An Azo‐Based Electrode for All‐Around High‐Performance Flexible Supercapacitors

The photo‐rechargeable supercapacitor enables the self‐powering of flexible wearable electronics. However, flexible wearable electronics require supercapacitors not only with excellent flexibility but also with high energy density. P‐diaminoazobenzene (P‐Azo) as a new type of organic electrode material with NN is directly connected to the benzene ring and forms a large π‐conjugated system, which makes it have a lower lowest unoccupied molecular orbital (LUMO) energy level, is beneficial to transfer of electrons, and increases the conductivity of organic molecules. In addition, NN can realize the transfer of two electrons, which makes P‐Azo have a higher energy density. Asymmetric flexible supercapacitors are fabricated by assembling P‐Azo, activated carbon, and an adhesive electrolyte, with 425.2 mW h cm−2 (55.19 Wh kg−1) energy density at a power density of 80 mW cm−2 (10.38 W kg−1), and 90.7% capacitance retention after 80 000 cycles of bending. In this work, supercapacitors and perovskite submodules are coupled to prepare a photo‐rechargeable supercapacitor to achieve a 7% overall energy‐conversion efficiency. Therefore, this supercapacitor paves a practical route for powering future wearable electronics.

DOI: 10.1002/smsc.202200101 The photo-rechargeable supercapacitor enables the self-powering of flexible wearable electronics. However, flexible wearable electronics require supercapacitors not only with excellent flexibility but also with high energy density. P-diaminoazobenzene (P-Azo) as a new type of organic electrode material with N═N is directly connected to the benzene ring and forms a large π-conjugated system, which makes it have a lower lowest unoccupied molecular orbital (LUMO) energy level, is beneficial to transfer of electrons, and increases the conductivity of organic molecules. In addition, N═N can realize the transfer of two electrons, which makes P-Azo have a higher energy density. Asymmetric flexible supercapacitors are fabricated by assembling P-Azo, activated carbon, and an adhesive electrolyte, with 425.2 mW h cm À2 (55. 19 Wh kg À1 ) energy density at a power density of 80 mW cm À2 (10.38 W kg À1 ), and 90.7% capacitance retention after 80 000 cycles of bending. In this work, supercapacitors and perovskite submodules are coupled to prepare a photo-rechargeable supercapacitor to achieve a 7% overall energy-conversion efficiency. Therefore, this supercapacitor paves a practical route for powering future wearable electronics.
imine compounds (C═N), and carbonyl compounds (C═O). [17] S-S has a higher theoretical specific capacity, but the small organic sulfide molecules are easily soluble in the electrolyte during the redox process, reducing the electrical properties of the electrodes. [18] The most commonly used organic radical is 2,2',6,6'-tetramethylpiperidine-1-oxyl radical (TEMPO), which has a higher rate due to its high conductivity. However, there are fewer active sites in the organic radical molecule, so its energy density is lower. [19] The current organic electrode materials mainly focus on imines and carbonyl compounds, through the reversible electrochemical reaction of C═N or C═O with cations, to achieve charge storage and release. The flexible free-standing supercapacitor was prepared by 2,4,6-trimethoxy-1,3,5-benzene-tricarbaldehyde and 2,6-diaminoanthraquinone Schiff 's base reaction, the device had an areal specific capacity of 1600 mF cm À2 at a current density of 3.3 mA cm À2 . [20] The flexible porous anthraquinone polymer prepared by nucleophilic reaction of 2,6-diaminoanthraquinone and aryl bromides has a mass specific capacity of 576 F g À1 at a current density of 1 A g À1 . [21] In addition to the aforementioned four types of organic electrode materials, azo compounds, as a new type of organic electrode materials, whose active sites N═N can interact with metal ions through reversible interactions that can achieve high-capacity charge storage. [15,17] Among many azo compounds, P-diaminoazobenzene (P-Azo) with an active site (N═N) is directly connected to the benzene ring and forms a large π-conjugated system, which makes it have a lowest unoccupied molecular orbital (LUMO), which is beneficial to electrons transfer and increases the conductivity of organic molecules. [22] In addition, the active site (N═N) of the P-Azo can realize the transfer of two electrons, which makes the P-diaminoazobenzene to have a higher energy density. Here, for the first time, we used P-Azo as electrode materials and successfully fabricated flexible supercapacitors with high energy density, high power density, and fatigue resistance. Based on the aforementioned performance of these supercapacitors, they are coupled with PSC to prepare a photo-rechargeable supercapacitor. Utilizing photogenerated charge storage in PSC via N═N redox, the prepared photo-rechargeable supercapacitor achieves a 7% overall energy-conversion efficiency.

Electrochemical Performance of P-Azo
The N═N in p-diaminoazobenzene (P-Azo) acts as an active site for redox reactions. [18] Differing from anthraquinone organic molecules with one electron transfer per active site, the azo molecules perform two electron transfers per redox-active site. The theoretical specific capacity of the P-Azo is calculated using where n is the number of electrons transferred by the redox reaction, F is the faraday constant, and M w is the molecular weight of the organic molecule. According to Equation (1), the theoretical specific capacity of P-Azo is 252.55 mA h g À1 . This high specific capacity of P-Azo was used along with activated carbon to assemble high-energy-density, high-power-density, and fatigueresistant all-around high-performance asymmetric flexible supercapacitors, which were fabricated into photo-rechargeable supercapacitors ( Figure 1). Flexible electrodes were prepared by coating P-Azo with a carbon cloth surface. Scanning electron microscope (SEM) images ( Figure S1a, Supporting Information) and transmission electron microscope (TEM) images ( Figure S1b, Supporting Information) showed that P-Azo was coated on the surface of carbon cloth. The energy-dispersive X-ray spectroscopy (EDX) mapping images showed that C and N were uniformly distributed on the electrode surface ( Figure S1c, Supporting Information), and X-ray photoelectron spectroscopy (XPS) spectra also confirmed the existence of C and N ( Figure S2, Supporting Information).
In the electrolyte, different cations have different physical and chemical properties, such as binding energy, ionic radius, number of transferred electrons, etc., which affect the electrical www.advancedsciencenews.com www.small-science-journal.com properties such as redox potential and specific capacity. The cyclic voltammetry (CV) curves of P-Azo in 1 mol L À1 HCl, LiCl, NaCl, MgCl 2 , ZnCl 2 , and AlCl 3 electrolytes were generated ( Figure 2a). In the different electrolytes, P-Azo shows different oxidation peaks, which are located at 0.197 (Figure 2b), À0.012 ( Figure S3a, Supporting Information), 0.017 ( Figure S3b, Supporting Information), 0.098 (Figure 2c), 0.247 (Figure 2d), and À0.35 V ( Figure S3c, Supporting Information). It is worth mentioning that P-Azo also showed obvious reduction peaks in HCl, MgCl 2 , and ZnCl 2 , indicating that N═N undergoes reversible redox reactions in these solutions. However, in Li þ , Na þ , and Al 3þ solutions, there is no obvious reduction peak, which indicates the reversibility of N═N is poor in the aqueous solutions of these three ions.
The oxidation peak current of P-Azo showed an increasing trend with the increase in scan rate. The surface capacitiveand diffusion-controlled contributions of P-Azo during charge and discharge can be calculated from curve fits using a power law [23] log where i is the peak current and v is the scan rate. The value of b is obtained by plotting logi versus logv and calculating the slope. If b is close to 0.5, it means that the charge-discharge process is mainly controlled by diffusion; if b is close to 1, the chargedischarge process is mainly controlled by surface diffusion. Figure 2e shows the regression equation fitted to log i versus log v for HCl, MgCl 2, and ZnCl 2 solutions, and the slopes, b, have been calculated (Table S1, Supporting Information). It can be observed that P-Azo electrodes have different slopes in different electrolytes. The b values of the P-Azo electrode at the oxidation and reduction peaks in MgCl 2 are 0.57 and 0.62, respectively, indicating that the redox reaction of P-Azo electrodes is mainly controlled by diffusion in the MgCl 2 solution. In the HCl and ZnCl 2 solutions, the P-Azo electrodes exhibit larger b values, indicating that the diffusion-controlled contribution of the electrode in these two solutions is gradually weakened. The previous results show that the nitrogen anions formed after oxidation of www.advancedsciencenews.com www.small-science-journal.com N═N have different binding forces with different cations. [24] The electrochemical impedance spectroscopy (EIS) curves show that P-Azo has different diffusion resistances in different solutions ( Figure S4, Supporting Information), which further confirms that the P-Azo has different binding forces in different cation solutions.
To quantitatively calculate the proportion of capacitive control and diffusion control, the capacitive contribution rate of the electrode is calculated using Equation (3) [23] i where k 1 v is the capacitive contribution current and k 2 v 1/2 is the diffusion contribution current. The capacitance contribution ratios of the P-Azo were calculated at the scan rate of 2-10 mV s À1 . At the scan rate of 2 mV s À1 , the capacitance contribution of the P-Azo electrodes in the MgCl 2 solution was only 29% ( Figure S5a, Supporting Information), which was lower than those in the HCl solution (48.4%) ( Figure S5b, Supporting Information) and ZnCl 2 solution (37.5%) ( Figure S5c, Supporting Information), further confirming that the redox reaction of the electrode in MgCl 2 solution is mainly controlled by diffusion. At an increased electrode scan rate of 1000 mV s À1 , the shape of the CV curve is still well maintained ( Figure S6, Supporting Information), indicating that P-Azo electrodes have fast ion-diffusion and charge-storage capability. [25] Galvanostatic charge-discharge (GCD) curves were obtained to evaluate the energy-storage capability of the P-Azo in different solutions. At various charge and discharge current densities of the electrode, the GCD curves of the P-Azo were generated in 1 mol L À1 HCl (Figure 2f Figure S7c, Supporting Information) solutions. The P-Azo has a longer charge-discharge time in MgCl 2 and ZnCl 2 , indicating that N═N has a better storage capacity for Mg 2þ and Zn 2þ . In MgCl 2 solution, the electrode has an areal specific capacity of 456.98 mF cm À2 at a current density of 0.25 mA cm À2 (Figure 2i), which is higher than the 333.87 mF cm À2 in ZnCl 2 solution, 128.82 mF cm À2 in HCl solution, 90.64 mF cm À2 in LiCl solution, and 27.53 mF cm À2 in NaCl solution. This result shows that, compared with other kinds of electrolytes, P-Azo has a higher charge-storage capacity in MgCl 2 .

Electrochemical Reaction Mechanism of Organic Electrodes
In the Mg 2þ electrolyte, the adsorption and desorption of ions occur through the redox of the N═N bond. To analyze the structural changes of P-Azo during the charging and discharging process, ex situ Raman spectroscopy was performed to investigate the N═N redox reaction. When the P-Azo gains two electrons during discharge and is opened to form a nitrogen anion and a coordination bond with Mg 2þ (Figure 3a), the N═N Raman absorption peak at 1455 cm À1 gradually disappears (Figure 3b). [14] When the electrode loses electrons after charge, the absorption peak reappears at 1455 cm À1 and the peak intensity gradually increases, indicating that the N═N structure is formed again, which is consistent with previous test results. [26] To further prove that the electrons of N═N become nitrogen negative ions during the charging and discharging process, the ultraviolet spectrum of the MgCl 2 electrolyte was acquired after 500 cycles of charge and discharge, and for comparison, the ultraviolet spectrum of P-Azo in aqueous solution (10 À5 mol L À1 ) was also obtained. Among them, P-Azo has a strong π-π* transition absorption peak at 392 nm in a 10 À5 mol L À1 aqueous solution ( Figure S8, Supporting Information), [27] while the π-π* transition absorption peak intensity in the electrolyte after 500 charge and discharge cycles is significantly reduced, indicating that the organic electrode molecules dissolved in MgCl 2 mainly exist in the form of nitrogen anions.

Preparation of Photo-Rechargeable Supercapacitors
Based on the excellent electrochemical performance of the P-Azo/MgCl 2 /AC FSC, the anode and cathode of the PSC are connected to the anode and cathode of the supercapacitor, respectively, to prepare photo-rechargeable supercapacitors ( Figure 6a). Under illumination, the electrons and holes generated by the PSC flow into the cathode and anode of the supercapacitor to realize charge storage. The obtained PSC (FA 0.91 Cs 0.09 PbI 3 ) exhibited an open-circuit voltage of 1.6 V, a short-circuit current of 11.53 mA cm À2 , and a power conversion efficiency (PCE) of 14.7% ( Figure S12, Supporting Information).
To evaluate the photoelectric conversion performance of the photo-rechargeable supercapacitor, the charging curve of the supercapacitor was measured under sunlight at an intensity of 100 mW cm À2 , and then the light source was turned off to obtain the discharge curves under different current densities in the dark. When the discharge current of the P-Azo/MgCl 2 /AC device is 10 mA cm À2 , the charge time and discharge time are relatively close (Figure 6b), and the energy stored in the FSC is 52.89 mW h cm À2 . When the discharge current of the FSC is 0.5 mA cm À2 , the discharge time is much higher than the charging time, and the energy stored in the FSC is 47.77 mW h cm À2 . This shows that the FSC enables fast charge storage at high current densities. The formula for calculating the overall conversion efficiency of a photo-rechargeable supercapacitor is as follows [1] where E light is the light input energy, E output is the energy stored in the supercapacitor, E sc is the areal energy density of the supercapacitor, A sc is the effective area of the supercapacitor, P i is the light intensity (100 mW cm À2 ), Δt is the light charging time, and A (cm 2 ) is the effective light area of the solar cell. At a discharge current density of 1 mA cm À2 , the overall conversion efficiency of the photo-rechargeable supercapacitor is 7%, which is comparable to that of similar photo-rechargeable supercapacitor reported so far. [46] In the current density range of 0.5-10 mA cm À2 , the overall conversion efficiency of the photo-rechargeable supercapacitors varies between 5.2% and 7% ( Figure S13, Supporting Information), indicating that supercapacitors have good charge-storage capacity. This excellent electrochemical performance of photoelectric storage is due to the high energy density of these supercapacitors. These photorechargeable supercapacitors show great application potential in the fields of flexible wearable electronic devices, satellites, hybrid electric vehicles, robots, notebook computers, drones, and distributed photovoltaic power generation.

Conclusion
We first apply P-diaminoazobenzene (P-Azo) as a new type of organic electrode material to prepare high-energy-density, high-power-density, and fatigue-resistant all-around highperformance asymmetric flexible supercapacitors. The molecular structure of P-Azo not only increases the conductivity of organic molecules but also makes it have a high energy density. P-Azo is assembled with activated carbon into asymmetric flexible supercapacitors to achieve high areal specific capacity (1195.875 mF cm À2 at 0.1 mA cm À2 ) and energy density (425.2 mW h cm À2 at 80 mW cm À2 , 10.44 mW h cm À2 at 8000 mW cm À2 ). In addition, the flexible supercapacitor still has stable capacitance retention after bending at different bending angles and 80 000 cycles. Supercapacitors and perovskite submodules are coupled to prepare a photo-rechargeable supercapacitor to achieve a 7% overall energy conversion efficiency. Therefore, this flexible supercapacitor paves a practical route for powering future wearable electronics.

Experimental Section
Preparation of Organic Electrodes: P-diaminoazobenzene, acetylene black, and polyvinylidene fluoride were uniformly mixed in N-methyl pyrrolidone in a ratio of 7:2:1 by mass, then coated onto carbon cloth, and dried in a drying oven at 70°C. The loading of active material in the electrode was 2 mg cm À2 .
Fabrication of Flexible Supercapacitors: First, 4 g of MgCl 2 was dissolved in 10 g of deionized water. Then, 4 g of acrylamide and 100 μL of photoinitiator 2-hydroxy-2-methyl-1-phenyl-1-acetone were added to it. The polyacrylamide ion gel electrolyte was obtained by free-radical polymerization under ultraviolet irradiation for 5 min.
The organic electrode was used as the negative electrode, the activated carbon was used as the positive electrode, and the polyacrylamide ion gel was used as the electrolyte, all of which were assembled into an all-solidstate flexible supercapacitor.
Preparation of Photo-Rechargeable Supercapacitor: Fabrication of PSCs was performed by our reported method. [47] To form the photorechargeable supercapacitor, the cathode of the PSC was connected to the cathode of the supercapacitor with a wire, and the anode of the PSC was connected to the anode of the supercapacitor. An electrochemical workstation was used to test the charge-discharge curve of the supercapacitor.
Electrochemical Performance Test: The electrochemical performance of the electrodes was tested using a three-electrode system with Pt as the counter electrode, Ag/AgCl as the reference electrode, and either 1 mol L À1 HCl, 1 mol L À1 LiCl, 1 mol L À1 NaCl, 1 mol L À1 MgCl 2 , 1 mol L À1 ZnCl 2 , or 1 mol L À1 of AlCl 3 as the electrolyte. CV curves, GCD curves, and EIS curves of the test electrode were obtained. The electrochemical performance of the all-solid-state flexible supercapacitor was tested using a two-electrode system.
The areal specific capacity of the electrodes was calculated using Equation (5) where C is areal specific capacity, I is the charge-discharge current of the electrode, Δt is the charge-discharge time, ΔV is the electrochemical window of the electrode, and A is the area of the electrode. The areal energy densities (E) of the electrodes were calculated using Equation (6) The areal power densities (P) of the electrodes were calculated using Equation (7) P ¼ E Δt Â 3600 (7)

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