Electrochemical study of electroactive poly (5-hydroxy-1,4-naphthoquinone-co-5-hydroxy-3-acetic acid-1,4-naphthoquinone) by impedance spectroscopy

Journal of Chemistry, Vol. 42 (4), P. 497 - 500, 2004<br /> <br /> <br /> <br /> ELECTROCHEMICAL STUDY OF CONDUCTING ELECTROACTIVE<br /> POLYMER POLY(5-HYDROXY-1,4-NAPHTHOQUINONE -co- 5-<br /> HYDROXY-3-ACETIC ACID-1,4-NAPHTHOQUINONE) BY<br /> ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY<br /> Received 3rd -Sept.- 2003<br /> TRAN §AI LAM<br /> Faculty of Chemical Technology, Hanoi University of Technology<br /> <br /> SUMMARY<br /> The conducting polymer poly(5-hydroxy-1,4-naphthoquinone-co-5-hydroxy-3-acetic acid-1,4-<br /> naphthoquinone) with electroactive quinone group was investigated by Electrochemical<br /> Impedance Spectroscopy (EIS) in aqueous medium. The evaluations of parameters of equivalent<br /> circuit in term of applied potential were considered. The simulated EIS data presented a good<br /> agreement with the results obtained by other methods such as Cyclic and Differential Pulse<br /> Voltammetries.<br /> <br /> I - INTRODUCTION frequently used is Randles circuit [3].<br /> For an ECP the behavior in the high<br /> Electrochemical methods such as Cyclic<br /> frequency (HF) and in the low frequency (LF)<br /> Voltammetry (CV), Differential Pulse Voltam-<br /> regions are similar to that of simple redox<br /> metry (DPV), Square Wave Voltammetry<br /> system. However, when the frequency is<br /> (SWV), through large perturbations (by<br /> decreased to very low values (~ mHz) the<br /> imposing potential steps or potential sweeps) on<br /> diffusion will become limited in favor of a<br /> the studied system, lead the system to the state<br /> charge accumulation into the polymer. The<br /> far from equilibrium. The obtained response is<br /> impedance approaches a purely capacitive<br /> normally transient. The principle of EIS is to<br /> introduce a small perturbation by the means of response with a phase angle of /2. This<br /> sinusoidal signal of small amplitude in order to limiting capacitance CL is associated with<br /> obtain the response in quasi- stationary state. limiting resistance RL in the following equation:<br /> This method has many advantages, among them CL= d2/ 3D.RL (d - film's thickness, D - diffusion<br /> the most important one is the ability to have coefficient).<br /> precise measurements because the response may Modified Randles equivalent circuit for ECP<br /> be indefinitely steady and the ability to<br /> distinguish processes that may occur in one The model developed by Ho et al. [4] and<br /> reaction, especially when their time constants independently by Glarum [5] for thin films of<br /> are well separated [1, 2]. intercalation materials, in which lithium<br /> injection and diffusion occur, was successfully<br /> Randles equivalent circuit for electronically used to analyze the impedance response of ECP,<br /> conducting polymers (ECP) in which the doping/dedoping processes involve<br /> In general, an electrochemical cell can be injection-removal of electrons in the polymer<br /> represented in terms of an equivalent circuit. A film balanced by counter ion diffusion from/to<br /> <br /> 497<br /> electrolyte. A more advanced model for ECP NaCl; 0.0027 M KCl; 0.0081 M Na2HPO4;<br /> has been suggested by Albery [6], Mathias and 0.00147 M KH2PO4, pH 7.4) from Dulbecco.<br /> Haas [7], who considered concurrent ionic and AC impedance measurements were<br /> electronic transport in the system. One can performed in aqueous buffer PBS, using a tree-<br /> imagine the impedance as a microstructure electrode cell, with frequency response analyzer<br /> composed of pores and cavities, represented by FRA of AUTOLAB. The reference was<br /> a transmission line. saturated calomel electrode (SCE), the auxiliary<br /> In present report, we will use Vorotynsev- was Pt sheet. The working electrode was carbon<br /> Deslouis model, initially developed for the disk (Tokai), sealed in Teflon (S = 0.07 cm2).<br /> study of metal protection by ECP [8]. In our These electrodes were potentiostatically charged<br /> case we can assume Rp and the circuit can for 120 s before the impedance measurements.<br /> be shown in figure 1. The amplitude is 10 mV, the frequency is from<br /> 50 kHz to 10 mHz, 5 points of frequency per<br /> Resistance Electrolyte/Film<br /> Film decade. The data were fitted with Vorotynsev-<br /> Electrolyte Interface Deslouis model by Simplex algorithm [10].<br /> Cef Cd<br /> III - RESULTS AND DISCUSSION<br /> Re<br /> 1. EIS spectra<br /> Ref Rt ZW<br /> Typical EIS results of polymer film at<br /> different potentials, corresponding to the<br /> Figure 1: Vorotynsev-Deslouis model used in<br /> electroactive domain of quinone group (from 0<br /> this study for simulation of experimental EIS<br /> to -0.8V vs. SCE) with potential step of 0.1 V,<br /> data<br /> are presented in figure 2.<br /> In this model, Re is electrolyte resistance; The EIS spectra comprise 3 parties. A<br /> Cef and Ref are the capacity of space charge capacitive component at high frequencies<br /> between electrolyte and polymer film and the followed by poorly defined diffusion process at<br /> charge transfer resistance associated with low frequencies. At very low frequencies, a<br /> charge insertion due to doping/dedoping au deviation from straight line is observed,<br /> processes on electrolyte-film interface. Cd and possibly due to non-uniform film thickness.<br /> Rt are double layer capacity and charge transfer Representation in Bode plot (logZ-logf) clearly<br /> resistance of the polymer film. W is Warburg demonstrated 3 these domains.<br /> diffusion impedance of the polymer film with<br /> the effect of charge saturation, which strongly The values of the circuit elements in<br /> depends on the film. function of applied potentials are summarized in<br /> table 1.<br /> II - EXPERIMENT We will try to consider the variations of<br /> these parameters with applied potentials. Firstly,<br /> 5-hydroxy-1,4-naphthoquinone (JUG) from the capacitance Ce/f, corresponding to charge<br /> Fluka (98%) was used as received. 5-hydroxy- accumulation on electrolyte-film interface<br /> 3-acetic acid-1,4-naphthoquinone (JUGA) was increases with increased potentials (in oxidation<br /> synthesized in one step from JUG and cycle) (Fig. 3a). This tendency can be explained<br /> thioglycolic acid (from Acros). A detailed if the decrease in diffusion layer formed<br /> procedure has been described elsewhere [9]. between the counterions and oxidized polymer<br /> Poly (JUG-co-JUGA) was electro-synthesized film is taken into account. Secondly, double<br /> by CV, on Autolab, model PGSTAT 30 (50 layer capacitance Cd of the polymer film goes<br /> cycles, potential domain 0.40 - 1.05 V vs.SCE, through a maximum around –0.4 V, which is<br /> scan rate 50 mV.s-1). the potential, corresponding to the current peak,<br /> Aqueous solution buffer is PBS (0.137 M observed in CV’s method (Fig. 3b). Thirdly, we<br /> <br /> 498<br />  1.1 Hz 220<br /> 120<br /> 1.1 Hz<br /> 200<br /> 1.1 Hz 1.1 Hz<br /> 100 180<br /> <br /> 160 1.1 Hz<br /> 80 1.1 Hz 140<br /> 1.1 Hz<br /> 1.1 Hz 120<br /> <br /> <br /> <br /> <br /> Z "( .cm 2)<br /> Z"( .cm 2)<br /> <br /> <br /> <br /> <br /> 60<br /> 100<br /> <br /> 80<br /> 40<br /> 0V 60<br /> -0.5V<br /> -0.2 V<br /> 20 236 Hz -0.3 V 40 -0.6V<br /> -0.4 V 236Hz -0.7V<br /> 50k 20 50k -0.8V<br /> 0 0<br /> 0 20 40 60 80 100 120<br /> 0 20 40 60 80 100 120 140 160 180 200 220<br /> 2<br /> Z'( .cm ) 2<br /> Z'( .cm )<br /> a) b)<br /> Figure 2: EIS spectra in Nyquist plot for poly(JUG-co-JUGA) film in PBS solution at different<br /> potentials: a) from 0 to -0.4V; b) from -0.5 to -0.8 V (vs.SCE)<br /> <br /> Table 1: Values of parameters of equivalent circuit in function of applied potentials<br /> <br /> E, V/SCE Re/f, .cm2 Ce/f, F.cm-2 Cd, F.cm-2 Dd×10-10, cm2.s-1 Rt, .cm2<br /> -0.7 V 26.0 0.8 8 2.3 194.8<br /> -0.6 V 29.0 1.0 12 2.5A 185.1<br /> -0.5 V 28.0 1.2 16 2.5 151.1<br /> -0.4 V 31.6 1.6 50 2.0 204.2<br /> -0.3 V 30.6 1.7 22.4 1.7 225.6<br /> -0.2 V 34.0 2.0 30 1.6 251.7<br /> <br /> note that diffusion coefficient Dd depends Q•- may exchange electrons at electrode-film<br /> weakly on potentials: with the film thickness of interface:<br /> ca. 100 nm (estimated by Scanning Electron<br /> kf<br /> Microscopy), Dd is approximately of 2.10-10 Q+ e Q<br /> cm2.s-1. Finally, the variations of charge transfer<br /> resistance in both oxidation (Fig. 4a) and kb<br /> reduction (Fig. 4b) cycles go through a The faradic current is calculated from the<br /> minimum at -0.5 V and -0.4 V, to which following equation:<br /> correspond perfectly the current peaks, obtained IF = F.A.(kf×CQ kb×CQ•-)<br /> in CV’s. Where IF is faradic current; F is Faraday<br /> The behavior of charge transfer resistance constant, A is surface area of electrode, kf and kb<br /> can be explained by a simple model, initially are the forward and backward reaction rates<br /> developed by Gabrielli [11] for reversible redox respectively, which follow the Tafel law: kf = k0f<br /> system. Effectively, in electroactive domain of exp(bfE); kb = k0b exp(-bbE), bf and bb are the<br /> quinone groups, the electroactive species Q and Tafel coefficients: bf = aF/RT and bb = (1-<br /> <br /> 499<br />  a)F/RT), CQ, CQ•-: the concentrations of Q and Q•-).<br /> 2.5<br /> 50<br /> <br /> <br /> <br /> 2.0 40<br /> Ce/f( F.cm )<br /> <br /> <br /> <br /> <br /> 30<br /> -2<br /> <br /> <br /> <br /> <br /> Cd ( F.cm )<br /> -2<br /> 1.5<br /> 20<br /> <br /> <br /> <br /> 1.0 10<br /> <br /> E<br /> E 0<br /> <br /> 0.5<br /> -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2<br /> <br /> E/ V(SCE) a) E/ V(SCE) b)<br /> Figure 3: Evaluation of capacities in oxidation cycle: a) Ce/f ; b) Cd<br /> 260<br /> 340 Rt<br /> Rt<br /> 240 320<br /> <br /> <br /> 220 300<br /> R ( .cm2)<br /> <br /> <br /> <br /> <br /> 280<br /> R ( .cm2)<br /> <br /> <br /> <br /> <br /> 200<br /> <br /> 260<br /> 180<br /> 240<br /> <br /> 160 E<br /> E 220<br /> <br /> <br /> 140 200<br /> -0.8 -0.7 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0 -0.6 -0.5 -0.4 -0.3 -0.2 -0.1 0.0<br /> E/V (SCE) a) E/V (SCE) b)<br /> Figure 4: Evaluation of charge transfer resistance in function of potential:<br /> a) in oxidation cycle ; b) in reduction cycle<br /> <br /> When EIS measurements are performed, the<br /> 1 1<br /> electrode potential is perturbed by a small Hence: Rt = K. + . Charge transfer<br /> sinusoidal amplitude ( E.exp(j t)). The k f kb<br /> corresponding faradic current is equal to: resistance evaluation in function of potential<br /> leads to investigate the following equation:<br /> IF = R . E + IF = R . E + F.A.(kf×CQ<br /> t<br /> -1<br /> t<br /> -1<br /> <br /> kb×CQ•-) dRt<br /> = K(-kf0.E.bf.exp(bfE) + kb0.E.bb.exp(-bbE))<br /> Where charge transfer resistance is determined dE<br /> by the following equation: With the extreme conditions: E + , kf<br /> 1<br /> Rt-1 = F.A.(bf×kf×CQ + bb×kb×CQ•-) = + Rt ;E kb + Rt<br /> kb<br /> F 2 . A k f .kb 1<br /> .C* (with C* = CQ + CQ•-)<br /> RT k f + kb kf<br /> <br /> 500<br /> i.e. Rt = f(E) pass through a minimum. 4. C. Ho, I. D. Raistrick, R.A. Huggins, J.<br /> Electrochem. Soc., Vol. 127, No. 2, P. 343<br /> IV - CONCLUSION (1980).<br /> 5. S. H. Glarum, J. H. Marshall. J.<br /> In this study, AC impedance was used for Electrochem. 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