Pseudocapacitive Energy Storage in Schiff Base Polymer with

Feb 26, 2018 - Salphen-type nickel Schiff base Ni(salphen), Ni(CH3-salphen) and Ni(CH3O-salphen) are synthesized and electropolymerized on stable ITO ...
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Pseudocapacitive Energy Storage in Schiff Base Polymer with Salphen-Type Ligands Fuhai Deng, Xinping Li, Feixiang Ding, Bangbang Niu, and Jianling Li* School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, China S Supporting Information *

ABSTRACT: Salphen-type nickel Schiff bases Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen) are synthesized and electropolymerized on stable ITO electrode, respectively. The morphologies of the three polymer electrodes were evaluated by field emission scanning electron microscopy (FESEM). X-ray photoelectron spectroscopy (XPS) measurements were carried out to shed light on the polymerization mode and energy storage mechanism. Meanwhile, kinetic analysis of the redox reactions was used to verify the pseudocapacitive mechanisms of charge storage. The result signals that the polymerization mode and the mechanism of energy storage are related to the reversible conversion of the azomethine nitrogen group (−NCH−) in the six-membered ring of Schiff base instead of the Ni2+/Ni3+ process. Meanwhile, the azomethine nitrogen group was found to be directly affected by the addition of the electron-donating group methyl and methoxy so that additional peaks of the CV curve are generated, making polyNi(CH3-salphen) and polyNi(CH3O-salphen) have higher doping level, charge transfer ability, and better pseudocapacitive energy storage property than the pristine polyNi(salphen) polymer. At the current density of 0.05 mA cm−2, the specific capacity of the polyNi(CH3Osalphen) electrode was about 216 F g−1, higher than the specific capacity of 85 F g−1 for polyNi(salphen) and 133 F g−1 for polyNi(CH3-salphen). In the meantime, the conductivity of polyNi(CH3O-salphen) is 108.7 S cm−1 higher than that of the other two polymers. Therefore, the addition of the stronger methoxy group for electron-donating substituents makes polyNi(CH3Osalphen) have more excellent electrochemical kinetics and pseudocapacitive characteristics. paths for electrons and ions.6 In the meantime, conducting polymers with low fabrication costs, high specific capacitance and power, light weight, and enhanced flexibility, which offer capacitance through redox reactions that occur not only on the surface but also throughout the entire bulk, have been considered as hopeful pseudocapacitive electrode materials for supercapacitors.7−10 As one of the conducting polymers, these polymers are widely applied in electrochemical catalyst,11 organic catalyst,12 analytical fields,13 and biological fields.14 Transition metal with tetradentate N 2 O 2 salphen-type (salphen = N,N′-bis(salicylidene)-o-phenylenediamine) Schiff base derived from salicylidene and derivatives, such as Ni(salphen), Ni(CH3salphen), and Ni(CH3O-salphen), which is a structural unit of the polymers, shown in Figure 1, has been studied for energy storage.15 Moreover, their polymeric transition metal complexes, also known as polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen), can be used as electrode materials for supercapacitor due to the reversible redox reactions carried out in the surface or near surface of the polymer.

1. INTRODUCTION In recent years, a variety of new energy technologies, such as wind energy, solar energy, tidal energy, and so on, have been paid wide attention. However, the distribution of new energy is discontinuous and inhomogeneous in time, season, and region and cannot be fully utilized.1,2 To make use of them steadily, it needs to be converted into stored energy forms, such as electrical energy, for transportation and release when needed. As a result, new energy storage devices, such as lithium ion batteries, supercapacitors, and fuel cells, have been greatly promoted in recent years.3 At the same time, supercapacitors are distinguished from energy storage devices because of their high power density, fast charge and discharge capability, and ultralong cycle stability. Supercapacitors, including electrochemical double-layer capacitors (EDLCs) storing energy using ion adsorption4 and pseudocapacitors achieved through fast surface or near-surface redox reactions,5 have gained much attention in recent years. EDLCs have high power density and excellent cycle performance, making it a promising technology for electrochemical energy storage. Although such EDLC materials for supercapacitors have been used, they are not enough to meet the need of the expected rise in high capacity for applications. Pseudocapacitance can provide high energy density and power density due to the relatively short diffusion © XXXX American Chemical Society

Received: January 14, 2018 Revised: February 26, 2018 Published: February 26, 2018 A

DOI: 10.1021/acs.jpcc.8b00434 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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Figure 1. Molecular structure of Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen).

Figure 2. CV plots for continuous electropolymerization of (a) Ni(salphen), (b) Ni(CH3-salphen), and (c) Ni(CH3O-salphen) on the ITO electrode. Coulometric assay of (d) consumed charge Q (mC), (e) surface coverage Γ (mol/cm2), and (f) doping level n in 10 cycles.

nitrate hexahydrate (Ni(NO3)2·6H2O) (A.R. grade), tetrabutylammonium perchlorate (TBAP, C.P. grade), and N-methyl-2pyrrolidone (NMP, C.P. grade) were purchased from Macklin Biochemical Co., Ltd. UV−Vis spectroscopy measurements were carried out using a PERSEE TU-1901, and Fourier transform infrared (FTIR) measurements were performed on an IR spectrometer (Shimadzu, FTIR-8400S). The XPS (AXIS UltraDLD), using a monochromatized Al Kα radiation source, was carried out. The containment carbon (C 1s = 284.8 eV) was used for calibrating the binding energies. The field emission scanning electron microscope (FESEM) (Zeiss Supra55 microscope) with an accelerating voltage of 15 kV was used to evaluate surface morphologies of the composites. The direct current sheet resistance measurements of samples were carried out with the four-probe method using a SZT-2C (TongChuang, China). Cvoltammogram (CV), electrochemical impedance spectroscopy (EIS), and galvanostatic charge/ discharge (GCD) technologies were carried out by the way of a VMP2 electrochemical workstation (Princeton Applied Research VersaSTAT3). 2.2. Material Synthesis. To obtain the Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen) monomers, 47.5 mmol of 1,2-phenylenediamine and 95 mmol of salicylaldehyde, 2-hydroxy-3-methoxybenzaldehyde, and 2-hydroxy-3methylbenzaldehyde were dispersed in ethanolic solution, respectively. Then it was stirred continuously at 65 °C for 1 h in round flask. On completion of the reaction, the solids formed were filtered off and dried in a vacuum oven at 80 °C overnight. H2(salphen), H2(CH3-salphen), and H2(CH3Osalphen) monomers were obtained. The dried powder was

Although the influence of the ligand structure, namely, the introduction of electron-donating methyl and methoxy substituents in the ortho-positions of the phenyl rings of the salphen-type Schiff base, on the electrochemical properties of these polymers, that defines their performance in the energy storage applications, has been systematically investigated,16 the role of the transition metal in salphen-type Schiff base polymer during their redox properties and the polymerization mode and energy storage mechanism have still been controversial. At the same time, research dealing with the role of electron donor substituents in salphen-type Schiff base polymer on the pseudocapacitive energy storage has rarely been reported. In this paper, salphen-type ligands for nickel Schiff base Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen) are synthesized and electropolymerized on stable ITO electrode. These polymer materials are designed as cathodes for Et3MeNBF4-based pseudocapacitors for investigating charge/ discharge kinetics. Then the influences of electron donor substituents in salphen-type Schiff base polymer of as-grown polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3Osalphen) on ITO electrode are demonstrated and discussed. Also, the investigation of the polymerization mode and energy storage mechanism by reversible redox reactions is conducted.

2. EXPERIMENTAL SECTION 2.1. Reagents and Instrumentation. Acetonitrile, 1,2phenylenediamine, triethylmethylammonium tetrafluoroborate (Et3MeNBF4, C.P. grade), salicylaldehyde, 2-hydroxy-3-methoxybenzaldehyde, 2-hydroxy-3-nethylbenzaldehyde, nickel B

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Figure 3. Plane-view (a, c, and e) and cross-sectional view (b, d, and f) SEM images of (a,b) polyNi(salphen), (c,d) polyNi(CH3O-salphen), and (e,f) polyNi(CH3O-salphen) on ITO electrode after electropolymerization.

mixed with stoichiometric Ni(NO3)2·6H2O and stirred for 1.5 h at 70 °C in a round flask and then cooled to room temperature in the indoor environment to obtain the final product. In short, synthesis process processes can be described in Figure S1. Moreover, the detailed characterizations of the H2(salphen), H2(CH3-salphen), and H2(CH3O-salphen) ligand and the formed Ni(salphen), Ni(CH3-salphen), and Ni(CH3Osalphen) monomers have been given in Figure S2. All of the electrochemical measurements were performed in an airtight three-electrode compartment cell, where the counter and reference electrodes were carbon sheet and Ag/AgCl, respectively. Schiff base polymer with salphen-type ligands was synthesized by the CV method with the scan rate 20 mV s−1 from 0 to 1.3 V for 10 cycle numbers on a conductive substrate, where the electrolyte was made with acetonitrile solution which contained 1.0 mmol L−1 Ni(salphen), Ni(CH3salphen), and Ni(CH3O-salphen) monomers and 0.1 mol L−1 TBAP, respectively. Indium tin oxide (ITO) coated glass (area = 1.0 cm2) was used as the working electrode. After that, CV, EIS, and GCD electrochemical measurements were carried out in a three-electrode system in Et3MeNBF4 solution (1.0 mol L−1) as the electrolyte at 25 °C, where the working electrodes were polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) electropolymerized on ITO.

3. RESULTS AND DISCUSSION As shown in Figure 2a−c, the continuous cyclic voltammetry polymerization curves, from the 1st to 10th sequential scans, of Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen), on ITO electrode were obtained, respectively. Electropolymerization products were polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen). Obviously, the well-fined redox peaks are obtained in 10 cycles. Besides, with the increase in the number of polymerization cycles, the anode and cathode peak currents gradually increase, which is ascribed to the accumulation and stabilization of the polymer on the ITO electrode. In detail, as seen in Figure 1a, one redox couple occurs whose peak potential is about 1.0 V. However, for Ni(CH3-salphen) shown in Figrure 1b, it is notable that there are two pairs of obvious redox couples at peak potentials of 0.35 and 0.9 V, respectively. Notably, compared to Figure 1a, the production of an additional redox peak around 0.35 V is ascribed to the electron-donating methyl group in Ni(CH3salphen). Similarly, for the Ni(CH3O-salphen) (Figrure 1c), due to the addition of the electron-donating methoxy group which is stronger than the methyl group, there are also two pairs of redox peaks in the polymerization curve. However, the two peak potentials are relatively closed, and the peak potentials are 0.7 and 0.85 V. Besides, with the increase of the number of polymerization cycles the two peaks have the C

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Figure 3e are significantly different. As can be seen from Figure 3c, the part of the polymer near the base has a tightly folded laminar layer. The pleated film layer grows along the base, and the overall morphologies are similar to the reduced graphene oxide.20 Thus, the salphen-type Schiff base polymer polyNi(CH3-salphen) with these morphologies can provide a larger specific surface area, allowing the electrolyte to be sufficiently contacted with the polymer. Also, from Figure 3e, it can be observed that the polymer film as a whole has a porous structure, and the loose porosity facilitates full contact of the electrolyte with the polymer. In summary, compared with polyNi(salphen), the morphology of polyNi(CH3-salphen) and polyNi(CH3O-salphen) has obvious features, and it can provide a larger area of contact with the electrolyte to provide greater capacity, which is ascribed to the electron-donating methyl group in Ni(CH3-salphen) and methoxy group in Ni(CH3Osalphen). The results are in good consistent with what is observed from the doping level of polyNi(salphen) grown on different electrodes. In addition, the thicknesses of the polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3Osalphen) polymers were calculated to be 1.327, 1.22, and 1.385 μm, respectively. To gain a better understanding of the formation of the polymer and the energy storage mechanism, surface analysis of the polymer films was carried out using X-ray photoelectron spectroscopy (Figure 4). Obviously, the Ni 2p, O 1s, N 1s, and

tendency of merging into a wide peak with 0.8 V. It is worthwhile mentioning that the polymerization curves of the Ni(salphen), Ni(CH3-salphen), and Ni(CH3O-salphen), in addition to the above-discussed redox couple, have additional anode peaks in the potential range of 1.1−1.2 V which is attributed to the cross-linking of the polymer.17 In the meantime, for all monomers, there are no obvious anode peaks in the first scan compared to other sequential scans with well-defined redox peaks which are obtained in 10 cycles. This phenomenon can be explained by the initial activation during the nucleation process of the polymer on the ITO electrode. Furthermore, as can be seen from Figure 2d, the consumed charge (Q) increases with the scan number by a linear variation, indicating that the polymer is gradually generated as the charge is consumed. The mass of the corresponding polymer can be calculated by the consumed charge of monomers in the electropolymerization process through the double coulometric assay.18 By calculating the consumed charge (Q) after ten cycles of polymerization, the values of 30.28 mC for Ni(salphen) and 16.76 mC and 16.53 mC for Ni(CH3-salphen) and Ni(CH3Osalphen), respectively, were obtained. This parameter indicates that the addition of the electron-donating methoxy group which is stronger than the methyl group makes the salphen-type Schiff base polymerize faster, suggesting a more considerable charge trapping over the time scale of the CV experiment for Ni(CH3O-salphen). On the other hand, in Figure 2e, we show Γ (mol/cm2) vs scan number, which is consistent with the consumption of the charge parameters. By analyzing the charge of each scan in 10 sequential scans, it can be concluded that Γ increases from 0.65 × 10−5 mol cm−2 in the first scan to 8.22 × 10−5 mol cm−2 in the 10th scan for Ni (salphen), from 0.71 × 10−5 mol cm−2 in the first scan to 7.53 × 10−5 mol cm−2 in the 10th scan for Ni(CH3-salphen), and from 1.18 × 10−5 mol cm−2 in the first scan to 1.32 × 10−4 mol cm−2 in the 10th scan for Ni(CH3O-salphen). Γ increases almost linearly, and the mass of the polymer should also increase linearly with the calculation of the polymer mass m = MΓA, where the M is the relative molecular mass of monomer and A the working area of the ITO electrode. Afterward, as shown in Figure 2f, the doping level of all monomers electropolymerization on different electrodes, which is calculated by the ratio of total redox charge and total synthesis charge,19 can be used to evaluate the electrochemical activity of the conducting polymer. It is obvious that the doping levels of Ni(CH3-salphen) and Ni(CH3Osalphen) have higher values than that of Ni(salphen). Among them, the value of n for Ni(CH3O-salphen) has the greatest value, which demonstrates that polyNi(CH3O-salphen) has better electrochemical and kinetic properties. It can be inferred that the introduction of the electron-donating methoxy group which is stronger than the methyl group indeed enhances the electrochemical activity of the salphen-type Schiff base ligand in the polymerization process. Figure 3 shows the SEM images and corresponding cross sections of polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) polymers on ITO electrodes, respectively. The thickness of the polymer film was calculated by a cross-section view to calculate the conductivity. As shown in Figure 3a, it can be observed that the polymer is closely packed with 150−200 nm in oat grains with small gaps between the particles, which is not conducive to more complete penetration of electrolyte ions into the entire polymer. However, in comparison, the morphologies of polyNi(salphen) and polyNi(CH3-salphen) in Figure 3c and polyNi(CH3O-salphen) in

Figure 4. XPS spectra of Ni(CH3O-salphen) and electropolymerization of Ni(CH3O-salphen) in the oxidation state, prepared polyNi(CH3O-salphen), and its oxidation states.

C 1s photoemission peaks in Figure 4 are found for the asprepared samples, which is attributed to the presence of Schiff base metal complexes and consistent with the literature.21 Apart from the typical peaks of the Schiff base metal complexes, an extra Cl 2p photoemission peak at the oxidative polymerization of the monomers was observed and is labeled as Poly-CVCharged, respectively, which can be attributed to the presence of charge-compensating ions from the tetrabutylammonium perchlorate (TBAP) during the electropolymerization of Ni(CH3O-salphen) in oxidation states. Besides, a F signal (F 1s) emerged in the XPS spectra of prepared samples polyNi(CH3O-salphen) in oxidation states, indicating that the BF−4 in electrolyte was transferred into the electrode. In order to deeply investigate the process of the polymerization mode and energy storage mechanism, high-resolution XPS spectra of N 1s, O 1s, and Ni 2p spectra in different states for Ni(CH3O-salphen) were shown in Figure 5. In the N 1s core levels (Figure 5a), there is a strong N 1s peak at 399.9 eV, D

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Figure 5. High-resolution XPS spectra of (a) N 1s, (b) O 1s, and (c) Ni 2p spectra in different states for Ni(CH3O-salphen).

Figure 6. Schematic illustration of the polyNi(CH3O-salphen) as the cathode and the way of the polymerization process of Ni(CH3O-salphen).

(oxygen-containing species),25 correspondingly. The binding energy at 531.5 eV in the neutral state shifts to a lower binding energy at 531.3 eV. It is signaling that electron density at the oxygen and nitrogen atom is biased from neutral states to oxidation states due to the change of the local environment change. On the other hand, compared with the XPS peak signal area of the O−C species in Schiff base metal complexes of the oxygen element in the electropolymerization of Ni(CH3Osalphen) in oxidation states, XPS peak signal areas of the Ni(CH3O-salphen) and polyNi(CH3O-salphen) are far higher. Compared with high-resolution XPS spectra of N 1s and O 1s in polyNi(CH3O-salphen) (Figure 5a,b) and polyNi(CH3Osalphen) in oxidation states, there are no obvious changes in the binding energies except for similar changes in the XPS peak signal area of the Ni(CH3O-salphen) in the previous discussion, which can be ascribed to spontaneous reduction owing to the external positive potential of the electrode being switched off. This suggests that an energy storage mechanism is similar to the mechanism of polymerization. In the case of the fitted Ni 2p core level (Figure 5c), the Ni 2p spectrum exhibits two

representing the azomethine nitrogen group (−NCH−) coordinated with the metal ion, and the peak appears at 402.2 eV, corresponding to the π−π* type due to an additional π bond between the nitrogen atom and the carbon atom (C− N*).22,23 Moreover, during the electropolymerization of Ni(CH3O-salphen) in oxidation states, the N 1s core levels show a slight change for the azomethine nitrogen group (−N CH−) and the C−N* species in Schiff base metal complexes and the binding energy of the azomethine nitrogen group, and the C−N* species shifts to a higher binding energy at 400 and 402.3 eV, respectively. What is more, the decreased XPS peak signal area of the azomethine nitrogen group peak, which is from 89.08% of the nitrogen element in Ni(CH3O-salphen) and 63.92% in polyNi(CH3O-salphen) to 39.78% in the electropolymerization of Ni(CH3O-salphen) in oxidation states, is observed. Similarly, the deconvoluted O 1s core level (Figure 5b) shows three peaks at 531.5, 533.3, and 534.6 eV which can be identified as the metal−oxygen bonding (oxygen bonding with Ni), the O−C species24 in Schiff base metal complexes, and the adsorbed surface oxidized species E

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Figure 7. CV plots of polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) electrodes in the range of 0−1.2 V at the scan rate of (a) 20 mV s−1 and (b) 200 mV s−1. (c) GCD plots at 1.0 mA cm−2. (d) Specific capacitances of electrodes at current densities from 0.05 mA cm−2 to 20 mA cm−2.

atom in the macrocycle is the source of electrons in the transfer of electrons in the π-conjugated system within the macrocyclic molecule. At the same time, the nickel element has no valence changes during the entire process, which can deduce that the nickel element merely provides a connecting bridge to make the electron transfer more smooth, rather than some references to the nickel element through the Ni2+/Ni3+ process during the continuous electropolymerization of polymer. In short, the redox processes can be described below:

contributions, Ni 2p1/2 and Ni 2p3/2 (resulting from the spin− orbit splitting), located at, respectively, 872.7 and 855.6 eV, which indicates that nickel metal is indeed present in the +2 oxidation state. Notably, there is no obvious change in nickel valence during the continuous electropolymerization of Ni(CH3O-salphen) grown on the electrode and energy storage. In addition, the absence of any shake up and satellite peak in the Ni 2p core level spectra illustrates that the Schiff base metal complexes actually exist in square planar geometry instead of in octahedral geometry.26 In summary, the polymerization of the salphen-type Schiff base can be strongly related to the azomethine nitrogen group (−NCH−). In detail, the monomers Ni(CH3O-salphen) were adsorbed on the electrode surface and converted to an electrochemically active state under the positive potential in the range 0.0−1.3 V, accompanied with reversible conversion of the six-membered ring with the azomethine nitrogen group (−N CH−) in Ni(CH3O-salphen). Thus, with the transfer of electrons on the transfer path in the π-conjugated system within the macrocycle, a continuous conjugate system is established so that the polymer monomers can be polymerized together by redox polymerization continuously. Finally, the metal−ligand, metal−metal, and π bond in polyNi(CH3O-salphen) form a weak coordinating bond together with the insertion of the compensating ion perchlorate in the supporting electrolyte TBAP in the polymerization solution, and then the monomers are connected with each other in the way of chain growth law to form a certain density of conductive polymer polyNi(CH3Osalphen), as shown in Figure 6. It is noteworthy that the oxygen

polyNi(CH3O‐salphen) + nBF−4 ⇔ polyNi(CH3O‐salphen)(BF4 )n + ne−

(1-1)

On the other hand, compared with high-resolution XPS spectra of N 1s in polyNi(salphen) and polyNi(CH3-salphen) (Figure S4a and Figure S6a), an increase of the XPS peak signal area of the azomethine nitrogen group (−NCH−) of the N 1s, from 85.25% in polyNi(salphen) to 72.60% and 63.90% in polyNi(CH3-salphen) and polyNi(CH3-salphen), respectively, is observed. A decrease in the azomethine nitrogen group (−NCH−) peaks is mainly caused by strong electrondonating methoxy group,27 and this is consistent with the observation from SEM images given in Figure 3 and analysis of doping level. Eventually, the kinetic behavior and electrochemical response of the composites must be influenced. To get a better understanding of the electrochemical performance of the as-prepared polymer, CV and GCD technologies were carried out in 1 mol L−1 Et3MeNBF4 in acetonitrile electrolyte solution with a three-electrode system. F

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Figure 8. Log(i) versus log(v) plot of the anodic and cathodic current response for sweep rates from 1 to 500 mV s−1: (a) polyNi(salphen), (b) first peak of polyNi(CH3-salphen), (c) second peak of polyNi(CH3-salphen), and (d) polyNi(CH3O-salphen).

and discharging time since an electrochemically inactive region is attributed to EDLC capacitance, whereas in the second stage, a much longer charging time is due to pseudocapacitance of the polyNi(salphen) component in electrodes, in accordance with the CV plot. Similarly, for polyNi(CH3-salphen), the CV curve can be divided into three parts to discuss: 0−0.3 V, containing the voltage range of the first pair of redox peaks, so the slope of the discharge curve is larger. The 0.3−0.6 V inert region for its smaller slope corresponds to short charging and discharging time. The 0.6−1.2 V part contains the voltage range of the second pair of redox peaks, so the slope of the charge− discharge curve is larger, corresponding to a larger capacity. The mechanism of contributing capacity in the active region is pseudocapacitive behavior. Similar to polyNi(salphen), polyNi(CH3O-salphen) can also be divided into two parts: 0.6−1.2 V corresponds to the EDLC capacitance, and 0.6−1.2 V corresponds to the capacitance contributed by the pseudocapacitance. Figure 7d shows the specific capacity curves for polymers at different current densities. It can be seen from the figure that the specific capacitance of polyNi(CH3O-salphen) is about 216 F g−1 at a current density of 0.05 mA cm−2, which is higher than polyNi(salphen) (85 F g−1) and polyNi(CH3salphen) (133 F g−1). This result is well consistent with the above analysis of the CV curve and the degree of doping during electropolymerization. To better understand polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) electrodes in electrochemical kinetics behavior, a plot of log(i) versus log(ν) from 1 to 500 mV s−1 for the anodic and cathodic peak is shown in Figure 8.

Figure 7a,b illustrates the CV plots of the polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) electrodes at a scan rate of 20 mV s−1 and 200 mV s−1 from 0 to 1.2 V, respectively. From the CV curve, it is obvious that the redox peak potential of the polyNi(salphen) electrode is about 0.85 V, and only a pair of redox peaks appeares in the polymerization curve (Figure 2a). For polyNi(CH3-salphen), the redox peaks appear at 0.2, 0.65, and 0.85 V, respectively, and the peaks at 0.65 and 0.85 V merge into a broad peak with the increase of scanning speed, around 0.7 V. Compared to the polyNi (CH3salphen) polymerization curve in Figure 2b, the peak potentials appeared to coincide with each other, and the CV curves obtained at low sweep rates had one extra peak. The number of redox peaks directly determines the charge and capacitance characteristics. Similarly, for polyNi(CH3O-salphen), an additional pair of redox peaks appears in the curve but quickly merge into a broader peak at 0.6 V with the increase of sweep rate. In addition, compared with the integral areas of the CV curves of the polymer electrodes, it can be seen that the polyNi(CH3O-salphen) is the largest, and polyNi(salphen) is the smallest, corresponding to the value of specific capacitance. This shows that the addition of the electron-donating group optimizes the capacitive properties, and the more electrondonating substituents, the more pronounced. Simultaneously, the GCD test was conducted at a current density of 1 mA cm−2 from 1 to 1.2 V, as shown in Figure 7c. It is obviously observed that the GCD curve of the polyNi(salphen) electrodes exhibits two distinct voltage stages: 0−0.6 V and 0.6−1.2 V, respectively. At the first stage, 0−0.6 V, relatively quick potential changes correspond to short charging G

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Figure 9. (a) EIS plots for electrodes at 0.8 V. (b) Conductivity of the three polymers.

polyNi(CH3O-salphen) is up to 108.7 S cm−1, and the minimum value of polyNi(CH3-salphen) is 21.7 S cm−1, which is consistent with the data of EIS impedance. Therefore, it can be seen that the addition of the electron-donating methoxy group makes Salphen-type ligands for Schiff base polymer electrode show better performance.

Assuming both are in accordance with the following relationship: i = avb

(1-2)

b in the equation is obtained by fitting the curve. The b value of 0.5 indicates current that is limited by semi-infinite linear diffusion processes corresponding to battery-like behavior, while b value of 1 indicates that the current is surface-controlled associated with capacitive behavior, in other words, is often discussed in pseudocapacitive energy storage.28 As shown in Figure 8, when the sweep rates range from 1 to 20 mV s−1, the curve of each electrode is fitted, and the b value is 1. However, as the scan rate decreases, the value of b gradually decreases, and polyNi(CH3-salphen) and polyNi(CH3O-salphen) are still in high value by comparing polyNi(salphen) with high scanning speed, which means that the value of the contribution of pseudocapacitance still accounts for a large proportion. As a result, at large charge and discharge current densities, the capacitance decays slowly along with the power density to maintain a high level. Therefore, it can be inferred that the addition of electron-donating substituents makes the salphentype ligands for Schiff base polymer demenstrate better capacitive properties. Figure 9a displays the EIS plots for polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) electrodes at 0.8 V. As can be seen from the figure, the Rct values of polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3Osalphen) electrodes are 5.13, 4.78, and 4.79 Ohm, respectively. Rct reflects the total reaction resistance, and the magnitude of Rct indicates the difficulty of electrode reaction. It is obvious that polyNi(salphen) electrode exhibits higher Rct among the electrodes. In addition, the impedance spectrum of RH is the impedance value of the real axis intersection, and it represents the equivalent series resistance containing the solution resistance and electronic resistance of the electrode. The data reveal that RH is about 29.71, 30.28, and 26.84 Ohm for polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3Osalphen) electrodes, respectively. For the size of the Ohm resistance value associated with the electrical conductivity of the material, the smaller the value, the greater the conductivity. Figure 9b is a histogram of the conductivity of the polymer electrodes obtained through the formula, in which the fourprobe method was conducted to measure the sheet resistance of the electrodes, and then the thickness of the polymer film obtained by the section of the SEM was used. It can be seen from the bar chart (Figure 9b) that the conductivity of

4. CONCLUTIONS In summary, polyNi(salphen), polyNi(CH3-salphen), and polyNi(CH3O-salphen) were successfully fabricated onto a stable ITO electrode via a continuous CV method to assess the effect of electron-donating substituents on salphen-type Schiff base polymer electrode in pseudocapacitive energy storage. The electron-donating substituents affect the azomethine nitrogen group (−NCH−) in salphen-type Schiff base so that polyNi(CH3-salphen) and polyNi(CH3O-salphen) have a higher doping level, charge transfer ability, electrochemical kinetics, and better energy storage characteristics than the pristine polymer. Among them, the addition of a methoxy group in which a strong electron-donating substituent makes polyNi(CH3O-salphen) shows more excellent conductivity as well as capacitive characteristics. The specific capacitance of polyNi(CH3O-salphen) was about 216 F g−1 at a current density of 0.05 mA cm−2, higher than 133 F g−1 for polyNi(CH3-salphen) and 85 F g−1 for polyNi(salphen), respectively. Moreover, the conductivity of polyNi(CH3Osalphen) reaches 108.7 S cm−1, which is higher than that of polyNi(CH3-salphen) (21.7 S cm−1) and polyNi(salphen) (68.96 S cm−1). Additionally, XPS measurements were carried out to shed light on the polymerization mode and energy storage mechanism. It can be concluded that the mechanism of polymerization and energy storage mechanism are strongly associated with the azomethine nitrogen group (−NCH−) in salphen-type Schiff base. In detail, with the transfer of electrons on the transfer path in the π-conjugated system within the macrocycle, a continuous conjugate system is established to form a certain density of conductive polymer. In the meantime, the metal ion located in the polymer acts as a bridge between ligand moieties instead of the Ni2+/Ni3+ process.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.8b00434. Schematic diagram of chemical reaction process in experimental part, UV−vis spectra and IR spectra of H

DOI: 10.1021/acs.jpcc.8b00434 J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

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synthesized pure salphen-type Schiff base, and XPS spectra (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Jianling Li: 0000-0002-3915-9540 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by National Natural Science Foundation of China (No. 51372021) and National Natural Science Foundation of China (No. 51772025, No. 51572024).

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DOI: 10.1021/acs.jpcc.8b00434 J. Phys. Chem. C XXXX, XXX, XXX−XXX