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Novel electroactive nanocomposite of POAP for highly efficient energy storage and electrocatalyst: Electrosynthesis and electrochemical performance
Journal of Colloid and Interface Science 484 (2016) 308–313
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Novel electroactive nanocomposite of POAP for highly efficient energy storage and electrocatalyst: Electrosynthesis and electrochemical performance Maryam Naseri a, Lida Fotouhi a,⇑, Ali Ehsani b,⇑, Hamid Mohammad Shiri c a b c
Department of Chemistry, Alzahra University, Tehran, Iran Department of Chemistry, Faculty of Science, University of Qom, Qom, Iran Department of Chemistry, Payame Noor University, Iran
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 27 June 2016 Revised 8 August 2016 Accepted 27 August 2016 Available online 31 August 2016 Keywords: POAP ZnO Electrosynthesis Nanocomposite
a b s t r a c t In this work, we presented facile route for fabrication of poly ortho aminophenol (POAP) POAP and POAP/ ZnO nanocomposite on the surface of the working electrode. The fractal dimension of nanocomposite films in the presence of counter ions was investigated. Surface morphology of the composite film was studied by surface microscopy techniques (SEM). The presence of ZnO in the films was confirmed by EDS analysis. The results indicate that a strong interaction exist at the interface of POAP and nanoZnO. Different electrochemical methods including galvanostatic charge discharge experiments and cyclic voltammetry have been applied to study the system performance. This work introduces new nanocomposite materials for electrochemical redox capacitors with such advantages as the ease of synthesis, high active surface area and stability in an aqueous electrolyte. Furthermore, comparison with a Ni-POAP the Ni-POAP/ZnO electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol in alkaline solution. It is observed that the in the presence of ZnO nanoparticles current density of electro-oxidation of methanol is almost constant in 400 cycles due to the stability of electrocatalyst in this cycle number and indicating that methanol reacted with the surface and no poisoning effect on the surface was observed. Ó 2016 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (L. Fotouhi), [email protected] (A. Ehsani). http://dx.doi.org/10.1016/j.jcis.2016.08.071 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
1. Introduction Conductive polymers such as polypyrrole, polythiophenes, polyaniline or poly ortho aminophenol represent a group of conjugated p-electron compounds, which process a combination of various electrical, optical and other semiconducting properties as organic semiconductors, whose electronic energy levels are by far different in comparison with inorganic semiconductors [1–8]. Polyaniline (PANI) is one of the most attractive conducting polymers due to its low cost, high environmental stability, good electrical conductivity and potential applications in molecular electronics. Aminophenols are interesting members of the class of substituted anilines [8–15]. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. The POAP film coated electrode displays electrochromic character when the potential is driven from 0.1 to 0.7 V vs. SCE. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7 [15–20]. The development of conducting polymer/inorganic nanoparticle composites with unique physical properties has recently attracted intensive research. The electrical properties of conductive polymer can be modified by the addition of inorganic fillers and carbon based materials [21–23]. One of the most attractive areas of research involves nanoscale fillers, given the intriguing properties arising from their nanosize and large surface areas. The insertion of nanoscale fillers may improve the electrical and dielectric properties of host conductive polymer materials [23]. Our goals in this paper were increasing the capacitance of POAP electrode by using ZnO nanostructures to form a composite electrode and moreover increase the cycle ability of the electrode. The complementary properties of both components generate a synergistic effect to enhance the electrochemical performance. It is an aim of the present work to investigate POAP/ZnO composite as supercapacitor electrode materials with several analytical tools such as field emission scanning electron microscopy, cyclic voltammetry and galvanostatic charge–discharge. The electrochemical tests demonstrate that the POAP/ZnO composite is a promising material in the application of energy storage electrode. Furthermore, electrochemical oxidation of methanol on a Ni-POAP/ZnO composite film in a solution of 0.1 M NaOH aiming at the elucidation of the mechanism and the usefulness of the electrocatalytic process has been investigated.
inverted to NiO by potential cycling (40 cycles at a scan rate of 100 mV/s between o and 1 V v.s Ag/AgCl). Morphology and particle dispersion was investigated by scanning electron microscopy (SEM). All studies were carried out at 298 ± 2 K. 3. Results and discussion According to our previous works [17,18] cyclic voltammetry (CV) was used to deposit POAP coatings from ortho aminophenol monomer solution. Fig. 1 shows the typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ZnO nanoparticles. As seen in the Fig. 1, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan. Under ultrasonic irradiation, the aggregates of nanocrystalline ZnO are broken down, while at the same time, OAP monomer is polymerized and the synthesized POAP is absorbed on the surface of nanocrystalline ZnO particles, which forms the conductive POAP. The surfactant molecules are absorbed on the surface of the composite particles and have a stabilizing effect. After electrosynthesis of POAP/ZnO, incorporation of ZnO in the polymer film is confirmed by the SEM and EDS analysis in Figs. 2 and 3 respectively, which shows the presence of Zn located at 1.10, 8.7 and 9.5 keV. Signal peak of carbon (C) at
Fig. 1. Typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ZnO nanoparticles.
2. Experimental All reagents were purchased from the Merck and Aldrich chemical companies and used without further purification. All electrochemical experiments were carried out by a Potentiostat/ galvanostat (Ivium V21508, Vertex) and Zahner Zennium. Saturated Ag/AgCl electrode, a Pt wire and a carbon paste electrode were employed as the reference, counter and working electrodes, respectively. POAP and composites of POAP/ZnO were formed on the surface of the modified working electrode using a ortho aminophenol monomer solution (0.01 M ortho aminophenol in 0.1 M HCl and 0.005 M SDS) and (0.01 M ortho aminophenol in 0.1 M HCl and 0.005 M SDS, and 0.001 g ZnO nanoparticles) respectively. The electropolymerization was carried out by potential cycling (50 cycles at a scan rate of 50 mV/s). The freshly electropolymerised working electrode was immerged in an aqueous solution of 0.1 M NiCl2 for 30 min then immerged in a aqueous solution of 0.1 M NaOH and Ni(II) was
309
Fig. 2. SEM image of electrosynthesized POAP/ZnO nanocomposite film.
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M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
Fig. 3. EDS spectrum of nanocomposite film.
0.25 keV and oxygen (O) at 0.53 keV characteristic of the POAP polymer. Because the gold is used as coating the mounted electrode in SEM analysis we observe also two peak at 2.07 and 9.8 keV of the gold. To elucidate the effect of ZnO on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4, as shown in Fig. 4. The cyclic voltammograms of the POAP/ZnO films showed a couple of strong and broad oxidation and reduction waves. Their wave currents were higher than that of pure POAP films electrodes. The voltammetric behavior of both films is similar and the CV curves show the voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between POAP/ZnO electrode and pure POAP electrode show that a POAP/ZnO electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded ZnO. This behavior occurs at all cycles and the higher current observed when ZnO was in solution, indicates that the polymerization process is catalyzed by the presence of the ZnO. One possible explanation for the higher current is the increase in the concentration of OAP monomers near the electrode surface due to the influence of ZnO. This is supported by the fact that the adsorption of positively charged ions of monomer on the surface of the nanoparticles is likely due to their negative zeta-potential
value in aqueous solutions. The z-potential values becomes slightly less negative when the positively charged OAP monomers are adsorbed on the surface of the ZnO and they became carriers of OAP increasing their concentration near the electrode surface and therefore the polymerization rate. Other reasons for the current increase could be that the increase of available area due to the incorporation of the ZnO. Fig. 5 shows the cyclic voltammograms of POAP/ZnO films at different scan rates in acidic solution. A pair of peaks signifying polymer,s redox processes are present in studies. In various scan rates by ploting ipeak vs scan rate in a log-log diagram, we obtain a from the slope of a linear fit to the data. The parameter a is related to the fractal dimension (Df) of the surface through [12,18].
Fig. 4. Cyclic voltammograms of the POAP and POAP/ZnO films in the monomer free solution (0.5 M HClO4).
Fig. 5. Cyclic voltammograms of POAP/ZnO films in different scan rates (25– 400mV s1).
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
I p / #a
a¼
ð1Þ
Df 1 2
ð2Þ
Substituting a value in the Eq. (2) we obtain the fractal dimension of the composite film around 2.49. Fractal geometry is a mathematical concept that describes objects of irregular shape. Some natural geometrical shapes, that can be irregular, tortuous, and rough or fragmented, can be described using concepts of fractal geometry as long as the requirement of self-similarity is satisfied [19]. Moreover, fractal geometry provides a powerful opportunity to investigate surface roughness via geometrical model [19]. The term fractal indicates the fact that the material of interest has a fractional dimension, not a whole number value, and the term was specifically applied for temporal and spatial phenomena that exhibit partial correlations over many scales. Fractal dimension (Df) is a quantitative parameter for analysis of fractal objects, which is widely used for different purposes. In addition, it is one of the most important and useful parameters for analysis of structure of rough surfaces.
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To further investigate the UV–vis spectra for POAP and POAP/ ZnO are shown in Fig. 6. Clearly, the prepared POAP/nanocrystalline ZnO composites not only can strongly absorb the UV light but also can absorb the visible and near-IR light. It is interesting to note that, in the presence of ZnO nanoparticles, the UV absorption is changed. This result indicates there is strong interaction between POAP and ZnO nanoparticles. Galvanostatic charge/discharge method has been used to highlight the capacitance characteristic of POAP/ZnO composite electrode. Fig. 7 shows the charge/discharge behavior of POAP and POAP/ZnO electrodes in the potential range from 0.0 to 0.6 V. As it can be seen, a triangular shape between this potential ranges is observed that indicating good columbic efficiency and ideal capacitive behavior of POAP/ZnO as electrode for application in pseudo supercapacitor. One of the most important parameters for practical application is cycling stability. Fig. 8 shows the stability POAP/ZnO composite electrodes when cycled at a current 1 mA for 1000 cycles. Stability of electrodes is compared in terms of losing their capacities as stability percentage. The stability of electrode calculated from following equation:
Stability ¼ Cn =C1 100
ð3Þ
In that Cn is the capacitance of electrode in each cycles and C1 is capacitance of electrode in the first cycle. Results shows, using POAP/ZnO in POAP caused an excellent retention in stability percentage of composite electrode suggesting the good stability toward long time charge-discharge applications. While the POAP electrode loses its stability fast, composite electrode maintains its stability and saves more that 91% of its capacitance of the first cycle under consecutive cycles after 1000 cycles. Specific capacitance (C) of composite electrode is calculated from galvanostatic charge/discharge curves using the following equation [15]:
C¼
I Dt DVm
ð4Þ
Fig. 6. UV spectrum of POAP and POAP/ZnO composite film.
Fig. 7. Galvanostatic charge and discharge measurements of (a) POAP and POAP/ZnO electrode in 0.1 M HClO4 solution at 0.005 mA,(b) different current (0.005(3)–0.01(1) mA) for composite and (c) during consecutive charge-discharge of composite film.
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Fig. 8. Cycle stability of POAP/ZnO composite electrode.
Fig. 9. Cyclic voltammograms of Ni-POAP (1) and Ni-POAP/ZnO (2) electrode in 0.1 M NaOH solution in the presence of 0.01 M methanol at a potential sweep rate of 10 mV s1.
where I is the current loaded, m is reactive material mass, V is the potential change during discharge process and t is the discharge time. The mass of deposited polymer on electrode was calculated from the charge (Q) passed during characteristic CV of electrode (Fig. 4) based on Faraday’s law: Q = znF, where Q is the difference
of passed charge through polymer electrodes, z is the number of exchanged electrons and n is the number of moles. By substituting the obtained mass value in equation 4, the specific capacitance of POAP/ZnO electrode was found to be 223 F g1. Fig. 9 shows cyclic voltammograms of Ni-POAP and Ni-POAP/ ZnO electrode in the presence of 0.01 M methanol at a potential sweep rate of 10 mV s1. The larger methanol response at the NiPOAP/ZnO in respect to Ni-POAP electrode is proposed to be the Ni-POAP/ZnO enhances the electrochemical activity of Ni2+/Ni3+ redox through fine dispersion of the particles into the conductive polymer matrix to result in a drastic increase in surface area. The electrocatalytic behavior of any material depends on various factors such as: (i) the position of the energy levels of the reactive species and the electrode material; (ii) the charge-transfer process across the electrode/electrolyte interface; (iii) the diffusion of the reactants into/near the electrode surface; and the surface morphology of the electrode. These results may be explained on the basis of electrochemical reactions at a semiconductor electrode. Here, POAP is considered as a p-type semiconductor. The impurity doping level will be situated above the uppermost valence level since POAP is a p-type material. Since the electrode is in an anodic condition, electrons are abstracted from the electrolyte and these combine with holes in the semiconductor. In the present study, it is therefore clear that charge transfer at the electrode/electrolyte interface has the most influence on the electro-oxidation of methanol when using a semi-conducting polymer electrode. It should also be noted that such charge transfer processes are also important in electrochromism and photoelectrochemical effects [11]. In the first step of charge transport within POAP films, which are considered to be p-type semiconductors where holes are majority carriers, there are two processes of importance, inter chain and intra chain transport. The intra chain transport of charge takes place along the main chain, which is facilitated due to extended conjugation. The inter chain charge transport occurs mainly across one chain to the other by hopping mechanism. Fig. 10a shows cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol at a potential sweep rate of 10 mV s1. At the surface of the Ni-POAP/ZnO electrode, oxidation of methanol appeared as a typical electrocatalytic response in alkaline media by Ni(OH)2/ NiOOH [24]. The anodic electrochemical reactions can be summarized as follows [24–29]:
Fig. 10. (a) Cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol (0–0.05 M) at a potential sweep rate of 10 mV s1, (b) cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of 0.01 M methanol at different potential sweep rates.
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
313
Plotting the net currents versus the square roots of time results in linear dependencies. Therefore, a diffusion-controlled process is dominant for electrooxidation of methanol. 4. Conclusion
Fig. 11. Chronoamperogram of Ni-POAQP/ZnO electrode in 0.1 M NaOH solution with different concentrations of methanol. 2þ
Ni
þ 2OH ! NiðOHÞ2
ð5Þ
NiðOHÞ2 þ OH ! NiO OH þ H2 O þ e
ð6Þ
NiO OH þ OH ! NiO2 þ H2 O þ e
ð7Þ
Whereas the reduction reaction observed can be simplified as:
NiO2 þ 2H2 O þ 2e ! NiðOHÞ2 þ 2OH
ð8Þ
Eqs. (5)–(7) suggest that the product obtained was a mixture of NiO2 and NiO–OH. Methanol is oxidized on the modified surface according to following reaction: III
II
Methanol þ Ni O OH ! Ni ðOHÞ2 þ product
ð9Þ
The anodic current in the positive sweep was proportional to the bulk concentration of methanol and any increase in the concentration of methanol caused an almost proportional linear enhancement of the anodic current. So, catalytic electrooxidation of methanol on Ni-POAP/ZnO seems to be certain. It is observed that the current density of electro-oxidation of methanol is almost constant in 400 cycles due to the stability of electrocatalyst in this cycle number and indicating that methanol reacted with the surface and no poisoning effect on the surface was observed. Cyclic voltammograms of Ni-POAP/ZnO in the presence of 0.01 M methanol at various potential sweep rates are illustrated in Fig. 10b. The cathodic peak was not observed in low scan rates, but appeared upon increasing the sweep rate. The phenomenon indicates that the electrooxidation of nickel species to higher valence state is much faster than the catalytic oxidation of methanol. This reveals that the oxidation of methanol on Ni may belong to a slow process. Meanwhile, the anodic peak currents that are linearly proportional to the square root of scan rate suggest that the overall oxidation of methanol at this electrode is controlled by the diffusion of ethanol from solution to the surface redox sites. The value of electron transfer coefficient for the reaction which is totally irreversible-diffusion controlled can be obtained from the following equation [28]
Ep ¼ ðRT=naFÞ ln v þ constant
ð10Þ
Using the dependency of anodic peak potential on the natural logarithm of the potential sweep rate, the value of electron transfer coefficient was obtained as 0.17. Chronoamperograms were recorded by setting the working electrode potentials to the desired values and measuring the catalytic rate constant on the Ni-POAP/ZnO electrode surface. Fig. 11 shows chronoamperograms for the Ni-POAP/ZnO electrode in the presence of methanol over the concentration range 0.006–0.05.
In this work, we presented cyclic voltammetry technique for fabrication of POAP and POAP/ZnO composite on the carbon paste electrode. Our goals in this paper were increasing the capacitance of POAP electrode by using ZnO nanostructures to form a composite electrode and moreover increase the cycle ability of the electrode. The complementary properties of both components generate a synergistic effect to enhance the electrochemical performance. The electrochemical tests demonstrate that the POAP/ZnO composite is a promising material in the application of energy storage electrode. According to common electrochemical results, NiPOAP/ZnO films have high fractal dimension and large specific surface area, which can be employed as an excellent electrode material in electrocatalytic process. Comparison with a Ni-POAP the Ni-POAP/ZnO electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol. Acknowledgements The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work. We also thank the Research Council of Alzahra University. References [1] X-Y. Peng, X-X. Liu, P-J. Hua, D. Diamond, K-T. Lau, J. Solid State Electrochem. 14 (2010) 1–7. [2] N. Ajami, A. Ehsani, F. Babaei, R. Safari, J. Mol. Liuid 215 (2016) 24–30. [3] J. Shabani-Shayeh, A. Ehsani, M.R. Ganjali, P. Norouzi, B. Jaleh, Appl. Surf. Sci. 353 (2015) 594. [4] A. Ehsani, M. Mahjani, M. Bordbar, S. Adeli, J. Electroanal. Chem. 710 (2013) 29. [5] J. Torabian, M.G. Mahjani, H. Mohammad Shiri, A. Ehsani, J. Shabani Shayehd, RSC Adv. 6 (2016) 41045–41052. [6] N. Salehifar, J. Shabani Shayeh, S.O. Ranaei Siadat, K. Niknam, A. Ehsani, S. Kazemi Movahhed, RSC Adv. 5 (2015) 96130–96137. [7] R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608–622. [8] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477–479. [9] M.J. Sailor, E.J. Ginsburg, C.B. Gorman, A. Kumar, R.H. Grubbs, N.S. Lewis, Science 249 (1990) 1146–1149. [10] J. Shabani-Shayeh, A. Ehsani, A. Nikkar, P. Norouzi, M.R. Ganjali, M. Wojdyla, New. J. Chem. 39 (2015) 9454. [11] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Electrochim. Acta 71 (2012) 128. [12] H. Mohammad Shiri, A. Ehsani, J. Colloid Interface 473 (2016) 126–131. [13] J. Torabian, M.G. Mahjani, H. Mohammad Shiri, A. Ehsani, J. Shabani Shayehd, RSC Adv. 6 (2016) 41045–41052. [14] H. Mohammad Shiri, A. Ehsani, J. Colloid Interface Sci. 473 (2016) 126. [15] J. Shabani Shayeh, P. Norouzi, M.R. Ganjali, RSC Adv. 5 (2015) 20446. [16] A. Ehsani, M.G. Mahjani, F. Babaei, H. Mostaanzadeh, RSC Adv. 5 (2015) 30394– 30404. [17] A. Ehsani, M.G. Mahjani, M. Jafarian, Synth. Met. 62 (2012) 199–204. [18] A. Ehsani, Prog. Org. Coat. 78 (2015) 133–139. [19] A. Ehsani, M.G. Mahjani, S. Adeli, S. Moradkhani, Prog. Org. Coat. 77 (2014) 1674–1681. [20] M. Naseri, L. Fotouhi, A. Ehsani, F. Babaei, New. J. Chem. 40 (2016) 2565–2573. [21] K. Kakaei, M. Hamidi, S. Husseindoost, J. Colloid Interface 479 (2016) 121–126. [22] A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. Kazemi, J. Colloid Interface 478 (2016) 181–187. [23] A. Ehsani, M.G. Mahjani, M. Bordbar, R. Moshrefi, Synth. Met. 165 (2013) 51– 55. [24] M. Fleischmann, K. Korinek, D. Pletcher, J. Chem. Soc., Perkin Trans. 2 (10) (1972) 1396–1402. [25] R. Ojani, J.B. Raoof, S. Fathi, Electrochim. Acta 54 (2009) 2190. [26] F.D. Eramo, J.M. Marioli, A.A. Arevalo, L.E. Sereno, Electroanalysis 11 (1999) 481. [27] M. Jafarian, M.G. Mahjani, H. Heli, F. Gobal, H. Khajehsharifi, M.H. Hamedi, Electrochim. Acta 48 (2003) 3423–3429. [28] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2001. [29] A. Ehsani, A. Vaziri-Rad, F. Babaei, H. Mohammad Shiri, Electrochim. Acta 159 (2015) 140–148.
Contents lists available at ScienceDirect
Journal of Colloid and Interface Science journal homepage: www.elsevier.com/locate/jcis
Regular Article
Novel electroactive nanocomposite of POAP for highly efficient energy storage and electrocatalyst: Electrosynthesis and electrochemical performance Maryam Naseri a, Lida Fotouhi a,⇑, Ali Ehsani b,⇑, Hamid Mohammad Shiri c a b c
Department of Chemistry, Alzahra University, Tehran, Iran Department of Chemistry, Faculty of Science, University of Qom, Qom, Iran Department of Chemistry, Payame Noor University, Iran
g r a p h i c a l a b s t r a c t
a r t i c l e
i n f o
Article history: Received 27 June 2016 Revised 8 August 2016 Accepted 27 August 2016 Available online 31 August 2016 Keywords: POAP ZnO Electrosynthesis Nanocomposite
a b s t r a c t In this work, we presented facile route for fabrication of poly ortho aminophenol (POAP) POAP and POAP/ ZnO nanocomposite on the surface of the working electrode. The fractal dimension of nanocomposite films in the presence of counter ions was investigated. Surface morphology of the composite film was studied by surface microscopy techniques (SEM). The presence of ZnO in the films was confirmed by EDS analysis. The results indicate that a strong interaction exist at the interface of POAP and nanoZnO. Different electrochemical methods including galvanostatic charge discharge experiments and cyclic voltammetry have been applied to study the system performance. This work introduces new nanocomposite materials for electrochemical redox capacitors with such advantages as the ease of synthesis, high active surface area and stability in an aqueous electrolyte. Furthermore, comparison with a Ni-POAP the Ni-POAP/ZnO electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol in alkaline solution. It is observed that the in the presence of ZnO nanoparticles current density of electro-oxidation of methanol is almost constant in 400 cycles due to the stability of electrocatalyst in this cycle number and indicating that methanol reacted with the surface and no poisoning effect on the surface was observed. Ó 2016 Elsevier Inc. All rights reserved.
⇑ Corresponding authors. E-mail addresses: [email protected] (L. Fotouhi), [email protected] (A. Ehsani). http://dx.doi.org/10.1016/j.jcis.2016.08.071 0021-9797/Ó 2016 Elsevier Inc. All rights reserved.
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
1. Introduction Conductive polymers such as polypyrrole, polythiophenes, polyaniline or poly ortho aminophenol represent a group of conjugated p-electron compounds, which process a combination of various electrical, optical and other semiconducting properties as organic semiconductors, whose electronic energy levels are by far different in comparison with inorganic semiconductors [1–8]. Polyaniline (PANI) is one of the most attractive conducting polymers due to its low cost, high environmental stability, good electrical conductivity and potential applications in molecular electronics. Aminophenols are interesting members of the class of substituted anilines [8–15]. The hydroxyl group in the phenyl ring can be oxidized to quinine and quinine can be reduced again. POAP gives a surface film of interesting electrochemical and electrochromic properties when it is electropolymerised in acidic solution. The POAP film coated electrode displays electrochromic character when the potential is driven from 0.1 to 0.7 V vs. SCE. This film is electroactive in aqueous and non-aqueous solutions containing protons but no response is observed at pH-value higher than pH 7 [15–20]. The development of conducting polymer/inorganic nanoparticle composites with unique physical properties has recently attracted intensive research. The electrical properties of conductive polymer can be modified by the addition of inorganic fillers and carbon based materials [21–23]. One of the most attractive areas of research involves nanoscale fillers, given the intriguing properties arising from their nanosize and large surface areas. The insertion of nanoscale fillers may improve the electrical and dielectric properties of host conductive polymer materials [23]. Our goals in this paper were increasing the capacitance of POAP electrode by using ZnO nanostructures to form a composite electrode and moreover increase the cycle ability of the electrode. The complementary properties of both components generate a synergistic effect to enhance the electrochemical performance. It is an aim of the present work to investigate POAP/ZnO composite as supercapacitor electrode materials with several analytical tools such as field emission scanning electron microscopy, cyclic voltammetry and galvanostatic charge–discharge. The electrochemical tests demonstrate that the POAP/ZnO composite is a promising material in the application of energy storage electrode. Furthermore, electrochemical oxidation of methanol on a Ni-POAP/ZnO composite film in a solution of 0.1 M NaOH aiming at the elucidation of the mechanism and the usefulness of the electrocatalytic process has been investigated.
inverted to NiO by potential cycling (40 cycles at a scan rate of 100 mV/s between o and 1 V v.s Ag/AgCl). Morphology and particle dispersion was investigated by scanning electron microscopy (SEM). All studies were carried out at 298 ± 2 K. 3. Results and discussion According to our previous works [17,18] cyclic voltammetry (CV) was used to deposit POAP coatings from ortho aminophenol monomer solution. Fig. 1 shows the typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ZnO nanoparticles. As seen in the Fig. 1, OAP is oxidized irreversibly at around 600 mV without corresponding cathodic processes in the reverse scan. Under ultrasonic irradiation, the aggregates of nanocrystalline ZnO are broken down, while at the same time, OAP monomer is polymerized and the synthesized POAP is absorbed on the surface of nanocrystalline ZnO particles, which forms the conductive POAP. The surfactant molecules are absorbed on the surface of the composite particles and have a stabilizing effect. After electrosynthesis of POAP/ZnO, incorporation of ZnO in the polymer film is confirmed by the SEM and EDS analysis in Figs. 2 and 3 respectively, which shows the presence of Zn located at 1.10, 8.7 and 9.5 keV. Signal peak of carbon (C) at
Fig. 1. Typical multi-sweep cyclic voltammograms during electropolymerization in the presence of ZnO nanoparticles.
2. Experimental All reagents were purchased from the Merck and Aldrich chemical companies and used without further purification. All electrochemical experiments were carried out by a Potentiostat/ galvanostat (Ivium V21508, Vertex) and Zahner Zennium. Saturated Ag/AgCl electrode, a Pt wire and a carbon paste electrode were employed as the reference, counter and working electrodes, respectively. POAP and composites of POAP/ZnO were formed on the surface of the modified working electrode using a ortho aminophenol monomer solution (0.01 M ortho aminophenol in 0.1 M HCl and 0.005 M SDS) and (0.01 M ortho aminophenol in 0.1 M HCl and 0.005 M SDS, and 0.001 g ZnO nanoparticles) respectively. The electropolymerization was carried out by potential cycling (50 cycles at a scan rate of 50 mV/s). The freshly electropolymerised working electrode was immerged in an aqueous solution of 0.1 M NiCl2 for 30 min then immerged in a aqueous solution of 0.1 M NaOH and Ni(II) was
309
Fig. 2. SEM image of electrosynthesized POAP/ZnO nanocomposite film.
310
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
Fig. 3. EDS spectrum of nanocomposite film.
0.25 keV and oxygen (O) at 0.53 keV characteristic of the POAP polymer. Because the gold is used as coating the mounted electrode in SEM analysis we observe also two peak at 2.07 and 9.8 keV of the gold. To elucidate the effect of ZnO on the property of POAP films, electrochemical performance of composite films was evaluated by carrying out CV measurements in 0.5 M HClO4, as shown in Fig. 4. The cyclic voltammograms of the POAP/ZnO films showed a couple of strong and broad oxidation and reduction waves. Their wave currents were higher than that of pure POAP films electrodes. The voltammetric behavior of both films is similar and the CV curves show the voltammograms reveal the electrodes are stable in HClO4 solution within the sweeping potential range. Close comparisons of CV curves between POAP/ZnO electrode and pure POAP electrode show that a POAP/ZnO electrode not only displays a higher background current in the potential sweep but also there exist faradic currents, which are believed to arise from the contribution of the loaded ZnO. This behavior occurs at all cycles and the higher current observed when ZnO was in solution, indicates that the polymerization process is catalyzed by the presence of the ZnO. One possible explanation for the higher current is the increase in the concentration of OAP monomers near the electrode surface due to the influence of ZnO. This is supported by the fact that the adsorption of positively charged ions of monomer on the surface of the nanoparticles is likely due to their negative zeta-potential
value in aqueous solutions. The z-potential values becomes slightly less negative when the positively charged OAP monomers are adsorbed on the surface of the ZnO and they became carriers of OAP increasing their concentration near the electrode surface and therefore the polymerization rate. Other reasons for the current increase could be that the increase of available area due to the incorporation of the ZnO. Fig. 5 shows the cyclic voltammograms of POAP/ZnO films at different scan rates in acidic solution. A pair of peaks signifying polymer,s redox processes are present in studies. In various scan rates by ploting ipeak vs scan rate in a log-log diagram, we obtain a from the slope of a linear fit to the data. The parameter a is related to the fractal dimension (Df) of the surface through [12,18].
Fig. 4. Cyclic voltammograms of the POAP and POAP/ZnO films in the monomer free solution (0.5 M HClO4).
Fig. 5. Cyclic voltammograms of POAP/ZnO films in different scan rates (25– 400mV s1).
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
I p / #a
a¼
ð1Þ
Df 1 2
ð2Þ
Substituting a value in the Eq. (2) we obtain the fractal dimension of the composite film around 2.49. Fractal geometry is a mathematical concept that describes objects of irregular shape. Some natural geometrical shapes, that can be irregular, tortuous, and rough or fragmented, can be described using concepts of fractal geometry as long as the requirement of self-similarity is satisfied [19]. Moreover, fractal geometry provides a powerful opportunity to investigate surface roughness via geometrical model [19]. The term fractal indicates the fact that the material of interest has a fractional dimension, not a whole number value, and the term was specifically applied for temporal and spatial phenomena that exhibit partial correlations over many scales. Fractal dimension (Df) is a quantitative parameter for analysis of fractal objects, which is widely used for different purposes. In addition, it is one of the most important and useful parameters for analysis of structure of rough surfaces.
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To further investigate the UV–vis spectra for POAP and POAP/ ZnO are shown in Fig. 6. Clearly, the prepared POAP/nanocrystalline ZnO composites not only can strongly absorb the UV light but also can absorb the visible and near-IR light. It is interesting to note that, in the presence of ZnO nanoparticles, the UV absorption is changed. This result indicates there is strong interaction between POAP and ZnO nanoparticles. Galvanostatic charge/discharge method has been used to highlight the capacitance characteristic of POAP/ZnO composite electrode. Fig. 7 shows the charge/discharge behavior of POAP and POAP/ZnO electrodes in the potential range from 0.0 to 0.6 V. As it can be seen, a triangular shape between this potential ranges is observed that indicating good columbic efficiency and ideal capacitive behavior of POAP/ZnO as electrode for application in pseudo supercapacitor. One of the most important parameters for practical application is cycling stability. Fig. 8 shows the stability POAP/ZnO composite electrodes when cycled at a current 1 mA for 1000 cycles. Stability of electrodes is compared in terms of losing their capacities as stability percentage. The stability of electrode calculated from following equation:
Stability ¼ Cn =C1 100
ð3Þ
In that Cn is the capacitance of electrode in each cycles and C1 is capacitance of electrode in the first cycle. Results shows, using POAP/ZnO in POAP caused an excellent retention in stability percentage of composite electrode suggesting the good stability toward long time charge-discharge applications. While the POAP electrode loses its stability fast, composite electrode maintains its stability and saves more that 91% of its capacitance of the first cycle under consecutive cycles after 1000 cycles. Specific capacitance (C) of composite electrode is calculated from galvanostatic charge/discharge curves using the following equation [15]:
C¼
I Dt DVm
ð4Þ
Fig. 6. UV spectrum of POAP and POAP/ZnO composite film.
Fig. 7. Galvanostatic charge and discharge measurements of (a) POAP and POAP/ZnO electrode in 0.1 M HClO4 solution at 0.005 mA,(b) different current (0.005(3)–0.01(1) mA) for composite and (c) during consecutive charge-discharge of composite film.
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M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
Fig. 8. Cycle stability of POAP/ZnO composite electrode.
Fig. 9. Cyclic voltammograms of Ni-POAP (1) and Ni-POAP/ZnO (2) electrode in 0.1 M NaOH solution in the presence of 0.01 M methanol at a potential sweep rate of 10 mV s1.
where I is the current loaded, m is reactive material mass, V is the potential change during discharge process and t is the discharge time. The mass of deposited polymer on electrode was calculated from the charge (Q) passed during characteristic CV of electrode (Fig. 4) based on Faraday’s law: Q = znF, where Q is the difference
of passed charge through polymer electrodes, z is the number of exchanged electrons and n is the number of moles. By substituting the obtained mass value in equation 4, the specific capacitance of POAP/ZnO electrode was found to be 223 F g1. Fig. 9 shows cyclic voltammograms of Ni-POAP and Ni-POAP/ ZnO electrode in the presence of 0.01 M methanol at a potential sweep rate of 10 mV s1. The larger methanol response at the NiPOAP/ZnO in respect to Ni-POAP electrode is proposed to be the Ni-POAP/ZnO enhances the electrochemical activity of Ni2+/Ni3+ redox through fine dispersion of the particles into the conductive polymer matrix to result in a drastic increase in surface area. The electrocatalytic behavior of any material depends on various factors such as: (i) the position of the energy levels of the reactive species and the electrode material; (ii) the charge-transfer process across the electrode/electrolyte interface; (iii) the diffusion of the reactants into/near the electrode surface; and the surface morphology of the electrode. These results may be explained on the basis of electrochemical reactions at a semiconductor electrode. Here, POAP is considered as a p-type semiconductor. The impurity doping level will be situated above the uppermost valence level since POAP is a p-type material. Since the electrode is in an anodic condition, electrons are abstracted from the electrolyte and these combine with holes in the semiconductor. In the present study, it is therefore clear that charge transfer at the electrode/electrolyte interface has the most influence on the electro-oxidation of methanol when using a semi-conducting polymer electrode. It should also be noted that such charge transfer processes are also important in electrochromism and photoelectrochemical effects [11]. In the first step of charge transport within POAP films, which are considered to be p-type semiconductors where holes are majority carriers, there are two processes of importance, inter chain and intra chain transport. The intra chain transport of charge takes place along the main chain, which is facilitated due to extended conjugation. The inter chain charge transport occurs mainly across one chain to the other by hopping mechanism. Fig. 10a shows cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol at a potential sweep rate of 10 mV s1. At the surface of the Ni-POAP/ZnO electrode, oxidation of methanol appeared as a typical electrocatalytic response in alkaline media by Ni(OH)2/ NiOOH [24]. The anodic electrochemical reactions can be summarized as follows [24–29]:
Fig. 10. (a) Cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of various concentrations of methanol (0–0.05 M) at a potential sweep rate of 10 mV s1, (b) cyclic voltammograms of Ni-POAP/ZnO electrode in 0.1 M NaOH solution in the presence of 0.01 M methanol at different potential sweep rates.
M. Naseri et al. / Journal of Colloid and Interface Science 484 (2016) 308–313
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Plotting the net currents versus the square roots of time results in linear dependencies. Therefore, a diffusion-controlled process is dominant for electrooxidation of methanol. 4. Conclusion
Fig. 11. Chronoamperogram of Ni-POAQP/ZnO electrode in 0.1 M NaOH solution with different concentrations of methanol. 2þ
Ni
þ 2OH ! NiðOHÞ2
ð5Þ
NiðOHÞ2 þ OH ! NiO OH þ H2 O þ e
ð6Þ
NiO OH þ OH ! NiO2 þ H2 O þ e
ð7Þ
Whereas the reduction reaction observed can be simplified as:
NiO2 þ 2H2 O þ 2e ! NiðOHÞ2 þ 2OH
ð8Þ
Eqs. (5)–(7) suggest that the product obtained was a mixture of NiO2 and NiO–OH. Methanol is oxidized on the modified surface according to following reaction: III
II
Methanol þ Ni O OH ! Ni ðOHÞ2 þ product
ð9Þ
The anodic current in the positive sweep was proportional to the bulk concentration of methanol and any increase in the concentration of methanol caused an almost proportional linear enhancement of the anodic current. So, catalytic electrooxidation of methanol on Ni-POAP/ZnO seems to be certain. It is observed that the current density of electro-oxidation of methanol is almost constant in 400 cycles due to the stability of electrocatalyst in this cycle number and indicating that methanol reacted with the surface and no poisoning effect on the surface was observed. Cyclic voltammograms of Ni-POAP/ZnO in the presence of 0.01 M methanol at various potential sweep rates are illustrated in Fig. 10b. The cathodic peak was not observed in low scan rates, but appeared upon increasing the sweep rate. The phenomenon indicates that the electrooxidation of nickel species to higher valence state is much faster than the catalytic oxidation of methanol. This reveals that the oxidation of methanol on Ni may belong to a slow process. Meanwhile, the anodic peak currents that are linearly proportional to the square root of scan rate suggest that the overall oxidation of methanol at this electrode is controlled by the diffusion of ethanol from solution to the surface redox sites. The value of electron transfer coefficient for the reaction which is totally irreversible-diffusion controlled can be obtained from the following equation [28]
Ep ¼ ðRT=naFÞ ln v þ constant
ð10Þ
Using the dependency of anodic peak potential on the natural logarithm of the potential sweep rate, the value of electron transfer coefficient was obtained as 0.17. Chronoamperograms were recorded by setting the working electrode potentials to the desired values and measuring the catalytic rate constant on the Ni-POAP/ZnO electrode surface. Fig. 11 shows chronoamperograms for the Ni-POAP/ZnO electrode in the presence of methanol over the concentration range 0.006–0.05.
In this work, we presented cyclic voltammetry technique for fabrication of POAP and POAP/ZnO composite on the carbon paste electrode. Our goals in this paper were increasing the capacitance of POAP electrode by using ZnO nanostructures to form a composite electrode and moreover increase the cycle ability of the electrode. The complementary properties of both components generate a synergistic effect to enhance the electrochemical performance. The electrochemical tests demonstrate that the POAP/ZnO composite is a promising material in the application of energy storage electrode. According to common electrochemical results, NiPOAP/ZnO films have high fractal dimension and large specific surface area, which can be employed as an excellent electrode material in electrocatalytic process. Comparison with a Ni-POAP the Ni-POAP/ZnO electrode shows a better catalytic performance for the electrocatalytic oxidation of methanol. Acknowledgements The authors would like to express their deep gratitude to the Iranian Nano Council for supporting this work. We also thank the Research Council of Alzahra University. References [1] X-Y. Peng, X-X. Liu, P-J. Hua, D. Diamond, K-T. Lau, J. Solid State Electrochem. 14 (2010) 1–7. [2] N. Ajami, A. Ehsani, F. Babaei, R. Safari, J. Mol. Liuid 215 (2016) 24–30. [3] J. Shabani-Shayeh, A. Ehsani, M.R. Ganjali, P. Norouzi, B. Jaleh, Appl. Surf. Sci. 353 (2015) 594. [4] A. Ehsani, M. Mahjani, M. Bordbar, S. Adeli, J. Electroanal. Chem. 710 (2013) 29. [5] J. Torabian, M.G. Mahjani, H. Mohammad Shiri, A. Ehsani, J. Shabani Shayehd, RSC Adv. 6 (2016) 41045–41052. [6] N. Salehifar, J. Shabani Shayeh, S.O. Ranaei Siadat, K. Niknam, A. Ehsani, S. Kazemi Movahhed, RSC Adv. 5 (2015) 96130–96137. [7] R. Gangopadhyay, A. De, Chem. Mater. 12 (2000) 608–622. [8] G. Gustafsson, Y. Cao, G.M. Treacy, F. Klavetter, N. Colaneri, A.J. Heeger, Nature 357 (1992) 477–479. [9] M.J. Sailor, E.J. Ginsburg, C.B. Gorman, A. Kumar, R.H. Grubbs, N.S. Lewis, Science 249 (1990) 1146–1149. [10] J. Shabani-Shayeh, A. Ehsani, A. Nikkar, P. Norouzi, M.R. Ganjali, M. Wojdyla, New. J. Chem. 39 (2015) 9454. [11] A. Ehsani, M.G. Mahjani, M. Jafarian, A. Naeemy, Electrochim. Acta 71 (2012) 128. [12] H. Mohammad Shiri, A. Ehsani, J. Colloid Interface 473 (2016) 126–131. [13] J. Torabian, M.G. Mahjani, H. Mohammad Shiri, A. Ehsani, J. Shabani Shayehd, RSC Adv. 6 (2016) 41045–41052. [14] H. Mohammad Shiri, A. Ehsani, J. Colloid Interface Sci. 473 (2016) 126. [15] J. Shabani Shayeh, P. Norouzi, M.R. Ganjali, RSC Adv. 5 (2015) 20446. [16] A. Ehsani, M.G. Mahjani, F. Babaei, H. Mostaanzadeh, RSC Adv. 5 (2015) 30394– 30404. [17] A. Ehsani, M.G. Mahjani, M. Jafarian, Synth. Met. 62 (2012) 199–204. [18] A. Ehsani, Prog. Org. Coat. 78 (2015) 133–139. [19] A. Ehsani, M.G. Mahjani, S. Adeli, S. Moradkhani, Prog. Org. Coat. 77 (2014) 1674–1681. [20] M. Naseri, L. Fotouhi, A. Ehsani, F. Babaei, New. J. Chem. 40 (2016) 2565–2573. [21] K. Kakaei, M. Hamidi, S. Husseindoost, J. Colloid Interface 479 (2016) 121–126. [22] A. Ehsani, H. Mohammad Shiri, E. Kowsari, R. Safari, J. Torabian, S. Kazemi, J. Colloid Interface 478 (2016) 181–187. [23] A. Ehsani, M.G. Mahjani, M. Bordbar, R. Moshrefi, Synth. Met. 165 (2013) 51– 55. [24] M. Fleischmann, K. Korinek, D. Pletcher, J. Chem. Soc., Perkin Trans. 2 (10) (1972) 1396–1402. [25] R. Ojani, J.B. Raoof, S. Fathi, Electrochim. Acta 54 (2009) 2190. [26] F.D. Eramo, J.M. Marioli, A.A. Arevalo, L.E. Sereno, Electroanalysis 11 (1999) 481. [27] M. Jafarian, M.G. Mahjani, H. Heli, F. Gobal, H. Khajehsharifi, M.H. Hamedi, Electrochim. Acta 48 (2003) 3423–3429. [28] A.J. Bard, L.R. Faulkner, Electrochemical Methods, Wiley, New York, 2001. [29] A. Ehsani, A. Vaziri-Rad, F. Babaei, H. Mohammad Shiri, Electrochim. Acta 159 (2015) 140–148.