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Enhancing electrocatalytic activity of perovskite oxides by tuning cation deficiency for oxygen reduction and evolution reactions Yinlong Zhu, Wei Zhou, Jie Yu, Yubo Chen, Meilin Liu, and Zongping Shao Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b04457 • Publication Date (Web): 27 Feb 2016 Downloaded from http://pubs.acs.org on February 28, 2016

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Chemistry of Materials

Yinlong Zhu,a Wei Zhou,*,a Jie Yu,a Yubo Chen,a Meilin Liu,*,b and Zongping Shaoc,d a

State Key Laboratory of Materials -Oriented Chemical Engineering, College of Chemistry & Chemical Engineering, Nanjing Tech University, No. 5 Xin Mofan Road, Nanjing 210009, P.R. China E-mail: zhouwei1982@n jtech.edu.cn b Center for Innovative Fuel Cell and Battery Technologies, School of Materials Science and Engineering, Georg ia Institute of Technology, Atlanta , GA 30332-0245 , USA E-mail: [email protected] c State Key Laboratory of Materials-Oriented Chemical Engineering, College of Energy, Nanjing Tech Un iversity, No. 5 Xin Mofan Road, Nan jing 210009, P.R. China d Department of Chemical Engineering, Cu rtin University, Perth, Western Australia 6845, Australia

ABSTRACT: Development of cost-effective and efficient electrocatalysts for oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is of prime importance to emerging renewable energy technologies. Here we report a simple and effective strategy for enhancing ORR and OER electrocatalytic activity in alkaline solutions by introducing A-site cation deficiency in LaFeO3 perovskite; the enhancement effect is more pronounced for the OER than the ORR. Among the A-site cation deficient perovskites studied, La0.95FeO3-δ (L0.95F) demonstrates the highest ORR and OER activity and, hence, the best bifunctionality. The dramatic enhancement is attributed to the creation of surface oxygen vacancies and small amount of Fe 4+ species. This work highlights the importance of tuning cation deficiency in perovskites as an effective strategy for enhancing ORR and OER activity for applications in various oxygen-based energy storage and conversion processes .

1. INTRODUCTION

catalytic oxidation, sensors, visible-light photocatalysis, and solid oxide fuel cells (SOFCs).17-20 Unfortunately, LaFeO3 perovskite was reported to have poor ORR and OER activity in alkaline solution based on previous experimental measurements and density functional theory (DFT) calculations.21-23 To address this issue, a few attempts had been made to enhance the ORR and OER activity of LaFeO3.24-26 For example, Zhang et al. has successfully prepared a 3D ordered macroporous LaFeO3 perovskite as efficient ORR and OER electrocatalyst for Li-O2 batteries by a colloidal crystal template method.24 Shao-Horn et al. reported a detailed study of ultrathin LaFeO3 films grown on Nb-doped SrTiO3 through pulsed laser deposition and found thicker films exhibited both photo-electrochemical ORR and OER activity.25 Moreover, the OER activity of LaFeO3 was promoted by coating an amorphous cobalt-phosphate (CoPi).26 Despite these efforts, enhancement in bifunctional ORR and OER activity of LaFeO3 is still very limited, and the adopted preparation methods and fabrication processes are complex, such as templated synthesis, electrodeposition, and pulsed laser deposition, which are difficult for cost-effective and largescale fabrication. Thus, simple and economical alternative strategies are still needed for fabrication of LaFeO3-based bifunctional catalysts with enhanced ORR and OER activity. As we know, an ideal ABO3 perovskite structure contains equivalent amount of cations in their A and B sites (i.e., the A/B cation ratio=1). In fact, however, the perovskite lattice structure can still be stable in many cases when the A/B cation ratio is deviated significantly from unit. In particular, it has been demonstrated that the introduction of A-site cation deficiencies into the lattice structure of perovskites can significantly alter the physical and chemical properties.27-30

The imminent global energy crisis, coupled with the depletion of fossil fuels and the associated environmental pollution issues, has stimulated intense interests in searching for clean and sustainable energy conversion and storage systems.1 The discovery of electrocatalysts for efficient oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is vital to the development of a new generation of energy technologies (e.g., fuel cells, metal-air batteries, hydrogen production from water, and solar fuel synthesis)2-7 because both ORR and OER are inherently sluggish and often require large overpotentials to drive a complex four-electron process.8 At present, precious metals such as platinum (Pt) and its alloys are recognized as the most active electrocatalysts for ORR, but have poor electrocatalytic activity for OER. In contrast, iridium and ruthenium oxide-based electrocatalysts (IrO2 and RuO2) have extraordinary OER activity but display poor ORR activity. Furthermore, the high cost and scarcity of these materials have severely limited their application in practical devices.9 Therefore, it is highly desirable (but very challenging) to develop low-cost and efficient bifunctional electrocatalysts for both ORR and OER. Perovskite-type oxides with a general formula of ABO3, where A is a rare-earth or alkaline earth element and B is a transition metal element, have demonstrated great potential as electrocatalysts for ORR and/or OER because of their high specific catalytic activity, abundant varieties with fascinating physical-chemical properties, low cost, and environmental friendliness.10-16 In particular, LaFeO3 is one of the most important parent oxides in the family of perovskite, which has widely been reported for various applications, including

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spectrometer equipped with an Al Kα X-ray source). The binding energies were calibrated to the adventitious C 1s peak at 284.6 eV and data were fitted by the public software package XPSPEAK. The specific surface areas of the catalysts were obtained with a Brunauer Emmet Teller (BET) analysis system with a N2 adsorptive medium. Mössbauer spectroscopy of La1-xFeO3-δ was recorded using a conventional constant acceleration spectrometer (OXFORD-MS500) with a gray source of 25 mCi 57Co in a palladium matrix moving at room temperature. The absorber was kept static in a temperaturecontrollable cryostat. All isomer shifts are quoted relative to aFe at room temperature. The Mössbauer spectrum was fitted using the MOSSWINN 3.0 program. The morphologies of the catalysts were observed using an environmental scanning electron microscope (ESEM, QUANTA-2000). The oxygen non-stoichiometry and the average of valence states of Co at room temperature were determined by iodometric titration. Briefly, approximately 0.1 g of powder was dissolved in a 6 mol-1 HCl solution under the protection of a nitrogen atmosphere to prevent the oxidation of I- ions (from KI) by air, followed by titration with a standard thiosulfate (S 2O32-) solution. Electrode preparation. Electrodes were prepared by a controlled drop-casting approach involving a rotating disk electrode (RDE) made of glassy carbon (GC, 0.196 cm2, Pine Research Instrumentation). The GC electrode was prepolished with 50 nm α-Al2O3 slurries on a polishing cloth and sonicated in ethanol for 5 min. The electrodes were finally rinsed with deionized water and dried before each test. The preparation method of the working GC electrodes containing the investigated catalysts was stated as follows. To remove any electrode conductivity limitations present in the thin film electrodes, all the catalysts were mixed with as -received conductive carbon (Super P Li) at a mass ratio of 1:1. Briefly, the electrocatalyst suspensions were prepared by sonication of a mixture of oxide (10 mg), conductive carbon (10 mg), Nafion solution (5 wt%, 100 μL) and ethanol (1 mL) for at least 1 h to generate a homogeneous ink. Next, a 5 μL aliquot of as-prepared catalyst ink was dropped on the surface of the GC substrate, yielding an approximate catalyst loading of 0.464 mg total cm-2 (0.232 mg cat cm-2) and left to dry for the electrochemical tests. Electrochemical measurements. Electrochemical measurements were conducted at room temperature in a standard three-electrode electrochemical cell (Pine Research Instrumentation) in an RDE configuration with a CHI 760E bipotentiostat. A Pt foil and Ag/AgCl (3.5 M KCl) were used as the counter and reference electrodes, respectively. The potentials reported in our work are referenced to the reversible hydrogen electrode (RHE), calibrated as described in Figure S4. All potential values are iR-corrected to compensate for the effect of solution resistance, which were calculated by the following equation: EiR-corrected = E-iR, where i is the current, and R is the uncompensated ohmic electrolyte resistance (~45 Ω) measured via high frequency ac impedance in O2-saturated 0.1 M KOH. The electrolyte was 0.1 M KOH aqueous solution, which was saturated with O2 for ~30 min prior to each test and maintained under O2 atmosphere through the whole test. Prior to electrochemical test, the electrodes were electrochemically activated and removing bubbles by running cycle voltammetry (CV) for at least five times until reproducible curves were observed. Linear sweep voltammetry (LSVs) curves were performed by using RDE at different rotation speeds (2000,

For example, many A-site cation deficient perovskite oxides were actively developed as cathodes for SOFCs, considering that the additional oxygen vacancies produced from the A-site cation deficiency can facilitate the transport of oxygen ions and thus improve the ORR activity at high temperatures. 31-33 More importantly, a number of previous studies have shown that moderate oxygen vacancies in oxides can enhance electrocatalytic activity toward the ORR and OER at room temperatures.34-39 These results indicate that the catalytic activity of perovskite oxides for ORR and OER may be tuned through tailoring A-site cation deficiency in perovskites. However, very little information is available about the effect of A-site cation deficiency in perovskite on the ORR and OER activity in alkaline solution at room temperatures. Herein, we report an effective strategy for significantly enhancing the ORR and OER electrocatalytic activity of LaFeO3 by simply introducing A-site cation deficiency (without other modifications). To the best of our knowledge, the A-site cation deficient perovskites, e.g. La1-xFeO3-δ (x=0.02, 0.05, 0.1), have never before been reported as efficient bifunctional catalysts for ORR and OER in an alkaline solution. Among all La 1-xFeO3-δ perovskites studied, La0.95FeO3-δ (denoted as L0.95F) showed the best ORR and OER activity and, hence, the best bifunctionality. The enhanced ORR and OER activity of La 1-xFeO3-δ perovskites (as compared to pristine LaFeO3) could be attributed to the creation of surface oxygen vacancies and small amount of Fe 4+ species. These findings represent a paradigm shift in knowledge-based rational design of perovskite-type catalysts with high ORR and OER activity for various applications. 2. EXPERIMENTAL S ECTION Catalyst synthesis. La1-xFeO3-δ (x=0, 0.02, 0.05, 0.1, 0.2, denoted as LF, L0.98F, L0.95F, L0.9F and L0.8F, respectively) perovskites were synthesized by a standard combined EDTAcitrate complexing sol-gel process. Stoichiometric amounts of La(NO3)3·6H2O, Fe(NO3)3·9H2O (all analytical grade, Sinopharm Chemical Reagent Co., Ltd.) were mixed in deionized water. EDTA and citric acid were then added as complexing agents in sequence at a mole ratio of 1:1:2 for total metal ions: EDTA: citric acid. To ensure complete complexation, the pH of the solution was adjusted to ~6 by the addition of an NH3 aqueous solution. A transparent gel was obtained by heating at 90 °C under stirring. The gel was then heated in the furnace at 250 °C for 5 h in air to form a solid precursor. Finally, the solid precursors of La1-xFeO3-δ were all calcined in air at 800 °C for 5 h. Basic characterizations. The prepared catalysts were characterized at room temperature (RT) using powder diffraction for phase identification and phase purity evaluation. A diffractometer (Rigaku Smartlab, Cu Kα radiation) in Bragg-Brentano reflection geometry was used. For more detailed structural analyses, Rietveld refinement (with the GSAS program and the EXPGUI interface) was used to determine the space group and the lattice parameters. The diffraction patterns were recorded by continuous scanning in the 2θ range of 20-90°with an interval of 0.02°. Inductively coupled plasma-mass spectroscopy (ICP-MS) was performed with a Varian Vista-Pro instrument to determine the compositions of oxides. The chemical compositions and surface element states were determined by X-ray photoelectron spectroscopy (XPS, PHI5000 VersaProbe


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1600, 1200, 800, 400 rp m) in O2-saturated 0.1 M KOH at a scan rate of 5 mV s -1 from 0.2 to -0.8 V (ORR) or from 0.2 to 1.0 V (OER) versus Ag|AgCl (3.5 M KCl). In the OER measurements, electrochemical impedance spectra (EIS) were recorded at 0.7 V vs. Ag/AgCl with frequencies ranging from 100 kHz to 0.1 Hz under an AC voltage of 5 mV. In the ORR measurements, the kinetic parameters, such as electron transfer number and kinetic current density, can be determined from the following Koutecky-Levich (K-L) equation:

bulk compositions of the prepared samples determined by inductively coupled plasma mass spectroscopy (ICP-MS), along with the oxygen non-stoichiometry (δ, representing the total oxygen vacancy) and the iron oxidation state determined by iodometric titration. The bulk compositions of the La1xFeO3-δ samples are in good agreement with the intended compositions. The results of iodometric titration indicate that the A-site cation deficient La1-xFeO3-δ perovskites contain oxygen vacancies and Fe ions of higher oxidation state, which are the key factors that enhance the ORR and OER activity, as to be discussed below.

1 1 1 1 1     0 2/3 -1/6 0 1/2 J J k J L nFkC 0.62nFDO2 v C w

(1) where J corresponds to the measured current density, Jk and JL are the kinetic and diffusion-limiting current densities, respectively, n is the electron transfer number, F is the Faraday constant, C°is the saturated concentration of oxygen in 0.1 M KOH, ω is the rotating rate (rad s −1), DO2 is the diffusion coefficient of oxygen, ν is the kinetic viscosity of the solution, and k is the rate constant for oxygen reduction. 3. RESULTS AND DISCUSSION Figure 1 shows the X-ray diffraction (XRD) patterns of the prepared La1-xFeO3-δ (x=0, 0.02, 0.05, 0.1, denoted as LF, L0.98F, L0.95F and L0.9F, respectively) powders. All samples of different compositions exhibited a pure orthorhombic perovskite structure with a space group of Pnma (JCPDS, No 88-0641).40 For the A-site cation deficient La1xFeO3-δ perovskites, the main peaks shifted slightly to higher angles compared to those for the pristine LF, implying a lattice shrinkage due to the creation of some smaller Fe cation (from 0.645 Å for Fe3+ to 0.585 Å for Fe4+ ).41,42 The structure of the prepared La1-xFeO3-δ perovskites was further confirmed by the Rietveld refinement (Figure S1 and Table S1). When we increased the amount of A-site cation deficiency in the LF to 20% (x=0.2), an impurity phase, Fe2O3, appeared, as shown in Figure S2. To avoid complications due to the effect of impurity phases on electrocatalytic activity, the deficiency in La was limited to 10% in our study, i.e., samples with a composition of La1-xFeO3-δ (x≤0.1). Shown in Table S2 are the

Figure 2. SEM images of the (a) LF, (b) L0.98F, (c) L0.95F, and (d) L0.9F powders. Insets are the magnified images Shown in Figure 2 is some typical surface morphology of the La1-xFeO3-δ samples as revealed using scanning electron microscopy (SEM). It is clearly seen that particles with a size of about several hundred nanometers were agglomerated into larger aggregates in all La1-xFeO3-δ samples. It should be noted that the A-site cation deficient La1-xFeO3-δ samples appear to have slightly larger particle size than the pristine LF and, accordingly, have smaller surface areas, as estimated from Brunauer-Emmett-Teller (BET) measurements (Figure S3). Besides, of all the A-site cation deficient La1-xFeO3-δ samples, L0.95F had the smallest particle size and the largest surface areas, which would benefit ORR and OER because of more active sites. To evaluate the electrocatalytic activity of La 1-xFeO3-δ for ORR, rotating disk electrode (RDE) experiments were conducted in 0.1 M KOH solution. All potentials were referenced to a reversible hydrogen electrode (RHE, see Figure S4 for RHE calibration). Figure 3a shows the linear sweep voltammetry (LSV) curves of the La 1-xFeO3-δ catalysts on the GC electrode at 1600 rpm in an O2-saturated 0.1 M KOH solution at a scan rate of 5 mV s -1. The A-site cation deficient La1-xFeO3-δ (i.e., L0.98F, L0.95F and L0.9F) catalysts exhibited higher limiting current density and more positive onset potential than the pristine LF, suggesting better ORR activity. It is noted that the contributions from the glassy carbon (GC) electrode and the GC-supported carbon electrode for ORR are not negligible under the same testing conditions (Figure 3a). However, the electrochemical behavior of each catalyst-loaded GC electrode was dominated by the catalyst; thus, the LSV responses reflect directly the ORR activity of the catalyst loaded on the GC electrode. For further insight into the ORR kinetics and electrocatalytic processes on the catalysts, we carried out a more detailed study of the RDE

Figure 1. XRD patterns of LF, L0.98F, L0.95F, and L0.9F powders, with an expanded region of 2θ=31.8-32.8°shown on the right.



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pristine LF, implying much better OER catalytic activity. It is important and meaningful to compare the potential for achieving the current density of 10 mA cm-2 (denoted as EJ10), which is a metric relevant to solar fuel synthesis. 43 Remarkably, the L0.95F catalyst demonstrated such a current density at a small potential of only ~1.64 V, which is significantly lower than ~1.74 V for the pristine LF. According to the criterion of EJ10, the L0.95F catalyst also showed higher activity than some currently reported wellknown perovskite-type OER catalysts under identical experimental conditions (Figure S7), such as Ba0.5Sr0.5Co 0.8Fe0.2O3-δ (BSCF), SrNb 0.1Co 0.7Fe0.2O3-δ (SNCF), PrBaCo 2O5+δ (PBC) and LaNiO3-δ (LN).10,11,44,45 Each crystal structure was identified by their respective XRD patterns (Figure S8). Furthermore, the mass activity (MA, normalized to the oxide mass loading) and specific activity (SA, normalized to real oxide surface area as estimated from BET measurements) were also calculated and shown in Figure 4b. At 1.63 V, the A-site cation deficient La 1-xFeO3-δ catalysts delivered much better mass activity and specific activity than the pristine LF. Especially, the L0.95F catalyst exhibited a MA of 23.56 A g -1ox and SA of 0.364 mA cm-2ox, which are ~4.3 and ~6.0 times higher than those for LF (5.45 A g -1ox and 0.061 mA cm-2ox). In addition, enhanced kinetics of A-site cation deficient La1-xFeO3-δ catalysts toward OER was proved by the Tafel plots and electrochemical impedance spectroscopy (EIS) analysis. The A-site cation deficient La 1xFeO3-δ catalysts exhibited a much smaller Tafel slope (Figure 4c), a smaller diameter of the semicircle in the EIS (Figure 4d), and thus more rapid OER rates and better charge transfer abilities than LF. It is worth noting that the order of OER activity for the La 1-xFeO3-δ resembles the trend of ORR activity: L0.95F>L0.9F≈L0.98F>LF. However, the enhancement effect in OER activity is more pronounced as compared to the ORR activity. Thus, introducing A-site cation

Figure 3. a) LSV curves for the ORR on the RDE (1600 rp m) comprised of LF, L0.98F, L0.95F, and L0.9F catalysts in O2saturated 0.1M KOH solution (scan rate: 5 mV s -1), together with LSV curves for a glassy carbon (GC) electrode and GCsupported carbon electrode. b) K-L plots at 0.25 V for LF, L0.98F, L0.95F, and L0.9F catalysts on the basis of the RDE data in Figure S5 of the Supporting Information. c) The electron transfer number (n) and kinetic current density (Jk ) derived from the K-L plots at 0.25 V for LF, L0.98F, L0.95F, and L0.9F catalysts. The error bars represent standard deviations from at least three independent measurements . d) Tafel plots of LF, L0.98F, L0.95F, and L0.9F catalysts. experiments at different rotation speeds (400-2000 rp m, Figure S5 of the Supporting Information). As calculated from the slope of the Koutecky-Levich (K-L) plots (Figure 3b), the electron transfer number (n) at 0.25 V is 2.52, 3.64, 3.96, and 3.87 for LF, L0.98F, L0.95F, and L0.9F, respectively (Figure 3c). This result demonstrates a dominant 4-electron ORR process (O2+H2O+4e-=4OH-) for the A-site cation deficient La1-xFeO3-δ catalysts, while a dominant 2-electron ORR process (O2+H2O+2e-=HO2-+OH-, HO2-+H2O+2e-=3OH-) for the pristine LF catalyst. A dominative selectivity of 4-electron transfer process for the A-site cation deficient La 1-xFeO3-δ catalysts is attributed to the creation of more oxygen vacancies in the defected structure, which may facilitate O2 adsorption and charge transfer.34-39. Furthermore, the ORR activity of the catalysts can be reflected by the kinetic current density (Jk ). As seen from Figure 3c and Figure S6, the Jk values of the catalysts at 0.25 V follow the order, in the sequence of L0.95F>L0.9F≈L0.98F>LF. To gain more insights into the La1-xFeO3-δ catalysts, Tafel plots of these catalysts are also drawn in Figure 3d. The Tafel slopes for the A-site cation deficient La1-xFeO3-δ catalysts are smaller than that of the pristine LF, implying much more rapid ORR rates of the former. Clearly, all these data unambiguously suggest that the introduction of A-site cation deficiency into LF perovskite effectively enhanced the ORR activity. In particular, the L0.95F exhibited the highest ORR activity among the A-sitedeficient La1-xFeO3-δ catalysts studied. To further exploit the potential application of A-site cation deficient La1-xFeO3-δ perovskites as bifunctional catalysts for oxygen electrocatalysis, their OER activity was also evaluated by recording LSV curves within the OER potential window. As shown in Figure 4a, the LSV curves of A-site cation deficient La1-xFeO3-δ exhibited similar earlier onset potential (~1.55 V) and greater catalytic current as compared with

Figure 4. a) LSV curves for the OER on the RDE (1600 rp m) comprised of LF, L0.98F, L0.95F, and L0.9F catalysts in O2saturated 0.1M KOH solution (scan rate: 5 mV s -1). b) OER mass activity (MA) and specific activity (SA) of LF, L0.98F, L0.95F, and L0.9F catalysts at 1.63 V. The error bars represent standard deviations from at least three independent measurements. c) Tafel plots of LF, L0.98F, L0.95F, and L0.9F catalysts. d) EIS of the LF, L0.98F, L0.95F, and L0.9F electrodes recorded at 1.65 V (no iR-corrected) under the influence of an AC voltage of 10 mV. The semicircular diameter in the EIS is indicative of the charge transfer resistance (Rct) value.


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deficiency in LF perovskite not only improves ORR activity, but also enhances OER activity to a great extent. The bifunctional catalytic ability of A-site cation deficient La1-xFeO3-δ catalysts toward both ORR and OER was further compared in Figure S9 in a wide range of potential window. To assess the overall bifunctionality of the catalyst, the difference in potential between the OER current density at 10 mA cm-2 and the ORR current density at -1 mA cm-2 was calculated according to the equation ΔE= EJ10,OER - EJ-1,ORR.46,47 A smaller ΔE value means better bifunctional catalytic activity and more potential for practical applications of the catalyst. From Figure S10 and Table S3, The ΔE values are 1.25, 1.10, 1.06, 1.09 V for LF, L0.98F, L0.95F, and L0.9F, respectively. Obviously, the A-site cation deficient La 1-xFeO3-δ catalysts showed much smaller ΔE values compared to pristine LF, and the ΔE of L0.95F was the smallest. Notably, the ΔE value of L0.95F at either 5 mV/s or 10 mV/s scan rate for LSV test (Figure S10) compared favourably to the precious-metalbased (i.e., IrO2, RuO2, and Pt/C) and some other reported excellent perovskite-based bifunctional catalysts in the literatures (Table S4). This result further demonstrated the superiority of introducing A-site cation deficiency into LF perovskite with significantly improved bifunctionality. The above electrochemistry analysis reveals that the introduction of A-site cation deficiency into LaFeO3 (LF) perovskite can significantly enhance the ORR and OER activity. Understanding the source of the excellent ORR and OER activity of A-site cation deficient La 1-xFeO3-δ perovskites can provide guidelines for further development in bifunctional catalysts. In general, a catalyst with more active sites leads to better catalytic activity. However, the LF with higher surface areas or smaller particle size (relative to A-site cation deficient La1-xFeO3-δ as shown in Figure 2 and S3) did not exhibit better catalytic activity, which rules out a dominant number of active sites influence. Instead, the significantly enhanced ORR and OER activity of A-site cation deficient La 1-xFeO3-δ perovskites can be attributed to the following two key factors. Firstly, the surface oxygen vacancies as reflected by the highly oxidative oxygen species (O22-/O-) were created in A-site cation deficient La1-xFeO3-δ perovskites. As mentioned above, oxygen vacancies, which are favorable to for ORR and OER catalysis,34-39 existed in A-site cation deficient La 1-xFeO3-δ perovskites as determined by bulk iodometric titration method. Because the catalytic reactions (e.g. ORR and OER) occur at the surface,48,49 it is worthwhile to study the surface oxygen vacancies. Previous studies have shown that the highly oxidative oxygen species (O22-/O-) formed on the surface of catalysts were closely related to the surface oxygen vacancies 50,51 and are active for catalyzing OER.14,52-54 Figure 5a shows the O 1s X-ray photoelectron spectroscopy (XPS) spectra of LF, L0.98F, L0.95F, and L0.9F, which can be deconvoluted into four characteristic peaks of lattice oxygen species (~529.2 eV for O2-), highly oxidative oxygen species (~530.5 eV for O22-/O-), hydroxyl groups or the surfaceadsorbed oxygen (~531.5 eV for -OH or O2 ), and adsorbed molecular water (~532.9 eV for H2O). The relative concentration of the different kinds of oxygen species estimated from the relative area of these fitted subpeaks is listed in Table S5. As can be seen, the relative concentration of O22-/O- species on the three A-site cation deficient La 12xFeO3-δ surfaces is similar. However, there is no O2 /O species existing on the surface of LF. This result clearly suggests that the surface oxygen vacancies (O22-/O- species) in

Figure 5. a) XPS spectra of O 1s species on the surface of LF, L0.98F, L0.95F, and L0.9F samples. b) Room temperature Mössbauer spectra of LF, L0.98F, L0.95F, and L0.9F samples. c) Schematic illustration of the formation of oxygen vacancy and Fe4+ in A-site-deficient La1-xFeO3-δ perovskites. the A-site cation deficient La 1-xFeO3-δ perovskites played a dominant role in the enhancement of ORR and OER activity. Secondly, small amount of Fe 4+ species with an optimal e g orbital filling (t 2g3eg1) existed in the A-site cation deficient La1xFeO3-δ perovskites. Recently, a well-known descriptor of ORR and OER activity has been proposed by Shao-Horn et al. that perovskites containing transition-metals with an e g =1 electron configuration showed a high ORR and OER activity in alkaline solution.10,11 Therefore, it is necessary to consider the electronic state of transition-metals in the B-site of perovskites. We first tried to identify the oxidation state of Fe in the La1-xFeO3-δ perovskites by XPS characterization. Although Fe 2p 3/2 binding energy (at 710.8 eV) and shake-up contribution (at 720 eV) are consistent with Fe 3+ , that is, the main species (Fe 3+ ) in all the samples (Figure S11), the slight positive shift of Fe 2p 3/2 peak also suggests the presence of iron in higher oxidation state, which will be further identified accurately by means of other analytical techniques, e.g., Mössbauer spectroscopy. Mössbauer spectroscopy is sensitive to iron element and widely used to examine the electronic structure of iron. Figure 5b shows the room Mössbauer spectra of LF, L0.98F, L0.95F, and L0.9F samples and Mössbauer parameters obtained by Lorentzian curves fit are listed in Table S6. The Mössbauer spectrum of pristine LF showed a single Fe3+ sextet component with sharp lines, while additional Fe4+ occurred in the A-site cation deficient La 14+ ions are basically in high-spin state xFeO3-δ. Considering Fe 55-57 in perovskites, therefore, Fe4+ (t2g3eg1 configuration) in Asite cation deficient La1-xFeO3-δ is another factor to enhance ORR and OER activity. To sum up, the combination of above two beneficial factors (with the schematic illustration shown in Figure 5c) is responsible for the significantly enhanced ORR and OER activity of A-site cation deficient La 1-xFeO3-δ perovskites. Notably, L0.95F exhibited better ORR and OER activity than other A-site cation deficient perovskites (i.e., L0.9F and L0.98F); the enhanced electrocatalytic activity may be attributed to the larger surface area (Figure S3), slightly more surface oxygen vacancies (Figure 5a and Table S5) and Fe4+ ions (Figure 5b and Table S6) as discussed earlier.


Chemistry of Materials 4. CONCLUSIONS

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In summary, we have developed a simple and cost-effective process for preparation of A-site cation deficient La1-xFeO3-δ (x=0.02, 0.05, 0.1) as highly-efficient bifunctional electrocatalysts for ORR and OER in alkaline solutions. Electrochemical investigations reveal that the ORR and OER activity can be tuned by tailoring the A-site cation deficiency in LaFeO3 (LF) perovskite. The enhancement in electrocatalytic activity is especially obvious in the OER process. Among the A-site cation deficient La 1-xFeO3-δ perovskites studied, the L0.95F showed the highest ORR and OER activity and, hence, the best bifunctionality (or the smallest ΔE value). In fact, the ΔE value of L0.95F (~1.06 V) compares favorably to those of the precious -metal-based and some other excellent perovskite-based bifunctional catalysts reported in the literatures. The creation of surface oxygen vacancies and small amount of Fe 4+ species in the A-site cation deficient La 1-xFeO3-δ perovskites are responsible for the observed enhanced ORR and OER activity. These results demonstrate that the family of A-site cation deficient La 1xFeO3-δ perovskites holds promise for applications in oxygenbased energy storage and conversion technologies, such as fuel cells, metal-air batteries, water splitting, and solar fuel synthesis. Further and in particular, the important insight we gained from the study of La 1-xFeO3-δ is applicable to a wide variety of other 3d transition-metal oxides for oxygen electrocatalysis.

Supporting Information. Additional XRD, BET, ICP, XPS spectra of Fe, O 1s XPS peak deconvolution results, M össbauer parameters, and electrochemistry data. This material is available free of charge via the Internet at http://pubs.acs.org.

* Email address: [email protected] * Email address: [email protected]

The authors declare no competing financial interest.

This work was financially supported by the Key Projects in Nature Science Foundation of Jiangsu Province under contract No. BK2011030, the“National Nature Science Foundation of China under contract No. 21576135, the M ajor Project of Educational Commission of Jiangsu Province of China under contract No. 13KJA430004, the Priority Academic Program Development of Jiangsu Higher Education Institutions, the Program for Jiangsu Specially-Appointed Professors and the Youth Fund in Jiangsu Province under contract No. BK20150945, and the US Department of Energy ARPA-E REBELS Program under award number DE-AR0000502.

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