable reversible redox activity through phenol–quinone conversion. Electrostatic potential (ESP) maps revealed that the –C = O group exhibited electron-withdrawing behavior, while the nucleophilic –SO_3_^−^ and –OH groups reflect the proton affinity of molecules (Fig. S4).Fig. 1CV profiles of fabricated ZICs with different electrolytes. a Photo of aqueous solutions (top) and ZS electrolytes (bottom) containing triphenylmethane compounds. CV profiles of b the baseline ZIC with ZS electrolyte and RZICs incorporating c PV, d MCP, e XO, and f BCP dyes at 10–50 mV s^-1^. g Square scheme for PET processes, with the colorimetric indicator represented as “Ind”. h Illustration of proposed PT reactions within the electrolyte (left) as well as ET reactions at the interface (right) of the RZIC, prepared with the BCP-containing electrolyte during the discharge process
Compared to the baseline ZIC prepared with ZS electrolyte (Fig. 1b), RZICs were successfully fabricated by redox electrolytes containing triphenylmethane dyes (Fig. 1c-f). CV analysis showed that while the ZS-based ZIC presented quasi-rectangular profiles, the dye-containing electrolytes exhibited distinct redox peaks, indicative of additional Faradaic contributions (Figs. S5 and S6). The electrochemical reversibility was found to depend on the molecular structure of dyes [8]. It is suggested that the halogen ligands enhance reversible charge storage, whereas sulfonate groups improve energy performance by increasing the aqueous solubility of redox-active species [8, 34]. As a result, the PV-containing electrolyte yielded the largest CV integrated area, suggesting high charge storage capacity. However, it exhibited irreversible reduction processes (Fig. 1c), likely due to dissolved oxygen interference [35]. The RZICs prepared by BCP-containing electrolyte exhibited aligned cathodic and anodic profiles due to the improved electrochemical reversibility relative to those with the PV-containing electrolyte (Figs. S7 and S8). The capacitive (k_1_v) and diffusion-controlled (k_2_v^1/2^) charge storage contributions were quantified using Dunn’s method [24], where v represents the scan rate of CVs and k denotes the proportional constant for each process (Fig. S9). The results show that the k_1_v contributions increased from 72%–92% for ZS electrolyte to 78%–95% for BCP-containing electrolyte across the same rate range, indicating the enhanced capacitive charge transfer for RZICs. Furthermore, increasing the BCP concentration in RZICs leads to a larger CV integrated area, attributed to the phenol–quinone redox transformation (Fig. S10). It is suggested that the RZICs, prepared with the colorimetric indicator, demonstrate the PET behavior [36, 37] throughout the electrochemical processes. The mechanism is illustrated in Fig. 1g, where the vertical line represents proton transfer (PT) within the electrolyte, enabling real-time visualization of electrochemical dynamics. The horizontal line depicts electron transfer (ET) at the electrode surface, which enhances the capacity of RZICs. The PET reactions occur simultaneously at the electric double layer, specifically in the electronic-ionic mixed conduction zone, as shown in Fig. 1h.
The electrochemical and coordination behavior of triphenylmethane-driven redox electrolytes was analyzed. Figure 2a demonstrates the cycling stability of symmetric Zn cells tested at 2 mA cm^−2^ and 1 mAh cm^−2^. The results showed that the ZS electrolyte experienced a short-circuited life of 94 h, while redox electrolytes, prepared by PV, MCP, XO, and BCP dyes, demonstrated extended cycle lives to 1,135, 2,188, 2,683, and 4,000 h, respectively. The chronopotentiometric analysis reveals that triphenylmethane-driven redox electrolytes showed a reduction of overpotential compared to the ZS electrolyte (Fig. S11), attributed to enhanced charge transfer that leads to finer Zn grains [38, 39]. Furthermore, the rate tests revealed that the redox electrolytes maintained stable operation at high currents, whereas the ZS electrolyte degraded at 10 mA cm^−2^ due to accelerated dendritic growth and water electrolysis [40, 41] (Fig. 2b). Under extreme current and capacity conditions of 30 mA cm^−2^ and 30 mAh cm^−2^, the redox electrolytes exhibited smoother voltage profiles and longer cycle life compared to the ZS electrolyte (Fig. S12). SEM images showed that the cycled Zn anode from ZS electrolyte exhibited granular dendrites with irregular platelets, while the incorporation of triphenylmethane dyes generated smooth and compact Zn deposition over plating/stripping (Fig. S13). Additionally, the BCP-containing electrolyte inhibited the formation of byproducts as confirmed by the XRD analysis (Fig. S14). Chronoamperometry tests indicated that the responding currents were reduced for triphenylmethane-driven redox electrolytes (Fig. S15) due to diminished nucleation barrier [42]. Figure 2c presents the Coulombic efficiency (CE) measurements using Zn||Ti cells. The ZS electrolyte experienced CE fluctuations around 50 cycles, while the redox electrolytes maintained stable CE profiles, validating the effectiveness of colorimetric indicators for mitigating parasitic reactions.Fig. 2. Characterizations of Zn stability and ZnSO_4_-based electrolytes. a Cycling behavior and b rate performance of Zn||Zn cells, as well as c Coulombic efficiencies of Zn||Ti cells in the ZS and redox electrolytes. d HOMO and LUMO energy levels of triphenylmethane molecules. e UV–vis spectra and photographs of the solutions with different BCP: ZnSO_4_ ratios. f Operational scheme of anodic interphase in the ZS electrolyte (left) and redox electrolyte (right)
Energy-level calculations provide insights into how triphenylmethane molecules suppress water electrolysis while delivering a colorimetric response (Fig. 2d). With lower LUMO energies than H_2_O, the anionic dyes create a thermodynamic driving force for preferential electron transfer to the Zn anode. The reduced orbital energy gap allows the dyes to outcompete water for surface interactions, minimizing hydrogen evolution and anode corrosion. Contact angle measurements provided interfacial compatibility, with smaller angles observed for triphenylmethane-driven redox electrolytes (Fig. S16). UV–vis analysis demonstrated molecular-level evidence of BCP-Zn^2+^ coordination. In aqueous solution, the BCP dye displayed the absorption bands at 305, 380, and 590 nm, attributed to the π–π*, n–π*, and charge transfer of phenol and benzenoid systems [43–45]. Titration with the ZS electrolyte leads to a decrease in the 590 and 305 nm bands, along with an increase in the 380 nm band due to the formation of a Zn^2+^-BCP complex (Fig. 2e). This behavior highlights the dynamic interaction between BCP anions and metal cations. The cation-dependent color shifts make triphenylmethane dyes suitable for use as metal-ion indicators [45]. The findings suggest that the colorimetric indicators help minimize anode degradation by coordination with cationic Zn^2+^ in the solvation sheath, thereby lowering the desolvation energy and promoting uniform zinc deposition (Fig. 2f). In addition, the dissociation of hydroxyl and sulfonate groups acts as a pH buffer, suppressing hydroxide accumulation and preventing byproduct formation [46]. The structural variations triggered by the PT process enable the monitoring of proton activity in aqueous electrolytes through their electrochromic behavior.
Analysis of Proton–Electron Transfer Behavior
The ionic behavior of triphenylmethane dyes was demonstrated through conductivity measurements. The results indicated that BCP-containing aqueous solutions exhibited a linear increase in conductivity (Fig. S17), while ZS electrolytes with a high BCP concentration showed a decrease due to the complexation effect (Fig. 3a). The electrochromic structural responses of the BCP-containing electrolyte were analyzed using in situ electrochemical Raman spectroscopy (Fig. 3b). No spectral changes were observed in the ZS electrolyte, indicating a lack of proton–electron transfer behavior. In contrast, the BCP-containing electrolyte displayed a C-H stretching peak at 1330 cm^−1^ and varying intensities at 1610 and 1580 cm^−1^ during the electrochemical process, reflecting the protonation–deprotonation behavior of BCP dye that leads to a quinone–phenolate hybrid structure with enhanced electron delocalization [47, 48]. EIS analysis demonstrates enhanced ion transport kinetics in the BCP-containing electrolyte, which exhibits lower charge transfer resistance compared to the ZS electrolyte (Fig. S18). Figure 3c illustrates the proposed structural transformations driving the electrochromism. The electrochromic behavior involves two processes: the redox process of the dyes (right side) at the interface, which features the PET reaction, and the acid–base equilibrium (left side) within the electrolyte that does not involve the ET behavior. As a buffer, the BCP dye exists in equilibrium between its protonated form (HBCP) and deprotonated form (BCP^−^) during the PT process, as described by the dissociation reaction:Fig. 3. Analysis of colorimetric indicators in electrolytes. a Conductivity measurement and photograph of ZS electrolytes with varied BCP concentrations. b In situ Raman analysis of the electrolytes during Zn plating in the ZS (left) and BCP-containing (right) electrolytes. c Schematic illustration of BCP’s structural transformation in acid–base equilibrium and charge transfer processes. LSV curves at 3 mV s^−1^ of Zn-Ti cells during d OER and e HER processes, prepared with the ZS and BCP-containing electrolytes. f Photographs of H-cells utilizing the BCP-containing electrolyte over the charge–discharge process
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{HBCP}}\rightleftharpoons {\mathrm{H}}^{+}{\mathrm{+}{\mathrm{BCP}}}^{-}$$\end{document}The acid–base dissociation of colorimetric indicators induces a tautomeric transition, resulting in distinct structural rearrangements (Fig. S19). The dissociation constant (K_a_) quantifies the equilibrium between protonated and deprotonated species in the PT process as [49, 50]:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{K}}_{\mathrm{a}}\mathrm{=}\frac{{\mathrm{c}}_{{\mathrm{BCP}}-}\cdot {\mathrm{c}}_{{\mathrm{H}}\mathrm{+}}}{{\mathrm{c}}_{\mathrm{HBCP}}}$$\end{document}The pKa measures acid strength and is the negative logarithm of Ka, reflecting the acid’s ability to release protons:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$${\mathrm{pK}}_{\mathrm{a}}\mathrm{=}-{\mathrm{lg}}\frac{{\mathrm{c}}_{{\mathrm{BCP}}-}\cdot {\mathrm{c}}_{{\mathrm{H}}\mathrm{+}}}{{\mathrm{c}}_{\mathrm{HBCP}}}$$\end{document}With the acid (HBCP) and its conjugate base (BCP⁻) coexisting, the pH-pKa relationship is described by:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{pH} = {\mathrm{pK}}_{\mathrm{a}}\left({\mathrm{HBCP}}\right)-{\mathrm{lg}}\frac{{\mathrm{c}}_{\mathrm{HBCP}}}{{\mathrm{c}}_{{\mathrm{BCP}}-}}$$\end{document}As the pH of the electrolyte decreases, the BCP^−^ undergoes protonation to form the HBCP species. Conversely, at elevated pH levels, the HBCP dissociates to release protons, which mitigates excessive acidity. This protonation–deprotonation behavior influences the electron distribution within the molecule, leading to observable colorimetric changes. Furthermore, the proton-exchange equilibrium dynamically buffers pH fluctuations by neutralizing acid/base perturbations (Fig. S20). Immersing metallic Zn in dye-containing solutions for 72 h induces visible color changes due to alterations in the local chemical environment (Fig. S21). The BCP-containing electrolyte demonstrates an enhanced voltage window compared to the ZS electrolyte, as evidenced by the measurements of the oxygen evolution reaction (OER) (Fig. 3d) and hydrogen evolution reaction (HER) (Fig. 3e). Tafel analysis reveals a steeper slope for the BCP-containing electrolyte, confirming the suppression of HER and OER processes (Fig. S22). Real-time AC//Zn H-cell observations reveal dynamic color changes: the electrolyte is initially light pink, the anode turns yellow upon charging, and upon discharging, the cathode remains yellow while the anode reverts to pink (Fig. 3f and Video S1). Repeated cycling intensifies the yellowing color at the cathode part due to pH fluctuations.
Optimizing the Performance of RZICs
Optimizing the operational voltage windows strikes a balance between energy density and electrochemical stability in energy storage devices. The pH buffering behavior of the indicator, facilitated by the PT process, enables the increase of the voltage window for RZICs (Fig. 4a). The ESP distributions of BCP molecules in their monoanionic (left) and protonated (right) forms are shown in Fig. 4b, reflecting their acid–base balances. The CV profiles in Fig. 4c–f compare the baseline ZIC and RZIC at 1 mV s^−1^ in different electrochemical ranges. Both ZS and BCP-containing electrolytes exhibit capacitive behavior in the low-voltage region (0.1–1.0 V) during cathodic scans (Fig. 4c, d). However, the BCP electrolyte demonstrates superior electrochemical activity in the anodic region (1.4–1.6 V), with a ~ 52.9% larger integrated CV area and well-defined redox peaks (Fig. 4e, f), suggesting enhanced charge storage capabilities. Voltage window optimization was quantified using S-value analysis as [51, 52]:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\mathrm{S} = \frac{{\mathrm{A}}_{\mathrm{p}}}{{\mathrm{A}}_{\mathrm{n}}}\mathrm{-1=}\frac{{\mathrm{Q}}_{\mathrm{p}}}{{\mathrm{Q}}_{\mathrm{n}}}\mathrm{-1}$$\end{document}where A_p_ and A_n_ are the areas of the blue and green regions, respectively, and Q_p_ and Q_n_ are the corresponding charges. A threshold of S < 0.1 defines the stable voltage range [51], yielding an optimal voltage range of 0.30–1.65 V for the baseline ZIC and 0.20–1.70 V for RZIC prepared by BCP-containing electrolyte (Fig. 4g).Fig. 4S-value analysis of fabricated ZICs with different electrolytes. a Scheme of the electrochemical window. b Calculated ESP distributions for the BCP molecule in deprotonated (left) and protonated (right) forms. CV profiles at 1 mV s^−1^, measured from c and d cathodic scan (1.0 to 0.1 V) and e, f anodic scan (1.4 to 1.9 V) in c, e ZS and d, f BCP-containing electrolytes. g Schematic representation of CV calculations and the relationship between S-value and vertex voltage for fabricated ZICs
Further refinement using the Weingarth criterion (d2S/d**U2 ≤ 0.05) [53] enables differentiation between double-layer capacitive and faradaic contributions while minimizing background interference. The second derivative at a given vertex potential (marked in Fig. 4g) is calculated as:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta_{1} = \frac{{S_{2} - S_{1} }}{0.05V}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\Delta_{2} = \frac{{S_{3} - S_{2} }}{0.05V}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\frac{{d^{2} S}}{{dU^{2} }} = \frac{{\Delta_{2} - \Delta_{1} }}{0.05V} \le 0.05$$\end{document}This stringent analysis refined the stable voltage windows to 0.45–1.50 V for ZS electrolyte and 0.20–1.60 V for BCP-containing electrolyte. S-value profiles show that the BCP-containing electrolyte exhibits gentler increases above 1.6 V and below 0.2 V, whereas the ZS electrolyte displays abrupt deviations. The results demonstrated that the redox electrolytes, prepared by colorimetric indicators, enhance charge storage capacity while broadening the electrochemical window.
The introduction of colorimetric indicators enhances the adsorption behavior and redox kinetics of RZICs. As shown in Fig. 5a, triphenylmethane molecules (e.g., PV and BCP), featuring conjugated aromatic structures, exhibit lower adsorption energy on AC materials compared to H_2_O and SO_4_^2−^. This preferential adsorption promotes efficient charge storage through enhanced redox reactions with ET behavior at the electrode–electrolyte interface [54]. Unlike conventional redox mediators [55, 56]—such as inorganic species and organic polyanion compounds—triphenylmethane indicators inhibit ion migration via strong π-π interactions with the carbon electrode, thereby suppressing parasitic shuttling while preserving high electron transfer efficiency [57, 58]. Differential charge density analysis reveals that the BCP molecule exhibits pronounced charge localization around its bromine substituent, resulting in a more pronounced charge density difference than the PV (Fig. S23). These benefits were demonstrated by self-discharge tests on fabricated ZICs using ZS and BCP-containing electrolytes. After charging to the cutoff voltage at 1 A g^−1^ and resting for 24 h under open-circuit conditions, the baseline ZIC showed a capacity retention of 71.5% (Fig. 5b), whereas the RZIC prepared by BCP-containing electrolyte maintained 72.8% with reduced self-discharge (Fig. 5c). Long-term cycling tests at 1.0 A g^−1^ showed that the ZS, PV-, and BCP-containing electrolytes retained capacities of 118.9, 135.1, and 152.4 mAh g^−1^ after 7,000, 15,000, and 20,000 cycles, respectively, corresponding to retention rates of 78.1%, 75.5%, and 87.7% relative to their initial capacities (Figs. 5d and S24). Furthermore, charge–discharge profiles demonstrated that RZICs with PV- and BCP-containing electrolytes exhibited improved discharge capacity with overlapping voltage curves over extended cycling (Fig. S25). Additionally, the RZICs delivered higher capacity and improved retention at elevated current densities (Fig. 5e).Fig. 5. Adsorption of triphenylmethane molecules and electrochemical characterization of fabricated ZICs. a DFT analysis for the adsorption of H_2_O, SO_4_^2−^, PV, and BCP molecules on the AC materials, along with the adsorption energies and differential charge density (yellow representing charge accumulation and blue representing charge depletion). Self-discharge behavior of b the baseline ZIC with ZS electrolyte and c the RZIC with BCP-containing electrolyte. d Cycling stability, e rate capability, and f corresponding CE and EE plots at different currents for the baseline ZIC with ZS electrolyte and RZIC with BCP-containing electrolyte
Redox-enhanced electrolytic systems combine high energy density with rapid kinetics, yet their practical performance hinges on efficiency metrics beyond Coulombic efficiency (CE) reversibility. Figure 5f compares the CE, voltage efficiency (VE), and energy efficiency (EE) of fabricated ZICs at varying currents, calculated as [59]:
\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$CE = \frac{{Q_{dis} }}{{Q_{ch} }}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$VE = \frac{{\overline{{V_{dis} }} }}{{\overline{{V_{dis} }} }} = \frac{{\frac{1}{{Q_{dis} }}\int_{q \le 0} {Vdq} }}{{\frac{1}{{Q_{ch} }}\int_{q \ge 0} {Vdq} }}$$\end{document} \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$EE = CE \times VE = \frac{{\xi_{dis} }}{{\xi_{dis} }} = \frac{{\mathop \smallint \nolimits_{q \le 0} Vdq}}{{\mathop \smallint \nolimits_{q \ge 0} Vdq}}$$\end{document}While CE quantifies charge reversibility (Q_dis_/Q_ch_), VE measures voltage hysteresis (V̄_dis_/V̄_ch_), and EE reflects energy utilization (ξ_dis_/ξ_ch_). It has been demonstrated that EE proves more sensitive than CE in assessing cycling stability for redox-enhanced electrochemical capacitors, as it considers both charge and voltage degradation effects [60]. The calculations showed that the RZIC employing BCP-containing electrolyte demonstrated more stable CE and higher EE than the baseline ZIC across all current densities (Fig. 5f). Rate-dependent charge–discharge profiles confirmed that the BCP-containing electrolyte mitigated voltage decay in comparison with the ZS electrolyte (Fig. S26). The RZICs developed in this work exhibited electrochemical performance comparable to that of redox-enhanced hybrid capacitors reported in the literature (Table S2). The results highlight the synergy between redox-mediated electrolyte design and electrochromic behavior facilitated by colorimetric indicators.
Conclusions
This work developed state-of-the-art triphenylmethane dyes as the colorimetric indicators within redox electrolytes for RZICs. The electrochromic structural responsiveness, driven by the PET behavior, was analyzed using UV–vis and electrochemical Raman techniques. The acid–base dissociation of the dyes facilitates the pH buffering behavior via the PT process, allowing for real-time monitoring of electrolyte conditions. The phenol–quinone redox transformation, coupled with metal cation coordination, promotes interfacial ET reactions with indicative electrochemical signatures. Electrochemical results revealed that the incorporation of triphenylmethane indicators extended the cycling lifespan of Zn||Zn cells from 94 to 4,000 h. The RZICs with BCP-containing electrolyte demonstrated a wider voltage window of 0.2–1.6 V with stable operation due to tunable proton regulation and suppression of water electrolysis. The use of BCP-containing electrolyte achieved a higher capacity of 152.4 mAh g^−1^ with improved cycling stability after 20,000 cycles compared to the baseline ZIC. These findings introduce new pathways for the development of functional electrolytes for redox-enhanced electrochemical capacitors.
Supplementary Information
Below is the link to the electronic supplementary material.Supplementary file1 (MP4 3852 KB)Supplementary file2 (DOCX 3003 KB)
The reference list from the paper itself. Each links out to its DOI / PubMed record.
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