Enhanced catalytic activity of perovskite La_1−xSr_xMnO_3+δ for the oxygen reduction reaction

Abstract

Fuel cells are considered promising next-generation energy conversion devices because of their high efficiency and environment-friendly attributes. However, a large overpotential for the oxygen reduction reaction (ORR) at the cathode minimizes the energy efficiency of these devices. In this context, ORR catalysts based on 3d transition metal oxides have attracted significant attention as affordable and highly active catalysts with potential to replace expensive noble metal catalysts (e.g., Pt). In this study, the ORR catalytic activities of ‘oxygen-excess’ perovskite La_1−xSr_xMnO_3+δ (x = 0, 0.1, 0.2, 0.3, and 0.4), with both higher activity and stability compared to a regular perovskite, i.e., La_0.8Sr_0.2MnO₃, were investigated and the performances were evaluated with respect to changes in the Mn valence number, amount of cation vacancies and Mn–O bond length. Sr substitution can affect the formation of Mn⁴⁺ ions and modulate the cation vacancies, thus changing the length of the Mn–O bond. The lowest overpotential was observed for La_0.8Sr_0.2MnO_3+δ, where the Mn–O bond length was the shortest, resulting in the highest activity. Overall, this work provides guidelines to improve the ORR catalytic activity of Mn-based perovskites.

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1. Introduction

The rapidly increasing global environmental pollution caused by the massive use of fossil fuels necessitates the development and use of clean and renewable energy resources.^1,2 Accordingly, various energy storage and conversion devices have been proposed in the last few decades.^3–7 Among them, fuel cells have attracted considerable attention as next-generation clean energy conversion devices because of their high energy conversion efficiency (60–70%) and environment-friendly attributes.⁸ In fuel cells, the chemical energy stored in fuels, such as hydrogen, methanol, and ethanol, is directly converted into electrical energy at the anode, while a simultaneous oxygen reduction reaction (ORR) occurs at the cathode. Both the fuel oxidation reaction and ORR must be carried out in the presence of a catalyst that significantly lowers the overpotential, leading to an efficient energy conversion.^9,10 However, ORR catalysts must be carefully designed as the ORR is a kinetically slow process;^11,12 thus, catalysts that exhibit both high activity and stability are generally expensive, and those with good attributes yet low costs are yet to be found.

For the ORR in alkaline solutions, two pathways are possible as shown below.¹³

4-Electron pathway (eqn (1)):

O₂ + 2H₂O + 4e⁻ ↔ 4OH⁻ (1)

2-Electron pathway (eqn (2)):

O₂ + H₂O + 2e⁻ → HO₂⁻ + OH⁻ (2)

Noble metals, such as Pt and Pd, have been widely used as ORR catalysts with excellent catalytic activity.^14,15 However, these are expensive and constrained by limited resources. Alternative catalysts, which are abundant and thus cheap yet similarly effective to or more effective than the precious metals in catalyzing the ORR, can solve the cost issue associated with ORR catalysts.^16,17 Transition metal oxides are considered as the most promising alternative catalysts.^18,19 In particular, perovskites with a general formula of ABO₃ are of great interest because of their high catalytic activity, significantly low cost, and compositional flexibility.^20,21 Hyodo et al. reported the effects of A- and B-site cations on the ORR catalytic activity using various types of perovskites.²² For the A-site cations, La-based perovskites showed a higher ORR catalytic activity than Pr-, Nd-, and Sm-based perovskites. Regarding the B-site cations, Mn, Fe, Co, and Ni with partially filled 3d orbitals showed high ORR catalytic activities. Therefore, LaMnO₃ has attracted considerable attention as an ORR catalyst. Notably, the perovskite structure allows the formation of non-stoichiometric compounds, such as oxygen-deficient LaMnO_3−δ and ‘oxygen-excess’ LaMnO_3+δ. It is worth noting that ‘oxygen-excess’ does not mean the existence of interstitial oxygen ions. Roosmalen et al. proposed that the ‘oxygen-excess’ originates from the existence of cation vacancies at La and Mn sites, and the amounts of La and Mn vacancies should be equal.²³ The real chemical formula of LaMnO_3+δ should be La_1−γMnO_1−γO₃(γ = δ/(3 + δ)), but for simplicity the former expression is used as the name of the sample. Elemental substitution is a method frequently used to modulate the defect density and electronic structure of LaMnO₃ to improve its ORR catalytic activity.^24,25 Commonly used elements for replacing La at the A-site are alkaline earth metals, such as Ca and Sr, and those for replacing Mn at the B-site are 3d transition metals, such as Ni, Fe, and Co. Furthermore, the partial replacement of La with Sr has been proven effective in improving the ORR catalytic activity via modulation of the Mn valence state and O vacancy concentration.^26,27 Most current studies on LaMnO₃ have been concerned with either stoichiometric LaMnO₃ or oxygen-deficient LaMnO_3−δ; however, the exploration of ‘oxygen-excess’ LaMnO_3+δ and its electronic structure modulation induced by Sr replacement is scarce.

In this study, we investigated the effects of Sr substitution in nonstoichiometric LaMnO_3+δ on the ORR catalytic activity in an oxygen-saturated 0.1 M KOH aqueous solution. We found that the Sr-substituted and ‘oxygen-excess’ La_1−xSr_xMnO_3+δ compounds (x = 0, 0.1, 0.2, 0.3, and 0.4) showed higher catalytic activity and stability than the normal Sr-substituted perovskite (i.e., La_0.8Sr_0.2MnO₃). Through a combination of spectroscopic and electrochemical studies, we specifically examined the intrinsic reasons for the ORR catalytic activity difference of the La_1−xSr_xMnO_3+δ series. The catalytic activity dependence on the Mn valence state, cation vacancies and Mn–O bond length is discussed.

2. Experimental

2.1. Synthesis of La_1−xSr_xMnO_3+δ

The La_1−xSr_xMnO_3+δ (x = 0, 0.1, 0.2, 0.3, and 0.4) compounds were synthesized by a sol–gel process. First, La(NO₃)₃·6H₂O (Nacalai Tesque, Inc. 99.9%), Sr(NO₃)₂ (Sigma-Aldrich, 99.0%), and Mn(NO₃)₂·6H₂O (Nacalai Tesque, Inc. 98.0%) were weighed according to their stoichiometric ratios and dissolved in 30 mL deionized water. After stirring the solutions for 2 h, 4.61 g of citric acid (Nacalai Tesque, Inc., 99.5%) was added to each solution, which were then stirred for another 2 h. Subsequently, the solutions were heated to 80 °C and maintained at that temperature for 12 h to obtain yellow gel precursors. The final products were obtained after the calcination of the gel precursors at 750 °C for 5 h. For comparison, stoichiometric La_0.8Sr_0.2MnO₃ (99%) powder was bought from Sigma-Aldrich.

2.2. Characterization

The crystal structure was examined by XRD measurements (Ultima IV, Rigaku Co., Ltd) of the powder samples using a molybdenum X-ray tube (λ = 0.7107 Å). Morphologies were observed by scanning electron microscopy (SEM, JEOL JSM-6010 LA) and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The valence state of Mn was analyzed by XPS using a ULVAC-PHI Quantera instrument. ICP-AES was conducted to determine the ratios of La, Sr, and Mn using a sample dissolved in a 1 M HCl aqueous solution. The sum of 2[Mn³⁺] and [Mn⁴⁺] was determined by iodometry using a titration device (COM-A19S, Hiranuma Co., Ltd). In detail, 10 mg of the sample was mixed with 40 mL of 0.1 g mL⁻¹ KI solution. Then, 3 mL of 6 mol L⁻¹ HCl was added to dissolve the sample. All the Mn³⁺ and Mn⁴⁺ ions will be reduced to Mn²⁺ by I⁻ as shown below:²⁸

2Mn³⁺ + 3I⁻ → 2Mn²⁺ + I₃⁻ (3)

Mn⁴⁺ + 3I⁻ → Mn²⁺ + I₃⁻ (4)

Then titration was performed using 10 mmol L⁻¹ Na₂S₂O₃ solution assuming the following reaction:

I₃⁻ + 2S₂O₃²⁻ → 3I⁻ + S₄O₆²⁻ (5)

The numbers of La³⁺, Sr²⁺, Mn³⁺, Mn⁴⁺, La and Mn vacancies (V_A-site and V_B-site) were estimated by considering the results of ICP, iodometry, and charge balance. XAS was performed in the transmission mode to obtain Mn K-edge absorption spectra at SPring-8 in Japan with a Si(111) double-crystal monochromator operated at 8 GeV. The Athena program was used for the background subtraction and normalization of the XAS data, and the Artemis program integrated with the IFEFFIT package was used for shell fitting of the XAS spectra in the R space.²⁹

2.3. Electrochemical measurements

The catalyst ink used for testing was prepared as follows. First, 10 mL of Nafion solution (5 mass% in a mixture of lower aliphatic alcohols and water, 45% water, Sigma-Aldrich) and 20 mL of 0.1 M KOH aqueous solution were mixed to obtain a K⁺-exchanged Nafion solution. Then, 0.3 mL of the K⁺-exchanged Nafion solution, 50 mg of the perovskite catalyst, and 10 mg of acetylene black were mixed in a 10 mL volumetric flask, and the flask was filled to 10 mL with a 6 : 4 (v/v) mixture of water and ethanol.

Electrochemical measurements were conducted using a three-electrode cell with a rotating ring disk electrode (RRDE) system (RRDE-3A, BAS Inc.) at a speed of 1600 rotations per min in 0.1 M KOH aqueous solution. The RRDE consisted of a glassy carbon disk (area ∼ 0.126 cm²) and a platinum ring (area ∼ 0.188 cm²). A Pt wire was used as the counter electrode, and a Hg/HgO (0.1 M KOH aqueous solution) electrode was used as the reference electrode.

Before the electrochemical measurements, oxygen was purged into the electrolyte solution for 20 min to obtain an oxygen-saturated solution. Note that, although oxygen bubbling was stopped after achieving the saturation in the solution, the oxygen flow was maintained above the solution during the measurements. LSV was performed at a scan rate of 10 mV s⁻¹ in the potential range from 0.3 to −0.6 V vs. Hg/HgO (0.1 M aq. KOH). The potential of the ring electrode was set to 0.4 V vs. Hg/HgO to oxidize HO₂⁻ formed on the disk electrode.

The production fraction of HO₂⁻ and electron transfer number (n) were calculated using the following equations:

where I_d is the disk current, I_r is the ring current, and N (= 0.424) is the capture rate of the RRDE electrode in this study.

All the potential values measured vs. Hg/HgO (0.1 M aq. KOH), i.e., E (V vs. Hg/HgO), were converted to the potential values vs. reversible hydrogen electrode, E (V vs. RHE), using the following relationship:

E (V vs. RHE) = E (V vs. Hg/HgO) + 0.926 (8)

3. Results and discussion

3.1 Structure, morphology, and composition

The crystal structures of the prepared La_1−xSr_xMnO_3+δ compounds were analyzed by X-ray diffraction (XRD) (Fig. 1a). The diffraction peaks for all samples with different amounts of Sr matched well with those for the reported perovskite-type structure of LaMnO₃ (ICSD#75070).³⁰ The lattice parameters obtained from the XRD patterns are listed in Table S1 (ESI†). The crystal system is rhombohedral and the space group is R3̄c, which has been reported to have a higher ORR catalytic activity than the orthorhombic structure.³¹ In Fig. 1b–f, the d-spacing obtained from the high-resolution TEM images also confirmed the rhombohedral structure of these samples. In the XRD profile of La_0.6Sr_0.4MnO_3+δ, minor impurity peaks appeared at approximately 12°, which corresponds to SrCO₃ (ICSD#15195), possibly formed by a reaction of Sr with citric acid during synthesis. Highly pure samples could not be obtained above x = 0.6, as shown in Fig. S1 (ESI†). SEM images and TEM images in Fig. S2 and S3 (ESI†) suggest that all the samples have similar morphologies. The primary particle diameters of all the samples are mostly in the range of 10–20 nm as shown in Fig. S3 (ESI†), indicating that Sr-substitution does not change the morphology and particle size obviously.

Fig. 1 (a) XRD patterns measured with Mo Kα radiation (wavelength: 0.7107 Å). The peak at 12° (marked with **) corresponds to an impurity peak of SrCO₃. (b–f) High-resolution TEM images with lattice spacings.

Inductively coupled plasma atomic emission spectroscopy (ICP-AES) and iodometry were performed to determine the numbers of La, Sr, and Mn (Mn³⁺ and Mn⁴⁺) assuming a chemical formula of La_1−x−y(V_A-site)_ySr_xMn_a³⁺Mn_b⁴⁺(V_B-site)_yO₃,²³ as shown in Table 1. The amount of cation vacancies is decreased, while the fraction of Mn⁴⁺ monotonically increased from 0.341 to 0.680 to maintain electrical neutrality as the amount of Sr is increased from 0 to 0.4.

Table 1 Actual composition analysis using the chemical formula of La_1−x−y(V_A-site)_ySr_xMn_a³⁺Mn_b⁴⁺(V_B-site)_yO₃

Sample name	La	Sr	V A-site	Mn^3+	Mn^4+	V B-site
LaMnO3+δ	0.943 ± 0.002	0	0.057 ± 0.012	0.602 ± 0.012	0.341 ± 0.012	0.057 ± 0.012
La0.9Sr0.1MnO3+δ	0.856 ± 0.001	0.094 ± 0.001	0.050 ± 0.014	0.553 ± 0.014	0.397 ± 0.014	0.050 ± 0.014
La0.8Sr0.2MnO3+δ	0.749 ± 0.002	0.203 ± 0.002	0.048 ± 0.011	0.457 ± 0.011	0.494 ± 0.011	0.048 ± 0.011
La0.7Sr0.3MnO3+δ	0.640 ± 0.002	0.315 ± 0.002	0.045 ± 0.015	0.369 ± 0.015	0.586 ± 0.015	0.045 ± 0.015
La0.6Sr0.4MnO3+δ	0.545 ± 0.001	0.410 ± 0.001	0.044 ± 0.012	0.275 ± 0.012	0.680 ± 0.012	0.044 ± 0.012

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3.2 Energy spectroscopies

X-Ray photoelectron spectroscopy (XPS) was conducted to analyze the chemical environment of Mn and O. As can be seen from the Mn 2p XPS profiles in Fig. 2a, Mn 2p_3/2 and Mn 2p_1/2 peaks are centered at 641.5 and 653.3 eV, respectively. The spin–orbit splitting value (ΔE) between Mn 2p_3/2 and Mn 2p_1/2 is 11.8 eV. Through XPS fitting, both the Mn 2p_1/2 and 2p_3/2 peaks can be deconvoluted into three peaks, comprising a primary and a satellite peak for Mn³⁺ and a peak for Mn⁴⁺.^32,33 The obtained binding energies are listed in Table S2 (ESI†). In all the samples, Mn exists in both +3 and +4 valence states, while the valence number of Mn can be considered +3 in the completely stoichiometric LaMnO₃. The atomic ratios of Mn³⁺ and Mn⁴⁺ obtained via XPS fitting are listed in Table 2. The percentage of Mn⁴⁺ increased from 32.3% to 67.4%, which is in good agreement with the titration results shown in Table 1. O 1s XPS profiles are shown in Fig. 2b, which can be assigned to two peaks: a low binding energy peak, which represents the lattice oxygen O_L, and a high energy peak, which can be attributed to the surface adsorbed oxygen species O_S.^34,35 The La_0.8Sr_0.2MnO_3+δ sample shows the highest surface adsorbed oxygen species concentration as shown in Table 2. This may partially contribute to ORR catalytic activity.

Fig. 2 (a) XPS Mn 2p spectra and (b) XPS O 1s spectra of La_1−xSr_xMnO_3+δ.

Table 2 Atomic concentration ratios of Mn³⁺ and Mn⁴⁺; O_S and O_L estimated by fitting the XPS profiles shown in Fig. 2

Sample	Mn^3+ (%)	Mn^4+ (%)	OS (%)	OL (%)
LaMnO3+δ	67.8	32.3	37.8	62.2
La0.9Sr0.1MnO3+δ	49.4	50.6	40.3	59.7
La0.8Sr0.2MnO3+δ	41.1	58.9	45.5	54.5
La0.7Sr0.3MnO3+δ	34.7	65.3	37.4	62.6
La0.6Sr0.4MnO3+δ	32.6	67.4	44.1	55.9

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X-Ray absorption spectroscopy (XAS) is a powerful tool to analyze the oxidation states and bond conditions. Fig. 3a presents the X-ray absorption near-edge spectra (XANES, Mn K-edge) of the La_1−xSr_xMnO_3+δ samples. The spectra of all the samples are located between the spectra of standard Mn₂O₃ and MnO₂, which again indicates that the valence states of Mn in all these samples are between +3 and +4, consistent with the XPS and ICP/iodometry analyses. Fig. 3b shows the extended X-ray absorption fine structure (EXAFS) spectra of the Mn K-edge in the R space. The first peak between 1.0 and 2.0 Å is related to the Mn–O bond.^36,37 To evaluate the Mn–O bond length, EXAFS fitting was performed by assuming a rhombohedral LaMnO₃ structure with the first nearest shell model using the Artemis program integrated with the IFEFFIT package in the 1.0–2.0 Å range.²⁹ The Mn–O bond lengths, determined from the fitting (cf. Table S3, ESI†), are shown in Fig. 3c. When x was increased from 0 to 0.2, the Mn–O bond length gradually decreased and reached a minimum at x = 0.2. The shortest bond length may be a result of the combination of an increase in the oxidation state of Mn and a decrease in Mn vacancies as shown in Tables 1 and 2. In addition, the shortest Mn–O bond length of the La_0.8Sr_0.2MnO_3+δ sample may result in the strongest interaction between Mn and O, which can explain the highest surface adsorbed oxygen species concentration obtained from O 1s XPS spectral fitting results.

Fig. 3 (a) Normalized Mn K-edge XANES profiles of La_1−xSr_xMnO_3+δ, Mn₂O₃ and MnO₂. (b) Fourier transform of the Mn K-edge EXAFS spectra of La_1−xSr_xMnO_3+δ in the R space. (c) Fitted Mn–O bond length in La_1−xSr_xMnO_3+δ. Error bars show the standard deviations.

3.3 ORR catalytic properties

The ORR catalytic activity of La_1−xSr_xMnO_3+δ (x = 0, 0.1, 0.2, 0.3, and 0.4) was evaluated using a rotating ring disk electrode (RRDE) system in a 0.1 M KOH aqueous solution (Fig. 4). A comparison of the linear sweep voltammograms (LSVs) in Fig. 4a suggests that La_0.8Sr_0.2MnO_3+δ exhibits the highest current density among all perovskites in the La_1−xSr_xMnO_3+δ series. The current density is not linearly correlated with the value of x, and it decreases in the following order: x = 0.2 > x = 0.4 > x = 0.3 > x = 0.1 > x = 0. The overpotentials (E_over) determined at −2 mA cm⁻² and the onset potentials (E_onset) determined at −0.1 mA cm⁻² obtained from the LSVs are listed in Table 3. E_over, E_onset, and the current density are all correlated; a lower E_over corresponds to a higher current density. Therefore, La_0.8Sr_0.2MnO_3+δ exhibited both the lowest E_over and the highest current density, indicating the highest catalytic activity among others with different Sr substitution. In the ORR, the 4-electron pathway is preferred over the 2-electron pathway because of the higher energy conversion efficiency of the former pathway. The production fraction of HO₂⁻ (HO₂⁻%) and electron transfer number (n)³⁸ were calculated using eqn (6) and (7) (see the Experimental section for details) using the disk and ring currents (Fig. S4, ESI†), as shown in Fig. 4b and c, respectively. LaMnO_3+δ produces >10% HO₂⁻ at 0.4 V vs. RHE through the 2-electron pathway, resulting in the lowest value of n (3.78) in the series. With 10% Sr substitution (i.e., La_0.9Sr_0.1MnO_3+δ), the HO₂⁻% decreases significantly to <5%, and n is close to 4. In particular, the HO₂⁻% and n for La_0.8Sr_0.2MnO_3+δ are very close to those of Pt, indicating the excellent 4-electron selectivity of the catalyst. As shown in Fig. 3c and Table 3, the trends for the changes in overpotential and Mn–O bond length are fully consistent. The above trends are in agreement with the report by Suntivich et al.³¹ who stated that the ORR catalytic activity is related to the B–O covalency in the ABO₃ perovskite, where a strong covalency of the B–O bond can increase the driving force of the rate-limiting O²⁻/OH⁻ exchange reaction occurring on the surface B ions. According to the EXAFS fitting results, the La_0.8Sr_0.2MnO_3+δ sample had the shortest Mn–O bond length with a high degree of covalency, resulting in the highest ORR catalytic activity in the series.

Fig. 4 (a) LSV curves obtained from the measurements in 0.1 M KOH aqueous solution using the disk electrode modified with different catalysts with a scan rate of 10 mV s⁻¹ at 1600 rpm. Pt/C served as the reference catalyst. (b) Fraction of HO₂⁻, (c) electron transfer number (n), and (d) LSV curves measured using La_0.8Sr_0.2MnO_3+δ during 500 cycles of cyclic voltammetry.

Table 3 Overpotential and onset potential at current densities of −2 and −0.1 mA cm⁻², respectively, obtained from the LSV curves of the ORR using La_1−xSr_xMnO_3+δ and Pt/C as catalysts

Sample	E over (V)	E onset (V vs. RHE)
LaMnO3+δ	0.494	0.900
La0.9Sr0.1MnO3+δ	0.456	0.961
La0.8Sr0.2MnO3+δ	0.398	1.135
La0.7Sr0.3MnO3+δ	0.444	1.035
La0.6Sr0.4MnO3+δ	0.421	1.069
Pt/C	0.355	1.085

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Durability of the catalysts is another important aspect for ORR catalysis. The LSV curves for the best sample, La_0.8Sr_0.2MnO_3+δ, measured during 500 cycles of cyclic voltammetry, are shown in Fig. 4d. For comparison, the LSV curves for La_0.8Sr_0.2MnO₃ over 500 cycles are shown in Fig. S5 (ESI†). After 500 cycles, the current density for La_0.8Sr_0.2MnO₃ was significantly lower than the initial value, indicating poor stability. In contrast, for the cycling using La_0.8Sr_0.2MnO_3+δ, the decrease and increase in the current density and overpotential, respectively, were smaller and gradual. Even after 500 cycles, the overpotential at −2 mA cm⁻² increased only by 5.5% compared to the initial value, indicating excellent stability. In addition, the Mn 2p XPS profiles for La_0.8Sr_0.2MnO_3+δ before and after 500 cycles of cyclic voltammetry are shown in Fig. S6 (ESI†), and the atomic ratios of Mn³⁺ and Mn⁴⁺ estimated using the XPS profiles are listed in Table S4 (ESI†). After 500 cycles, the ratio of Mn⁴⁺ was decreased, suggesting a gradual reduction of Mn⁴⁺ to Mn³⁺ during 500 cycles. Thus, the slight increase of the overpotential for the ORR of La_0.8Sr_0.2MnO_3+δ of about 5.5% after 500 cycles as shown in Fig. 4d could have been caused by the decrease in the Mn valence number.

4. Conclusions

In this study, we synthesized a series of perovskites with a general formula of La_1−xSr_xMnO_3+δ (x = 0, 0.1, 0.2, 0.3, and 0.4) via Sr substitution of ‘oxygen-excess’ LaMnO_3+δ and investigated the effect of Sr substitution on the ORR catalytic activity of the resultant catalysts. Sr substitution leads to an increase in the Mn valence state and a decrease in the amount of cation vacancies, which were verified by iodometry and XPS. The opposite trends in the valence state of Mn and the amount of Mn vacancies may have led to a minimum in the Mn–O bond length. The ORR catalytic activity was found to be correlated with the Mn–O bond length; the shortest Mn–O bond length (and the highest covalency) of La_0.8Sr_0.2MnO_3+δ among others in the series was responsible for the highest ORR catalytic activity of this catalyst. The high catalytic activity, high 4-electron selectivity close to Pt and the excellent stability make this material a promising ORR catalyst that can replace precious metals.

Author contributions

Wencong Wang: experiment, analysis, and writing. Wei Liu: XAFS analysis. Masao Kamiko: XPS experimental support. Shunsuke Yagi: supervision, writing, reviewing, and editing.

Funding sources

This research was financially supported by a Grant-in-Aid for the University of Tokyo Excellent Young Researcher and Grant-in-Aid for Scientific Research (20H05180, 22H04497) commissioned by the Japan Society for the Promotion of Science. X-Ray absorption spectroscopy (XAS) data were obtained at the BL14B2 beamline (Proposal No. 2021B1902) of SPring-8 with support from the Japan Synchrotron Radiation Research Institute.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that influenced the work reported in this study.

Acknowledgements

We thank Prof. Toru H. Okabe and Dr Takanari Ouchi for their support with the ICP-AES measurements. We also thank Prof. Ikuya Yamada and Dr Yuichi Okazaki for giving an opportunity to measure the XAS data.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d2nj02619h

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