Article pubs.acs.org/JPCC
Oxygen Evolution Activity and Stability of Ba6Mn5O16, Sr4Mn2CoO9, and Sr6Co5O15: The Influence of Transition Metal Coordination Alexis Grimaud,† Christopher E. Carlton,† Marcel Risch,† Wesley T. Hong,†,‡ Kevin J. May,†,§ and Yang Shao-Horn*,†,‡,§ †
Electrochemical Energy Laboratory, ‡Department of Materials Science and Engineering, and §Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, 02139, United States S Supporting Information *
ABSTRACT: Several coordination motifs of cobalt and manganese ions were obtained in various transition metal oxides, which enabled different oxidation and spin states. Combined high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) confirmed the presence of coordination environments such as Co2+ in disordered prisms and Co4+/Mn4+ in face-shared octahedra. The influence of cobalt and manganese coordination on the oxygen evolution reaction (OER) activity and oxide stability in alkaline solution was studied. Under cycling, the surface of perovskites that consists of Co2+ in prisms was amorphized and the activity was similar to that of LaCoO3, which has a stable surface composed of Co3+ in octahedral coordination. These findings highlight the critical role of the electronic structure of transition metal oxides on the OER activity and stability.
1. INTRODUCTION The discovery of highly active and cost-effective catalysts for the oxygen evolution reaction (OER) is a key challenge for many energy storage devices.1−3 AMO3 perovskite oxides made of alkaline or rare-earth ions in the A-site and first row transition metal (TM) cations in the M-site have been shown to be efficient catalysts in alkaline solution,4−9 the best of which exhibit surface area normalized activities comparable to IrO2 and RuO2.8−10 We recently reported a design principle for the OER activity of perovskites based on the eg orbital occupancy of TM ions, demonstrating high OER activities obtainable with occupancies near unity.8 As the proposed design principle is established from OER data of corner-shared pseudocubic perovskites8 that only have TM ions in octahedral (Oh) symmetry, the effect of altering the energy and filling of frontier orbitals using the crystal field splitting of other TM coordination on the OER activity remains largely unknown and only few studies were published on binary transition metal oxides.11,12 In this study, we modify the coordination of cobalt and manganese ions within perovskites and related layered oxides from which the oxidation and spin state can be altered and examine the influence of oxidation and spin state on the OER activity and stability. Here, we take advantage of the structural flexibility of the perovskite family to stabilize various cobalt and manganese coordination and oxidation states. By increasing the A-site ionic radius using barium or strontium (Supporting Information Table S1), the cubic perovskite with corner-sharing octahedra (Oh) (0.98 ≤ t ≤ 1.03) are destabilized relative to hexagonal perovskites with face-sharing octahedra (f-s) (t ≥ 1.03), where t is defined as [(rA + rO)/(√2(rM + rO))], and rA, rM, and rO are © 2013 American Chemical Society
the A- and M-site and oxygen ionic radius, respectively. Two distinct TM coordinations can be obtained in the hexagonal perovskites: face-shared (f-s) and prisms (P).13,14 Although Co2+ can be found in 6-fold coordination in few oxides such as CoFe2O4, CoO or double perovskites A2+2Co2+M6+O6,15−17 it is rare to have Co2+ in octahedral coordination in the perovskite-related structures. The large distorted prisms (P) can stabilize Co2+ in a 6-fold coordination,18−21 in contrast to other Co2+-containing oxides such as YBaCo4O7 and spinel Co3O4,22,23 where Co2+ is in 4-fold coordination. The small face-shared octahedra20,24 may stabilize Co4+ and Mn4+ ions but this still requires experimental verification. Cobalt and manganese ions in f-s and P arrangement in perovskites can have considerably different energy levels and frontier orbital filling relative to the corner-sharing (c-s) arrangement in pseudocubic perovskites,20 as shown in Figure 1. These orbital energy levels and their filling can greatly influence the activity and stability of the oxides for OER. In this paper, we use combined high-resolution transmission electron microscopy (HRTEM) and X-ray absorption spectroscopy (XAS) to show that Mn4+ and Co4+ can be stabilized in f-s arrangement and Co2+ in P arrangement in the nearsurface regions. The OER activities of Mn4+ and Co4+ in f-s and Co2+ in P arrangement of cobalt- and manganese-based perovskites are compared subsequently with Mn3+/Mn4+ and Co3+ in the c-s arrangement for pseudocubic perovskites in alkaline solution at pH 13. Received: August 27, 2013 Revised: November 19, 2013 Published: November 27, 2013 25926
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
Figure 1. Schematic representation of the d-block orbitals of Mn4+ and low-spin (LS)25 Co4+ in f-s,26 HS Co2+ in distorted P,26 and intermediate-spin (IS) Co3+ in c-s.
2. EXPERIMENTAL SECTION 2.1. Materials Synthesis. The compounds Ba6Mn5O16, Sr4Mn2CoO9, Sr6Co5O16, and CaMnO3 were prepared by a conventional solid-state reaction. Stoichiometric amounts of BaCO3 (Alfa-Aesar 99.95%), SrCO3 (Alfa-Aesar 99%), MnO2 (Alfa-Aesar 99.9%) and Co3O4 (Alfa-Aesar 99.7%) previously dehydrated were thoroughly grounded and fired in air at 1000 °C for 10 h. Products were ground and annealed in air at 1100 °C for 12 h then 900 °C for 48 h for Sr6Co5O16, 900 °C for 20 h for Ba6Mn5O16, 1275 °C for 90 h for Sr4Mn2CoO9, and 1100 °C for 10 h under air followed by 1000 °C for 10 h under oxygen for CaMnO3. LaCoO3 and LaMnO3+δ used for the OER measurements were synthesized using a coprecipitation route. Cobalt, manganese, and lanthanum nitrate precursors (AlfaAesar, 99.98%) were mixed at stoichiometric ratios in deionized water at a concentration of ∼0.1 M. Solid precipitate was formed by titrating with 1.2 M tetramethylammonium hydroxide (Alfa-Aesar) and filtered out and dried at 200 °C for 12h, then calcined at 1000 °C in dry air (Airgas) for 12 h. LaCoO3 and LaMnO3+δ used for the XAS measurements were synthesized by a conventional solid-state reaction starting from stoichiometric amounts of La2O3, Co3O4 and MnO2. Final heat treatment of 1000 °C for 20 and 40 h with intermediate grinding states was used for LaCoO3 and LaMnO3+δ, respectively. Commercially available powder of MnO2 with rutile structure (Alfa-Aesar 99.9%) and Co3O4 with spinel structure (Alfa-Aesar 99.7%) were heat-treated before any measurements. The LaMnO3+δ overstoichiometry (LaMnO3.08) was determined by iodometric titration.27 2.2. Laboratory X-ray Diffraction. XRD measurements were performed using a PANAlytical X’Pert Pro powder diffractometer in the Bragg−Brentano geometry using a Copper Kα1,2 radiation. Data were collected using the X’Celerator detector in the 8−80° window in the 2θ range. The lattice parameters were determined after a refinement of the XRD patterns with the LeBail method using the Fullprof program.28 2.3. Electrode Preparation and Electrochemical Measurements. Electrodes were prepared by drop-casting an ink-containing catalyst powder on a glassy carbon electrode, as described previously.29 The glassy carbon surface was loaded with 0.25 mgoxide/cm2 and a mass ratio of 5:1:1 of oxide catalyst to acetylene black (AB) carbon to Nafion. OER measurements
were performed with a rotating disk electrode setup using a glass electrochemical cell with a Pt counter electrode and Ag/ AgCl reference electrodes. The potential of the latter were in the range 0.955−0.961 V relative to hydrogen redox on Pt in H2 saturated electrolyte, that is, a reversible hydrogen electrode (RHE). A 0.1 M KOH (99.99% purity, Sigma Aldrich) electrolyte was prepared with deionized water (>18 MΩ cm). Oxygen was bubbled to ensure O2/HO− equilibrium at 1.23 V versus RHE. The potential was controlled using a Biologic SP300 potentiostat. A scan rate of 10 mV/s was used for all cyclic voltammetry and rotation was set to 1600 rpm. Ohmic losses were corrected by subtracting the ohmic drop from the measured potential, using an electrolyte resistance determined by high-frequency AC impedance; iR corrected potentials are denoted E − iR where i is the current and R the electrolyte resistance. The analysis of OER kinetic currents was capacitance-corrected by taking the average forward and backward scans and then iR-corrected. 2.4. Surface Normalization. Specific surface areas were determined for each oxide powder using single point Brunauer, Emmet, and Teller (BET) analysis provided by a Quantachrome ChemBET Pulsar apparatus. 2.5. High-Resolution Transmission Electron Microscopy. The sample was examined with a JEOL 2010f transmission electron microscope. The microscope is equipped with a field emission electron gun and can reach a point-topoint resolution of 1.9 Å at the operating voltage (200 kV). Fourier analysis was performed using the Gatan Digital Micrograph software v2.01 (Gatan Inc.). Samples of cycled electrode were prepared by swabbing the glassy carbon surface, previously cleaned with ethanol. 2.6. X-ray Absorption Spectroscopy. X-ray absorption measurements (XAS) at the metal L-edge and the O K-edge were collected in both total electron yield (TEY) and total fluorescence yield (TFY) modes at the Advanced Light Source (Beamline 8.0.1) at Lawrence Berkeley National Laboratory. TEY measurements were collected by registering the sample current normalized to the photon flux. Experiments were performed at room temperature and with the linear polarization of the incident beam 45° to the sample surfaces. Measurements were collected under UHV. As an instrumental correction for better comparison with previously reported studies done at different beamlines, the reported incident photon energies are shifted by +5 eV. Electron escape depths of ∼2 nm for Mn 25927
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
Figure 2. HRTEM images of surface regions of (a) Ba6Mn5O16, (b) Sr4Mn2CoO9, and (c) Sr6Co5O15 with fast Fourier transformation analysis, as well as a schematic representation of their structure and local coordination (previously reported18,25,26) projected along the same zone axis.
LMM electrons (K.E. ∼ 500 eV) in Ba6Mn5O16 and Sr4Mn2CoO9 to ∼3 nm for Sr6Co5O15 and Sr4Mn2CoO9 for Co LMM electrons (K.E. ∼ 620 eV) were estimated using the formula proposed by Elam et al.30 d = (0.011/ρ)[E]3/2 with d as the escape depth in angstroms, ρ is the oxide density in grams per cubic centimeter, and E is the energy of the Auger electrons.
3. RESULTS AND DISCUSSION 3.1. Structural Characterization. Three transition metal oxides known for f-s arrangement, Ba6Mn5O16, Sr4Mn2CoO9 and Sr6Co5O15, were synthesized by solid-state synthesis routes.18,25,26 The crystal structure and local transition metal coordination of these oxides was previously refined by various authors.18,25,26 The XRD patterns (Figure S1 of the Supporting Information) confirmed the phase purity and the refined space groups and lattice parameters were found to be similar to those reported previously reported.18,25,26 The symmetry of these oxides was further confirmed by HRTEM (Figure 2). The surfaces of Sr4Mn2CoO9 and Sr6Co5O15 were found close to be perfectly crystalline (Figure 2b,c) while amorphous regions were observed on the surfaces of Ba6Mn5O16 that were perpendicular to the basal plane (Figure 2a and Figure S2a of the Supporting Information). 3.2. XAS L-edge study. XAS Mn L2,3-edge spectra provided the evidence that Mn ions in Ba6Mn5O16 and Sr4Mn2CoO9 are Mn4+ in the near-surface area, as suggested previously for f-s arrangement in the bulk of these hexagonal perovskites.17,25 The spectra were collected in TEY with an estimated escape depth of ≤3 nm (see Experimental Section). Two sharp features in the Mn L3-edge spectra of Ba6Mn5O16 and Sr4Mn2CoO9 in the energy range from 635 to 645 eV are characteristic of Mn4+31 as shown in Figure 3a. In contrast, the LaMnO3.08 spectrum possesses one main broad peak at 641 eV, characteristic of Mn3+, and a shoulder at 639 eV presumably owing to the overstoichiometry (δ ∼ 0.08), and the presence of a small amount of Mn4+. Having Mn4+ in Ba6Mn5O16 and Sr4Mn2CoO9 is further supported by their higher photon
Figure 3. X-ray absorption (a) at the manganese L-edge for LaMnO3+δ (with δ = 0.08 ± 0.01), Ba6Mn5O16, and Sr4Mn2CoO9 and (b) at the cobalt L-edge for LaCoO3, Sr4Mn2CoO9, and Sr6Co5O15. These XAS spectra were measured in the TEY mode with an escape depth of ≤3 nm (see Experimental Section).
energy observed at the L2-edge relative to LaMnO3+δ (Figure 3a) and by an increase of the pre-edge peak in the O K-edge spectra (A in Supporting Information Figure S3), indicating a 25928
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
stronger hybridization of the oxygen 2p-metal 3d bands (Figure S3 of the Supporting Information). XAS Co L2,3-edge spectra suggested that cobalt ions are Co2+ for Sr4Mn2CoO9 while both Co4+ and Co2+ were observed for Sr6Co5O15. The XAS spectra of Sr4Mn2CoO9 and Sr6Co5O15 are shown in Figure 3b, which is compared with the spectrum of LaCoO3 serving as a reference for Co3+. The L3-edge (775− 785 eV) of Sr6Co5O15 has its main feature at higher energy than LaCoO3, suggesting the presence of Co4+ in the LS state in the smallest f-s coordination. The complex feature observed at ∼778 eV in the spectra of Sr4Mn2CoO9 and Sr6Co5O15 is characteristic of Co2+, most likely populating the largest distorted prisms.20,25 However, the assignment of the main feature at ∼780 eV of Sr4Mn2CoO9 and Sr6Co5O15 to Co2+ ions in the distorted prism is not straightforward. No energy shift was observed for this feature between the TFY and the TEY mode (Figure S4b of the Supporting Information), which ruled out preferential oxidation of Co on the surface. Therefore, it is proposed that this feature is characteristic of Co2+ in the P arrangement, which differs from Co2+ in the c-s arrangement due to different d-states.20 3.3. Oxygen Evolution Reaction Activity. The intrinsic OER activity of Ba6Mn5O16 with Mn4+ in the f-s coordination was found to be comparable to LaMnO3+δ with Mn3+ in the c-s coordination and CaMnO3 with Mn4+ in the c-s coordination. Cyclic voltammetric (CV) scans of manganese-containing perovskites supported on glassy carbon electrodes (GCE) in 0.1 M KOH electrolyte are shown in Figure 4a. The intrinsic OER activity was defined as the average current from the forward-going and backward-going scans, which were normalized to the particle surface area determined from BET. No significant difference beyond experimental uncertainty was noted in the OER intrinsic activity of Ba6Mn5O16 and CaMnO3, which were comparable to other Mn4+-containing oxides such as MnO2 (Figure S5 of the Supporting Information). The corner-shared perovskite LaMnO3+δ have already been shown to be structurally stable under OER conditions in alkaline solution,32 but TM oxidation is conceivable. The former observation suggests that the activities of Mn4+-oxygen bonds for OER are not markedly influenced by the local coordination (c-s versus f-s). In contrast to previous density functional theory (DFT) studies,7 where increasing the Mn oxidation state from Mn3+ to Mn4+ in the perovskite structure was predicted to enhance the OER activity considerably, CaMnO 3 and LaMnO3+δ exhibited relatively similar OER activities (Figure S5 of the Supporting Information) in agreement with previous experimental findings.8 Lastly, Ba6Mn5O16 was found to be stable during OER, similar to LaMnO3+δ32 and CaMnO3; no significant changes were found in the CV scans (Figure S6 of the Supporting Information) or in the surface atomic structure from TEM imaging (Figure S7 of the Supporting Information). The intrinsic OER activity of Sr4Mn2CoO9 with Co2+ in the P coordination and Mn4+ in the f-s coordination was higher than that of Ba6Mn5O16 with Mn4+ in the f-s coordination. The onset OER voltage observed for Sr4Mn2CoO9 was considerably lower than for Ba6Mn5O16, as shown in Figure 4a. The OER current normalized to the surface area of Sr4Mn2CoO9 is at least 1 order of magnitude higher than that of Ba6Mn5O16 (Figure 4c). Knowing the low OER activity of Mn4+ in f-s coordination for Ba6Mn5O16, the large activity of Sr4Mn2CoO9 can be attributed to HS Co2+ in the P coordination. The OER activities of Sr4Mn2CoO9 and Sr6Co4CoO15 obtained from the second cycle were found comparable, as shown in Figure 4b,c.
Figure 4. (a) Cyclic voltammograms (10 mV/s) of LaMnO3+δ, Ba 6 Mn 5 O 16 , CaMnO 3 , and Sr 4 Mn 2 CoO 9 , (b) of LaCoO 3 , Sr4Mn2CoO9, and Sr6Co5O15, and (c) Tafel plot for Ba6Mn5O16, Sr4Mn2CoO9, Co3O4, LaCoO3, Sr6Co5O15, and La0.4Sr0.6CoO3. All measurements were performed using an ink containing AB carbon, Nafion, and oxide particles with a loading of 0.25 mg/cm2 disk supported on a glassy carbon electrode in O2-saturated 0.1 M KOH electrolyte. Standard deviations calculated from measurements of at least three electrodes were used as error bars. Data presented were obtained from the second cycle. The specific surface area of oxides is given in Supporting Information Table S2.
This observation may suggest that HS Co2+ in distorted prisms (P coordination) is more active than Co4+ in distorted octahedra (f-s coordination) but further studies are needed to provide further evidence. 3.4. Stability of the Cobalt-Containing Hexagonal Perovskites under OER Conditions. The OER currents of Sr4Mn2CoO9 were found to gradually increase with increasing CV scans (Figure S6b of Supporting Information), which suggests that the surfaces of Sr4Mn2CoO9 and HS Co2+ in P coordination might not be stable during OER. This observation is in contrast to the hexagonal perovskite Ca3Co2O6 (Figure S8 of the Supporting Information) and the layered compound LiCoO2,33 where a large oxidation wave, indicative of instability 25929
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
of bulk oxide particles during OER, was observed in the first CV. HRTEM imaging of Sr4Mn2CoO9 particle surfaces after 25 CV cycles revealed surface amorphization confined in the nearsurface regions of ∼7−10 nm (Figure S7a of the Supporting Information), which was accompanied by an increase of the pseudocapacitance during cycling (Figure S6b of the Supporting Information). The increased OER currents of Sr4Mn2CoO9 with CV cycles can be attributed to increasing electrochemically active surface area (EASA), and changes in the cobalt coordination. Similarly to what was previously observed for Ba0.5Sr0.5Co0.8Fe0.2O3‑δ,32 a diffuse ring with a dspacing ∼2.8 Å is observed for Sr4Mn2CoO9 after cycling (Figure S7a of the Supporting Information). This can be attributed to the destabilization of Co2+ in distorted prisms to a local structure of layered edge-shared octahedra similar to cobalt oxyhydroxides or Co-Pi structure.34,35 The increased EASA suggests that not only the surface of the amorphous structure formed during CV cycles is accessible for the electrolyte but presumably the whole amorphous volume. Although a limited increase in the OER current was observed for Sr6Co5O16 during CV scans (Figure S6c of the Supporting Information), cycled Sr6Co5O15 particle surfaces were amorphized in the near-surface regions of ∼1−2 nm (Figure S7b of the Supporting Information), which is in contrast to LaCoO3 staying crystalline during OER.32,34 Similarly to Sr4Mn2CoO9, the diffuse ring with a d-spacing ∼2.7 Å suggests the formation of edge-shared octahedra under cycling. Nevertheless, a limited increase of the EASA was observed, which is consistent with the limited formation of the amorphized region (Figure S6c of the Supporting Information). These observations suggest that Co2+ in P coordination and/or Co4+ in the f-s coordination might not be stable during OER, which transformed upon surface amorphization to a different cobalt coordination.36 Interestingly, this was previously observed for MnOx catalysts which form Mn−Mn edge-shared bridges under OER conditions, this coordination possessing a higher activity than β-MnO2 studied in this work.11,12 The redox peaks observed at ∼1.45 V versus RHE in the capacitive region for Sr4Mn2CoO9 and Sr6Co5O15 might suggest that Co2+ is oxidized at high potential on the surface of the catalysts (Figure S6 of the Supporting Information). Nevertheless, the activity reported after two cycles is intrinsic to the pristine powder as the formation of the amorphous layer only occurs after few cycles (∼5−10 cycles). 3.5. Electronic Structure Influence on the OER Activity. We propose a classification of the manganese and cobalt activity in several crystallographic sites as well as several oxidation and spin states (Figure 5). It appears that Mn3+ in c-s
octahedra is the least active configuration (LaMnO3+δ). By oxidizing to Mn4+ (MnO2, CaMnO3, and Ba6Mn5O16), the OER activity is slightly increased but remains limited and no clear distinction can be made between the activity of c-s and f-s octahedra. The coordination and the oxidation state of manganese thus plays a limited role on the OER activity of manganese oxides, which rules out the effect of d-block splitting and orbital overlap on the OER activity of manganese oxides. One can argue that Mn4+ and Mn3+ cations, characterized by only one allowed spin state configuration in 6-fold symmetry at room temperature (low- and high-spin, respectively), are not versatile enough to obtain high OER activity. One should note that different processes during the OER were observed for corner-sharing MnO6 octahedra versus layered edge-sharing MnO6 octahedra.11,12 This might indicate that the mechanism is similar for corner-shared and face-shared coordination. This is further supported by the nearly identical Tafel slopes found for these manganese oxides (Supporting Information Figure S5). We have previously demonstrated that the σ* electronic states act as a primary descriptor for the OER activity of pseudocubic perovskites made of transition metal in Oh symmetry (c-s arrangement); this work shows that this descriptor can be used to understand the OER activity of other symmetries. The σ*-parentage orbitals of Co2+ for Sr4Mn2CoO9 and Sr6Co5O15 have filling equal to 2 (see Table S3 of the Supporting Information), which is above the optimal filling for high activity reported by Suntivich et al., ∼1.2.8 The combination of HS Co2+ in tetrahedra (three electrons in the t2 orbitals) and LS Co3+ in Oh (0 electron in the eg orbitals) in Co3O4 gives the lowest OER activity of the cobalt oxides measured in this study. This further demonstrates the importance of the σ*-state electrons for a given oxidation state. It also demonstrates that oxygen is too weakly bonded to the surface of Co2+ containing transition metal oxides (right side of the volcano). However, not only is the number of σ* electrons crucial, but the d-block (crystal field) splitting also presumably influences the OER activity by modifying the relative energy of σ* states. As previously discussed, transition metals with high oxidation states are preferentially stabilized in the f-s symmetry due to the small metal−oxygen bond distance. The large crystal field value associated with this small bond distance not only inhibit the stabilization of intermediate or high spin state Co4+, but might also alter the charge transfer between the 2e orbitals of Co4+ in f-s and the adsorbate 2p orbitals, which is consistent with the similar activity measured for Sr4Mn2CoO9 and Sr6Co5O15. In contrast, by increasing the oxidation state of cobalt from Co3+ to Co4+ in the same coordination (i.e., c-s coordination for LaCoO 3 and La0.4Sr0.6CoO3), the measured activity increases. This result is in line with the calculated activity of Co4+ in Oh symmetry (c-s in SrCoO3) by Man et al. that is notably higher than for Co3+ in c-s LaCoO3.7 This further demonstrates the importance of the d-block splitting for a given oxidation state. A stronger overlap of O 2p and Co 3d orbitals was observed in the XAS O K-edge for Sr4Mn2CoO9 and Sr6Co5O15 in comparison to LaCoO3 (i.e., pre-edge region 520−525 eV in Figure S3 of the Supporting Information), despite having comparable catalytic activity. This suggests that the orbital overlap plays a less significant role for the charge transfer than the σ* orbital filling and the d-block splitting. As previously described, the surfaces made of Co2+ in distorted prism (i.e., Sr4Mn2CoO9 and Sr6Co5O15) are destabilized under OER conditions, whereas the surface of c-s
Figure 5. Evolution of the current density at 1.55 V versus RHE in 0.1 M KOH obtained from the second cycle for the studied manganese and cobalt oxides with several environments. 25930
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
LaCoO3 (Co3+ in Oh) was already proven to be stable versus cycling. The catalyst’s stability could be governed by the magnitude of the d-block splitting; further experiments will be required to examine this effect.
Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (9) Grimaud, A.; May, K. J.; Carlton, C. E.; Lee, Y. -L.; Risch, M.; Hong, W. T.; Zhou, J.; Shao-Horn, Y. Double Perovskites as a Family oh Highly Active Catalysts for Oxygen Evolution in Alkaline Solution. Nature Commun. 2013, 4, 2439. (10) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399−404. (11) Bergmann, A.; Zaharieva, I.; Dau, H.; Strasser, P. Electrochemical Water Splitting by Layered and 3d Cross-linked Manganese Oxides: Correlating Structural Motifs and Catalytic Activity. Energy. Environ. Sci. 2013, 6, 2745−2755. (12) Mette, K.; Bergmann, A.; Tessonnier, J. P.; Havecker, M.; Yao, L. D.; Ressler, T.; Strasser, P.; Berhens, M. Nanostructured Manganese Oxide Supported on Carbon Nanotubes for Electrocatalytic Water Splitting. ChemCatChem. 2012, 4, 851−862. (13) Darriet, J.; Subramanian, M. A. Structural Relationships Between Compounds Based on the Stacking of Mixed Layer Related to Hexagonal Type Perovskites-Type Structures. J. Mater. Chem. 1995, 5, 543−552. (14) zur Loye, H.-C.; Zhao, Q.; Bugaris, D. E.; Chance, W. M. 2HPerovskites Related Oxides: Synthesis, Structures, and Predictions. CrystEngComm. 2012, 14, 23. (15) Sawatzky, G. A.; Van Der Woude, F.; Morrish, A. H. Cation Distributions in Octahedral and Tetrahedral Sites of the Ferrimagnetic Spinel CoFe2O4. J. Appl. Phys. 1968, 39, 1204. (16) Burnus, T.; Hu, Z.; Haverkot, M. H.; Cezar, J. C.; Flahaut, D.; Hardy, V.; Maignan, A.; Brookes, N. B.; Tanaka, A.; Hsieh, H. H.; et al. Valence, Spin and Orbital State of Co Ions in One Dimensional Ca3Co2O6: An X-ray Absorption and Magnetic Circular Dichroism Study. Phys. Rev. B. 2006, 74, 245111. (17) Viola, M. C.; Martinez-Lope, M. J.; Alonso, J. A.; Martinez, J. L.; De Paoli, J. M.; Pagola, S.; Pedrogosa, J. C.; Fernandez-Diaz, M. T.; Carbonio, R. E. Structure and Magnetic Properties of Sr2CoWO6: An Ordered Double Perovskite Containing Co2+ (HS) with Unquenched Orbital Magnetic Moment. Chem. Mater. 2003, 15, 1655−1663. (18) Boulahya, K.; Parras, M.; Gonzalez-Calbet, J. M.; Martinez, J. L. Synthesis, Structural Characterization, and Magnetic Study of Sr4Mn2CoO9. Chem. Mater. 2003, 15, 3537−3542. (19) Zhao, Q.; Darriet, J.; Whangbo, M. -H.; Ye, L.; Stackhouse, C.; zur Loye, H.−C. Intriguing Interconnections Amoung Phase Transition, Magnetic Moment and Valence Disproportionation in 2H-Perovskites Related Oxides. J. Am. Chem. Soc. 2011, 133, 20981− 20994. (20) Whangbo, M.−H.; Koo, H. -J; Lee, K.-S.; Gourdon, O.; Evain, M.; Jobic, S.; Brec, R. On the Structural and Electronic Factors Governing the Magnetic Properties of the of the Hexagonal Perovskite-Type Oxides AxBO3 (A = Ca, Sr, Ba; B = Co, Ni). J. Solid State Chem. 2001, 160, 239−246. (21) Zhang, Y.; Kan, E.; Xiang, H.; Villesuzanne, A.; Whangbo, M.-H. Density Functional Analysis of the Interplay Between Jahn-Teller Unstability, Uniaxial Magnetism, Spin Arrangement, Metal-Metal Interaction, and Spin-Orbit Coupling in Ca3CoMO6 (M = Co, Rh, Ir). Inorg. Chem. 2011, 50, 1758−1766. (22) Maignan, A.; Caignaert, V.; Hebert, S.; Pralong, V.; Hetjmanek, J.; Khomskii, D. Spin, Charge, and Lattice Coupling in Triangular and Kagome Sublattices of CoO4 Tetrahedra: YBaCo4O7+δ (δ = 0, 1). Phys. Rev. B. 2006, 74, 165110. (23) Li, Y.; Cheng, J.; Song, J. A.; Fernandez-Diaz, M. T.; Goodenough, J. B. Characterization of the Double Perovskite Ba2BixSc2Co1.8‑xO6‑δ (x = 0.1, 0.2). Chem. Mater. 2012, 24, 4114−4122. (24) Villesuzanne, A.; Whangbo, M.−H. Comparative Electronic Band Structure Study of the Intrachain Ferromagnetic Versus Antiferromagnetic Coupling in the Magnetic Oxides Ca3Co2O6 and Ca3FeRhO6. Inorg. Chem. 2005, 44, 6339−6345. (25) Boulahya, K.; Parras, M.; Gonzalez-Calbet, J. M.; Martinez, J. L. Synthesis and Structural Characterization of Ba6Mn5O16: The First
4. CONCLUSION In summary, we show the influence of transition metal coordination on the OER activity and stability of perovskites. The OER activities of cobalt-based perovskites are higher than perovskites containing manganese, which can be explained by the greater flexibility of cobalt ions to adopt several oxidation and spin configurations in the perovskite framework. The destabilization of Co2+ in distorted prisms on the surface of hexagonal perovksites demonstrates the critical role of the electronic structure of transition metal oxides on the OER activity and stability.
■
ASSOCIATED CONTENT
S Supporting Information *
Additional electrochemical data XAS and HRTEM images. This material is available free of charge via the Internet at http:// pubs.acs.org.
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS This work is supported by the U.S. Department of Energy Hydrogen Initiative program under award DE-FG0205ER15728 and by the Office of naval Research (ONR) under contract numbers N00014-12-1-0096. The Advanced Light Source is supported by the Director, Office of Science, Office of Basic Energy Sciences, of the U.S. Department of Energy under contract No. DE-AC02-05CH11231. The authors would like to acknowledge Wanli Yang, Kelsey Stoerzinger, and Dr. Paul Olalde-Velasco for XAS measurements as well as David Kwabi for fruitful discussion.
■
REFERENCES
(1) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729−15735. (2) Dau, H.; Limberg, C.; Reier, T.; Risch, M.; Roggan, S.; Strasser, P. The Mechanism of Water Oxidation: From Electrolysis via Homogeneous to Biological Catalysis. ChemCatChem 2010, 2, 724− 761. (3) Gray, H. B. Powering the Planet with Solar Fuel. Nature Chem. 2009, 1, 7−7. (4) Bockris, J. O. M.; Otagawa, T. Mechanism of Oxygen Evolution on Perovskites. J. Phys. Chem. 1983, 87, 2960. (5) Bockris, J. O. M.; Otagawa, T. The Electrocatalysis of Oxygen Evolution on Perovskites. J. Electrochem. Soc. 1984, 131, 290−302. (6) Rossmeisl, J.; Qu, Z. W.; Zhu, H.; Kroes, G. J.; Nørskov, J. K. Electrolysis of Water on Oxide Surfaces. J. Electroanal. Chem. 2007, 607, 83−89. (7) Man, I. C.; Su, H.-Y.; Calle-Vallejo, F.; Hansen, H. A.; Martínez, J. I.; Inoglu, N. G.; Kitchin, J.; Jaramillo, T. F.; Nørskov, J. K.; Rossmeisl, J. Universality in Oxygen Evolution Electrocatalysis on Oxide Surfaces. ChemCatChem 2011, 3, 1159−1165. (8) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution 25931
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
The Journal of Physical Chemistry C
Article
Layered Oxide Isostructural to Cs6Ni5F16. Chem. Mater. 2002, 14, 4006−4008. (26) Harrison, W. T. A.; Hegwood, S. L.; Jacobson, A. J. A Powder Neutron Diffraction Determination of the Structure of Sr6Co5O15, Formerly Described as the Low-Temperature Hexagonal Form of SrCoO3‑x. Chem. Soc., Chem. Comm. 1995, 14, 4006−4008. (27) Vásquez-Vásquez, C.; Carmen Blanco, M.; Arturo LópezQuintela, M.; Sánchez, R. D.; Rivas, J.; Oseroff, S. B. Characterization of La0.67Ca0.33MnO3±δ Particles Prepared by the Sol-Gel Route. J. Mater. Chem. 1998, 8, 991−1000. (28) Roisnel, T.; Rodriguez-Carvajal, J. WinPLOTR: A Windows Tool for Powder Diffraction Pattern Analysis. Mater. Sci. Forum 2001, 378−389, 118−123. (29) Suntivich, J.; Gasteiger, H. A.; Yabuuchi, N.; Shao-Horn, Y. Electrocatalytic Measurement Methodology of Oxide Catalysts Using a Thin-Film Rotating Disk Electrode. J. Electrochem. Soc. 2010, 157, B1253−B1258. (30) Elam, W. T.; Kirkland, J. P.; Neiser, R. A.; Wolf, P. D. Depth Dependence for EXAFS Detected via Electron Yield in He and Vacuum. Phys. Rev. B 1988, 28, 26−30. (31) Ammundsen, B.; Paulsen, J.; Liu, R.-S.; Shen, C.-H.; Chen, J.M.; Yang, L.-Y; Lee, J.-F. Local Structure and First Cycle Redox Mechanism of Layered Li1.2Cr0.4Mn0.4O2 Cathode Material. J. Electrochem. Soc. 2002, 149, A431−A436. (32) May, K. J.; Carlton, C. E.; Stoerzinger, K. E.; Risch, M.; Suntivich, J.; Lee, Y.-L.; Grimaud, A.; Shao-Horn, Y. Influence of Oxygen Evolution During Water Oxidation on the Surface of Perovskite Oxide Catalysts. J. Phys. Chem. Lett. 2012, 3, 3264−3270. (33) Lee, S. W.; Carlton, C. E.; Risch, M.; Surendranath, Y.; Chen, S.; Furutsuki, S.; Yamada, A.; Nocera, D. G.; Shao-Horn, Y. The Nature of Lithium Battery Materials Under Oxygen Evolution Reaction Conditions. J. Am. Chem. Soc. 2012, 134, 16959−16962. (34) Risch, M.; Grimaud, G.; May, K. J.; Stoerzinger, K. A.; Chen, T. J.; Mansour, A. N.; Shao-Horn, Y. Structural Changes of Cobalt-Based Perovskites Upon Water Oxidation Investigated by EXAFS. J. Phys. Chem. C 2013, 117, 8628−8635. (35) Kanan, M. W.; Nocera, D. G. In Situ Formation of an OxygenEvolution Catalyst in Neutral Water Containing Phosphate and Co2+. Science 2008, 321, 1072−1075. (36) Risch, M.; Shevchenko, D.; Anderlund, M. F.; Styring, S.; Heidkamp, J.; Lange, K. M.; Thapper, A.; Zaharieva, I. Atomic Sructure of Cobalt-Oxide Nanoparticles Active in Light-Driven Catalysis of Water Oxidation. Int. J. Hydrogen Energy 2012, 37, 8878−8888.
25932
dx.doi.org/10.1021/jp408585z | J. Phys. Chem. C 2013, 117, 25926−25932
