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Influence of surface charges/chemistry on the catalysis of perovskite complexes Seungkyu Park, Gyutae Nam, Jang-Soo Lee, Jaephil Cho, and Jae-Il Jung ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b04442 • Publication Date (Web): 06 Aug 2018 Downloaded from http://pubs.acs.org on August 6, 2018

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ACS Applied Materials & Interfaces

Influence of Surface Charges/Chemistry on the Catalysis of Perovskite Complexes Seungkyu Park1‡, Gyutae Nam1‡, Jang-Soo Lee2, Jaephil Cho1* and Jae-Il Jung1* 1

Department of Energy Engineering, School of Energy and Chemical Engineering Ulsan National Institute of Science & Technology (UNIST) 50, UNIST-gil, Ulsan 44919, Korea 2

LG Chem. Research Park, Daejeon, Korea.

Abstract The electrochemical performance of perovskite complex was found out to depend greatly on the different locations of the identical particle, which represent different surface charges accordingly. The surface charges were evaluated by Zeta potential (ζ) for the intrinsic BSCF5582 (Ba0.5Sr0.5Co0.8Fe0.2O3−δ: BSCF5582), ball-milled (BM-BSCF5582) and heattreated in oxygen atmosphere after ball-milling (48h O2 -BM-BSCF5582), the mean ζ of which represent -11.1, 21.2, and -6.1 mV, respectively, which reflects well on different surface chemistries. When the bonding structures at the different stratified layers and the overall crystalline morphologies were analyzed via XPS and HR-TEM, respectively, the crystalline- and bonding-structure at the 50 nm depth of BSCF5582 is nearly identical to that of BM-BSCF5582 at the surface. As ball-milling proceeds, not only particles are comminuted, but also the amorphous surface is broken open, leading to the revelation of inner and naïve cubic crystalline phase surfaces, and affecting positively and negatively on the catalytic activities of OER and ORR, respectively, in significant scales.

*

Corresponding author : Jaephil Cho & Jae-Il Jung E-mail: [email protected]& [email protected]

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Keywords: Perovskites, Surface charge, Ball-milling, Oxygen reduction reaction, Oxygen evolution reaction

1.

Introduction

Nowadays, the development of bi-functional electrocatalyst is strongly demanded to develop the next-generational rechargeable energy conversion/storage systems. A wide spectrum of oxide kinds have been suggested as the promising candidates of the efficient bifunctional electrocatalysts for ESS (Energy Storage System) applications,1-16 among which the perovskite-based catalysts have drawn considerable attentions due to the relatively cheap prices, the flexibility of monitoring properties and outstanding electrochemical performance. Especially, Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF5582) was introduced to have the highest OER activity with the same normalized catalytic activity of IrO2 with eg~1.2 of t2g5eg˜1.2 as the band descriptor of Co cation electronic configuration.15,16 While the enhancement of catalytic activity on the identical mass are in general pursued by increasing surface area towards nanostructured perovskite catalysts,16-20 the electrochemical activity of perovskite catalysts can also be monitored by other factors such as oxygen vacancy, surface layer structure/chemistry, surface area,21-23 among which oxygen vacancy distributed on the surface has been considered as a primary active site, promoting the electro-catalytic activities both in the ORR (Oxygen Reduction Reactions) and OER (Oxygen Evolution Reactions) processes.24-26 The adsorption rate of a gas molecule onto the catalyst surface is proportional to the concentration of available active sites.21,27 Chen group reported that both the ORR and OER electro-catalytic activities of MnO2 were enhanced significantly by the newly formed pyrolusite lattice as an oxygen-vacancy-induced structure, when it underwent heat-treatment


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in argon atmosphere.28,29 Aguadero et al reported that the richly formed oxygen vacancy on the surface of perovskite catalysts contributes greatly to an increased oxidizing electrocatalytic activity of the organic substrates, when (La,Sr)0.5(Mn,Co)0.5O3−d was heattreated at 150oC under a vacuum condition.

30

It signifies that the successful monitoring over

the surface morphology/chemistry would contribute to enhancing significantly the electrocatalytic performance of bi-functional perovskite electrocatalyst.22,23 Coincidently, our group confirmed the significant enhancement of the electrochemical performance in BSCF5582 catalyst, which was heat-treated in oxygen atmosphere (O2-BSCF5582) (Figure S1).23 On the other hand, when BSCF5582 underwent heat-treatment at 950 oC in argon condition, ~30 nm amorphous surface layer has grown up to 200 nm, which led to the obstruction of the electrocatalytic reactions.22,23 So, it is significant to investigate into the role of active sites, which are buried underneath the amorphous surface layer, while the active sites are closely related with oxygen vacancies or the defect complexes of B-site cations.22,23,31 Oxygen vacancy itself does not have charge, but could be understood as positive (pseudo positive, Vo••) surrounded by free electron (2e−), when the oxygen atom at the oxygen lattice site (Oo) escapes into the atmosphere as oxygen gas (1/2O2), which can be described as Oo → Vo•• + 2e− + 1/2O2. However, the role of active sites during the electrocatalytic process, particularly correlated with oxygen vacancy (Vo••) within crystalline structure, has neither yet been brought onto serious consideration nor clarified upon an evidential study, while a significant amount of oxygen vacancy concentration is included in BSCF5582 (Ba0.5Sr0.5Co0.8Fe0.2O3−δ) with 3-δ = 2.53.18,27 It is also a challenging effort to investigate into it systematically, in the consideration that the naïve crystalline structures include a significant concentration of



oxygen vacancies (Vo••), which are usually buried underneath the amorphous surface layers. So, in order to observe naïve crystalline structures mainly contribute to electrocatalytic process, it is essential to obtain the proper condition, where the effect of amorphous surface layers is suppressed as much as possible. In this report, we adopted ball-milling process, which would break open the intrinsic amorphous surface layer and reveal crystalline structures to the surface, as described in the schematic diagram of Figure 1. As ball milling proceeds, not only the particles are comminuted towards nanosize, but also the crystalline structures underneath the amorphous surface layers are sure to be open to the surface, participating directly in the catalytic process. The electrochemical performance would be explained in correlation with the coincident ζ changes, which accompany the changes of surface structure and chemistry.

2.

Experimental Section

Synthesis Ba0.5Sr0.5Co0.8Fe0.2O3-δ powders were prepared using polymerized complex methods.27-29 The starting materials consisted of barium nitrate (Ba(NO3)2, ≥99.0% purity, Alfar Aesar Co.), strontium nitrate (Sr(NO3)2, ≥99.0% purity, Aldrich Chemical Co.), cobalt(II) nitrate (Co(NO3)2·6H2O, ≥99.0% purity, Alfar Aesar Co.), and iron(III) nitrate (Fe(NO3)3·9H2O, ≥98.0% purity, Alfar Aesar Co.). A 0.04 mol quantity of ethylenediamine tetraacetic acid

(EDTA) was mixed with 40 ml of a 1M NH4OH solution to make a NH4-EDTA buffer solution. Equal molar amounts of barium nitrate (0.01 mol) and strontium nitrate (0.01 mol), 0.16 mol of Co(NO3)2·6H2O and 0.04 mol of Fe(NO3)3·9H2O were added to the buffer solution to make the required stoichiometry of Ba0.5Sr0.5Co0.8Fe0.2O3-δ. Anhydrous citric acid


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(0.06 mol) was added, and the pH value was adjusted to 8 using 1N NH4OH solution. Each solution was kept on a hot plate at 180 °C and stirred until gelation occurred. After 24 h, the gelled samples were baked in a drying oven at 200 °C for 6 h. The as-produced powders were then calcined at 1050 °C for 5 h in air for the intrinsic BSCF5582. The intrinsic BSCF5582 powder was mixed into ethanol, and the mixtures were ball milled with Al2O3 media for 48 h in a polyethylene bottle, followed by dying at 80 oC in a vacuum oven for 24 h for BMBSCF5582. BM-BSCF5582 was annealed at 950 °C for 5 h and 48 h for 5h O2-BMBSCF5582 and 48h O2-BM-BSCF5582, respectively, under the constant flow of 10 ml/min of O2 gas within the closed tube furnace, and were cooled down to RT.

Preparation of catalysts inks

5 µL of the catalysts ink was carefully dropped onto a polished glassy carbon (GC) electrode of 4 mm diameter (RRDE Pt Ring/GC Disk Electrode, cat. NO. 011162, ALS Co., Ltd.). Glassy carbon electrodes were polished with 0.05 µm polishing alumina to maintain a good condition of working electrode (PK-3 Electrode Polishing kit, ALS Co., Ltd.). Catalystcoated GC electrodes were then dried under vacuum at room temperature for 3 h. Perovskite oxides or IrO2 were mixed with KB (commercial Ketjenblack EC-600JD) at a mass ratio of 4:1. Catalyst inks were prepared by dispersing 20 mg perovskite oxides and KB mixed powder in ethanol (≥99.5% purity) 0.8ml and 0.2 ml of 5wt % Nafion (Aldrich) solution. Catalyst ink for 20wt% Pt on 80wt% Vulcan XC-72 composites (Pt/C 20%) was prepared using the same solution mixture (ethanol and Nafion) without KB.

Electrochemical characterization



Electrochemical measurements were performed with Rotating ring-disk electrode (RRDE) (ALS Co., Ltd.) with a 4 mm diameter working electrode where Pt wire and Hg/HgO were used as a counter and reference electrodes, respectively. 0.1 M KOH solution was used as an electrolyte; pure oxygen gas (99.9%) was fluxed into solution as a purging process for 30 min before each RDE experiment to make the electrolyte saturated with oxygen. Electrochemical characterization of as-prepared catalysts were conducted using bi-potentiostat (Iviumstat) with a scan rate of 10 mV/s at 1600 rpm with a potential range from 0.15 to -0.8 V vs. Hg/HgO under the saturated oxygen gas, and sufficient ring potential of 0.4 V was biased to oxidize immediately during ORR. Half-cell experiments for ORR using RRDE were carried out under the saturated argon gas for background measurements separately. Potential/V vs. RHE was obtained by adding 0.92 V to Potential/V vs. Hg/HgO. The collection efficiency (N) was determined under Ar atmosphere using 10 mM K3[Fe(CN)6], which is around 0.41. This value is very close to its theoretical value, 0.42. Hydrogen peroxide yields and the number of electrons transferred (n) were calculated using the equations as below.22,23

HO2 (% ) = 200

Ir N Id +

n=4

Ir N

Id Id +

Ir N

For OER performance measurement, Rotating disk electrode (RDE) experiments were performed for 80wt% BSCF5582, BM-BSCF5582, 5h O2-BM-BSCF5582 and 48h O2-BMBSCF5582 on 20wt% Ketjenblack composites (BSCF5582) and 80wt% IrO2 on 20% Ketjenblack composites (IrO2). All half-cell experiment for OER using a rotating disk


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electrode (RDE) (ALS Co., Ltd.) was carried out under the same conditions with a 4 mm diameter working electrode, where Pt wire and Hg/HgO were used as a counter and reference electrodes, respectively. 0.10 M KOH was used as an electrolyte; pure oxygen gas (99.9%) was purged for 30 min before each RDE experiment to make the electrolyte saturated with oxygen.

Electrochemical characterization of as-prepared catalysts was conducted using a bi-

potentiostat (Iviumstat) with a scan rate of 10 mV/s at 1600 rpm with the potential range from 0.35 to 0.9 V vs. Hg/HgO.

Material Characterization

XRD: The crystal structures of the materials were analyzed by X-ray diffractometer (XRD) (D/Max2000, Rigaku) using Cu-Ka radiation with a scan range of 20°–800°, a step size of 0.02°, and a counting time of 5 s. Lattice parameters were determined using a leastsquares method. Particle size measurement: Particle size was measured by Particle Size Analyzer (Cilas 1090, Cilas) which ensures size measurement ranging from 0.04 to 500 microns. Powder was well-dispersed using ultra-sonication for 30 minute in ethanol solvent. The particle size of each sample means D50 value. Zeta potential (ζ ζ, mV): Zeta potential (ζ, mV) was measured by Zetasizer (Nano ZS, Malvern) with ethanol solvent after each sample (82, 82BM, O2-5hr, O2-48hr) was ultrasonicated for 2 h for the sake of dispersion. Each data is obtained as an average value, after 10 times of measurements were executed, where each measurement was carried out for 10 seconds.



Full cell: The full-cell is structured with 0.8 g Zn foil as anode, 6 M KOH electrolyte, Ni mesh as current collector and air cell. Air electrode was fabricated with 60% of active carbon, 5% of catalyst, 3% of KB as conductor, and 32% of PTFE as binder. TEM analysis: The surface micor-structures were analyzed on the particles using a TEM instrument (JEOL JEM-2100F) operated at a voltage oft 200 kV, after the TEM sample were prepared by the dry colloid method. XPS analysis: Chemical states on the surfaces were analyzed by X-ray Photoelectron spectroscopy (XPS) (K-alpha, ThermoFisher). Depth profile was obtained by etching surface for 100 seconds by the depth of 10 nm for each measurement.

3.

Results and Discussion

The surface morphologies for the intrinsic BSCF5582 perovskite catalysts (BSCF5582), the ball-milled BSCF5582 for 48hr (BM-BSCF5582), and the heat-treated BSCF5582 for 48 h in oxygen atmosphere after ball-milling for 48 h (48h O2-BM-BSCF5582) are represented in the HR-TEM images of Figure 2 (a), (d) and (g), respectively (Figure S2). The crystalline bonding structures at stratified layers on the identical spot were evaluated via the O1s XPS spectra (Binding Energy (B.E.) from 527 eV to 535 eV) by the steps of 10 nm in-depth (Figure S3, S4 and S5). Figure 2 (b) and (c) represent the O1s XPS spectra of the surface (the amorphous surface) and the 50 nm deep inside from the surface (inside) of BSCF5582, respectively; Figure 2 (e) and (f) represent the O1s XPS spectra of the surface (BM-surface) and the 50 nm deep inside from the surface (BM-inside) of BM-BSCF5582, respectively; Figure 2 (h) and (i) represent the O1s XPS spectra of the surface (48h O2-BM-surface) and the 50 nm deep inside from the surface (48h O2-BM-inside) of 48h O2-BM-BSCF5582, respectively. When the O1s spectrum of BSCF perovskite ceramic is de-convoluted, it usually


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consists of three spectra, lattice oxygen species (O2-), adsorbed oxygen species (O22-/O-) and decomposed carbonate (CO32-), with the BE values of 528.8, 529.5 and 531.4 eV, respectively.32-34 The amorphous surface layer consists mostly of decomposed carbonate (CO32-) peak at 531.4 eV without any lattice oxygen species (O2-) peak at 528.8 eV. According as the analysis profile approaches the inside of BSCF5582, there is a significant peak area decrease at 531.4 eV, while there are some substantial peak area at 528.8 eV peak, as summarized in Table S1 (Figure S2, S3 and S4, and Table S2, S3). The ball-milling process plays the role of opening up the amorphous surface layer, bringing up the crystalline lattice to the surface, and affecting on surface charges (Figure S2 and S4, and Table S3). The lattice includes not only B-site sublattice octahedron but also Vo•• as plausible catalytic active sites, which is expected to contribute significantly to positively charged (psedo-charged) surface being surrounded by negative free electrons. Besides, the crystalline bonding structures of the surface between BM-BSCF5582 and 48h O2-BM-BSCF5582 differ distinctively, while the surface morphologies of BM-BSCF5582 and 48h O2-BM-BSCF5582 are nearly identical according to the microstructural analyses in the Figure 2

(d) and (g),

respectively (Table S1, S3 and S4, Figure S2, S4 and S5). So that, it is reasonable to expect that the different crystalline bonding structures would contribute differently to the electrochemical performances. As a way of evaluating oxygen electrocatalysis for BSCF5582, BM-BSCF5582, 5h O2BM-BSCF5582 and 48h O2-BM-BSCF5582, which have the BET surface areas of 0.2 m2g-1, 21.5 m2g-1, 25.8 m2g-1 and 26.9 m2g-1, respectively (Figure S6, S7 and Table S5), the capacitance-corrected ORR activities were measured after mixing the perovskite catalysts as shown in Figure 3 (a-c). The linear sweep voltammograms of Figure 3(a) show that the ORR onset potentials of BSCF5582, BM-BSCF5582, 5h O2-BM-BSCF5582, and 48h O2-BM-



BSCF5582 converge to 0.78 V vs RHE, while Pt/C 20% is at 0.86 V vs RHE. Comparing with the limiting currents of Pt/C 20% with -5.8 mAcm-2, the values increased significantly from -5.3 mAcm-2 of BSCF5582 to -4.1 mAcm-2 of BM-BSCF5582, as ball-milling proceeded, which is followed by the decreasing values systematically to -5.3 and -5.8 mAcm2

for 5h O2-BM-BSCF5582 and 48h O2-BM-BSCF5582, respectively, in accordance with

different delay times in oxygen atmosphere (Figure 3 (a), S8 and S9). Figure 3(b) showed that, while the peroxide yield (H2O2 %) at kinetic region increased from 13 % for BSCF5582 to 23 % for BM-BSCF5582; as the heat-treatment proceeded, it decreased to 18 % and 10 % for 5h O2-BM-BSCF5582, 48h O2-BM-BSCF5582, respectively (Figure S10 (b)). This suggests that, instead of the increasing surface area (As) via ball-milling process, the successful control of surface charge and structure would rather enhance the 4-electron ORR process and decrease the peroxide yield of BSCF-based perovskite catalysts.22,23 As the heat-treatment proceeds after ball-milling process, the ORR activity is enhanced enough to surpass that of the intrinsic BSCF5582 and approach to that of Pt/C 20%, though there was little change of As (Table S5). It indicates that it is not just the surface area (As), but the crystalline bonding structures, defect kinds and/or defect concentrations, which rather affect more decisively on the ORR catalytic performance. The overall trend of OER activities conforms to the reversal one of ORR activities, as summarized in Figure 3 (d), (e) and (f) (Figure S11). The OER overpotentials at 2 A/g are 1.62V, 1.51 V, 1.52 V, 1.53 V and 1.65 V for BSCF5582, BM-BSCF5582, 5h O2-BMBSCF5582, 48h O2-BM-BSCF5582 and IrO2, respectively, where, the gravimetric activity represents highest at BM-BSCF5582, followed by the systematic decrease of gravimetric activities in accordance with the increasing delay times of heat-treatment (Figure 3 (f)). The geometric and gravimetric OER activities of BM-BSCF5582 were nearly 20 times higher and


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30 times higher than that of the commercial IrO2, respectively, as represented in Figure 3 (e) and (f). This result summarizes that the surface charge and structure are rather critically contributive to the oxygen electrochemical performances of both ORR and OER, in the consideration that heat-treatment process after ball-milling is dominantly effective in the electrocatalytic activities. The characteristics of the surfaces were evaluated by zeta-potential (ζ) analysis, as represented in Figure 4 (a). The mean ζ increases from -11.1 mV of BSCF55582 to 21.2 mV of BM-BSCF5582, followed by the incremental decrease to 5.2 mV and -6.1 mV of O2 5hBM-BSCF5582 and O2 48h-BM-BSCF5582, respectively, as summarized in Figure 4 (b). The positive mean ζ after ball-milling comes mostly from the positive Zeta layer, where positively charged ions are clustered around the negative surface of the particle, and the charge is supposed to be determined by the free electron (2e-) around the pseudo positive oxygen vacancy (Vo••), as described in Figure 4 (c). And, ζ values recover back negatively towards that of BSCF5582, the clear-cut coincidence with catalytic activities. Upon this argument, we suggest the catalytic mechanisms of ORR and OER as in the schematic diagrams of Figure S12 (a) and (b), respectively, when the perovskite ceramic oxides are used as catalysts at the cathode of Zn-air batteries. During ORR, the supplied e- would be involved not only in reducing both O2 (g) and water molecules (H2O) into hydroxide molecules (OH-), but also in the reaction between Vo•• and O2 (g) into VoX, on the condition that O2 (g) is constantly supplied (Vo•• + 2e- + 1/2O2 → Oo) to the aqueous KOH electrolyte (Figure S12 (a)). In contrast with the present investigation on ORR, there are quite a few reports that dwell on the beneficent effect of Vo••



on the OER catalysis,4,24,35,36 which coincides with our experimental results (Figure S13). Additionally, as a flux of electrons is provided from the catalytic surface to the outside operating system, the surface becomes depleted with free electrons (e-), leaving behind Vo••surface during OER process in Zn-air batteries, as described in Figure S12 (b). This approach supports the argument that, as surface gets depleted of free electrons leaving behind Vo••surface during OER process, the higher [Vo••surface], the more enhanced OER catalysis by reacting with OH-. Figure 5 is an I-V scanning measurement of zinc-air full-cell, reading cathodic and anodic polarization (Figure S14). The charge and discharge functionalities of O2 48h-BMBSCF5582, with 1.80 V and 1.11 V, respectively, at a charge/discharge current density of 40 mA/cm2, represents superiority to those of Pt/C-IrO2 composite with 1.86 V and 1.05 V. Moreover, the overpotential, which is the most decisive parameter in determining bifunctional electrocatalyst, can be evaluated by the value difference (∆V) between cathodic and anodic plots. O2 48h-BM-BSCF5582, with ∆V=0.69V at a charge/discharge current density of 40 mA/cm2, outperforms Pt/C-IrO2, BM-BSCF5582 and BSCF5582, which have the values of ∆V with ∆V=0.81 V, ∆V=0.81 V and ∆V=0.83 V, respectively.37 In particular, as current density increases up to 53 mA/cm2, O2 48h-BM-BSCF5582 reaches ∆V=1.00 V, while Pt/C-IrO2 represents ∆V=1.55 V, levelling off above 53 mA/cm2. It signifies that the overpotential of O2 48h-BM-BSCF5582 is far superior to that of Pt/C-IrO2. It signifies that the successful engineering upon the solid understanding of surface charge and defect chemistry is an indispensable ingredient in designing the bi-functional electrocatalyst out of the identical material system. In addition the rechargeable zinc-air battery profiles conducted at a current density of 20 mA/cm2 (Figure S15). Even though O2 5h-BM-BSCF5582


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performed at a lower OER overpotential than O2 48h-BM-BSCF5582 (Figure S15b), eventually O2 48h-BM-BSCF5582 resulted in better stability than two other catalysts (O2 5hBM-BSCF5582 and BM-BSCF5582).

4.

Conclusions

The report enables to envision another task of ball-milling, besides the comminution of ceramic powders towards nano-size particles. According to our investigation, when ballmilling proceeded on BSCF5582 ceramic powders (BM-BSCF5582), there was not only the increase of As, but also surface charge. Furthermore, throughout the heat treatment process, which is executed on BM-BSCF5582, both ORR and OER electrochemical performances are found out to be affirmatively dependent on surface charge, defect kinds and crystalline bonding rather than surface area (As). The overall catalytic performance was in harmony with ζ value changes, where the mean ζ value of BSCF5582 with -11.1 mV increases to 21.2 mV of BM-BSCF5582, followed by the incremental decrease to 5.2 mV and -6.1 mV of O2 5hBM-BSCF5582 and O2 48h-BM-BSCF5582, respectively. The surface morphologies and crystalline structures were also analyzed by HR-TEM and the O1s XPS spectra. It signifies that ball-milling process breaks open the intrinsic amorphous surface layer and reveals crystalline structures, which are distributed with a high [Vo••surface] on the surface. We investigated systematically on the role of Vo••surface for catalysis reaction via ball-milling process, where we suggested the supposition that Vo••surface would contribute to OER catalysis,



on the contrary it would suppress ORR. It will provide a new insight into how to make use of ball-milling process, and design properly the bi-functional oxide-based electrocatalysts from the aspect of Vo••surface.

Figure 1. Schematic diagrams of describing the effect of ball milling on the surface charge changes of comminuted BSCF5582 particles, which represents that it consists of two different surfaces; one is amorphous surface layer, and the other is naïve cubic crystalline phase. Figure 2. The HR-TEM images on the surface morphologies of (a) the intrinsic BSCF5582 perovskite catalysts (BSCF5582), (d) the ball-milled BSCF5582 for 48hr (BM-BSCF5582), and (g) the heat-treated BSCF5582 for 48 h in oxygen atmosphere after ball-milling for 48 h (48h O2-BM-BSCF5582), and the O1s XPS spectra of (b) the surface (the amorphous surface) and (c) the 50 nm deep inside from the surface of BSCF5582 (inside), (e) the surface (BMsurface) and (f) the 50 nm deep inside from the surface of BM-BSCF5582 (BM-inside), and (h) the surface (48h O2-BM-surface) and (i) the 50 nm deep inside from the surface (48h O2BM-inside) of 48h O2-BM-BSCF5582. Figure 3. The properties of ORR and OER activities (capacitance-corrected) of intrinsic BSCF5582, BM-BSCF5582, O2 5h-BM-BSCF5582, O2 48h-BM-BSCF5582; (a) disc, (b) peroxide percentage (HO2- %) and (c) Tafel plots of the gravimetric ORR activities as a function of potential in oxygen saturated 0.1 M KOH electrolyte at 10 mVs-1 scan rate at 1600 rpm. And, (d) linear sweep voltammetry for OER measurement at 10mVs-1, (e) OER currents at selected potentials and (f) Tafel plot of the gravimetric OER activities. The oxide electrode composites consist of 80 wt% oxide materials and 20 wt% KB, and Pt/C 20% consists of 20 wt% Pt and 80 wt% Vulcan XC-72(E-tek). Pt/C 20% (black line), intrinsic BSCF5582 (blue line), BM-BSCF5582 (green line), O2 5h-BM-BSCF5582 (orange line), O2 48h-BMBSCF5582 (red line), and IrO2 (purple line). The all oxide electrode composites contain 0.64


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mgox/cm2disk, 0.16 mgKB/cm2disk, 0.35 mgNafion/cm2disk, respectively, while Pt/C 20% electrode contains 0.16 mgPt/cm2disk, 0.64 mgXC-72/cm2disk, 0.35 mgNafion/cm2disk, respectively. At least three independent measurements were executed in order to confirm the repeatability of the experimental results for each sample. Figure 4. The plots of (a) zeta potential (ζ, mV) graphs, (b) summarized histograms for average zeta potentials and Number of electrons (n) for intrinsic BSCF5582 (blue), BMBSCF5582 (green), O2 5h-BM-BSCF5582 (orange), O2 48h-BM-BSCF5582 (red), and (c) the schematic diagrams of describing the different surface charges on different surfaces (amorphous surface layer and naïve cubic crystalline phase) within the identical perovskite catalyst, when it is communited by ball-milling process. Figure 5. Discharge and charge polarization curves of Zn-air full-cell batteries (6M KOH), consisting of intrinsic BSCF5582 (blue line), BM-BSCF5582 (green line), O2 48h-BMBSCF5582 (red line), Pt/C-IrO2 50% (black line) as a catalyst for each full-cell under the scan rate of 4 mA/s, where ambient air was conditioned on the cathode side.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/acsami.xxxxxxx. A schematic diagram of the seawater battery cell, SEM, HRTEM, EDS, XRD, TOF-SIMS study Corresponding Author *E-mail: [email protected], [email protected]

Author Contributions ‡These authors contributed equally to this article.

Acknowledgements



This research was supported by Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education.(NRF016R1D1A1B03933064)

Figures

Figure 1. Schematic diagrams of describing the effect of ball milling on the surface charge changes of comminuted BSCF5582 particles, which represents that it consists of two different surfaces; one is amorphous surface layer, and the other is naïve cubic crystalline phase.


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Figure 2. The HR-TEM images on the surface morphologies of (a) the intrinsic BSCF5582 perovskite catalysts (BSCF5582), (d) the ball-milled BSCF5582 for 48hr (BM-BSCF5582), and (g) the heat-treated BSCF5582 for 48 h in oxygen atmosphere after ball-milling for 48 h (48h O2-BM-BSCF5582). And, the O1s XPS spectra of (b) the surface (the amorphous surface) and (c) the 50 nm deep inside from the surface of BSCF5582 (inside), (e) the surface (BMsurface) and (f) the 50 nm deep inside from the surface of BM-BSCF5582 (BM-inside), and (h) the surface (48h O2-BM-surface) and (i) the 50 nm deep inside from the surface (48h O2BM-inside) of 48h O2-BM-BSCF5582.



Figure 3. The properties of ORR and OER activities (capacitance-corrected) of intrinsic BSCF5582, BM-BSCF5582, O2 5h-BM-BSCF5582, O2 48h-BM-BSCF5582; (a) disc, (b) peroxide percentage (HO2- %) and (c) Tafel plots of the gravimetric ORR activities as a function of potential in oxygen saturated 0.1 M KOH electrolyte at 10 mVs-1 scan rate at 1600 rpm. And, (d) linear sweep voltammetry for OER measurement at 10mVs-1, (e) OER currents at selected potentials and (f) Tafel plot of the gravimetric OER activities. The oxide electrode composites consist of 80 wt% oxide materials and 20 wt% KB, and Pt/C 20% consists of 20 wt% Pt and 80 wt% Vulcan XC-72(E-tek). Pt/C 20% (black line), intrinsic BSCF5582 (blue


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line), BM-BSCF5582 (green line), O2 5h-BM-BSCF5582 (orange line), O2 48h-BMBSCF5582 (red line), and IrO2 (purple line). The all oxide electrode composites contain 0.64 mgox/cm2disk, 0.16 mgKB/cm2disk, 0.35 mgNafion/cm2disk, respectively, while Pt/C 20% electrode contains 0.16 mgPt/cm2disk, 0.64 mgXC-72/cm2disk, 0.35 mgNafion/cm2disk, respectively. At least three independent measurements were executed in order to confirm the repeatability of the experimental results for each sample.



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Figure 4. The plots of (a) zeta potential (ζ, mV) graphs, (b) summarized histograms for average zeta potentials and Number of electrons (n) for intrinsic BSCF5582 (blue), BM-BSCF5582 (green), O2 5h-BM-BSCF5582 (orange), O2 48h-BM-BSCF5582 (red), and (c) the schematic diagrams of describing the different surface charges on different surfaces (amorphous surface layer and naïve cubic crystalline phase) within the identical perovskite catalyst, when it is communited by ball-milling process.

20


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Figure 5. Discharge and charge polarization curves of Zn-air full-cell batteries (6M KOH), consisting of intrinsic BSCF5582 (blue line), BM-BSCF5582 (green line), O2 48h-BMBSCF5582 (red line), Pt/C-IrO2 50% (black line) as a catalyst for each full-cell under the scan rate of 4 mA/s, where ambient air was conditioned on the cathode side.

21



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Table of contents The simultaneous analysis of ball-milling and heat-treatment reveals a distinct mechanism of the catalytic activity from the aspect of naïve crystalline of BSCF ceramics.

Keywords: Perovskites, Surface charge, Ball-milling, Oxygen reduction reaction, Oxygen evolution reaction Seungkyu Park1‡, Gyutae Nam1‡, Jang-Soo Lee2, Jaephil Cho1*and Jae-Il Jung1*

TOC figure

(3.5 cm × 8.0 cm)

*

Corresponding author : Jaephil Cho & Jae-Il Jung E-mail: [email protected]; [email protected] 22


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