Optimizing nanoparticle perovskite for bifunctional oxygen electrocatalysis

Abstract

Highly efficient bifunctional oxygen electrocatalysts are indispensable for the development of highly efficient regenerative fuel cells and rechargeable metal-air batteries, which could power future electric vehicles. Although perovskite oxides are known to have high intrinsic activity, large particle sizes rendered from traditional synthesis routes limit their practical use due to low mass activity. We report the synthesis of nano-sized perovskite particles with a nominal composition of La_x(Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ (BSCF), where lanthanum concentration and calcination temperature were controlled to influence oxide defect chemistry and particle growth. This approach produced bifunctional perovskite electrocatalysts ∼50 nm in size with supreme activity and stability for both the oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). The electrocatalysts preferentially reduced oxygen to water (<5% peroxide yield), exhibited more than 20 times higher gravimetric activity (A g⁻¹) than IrO₂ in OER half-cell tests (0.1 M KOH), and surpassed the charge/discharge performance of Pt/C (20 wt%) in zinc-air full cell tests (6 M KOH). Our work provides a general strategy for designing perovskite oxides as inexpensive, stable and highly active bifunctional electrocatalysts for future electrochemical energy storage and conversion devices.

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Broader context

The world-wide interest in reducing the dependency on fossil fuels demands the development of energy storage systems with high power density from abundant materials, which would enable wide-spread industrial deployment of grid-scale renewable energy systems, as well as the progressive advancement of high-powered electric vehicles (EVs). Perovskite oxide ceramics have attracted significant attention as strong candidates as bi-functional electrocatalysts for metal-air batteries. There has been consistent investigation of the viability of bi-functional electrocatalysts, because energy storage systems cannot operate rechargeably without suitable bi-functional electrocatalysts. Among various electrocatalysts for both oxygen evolution and reduction, making nanoparticles from these materials for practical applications is a great challenge. Newly introduced perovskite electrocatalysts ∼50 nm in size preferentially reduced oxygen to water (<5% peroxide yield), exhibited more than 20 times higher gravimetric activity (A g⁻¹) than IrO₂ in OER half-cell tests, and surpassed the charge/discharge performance of Pt/C (20 wt%) in zinc-air full cell tests. This study describes substantially the systematic engineering of perovskite ceramics into such bifunctional nanosized electrocatalysts with high stability and activity, which is also explained in detail from the aspect of defect chemistry.

The world-wide interest in reducing the dependency on fossil fuels demands the development of energy storage systems with high power density from abundant materials, which would enable wide-spread industrial deployment of grid-scale renewable energy systems, as well as the progressive advancement of high-powered electric vehicles (EVs).^1–3 Even though Li ion batteries (LIBs) are traditionally considered the most promising candidates for EV applications due to their high cycle capability and energy efficiency, their insufficient gravimetric storage capacity (100–200 W h kg⁻¹) limits long-term applications in EVs. Alternatively, rechargeable zinc-air batteries are being spotlighted as strong candidates due to their extremely high gravimetric energy density (470 W h kg⁻¹), low cost and environmentally friendly operation.^1,4 In addition, zinc-air batteries are compact and light-weight because they employ a lighter air cathode, which makes use of oxygen from the air upon discharge. However, the rechargeable zinc-air batteries are still in their early stages of development because there are still some critical issues to overcome. For example, the compact and rechargeable zinc-air batteries operating with two-electrodes have to overcome limited life, high cost and lack of bi-functional catalysts.⁵ In order to fabricate available rechargeable zinc-air batteries, the development of metal oxide-based bi-functional catalysts that can catalyze satisfactorily both the oxygen reduction reaction (ORR) upon discharge and the oxygen evolution reaction (OER) upon charging is urgently demanded. Unfortunately, the OER and ORR mechanisms for oxides are not well understood, neither are potential modifications of the surface during catalytic turnover.⁶ The bi-functionality of metal oxide catalysts has been proposed to be controlled by the facility of adopting different valence states through redox couples at the overpotentials of ORR and OER.^7–10 Among various metal oxide-based bi-functional electrocatalysts, La_xCa_1−xMO₃ (M = Co, Ni, Mn) perovskite oxides have attracted consistent attention as potential bi-functional catalysts for metal-air batteries and fuel-cell systems.^11–16 Further improvement of activity was achieved by using composite electrodes, e.g. a core-corona structural bifunctional catalyst (CCBC) that consists of LiNiO₃ as the core supported on the nitrogen-doped carbon nanotube, which can catalyze OER and ORR, respectively, in zinc-air batteries.¹⁷ Generally, composite electrode systems are known to support higher bifunctional activity and durability compared with those containing only a single material.¹⁷ Recently, Suntivich et al. introduced Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ (BSCF) as an excellent OER catalyst, that has Co cations with the t_2g⁵e_g^∼1.2 electronic configuration and exhibits the highest OER activity among perovskite oxides, and whose performance is one order higher than IrO₂ in OER activity.^18,19 They also explained that any transition-metal-oxide perovskite, which has an e_g-filling (σ*-orbital occupation) slightly below 1 shows the maximum ORR activity.^20–22

BSCF has a complex defect chemistry and demonstrates diverse structural transitions according to heat treatment and/or dopant addition.^23–26 It is a great challenge to take advantage of the dynamic defect chemistry of BSCF perovskite catalysts through diverse directions and methods to design active and stable bifunctional electrocatalysts for industrial applications. The electrochemical performance of perovskite catalysts in metal-air batteries can be significantly enhanced not only by modifying the surface chemistry and structure (yielding high intrinsic activity at a fixed loading), but also by reducing the particle size to the nanoscale (yielding gravimetric mass activity at a fixed composition) using the synergic effect of synthesis temperature and doping conditions.²⁷ A series of studies were reported in which La(Ca,Co)O₃ perovskite particles were heat-treated and comminuted to obtain the increased concentrations of active sites via increased surface areas in bi-functional oxygen electrocatalysts.^14–16 As an example, Bursell et al. synthesized a 70 nm sized LaCaMnO₃ perovskite by quenching the sample to R.T. after calcining at 700 °C in air.¹⁴ Such an effort for improving catalytic activity by synthesizing nanoparticles was also observed in the SOFC system.²⁸ For instance, Ortiz-Vitoriano et al. synthesized 150 nm sized clusters of La_0.8Ca_0.2Fe_0.8Ni_0.2O_3−δ oxide particles via freeze drying and liquid mixing methods, which would be used as the cathode material in SOFCs.²⁸ Systematic studies on the nanoscale perovskite electrocatalyst particles that could be used in rechargeable metal-air batteries have been rarely reported.

In particular, BSCF perovskites, which are nowadays spotlighted as strong candidates as bi-functional electrocatalysts both in ORR and OER, are worthwhile to be investigated deeply in pursuit of nanosized particles as a way of achieving a breakthrough in the area of rechargeable metal-air batteries.^18,20,27,29 From this aspect, this report could be a pioneering progenitor in reporting the systematic engineering of BSCF perovskite oxides into such outstanding bifunctional nanosized electrocatalysts with high stability and activity. In this work, we introduce ∼50 nm sized La-doped BSCF perovskite catalysts, which display outstanding OER and ORR performances in half-cell tests, and surpasses Pt/C 20% as air-electrode catalysts in Zn-air full cell tests.

Experimental

Catalyst preparation

La_x(Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ (La_x-5582) (x = 0.1, 0.3, 0.5 and 0.7) powders were prepared using polymerized complex methods described previously.¹ 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.), lanthanum(III) nitrate hexahydrate (La(NO₃)₃·6H₂O, ≥99.9% purity, Alfar Aesar 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 1 M NH₄OH solution to make a NH₄–EDTA buffer solution. Molar amounts of lanthanum(III) nitrate hexahydrate (x = 0.002, 0.006, 0.010 and 0.014 mol), barium nitrate (1−x = 0.009, 0.007, 0.005 and 0.003 mol) and strontium nitrate (1−x = 0.009, 0.007, 0.005 and 0.003 mol) as well as 0.016 mol of Co(NO₃)₂·6H₂O and 0.004 mol of Fe(NO₃)₃·9H₂O were added to the buffer solution to make the required stoichiometries of La_x(Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ (La_x-5582). Anhydrous citric acid (0.06 mol) was added, and the pH value was adjusted to 8 using 1 N NH₄OH solution. Each solution was kept on a hot plate at 100 °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 in air at each designated temperature ranging from 600 °C to 1200 °C with the incremental temperature of 50 °C for 5 h in order to synthesize the La_x-5582 at each temperature.

The calcined cakes of perovskite catalysts were ground using mortars to be dispersed better in aqueous solutions. The mixtures of desired catalyst inks were prepared by physically mixing 16 mg of catalyst powders with 0.2 mL of 5 wt% Nafion (Aldrich) solution and 0.8 mL of ethanol solution (≥99.5% purity), followed by ultrasonicating the mixtures for 3 h in order to form homogeneous catalyst inks.

The mixtures of catalyst inks were prepared by physically mixing 16 mg of catalyst powders and 4 mg of KB (commercial Ketjenblack EC-600JD) with 0.2 mL of 5 wt% Nafion (Aldrich) solution and 0.8 mL of ethanol solution (≥99.5% purity), followed by ultrasonicating the mixtures for 3 h in order to form homogeneous catalyst inks. 5 μL of catalyst inks were carefully dropped onto polished glassy carbon (GC) electrodes 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 the working electrode (PK-3 electrode polishing kit, ALS Co., Ltd). Catalyst-coated GC electrodes were then dried under vacuum at room temperature for 3 h. In this study, the loading amount of all catalysts is 0.639 mg_ox cm⁻², except for Pt/C 20% used as a reference catalyst ink. All the composite materials are composed of catalyst materials (La_x-5582, and IrO₂) 80 wt%/KB600 20 wt%, so material names are simply mentioned in this study as La_0.1-1 μm, La_0.7-100 nm, La_0.7-50 nm and IrO₂. The Pt/C 20% catalyst ink was made by dispersing Vulcan XC-72 catalysts (Premetek Co.) in a water–ethanol solution, of which 0.157 mg_Pt cm⁻² was applied to electrodes. (Note this is an optimized procedure of the one reported in ref. 29). For precious metal-based oxide catalysts, IrO₂ (∼5 μm, iridium(IV) oxide, Sigma-Aldrich 99.9%) was used as a reference material.

Catalyst evaluation

Rotating ring-disk electrode (RRDE) experiments were performed for 80 wt% La_x-5582 on 0 wt% Ketjenblack composites (BSCF5582) and 20 wt% Pt on 80 wt% Vulcan XC-72 composites (Pt/C 20%). All half-cell experiments for ORR using a rotating ring-disk electrode (RRDE) (ALS Co., Ltd) were carried out under the same conditions with a 4 mm diameter working electrode where a Pt wire and Hg/HgO were used as 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 the as-prepared catalysts was conducted using a bi-potentiostat (Iviumstat) at a scan rate of 10 mV s⁻¹ at 1600 rpm within the potential range from 0.15 to −0.7 V vs. Hg/HgO under the saturated oxygen gas, and a sufficient ring potential of 0.4 V was biased to oxidize immediately during ORR. Half-cell experiments for ORR using a 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 is yielded and the number of electrons transferred (n) were calculated using the equations as below

where the experimentally determined collection efficiency (N) was 0.41.

Rotating disk electrode (RDE) experiments for OER were performed for 80 wt% La_x-5582 on 20 wt% Ketjenblack composites (BSCF5582) and 80 wt% IrO₂ on 20% Ketjenblack composites (IrO₂). All half-cell experiments for OER using a rotating disk electrode (RDE) (ALS Co., Ltd) were carried out under the same conditions with a 4 mm diameter working electrode where a Pt wire and Hg/HgO were used as 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 the as-prepared catalysts was conducted using a bi-potentiostat (Iviumstat) at a scan rate of 10 mV s⁻¹ at 1600 rpm within the potential range from 0.35 to 0.9 V vs. Hg/HgO.

Rotating disk electrode (RDE) experiments were executed for 80 wt% La_0.7-50 nm on 20 wt% Ketjenblack composites to measure the lifetime of ORR and OER catalysts as chronoamperometric analysis. All half-cell experiments for ORR and OER using a rotating disk electrode (RDE) (ALS Co., Ltd) were carried out under the same conditions with a 4 mm diameter working electrode where a Pt wire and Hg/HgO were used as 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. Chronoamperometric characterization of the as-prepared catalysts was executed using a bi-potentiostat (Iviumstat) at 1600 rpm at a scan rate of 10 mV s⁻¹ at fixed potentials of 0.7 V (vs. RHE) and 1.5 V (vs. RHE) for ORR and OER, respectively.

Material characterization

The crystal structures of the materials were analyzed using an X-ray diffractometer (XRD) (D/Max2000, Rigaku) using Cu-Ka radiation, a scan range of 10°–100°, a step size of 0.02°, and a counting time of 5 s. Lattice parameters were determined using a least-squares method. The material morphology and structure were analyzed using a SEM (S-4800, Hitachi) operating at 10 kV and a TEM (JEOL JEM-2100F) operating at 200 kV. The particle size distributions for histograms were analyzed by counting 80–100 identical particles under a SEM (FEI) operating at 10 kV. The Fe and Co K-edge X-ray absorption spectra of the perovskite La-doped BSCF5582 and a 100× electrochemically-cycled sample, X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) were collected at the BL10C beam line using the Pohang light source (PLS-II) with top-up mode operation under a ring current of 300 mA at 3.0 GeV. The monochromatic X-ray beam could be obtained using a liquid-nitrogen cooled Si(111) and Si(311) double crystal monochromator (Bruker ASC) with in situ exchange under the vacuum from high intense X-ray photons of a multipole wiggler source. The Si(111) crystal pair has been used for Fe (absorption edge energy, 7112 eV), Co (7709 eV) K-edge XAFS measurements. The X-ray absorption spectroscopic data were recorded for the uniformly dispersed powder samples with an appropriate thickness on the polyimide film, in transmission mode with N₂ gas-filled ionization chambers as photon detectors. Higher order harmonic contaminations were eliminated by detuning to reduce the incident X-ray intensity by ∼40%. Energy calibration has been simultaneously carried out for each measurement with reference metal foils placed in front of the third ion chamber. The data reductions of the experimental spectra to normalized XANES and Fourier-transformed radial distribution function (RDF) were performed through the standard XAFS procedure.

Results and discussion

Upon exploring the overall range of the calcination temperatures and La substitution, x, in La_x(Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ, (La-BSCF), we discovered an interesting pattern of perovskite phase existence and particle growth (Fig. S1, ESI†). The phase diagram in Fig. 1 shows that perovskite catalysts exist as pure cubic phases in the range between the higher critical temperature (T_h.c.) and the lower critical temperature (T_l.c.). Above the higher critical temperature (T_h.c.), the rhombohedral LaCoO₃ phase co-exists in the matrix of a cubic perovskite phase, and above the lower critical temperature (T_l.c.), a cubic perovskite phase appears where the overall particle size decreases with the increasing substitution of La into La-BSCF (Fig. S1–S4, ESI†). As the calcination temperature decreases at an La stoichiometry x = 0.7, the calcined particle size decreases systematically from 100 nm at 900 °C to 50 nm at 700 °C in the HRTEM images of Fig. S5 and S8 (ESI†). Generally, as La₂O₃ is doped into BSCF, the La cation is expected to occupy the lattice site of the A-site cations (Ba or Sr) and give rise to the oxidation of the B-site cation, preferentially in the Co-octahedral sublattice (Fig. S1, eqn (S1) and (S2), ESI†).²⁹ As the La concentration (x) increased from 0.1 to 0.7 at 900 °C, the Co K-edge XANES edge shifts towards the higher energy region, while the Fe K-edge XANES peak does not change, which demonstrates the preferential oxidation of Co sublattice octahedra with increasing La concentration (Fig. 2d).

Fig. 1 (a) Schematic diagrams phase against calcination temperature (°C) and La concentration (x) in La_x(Ba_0.5Sr_0.5)_1−xCo_0.8Fe_0.2O_3−δ that describe the zone of stable cubic perovskite phases between high critical temperatures (T_h.c.) and low critical temperatures (T_l.c.). G.G. denotes grain growth. The arrows point in the direction of the increasing particle size (see discussion in text). (b) The particle size as a function of x and temperature.

Fig. 2 (a) BFTEM image of La_0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-50 nm calcined at 700 °C for 5 h in air (La_0.7-50 nm), (b) an HAADF STEM image showing the atomic arrangements of the cubic perovskite structure in the 〈100〉 direction and a schematic of a perovskite unit cell, (c) X-ray diffraction patterns for La_0.7-50 nm, La_0.7-100 nm and La_0.1-1 μm, (d) Normalized Co and Fe K-edge XANES spectra for La_{0.7_900°C} (La_0.7-100 nm) (filled circles) and La_{0.1_900°C} (La_0.1-1 μm) (open circles) and references, (e) RDFs of Fourier-transformed k³-weighted EXAFS spectra for La_{0.7_700°C} (La_0.7-50 nm), La_{0.7_800°C} (La_0.7-70 nm), La_{0.7_900°C} (La_0.7-100 nm) and La_{0.7_1000°C}.

The interplay of defects with positive and negative effective charges is important to understand the structure and size of the oxide particles. The defect complexes formed above T_h.c. in this study involve association between La³⁺ cations (positive effective charge) and cobalt cations (effective negative charge), forming into a neutral complex defect (Fig. S1, eqn (S3), ESI†).^29–31 With increasing concentration, these types of defects may arrange and form intergrown microdomains of a closely related perovskite phase, rhombohedral LaCoO₃ perovskite phase, which is analyzed in detail by X-ray diffraction, TEM EDS, TEM,^29,32,33 and analysis of the extended X-ray absorption fine structure (EXAFS). Within the range between T_h.c. (∼1050 °C) and T_l.c, a cubic perovskite phase exists uniformly as inferred from XRD (Fig. S1, ESI†). The local structure was investigated by analysis and fitting the Fourier-transform (FT) of the k³-weighted EXAFS, where the peak position correlates with interatomic distances and the peak height correlates with the number of atoms sharing the same mean interatomic distance and the distribution of these distances. Therefore, the decrease of the EXAFS FT peak intensity at ∼1.5 Å reduced distance indicated an increase of oxygen vacancies, and the EXAFS FT peak at ∼3.5 Å suggested an effective decrease of the long-range order for Co–O–Co and Co–O–A bond arrays (separate peaks not resolved in Fig. 2e and Fig. S6, but in the simulation results of Table S1, ESI†).³⁴ Moreover, the EXAFS FT separation between peak positions for the Co and Fe lattices increased with annealing temperature (Fig. 2e and Table S1, ESI†). This suggests that the sublattices are in the same phase at low temperature, while, at higher temperature, Co–O–Fe cation ordering (with short Co–O and long Fe–O) or separate formation of the Co-rich phase could have started to develop in the perovskite structures (Fig. S6, S7 and Table S1, ESI†). In support of this, a secondary phase was detected by XRD above T_h.c. (Fig. S1, ESI†).

The conditions for the formation of the cubic perovskites can be understood in terms of the activation energy (Q) obtained from the Arrhenius plot of the grain size (Fig. S8, ESI†). With increasing La concentration (x), Q increased from 1.64 eV per atom at x = 0.1 to 2.60 eV per atom-K at x = 0.3 as an apex, from which Q decreased to 0.34 eV per atom-K at x = 0.7 (Fig. S8 and eqn (S4), ESI†). Thus, a higher Q results in a wider range of calcination temperatures that produces a stable perovskite cubic phase. When Q is reduced, both the particle size and the synthesis temperature decrease coincidentally.^35,36 Considering that Q is likely dependent not only on the concentration of oxygen vacancies but also on the crystal structure, we suggest that the diffusion paths required for ions to migrate during the calcination process are blocked with increasing La concentrations, abating the particle growth rate.

Using the newly developed phase diagram (Fig. 1), we selected several cubic perovskites of the La-BSCF family with promising properties for oxygen electrocatalysis. For clarity of presentation, they will be denoted by their La concentration and particle size. La_0.7-50 nm was prepared by calcining at 700 °C for 5 h in air, and La_0.7-100 nm and La_0.1-1 μm were prepared at 900 °C in air, resulting in BET surface areas of 21.3 m² g⁻¹, 4.2 m² g⁻¹ and 1.2 m² g⁻¹, respectively (Table S2, ESI†). Fig. 3(a–d) show the capacitance-corrected ORR activities of La_0.1-1 μm, La_0.7-100 nm and La_0.7-50 nm mixed with 20 wt% Ketjenblack carbon (KB) using rotating ring-disk electrodes (RRDEs), in comparison with the reference Pt/C 20% that consists of 20 wt% Pt and 80 wt% Vulcan XC-72 (E-tek) (Fig. S9, ESI†). The linear sweep voltammograms in Fig. 3(a) show that the onset potentials for ORR of La_0.1-1 μm, La_0.7-100 nm and La_0.7-50 nm are 0.70, 0.72 and 0.72 V vs. RHE, respectively, while Pt/C 20% is at 1.01 V vs. RHE. Comparing the limiting currents of Pt/C 20% with -6.1 mA cm⁻², the values decreased significantly from −4.2 mA cm⁻² of La_0.1-1 μm to −5.9 mA cm⁻² of La_0.7-50 nm, as the particle size approached the nano scale, indicating optimal coverage and mass transport of the nanoparticles. The ORR overpotentials at 2 A g⁻¹ are 0.70 V, 0.71 V, 0.66 V and 1.01 V for La_0.7-50 nm, La_0.7-100 nm, La_0.1-0.1 μm and Pt/C, respectively (Fig. 3d). The gravimetric ORR activities were the highest reported among crystalline perovskite oxides (Table S4, ESI†). In addition, Fig. 3(b) shows that the peroxide yield (HO₂⁻%) in the kinetic region is suppressed on a greater scale from 15% for La_0.1-1 μm, 7% of La_0.7-100 nm to 5% for La_0.7-50 nm. This demonstrated that La_0.7-50 nm is dominated by reduction of oxygen to water, which is quite comparable to Pt/C 20% (Fig. S10–S12, ESI†). This suggests that successful control of the particle size at the nanoscale can significantly enhances the gravimetric ORR activity and decrease the peroxide yield of BSCF-based perovskite catalysts.

Fig. 3 The properties of ORR and OER activities (capacitance-corrected) of La_0.1(Ba_0.5Sr_0.5)_0.9Co_0.8Fe_0.2O_3−δ (La_0.1-1 μm), La_0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-100 nm (La_0.7-100 nm) and La_0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-50 nm (La_0.7-50 nm) (a) disc and (c) ring currents (voltammograms) of the ORR and the determined (b) peroxide percentage (HO₂⁻%) and (d) Tafel plots of the gravimetric ORR activities as a function of potential in oxygen saturated 0.1 M KOH electrolyte at 10 mV s⁻¹ scan rate at 1600 rpm. (e) OER currents at select 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), IrO₂ (brown line), La_0.1-1 μm (green line), La_0.7-100 nm (blue line) and La_0.7-50 nm (red line). The all oxide electrode composites contain 0.64 mg_ox cm_disk⁻², 0.16 mg_KB cm_disk⁻², 0.35 mg_Nafion cm_disk⁻², respectively, while the Pt/C 20% electrode contains 0.16 mg_Pt cm_disk⁻², 0.64 mg_XC-72 cm_disk⁻², 0.35 mg_Nafion cm_disk⁻², respectively. At least three independent measurements were executed in order to confirm the repeatability of the experimental results for each sample.

The gravimetric OER activity is likewise enhanced when the particle size approached the nanoscale (Fig. S13, ESI†). The OER overpotentials at 2 A g⁻¹ are 1.54 V, 1.56 V, 1.58 V and 1.67 V for La_0.7-50 nm, La_0.7-100 nm, La_0.1-0.1 μm and IrO₂, respectively. Again, these gravimetric activities are among the highest reported for crystalline perovskite oxides (Table S4, ESI†). In addition, Fig. 3(e) and (f) show that the geometric and gravimetric OER activities of La_0.7-50 nm were nearly five times higher and 20 time higher, respectively, compared to commercial IrO₂, despite having a specific activity slightly lower than IrO₂ (Fig. S13, ESI†). This suggests that the significant enhancement of the gravimetric OER activity of La_0.7-50 nm is attributable to an increased number of active sites through the increased surface area, not to optimization of surface chemistry of the BSCF perovskite catalysts. The specific activity of these catalysts decreases upon approaching nanoparticles (Fig. S13, ESI†), which is not understood. In summary, the successful synthesis of nanoscale perovskite catalysts contributed on a large scale to the simultaneous enhancement of both gravimetric ORR and OER activities for BSCF-based perovskite electrocatalysts in half-cell tests, particularly without any supportive reinforcement such as N-doped particles, CNTs and graphite sources. This achievement is of great significance from the aspect of real application, for instance rechargeable Zn-air batteries are more robust against the structural degradation that is related with carbon decomposition.^17,37–40

The availability of this catalyst for practical application was addressed by zinc-air full-cell tests, as shown in Fig. 4, where the experiments were performed under 10.5 mA cm⁻² (2 A g_ox⁻¹ or 10 A g_Pt⁻¹) in 6 M KOH, making use of ambient air on the cathode side (Fig. S16, ESI†). We performed up to 100 cycles as either short or long duration cycling. For short duration cycling, one cycle consists of 10 min of discharging, followed by 10 min of charging (Fig. 4), while for long duration cycling, one cycle consists of 60 min of discharging, followed by 60 min of charging (Fig. S17, ESI†). For Pt/C 20%, the overpotential difference between charge and discharge (Δη) increased from 0.80 V during the 1st cycle (Δη₁) to 1.50 V during the 100th cycle (Δη₂) (Δ = Δη₂ − Δη₁ = 0.7 V), while the potential difference for La_0.7-50 nm only varied between 0.75 V and 1.0 V (Δ = Δη₂ − Δη₁ = 0.25 V), indicating superior activity and stability of the latter (Fig. 4a and b). While we caution that full cell tests are difficult to compare, the overpotential difference and its minor increase with cycling (i.e. high stability) are as good as or better than other Zn-air battery chemistries in the literature (Table S5, ESI†).

Fig. 4 The repeated charge and discharge tests as a Zn-air full-cell under 10.5 mA cm⁻² in 6 M KOH with (a) Pt/C 20% and (b) La _0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-50 nm (La_0.7-50 nm) catalysts as short time cycles, where 1 cycle consists of the 300 s of discharging, followed by 300 s of charging, and comparison of the first and last 5 cycles for (c) Pt/C 20% and (d) La_0.7-50 nm.

Excellent stability of La_0.7-50 nm is also supported by HRTEM analysis and XAFS. HRTEM before and after the 100th cycle of the short time full-cell tests clearly reveals the absence of structural degradation or transformation such as surface amorphization, and neither was there any micro-structural change after and before the 100th cycle in the short time full-cell tests of Pt/C 20% as shown in Fig. 5(c and d) and Fig. S18 (ESI†). XAFS at both the Co and Fe K-edges of La_0.7-50 nm in Fig. 5(e) and (f) clearly shows a reduction of the edge position by 0.4 eV after 100 cycles relative to the pristine sample, as observed previously for BSCF.⁴¹ In contrast to BSCF, the absence of new peaks in the RDF indicates that the perovskite structure is retained during cycling (Table S3, ESI†).⁴¹ We note that the FT peaks were associated with the Fe–O–A and Fe–O–B decrease, which would indicate a reduction in the order of the Fe sublattice, being the subject of ongoing research. On the other hand, the gradual increase of Δη during full-cell testing did not necessarily come from the degradation or phase change of catalyst particles itself, but could be from that of the carbon support. However, a more detailed investigation is required to fully explain this supposition.

Fig. 5 HRTEM images on the surfaces of La_0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-50 nm particles (a) before and (b) after 100th cycle of the short time full-cell tests, and Pt/C 20% particles (c) before and (d) after 100th cycle of the short time full-cell tests. (The insets of Fig. 5(a)–(d) indicate fast Fourier transform (FFT) of HRTEM images of Fig. 5(a)–(d).) (e) Co and (f) Fe K-edge XAFS spectra of the sample after 100 electrochemical cycles for La_0.7(Ba_0.5Sr_0.5)_0.3Co_0.8Fe_0.2O_3−δ-50 nm particles. (The insets of Fig. 5(e) and (f) describe the distinctive reduction of the order of the Fe sublattice (Fe–O–A and Fe–O–B), when compared with Co sublattice.)

The outstanding gravimetric activity of La_0.7-50 nm in both OER/ORR half-cell and zinc-air full-cell tests as a bi-functional catalyst is attributable to the favorable electronic structure of B-site cations in the BSCF-based perovskite catalyst, as well as to the increased surface area of the nano-sized perovskite catalyst. The successful synthesis of the nano-sized La-doped BSCF-based perovskite oxides in this study was contrived on the basis of a deep understanding of the particle growth mechanism considering the perovskite defect chemistry. This approach resulted in the synthesis of 50 nm sized La-doped BSCF particles with a cubic perovskite structure. These particles represented outstanding ORR/OER performances in both half-cell and Zn-air full cell tests compared with Pt/C 20%. This impressive performance of the La_0.7-50 nm perovskite catalyst harbingers the conceivable application of BSCF-based bifunctional electrocatalysts in metal-air battery systems, which will result in rechargeable metal-air battery systems with high energy and power density. Our synthesis approach opens the way for the development of further bi-functional nano-sized perovskite catalysts with high performance and stability. Such catalysts are urgently needed for the aggressive advancement of the renewable energy industry, particularly for rechargeable air batteries.

Authors Contributions

J. I. J. and M. R. contributed equally as co-first authors.

Acknowledgements

This research was supported by the next-generation battery R&D program of MOTIE/KEIT, Korea (10042575), and was also partially funded by the Cooperative Agreement between the Masdar Institute of Science and Technology (Masdar Institute), Abu Dhabi, UAE and the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA - Reference 02/MI/MIT/CP/11/07633/GEN/G/00, and the Skoltech-MIT Center for Electrochemical Energy Storage, USA.

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Footnote

† Electronic supplementary information (ESI) available: Table S1, and Fig. S1–S6. See DOI: 10.1039/c5ee03124a

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