EIS ANALYSIS ON THE ANODIC PROCESS OF ZINC IN AN ALKALINE SOLUTION

ACTA METALLURGICA SINICA (ENGLISH LETTERS)Vol.17 No.6 pp 912-922 December 2004EIS ANALYSIS ON THE ANODIC PROCESS OF ZINC IN ANALKALINE SOLUTIONY. Zheng", J.M. Wang", H. Chen", J.Q. Zhang'.2) and C.N. Cao'.2)1) Department of Chemistry, Zhejiang University, Hangzhou 310027, China2) Chinese State Key Laboratory for Corrosion and Protection, Instute of Metal Reseach, The ChineseAcademy of Sciences, Shenyang 110016, ChinaManuscript received 28 August 2003The EIS on the anodic process of pure Zn in an alkaline solution were performed and fittedusing the expression of Faradic admittance, based on which the mechanism of the processwas proposed. The results showed that besides electrode potential E the electrochemicalreaction rate was affected by the adsorption of Zn(OH)as on the electrode surface and thediffusion of in the electrolyte in active region, and only by the covering of passivation layeron the electrode surface in both transitive and passivation regions. The equivalent circuitsapplied in these various circumstances were proposed and the variation of some parametersand state variables was also discussed.KEY WORDS Zn, alkaline solution, anodic process, EIS1. IntroductionDue to its low equilibrium potential, good reversibility, high volumetric energy density, low costand low toxicity, zinc has a wide variety of applications as a negative electrode material in Zn-Mn,Zn-Ag, Zn-Ni and Zn-air batteriesl-19. Increasing efforts have recently been devoted to the developmentof secondary zinc batterislo), which makes it a promising candidate in electric vehicle applications, aswell as in military and commercial aircraft.However, these secondary zinc based cells are usually limited in the performance and widespreadcommercialization by the poor cycling characteristics, which is mainly resulted from the high solubilityof the zinc discharge products in KOH electrolyte(912 2127. Many fundamental researches suggested thatthe specific power and capacity of both primary and secondary alkaline Zn batteries are determined bythe characteristics of Zn electrodissolution, and the kinetics and mechanisms of active electrodissolutionand passivation of Zn have been studied by several groupsl283). So knowledge of the anodic behavior ofZn electrode in alkaline solution is required for optimizing the electrode design.Electrochemical impedance spectroscopy (EIS) has frequently been applied in the investigation ofelectrode (interface reaction) or transport processes as well as in fields like characterization of materialsand corrosion sciencelB639. One advantage of this technique is the possibility to distinguish relaxationprocesses with different time constants taking place on the surface. During recent decades，electro-chemists tried to integrate impedance spectra measured wi中国煤化工-of electrode processbesides the common means of equivalent circuit. Theadmittance for theelectrode processes whose rates depend upon electrode purmcs。cin orTUiuiHCNMHther state variablesvarying with E was derived based on the stability condition under potentiostatic control and the913characteristics of linear systems by Caol4041. Therefore, considering that most investigations have beencarried out in concentrated alkaline solutions (7- 10mol/L KOH), for the low equilibrium potential of Znelectrode and the high ionic conductivity of the electrolyte solution under these conditions, in the presentwork a possible reaction model describing the anodic processes of zinc in a less concentrated alkalinesolution (4mol/L KOH), which is a electrolyte close to that used in the secondary zinc-based batteries, isproposed by analyzing the experimental impedance spectra using the above-mentioned theory.2. ExperimentalElectrochemical measurements were taken in a classical three-electrode glass cell. The workingelectrode was made of pure zinc rods，10mm in diameter. Before each test the electrode surface waspolished by 1500# waterproof abrasive paper, degreased ia acetone and rinsed in deionized water,followed by immediate immersion into the test solution for measurement. The counter electrode was aplatinum foil and the reference electrode a Hg/HgO electrode. The measuring system consisting of apotentiostat (EG&G model 273) and a lock-in amplifier (model 5210) was controlled by a micro-computer with certain software. EIS were obtained in the frequency range from 120kHz to 0.005Hz andwith the a.c. signal amplitude of 5mV. Each measurement was taken until the electrode reaching a steadystate. The experiments were conducted at a constant temperature of25+1°C.3. Results and Discussions3.I The anodic polarization curveThe typical anodic polarization curve of zinc electrode in 4mol/L KOH is presented in Fig1. It can beseen that the current for the electrodissolution-200reaction shows at first an increase withincreasing anodic potential (active region) and.150then a dramaticdecrease with furtherpolarization (ransitive region) followed by aslow increase again with the continuousN -100increase of anodic potential (passivationregion). A simple theory to account for thisbehaviour assumes that the electrodissolutioncurrent decreases because of the gradual-1-0.80.6formation of passivation layer on the elec-EV vs. Hg/HgOtrode surface. The passivation occurs whenFig.1 Anodic polarization curve of Zn in 4mol/L KOH at 25CAE>200mV (AE= E -Eoc), and when(scaning rate: 0. lmV/s).AE> 350mV the rate of electrodissolutionreaction increases slowly again. Therefore, it can be determined that there is different reaction mechanismin various anodic regions, which will be discussed later by the interpretation of the EIS obtained.3.2 The impedance spectra in active region中国煤化工The EIS of the Zn electrode were performed in the .MYHCNMHGndsomeofthemaredisplayed in Fig.2. These EIS consist of three parts: (i) a high-frequency capacitive loop representing thecharge transfer resistance (R) in parallel with the double layer capacitance (Ca); (i) a middl-frequency914.7 r0.6 r- Fiting resultsb)|0.6-■Experimental data0.5 t.5-.4 t.4-.3-0.2N 0.2-0.1 t21-.o0).0 F0.46.80. 0.0)2.4.6之. Qcmz, ocm?0.30(C)d)|0.250.20.2-0.10N 0.10..0-000.00.10.3-0.05.0.2z, a cm?之, a cm?Fig.2 Some eis of Zn electrode in the range of OE: (a) OE= -40mV; (b) OE=80mV; () OE=100mV; (d)AE=1 60mV.inductive loop corresponding to the adsorption ofOH species on Zn'"; (ii) a low-frequency portion typicalfor a finite layer diffusion processt2. The characteristics of Zn electrode here can be represented by theequivalent circuit shown in Fig3, where R。is the total ohmic resistance of the solution, Ca the double layercapacity, R, the charge-transfer resistance of the electrode,RL and L are the equivalent componentscorresponding with the covering degree of OH species on the electrode surface, and Ws is the finite layerdiffusion impedance. Nevertheless, the EIS of Zn electrode at△E= =200mV shows no middl-frequencyinductive loop but another unobvious capacitive loop instead, as shown in Fig4, and the equivalent circuitused here is revised to the one shown in Fig.5 correspondingly, where R。and C, are the equivalent com-ponents corresponding with the covering degree of species on the electrode surface. All these EIS can befitted well using these two circuits respectively.According to Cao's theoryf94l, without taking the diffusion impedance into account the general expres-sion of the Faradaic admittance for the electrode process whose rate depends upon electrode potential E andanother state variable X varing with E isBYp=R a+中国煤化工THCNMHGwhere(alpR,"(JE'915Hi=a=(3R、axE=_dXdtRB=mb=(dIE)。dE.)ssFig.3 Equivalent circuit for Zn electrode inEEthe range of AE = 40-160mV.0.5j=√-1.. Fiting resultssE=200mV■Experimental data0.The subscript‘ ss’denotes steady state and w isthe circular frequency. Defning T=1/a , named timeconstant, with dimension of sec, Eq.(1) can be changeda‘0.2 t。to the following form”0.dX0Y=(2)R 1+ jo/a~ R1+ jOT .0.10.2 0.30.50.6=8o+1+ jorz, acm2Fig.4 EIS of Zn electrode at OE=200mV.where go=1/R: ，which means the electrochemicalreaction conductance on unit area with dimension of!cQ:*xcm2 or Sxcm', andR，dX_ dE、dX(3)dEwhich has the same dimension with go- As the values ofR 4Fthe equivalent elements can be obtained through ftting,Gsome parameters can be calculated with thoseFig.5 Equivalent circuit for Zn electrode atequations above. Some results are tabulated in Table 1,△E =200mV.which shows that T>0，i.e. a>0， which fulflls thecondition of stabilityl04z2. The continuous decrease of R; means that the resistance of the electrochemicalreaction decreases, which is indicated by the dramatic increase of electrodissolution reaction current inactive region.According to the interpretation of EIS the electrodissolution mechanism of Zn before the happeningof passivation in alkaline solutions is considered probably asZn+OH~口Zn(O1中国煤化工(4)Zn(OH)ads +OH~ - →ZnTY.HCNMHG(5)Zn(OH)2ads + 20H~口Zn(OH)2(6)916Table 1 Some parameters obtained by EIS ftting in active regionE, mVR。n .cm2r,sgo 02.cm2g, s-.cm2400.2410.05144.1491.705800.2230.02774.4923.6601000.1650.01226.0763.2571600.1110.03239.0217.0132000.02210.027245.303-30.213among which the rate- determining step is the electrochemical reaction (5). Therefore, in the presentwork the state variable X is the covering rate of the intermediate Zn(OH)xs on the electrode surface (0),and the time constant l means the transformation time needed for one Zn(OH)as molecule on unit surfacesite. Noting that the electrochemical reaction ratev=do, from Eq.(3) we obtaindta d6dO)v、ddθ8=(a)sdE= F(l=-(_dt>)dE(7)'s dEa 7ss dEFrom Table 1，it can be seen that g>0 in the range of△E=40- 160mV, indicating that θ increaseswith the increase of the anodic polarization potential E according to Eq.(7). This phenomenon can beexplained by the continuous accumulation and adsorption of Zn(OH)as on electrode surface during theanodic polarization for Eq.(5) is the r.d.s. of the whole anodic process. However, the value of g turns intoa negative number and the middle-frequency inductive loop in the EIS turns into a capacitive loopcorrespondingly at△E=200mV, indicating that under a relatively high potential in active region θ beginsto decrease dramatically. The reason may be that under a relatively strong polarization the consumptionof electrolyte makes the OH - near electrode surface insufficient for the reaction to form Zn (OH)asconveniently. In this case，the adsorption bond between metal atom and transforms to chemical bondand the major electrochemical reaction product is inclined to be the compound molecule of thepassivation layer for requiring less OH . The decrease of 0 indicates that the Zn electrode has the trendto be passivated OE> >200mV.Furthermore, with the increase of E the time constant τ decreases at the beginning and thenincreases. Such a variation can be explained by the similar variation of 0 with E discussed above be-cause the consumption rate of Zn(OH)a$s is decided by the covering degree θ of Zn(OH)x on the elec-trode surface according to Eq.(5).3.3 The impedance spectra in transitive regionThe EIS of the Zn electrode were also performed中国煤化工20mV and some ofthem are displayed in Fig.6. Compared with those in activ:M.HCN MH G in transitive regionshows great difference. The EIS consist of two parts here:" a uny capaciuve Io0p still representing R: inparallel with Ca and another large capacitive loop corresponding to the covering of the passivation layeron the electrode surface. Under these potentials there is no impedance representing the diffusion process9175a)3-&2飞2NNi,一- Ftting resultsExperimental data02.43-2-1z, 2 cm2z.2 cm2(d)0.820 I0.615 t5 0.410Nj0.25F0.-0.8-0.6-0.4-0.2-15-10-5z,。cm2z之, a cm2Fig.6 Some eis of Zn electrode in the range of DE: (a) OE=250mV; (b) 0E=280mV; (C) AE=300mV; (d)AE=320mV.appeared in the frequency range tested because most of the current is consumed for the formation of passivation layer here, which decreases the electrodissolution rate of Zn and reduces the influence on thewhole electrochemical process by diffusion of OH. The EIS displayed in Fig.6 can be fitted well usingthe equivalent circuit shown in Fig.7, where R。and Cs are the equivalent components corresponding withthe covering degree 02 of the passivation layer on the electrode surface. Some ftting results are tabulatedin Table 2. The increase of R means that the resistance of the electrochemical reaction increases, whichis indicated by the dramatic decrease of electrodissolution reaction current in transitive region.In Fig.6 the low frequency parts of some spectroscopy are appeared in the second quadrant. Such aphenomenon means that the polarization resistance Rp defined as RF= (Z)mo，varies from positive tonegative with the increase of the potential E. A negative R, means that the equivalent resistance Rs isnegative too4o, which has been proved reasonable for it fulfills the condition of stabilityl42I. The data ofR。obtained by ftting is also listed in Table 2 as a proof.In this case, Eq.(7) should be changed to the following form because v=d02中国煤化工aIp、 d6dvdθ2MHCNMHG10,= F(一山4=-F.a8)8=o,'s dE”“882)ss dEdE918.From Table 2 it can be seen that g 200mV.(2) In transitive region, the electrochemical reaction rate is not affected by the diffusion of in theelectrolyte again but the covering of passivation layer on the electrode surface. The increase of R: meansthat the resistance of the electrochemical reaction increases, which is indicated by the dramatic decreaseof electrodissolution reaction current. The covering degree of passivation layer on the electrode surfaceincreases with the anodic polarization in the range of△E= 250 - -320mV，and its growth rate increasesrapidly at the initial stage and progressively decreases afterwardsnder 9 ralatively high potential(about 350mV in the present work) the EIS shows a special中国煤化工both ininties.(3) In passivation region, the electrochemical reJYHCNMH Gy the covering ofpassivation layer on the electrode surface. The progressively decrease of R; ilustrates that the resistanceof the electrodissolution reaction decreases, which is proved by the slow increase of electrodissolution921reaction current. The covering degree of passivation layer on the. electrode surface continuouslyincreases with the anodic polarization in the range of OE -400- 625mV because it is impossible to form acompact and complete passivation layer in the real situation. 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