Electrochemical Polymerization and Energy Storage for Poly[Ni(salen

Apr 14, 2014 - field emission scanning electron microscopy and transmission electron microscope ... storage mechanism were investigated through the me...
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Electrochemical Polymerization and Energy Storage for Poly[Ni(salen)] as Supercapacitor Electrode Material Gang Yan,† Jianling Li,*,† Yakun Zhang,† Fei Gao,† and Feiyu Kang‡ †

State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing, No. 30 College Road, Beijing 100083, China ‡ Lab of Advanced Materials, Department of Materials Science and Engineering, Tsinghua University, Beijing 100084, China

ABSTRACT: Nanobelt-like poly[Ni(salen)] obtained by electrodeposition of potentiostatic method has been characterized by field emission scanning electron microscopy and transmission electron microscope. The polymerization mode and the energy storage mechanism were investigated through the methyl replacement in the para-position of phenyl rings in Ni(salen) monomer and the removal of center metal ion Ni. The electrode samples were characterized by cyclic voltammetry and galvanostatic charge/discharge methods.

1. INTRODUCTION Electric double layer capacitors (EDLCs) exhibit very good reversibility, relatively low-temperature coefficient, high specific power and very long cycle life.1−4 So far, specific energy delivered by carbon-based supercapacitors is the only parameter that still requires significant improvement. A powerful method to further extend the limit and significantly promote the energy density of the existing EDLCs is to combine the capacitive charging of the electric double layer with a fast and reversible faradaic charge-transfer reaction at the electrode interface with the overall capacitance of the composite electrode being the sum of the double layer capacitance and the redox capacitance (called pseudocapacitance).5,6 Conducting polymers with faradaic reaction of high specific capacitance are good candidates to combine with the material of electric double layer capacitance.7−12 As one kind of conducting polymers, transition metal complexes with tetradentate N2O2 Schiff-base ligand from salicylaldehyde and derivatives have been investigated for specific application13−15 and N,N′-ethylenbis(salicylideneaminato) nickel(II) ([Ni(salen)]) is the archetype of Schiff base metal complexes. Their anodic polymerization products possess reversible redox behavior and can be used as supercapacitor electrode materials.16−18 However, their redox properties have been in debate focusing on whether they are ligand centered or metal centered. In this paper, we first prepared the nanobelt redox polymer of Ni(salen) by © 2014 American Chemical Society

electrochemical method. Also, the investigation on its structure and energy storage mechanism by redox switching of its polymerization products was conducted. Potentiostatic method, which has more available operational parameters on growth and obtains higher electroactivity, is widely used for preparation of conducting polymers, metals, and metal oxides. In this work, potentiostatic method was introduced to polymerize Ni(salen). Multiwalled carbon nanotube (MWCNT) matrix was chosen to be the carrier of poly[Ni(salen)] because of its high conductivity and the mesoporous network that allows the polymer to adapt to volume changes during redox switching.

2. EXPERIMENTAL SECTION 2.1. Materials. Acetonitrile (AN, >99.9%, A.R. grade) was purchased from Guangdong Xilong Chemical Co., Ltd., while tetrabutylammonium perchlorate (TBAP, >99.9%, C.P. grade) and triethylmethylammonium tetrafluoroborate (Et3MeNBF4, >99.9%, C.P. grade), Zhong Sheng Hua Teng Co., Ltd. They were used as received. The Ni(salen) monomer was synthesized following the procedure in the literature19 and recrystallized in AN. Received: January 9, 2014 Revised: April 1, 2014 Published: April 14, 2014 9911

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nm, and the larger length−width ratio makes the appearance of this structure look like a corn leaf, which is shown in Figure 1b. To further characterize the structure and morphology, the TEM images were used to confirm the belt-like structures as observed in the SEM images. Some nanobelts were broken by the ultrasonic irradiation during the TEM sample preparation inevitably. Representative microstructures of prepared poly[Ni(salen)] nanobelts are presented in Figure 2. The TEM

The support electrode was prepared by the homodispersion of the MWCNTs in N-methyl-2-pyrrolidone (NMP) through sonicating. The slurry was coated on a Ti sheet (current collector) with the coating mass of 0.3 mg. 2.2. Fabrication and Instruments. The surface morphologies of the bare support and poly[Ni(salen)] coated samples were characterized by field emission scanning electron microscopy (FESEM, Zeiss SuprA55 microscope) and transition electron microscopy (TEM, JEOL JEM 2011). The electrode position of Ni(salen) and electrochemical measurements were carried out on a VMP2 electrochemical workstation with EC-Lab software (version 10.02) made by Princeton. The electropolymerization was conducted in a closed threeelectrode compartment cell. The electrode position of Ni(salen) on the resulting electrode (1 cm × 1 cm) was performed in AN solution containing 1.0 mmol L−1 Ni(salen) complex monomer and 0.1 mol L−1 TBAP by potentiostat electrode position under a certain potential for some time using an activated carbon sheet (1.5 cm × 2.5 cm) and a capillary Ag/ AgCl wire as the counter and reference electrodes, respectively. The poly[Ni(salen)] electrodes were then washed in acetonitrile in order to remove any soluble species from the film and were tested in a monomer-free solution of 1 mol L−1 Et3MeNBF4 in acetonitrile. Cyclic voltammetry (CV) and galvanostatic charge/discharge tests were measured with the same counter and reference electrodes. All potentials in this article are given versus Ag/AgCl.20 The ferrocene/ferrocenium couple (Fc/Fc+) was used as internal standard: under the experiment conditions used, E1/2 for the couple was 0.47 V in AN. The specific capacitance of the electrode can be calculated using

Cm = Cs =

it Vm

Figure 2. TEM image of poly[Ni(salen)]. (a) Grow as free; (b) inner a MWCNT.

micrograph in Figure 2a shows the typical poly[Ni(salen)] out of the carbon nanotubes, while Figure 2b reveals the HRTEM microstructure of poly[Ni(salen)] inner a carbon nanotube. The high-resolution electron microscopy result shows that the d-spacing of 0.255 nm is in well accordance with the d-spacing value d1000 of β-Ni(salen) induced by KCl or NaCl surfaces,21 and the deviation less than 1.2%. On this basis, it could be conjectured that Ni(salen) polymerized following a ligand− radical coupling polymerization mode, forming from the generation of the C−C bonds between the phenyl rings of the ligands of individual polymer chain fragments as chain structure, seen in Figure 3. 3.2. Polymerization Mode. In order to clarify the polymerization mode in the oxidation of Ni(salen), the oxidative electrochemistry of a kind of nickel(II) Schiff base complex was investigated. The salicylaldimine chelates of the complex are replaced with 5-methylsalicylaldehydine chelates as shown in Figure 4. The 5-methylsalicylaldehydine chelates used in this study were chosen because of their similarity to the salicylaldimine chelate in structure without the methyl replacement in the para-position of phenyl rings. It is obviously known that alkyl substitution at the para-positions inhibits the oxidative coupling of phenols at these sites. Monomer samples with different proportion of Ni(salen) and Ni(CH3-salen) were potentiostatically polymerized and tested under the same conditions (0.85 V, 20 min). The mole ratios of Ni(salen) to Ni(CH3-salen) in the samples are 1:0, 7:3, 5:5, 3:7, and 0:1, and the corresponding samples are labeled as S1−0, S7−3, S5−5, S3−7 and S0−1, respectively. The cyclic voltammograms of the different composite electrodes with a potential window from 0.0 to 1.2 V (vs AgCl/Ag) are shown in Figure 5. As the monomer was coated on the MWCNT electrode with an increase of Ni(salen) component, oxidation and deoxidization peaks of the composite electrode became gradually obvious with an increase in current densities. CV plot of sample S0−1 exhibits no redox peak and the CV curve is almost the same with that of the bare MWCMT electrode. Thus, it is reasonable to give the verdict that the Ni(CH3-salen) complex does not undergo oxidative

(1)

it Vs

(2) −1

In eq 1, Cm is the specific capacitance (F g ), i is the response current (mA), t is discharge time (s), V is charge−discharge windows (V), and m is the mass of the electroactive materials in the electrodes (g). In eq 2, Cs is the specific capacitance (mF cm−2), i is the response current (mA), t is the discharge time (s), V is charge−discharge windows (V), and s is the area of the electroactive materials in the electrode (cm2).

3. RESULTS AND DISCUSSION 3.1. Nanobelt of Poly[Ni(salen)]. The surface morphology of poly[Ni(salen)] film by potentiostatic method is shown in Figure 1a by applying a potential of 0.85 V for 20 min. This represents the belt kind structure with an average width of 100

Figure 1. (a) SEM image of poly[Ni(salen)] on MWCNTs electrode; (b) leaves of corn. 9912

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Figure 3. Polymerization process of Ni(salen) as “chain” structure.

window between 0.0 and 1.0 V. Consistent with the electrochemical results of CV plots, the specific capacitances calculated from the galvanostatic charge−discharge test show an increase with the increase of Ni(salen) content, as shown in Figure 6. The specific capacitance is 272, 152, 102, and 56 F g−1

Figure 4. Chemical structure of Ni(salen) and Ni(CH3-salen).

Figure 6. Specific capacitances of MWCNT−polymer electrodes as a function of discharge current densities.

for S1−0, S7−3, S5−5, and S3−7, respectively at the current density of 0.05 mA cm−2. It can be seen that sample S1−0 exhibits higher specific capacitance than other samples at all discharge current densities. The specific capacitance decreases at almost the same speed until the discharge current density is up to 8 mA cm−2. With a further increase in the discharge current density up to 20 mA cm−2, the S1−0 electrode shows a bigger decrease in specific capacitance than the other electrodes. This illustrates that the polymer prepared on the MWCNT electrode only refers to Ni(salen) while Ni(CH3salen) takes no part in the polymerization process, and all the capacitance contribution comes from poly[Ni(salen)]. Figure 7 shows the relative change of specific capacitance between mixed samples and pure Ni(salen). It is clearly that the ratio of specific capacitance is slightly lower than the monomer proportion of Ni(salen). This is due to the no polymerization of Ni(CH3-salen). By adding Ni(CH3-salen) to the polymerization system, the diffusion rate of the monomer is the same because of the constant total monomer concentration in the polymerization system, but Ni(salen), which can access to the electrode surface and participate in polymerization, is reduced.

Figure 5. Cyclic voltammograms of bare MWCNT electrode and MWCNT-polymer electrodes in 1 mol L−1 Et3MeNBF4/AN in the range of 0−1.2 V (scan rate 100 mV s−1).

polymerization at the electrode. The result is also in accordance with no evidence of [Ni(Me, R2, Me-mal-H-salen)] (R2 = H, (CO)Ph) and [Ni(Me, H, Me-mal2en)] for cyclic voltammetric polymerization at the electrode.22 This thereby allows us to address the question of ligand-localized oxidation against metallocalized and simplifies the electrochemical behavior of these complexes. In the CV plot of sample S3−7, the reduction peak appears, whereas the oxidation peak is not quite obvious. With a further increase in the content of Ni(salen), the composite electrodes show apparent increase in current while maintaining the distinct redox peaks in CV. It suggests that the increase in Ni(CH3-salen) in polymerization samples will induce the decrease in electrochemical performance of the electrodes. The charge/discharge cycling was carried out at different current densities from 0.02 to 20.0 mA cm−2 in a potential 9913

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After 0.85 V potential is applied, the charge variation trend of both electrodes is similar. It indicates that both Ni(salen) and H2(salen) undergo polymerization, and this further supports the ligand-localized oxidation model. However, the linear improvement in the charge of the Ni(salen) polymerization over H2(salen) polymerization is clearly seen in Figure 8. Charge depleted for Ni(salen) polymerization is 2.9 times as that of H2(salen) with the value of 88.5 mC and 30.3 mC, respectively. This phenomenon illustrates an easier polymerization of Ni(salen) over H2(salen). The CV plots of the composite electrodes possess a couple of characteristic redox peaks in Figure 9 as well as the electric double-layer current attributed to MWCNTs. The redox peaks of poly[Ni(salen)] is distinct at all the scan rates from 5 to 500 mV s−1, shown in Figure 9a,c, with an enlargement of the redox peak potential difference by the increase in scan rate because of the smaller speed of anion diffusion inside the polymer film in contrast with the electron-withdrawing/donating interaction of the salen ligand in poly[Ni(salen)]. From Figure 9b,d, the reversible redox peaks of poly[H2(salen)] are clearly seen between 0.8−0.9 , but the peak current is much smaller than poly[Ni(salen)]. As shown in Figure 10, the current responses at 100 mV s−1 of both composites within the potential range of 0.0−1.2 V are larger than those of MWCNTs, and the rise of electric doublelayer current within a low potential range of 0.0−0.5 V for both composites compared with MWCNTs means the electric double-layer reaction is not limited by the coating polymer. However, the rise of redox reaction current for poly[Ni(salen)] within a high potential range of 0.5−1.2 V is much larger than that for poly[H2(salen)]. The area of responsive currents of poly[H2(salen)] electrode during the scan range 0.0−1.2 V is almost the same with that of the MWCNT electrode. The difference in current rise between poly[Ni(salen)] and poly[H2(salen)] means the charge transfer ability of poly[H2(salen)] is much poorer than that of poly[Ni(salen)]. In poly[H2(salen)], the charge transfer is considered to be an electron exchange through a set of conjugated π-bonds. On the basis of the same π-bond state between poly[H2(salen)] and poly[Ni(salen)], the results clearly demonstrates the apparent redox peak in the CV curve of poly[Ni(salen)] is caused by valence change of the center metal Ni. Furthermore, the big redox peak current states the oxidation state variation of poly[Ni(salen)] a quick process with good reversibility. Accompanied by the redox process of polymer, the anions around the electrolyte get into the polymer for charge compensation in order to maintain the electrical neutral. Under semi-infinite diffusion conditions, the peak current (jp) for a reversible electrode action is given by the Randles−Sevick equation, which can be used to estimate the charge transport diffusion coefficient (D). The jp ∼ v1/2 plots of composites present approximately linear dependence, shown in Figure 11. It demonstrates that both electrodes show two stages with different charge diffusion velocities during the scan rate range from 5 to 500 mV s−1. Both the redox charge transport of polymer and electron charge of MWCNTs contribute to the value of D. Values of D can be estimated from Figure 11 and are listed in Table 1. It clearly reflects the influence of center metal Ni on the kinetics of composites. The surface coverage of poly[Ni(salen)] is three times more than that of poly[H2(salen)], and 1000 times for the value of D. The order of magnitude are 10−11 ∼ 10−12 and 10−9 for poly[Ni(salen)] and poly-

Figure 7. Specific capacitance variation of poly[Ni(salen)] from mixed samples relative to pure Ni(salen).

Furthermore, it leads to a shorter polymer chain and eventually a decrease in specific capacitance. The influence of polymer chain on specific capacitance in return reflects that the specific capacitance is partly provided by the conjugated π-bonds formed from the C−C bonds between the phenyl rings of the ligands. 3.3. Energy Storage Mechanism. The mechanism of charge transport for poly[M(Schiff)] complexes also remains a matter of controversy. In the first model, poly[M(Schiff)] is regarded as a redox conductor, and the charge transfer is considered to be an electron exchange between the neighboring metal centers through a set of conjugated π-bonds of the ligand environment (redox conduction). The other model thinks poly[M(Schiff)] as a polaronic conductor in which the metal takes no part in the charge transfer process and never changes its charge. In order to clarify the role of the metal center in the redox process of the poly[Ni(salen)], the oxidative electrochemistry of Ni(salen) and H2(salen) were investigated. Figure 8 shows a comparative study of the charge variations during the constant-potential polymerization for H2(salen) and

Figure 8. Charge variation during the constant-potential polymerization at 0.85 V for 20 min. (Left) H2(salen); (right) Ni(salen).

Ni(salen) in an AN solution with 0.1 mol L−1 TBAP. Before polymerization, the concentration of both Ni(salen) and H2(salen) is 1 mmol L−1. A plot of MWCNT refers to the charge variation of MWCNT electrode under 0.85 V for 20 min in a blank solution that contains no Ni(salen) or H2(salen), aiming to remove the disturber from double charge and discharge current of MWCNT electrode during the polymerization. 9914

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Figure 9. Cyclic voltammograms of polymer coated MWCNT electrodes in the range of 0−1.2 V at different scan rates from 5 to 500 mV s−1. (a,c) Poly[Ni(salen)]; (b,d) poly[H2(salen)].

conjugate π-bond, thus greatly improving the charge transfer coefficient. Figure 12 shows the galvanostatic charge/discharge curves of the composite and MWCNT electrodes at the current density of 0.05 mA cm−2. MWCNTs have symmetric change for charge/discharge course that is the characteristic of high Coulombic efficiency. Both composites possess longer charge/ discharge time than MWCNTs, however, poly[Ni(salen)] contributes much larger pseudocapacitance caused by the reversible redox switching between neutral and oxidative states on the basis of electric double-layer capacitance of MWCNTs. Considering the effective active surface area supported by MWCNTs is the same, the pristine properties of conducting polymer become the important factor for the improvement of charge storage ability. From the charge/discharge curve of poly[Ni(salen)] composite electrode, it is clearly seen that the potential change obviously divides into two parts: at a lower potential 0.0−0.7 V, the potential changes quickly with time. During this potential range, poly[Ni(salen)] is in electrochemical inert zone, in according with the CV plot, thus the electrode capacitance is mainly provided by the electric double layer. When the potential increases up to 0.7 V, the change of potential occurs slowly and the electrode capacitance is provided both by the polymer poly[Ni(salen)] and the MWCNTs, with poly[Ni(salen)] as the main contributor. It could be interpreted also from the CV test that the reversible transition between neutral state and oxidation state of poly[Ni(salen)] is accompanied by the injection/ejection of counterions, which is corresponding to the Ni2+/3+ redox processes

Figure 10. CV plots of polymer-coated MWCNT electrodes at 100 mV s−1.

poly[Ni(salen)] + n BF−4

Figure 11. Plots of peak current versus v1/2 of composites.

⇔ poly[Ni(salen)]ox(BF4)n + ne−

[H2(salen)], respectively. This could be explained by the effect of metal center Ni, which changes between different valences and provides a new path for charge transfer besides the

From the specific capacitance changes based on area after polymerization in Figure 12, it clearly demonstrates the big difference in capacitance provided by polymers between 9915

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Table 1. Values of D of Poly[H2(salen)] and Poly[Ni(salen)] Obtained from CV on the Basis of Randles-Sevcik Equation poly[H2(salen)] D (cm2 s−1) −1

5−80 mV s 100−500 mV s−1 Γ(μmol cm−2)

anodic

poly[Ni(salen)] cathodic

−12

3.06 × 10 2.51 × 10−11 0.101

anodic

−12

cathodic −9

1.76 × 10 1.74 × 10−11

1.87 × 10 7.83 × 10−9 0.349

2.62 × 10−9 8.55 × 10−9

4. CONCLUSIONS Nanobelt structure of poly[Ni(salen)] was obtained by electrode position of potentiostatic method. The verification and validation of the polymerization mode and the energy storage mechanism were carried out. CV plots and specific capacitance from constant-current discharge of composite electrodes polymerized from monomers with different proportion of Ni(salen) and Ni(CH3-salen) illustrate that only Ni(salen) monomers participate in the polymerization, and then provide specific capacitance. Hence, the polymerization of Ni(salen) is following a ligand-radical coupling polymerization mode, which results from the generation of the C−C bonds between the phenyl rings of the ligands of individual polymer chain fragments as chain structure. Poly[Ni(salen)] contributes much larger pseudocapacitance by the reversible redox switching between neutral and oxidative states on the basis of electric double-layer capacitance of MWCNTs, which also proves the energy storage mechanism of poly[Ni(salen)]. The overwhelming majority capacitance of poly[Ni(salen)] is provided by the center metal Ni.

Figure 12. Charge and discharge curves of composite electrodes.

poly[H2(salen)] and poly[Ni(salen)]. Capacitance provided by poly[H2(salen)] is 3.2 mF cm−2, whereas poly[Ni(salen)] brings about a 21.6 mF cm−2 increase in capacitance at the same current density of 0.02 mA cm−2. Namely, the capacitance increase caused by poly[Ni(salen)] is 6.75 times as that caused by poly[H2(salen)] at this current density. From the ratio curve between poly[H2(salen)]and poly[Ni(salen)] in Figure 13, the

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel./Fax: +86-10-62332651. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (No.51372021), National High Technology Research and Development Program of China (863 Program, 2012AA110302), and the National Natural Science Foundation of China (No.51172023).

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Figure 13. Capacitance provided by the polymer and the ratio of capacitance between poly[H2(salen)] and poly[Ni(salen)].

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