Unraveling Factors Leading to High Pseudocapacitance of Redox

Dec 27, 2018 - Graphene sheets functionalized by redox-active small aromatics can exhibit enhanced capacitance because of the introduced faradaic proc...
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Unraveling Factors Leading to High Pseudocapacitance of RedoxActive Small Aromatics on Graphene Yi Zhao,† Xiaoxu Wang,‡ Na Wang,† Ming Li,† Qi Li,† and Jinzhang Liu*,† †

School of Materials Science and Engineering, Beihang University, Beijing 100191, China Beijing Computing Center, Beijing 100094, China

‡

J. Phys. Chem. C Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/07/19. For personal use only.

S Supporting Information *

ABSTRACT: Graphene sheets functionalized by redox-active small aromatics can exhibit enhanced capacitance because of the introduced faradaic process. However, the immense number of possible molecules for energy storage makes the selection of the appropriate ones difficult. This study combines experiment and theory to unveil factors behind the different pseudocapacitance contributions of some aromatic isomers adsorbed onto graphene, aiming to provide a guideline for computationally screening out optimal molecules for supercapacitor electrodes. Eight kinds of molecules containing amino groups are intentionally selected to functionalize N-doped graphene (NG) and their electrochemical properties are compared. The highest occupied molecular orbital level of a molecule is found to play an important role in rendering a high pseudocapacitance. Also, remarkable efficacies from two kinds of molecules, 4-aminophenol and 1,5-naphthalenediamine (1,5-NAPD), are unveiled, and the role of the amino group in charge storage is discussed. As a result, the graphene film absorbed with 1,5-NAPD molecules shows a high specific gravimetric capacitance of 877 F g−1 within the voltage window of 1 V, corresponding to a high areal specific capacitance of 1.14 F cm−2 from the thin film with a mass loading of 1.3 mg cm−2. Also, the 1,5-NAPD/NG film shows good cycling stability, achieving 105% capacitance retention after 5000 charge−discharge cycles.

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INTRODUCTION Electrochemical energy storage materials have long been a focus of research because of their indispensable role in developing high-performance batteries and supercapacitors. Electrical double-layer supercapacitors store charge via physical adsorption of ions from organic electrolytes. This technology offers a high charge−discharge rate, high power density, and long cycling lifetime but has the drawback of a low energy density. Although graphene has been considered as an ideal electrode material, the reported specific capacitance of graphene-based electrodes is by far unsatisfactory. Many strategies for improving the capacitance of graphene-based electrodes have been reported, including anti-restacking approaches,1,2 three-dimensional holey graphene frameworks,3,4 chemically modified graphene aerogels,5 and Ndoped graphene,6,7 yet their efficacies are not remarkable. Pseudocapacitors rely on reversible chemical reactions to store charge, which have the advantage of higher specific capacitance. Although N, S, or P doping in graphene can introduce extra pseudocapacitance because of the oxygenated bonds at these doping sites, so far the maximum specific capacitance has not exceeded 450 F g−1 due to the limited contribution of pseudocapacitance6,8,9 Conventional pseudocapacitive materials include metal oxides and some conducting polymers, and aqueous electrolytes are used.10,11 There are © XXXX American Chemical Society

over 10 kinds of metal oxides and several kinds of polymers that can be used for pseudocapacitive electrodes, and many researchers combined them with graphene to make supercapacitors.12−14 However, the technology of a pseudosupercapacitor is by far immature in terms of a high energy density, long lifetime, and economical issues. Therefore, exploring new electrode materials is the key to the development of advanced supercapacitors. Graphene sheets functionalized by redox-active small aromatics can exhibit an enhanced overall capacitance due to the introduced pseudocapacitance.15,16 The quantum capacitance of pristine graphene is ∼21 μF cm−2,17 which determines the theoretical specific capacitance of 550 F g−1. With the added pseudocapacitance, the specific capacitance of a graphene sheet can be much higher than this theoretical value. So far, organic materials for supercapacitor electrodes have been limited to a few conductive polymers, such as polyaniline (PANI) and polypyrrole,18,19 which have the drawbacks of a narrow voltage window (∼0.8 V) and poor cycling stability. Differing from such polymers, small organic molecules are diverse and can provide more opportunities to Received: August 27, 2018 Revised: December 2, 2018 Published: December 27, 2018 A

DOI: 10.1021/acs.jpcc.8b08348 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. (a) Cross-sectional SEM image of a 4,4′-TDA/NG film, showing rumpled graphene sheets. (b) Illustration of the graphene film functionalized by small aromatic molecules. (c) High-resolution XPS C 1s spectrum of the functionalized NG film. (d) Survey XPS spectra of NG and 4,4′-TDA/NG films, respectively. (e) The S 2p XPS peak. (f) High-resolution XPS C 1s spectrum of a blank NG film.

the composite film was squeezed by two glass slides and then transferred into a Teflon-lined autoclave containing diluted ammonia solution for hydrothermal reduction at 180 °C. After maintaining this temperature for 3 h, the autoclave was cooled down and the paper-like NG film was washed with deionized (DI) water. Functionalizing NG with Small Aromatic Molecules. Eight kinds of small organic molecules, including 4-aminophenol (AP), 2-AP, 1,5-naphthalenediamine (NAPD), 1,8NAPD, 4,4′-oxydianiline (ODA), 3,4′-ODA, 4,4′-thiodianiline (TDA), and 2,2′-TDA, were used to functionalize NG films with a mass loading ∼1 mg cm−2. Typically, the organic compound was dissolved in DI water solution at a concentration of 0.06 M. The NG film was immersed into the solution at 80 °C for 6 h to allow aromatic molecules to be spontaneously adsorbed onto graphene via π−π interaction. Then, the functionalized NG (FNG) film was repeatedly washed with DI water. To measure the areal mass loading, a regularly shaped FNG film was freeze-dried and weighed by an analytical balance with 0.01 mg resolution. NG films functionalized by organic molecules, such as 4-AP, are named as 4-AP/NG. The other seven samples are named similarly. Electrochemical Characterization. The freestanding FNG film was cut into rectangular pieces as supercapacitor electrodes (Figure S1a). These films together with 200 nmthick porous separator films were soaked in 1 M H2SO4 aqueous solution for 2 h prior to device fabrication. Symmetric cells were assembled by using two Pt foils to squeeze the two FNG films with a separator film in between. The overlapping area between the two electrode films was 1 × 1 cm2. The sandwich-structured symmetric cell was tested on an electrochemical workstation. For the electrochemical test using a three-electrode system with 1 M H2SO4 electrolyte, the work electrode was prepared by a standard slurry-coating process. First, the NG film was ultrasonically shredded into tiny pieces in a beaker containing water. Second, the black dispersion was centrifuged and freeze dried, leaving a powdery product of NG. Third, the black powder was mixed with acetylene black and

make supercapacitors with better performance. Redox-active organic molecules, mostly quinones, have been used to develop ionic batteries20,21 or redox flow batteries,22,23 but rarely applied in supercapacitors. Although some researchers polymerized small organic molecules to make supercapacitor electrodes,24,25 their electrochemical performances were not outstanding compared with that of conventional PANI-based electrodes. The advantage of using small organic molecules is that in a monolayer of molecules attached to the graphene surface, each molecule can be exploited to store energy via the faradaic process. However, the number of possible aromatics that can be explored as useful materials for supercapacitors is immense. There are a few reports on the combination of quinones, phenols, and carbon materials for supercapacitors, by exploiting the redox of carbonyl and aldehyde groups, respectively.15,26,27 Herein, we first looked into aminocontaining aromatics to investigate the correlation between pseudocapacitance and the electronic structure of a molecule. This can provide a theoretical clue for the rational selection of small aromatics with high pseudocapacitance, greatly reducing the number of candidates to be tested as well as saving valuable time and cost. Moreover, two kinds of small aromatic molecules with superior pseudocapacitance were discovered in this work.

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EXPERIMENTAL AND THEORETICAL METHODS Material Preparation. Organic compounds were purchased from Shanghai Aladdin Bio-Chem Co and used as received. The graphene oxide (GO) was synthesized by using a modified Hummer’s method reported elsewhere.28 In this work, we prepared paper-like nitrogen-doped graphene (NG) films with an areal mass loading of ∼1 mg cm−2 to investigate the effect of organic molecule functionalization on the capacitance enhancement. In brief, a viscous aqueous solution containing GO, NaCl, urea, and (NH4)2CO3 with a mass ratio of 1:3:3:3 was blade-coated onto glass slides to prepare hydrogel-like films, which were subsequently immersed into acetone for extracting water. After being naturally dried in air, B

DOI: 10.1021/acs.jpcc.8b08348 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 2. (a) CV loops at 10 mV s−1 collected from a three-electrode system using NG, 2-AP/NG, and 4-AP/NG as work electrodes, respectively. Using a two-electrode symmetric configuration, CV loops corresponding to the three materials are shown in (b). (c) GCD curves at 1 A g−1 from symmetric cells based on the three electrode films, respectively. (d) Specific capacitances of the three films at various current densities. (e) Frontier molecular orbital (MO) diagrams of 4-AP and 2-AP molecules. (f) Energy-level diagram.

poly(tetrafluoroethylene) as a binder at a mass ratio of 8:1:1 in N-methyl pyrrolidone solution to form slurry, which was subsequently coated onto the Pt foil and then dried. A Pt wire was used as the counter electrode, and the reference electrode was Ag/AgCl. Computational Details. The organic molecules with optimized structures were computed by using the Gaussian 09 software package. The solvation model SMD and metaGGA density functional M06 with the 6-31+G(d,p) allelectron basis set and ultrafine integration grids were used.29,30 The binding energies and density of states for organic molecules on graphene were computed using the VASP package. First-principles simulations were performed using the density functional theory (DFT)-based projector augmentedwave method as implemented in the VASP code. Using the plane-wave-based total energy minimization, all structures were completely relaxed till the force on each atom was less than 0.01 eV Å−1. To account for the weak van der Waals forces, we considered the Grimme DFT-D3 dispersion correction, which uses a pairwise force field for describing the van der Waals interactions.

adsorbing 4-AP or 1,5-NAPD molecules, the thickness was increased to ∼1.5 nm, indicating a monolayer of molecules on the surface (Figure S2b,c). In Figure 1d, two survey XPS spectra of NG and 4,4′-TDA/NG films show C 1s, N 1s, and O 1s XPS peaks. Notably, the XPS spectrum of 4,4′-TAD/NG film contains S 2s and S 2p XPS peaks contributed by 4,4′TAD molecules. Our XPS elemental analysis reveals that the NG film contains 75.4 atom % C, 4.9 atom % N, and 19.7 atom % O. The 4,4′-TDA/NG sample contains 3.0 atom % S, and the high-resolution S 2p XPS spectrum is shown in Figure 1e. Owing to the amino groups in 4,4′-TDA, the N 1s XPS peak of 4,4′-TDA/NG is more pronounced, leading to a high N content of 8.9 atom %. The high-resolution C 1s XPS spectra of 4,4′-TDA/NG and NG films are shown in Figure 1c,f, respectively. For other samples without the sulfur element, our XPS analysis also shows increased nitrogen content in the NG film after the adsorption of amino-contained molecules (Figure S2d−f). NG films functionalized by different molecules were used as supercapacitor electrodes, and the pseudocapacitance contributions of different molecules are compared. Comparison between 4-AP and 2-AP Molecules with Regard to Pseudocapacitance Contribution. 4-AP and 2AP were first selected for a comparative study. Figure 2a shows cyclic voltammetry (CV) loops measured at 10 mV s−1 within the voltage range of −0.2−0.8 V, corresponding to NG, 2-AP/ NG, and 4-AP/NG films tested in a three-electrode cell. The CV loop of the NG film is in a quasi-rectangular shape and narrower than the other two CV loops containing multiple redox peaks, owing to the organic molecules on the graphene surface. The three CV loops in Figure 2b were measured from symmetric cells based on NG, 2-AP/NG, and 4-AP/NG films, respectively. According to the area of the CV loop, it can be concluded that the capacitance sequence is 4-AP/NG > 2-AP/ NG > NG. In Figure 2c, galvanostatic charge−discharge (GCD) curves at 1 A g−1 were collected from three symmetric

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RESULTS AND DISCUSSION Structural Characterization. Figure 1a shows a typical cross-sectional scanning electron microscopy (SEM) image of the NG film functionalized by 4,4′-TDA, named as 4,4′-TDA/ NG. Between NG sheets, there are no solid agglomerations of organic molecules (Figures 1a and S1). Figure 1b illustrates rumpled graphene sheets with aromatic molecules on the surface. The presence of organic molecules on a graphene surface can be confirmed by using X-ray photoelectron spectroscopy (XPS) to study the surface chemistry and atomic force microscopy (AFM) to measure the thickness change of a sheet. For instance, the thickness of a partially reduced graphene oxide (rGO) sheet was 0.9 nm (Figure S2a). After C

DOI: 10.1021/acs.jpcc.8b08348 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 3. (a) CV loops at 10 mV s−1 collected from a three-electrode system using 1,5-NAPD/NG and 1,8-NAPD/NG as work electrodes, respectively. Using a two-electrode symmetric configuration, CV loops corresponding to NAPD/NG and 1,8-NAPD/NG films are shown in (b). (c) GCD curves collected from symmetric cells based on NAPD/NG and 1,8-NAPD/NG films, respectively. (d) Specific capacitances of the two electrode films at various current densities. (e) Frontier MO diagrams of 1,5-NAPD and 1,8-NAPD molecules. (f) Energy-level diagram.

doping.34 Hwang et al. reported that the WF of the NG film with 4.5 atom % nitrogen was ∼4.32 eV.35 Therefore, in Figure 2f, the energy level of NG is set at −4.3 eV. The calculated EHOMO values of 4-AP and 2-AP molecules are −5.44 and −5.86 eV, respectively. Thus, the 4-AP molecule has a higher pseudocapacitance as well as EHOMO than the 2-AP molecule. To verify the trend that among positional isomers, the one with a higher HOMO level has a higher pseudocapacitance, we further looked into NAPD isomers that contain two benzene rings and two −NH2 in a single molecule. Comparison between 1,5-NAPD and 1,8-NAPD Molecules with Regard to Pseudocapacitance Contribution. Figure 3a shows two CV loops at 10 mV s−1 collected from a three-electrode cell using 1,5-NAPD/NG and 1,8NAPD/NG films as work electrodes, respectively. The 1,5NAPD molecules show two redox peaks at 0.1 and 0.3 V in the CV curve, respectively, whereas the 1,8-NAPD molecules show only one broad redox peak at ∼0.35 V. The other two CV loops in Figure 3b were measured from symmetric cells based on 1,5-NAPD/NG and 1,8-NAPD/NG films, respectively. GCD curves at 1 A g−1 from the two devices are displayed in Figure 3c, from which the Coulombic efficiencies of 1,8NAPD/NG and 1,5-NAPD/NG devices are obtained as 95 and 92%, respectively. More CV loops and GCD curves of the 1,5-NAPD/NG device can be found in Figure S3d−f. Specific capacitance values of 1,5-NAPD/NG and 1,8-NAPD/NG films deduced from GCD curves at different current densities are plotted in Figure 3d. The specific capacitance of 1,5-NAPD/ NG reached 877 F g−1, much higher than that of the 1,8NAPD/NG film (678 F g−1). The mass loading of the NG film was increased by 30 and 40% after adsorbing 1,5-NAPD and 1,8-NAPD molecules, respectively, and the mass gain was included in the calculation of gravimetric specific capacitance. It should be addressed that the areal specific capacitance of an NG film can be increased more than 3-fold after being

cells based on NG, 2-AP/NG, and 4-AP/NG electrodes, respectively. From GCD curves at different current densities, the specific capacitance values of different electrodes were calculated, as shown in Figure 2d. The blank NG film with 4.9 atom % nitrogen and a mass loading density of ∼1.0 mg cm−2 showed specific capacitances up to 350 F g−1. Without Ndoping, the maximum specific capacitance of such a graphene film would be less than 300 F g−1.31 Although it is well known that N-doping in graphene can enhance the specific capacitance due to the enhanced quantum capacitance and introduced pseudocapacitance from N−H bonds, its efficacy is not remarkable.7 In our work, the maximum specific capacitance values of 2-AP/NG and 4-AP/NG films were increased to 582 and 756 F g−1 at 1 A g−1, with the Coulombic efficiencies of 91 and 93%, respectively. It indicates that the gravimetric specific capacitance of an NG film can be more than doubled by adsorbing 4-AP molecules. Considering the 30% mass gain after adsorbing 4-AP molecules, the areal specific capacitances of the 4-AP/NG film was increased by 2.8 times, as from 0.35 to 1.0 F cm−2 (Figure S4). More electrochemical measurements of the 4-AP/NG device can be found in Figure S3a−c. To elucidate why the 4-AP/NG has a higher specific capacitance than 2-AP/NG, we performed quantum chemistry calculation. Figure 2e shows frontier molecular orbital (MO) diagrams of 4-AP and 2-AP. Energy levels of the highest occupied MO (HOMO) and the lowest unoccupied MO (LUMO) for the two molecules are given in Figure 2f. The energy of HOMO, noted as EHOMO, reflects a tendency toward the donation of electrons by a molecule. The higher the EHOMO, the better the tendency of donating electrons. ELUMO indicates the ability to accept electrons by the molecule. The work function (WF) of pristine graphene was reported to be ∼4.5 eV.32 The WF of one to three layers of rGO was measured to be 4.46 eV.33 However, the WF of graphene can be decreased by nitrogen D

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Figure 4. (a) Molecular structures of four organic compounds. (b) CV loops from symmetric cells based on NG films functionalized by ODA isomers. The other two CV loops corresponding to TDA/NG films are shown in (c). (d) GCD curves at 1 A g−1 from symmetric cells based on four different electrode films, respectively. (e) Specific capacitances of the four electrode films at various current densities. (f) Energy-level diagram.

as that of a single 1-NAP molecule containing only one −NH2. For the 1,8-NAPD molecule, although it also contains two −NH2 and its EHOMO is very close to that of 1,5-NAPD, the two amino groups are adjacent in the molecular structure (Figure 3e), which would lead to electrostatic interference in the charge storage process and cause a relatively low specific capacitance. However, the two −NH2 in the 1,5-NAPD molecule are diagonally separated and their electrostatic interaction is largely reduced, allowing faradaic charge storage at each side to proceed independently. ODA and TDA Isomers with Similar Geometries: The Correlation between Pseudocapacitance and the HOMO Level of the Molecule. To verify the ubiquity of the relationship between EHOMO and pseudocapacitance contribution of a redox-active molecule, we used ODA and TDA isomers, as shown in Figure 4a, for further study. Starting from 4,4′-ODA , variables of molecular parameters were elaborately set as follows: (1) Changing the position of a single amino group, as 4,4′-ODA → 3,4′-ODA; (2) replacing the central O with S, as 4,4′-ODA → 4,4′-TDA. This is to alter the energy levels, while maintaining the molecular geometry intact; (3) changing the positions of two amino groups, as 4,4′-TDA → 2,2′-TDA. Therefore, three pairs of molecules are compared. Frontier MO diagrams of the four kinds of molecules are shown in Figure S6. CV loops at 10 mV s−1 from four different symmetric cells are shown in Figure 4b,c. The CV loop of the 4,4′-ODA/NG cell shows more pronounced redox peaks and a larger area compared with that of the 3,4′-ODA/NG cell. In Figure 4c, although the CV loop of 4,4′-TDA/NG shows no distinct redox peaks, its area is larger than that of 2,2′-TDA/NG. From GCD curves at 1 A g−1 (Figure 4d), the capacitance sequence can be obtained as

functionalized by 1,5-NAPD molecules, as from 0.35 to 1.14 F cm−2 (Figure S4). Furthermore, our calculation suggests that the EHOMO level of 1,5-NAPD at −5.18 eV is also higher than that of 1,8-NAPD at −5.34 eV of (Figure 3e). Therefore, the trend shown by AP isomers, i.e., a higher EHOMO level of a molecule, is more favorable to pseudocapacitance contribution and is also applicable to NAPD isomers. By using the specific capacitance calculated from GCD curves, the highest energy density of the 1,5-NAPD/NG cell is estimated to be 31 Wh kg−1, which is comparable to that of lead−acid and Ni−Cd batteries (20−60 Wh kg−1). The number of amino groups in a single molecule may play a role by influencing the pseudocapacitance. In this regard, we compared 1,5-NAPD with 1-naphthylamine (1-NAP) for their efficacies of pseudocapacitance enhancement. As a result, the maximum specific capacitance of the 1-NAP/NG film was 505 F g−1 (Figure S5), much lower than that of the 1,5-NAPD/NG film (877 F g−1). Both 1,5-NAPD/NG and 1-NAP/NG films show an equivalent weight gain percentage of 30% after functionalization. Using the molecular weight of 158.2 for 1,5NAPD and 143.2 for 1-NAP, and the original capacitance of 350 F g −1 of a blank NG film, we estimated the pseudocapacitance of a single molecule using ΔC/N, where ΔC is the areal capacitance gain and N is the number of molecules. Here, we assume that all organic molecules adsorbed onto the graphene surface can participate in the faradaic process. This is based on our AFM observation that these surface-adsorbed molecules are in the form of monolayers. It is conceivable that each molecule would be exposed to the electrolyte during the charge−discharge process. Hence, the pseudocapacitance of a single 1,5-NAPD molecule that contains two −NH2 is about 2.85 times as high E

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Graphene. It is worth noting that the 1,5-NAPD/NG film exhibits a remarkably high specific capacitance of 877 F g−1, surpassing most PANI-based electrodes or small-moleculefunctionalized graphene in previous reports. For example, Du et al. prepared a PANI/graphene hydrogel, showing 530 F g−1; and37 Cong et al. prepared a PANI/graphene paper that reached 763 F g−1.18 For the combination of graphene and quinones, graphene hydrogels functionalized by hydroquinone26 or the anthraquinone derivative alizarin24 showed specific capacitances of 441 and 350 F g−1, respectively. More comparisons can be found in Table S1. The faradaic process at an amino group can be expressed as follows: (i) −NH2 − H+ − e− → −NH; (ii) −NH2 − 2H+ − 2e− → −N. The −NH2 releases one/two hydrogen ions, and one/two electrons are transferred from the HOMO level of the molecule to the graphene energy level, causing a current rise in the circuit as exhibited by a peak in the CV curve. Therefore, reducing the energy gap between Egraphene and EHOMO can facilitate the extraction of electrons from −NH2 as well as increase the probability of (ii) redox occurrence. According to the probability nature, only a certain percentage of amino groups release two H+ during the charging process, depending on the occurrence probability of (ii) redox. Since the two redox processes coexist, here we use α and β% to stand for proportions of (i) and (ii) processes contributed to the pseudocapacitance, respectively. The proportion β can be increased by lifting the HOMO level. Hence, even if the molecule contains a couple of −NH2, a low HOMO level could decrease the occurrence probability of (ii) redox as well as impede the pseudocapacitance contribution. Now, we estimate the number of electrons that can be stored via a single organic molecule on the graphene surface. The areal mass loading of a NG film, 1 mg cm−2, is increased to 1.3 mg cm−2 after adsorbing 1,5-NAPD molecules, and the areal capacitance was increased from 0.35 to 1.14 F cm−2. Thus, the number of adsorbed molecules is 1.14 × 1018, and each molecule has a capacitance contribution of 6.9 × 10−18 F. Note that the redox process will be terminated when the voltage is beyond 0.8 V (Figure 3b). According to Q = CV, where Q is the quantity of electric charge and V is the voltage, we can deduce that in experiment, 3.4 electrons are stored via a single 1,5-NAPD molecule. This gives occurrence probabilities of 30% for the above-mentioned (i) redox process and 60% for the (ii) redox process. Similarly, we can estimate that 1.9 electrons are stored via a single 4-AP molecule in experiment, although in theory this molecule containing one −OH and one −NH2 can store a maximum of three electrons through the faradaic process. To have an insight into the pseudocapacitance of aminocontained organic molecules, we calculated the partial density of states (p-DOS) for a single 1,5-NAPD molecule adsorbed onto graphene. Figure 5a shows charge density difference maps viewed from the top and the side, respectively. The two amino groups are enclosed by two red circles. The yellow-colored domains indicate sites for donating electrons, and the cyancolored domains are prone to accept electrons. Figure 5b shows the total DOS for pristine graphene and a single 1,5NAPD molecule adsorbed onto graphene. There are extra peaks below the Fermi level after adsorption by 1,5-NAPD. These empty bands will help store more charges, enhancing the capacitance. Figure 5c shows the partial DOS for C p, N p, and H s orbitals of the organic molecule, according to which we can conclude that the N p orbitals in amino groups have

4,4′-ODA/NG > 4,4′-TDA/NG > 3,4′-TDA/NG > 2,2′TDA/NG. NG films after the adsorption of 4,4′-ODA, 4,4′TDA, 3,4′-TDA, and 2,2′-TDA molecules showed mass gains of 17, 23, 20, and 20%, respectively. Specific capacitance values at different current densities are plotted in Figure 4e. For the first pair, at 1 A g−1, the 4,4′-ODA/NG film shows a specific capacitance of 590 F g−1, much higher than that of 3,4′-ODA/ NG (450 F g−1). Coincidently, the EHOMO of 4,4′-ODA at −5.59 eV is higher than that of 3,4′-ODA (−5.96 eV), as shown in Figure 4f. For the second pair, the maximum specific capacitance of 4,4′-TDA/NG is a bit lower than that of 4,4′ODA/NG, as 546 versus 590 F g−1. Correspondingly, the EHOMO of 4,4′-TDA is slightly lower than that of 4,4′-ODA, as −5.593 versus −0.679 eV. Hence, a similar trend is also found from the two TDA isomers, 2,2′-TDA versus 4,4′-TDA. For structural isomers of the same kind of amino-contained small aromatics, our study suggests that a higher EHOMO level of the molecule helps enhance the pseudocapacitance. Now, we look into molecules of different kinds. The HOMO level of 1,8-NAPD at −5.34 eV is higher than that of 4-AP at −5.44 eV, whereas the specific capacitance of the 1,8-NAPD/NG film is lower than that of the 4-AP/NG film. Therefore, it is more complicated to rank the pseudocapacitance contribution of different kinds of small aromatics. Our quantum chemistry calculation is based on isolated molecules at a relaxed status. For two isomers attached to graphene, their MO levels would be altered during the redox process, and the energy-level diagram of the molecule−graphene junction is analogous to that of a biased Schottky junction. However, the height sequence of their HOMO levels is unlikely to change, as their MO levels are synchronously elevated or lowered in the charge−discharge process. Stable Adsorption of the Selected Aromatic Molecules on Graphene. For the eight kinds of organic molecules, the adsorption energy for each molecule on graphene was computed by using ab initio molecular dynamics and the values are summarized in Table 1. Compared with the Table 1. Computed Adsorption Energies of Selected Organic Molecules on Graphene molecule

ΔE (kcal mol−1)

molecule

ΔE (kcal mol−1)

4-AP 2-AP 1,5-NAPD 1,8-NAPD

−11.28 −10.03 −18.13 −15.70

4,4′-ODA 3,4′-ODA 4,4′-TDA 2,2′-TDA

−10.31 −13.97 −10.53 −12.45

adsorption energy of ethanol on graphene, as −7.9 kcal mol−1,36 the interactions between these aromatic molecules and graphene are relatively strong. The 1,5-NAPD molecule on graphene has the highest adsorption energy, and the 2-AP has the lowest binding energy with graphene. For simplicity, we used pristine graphene for calculation. In reality, our reduced GO is defective and its binding strength with organic molecules would be stronger than the calculated values. It is worth mentioning that many of these organic compounds have colors when dissolved in water. For example, the aqueous solution of 1,5-NAPD is in dark purple. However, the electrolyte in a three-electrode cell remained clear when testing our functionalized NG films, indicating that no organic molecules were detached from the graphene film. Insights into the High Pseudocapacitance and High Cycling Stability of 1,5-NAPD and 4-AP Molecules on F

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Figure 5. (a) Charge density difference maps of a single 1,5-NAPD molecule on graphene. (b) Total DOS for pristine graphene and a single 1,5NAPD molecule adsorbed onto graphene. (c) Partial DOS for C p, N p, and H s orbitals of the molecule.

the most contribution to the empty bands in the DOS of 1,5NAPD/graphene. This suggests that the pseudocapacitance originates from the amino groups of the organic molecule, consistent with the proposed mechanism. Using the Bader charge analysis, we were able to calculate the change of valence electrons of the amino group of a 1,5-NAPD molecule before and after being adsorbed onto graphene. As shown in Table 2,

in isolation on graphene

−NH2

N1

H 1−1

H 1−2

N2

H 2−1

H 2−2

6.221 6.233

0.561 0.560

0.575 0.563

6.229 6.260

0.578 0.517

0.564 0.549

CONCLUSIONS

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ASSOCIATED CONTENT

In summary, we investigated the molecular factors linked to the pseudocapacitance contribution of amino-contained small aromatics adsorbed onto graphene. Eight kinds of organic compounds with redox activity were selected to functionalize NG films, and their efficacies of capacitance enhancement were correlated with density functional theory calculations to give an in-depth understanding of the factors behind their different supercapacitance contributions. First, for structural isomers, a higher EHOMO level of the molecule leads to a higher pseudocapacitance, because it reduces the energy barrier between the molecule and graphene to promote the occurrence of the above-mentioned (ii) process that is responsible for the high capacitance. Second, small aromatics containing only amino groups have a better cycling performance than those containing hydroxyl groups. 4-AP and 1,5NAPD are found to be excellent in enhancing the capacitance of graphene-based electrodes. In particular, the 1,5-NAPD/NG film exhibited a high specific capacitance of 877 F g−1, and its capacitance retention was 105% after 5000 charge−discharge cycles. The symmetric cell based on 1,5-NAPD/NG films shows a maximum energy density of 31 kWh kg−1. This work provides a theoretical clue for screening out optimal aminocontained aromatics from a large variety of possible ones, aiming to efficiently exploit the redox of amino groups for making high-performance supercapacitors.

Table 2. Bader Charge Analysis for the Variation of Valence Electrons in Two Amino Groups of a 1,5-NAPD Molecule and after being Adsorbed onto Graphene −NH2

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the two N atoms gain valence electrons, whereas H atoms lose valence electrons after the adsorption. Therefore, the two N atoms in a graphene-supported 1,5-NAPD molecule are more negative and prone to interact with H+ from the electrolyte, facilitating the faradaic process. Cycling stabilities of 1,5-NAPD/NG and 4-AP/NG films, both exhibiting high pseudocapacitance, were tested. After cycling the CV test at 50 mV s−1 for 5000 cycles, the capacitance retentions of 1,5-NAPD/NG and 4-AP/NG electrodes are 105 and 89%, respectively, as shown in Figure S7. The slightly increased capacitance of 1,5-NAPD/NG electrode against the cycling number could be attributed to the continuous activation of organic molecules on the graphene surface that slightly reduces the energy gap between Egraphene and EHOMO of the molecule, making the amino group more prone to lose two H+. The 4-AP molecules containing −OH are less stable upon cycling. Further calculation shows that the redox of a 1,5-NAPD molecule involves larger energy change compared with that of 4-AP (Figure S8). It means that the larger energy change required to drive the redox process is responsible for the better cycling stability of the molecule.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b08348. Experiments and the calculation of capacitance, characterizations of functionalized NG films, DFT calculation results for some small aromatics, and comparisons of our supercapacitor performances with others that were reported previously (PDF) G

DOI: 10.1021/acs.jpcc.8b08348 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86 10 8231 7132. ORCID

Jinzhang Liu: 0000-0003-4788-3560 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by “The Fundamental Research Funds for Central Universities” through BUAA and the Academic Excellence Foundation of BUAA for Ph.D. students.

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DOI: 10.1021/acs.jpcc.8b08348 J. Phys. Chem. C XXXX, XXX, XXX−XXX