Strong Lanthanoid Substitution Effect on Electrocatalytic Activity of

15 Mar 2018 - The oxygen reduction reaction (ORR) catalytic activity was systematically studied on the BaLnMn2O5 series (Ln = Y, Gd, Nd, and La) with ...
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Strong Lanthanoid Substitution Effect on Electrocatalytic Activity of Double-Perovskite-Type BaLnMn2O5 (Ln = Y, Gd, Nd, and La) for Oxygen Reduction Reaction Etsushi Tsuji,*,†,§ Teruki Motohashi,*,‡ Hiroyuki Noda,§ Yoshitaka Aoki,§,∥ and Hiroki Habazaki§,∥ †

Department of Chemistry and Biotechnology, Graduate School of Engineering, Tottori University, Tottori 680-8552, Japan Department of Materials and Life Chemistry, Kanagawa University, Yokohama, Kanagawa 221-8686, Japan § Graduate School of Chemical Sciences and Engineering and ∥Division of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan

J. Phys. Chem. C 2018.122:7081-7087. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/24/18. For personal use only.

‡

S Supporting Information *

ABSTRACT: The oxygen reduction reaction (ORR) catalytic activity was systematically studied on the BaLnMn2O5 series (Ln = Y, Gd, Nd, and La) with a layered double-perovskitetype structure in an alkaline aqueous solution. The onset ORR potential and the number of electrons involved in ORR were found to be strongly Ln-dependent: both values were significantly higher for larger Ln = La and Nd than for smaller Ln = Gd and Y, despite similarities in their chemical compositions and crystal structures. The enhanced ORR activity of the Ln = La and Nd compounds is likely attributed to their stronger affinity to oxygen species, consistent with the greater oxygen storage capability of these compounds, as revealed by the water dissolution reaction at elevated temperatures. correlates with the eg-electron filling at the TM site, that is, perovskite catalysts with nearly one eg-electron number tend to show high ORR activity.7 In addition, the ORR activity is suggested to improve in TM oxides with covalent TM−O bonds. The more covalent bonding nature should facilitate the exchange reaction between oxygen molecules and adsorbed hydroxide ions on the surface TM sites during ORR.7 On the other hand, other researchers discussed the composition/ structure-vs-activity relationship in terms of the electronic states of surface TM sites.12−16 Celorrio et al. reported that the ORR catalytic activity of perovskite-type manganese oxides is related to various factors, such as electron density at the surface Mn sites14,15 and Mn−O bond length.16 Although the aforementioned reports dealt with perovskite catalysts containing various TMs having the identical electron filling or fixed TMs with different formal oxidation states, there have been only a few studies on a TM oxide series containing various isovalent A-site cations without changing the coordination environment at the TM sites. The present work focused on a series of manganese oxide perovskites, BaLnMn2O5+δ with Ln = La, Nd, Gd, and Y, all of which are trivalent. These oxides are categorized as A-site

1. INTRODUCTION There have been increased demands for the development of highly efficient and affordable energy storage devices as alternatives to lithium-ion secondary batteries for electrical vehicles, portable electronic devices, and so on. Recently, metal−air secondary batteries have attracted much attention owing to their higher theoretical energy density.1 For this technology, active cathode catalysts play an important role in the overall performance because a major part of energy losses comes from overpotentials of oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) in the discharging and charging processes, respectively.2,3 Although platinum and platinum alloy nanoparticles are known to exhibit high ORR catalytic activity,4−6 the scarcity of platinum prohibits its largescale uses. Therefore, the search for cost-effective ORR catalysts is one of the major challenges in the realization of practicable metal−air secondary batteries, and transition-metal (TM) oxides with a perovskite-type structure (ABO3) have been regarded as promising candidates for industrially producible cathode catalysts.7,8 Notably, manganese oxides, such as LaMnO3+δ, La1−xCaxMnO3, and La1−xSrxMnO3, are found to exhibit relatively high ORR activity.7−16 To gain insight into the ORR activity of perovskite-type TM oxides, the interplay between the catalytic activity and the electronic states of constituent TMs was systematically studied. Suntivich et al. reported that the ORR activity primarily © 2018 American Chemical Society

Received: December 25, 2017 Revised: March 8, 2018 Published: March 15, 2018 7081

DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087

Article

The Journal of Physical Chemistry C

tetragonal unit cell (Figure S1). The lattice parameters for the products are in agreement with those in the literature. As expected, these values systematically increase with the increasing ionic radius of Ln3+ in the order of Ln = Y, Gd, Nd, and La. The oxygen content (5+δ) values determined by iodometry were close to 5.00 for all of the Ln samples.19 The grain morphology observations were performed on the sample series using a scanning electron microscope (JEOL, JSM6500F) operated at 10 kV. Because we were unable to prepare sufficient amounts of powders needed for accurate measurements of nitrogen gas adsorption/desorption isotherms, the specific surface area (A) of each catalyst was estimated from the scanning electron microscopy (SEM) images employing the following equation7,8

ordered double perovskites, which contain smaller Ln and larger Ba ions in alternate layers, as illustrated in Figure 1.17−19

A=

(∑ 4π r 2)/[ρ ∑ (4/3)ρπ r 3]

= (6 ∑ d 2)/(ρ ∑ d3)

Figure 1. Schematic illustration of crystal structures of doubleperovskite-type BaLnMn2O5 with δ = 0 and 1. The illustration was drawn with the VESTA software21 based on the structural model reported in the literature.

(1)

where ρ is the theoretical bulk density calculated from the crystallographic data, and d is the diameter of at least 60 individual particles determined by SEM. Working electrodes for electrochemical measurements were prepared as follows: 50 mg of the oxide catalysts, 10 mg of acetylene black (AB), and 200 mg of Na+-exchanged Nafion solution were dispersed ultrasonically in 4.7 mL of ethanol to obtain a homogenous black suspension. The AB additive was obtained by immersing a commercial AB powder (Strem Chemicals, 99.99%) into concentrated nitric acid at 80 °C overnight, then filtering, and drying at 100 °C overnight. The Na+-exchanged Nafion solution was obtained by mixing a 5 wt % Nafion solution (Wako Pure Chemical) with an appropriate amount of 0.1 mol dm−3 NaOH ethanol solution. Then, 20 μL of the suspension was pipetted onto the surface of a 5 mm diameter glassy carbon (GC) electrode with a rotation rate of 500 rpm. In all of the experiments, special-grade chemicals were used without further purification, and deionized water was purified with a Milli-Q water purification system. Electrochemical measurements were carried out utilizing a potentiostat (Hokuto Denko, HZ-7000) combined with a rotating ring-disk electrode (RRDE; Pine Instrument Co., AFMSRCE). A platinum sheet and an Hg/HgO/KOH electrode were used as counter and reference electrodes, respectively. The working electrode was a GC disc electrode loaded with 1.0 mg cm−2 of the catalyst. All of the measurements were conducted at room temperature (∼25 °C) in a 4 mol dm−3 KOH aqueous solution (pH 14) saturated with high-purity argon (99.999%) or oxygen (99.5%), which was bubbled through for 15 min at 100 sccm. Such ORR data in a concentrated alkaline electrolyte are informative to evaluate the performance in practical metal−air secondary batteries.22 The potential U was converted from the Hg/HgO reference scale to the reversible hydrogen electrode (RHE) using the following equation

The oxygen sites within the Ln layer are readily filled/unfilled at elevated temperatures depending on the surrounding atmosphere, resulting in a large oxygen nonstoichiometry in the range of 0 ≤ δ ≤ 1. The deoxygenated δ ≈ 0 form can act as a reductant owing to its strong affinity to oxygen. In fact, BaLnMn2O5 was found to show a capability to produce hydrogen gas through the water dissolution at 500 °C.20 Remarkably, the reactivity significantly depends upon the Ln species: the larger the ionic size of Ln3+, the higher reactivity the BaLnMn2O5 samples exhibit. This finding implies that the redox characteristics of BaLnMn2O5+δ can be widely controlled through isovalent substitutions neighboring the active sites, even though the redox species (manganese) has remained untouched. Then, it is of particularly interest to ascertain whether the BaLnMn2O5 series shows strongly Ln-dependent ORR performance in alkaline aqueous solutions, similarly to its oxygenation/deoxygenation reactions at 500 °C. Here, we report a systematic study on the ORR catalytic activity of the BaLnMn2O5+δ series (Ln = La, Nd, Gd, and Y; δ ≈ 0) in alkaline aqueous solutions. Our catalytic activity test evidenced that the onset potential and the number of electrons involved in ORR are significantly higher for Ln = La and Nd than for Ln = Gd and Y, implying a large impact of the Ln species on the ORR catalytic activity. The Ln-dependent ORR performance of the BaLnMn2O5 series will be discussed taking into account the result of catalytic activity test for the reduction of hydrogen peroxide ions (HO2−).

2. EXPERIMENTAL SECTION Samples of BaLnMn2O5+δ (Ln = Y, Gd, Nd, and La) were synthesized via a citrate precursor route combined with the oxygen-pressure-controlled encapsulation technique reported previously.19,20 Details of the sample synthesis are given in the Supporting Information. Phase purity of the catalysts was checked by powder X-ray diffraction (PXRD; Rigaku, Ultima IV). The data were collected using Cu Kα radiation (40 kV, 40 mA) in a 2θ range of 5−90°. The scan rate and Δ2θ step were 5° min−1 and 0.02°, respectively. The XRD analysis indicated that the resultant BaLnMn2O5+δ products are essentially phasepure of the fully deoxygenated δ ≈ 0 (“O5”) form with a

U vs RHE = U vs Hg/HgO + 0.098 + 0.059 × pH

(2)

In each measurement, the potential of the catalyst-loaded GC disc electrode was swept from 1.10 to 0.60 V vs RHE with a potential sweep rate of 1 mV s−1 and a rotational rate of 1600 rpm in the oxygen-saturated electrolyte. The current density was iR-corrected with a solution resistance (R = 3.2 Ω). The catalytic activity tests for the reduction of hydrogen peroxide ions (HO2−) were also performed utilizing a similar 7082

DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087

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The Journal of Physical Chemistry C

Figure 2. Scanning electron micrographs of the products of (a) BaYMn2O5, (b) BaGdMn2O5, (c) BaNdMn2O5, and (d) BaLaMn2O5.

solid curves, a large cathodic current is observed at the potential region below 0.90 V vs RHE, indicating the ORR catalytic activity. To investigate the stability of the deoxygenated O5 form of the double perovskites during ORR, the BaLaMn2O5 catalyst was characterized after a prolonged application of constant potential at 0.60 V vs RHE for 8 h in the oxygensaturated KOH aqueous solution. The XRD analysis of the catalyst indicated that neither decomposed products nor partially/fully oxygenated O5.5/O6 phases were formed by ORR. In addition, the ORR current density vs potential curves essentially remained unchanged before and after the stability test (Figures S2 and S3). These results suggest that the BaLnMn2O5 catalysts show a sufficient stability against ORR operations in a concentrated alkaline electrolyte. In the linear sweep voltammograms of the BaLnMn2O5 catalysts, the development of the ORR current shifts to a positive potential direction as the ionic radius of Ln3+ increases. To discuss the Ln-dependent ORR property in detail, the current density is recalculated using the specific surface areas of the catalysts (Table 1). The onset potential is defined as the potential to give an ORR current of 10 μA cm−2oxide and plotted against the Ln3+ radius in Figure 3b. The onset potential linearly increases with the increasing Ln3+ radius in the order of Ln = Y, Gd, and Nd and tends to be constant for Nd and La. Importantly, the Tafel slope at a low overpotential region (≈0.90 V vs RHE) significantly depends upon the Ln species. As presented in Table 2, the values for Tafel slope are divided into two groups: one with relatively low values (43 and 46 mV dec−1) for larger Ln = La and Nd and the other with high values (53 and 52 mV dec−1) for smaller Ln = Gd and Y. Such a distinct jump in the Tafel slope implies different ratedetermining steps in the ORR process,25−27 despite similar chemical compositions and crystal structures of the catalysts. Notably, the kinetically limited current density jk obtained from Koutecky−Levich plots shows a similar trend, that is, the jk values are higher for Ln = Nd and La than for Ln = Y and Nd (Figures S4 and S5). The number of electrons transferred during ORR is determined using the ring and disc current values of the

electrochemical setup. The electrolyte was a mixture aqueous solution of 0.04 mol dm−3 H2O2 and 4 mol dm−3 KOH saturated with argon. The constant potential was applied to the working ring electrode made of platinum while the potential of the working electrode was similarly swept. The hydrogen peroxide ions were assumed to decompose in an alkaline aqueous solution according to the following reaction23,24 HO2− + OH− → O2 + H 2O + 2e− E0 = 0.748 V vs RHE (3)

Thus, the potential applied to the ring electrode was set at 1.05 V vs RHE, which is sufficiently high to completely decompose hydrogen peroxide ions.

3. RESULTS AND DISCUSSION Figure 2 shows SEM images of the BaLnMn2O5 products with Ln = Y, Gd, Nd, and La. All of the four products consist of coarse grains with sizes of 1−2 μm. As summarized in Table 1, Table 1. Average Grain Diameters and Specific Surface Areas of the BaLnMn2O5 Catalysts (Ln = Y, Gd, Nd, and La)a samples

average grain diameter (μm)

surface area (m2 g−1)

BaYMn2O5 BaGdMn2O5 BaNdMn2O5 BaLaMn2O5

1.41 2.34 1.14 0.93

0.54 0.29 0.57 0.75

a

The specific surface areas were calculated using the grain diameter values estimated from the SEM images.

the average grain diameters of the Ln = Y, Nd, and La products are comparable to each other with similar values for the specific surface area (A) as large as 0.54−0.75 m2 g−1, although the Ln = Gd product possesses a slightly smaller A value (0.29 m2 g−1) because of its larger grain size. Disc and ring current densities vs potential curves for the BaLnMn2O5 catalysts are shown in Figure 3a. In these curves, the disc and ring current densities are normalized with geometrical surface areas of the GC disc and the platinum ring, respectively. For the disc current density denoted with 7083

DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087

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Figure 3. (a) Linear sweep voltammograms of the platinum ring electrode and the BaLnMn2O5 disc electrode for ORR. The current density values were normalized with the geometric surface areas of the ring and disc electrodes. (b) The onset ORR potential vs ionic radius of Ln3+ relation and (c) electron transfer number of BaLnMn2O5 (Ln = Y, Gd, Nd, and La) for ORR. Error bars represent standard deviations from at least four independent measurements.

followed by disproportionation of two hydrogen peroxide ions (eq 8).

Table 2. Values for Tafel Slope of the BaLnMn2O5 Catalysts (Ln = Y, Gd, Nd, and La) samples

Tafel slope (mV dec−1)

BaYMn2O5 BaGdMn2O5 BaNdMn2O5 BaLaMn2O5

52 53 46 43

O2 + 2H 2O + 4e− → 4OH−

(5)

O2 + H 2O + 2e− → HO2− + OH−

(6)

HO2− + H 2O + 2e− → 3OH−

(7)

−

−

2HO2 → 2OH + O2

(8)

The nearly ideal n values (≈4.0) obtained for Ln = La and Nd (Figure 3c) indicate that these compounds can catalyze ORR mainly through the direct and/or indirect 4e− pathways. For Ln = Gd and Y, on the other hand, a significant number of HO2− ions formed by the 2e− oxygen reduction (eq 6) should be subsequently decomposed through the aforementioned disproportionation reaction (eq 8), resulting in much smaller n values. Thus, the magnitude of n reflects the selectivity for the two types of dissolution processes of HO2− ions and thereby the electron transfer efficiency of the catalyst toward the reactants. To confirm the above argument, the catalytic activity test for the reduction of hydrogen peroxide ions (eq 7) was conducted. Figure 4 shows the current density vs potential curves of the

RRDE experiments. The electron transfer number, n, can be calculated by the following equation23,24 4Idisc n= Idisc + Iring /N (4) where Idisc is the disc current, Iring is the ring current, and N is the capture coefficient typically assumed to be 0.0995. The n vs potential plots for the four catalysts are shown in Figure 3c. For all of the catalysts, the n value gradually decreases with the lowering applied potential. This behavior is related to the increased ring current density at potentials below 0.85 V vs RHE (see dotted curves in Figure 3a) and may be attributed to the formation of HO2− ions through two-electron reduction of oxygen molecules. The most interesting feature is that the magnitude of n strongly depends upon the Ln3+ species. Similarly to the results of Tafel plots (Table 2) and Koutecky− Levich plots (Figures S4 and S5), the catalysts are divided into two groups: one for larger Ln = La and Nd showing nearly ideal n values (≈4.0) at 0.80−0.85 V vs RHE expected for the complete ORR and the other for Ln = Gd and Y with rather smaller n values (≈3.6). The larger n values for Ln = La and Nd than for Ln = Gd and Y clearly suggest the higher ORR catalytic activity for the former, consistent with the trend of the onset ORR potential presented in Figure 3b. It has been generally assumed that ORR in an alkaline electrolyte proceeds through the following three types of pathways:23,24 (i) a direct four-electron (4e−) reduction pathway (eq 5); (ii) an indirect 4e−-reduction pathway involving a two-electron (2e−) reduction reaction of an oxygen molecule (eq 6) followed by a 2e− reduction reaction of a hydrogen peroxide ion (eq 7); (iii) a 2e− reduction pathway involving the 2e− reduction of an oxygen molecule (eq 6)

Figure 4. Linear sweep voltammograms of the BaLnMn2O5 electrode for the reduction of HO2−. 7084

DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087

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Figure 5. Schematic illustration of the ORR mechanistic models for (a) BaLnMn2O5 with smaller Ln = Y and Gd and (b) larger Ln = Nd and La.

remarkable oxygen storage capability originating from its strong affinity to oxygen, it would be reasonable to assume that the ORR catalytic activity of the BaLnMn2O5 series is related to the oxygen intake/release property. Interestingly, the oxygen affinities of the BaLnMn2O5 series at elevated temperatures are also significantly Ln-dependent.19,20 The theoretically estimated Gibbs energies of the O5.0-to-O5.5 oxygenation reaction are ΔG°(O5.0/O5.5) = −85.4 and −123.9 kJ mol−1 at 800 K for Ln = Y and La, respectively, implying that the process is energetically more favorable for Ln = La than that for Ln = Y. 20 Because the Mn−O bonding nature cannot be distinguished among the BaLnMn2O5 series, the Ln-dependent characteristics may rather be explained focusing on the slightly smaller electronegativity of La than that of Y, emphasizing a primary role of electrostatics on the redox energetics. Taking into account the fact that the ORR performance is similarly Lndependent, the Ln site neighboring the catalytic active site plays an important role also in the ORR characteristics. On the basis of the experimental results, a possible ORR mechanistic model has been constructed and schematically illustrated in Figure 5. For BaLnMn2O5 with smaller Ln = Gd and Y (Figure 5a), the catalyst−reactant interactions are relatively weak such that a significant number of HO2− ions formed by the 2e− oxygen reduction are expensed by nonreductive disproportionation reaction, resulting in much smaller n values. For BaLnMn2O5 with larger Ln = La and Nd (Figure 5b), on the other hand, the stronger affinity to the reactants facilitates the electron transfer, leading to the direct and/or indirect 4e− reduction processes as predominant pathways.

BaLnMn2O5 series for the reduction of hydrogen peroxide ions. The current density is normalized with the geometrical surface area of the GC disc. For all of the catalysts, the reduction current is observed at potentials below 0.95 V vs RHE. It can be seen that the magnitude of the current density is significantly Ln-dependent showing a similar feature to the onset ORR potential, that is, the current density tends to increase with the increasing ionic radius of Ln3+. This feature is also in good agreement with the plots of the electron transfer number presented in Figure 3c. As mentioned in the Introduction, highly covalent TM−O bonds would facilitate the exchange reaction between oxygen molecules and adsorbed hydroxide ions on the surface TM sites, leading to the significant ORR activity.7 Nevertheless, a comparison of the electronic structure by means of crystal orbital Hamilton population revealed that the Mn−O bonding nature is very similar between the Ln = La and Y compounds.20 This means that the significantly different ORR catalytic activity of these compounds is hardly explained on the basis of the TM−O covalency. On the other hand, a systematic study on perovskite-type AMnO3 (A = Y, Ca, La and Sr) by Celorrio et al.16 revealed a similar trend to the case of our BaLnMn2O5 series, that is, LaMnO3 shows much higher ORR activity than YMnO3, suggesting the importance of the coordination environment at the Mn sites. They also reported that these manganese oxides exhibited A-site-rich surface although the extent of segregation was weakly dependent on the A-site nature. The X-ray photoelectron spectroscopy (XPS) experiments on the BaYMn2O5 and BaLaMn2O5 catalysts also revealed their A-site-rich compositions (Figure S6 and Table S1). As the compositional ratios between Ba + Ln and Mn sites are comparable to each other for Ln = Y and La, we tentatively suggest that the A-site segregation seems to be less important to discuss the distinct ORR activities of the two. More detailed investigations focusing on the surface nature are necessary to gain deeper insight into the influence of surface segregation. Another interpretation from the adsorption characteristics may be applicable. In fact, the previous ORR mechanistic studies on TM-based catalysts, such as oxides and functionalized graphitic materials, indicated that the ORR/OER catalytic activities as well as the electron transfer number are strongly influenced by adsorption energies of the reactants (oxygen, HO•, and HOO•).28−30 In addition, for various perovskite oxides, the oxygen surface binding energy also correlates well with reaction energy barriers of ORR relevant surface reactions, such as oxygen surface exchange, oxygen vacancy formation, and oxygen/vacancy hopping, at elevated temperatures.31,32 Bearing in mind that BaLnMn2O5 possesses

4. CONCLUSIONS The present work demonstrated that the ORR activity of the BaLnMn2O5 series (Ln = La, Nd, Gd, and Y) in alkaline aqueous solutions is strongly Ln-dependent: the onset potential and the number of electrons involved in ORR are significantly higher for larger Ln = La and Nd than for smaller Ln = Gd and Y, despite similarities in their chemical compositions and crystal structures. The catalytic activity of the HO2− reduction was also found to enhance for the catalysts with larger Ln3+ ions, leading to higher selectivity of the direct/indirect four-electron reduction pathways against the two-electron reduction pathway followed by the HO2− disproportionation. The enhanced ORR activity of the Ln = La and Nd compounds is likely attributed to their stronger affinity to oxygen species, consistent with the greater oxygen storage capability of these compounds, as revealed by the water dissolution reaction at elevated temperatures. 7085

DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.jpcc.7b12678. Details of synthesis and XPS measurements, XRD patterns for the BaLnMn2O5 (Ln = Y, Gd, Nd, and La), current density vs time plot of the BaLaMn2O5 at 0.6 V vs RHE, XRD patterns for BaLaMn2O5 before and after a prolonged application of constant potential at 0.6 V vs RHE for 8 h, Koutecky−Levich plots and kinetically limited current density of BaLnMn2O5 (Ln = Y, Gd, Nd, and La), XPS spectra and resultant ratio between Ba + Ln and Mn sites in BaLnMn2O5 with Ln = Y and La (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Phone/Fax: +81-857-315257 (E.T.). *E-mail: [email protected]. Phone: +81-45-481-5661. Fax: +81-45-413-9770 (T.M.). ORCID

Etsushi Tsuji: 0000-0002-2783-7017 Teruki Motohashi: 0000-0002-4568-5600 Yoshitaka Aoki: 0000-0001-5614-1636 Hiroki Habazaki: 0000-0002-7172-8811 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the “Research and Development Initiative for Science Innovation of New Generation Battery (RISING Project)” of the New Energy and Industrial Technology Development Organization (NEDO), Japan. A part of this work was conducted at “Joint-use Facilities: Laboratory of Nano-Micro Material Analysis”, Hokkaido University, supported by “Nanotechnology Platform” Program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

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REFERENCES

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DOI: 10.1021/acs.jpcc.7b12678 J. Phys. Chem. C 2018, 122, 7081−7087