Calcium-Doped Y114 Layered Cobalt Perovskite, a ...

Advanced Engineering Forum ISSN: 2234-991X, Vol. 27, pp 63-73 doi:10.4028/www.scientific.net/AEF.27.63 © 2018 Trans Tech Publications, Switzerland

Submitted: 2017-02-28 Revised: 2017-05-24 Accepted: 2017-06-07 Online: 2018-04-27

Calcium-Doped Y114 Layered Cobalt Perovskite, a Promising Anodic Material for Direct Methanol Fuel Cell CRAIA-JOLDES Victor-Daniel1,a, DAN Mircea Laurentiu1,b*, DUCA Delia-Andrada1,c,VASZILCSIN Nicolae1,d 1

University Politehnica Timisoara, Faculty of Industrial Chemistry and Environmental Engineering, 6 V. Parvan Blvd., 300223, Timisoara, Romania

a

[email protected], [email protected], [email protected], [email protected]

Keywords: cobalt layered perovskite, methanol electrooxidation, chronoelectrochemical methods

Abstract. In this paper, calcium doped cobalt layered perovskite type 114 electrode electrocatalytic activity for methanol oxidation reaction (MOR) in aqueous alkaline solution (1 M KOH) has been investigated in order to find the relationship between methanol concentration in the solution and oxidation potential, current density, and oxidation efficiency. For a complete characterization of MOR on this electrode, several electrochemical methods were used: chronoamperometry, chronopotentiometry, chronocoulometry. Also, in order to understand the oxidation mechanism, kinetic parameters were determined using Tafel method and electrochemical impedance spectroscopy (EIS) was performed. MOR on layered cobalt perovskite electrodes becomes a serious issue, especially due to their use as anode in alkaline fuel cells. Introduction Methanol is the most common small alcohol which can be used in direct alcohol fuel cells DAFCs. Also, it is easily stored and handled, and widely available [1]. Methanol, the main raw material of direct methanol fuel cells (DMFCs), can be easily generated from different sources, e.g. natural gas, oil, coal or biomass [2]. Although, DMFC have been discovered more than 25 years ago, the development of them still requires increased attention and effort. MOR efficiency in alkaline electrolyte is better than in acid solutions [3]. A large number of researches have been dedicated for the achievement of new electrodes with high catalytic activity for MOR in alkaline media. Noble metals (Pt, Pd) and their alloys are recognized as the best catalysts in alkaline media for this type of fuel cells [4]. The major disadvantage is that these electrodes are expensive and CO adsorption onto metal surface reduces their long term utility [5]. New electrodes materials with high catalytic activity for MOR are of continued interest [6]. The new electrode materials family, oxide-ion conducting perovskites as SrPdO3, SrRuO3, La0.8Ce0.2CoO3, La0.8Sr0.2CoO3 have demonstrated electrocatalytic activity towards direct methanol oxidation during cyclic voltammetry measurements [7]. Lanthanum-based perovskite-type oxides, among which La2-xSrxNiO4 (0 ≤ x ≤ 1), known as functional materials with a wide range of applications, have been used as electrocatalysts for alkaline fuel cells and as environmental catalysts: hydrocarbon oxidation, CO oxidation and NOx reduction [8]. Y0.5Ca0.5BaCo4O7 belongs of a new class of transitional metals mixed oxides, named layered cobalt perovskites type 114. These compounds were extensively studied during the last period due to their structural, magnetic and electrochemical properties [9,10]. Based on these properties, layered cobalt perovskites can be used as membranes with high oxygen permeability, oxygen sensors and also fuel cells electrodes. Y0.5Ca0.5BaCo4O7 was first synthesized by M. Valldor by partial substitution of Y3+ with low valence Ca2+ cations in perovskite structure [11]. Previous studies have demonstrated a correlation between the compound structure and his properties, especially due to the variable cobalt ions valence and it was found that oxygen adsorption properties can be modified greatly by Y3+ ions substitution in the original structure of YBaCo4O7.

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Advanced Engineering Forum Vol. 27

Y0.5Ca0.5BaCo4O7 perovskite has been previously tested as catalyst for sulphite anodic oxidation in alkaline solution [12]. Also, a complete voltammetric study of the electrocatalytic oxidation of methanol on Y0.5Ca0.5BaCo4O7 electrode was published by the authors of this paper [13]. This paper presents complete characterization of processes occurring at the interface Y0.5Ca0.5BaCo4O7/methanol in alkaline media by EIS and Tafel method. Further, using chronoelectrochemical methods, optimum characteristic parameters, from which the most important is the efficiency of the oxidation process, were determined. Materials and Experimental Procedure Y0.5Ca0.5BaCo4O7 perovskite was obtained using solid state reaction, mixing the precursors Y2O3, CaCO3, BaCO3 and CoO1.38 (reagents, p.a. 99.99% Normapur), according to the stoichiometric cation ratio. After decarbonation at 600°C, the powder was reground, fired in air for 48 h at 1100°C and then removed rapidly from furnace and set to ambient temperature. The mixture was reground again, pressed into discs (1 cm2) and sintered at 1100°C for 24 h in air. The structure of obtained Y0.5Ca0.5BaCo4O7 pure compund was checked by X-Ray powder diffraction (Rigaku Ultima IV). 1 M KOH solution (prepared using Merck KOH, p.a.) ensured the alkaline media used in all experimental studies. Different concentrations of methanol were added: 0.06, 0.12, 0.25, 0.5, 1 and 2 M, all prepared from Sigma-Aldrich reagent p.a. min 99.8%. Electrochemical tests were performed at room temperature using a SP-150 potentiostat/galvanostat (Bio-Logic, SAS, France). A 100 mL typical glass cell was equipped with three electrodes: working electrodes consisting of perovskite samples, Ag/AgCl reference electrode and two graphite rods used as counter electrodes. All potentials are given versus the reference electrode (Eref = 0.197 V vs NHE). For performed experiments, the exposed surface of working electrode was 0.2 cm2. Results and Discussion In Fig. 1a, linear voltammograms recorded on Y0.5Ca0.5BaCo4O7 electrode in alkaline media with different concentrations of methanol, at 1 mV s-1 scan rate are presented. Analyzing voltammograms from Fig. 1, it can be observed that the specific potential domain for MOR, between +0.25 and +1.30 V, is depending on the methanol concentration added in alkaline electrolyte.

a) b) -1 Fig. 1. LVs at 1 mV s (a) and Tafel plots (b) recorded on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentration.


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Mechanism of MOR on Y0.5Ca0.5BaCo4O7 perovskite is complex and not entirely elucidated. It is probably mediated by the Co(III)/Co(II) redox couple from inside of perovskite according with Eq. 1-5 [13]: Co(II) → Co(III) + eCo(III) + OH- ↔ Co(III)·OH + eCo(III) + CH3OH ↔ Co(III)·CH3OH Co(III)·CH3OH + Co(III)·OH + OH- → Co(III)·CH2O + Co(II) + 2H2O Co(III)·CH2O + Co(III)·OH + OH- → products

(1) (2) (3) (4) (5)

Based on LVs from Fig. 1a, kinetic parameters (transfer coefficient - α and exchange current density - io) for MOR in alkaline solution with different methanol concentration on Y0.5Ca0.5BaCo4O7 electrode have been calculated using Tafel method. The results are gathered in table 1. Table 1. The kinetic parameters for methanol oxidation in test solutions. MeOH concentration [M] 0.06 0.12 0.25 0.5 1 2

b [mV dec-1] 402 419 443 493 481 544

α 0.15 0.14 0.13 0.12 0.12 0.11

io [A m-2] 6.06 5.89 5.73 5.93 5.87 5.65

Transfer coefficient values are similar with those obtained for sulphite to sulphate oxidation in alkaline media which proves the complexity of the anodic mechanisms on this layered perovskite electrode type [12]. In alkaline media, electrochemical behaviour of this kind of perovskite can be described by the reversible reaction given by Eq. 6. Y0.5Ca0.5BaCo4O7 + 2δHO- ↔ Y0.5Ca0.5BaCo4O7+δ + δH2O + 2δe-

(6)

Due to the structure flexibility, Y0.5Ca0.5BaCo4O7 can realise an excess of (7+δ) or deficit of (7-δ) of oxygen ions into its structure, property common to others 114 layered cobalt perovskite [9]. After previously described preparation of the electrode, δ = 0 and cobalt average oxidation number is +2.375, in the crystal structure of Y0.5Ca0.5BaCo4O7±δ being present both Co(II) and Co(III) ions. Since Co(II)/Co(III) redox couple from the perovskite structure mediates MOR, the diminution of oxygen content by electrochemical reduction in alkaline media would favours electrooxidation processes occurring on the electrode surface by decreasing the cobalt average oxidation number. Proceeding from these considerations, for an accurate electrochemical characterization of MOR, has been started from the preliminary stage of Y0.5Ca0.5BaCo4O7 surface activation by chronoamperometry at Co(III) to Co(II) reduction potential. The chronoamperometric studies were carried out for 30 minutes at Ered = -0.50 V in 1 M KOH without and with different methanol concentrations, as it is shown in Fig. 2.

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Fig. 2. Y0.5Ca0.5BaCo4O7 electrode surface activation in test solutions by chronoamperometric measurements at -0.5V. After partially removing of the oxygen ions by electrochemical reduction, followed the study of open circuit potential (OCP) values by chronopotentiometry at I = 0 A for 60 minutes, that is sufficient time to install a quasi-equilibrium state at electrode/electrolyte solution interface. In Fig. 3, the Y0.5Ca0.5BaCo4O7 electrode OCP variations in alkaline media are shown. Oxygen elimination process from the perovskitic structure is favoured by the direct interface contact with the alkaline electrolyte solution.

Fig. 3. EOCP variation after surface activation in test solutions. In Table 2, the values obtained for OCP before (Ei-OCP) and after activation surface (Ered-OCP) are presented. Table 2. Open circuit potential values for Y0.5Ca0.5BaCo4O7 electrode in test solutions before (Ei-OCP) and after surface activation (Ered-OCP). MeOH concentration [M] 0 0.06 0.12 0.25 0.5 1 2

Ei-OCP [V] 0.35 0.35 0.31 0.28 0.24 0.21 0.17

Ered-OCP [V] 0.22 0.22 0.20 0.17 0.13 0.07 0.03


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From EOCP values, it can be observed a shift of about 100 mV towards more negative potentials after surface activation, which correlated with literature data indicates the perovskite structure has a decrease in the amount of oxygen ions [10]. Chronoamperometric and cronocoulometric studies had as starting point the linear voltamograms shown in Fig. 1a. Analyzing these curves, three potential values corresponding to the methanol oxidation plateau were chosen in order to carry out the chronoamperometric measurements: +0.50, +0.75 and +1.00 V. In Fig. 4, the chronoamperometric measurements recorded for 60 minutes for MOR on Y0.5Ca0.5BaCo4O7 in alkaline solutions with different methanol concentrations at Eox = +0.50 V are presented.

Fig. 4. Chronoamperometric measurements for methanol oxidation in test solutions, at +0.50 V. Similarly, in Fig. 5, chronoamperograms plotted on working electrode in alkaline electrolyte with 2 M methanol, at four potential values are depicted. Eox = +0.25 V has been chosen to establish that despite the fact that from Fig. 1a it may look like MOR occurs at this value, actually the process is very slow. On the chronoamperogram it can be seen the current density at this value is very close zero.

Fig. 5. Chronoamperometric measurements for methanol oxidation in 1 M KOH (BS) with 2 M methanol, at different oxidation potential values. Methanol electrooxidation current densities recorded at four oxidation time-frames (15, 30, 45 and 60 minutes) are shown in Table 3 for all methanol concentrations in alkaline electrolyte solutions, for three oxidation potential values: +0.50, +0.75 and +1.00 V.

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Table 3. Chronoamperometric measurements for methanol electrooxidation in test solutions. MeOH concentration [M] 0.06 0.12 0.25 0.5 1 2

t [min]

EOx [V]

15

30

0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00

5.8 28.4 64.0 6.5 28.3 71.3 4.3 25.1 54.9 4.4 24.8 47.9 3.9 23.6 44.4 4.0 21.4 39.5

5.6 27.8 60.1 5.9 28.3 69.6 4.1 24.8 52.8 4.4 24.6 45.8 4.1 23.4 42.6 4.1 20.9 37.4

-2

i [A m ]

45

60

2.6 13.0 29.8 5.5 28.0 65.1 4.0 24.6 51.4 4.3 24.4 44.6 4.1 23.1 41.4 4.3 20.5 36.1

4.5 25.4 57.6 5.3 27.6 59.5 3.9 24.5 50.1 4.3 24.1 43.9 4.1 22.8 40.5 4.3 20.0 35.1

Taking into account the values of presented data in Table 3, it can be stated that MOR current densities values are dependent on methanol concentration in alkaline media. The increase of methanol concentration from 0.12 M to 0.2 M results in a decrease of about 20% of the current density when MOR is conducted at Eox = +0.75 V, respectively of 40% at Eox = +1.00 V. The approximate constant value of current densities in time during chronoamperometric studies in alkaline solution confirms that methanol concentration used in experimental studies is high enough, its slightly decrease is depending on the decrease of methanol concentration near the interface. Simultaneously with chronoamperometric measurements, chronocoulometric studies have been recorded for one hour in order to estimate, using Faraday's laws, the number of methanol moles changed in the anodic reaction (δ) for different products and methanol electrochemical transformation degree (r) [14]. Chronocoulometric data versus time and potential for 2 M methanol (the maximum concentration used in experimental tests) in alkaline solution (1 M KOH), are presented in Fig. 6.

Fig. 6. Transformation degree (r) and number of oxidized methanol moles in 1 M KOH with 2 M methanol, at different oxidation potential values.


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In Table 4, the transformation degrees of MOR in alkaline electrolyte solutions, at same electrolysis four time-frames as in chronoamperometric studies, versus potential and methanol concentrations are shown. Table 4. Chronocoulometric measurements for methanol electrooxidation in test solutions. MeOH concentration [M] 0.06 0.12 0.25 0.5 1 2

t [min]

EOx [V]

15

30

0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00 0.50 0.75 1.00

0.08 0.35 0.87 0.04 0.17 0.43 0.01 0.07 0.17 0.01 0.036 0.078 0.003 0.02 0.04 0.001 0.01 0.02

0.15 0.70 1.62 0.08 0.35 0.88 0.02 0.15 0.33 0.01 0.074 0.15 0.006 0.04 0.07 0.003 0.02 0.03

r [%]

45

60

0.21 1.04 2.38 0.11 0.53 1.30 0.04 0.22 0.49 0.02 0.11 0.21 0.009 0.05 0.10 0.005 0.03 0.04

0.27 1.37 3.08 0.15 0.70 1.68 0.05 0.29 0.64 0.03 0.15 0.28 0.012 0.07 0.13 0.007 0.04 0.06

Further, Y0.5Ca0.5BaCo4O7 perovskite behavior as catalyst for methanol oxidation has been studied by EIS. As well, this technique analyzes the electrochemical processes occurring at the electrode interface. EIS measurements have been recorded between 100 kHz and 100 mHz at amplitude of 10 mV at the three electrode potential characteristic for MOR: +0.50, +0.75 and +1.00 V. In Fig. 7-9, EIS spectra in the Nyquist complex plane representation and Bode plots for each case studied are presented.

a) b) Fig. 7. Nyquist (a) and Bode plots (b) recorded on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentrations, at +0.50 V.

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a) b) Fig. 8. Nyquist (a) and Bode plots (b) recorded on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentrations, at +0.75 V.

a) b) Fig. 9. Nyquist (a) and Bode plots (b) recorded on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentrations, at +1.00 V. As it can be seen, EIS spectra depend on the electrode potential. Thus, if methanol oxidation process is carried out at +0.50 V and +0.75 V, Nyquist plots consist of two semicircles. If experimental potential is +1.00 V, the Nyquist plots are represented as a single semicircle which confirms that the MOR on Y0.5Ca0.5BaCo4O7 perovskite electrode is controlled by the charge transfer step. EIS data were fitted using a complex non-linear least squares (CNLS) procedure with the equivalent electrical circuits shown in Fig. 10 a and b. The circuit elements are represented by: Rs is ohmic resistance of electrolyte solution; Rct is the charge transfer resistance, specific for MOR on electrode surface; CPE is a constant phase element, used instead of a Cdl element due to the surface inhomogeneity of perovskite electrode [15]. The double layer capacitance is expressed as CPE capacitance parameter T. The two R-CPE networks in equivalent electrical circuit, characteristic for EIS data recorded at +0.50 V and +0.75 V, is due to a mixed oxidation process: Co(II) from inside oxidation and methanol oxidation process, described by Eq. (1) - (5) in the electrochemical mechanism presented previously. At low frequency spectra, the capacitive element (CPE2) may be associated with the adsorption on electrode surface of different organic intermediate species generated during the methanol oxidation complex process. Consequently, MOR is limited by this adsorption process [16].


Rs

CPE1

CPE2

R1

R2

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Rs

CPE1 R1

a) b) Fig.10. Equivalent electrical circuits for modeling methanol and perovskite oxidation in test solutions at +0.50 V and +0.75 V (a) and +1.00 V (b). The fitting results are shown as continuous line in Fig. 7 - 9 and the corresponding values of the circuit elements are given in Tables 5-7 for MOR on Y0.5Ca0.5BaCo4O7 perovskite electrode in alkaline solution with different methanol concentrations. Table 5. Calculated values of the circuit elements on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentration, at +0.50 V. Electrolyte 1 M KOH (BS) BS + 0.06 M MeOH BS + 0.12 M MeOH BS + 0.25 M MeOH BS + 0.5 M MeOH BS + 1 M MeOH BS + 2 M MeOH

Rs [Ω cm2] 13.4 13.4 14.4 15.2 16.4 17.7 20.5

T1 . 104 [F cm-2 sn-1] 3.51 2.91 2.54 2.57 2.61 3.67 6.82

n1 0.73 0.74 0.78 0.84 0.81 0.89 0.91

Rct1 [Ωcm2] 127 167 223 247 329 127 84

T2 . 103 [F cm-2 sn-1] 7.33 6.43 5.15 3.91 3.30 1.85 1.59

n2 0.27 0.29 0.32 0.37 0.41 0.44 0.46

Rct2 [Ωcm2] 459 658 904 1443 1597 3445 3257

Chi2 . 103 0.25 0.26 0.30 0.41 1.06 1.59 2.22

Table 6. Calculated values of the circuit elements on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentration, at +0.75 V. Electrolyte 1 M KOH (BS) BS + 0.06 M MeOH BS + 0.12 M MeOH BS + 0.25 M MeOH BS + 0.5 M MeOH BS + 1 M MeOH BS + 2 M MeOH

Rs [Ω cm2] 13.6 12.9 13.9 14.3 14.8 15.3 18.2

T1 . 104 [F cm-2 sn-1] 7.17 4.04 2.90 2.95 3.44 4.43 4.99

n1 0.77 0.78 0.83 0.90 0.88 0.92 0.95

Rct1 [Ωcm2] 41 58 75 100 85 49 35

T2 . 103 [F cm-2 sn-1] 6.46 5.99 4.82 5.65 4.46 4.79 4.00

n2 0.32 0.32 0.34 0.34 0.37 0.34 0.36

Rct2 [Ωcm2] 157 176 202 269 285 480 521

Chi2 . 103 0.45 0.20 0.34 0.59 0.58 1.06 2.01

Table 7. Calculated values of the circuit elements on Y0.5Ca0.5BaCo4O7 electrode in 1 M KOH solution with different methanol concentration, at +1.00 V. Electrolyte 1 M KOH (BS) BS + 0.06 M MeOH BS + 0.12 M MeOH BS + 0.25 M MeOH BS + 0.5 M MeOH BS + 1 M MeOH BS + 2 M MeOH

Rs [Ω cm2] 13.15 13.5 14.35 17.08 16.26 17.9 21.54

T . 103 [F cm-2 sn-1] 5.69 4.23 3.52 2.88 2.78 2.66 2.59

n 0.45 0.46 0.47 0.48 0.49 0.50 0.51

Rct [Ωcm2] 58 64 74 95 106 118 132

Chi2 . 103 3.06 1.27 1.24 2.64 2.22 2.91 4.81

In Table 8 are presented the total charge transfer resistance values for MOR on Y0.5Ca0.5BaCo4O7 electrode at different oxidation potential in alkaline electrolyte with all methanol concentrations used in electrochemical tests.

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Table 8. Calculated total charge transfer resistances (Rct) for MOR on Y0.5Ca0.5BaCo4O7 electrode at different oxidation potentials. Electrolyte

+0.50

1 M KOH (BS) BS + 0.06 M MeOH BS + 0.12 M MeOH BS + 0.25 M MeOH BS + 0.5 M MeOH BS + 1 M MeOH BS + 2 M MeOH

586 825 1127 1690 1926 3572 3341

EOx [V] +0.75 Rct [Ωcm2] 198 234 277 369 370 529 556

+1.00 58 64 74 95 106 118 132

Analysing the results from Table 8, it can be observed that Rct values decrease with the increasing of the oxidation potential and with the increase of methanol concentration, indicating that MOR takes place with higher rate at +1.00 V at a low methanol concentration in alkaline electrolyte solution. Conclusions The results presented in this paper exhibit a complete characterization of methanol oxidation process on Y0.5Ca0.5BaCo4O7 electrode, correlating kinetic parameters obtained from Tafel method and EIS results with chronoamperometry, cronopotentiometry and cronocoulometry data. Kinetic parameters obtained with Tafel method indicate that MOR take place relatively rapid, the exchange current density being approximately 6 A m-2 for all methanol concentration in alkaline electrolyte solution. Anodic transfer coefficient reduced values (α ϵ [0.11÷0,15]) indicates methanol oxidation occurs far from the electrode surface, toward solution bulk. It has been found that the transformation degree values of methanol in alkaline solution depend on the potential value at which this process is conducted and the concentration of methanol in the electrolyte. The optimum potential is +1.00 V for methanol low concentrations, 0.06 M - 0.25 M, added in alkaline media. Chronoamperometric and chronocoulometric studies have showed an increased rate for methanol oxidation when increasing anodic polarization (high overpotential) and at lower methanol concentrations in alkaline media. Also, this variation has been confirmed by EIS measurements. Experimental data have confirmed the possibility to oxidize methanol in alkaline media on a cobalt layer perovskite type 114 electrode. Acknowledgement This work was supported by University Politehnica Timisoara in the frame of PhD studies. References [1] L.K.Verma, Studies on methanol fuel cell, J. Power Sources, 86 (2000) 464-468. [2] S.S. Mahapatra, J. Datta, Characterization of Pt-Pd/C Electrocatalyst for Methanol Oxidation in Alkaline Medium, International Journal of Electrochemistry 2011 (2011) 1-16. [3] H.B. Hassan , Z. Abdel Hamid , R. M. El-Sherif, Electrooxidation of methanol and ethanol on carbon electrodeposited Ni–MgO nanocomposite, Chinese J. Catal. 37 (4) (2016) 616-627. [4] C. Xu, L. Cheng, P. Shen, Y. Liu, Methanol and ethanol electrooxidation on Pt and Pd supported on carbon microspheres in alkaline media, Electrochem. Commun. 9(5) (2007) 997-1001.


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