Materials Science Forum Vols. 514-516 (2006) pp 377-381 Online available since 2006/May/15 at www.scientific.net © (2006) Trans Tech Publications, Switzerland doi:10.4028/www.scientific.net/MSF.514-516.377

Oxygen Evolution on Perovskite-type Cobaltite Anodes: an Assessment of Materials Science-related Aspects A.V. Kovalevsky1,a, D.V. Sviridov2,b, V.V. Kharton1,c, E.N. Naumovich1,d, J.R. Frade1,e 1.

Department of Ceramics and Glass Engineering, CICECO, University of Aveiro, 3810-193 Aveiro, Portugal

2

a

Institute of Physicochemical Problems, Belarus State University, 14 Leningradskaya Str., 220050 Minsk, Belarus [email protected], b [email protected], c [email protected], d [email protected], e

[email protected]

Keywords: oxygen evolution, electrochemical activity, perovskite, oxygen deficiency, electronic conductivity, cobaltite Abstract. Ceramic anodes, made of perovskite-type rare-earth and strontium cobaltites substituted in both sublattices, exhibit a high electrocatalytic activity towards oxygen evolution in alkaline media. This work analyzes the relationships between cation composition, defect structure, electronic conductivity and electrochemical performance for a wide group of perovskite-like cobaltites, including Ln1-yAyCoO3-δ (Ln= Pr, Nd, Sm; A= Sr, Ca; y= 0-0.4), La1-x-ySrxBiyCoO3-δ (x= 0-0.6, y= 0-0.1), La0.7-xSr0.3CoO3-δ (x= 0-0.10), Sr1-xBaxCoO3-δ (x= 0.1-0.2) and SrCo1-yMyO3-δ (M=Fe, Ni, Ti, Cu; y= 0.1-0.6). The materials were prepared by the standard ceramic technique and characterized employing XRD, TGA, iodometric titration, and total conductivity measurements. A relatively high electrochemical performance in alkaline solutions was observed for (La,Sr)CoO3-based compositions with a moderate A-site deficiency. For SrCoO3-based materials, an increase in the oxygen evolution rate was found when co-substituting cobalt with several transition metal cations, such as Fe3+/4+ and Cu2+/3+. The results show that, in general, the key composition-related factors influencing electrochemical activity in alkaline media include the oxygen vacancy concentration, the average positive charge density in the crystal lattice, and possible blocking of active sites on the electrode surface. Introduction Numerous perovskite-type oxides with high electronic conductivity show a substantial electrocatalytic activity towards anodic oxygen evolution and cathodic oxygen reduction reactions [1-3]. These properties are very important for numerous applications, including water electrolyzers, secondary metal-air batteries and chloralkali cells. For the commercial electrolyzers, nickel or nickel plated steel are usually used as anode materials. However, nickel-based anodes are characterized with high oxygen-evolution overpotentials [4], exhibiting a gradual increase in the course of electrolysis. Metal oxides, such as IrO2 and RuO2, are more active towards oxygen evolution compared to metals, but these materials are very expensive [5]. On the contrary, perovskite-type oxides containing Co, Fe, Ni and Mn ions in the lattice have moderate cost and exhibit an acceptable long-term stability in alkaline media [6,7]. The present work was focused on the assessment of materials science-related aspects, such as cation composition, defect structure and electronic conductivity, which may affect electrochemical activity of Co-containing perovskites. As a rule, high level of electrical conductivity in cobaltites can be achieved by acceptor-type doping in A-perovskite sublattice using alkaline-earth metal cations, such as Sr2+, Ca2+ and Ba2+, which leads to the formation of oxygen vacancies and/or electronic defects for charge compensation. The All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP, www.ttp.net. (ID: 82.130.64.38-20/03/12,12:17:26)

378

Advanced Materials Forum III

electronic conductivity of SrCoO3-δ also can be improved by substitution of cobalt with transition metal cations stabilizing the cubic perovskite lattice and increasing the concentration of p-type electronic charge carriers. Thus the model compositions of ceramic anodes were selected from the Ln1systems La1-x-ySrxBiyCoO3-δ (x= 0-0.6, y= 0-0.1), La0.7-xSr0.3CoO3-δ (x= 0.05- 0.1), A CoO (Ln= Pr, Nd, Sm; A= Sr, Ca; y= 0-0.4), SrCo M O (M= Fe, Ni, Ti, Cu; y= 0.1-0.6) 3-δ 1-y y 3-δ y y and Sr1-xBaxCoO3-δ (x= 0.1-0.2), characterized in previous works. For the sake of simplicity, the evaluation of electrocatalytic activity was based on the measurements of current-potential dependencies without IR correction; although the results cannot be used for analysis of the reaction mechanism, these data enable to select families of the most active compositions and general qualitative trends necessary for the developments of high-performance ceramic anodes. Experimental All materials studied in this work were prepared by the standard ceramic technique. The description of processing conditions and experimental procedures used for characterization, and physicochemical properties of the ceramic materials were reported elsewhere [8-10]. The oxygen nonstoichiometry (δ) of selected compositions was determined by thermogravimetry and iodometric titration [9]. For the investigation of electrocatalytic activity, dense ceramic disks were mechanically polished, followed by ultrasonic washing with acetone and distilled water. The electrochemical measurements were performed in an aqueous 1M NaOH solution at room temperature under potentiostatic conditions using a standard three-electrode scheme of polarization. A Pt-foil and an Ag/AgCl/KCl (saturated) electrode were used as the counter and reference electrodes, respectively. The current-voltage curves presented in this paper are shown without correction for the ohmic drop. Detailed description of experimental techniques and equipment, used for electrochemical characterization, can be found in [1]. Results and discussion Table 1 lists the lattice parameters, unit cell volume (V), oxygen nonstoichiometry and electrical conductivity for the materials equilibrated with atmospheric oxygen at low temperatures, and the anodic current densities for oxygen evolution under a fixed potential. Table 1. Properties of perovskite-type electrode materials Electrode composition

La0.65Sr0.3CoO3-δ La0.6Sr0.3CoO3-δ La0.4Sr0.6CoO3-δ La0.9Bi0.1CoO3-δ La0.6Sr0.35Bi0.05CoO3-δ La0.75Sr0.2Bi0.05CoO3-δ La0.7Sr0.2Bi0.1CoO3-δ La0.55Sr0.35Bi0.1CoO3-δ PrCoO3-δ Pr0.8Sr0.2CoO3-δ NdCoO3-δ Nd0.8Sr0.2CoO3-δ Nd0.8Ca0.2CoO3-δ Nd0.7Sr0.3CoO3-δ Sm0.6Sr0.4CoO3-δ Sr0.9Ba0.1CoO3-δ Sr0.8Ba0.2CoO3-δ SrCo0.9Fe0.1O3-δ SrCo0.7Ni0.3O3-δ SrCo0.7Ti0.3O3-δ SrCo0.6Ni0.4O3-δ SrCo0.4Fe0.4Cu0.2O3-δ

Current density* [mA/cm2] 407 167 22.1 2.3 229 25.8 112 6.6 5.4 101 3.3 11.9 37.9 28.9 35.6 16.8 1.1 128 2.6 4.4 6.6 90.4

Unit cell parameters**

δ

0.05 0.09 0.21 – – – – – -0.23 -0.03 0.00 0.02 – 0.06 – – – – – – – –

a [nm] 0.5403 0.5404 0.3830 0.5379 0.5401 0.5398 0.5398 0.5406 0.7561 0.7595 0.7548 0.5340 0.7542 0.5363 0.7587 0.5478 0.3913 – 0.3866 0.3888 0.3894 0.3872

c [nm] – – – – – – – – – – – – – – – 0.4213 – – – – – –

α [°] 60.42 60.40 – 60.81 60.51 60.59 60.59 60.48 – – – 59.40 – 60.22 – – – – – – – –

V×102, [nm3] 5.576 5.580 5.618 5.502 5.570 5.561 5.561 5.586 5.403 5.476 5.375 5.384 5.362 5.454 5.459 – 5.991 – 5.779 5.877 5.905 5.805

Total conductivity [S/cm] 3.6×103 3.0×103 – 7.8×10-2 2.2×102 1.3×102 45 85 4.0×10-4 9.1×102 <1×10-3 7.9×102 1.4 1.7×103 1.4×103 3.7×10-1 4.3×10-1 2.1 – 1.4×10-1 – 16

Materials Science Forum Vols. 514-516

379

* The electrode potential is 1.5 V vs. Ag/AgCl reference electrode, without IR correction. ** The parameters a, a and c, a and α relate to the cubic, tetragonal and rhombohedral unit cells, respectively.

All data are given for 294±2 K. The X-ray diffraction (XRD) analysis showed formation of solid solutions with cubic or rhombohedrally distorted perovskite structure for most materials. In all cases, the total conductivity is predominantly p-type electronic [8-11]. Obviously, increasing electronic transport leads to a decreasing contribution of bulk electrode resistivity to the overall potential drop, and may also facilitate interfacial electron transfer. The highest conductivity values are characteristic for the materials derived from strontium-substituted lanthanide cobaltites (Table 1). One mechanism of oxygen evolution on the perovskite electrodes in alkaline media was proposed in Ref. [1]. This mechanism involves H2O dissociation on the reaction sites, namely the transition metal cations Bz+, with the formation of adsorbed OH radical, Bz+-OH. Breaking of the Bz+−OH bond, followed by the formation of hydrogen peroxide and oxygen evolution, is considered as the rate-determining step [1]. A slightly different mechanism suggested in Ref.[2] predicts an increase in the oxygen evolution rate when positive charge density on the transition metal cation increases. In all cases, electronic configuration of Bz+ ions has a key importance for the electrocatalytic activity. The current-potential dependencies for selected Co-containing electrodes are presented in Figs. 1 and 2. The maximum electrochemical performance is characteristic for Sr-doped LaCoO3-δ, in agreement with literature [1]. These materials exhibit also a very high electronic conductivity (Table 1) and a substantial oxygen-ionic diffusivity at elevated temperatures [9-11]. Doping with strontium leads to a drastic increase of the electrocatalytic activity (Fig. 2). 0.0

0.0

La0.65Sr0.3CoO3-δ La0.6Sr0.3CoO3-δ

La0.65Sr0.3CoO3-δ La0.6Sr0.35Bi0.05CoO3-δ

-1.0

log i (A/cm2)

log i (A/cm2)

-1.0

-2.0

-2.0

400

SrCo0.4Fe0.4Cu0.2O3-δ Pr0.8Sr0.2CoO3-δ Nd0.8Ca0.2CoO3-δ

i, A/cm2

La0.4Sr0.6CoO3-δ La0.6Sr0.35Bi0.05CoO3-δ

-3.0

La0.75Sr0.2Bi0.05CoO3-δ

200

La0.7Sr0.2Bi0.1CoO3-δ 0 1

-3.0

600

900

1200

1500

1800

E, mV Fig. 1. Current-potential dependencies for oxygen evolution on selected co-containing electrodes,.

-4.0

600

2

La:Sr ratio

900

3

4

1200

La0.55Sr0.35Bi0.1CoO3-δ

1500

1800

E, mV Fig.2. Current-potential dependencies for LaCoO3-based electrodes. Inset illustrates relationship between the La:Sr ratio and anodic current at 1.5 V without IR correction.

The observed behavior may be associated with changes of the average strength of Bz+−OH bonds, which depends, in particular, on the concentrations of dopant cations and oxygen vacancies. Acceptor-type doping of cobaltites increases the Co4+ content and oxygen deficiency ([9,11] and Table 1). The latter is expected to decrease oxygen bonding strength to the lattice [12]. Another factor influencing metal-oxygen bond energy is the unit cell volume normalized to formula unit. The cell volume increases on the incorporation with Sr2+ having a larger size with respect to rareearth cations. If assuming that the bismuth cations play a rather passive role, the maximum electrocatalytic activity is observed when La:Sr concentration ratios are in the approximate range 1.8-2.4 (inset in Fig. 2).

380

Advanced Materials Forum III

The creation of 5% cation vacancies in the A sublattice of La0.7Sr0.3CoO3-δ enhances the oxygen evolution rate (Fig. 2). Again, this tendency may be explained in terms of increasing oxygen nonstoichiometry, as the dominant charge compensation mechanism for the A-site cation vacancy formation is via increasing δ values (Table 1 and [9]). Similar phenomena can also be expected for the Bi-containing cobaltites, where bismuth oxide volatilization in the course of ceramics sintering may lead to a minor A-site deficiency. Note that a decrease of the electrochemical activity is observed when the cation nonstoichiometry is excessive, most likely due to partial decomposition of the perovskite phase. -0.5 -1.0

Pr0.8Sr0.2CoO3-δ Nd0.8Sr0.2CoO3-δ Nd0.7Sr0.3CoO3-δ

A

-1.5 -2.0

log i (A/cm2)

-2.5 -3.0

La0.9Bi0.1CoO3-δ PrCoO3-δ NdCoO3-δ

-3.5 -4.0 -0.5

Nd0.8Ca0.2CoO3-δ Sr0.9Ba0.1CoO3-δ Sr0.8Ba0.2CoO3-δ

-1.0 -1.5

B

-2.0 -2.5 -3.0 -3.5 600

900

1200

1500

1800

E, mV Fig.3. Oxygen evolution currents for selected rare-earth and strontium-based cobaltites.

The trends identified for Nd1-xSrxCoO3-δ and Pr1-xSrxCoO3±δ (Fig. 3A) are quite similar with respect to lanthanum-strontium cobaltites. One should mention, however, that Pr1-xSrxCoO3±δ demonstrates higher oxygen evolution currents compared to Nd1Both PrCoO3-δ and xSrxCoO3-δ. Pr0.8Sr0.2CoO3-δ are characterized as oxygenhyperstoichiometric compounds (Table 1); oxygen excess may be compensated either by deficiency in both cation sublattices or by a minor segregation of praseodymium oxide at the grain boundaries. In all cases, tetravalent Pr4+ cations should present. These may contribute to the positive charge density and, thus, to the catalytic activity, in agreement with the electrochemical reaction mechanism [2]. The activity of Ca-substituted NdCoO3-δ is considerably higher that that of the Sr-containing analogue (Fig. 3B). Analogously, the incorporation of large Ba2+ into the A sublattice of strontium cobaltite has a negative effect on the electrochemical activity. The stability of perovskites in alkaline media is expected to follow an

Materials Science Forum Vols. 514-516

381

-0.5

opposite sequence.It seems, therefore, that the formation of hydroxides or carbonates on the electrode surface may lead to partial -1.0 blocking. The same factor may be responsible -1.5 for the fact that the electrochemical activity of SrCoO3-based compositions is much -2.0 lower that that of the electrodes derived from (La,Sr)CoO3-δ (Figs. 1 and 4). In the former case, a significant improvement can be -2.5 achieved by co-doping with several transition metal cations, such as iron and copper. -3.0 In summary, the maximum electrochemical performance was found for -3.5 A-site deficient and Bi-containing 600 900 1200 1500 1800 perovskites based on La1-xSrxCoO3-δ, where E, mV the concentrations of lanthanum and Fig. 4. Current-potential dependencies for SrCoO3-based strontium cations are comparable. anodes. Among other relevant factors, one should note the concentration of oxygen vacancies, ionic radius of A-site dopants reactive with the solution components, and the average charge density of both cation sublattices. The role of the latter may be, however, negative when the coulombic interaction increases the B-OH bond strength, especially due to increasing B-site cation charge. Since A-site cation vacancies may be formed in the Bi-containing materials due to Bi2O3 volatilization, creation of a moderate nonstoichiometry in the A sublattice and doping with calcium instead of strontium may be of definite interest for the anode composition optimization. Apparently, the surface concentration of active sites may also be increased by the incorporation of several transition metal cations into the Co sublattice.

log i (A/cm2)

SrCo0.7Ti0.3O3-δ SrCo0.7Ni0.3O3-δ SrCo0.6Ni0.4O3-δ SrCo0.4Fe0.4Cu0.2O3-δ

Acknowledgements. This work was supported by the FCT, Portugal (POCTI program and Project SFRH/BPD/15003/2004) and the NATO Science for Peace program (project 978002). References [1] J. O’M. Bockris, T. Otagawa, J. Electrochem. Soc. 131-2 (1984), p.290. [2] Y. Matsumoto, S. Yamada, T. Nishida, E. Sato, J. Electrochem. Soc. 127 (1980), p.2360. [3] Y. Shimizu, K. Uemura, H. Matsuda, N. Miura, N. Yamazoe, J. Electrochem. Soc. 137 (1990), p.3430. [4] D.E. Hall, J. Electrochem Soc. 132-2 (1985), p. 41C. [5] K. Kinoshita: Electrochemical Oxygen Technology (Wiley, New York, 1992). [6] S. Trasatti, in: J. Lipkowsky, P.N. Ross (eds), The electrochemistry of novel materials, VCH Publishers, New York, 1994, p. 207. [7] S.K. Tiwari, S.P. Singh, R.H. Singh, J. Electrochem. Soc. 143-5 (1996), p. 1505. [8] V.V. Kharton, P.P. Zhuk, A.K. Demin, A.V. Nikolaev, A.A.Tonoyan, A.A.Vecher, Inorganic Materials 28 (1992), p. 1406. [9] V.V. Kharton, Ph.D. Thesis, Belarus State University, Minsk (1993). [10] V.V. Kharton, A.A. Yaremchenko, E.N. Naumovich, J. Solid State Electrochem 3 (1999), p. 303. [11] V.V.Kharton, E.N.Naumovich, A.A.Vecher, A.V.Nikolaev, J. Solid State Chem., 120 (1995), p. 128. [12] T. Nakamura, M. Misono, Y. Yoneda, Bull. Chem. Soc. Jpn. 55 (1982), p. 394. [13] K.D. Kreuer, Proc. Symp. “Ionic and mixed conducting ceramics”, The Electrochemical Society, Pennington N.J. (1988), p. 17.

Advanced Materials Forum III 10.4028/www.scientific.net/MSF.514-516

Oxygen Evolution on Perovskite-Type Cobaltite Anodes: An Assessment of Materials Science-Related Aspects 10.4028/www.scientific.net/MSF.514-516.377 DOI References [1] J. O’M. Bockris, T. Otagawa, J. Electrochem. Soc. 131-2 (1984), p.290. doi:10.1149/1.2115565 [2] Y. Matsumoto, S. Yamada, T. Nishida, E. Sato, J. Electrochem. Soc. 127 (1980), p.2360. doi:10.1149/1.2129415 [4] D.E. Hall, J. Electrochem Soc. 132-2 (1985), p. 41C. doi:10.1149/1.2113856