Anthraquinone‐Functionalized Polydiacetylene Supercapacitors

Organic supercapacitors are considered attractive alternatives to traditional inorganic‐based charge storage devices due to their synthetic versatility, low cost, and environment‐friendliness features. Photopolymerized anthraquinone‐polydiacetylene is employed as a core component in high‐performance asymmetric supercapacitors (ASCs). Specifically, interspersed polydiacetylene‐anthraquinone/polyaniline (PANI) electrodes are prepared via drop‐casting and used as cathodes in devices employing polypyrrole/reduced graphene oxide anodes using aqueous or ionic liquid electrolytes. The excellent electrochemical properties of the polydiacetylene‐anthraquinone/PANI electrodes, specifically high capacitance (specific capacitance ≈720 F g−1 at 1 A g−1), long discharge time, and cycling stability, are ascribed to the superior redox profile of the anthraquinone and ambipolar charge transport associated with the polydiacetylene framework. The asymmetric supercapacitor prepared using the polydiacetylene‐anthraquinone/PANI electrodes displays a high energy density of 36 Wh kg−1 at a power density of 995 W kg−1, underscoring possible utilization of the anthraquinone‐polydiacetylene derivative in practical energy storage devices.


Introduction
Supercapacitors are key components in charge storage technologies due to their high-power density, moderately high energy density which is better than conventional ceramic capacitors, maintenance-free nature, and stability. Organic supercapacitors have gained particular interest due to their less adverse environmental impact, structural and functional variability, and DOI: 10.1002/adsu.202300035 pseudocapacitive properties often enhancing their electrochemical profiles and charge storage capacities. Organic supercapacitor technologies have yet to make practical breakthroughs due to their limited performance as compared to transition metal-based devices and challenges in maintaining long-term stability of these devices.
Quinone-based aromatic molecules have been touted as promising constituents in organic supercapacitors. [1] Quinones exhibit rapid reversible redox reactions that are important to attain high coulombic efficiency. [1a] Anthraquinones in particular have been used as components in supercapacitor electrodes. Luo et al., for example, employed condensation of 2,6-diaminoanthraquinone with nitrogen-rich cyanuric chloride to produce an electrode displaying specific capacitance of 184 F g −1 . [2] Xie et al. functionalized sulfur/nitrogen-doped biomass carbon with covalently grafted anthraquinone exhibiting high specific capacitance of more than 320 F g −1 recorded at 5 mV s −1 . [3] Anthraquinones, however, have encountered limitations in organic supercapacitor, including low intrinsic electrical conductivity, self-discharge, and dissolution in the electrolyte throughout the charge-discharge processes. [1a] Polydiacetylenes (PDAs) are a class of conjugated polymers exhibiting unique chromatic properties. [4] Specifically, the delocalized electrons in photopolymerized conjugated PDA networks have contributed to varied applications, particularly bio-and chemosensing. [4b,5] PDA systems have been employed in electrochemical systems, specifically a recent demonstration of high performance PDA-perylenediimide organic supercapacitor. [6] Indeed, the conjugated system which contributes to electrical conductivity may enhance charge storage properties in supercapacitor devices. [7] Porous polymers have been also used in organic electrodes used for energy storage devices. [8] Anthraquinone-enriched conjugated microporous polymers were employed as backbones in such porous electrodes for supercapacitors. [9] Other reports depict the use of porous anthraquinone in other applications such as CO 2 uptake and Li-ion batteries. [10] Here, we utilized polydiacetylene-functionalized anthraquinone prepared through photopolymerization of the monomer bis-diacetylene-anthraquinone as a core electrode constituent in asymmetric supercapacitors (ASCs), displaying excellent electrochemical properties. Specifically, an electrode comprising polydiacetylene-anthraquinone interspersed with the conductive polymer polyaniline (PANI) featured high specific capacitance, good cycling stability, and low intrinsic resistance. Asymmetric devices comprising polymerized bis-diacetylene-anthraquinone and PANI as the cathode and polypyrrole/reduced-graphene oxide as the anode yielded superior electrochemical properties, particularly high power, and high energy densities. The excellent performance of the devices is attributed to the redox properties of the anthraquinone moieties complemented by the double-layer charge storage afforded by the PDA backbone. The -conjugated PDA network additionally enhances the electrical conductivity in the electrode, likely producing a synergistic effect together with the PANI domains. Overall, this work presents a new, powerful organic supercapacitor design which may be employed in practical charge storage devices.

Design and Electrochemical Characterization of the Anthraquinone-Polydiacetylene Electrode
The chemical structure of bis-anthraquinone-diacetylene (bis-ADA) is shown in Figure 1A, highlighting the two quinones moieties partaking in redox reactions that are key to the electrochemical properties of the molecule (fourier transform infrared (FTIR) and X-ray photoelectron spectroscopy characterization of bis-ADA are provided in Figures S1 and S2, Supporting Information, respectively). Chloroform-dissolved bis-ADA monomers were drop-casted on a graphite sheet and photopolymerized (254 nm, 60 s), yielding the conjugated polydiacetylene network. [11] The representative scanning electron microscopy (SEM) image in Figure 1B shows the assembly of bis-anthraquinone-polydiacetylene (bis-APDA) microflakes furnishing porosity and high surface area that are important for the electrochemical performance. Figure 1C depicts the cyclic voltammetry (CV) curve recorded at a scan rate of 5 mV s −1 for a graphitic electrode coated with bis-APDA, specifically indicating the two redox peaks corresponding to the quinone moieties. The rectangular shape of the bis-APDA CV curve attests to the charge storage capabilities of the polymer and its utilization in supercapacitor systems. Specifically, the anthraquinone core likely serves as a conduit for redox reactions (contributing to pseudo-capacitance), while the polydiacetylene backbone may have electric double layer capacitance (EDLC) type charge storage capabilities. [12] Indeed, PDA networks exhibit ambipolar charge transport, which may further enhance the electrochemical charge storage performance. [13] To employ bis-APDA in a high-performance supercapacitor, we prepared electrodes comprising bis-APDA interspersed with PANI, a conductive polymer widely used in electrochemical applications (Figure 2). [14] SEM analysis of PANI and bis-APDA/PANI composite underscores the pronounced porosity of the electrode-deposited films ( Figure  S3, Supporting Information).
The CV curves in Figure 2A underscore the enhanced charge storage capacity of mixed bis-APDA/PANI electrodes in comparison with electrodes comprising only bis-APDA or PANI. Indeed, the bar diagram in Figure 2B, showing the specific capacitance calculated from the CV curves, reveals that an electrode comprising bis-APDA and PANI at a 1:4 weight Figure 2. Electrochemical properties of the bis-anthraquinone-polydiacetylene/polyaniline electrodes. A) CV curves recorded for electrodes comprising different weight ratios of bis-APDA and PANI, at a scan rate of 5 mV s −1 . B) The specific capacitance calculated from the CV curves at 5 mV s −1 . C) CV curves of bis-APDA/PANI (1:4 weight ratio) recorded at different scan rates. D) Relationships between the absolute peak current (i p ) and scan rate (v) for the oxidation peak (black) and the reduction peak (red) calculated for the 1st electron transfer for anthraquinone moieties (at potential 0.5 and 0.39 V, respectively) showing linear relation within the scan rate range employed. E) Galvanostatic charge/discharge curves recorded in different current densities. F) Specific capacitance as a function of current density, calculated from the GCD curves. ratio exhibited the highest specific capacitance (720 F g −1 at a scan rate of 5 mV s −1 ). The electrochemical properties of pristine polydiacetylene-anthraquinone and PANI individually are shown in Figure S5, Supporting Information. Table S1 in the Supporting Information presents the specific capacitance for Bis-APDA, PANI, and composites calculated based on averaging the weight ratios of the individual components, as well as the experimentally obtained values, underscoring the synergistic effects of bis-APDA and PANI in the mixture, and the optimal 1:4 weight ratio. Nyquist plots ( Figure S6, Supporting Information) recorded for the electrodes comprising different bis-APDA/PANI ratios yielded the same outcome.
The CV curves, recorded in 5-200 mV s −1 for a bis-APDA/PANI electrode exhibiting a 1:4 weight ratio in 1 m H 2 SO 4 electrolyte, are presented in Figure 2C. The CV curves feature the distinct reduction and oxidation peaks of anthraquinone at 0.5 and 0.85 V, respectively. [15] Notably, even at higher scan rates, the redox peaks are still apparent indicating the electrode exhibits good pseudocapacitive properties. Figure 2D depicts the oxidation and reduction peak currents for the 1st electron transfer of anthraquinone at 0.5 and 0.39 V respectively, recorded in different scan rates. The linear dependence observed for both reduction current (I po ) and oxidation current (I pr ) account for the reversibility of the redox reactions. [16] Another set of redox peaks (the oxidation and reduction peak currents at 0.85 and 0.72 V) correspond to anthraquinone moieties at higher potentials, as the redox process for anthraquinone is a two-electron process. An additional small contribution from the inherent redox peaks of PANI at 0.27 and 0.62 V (oxidation peaks) and 0.16 and 0.51 V (reduction peaks) can be also discerned.
The galvanostatic charge-discharge (GCD) curves measured for the bis-APDA/PANI electrode (1:4 weight ratio) in constant currents within a potential range of 0-1.0 V further highlight the excellent capacitive properties of the electrode ( Figure 2E). The linear regions of the GCD curves are attributed to the EDL behavior of the electrode, whereas the shoulders correspond to the quinone redox reactions, reflecting the pseudocapacitive contributions. [17] Figure 2F depicts the specific capacitance values with respect to current density, calculated from the GCD curves. The specific capacitance values in Figure 2F are higher than many reported organic supercapacitor electrodes. [18] Specifically, the high maximal specific capacitance calculated at 1 A g −1 -720 F g −1 -likely underscores the pseudocapacitive properties of the bis-APDA/PANI electrode compared to other systems. [2,6a,18a,19] The graph in Figure 2F also demonstrates considerable capacitance retention at a high current density of 10 A g −1 , at more than 50% of the capacitance recorded at 1 A g −1 . A 92% capacitance retention after 5000 cycles ( Figure  S8, Supporting Information) indicates very high stability of the bis-APDA/PANI electrode.

Anthraquinone-Polydiacetylene/Polyaniline Electrode in Asymmetric Supercapacitors
Figures 3 and 4 present electrochemical characterization of asymmetric supercapacitors in which bis-APDA/PANI (1:4 weight ratio) constituted the cathode and a polypyrrole (PPy)/reduced graphene oxide (rGO) served as the anode. [20] Structural and electrochemical characterization of the rGO/PPy electrode are provided in Figures S9 and S10 in the Supporting Information. Figure 3 depicts the electrochemical properties of the bis-APDA/PANI-PPy/rGO device using 1 H 2 SO 4 aqueous electrolyte. Figure 3A shows the CV curves recorded in a 1.2 V window at different scan rates. The nonrectangular shapes of the CV curves in Figure 3A reflect the significant contribution of the redox reactions in bis-APDA/PANI. [15b,21] Figure 3B portrays the specific capacitance with respect to the inverse square root of the scan rate, calculated from the CV curves in Figure 3A. Notably, the specific capacitance of ≈140 F g −1 at 5 mV s −1 is on par or higher than many reported organic supercapacitors. [3,18c] Using the method reported by Trasatti and Lee, [22] we estimated the capacitance contributions from the EDLC and pseudocapacitance using Equation (1) [23] in which a represents the EDLC contribution and b reflects the pseudocapacitance contribution in the device. Through extrapolating the linear region of the graph in lower scan rates in which the Faradaic contributions to the specific capacitance are substantial, Figure 3B reveals that pseudocapacitance contributes around 40% of the total specific capacitance of the device. The significant pseudocapacitance likely accounts for the redox reactions at the anthraquinone units of bis-APDA. Furthermore, the interspersed PANI domains furnish enhanced conductivity and PANI-associated redox underscores synergistic effects. The GCD curves in Figure 3C, recorded in a 1.2 V voltage window, further reflect the pseudocapacitive properties of the asymmetric bis-APDA/PANI-PPy/rGO supercapacitor, furthermore indicating low resistance losses. [24] Figure 3D presents the specific capacitance calculated from the GCD curves, indicating a maximal specific capacitance of 145 F g −1 at 1 A g −1 , superior than many previously reported quinone-based supercapacitors. [2,3,25] The gradual decrease of the specific capacitance in higher currents (20% capacitance retention at 10 A g −1 ) is also indicative of a considerable pseudo-capacitance contribution. The cycling experiment in Figure 3E reveals high capacitance retention of around 80% after 5000 cycles, and the Nyquist plot in Figure 3F further underlies low series resistance (≈0.7 Ω) and charge transfer resistance (≈5.5 Ω), that are better compared to various reported organic supercapacitors. [26] The linear region in Figure 3F in the low frequency region yields an angle of ≈72°with the Z ' (Ω) axis, signifying a pronounced capacitive behavior. [27] To prepare an asymmetric supercapacitor operating in a wider potential window, we constructed a bis-APDA/PANI-PPy/rGO device utilizing a nonaqueous, ionic liquid electrolyte (1-ethyl-3-methylimidazolium hydrogen sulfate (EMIM + HSO 4 − ) in dimethylformamide (DMF); Figure 4). Figure 4A presents the CV curves recorded at different scan rates in a voltage window of 2 V. The CV curves in Figure 4A are nearly rectangular, attributed to the adsorption/desorption of electrolyte ions accompanying the formation of an electrochemical double layer at the electrode-electrolyte interface. [28] Faradaic redox reactions, contributing to the small humps in the CV curves, are observed in low scan rates (i.e., 5 mV s −1 ), accounting for electrolyte ion diffusion and interactions with the electrodes. We further tested the device in extended potential windows up to 2.3 V (Figure 4B), confirming stability. Beyond this voltage value, the electrolyte becomes unstable due to dissociation of DMF. [29] Figure 4C depicts the GCD curves recorded in the range of 1-10 A g −1 within the 2 V potential window. The almost linear GCD curves account for the formation of a double layer at the electrode/electrolyte interface, with lower pseudocapacitive contribution due to the viscous nature and large ionic sizes of the ionic liquids. [30] In addition, minor low IR drops are apparent in the GCD curves, indicating low series resistance of the device. [28] Figure 4D presents the specific capacitance calculated at different current densities, obtained from the GCD curves in Figure 4C. The maximal specific capacitance of 67 F g −1 is high compared to other organic supercapacitor devices utilizing ionic liquid electrolytes, ascribed to the improved conductivity and enhanced surface area furnished by the bis-APDA/PANI mixture. Furthermore, at a high current density (5 A g −1 ), 70% capacitance was retained, indicating good rate capability. The cycling experiment in Figure 4E further attests to a high, 80% capacitance retention after 5000 cycles. [31] The Ragone plot in Figure 4F highlights the high energy and power densities of the bis-APDA/PANI-PPy/rGO asymmetric supercapacitor. Specifically, the device generated a high energy density of 36.2 Wh kg −1 at a power of 995 W kg −1 which is superior compared to many reported systems. [2,3,18,25,32]  The pronounced energy and power densities accomplished in the bis-APDA/PANI-PPy/rGO device in both aqueous and nonaqueous ionic liquid electrolytes reflect the combined contributions of bis-APDA and PANI, providing high charge storage capacity and fast discharge. The excellent electrochemical properties of the electrode reflect both the redox reactions in the anthraquinone in Bis-APDA and the EDLC contributions of the polydiacetylene network, combined with the efficient electron transport properties of PANI. Specifically, in bis-APDA/PANI-PPy/rGO device operating in an aqueous electrolyte, the redox reactions, higher ion concentrations in the aqueous electrolyte, and lower resistance provided high charge storage as well as fast discharge contributing to high power properties. In the case of the nonaqueous ionic liquid electrolyte, the higher energy density arises from the EDLC mechanism afforded by both the Bis-APDA flakes and PANI units. Figure 5 shows the application of the asymmetric bis-APDA/PANI-PPy/rGO device utilizing ionic liquid electrolyte, in series arrangement in the electrical circuit, as a power source for a blue light-emitting diode (LED). The photograph in Figure 5B demonstrates the initial light intensity which is retained after 3 min, reflecting the high energy density of the device.

Conclusions
We synthesized a diacetylene-functionalized anthraquinonebis-diacetylene-anthraquinone-as a core electrode constituent in asymmetric supercapacitors, exhibiting excellent electrochemical properties. Specifically, an electrode comprising photopolymerized bis-diacetylene-anthraquinone interspersed with the conductive polymer PANI featured high specific capacitance, good cycling stability, and low intrinsic resistance due to its high redox contribution from anthraquinone moieties and complementing double-layer charge storage afforded PDA backbone. The APDA-PANI electrode generated a pronounced specific capacitance of 720 F g −1 at 1 A g −1 current density. Asymmetric devices comprising polymerized bis-diacetylene-anthraquinone and PANI as the cathode and PPy/rGO as the anode yielded superior electrochemical properties displaying a wide operating potential window of 2 V (extendable up to 2.3 V), with a high energy density of 36.2 Wh kg −1 at a power of 995 W kg −1 . Overall, this work presents new organic supercapacitor designs, taking

Synthesis of bis-ADA:
The synthesis of polyaniline was performed following the chemical route published earlier. [11] Synthesis of PANI: The synthesis of PANI was performed following the chemical route published earlier with a little modification in the process.
[14b] Briefly, 3.1 mL of aniline was dissolved in aqueous solution of 20 mL of concentrated HCl containing 190 mL of Deionised (DI) water. The solution was left under continuous stirring for 24 h at room temperature. Subsequently, an aqueous solution of ammonium persulfate (APS) was prepared by adding 7.6 g of APS in 10 mL of DI water. This APS solution was added slowly to the prepared aniline solution and stirred for 24 h in an ice bath. The obtained deep green precipitate was filtered and washed several times with ethanol and DI water. Finally, the product was dried at 60°C for 48 h.
Synthesis of Reduced Graphene Oxide: Graphene oxide was synthesized using the protocol reported earlier. [33] Subsequent reduction by L(+)ascorbic acid was carried out by dispersing the material in ethanol and stirring for 12 h. After the reduction, the material was collected by centrifugation and dried for 48 h at 70°C.

Synthesis of Polypyrrole:
Polypyrrole was synthesized using the chemical polymerization method using ferric chloride (FeCl 3 ) as the oxidizing agent. Briefly, 3 mL of pyrrole was dissolved using distilled water with the abovementioned oxidizing agent in a weight ratio of 1:2.4 (monomer: oxidizing agent) and kept stirring for 14 h at a temperature of (0-5°C). This resulted in black precipitation which was collected using filtration and washed several times with ethanol and DI water. Finally, the product was dried at 60°C for 48 h.
Characterization: SEM images were recorded on Verios 460 L FEI (Czech Republic) after a 5 nm of gold coating. The images were viewed at different magnifications, at an acceleration voltage of 5 kV. Electrochemical measurements were performed in a three-electrode configuration, in which the graphite sheet current collector coated with sample acted as the working electrode, platinum wire as the counter electrode, and Ag/AgCl as the reference electrode. 1 m H 2 SO 4 was used as the electrolyte and the measurements were performed at different scan rates and current densities. The electrochemical measurements of the asymmetric supercapacitor device were performed in a two-electrode configuration. For electrochemical impedance spectroscopy measurement, a 5 mV ac signal in the frequency range of 1 MHz to 50 mHz was used. All the experiments were performed on a BioLogic SP-150 instrument (Seyssinet-Pariset, France).
Preparation of Electrodes and Electrolytes: UV Induced Polymerized bis-ADA/Polyaniline Composite Electrodes: 15 mg bis-ADA was dissolved in 2 mL of toluene. The solution was then drop-casted on the surface of the graphite sheet current collector and dried for 2 h at room temperature. The resultant coated electrodes were further irradiated for 3 min with a UV lamp (254 nm, 16 W) to induce PDA cross-linking in ambient atmosphere. The color of the deposited composite film changed from yellowish-green color to dark brown. Then the prepared electrodes were further annealed on a hot plate (70°C for 6 h). In the case of rGO/PPy composite, materials were mixed well with perfluorinated resin solution containing Nafion in a material: Nafion weight ratio of 95:5 in ethanol and drop-casted onto the graphite sheet. These electrodes were used as electrodes after drying overnight at 70°C. For optimizing the electrode all the measurements were performed in 1 m aqueous H 2 SO 4 solution as electrolyte. In all the cases, the areal mass loading of the electrodes was ≈1 mg cm −2 .
Preparation of the ASC: The device was prepared using two 1 × 1 cm 2 electrodes drop casted similarly as stated earlier and dried overnight. The mass loading of the device was kept 1.6 mg in total following the mass loading is calculated considering the charge balance from both the electrode. For aqueous ASC 1 m H 2 SO 4 in DI and for nonaqueous ASC 1 m 1-ethyl-3-methylimidazolium hydrogen sulfate in DMF were taken as the electrolyte respectively. Whatman grade GF/A fine retention glass microfiber filter with a particle retention of 1.6 μm and thickness of 260 μm was as the separator. All the device components were then packed in CR2032 button cell to make a device.
Electrochemical Data Analysis: The specific capacitance (C) from CV curve for electrodes in three electrode configurations were estimated from Equation (2) whereas the specific capacitance device is calculated from Equation (3) where I is the current at a specific potential, ΔV is the potential/voltage window, m is mass of the active electrode material, and v is scan rate at which CV is performed. The specific capacitance from the galvanostatic charge/discharge curves from three electrodes, as well as device, were calculated using Equation (4) where ( I m ) is the specific charge-discharge current, ΔV is the potential/voltage window, and Δt is the discharge time of the electrode/device.
The energy density (E) of the electrode was calculated using Equation (5) and power density (P) was calculated using Equation (6) where all the notation has been followed as earlier.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.