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Synthesis and electrochemical properties of poly(3,4-dihydroxystyrene) and its composites with conducting polymers
Synthetic Metals 256 (2019) 116151
Contents lists available at ScienceDirect
Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and electrochemical properties of poly(3,4-dihydroxystyrene) and its composites with conducting polymers
T
D.A. Lukyanov, R.V. Apraksin, A.N. Yankin, P.S. Vlasov, O.V. Levin, E.G. Tolstopjatova, ⁎ V.V. Kondratiev Saint Petersburg State University, 199034, Saint Petersburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Composite materials Conducting polymers Poly-3,4-ethylenedioxythiophene Poly(3,4-dihydroxystyrene) Cyclic voltammetry Electrochemical capacitors
The present study reports electrochemical performance of electrode materials based on poly(3,4-dihydroxystyrene) (PDHS) and its composites with conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), deposited on glassy carbon electrodes, in diluted sulfuric acid. Poly(3,4-dihydroxystyrene), bearing redox active hydroquinone groups, was employed to the first time as an electrode material in combination with carbon, binder and conducting polymer. It was found that this quinone-based composite exhibits moderate electrochemical characteristics with specific capacity of about 50–54 mA h g−1. The comparison of rate constants obtained for GC/PDHS and GC/PEDOT/PDHS electrodes confirms the catalytic effect of conducting polymer PEDOT on the redox transformation of PDHS.
1. Introduction Inorganic metal-containing compounds are prevailing among currently available rechargeable materials for electrochemical power sources. Basically they are intercalated metal oxides or other rare-metal compounds [1,2], sources of which may be depleted in the future. The employment of organic materials was recently proposed as a promising solution for energy storage materials [3–9]. Organic compounds, containing quinone and hydroquinone moieties, were recognized as redoxactive organic materials that possess stable and reversible electrochemical characteristics and can provide high capacities. Quinones can be considered as potentially suitable components of energy storage materials for supercapacitors and metal-ion batteries [4–9]. Many low molecular quinones and hydroquinone derivatives were recently proposed as materials for batteries [7–9], however, their low conductivity and gradual dissolution in electrolytes leads to non-complete utilization of specific capacity and low cycling stability. Therefore, conductive additives, such as carbon black, or incorporation of quinones into the matrix of conducting polymer with high stability are needed to improve their performance. In this work the new polymer compound poly(3,4-dihydroxystyrene) (PDHS) carrying the hydroxyquinone moieties was synthesized and investigated as a possible component of organic electrode material for lithium-ion batteries alone and in composite with conducting polymer poly(3,4-ethylendioxithiphene) (PEDOT). PEDOT was chosen as a
⁎
component of the composite, being a material with stable pseudocapacitive response [10], and a multi-purpose conductive matrix which has also catalytic effect on the redox processes with molecular forms of quinones [11,12]. 2. Experimental 3,4-Ethylenedioxythiophene (EDOT, 98%) was purchased from Aldrich. Acetonitrile (HPLC grade) was from abcr GmbH, Germany. Propylene carbonate (PC) was from Aldrich. Conductive carbon black «Super P» was purchased from Timcal Inc. (Belgium). Sulfuric acid was from Neva Reactive Co., Russia. Sulfuric acid solutions were prepared on deionized water of resistivity not less than 18 MΩ, obtained by means of Millipore Direct-Q UV. 2.1. Synthesis of poly(3,4-dihydroxystyrene) Poly(3,4-dihydroxystyrene) (PDHS) was obtained from veratraldehyde using the following reaction sequence (1). The 3,4-dimethoxystyrene obtained from the former aldehyde using methylenephosphoniumylide was polymerized in DMF with an azobisisobutyronitrile (AIBN) as an initiating agent, affording the poly (3,4-dimethoxystyrene) in 81% yield with M w =130,000 g/mol and Đ = 1.64 and with NMR data acquired in agreement with literature [13]. Poly(3,4-dimethoxystyrene) was subjected to the demethylation
Corresponding author. E-mail address: [email protected] (V.V. Kondratiev).
https://doi.org/10.1016/j.synthmet.2019.116151 Received 10 June 2019; Received in revised form 23 July 2019; Accepted 17 August 2019 Available online 28 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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with BBr3, giving the desired product, poly(3,4-dihydroxystyrene). Poly (3,4-dimethoxystyrene) (1.71 g, 10.5 mmol) was dissolved in dry CH2Cl2 (50 ml). BBr3 (3.2 ml, 8.5 g, 35 mmol) was added dropwise to a vigorously stirred ice-cooled solution via syringe under Ar. Mixture was allowed to warm to RT, shaken for 6 h and then stirred overnight. Mixture was poured in 0.1 M HCl (200 ml), stirred for 20 min and filtered. Solid was washed with 0.1 M HCl for 3 times (20 min each washing), washed with water and dried in vacuo at 50 °C. Precipitation of the product from 1,4-dioxane with CH2Cl2, followed by drying in vacuo at 50 °C, gave the desired product (1.41 g, 10.5 mmol, 100%) as a creamy solid. The obtained product was studied by NMR and FTIR spectroscopy. The following data, confirming the chemical nature of the product, were obtained: 1H NMR (400 MHz, DMSO-d6) δ ppm: 8.32 (s, 2 H), 6.65-5.45 (m, 3 H), 2.15-0.60 (m, 3 H) (lit. [14]: 8.36 (s, 2 H), 6.75.5 (bm, 3 H), 2.2-0.5 (bd, 3 H)); FTIR (KBr) ῦ cm−1: 3433, 1608, 1517, 1444, 1368, 1279, 1187, 1107.
electrode cell using an AUTOLAB PGSTAT-302 potentiostat. The potential scan rates were varied within range of 5–500 mV s−1. A glassy carbon (GC) electrode (0.07 cm2) was used as the working electrode, Pt foil was used as an auxiliary electrode. The potential values were measured relative to Ag/AgCl reference electrode. The measurements were performed in aqueous solution of 0.5 M H2SO4 at room temperature (20 ± 2 °C). 2.4. Material characterization 1 H NMR measurements were performed on a Bruker-Avance 400 instrument at ambient temperature. FTIR spectra were recorded on Shimadzu IRAffinity-1 spectrometer using KBr pellets. The morphology of films was characterized by SEM at accelerating voltage of 21 kV on an Carl Zeiss SUPRA 40 VP instrument.
(1) 2.2. Electrode preparation
3. Results and discussion
As PDHS has low conductivity, conductive additives (carbon black, PEDOT) were necessary for its testing as a rechargeable electrode material. Two types of electrodes were deposited on glassy carbon (GC) substrate: GC/PDHS and GC/PEDOT/PDHS. GC/PDHS electrodes were prepared by drop-casting acetonitrile dispersion containing 50 wt% of PDHS as active material and 50 wt % of carbon black (CB) as a conductive additive onto GC electrodes and then dried under mild heating (up to 90 °C). GC/PEDOT/PDHS composite electrodes were obtained by electrochemical synthesis. PEDOT/PDHS composite films were deposited on GC electrode from the solution of 0.05 M EDOT + 0.5 M LiClO4 + 0.3 M PDHS in propylene carbonate (PC) in potentiodynamic conditions in the potential range from -0.3 to 1.4 V with a potential scan rate of 50 mV s−1. The SEM images of two types of electrodes exhibited remarkably different morphologies (Fig. 1). More dense, compact layer was observed for GC/PDHS electrode, whereas for GC/PEDOT/PDHS electrode porous globular structure can be seen.
Comparative study of redox processes in PDHS was conducted on GC/PDHS and GC/PEDOT/PDHS electrodes in dilute sulfuric acid, which is the most frequently used acidic electrolyte for testing of quinone-based electrode materials. Typical cyclic voltammograms (CVs) of GC/PDHS electrodes in solution of 0.5 M H2SO4 are shown in Fig. 2a,b. Well-defined couple of redox peaks at GC/PDHS electrodes associated with hydroquinonequinone redox transformation (reaction 2) is observed.
(2) The single couple of symmetrical redox peaks on CV for polymer film with many repeated redox-centers implies that multielectron transfer takes place at the same formal electrode potential, as it was observed. Gradual decrease of peak currents was observed during several initial cycles, more stable behavior was observed at prolonged cycling, as it shown in Fig. 2a. The observed high value of peak-to-peak separation (about 250 mV) indicates on the irreversible two-electron electrode process. The formal potential of redox couple E0’ was 0.570 V.
2.3. Electrochemical measurements Cyclic voltammetry experiments were performed in a three-
Fig. 1. SEM images of two types of electrodes: a - GC/PDHS, b - GC/PEDOT/PDHS.
2
Synthetic Metals 256 (2019) 116151
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Fig. 2. CVs of GC/PDHS electrodes in 0.5 M H2SO4: a – gradual stabilization of currents at v = 50 mV s−1, b – CVs of GC/PDHS at different scan rates, c – lnIp – lnv dependences, d – CVs of GC/PDHS electrodes in 0.5 M H2SO4 at different scan rates (curves are shown for 3 consecutive cycles).
formation was observed with appearance of the specific pair of peaks related to the redox process in PDHS located at approx. 0.9 V (the increase of peak currents is shown by arrows). Electrodeposited GC/PEDOT/PDHS composite electrodes were further investigated in 0.5 M H2SO4 (Fig. 4). Here, the potential scan range has been expanded compared to GC/PDHS to match the range of PEDOT electroactivity in aqueous solutions, but the positive potential limit was +0.9 V to prevent overoxidation of PDHS and PEDOT. As seen from Fig. 4a, the voltammetric responses of GC/PEDOT/PDHS electrodes combine the contribution of PEDOT film (quasi-rectangular shape of response) and the pair of peaks related to the redox transformation of quinone-hydroquinone groups of PDHS). As follows from the comparison of CVs of PEDOT and PEDOT/PDHS films, constant current values of PEDOT response may be assumed for the whole potential range and extracted from the PEDOT/PDHS composite response. It allowed us to estimate the currents originating from the redox-process of PDHS component. The specific capacitance of PDHS component in PEDOT/PDHS composite was estimated from the charge Q calculated by the integration of current peaks. Specific capacitance normalized to the mass of PDHS was found of about 54 mA h g−1. The shape of peaks associated with hydroquinone-quinone redox couple in GC/PEDOT/PDHS electrodes drastically changed in comparison with GC/PDHS electrodes. The observed peak-to-peak separation was of about 100 mV and it was less dependent on the scan rate (Fig. 4b), which indicates on more reversible kinetics of redox process in the presence of PEDOT. It is in agreement with previously obtained data on the catalytic effect of PEDOT on the redox processes of molecular forms of quinones [11,12]. The formal potential of redox couple of PDHS in PEDOT/PDHS composite was calculated (E0’ = 0.587 V) and this value is close to the formal potential of redox couple of PDHS without PEDOT (0.570 V). The shift in the formal potential of redox couple indicates that an interaction between PDHS and PEDOT may take place, probably the donor-acceptor one. The dependences of peak currents (Ip,a and Ip,с) on
Fig. 3. Electropolymerization of PEDOT/PDHS on a GC electrode from the solution of 0.05 M EDOT + 0.5 M LiClO4 + 0.3 M PDHS in PC (v = 50 mV s−1).
CVs at different scan rates are shown in Fig. 2b, the increase of peak-to peak separation at higher scan rates is observed. Good linear dependences of peak currents (Ip,a and Ip,с) on the scan rate in bilogarithmic coordinates with slope close to unity (1.05) were observed (Fig. 2c), that is consistent with irreversible surface redox-process without diffusion limitations of charge transport. It allows to conclude that electrode process in GC/PDHS film electrode proceeds without diffusion limitations at the scan rates under study. The capacity of GC/PDHS electrode calculated from the stabilized 5th CV cycle, normalized to the mass of polyquinone, was about 50 mA h g−1. The decrease of the reversible capacity of PDHS film during the first several cycles (Fig. 2a) shows that diffusion issues related to the release of PDHS are not fully inhibited. However, after several consecutive cycles the electrochemical response of GC/PDHS has become satisfactorily stable (Fig. 2d). CVs during the electrochemical synthesis of PEDOT/PDHS composite polymer films on GC electrode in potentiodynamic conditions are shown in Fig. 3. The gradual increase of currents related to PEDOT film 3
Synthetic Metals 256 (2019) 116151
D.A. Lukyanov, et al.
Fig. 4. CVs of GC/PEDOT/PDHS electrodes in 0.5 M H2SO4: a – GC/PEDOT/PDHS compared with GC/PEDOT, v = 50 mV s−1, b – CVs at different scan rates, c – lnIp–lnv dependences.
Ep = E 0 ±
RT zFv ln zF RTk 0
(3)
0
where k is apparent rate constant, α is the transfer coefficient, z is the number of charges transferred, and E0’ is the formal potential, the plus sign signifies reduction and the minus sign oxidation. The values of transfer coefficient α were obtained from the slope of linear dependence of Ep vs. ln v. The estimated values of apparent rate constant were found k0GC = 0.011 s−1 and k0GC/PEDOT = 0.27 s−1. The comparison of rate constants obtained for GC/PDHS and GC/ PEDOT/PDHS electrodes confirms the catalytic effect of PEDOT on the redox transformation of PDHS. The value of k0GC/PEDOT for GC/PEDOT/ PDHS system is much higher than value of k0GC for GC/PDHS. 4. Conclusion Poly(3,4-dihydroxystyrene) was employed to the first time as rechargeable electrode material in combination with carbon and conducting polymer. The survey of electrochemical performance of electrode materials based on poly(3,4-dihydroxystyrene) and its composite with PEDOT was performed by cyclic voltammetry. It was found that PDHS exhibits moderate electrochemical performance with specific capacity of about 50–54 mA h g−1. The comparison of rate constants obtained for GC/PDHS and GC/PEDOT/PDHS electrodes confirms the catalytic effect of conducting polymer PEDOT on the redox transformation of PDHS. However, optimization of electrode compositions and electrolytes is needed for improvement of energy storage properties of polymer composites, and it is now in progress.
Fig. 5. Normalized capacities vs. number of cycle for GC/PDHS(1) and GC/ PEDOT/PDHS(2) electrodes in 0.5 M H2SO4.
the scan rate are shown in Fig. 4c. The change of electrode capacities with number of cycles for GC/ PDHS and GC/PEDOT/PDHS electrodes is shown in Fig. 5. As seen from Fig. 5, in both cases unstable capacitive behavior of electrodes is observed at several initial cycles, but it is getting more stable at prolonged cycling. Probably, in both cases we have the slow release of oligomers from the film electrodes, which is less pronounced for GC/PEDOT/PDHS electrodes due to the inclusion (trapping) of PDHS in-the PEDOT polymer film. The apparent rate constants of redox process for GC/PDHS and GC/ PEDOT/PDHS electrodes were estimated by using the Laviron equation [15] assuming the number of electrons for each of two hydroquinonequinone redox couples in PDHS equal 2. From the scan rate dependence of peak potential (cathodic) the apparent rate constant was calculated in accord with the equation:
Acknowledgements This work was supported by Saint Petersburg State University (grant № 26455158). The authors are also thankful to the colleagues from Interdisciplinary Resource Center for Nanotechnology of SaintPetersburg State University for providing SEM analysis.
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References
[9] K. Takada, P. Gopalan, C.K. Ober, H.D. Abruna, Synthesis, characterization, and redox reactivity of novel quinone-containing polymer, Chem. Mater. 13 (2001) 2928–2932, https://doi.org/10.1021/cm010159q. [10] G.A. Snook, C. Peng, D.J. Fray, G.Z. Chen, Achieving high electrode specific capacitance with materials of low mass specific capacitance: potentiostatically grown thick micro-nanoporous PEDOT films, Electrochem. Commun. 9 (2007) 83–89, https://doi.org/10.1016/j.elecom.2006.08.037. [11] I.C. Monge-Romero, M.F. Cuarez-Herrera, Electrocatalysis of the hydroquinone/ benzoquinone redox couple at platinum electrodes covered by a thin film of poly (3,4-ethylenedioxythiophene), Synth. Met. 175 (2013) 36–41, https://doi.org/10. 1016/j.synthmet.2013.04.027. [12] S. Miao, Е.G. Tolstopyatova, V.V. Kondratiev, Redox processes involving quinines on poly-3,4-ethylenedioxythiophene-modified glassy carbon surface, Russ. J. General Chem. 89 (2019) 266–270, https://doi.org/10.1134/ S1070363219020166. [13] G. Westwood, T.N. Horton, J.J. Wilker, Simplified polymer mimics of cross-linking adhesive proteins, Macromolecules 40 (2007) 3960–3964, https://doi.org/10. 1021/ma0703002. [14] C. Nagamani, U. Viswanathan, C. Versek, M.T. Tuominen, S.M. Auerbach, S. Thayumanavan, Importance of dynamic hydrogen bonds and reorientation barriers in proton transport, Chem. Commun. 47 (2011) 6638–6640, https://doi.org/ 10.1039/C1CC11207D. [15] E. Laviron, General expression of the linear potential sweep voltammogram in the case of diffusionless electrochemical systems, J. Electroanal. Chem. 101 (1979) 19–26, https://doi.org/10.1016/S0022-0728(79)80075-3.
[1] M. Armand, J.M. Tarascon, Building better batteries, Nature 451 (2008) 652–657, https://doi.org/10.1038/451652a. [2] B. Scrosati, J. Garche, Lithium batteries: status, prospects and future, J. Power Sources 195 (2010) 2419–2430, https://doi.org/10.1016/j.jpowsour.2009.11.048. [3] S. Muench, A. Wild, C. Friebe, B. Häupler, T. Janoschka, U.S. Schubert, Polymerbased organic batteries, Chem. Rev. 116 (2016) 9438–9484, https://doi.org/10. 1021/acs.chemrev.6b00070. [4] Y. Jing, Y. Liang, S. Gheytani, Y. Yao, Cross-conjugated oligomeric quinones for high performance organic batteries, Nano Energy 37 (2017) 46–52, https://doi.org/ 10.1016/j.nanoen.2017.04.055. [5] Y. Liang, Z. Tao, J. Chen, Organic electrode materials for rechargeable lithium batteries, Adv. Energy Mater. 2 (2012) 742–769, https://doi.org/10.1002/aenm. 201100795. [6] T. Takeda, R. Taniki, A. Masuda, I. Honma, T. Akutagawa, Electron-deficient anthraquinone derivatives as cathodic material for lithium ion batteries, J. Power Sources 328 (2016) 228–234, https://doi.org/10.1016/j.jpowsour.2016.08.022. [7] K. Pirnat, G. Mali, M. Gaberscek, R. Dominko, Quinone-formaldehyde polymer as an active material in Li-ion batteries, J. Power Sources 315 (2016) 169–178, https:// doi.org/10.1016/j.jpowsour.2016.03.010. [8] M. Sterby, R. Emanuelsson, X. Huang, A. Gogoll, M. Strømme, M. Sjödin, Characterization of PEDOT-quinone conducting redox polymers for water based secondary batteries, Electrochim. Acta 235 (2017) 356–364, https://doi.org/10. 1016/j.electacta.2017.03.068.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Synthesis and electrochemical properties of poly(3,4-dihydroxystyrene) and its composites with conducting polymers
T
D.A. Lukyanov, R.V. Apraksin, A.N. Yankin, P.S. Vlasov, O.V. Levin, E.G. Tolstopjatova, ⁎ V.V. Kondratiev Saint Petersburg State University, 199034, Saint Petersburg, Russia
ARTICLE INFO
ABSTRACT
Keywords: Composite materials Conducting polymers Poly-3,4-ethylenedioxythiophene Poly(3,4-dihydroxystyrene) Cyclic voltammetry Electrochemical capacitors
The present study reports electrochemical performance of electrode materials based on poly(3,4-dihydroxystyrene) (PDHS) and its composites with conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT), deposited on glassy carbon electrodes, in diluted sulfuric acid. Poly(3,4-dihydroxystyrene), bearing redox active hydroquinone groups, was employed to the first time as an electrode material in combination with carbon, binder and conducting polymer. It was found that this quinone-based composite exhibits moderate electrochemical characteristics with specific capacity of about 50–54 mA h g−1. The comparison of rate constants obtained for GC/PDHS and GC/PEDOT/PDHS electrodes confirms the catalytic effect of conducting polymer PEDOT on the redox transformation of PDHS.
1. Introduction Inorganic metal-containing compounds are prevailing among currently available rechargeable materials for electrochemical power sources. Basically they are intercalated metal oxides or other rare-metal compounds [1,2], sources of which may be depleted in the future. The employment of organic materials was recently proposed as a promising solution for energy storage materials [3–9]. Organic compounds, containing quinone and hydroquinone moieties, were recognized as redoxactive organic materials that possess stable and reversible electrochemical characteristics and can provide high capacities. Quinones can be considered as potentially suitable components of energy storage materials for supercapacitors and metal-ion batteries [4–9]. Many low molecular quinones and hydroquinone derivatives were recently proposed as materials for batteries [7–9], however, their low conductivity and gradual dissolution in electrolytes leads to non-complete utilization of specific capacity and low cycling stability. Therefore, conductive additives, such as carbon black, or incorporation of quinones into the matrix of conducting polymer with high stability are needed to improve their performance. In this work the new polymer compound poly(3,4-dihydroxystyrene) (PDHS) carrying the hydroxyquinone moieties was synthesized and investigated as a possible component of organic electrode material for lithium-ion batteries alone and in composite with conducting polymer poly(3,4-ethylendioxithiphene) (PEDOT). PEDOT was chosen as a
⁎
component of the composite, being a material with stable pseudocapacitive response [10], and a multi-purpose conductive matrix which has also catalytic effect on the redox processes with molecular forms of quinones [11,12]. 2. Experimental 3,4-Ethylenedioxythiophene (EDOT, 98%) was purchased from Aldrich. Acetonitrile (HPLC grade) was from abcr GmbH, Germany. Propylene carbonate (PC) was from Aldrich. Conductive carbon black «Super P» was purchased from Timcal Inc. (Belgium). Sulfuric acid was from Neva Reactive Co., Russia. Sulfuric acid solutions were prepared on deionized water of resistivity not less than 18 MΩ, obtained by means of Millipore Direct-Q UV. 2.1. Synthesis of poly(3,4-dihydroxystyrene) Poly(3,4-dihydroxystyrene) (PDHS) was obtained from veratraldehyde using the following reaction sequence (1). The 3,4-dimethoxystyrene obtained from the former aldehyde using methylenephosphoniumylide was polymerized in DMF with an azobisisobutyronitrile (AIBN) as an initiating agent, affording the poly (3,4-dimethoxystyrene) in 81% yield with M w =130,000 g/mol and Đ = 1.64 and with NMR data acquired in agreement with literature [13]. Poly(3,4-dimethoxystyrene) was subjected to the demethylation
Corresponding author. E-mail address: [email protected] (V.V. Kondratiev).
https://doi.org/10.1016/j.synthmet.2019.116151 Received 10 June 2019; Received in revised form 23 July 2019; Accepted 17 August 2019 Available online 28 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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with BBr3, giving the desired product, poly(3,4-dihydroxystyrene). Poly (3,4-dimethoxystyrene) (1.71 g, 10.5 mmol) was dissolved in dry CH2Cl2 (50 ml). BBr3 (3.2 ml, 8.5 g, 35 mmol) was added dropwise to a vigorously stirred ice-cooled solution via syringe under Ar. Mixture was allowed to warm to RT, shaken for 6 h and then stirred overnight. Mixture was poured in 0.1 M HCl (200 ml), stirred for 20 min and filtered. Solid was washed with 0.1 M HCl for 3 times (20 min each washing), washed with water and dried in vacuo at 50 °C. Precipitation of the product from 1,4-dioxane with CH2Cl2, followed by drying in vacuo at 50 °C, gave the desired product (1.41 g, 10.5 mmol, 100%) as a creamy solid. The obtained product was studied by NMR and FTIR spectroscopy. The following data, confirming the chemical nature of the product, were obtained: 1H NMR (400 MHz, DMSO-d6) δ ppm: 8.32 (s, 2 H), 6.65-5.45 (m, 3 H), 2.15-0.60 (m, 3 H) (lit. [14]: 8.36 (s, 2 H), 6.75.5 (bm, 3 H), 2.2-0.5 (bd, 3 H)); FTIR (KBr) ῦ cm−1: 3433, 1608, 1517, 1444, 1368, 1279, 1187, 1107.
electrode cell using an AUTOLAB PGSTAT-302 potentiostat. The potential scan rates were varied within range of 5–500 mV s−1. A glassy carbon (GC) electrode (0.07 cm2) was used as the working electrode, Pt foil was used as an auxiliary electrode. The potential values were measured relative to Ag/AgCl reference electrode. The measurements were performed in aqueous solution of 0.5 M H2SO4 at room temperature (20 ± 2 °C). 2.4. Material characterization 1 H NMR measurements were performed on a Bruker-Avance 400 instrument at ambient temperature. FTIR spectra were recorded on Shimadzu IRAffinity-1 spectrometer using KBr pellets. The morphology of films was characterized by SEM at accelerating voltage of 21 kV on an Carl Zeiss SUPRA 40 VP instrument.
(1) 2.2. Electrode preparation
3. Results and discussion
As PDHS has low conductivity, conductive additives (carbon black, PEDOT) were necessary for its testing as a rechargeable electrode material. Two types of electrodes were deposited on glassy carbon (GC) substrate: GC/PDHS and GC/PEDOT/PDHS. GC/PDHS electrodes were prepared by drop-casting acetonitrile dispersion containing 50 wt% of PDHS as active material and 50 wt % of carbon black (CB) as a conductive additive onto GC electrodes and then dried under mild heating (up to 90 °C). GC/PEDOT/PDHS composite electrodes were obtained by electrochemical synthesis. PEDOT/PDHS composite films were deposited on GC electrode from the solution of 0.05 M EDOT + 0.5 M LiClO4 + 0.3 M PDHS in propylene carbonate (PC) in potentiodynamic conditions in the potential range from -0.3 to 1.4 V with a potential scan rate of 50 mV s−1. The SEM images of two types of electrodes exhibited remarkably different morphologies (Fig. 1). More dense, compact layer was observed for GC/PDHS electrode, whereas for GC/PEDOT/PDHS electrode porous globular structure can be seen.
Comparative study of redox processes in PDHS was conducted on GC/PDHS and GC/PEDOT/PDHS electrodes in dilute sulfuric acid, which is the most frequently used acidic electrolyte for testing of quinone-based electrode materials. Typical cyclic voltammograms (CVs) of GC/PDHS electrodes in solution of 0.5 M H2SO4 are shown in Fig. 2a,b. Well-defined couple of redox peaks at GC/PDHS electrodes associated with hydroquinonequinone redox transformation (reaction 2) is observed.
(2) The single couple of symmetrical redox peaks on CV for polymer film with many repeated redox-centers implies that multielectron transfer takes place at the same formal electrode potential, as it was observed. Gradual decrease of peak currents was observed during several initial cycles, more stable behavior was observed at prolonged cycling, as it shown in Fig. 2a. The observed high value of peak-to-peak separation (about 250 mV) indicates on the irreversible two-electron electrode process. The formal potential of redox couple E0’ was 0.570 V.
2.3. Electrochemical measurements Cyclic voltammetry experiments were performed in a three-
Fig. 1. SEM images of two types of electrodes: a - GC/PDHS, b - GC/PEDOT/PDHS.
2
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D.A. Lukyanov, et al.
Fig. 2. CVs of GC/PDHS electrodes in 0.5 M H2SO4: a – gradual stabilization of currents at v = 50 mV s−1, b – CVs of GC/PDHS at different scan rates, c – lnIp – lnv dependences, d – CVs of GC/PDHS electrodes in 0.5 M H2SO4 at different scan rates (curves are shown for 3 consecutive cycles).
formation was observed with appearance of the specific pair of peaks related to the redox process in PDHS located at approx. 0.9 V (the increase of peak currents is shown by arrows). Electrodeposited GC/PEDOT/PDHS composite electrodes were further investigated in 0.5 M H2SO4 (Fig. 4). Here, the potential scan range has been expanded compared to GC/PDHS to match the range of PEDOT electroactivity in aqueous solutions, but the positive potential limit was +0.9 V to prevent overoxidation of PDHS and PEDOT. As seen from Fig. 4a, the voltammetric responses of GC/PEDOT/PDHS electrodes combine the contribution of PEDOT film (quasi-rectangular shape of response) and the pair of peaks related to the redox transformation of quinone-hydroquinone groups of PDHS). As follows from the comparison of CVs of PEDOT and PEDOT/PDHS films, constant current values of PEDOT response may be assumed for the whole potential range and extracted from the PEDOT/PDHS composite response. It allowed us to estimate the currents originating from the redox-process of PDHS component. The specific capacitance of PDHS component in PEDOT/PDHS composite was estimated from the charge Q calculated by the integration of current peaks. Specific capacitance normalized to the mass of PDHS was found of about 54 mA h g−1. The shape of peaks associated with hydroquinone-quinone redox couple in GC/PEDOT/PDHS electrodes drastically changed in comparison with GC/PDHS electrodes. The observed peak-to-peak separation was of about 100 mV and it was less dependent on the scan rate (Fig. 4b), which indicates on more reversible kinetics of redox process in the presence of PEDOT. It is in agreement with previously obtained data on the catalytic effect of PEDOT on the redox processes of molecular forms of quinones [11,12]. The formal potential of redox couple of PDHS in PEDOT/PDHS composite was calculated (E0’ = 0.587 V) and this value is close to the formal potential of redox couple of PDHS without PEDOT (0.570 V). The shift in the formal potential of redox couple indicates that an interaction between PDHS and PEDOT may take place, probably the donor-acceptor one. The dependences of peak currents (Ip,a and Ip,с) on
Fig. 3. Electropolymerization of PEDOT/PDHS on a GC electrode from the solution of 0.05 M EDOT + 0.5 M LiClO4 + 0.3 M PDHS in PC (v = 50 mV s−1).
CVs at different scan rates are shown in Fig. 2b, the increase of peak-to peak separation at higher scan rates is observed. Good linear dependences of peak currents (Ip,a and Ip,с) on the scan rate in bilogarithmic coordinates with slope close to unity (1.05) were observed (Fig. 2c), that is consistent with irreversible surface redox-process without diffusion limitations of charge transport. It allows to conclude that electrode process in GC/PDHS film electrode proceeds without diffusion limitations at the scan rates under study. The capacity of GC/PDHS electrode calculated from the stabilized 5th CV cycle, normalized to the mass of polyquinone, was about 50 mA h g−1. The decrease of the reversible capacity of PDHS film during the first several cycles (Fig. 2a) shows that diffusion issues related to the release of PDHS are not fully inhibited. However, after several consecutive cycles the electrochemical response of GC/PDHS has become satisfactorily stable (Fig. 2d). CVs during the electrochemical synthesis of PEDOT/PDHS composite polymer films on GC electrode in potentiodynamic conditions are shown in Fig. 3. The gradual increase of currents related to PEDOT film 3
Synthetic Metals 256 (2019) 116151
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Fig. 4. CVs of GC/PEDOT/PDHS electrodes in 0.5 M H2SO4: a – GC/PEDOT/PDHS compared with GC/PEDOT, v = 50 mV s−1, b – CVs at different scan rates, c – lnIp–lnv dependences.
Ep = E 0 ±
RT zFv ln zF RTk 0
(3)
0
where k is apparent rate constant, α is the transfer coefficient, z is the number of charges transferred, and E0’ is the formal potential, the plus sign signifies reduction and the minus sign oxidation. The values of transfer coefficient α were obtained from the slope of linear dependence of Ep vs. ln v. The estimated values of apparent rate constant were found k0GC = 0.011 s−1 and k0GC/PEDOT = 0.27 s−1. The comparison of rate constants obtained for GC/PDHS and GC/ PEDOT/PDHS electrodes confirms the catalytic effect of PEDOT on the redox transformation of PDHS. The value of k0GC/PEDOT for GC/PEDOT/ PDHS system is much higher than value of k0GC for GC/PDHS. 4. Conclusion Poly(3,4-dihydroxystyrene) was employed to the first time as rechargeable electrode material in combination with carbon and conducting polymer. The survey of electrochemical performance of electrode materials based on poly(3,4-dihydroxystyrene) and its composite with PEDOT was performed by cyclic voltammetry. It was found that PDHS exhibits moderate electrochemical performance with specific capacity of about 50–54 mA h g−1. The comparison of rate constants obtained for GC/PDHS and GC/PEDOT/PDHS electrodes confirms the catalytic effect of conducting polymer PEDOT on the redox transformation of PDHS. However, optimization of electrode compositions and electrolytes is needed for improvement of energy storage properties of polymer composites, and it is now in progress.
Fig. 5. Normalized capacities vs. number of cycle for GC/PDHS(1) and GC/ PEDOT/PDHS(2) electrodes in 0.5 M H2SO4.
the scan rate are shown in Fig. 4c. The change of electrode capacities with number of cycles for GC/ PDHS and GC/PEDOT/PDHS electrodes is shown in Fig. 5. As seen from Fig. 5, in both cases unstable capacitive behavior of electrodes is observed at several initial cycles, but it is getting more stable at prolonged cycling. Probably, in both cases we have the slow release of oligomers from the film electrodes, which is less pronounced for GC/PEDOT/PDHS electrodes due to the inclusion (trapping) of PDHS in-the PEDOT polymer film. The apparent rate constants of redox process for GC/PDHS and GC/ PEDOT/PDHS electrodes were estimated by using the Laviron equation [15] assuming the number of electrons for each of two hydroquinonequinone redox couples in PDHS equal 2. From the scan rate dependence of peak potential (cathodic) the apparent rate constant was calculated in accord with the equation:
Acknowledgements This work was supported by Saint Petersburg State University (grant № 26455158). The authors are also thankful to the colleagues from Interdisciplinary Resource Center for Nanotechnology of SaintPetersburg State University for providing SEM analysis.
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