Oxygen Reduction Activity and Interfacial Structures of La_0.8Sr_0.2CoO₃ at Initial Electrochemical Process in an Alkaline Solution

Abstract

Oxygen reduction and evolution reactions (ORR and OER, respectively) of perovskite-type La_0.8Sr_0.2CoO₃ were characterized using two-dimensional model electrodes with different reaction planes (001), (110), and (111). Synthesized by pulsed laser deposition, these thin (30 nm) and flat (roughness < 1 nm) electrodes can reveal the reaction plane dependence of the ORR activity. From steady-state polarization measurements in KOH (aq.), the ORR activity was the highest on the (001) film during the first ORR/OER cycle, and it decreased significantly during the second cycle. In-situ synchrotron X-ray diffraction clarified crystal structure changes in the bulk and surface regions of La_0.8Sr_0.2CoO₃, and these changes are associated with forming oxygen defects during the initial electrochemical process. Furthermore, the La_0.8Sr_0.2CoO₃ surface partially decomposed upon reacting with the aqueous solution, as clarified by hard X-ray photoemission spectroscopy. Therefore, the interfacial structures formed in the electrochemical reaction field is important for enhancing ORR and OER activities.

1. Introduction

Rechargeable metal–air batteries have attracted much attention as next-generation energy devices to deliver remarkably high energy densities.^1–3 Perovskite-type metal oxides are promising air electrodes for these batteries because of their high reactivity in both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in alkaline aqueous solutions.^4–6 However, the actual cell potentials are generally much lower than the theoretical values due to the overpotentials derived from both anode and cathode polarizations. In addition, the deterioration of catalysts and oxidation of carbons increase the overpotential during charge-discharge cycles. Therefore, both the power characteristics and stability of air electrodes should be improved for practical use.

ORR and OER occur through terminating oxygen atoms at the electrode surface. The electrochemical activity of the oxygen electrodes is affected by the bonding conditions between the surface oxygen and neighboring cations.^7,8 Many studies have shown that the bonding conditions change with various parameters, such as lattice planes in contact with the electrolyte,^9,10 metal species,^5,11–13 lattice distortion, and oxygen vacancies.^6,10 The present authors have clarified changes of crystal structure at the surfaces of intercalation electrodes during the initial electrochemical reaction process:^14–16 the surface structures drastically change upon soaking in the electrolyte and are reconstructed during the initial electrochemical process. However, most prior investigations focused on the crystal structure of bulk materials prior to electrochemical reactions, while little is known about the surface structure of oxygen electrodes after their placement in aqueous alkaline solutions.

Epitaxial film electrodes are advantageous for studying the mechanisms of ORR and OER, because their two-dimensional interface restricts the reaction fields and allows direct observation of the lattice plane dependence of the reaction. Moreover, on a flat electrode surface, researchers can examine structural changes occurring exclusively at the surface where the electrochemical reaction is initiated. In this study, the ORR and OER processes of La_0.8Sr_0.2CoO₃, a known highly active oxygen electrode catalyst,⁴ was investigated in alkaline aqueous solution using epitaxial film electrodes with different reaction planes of (001), (110), and (111). The charge transfer process of ORR on each reaction plane was quantitatively characterized using a steady-state polarization technique. Interfacial structure changes during electrochemical cycling were investigated for the La_0.8Sr_0.2CoO₃ (001) film using in-situ X-ray diffraction (XRD), X-ray absorption near edge structure (XANES), and hard X-ray photoemission spectroscopy (HAXPES). Structural changes at the La_0.8Sr_0.2CoO₃/electrolyte interface may significantly influence the reaction kinetics and stability of the La_0.8Sr_0.2CoO₃ surface.

2. Materials and Methods

Epitaxial La_0.8Sr_0.2CoO₃ (001) thin films were grown on Nb-doped SrTiO₃ (001), (110), (111), and MgO (001) substrates using a KrF excimer laser with a wavelength of 248 nm and a pulsed laser deposition (PLD) apparatus (PLD 3000, PVD Products, Inc.). The synthesis conditions were described in detail elsewhere.^9,17 The crystal structure, film thickness, density, and roughness of the synthesized films were characterized using XRD and X-ray reflectivity (XRR) measurements on a thin-film X-ray diffractometer (ATX-G, Rigaku Corp.). Activities of electrochemical OER and ORR were investigated using Tafel plots obtained by measuring the steady-state polarization curves.^18–20 Figure 1 is a schematic of the three-electrode cell consisting of a La_0.8Sr_0.2CoO₃ film working electrode, a Pt wire (Nilaco Corp.) counter electrode, a Hg/HgO reference electrode (ALS Co. Ltd.), and 1 M KOH aq. as the electrolyte. Prior to electrochemical measurements, the aqueous KOH electrolyte was bubbled with O₂ gas for 1 h to saturate it with oxygen. Polarization curves were obtained by measuring the currents 180 s after setting the electrode at the given potential (0 to −0.4 V vs. Hg/HgO).

Figure 1.

Schematic of the three-electrode cell consisting of a La_0.8Sr_0.2CoO₃ film working electrode, 1 M KOH aqueous electrolyte, Pt wire counter electrode, and Hg/HgO reference electrode.

In-situ synchrotron XRD patterns were collected on an κ-type six-circle diffractometer (Newport) installed on the bending-magnet beamline BL14B1 at SPring-8 with an X-ray wavelength of 0.82552 Å (15 keV), using an in-situ spectroelectrochemical cell designed for our experiments.^21–24 The cell voltage increased from the initial value (∼0 V) to 0.7 V and then decreased to −0.5 V. The reaction was potentiostatically controlled using a potentiostat/galvanostat (CompactStat; IVIUM). Once the cell potential was stabilized, in-plane and out-of-plane XRD measurements were performed. Afterwards, the cell potential was set at a fixed value for the next measurement. For the in-situ XRD measurements, a reciprocal coordinate system (H,K,L) was used, with two components (H and K) parallel to the surface and the third component (L) along the surface normal, as generally used for surface diffraction techniques. The reciprocal lattice space model is described by a cubic system for La_0.8Sr_0.2CoO₃(001)/MgO(001). The relationship between (H,K,L)_cubic for the cubic lattice and (h,k,l)_tetra for the tetragonal lattice is given by the transformations H = h, K = k, and L = l. Diffraction peaks of out-of-plane (0,0,2)_cubic and (2,0,2)_cubic and in-plane (2,0,0)_cubic were collected for the La_0.8Sr_0.2CoO₃ (001) film. HKL scans were performed with respect to the unit cell of the MgO(001) substrate. The incident angles for the in-plane measurement were controlled around the critical angle of La_0.8Sr_0.2CoO₃ (θ_c = 0.20°), namely at 0.14° and 0.24° for the radiation used in this study (15 keV). X-rays with incident angles below the critical angle penetrate only a few nanometers into the sample surface, allowing collection of diffractions from the surface without those from the bulk. During measurements at a low incident angle, the X-rays could pass through the electrolyte over a long distance. To avoid the effect of X-ray adsorption by the electrolyte, the volume of electrolyte in the electrochemical cell was fixed during such measurements. Thus, the in-plane and out-of-plane XRD provided the crystal structure changes at the surface and in the entire regions of the La_0.8Sr_0.2CoO₃ (001) film during the OER/ORR processes.

XANES and HAXPES measurements were performed to evaluate changes in the valence state of the La_0.8Sr_0.2CoO₃ films and chemical species formed at the surface. The XANES spectra were collected at pristine and after ORR and OER conditions, in the fluorescence mode using a two-dimensional pixel array detector (PILATUS) installed in beamline BL14B2 at SPring-8, Japan.^25,26 XANES spectra were collected for the Co-K edge with an X-ray beam monochromated by Si(111) at an oblique incidence of 4° using a 2θ–θ stage. The XANES spectra were collected at pristine and after ORR and OER conditions, in the fluorescence mode using a two-dimensional pixel array detector (PILATUS) installed in beamline BL14B2 at SPring-8, Japan.^25,26 XANES spectra were collected for the Co-K edge with an X-ray beam monochromated by Si(111) at an oblique incidence of 4° using a 2θ–θ stage. The oxidation states of La, Co, and O ions in the films were determined from HAXPES spectra acquired at SPring-8 BL46XU using a hemispherical electron energy analyzer (SCIENTA, R-4000) with an incident photon energy level of approximately 7940 eV and a photoelectron take-off angle of 80°, which can provide the structural changes in the entire regions of the 30 nm-thick La_0.8Sr_0.2CoO₃ (001) film. The binding energy was calibrated according to the Au 4f7/2 core level spectrum.

3. Results and Discussion

Figure 2a shows the out-of-plane XRD patterns of La_0.8Sr_0.2CoO₃ films synthesized on SrTiO₃ substrates with different orientations. The film on the SrTiO₃ (001) substrate showed diffraction peaks at 23.5°, 48.2°, and 75.6°, which were respectively indexed as 001, 002, and 003 based on a pseudo-tetragonal lattice, indicating a 001 film orientation. The La_0.8Sr_0.2CoO₃ films on the SrTiO₃ (110) and (111) planes exhibited hh0 and hhh diffraction peaks, respectively. The lattice parameters along the out-of-plane direction (c_tetra) were calculated to be 0.3776(4), 0.3816(12), and 0.3807(3) nm for the La_0.8Sr_0.2CoO₃ (001), (110), and (111) films, respectively. The in-plane XRD patterns of the films are shown in Fig. 2b. No distinct diffraction peaks associated with La_0.8Sr_0.2CoO₃ were observed. The in-plane diffraction peaks of the La_0.8Sr_0.2CoO₃ films may overlap those of the SrTiO₃ substrate, because of in-plane expansion of the lattice in the La_0.8Sr_0.2CoO₃ film to match the SrTiO₃ substrate (c_cubic = 0.3905 nm). This tetragonal distortion is consistent with that previously reported for an epitaxial LaCoO₃ (001) film on SrTiO₃ (001) (c_tetra = 0.3785 nm and a_tetra = 0.3896 nm).²⁷ The observed and calculated XRR spectra of La_0.8Sr_0.2CoO₃ (001), (110), and (111) films are shown in Fig. 2c. The spectra are plotted as a function of Q_z = 4π sin θ/λ, where λ is the wavelength of the X-rays (0.1541 nm) and θ is the angle of incidence. A three-layer model composed of the SrTiO₃ substrate, La_0.8Sr_0.2CoO₃ film, and surface layer provides the best fit to the reflectivity curves. The resulting thicknesses and surface roughness values of the La_0.8Sr_0.2CoO₃ films are summarized in Table 1. The thicknesses are 24.3, 23.7 and 23.1 nm for the La_0.8Sr_0.2CoO₃ (100), (110), and (111) layers; and those for the surface layers are 0.7, 0.9, and 0.9 nm, respectively. The surface roughness values for all the La_0.8Sr_0.2CoO₃ films are less than 1.0 nm. The surface layer possessed a lower density (3.7 to 3.8 g cm⁻³) than the La_0.8Sr_0.2CoO₃ layer (6.9 to 7.1 g cm⁻³). It is possible that impurity phases such as SrCO₃ (3.44 g cm⁻³) or Sr(OH)₂ (3.84 g cm⁻³) were formed on the La_0.8Sr_0.2CoO₃ surfaces due to reactions with moisture and carbon dioxide in ambient air. These results reveal that the three La_0.8Sr_0.2CoO₃ films provide simple reaction fields with similar surface areas for OER and ORR.

Figure 2.

(a) Out-of-plane and (b) in-plane XRD patterns and (c) observed and calculated XRR spectra of La_0.8Sr_0.2CoO₃ films synthesized on SrTiO₃ (001), (110), and (111) substrates. The fitting model used for XRR analyses are shown as inset in Fig. 2(c). STO = SrTiO₃.

Table 1. Refined density (d), thickness (t), and roughness (r) of La_0.8Sr_0.2CoO₃ (001), (110), and (111) films from XRR analyses.

	Surface layer			La0.8Sr0.2CoO3			SrTiO3
Substrate	d/g cm−3	t/nm	r/nm	d/g cm−3	t/nm	r/nm	d/g cm−3	r/nm
SrTiO3(001)	3.8	0.7	0.4	7.1	24.3	0.4	5.1*	0.4
SrTiO3(110)	3.7	0.9	0.5	7.1	23.7	0.5	5.1*	1.0
SrTiO3(111)	3.8	0.9	0.5	7.1	23.1	0.8	5.1*	1.0

*The density of SrTiO₃ was fixed for refinement.

Figure 3 depicts the Tafel plots of the La_0.8Sr_0.2CoO₃ (001), (110), and (111) films from steady-state polarization curves collected during the first and second ORR processes. The cathodic current increased when the applied voltage was changed from 0 to −0.4 V vs. Hg/HgO due to the ORR (total reaction: O₂ + 2H₂O + 4e⁻ → 4OH⁻). All the plots show a linear relationship between the logarithm of the current density and the electrode potential, which follows the Tafel equation. The Tafel slopes for the first ORR were determined to be −57, −81, and −118 mV dec⁻¹ for the (001), (110), and (111) films, respectively. The (001) film with the smallest Tafel slope had a higher charge transfer rate for the ORR than the (110) and (111) films. Considering the similar specific surface areas of the three La_0.8Sr_0.2CoO₃ films, the difference in their ORR activity should be associated with the lattice orientation of the films. In the second ORR process, the Tafel slopes changed to −74, −113, and −121 mV dec⁻¹ for the (001), (110), and (111) films, respectively. The charge transfer rates of the ORR activities decreased after the first ORR and OER processes for all film electrodes. These results indicate that the (001) plane exhibits higher ORR activity than the (110) and (111) planes in both the first and second cycles.

Figure 3.

Tafel plots of La_0.8Sr_0.2CoO₃ films with (a) (001), (b) (110), and (c) (111) orientations.

Previously, we reported the orientation dependence of OER and ORR on La_0.8Sr_0.2CoO₃ air electrodes:¹⁷ the (110) film had a higher activity for OER and ORR than the (001) and (111) films from cyclic voltammetry measurements using a two-electrode type cell. The conflicting results between the two studies can be deduced as follows. The positions of the reduction and oxidation peaks in the cyclic voltammograms change according to the charge and mass transfer rates. ORR activity cannot be evaluated by separating the mass transfer rate from the charge transfer rate. Considering that no bubbling process was conducted in the previous study to achieve O₂ saturation prior to the cyclic voltammetry measurements, the peak positions might change depending on the mass transfer of O₂ molecules and/or OH⁻ anions at the interface. In this study, we measured the steady-state polarization curves at the initial cycles immediately after saturating the electrolyte with O₂. Considering the clearly linear Tafel plots here, we have successfully focused on the charge transfer rate of ORR at the specific film orientations of (001), (110), and (111). The differences in the steady-state potentials between the first and second cycles are greater than 10 mV for all samples. The increase in resistivity is calculated to over 4.8 × 10⁷ Ω m using the difference in the steady-state potential (>10 mV), the film thickness (30 nm), and the current density (0.7 µA cm²). This value is much higher than the resistivity of electronic conduction in cobalt-based perovskites, which supports that the charge transfer process is the rate-determining step of the ORR.

To further understand the degradation of ORR activity during the initial cycles, the crystal structure changes of a La_0.8Sr_0.2CoO₃ (001) film fabricated on MgO(001) were investigated using in-situ synchrotron XRD. Figure 4 shows the out-of-plane and in-plane XRD patterns and XRR analysis results. The out-of-plane XRD pattern shows 00l diffraction peaks, indicating a (001) orientation. In-plane diffraction peaks were also observed at 23.2°, 46.6°, and 73.8°, which were indexed as 100, 200, and 300, respectively. The four-fold symmetry observed in the in-plane ϕ-scan XRD pattern was consistent with the perovskite type with a pseudo-tetragonal lattice. These results confirm the epitaxial growth of La_0.8Sr_0.2CoO₃ (001) on the MgO (001) substrate. The out-of-plane and in-plane cell parameters of the La_0.8Sr_0.2CoO₃ film were c_tetra = 0.3821 nm and a_tetra = 0.3865 nm, respectively. The lattice expanded to match that of the MgO substrate, which has a larger cell parameter (0.4212 nm) than La_0.8Sr_0.2CoO₃ (0.380 nm for polycrystalline La_0.8Sr_0.2CoO₃ with a pseudocubic lattice).²⁸ Owing to the large difference between the lattices of La_0.8Sr_0.2CoO₃ and MgO, the in-plane diffraction peaks of La_0.8Sr_0.2CoO₃ was observed separately from those of the MgO substrate. This enabled us to detect structural changes along the in-plane directions during in-situ XRD measurements. XRR analysis using a three-layer model provided good fitting results. The thickness and surface roughness of the La_0.8Sr_0.2CoO₃ (001) film were calculated to be 30.7 and 0.8 nm, respectively. The thickness and SLD of the surface layer were 1.4 nm and 3.7 g cm⁻³, respectively. These values are similar to those of the surface layer formed on La_0.8Sr_0.2CoO₃ (001)/SrTiO₃(001) (Table 1). Thus, a high-quality La_0.8Sr_0.2CoO₃ (001) film was grown on the MgO (001) substrate just like that grown on the SrTiO₃ (001) substrate. In contrast, the La_0.8Sr_0.2CoO₃ (110) and (111) films could not be successfully grown on MgO (110) and (111) substrates, respectively, which may be due to the large difference in lattice size. The OER and ORR activities of La_0.8Sr_0.2CoO₃ (001)/MgO(001) were confirmed by cyclic voltammetry (Supporting Information, S1).

Figure 4.

(a) Out-of-plane, (b) in-plane, and (c) in-plane φ scan XRD patterns and (d) observed and calculated XRR spectra of a La_0.8Sr_0.2CoO₃ film grown on MgO (001) substrate. LSC = La_0.8Sr_0.2CoO₃.

In-situ XRD measurements were conducted using La_0.8Sr_0.2CoO₃(001)/MgO(001) under the initial ORR and OER conditions. Figure 5 shows the in-situ XRD patterns, d-values, and peak intensities of the out-of-plane 002 reflection for the 30 nm-thick La_0.8Sr_0.2CoO₃ (001) film. No significant changes were observed after cell construction (open circuit voltage (OCV) condition). At the first OER condition (0.7 V vs. the Pt quasi-reference electrode (PtQRE)), the peak intensity decreased with no change in peak position. We speculated that atomic displacements led to a long-range disorder in the La_0.8Sr_0.2CoO₃ lattice without changing the chemical composition. The OER process consists of several elementary steps such as the diffusion of OH⁻ ions in the electrolyte and their adsorption at the La_0.8Sr_0.2CoO₃ surface, charge transfer between the La_0.8Sr_0.2CoO₃ and OH⁻ ions, cleavage of O–H bond, and desorption of O₂ molecules. The La_0.8Sr_0.2CoO₃ (001) surface interacts with the adsorbed species during the OER, which could lead to a change in the crystal structure throughout the thin La_0.8Sr_0.2CoO₃ film. During the first ORR (−0.5 V), the peak intensity increased with a peak shift to lower angles. This shift corresponds to the increase in the lattice parameter c_tetra of the La_0.8Sr_0.2CoO₃ (001) film from 0.3828 to 0.3849 nm. The lattice expansion in the out-of-plane direction suggests that the chemical composition of the La_0.8Sr_0.2CoO₃ (001) film changed with the applied voltage. Similar changes were observed for the out-of-plane 202 peak during the first OER and ORR processes (see Supporting Information, S2). Generally, the lattice of perovskite oxides expands with increasing oxygen defects.^29,30 The cell parameter of La_2/3Sr_1/3CoO_3−∂ increased from 0.3811 to 0.4016 nm when the oxygen defect ∂ increased from 0 to 0.146.³¹ Thus, the lattice expansion indicates oxygen defect formation in the La_0.8Sr_0.2CoO₃ films during the first ORR process. Such formation of oxygen defects could be accompanied by the reduction of Co⁴⁺ to Co³⁺ in the near-surface region at low voltages:   

\begin{align} &\text{La$_{0.8}$Sr$_{0.2}$CoO$_{3}$} + \text{$x$H$_{2}$O} + \text{2$x$e$^{-}$}\notag\\ &\quad\to \text{La$_{0.8}$Sr$_{0.2}$CoO$_{3-x}$} + \text{2$x$OH$^{-}$} \end{align} (1)

The oxygen-deficient phase participates in the ORR from O₂ to 2O²⁻ at the top surface. Although the 002 peak shifted back to higher angles at the second OER (0.7 V), the c_tetra value of 0.3831 nm was larger than that at the first OER. During the second OER, the oxygen atoms partially reinserted into the La_0.8Sr_0.2CoO₃ lattice to cancel the oxygen deficiency. In the third OER, the 002 peak appeared at the same angle as that in the second cycle. These results indicate that the OER proceeded at the partially oxygen-deficient La_0.8Sr_0.2CoO₃ surface after the first ORR. In the third ORR, the 002 peak shifted to lower angles while increasing its intensity, which is similar to that in the first ORR. Oxygen atoms were removed from La_0.8Sr_0.2CoO₃ during the ORR and re-inserted into La_0.8Sr_0.2CoO₃ during the OER after the initial cycles. In contrast, the peak position was located at slightly higher angles than that of the first ORR. The amount of oxygen deficiency became smaller in the third ORR compared to that in the first ORR. These structural changes in the crystal affect electronic structures at the La_0.8Sr_0.2CoO₃ surface, which may lower the OER and ORR activities after the initial cycle. Furthermore, it should be noted that the relative intensities of the 002 peak decreased to 61 % and 69 % of that at the pristine condition at the third OER and ORR, respectively. This gradual decrease in the peak intensity may be associated with irreversible structural changes during the OER and ORR cycles. This suggests another deterioration mechanism of the La_0.8Sr_0.2CoO₃ film, such as decomposition to La(OH)₃ and Co(OH)_x by reactions with chemical species in the strongly basic KOH aq.³²

Figure 5.

In-situ XRD patterns, d-values, and peak intensities of out-of-plane 002 reflection for La_0.8Sr_0.2CoO₃ (001) film from the pristine (dry) to the third ORR conditions.

Figure 6 shows the XRD patterns of the in-plane 200 reflection for the 30 nm-thick La_0.8Sr_0.2CoO₃ (001) film collected using two X-ray incident angles of 0.24° and 0.14°. The corresponding penetration depths of X-rays inside the La_0.8Sr_0.2CoO₃ film were calculated to be 64 and 2.7 nm, respectively, from the critical angle of La_0.8Sr_0.2CoO₃ (0.20°, estimated for 15 keV X-rays). The in-plane XRD patterns collected using an incident angle of 0.24° reflect crystal structures of the entire La_0.8Sr_0.2CoO₃ film. The 200 diffraction peak showed no significant change in its position during the OER and ORR processes. This behavior is often observed in the in-plane diffraction peaks of epitaxial film electrodes, because the lattice change is strongly constrained by the substrate lattice.^14–16 In contrast, the peak intensity changed during the OER and ORR processes, which corresponds to changing atomic positions in the restricted La_0.8Sr_0.2CoO₃ lattice. The peak intensity decreased at the first OER and increased at the first ORR, which is consistent with that observed for the out-of-plane 002 peak. This indicates that similar atomic displacements occurred along the out-of-plane and in-plane directions in the La_0.8Sr_0.2CoO₃ bulk. The in-plane XRD patterns collected using an incident angle of 0.14° mainly contain structural information about the surface region of the La_0.8Sr_0.2CoO₃ film. Similar to the bulk region, the 200 diffraction peak showed no significant position change during the OER and ORR processes. The La_0.8Sr_0.2CoO₃ lattice in the surface region was also restricted in its volume change, which may be due to the small thickness of 30 nm. In contrast to the bulk region, the peak intensity increased during the first OER and decreased during the first ORR processes. No significant change in the 200 peak was observed from the first ORR to the second OER. Although surface structural changes could not be detected along the out-of-plane direction, we speculated that the La_0.8Sr_0.2CoO₃ surface has different atomic arrangements from the La_0.8Sr_0.2CoO₃ bulk, which could be directly associated with the electrochemical activity of the ORR and OER at the La_0.8Sr_0.2CoO₃/electrolyte interface.

Figure 6.

In-situ XRD patterns of in-plane 200 reflection for La_0.8Sr_0.2CoO₃ (001) film collected using different X-ray incident angles: (a) L = 0.06 (0.24°) and (b) L = 0.04 (0.14°).

The in-situ XRD results clarified the intensity decrease in the out-of-plane 002 peak by suggesting decomposition of the La_0.8Sr_0.2CoO₃ layer to produce a surface impurity layer. The bulk and surface chemical species of a 30 nm-thick La_0.8Sr_0.2CoO₃ (001) film were investigated by HAXPES at different take-off angles (TOAs). Figure 7 depicts the La 3d_5/2, Co 2p_3/2, and O 1s XPS data of the La_0.8Sr_0.2CoO₃ (001) film deposited on MgO (001), collected at the pristine state and after the first OER and ORR conditions. In the La 3d_5/2 region, there were two peaks at 832.9 and 837.3 eV derived from the final-state doublet of La 3d_5/2, similar to the behavior observed for La_0.5Sr_0.5CoO_3−δ.³³ The intensity ratio of La_II to La_I increased from the pristine to the first OER conditions. The La 3d_5/2 spectra collected at TOA = 15° showed higher La_II/La_I ratios than those collected at TOA = 80°. This reveals the formation of a surface phase that exhibits La 3d_5/2 peaks at higher binding energies. It has been reported that La_1−xCa_xCoO₃ decomposes to form La(OH)₃ during the OER reaction in an alkaline solution.³⁴ The La 3d_5/2 peaks of La(OH)₃ were reported to appear at 835.1 and 839.0 eV,³² in agreement with the increased peak intensity in the La_II region between 835 and 840 eV. No significant changes in the La_II/La_I ratio were observed in either the bulk or surface regions during the first ORR. The La(OH)₃ layer formed at the La_0.8Sr_0.2CoO₃ surface during the first OER was electrochemically stable in the subsequent processes. The O 1s spectrum of the pristine La_0.8Sr_0.2CoO₃ film mainly consists of two peaks at 528.7 and 532.7 eV, which correspond to lattice oxygens in La_0.8Sr_0.2CoO₃ and SrCO₃, respectively.³⁵ The intensity of the 532.7 eV peak was higher in the spectrum collected at TOA = 15° than that collected at 80°. The SrCO₃ impurity was formed as a surface layer by the chemical reaction of La_0.8Sr_0.2CoO₃ with air species, which agrees with the XRR analysis results (Fig. 2c and Table 1). This peak disappeared during the first OER owing to the dissolution of SrCO₃ into the electrolyte. A new surface layer with an O 1s peak at 532.0 eV was observed in the first OER. This peak can be identified as La(OH)₃,³⁴ as detected in the La 3d_5/2 spectra. The OH⁻ ions and O₂ molecules must diffuse into the surface La(OH)₃ layer during the OER and ORR. The 528.7 eV peak derived from the lattice oxygen in La_0.8Sr_0.2CoO₃ showed no significant changes after the OER and ORR processes in both the bulk and surface regions. This result indicates that oxygen atoms are not involved in charge compensation for oxygen desertion/insertion in the La_0.8Sr_0.2CoO₃ film. XPS data in the Co 2p_3/2 region show a broad peak with a shoulder at high binding energies, making it difficult to identify peak position changes under the OER and ORR conditions. The peak top position slightly shifted to lower binding energies after the ORR, implying the reduction of cobalt ions in the oxygen-deficient La_0.8Sr_0.2CoO₃ formed under ORR conditions.³³ It is difficult to discuss the oxidation states of cobalt ions and the formation of impurity phases. The cobalt reduction was supported by ex-situ XANES spectra, where the Co-K absorption edge for the La_0.8Sr_0.2CoO₃ (001) film shifted to lower energies after the ORR treatment (Supporting Information, S3).

Figure 7.

(a, b) La 3d_5/2, (c, d) Co 2p_3/2, and (e, f) O 1s HAXPES data of a La_0.8Sr_0.2CoO₃ (001) film deposited on MgO (001) at pristine and after the first OER and ORR conditions. Different take-off-angles (TOA) of photoelectrons, namely (a, c, e) 80° and (b, d, f) 15°, were used to investigate electronic states in the whole and surface regions of the La_0.8Sr_0.2CoO₃ film.

Figure 8 is a schematic representation of the structural changes at the La_0.8Sr_0.2CoO₃(001)/KOH aq. interface during the initial OER and ORR based on our electrochemical, in-situ XRD, and ex-situ HAXPES measurements. The pristine La_0.8Sr_0.2CoO₃(001) surface is covered with surface impurity phases such as SrCO₃ and Sr(OH)₂, which are products of chemical reactions of La_0.8Sr_0.2CoO₃ with carbon dioxide and moisture in air. These impurities dissolve into the aqueous solution after constructing the electrochemical cell, and a new La(OH)₃ surface layer is formed under the strong alkaline conditions. During the first OER process, OH⁻ ions penetrate the La(OH)₃ surface layer and are oxidized to generate O₂ molecules. The atomic arrangement of La_0.8Sr_0.2CoO₃(001) changed in the surface regions, which could be associated with the absorption and desorption of the reaction species. Although crystal structure in the bulk region does not significantly change in the first OER condition, the La_0.8Sr_0.2CoO₃ lattice expands during the first ORR due to the formation of oxygen deficiencies in the lattice. That is, lattice oxygen is extracted from La_0.8Sr_0.2CoO₃ and OH⁻ is formed according to reaction (1). This leads to further changes in the surface structure that affect the ORR activity. Although La_0.8Sr_0.2CoO₃ (001) showed some degree of reversible structural changes in the OER and ORR in the following cycles, the structures were different from those in the first cycle. Our electrochemical investigation revealed a significant decrease in the charge transfer rate of ORR at the La_0.8Sr_0.2CoO₃ (001) surface during the second cycle. The design of surface structure at the electrochemical interface is therefore a key issue for improving the ORR activity of perovskite-type catalysts in metal–air batteries.

Figure 8.

Schematic representation of structural changes at La_0.8Sr_0.2CoO₃/KOH aq. interface during the initial OER and ORR.

4. Conclusion

The lattice plane dependence of ORR properties for the La_0.8Sr_0.2CoO₃ air electrodes was investigated using two-dimensional thin-film electrodes with (001), (110), and (111) orientations. Each film electrode was 30 nm in thickness with a surface roughness of less than 1.0 nm, providing a suitable reaction field for studying the lattice plane dependence. Steady-state polarization measurements in a KOH aqueous solution showed the smallest charge transfer resistance of ORR at the (001) surface in the first cycle, which is an experimental demonstration of the different ORR activities depending on the reaction plane. However, the ORR activity decreased significantly during the second cycle. The crystal structure of the La_0.8Sr_0.2CoO₃ surface changed with the formation of oxygen defects due to oxygen desertion from and insertion into the La_0.8Sr_0.2CoO₃ lattice. Furthermore, the La_0.8Sr_0.2CoO₃ surface was partially decomposed by reacting with chemical species in the aqueous solution to form an interphase between La_0.8Sr_0.2CoO₃ and the solution. Our findings reveal that controlling the interfacial structure formed in the early stages of the ORR and OER is as important as designing the unreacted surface structure for developing perovskite-type air electrodes with high ORR activity.

Acknowledgments

This study, conducted in collaboration with the Genesis Research Institute, was partly supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 19H05793) and a Grant-in-Aid for Scientific Research (A) (No. 22245035) from the Japan Society for the Promotion of Science. The XRD, HAXPES, and XAFS experiments using synchrotron radiation were performed as projects approved by the Japan Synchrotron Radiation Research Institute (JASRI) (proposal Nos. 2010B1817, 2010B3675, 2011A1866, and 2018B3641).

Data Availability Statement

The data that support the findings of this study are openly available under the terms of the designated Creative Commons License in J-STAGE Data at https://doi.org/10.50892/data.electrochemistry.20966677.

CRediT Authorship Contribution Statement

Akira Matsuzaki: Data curation (Lead), Formal analysis (Supporting), Investigation (Equal)

Masaaki Hirayama: Funding acquisition (Equal), Investigation (Lead), Methodology (Lead), Supervision (Lead), Visualization (Lead), Writing – original draft (Lead), Writing – review & editing (Lead)

Shouya Ohguchi: Data curation (Supporting), Investigation (Supporting)

Mamoru Komo: Data curation (Supporting), Investigation (Supporting)

Atsunori Ikezawa: Investigation (Equal), Methodology (Equal), Writing – review & editing (Equal)

Kota Suzuki: Investigation (Supporting), Methodology (Supporting), Writing – review & editing (Supporting)

Kazuhisa Tamura: Data curation (Equal), Investigation (Supporting), Methodology (Equal), Writing – review & editing (Supporting)

Hajime Arai: Methodology (Equal), Writing – review & editing (Equal)

Ryoji Kanno: Funding acquisition (Equal), Methodology (Equal), Supervision (Supporting), Writing – review & editing (Supporting)

Conflict of Interest

The authors declare no conflict of interest in the manuscript.

Funding

Japan Society for the Promotion of Science: 19H05793

Japan Society for the Promotion of Science: 22245035

Footnotes

A part of this paper has been presented in the ECSJ Fall Meeting in 2019 (Presentation #2J13).

M. Hirayama, A. Ikezawa, K. Suzuki, K. Tamura, and H. Arai: ECSJ Active Members

R. Kanno: ECSJ Fellow

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