Durability of Carbon-Supported La-Mn-Based Perovskite-Type Oxides as Oxygen Reduction Catalysts in Strong Alkaline Solution

 Figure 1a shows the procedure for preparing the precursor of the carbon-supported perovskite-type oxides (LaMnO₃ and LaMn_0.6Fe_0.4O₃) by using an RM method. In the first step, 1 ml of a mixed aqueous solution of a stoichiometric amount of La(NO₃)₃ (0.2 mmol), Mn(NO₃)₂ (0.12 mmol), and Fe(NO₃)₃ (0.08 mmol) was titrated into a mixed solution of cyclohexane (18 g) and hexaethyleneglycol nonylphenyl ether (NP-6, 9 g) in order to obtain the RM solution containing the metal nitrates (abbreviated as RM-A). Simultaneously, an RM solution containing a 10% tetramethylammonium hydroxide (abbreviated as TMAH) solution (abbreviated as RM-B) was prepared by mixing cylohexane (54 g), NP-6 (27 g), and 10% TMAH solution (3 ml). For RM-A and RM-B, the weight ratios of NP-6 to the aqueous solution and cyclohexane to NP-6 were fixed at 9 and 2, respectively. The RM-A and RM-B were then mixed together, leading to a brown RM solution. The brown RM solution appeared to contain mixed hydroxides of La, Mn, and Fe inside the RM.

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For preparing carbon-supported Ca- and Fe-partially substituted LaMnO₃ (La_0.4Ca_0.6Mn_0.6Fe_0.4O₃), manganese hydroxide of high valence was applied because some of the B-site cations in La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ should be transformed from trivalent to tetravalent in order to compensate the charge balance of the oxide. The procedure for preparing the precursor of the carbon-supported La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ using the RM method is shown in Fig. 1b. In the first step, 0.85 ml of a mixed aqueous solution of La(NO₃)₃ (0.08 mmol), Ca(NO₃)₂ (0.12 mmol), Mn(NO₃)₂ (0.06 mmol), and Fe(NO₃)₃ (0.08 mmol) was titrated into a mixed solution of cyclohexane (15.3 g) and NP-6 (7.65 g), in order to obtain the RM solution containing the metal nitrates (abbreviated as RM-A'). Simultaneously, the RM containing KMnO₄ solution (RM-C) and the RM-B were prepared by mixing 0.3 ml of an aqueous solution of KMnO₄ (0.06 mmol), cyclohexane (10 g), and NP-6 (2.7 g). The RM-B and RM-C were then mixed together, and an RM solution containing a mixed aqueous solution of TMAH and KMnO₄ was obtained. The RM solution obtained was mixed with the RM-A', which was accompanied until a color of the RM solution changed in brown. It appeared that the mixed hydroxides of La, Ca, and Fe were obtained in the RM. At the same time, manganese hydroxides of high valence seemed to precipitate through the oxidation–reduction reaction between Mn₂₊ (Mn(NO₃)₂) and Mn₇₊ (KMnO₄) in the RM.

Carbon black (EC600-JD, 1280 m₂/g, Ketjen Black international Co.) was then added into the RM solutions containing the precursors of the perovskite-type oxides, which was obtained as shown in Figs. 1a and 1b. After ultrasonic blending was used to disperse the carbon black powder into the RM solutions, 200 ml of ethanol was added to the resultant RM solutions to break the RMs in order to load the metal hydroxides on the carbon black. The precipitates obtained were collected on the filter paper, and then the precipitates were washed with 500 ml of ethanol. After the precipitates were dried at 120°C for more than 6 h, the resultant powders were calcined at 550–700°C for 5 h in an N₂ flow to prevent the carbon from combusting. Then, 200 mg of the carbon-supported perovskite-type oxides were dispersed in 30 ml of deionized water with a PTFE dispersion (32J, Daikin Kogyo Co., Ltd.) and 0.5 ml of n-butanol under vigorous stirring. The precipitates obtained were collected on the filter paper and dried at 120°C for more than 3 h. The powders for the catalyst layers were thus obtained. In the powders, the content of PTFE was fixed at 15 wt %.

A mixture of acetylene black and 30 wt % PTFE was compressed into a thin sheet at 16 MPa on a Ni mesh (100 mesh) to act as a gas diffusion layer. The powder for the catalyst layer was then fabricated as a thin film on the gas diffusion layer under a pressure of 32 MPa. Finally, the catalyst layer and gas diffusion layer stacked on the Ni mesh were adhered to each other by a hot-press method at 365°C under a pressure of 64 MPa to furnish the GDEs.

The GDE obtained was welded with a Cu wire and mounted on the window of a PTFE cell, inside which 9 mol/l NaOH solution was placed as an electrolyte. The cell assembly was soaked in a water bath and held at 80°C. A Pt plate and an Hg/HgO electrode were used as a counter electrode and a reference electrode, respectively.

Cathodic polarization of the GDE for the oxygen reduction reaction was carried out using a potentiostat (HA-305G, Hokuto Denko Co., Ltd.) under O₂ flow. When comparing the durability of oxides from different procedure, the cathodic polarization of GDE was carried out at a current density of 500 mA cm₋₂. When comparing the composition of oxides, the cathodic polarization of GDE was carried out at −100 mV versus Hg/HgO for 3 h.

The amount of decomposed perovskite-type oxides were quantified by means of an absorptiometric method. GDEs after the cathodic polarization were washed with deionized water and the catalyst layer was then removed. The catalyst layer was subsequently immersed in a 30% H₂SO₄ aqueous solution in order to dissolve the perovskite-type oxide remaining in the catalyst layer. After the remaining carbon black in the solution was removed with the filter paper, an excess amount of NaBiO₃ was added to the filtrate in order to oxidize the Mn₃₊ or Mn₄₊ in the filtrate to . Then, the absorbance of the solution at 525 nm was measured by means of a UV-Vis spectrometer (U-1000, Hitachi Ltd.) to quantify the molarity of in the filtrate. Quartz cuvettes of path length 1 cm were used for the measurement. At first, the absorbance of calibration standards (2–10 μmol/l of KMnO₄ solutions) at 525 nm was measured in order to prepare the standard curve for . Then the absorbance of the filtrate at 525 nm was measured. The molarity of the in the filtrate was determined by the obtained absorbance and the calibration curve. The amount of Mn in the perovskite and the decomposed amount of perovskite-type oxide were thus calculated from the molarity of in the filtrate.

To investigate the durability of the carbon-supported LaMnO₃ nanoparticles prepared by the RM method, the cathodic polarization of GDE was carried out at a current density of 500 mA cm₋₂ in 9 M NaOH at 80°C under an O₂ flow, and the change of crystalline structure of LaMnO₃ was investigated by means of an x-ray diffractometer with nickel-filtered Cu Kα (1.5418 Å) source (RINT2100, Rigaku Denki Co. Ltd., Japan). As a comparison, the carbon-supported LaMnO₃ prepared by the mechanical mixing of LaMnO₃ with the carbon support was investigated under the same conditions. In the sample of the mechanical mixing of LaMnO₃ with the carbon support, LaMnO₃ was prepared by the thermal decomposition of La, Mn-malic complex at 850°C. ³³ Figures 2a and 2b show changes in the XRD pattern of the GDE using carbon-supported LaMnO₃ prepared by the RM method, and by the mechanical mixing of LaMnO₃ with the carbon support, respectively. In the XRD pattern of the GDE using carbon-supported LaMnO₃ prepared by the RM method, although there were no impurity phases except for the peaks of LaMnO₃ before the cathodic polarization, we found that La(OH)₃ coexisted with LaMnO₃ after the cathodic polarization, as shown in Fig. 2a. This result indicates the decomposition of the LaMnO₃ phase by the cathodic polarization. In addition to that, the change of coloring light blue was seen in the electrolyte after the cathodic polarization. This means that water-soluble metal ions were dissolved in the electrolyte. It seems that the dissolved ions are derived from Mn in the LaMnO₃, because the La₃₊ ion cannot form water-soluble ions in the strong alkaline solutions. ³³ Among many kinds of manganese compound, (Mn₇₊), (Mn₆₊), (Mn₅₊), and (Mn₂₊) exist stably in the strong alkaline solution. ^{33, 34} In the LaMnO₃ phase, it can be considered that valence state of Mn becomes not more than tetravalent under the cathodic polarization, because the valence state of Mn in the LaMnO₃ phase seems to be trivalent or tetravalent. Therefore, the dissolved ion in the electrolyte seems to be the ion, which is water-soluble divalent manganese ion. ^{35, 36} Taking into account the above considerations, it can be considered that Mn₃₊ or Mn₄₊ in LaMnO₃ was reduced to Mn₂₊ by the cathodic polarization, and that the reduced Mn₂₊ ions were dissolved in the alkaline electrolyte.

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On the other hand, the catalyst prepared by the mechanical mixing of LaMnO₃ with the carbon support was quite stable against the cathodic polarization in the strong alkaline solution, as shown in Fig. 2b. We propose that the difference in durability of the above two samples was caused by a difference in the dispersion of LaMnO₃ on the carbon support. Therefore, we observed carbon-supported LaMnO₃ powders using FE-SEM (JSM-6340F, JEOL Ltd., Japan). Figures 3 and 4 show the SEM images of carbon-supported LaMnO₃ prepared by the RM method and by the mechanical mixing of LaMnO₃ with the carbon support, respectively. In both Figs. 3 and 4, a secondary electron and a reflected electron images are arranged as (a) and (b), respectively. In the reflected electron images, bright spots and dark areas correspond to LaMnO₃ particles and the carbon support, respectively, because the intensity of the reflected electrons tends to increase with increasing atomic number. In the carbon-supported LaMnO₃ prepared by the mechanical mixing of LaMnO₃ with the carbon support (Fig. 3), agglomeration of the LaMnO₃ particles in the range of 1–20 μm can be seen in the reflected electron image. Conversely, in the carbon-supported LaMnO₃ prepared by the RM method (Fig. 4), agglomerations of LaMnO₃ particles are low in μm level, as compared to the carbon-supported LaMnO₃ prepared by mixing the LaMnO₃ with the carbon support. The difference in the dispersion of the LaMnO₃ for the two samples is due to the loading process on the carbon support. In the carbon-supported LaMnO₃ prepared by mixing of the LaMnO₃ with the carbon support, it seemed that the LaMnO₃ could not be dispersed on the carbon matrices because the LaMnO₃ particles were already agglomerated before the loading. However, in the case of the carbon-supported LaMnO₃ prepared by the RM method, precursors of the LaMnO₃ were deposited on the carbon support in solution before calcination. Therefore, agglomeration of the LaMnO₃ particles may be prevented by the carbon support. Taking into account the differences in the dispersion of LaMnO₃ on the carbon support as shown in the SEM images, it appeared that the number of the oxygen reduction reaction sites on the LaMnO₃ prepared by the RM method was larger than that of the carbon-supported LaMnO₃ prepared by the mechanical mixing of LaMnO₃ with the carbon support. Therefore, the amount of decomposition of LaMnO₃ in the sample prepared by the RM method was larger than the amount in the samples of mixing agglomerated LaMnO₃ with the carbon support.

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To investigate the durability of the carbon-supported LaMnO₃ prepared by the RM method more thoroughly, the dependence of the durability on the calcination temperature was investigated. Table I summarizes the decomposed amount of carbon-supported LaMnO₃ after polarization at −100 mV versus Hg/HgO for 3 h in 80°C 9 M NaOH solution. In the case of the carbon-supported LaMnO₃ calcined at 550°C, we found that the color of the electrolyte after polarization was changed from transparent to light blue. The formation of a light blue solution seemed to indicate the presence of ions, which are formed by the reduction of Mn₃₊ or Mn₄₊ in the LaMnO₃ lattice to Mn₂₊.From the measurements of the amount of decomposed LaMnO₃, we found that 45% of the LaMnO₃ was decomposed after polarization at −100 mV for 3 h. However, as shown in Table I, the decomposed amount of the carbon-supported LaMnO₃ decreased with an increase in the calcination temperature. It seemed that the LaMnO₃ particles on the carbon support agglomerated more with an increase in the calcination temperature. Thus, the number of oxygen reduction reaction sites decreased with an increase in the calcination temperature, leading to a decrease in the amount of decomposed LaMnO₃.

 As mentioned in the previous section, the durability of the carbon-supported LaMnO₃ prepared by the RM method against cathodic polarization increased with an increase in the calcination temperature. However, increasing the calcination temperature of LaMnO₃ is not desirable, because high calcination temperatures cause agglomeration of the LaMnO₃ particles, leading to a decrease in the oxygen reduction activity. Therefore we aimed to improve the durability of the carbon-supported LaMnO₃ by not using a high calcination temperature, but through the investigation of the composition of the perovskite-type oxide.

As for the stability of LaBO₃ (B = V, Cr, Mn, Fe, Co, Ni) against the reducing of B_3+/4+ to B₂₊,Nakamura et al. investigated under low oxygen partial pressure at high temperature using a TG/DTA and powder x-ray diffractions. ³⁷ As a result, they found that LaCrO₃ and LaFeO₃ are more stable than LaMnO₃ under reducing atmosphere. Therefore, we deduced that LaCrO₃ and LaFeO₃ are more stable than LaMnO₃ under the cathodic polarization in the strong alkaline solution. Among LaCrO₃ and LaFeO₃, LaCrO₃ is difficult to prepare using the RM method, because Cr(OH)₃ is dissolved in the strong alkaline solution. ³⁴ Therefore, we choose LaFeO₃ as a candidate for new component for oxygen reduction catalyst.

At first, difference of the durability under cathodic polarization between LaMnO₃ and LaFeO₃ was investigated. To evaluate the durability of LaBO₃, a linear sweep voltammetry (LSV) for the LaMnO₃ and LaFeO₃ disks was carried out at 80°C in 9 M NaOH aqueous solution under N₂ bubbling. The sweep rate of LSV measurements was −2 mV/s. The sample disk was fabricated by sintering LaBO₃ pellets at 1000°C for 5 h in air. Figure 5 shows the linear sweep voltammetry curves for the LaMnO₃ and the LaFeO₃ disks. In Fig. 5, negative current denotes the reduction of perovskite-type oxides because dissolved oxygen in the electrolyte was removed by the N₂ bubbling. For the LaMnO₃ disk, a small negative current was observed in the range of 0−0.3 V versus Hg/HgO. And a large negative current was observed at the potential of lower than −0.3 V versus Hg/HgO. This result indicates that the LaMnO₃ can be reduced slowly even at the small cathodic polarization. On the contrary, in the case of the LaFeO₃ disk, a negative current was not seen in the range of 0–0.35V versus Hg/HgO. Therefore we presumed that the partial substitution of Fe at the B-sites of LaMnO₃ could stabilize the perovskite phase against cathodic polarization.

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Thus, carbon-supported LaMn_0.6Fe_0.4O₃ was prepared by using the RM method, and its durability against cathodic polarization was examined. In addition, it has been reported that the partial substitution of Ca or Sr at the A-sites of perovskite-type oxides improves their oxygen reduction activity. ²⁸ The Ca- or Sr-partial substitution increases the valence state of the B-sites to compensate the charge valance of the perovskite phase. Such an increase in the valence state of the B-sites has been confirmed by means of the XPS measurement. ^{38, 39} An increase in the valence state of the B-sites could possibly increase the durability of La-Mn-based perovskite-type oxides against the cathodic polarization. Thus, the durability of La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ was also investigated. Figure 6 shows the XRD patterns of the GDE using carbon-supported LaMnO₃, LaMn_0.6Fe_0.4O₃ and La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ after polarization at −100 mV versus Hg/HgO for 120 h at 80°C in 9 M NaOH solution. Furthermore, Table II summarizes the average valence state of the B-sites of the oxides, the amount of decomposed perovskite-type oxides after polarization for 120 h, and the current density under polarization at −100 mV versus Hg/HgO. The average valence state of the B-sites of the perovskite-type oxides was quantified by iodometric titration. ⁴⁰ The current density summarized in Table II indicates the oxygen reduction activity of the carbon-supported oxides. In each specimen, carbon-supported oxides (30 wt %) were prepared using the RM method, and was calcined at 700°C. In the case of carbon-supported LaMnO₃, the formation of a large amount of La(OH)₃ was confirmed by the XRD pattern (Fig. 6a). Such formation of La(OH)₃ appeared to be caused by the decomposition of LaMnO₃ under cathodic polarization. The amount of decomposed LaMnO₃ was quantified as 65 mol %.Conversely, in the case of carbon-supported LaMn_0.6Fe_0.4O₃, the relative peak intensity of the La(OH)₃ to the perovskite-type oxide in the XRD patterns in Fig. 6 was small when compared with the carbon-supported LaMnO₃. In addition, the decomposed amount of oxide was significantly decreased as compared to the carbon-supported LaMnO₃. Such phenomena indicate that the partial substitution of Fe at the B-sites of LaMnO₃ was effective at increasing the electrochemical durability against the cathodic polarization in the strong alkaline solutions. However, we found that the oxygen reduction activity of the carbon-supported LaMn_0.6Fe_0.4O₃ was inferior to carbon-supported LaMnO₃, as shown in the current density at −100 mV versus Hg/HgO in Table II. Thus far, it has been reported that LaFeO₃ is less active than LaMnO₃. ^{32, 41} In addition, it has been claimed that the catalytic active sites of perovskite-type oxides for oxygen reduction are B-site cations. ⁴² Therefore, one can say that Fe cations in the perovskite-type oxides are less active than Mn cations in the perovskite-type oxides. Additionally, the oxygen reduction activity of carbon-supported LaMn_0.6Fe_0.4O₃ was less than that of carbon-supported LaMnO₃.

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When Ca was partially substituted at 60% of the A-sites of LaMn_0.6Fe_0.4O₃ (La_0.4Ca_0.6Mn_0.6Fe_0.4O₃), the average valence state of the B-sites was increased by about 35% as compared with LaMnO₃ and LaMn_0.6Fe_0.4O₃ (Table II). When we compared the oxygen reduction activity of carbon-supported La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ with those of carbon-supported LaMnO₃ and LaMn_0.6Fe_0.4O₃, we found that carbon-supported La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ was far superior to the others. Such a tendency is in good agreement with the tendencies previously reported. ⁴³ The oxygen reduction activity of the La-Mn-based perovskite oxides seem to depend on the content of tetravalent cations in the B-sites. However, the correlation between the content of the tetravelent cations in the B-sites and the oxygen reduction activity has not yet been elucidated. Further investigations are required in this area. In terms of durability of the oxide La_0.4Ca_0.6Mn_0.6Fe_0.4O₃, the relative peak intensity of La(OH)₃ to the perovskite-type oxide of was far reduced when compared with the carbon-supported LaMn_0.6Fe_0.4O₃, as shown in Fig. 6. Furthermore, the decomposed amount on the oxides was restrained up to 5.5%.These results indicated that the high valence state of the B-sites, caused by the substitution of Ca at the A-sites, was effective at increasing the durability against the cathodic polarization in the strong alkaline solutions. As shown above, Mn₃₊ or Fe₃₊ cation in the perovskite lattice tend to be reduced to water-soluble species such as (Mn₂₊) or (Fe₂₊) under the cathodic polarization. However, when the B-sites of LaMn_0.6Fe_0.4O₃ are shifted to high valence states by partially substituting Ca at the A-sites, it appeared that B-site cation cannot be easily reduced to water-soluble species due to the higher valence state of the B-sites as compared with LaMn_0.6Fe_0.4O₃. From the above results, we can state that the partial substitution of Ca at the A-sites of perovskite-type oxides is effective at increasing both the oxygen reduction activity and durability against the cathodic polarization.

In order to investigate the dynamics of the decomposition of oxides, the decomposition amount of oxides were measured after various polarization terms. Figure 7 shows the dependence of the decomposition amount of oxide for the carbon-supported LaMnO₃, LaMn_0.6Fe_0.4O₃, and La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ on the polarization term. It was remarkable for the carbon-supported LaMnO₃ that the decomposition of oxide is fast in the early stage, and that the decomposition of oxide slowed gradually. When LaMnO₃ is decomposed under the cathodic polarization, LaMnO₃ can be decomposed to water-soluble and La(OH)₃. Although is soluble to the electrolyte, La(OH)₃ is indissoluble to the strong alkaline solution. Therefore, La(OH)₃ was deposited on the LaMnO₃ particles which have not been decomposed, leading to the eclipse of LaMnO₃ surface. For the similar reasons, the decomposition of carbon-supported LaMn_0.6Fe_0.4O₃ was decomposed gradually. On the contrary, La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ was fairly stable after the polarization for 120 h because La_0.4Ca_0.6Mn_0.6Fe_0.4O₃ is durable against the cathodic polarization as mentioned above.

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