Highly boosting the supercapacitor performance by polydopamine-induced surface modification of carbon materials and use of hydroquinone as an electrolyte additive

Electrochimica Acta 339 (2020) 135940

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Highly boosting the supercapacitor performance by polydopamine-induced surface modification of carbon materials and use of hydroquinone as an electrolyte additive Zhong Jie Zhang a, *, Guo Liang Deng a, Xuan Huang a, Xin Wang a, Jun Min Xue a, Xiang Ying Chen b, c, ** a College of Chemistry & Chemical Engineering, Anhui Province Key Laboratory of Environment-friendly Polymer Materials, Anhui University, Hefei, 230601, Anhui, PR China b Anhui Province Key Laboratory of Green Manufacturing of Power Battery, Tianneng Battery Group (Anhui Company), Jieshou, 236500, Anhui, PR China c School of Chemistry and Chemical Engineering, Hefei University of Technology, Hefei, 230009, Anhui, PR China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 23 January 2020 Received in revised form 19 February 2020 Accepted 20 February 2020 Available online 21 February 2020

In this work, we use polydopamine (PDA) to modify the surface of carbon materials, and then adopt hydroquinone (HQ) as a redox additive to jointly improve the performance of supercapacitors. This noncovalent bond PDA coating produce functional groups such as amino and hydroxyl, which can interact with HQ in H2SO4 system by hydrogen bonding. The carbon materials solely coated with PDA (without any HQ) deliver favorable capacitance of 218 F g
1 and energy density of 7.59 Wh kg
1; whereas the one not only coated with PDA but also using HQ as redox additive, higher capacitance of 557 F g
1 and energy density of 19.36 Wh kg
1 are achieved, realizing the energy density increases by 2.55 times. In addition, for carbon materials not covered with PDA, HQ kinetics is diffusion controlled; for carbon materials covered with PDA, HQ kinetics is surface controlled. Obviously, this work provides a synergistic approach to collectively boosting the performance of supercapacitors. © 2020 Published by Elsevier Ltd.

Keywords: Polydopamine Surface modification Hydroquinone Redox additive Supercapacitor

1. Introduction Owing to large surface area, good thermal/electrical stability and moderate cost, activated carbons have been the choice of the mostly used electrode materials for supercapacitor applications, exhibiting high capacitance, long recycling etc [1,2]. However, it is worth our attention that, because activated carbons are derived from physical/chemical activation of various carbon sources, they commonly deliver unfavorable energy storage in light of narrow interconnected pore structures and short path lengths, preventing them from working to their fullest extent of charge storage capacity [3]. In order to solve this problem, surface modification towards carbon material is appealing and effective via either non-covalent or covalent functionalization approach [4]. In details, non-

* Corresponding author. ** Corresponding author. Anhui Province Key Laboratory of Green Manufacturing of Power Battery, Tianneng Battery Group (Anhui Company), Jieshou, 236500, Anhui, PR China. E-mail addresses: [email protected] (Z.J. Zhang), [email protected] (X.Y. Chen). https://doi.org/10.1016/j.electacta.2020.135940 0013-4686/© 2020 Published by Elsevier Ltd.

covalent functionalization relies on supramolecular complexation using different adsorption forces, such as hydrogen bonds, van der Waals force, electrostatic force and p-stacking interactions, while covalent functionalization on the formation of covalent bonding between functional entities and carbon matrix (stronger than that of the non-covalent interaction) [5]. By surface modification of carbon materials, many properties including wettability, electrical conductivity and dispersion that are significantly different from the unmodified ones [6] are thus produced. Relatively speaking, direct coating of organic molecules or polymers on the surface of carbon materials involving non-covalent approach is more appreciable, which can minimize the effect imposed on the carbon’s pristine structure (almost maintaining structural integrity) [7]. Among these, polydopamine (PDA) chemistry has caused widespread attention [8,9] because dopamine molecules can easily self-polymerize under weakly alkaline [10] or acidic [11] conditions, making PDA coatings easy to deposit on various surfaces [12] typically as MWCNTs [13]. What’s more, the PDA coating contains high-density catechol and imine functions [14], which can serve as a versatile and robust platform for the second reaction, thereby stimulating further modifications for

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Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

specific applications especially the field of energy storage. For instance, the deposition of PDA coating onto interconnected porous carbon nanosheet (IPCN) has resulted in the specific capacitance increases by40% as compared to that of the unmodified electrode and the additional pseudocapacitance comes from the catechol groups of PDA [15]. Polydopamine modified reduced graphene oxides (PDA-rGO) delivers a capacitance of 120 F g
1 at 2 A g
1 together with superior capacitance retention of ~99% after 10,000 cycles, in which the addition of PDA well prevents the stacking of rGO and enhances the wettability of composite [16]. Lee et al. figured out that PDA have acted as dual roles toward high-voltage lithium-ion batteries, therein one is the PDA coating onto LiNi1/ 3Co1/3Mn1/3O2/artificial graphite cell (suppressing the oxidative decomposition of the electrolyte during high-voltage operation up to 4.5 V) and the other works as electrolyte additive for improving the capacity retention of the cell from 136 to 147 mAh g
1 after 100 cycles at a rate of 1C [17]. As a consequence, bio-inspired PDA molecules have advanced an effective strategy for the surface modification of carbon materials, thereby creating a multifunctional nanoplatform that can be further modified for various applications. In the meantime, another simple and effective way to increase the energy of supercapacitors is to add a small amount of redox additives to the electrolyte [18,19]. In recent years, our research group has made many useful attempts and explorations around this aspect, and achieved good experimental results. In addition to simply adding one substance as pyrocatechol violet [20], Fe2þ ions [21] and rutin [22], we can also add two substances as 1, 4dihydroxyanthraquinone and hydroquinone [23] and Na2MoO4 and KI [24], which is mainly to take advantage of the synergy between them. Of course, other better works concerning redox additives for supercapacitor application are also emerging. For example, Niu et al. presented 3,4,9,10-perylene tetracarboxylic acid (PTCA) confined on the graphene electrode surface, disclosing a wide voltage window (1.2 V) and capacitance of 143 F g
1 at an ultrafast density of 1000 A g
1 (at least 1 order of magnitude faster than present speeds) [25]; Stucky and co-workers have developed a dual-redox supercapacitors consisting of bromide catholyte and ethyl viologen anolyte with the addition of tetrabutylammonium bromide, which can enhance energy by factors of 11 and 3.5, respectively, with a specific energy of ~64 Wh kg
1 [26]. However, to date, there are still many defects in using rodox additives to improve supercapacitor performance, such as low rate capability, poor reversibility and unfavorable coulombic efficiency. The reasons involved are many, one of which may be that the electrode materials and the redox additive have not been reasonably designed, especially the mutual relationship between them is not fully considered. Encouragingly, Mai et al. [27] introduced an effective strategy that utilizes Cu2þ reduction with carbon-oxygen surface groups, easily generating an additional redox CuCl layer, which can provide a 10-fold increase in capacitance (4700 F g
1) compared with conventional electrolyte. In this work, based on the strategy of using non-covalent bond surface modification, we directly used PDA to modify the commercial activated carbon, resulting in a large number of functional groups on its surface. Then, on the premise of fully considering the relationship between the surface functional groups of the electrode material and the redox additive, an appropriate amount of hydroquinone (HQ) was introduced into the H2SO4 system as a strategy to effectively improve the supercapacitor performance. Furthermore, the focus of our attention is the effect of the introduction of PDA on the porosity, conductivity, surface functionality, and supercapacitor properties of carbon materials.

2. Experimental section All chemicals are of analytical grade from Sinopharm Chemical Reagent Co., Ltd except for activated carbon (C-blank) purchased from Nanjing XFNANO Materials Tech Co., Ltd. 2.1. Typical procedure for synthesizing the C-PDA-1-1 sample Dopamine (0.2 g) was immersed into 1 L tris(hydroxymethyl) aminomethane hydrochloride solution (Tris-HCl, pH ¼ 8.5) for 2 h under magnetic stirring; Subsequently, activated carbon materials (0.2 g) was added. After stirring for 24 h, the solid sample was filtered, washed with deionized water and then dried at 80  C for 6 h under vacuum, yielding the C-PDA-1-1 sample. As for the C-PDA-1-3, C-PDA-1-5 samples, the synthesis procedure are almost invariable to the above except for the use of dopamine as 0.6 and 1.0 g, respectively. 2.2. Typical procedure for synthesizing the mixed electrolyte To 1 mol L
1 H2SO4 solution, hydroquinone (HQ) was introduced as 30 mmol L
1 under magnetic stirring at room temperature, thus producing the mixed electrolyte. 2.3. Structure characterization High-resolution transmission electron microscope (HRTEM) images, elemental mappings and EDAX spectrum were performed with a JEM-2100 F unit. The specific surface area and pore structure of the carbon sample were determined by N2 adsorptiondesorption isotherms at 77 K (Quantachrome Autosorb-iQ). The specific surface area was calculated by the BET (Brunauer-EmmettTeller) method. Pore size distribution was calculated by using a slit/ cylindrical nonlocal density functional theory (NLDFT) model. X-ray photoelectron spectra (XPS) were obtained using a VG Instruments ESCALAB MK II X-ray photoelectron spectrometer with an excitation source of Al Ka (1253.6 eV). The electrical conductivity was determined by measuring the impedance response of the test cell (1  1 cm2) using a ST2258C digital four-probe tester (Suzhou JingGe Electronic Co., Ltd) over the frequency range 1 Hz to 1 MHz. 2.4. Electrochemical measurements conducted in a three-electrode system Typically, the carbon sample (80 wt %), graphite (15 wt %) and polytetrafluoroethylene (5 wt %) were mixed in ethanol. The mixed slurry (active carbon materials of 3e4 mg) was coated onto platinum net (~1 cm2) to prepare the working electrode, and the electrode was dried at 110  C in an oven for 12 h. The three electrode system was executed in the prepared electrolyte (1 mol L
1 H2SO4 solution) with a counter electrode of platinum foil (6 cm2) and a reference electrode of saturated calomel electrode (SCE). All tests were carried out on a CHI 760 E (ChenHua Instruments Co. Ltd., Shanghai). The electrochemical performances of the samples were evaluated by cyclic voltammetry (CV), galvanostatic chargedischarge (GCD), and electrochemical impedance spectroscopy (EIS) techniques. The EIS measurements were carried out in the frequency range from 100 kHz to 0.01 Hz at open circuit potential with an ac perturbation of 5 mV. Specific capacitances derived from galvanostatic tests can be calculated from the equation:

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

C¼

Idt mdU

3

(1)

where C (F g
1) is the specific capacitance; I (A) is the discharge current; t (s) is the discharge time; U (V) is the potential; and m (g) is the mass of active materials loaded in working electrode. 2.5. Electrochemical measurements conducted in a two-electrode system The electrochemical experiments in two-electrode system were performed in a CR2032 corn-type cell. The carbon electrode was used as the working electrode and another carbon electrode with the same mass of active material was used as the counter electrode. A glassfiber membrane was used as the separator for the symmetric supercapacitor. Note that the mass of active carbon materials on each electrode is 1e2 mg. Specific capacitances derived from galvanostatic tests can be calculated from the equation:

C¼

2Idt mdU

(2)

where I (A) is the constant current; m (g) is the mass of active material loaded on the single electrode; U is the voltage window (V); C (F g
1) is the specific capacitance. Specific energy density (E) and specific power density (P) derived from galvanostatic tests can be calculated from the equations:

E¼

1 1   C DU 2 3:6 8

(3)

P¼

3600E Dt

(4)

E (Wh kg
1) is the energy density; C (F g
1) is the specific capacitance; U (V) is the potential; and P (W kg
1) is the power density and t (s) is the discharge time. 3. Results and discussion Using dopamine as a monomer, oxidative self-polymerization is used to achieve the preparation of polydopamine in Tris-HCl solution, which has proven to be a simple and easy route. The present synthesis process for carbon-PDA composite as well as the unit structures of DA and PDA are given in Fig. 1a. It should be pointed out that there is still a lot of controversy about the structure of PDA so far, which has a large degree of uncertainty [10,28]. Moreover, the carbon materials using PDA for surface coating have mainly some ordered structures, such as carbon nanotubes [29], and porous carbon nanosheets [15]. However, compared with other types of carbon materials, such as graphene, carbon nanotubes, C60, etc., activated carbon materials have several obvious advantages, mainly including low cost, wide sources, and stable structure (of course, for disordered activated carbon, uncontrollable pore structure is also a significant disadvantage). Moreover, for activated carbon with disordered structure, related research is less. Herein, as shown in Fig. 1b, the activated carbon we studied obviously has an irregular shape with a size of about tens of micrometers; moreover, it has a very obvious disordered and micrometer porous structure. Taking the C-PDA-1-3 sample as an example, elemental mapping results clearly show that it is fully composed of C/O/N species. This also indicates that the PDA has completed an effective self-polymerization reaction on the surface

Fig. 1. (a) Synthesis process for carbon-PDA composite as well as the unit structures of DA and PDA; (b, c) HTREM images and elemental mappings of the C-PDA-1-3 sample. Note: The blue curves in the element mapping test is only used to describe the appearance of the carbon material (not representing the PDA).

of activated carbon, achieved uniform coating, and can stably exist in ethanol solvent without being dissolved. As we all know, the porosity of carbon materials usually includes three aspects, namely BET specific surface area, pore volume and pore size distribution (PSD). These three factors all play a decisive role in the performance of supercapacitors. Here, we used N2 adsorption-desorption technology to measure the porosity of carbon materials before and after PDA coating and the results are shown in Fig. 2. Fig. 2a shows the isotherms of the four samples, the C-blank, C-PDA-1-1/1e3/1e5 samples. Obviously, they are similar in appearance, except for the difference in the number of adsorbed volumes. In the low pressure region, they all display fast vertical adsorption characteristics, while in the middle and high pressure regions, no obvious hysteresis loops (capillary condensation) are found, indicating that all four samples can be attributed to the typeI porous characteristics. Obviously, the proper amount of PDA coating does not significantly change the microporous characteristics of carbon materials. In fact, this view can be proved by the following experimental results: When the PDA is coated on the surface of carbon nanotubes, the results of TEM observation show that the PDA is axially coated around the carbon nanotubes to form a coaxial nanocable structure; Moreover, these PDA materials are obviously amorphous and micropore-sized (<2 nm) [30]. Fig. 2b is the PSD results of these four samples, and the difference between them is not remarkable, all showing three typical distribution peaks at 0.95, 1.22 and 2.00 nm, respectively. BET surface area and pore volume of the pristine C-blank sample is 1932 m2 g
1 and 0.95 cm3 g
1, respectively. For the CPDA-1-1/1e3/1e5 samples, their BET surface areas are 1863, 1807 and 1719 m2 g
1 and pore volumes of 0.90, 0.87 and 0.80 cm3 g
1, respectively. The corresponding trend is shown in Fig. 2c.

4

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

3

Volume Adsorbed cm /g

700

500 400 300

-1

2

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

200 100 0

0.0

0.2 0.4 0.6 0.8 Relative Pressure P/P0

1.0

2000

1.10

1900 (c)

1.05

1800

1.00

1700

0.95 0.90

1600

0.85

5 -1 -3 nk -1A-1 A-1 A bla D D D P C C-P C-P C-

-1

0.80

1400

3

1500

Pore volume /cm g

Surface area / m g

(a)

600

0.75

Fig. 2. The C-blank, C-PDA-1-1/1e3/1e5 samples: (a) Isotherms; (b) PSD curves; (c) BET surface areas and pore volumes; (d) Proportion of micropore and meso/macropore.

the functional groups on the surface. Fig. 3a reveals XPS survey of the C-blank, C-PDA-1-1/1e3/1e5 samples. Except for the C-blank sample, the other three samples all contain a small amount of nitrogen (mainly due to the low content of N within dopamine itself), which is obviously very reasonable, and is introduced due to the surface coating of PDA by the strong p-p interaction between them [14]. Next, we used the XPSPEAK software to perform a more reasonable and scientific fitting of the oxygen elements of the four samples, and the results are shown in Fig. 3bee. In general, the O1s of these samples can be fitted to four independent peaks in the range of 527e541 eV, mainly according to references [29,33,34]. Among them, some of the oxygen-containing functional groups are

200 400 600 800 1000 1200 1400 Binding energy / eV

(e) O1s

O2 O1

C-PDA-1-5 O3 O4

526 528 530 532 534 536 538 540 542 Binding energy (eV)

O1s

(c)

O3 O2 O4

O1

526 528 530 532 534 536 538 540 542 Binding energy (eV)

N1s

N2 N1

392 394 396 398 400 402 404 406 408 Binding energy (eV)

O3

O1

528

C-PDA-1-1

(f)

O2

O4

531 534 537 540 Binding energy (eV)

N1s

N1

N2

392 394 396 398 400 402 404 406 408 Binding energy (eV)

C-PDA-1-3

O1s

O3

O1

525

543

528

O4

531 534 537 540 Binding energy (eV)

N1s

543

C-PDA-1-5

(h)

C-PDA-1-3

(g)

O2

(d)

C-PDA-1-1

O1s

Intensity (a.u.)

C-blank

(b)

Intensity (a.u.)

N 1s

0

Intensity (a.u.)

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

Intensity (a.u.)

Intensity / a.u.

O 1s

Intensity (a.u.)

C 1s

Intensity (a.u.)

(a) survey

Intensity (a.u.)

Obviously, coating PDA on the surface of carbon material will reduce its porosity, primarily due to the formation of a thin film on the surface of PDA [14,31]; moreover, as the amount of PDA increases, the porosity of carbon material will decrease more severely. Besides, the proportion of micropore and meso/macropore of these samples are also calculated, as presented in Fig. 2d. This may be determined by the surface porosity of the film formed by the PDA [32]. By coating the surface of carbon material with PDA, a variety of functional groups can be introduced, and the type and number of these functional groups will greatly determine the structure and corresponding properties of the composite material. Therefore, it is quite necessary to use XPS as a detection method to characterize

N1

N2

392 394 396 398 400 402 404 406 Binding energy (eV)

Fig. 3. The C-blank, C-PDA-1-1/1e3/1e5 samples: (a) XPS survey; (bee) O 1s; (feh) N 1s.

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

5

Table 1 XPS data and the molar percentage of C, O and N species. Element

O 1s O 1s O 1s O 1s O 1s O 1s O 1s N 1s N 1s

Binding energy (eV)

Assignment

531.9 532.6 533.1 533.7 534.1 535.2 536.4 399.5 401.3

C/]/O (carboxyl) C/]/O (ester, amide) CeOH OeC O/]/CeO COOH O2ads, H2Oads eN/]/ eNH2

Elemental composition C O N

consistent with the structure diagram of the PDA in Fig. 1a, while the other parts are inconsistent. This may be due to many factors such as the complex and diverse polymerization of the PDA itself and its interaction with the functional groups on the surface of the carbon material [35]. As for N1s, it can be fit into two independent peaks, as shown in Fig. 3feh, which are basically indexed as eN] and eNH2 [36,37]. These N1s fitting are well consistent with the structural primitives of PDA in Fig. 1a. This result is very different from the traditional nitrogen doping system, because nitrogen doping of carbon materials usually produces 4 types of doping components, including N-6 (pyridine N), N-5 (pyridone/pyrrolic N), N-Q (quaternary N), and N-X (oxidized pyridine N) [38e40]. Thus, it is proved that the PDA is coated on the surface of the carbon material in this work, instead of being doped in the bulk phase. Previous research work have revealed both dopamine and PDA exhibit strong affinity to the sidewalls of carbonacesous through p-p stacking [41,42]. For convenience, specific binding energy, assignment and molar percentage (%) of O1s of these samples are summarized in Table 1. It can be seen that with the increase in the amount of PDA, the type

Molar percentage (%) C-blank

C-PDA-1-1

C-PDA-1-3

C-PDA-1-5

17.46 / 27.38 / 38.54 16.62 / / /

14.17 / 42.89 37.66 / 5.28 / 14.28 85.72

15.94 / / 47.79 27.04 9.32 / 47.27 52.73

/ 25.67 / 29.74 / 21.86 22.73 44.01 55.99

91.12 8.88 /

83.63 14.34 2.03

83.54 14.19 2.27

83.38 14.02 2.60

and number of functionalities on the surface of the carbon material have changed to a large extent. However, from the total point of view, the nitrogen content in the product is basically proportional to the amount of PDA, which is also a reasonable result. As mentioned earlier, when PDA is formed into thin film by a non-covalent bond on the surface of a carbon material, many different types of functional groups are generated on the surface of the obtained carbon-PDA composite. Here, for the sake of simplicity, we have only selected amino (eNH2) and hydroxyl (eOH) groups as typical representatives. Next, we will examine the changes in the series of structure, conductivity, and electrochemical interaction between HQ before and after PDA coating on the surface of carbon materials. From step (1) in Fig. 4, we can clearly see that as the amount of PDA increases, the electrical conductivity of the obtained carbon-PDA composites also gradually increases. Using the digital four-probe tester toward powder compressed pellet, the conductivity of the C-blank sample is 6.5 mS cm
1, and the conductivities of the C-PDA-1-1/1e3/1e5 samples are 8.8, 10.4, and 12.9 mS cm
1, respectively. As a matter of fact, as we know, PDA film exhibits a special zwitterionicity: PDA layer is negatively

Fig. 4. Schematic illustration towards the carbon-PDA composite: (1) Increasing electrical conductivity; (2) Interaction with HQ; (3) Redox reaction of HQ; (4) Redox reaction of PDA.

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940 1.0

(a)

10 0

-10 -20 -1

100 m Vs

-30 0.0

0.2 0.4 0.6 0.8 Potential / V vs SCE

(c)

Cu rr 30 5 en 2 0 td 2 en 15 sit 10 y/ 5 A -0 g 1

400 350 300 250 200 150 10 50 0 0 C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

(b)

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

0.8 0.6

-1

2Ag

0.4 0.2 0.0

1.0

Specific capa citance / F g -1

-40

1 M H2SO4

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

Potential / V vs SCE

20

0

100 200 300 400 500 600 700 Time / s

2.5 2.0 -Z'' / ohm

30 -1

charged and allows good permeability of positively charged molecules at high pH (exceeding 4) [43]. For example, Zhao reported that proper amount of PDA can improve the conductivity polypyrrole (PPy) from 0.04 to 3.8 S cm
1 (with the molar ratio of PDA/ PPy as 0.1) [44]. Similarly, for graphene oxide paper, adding PDA can also make it more conductive [45]. In fact, PDA is more concerned about its adhesion; by introducing a PDA film on the surface of the base material, it can use its adhesion to build a richer reaction platform and form a more diverse composite system. Here, we chose HQ as the linking molecule, mainly based on the following two considerations: first, HQ is an excellent electrolyte additive that can significantly increase the specific capacitance, and this has been widely proven [46]; HQ has a hydroxyl functional group, and it is possible to interact with the functional group on the surface of carbon-PDA. Specifically, the hydroxyl group of HQ can form a wide range of hydrogen bonds with the amino group and hydroxyl group on the surface of the PDA film [47], as depicted in step (2) of Fig. 4. Obviously, the use of thin layer of PDA adhesion and hydrogen bonding with HQ can jointly enhance the interaction between the two. And the increase of this force is helpful for the subsequent improvement of electrochemical properties, including specific capacitance, energy density, cycle stability, etc. In an acidic medium, para-substituted HQ is the most active because the distance between the hydroxyl groups on the benzene ring is the furthest. HQ can reversibly form quinones by gaining and losing two protons and electrons, which involves more complicated processes such as charge transfer and deprotonation [19], as shown in step (3) of Fig. 4. On the other hand, it is interesting that PDA can actually undergo a reversible redox reaction, but this is something that people have paid less attention to in the past. Due to the strong ionization of phenolic hydroxyl and amide groups, PDA molecules have a variety of existence states in different pH solutions. In the acidic H2SO4 electrolyte in this article, the PDA will first protonate the amino group, and then through the gain and loss of protons and electrons, a reversible redox reaction is generated. The corresponding equation is shown in step (4) in Fig. 4 [48]. The protonation of such amino groups in acidic systems is also present in previously reported systems of p-aminophenol [49]. Obviously, this reversible reaction gives us an interesting hint, that is, PDA can not only be used as an adhesive in the traditional sense, but also generate additional pseudo-capacitance through redox reaction, which is conducive to the improvement of supercapacitor performance [31]. Next, we first performed a three-electrode test in 1 mol L
1 H2SO4 solution (without adding any HQ) on the electrochemical performance of the C-blank, C-PDA-1-1/1e3/1e5 samples, which are conventional strategies for supercapacitor tests. Generally, according to different energy storage methods, supercapacitors can be divided into two types, namely electrical double layer capacitor (EDLC) and pseudo-capacitor. The former’s capacitance comes from the electrostatic charge accumulated between the carbon material electrode and the electrolyte interface; the latter is related to the Faraday charge transfer or redox reaction of the active material [1]. Obviously, the C-blank sample belongs to EDLC type, and its CV curve should be a regular rectangle in theory. However, the CV curve of the sample deviates from the theoretical shape at a scan rate of 100 mV s
1, as shown in Fig. 5a, indicating that some functional groups on the surface of the sample still generate a certain amount of pseudo-capacitance. This is consistent with the oxygen functionality in the data of Table 1. For the C-PDA-1-1/1e3/ 1e5 samples, the integrated area surrounded by their CV curves is significantly increased, which is obviously caused by the PDA’s surface coating on the carbon material. However, it is worth noting that the C-PDA-1-3 sample has the largest CV integral area, that is,

Current density / A g

6

1.5 1.0

(d) C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

0.5 0.0 0.5

1.0

1.5 2.0 Z' / ohm

2.5

3.0

Fig. 5. The C-blank, C-PDA-1-1/1e3/1e5 samples when tested in a three-electrode configuration using 1 mol L
1 H2SO4 solution as aqueous electrolyte: (a) CV curves at 100 mV s
1; (b) GCD curves at 2 A g
1; (c) Specific capacitances; (d) Nyquist plots.

its specific capacitance is the largest, because there is a proportional relationship between the two. This should be the result of the balance and optimization of the porosity, surface functional groups and conductivity of the sample. Another thing worth pointing out is that under acidic conditions, the position of the redox peak of the PDA reaction is not located between 0 and 1 V [48], so the redox peak of the present composite material formed by the PDA coated carbon material is not obvious. Similarly, the GCD results of these four samples also show a similar trend to CV, as given in Fig. 5b, that is, the C-PDA-1-3 sample has the largest discharge time. Using Equation (1), a series of specific capacitance (Cs) data can be obtained by calculating the GCD curves, as shown in Fig. 5c. At the current density of 1 A g
1, the Cs of the C-blank sample is 137 F g
1, whereas, for the C-PDA-11/1e3/1e5 samples, their Cs have been effectively increased, reaching up to 228, 358 and 291 F g
1, respectively. Obviously, the C-PDA-1-3 sample has the largest Cs, and even if the current density is increased to 30 A g
1, it still has a Cs value of 195 F g
1, thus exhibiting excellent capacitance performance. Nyquist plot is also one of the important methods used to characterize the performance of supercapacitors. In Fig. 5d, all samples show a straight line in the low-frequency region and an inconspicuous arc in the high frequency region. As reported, the high frequency loop is associated with the electronic resistance of electrode materials [50] or electrolyte resistance [51]. As a consequence, the C-PDA-1-3 sample exhibits the lowest electronic resistance, well according with the previous capacitive results. Furthermore, HQ serving as redox additive was introduced into 1 mol L
1 H2SO4 solution, resulting in the C-blank-HQ, C-PDA-1-1/ 1e3/1-5-HQ samples, and the electrochemical behaviors conducted in a three-electrode configuration are displayed in Fig. 6. In details, Fig. 6a indicates the whole CV curves showing apparent oxidationreduction peaks when added with HQ, quite different from those without HQ (shown in Fig. 5a). This is caused by the redox reaction of HQ. On the other hand, peak separation between oxidationreduction peaks (DEP) is also crucial for estimating the electron transfer kinetics or electrochemical reversibility. For electrochemical reversible reactions, electrons are transferred to the electrode faster than the scan rate of voltammetry. And for surfacebound redox materials, DEP should ideally be close to 0 mV [52]. Herein, in terms of the CV curves at 20 mV s
1, the DEP values for the C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples are 0.31, 0.42,

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

1.0

(a)

C-blank-HQ C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

0.2 0.4 0.6 0.8 Potential / V vs SCE

(c)

0.6 0.4 0.2 -1

2Ag

0.0

1.0

0

800 400 200 0

C-blank-HQ C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

300

600 900 1200 1500 1800 Time / s

2.5

1000 600

Cu rr 80 en 60 td en 40 sit y / 20 A g 1

C-blank-HQ C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

(b)

0.8

2.0

-Z''/ohm

-1

20 mV s

0.0

(capacitive). For a redox reaction limited by semi-infinite linear diffusion, the current response varies with v1/2; for a capacitive process, the current varies directly with v. Thus for any material, the following general relationship can be written for the current at a specific potential [55]: iðVÞ ¼ k1 v1=2 þ k2 v. Next, we fit the relationship between the peak intensity of the CV curve and the scan rate of the four samples, and take the one with larger correlation as the best, as shown in Fig. 7 b,d,f,h. To our surprise, the fitting results can be roughly divided into two categories. The peak intensity of the C-blank-HQ sample is proportional to the square root of the scan rate, while those of the other CPDA-1-1/1e3/1-5-HQ samples are proportional to pure scan rate. That is to say, for the case of C-blank-HQ, the redox reaction of HQ is limited by semi-infinite linear diffusion on the surface of carbon electrode, and this experimental result complies with the previous literatures [47,56,57]. As for the C-PDA-1-1/1e3/1-5-HQ samples, the redox of HQ are surface controlled (capacitive). The main reasons for the two different types of kinetics are as follows: because the C-blank-HQ sample is not coated with PDA, HQ mainly enters the internal microporous structure of the carbon material by diffusion; whilst for the other three samples, due to their surface covered with a thin film of PDA, HQ must first effectively cover the surface of the PDA film before entering the carbon micropores, that is, to achieve the kinetics of surface control. In order to more accurately determine the performance of supercapacitors, we further adopted a two-electrode system when. It should be pointed out that because the two electrode configuration are much closer to practicality in terms of assembly and testing, the measured data is becoming more authentic. However, compared with the three electrode configuration, the wetting between the electrolyte and the electrode material is poor in the two electrode one, and therein various resistances will increase, which usually results in a relatively lower supercapacitor data [58]. Fig. 8a and b shows the typical CV curves at 100 mV s
1 and GCD curves at 2 A g
1 concerning the C-blank, C-PDA-1-1/1e3/1e5 samples, respectively, which are almost the same in shapes to those in Fig. 5a

Potential / V vs SCE

75 60 45 30 15 0 -15 -30 -45 -60

Specific ca pacitance / F g -1

Current density / A g

-1

0.52 and 0.54 V, respectively. Based on these data, it can be seen that the redox reaction of HQ on the carbon material surface in this experiment has poor reversibility. Fig. 6b is typical GCD curves of these samples. It can be found that they all have a very obvious charging and discharging platform, and the corresponding voltage scope is 0.3e0.5 V, well consistent with previous reports [47,53]. Moreover, Cs of these samples are also presented in Fig. 6c. On the whole, the addition of HQ to the H2SO4 electrolyte can significantly increase Cs, in contrast to those without any HQ in Fig. 5c, which is certainly due to the pseudocapacitive effect produced by HQ. Specifically, the Cs values for the C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples are 360, 541, 899 and 667 F g
1 when tested at 2 A g
1, respectively. As expected, the C-PDA-1-3 sample delivers the largest Cs value; at the same time, the rate capability of this sample is also outstanding, because even if the current density is increased to 80 A g
1, it can still maintain a high Cs of 488 F g
1 (capacitance retention as 54.28%). As for the Nyquist plots of these samples, as shown in Fig. 6d, their change rule is basically similar to that without HQ (Fig. 5d). In order to further study the electrochemical behavior of HQ on the surface of carbon electrode materials, we tested a series of low scan rate CV curves, as shown in Fig. 7 a,c,e,g. When the scan rate is increased from 5 to 20 mV s
1, the corresponding redox peaks extend in the positive and negative directions, respectively. In other words, the DEp value of each pair of redox peaks is gradually increasing. Taking the C-blank-HQ sample as an example, as indicated in Fig. 7a, the DEp value toward 5, 7, 10 and 20 mV s
1 are 0.21, 0.25, 0.30 and 0.31 V, respectively. DEP increases with the increase of scanning rate, hence revealing that the electron transfer process to the electrode becomes quasi-reversible; that is to say, the electron transfer rate becomes slower than that of voltammetry [54]. In addition, the quantitative analysis of the relationship between peak current (ip) and scan rate (v) can better illustrate the kinetics of electrode material. The response of the current to the applied scan rate will vary to large extent depending on whether the redox reaction is diffusion controlled or surface controlled

7

1.5

(d) C-blank-HQ C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

1.0 0.5 0.0 0.5

1.0

1.5 2.0 Z'/ohm

2.5

3.0

Fig. 6. The C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples when tested in a three-electrode configuration using 1 mol L
1 H2SO4 solution as aqueous electrolyte: (a) CV curves at 20 mV s
1; (b) GCD curves at 2 A g
1; (c) Specific capacitances; (d) Nyquist plots.

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

0.8

0 -10

2.0

C-PDA-1-1-HQ

0.0

2

R R=0.9907 C-blank-HQ

2.5

3.0 3.5 4.0 -1 1/2 (Scan rates / mV s )

0.2

0.4 0.6 Potential / V

0.8

1.0

(d)

20

2

R O=0.9986

10 C-PDA-1-1-HQ

0 -10

2

R R=0.9988

-20 -30

4.5

-1

C-PDA-1-3-HQ

0.0

0.2

0.4 0.6 Potential / V

0.8

Current density / A g

-1

5 mV s -1 7 mV s -1 10 mV s -1 20 mV s

4

8 12 16 -1 Scan rates / mV s

20

(g)

20 0 -1

5 mV s -1 7 mV s -1 10 mV s -1 20 mV s

-40 -60

1.0

C-PDA-1-5-HQ

0.0

0.2

0.4 0.6 Potential / V

0.8

1.0

60 (f)

50

2

R O=0.9978

25 0

C-PDA-1-3-HQ

-25

2

R R=0.9992

-50 -75

40

-20

75

30

R O=0.9915

5 mV s -1 7 mV s -1 10 mV s -1 20 mV s

-30

1.0

2

10

-20

-1

-20

(b)

20

-1

-10

60 (e)

-1

0.4 0.6 Potential / V

0

75 60 45 30 15 0 -15 -30 -45 -60

4

8 12 16-1 Scan rates / mV s

20

Current density / A g

-1

Current density / A g

0.2

10

Current density / A g

C-blank-HQ

0.0

30

5 mV s -1 7 mV s -1 10 mV s -1 20 mV s

(c)

20

-1

-1

Current density / A g

-1

30

Current density / A g

(a)

-1

25 20 15 10 5 0 -5 -10 -15 -20

Current density / A g

Current density / A g

-1

8

(h)

40

2

R O=0.9982

20 C-PDA-1-5-HQ

0

-20

2

R R=0.9978

-40 -60

4

8 12 16 -1 Scan rates / mV s

20

20 15 10 5 0 -5 -10 -15 -20

1.0

-1

100 mVs

in 1M H2SO4

-1

240

0.4 0.6 Potential / V

0.8

160

0

40 30

-80 100

(e)

25 20 15

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

10 5 0

0

5

10

15 20 Z' / ohm

25

300

400

500

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

1000 10000 -1 Power density / W kg C-PDA-1-1 (f) C-PDA-1-5 C-PDA-1-3 C-blank

-60 -40 -20 0 0.01

30

200

(d)

1 5 10 15 20 25 -1 Current density / A g

100

Time / s

10

80

30

-Z'' / ohm

0.0

1.0

120

0

-1

2Ag

0.4 0.2

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

(c)

200

0.6

-1

0.2

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

(b)

0.8

Energy density / A g

0.0

Specific capacitance / F g

C-blank C-PDA-1-1 C-PDA-1-3 C-PDA-1-5

Potential / V

(a)

Phase angle / degree

Current density / A g

-1

Fig. 7. The C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples when tested in a three-electrode configuration using 1 mol L
1 H2SO4 solution as aqueous electrolyte: (a, c, e, g) CV curves at various scan rates; (b, d, f, h) Relationship between peak intensity and scan rate.

0.1 1 Frequency / Hz

10

Fig. 8. The C-blank, C-PDA-1-1/1e3/1e5 samples when tested in a two-electrode configuration: (a) CV curves at 100 mV s
1; (b) GCD curves at 2 A g
1; (c) Specific capacitances; (d) Ragone plots; (e) Nyquist plots; (f) Bode plots.

and b, expect for the change of data. The corresponding Cs achieved from GCD curves are given in Fig. 8c. It is apparently seen that even if in the two electrode configuration, these samples still exhibit favorable capacitances. At the current density of 1 A g
1, the C-blank sample delivers a capacitance of 84 F g
1, and for the C-PDA-1-1/1e3/1e5 samples, their Cs are of 139, 218 and 181 F g
1, respectively. In particular, the C-PDA-1-3 sample remains high Cs of 119 F g
1 when extending the current density up to 30 A g
1. Besides, to compare different energy delivery capabilities, Ragone plots showing the function of energy density vs power density are used very often. Based on the mass of carbon material in the electrode, we calculated Ragone plots for different samples using Equations (3) and (4). Similar to the improvement trend of Cs aforementioned, the energy density of

these samples also gradually increases with the increase of PDA usage. For instance, at the power density of 0.5 kW kg
1, energy density of the C-blank sample is 2.91 Wh kg
1, and those of the CPDA-1-1/1e3/1e5 samples reach up to 4.83, 7.59 and 6.27 Wh kg
1, respectively. For Nyquist plots, these samples also exhibit similar trend to the above results. There exits inconspicuous arcs in the high frequency region, as shown in Fig. 8e, and the C-PDA-1-3 sample has the smallest one, definitely revealing its lowest resistance. Moreover, the dependence of phase angle on the frequency is estimated and proposed in Fig. 8f. The characteristic frequency with the phase angle of 45 is usually nominated as the knee frequency (f0), where the capacitive and the resistive impedances are equal. Beyond this point, the higher frequency is, the more resistive occurs. The relaxation time (t0 ¼ 1/f0) represents the shortest time required to release all energy from the device with an efficiency greater than 50% [59]. Herein, the t0 data regarding the C-blank, C-PDA-1-1/ 1e3/1e5 samples are of 45.7, 22.4, 1.8 and 15.6 s, respectively. Clearly, the C-PDA-1-3 sample delivers the minimum t0; and this reveals that it has got faster frequency response capability, as well as shorter ionic transport path and higher electrical conductivity [60]. With respect to the C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples, they were also measured in a two-electrode configuration. The resulting CV curves at 100 mV s
1 and GCD curves at 2 A g
1 are demonstrated in Fig. 9a and b, which are to some extent different from those without any HQ shown in Fig. 6a and b, primarily owing to the worse interaction between carbon electrode materials and HQ in H2SO4 solution. Even so, they still deliver appreciable Cs, and at 2 A g
1, the C-blank-HQ sample reaches 216 F g
1. Under the same current density, the C-PDA-1-1/1e3/1-5-HQ samples’ Cs are of 330, 557 and 406 F g
1, respectively, as shown in Fig. 9c. Especially, regarding the C-PDA-1-3-HQ sample, it also shows good capacitance retention, which still reaches up to 375 F g
1 even at high current density of 50 A g
1. Accordingly, these samples also have achieved good energy densities and power densities, as depicted in the Ragone plots in Fig. 9d. At high power density of 1 kW kg
1, the C-blank-HQ, CPDA-1-1/1e3/1-5-HQ samples’ energy densities are of 7.49, 11.45, 19.36 and 14.13 Wh kg
1, respectively. In particular, what deserves our attention is that the C-PDA-1-3-HQ sample can maintain a high energy density of 13.01 Wh kg
1 under a large power density of 25 kW kg
1. Cyclic stability is also an important indicator for the investigation of supercapacitors. In up to 10,000 continuous charge-

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

-1

1.0

10 m V s

(a)

12 8

Potential / V

Current density / A g

-1

16

4 C-blank-HQ C-PDA-1-1-HQ

0 -4 -8

C-blank-HQ C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

(b)

0.8 0.6

-1

2Ag

0.4 0.2

C-PDA-1-3-HQ C-PDA-1-5-HQ

-12 0.0

0.2

0.4 0.6 Potential / V

0.8

0.0

1.0

0

200

400 600 Time / s

800

1000

(c)

20 30 40 -1 50 Current density / A g

C-blank C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

600 Specfic capacitance / F g

-1

(e)

500

C-PDA-1-3-HQ

98%

400 C-PDA-1-5-HQ

95%

C-PDA-1-1-HQ

93%

300 200 C-blank-HQ

90%

100 0

2k

4k 6k 8k Cycle number

10k

(d)

10

1

0.1

C-blank C-PDA-1-1-HQ C-PDA-1-3-HQ C-PDA-1-5-HQ

1000 10000 -1 Power density / W kg

Potential / V

10

600 500 400 300 200 100 0

Specific capacitance / F g -1 Energy density / A g

-1

-16

9

12k

1.0 1st 0.8 (f) 1st 10000th 0.6 10000th 0.4 C-PAD-1-3-HQ C-blank-HQ 0.2 6 A g-1 6 A g-1 0.0 1.0 0 10 20 30 40 50 0 20 40 60 80 100120140 0.8 1st 1st th 10000 10000th 0.6 0.4 C-PAD-1-1-HQ C-PAD-1-5-HQ 0.2 6 A g-1 6 A g-1 0.0 0 20 40 60 80 0 20 40 60 80 100 Time / s

Fig. 9. The C-blank-HQ, C-PDA-1-1/1e3/1-5-HQ samples when tested in a two-electrode configuration: (a) CV curves at 100 mV s
1; (b) GCD curves at 2 A g
1; (c) Specific capacitances; (d) Ragone plots; (e) Cycling stability; (f) the 1st and 10000th GCD curves.

discharge cycle tests, these samples have showed excellent stability, as indicated in Fig. 9e. In general, coating the surface of the carbon material with PDA can effectively improve the cycle stability of supercapacitor, which is caused by the increased conductivity and reduced resistance after the PDA coating. Similar examples include double polymer sheathed carbon nanotube supercapacitors [61] and conductive polymer electrodes by deposition of a thin carbonaceous shell onto their surface [62]. In particular, the C-PDA1-3-HQ sample coated with PDA can maintain a capacitance retention rate of up to 98% after 10,000 cycles, which is also a very useful feature for supercapacitor application. Moreover, these samples’ excellent cycling stabilities within 10,000 cycles are also evinced by their 1st and 10000th GCD curves, as given in Fig. 9f. Therefore, we can conclude that PDA coating on the surface of

carbon materials can significantly improve a series of performance of the supercapacitors, which can guide and lead the further development of the potential of PDA in the future. For comparison, a list of supercapacitor performances under two-electrode configuration are summarized in Table 2. Overall, the Cs, energy density in this experiment exceeds the general level, regardless of whether HQ as redox additive or N-doping is adopted. In the best case of the C-PDA-1-3 sample, after adding HQ into 1 M H2SO4 solution, its the energy density increases by 2.55 times, also exhibiting excellent cycling stability. Obviously, the surface coating of carbon materials by PDA in this experiment played a key role in improving the electrochemical performance. Note that the energy densities reported in Ref. 64, 65 are achieved on the prerequisite of their high voltage scope (3.0 or 3.5 V) derived from the

10

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

Table 2 Comparison of supercapacitor performances under two-electrode configuration. Electrode materials

Electrolyte

Redox additive

Cs

Energy density

Ref.

carbon nanosheets activated carbon activated carbon N-doped carbon/CNT activated carbon N- doped carbon N- doped carbon sheets N-doped Carbon fibers N-doped Carbon fibers activated carbon Microporous Carbon C-PDA-1-3 C-PDA-1-3-HQ

1 M H2SO4 1 M H2SO4 PVA-H2SO4 1 M H2SO4 PVA-H2SO4 [EMIm]BF4-AN (3.0 V) EMIMBF4 (3.5 V) 6 M KOH 1 M Na2SO4 PVA/PVP/EMIHSO4 2 M H2SO4 1 M H2SO4 1 M H2SO4

HQ HQ HQ / HQ / / / / HQ HQ / HQ

72 F g
1 at 2 A g
1 253 F g
1 at 0.5 A g
1 420 F g
1 at 1 A g
1 45 F g
1 at 1 A g
1 474 F g
1 at 1 A g
1 65 F g
1 at 10 A g
1 220 F g
1 at 1 A g
1 202 F g
1 at 1 A g
1 170 F g
1 at 1 A g
1 485 F g
1 at 0.84 A g
1 1177 F g
1 at 1 A g
1 218 F g
1 at 1 A g
1 557 F g
1 at 2 A g
1

10.0 Wh kg
1 at 1 kW kg
1 8.8 Wh kg
1 at 0.25 kW kg
1 11.0 Wh kg
1 at 1 kW kg
1 1.0 Wh kg
1 at 1 kW kg
1 11.31 Wh kg
1 at 0.25 kW kg
1 40.13 Wh kg
1 at 4.5 kW kg
1 100 Wh kg
1 at 1 kW kg
1 6.3 Wh kg
1 at 1 kW kg
1 10.42 Wh kg
1 at 1 kW kg
1 24.3 Wh kg
1 at 1 kW kg
1 163 Wh kg
1 at 6 kW kg
1 7.59 Wh kg
1 at 0.5 kW kg
1 19.36 Wh kg
1 at 1 kW kg
1

[23] [47] [56] [63] [64] [65] [66] [67] [68] [69] [70] this work this work

ionic liquid effect, however, in present work, only 1 V is realized for the case of 1 M H2SO4 solution. Here is an interesting scientific question. For carbon materials, is N-doping or surface coating better for improving supercapacitor performance? Cao et al. answered this question very well. It is revealed that nitrogen in the bulk carbon network (upon heat treatment of the hexamine-coated carbon) shows increased conductivity, while nitrogen coated on the surface of porous carbon (using hexamine) is more effective at increasing capacitance. And hexamine coating also has a low diffusion resistance and charge transfer resistance, resulting in enhanced performance at high charge and discharge rates [71]. Of course, the electrochemical properties in this experiment are still relatively unsatisfactory, largely due to the fact that the carbon materials used in this experiment are basically based on a single microporous structure. As we all know, the carbon materials that are more suitable for electrochemical energy storage should have a hierarchical structure [72,73], thus this greatly limits the present performance of supercapacitors from a structural point of view. Therefore, how to post-process such commercialized activated carbon materials to have a hierarchical porous structure will be the focus of future experimental research. In addition, how to use the binary system redox additives such as the PPDE-LiCl-EV system [74] or explore the synergistic effect between the coating agent and the carbon material such as the composite of MWCNTs-HQ [75] will also be the direction of future research.

(5) After adding HQ into 1 M H2SO4 solution, the energy density of carbon materials increases by 2.55 times, also exhibiting excellent cycling stability (capacitance retention up to 98% after 10,000 cycles).

Declaration of competing interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Zhong Jie Zhang: Conceptualization, Writing - review & editing. Guo Liang Deng: Data curation. Xuan Huang: Data curation. Xin Wang: Formal analysis. Jun Min Xue: Formal analysis. Xiang Ying Chen: Methodology, Writing - review & editing. Acknowledgments The authors gratefully thank financial support from National Natural Science Foundation of China (51602003), Startup Foundation for Doctors of Anhui University (J01003211), and University Student Innovation Experiment Project of Anhui University (201910357028). References

4. Conclusions In summary, PDA is firstly coated on the surface of activated carbon materials via non-covalent bond and moreover, HQ is introduced into the H2SO4 system to collectively boost the supercapacitor performance. Some of the favorable results are concluded as follows: (1) Coating PDA on the surface of carbon material will reduce its porosity; moreover, as the amount of PDA increases, the porosity of carbon material will decrease more severely. (2) The thin layer of PDA on the surface of the carbon material can generate many nitrogen-containing functional groups, which are combined with HQ through hydrogen bonding. (3) Not only PDA on the surface of carbon materials, but also HQ in H2SO4 solution, can undergo redox reactions and generate pseudo-capacitance. (4) For carbon materials not covered with PDA, HQ kinetics is diffusion controlled; for carbon materials covered with PDA, HQ kinetics is surface controlled.

[1] Y. Zhai, Y. Dou, D. Zhao, P.F. Fulvio, R.T. Mayes, S. Dai, Carbon materials for chemical capacitive energy storage, Adv. Mater. 23 (2011) 4828e4850. guin, V. Presser, A. Balducci, E. Frackowiak, Carbons and electrolytes for [2] F. Be advanced supercapacitors, Adv. Mater. 26 (2014) 2219e2251. [3] H.A.A. Bashid, H.N. Lim, S.M. Hafiz, Y. Andou, M. Altarwneh, Z.T. Jiang, N.M. Huang, Modification of Carbon-Based Electroactive Materials for Supercapacitor Applications, Carbon-Based Polymer Nanocomposites for Environmental and Energy Applications, 2018, pp. 393e413. [4] S. Mallakpour, S. Soltanian, Surface functionalization of carbon nanotubes: fabrication and applications, RSC Adv. 6 (2016) 109916e109935. [5] M. Majumder, A.K. Thakur, Graphene and its modifications for supercapacitor applications, in: S. Sahoo, S. Tiwari, G. Nayak (Eds.), Surface Engineering of Graphene. Carbon Nanostructures, Springer, Cham, 2019. [6] F. Yan, Y. Jiang, X. Sun, Z. Bai, Y. Zhang, X. Zhou, Surface modification and chemical functionalization of carbon dots: a review, Microchim. Acta 185 (2018) 424. [7] M.S. Ata, R. Poon, A.M. Syed, J. Milne, I. Zhitomirsky, New developments in non-covalent surface modification, dispersion and electrophoretic deposition of carbon nanotubes, Carbon 130 (2018) 584e598. [8] H. Lee, S.M. Dellatore, W.M. Miller, P.B. Messersmith, Mussel-inspired surface chemistry for multifunctional coatings, Science 318 (2007) 426e430. [9] Y. Liu, K. Ai, L. Lu, Polydopamine and its derivative materials: synthesis and promising applications in energy, environmental, and biomedical fields, Chem. Rev. 114 (2014) 5057e5115. [10] A. Postma, Y. Yan, Y. Wang, A.N. Zelikin, E. Tjipto, F. Caruso, Self-

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940

[11] [12] [13]

[14]

[15]

[16]

[17]

[18]

[19] [20]

[21]

[22]

[23]

[24]

[25]

[26]

[27]

[28] [29]

[30]

[31]

[32]

[33]

[34] [35]

[36]

[37]

polymerization of dopamine as a versatile and robust technique to prepare polymer capsules, Chem. Mater. 21 (2009) 3042e3044. T. Chen, T. Liu, T. Su, J. Liang, Self-polymerization of dopamine in acidic environments without oxygen, Langmuir 33 (2017) 5863e5871. J. Ryu, P.B. Messersmith, H. Lee, Polydopamine surface chemistry: a decade of discovery, ACS Appl. Mater. Interfaces 10 (2018) 7523e7540. H. Long, D.D. Frari, A. Martin, J. Didierjean, V. Ball, M. Michel, H.I.E. Ahrach, Polydopamine as a promising candidate for the design of high performance and corrosion-tolerant polymer electrolyte fuel cell electrodes, J. Power Sources 307 (2016) 569e577. Y. Song, G. Ye, F. Wu, Z. Wang, S. Liu, M. Kopec, Z. Wang, J. Chen, J. Wang, K. Matyjaszewski, Bioinspired polydopamine (PDA) chemistry meets ordered mesoporous carbons (OMCs): a benign surface modification strategy for versatile functionalization, Chem. Mater. 28 (2016) 5013e5021. Y.A. Lee, Ji Lee, D. Kim, C. Yoo, S. Park, J. Yoo, S. Kim, B. Kim, W. Cho, H. Yoon, Mussel-inspired surface functionalization of porous carbon nanosheets using polydopamine and Fe3þ/tannic acid layers for high-performance electrochemical capacitors, J. Mater. Chem. A 5 (2017) 25368e25377. S. Dong, Z. Xie, Y. Fang, K. Zhu, Y. Gao, G. Wang, J. Yan, K. Cheng, K. Ye, D. Cao, Polydopamine-modified reduced graphene oxides as a capable electrode for High-performance supercapacitor, ChemSel 4 (2019) 2711e2715. H. Lee, T. Han, K. Cho, M. Ryou, Y. Lee, Dopamine as a novel electrolyte additive for high-voltage lithium-ion batteries, ACS Appl. Mater. Interfaces 8 (2016) 21366e21372. S.E. Chun, B. Evanko, X. Wang, D. Vonlanthen, X. Ji, G.D. Stucky, S.W. Boettcher, Design of aqueous redox-enhanced electrochemical capacitors with high specific energies and slow self-discharge, Nat. Commun. 6 (2015) 7818, https://doi.org/10.1038/ncomms8818. E. Frackowiak, M. Meller, J. Menzel, D. Gastol, K. Fic, Redox-active electrolyte for supercapacitor application, Faraday Discuss 172 (2014) 179e198. Q. Wang, Y. Nie, X. Chen, Z. Xiao, Z. Zhang, Use of pyrocatechol violet as an effective redox additive for highly promoting the supercapacitor performances, J. Power Sources 323 (2016) 8e16. X. Sun, W. Hu, D. Xu, X. Chen, P. Cui, Integration of redox additive in H2SO4 solution and the adjustment of potential windows for improving the capacitive performances of supercapacitors, Ind. Eng. Chem. Res. 56 (2017) 2433e2443. Y. Nie, Q. Wang, X. Chen, Z. Zhang, Nitrogen and oxygen functionalized hollow carbon materials: the capacitive enhancement by simply incorporating novel redox additives into H2SO4 electrolyte, J. Power Sources 320 (2016) 140e152. D. Xu, N. Sun, W. Hu, X. Chen, Carbon nanosheets-based supercapacitors: design of dual redox additives of 1, 4-dihydroxyanthraquinone and hydroquinone for improved performance, J. Power Sources 357 (2017) 107e116. D. Xu, W. Hu, N. Sun, P. Cui, X. Chen, Redox additives of Na2MoO4 and KI: synergistic effect and the Improved capacitive performances for carbon-based supercapacitors, J. Power Sources 341 (2017) 448e456. S. Gan, L. Zhong, L. Gao, D. Han, L. Niu, Electrochemically driven surface confined acid/base reaction for an ultrafast Hþ supercapacitor, J. Am. Chem. Soc. 138 (2016) 1490e1493. S. Yoo, B. Evanko, X. Wang, M. Romelczyk, A. Taylor, X. Ji, S. Boettcher, G. Stucky, Fundamentally addressing bromine storage through reversible solid-state confinement in porous carbon electrodes: design of a Highperformance dual-redox electrochemical capacitor, J. Am. Chem. Soc. 139 (2017) 9985e9993. L. Mai, A. Minhas-Khan, X. Tian, K.M. Hercule, Y. Zhao, X. Lin, X. Xu, Synergistic interaction between redox-active electrolyte and binder-free functionalized carbon for ultrahigh supercapacitor performance, Nat. Commun. 4 (2013) 2923, https://doi.org/10.1038/ncomms3923. D.R. Dreyer, D.J. Miller, B.D. Freeman, D.R. Paul, C.W. Bielawski, Elucidating the structure of poly(dopamine), Langmuir 28 (2012) 6428e6435. K. Qu, Y. Zheng, Y. Jiao, X. Zhang, S. Dai, S. Qiao, Polydopamine-inspired, dual heteroatom-doped carbon nanotubes for highly efficient overall water splitting, Adv. Energy Mater. (2017) 1602068. B. Fei, B. Qian, Z. Yang, R. Wang, W. Liu, C. Mak, J. Xin, Coating carbon nanotubes by spontaneous oxidative polymerization of dopamine, Carbon 46 (2008) 1792e1828. Z. Zhang, Q. Song, W. He, P. Liu, Y. Xiao, J. Liang, X. Chen, Dual surface modification of carbon materials by polydopamine and phosphomolybdic acid for supercapacitor application, Dalton Trans. 48 (2019) 17321e17330. Z. Xi, Y. Xu, L. Zhu, Y. Wang, B. Zhu, A facile method of surface modification for hydrophobic polymer membranes based on the adhesive behavior of poly(DOPA) and poly(dopamine), J. Membr. Sci. 327 (2009) 244e253. T.I.T. Okpalugo, P. Papakonstantinou, H. Murphy, J. McLanghlin, N.M.D. Brown, High resolution XPS characterization of chemical functionalized MWCNTs and SWCNTs, Carbon 43 (2005) 153e161. W. Shen, Z. Li, Y. Liu, Surface chemical functional groups modification of porous carbon, Recent Pat. Chem. Eng. 1 (2008) 27e40. N.D.F. Vecchia, R. Avolio, M. Alfe, M.E. Errico, A. Napolitano, M. d’Ischia, Building-block diversity in polydopamine underpins a multifunctional eumelanin-type platform tunable through a quinone control point, Adv. Funct. Mater. 23 (2012) 1331e1340. R.A. Zangmeister, T.A. Morris, M.J. Tarlov, Characterization of polydopamine thin films deposited at short times by autoxidation of dopamine, Langmuir 29 (2013) 8619e8628. C. Chao, J. Liu, Y. Zhang, B. Zhang, Y. Zhang, X. Xiang, R. Chen, Surface

[38]

[39]

[40]

[41] [42]

[43]

[44]

[45]

[46]

[47]

[48]

[49]

[50]

[51]

[52]

[53]

[54]

[55] [56]

[57]

[58] [59] [60]

[61]

[62]

[63]

[64]

[65]

11

modification of halloysite nanotubes with dopamine for enzyme immobilization, ACS Appl. Mater. Interfaces 5 (2013) 10559e10564. X. Chen, C. Chen, Z. Zhang, D. Xie, X. Deng, J. Liu, Nitrogen-doped porous carbon for supercapacitor with long-term electrochemical stability, J. Power Sources 230 (2013) 50e58. H. Liu, M. Wang, D. Zhai, X. Chen, Z. Zhang, Design and theoretical study of carbon-based supercapacitors especially exhibiting superior rate capability by the synergistic effect of nitrogen and phosphor dopants, Carbon 155 (2019) 223e232. M. Wang, H. Liu, D. Zhai, X. Chen, Z. Zhang, In-situ synthesis of highly nitrogen, sulfur co-doped carbon nanosheets from melamine-formaldehydethiourea resin with improved cycling stability and energy density for supercapacitors, J. Power Sources 416 (2019) 79e88. J. Sedo, J. Saiz-Poseu, F. Busque, D. Ruiz-Monlina, Catechol-based biomimetic functional materials, Adv. Mater. 25 (2013) 653e701. N. Orishchin, C. Crane, M. Brownell, T. Wang, S. Jenkins, M. Zou, A. Nair, Rapid deposition of uniform polydopamine coatings on nanoparticle surfaces with controllable thickness, Langmuir 33 (2017) 6046e6053. Y. Li, S. Shi, H. Cao, Z. Zhao, C. Su, H. Wen, Improvement of the antifouling performance and stability of an anion exchange membrane by surface modification with graphene oxide (GO) and polydopamine (PDA), J. Membr. Sci. 566 (2018) 44e53. W. Zhang, F. Yang, Z. Pan, J. Zhang, B. Zhao, Bio-inspired dopamine functionalization of polypyrrole for improved adhesion and conductivity, Macromol. Rapid Commun. 35 (2014) 350e354. W. Lee, J. Lee, B. Jung, J. Byun, J. Yi, S. Lee, B. Kim, Simultaneous enhancement of mechanical, electrical and thermal properties of graphene oxide paper by embedding dopamine, Carbon 65 (2013) 296e304. n, C. Blanco, M. Granda, R. Mene ndez, R. Santamaría, Towards a S. Rolda further generation of high-energy carbon-based capacitors by using redoxactive electrolytes, Angew. Chem. Int. Ed. 50 (2011) 1699e1701. D. Zhai, M. Wang, H. Liu, D. Wu, X. Chen, Z. Zhang, Integrating surface functionalization and redox additives to improve surface reactivity for high performance supercapacitors, Electrochim. Acta 323 (2019), 134810. H. Zhang, F. Zhang, S. Li, H. Li, A multilayer gold nanoparticle-polydopamine hybrid membrane-modified indium tin oxide electrode for selective sensing of dopamine in the present of ascorbic acid, Ionics 23 (2017) 2475e2487. Z. Zhang, S. Sun, Understanding the redox effects of amine and hydroxyl groups of p-aminophenol upon the capacitive performance in KOH and H2SO4 electrolyte, J. Electroanal. Chem. 778 (2016) 80e86. Y. Wang, Z. Shi, Y. Huang, Y. Ma, C. Wang, M. Chen, Y. Chen, Supercapacitor devices based on graphene materials, J. Phys. Chem. C 113 (2009) 13103e13107. B. Ai, O. Munteshari, J. Lau, B. Dunn, L. Pilon, Physical interpretations of Nyquist Plots for EDLC electrodes and devices, J. Phys. Chem. C 122 (2018) 194e206. C. Singh, A. Paul, Physisorbed hydroquinone on activated charcoal as a supercapacitor: an application of proton-coupled electron transfer, J. Phys. Chem. C 119 (2015) 11382e11390. Z. Zhang, X. Chen, Illustrating the effect of electron withdrawing and electron donating groups adherent to p-hydroquinone on supercapacitor performance: the cases of sulfonic acid and methoxyl groups, Electrochim. Acta 282 (2018) 563e574. C. Costentin, J.M. Saveant, Theoretical and mechanistic aspects of protoncoupled electron transfer in electrochemistry, Curr. Opin. Electrochem. 1 (2017) 104e109. V. Augustyn, P. Simon, B. Dunn, Pseudocapacitive oxide materials for high-rate electrochemical energy storage, Energy Environ. Sci. 7 (2014) 1597e1614. J. Zhong, L. Fan, X. Wu, J. Wu, G. Liu, J. Lin, M. Huang, Y. Wei, Improved energy density of quasi-solid-state supercapacitors using sandwich-type redox-active gel polymer electrolytes, Electrochim. Acta 166 (2015) 150e156. J. Park, V. Kumar, X. Wang, P.S. Lee, W. Kim, Investigation of charge transfer kinetics at carbon/hydroquinone interfaces for redox-active-electrolyte supercapacitors, ACS Appl. Mater. Interfaces 9 (2017) 33728e33734. M. Stoller, S. Park, Y. Zhu, J. An, R. Ruoff, Graphene-based ultracapacitors, Nano Lett. 8 (2008) 3498e3502. S. Zhang, N. Pan, Supercapacitors performance evaluation, Adv. Energy Mater. 5 (2015), 1401401. T. Purkait, G. Singh, D. Kumar, M. Singh, R. S. Dey, High-performance flexible supercapacitors based on electrochemically tailored three-dimensional reduced graphene oxide networks, Sci. Rep. 8 (018) 640. W. Zhao, S. Wang, C. Wang, S. Wu, W. Xu, M. Zou, S. Ouyang, A. Cao, Y. Li, Double polymer sheathed carbon nanotube supercapacitors show enhanced cycling stability, Nanoscale 8 (2016) 626e633. T. Liu, L. Finn, M. Yu, H. Wang, T. Zhai, X. Lu, Y. Tong, Y. Li, Polyaniline and polypyrrole Pseudocapacitor Electrodes with eexcellent cycling stability, Nano Lett. 14 (2014) 2522e2527. M. Sevilla, L. Yu, L. Zhao, C.O. Ania, M.M. Titiricic, Surface modification of CNTs with N-doped carbon: an effective way of enhancing their performance in supercapacitors, ACS Sustain. Chem. Eng. 2 (2014) 1049e1055. H. Yu, J. Wu, L. Fan, Y. Lin, K. Xu, Z. Tang, C. Cheng, S. Tang, J. Lin, M. Huang, Z. Lan, A novel redox-mediated gel polymer electrolyte for high-performance supercapacitor, J. Power Sources 198 (2012) 402e407. M. Zhong, M. Wang, H. Liu, Y. Zhu, X. Chen, Z. Zhang, Hierarchically N/Oenriched nanoporous carbon for supercapacitor application: simply

12

[66]

[67]

[68]

[69]

[70]

Z.J. Zhang et al. / Electrochimica Acta 339 (2020) 135940 adjusting the composition of deep eutectic solvent as well as the ratio with phenol-formaldehyde resin, J. Power Sources 438 (2019), 226982. J. Hou, C. Cao, F. Idrees, X. Ma, Hierarchical porous nitrogen-cdoped Carbon nanosheets derived from usilk for Ultrahigh-capacity battery anodes and supercapacitors, ACS Nano 9 (2015) 2556e2564. L. Chen, X. Zhang, H. Liang, M. Kong, Q. Guan, P. Chen, Z. Wu, S. Yu, Synthesis of nitrogen-doped porous carbon nanofibers as an efficient electrode material for supercapacitors, ACS Nano 6 (2015) 27092e27102. H. Liu, M. Wang, D. Zhai, J. Liu, X. Chen, Z. Zhang, Urea-modified phenolformaldehyde resins for the template-assisted synthesis of nitrogen-doped carbon nanosheets as electrode material for supercapacitors, ChemElectroChem 6 (2019) 885e891. N. Yadav, N. Yadav, S.A. Hashmi, Ionic liquid incorporated, redox-active blend polymer electrolyte for high energy density quasi-solid-state carbon supercapacitor, J. Power Sources 451 (2020), 227771. C. Singh, A. Paul, Immense microporous carbon@hydroquinone metamorphosed from nonporous carbon as a supercapacitor with remarkable energy density and cyclic stability, ACS Sustain. Chem. Eng. 6 (2018)

11367e11379. [71] S. Candelaria, E. Uchaker, G. Cao, Comparison of surface and bulk nitrogen modification in highly porous carbon for enhanced supercapacitors, Sci. China Mater. 58 (2015) 521e533. [72] S. Dong, X. He, H. Zhang, X. Xie, M. Yu, C. Yu, N. Xiao, J. Qiu, Surface modification of biomass-derived hard carbon by grafting porous carbon nanosheets for high-performance supercapacitors, J. Mater. Chem. A 6 (2018) 15954e15960. [73] H. Liu, Q. Tang, Z. Qiu, X. Chen, Z. Zhang, Tuning nitrogen species in twodimensional carbon through pore structure change for high supercapacitor performance, ChemElectroChem 6 (2019) 5220e5228. [74] S. Hyeon, J. Seo, J. Bae, W. Kim, C. Chung, Faradaic reaction of dual-redox additive in zwitterionic gel electrolyte boosts the performance of flflexible supercapacitors, Electrochim. Acta 319 (2019) 672e681. [75] D. Potphode, L. Sinha, P. Shirage, Redox additive enhanced capacitance: multiwalled carbon nanotubes/polyaniline nanocomposite based symmetric supercapacitors for rapid charge storage, Appl. Surf. Sci. 469 (2019) 162e172.