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Oxygen evolution reaction on IrO2-based DSA® type electrodes: kinetics analysis of Tafel lines and EIS
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International Journal of Hydrogen Energy 29 (2004) 791 – 797
www.elsevier.com/locate/ijhydene
Oxygen evolution reaction on IrO2-based DSAJ type electrodes: kinetics analysis of Tafel lines and EIS Ji-Ming Hu∗ , Jian-Qing Zhang, Chu-Nan Cao Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Accepted 12 September 2003
Abstract The oxygen evolution reaction (OER) on IrO2 –Ta2 O5 mixed oxide electrodes in H2 SO4 solution was studied by performing quasi-stationary current-potential and electrochemical impedance spectroscopy (EIS) measurements. The uncompensated resistance corrected Tafel lines displayed two distinct linear regions, with one of the slope close to 60 mV dec−1 in the low potential region and the other close to 130 mV dec−1 in the high potential region. The kinetics equations based on the proposed OER mechanism were derived and were used to simulate the current-potential curve. A mathematical deduction of EIS data was also carried out based on one state-variable theory. Both the polarization curve and impedance spectra were well approximated by kinetics analysis. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Oxygen evolution reaction; IrO2 ; Tafel lines; Impedance; Mechanistic aspects
1. Introduction Ti based dimensionally stable anodes (DSAJ ) have been widely employed for oxygen evolution in the electrochemical industry [1,2]. An interest in IrO2 based metallic oxide catalysts for oxygen evolution has attracted researchers in the past decade [3]. This is because IrO2 catalyst exhibits high corrosion-resistant properties, but only slight inferiority in electrocatalytic activities than RuO2 . Various kinds of this type of anodes were developed, such as IrO2 + TiO2 [4], IrO2 + ZrO2 [5] and IrO2 + Nb2 O5 [6], among which the combination of 70% IrO2 + 30%Ta2 O5 (at mole fraction) has been reported to present the highest electrocatalytic activity and longest service life in acidic media [7,8]. Unlike the H2 or Cl2 evolution reactions on metal(s) or metallic oxide(s), the O2 evolution reaction (OER) represents a low irreversibility. Its complicated kinetics behavior makes the deep understanding of OER continues to be a challenge for electrochemists. Cyclic voltammetry (and even its integrated charge), quasi-stationary ∗
Corresponding author. Fax: +86-571-8795-1895. E-mail address: [email protected] (J.-M. Hu).
current-potential curves, and the double layer capacitance were all used to characterize the electrochemical performance of the oxide electrodes [9,10]. From a fundamental point of view, however, these above-mentioned studies could only provide apparent or shallow information on electro catalytic properties of the electrodes. Other works were carried out on the analysis of Tafel region of OER [11]. Double Tafel slopes were often observed in the whole potential region. Even more, the Tafel slope was also dependent to a great extent on the type, composition and physical properties of oxide electrodes. For instance, the low-potential Tafel slope for RuO2 or mixed RuO2 –TiO2 is typically 40 mV, compared to 60 mV for IrO2 . It was also interesting to Hnd that for a Hxed type of IrO2 –Ta2 O5 mixture diIerent preparation methods often lead to the diIerent Tafel slope values [12]. Although some eIorts have been paid on the explanation of double-Tafel slope behavior, little was carried out on the mathematical interpretation. Electrochemical impedance spectroscopy (EIS) has been used in characterization of the surface properties of metallic oxide electrodes during the past years [13–15]. However, very few works have specially focused on the faradaic impedance that provides the essential kinetics information
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.09.007
792
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
of electrode processes. Moreover, to our best knowledge, no work has been done on the mathematical deduction of faradaic impedance of OER on noble metal oxides derived from reaction mechanism. In the present paper, by using thermo-prepared Ti supported IrO2 –Ta2 O5 mixture as a model system, anodic polarization curve and EIS were investigated. The work emphasized on the simultaneously mathematical simulation of the polarization curve and faradaic impedance based on the mechanism of OER on these DSAJ type electrodes. 2. Experimentals The thermo-decomposition method was used to prepare the oxide coatings. Prior to use, a 20 × 20 × 1 mm titanium plate (TA1) was degreased, and etched in 3:0 mol dm−3 HCl solution, then rinsed with deionized water. The precursors were prepared by mixing H2 IrCl6 · 6H2 O and TaCl5 solution (with molar ratio of 7:6). H2 IrCl6 solution was commercially provided dissolved in hydrochloric acid, and TaCl5 in ethanol. Our previous work indicated that, compared to those from aqueous precursors, oxide anodes prepared from organic solvent system displayed better performance [16]. Therefore, in the present work the mixed chlorides were pre-dried at 80◦ C for 24 h to remove water and HC1, then were mechanically powdered. After that, the obtained powders were dissolved in 1:1 volume ratio alcohol and isopropanol mixed solvent in which the total metal concentration was kept around 0:2 mol dm−3 . Then the Ti substrates were painted with the coating solution by brushes. After being dried at 100◦ C, the samples were heated at the annealing temperature (450◦ C) for 10 min. The entire procedure was repeated for 10 times, after which the samples were heated at the same annealing temperature for 1 h. The total loading of the obtained oxide coating was around 10 g m−2 . All the electrochemical measurements were conducted in 0:5 mol dm−3 H2 SO4 solution at 25◦ C, and were operated using three-compartment all-glass cell. A platinum plate (∼ 2 cm2 ) was used as the counter electrode, and KC1 saturated calomel electrode (SCE) as the reference. PAR instrument was used throughout. The current–potential curves were carried out using Model 351 Corrosion Analysis Software with the scan rate of 0:5 mV s−1 . The ohmic drop (iRs ) was corrected by the EIS measurement. The solutions were pre-electrolyzed by double platinum electrodes at 1 mA cm−2 for 10 h before each measurement and stirred by bubbling nitrogen during the experiment. The EIS measurements were conducted on the same potentiostat/galvanostat in combination with a PAR model 5210 lock-in ampliHer covering the frequency region of 10 mHz– 100 kHz in the potential range between 1.26 and 1:55 V. A 5 mV amplitude of sinusoidal potential perturbation was employed.
3. Results and discussion 3.1. Tafel lines analysis Fig. 1 displays the iRs -corrected anodic polarization curve of IrO2 –Ta2 O5 electrodes in H2 SO4 solution. In the present paper the eIective potential, EeI , which shows the real potential value for the OER, is employed. EeI = Eappl − iRs ;
(1)
where Eappl is the applied potential, i is faradaic current, and Rs is the electrolyte resistance. The Tafel lines show one slope close to 60 mV dec−1 in the low current density region and another higher slope of nearly 120 mV dec−1 in the high current density region. The data of these two slopes were usually reported as the typical values in OER for IrO2 based oxide electrodes [17]. In the acid medium, the following reactions cycle was generally proposed as the mechanism for oxygen evolution on active oxide electrodes [11]: S + H2 O → S–OHads + H+ + e− ;
(R1)
S–OHads → S–Oads + H+ + e− ;
(R2)
S–Oads → S + ( 12 )O2 ;
(R3)
where S stands for active sites on oxides surface, and S– OHads , S–Oads are two adsorption intermediates. This mechanism predicts the following Tafel slope: 120 mV dec−1 if step (R1) is the rate determining step, rds, 40 mV dec−1 for step (R2) and 30 mV dec−1 for step (R3). Which step is the rds depends on the strength of the adsorption of the intermediates, which is in turn governed by the composition of the oxide layer [17]. This can interpret the diIerent Tafel slopes
Fig. 1. Steady state polarization curve for the Ti=IrO2 –Ta2 O5 oxide anode in H2 SO4 solution at 25◦ C.
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
for various oxide catalysts. However, the above-listed mechanism cannot result in a Tafel slope of 60 mV dec−1 on IrO2 based oxide catalysts at low current range. Therefore, the reaction paths need to be modiHed. If assuming that step (R1) may be substituted by the following two sub-reactions: S + H2 O →
S–OH∗ads
+
−
+H +e ;
S–OH∗ads → S–OHads ;
(R1 ) (R1 )
in which adsorption intermediates S–OH∗ads and S–OHads possess the same chemical structure, but have diIerent energy states. The similar mechanism of the OER has been proposed on the RuTiCeO2 oxide electrodes [18]. The dissociation of the surface complex, proposed by Krasil’shchikov, was previously used to interpret the Tafel coeOcient of 60 mV, observed for Ti=Co3 O4 electrode [11]. However, it seems that the negatively charged species can hardly exist on electrode surface at highly anodic potentials. The reactions (R1 ) and (R1 ) are mechanism steps, while the other steps (R2) and (R3) are the fast reactions. The concentration of H+ ion in solutions and the pressure of O2 are taken as constants and approximately equal units. On this basis, the net reaction rates, V1 –V4 , of steps (R1 ), (R1 ), (R2) and (R3) are, respectively, given by the following equations: 1 FE ; (2) V1 = +1 = k+1 (1 − 1 − 2 − 3 ) exp RT V2 = +2 = k+2 1 ; V3 = +3 − −3 2 FE ( 2 − 1)FE = k+3 2 exp − k−3 3 exp ; RT RT
(3)
(4)
(5)
where ±i , i = 1; 2; 3; 4, are the forward and reversed reaction rates for each step; k±i , i = 1; 2; 3; 4, are the forward and reversed rate coeOcients, E corresponds to electrode potential, 1 and 2 are the symmetry coeOcients for reactions (R1 ) and (R2), respectively; 1 , 2 and 3 are the surface coverage by the intermediates S–OH∗ads , S–OHads and S–Oads , respectively. The faradaic current can be expressed as iF = 2F(V1 + V3 ):
(6)
In the steady state, we have V 1 = V2 = V3 = V4 :
After this is done, one will Hnd that 1 k+2
s1 =
1 k+4
s2 =
(8)
+
=
K4 1 k+1 A1
K3 1 k+4 A0
s3 = with
;
+
;
(9)
1 1 k+3 A2
1 1 + k+2 k+4
+
K3 K4 1 k+1 A3
K3 1 + + k+4 A0
;
(10)
1 + K4 k+1
1 A1
K3 K4 1 1 1 + ; k+3 A2 k+1 A3 FE A0 = exp ; RT 1 FE A1 = exp ; RT 2 FE A2 = exp ; RT +
A3 = exp
(1 + 1 )FE RT
and K3 =
k−3 ; k+3
K4 =
k−4 ; k+4
The steady-state current is is given by
V4 = +4 − −4 = k+4 3 − k−4 (1 − 1 − 2 − 3 );
793
(7)
By using Eqs. (2)–(5) and (7) the steady-state values of the surface coverage of the corresponding intermediates,
s1 , s2 and s3 , at a certain potential value (E) can be determined.
is = 4Fk+2 s1 =
4F
(11)
The above equation is used to simulate the experimental i = i(E) curve. The best approximation of the polarization curve is shown in Fig. 2. It is noted that, according to the proposed mechanism, one has k+2 k+4 and k+1 k+3 : Then some kinetics parameters can be approximated (see Table 1). In Fig. 3, the variations of surface coverage of the three intermediates are given. At the low potentials s1 remains at very low values, leading to impeding in the chemical transition reaction from S–OH∗ads to S–OHads . Increase in s1 at the high potentials facilitates the chemical transfer reaction but retards the Hrst electron transfer reaction. In the whole potential range, s2 remains at very low values (¡ 10−2 ), in accordance with the proposed mechanism that
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J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
Fig. 2. Reaction mechanism based simulation of polarization curve. The parameters used are listed in Table 1.
the Hrst two steps are slow reactions and the following steps are fast ones. If assuming 1 = 2 , Eq. (11) can be simpliHed as follows: 4F is = : (12) K3 1 1+K4 1 1 + + + Kk3+1K4 A13 k+2 k+4 A0 k+1 A1 In the Tafel regions, the OER takes place at highly positive potentials (for example E ¿ 1:26 V vs SCE in the present work as shown in Fig. 1). Therefore the last item in the above equation, K3 K4 =(k+1 A3 ), can be neglected, i.e. the Tafel lines are determined by the following equation: is =
4F 1
k+2
+
K3 1 k+4 A0
+
1+K4 1 k+1 A1
:
(13)
(i) In the low-potential region is ≈
4Fk+4 A0 : K3
(14)
In this case the OER is completely controlled by step (R1 ). Eq. (14) predicts a Tafel slope of 60 mV dec−1 for T = 298 K, which is exactly in accordance with the experimental results.
Fig. 3. Variation of the calculated surface coverage of S–OH∗ads , S–OHads , and S–Oads species with electrode potential on the IrO2 based electrodes.
(ii) At high-potential region is ≈
4Fk+1 A1 : 1 + K4
(15)
In this case step (R1 ) becomes the rds of the OER. For 1 ( 2 ) = 0:473 (obtained from the simulation result, see Table 1) and T = 298 K, the Tafel line displays a slope of 125 mV dec−1 . This value is close to the experimental one (∼ 130 mV dec−1 ). 3.2. EIS analysis According to the above-proposed mechanism, the rate of the OER is kinetically controlled by the reactions (R1 ) and (R1 ), and the reactions (R2) and (R3) are fast-rate steps. Therefore, we can simply assume that apart from the potential only the Hrst intermediate, S–OH∗ads , makes a signiHcant contribution on the faradaic impedance of the OER. On the basis of Cao’s expression [19], the faradaic admittance (YF ) of irreversible electrode reactions with one state
Table 1 Kinetics parameters derived from the simulation of steady state current curve of the OER on IrO2 –Ta2 O5 oxide electrodes in 0:5 mol dm−3 H2 SO4 solution at room temperature k+1 (mol cm−2 s−1 )
k+2 (mol cm−2 s−1 )
k−4 (mol cm−2 s−1 )
(k−3 k−4 )/(k+3 k+4 )
1
2
5:3 × 10−19
8:7 × 10−5
1:9 × 10−8
1:8 × 1022
0.473
0.473
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
variable besides electrode potential is written as 1 B YF = + ; Rt a + j! where 1 @iF = ss; Rt @E
795
(16)
(17)
B = mb; @iF m= ss; @
· @ ss; b= @E · @
ss; a = − @
(18) (19) (20)
(21)
Rt is the transfer resistance of the electrode reactions and always has a positive value, and the subscript ‘ss’ denotes steady state. In this work, is the surface coverage of S–
·
OH∗ads intermediate. is the change rate of , d =dt. If we rewrite the kinetics equations of the OER based on one state variable model, we obtain 1 FE iF = 4Fk+1 (1 − ) exp (22) RT and
F 1 FE k+1 (1 − ) exp − k+2 ;
= q RT
·
(23)
where q is the charge required to complete deposition of the intermediate on electrode surface. Thus, from Eq. (16)–(23) the mathematical expression of faradaic impedance can be evolved. As we Hnd 1 FE
−k+1 exp RT B= ¡ 0; qRt then the faradaic impedance, ZF , can be equivalently written as 1 Ra ZF = = Rt + (24) YF 1 + j!Ra Ca with R2t |B| a − Rt |B| 1 Ca = 2 Rt |B| Ra =
(25) (26)
Ra and Ca are equivalent resistance and capacitance, respectively, associated with the adsorption of intermediate. Eq. (24) predicts an equivalent electrical circuit (EEC) for the OER shown in Fig. 4(a), in which Cdl is the double-layer capacitance. Generally, based on this circuit two capacitance loops will be displayed in the impedance diagrams.
Fig. 4. Equivalent electrical circuits (EEC) for oxide anodes during oxygen evolution: (a) model based on reaction mechanism; (b) model employed by literatures; (c) complete model, ZF in the dash rectangle represents the circuit of faradaic impedance of the OER.
Experimental impedance of Ti=IrO2 –Ta2 O5 anodes in H2 SO4 solution at diIerent anodic potentials is measured, and some representative results are shown in Fig. 5. Phenomenally, the complex plane in the whole frequency domain shows only one capacitance arc, decreasing with the electrode potential. The Bode plot Fig. 5(b) obviously indicates the existence of inductance, L, at high frequencies. The simulated value of L is in the order of magnitude of 10−6 H, in agreement with the inductance value of the wiring and measuring equipment components [20], which can help interpret the source of inductance. In the previous literature, the EEC shown in Fig. 4(b) was often used to simulate the impedance data for OER on metal oxide anodes [14]. The (Rf Cf ) and (Rt Cdl ) combinations were observed in high and low (and intermediate) frequency domains of impedance spectroscopy, respectively. Rf is the resistance of oxide layer and Cf the capacitance. The (Rf Cf ) combination is independent of the potential. The low-frequency loop is deHnitely related to the OER. This simple circuit
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J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
Fig. 6. Potential depedence of polarization resistance of the OER (Scatters), Rt derived from Eq. (17) (line 1) and the sum of Rt and Ra derived from Eq. (25) (line 2).
Fig. 5. Experimental EIS patterns for IrO2 based oxide electrodes in H2 SO4 solution at diIerent potentials: (a) Nyquist diagrams; and (b) Bode plots.
model only considers the inRuence of electrode potential, E. Therefore, the value of charge transfer resistance, Rt , equals the polarization resistance, Rp . However, as shown in Fig. 6 this traditional EEC (Fig. 4b) cannot provide a consistent relationship in Rt value with that calculated from Eq. (17). Fig. 6 shows Rp value obtained from the simulation by the above EEC approximately equals the sum of Rt and Ra theoretically calculated from Eqs. (17) and (25), respectively. This result indicates that this EEC misunderstands the mechanism of OER and leads to overlapping the (Ra Ca ) combination at the OER frequency domain. The modiHed circuit is shown in Fig. 4(c). This EEC is consistent with the mathematical expression of faradaic impedance shown in Eq. (24). Fig. 7 gives the calculated impedance spectra using the modiHed circuit. The values of Rt , Ra and Ca are obtained from Eqs. (17), (25) and (26), respectively, where the corresponding simulation values of kinetics parameters (k+1 , k+2 and 1 ) are used. The values of Rs , Rf , Cf and Cdl used for completing the circuit are obtained from the Htting by Boukamp’s EQUIVCRT program.
Fig. 7. Mathematically calculated EIS patterns based on reaction kinetics with one state variable besides electrode potential.
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
The results of the calculated spectra qualitatively agree with the experimental data. Similarly, only a single arc can be phenomenally observed in the OER frequency domain over the whole potential region. The above results indicate that the modeling of EIS considering the eIect of state variable besides electrode potential can obtain better approximation. 4. Conclusions Based on the proposed reaction mechanism, mathematical simulation of current-potential curve and impedance spectroscopy is deduced for the OER on IrO2 based oxide anodes. The model successfully interprets the double Tafel lines, and the mathematical expression of faradaic impedance based on one state variable besides electrode potential can Ht well the experimental EIS data. The analysis shows that the traditional equivalent circuit for OER on metal oxide anodes oversimpliHes the reaction mechanism and overemphasized the inRuence of adsorption contribution of intermediates. Acknowledgements The authors acknowledge Hnancial support from Special Funds of the Chinese State Basic Research Projects (No. 19990650) and National Natural Science Foundation of China (No. 50201015). The authors also gratefully acknowledge Hnancial support from China Postdoctoral Science Foundation and the Chinese State Key Laboratory for Corrosion and Protection. Appendix Determination of parameter q According to Castro’s proposition [21], the charge involved in the overlapped anodic peaks, previous to the oxygen evolution potential, q, was used to evaluate the real active area, Ar : (27) q = Ar Se F=N; where Se is the number of oxygen evolution sites per unit area (1:1 × 1015 sites cm−2 ), N the Avogadro’s number and F the Faraday constant. Ar was calculated from the Htted values of Cdl , assuming a double-layer capacitance of Cdl0 = 60 F cm−2 for a smooth electrode, i.e. Ar = Cdl =Cdl0 :
(28)
The average value of Cdl of the IrO2 –Ta2 O5 electrodes simulated in this work is around ∼ 1500 F cm−2 . Then the value of q is calculated to be ∼ 0:0048 C cm−2 . References [1] Beer HB. US patent US549194, 1966; US710551, 1968. [2] Trasatti S. Electrodes of conductive metallic oxides, part B. Amsterdam: Elsevier; 1981. [3] Ardizzone S, Carugati A, Trasatti S. Properties of thermally prepared iridium dioxide electrodes. J Electroanal Chem 1981;126:287.
797
[4] Kristof J, Mink J. Emission FTIR studies on the formation mechanism of IrO2 /TiO2 Based coatings. Electrochim Acta 1994;39:1531. [5] Benedetti A, Riello P, Battaglin G, De Battisti A, Barbieri A. Physicochemical properties of thermally prepared Ti-supported IrO2 +ZrO2 electrocatalysts. J Electroanal Chem 1994;376:195. [6] Terezo AJ, Pereira EC. Fractional factorial design applied to investigation properties of Ti/IrO2 –Nb2 O5 electrodes. Electrochim Acta 2000;45:4351. [7] Vercesi GP, Salamin JY, Comninellis Ch. Morphological and Microstructural study of the Ti/IrO2 +Ta2 O5 electrode: eIect of the preparation temperature. Electrochim Acta 1991;36:991. [8] Comninellis C, Vercesi GP. Characterization of DSAJ -type oxygen evolving electrodes: choice of a coating. J Appl Electrochem 1991;21:335. [9] Rojas MI, Eslandiu MJ, Avalle LB, Leiva EPM, Macagno VA. The oxygen and chlorine evolution reactions at titanium oxide electrodes modiHed with platinum. Electrochim Acta 1998;43:1785. [10] Hu JM, Zhang JQ, Meng HM, Cao CN. Microstructure, electrochemical Surface and electrocatalytic properties of IrO2 +Ta2 O5 oxide electrodes. J Mater Sci 2003;38:705. [11] Da Silva LM, Boodts JFC, De Faria LA. Oxygen evolution at RuO2 (x)+Co3 O4 (1 − x) electrodes from acid solution. Electrochim Acta 2000;46:1369. [12] Morimitsu M, Otogawa R, Matsunaga M. EIects of cathodizing on the morphology and composition of IrO2 -Ta2 O5 =Ti anodes. Electrochim Acta 2000;46:401. [13] Lassali TAF, Boodts JFC, Bulhoes LOS. Charging processes and electrocatalytic properties of IrO2 =TiO2 =SnO2 oxide Hlms investigated by in situ AC impedance measurements. Electrochim Acta 1999;44:4203. [14] Alves VA, Da Silva LA, Boodts JFC. Surface characterization of IrO2 =TiO2 =CeO2 oxide electrodes and Faradaic impedance investigation of the oxygen evolution reaction from alkaline solution. Electrochim Acta 1998;44:1525. [15] Hu JM, Meng HM, Zhang JQ, Cao CN. Degradation Mechanism of Long Service life Ti/IrO2 -Ta2 O5 Oxide Anodes in Sulphric Acid Solution. Corros Sci 2002;44:1655. [16] Hu JM, Meng HM, Zhang JQ, Cao CN. EIect of Preparation Conditions on the performance of Ti Based IrO2 +Ta2 O5 Anodes. Acta Metall Sinica 2002;38:69 [in Chinese]. [17] Martelli GN, Ornelas R, Faita G. Deactivation mechanism of oxygen evolving anodes at high current density. Electrochim Acta 1994;39:1551. [18] De Faria LA, Boodts JFC, Trasatti S. Electrocatalytic properties of ternary oxide mixtures of composition Ru0:3 Ti(0:7−x) Cex O2 : oxygen evolution from acidic solution. J Appl Electrochem 1996;26:1195. [19] Cao CN. On the impedance plane displays for irreversible electrode reactions based on the stability conditions of the steady-state—I. One state variable besides electrode potential. Electrochim Acta 1990;35:831. [20] Da Silva LA, Alves VA, Da Silva MAP, Trasatti S, Boodts JFC. Electrochemical impedence, SEM, EDX and voltammetric study of oxygen evolution on Ir+Ti+Pt ternary-oxide electrodes in alkaline solution. Electrochim Acta 1996;41:1279. [21] Castro EB, Gervasi CA. Electrodeposited Ni-Co-oxide electros: characterization and kinetics of the oxygen evolution reaction. Int J Hydrogen Energy 2000;25:1163.
International Journal of Hydrogen Energy 29 (2004) 791 – 797
www.elsevier.com/locate/ijhydene
Oxygen evolution reaction on IrO2-based DSAJ type electrodes: kinetics analysis of Tafel lines and EIS Ji-Ming Hu∗ , Jian-Qing Zhang, Chu-Nan Cao Department of Chemistry, Zhejiang University, Hangzhou 310027, PR China Accepted 12 September 2003
Abstract The oxygen evolution reaction (OER) on IrO2 –Ta2 O5 mixed oxide electrodes in H2 SO4 solution was studied by performing quasi-stationary current-potential and electrochemical impedance spectroscopy (EIS) measurements. The uncompensated resistance corrected Tafel lines displayed two distinct linear regions, with one of the slope close to 60 mV dec−1 in the low potential region and the other close to 130 mV dec−1 in the high potential region. The kinetics equations based on the proposed OER mechanism were derived and were used to simulate the current-potential curve. A mathematical deduction of EIS data was also carried out based on one state-variable theory. Both the polarization curve and impedance spectra were well approximated by kinetics analysis. ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. Keywords: Oxygen evolution reaction; IrO2 ; Tafel lines; Impedance; Mechanistic aspects
1. Introduction Ti based dimensionally stable anodes (DSAJ ) have been widely employed for oxygen evolution in the electrochemical industry [1,2]. An interest in IrO2 based metallic oxide catalysts for oxygen evolution has attracted researchers in the past decade [3]. This is because IrO2 catalyst exhibits high corrosion-resistant properties, but only slight inferiority in electrocatalytic activities than RuO2 . Various kinds of this type of anodes were developed, such as IrO2 + TiO2 [4], IrO2 + ZrO2 [5] and IrO2 + Nb2 O5 [6], among which the combination of 70% IrO2 + 30%Ta2 O5 (at mole fraction) has been reported to present the highest electrocatalytic activity and longest service life in acidic media [7,8]. Unlike the H2 or Cl2 evolution reactions on metal(s) or metallic oxide(s), the O2 evolution reaction (OER) represents a low irreversibility. Its complicated kinetics behavior makes the deep understanding of OER continues to be a challenge for electrochemists. Cyclic voltammetry (and even its integrated charge), quasi-stationary ∗
Corresponding author. Fax: +86-571-8795-1895. E-mail address: [email protected] (J.-M. Hu).
current-potential curves, and the double layer capacitance were all used to characterize the electrochemical performance of the oxide electrodes [9,10]. From a fundamental point of view, however, these above-mentioned studies could only provide apparent or shallow information on electro catalytic properties of the electrodes. Other works were carried out on the analysis of Tafel region of OER [11]. Double Tafel slopes were often observed in the whole potential region. Even more, the Tafel slope was also dependent to a great extent on the type, composition and physical properties of oxide electrodes. For instance, the low-potential Tafel slope for RuO2 or mixed RuO2 –TiO2 is typically 40 mV, compared to 60 mV for IrO2 . It was also interesting to Hnd that for a Hxed type of IrO2 –Ta2 O5 mixture diIerent preparation methods often lead to the diIerent Tafel slope values [12]. Although some eIorts have been paid on the explanation of double-Tafel slope behavior, little was carried out on the mathematical interpretation. Electrochemical impedance spectroscopy (EIS) has been used in characterization of the surface properties of metallic oxide electrodes during the past years [13–15]. However, very few works have specially focused on the faradaic impedance that provides the essential kinetics information
0360-3199/$ 30.00 ? 2003 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.ijhydene.2003.09.007
792
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
of electrode processes. Moreover, to our best knowledge, no work has been done on the mathematical deduction of faradaic impedance of OER on noble metal oxides derived from reaction mechanism. In the present paper, by using thermo-prepared Ti supported IrO2 –Ta2 O5 mixture as a model system, anodic polarization curve and EIS were investigated. The work emphasized on the simultaneously mathematical simulation of the polarization curve and faradaic impedance based on the mechanism of OER on these DSAJ type electrodes. 2. Experimentals The thermo-decomposition method was used to prepare the oxide coatings. Prior to use, a 20 × 20 × 1 mm titanium plate (TA1) was degreased, and etched in 3:0 mol dm−3 HCl solution, then rinsed with deionized water. The precursors were prepared by mixing H2 IrCl6 · 6H2 O and TaCl5 solution (with molar ratio of 7:6). H2 IrCl6 solution was commercially provided dissolved in hydrochloric acid, and TaCl5 in ethanol. Our previous work indicated that, compared to those from aqueous precursors, oxide anodes prepared from organic solvent system displayed better performance [16]. Therefore, in the present work the mixed chlorides were pre-dried at 80◦ C for 24 h to remove water and HC1, then were mechanically powdered. After that, the obtained powders were dissolved in 1:1 volume ratio alcohol and isopropanol mixed solvent in which the total metal concentration was kept around 0:2 mol dm−3 . Then the Ti substrates were painted with the coating solution by brushes. After being dried at 100◦ C, the samples were heated at the annealing temperature (450◦ C) for 10 min. The entire procedure was repeated for 10 times, after which the samples were heated at the same annealing temperature for 1 h. The total loading of the obtained oxide coating was around 10 g m−2 . All the electrochemical measurements were conducted in 0:5 mol dm−3 H2 SO4 solution at 25◦ C, and were operated using three-compartment all-glass cell. A platinum plate (∼ 2 cm2 ) was used as the counter electrode, and KC1 saturated calomel electrode (SCE) as the reference. PAR instrument was used throughout. The current–potential curves were carried out using Model 351 Corrosion Analysis Software with the scan rate of 0:5 mV s−1 . The ohmic drop (iRs ) was corrected by the EIS measurement. The solutions were pre-electrolyzed by double platinum electrodes at 1 mA cm−2 for 10 h before each measurement and stirred by bubbling nitrogen during the experiment. The EIS measurements were conducted on the same potentiostat/galvanostat in combination with a PAR model 5210 lock-in ampliHer covering the frequency region of 10 mHz– 100 kHz in the potential range between 1.26 and 1:55 V. A 5 mV amplitude of sinusoidal potential perturbation was employed.
3. Results and discussion 3.1. Tafel lines analysis Fig. 1 displays the iRs -corrected anodic polarization curve of IrO2 –Ta2 O5 electrodes in H2 SO4 solution. In the present paper the eIective potential, EeI , which shows the real potential value for the OER, is employed. EeI = Eappl − iRs ;
(1)
where Eappl is the applied potential, i is faradaic current, and Rs is the electrolyte resistance. The Tafel lines show one slope close to 60 mV dec−1 in the low current density region and another higher slope of nearly 120 mV dec−1 in the high current density region. The data of these two slopes were usually reported as the typical values in OER for IrO2 based oxide electrodes [17]. In the acid medium, the following reactions cycle was generally proposed as the mechanism for oxygen evolution on active oxide electrodes [11]: S + H2 O → S–OHads + H+ + e− ;
(R1)
S–OHads → S–Oads + H+ + e− ;
(R2)
S–Oads → S + ( 12 )O2 ;
(R3)
where S stands for active sites on oxides surface, and S– OHads , S–Oads are two adsorption intermediates. This mechanism predicts the following Tafel slope: 120 mV dec−1 if step (R1) is the rate determining step, rds, 40 mV dec−1 for step (R2) and 30 mV dec−1 for step (R3). Which step is the rds depends on the strength of the adsorption of the intermediates, which is in turn governed by the composition of the oxide layer [17]. This can interpret the diIerent Tafel slopes
Fig. 1. Steady state polarization curve for the Ti=IrO2 –Ta2 O5 oxide anode in H2 SO4 solution at 25◦ C.
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
for various oxide catalysts. However, the above-listed mechanism cannot result in a Tafel slope of 60 mV dec−1 on IrO2 based oxide catalysts at low current range. Therefore, the reaction paths need to be modiHed. If assuming that step (R1) may be substituted by the following two sub-reactions: S + H2 O →
S–OH∗ads
+
−
+H +e ;
S–OH∗ads → S–OHads ;
(R1 ) (R1 )
in which adsorption intermediates S–OH∗ads and S–OHads possess the same chemical structure, but have diIerent energy states. The similar mechanism of the OER has been proposed on the RuTiCeO2 oxide electrodes [18]. The dissociation of the surface complex, proposed by Krasil’shchikov, was previously used to interpret the Tafel coeOcient of 60 mV, observed for Ti=Co3 O4 electrode [11]. However, it seems that the negatively charged species can hardly exist on electrode surface at highly anodic potentials. The reactions (R1 ) and (R1 ) are mechanism steps, while the other steps (R2) and (R3) are the fast reactions. The concentration of H+ ion in solutions and the pressure of O2 are taken as constants and approximately equal units. On this basis, the net reaction rates, V1 –V4 , of steps (R1 ), (R1 ), (R2) and (R3) are, respectively, given by the following equations: 1 FE ; (2) V1 = +1 = k+1 (1 − 1 − 2 − 3 ) exp RT V2 = +2 = k+2 1 ; V3 = +3 − −3 2 FE ( 2 − 1)FE = k+3 2 exp − k−3 3 exp ; RT RT
(3)
(4)
(5)
where ±i , i = 1; 2; 3; 4, are the forward and reversed reaction rates for each step; k±i , i = 1; 2; 3; 4, are the forward and reversed rate coeOcients, E corresponds to electrode potential, 1 and 2 are the symmetry coeOcients for reactions (R1 ) and (R2), respectively; 1 , 2 and 3 are the surface coverage by the intermediates S–OH∗ads , S–OHads and S–Oads , respectively. The faradaic current can be expressed as iF = 2F(V1 + V3 ):
(6)
In the steady state, we have V 1 = V2 = V3 = V4 :
After this is done, one will Hnd that 1 k+2
s1 =
1 k+4
s2 =
(8)
+
=
K4 1 k+1 A1
K3 1 k+4 A0
s3 = with
;
+
;
(9)
1 1 k+3 A2
1 1 + k+2 k+4
+
K3 K4 1 k+1 A3
K3 1 + + k+4 A0
;
(10)
1 + K4 k+1
1 A1
K3 K4 1 1 1 + ; k+3 A2 k+1 A3 FE A0 = exp ; RT 1 FE A1 = exp ; RT 2 FE A2 = exp ; RT +
A3 = exp
(1 + 1 )FE RT
and K3 =
k−3 ; k+3
K4 =
k−4 ; k+4
The steady-state current is is given by
V4 = +4 − −4 = k+4 3 − k−4 (1 − 1 − 2 − 3 );
793
(7)
By using Eqs. (2)–(5) and (7) the steady-state values of the surface coverage of the corresponding intermediates,
s1 , s2 and s3 , at a certain potential value (E) can be determined.
is = 4Fk+2 s1 =
4F
(11)
The above equation is used to simulate the experimental i = i(E) curve. The best approximation of the polarization curve is shown in Fig. 2. It is noted that, according to the proposed mechanism, one has k+2 k+4 and k+1 k+3 : Then some kinetics parameters can be approximated (see Table 1). In Fig. 3, the variations of surface coverage of the three intermediates are given. At the low potentials s1 remains at very low values, leading to impeding in the chemical transition reaction from S–OH∗ads to S–OHads . Increase in s1 at the high potentials facilitates the chemical transfer reaction but retards the Hrst electron transfer reaction. In the whole potential range, s2 remains at very low values (¡ 10−2 ), in accordance with the proposed mechanism that
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J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
Fig. 2. Reaction mechanism based simulation of polarization curve. The parameters used are listed in Table 1.
the Hrst two steps are slow reactions and the following steps are fast ones. If assuming 1 = 2 , Eq. (11) can be simpliHed as follows: 4F is = : (12) K3 1 1+K4 1 1 + + + Kk3+1K4 A13 k+2 k+4 A0 k+1 A1 In the Tafel regions, the OER takes place at highly positive potentials (for example E ¿ 1:26 V vs SCE in the present work as shown in Fig. 1). Therefore the last item in the above equation, K3 K4 =(k+1 A3 ), can be neglected, i.e. the Tafel lines are determined by the following equation: is =
4F 1
k+2
+
K3 1 k+4 A0
+
1+K4 1 k+1 A1
:
(13)
(i) In the low-potential region is ≈
4Fk+4 A0 : K3
(14)
In this case the OER is completely controlled by step (R1 ). Eq. (14) predicts a Tafel slope of 60 mV dec−1 for T = 298 K, which is exactly in accordance with the experimental results.
Fig. 3. Variation of the calculated surface coverage of S–OH∗ads , S–OHads , and S–Oads species with electrode potential on the IrO2 based electrodes.
(ii) At high-potential region is ≈
4Fk+1 A1 : 1 + K4
(15)
In this case step (R1 ) becomes the rds of the OER. For 1 ( 2 ) = 0:473 (obtained from the simulation result, see Table 1) and T = 298 K, the Tafel line displays a slope of 125 mV dec−1 . This value is close to the experimental one (∼ 130 mV dec−1 ). 3.2. EIS analysis According to the above-proposed mechanism, the rate of the OER is kinetically controlled by the reactions (R1 ) and (R1 ), and the reactions (R2) and (R3) are fast-rate steps. Therefore, we can simply assume that apart from the potential only the Hrst intermediate, S–OH∗ads , makes a signiHcant contribution on the faradaic impedance of the OER. On the basis of Cao’s expression [19], the faradaic admittance (YF ) of irreversible electrode reactions with one state
Table 1 Kinetics parameters derived from the simulation of steady state current curve of the OER on IrO2 –Ta2 O5 oxide electrodes in 0:5 mol dm−3 H2 SO4 solution at room temperature k+1 (mol cm−2 s−1 )
k+2 (mol cm−2 s−1 )
k−4 (mol cm−2 s−1 )
(k−3 k−4 )/(k+3 k+4 )
1
2
5:3 × 10−19
8:7 × 10−5
1:9 × 10−8
1:8 × 1022
0.473
0.473
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
variable besides electrode potential is written as 1 B YF = + ; Rt a + j! where 1 @iF = ss; Rt @E
795
(16)
(17)
B = mb; @iF m= ss; @
· @ ss; b= @E · @
ss; a = − @
(18) (19) (20)
(21)
Rt is the transfer resistance of the electrode reactions and always has a positive value, and the subscript ‘ss’ denotes steady state. In this work, is the surface coverage of S–
·
OH∗ads intermediate. is the change rate of , d =dt. If we rewrite the kinetics equations of the OER based on one state variable model, we obtain 1 FE iF = 4Fk+1 (1 − ) exp (22) RT and
F 1 FE k+1 (1 − ) exp − k+2 ;
= q RT
·
(23)
where q is the charge required to complete deposition of the intermediate on electrode surface. Thus, from Eq. (16)–(23) the mathematical expression of faradaic impedance can be evolved. As we Hnd 1 FE
−k+1 exp RT B= ¡ 0; qRt then the faradaic impedance, ZF , can be equivalently written as 1 Ra ZF = = Rt + (24) YF 1 + j!Ra Ca with R2t |B| a − Rt |B| 1 Ca = 2 Rt |B| Ra =
(25) (26)
Ra and Ca are equivalent resistance and capacitance, respectively, associated with the adsorption of intermediate. Eq. (24) predicts an equivalent electrical circuit (EEC) for the OER shown in Fig. 4(a), in which Cdl is the double-layer capacitance. Generally, based on this circuit two capacitance loops will be displayed in the impedance diagrams.
Fig. 4. Equivalent electrical circuits (EEC) for oxide anodes during oxygen evolution: (a) model based on reaction mechanism; (b) model employed by literatures; (c) complete model, ZF in the dash rectangle represents the circuit of faradaic impedance of the OER.
Experimental impedance of Ti=IrO2 –Ta2 O5 anodes in H2 SO4 solution at diIerent anodic potentials is measured, and some representative results are shown in Fig. 5. Phenomenally, the complex plane in the whole frequency domain shows only one capacitance arc, decreasing with the electrode potential. The Bode plot Fig. 5(b) obviously indicates the existence of inductance, L, at high frequencies. The simulated value of L is in the order of magnitude of 10−6 H, in agreement with the inductance value of the wiring and measuring equipment components [20], which can help interpret the source of inductance. In the previous literature, the EEC shown in Fig. 4(b) was often used to simulate the impedance data for OER on metal oxide anodes [14]. The (Rf Cf ) and (Rt Cdl ) combinations were observed in high and low (and intermediate) frequency domains of impedance spectroscopy, respectively. Rf is the resistance of oxide layer and Cf the capacitance. The (Rf Cf ) combination is independent of the potential. The low-frequency loop is deHnitely related to the OER. This simple circuit
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J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
Fig. 6. Potential depedence of polarization resistance of the OER (Scatters), Rt derived from Eq. (17) (line 1) and the sum of Rt and Ra derived from Eq. (25) (line 2).
Fig. 5. Experimental EIS patterns for IrO2 based oxide electrodes in H2 SO4 solution at diIerent potentials: (a) Nyquist diagrams; and (b) Bode plots.
model only considers the inRuence of electrode potential, E. Therefore, the value of charge transfer resistance, Rt , equals the polarization resistance, Rp . However, as shown in Fig. 6 this traditional EEC (Fig. 4b) cannot provide a consistent relationship in Rt value with that calculated from Eq. (17). Fig. 6 shows Rp value obtained from the simulation by the above EEC approximately equals the sum of Rt and Ra theoretically calculated from Eqs. (17) and (25), respectively. This result indicates that this EEC misunderstands the mechanism of OER and leads to overlapping the (Ra Ca ) combination at the OER frequency domain. The modiHed circuit is shown in Fig. 4(c). This EEC is consistent with the mathematical expression of faradaic impedance shown in Eq. (24). Fig. 7 gives the calculated impedance spectra using the modiHed circuit. The values of Rt , Ra and Ca are obtained from Eqs. (17), (25) and (26), respectively, where the corresponding simulation values of kinetics parameters (k+1 , k+2 and 1 ) are used. The values of Rs , Rf , Cf and Cdl used for completing the circuit are obtained from the Htting by Boukamp’s EQUIVCRT program.
Fig. 7. Mathematically calculated EIS patterns based on reaction kinetics with one state variable besides electrode potential.
J.-M. Hu et al. / International Journal of Hydrogen Energy 29 (2004) 791 – 797
The results of the calculated spectra qualitatively agree with the experimental data. Similarly, only a single arc can be phenomenally observed in the OER frequency domain over the whole potential region. The above results indicate that the modeling of EIS considering the eIect of state variable besides electrode potential can obtain better approximation. 4. Conclusions Based on the proposed reaction mechanism, mathematical simulation of current-potential curve and impedance spectroscopy is deduced for the OER on IrO2 based oxide anodes. The model successfully interprets the double Tafel lines, and the mathematical expression of faradaic impedance based on one state variable besides electrode potential can Ht well the experimental EIS data. The analysis shows that the traditional equivalent circuit for OER on metal oxide anodes oversimpliHes the reaction mechanism and overemphasized the inRuence of adsorption contribution of intermediates. Acknowledgements The authors acknowledge Hnancial support from Special Funds of the Chinese State Basic Research Projects (No. 19990650) and National Natural Science Foundation of China (No. 50201015). The authors also gratefully acknowledge Hnancial support from China Postdoctoral Science Foundation and the Chinese State Key Laboratory for Corrosion and Protection. Appendix Determination of parameter q According to Castro’s proposition [21], the charge involved in the overlapped anodic peaks, previous to the oxygen evolution potential, q, was used to evaluate the real active area, Ar : (27) q = Ar Se F=N; where Se is the number of oxygen evolution sites per unit area (1:1 × 1015 sites cm−2 ), N the Avogadro’s number and F the Faraday constant. Ar was calculated from the Htted values of Cdl , assuming a double-layer capacitance of Cdl0 = 60 F cm−2 for a smooth electrode, i.e. Ar = Cdl =Cdl0 :
(28)
The average value of Cdl of the IrO2 –Ta2 O5 electrodes simulated in this work is around ∼ 1500 F cm−2 . Then the value of q is calculated to be ∼ 0:0048 C cm−2 . References [1] Beer HB. US patent US549194, 1966; US710551, 1968. [2] Trasatti S. Electrodes of conductive metallic oxides, part B. Amsterdam: Elsevier; 1981. [3] Ardizzone S, Carugati A, Trasatti S. Properties of thermally prepared iridium dioxide electrodes. J Electroanal Chem 1981;126:287.
797
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