Boosting the Capacitance of an Aqueous Zinc-Ion Hybrid Energy

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Boosting the Capacitance of An Aqueous Zinc-Ion Hybrid Energy Storage Device by Using Poly(3,3’dihydroxybenzidine)-Modified Nanoporous Carbon Cathode Na Wang, Tuo Xin, Yi Zhao, Qi Li, Mingjun Hu, and Jinzhang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.9b02935 • Publication Date (Web): 24 Jul 2019 Downloaded from pubs.acs.org on July 25, 2019

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ACS Sustainable Chemistry & Engineering

Boosting the Capacitance of An Aqueous Zinc-Ion Hybrid Energy Storage Device by Using Poly(3,3’dihydroxybenzidine)-Modified Nanoporous Carbon Cathode Na Wang, Tuo Xin, Yi Zhao, Qi Li, Mingjun Hu, Jinzhang Liu*

School of Materials Science and Engineering, Beihang University, Beijing 100083, China. *E-mail: [email protected] KEYWORDS: Zinc-ion energy storage device; Organic electrode; Activated carbon; Pseudocapacitance; Electropolymerization;

ABSTRACT

Aqueous electrochemical energy storage devices are highly safe, low cost, and environmentally benign, yet suffer from low energy storage capacity. Here, we devise a novel cathode material for making aqueous Zn-ion hybrid energy storage devices with

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high areal capacitance. A pseudocapacitive polymer, poly(3,3’-dihydroxybenzidine, DHB), is electrodeposited onto the surfaces of porous active carbon (AC) granules to increase the capacitance. This composite coating has high mass loading, leading to high areal capacitance in F cm-2 scale. The flexible sandwich-structured cell made by combing the poly(3,3’-DHB)/AC cathode and the Zn foil anode shows stable electrochemical performance upon bending. The areal capacitance of this cell is up to 1.3 F cm−2, and the maximum energy and power densities are 0.18 mWh cm-2 and 4.01 mW cm-2, respectively. Moreover, a Zn-ion micro cell is fabricated by combing two sets of carbon-paper-based finger electrodes, one is plated with Zn and the other is coated with the poly(3,3’-DHB)/AC composite. The in-plane micro cell shows a high areal capacitance of 1.1 F cm−2 and a high areal energy density of 152 µWh cm−2. Our research suggests a new approach to make high-capacitance Zn-ion hybrid energy storage devices with different forms to meet various applications.


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INTRODUCTION

Electrochemical energy storage devices play an important role in consumer electronics, and their safety, environmental benignity, and specific energy are important issues for merit evaluation.1,

2

The safety issue of rechargeable electrochemical cells with

flammable organic electrolytes has long been concerned, though the use of organic


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electrolyte promises higher operating voltage of the device. Aqueous electrolytes are superior to the organic ones in terms of low cost, fast ion mobility, and safety, hence aqueous rechargeable cells deserve more investigation.3-6 However, the water splitting voltage point at ~1.23 V leads to a relatively lower operating voltage of the cell, which limits the energy density. Therefore, exploring new approaches to increase the specific energy of an aqueous rechargeable cell is of great importance.

Supercapacitors have been regarded as one of the promising power sources owing to their fast charging and discharging rates, long cycling life, and high power density.7-9

Though

these

characteristics

cannot

be

found

from

conventional

rechargeable batteries, commercial supercapacitors in today’s market suffer from low energy density,10 because the energy storage relies on the physical adsorption of ions onto surface of porous activated carbon (AC) in the electrode. The hybrid electrochemical energy storage device with properties of both ionic battery and supercapacitor have been emerged, and the boundary between conventional supercapacitor and ionic battery is blurred.11 Very recently, aqueous Zn-ion


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supercapacitors comprising a Zn foil anode and an porous AC cathode were reported.12 The device showed a maximum capacity of 121 mAh g−1, corresponding to an energy density of 84 Wh kg−1. Zhang et al. reported Zn-ion hybrid micro-supercapacitors (MSCs) with the maximum areal capacitance of 1.3 F cm−2 at 0.16 mA cm−2 and the maximum energy density of 115.4 µWh cm−2.13 Though the Zn electrode with a low redox potential (-0.76 V for standard hydrogen electrode) and a high theoretical capacity (823 mAh g−1 for Zn/Zn2+) is superior to alkali metals such as Li, Na, and K, in terms of high safety, low cost, and multivalence, the specific capacitance of the Zn//AC device is determined by the specific surface area of porous AC in the cathode, as the charge storage is governed by the electric double-layer capacitance. Pseudocapacitors based on redoxactive electrodes rely on reversible chemical reactions for energy storage, which resemble ionic batteries.10,

14-16

So far, various pseudocapacitive electrode materials,

including transition metal compounds and conducing polymers, have been reported and most of them exhibit much higher specific capacitance than the carbonaceous materials such as porous AC and graphene.15, 16 For electrode coatings of inorganic compounds or conducting polymers, a high mass loading is required to achieve high capacitance.


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However, with thickening the electrode coating, a common problem is that the gravimetric specific capacitance of electrode material would drop rapidly and the resistance is increased. Therefore, in the literature, supercapacitors with low electrode mass loadings were studied, and their areal capacitances are generally far below 1 F cm-2.15

Herein, we report aqueous Zn-ion hybrid energy storage devices (ZIHESDs) based on a novel organic cathode material, poly(3,3’-dihydroxybenzidine, DHB), which is electrodeposited onto porous AC to introduce pseudocapacitance. The poly(3,3’-DHB)/AC composite coated onto either carbon cloth or carbon paper is combined with the Zn anode to make devices in different forms, including the flexible sandwich structured device and the in-plane micro device with interdigital finger electrodes. Owing to the high mass loading of the cathode material, both devices can achieve high areal capacitance in F cm-2 scale, which is crucial for delivering high energy storage capacity of the cell.

EXPERIMENTAL SECTION Preparation of the sandwich-structured ZIHESD

First, a slurry was prepared by mixing the porous AC powder (YP-80, Kuraray Co.), carbon black, and polyvinylidene fluoride at a weight ratio of 8:1:1 in 1-methyl-2-


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pyrrolidone solvent. The slurry was applied onto a rectangular-shaped carbon cloth with thickness of 300 μm, followed by a drying process. Second, the 3,3’-DHB molecules were dissolved into 50 mL of 1 M H2SO4 aqueous solution to form a saturated solution, which was filled into a three-electrode cell for electropolymerization. The AC-coated carbon cloth used as work electrode, a Pt foil as counter electrode, and a Ag/AgCl reference electrode were immersed into the solution. By running CV loops at 20 mV s-1 within the voltage range of –0.15 — 0.85 V for ~300 cycles, 3,3’-DHB molecules were polymerized and deposited onto the mesoporous AC base. Third, the poly(3,3’DHB)/AC cathode and a 20-μm-thick Zn foil anode were stacked together, with a 500 μm-thick porous glass fibre membrane in between. The electrolyte was a 2 M ZnSO4 solution. Finally, the device was sealed in an Al-plastic film pouch by using hot melting glue for electrochemical measurements.

Preparation of the small-size planar ZIHESD

Here, the carbon paper instead of woven carbon cloth was used as both the substrate and the current collector. A UV laser machine was employed to cut the substrate to


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make comb-shaped finger electrodes. First, in a 2 M ZnSO4 aqueous solution, Zn was electrodeposited onto one set of finger electrodes for 1000 s by using a potentiostatic deposition method. Second, a piece of carbon paper coated with poly(3,3’-DHB)/AC, of which the preparation is above detailed, was cut by using the laser beam to make the cathode finger electrodes. Third, the two sets of finger electrodes, one as anode the other as cathode, were assembled together and fixed by using adhesive tape to form an interdigital configuration. After applying a drop of 2 M ZnSO4 to wet the electrodes, the cell was sealed by using Al-plastic films for electrochemical tests.

RESULTS AND DISCUSSION The three-electrode setup for electrodepositing poly(3,3’-DHB) onto the carbon-clothsupported AC coating is illustrated in Figure 1a. The aqueous solution containing 1 M H2SO4 and ~ 0.03 M 3,3’-DHB molecules was initially clear, and then turned into brown color with running the cyclic voltammetry (CV) cycles at 20 mV s-1 for electropolymerization, as shown in Figures 1b and c. The area of CV loops kept increasing with adding the CV cycles, indicating the increased capacitance of the


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carbon substrate by electrodepositing poly(3,3’-DHB). The porous AC has a high specific surface area of 2230 m2 g-1 (Figure S2). It is believed that the poly(3,3’-DHB) was not only electrodeposited onto the outer surfaces, but also filled into the nanopores of AC granules. X-ray photoelectron spectroscopy (XPS, Thermo Scientific Escalab 250Xi) was used to characterize the surface chemistry of the poly(3,3’-DHB)/AC electrode. The survey XPS spectra collected from the poly(3,3’-DHB)/AC and the bare AC film are displayed in Figure 1e. Both XPS spectra contain C 1s and O 1s peaks, while the poly(3,3’-DHB)/AC sample shows additional N 1s signal due to amino groups in the polymer. The AC coating contains 5.7 atom % oxygen. The poly(3,3’-DHB)/AC contains 27.3 atom % oxygen and 6.6 atom % nitrogen, confirming the presence of polymer. The high-resolution C 1s XPS spectrum of this sample, as shown in Figure 1f, reveals C=O, C–O, C–C, and C=C bonds in the polymer. It should be addressed that partial poly(3,3’-DHB) was precipitated in the bath during the electropolymerization process. The flocculent precipitate was collected for further FT-IR analysis, as shown in Figures 1g. The absorption peaks located at 3355, 3158, and 1504 cm-1 are assigned to the stretching vibrations of N–H, C–C and C=C in the benzene ring, respectively.17-19


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The peaks at 1633 and 1284 cm-1 correspond to the stretching vibrations of C=N and C– N bonds, respectively.20-22 The other two peaks at 1405 and 1109 cm-1 can be attributed to C–O–H and C-O-C stretching vibrations, respectively.23

Figure 1. (a) Illustration for the experimental setup for electrodepositing poly(3,3’-DHB) onto the porous AC coating. (b and c) Photographs of the three-electrode cell before and after the electrodeposition process, respectively. (d) The evolution of CV loop with cycling the CV test at 20 mV s-1 for electropolymerization. (e) Survey XPS spectra of the bare AC base and the poly(3,3’-DHB)/AC cathode. (f) High-resolution C 1s XPS spectrum of poly(3,3’-DHB)/AC. (g) FT-IR spectrum of pure poly(3,3’-DHB).


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Figure 2a shows a scanning electron microscopy (SEM, Zeiss Supra55) image of the carbon cloth used as current collector in our work. The carbon fibres are about 8 μm in thickness, as shown in the inset. The SEM image in Figure 2b depicts the coating of porous AC. The sizes of AC granules are about several micrometers. Though this carbon material has high specific surface area, the nanopores cannot be observed by our SEM. After the deposition of poly(3,3’-DHB), the surface morphology of a single AC granule is depicted by the high-magnification SEM image in Figure 2c. A close-view SEM image in the inset shows the agglomeration of polymer nanoparticles over the AC surface. For the SEM image in Figure 2d, C and N element mappings over the same area were obtained by using the energy dispersive X-ray spectrometry attached to SEM, and the results are shown in Figures 2e and f, respectively. The distribution of N signal in Figure 2f indicates the even coating of poly(3,3’-DHB) over AC granules.


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Figure 2. SEM images and element mappings. (a) The carbon cloth. (b) The porous AC base. (c) A single AC granule coated with poly(3,3’-DHB). The inset shows fine features of the deposited polymer. (d) A low-magnification SEM image of the poly(3,3’-DHB)/AC coating. C and N elemental mappings from the same area are shown in (e) and (f), respectively.

The poly(3,3’-DHB)/AC, used as cathode, was combined with a Zn foil anode to make sandwich-structured ZIHESD. A glass fibre membrane soaked with 2 M ZnSO4 aqueous electrolyte was squeezed between the two electrodes. For comparison, we also made a device by using bare AC coating as the cathode to match Zn foil. In Figure 3a, the two CV loops at 5 mV s-1 and within the voltage range from 0.5 to 1.5 V were


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collected from the Zn//poly(3,3’-DHB)/AC cell and the Zn//AC cell, respectively. Both cathodes have identical AC mass loading of 4.4 mg cm-2, hence it indicates that the capacitance can be increased by introducing poly(3,3’-DHB) to the AC coating, as the area of a CV loop is proportional to the capacitance. The mass gain of polymer after the electrodeposition process is 1.3 mg cm-2. Therefore, the overall mass loading of the poly(3,3’-DHB)/AC coating is 5.7 mg cm-2, and the weight percentage of polymer is 23%. However, the mass gain of electrodeposited polymer is also a function of the mass loading of AC base. As shown in table S1, the higher the mass loading of AC, the higher the net weight of poly(3,3’-DHB). Figure 3b shows CV loops of the Zn//poly(3,3’DHB)/AC cell at different voltage scan rates. Correspondingly, galvanostatic charge/discharge (GCD) curves of this device at different current densities are shown in Figure 3c. Specific capacitances can be deduced from the GCD curves. Our calculation using the slope of discharge curve reveals that the capacitance of poly(3,3’-DHB)/AC is about three times as high as that of bare AC, provided that both have identical AC mass loading (Figure S3). It is worth noting that the areal capacitance of the device is a function of the mass loading of porous AC in cathode, though the added poly(3,3’-DHB)


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enhances the overall capacitance. For further study, porous AC coatings with different mass loadings were used as the base for electrodepositing poly(3,3’-DHB), and the areal capacitances of different Zn-ion hybrid energy storage device based on these cathode coatings were shown in Figure 3d. The higher the AC mass loading, the higher the areal capacitance. However, the maximum capacitance would reach a saturation level when the AC mass loading is beyond 5 mg cm-2, as can be concluded from Figure 3d by comparing the two curves corresponding to AC mass loadings of 6.92 and 5.11 mg cm-2, respectively. Note that the device with the thickest AC coating reached a maximum capacitance of 1.3 F cm-2 at 0.5 mA cm−2. Even at 8 mA cm−2, the capacitance was retained above 0.6 F cm−2. Ragone plots of these devices with different AC mass loadings are shown in Figure 3e. For the device with AC mass loading of 6.92 mg cm-2, it delivers a maximum energy density of 0.18 mW h cm-2, corresponding to a power density of 0.26 mW cm-2. In addition, the device exhibits a superior cycling stability with capacitance retention of 80% after 5000 CD cycles (Figure 3f). Note that the Lead-acid battery can only sustain 400-500 deep cycles.


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Figure 3. (a) Two CV loops from a Zn//poly(3,3’-DHB)/AC cell and a Zn//AC cell, both have identical AC mass loading. (b and c) CV and GCD curves collected from the Zn//poly(3,3’-DHB)/AC cell, respectively. (d) Dependence of the areal capacitance on current density for Zn//poly(3,3’-DHB)/AC cells with different AC mass loadings. (e) Ragone plots of Zn//poly(3,3’-DHB)/AC cells with different AC mass loadings in the cathode. (f) Cycling stability of a typical device.

The Zn//poly(3,3’-DHB)/AC cell encapsulated into a flat pouch is bendable, as shown in the inset in Figure 4a. Flexible energy storage devices have the potential for


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wearable electronics,24-26 and their electrochemical behaviors are required to be stable when bent at large curvature. In Figure 4a, the CV loops at 5 mV s-1 of this cell curved at different angles ranging from 30° to 120° are quite similar in both shape and area, indicating a minor influence of cell bending on the energy storage performance. We also tested the cycling performance of a bent cell, by recording the capacity variation with continuously charging and discharging the device for 2000 cycles. As shown in Figure S6, when the bending angle is below 60°, the capacity retention remains unaffected by bending. However, when the device is bent at lager angles like 90° and 120°, the capacity retentions are a bit lower. To meet different practical applications, individual unit can be connected either in series to boost the voltage or in parallel to multiply the capacitance. In Figure 4b, the CD cycle from two pouch cells connected in series is within a voltage window from 0.5 V to 3 V, which is twice as wide as that of a single cell. In Figure 4c, according to the time spans of two CD cycles, one from a single cell and the other from two cells in parallel, it can be deduced that the parallel connection leads to doubled capacitance. As a demonstration, Figure 4d shows a LED night light powered by two pouch cells in series.


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Figure 4. (a) CV loops at 5 mV s-1 measured from a flexible cell at different bending angles. (b) CD curves from a single cell and two cells in series, respectively. (c) The comparison of CD cycle between a single cell and two cells in parallel. (d) Optical photograph of two cells in tandem connection for driving a LED night light.

Small-size power sources are in high demand due to the trend of miniaturization of electronic devices.27-29 In addition to the conventional sandwich structure, small-size planar ZIHESDs based on the poly(3,3’-DHB)/AC cathode were fabricated by using a


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laser-cutting technique. Using a UV laser machine, we cut comb-shaped finger electrodes from a nonwoven carbon fibre paper. One set of finger electrodes were electrochemically plated with Zn, as depicted in Figure 5a, to be used as anode. It can be seen from the inset that densely-packed Zn nanosheets are deposited onto the carbon fibre. Moreover, the color of carbon paper was changed from dark gray to light grey after the electrodeposition of Zn. (Figure S7) The presence of Zn is also confirmed by our X-ray diffraction analysis (Figure S8). The Zn-coated carbon fibres are about 30 μm in thickness. However, the thickness of a bare carbon fibre is about 8 μm, as seen in the SEM image in Figure 5g. Therefore, the thickness of Zn coating over a carbon fibre is about 11 μm. To prepare the cathode, the carbon paper was firstly coated with porous AC, of which the mass loading was approximately 5.2 mg cm-2. After electrodepositing poly(3,3’-DHB) onto the AC coating, the substrate was cut by using UV laser beam to make finger electrodes. Two sets of finger electrodes, one was coated with Zn and the other was coated with poly(3,3’-DHB)/AC, were assembled together to form a microcell, as illustrated in Figure 5d. A photograph and a micrograph of the small-size planar ZIHESD are shown in Figures S9a and b, respectively. The device was soaked with


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ZnSO4 aqueous electrolyte before sealed for electrochemical test. Figure 5b shows CV loops of the cell at different voltage scan rates ranging from 0.2 to 2.0 mV s-1. Using the redox peaks over the CV loops, we were able to have an insight into the charge storage mechanism by using the relationship between the peak current i and the voltage scan rate ν, as 𝑖 = 𝑎𝑣𝑏, where a and b are adjustable parameters. When b is 0.5, it means the semi-infinite linear diffusion, corresponding to battery-like charge storage. When b is 1, it implies a surface-controlled process that is generally linked to the pseudocapacitive behaviour. According to the relationship between log(v) and log(i) in Figure 5c, the b values corresponding to peaks 1 and 2 are deduced to be 0.96 and 0.86, respectively, indicating that the pseudocapacitive charge storage is dominant. Figure 5e shows GCD curves of the device at different current densities, according to which the dependence of areal capacitance of the device on current density is deduced, as shown in Figure 5f. This device reaches a maximum areal capacitance of 1.1 F cm-2 at 0.5 mA cm-2, and retains 0.71 F cm-2 (64.8% capacitance retention) when the current is increased to 8 mA cm-2. Also, the device shows long-term cycling stability, with a capacitance retention of 80% after 3000 cycles, as shown in Figure 5h. The benefit of Zn-plated carbon fiber as


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the anode in the micro-size planar cell is that it can prolong the lifetime. For comparison, we made the other cell with using Zn-foil-based finger electrodes to match the other set of poly(3,3’-DHB)/AC finger electrodes. As shown in Figure S10, this cell only survived for 535 cycles, and then was disabled due to short circuit caused by long Zn dendrites across the gap between finger electrodes. For the electrodeposited zinc flakes over carbon fibers, they form a 3D structure that can effectively inhibit the growth of dendrites.30


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Figure 5. (a) SEM image of the Zn-plated carbon fibre paper. The high-magnification image in the inset reveals densely packed Zn nanosheets over the fibre. (b) CV loops at different scan rates. The relationship between the peak current and the voltage scan rate for two redox peaks is given in (c). (d) Illustration of a small-size planar ZIHESD. (e) GCD curves at different current densities. (f) Dependence of the areal capacitance on the current density. (g) SEM image of a bare carbon fibre paper. (h) Cycling stability of the device. (i) Ragone plot of a typical device. Energy and power densities of other selected devices in the literature are marked as well.

Figure 5i shows the Ragone plot of our small-size planar ZIHESD, revealing a high energy density of 152 μWh cm-2 and a maximum power density of 5 mW cm-2. For comparison, energy and power densities of other MSCs from the literature are marked as well. Examples include symmetric cells based on porous AC,31 rGO/carbon nanotube,32 N-rGO electrodes composite33 and the asymmetric cell with graphenequantum-dots//polyaniline configuration.34 With respect to the power density, our device


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based on a novel polymer cathode is also superior to other MSCs based on inorganic electrodes, such as the symmetric cells based on graphene/MnO2,35 Au/MnO2/Au,36 and the asymmetric one with GQDs//MnO2 configuration.37

In order to have an insight into the energy storage mechanism of the Zn//poly(3,3’DHB)/AC cell, we used ex situ X-ray photoelectron spectroscopy (XPS) to study the cathode at fully-charged (1.5 V) and fully-discharge(0.5 V) states, respectively. As shown in Figure 6a, the two survey XPS spectra contain C 1s, O 1s, and F 1s peaks. The F signal comes from the polyvinylidene fluoride binder that is incorporated in the AC coating. The fully charged cathode shows additional Zn signals, and its O 1s peak is also enhanced. Therefore, the interaction between Zn2+ and poly(3,3’-DHB) plays an important role in charge storage. Figure 6b illustrates the energy storage process. The SO24 - may also participate in the redox process, as the ZnSO4 electrolyte is superior to others such as ZnCl2 and Zn(CF3SO3)2 (zinc trifluoromethanesulfonate). Electrochemical performances of the device with using ZnCl2 and Zn(CF3SO3)2 aqueous electrolytes are shown in Figures S4 and S5, respectively. The suitable voltage window of ZnCl2 system is only 0.8 V, narrower than that of the ZnSO4 system. For the Zn(CF3SO3)2 system, though the voltage window can be remained in the range of 0.5 – 1.5 V, the capacity is lower than that of the ZnSO4 system. According to our


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FT-IR analysis (Figure 1g), the molecular structure of poly(3,3’-DHB) is proposed, and its charged and discharged status are shown in Figures 6c and d, respectively.

Figure 6. (a) Ex situ XPS spectra of the poly(3,3’-DHB)/AC cathode discharged to 0.5 V and charged to 1.5 V, respectively. (b) Illustration for the energy storage process of the device. (c and d) The proposed oxidation and reduction of poly(3,3’-DHB), respectively.

The merit of a redox-active material can be well understood by analyzing its capacitance contribution at molecular level, which is often applied to 2D materials.38, 39


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Since our pseudocapacitive polymer consists of repeating units, the theoretical specific capacitance is estimated to be 920 F/g. In addition, the capacitive contributions of the polymer are estimated by using the Dunn’s method to be 59%, 66%, and 74%, according to CV loops at 2, 3, and 4 mV s-1, respectively (Figure S11). Therefore, the capacitive behavior is dominant over the charge storage mechanism in our ZIHESDs.

CONCLUSIONS In summary, aqueous ZIHESDs incorporating a poly(3,3’-DHB)/AC cathode and a Zn metal anode were fabricated. Using a simple electrodeposition method, we coated poly(3,3’-DHB) onto porous AC granules to largely boost the capacitance. Two device forms, the flexible cell with sandwich structure and the in-plane micro cell with interdigital finger electrodes, were made. Both can reach high areal capacitance in F cm-2 scale owing to the high mass loading of the cathode material. For the sandwichstructured cell, the carbon-cloth-supported AC coatings with different areal mass loadings ranging from 2.6 to 6.9 mg cm-2 were coated with poly(3,3’-DHB), and were combined with the Zn foil anode to study the energy storage performance. The cell with


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the thickest AC coating in cathode exhibits a high areal capacitance of 1.3 F cm−2 at 0.5 mA cm−2 and a landmark energy density of 180 μWh cm-2. The capacitance retention was 80% over 5000 CD cycles. Also, the flexible cell shows stable electrochemical performance upon bending at large angles up to 120o. For the small-size planar ZIHESD, a high areal capacitance of 1.1 F cm-2 is achieved. The maximum energy and power densities of the planar micro ZIHESD are 150 μWh cm-2 and 5.1 mW cm-2, respectively. Our study suggests an efficient way to enhance the energy storage capacity of aqueous Zn-ion energy storage device by combing poly(3,3’-DHB) with porous AC to make the cathode.

ASSOCIATE CONTENT Supporting Information

The Supporting Information is available free of charge on the ACS Publications Website at DOI: xxxxx Calculation details for capacitance, energy and power densities; BET analysis of porous AC; Photographs of the micro-supercapacitor; XRD analysis.

AUTHOR INFORMATION


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Corresponding Author *E-mail: [email protected]

ACKNOWLEGEMENTS This work was financially supported by ‘The Fundamental Research Funds for Central Universities’ through Beihang University, and the National Natural Science Foundation of China (NSFC, Grant No. 51702009 and 21771017).

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TOC (For Table of Contents Use Only)


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A novel cathode based on poly(3,3’-dihydroxybenzidine) is used to match with Zn anode to make aqueous energy storage devices with high areal capacitance.


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Boosting the Capacitance of an Aqueous Zinc-Ion Hybrid Energy

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