Single crystalline Bi2Ru2O7 pyrochlore oxide nanoparticles as efficient bifunctional oxygen electrocatalyst for hybrid Na-air batteries

Accepted Manuscript Single crystalline Bi2Ru2O7 pyrochlore oxide nanoparticles as efficient bifunctional oxygen electrocatalyst for hybrid Na-air batteries Myeongjin Kim, Hyun Ju, Jooheon Kim PII: DOI: Reference:

S1385-8947(18)31922-3 https://doi.org/10.1016/j.cej.2018.09.204 CEJ 20043

To appear in:

Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

8 June 2018 26 September 2018 27 September 2018

Please cite this article as: M. Kim, H. Ju, J. Kim, Single crystalline Bi2Ru2O7 pyrochlore oxide nanoparticles as efficient bifunctional oxygen electrocatalyst for hybrid Na-air batteries, Chemical Engineering Journal (2018), doi: https://doi.org/10.1016/j.cej.2018.09.204

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Single crystalline Bi2Ru2O7 pyrochlore oxide nanoparticles as efficient bifunctional oxygen electrocatalyst for hybrid Na-air batteries Myeongjin Kim, Hyun Ju and Jooheon Kim* ((Optional Dedication)) Dr. Myeongjin Kim School of Chemical Engineering & Materials Science, Chung-Ang University, 211 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea E-mail: [email protected] Dr. Hyun Ju School of Chemical Engineering & Materials Science, Chung-Ang University, 211 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea E-mail: [email protected] Prof. Jooheon Kim* School of Chemical Engineering & Materials Science, Chung-Ang University, 211 Heukseok-dong, Dongjak-gu, Seoul 156-756, Republic of Korea E-mail: [email protected] Tel:+82-2-820-5763; Fax:+82-2-812-3495 Keywords : Na-air batteries; pyrochlore oxide; bismuth ruthenate oxide; bifunctional electrocatalyst; catalytic origin

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Abstract The sodium-air (Na–air) batteries are spotlighted as state-of-the-art electrical energy storage system, because of their high sodium-ion conductivity and specific energy density performance. However, the undesirable sluggish oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics limit the practical production of rechargeable Na-air batteries. Therefore, it is essential to develop highly effective bifunctional electrocatalysts for OER and ORR. Although the pyrochlore oxides (A2B2O7) exhibits great potential for highly-active bifunctional electrocatalyst, the lack of studies regarding to A-site cations have hindered the development of new pyrochlore catalysts with comprehensive understanding of catalytic activity. In this work, we report the use of a novel nanocrystalline bismuth ruthenate pyrochlore oxide (Bi2Ru2O7) as an effective oxygen electrocatalyst by using the favorable oxidation nature of Bi and Ru ions in Bi2Ru2O7. Further, the oxidized cations can donate the electrons to the surface and inner layers, providing the low-resistance pathway during OER and ORR. Finally, the bifunctional electrocatalytic activities of Bi2Ru2O7 are successfully translated to a practical device, an aqueous Na-air battery, for the first time.

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1. Introduction

Environmentally friendly energy storage systems are necessary to help reduce fossil fuel dependence and environmental contamination [1]. Among the diverse energy conversion and storage devices, to obtain the outstanding power and energy density performance, the metal–air batteries show great attention for effective electrical energy storage devices [2,3]. In particular, sodium-air (Na–air) batteries are of interest because of the favorable characteristics, including 1683 W h kg-1 of high theoretical specific energy density, high sodium-ion conductivity, high sodium abundance, low cost, long shelf-life and low environmental impact [4,5]. In addition, Na-air batteries have higher specific energy density (1,683 W h kg-1) than Li-ion batteries ((200–250) W h kg-1) and Zn-air batteries (1,084 W h kg-1) [6,7]. Unfortunately, the slow oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) kinetics limit not only to obtain the highly desirable bifunctional catalytic activity but also to produce the rechargeable Na-air batteries [7,8]. To date, various metal free-based materials, nonprecious metals, metal oxide materials have been developed for efficient bifunctional ORR and OER electrocatalysts [9-20]. The most promising candidate based on highly active catalytic properties and cycling stability are metal oxides, as they demonstrate comparable ORR and OER activities with the commercial Ir/C and Pt/C catalysts [21-22]. Along with metal oxide-based materials, pyrochlore oxides (A2B2O7), such as Pb2Ru2O7-x [23], Pb2[Ru2-xPbx4+]O6.5 [24], Y2[Ru2-xYx]O7-y [25] and Pb2Ru2O6.5 [26] have been extensively developed for achieving highly active bifunctional electrocatalysts [23-29]. The metallic conduction properties, specifically high charge transfer, of pyrochlore oxides have been reported to result in high catalytic behavior and electrical conductivity and these metallic conduction behaviors can be determined by the chemical bonding nature of A and B site cations [30]. However, A-site cations studied were limited to Pb2+ ion, which restricts the comprehensive study of the catalytic origin of the pyrochlore oxide and the studies of the development of additional A-site cations still remains big challenge [23,24]. Therefore, Bi3+ cation, which exhibits similar ionic radius as Pb2+, is regarded as a new A-site cation of the A2Ru2O7 pyrochlore oxide. Moreover, the similarity of Ru (2.2) and an A-site 3

Bi ion (2.02) Pauling electronegativities causes the high density of covalent chemical bond between Ru-O-Ru/Bi, resulting in the shared electrons in the Ru-O-Ru/Bi covalent bonds due to the delocalization of orbital [30]. The shared electrons in delocalized orbitals in Bi2Ru2O7 allows effective and rapid charge transfer behavior during electrocatalysis. Because of these favorable qualities, Bi2Ru2O7 should be developed as a novel bifunctional electrocatalyst to examine the pyrochlore structure and catalytic origin. Herein, the bismuth ruthenate (Bi2Ru2O7) pyrochlore oxide nanoparticles are synthesized as highly active and stable bifunctional oxygen electrocatalysts for use as air electrode in hybrid Na-air battery. The Bi2Ru2O7 shows promising bifunctionial catalytic behavior with 0.85 V of oxygen electrode activity (The OER and ORR potential difference at 10 and -3 mA cm-2 of current density, respectively) in an alkaline medium, which shows much lower value than that of Ir/C and Pt/C catalysts. These outstanding ORR and OER catalytic activities are investigated by ex situ X-ray absorption spectroscopy (XAS). Based on the XAS analysis, the positive peak shift of the Bi2Ru2O7 catalyst Ru K-edge and Bi LIII-edge XANES spectra imply the oxidation of the Bi and Ru ions, and these oxidized cations can donate the electrons to the surface. Further, the donated electrons can easily reach to the inner layers, with low-resistance pathway during OER and ORR. Therefore, the favorable oxidation nature of Bi and Ru ions in Bi2Ru2O7 during electrocatalysis leads to highly active bifunctional catalytic properties. Significantly, the Bi2Ru2O7 is firstly employed to aqueous Na-air battery air cathode and the Bi2Ru2O7-based Na-air cell exhibited low overpotential gap (0.211 V), and high round-trip efficiency (93.58 %) compared to that of Ir/C- and Pt/C-based cells with superior power density (156.32 mW g-1 of maximum specific power density at a current density of 120 mA g1),

and with excellent cycling stability up to 50 cycles.

2. Experimental

2.1 Synthesis of bismuth ruthenium pyrochlore oxide (Bi2Ru2O7)

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The buffer solution was made with a mixture of 1 M ammonia solution, 3.42 × 10–2 mol anhydrous ethylenediaminetetraacetic acid and 1.5 mL nitric acid at a solution pH of 7. To prepare the Bi2Ru2O7, 0.3173 g of bismuth (III) acetate, 0.2614 g of ruthenium (III) nitrosyl nitrate solution and 10 g anhydrous citric acid were dissolved and stirred with the buffer solution for 24 h at 150 °C. The gelled solution was dried in an oven at 200 °C for 12 h. The prepared dried powder were crystallized at 1320 °C for 8 h to produce single crystalline Bi2Ru2O7 nanoparticles.

2.2 Preparation and characterization of Na-air battery

To construct the aqueous Na-air battery, the air cathode was prepared by fabricating the catalyst ink solutions, which were consisted of the Bi2Ru2O7 (90 wt%) with polyvinylidene fluoride (10 wt%) as a binder and N-methyl-2-pyrrolidone as a solvent. The catalyst ink solution was sprayed onto one side of the Teflon-treated carbon paper. The area of the air electrode was 4 cm2, and the mass loading of the catalyst layer was 2.5 mg cm-2. The negative electrode was fabricated in a glove box as the pouch cell. The metallic sodium electrode attached with nickel mesh was inserted into the pouch cell, followed by injection of an organic electrolyte. Thereafter, the separator solid electrolyte membrane (NASICON) was introduced and the pouch cell was sealed in the globe box with one side of the solid electrolyte membrane exposed to air and the other side in contact with the organic electrolyte (1 M NaCF3SO3/TEGDME).

2.3 Characterization methods

The OER/ORR activities were measured using a rotating disk electrode (RDE) and a threeelectrode electrochemical cell. A Pt wire, Ag/AgCl, and glassy carbon rotating disk electrode were used as a counter, reference and working electrode. The electrolyte used in OER/ORR was 0.1 m KOH solution. Pure oxygen gas (99.9%) was purged for 30 min before RDE experiment to make the electrolyte saturated with oxygen. The catalyst (7.5 mg) was mixed with deionized water (0.1 mL), 5

ethanol (0.86 mL), and 5 wt% Nafion (0.038 mL, 5 wt% in isopropanol). The resulting slurry was ultra-sonicated for 30 min to generate a catalyst ink. The ink (10.0 μL) was pipetted onto the 0.2475 cm2 glassy carbon electrode. Pt on Vulcan carbon black (Pt/C, JM) and 20 wt% Ir on Vulcan (Ir/C, Premetek) were measured for comparison. The catalyst ink was prepared as follows. The Pt/C (or Ir/C) catalyst (5 mg) was mixed with deionized water (0.1 mL), ethanol (1.06 mL), and 5 wt% Nafion (0.04 mL, in isopropanol). The resulting slurry was ultrasonicated for 30 min to generate a catalyst ink. The ink (6.0 μL) was pipetted onto the 0.2475 cm2 glassy carbon electrode. All ORR and OER measurements are reported versus the reversible hydrogen electrode (RHE), and the obtained potentials (vs Ag/AgCl) were converted to RHE by using following equation: ERHE = EAg/AgCl + 0.0592 pH + EAg/AgCl0

(1)

Where, EAg/AgCl0 (in 1 M KCl) = +0.235 V, pH = 12.9 for 0.1 M KOH. Moreover, the further detailed characterization methods were summarized in supporting information.

3. Results and Discussion

Figure 1(a) represents the chemical route for the synthesis of the single crystalline bismuth ruthenate (Bi2Ru2O7) pyrochlore oxide. Bi2Ru2O7 was obtained from uniformly cross-linked Bi3+ and Ru4+ ions via sol-gel method with citric acid (C6H8O7) as a chelating agent. In order to obtain the single crystalline Bi2Ru2O7 nanoparticles, the cross-linked Bi3+ and Ru4+ ions were crystallized at 1320 °C. Figure 1(b) represents the X-ray diffraction (XRD) pattern of Bi2Ru2O7, indicating the space group of Fd3m symmetry with the cubic phase crystal structure. The X-ray adsorption near edge structure (XANES) was used to determine the bonding characteristics and local composition of Bi2Ru2O7 nanoparticles. Figure 1(c) shows the normalized Ru K-edge XANES spectra of Ru metal and RuO2, and Bi2Ru2O7 catalysts. The Ru K-edge spectra for Bi2Ru2O7 represents the two overlapped peaks, which are observed at the photon energy of 22139 and 22149 eV. The former indicates the bound of dipole-allowed transition from Ru 1s to 5p states and the latter represents to the continuum. Moreover, Figure 1(d) shows that the Bi LIII-edge XANES spectra for Bi2Ru2O7 shows the most 6

intense peak at a photon energy of 13442 eV due to the dipole-allowed transition from Bi 1s to 6P states. Figure 2 shows the high-resolution transmission electron microscopy (HR-TEM) and fieldemission scanning electron microscopy (FE-SEM) images of Bi2Ru2O7, in order to verify the morphology. Figure 2(a)–(c) indicate that the nanoparticles of the pyrochlore Bi2Ru2O7 were aggregated, with irregular polyhedral shapes of size below 200 nm. The X-ray spectrometry (EDS) Bi, Ru and O elemental mapping images for Bi2Ru2O7 indicate uniform distributions of each element throughout the Bi2Ru2O7 nanoparticles (Figure 2(d)–(f)). The cubic phase of the Bi2Ru2O7 nanoparticles were further characterized by selected–area electron diffraction (SAED) pattern (Figure 3(a)). The SAED pattern along the [1 1 0] main zone axis shows unit spots of (-1 1 1) and (-1 1 -1), indicating the typical cubic structure of the pyrochlore oxide. Moreover, as shown in Figure 3(b), 0.59 and 0.51 nm of lattice d-spacing values are corresponded to (1 1 1) and (0 0 2) diffraction planes, respectively. In the same manner, the SAED pattern (Figure 3(c)) along the [1 1 1] main zone axis indicates the typical cubic phases with the unit spots of (-2 0 2) and (0 -2 2). In addition, both (2 2 0) and (0 2 2) crystal planes showed the lattice fringes with d-spacing values of around 0.36 nm (Figure 3(d)). The bifunctional electrocatalytic activity of the Bi2Ru2O7 catalyst was evaluated in an O2saturated 0.1 M KOH solution using a rotating disk electrode (RDE). Moreover, the commercial Ir/C and Pt/C catalysts were also investigated to benchmark the bifunctional electrocatalytic activities. The ORR activity of the Bi2Ru2O7 catalyst was investigated by cyclic voltammetry (CV) in an O2- and N2saturated KOH solution. Figure 4(a) shows that obvious cathodic reduction peaks for ORR were observed in O2-saturated solution; however, there are no cathodic reduction peaks in N2-saturated KOH solution. The Bi2Ru2O7 catalyst exhibits ~0.9 V (versus RHE) of reduction onset potential, which can be ascribed to the electrocatalytic oxygen reduction on the electrode [31]. Linear sweep voltammetry (LSV) was used for additional characterization, and the ORR catalytic activity LSV polarization curves of Bi2Ru2O7 catalyst shows an onset potential of ~0.89 V and a limiting current density of -5.349 mA cm-2 (Figure 4(b)). In addition, Enhanced ORR activity of Bi2Ru2O7 was also 7

indicated by its reduction potential of 0.827 V at a current density of -3 mA cm-2 relative to that of Ir/C catalyst (0.637 V), indicating the outstanding oxygen reduction activity for the Bi2Ru2O7 catalyst [32]. Although the onset and reduction potential of Pt/C show more positive value than Bi2Ru2O7, the ORR catalytic activity for Bi2Ru2O7 is much outstanding compared with other previously reported metal- or metal oxide based, non-noble electrocatalysts (Table S1). To obtain deep insight into the ORR properties, LSV curves were recorded at different rotating speeds.

Figure 4(c) shows the LSV

curves for Bi2Ru2O7 catalyst, which reveal that the limiting current density increases with the increase of rotation speed. For comparison, Figure S1 shows the LSV curves at different rotating speeds for Pt/C catalyst. In addition, Figure S2 shows the corresponding Koutecky-Levich (K-L) plot for Bi2Ru2O7 and Pt/C catalyst. Based on the K-L plots, the first-order reaction kinetics toward the concentration of dissolved O2 can be obtained by the near parallelism of the fitting lines at different potentials from 0.2 to 0.7 V. The number of electrons involved per O2 in the ORR on various catalysts was determined by the Koutecky-Levich equation [33,34]: 1 l l l l = + = + J J L J K Bω1 / 2 J K

(2)

where, J, JL, JK, B and ω are the measured disk current density, diffusion limiting current density, kinetic limiting current density, B-factor and electrode rotation speed, respectively. Moreover, the Bfactor can be obtained by the following equation [35]: B = 0.62nFCoDo2/3ν-1/6

(3)

where, n, F, Do, Co and ν are the apparent number of electrons transferred in the reaction, Faraday constant (96,485 C mol-1), diffusion coefficient of O2 in 0.1 M KOH (1.9 x 10-5 cm2 s-1), concentration of O2 dissolved in 0.1 M KOH (1.2 x 10-6 mol cm-3) and kinetic viscosity of the solution (0.01 cm2 s-1), respectively. Figure 4(d) represents the electron transfer number (n) for the Bi2Ru2O7 and Pt/C catalysts obtained by the slopes of the K–L plots. The ORR on Bi2Ru2O7 yields an n ranging from 3.66 to 3.79 over the potential range from 0.2 to 0.7 V, which is similar to that of a Pt/C catalyst (n: 3.83–3.91), demonstrating the favorable four-electron reduction on the ORR property for Bi2Ru2O7. Figure 5(a) shows the OER polarization curves of Bi2Ru2O7, Pt/C, and Ir/C catalysts in O2-

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saturated 0.1 M KOH solution obtained by RDE. Although the OER onset potential of Bi2Ru2O7 is slightly higher than Ir/C, the overpotentials at a current density of 10 mA cm-2 for Bi2Ru2O7 and Ir/C shows similar values (0.448 and 0.372 V). The Tafel slope was calculated based on the OER polarization curves applying the Tafel equation (η = a + b∙log│J│, where J, b and η is the current density, Tafel slope and overpotential, respectively). Figure 5(b) shows that Bi2Ru2O7 reveals a Tafel slope of 108 mV dec-1, which is similar to that of the Ir/C catalyst (80 mV dec-1), suggesting the desirable rapid OER kinetics of Bi2Ru2O7 [36, 37]. Although the OER activity of the Bi2Ru2O7 is slightly inferior to that of the Ir/C, when considering the cost of the Ir/C, it could be a promising metal-free catalyst for the OER. Figure 5(c) shows the overall ORR and OER catalytic activities for the Bi2Ru2O7, Pt/C, and Ir/C catalysts by representing the overall polarization curves within the ORR and OER potential window. The oxygen electrode activity is obtained by calculating the OER and ORR potential difference at 10 and -3 mA cm-2 of current density, respectively (ΔE = EOER,J10 – EORR,J3)

[38-40]. A more desirable bifunctional catalyst is characterized by a lower ΔE value. Therefore,

Bi2Ru2O7 exhibits the most outstanding oxygen electrode activity because of the smallest ΔE value (0.85 V) compared with Pt/C (0.95 V) and Ir/C (0.97 V) catalysts (Figure 5(d)). Moreover, the ΔE value for Bi2Ru2O7 is comparable, or even much lower to those of the OER and ORR bifunctional electrocatalysts (e.g., metal oxides, pyrochlore oxides, perovskite oxides), which were previously reported data and this could be attributed to the enhancement of the bifunctional activity of Bi2Ru2O7 for OER and ORR (Table S2). In order to examine the origin of the high bifunctional catalytic activity of Bi2Ru2O7, the electron configurations of Bi and Ru ions in the Bi2Ru2O7 catalyst are analyzed by ex situ X-ray absorption spectroscopy (XAS). Figure 6(a) and (b) show the Ru K-edge and Bi LIII-edge XANES spectra for the Bi2Ru2O7 catalyst before and after ORR. The peaks for Ru K-edge and Bi LIII-edge XANES spectra of the Bi2Ru2O7 were shifted to higher photon energy values after completing the ORR, demonstrating that the A-site Bi and B-site Ru ions are oxidized and exhibit higher oxidation states. In addition, Ru and Bi ions exhibit higher oxidation states during the OER by confirming the positive peak shift of the photon energy (Figure 6(c) and (d)). According to the XAS results, the slight 9

variations of the Ru K-edge and Bi LIII-edge XANES spectra for the Bi2Ru2O7 catalyst suggest the substantial changes of Bi and Ru ions at the Bi2Ru2O7 surface. During the OER and ORR, the Bi and Ru ions are oxidized and the electrons are donated simultaneously. Therefore, the donated electrons can easily reach to the inner layers and surface of Bi2Ru2O7 and provide the low-resistance pathway during electrocatalysis due to the highly improved electron transport. As a result, the favorable oxidation nature of Bi and Ru ions in Bi2Ru2O7 occurring during the ORR and OER leads to highly efficient bifunctional catalytic activity. To evaluate the utility of the Bi2Ru2O7 pyrochlore oxides, Bi2Ru2O7 was applied as air cathode for the hybrid Na-air battery. The rechargeable aqueous Na-air battery was built with ceramic solid separator/electrolyte (NASICON) and catalyst air cathode in 0.1 M NaOH solution and Na metal anode in 1 M NaCF3SO3/TEGDME electrolyte solution [41]. The electrochemical reactions during charging and discharging at the cathode and anode sides can be described as follows Cathode : 1/2O2 + H2O + 2e- → 2OH-

E0 = +0.40 V

(4)

Anode : 2Na+ + 2e- → 2Na

E0 = -2.71 V

(5)

Overall : Na(s) + 1/2H2O(l) + 1/4O2(g) → Na+(aq) + OH-(aq)

E0 = 3.11 V

(6)

Therefore, the standard state cell potential of fabricated hybrid Na-air battery is 3.11 V [42]. However, because of the molar concentration of NaOH aqueous electrolyte (0.1 M), the cell potential at nonstandard state conditions for hybrid Na-air battery is calculated by following equation: 𝑅𝑇

0 𝐸𝑐𝑒𝑙𝑙 = 𝐸𝑐𝑒𝑙𝑙 ‒ (𝑛𝐹)𝑙𝑛𝑄

(7)

Where, Ecell, E0cell, R, T, F, n and Q is the cell potential at non-standard state conditions, standard state cell potential, gas constant (8.31 J mol-1 K-1), absolute temperature (K), Faraday’s constant (96485 C mol-1), number of moles of electrons transferred in the balanced equation for the reaction occurring in the cell and reaction quotient for the reaction, respectively. Based on the balanced equation (4) and (5), the number of moles of electrons transferred (n) is 2 and reaction quotient (Q) for the hybrid Na-air battery can be calculated to be 0.01 by considering the equation (6). Therefore, by the substitution of the obtained n and Q to the equation (7), the fabricated Na-air battery cell voltage is calculated to be 3.1692 (~3.17) V, which shows similar value with previous literature [43]. The charge/discharge 10

curves for bare carbon paper, Pt/C, Ir/C and Bi2Ru2O7 at a current density of 0.01 mA cm-2 are shown in Figure 7(a). Although the charge and discharge curves for all Na-air cells are deviated from the theoretical voltage value (~3.17 V), the Bi2Ru2O7 catalyst applied cell exhibited the most ideal charge/discharge profile compared with other catalysts employed cells. The terminated charge and discharge voltage, and overpotential gap of Bi2Ru2O7, bare carbon paper, Pt/C and Ir/C based Na-air cells are summarized in Figure 7(b). The Bi2Ru2O7 air cathode employed cell shows the most efficient charge/discharge behavior (overpotential gap : 0.211 V, round trip efficiency : 93.58 %) than bare carbon paper (0.719 V, 79.26 %), Ir/C (0.297 V, 90.9 %) and Pt/C (0.236 V, 92.91 %) due to the outstanding ORR and OER activity of Bi2Ru2O7. The cycle performance of Na-air cells were analyzed by charge/discharge testing at a current density of 0.01 mA cm-2 up to 50 cycles. Figure S3 represents the cycling stability of the bare carbon paper based Na-air cell, which shows stable cycle performance only up to the 25th cycle; however, after that it starts to lose the stable cycle performance. Meanwhile, Bi2Ru2O7 air cathode employed Na-air cell shows no obvious decrease of round trip efficiency, which indicates good cycle performance of the Bi2Ru2O7 (Figure 7(c)). The charge/discharge curves acquired with various current densities were obtained to investigate the rate capability of the Bi2Ru2O7 air electrode (Figure 7(d)). The overpotential gap between the terminated charge and discharge voltage was gradually increased with increasing current densities. Surprisingly, the Bi2Ru2O7 air electrode demonstrated the smallest decrease in the round trip efficiency along with a fivefold increase in the current density, revealing an outstanding rate performance of Bi2Ru2O7 (Figure 7(e)). The rate capability for Bi2Ru2O7 air cathode as a function of charge-discharge cycling was further analyzed by increasing the current density (Figure 7(f)). Although the Bi2Ru2O7 air cathode shows the gradual decrease of round-trip efficiency with increasing the current density, the round-trip efficiency for the Bi2Ru2O7 air electrode was recovered to 93.27 % even after high-rate cycles, representing the excellent rate performance. The power density of Na-air battery was calculated by following equation [5]: Ps = Is x Vad

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(8)

where, Ps, Is, and Vad are the power density, applied current density, and average discharge voltage, respectively. Figure S4 shows that the Bi2Ru2O7-based Na-air cell was found to deliver 156.32 mW g1

of maximum specific power density at a current density of 120 mA g-1. The comparison of the

electrochemical performance for previously reported aqueous or non-aqueous Na-air batteries and Bi2Ru2O7-based Na-air battery is summarized in Table S3. Based on these results, the Bi2Ru2O7 catalyst can be a promising candidate for use in the rechargeable aqueous Na-air battery, with its improved electrochemical performances, such as low overpotential gap, superior round-trip efficiency, high specific power density, and excellent charge-discharge cycling stability.

4. Conclusions

The bismuth ruthenate (Bi2Ru2O7) pyrochlore oxide nanoparticles were investigated as ORR and OER electrocatalysts for Na-air batteries. The prepared Bi2Ru2O7 exhibits high current density and excellent oxygen electrode activity (0.85 V) during OER and ORR, indicating the outstanding bifunctional catalytic activity. The favorable oxidation nature of Bi and Ru ions in Bi2Ru2O7 can be attributed to the highly active bifunctional catalytic activity of Bi2Ru2O7. The oxidized Bi and Ru ions can donate the electrons, and these electrons can easily migrate into surface and inner layers, with low-resistance pathway of electrons during the electrocatalytic reaction. As a result, significantly enhanced electrochemical performance of the hybrid Na-air battery was achieved by using a Bi2Ru2O7 air cathode with low overpotential gap (0.211 V), high round-trip efficiency (93.58 %) and power density (156.32 mW g-1 of maximum specific power density at a current density of 120 mA g-1), with excellent cycling stability up to 50 cycles. Therefore, we strongly believe that our work can suggest new insights and possibilities for effective catalytic-active pyrochlore oxides.

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Figure captions: Figure 1. (a) Schematic representation of the preparation of Bi2Ru2O7. (b). XRD pattern for Bi2Ru2O7. (c) Normalized Ru K-edge XANES spectra of Ru metal, RuO2 and Bi2Ru2O7. (d) Normalized Bi LIII-edge XANES spectra of Bi2O3, and Bi2Ru2O7. Figure 2. (a) Low-magnification HR-TEM image of Bi2Ru2O7. (b). High-magnification HR-TEM image of Bi2Ru2O7. (c) FE-SEM image of Bi2Ru2O7. (d). EDS Bi element mapping image for Bi2Ru2O7. (e) EDS Ru element mapping image for Bi2Ru2O7. (f) EDS O element mapping image for Bi2Ru2O7. Figure 3. (a) SAED patterns for Bi2Ru2O7 along the [1 1 0] zone axis. (b) Lattice structures of Bi2Ru2O7 examined by HR-TEM. (c) SAED patterns for Bi2Ru2O7 along the [1 1 1] zone axis. (d) Lattice structures of Bi2Ru2O7 examined by HR-TEM. Figure 4. (a) CV curves for Bi2Ru2O7 in N2 and O2 saturated 0.1 M KOH solution with a scan rate of 100 mV s-1. (b) ORR polarization curves for Bi2Ru2O7, Pt/C and Ir/C in O2-saturated 0.1 M KOH (Rotation rate : 1600 rpm, Scan rate : 10 mV s-1). (c) ORR polarization curves for Bi2Ru2O7 at different rotating speed with a scan rate of 10 mV s-1. (d) Electron transfer number for Bi2Ru2O7 and Pt/C at different potentials. Figure 5. (a) OER polarization curves for Bi2Ru2O7, Pt/C and Ir/C in O2-saturated 0.1 M KOH (Rotation rate : 1600 rpm, Scan rate : 10 mV s-1). (b). Tafel plots for Bi2Ru2O7 and Ir/C. (c) The overall polarization curves for Bi2Ru2O7, Pt/C and Ir/C catalysts within the ORR and OER potential window. (d) Comparison of oxygen electrode activity of Bi2Ru2O7, Pt/C and Ir/C. Figure 6. (a) Normalized Ru K-edge XANES spectra of Bi2Ru2O7 before and after ORR. (b) Normalized Bi LIII-edge XANES spectra of Bi2Ru2O7 before and after ORR. (c) Normalized Ru K-edge XANES spectra of Bi2Ru2O7 before and after OER. (d) Normalized Bi LIII-edge XANES spectra of Bi2Ru2O7 before and after OER. 18

Figure 7. (a) Charge-discharge curves of the Na-air cell using Bi2Ru2O7, Pt/C and Ir/C-coated carbon papers at a current density of 0.01 mA cm-2. (b). Comparison of the overpotential gap of the carbon paper, Bi2Ru2O7, Pt/C and Ir/C. (c) Cycling stability of Bi2Ru2O7 electrode up to 50 cycles with terminated charge and discharge voltage and round trip efficiency. (d) Chargedischarge curves of the Na-air cell using Bi2Ru2O7-coated carbon papers at different current densities. (e) Comparison of the round trip efficiency of the carbon paper, Bi2Ru2O7, Pt/C and Ir/C. (f) Cycling terminated charge and discharge voltage and round trip efficiency profile of Bi2Ru2O7 at different current densities.

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