Single crystalline pyrochlore nanoparticles with metallic conduction as efficient bi-functional oxygen electrocatalysts for Zn–air batteries

Abstract

Oxygen reduction reaction (ORR) or oxygen evolution reaction (OER) electrocatalysts including carbon-, non-precious metal-, metal alloy-, metal oxide-, and carbide/nitride-based materials are of great importance for energy conversion and storage technologies. Among them, metal oxides (e.g., perovskite and pyrochlore) are known to be promising candidates as electrocatalysts. Nevertheless, the intrinsic catalytic activities of pyrochlore oxides are still poorly understood because of the formation of undesirable phases derived from the synthesis processes. Herein, we present highly pure single crystalline pyrochlore nanoparticles with metallic conduction (Pb₂Ru₂O_6.5) as an efficient bi-functional oxygen electrocatalyst. Notably, it has been experimentally shown that the covalency of Ru–O bonds affects the ORR and OER activities by comparing the X-ray absorption near edge structure (XANES) spectra of the metallic Pb₂Ru₂O_6.5 and insulating Sm₂Ru₂O₇ for the first time. Moreover, we followed the interatomic distance changes of Ru–O bonds using in situ X-ray absorption spectroscopy (XAS) to investigate the structural stabilities of the pyrochlore catalysts during electrocatalysis. The highly efficient metallic Pb₂Ru₂O_6.5 exhibited outstanding bi-functional catalytic activities and stabilities for both ORR and OER in aqueous Zn–air batteries.

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Broader context

Metal–air batteries have received great attention as possible candidates for environment-friendly energy conversion and storage systems based upon their high specific energy density. Notably, Zn–air batteries are considered to be an alternative battery system because of their long shelf-life, low price and abundant zinc deposits. Currently, many researchers have been intensively studying rechargeable Zn–air batteries; however, the slow kinetics of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) hinders the charge and discharge processes. Thus, bi-functional electrocatalysts with high catalytic activity and stability are of great importance for rechargeable Zn–air batteries. To date, metal oxides have demonstrated outstanding electrochemical performances. In this work, we report metallic- and insulating-pyrochlore oxides (A₂Ru₂O_7−x, A = Pb and Sm) as bi-functional electrocatalysts for rechargeable Zn–air batteries. Interestingly, these two pyrochlore structures presented different chemical properties depending on the A- and B-site cations, and the origin of catalytic activities for ORR and OER was proposed by correlating the electronic structure and catalytic activity.

Environmentally friendly energy storage systems are required to reduce environmental contamination and fossil fuel dependence. Among the various energy conversion and storage technologies, metal–air batteries with high power and energy densities are promising candidates for the operation of portable devices and electric vehicles,^1,2 particularly Zn–air batteries due to their long shelf-life, low price and abundant zinc deposits.^3–7 Zn–air batteries have a higher specific energy density (ca. 1084 W h kg⁻¹) than Li-ion batteries (200–250 W h kg⁻¹).⁵ However, to achieve rechargeable Zn–air batteries, there are critical limitations on charging and discharging because of the sluggish rates of the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER).^8,9 Highly active and stable ORR or OER electrocatalysts including carbon-,^10–15 non-precious metal-,^15–18 metal alloy-,¹⁹ carbide/nitride-,^20–22 metal oxide-based materials^23–31 are key for enhancing the electrochemical performance of Zn–air batteries. However, the current electrocatalysts other than metal oxide-based materials showed a poor bi-functional electrocatalytic activity for both ORR and OER in rechargeable Zn–air batteries.

Along with the extensive research and development of metal oxide-based materials, perovskite oxides^23–31 have been intensively investigated as bi-functional electrocatalysts in rechargeable Zn–air batteries with high ORR (O₂ + 2H₂O + 4e⁻ → 4OH⁻) and OER (4OH⁻ → O₂ + 2H₂O + 4e⁻) activities due to their stable structures and chemical flexibilities.^32–35 Suntivich et al. demonstrated a volcano-shaped relationship between the e_g orbital filling of B-site cations and the intrinsic catalytic activities of perovskite oxides (ABO₃).^32,33 They also proposed the metal–oxygen covalency as a secondary factor influencing activity. Jung et al. explained that a catalytic activity of perovskite oxides is dependent on the concentration of oxygen vacancies and the valence state of B-site cations via ex situ X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) analyses.²⁵ However, these perovskite oxides showed lower ORR activities than other ORR catalysts. Another limitation of perovskite oxides is the formation of undesirable phases during heat treatment.³⁶

Furthermore, pyrochlore oxides (A₂B₂O_7−x) have been studied as possible electrocatalyst candidates for ORR and/or OER among metal oxide-based materials. Examples include Pb₂Ir_2−xPb_xO_7−y,³⁷ Bi₂[Ru_2−xBi_x]O_7−y,³⁸ Pb₂[Ru_2−xPb_x⁴⁺]O_6.5,^37,38 Pb₂Ru₂O_6.5,^39–41 Bi_2.4Ru_1.6O₇,⁴¹ Bi₂Ru₂O₇^39,42 and Pb₂Ru₂O_7−x⁴³ (Table S1, ESI†). It was reported that their high catalytic activity and electrical conductivity could be ascribed to the metallic properties of pyrochlore oxides. However, in terms of the structural integrity, previously reported pyrochlore catalysts showed undesirable phases and impurities such as RuO₂ derived from the synthesis processes (e.g., solid-state reaction and precipitation). Therefore, high phase purity is required to investigate the intrinsic catalytic activities of pyrochlore oxides. Oh et al. studied the electrochemical behavior of a mesoporous pyrochlore catalyst for aprotic Li–O₂ batteries and proposed a mechanism for Li₂O₂ formation.⁴⁴ However, the origin of outstanding catalytic activities and structural stabilities of pyrochlore oxides in aqueous Zn–air batteries are not clearly revealed, in part due to the difficulty in identification during electrocatalysis.

Herein, we present the highly pure single crystalline pyrochlore nanoparticles with metallic conduction (Pb₂Ru₂O_6.5) as an efficient electrocatalyst. Notably, the covalency near the surface of pyrochlore oxides is directly related to the ORR and OER activities.⁴¹ Accordingly, we provide experimental evidence that the covalency of Ru–O bonds affects the ORR and OER activities by comparing X-ray absorption near edge structure (XANES) spectra of the metallic Pb₂Ru₂O_6.5 and insulating Sm₂Ru₂O₇ for the first time. In addition, we use in situ X-ray absorption spectroscopy (XAS) to follow the distance changes between Ru at the B-site and oxygen in pyrochlore oxides in order to monitor the structural stabilities of the pyrochlore catalysts during ORR and OER in real-time. Furthermore, the highly efficient metallic Pb₂Ru₂O_6.5 nanoparticles demonstrate outstanding ORR and OER activities in half- and full-cells for primary and rechargeable Zn–air batteries.

Fig. 1a shows the synthetic procedure for the highly pure single crystalline pyrochlore oxide nanoparticles (A₂Ru₂O_7−x, A = Pb and Sm). We employed a sol–gel method in order to crosslink the A-site and B-site cations using a citric acid as a chelating agent. Then, the pyrochlore oxides were crystallized using a heat treatment method at temperatures of 650 °C for Pb₂Ru₂O_6.5 and 1050 °C for Sm₂Ru₂O₇, respectively. As shown in Fig. 1b, cubic phases of the pyrochlore oxides (A₂B₂O_7−x) without any impurities were successfully prepared, as confirmed by X-ray diffraction (XRD). The space group of cubic Pb₂Ru₂O_6.5 has F4̄3m symmetry. This structure includes 0.5 mol defects at the oxygen sites, which originate from the charge imbalance between Pb²⁺ and Ru^4/5+. On the other hand, there are no defects at the oxygen sites in the other sample because of the balanced oxidation states of Sm³⁺ and Ru⁴⁺; this sample is a cubic crystal with Fd3̄m symmetry. Fig. 1c exhibits the scanning electron microscopy (SEM) image of Pb₂Ru₂O_6.5. The primary particles of the pyrochlore oxides were aggregated, showing angular shapes with average sizes of ≤200 nm. To investigate the highly crystalline nanostructure of the pyrochlore particles, the fast Fourier transform (FFT) image of high resolution transmission electron microscopy (HR-TEM) was obtained by focused ion beam (FIB) sampling (Fig. 1d). The FFT analysis of Pb₂Ru₂O_6.5 was carried out in the [110] direction as the main zone axis. The FFT image showed cubic phases based on the unit spots of (−11−1) and (−111), with an interaxial angle of 70.53°. Also, high-angle annular dark field scanning TEM (STEM-HAADF) images with Pb₂Ru₂O_6.5d-spacing identified cubic pyrochlore oxide phases on the lattice scale (Fig. 1e). The lattice fringes with d-spacing values of 0.304 nm correspond to (111) crystal planes. The (002) crystal planes showed d-spacing values of around 0.51 nm. The Sm₂Ru₂O₇ exhibited the same cubic phase as Pb₂Ru₂O_6.5 (Fig. S1, ESI†). Notably, the FFT patterns of both samples showed the formation of single crystalline phases. Elemental composition and distribution of the pyrochlore oxides were analyzed in more detail by using STEM energy-dispersive X-ray spectroscopy (EDS) and elemental mapping (Fig. S2–S5, ESI†). The atomic ratios of A-site and B-site cations in the pyrochlore oxides were ca. 1, and these values corresponded exactly to the results from XRD analysis (Tables S2 and S3, ESI†). Collectively, these results revealed that cubic phases of the pyrochlore oxides with highly pure single crystalline nanoparticles were successfully synthesized.

Fig. 1 Preparation, morphology and structural characterization of highly pure single crystalline pyrochlore oxide nanoparticles (A₂B₂O_7−x, A = Pb and Sm, B = Ru). (a) Schematic representation of the preparation process for the highly pure single crystalline pyrochlore oxide nanoparticles. During the sol–gel process, A-site cations (blue spheres) are crosslinked with B-site cations (green spheres) by citric acid (black lines). After heat-treatment at different temperatures (650 °C for Pb₂Ru₂O_6.5 and 1050 °C for Sm₂Ru₂O₇), the powdered pyrochlore members (red brown solid figures) are synthesized. (b) XRD patterns of Pb₂Ru₂O_6.5 and Sm₂Ru₂O₇. (c) SEM image, (d) FFT image of HR-TEM along the [110] zone axis and (e) STEM-HAADF image of Pb₂Ru₂O_6.5. In (d), the indexed points refer to the lattice planes of (−11−1), (−220), (−111), and (002) in clockwise order. In (e), 0.304 and 0.510 nm denote the lattice spacing on the (111) and (002) planes, respectively.

Fig. 2 shows the ohmic- and capacitive-corrected electrocatalytic activities of the pyrochlore oxides as bi-functional electrocatalysts for ORR and OER. The linear scan voltammogram (LSV) curves for the ORR exhibited an exceptional onset potential for Pb₂Ru₂O_6.5 of 0.89 V vs. RHE (Fig. 2a). Commercial Pt/C was used as a reference, which showed an onset potential of 1.05 V. Compared with the limiting current density of Pt/C (−5.7 mA cm_disk⁻²), Pb₂Ru₂O_6.5 showed a very similar value of −5.7 mA cm_disk⁻² near 0.5 V. In the ORR, there are two pathways: direct four-electron and intermediate two-electron reactions. We can determine the reaction pathway by calculating the disk and ring current densities. As shown in Fig. 2b, the Pb₂Ru₂O_6.5 demonstrates a highly increased number of transferred electrons (n) of almost 4; compare this value to that of Sm₂Ru₂O₇ (≤3.5). The corresponding limiting current density for the resulting value (n = 4) is about −5.7 mA cm_disk⁻² which is consistent with experimentally obtained values for Pt/C and Pb₂Ru₂O_6.5 in Fig. 2a. This dominant four-electron pathway implies a superb catalytic activity of the Pb₂Ru₂O_6.5 for ORR.

Fig. 2 Electrocatalytic activities for ORR and OER on the highly pure single crystalline pyrochlore oxide nanoparticles (A₂B₂O_7−x, A = Pb and Sm, B = Ru). (a) Linear scan voltammogram (LSV) curves and (b) the number of transferred electrons in O₂-saturated 0.1 M KOH at a rotation speed of 1600 rpm and a scan rate of 10 mV s⁻¹ for Pb₂Ru₂O_6.5, Sm₂Ru₂O₇ and Pt/C at a rotating ring-disk electrode (RRDE). (c) Tafel plots of log I_k (A) vs. E (V vs. reversible hydrogen electrode (RHE)) for the pyrochlore catalysts and Pt/C at the potential range for the ORR. In (c), the limiting current density obtained at potential = 0.35 V was used to calculate i_k using the equation, i_k = i_L·i/(i_L − i). (d) Kinetic current density of the pyrochlore oxides and Pt/C at 0.85 V. (e) LSV curves of the pyrochlore oxides and RuO₂. (f) Tafel plots of log J_k (mA cm⁻²) vs. E (V vs. RHE) for the pyrochlore oxides and RuO₂ at the potential range for the OER.

To further understand the kinetic reactions, mass transfer-corrected Tafel plots for the ORR were measured for the pyrochlore catalysts and Pt/C (Fig. 2c). At low overpotential regions, the average Tafel slopes of pyrochlore was around 60 mV dec⁻¹, similar to that of Pt/C (60 mV dec⁻¹), implying that the rate determining step (RDS) in the ORR is the formation of superoxide via a one electron transfer reaction (O_2(ads.) + e⁻ → O_2(ads.)⁻) (Table S4, ESI†).⁴⁵ It also indicated mass and charge transfer-limited processes at low and high overpotential regions, respectively.⁴⁶ A higher kinetic current density of Pb₂Ru₂O_6.5 as compared to Sm₂Ru₂O₇ at 0.85 V is indicative of higher ORR activity (Fig. 2d). Pb₂Ru₂O_6.5 is among the most active oxides reported for ORR (Table S5, ESI†). Moreover, Pb₂Ru₂O_6.5 shows a higher OER activity with lower onset potentials than the Sm₂Ru₂O₇ and RuO₂ reference samples (Fig. 2e). Compared with the current density of RuO₂ at 1.55 V (1.42 mA cm_disk⁻²), Pb₂Ru₂O_6.5 showed a higher value of 2.96 mA cm_disk⁻², which could be ascribed to the significantly enhanced bi-functional catalytic activities (Table S6, ESI†). Moreover, the Tafel slope of the Pb₂Ru₂O_6.5 was 114.2 mV dec⁻¹ (Table S7, ESI†), similar to the OER catalyst standard of RuO₂ (115.9 mV dec⁻¹).

Fig. 3a shows a schematic representation of in situ XAS electrochemical cell used for real-time investigation of the electron configurations and local structures of Ru in the pyrochlore catalysts during ORR and OER. The normalized Ru K-edge XANES spectra of the Ru metal, RuO₂ and the pyrochlore catalysts are shown in Fig. 3b, which can initially explain the correlation between catalytic activity and the intrinsic electron configuration of Ru. In Pb₂Ru₂O_6.5, two distinct peaks were observed at the photon energies of 22 140 and 22 150 eV. This split maximum was caused by the dipole-allowed transition from Ru 1s to bound 5p states and continuum 5p states, respectively.^47,48 On the other hand, Sm₂Ru₂O₇ showed two overlapping peaks at the photon energies of 22 135 and 22 145 eV. Furthermore, Fig. 3c shows the O K-edge XANES spectra, in which the spectra are categorized into two types of pyrochlore catalysts, similar to the Ru K-edge XANES results. The double peaks at 525 and 529 eV were assigned to the number of holes in the t_2g and e_g orbitals of the Ru 4d states that originated from hybridization between Ru 4d and O 2p, respectively.^49,50 The intensity ratio of the t_2g and e_g orbitals was 2 : 4 for both pyrochlore oxides, corresponding to low spin states of Ru orbital. Based upon the chemical composition of the pyrochlore catalysts, the electronic configurations of Ru are estimated to be 4d^4.5 (t^4.5_2ge⁰_g) for Pb₂Ru₂O_6.5 and 4d⁴ (t⁴_2ge⁰_g) for Sm₂Ru₂O₇ with low spin states in common. The absolute intensity of Pb₂Ru₂O_6.5 was higher than Sm₂Ru₂O₇, indicating a more covalent Ru–O bonding characteristic because the higher bond covalency leads to more hole density in oxygen 2p orbital by stronger hybridization between Ru 4d and oxygen 2p orbitals. This could be explained by Fermi energy (E_F) differences between the e_g states of the Ru 4d (ca. 5 eV) and A-site metal states (ca. 6 eV). For instance, the E_F of the Pb 6p states (near to Ru 4d) are significantly lower than those of the Sm 4f states (far from Ru 4d).⁵¹ According to dynamic mean field theory (DMFT), there were quasiparticle (QP) peaks in Pb₂Ru₂O_6.5 that were attributed to low electron–electron correlation in the t_2g states of Ru 4d. However, Sm₂Ru₂O₇ did not show QP peaks, resulting from obvious splitting of the Hubbard band due to high electron–electron correlation in the Ru 4d states.⁵² Therefore, these differences were caused by the splitting degree of the Hubbard bands to upper and lower levels by the QP band at the E_F near the Ru 4d states,⁵³ which made Pb₂Ru₂O_6.5 good conductors and Sm₂Ru₂O₇ relative insulators.

Fig. 3 Ex situ XANES and in situ XAS analyses of the highly pure single crystalline pyrochlore oxide nanoparticles (A₂Ru₂O_7−x, A = Pb and Sm). (a) Schematic representation of the in situ XAS design combined with XAS and a three-electrode half-cell. The half-cell consists of the pyrochlore oxide-based air electrode as the working electrode, the Hg/HgO reference electrode, and a Pt wire as the counter electrode in a 0.1 M KOH electrolyte. Incident X-rays (I_o) are absorbed by the pyrochlore oxide-based air electrode and emit photons towards the fluorescence detector during the ORR and OER. (b) Normalized Ru K-edge XANES spectra of the Ru metal, RuO₂, Pb₂Ru₂O_6.5 and Sm₂Ru₂O₇ with their magnified inset. (c) Normalized O K-edge XANES total electron yield mode spectra of the pyrochlore catalysts. (d) Potentials at −25 and 25 μA cm⁻² as a function of the Ru–O covalency in the pyrochlore oxides for ORR and OER, respectively. (e) The corresponding interatomic distances for the reduced distances of the pyrochlore oxides at various applied potentials during electrocatalysis.

Furthermore, to quantify the covalency of Ru–O bonds on the surface of the pyrochlore catalysts, we calculated a degree of covalency using an equation of absorbance/(hole_eg + 1/4hole_t2g) as shown in Fig. 3d.²⁸ The excitations of O 1s → Ru 4d–O 2p bands in pre-edge of O K-edge spectra can be assigned to the absorbance of Ru covalency, and it was calculated by subtracting the integrated area of the fitted linear background from that of the pre-edge peak. Normalization of the integral by (hole_eg + 1/4hole_t2g) showed excellent agreement with the expected trends in a previous study.⁵⁴ The normalization factor is based on the assumption of a 2-fold higher transfer integral of the e_g symmetry states as compared to the t_2g states due to the angular overlap with the ligand field. This would lead to about 4 times higher number of holes in the e_g states of the O K-edge spectral intensity than that of the t_2g states. The quantified covalency values for Pb₂Ru₂O_6.5 and Sm₂Ru₂O₇ were around 0.65 and 0.39, respectively. These results implied that the surface of the metallic Pb₂Ru₂O_6.5 demonstrated a stronger covalency of Ru–O bonds than that of the insulating Sm₂Ru₂O₇, resulting in highly enhanced electrochemical performances. This might be attributed to the enhanced kinetics of O²⁻/OH⁻ exchange on the surface Ru ions and the deprotonation of the oxyhydroxide group to form peroxide ions, which were considered as the RDSs in the ORR and OER, respectively.^32,33

This could also be explained by the difference of Pauling electronegativity between Ru (2.2) and A-site cations of Pb (2.33) and Sm (1.17), respectively. Namely, the chemical bonding nature of Ru–O–Ru/Pb with similar electronegativity values is much more covalent than that of Ru–O–Ru/Sm bonding. The fact means that the electrons in the bonding characteristic of Ru–O–Ru/Pb can be more delocalized through long-range order than that of Ru–O–Ru/Sm. Consequently, the freer charges of Pb₂Ru₂O_6.5 than relatively trapped charges of Sm₂Ru₂O₇ can quickly respond to a charge-transfer reaction during ORR and OER. This result was consistent with the electrochemical performances of the pyrochlore oxides. The significance of the results is that we firstly revealed the effect of covalency on ORR and OER activities in these pyrochlore oxides.

To identify structural stabilities on the basis of intrinsic Ru–O bonding characteristics in the pyrochlore catalysts during electrocatalysis, we carried out in situ XAS to investigate redox reactions around Ru. By using in situ XAS, we experimentally observed the redox reactions of Ru, resulting from the changes in the distance between Ru and oxygen during electrocatalysis. Fig. 3e shows the calculated interatomic distances for Ru–O bonds of the pyrochlore catalysts at various applied potentials during chronoamperometric tests (Tables S8 and S9, ESI†). The potential ranges for ORR and OER are 0.7–0.3 and 1.3–1.7 V, respectively (Fig. S7, ESI†). Going to both the ORR and OER potentials from open circuit, the interatomic distance of the Ru–O bond for Pb₂Ru₂O_6.5 was slightly decreased from 1.967 Å to 1.948 Å (ORR at 0.3 V) and 1.953 Å (OER at 1.7 V), which is in good agreement with the partial oxidation of Ru. In contrast, Sm₂Ru₂O₇ showed partially reduced Ru corresponding to the increase of Ru–O bond distances from 1.988 Å to 2.001 Å (ORR at 0.3 V) and 2.004 Å (OER at 1.7 V). Note that Pb₂Ru₂O_6.5 and Sm₂Ru₂O₇ showed insignificant interatomic distance changes of Ru–O bonds (<0.02 Å) in the ORR and OER potential regions, implying a stable bulk structure of either pyrochlore oxide. The origin of significantly improved structural stabilities of the pyrochlore catalysts might be caused by the high crystallinity of the pure single crystalline nanoparticles.

In order to evaluate the activities of the pyrochlore catalysts, we performed primary and rechargeable Zn–air battery tests. The discharge polarization curves of the pyrochlore catalysts and Pt/C were obtained by increasing the current density to 300 mA cm⁻² (Fig. 4a). Initially, commercial Pt/C showed the lowest overpotential of the pyrochlore catalysts. The electrochemical performance of Pb₂Ru₂O_6.5 was comparable to that of Pt/C at current densities of >125 mA cm⁻². Pb₂Ru₂O_6.5 showed the highest peak power density of 195 mW cm⁻², which was higher than that of Pt/C (188 mW cm⁻²) (Fig. 4b). For practical applications, a current density of 20 mA cm⁻² was chosen as the discharge rate of the primary Zn–air batteries containing the pyrochlore catalysts and Pt/C (Fig. 4c). Pt/C showed the lowest overpotential and highest operating voltage at a current density of 20 mA cm⁻², which is in agreement with the results of the polarization test shown in Fig. 4a. However, Pb₂Ru₂O_6.5 showed significantly improved durability over 330 min, although it showed a lower plateau voltage, than Pt/C. These results implied that the Zn–air battery containing Pb₂Ru₂O_6.5 could show highly stable discharge performance with high power density due to the strong covalency of Ru on the surface of the metallic pyrochlore oxide. The Sm₂Ru₂O₇ showed slightly degraded power densities, resulting from the weak covalency of Ru in the insulating pyrochlore oxides. These results correspond well with the order estimated from in situ XAS analysis.

Fig. 4 Performance of primary and rechargeable Zn–air batteries based on the highly pure single crystalline pyrochlore oxide nanoparticles (A₂Ru₂O_7−x, A = Pb and Sm). (a) Current density–voltage curves, (b) power–current density curves and (c) discharge curves at a current density of 20 mA cm⁻² of primary Zn–air batteries using Pb₂Ru₂O_6.5, Sm₂Ru₂O₇ and Pt/C-based air electrodes as ORR catalysts. (d) Anodic and cathodic polarization curves of rechargeable Zn–air batteries with the pyrochlore oxide catalysts and a mixture of the Pt/C and IrO₂ catalyst. (e) Discharge and charge cycling curves of rechargeable Zn–air batteries based on Pb₂Ru₂O_6.5 and the mixture of Pt/C and IrO₂ at a current density of 10 mA cm⁻² in short cycle periods (600 s per cycle) and (f) long cycle periods (2 h per cycle) with ambient air.

Fig. 4d shows the anodic and cathodic polarization curves of the rechargeable Zn–air batteries. Upon discharging, the mixture of Pt/C and IrO₂ showed the lowest overpotential of 0.2 V at a current density of 50 mA cm⁻². Pb₂Ru₂O_6.5 showed a similar low overpotential (0.23 V). Upon charging, Pb₂Ru₂O_6.5 showed reliable bi-functional properties, with a substantially lower overpotential (0.52 V) than the mixture of Pt/C and IrO₂ (0.77 V). To explore the bi-functional catalytic properties, we performed short cycle period tests (600 s per cycle) at a current density of 10 mA cm⁻², which corresponded to a depth of discharge (DOD) of ∼1% (Fig. 4e). We calculated the operating voltage differences between charge and discharge after 200 cycles (Δη₂₀₀) for 33 h. The mixture of Pt/C and IrO₂ showed Δη₂₀₀ of 1.05 V, implying a larger overpotential compared to Pb₂Ru₂O_6.5 (0.77 V). These results implied that the rechargeable Zn–air batteries containing the Pb₂Ru₂O_6.5 demonstrated superior electrochemical performance and durability compared to that of the state-of-the-art catalyst of La_0.7-BSCF5582 perovskite with a larger overpotential (1.0 V @ 100 cycles).³⁵ Further, we increased the DOD to ∼7.5%, corresponding to a cycle period of 2 h, which was required to gain reliable rechargeability of the Zn–air batteries (Fig. 4f). Pb₂Ru₂O_6.5 showed a lower difference in the operating voltage between charge and discharge after 18 cycles (Δη₁₈ = 1.13 V) than the mixture of Pt/C and IrO₂ (1.42 V). The results were consistent with those obtained with a DOD of ∼1%, suggesting that the electrons were easily transferred through Ru in Pb₂Ru₂O_6.5. In contrast, Sm₂Ru₂O₇ showed slightly degraded overpotentials due to a lack of electron transport (Fig. S8, ESI†). We believe that these results correlate with cycle durability in the electrochemical reactions. This is evidenced in the Ru K-edge EXAFS/XANES spectra and XRD patterns after 100 cyclic voltammogram (CV) for the pyrochlore catalysts, confirming that only the insulating Sm₂Ru₂O₇ was partially reduced because of lower electrochemical stability compared to the metallic Pb₂Ru₂O_6.5 (Fig. S9, ESI†).

In this study, we have shown that highly efficient metallic pyrochlore oxide nanoparticles (Pb₂Ru₂O_6.5) exhibit outstanding activity as bi-functional electrocatalysts in aqueous Zn–air batteries for ORR and OER. Notably, the metallic Pb₂Ru₂O_6.5 demonstrated stronger covalency of Ru as compared to the insulating Sm₂Ru₂O₇, resulting in highly enhanced electrochemical performances for primary and rechargeable Zn–air batteries. Moreover, we provided experimental support for the structural stabilities of the pyrochlore oxides during ORR and OER by in situ XAS for the first time. The importance of the results reported herein is that the electrocatalytic activities of the pyrochlore oxides might be mainly attributed to the covalency of Ru, which explains the superior bi-functional activity of metallic Pb₂Ru₂O_6.5.

Acknowledgements

This research was supported by the next-generation battery R&D program of MOTIE/KEIT, Korea (10042575), and was also funded by the Skoltech-MIT Center for Electrochemical Energy Storage, USA.

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Footnote

† Electronic supplementary information (ESI) available: The details of the synthetic procedure and characterization methods, Fig. S1–S9, Tables S1–S8 and Note S1. See DOI: 10.1039/c6ee03046g

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