Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Alexis Grimaud; Kevin J. May; Christopher E. Carlton; Yueh-Lin Lee; Marcel Risch; Wesley T. Hong; Jigang Zhou; Yang Shao-Horn

doi:10.1038/ncomms3439

Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Grimaud, Alexis; May, Kevin J.; Carlton, Christopher E.; Lee, Yueh-Lin; Risch, Marcel; Hong, Wesley T.; Zhou, Jigang; Shao-Horn, Yang 2013-09-17 00:00:00 ARTICLE Received 28 May 2013 | Accepted 13 Aug 2013 | Published 17 Sep 2013 DOI: 10.1038/ncomms3439 Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution 1 1,2 1 1 1 1,3 Alexis Grimaud , Kevin J. May , Christopher E. Carlton , Yueh-Lin Lee , Marcel Risch , Wesley T. Hong , 4 1,2,3 Jigang Zhou & Yang Shao-Horn The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln Ba )CoO (Ln ¼ Pr, Sm, Gd and Ho) as a 0.5 0.5 3 d family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba Sr Co Fe O which readily amorphize during the 0.5 0.5 0.8 0.2 3 d oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies. 1 2 Electrochemical Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Canadian Light Source Inc., Saskatoon, Saskatchewan, Canada S7N 2V3. Correspondence and requests for materials should be addressed to Y.S.-H. (email: [email protected]). NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 he discovery of new cost-effective and highly active catalysts for electrochemical energy conversion and storage Ln Tis of prime importance to address climate change challenges and develop storage options for renewable energy Ba production. Among the electrochemical processes, the oxygen evolution reaction (OER) on water oxidation is efﬁciency-limiting 1,2 for direct solar and electrolytic water splitting (H O-H þ Co 2 2 3,4 ½O ) , rechargeable metal-air batteries (M O -M þ O ) and 2 x 2 x 2 regenerative fuel cells . Transition metal oxides such as ABO perovskites composed of rare and alkaline earth (A) and 3d transition metal cations (B) are of particular interest as they have intrinsic activities comparable to the gold standards of OER O C bc catalysts such as IrO and RuO (ref. 4). Sabatier’s principle, h 4v 2 2 which qualitatively describes that high catalytic activity can be obtained when adsorbed species bind to the surface neither too strongly nor too weakly, has been instrumental to fundamental 2 2 x –y understanding of OER mechanisms and the development of highly active catalysts. Unfortunately, the binding strength of 6,7 2 2 x –y oxygen molecules and reaction intermediate species is difﬁcult d , d 2 2 2 2 d x –y z –r 2 2 z –r to assess experimentally. Instead, experimental studies have 2 2 z –r correlated with the intrinsic OER activities of transition metal xy 8–10 d , d , d xy yz xz oxides with parameters that can be estimated and measured d , d yz xz from experiments such as the e orbital ﬁlling of transition metal ions . The use of e orbital ﬁlling as an OER activity descriptor identiﬁed an optimal e orbital occupancy near unity and led Figure 1 | Double perovskite crystal structure and cobalt crystal ﬁeld. to the identiﬁcation of Ba Sr Co Fe O (BSCF) with 0.5 0.5 0.8 0.2 3 d (a) Schematic representation of (Ln Ba )CoO double perovskites 0.5 0.5 3 d a record intrinsic OER activity . However, BSCF particles readily (LnBaCo O 0 with d ¼ 1 2d) showing the ordering of Ln and Ba cations 2 5þ d become amorphous under OER conditions, producing Co-O and the formation of oxygen deﬁciency in the LnO planes. This ordering 1 d 11,12 motifs having local order with edge-sharing octahedra similar is reﬂected by the doubling of the c parameter compared with ideal cubic 13–15 to electrodeposited Co oxides such as Co-Pi , during which perovskites, resulting in ap ap 2ap lattice parameters (ap being the the OER activity and electrochemical active surface area increase. lattice parameter of cubic perovskite indexed in Pm-3m space group). The Here we report that the intrinsic OER activities of structure has (b) octahedral (O ) and (c) square pyramidal (C ) symmetry h 4v (Ln Ba )CoO are among the highest reported to date, with 0.5 0.5 3 d for Co ions, with different crystal-ﬁeld splitting of d-electron states for the most active (Pr Ba )CoO in the series exhibiting activities 0.5 0.5 3 d the different coordination symmetries. greater than BSCF, the most active cubic perovskite. Unlike BSCF, these double perovskites are stable under OER conditions based on measurements from cyclic voltammetry (CV), galvanostatic testing and transmission electron microscopy (TEM) imaging. The physical collected from oxide thin ﬁlms supported on glassy carbon origin of the high activity and stability of these Co-based double electrodes in O -saturated 0.1 M KOH (Fig. 2a). perovskites is compared with Co-based pseudocubic perovskites and (Pr Ba )CoO was found to have the lowest voltage for 0.5 0.5 3 d discussed in term of e ﬁlling of Co ions and the O p-band centre the onset of oxidation current, indicative of the highest OER relative to the Fermi level as predicted from density functional activity. The intrinsic OER activities normalized to the true oxide theory (DFT). surface areas, which were estimated from Brunauer, Emmet and Teller (BET) measurements, are shown as a function of potential in Fig. 2b. Not only are the OER activities of these double Results perovskites one order of magnitude higher than that of LaCoO 10,11 OER activity measurements. In this study, we examine double (LCO), but they are also comparable to that of BSCF and perovskites with a formula unit of LnBaCo O (Ln¼ Pr, Sm, Gd SrCo Fe O (SCF) , the most active OER catalysts known 2 5þ d 0.8 0.2 3 d and Ho and Ln), which can be written as (Ln Ba )CoO to date. In addition, Tafel slopes of the double perovskites 0.5 0.5 3 d (d ¼ 1 2d) in the formula unit of pseudocubic perovskites. Because (B60 mV per decade) are comparable to those of of the large difference in ionic radius between lanthanides and bar- La Sr CoO (ref. 11) and Co-Pi reported previously. 1 x x 3 d ium, double perovskites differ from cubic perovskites by the ordering Moreover, substituting cobalt with iron in the double perovskite 16,17 of A-site cations along the c direction . The difference in ionic structure, such as (Pr Ba )Co Fe O (with x¼ 0.25 0.5 0.5 1 x x 3 d radius and polarizability between lanthanides and barium tends to and 0.5) had negligible effect on the OER activity 16,18,19 localize the oxygen vacancies in the lanthanide plane (Fig. 1a), (Supplementary Fig. S1). in contrast to the randomly distributed vacancies of pseudocubic (Ln Ba )CoO were found to be stable under OER 0.5 0.5 3 d 19,20 11,12 perovskites . The presence of oxygen vacancies induces a potentials unlike BSCF . No signiﬁcant changes in the CV stabilization of the d and d and the d molecularorbitalsinthe data or in the capacitive current were detected during repeated xz yz z t and e -parentage orbitals, respectively, which leads to the CV scans (Fig. 2c). This observation is in contrast to considerable 2g g formationofanoxygen-deﬁcient octahedral symmetry—the so- changes in the CV scans of LiCoPO (ref. 21), LiCoO (ref. 21) 4 2 11,12 called square pyramidal symmetry (C ) in Fig. 1c. The concentration and (B)SCF , where the near-surface regions or the bulk of 4v of oxygen vacancies and the cobalt oxidation state in double per- oxide particles become amorphous. The stability of the double ovskites can be tuned by the nature of the lanthanide and both are perovskites during OER was further supported by galvanostatic expected to inﬂuence the OER activity. measurements. The results obtained from (Pr Ba )CoO 0.5 0.5 3 d (Ln Ba )CoO (Ln¼ Pr, Sm, Gd and Ho) exhibited are shown in Supplementary Fig. S2 as an example, where no 0.5 0.5 3 d comparable OER activities from CV measurements, which were signiﬁcant changes occur in the voltage (Supplementary Fig. S2a) 2 NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 ARTICLE a [221] PBCO Dry Pr (110) Ho (102) Sm Gd [021] (200) PBCO THF (100) [110] PBCO 25c 1.2 1.4 1.6 (110) E - iR (V versus RHE) (002) (001)* LCO 1.65 Gd Sm [221] Ho PBCO 2 h (110) Pr (102) 1.60 1.55 BSCF Figure 3 | Structure of double perovskites on oxygen evolution. High- resolution TEM images (scale bars, 5 nm) and fast Fourier transforms 1.50 (FFTs) of (a) dry (Pr Ba )CoO powder, (b) (Pr Ba )CoO 0.5 0.5 3 d 0.5 0.5 3 d exposed to tetrahydrofuran (THF), (c) (Pr Ba )CoO cycled between 0.5 0.5 3 d 0.1 1 10 1.1 and 1.7 V versus reversible hydrogen electrode (RHE) at 10 mVs for –2 i (mA cm ) 25 times and (d) (Pr Ba )CoO galvanostatically tested at oxide 0.5 0.5 3 d 5mAcm for 2 h. (Pr Ba )CoO mixed with Naﬁon and disk 0.5 0.5 3 d acetylene black carbon supported on a glassy carbon electrode with a 2nd cycle (Pr Ba )CoO loading of 0.25 mg cm was tested in O -saturated 0.1 M 0.5 0.5 3– oxide 2 KOH electrolyte. FFTs images were indexed using Pmmm as space group 10th cycle and ap ap 2ap lattice parameters. Red circles indicate reﬂection that uniquely belongs to the double perovskite structure. 25th cycle imaging. Although the surfaces of as-prepared double perovskites are perfectly crystalline (Fig. 2a and Supplementary Fig. S3), exposing the oxides to tetrahydrofuran (THF) during the electrode preparation for OER measurements appeared to induce a small degree of amorphization, as shown in Fig. 3b and Supplementary Fig. S4. No further change in the surfaces of double perovskites was observed after OER measurements, where 1.2 1.4 1.6 TEM imaging showed that the surfaces of (Pr Ba )CoO 0.5 0.5 3 d E - iR (V versus RHE) after 25 CV scans (Fig. 3c and Supplementary Fig. S5a) and 2 h held at 5 mA cm (Fig. 3d and Supplementary Fig. S5b) oxide Figure 2 | Oxygen evolution activities of double perovskites. (a) Cyclic were comparable to those only exposed to THF (Fig. 3b). The voltamograms of (Ln Ba )CoO with Ln¼ Pr, Sm, Gd and Ho 0.5 0.5 3 d 0 stability of the double perovskite structure during OER measure- (LnBaCO O with d ¼ 1 2d), where measurements were performed 2 5 þ d ments was further supported by the presence of cation-ordering using ink-casted oxides (containing Naﬁon and acetylene black carbon) 2 reﬂections in the fast Fourier transforms (marked by * in Fig. 3c with an oxide loading of 0.25 mg cm supported on a glassy carbon disk and Supplementary Fig. S5a) and X-ray diffraction (XRD) pattern electrode in O -saturated 0.1 M KOH electrolyte) with a scanning rate of 1 of cycled electrodes (Supplementary Fig. S6), unique to the double 10 mVs .(b) Intrinsic OER activities normalized by the true oxide surface perovskite structure with space group Pmmm and having a unit area, which were obtained from averaging currents in the forward and cell of ap ap 2ap. This observation indicates that no leaching backward scans of the second cycle and subtracting the ohmic losses. The of praseodymium or barium occurs during oxygen evolution, OER activities of Ba Sr Co Fe O (BSCF) and LaCoO (LCO) 0.5 0.5 0.8 0.2 3 d 3 d unlike in BSCF , and that these double perovskites are stable are included for comparison. Error bars represent s.d. from at least four under the OER conditions used in this study. independent measurements. (c) The 2nd, 10th and 25th CV scans of (Pr Ba )CoO . 0.5 0.5 3 d Correlation between OER activity and estimated e ﬁlling.To or in the CVs (Supplementary Fig. S2b) recorded before and after estimate the e ﬁlling of cobalt ions in (Ln Ba )CoO double g 0.5 0.5 3 d galvanostatic testing of 5 mA cm for 2 h. Further evidence perovskites, the cobalt oxidation and spin states are needed. oxide of the double perovskite stability against OER came from TEM The cobalt oxidation state was determined by iodometric and NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 3 & 2013 Macmillan Publishers Limited. All rights reserved. E - iR (V versus RHE) –2 i (mA cm ) oxide –2 i (mA cm ) oxide ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 Mohr’s salt titrations, which was further conﬁrmed by Co K-edge high spin in the C symmetry because of polarizability and steric 4v 25 3þ 6 X-ray absorption spectroscopy (XAS, see Methods). The cobalt issues ;Co (3d ) having intermediate spin in the O and C ; h 4v 4þ 5 26 oxidation state was found to increase with increasing lanthanide and Co (3d ) stabilized in high spin in the O symmetry ionic radius in Fig. 4a, which is in agreement with previous work (Supplementary Fig. S9). Such assignments give rise to the e by Maignan et al. As Co K-edge XAS data showed comparable occupancy of cobalt in these double perovskites near unity, white line energies for Sm and Pr double perovskites, no having values from B1.1 to B1.3 (Supplementary Fig. S9). signiﬁcant change in the cobalt oxidation state was found for The exceptionally high OER activities of these double perovskites, these two double perovskites (Supplementary Figs S7a,S8). This is with their estimated e values near unity (Supplementary in contrast to the large difference obtained by chemical titration Fig. S10), support the design principle proposed by Suntivich (Fig. 4a). The poor sensitivity of Co K-edge XAS for the oxidation et al. , where the OER activity of perovskites peaks near e state of cobalt greater than 3þ can be attributed to higher occupancy of B1.2. hybridization of Co-O bonds (Estimating hybridization of transition-metal and oxygen states in perovskites from O K-edge X-ray absorption spectroscopy, manuscript in preparation) in the Correlation between OER activity and computed O p-band. Pr double perovskite than the Sm double perovskite, which is Despite its utility, the e -ﬁlling estimation of cobalt ions is 10,27 supported by O K-edge XAS measurements discussed below. inferred from previous studies and cannot be measured or Unfortunately, it is not straightforward to estimate the cobalt computed directly. The presence of multiple spin states of cobalt spin states in double perovskites because of the presence of O ions in these double perovskites further introduces ambiguities in 16,18–20 and C symmetries and multiple spin conﬁgurations the estimation of e ﬁlling. Recent studies have shown that the 4v (Fig. 1). Increasing the oxygen content from the Ho to Pr double computed O p-band centre relative to the Fermi level of 3 þ perovskite leads to a greater number of Co in O symmetry at perovskites scales linearly with the energy of oxygen vacancy the expense of C symmetry, whereas the accompanied increase formation and the oxygen surface exchange kinetics at elevated 4v 2 þ of the Co oxidation state reduces the number of Co and temperatures. We here report the O p-band centre relative to the 3 þ 4þ increases the number of Co and Co . By a ﬁrst approxima- Fermi level of perovskites as an alternative descriptor for the OER tion, these double perovskites were selected to have the following activities of perovskites in alkaline solution in an analogous role 20,23,24 2 þ 7 29,30 spin states based on previous studies :Co (3d ) having as the d-band centre for noble metal catalysts . The O p-band a b 3.4 –2.6 (Ln Ba )CoO 0.5 0.5 3– Pr (Ln Ba )CoO –2.4 0.5 0.5 3 3.2 Sm –2.2 Sm BSCF Gd Gd LCO 3.0 Pr –2.0 Ho 2.8 Ho –1.8 2.6 –1.6 2.5 2.75 3.0 Ho Gd Sm Pr Oxygen content (Ln Ba )CoO 0.5 0.5 3– cd 0.6 Pr Pr 0.4 Sm Sm Gd Gd 0.2 Ho Ho O K-edge 0.0 530 535 540 545 550 555 2.8 3.0 3.2 Energy (eV) Cobalt oxidation state Figure 4 | Inﬂuence of lanthanides on electronic structure. (a) Cobalt oxidation obtained by chemical titration (closed symbols) and by XAS at the Co K-edge (open symbols) plotted versus the oxygen content for (Ln Ba )CoO (LnBaCo O with d ¼ 1 2d and Ln¼ Pr, Sm, Gd and Ho), 0.5 0.5 3 d 2 5þ d Ba Sr Co Fe O (BSCF) and LaCoO (LCO). The oxygen vacancy concentration d was directly calculated from the cobalt oxidation state x by 0.5 0.5 0.8 0.2 3 d 3 d¼ (3.5 x)/2. The error bars in the oxidation state obtained by chemical titration represent the s.d. from at least three independent measurements. The errors in the oxidation state obtained by XAS were calculated by taking the root of the squared s.e. obtained by linear regression (see Methods). (b) The computed O p-band centre versus the Fermi level for the nonstoichiometric (Ln Ba )CoO double perovskites (having d constrained 0.5 0.5 3 d to the values obtained from chemical titration) and stoichiometric (Ln Ba )CoO in the constrained or relaxed structure (see Supplementary 0.5 0.5 3 Information). (c) O K-edge XAS data of (Ln Ba )CoO measured in the total electron yield mode. (d) Integrated intensities of the O K-edge pre-edge 0.5 0.5 3 d region normalized to oxygen content per formula unit and the nominal number of empty cobalt 3d states in both e and t symmetry. g 2g 4 NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. Cobalt oxidation state Normalized intensity (r.u.) O p-band center versus E (eV) Absorbance/e holes + t holes g 2g δ = 0.375 Constrained Relaxed δ = 0.25 Constrained Relaxed δ = 0.25 Constrained Relaxed δ = 0.125 Constrained Relaxed NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 ARTICLE centre was computed from DFT and details for the calculations a b 1.50 Pr can be found in the Supplementary Information. Owing to the Ho strong correlation error present in standard DFT-generalized BSCF82 Sm gradient approximation calculations of the transition metal 3d- M 3d Gd SCF82 1.55 band, which was recently shown to be important for describing the OER activities of Co oxides , the more delocalized nature of LSC46 the O p-band more accurately captures the electronic structure LCO O p-band O 2p characteristics of oxides critical to catalysis while still reﬂecting 1.60 BSCF46 center the metal d-character through hybridized density of states Stable Amorphized (Fig. 5a). –2 @ 0.5 mA cm oxide The O p-band centre relative to the Fermi level of 1.65 (Ln Ba )CoO double perovskites using the cobalt oxidation 0.5 0.5 3 d –2.5 –2.0 –1.5 state and oxygen nonstoichiometry determined from chemical O p-band relative to E (eV) titration (Fig. 4a) was found to linearly increase with increasing cobalt oxidation from Ho to Pr, as shown in Fig. 4b. This uplift of Figure 5 | Computed oxygen p-band centre for oxygen evolution. the O p-band centre with increasing cobalt oxidation would (a) Schematic representation of the O p-band for transition metal oxides enhance Co-O hybridization, which is supported by O K-edge and (b) evolution of the iR-corrected potential at 0.5 mA cm oxide XAS measurements. The hybridization of Co-O bonds was found versus the O p-band centre relative to E (eV) of (Ln Ba )CoO with F 0.5 0.5 3 d to increase from Ho to Pr double perovskite based on the O 11 11 Ln¼ Pr, Sm, Gd and Ho, for LaCoO (LCO) ,La Sr CoO (LSC46) , 3 0.4 0.6 3 d K-edge XAS measurements. The pre-edge feature can be 11 11 Ba Sr Co Fe O (BSCF82) ,Ba Sr Co Fe O (BSCF46) 0.5 0.5 0.8 0.2 3 d 0.5 0.5 0.4 0.6 3 d attributed to unoccupied O 2p states resulting from the mixing and SrCo Fe O (SCF82) . The O p-band centre relative to the Fermi 0.8 0.2 3 d between O 2p and metal 3d states, and its intensity is linear with level was computed by DFT for the fully oxidized and relaxed structure the number of holes in the O 2p-band . The intensity of the using the method described in Methods section. Error bars represent s.d. pre-edge region (two peaks near B532–534 eV) increases with from at least four independent measurements. increasing ionic radius of the lanthanide (Fig. 4c and Supplementary Fig. S7b). The hybridization of Co-O bonds of these double perovskites can be assessed by the integrated intensities of the pre-edge region normalized to oxygen content found for the oxides on the left branch in Fig. 5b during OER, per formula unit and the nominal number of empty cobalt 3d whereas rapid amorphization in the near-surface regions states in both e and t symmetry. The hybridization was shown accompanied with leaching of A-site ions, having local structures g 2g to increase and scale with increasing cobalt oxidation state resembling that of Co-Pi, was observed for oxides on the right 11,12 associated with greater ionic radius of lanthanide (Fig. 4d). The branch . trend can be attributed to decreasing charge transfer gap between oxygen 2p and cobalt 3d states associated with increasing cobalt oxidation state . Discussion Assuming the surface of the catalyst to be fully oxidized in We show double perovskites (Ln Ba )CoO (with Ln¼ Pr, 0.5 0.5 3 d KOH under OER conditions, DFT studies of the O p-band centre Sm, Gd and Ho) as a new family of highly active catalysts for were then performed on stoichiometric (Ln Ba )CoO double OER in alkaline electrolyte, with comparable activities and 0.5 0.5 3 perovskites (Supplementary Fig. S11). Having oxygen stoichio- enhanced stability relative BSCF—the most active catalyst 10,11 metry in the double perovskite structure without structural reported to date . Although the high activities of these relaxation gave rise to the same trend as that found for double perovskites with e ﬁlling of cobalt ions near unity is in nonstoichiometric (Ln Ba )CoO , but the O p-band centre agreement with the design principle of OER catalysts reported 0.5 0.5 3 d was much uplifted because of increasing oxygen content (Fig. 4b). recently , considerable ambiguities exist in the estimation In contrast, the O p-band centre peaks on (Gd Ba )CoO of e ﬁlling associated with the presence of two crystal ﬁelds 0.5 0.5 3 g in the relaxed structure. The change in the O p-band centre trend (octahedral O and square pyramidal C ), multiple Co oxidation h 4v 3 þ versus (Ln Ba )CoO series in the fully relaxed stoichiometric states and multiple spin states of Co ions in these double 0.5 0.5 3 double perovskite structure was found to result from distortion perovskites. In this study, we show that the computed O p-band and rotation of Co-O octahedra caused by A-site and B-site ionic centre relative to the Fermi level and parameters derived from radii mismatch (Supplementary Fig. S11), which alters the this can be used as descriptors to screen the OER activity and Co-O bond length and metal 3d–oxygen 2p orbital hybridiza- stability of oxides. Moving the computed O p-band centre closer tion , leading to an upshift of the O p-band centre of to the Fermi level can increase OER activities, but having (Ho Ba )CoO relative to the Fermi level relative to the other computed O p-band centre too close to the Fermi level decreases 0.5 0.5 3 double perovskites. oxide stability during OER. This trend is similar to the correlation The intrinsic OER activities of cobalt-based double perovskites previously established for oxygen reduction on perovskites at and pseudocubic perovskites, which can vary by B2 orders of elevated temperatures . The pseudocubic perovskites and double magnitude (Fig. 2b), were found to correlate with the computed perovskites with the O p-band centre very close to the Fermi level O p-band centre relative to the Fermi level of the fully oxidized not only have the highest OER activities in alkaline solution but and relaxed structure, as shown in Fig. 5b. It is appropriate to use also exhibit the highest activities for surface oxygen exchange the fully oxidized and relaxed structure in this analysis as the kinetics upon oxygen reduction at elevated temperatures , which surface of these oxides is expected to be fully oxidized in KOH highlights the importance of the oxide electronic structure on under OER conditions. Moving the O p-band centre closer to the oxygen electrocatalysis. Future spectroscopic experiments of these Fermi level from LCO to (Pr Ba )CoO greatly increases the oxides are needed to verify the computed O p-band centre trend 0.5 0.5 3 intrinsic OER activity in the left branch. However, further lifting in this study, and seek activity and stability descriptors that can be 11,28 the O p-band centre in BSCF ,Ba Sr Co Fe O measured experimentally. Our study highlights the importance of 0.5 0.5 0.4 0.6 3 11 11 (BSCF46) and SCF did not result in any increase in the controlling a transition metal oxide having the O p-band close to OER activity but decreased the oxide stability. No changes were the Fermi level as a promising strategy to create highly active NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 5 & 2013 Macmillan Publishers Limited. All rights reserved. E - iR (V versus RHE) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 oxide catalysts for OER to enable the development of efﬁcient, inﬂection points . The Co oxidation states were calculated by relating the edge position obtained by the integral method with the known oxidation states of single- rechargeable metal-air batteries, regenerative fuel cells and other 2þ 2þ 2.6þ phase Co O, Co (OH) ,Co O and LiCoO using linear regression. The 2 3 4 2 rechargeable air-based energy storage devices. errors in the edge position are estimated and the errors in the oxidation state were calculated by taking the root of the squared s.e. obtained by linear regression. Methods Synthesis and bulk characterization. Double perovskite catalysts 28,37 DFT studies. DFT calculations with the Hubbard U (U ¼ 3.3 eV) correction eff (Ln Ba )CoO (with Ln¼ Pr, Sm, Gd and Ho) and (Pr,Ba)Co Fe O 0.5 0.5 3 d 1 x x 3 d for the Co 3d electrons were performed with the Vienna Ab initio Simulation were synthesized using a conventional solid-state route. Stoichiometric amounts of 38,39 40 Package using the projector-augmented plane-wave method . Exchange- Ln O , BaCO and Co O and Fe O previously dehydrated were thoroughly 2 3 3 3 4 2 3 correlation was treated in the Perdew–Wang-91 generalized gradient grounded and ﬁred in air at 1,000 C for 12 h. Products were ground and approximation . Fully relaxed stoichiometric bulk perovskite calculations were annealed in air at 1,100 C for (Ln Ba )CoO catalysts and 1,200 C for 0.5 0.5 3 d simulated with 2 2 2 perovskite supercells. The double perovskites were (Pr Ba )Co Fe O (x ¼ 0.25 and 0.5) for 24 h with intermediate grindings. 0.5 0.5 1 x x 3 d simulated based on the reported ordered structures within the 2 2 2 perovskite All catalysts reported in this study are single phase, as analysed by XRD 16,18 supercell . An effective O p-band centre was determined by taking the centroid (Supplementary Fig. S14), with lattice parameters consistent with those reported of the projected density of states of O 2p states relative to the Fermi level. More previously . XRD measurements were performed using a PANalytical X’Pert Pro details of the calculations are provided in the Supplementary Information. powder diffractometer in the Bragg–Brentano geometry using a Copper K radiation, where data were collected using the X’Celerator detector in the 8–80 window in the 2y range. The speciﬁc surface area of each oxide sample was References determined using BET analysis on a Quantachrome ChemBET Pulsar from a single-point BET analysis performed after 12 h outgassing at 150 C, which is 1. Gray, H. B. Powering the planet with solar fuel. Nat. Chem. 1, 7–7 (2009). 2. Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar reported in Supplementary Table S1. The oxygen vacancy d was deduced by the determination of the cobalt oxidation energy utilization. Proc. Natl Acad. Sci. USA 104, 20142–22014 (2007). state by chemical titration. For Ln ¼ Pr, Gd and Sm, the samples were dissolved in 3. Dau, H. et al. 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K., Bligaard, T., Rossmeisl, J. & Christensen, C. H. Towards the program under award DE-FG02-05ER15728 and by the Ofﬁce of Naval Research (ONR) computational design of solid catalysts. Nat. Chem. 1, 37–46 (2009). under contract numbers N00014-12-1-0096 (MIT) and N00014-12-WX20818 30. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel- (NSWCCD). The National Synchrotron Light Source is supported by the U.S. cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004). Department of Energy, Division of Material Sciences and Division of Chemical Sciences 31. Garcı´a-Mota, M. et al. Importance of correlation in determining electrocatalytic under contract DE-AC02-98CH10886. The Ofﬁce of Naval Research and contributions oxygen evolution activity of cobalt oxides. J. Phys. Chem. C. 116, 21077–21082 from Participating Research Team members support Beamline X11. Canadian Light (2012). Source is supported by the NSERC, NRC, CIHR of Canada, the Province of 32. 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Y.-L.L. carried out the DFT calculation. A.G. and Y.- measurement methodology of oxide catalysts using a thin-ﬁlm rotating disk S.H. wrote the manuscript and all authors edited the manuscript. electrode. J. Electrochem. Soc. 157, B1263–B1268 (2010). 36. Dau, H., Liebisch, P. & Haumann, M. X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers – potential Additional information and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis. Anal. Bioanal. Chem. 376, 562–583 Supplementary Information accompanies this paper at http://www.nature.com/ (2003). naturecommunications 37. Lee, Y. L., Kleis, J., Rossmeisl, J. & Morgan, D. Ab initio energetics of Competing ﬁnancial interests: The authors declare no competing ﬁnancial interests. LaBO (001) (B¼ Mn, Fe, Co and Ni) for solid oxide fuel cell cathodes. Phys. Rev. B 80, 224101 (2009). Reprints and permission information is available online at http://npg.nature.com/ 38. Kresse, G. & Hafner, J. Ab initio molecular-dynamics for liquid metals. Phys. reprintsandpermissions/ Rev. B 47, 558–561 (1993). 39. Kresse, G. & Furthermu¨ller, J. Efﬁcient iterative schemes for ab initio total- How to cite this article: Grimaud, A. et al. Double perovskites as a family of highly energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 4:2439 (1996). doi: 10.1038/ncomms3439 (2013). NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 7 & 2013 Macmillan Publishers Limited. All rights reserved. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Nature Communications Springer Journals http://www.deepdyve.com/lp/springer-journals/double-perovskites-as-a-family-of-highly-active-catalysts-for-oxygen-DvIPnXrV2B

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Publisher: Springer Journals
Copyright: Copyright © 2013 by Nature Publishing Group, a division of Macmillan Publishers Limited. All Rights Reserved.
Subject: Science, Humanities and Social Sciences, multidisciplinary; Science, Humanities and Social Sciences, multidisciplinary; Science, multidisciplinary
eISSN: 2041-1723
DOI: 10.1038/ncomms3439
Publisher site: See Article on Publisher Site

Abstract

ARTICLE Received 28 May 2013 | Accepted 13 Aug 2013 | Published 17 Sep 2013 DOI: 10.1038/ncomms3439 Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution 1 1,2 1 1 1 1,3 Alexis Grimaud , Kevin J. May , Christopher E. Carlton , Yueh-Lin Lee , Marcel Risch , Wesley T. Hong , 4 1,2,3 Jigang Zhou & Yang Shao-Horn The electronic structure of transition metal oxides governs the catalysis of many central reactions for energy storage applications such as oxygen electrocatalysis. Here we exploit the versatility of the perovskite structure to search for oxide catalysts that are both active and stable. We report double perovskites (Ln Ba )CoO (Ln ¼ Pr, Sm, Gd and Ho) as a 0.5 0.5 3 d family of highly active catalysts for the oxygen evolution reaction upon water oxidation in alkaline solution. These double perovskites are stable unlike pseudocubic perovskites with comparable activities such as Ba Sr Co Fe O which readily amorphize during the 0.5 0.5 0.8 0.2 3 d oxygen evolution reaction. The high activity and stability of these double perovskites can be explained by having the O p-band centre neither too close nor too far from the Fermi level, which is computed from ab initio studies. 1 2 Electrochemical Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA. Canadian Light Source Inc., Saskatoon, Saskatchewan, Canada S7N 2V3. Correspondence and requests for materials should be addressed to Y.S.-H. (email: [email protected]). NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 1 & 2013 Macmillan Publishers Limited. All rights reserved. ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 he discovery of new cost-effective and highly active catalysts for electrochemical energy conversion and storage Ln Tis of prime importance to address climate change challenges and develop storage options for renewable energy Ba production. Among the electrochemical processes, the oxygen evolution reaction (OER) on water oxidation is efﬁciency-limiting 1,2 for direct solar and electrolytic water splitting (H O-H þ Co 2 2 3,4 ½O ) , rechargeable metal-air batteries (M O -M þ O ) and 2 x 2 x 2 regenerative fuel cells . Transition metal oxides such as ABO perovskites composed of rare and alkaline earth (A) and 3d transition metal cations (B) are of particular interest as they have intrinsic activities comparable to the gold standards of OER O C bc catalysts such as IrO and RuO (ref. 4). Sabatier’s principle, h 4v 2 2 which qualitatively describes that high catalytic activity can be obtained when adsorbed species bind to the surface neither too strongly nor too weakly, has been instrumental to fundamental 2 2 x –y understanding of OER mechanisms and the development of highly active catalysts. Unfortunately, the binding strength of 6,7 2 2 x –y oxygen molecules and reaction intermediate species is difﬁcult d , d 2 2 2 2 d x –y z –r 2 2 z –r to assess experimentally. Instead, experimental studies have 2 2 z –r correlated with the intrinsic OER activities of transition metal xy 8–10 d , d , d xy yz xz oxides with parameters that can be estimated and measured d , d yz xz from experiments such as the e orbital ﬁlling of transition metal ions . The use of e orbital ﬁlling as an OER activity descriptor identiﬁed an optimal e orbital occupancy near unity and led Figure 1 | Double perovskite crystal structure and cobalt crystal ﬁeld. to the identiﬁcation of Ba Sr Co Fe O (BSCF) with 0.5 0.5 0.8 0.2 3 d (a) Schematic representation of (Ln Ba )CoO double perovskites 0.5 0.5 3 d a record intrinsic OER activity . However, BSCF particles readily (LnBaCo O 0 with d ¼ 1 2d) showing the ordering of Ln and Ba cations 2 5þ d become amorphous under OER conditions, producing Co-O and the formation of oxygen deﬁciency in the LnO planes. This ordering 1 d 11,12 motifs having local order with edge-sharing octahedra similar is reﬂected by the doubling of the c parameter compared with ideal cubic 13–15 to electrodeposited Co oxides such as Co-Pi , during which perovskites, resulting in ap ap 2ap lattice parameters (ap being the the OER activity and electrochemical active surface area increase. lattice parameter of cubic perovskite indexed in Pm-3m space group). The Here we report that the intrinsic OER activities of structure has (b) octahedral (O ) and (c) square pyramidal (C ) symmetry h 4v (Ln Ba )CoO are among the highest reported to date, with 0.5 0.5 3 d for Co ions, with different crystal-ﬁeld splitting of d-electron states for the most active (Pr Ba )CoO in the series exhibiting activities 0.5 0.5 3 d the different coordination symmetries. greater than BSCF, the most active cubic perovskite. Unlike BSCF, these double perovskites are stable under OER conditions based on measurements from cyclic voltammetry (CV), galvanostatic testing and transmission electron microscopy (TEM) imaging. The physical collected from oxide thin ﬁlms supported on glassy carbon origin of the high activity and stability of these Co-based double electrodes in O -saturated 0.1 M KOH (Fig. 2a). perovskites is compared with Co-based pseudocubic perovskites and (Pr Ba )CoO was found to have the lowest voltage for 0.5 0.5 3 d discussed in term of e ﬁlling of Co ions and the O p-band centre the onset of oxidation current, indicative of the highest OER relative to the Fermi level as predicted from density functional activity. The intrinsic OER activities normalized to the true oxide theory (DFT). surface areas, which were estimated from Brunauer, Emmet and Teller (BET) measurements, are shown as a function of potential in Fig. 2b. Not only are the OER activities of these double Results perovskites one order of magnitude higher than that of LaCoO 10,11 OER activity measurements. In this study, we examine double (LCO), but they are also comparable to that of BSCF and perovskites with a formula unit of LnBaCo O (Ln¼ Pr, Sm, Gd SrCo Fe O (SCF) , the most active OER catalysts known 2 5þ d 0.8 0.2 3 d and Ho and Ln), which can be written as (Ln Ba )CoO to date. In addition, Tafel slopes of the double perovskites 0.5 0.5 3 d (d ¼ 1 2d) in the formula unit of pseudocubic perovskites. Because (B60 mV per decade) are comparable to those of of the large difference in ionic radius between lanthanides and bar- La Sr CoO (ref. 11) and Co-Pi reported previously. 1 x x 3 d ium, double perovskites differ from cubic perovskites by the ordering Moreover, substituting cobalt with iron in the double perovskite 16,17 of A-site cations along the c direction . The difference in ionic structure, such as (Pr Ba )Co Fe O (with x¼ 0.25 0.5 0.5 1 x x 3 d radius and polarizability between lanthanides and barium tends to and 0.5) had negligible effect on the OER activity 16,18,19 localize the oxygen vacancies in the lanthanide plane (Fig. 1a), (Supplementary Fig. S1). in contrast to the randomly distributed vacancies of pseudocubic (Ln Ba )CoO were found to be stable under OER 0.5 0.5 3 d 19,20 11,12 perovskites . The presence of oxygen vacancies induces a potentials unlike BSCF . No signiﬁcant changes in the CV stabilization of the d and d and the d molecularorbitalsinthe data or in the capacitive current were detected during repeated xz yz z t and e -parentage orbitals, respectively, which leads to the CV scans (Fig. 2c). This observation is in contrast to considerable 2g g formationofanoxygen-deﬁcient octahedral symmetry—the so- changes in the CV scans of LiCoPO (ref. 21), LiCoO (ref. 21) 4 2 11,12 called square pyramidal symmetry (C ) in Fig. 1c. The concentration and (B)SCF , where the near-surface regions or the bulk of 4v of oxygen vacancies and the cobalt oxidation state in double per- oxide particles become amorphous. The stability of the double ovskites can be tuned by the nature of the lanthanide and both are perovskites during OER was further supported by galvanostatic expected to inﬂuence the OER activity. measurements. The results obtained from (Pr Ba )CoO 0.5 0.5 3 d (Ln Ba )CoO (Ln¼ Pr, Sm, Gd and Ho) exhibited are shown in Supplementary Fig. S2 as an example, where no 0.5 0.5 3 d comparable OER activities from CV measurements, which were signiﬁcant changes occur in the voltage (Supplementary Fig. S2a) 2 NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 ARTICLE a [221] PBCO Dry Pr (110) Ho (102) Sm Gd [021] (200) PBCO THF (100) [110] PBCO 25c 1.2 1.4 1.6 (110) E - iR (V versus RHE) (002) (001)* LCO 1.65 Gd Sm [221] Ho PBCO 2 h (110) Pr (102) 1.60 1.55 BSCF Figure 3 | Structure of double perovskites on oxygen evolution. High- resolution TEM images (scale bars, 5 nm) and fast Fourier transforms 1.50 (FFTs) of (a) dry (Pr Ba )CoO powder, (b) (Pr Ba )CoO 0.5 0.5 3 d 0.5 0.5 3 d exposed to tetrahydrofuran (THF), (c) (Pr Ba )CoO cycled between 0.5 0.5 3 d 0.1 1 10 1.1 and 1.7 V versus reversible hydrogen electrode (RHE) at 10 mVs for –2 i (mA cm ) 25 times and (d) (Pr Ba )CoO galvanostatically tested at oxide 0.5 0.5 3 d 5mAcm for 2 h. (Pr Ba )CoO mixed with Naﬁon and disk 0.5 0.5 3 d acetylene black carbon supported on a glassy carbon electrode with a 2nd cycle (Pr Ba )CoO loading of 0.25 mg cm was tested in O -saturated 0.1 M 0.5 0.5 3– oxide 2 KOH electrolyte. FFTs images were indexed using Pmmm as space group 10th cycle and ap ap 2ap lattice parameters. Red circles indicate reﬂection that uniquely belongs to the double perovskite structure. 25th cycle imaging. Although the surfaces of as-prepared double perovskites are perfectly crystalline (Fig. 2a and Supplementary Fig. S3), exposing the oxides to tetrahydrofuran (THF) during the electrode preparation for OER measurements appeared to induce a small degree of amorphization, as shown in Fig. 3b and Supplementary Fig. S4. No further change in the surfaces of double perovskites was observed after OER measurements, where 1.2 1.4 1.6 TEM imaging showed that the surfaces of (Pr Ba )CoO 0.5 0.5 3 d E - iR (V versus RHE) after 25 CV scans (Fig. 3c and Supplementary Fig. S5a) and 2 h held at 5 mA cm (Fig. 3d and Supplementary Fig. S5b) oxide Figure 2 | Oxygen evolution activities of double perovskites. (a) Cyclic were comparable to those only exposed to THF (Fig. 3b). The voltamograms of (Ln Ba )CoO with Ln¼ Pr, Sm, Gd and Ho 0.5 0.5 3 d 0 stability of the double perovskite structure during OER measure- (LnBaCO O with d ¼ 1 2d), where measurements were performed 2 5 þ d ments was further supported by the presence of cation-ordering using ink-casted oxides (containing Naﬁon and acetylene black carbon) 2 reﬂections in the fast Fourier transforms (marked by * in Fig. 3c with an oxide loading of 0.25 mg cm supported on a glassy carbon disk and Supplementary Fig. S5a) and X-ray diffraction (XRD) pattern electrode in O -saturated 0.1 M KOH electrolyte) with a scanning rate of 1 of cycled electrodes (Supplementary Fig. S6), unique to the double 10 mVs .(b) Intrinsic OER activities normalized by the true oxide surface perovskite structure with space group Pmmm and having a unit area, which were obtained from averaging currents in the forward and cell of ap ap 2ap. This observation indicates that no leaching backward scans of the second cycle and subtracting the ohmic losses. The of praseodymium or barium occurs during oxygen evolution, OER activities of Ba Sr Co Fe O (BSCF) and LaCoO (LCO) 0.5 0.5 0.8 0.2 3 d 3 d unlike in BSCF , and that these double perovskites are stable are included for comparison. Error bars represent s.d. from at least four under the OER conditions used in this study. independent measurements. (c) The 2nd, 10th and 25th CV scans of (Pr Ba )CoO . 0.5 0.5 3 d Correlation between OER activity and estimated e ﬁlling.To or in the CVs (Supplementary Fig. S2b) recorded before and after estimate the e ﬁlling of cobalt ions in (Ln Ba )CoO double g 0.5 0.5 3 d galvanostatic testing of 5 mA cm for 2 h. Further evidence perovskites, the cobalt oxidation and spin states are needed. oxide of the double perovskite stability against OER came from TEM The cobalt oxidation state was determined by iodometric and NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 3 & 2013 Macmillan Publishers Limited. All rights reserved. E - iR (V versus RHE) –2 i (mA cm ) oxide –2 i (mA cm ) oxide ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 Mohr’s salt titrations, which was further conﬁrmed by Co K-edge high spin in the C symmetry because of polarizability and steric 4v 25 3þ 6 X-ray absorption spectroscopy (XAS, see Methods). The cobalt issues ;Co (3d ) having intermediate spin in the O and C ; h 4v 4þ 5 26 oxidation state was found to increase with increasing lanthanide and Co (3d ) stabilized in high spin in the O symmetry ionic radius in Fig. 4a, which is in agreement with previous work (Supplementary Fig. S9). Such assignments give rise to the e by Maignan et al. As Co K-edge XAS data showed comparable occupancy of cobalt in these double perovskites near unity, white line energies for Sm and Pr double perovskites, no having values from B1.1 to B1.3 (Supplementary Fig. S9). signiﬁcant change in the cobalt oxidation state was found for The exceptionally high OER activities of these double perovskites, these two double perovskites (Supplementary Figs S7a,S8). This is with their estimated e values near unity (Supplementary in contrast to the large difference obtained by chemical titration Fig. S10), support the design principle proposed by Suntivich (Fig. 4a). The poor sensitivity of Co K-edge XAS for the oxidation et al. , where the OER activity of perovskites peaks near e state of cobalt greater than 3þ can be attributed to higher occupancy of B1.2. hybridization of Co-O bonds (Estimating hybridization of transition-metal and oxygen states in perovskites from O K-edge X-ray absorption spectroscopy, manuscript in preparation) in the Correlation between OER activity and computed O p-band. Pr double perovskite than the Sm double perovskite, which is Despite its utility, the e -ﬁlling estimation of cobalt ions is 10,27 supported by O K-edge XAS measurements discussed below. inferred from previous studies and cannot be measured or Unfortunately, it is not straightforward to estimate the cobalt computed directly. The presence of multiple spin states of cobalt spin states in double perovskites because of the presence of O ions in these double perovskites further introduces ambiguities in 16,18–20 and C symmetries and multiple spin conﬁgurations the estimation of e ﬁlling. Recent studies have shown that the 4v (Fig. 1). Increasing the oxygen content from the Ho to Pr double computed O p-band centre relative to the Fermi level of 3 þ perovskite leads to a greater number of Co in O symmetry at perovskites scales linearly with the energy of oxygen vacancy the expense of C symmetry, whereas the accompanied increase formation and the oxygen surface exchange kinetics at elevated 4v 2 þ of the Co oxidation state reduces the number of Co and temperatures. We here report the O p-band centre relative to the 3 þ 4þ increases the number of Co and Co . By a ﬁrst approxima- Fermi level of perovskites as an alternative descriptor for the OER tion, these double perovskites were selected to have the following activities of perovskites in alkaline solution in an analogous role 20,23,24 2 þ 7 29,30 spin states based on previous studies :Co (3d ) having as the d-band centre for noble metal catalysts . The O p-band a b 3.4 –2.6 (Ln Ba )CoO 0.5 0.5 3– Pr (Ln Ba )CoO –2.4 0.5 0.5 3 3.2 Sm –2.2 Sm BSCF Gd Gd LCO 3.0 Pr –2.0 Ho 2.8 Ho –1.8 2.6 –1.6 2.5 2.75 3.0 Ho Gd Sm Pr Oxygen content (Ln Ba )CoO 0.5 0.5 3– cd 0.6 Pr Pr 0.4 Sm Sm Gd Gd 0.2 Ho Ho O K-edge 0.0 530 535 540 545 550 555 2.8 3.0 3.2 Energy (eV) Cobalt oxidation state Figure 4 | Inﬂuence of lanthanides on electronic structure. (a) Cobalt oxidation obtained by chemical titration (closed symbols) and by XAS at the Co K-edge (open symbols) plotted versus the oxygen content for (Ln Ba )CoO (LnBaCo O with d ¼ 1 2d and Ln¼ Pr, Sm, Gd and Ho), 0.5 0.5 3 d 2 5þ d Ba Sr Co Fe O (BSCF) and LaCoO (LCO). The oxygen vacancy concentration d was directly calculated from the cobalt oxidation state x by 0.5 0.5 0.8 0.2 3 d 3 d¼ (3.5 x)/2. The error bars in the oxidation state obtained by chemical titration represent the s.d. from at least three independent measurements. The errors in the oxidation state obtained by XAS were calculated by taking the root of the squared s.e. obtained by linear regression (see Methods). (b) The computed O p-band centre versus the Fermi level for the nonstoichiometric (Ln Ba )CoO double perovskites (having d constrained 0.5 0.5 3 d to the values obtained from chemical titration) and stoichiometric (Ln Ba )CoO in the constrained or relaxed structure (see Supplementary 0.5 0.5 3 Information). (c) O K-edge XAS data of (Ln Ba )CoO measured in the total electron yield mode. (d) Integrated intensities of the O K-edge pre-edge 0.5 0.5 3 d region normalized to oxygen content per formula unit and the nominal number of empty cobalt 3d states in both e and t symmetry. g 2g 4 NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. Cobalt oxidation state Normalized intensity (r.u.) O p-band center versus E (eV) Absorbance/e holes + t holes g 2g δ = 0.375 Constrained Relaxed δ = 0.25 Constrained Relaxed δ = 0.25 Constrained Relaxed δ = 0.125 Constrained Relaxed NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 ARTICLE centre was computed from DFT and details for the calculations a b 1.50 Pr can be found in the Supplementary Information. Owing to the Ho strong correlation error present in standard DFT-generalized BSCF82 Sm gradient approximation calculations of the transition metal 3d- M 3d Gd SCF82 1.55 band, which was recently shown to be important for describing the OER activities of Co oxides , the more delocalized nature of LSC46 the O p-band more accurately captures the electronic structure LCO O p-band O 2p characteristics of oxides critical to catalysis while still reﬂecting 1.60 BSCF46 center the metal d-character through hybridized density of states Stable Amorphized (Fig. 5a). –2 @ 0.5 mA cm oxide The O p-band centre relative to the Fermi level of 1.65 (Ln Ba )CoO double perovskites using the cobalt oxidation 0.5 0.5 3 d –2.5 –2.0 –1.5 state and oxygen nonstoichiometry determined from chemical O p-band relative to E (eV) titration (Fig. 4a) was found to linearly increase with increasing cobalt oxidation from Ho to Pr, as shown in Fig. 4b. This uplift of Figure 5 | Computed oxygen p-band centre for oxygen evolution. the O p-band centre with increasing cobalt oxidation would (a) Schematic representation of the O p-band for transition metal oxides enhance Co-O hybridization, which is supported by O K-edge and (b) evolution of the iR-corrected potential at 0.5 mA cm oxide XAS measurements. The hybridization of Co-O bonds was found versus the O p-band centre relative to E (eV) of (Ln Ba )CoO with F 0.5 0.5 3 d to increase from Ho to Pr double perovskite based on the O 11 11 Ln¼ Pr, Sm, Gd and Ho, for LaCoO (LCO) ,La Sr CoO (LSC46) , 3 0.4 0.6 3 d K-edge XAS measurements. The pre-edge feature can be 11 11 Ba Sr Co Fe O (BSCF82) ,Ba Sr Co Fe O (BSCF46) 0.5 0.5 0.8 0.2 3 d 0.5 0.5 0.4 0.6 3 d attributed to unoccupied O 2p states resulting from the mixing and SrCo Fe O (SCF82) . The O p-band centre relative to the Fermi 0.8 0.2 3 d between O 2p and metal 3d states, and its intensity is linear with level was computed by DFT for the fully oxidized and relaxed structure the number of holes in the O 2p-band . The intensity of the using the method described in Methods section. Error bars represent s.d. pre-edge region (two peaks near B532–534 eV) increases with from at least four independent measurements. increasing ionic radius of the lanthanide (Fig. 4c and Supplementary Fig. S7b). The hybridization of Co-O bonds of these double perovskites can be assessed by the integrated intensities of the pre-edge region normalized to oxygen content found for the oxides on the left branch in Fig. 5b during OER, per formula unit and the nominal number of empty cobalt 3d whereas rapid amorphization in the near-surface regions states in both e and t symmetry. The hybridization was shown accompanied with leaching of A-site ions, having local structures g 2g to increase and scale with increasing cobalt oxidation state resembling that of Co-Pi, was observed for oxides on the right 11,12 associated with greater ionic radius of lanthanide (Fig. 4d). The branch . trend can be attributed to decreasing charge transfer gap between oxygen 2p and cobalt 3d states associated with increasing cobalt oxidation state . Discussion Assuming the surface of the catalyst to be fully oxidized in We show double perovskites (Ln Ba )CoO (with Ln¼ Pr, 0.5 0.5 3 d KOH under OER conditions, DFT studies of the O p-band centre Sm, Gd and Ho) as a new family of highly active catalysts for were then performed on stoichiometric (Ln Ba )CoO double OER in alkaline electrolyte, with comparable activities and 0.5 0.5 3 perovskites (Supplementary Fig. S11). Having oxygen stoichio- enhanced stability relative BSCF—the most active catalyst 10,11 metry in the double perovskite structure without structural reported to date . Although the high activities of these relaxation gave rise to the same trend as that found for double perovskites with e ﬁlling of cobalt ions near unity is in nonstoichiometric (Ln Ba )CoO , but the O p-band centre agreement with the design principle of OER catalysts reported 0.5 0.5 3 d was much uplifted because of increasing oxygen content (Fig. 4b). recently , considerable ambiguities exist in the estimation In contrast, the O p-band centre peaks on (Gd Ba )CoO of e ﬁlling associated with the presence of two crystal ﬁelds 0.5 0.5 3 g in the relaxed structure. The change in the O p-band centre trend (octahedral O and square pyramidal C ), multiple Co oxidation h 4v 3 þ versus (Ln Ba )CoO series in the fully relaxed stoichiometric states and multiple spin states of Co ions in these double 0.5 0.5 3 double perovskite structure was found to result from distortion perovskites. In this study, we show that the computed O p-band and rotation of Co-O octahedra caused by A-site and B-site ionic centre relative to the Fermi level and parameters derived from radii mismatch (Supplementary Fig. S11), which alters the this can be used as descriptors to screen the OER activity and Co-O bond length and metal 3d–oxygen 2p orbital hybridiza- stability of oxides. Moving the computed O p-band centre closer tion , leading to an upshift of the O p-band centre of to the Fermi level can increase OER activities, but having (Ho Ba )CoO relative to the Fermi level relative to the other computed O p-band centre too close to the Fermi level decreases 0.5 0.5 3 double perovskites. oxide stability during OER. This trend is similar to the correlation The intrinsic OER activities of cobalt-based double perovskites previously established for oxygen reduction on perovskites at and pseudocubic perovskites, which can vary by B2 orders of elevated temperatures . The pseudocubic perovskites and double magnitude (Fig. 2b), were found to correlate with the computed perovskites with the O p-band centre very close to the Fermi level O p-band centre relative to the Fermi level of the fully oxidized not only have the highest OER activities in alkaline solution but and relaxed structure, as shown in Fig. 5b. It is appropriate to use also exhibit the highest activities for surface oxygen exchange the fully oxidized and relaxed structure in this analysis as the kinetics upon oxygen reduction at elevated temperatures , which surface of these oxides is expected to be fully oxidized in KOH highlights the importance of the oxide electronic structure on under OER conditions. Moving the O p-band centre closer to the oxygen electrocatalysis. Future spectroscopic experiments of these Fermi level from LCO to (Pr Ba )CoO greatly increases the oxides are needed to verify the computed O p-band centre trend 0.5 0.5 3 intrinsic OER activity in the left branch. However, further lifting in this study, and seek activity and stability descriptors that can be 11,28 the O p-band centre in BSCF ,Ba Sr Co Fe O measured experimentally. Our study highlights the importance of 0.5 0.5 0.4 0.6 3 11 11 (BSCF46) and SCF did not result in any increase in the controlling a transition metal oxide having the O p-band close to OER activity but decreased the oxide stability. No changes were the Fermi level as a promising strategy to create highly active NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 5 & 2013 Macmillan Publishers Limited. All rights reserved. E - iR (V versus RHE) ARTICLE NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 oxide catalysts for OER to enable the development of efﬁcient, inﬂection points . The Co oxidation states were calculated by relating the edge position obtained by the integral method with the known oxidation states of single- rechargeable metal-air batteries, regenerative fuel cells and other 2þ 2þ 2.6þ phase Co O, Co (OH) ,Co O and LiCoO using linear regression. The 2 3 4 2 rechargeable air-based energy storage devices. errors in the edge position are estimated and the errors in the oxidation state were calculated by taking the root of the squared s.e. obtained by linear regression. Methods Synthesis and bulk characterization. Double perovskite catalysts 28,37 DFT studies. DFT calculations with the Hubbard U (U ¼ 3.3 eV) correction eff (Ln Ba )CoO (with Ln¼ Pr, Sm, Gd and Ho) and (Pr,Ba)Co Fe O 0.5 0.5 3 d 1 x x 3 d for the Co 3d electrons were performed with the Vienna Ab initio Simulation were synthesized using a conventional solid-state route. Stoichiometric amounts of 38,39 40 Package using the projector-augmented plane-wave method . Exchange- Ln O , BaCO and Co O and Fe O previously dehydrated were thoroughly 2 3 3 3 4 2 3 correlation was treated in the Perdew–Wang-91 generalized gradient grounded and ﬁred in air at 1,000 C for 12 h. Products were ground and approximation . Fully relaxed stoichiometric bulk perovskite calculations were annealed in air at 1,100 C for (Ln Ba )CoO catalysts and 1,200 C for 0.5 0.5 3 d simulated with 2 2 2 perovskite supercells. The double perovskites were (Pr Ba )Co Fe O (x ¼ 0.25 and 0.5) for 24 h with intermediate grindings. 0.5 0.5 1 x x 3 d simulated based on the reported ordered structures within the 2 2 2 perovskite All catalysts reported in this study are single phase, as analysed by XRD 16,18 supercell . An effective O p-band centre was determined by taking the centroid (Supplementary Fig. S14), with lattice parameters consistent with those reported of the projected density of states of O 2p states relative to the Fermi level. More previously . XRD measurements were performed using a PANalytical X’Pert Pro details of the calculations are provided in the Supplementary Information. powder diffractometer in the Bragg–Brentano geometry using a Copper K radiation, where data were collected using the X’Celerator detector in the 8–80 window in the 2y range. The speciﬁc surface area of each oxide sample was References determined using BET analysis on a Quantachrome ChemBET Pulsar from a single-point BET analysis performed after 12 h outgassing at 150 C, which is 1. Gray, H. B. Powering the planet with solar fuel. Nat. Chem. 1, 7–7 (2009). 2. Lewis, N. S. & Nocera, D. G. Powering the planet: chemical challenges in solar reported in Supplementary Table S1. The oxygen vacancy d was deduced by the determination of the cobalt oxidation energy utilization. Proc. Natl Acad. Sci. USA 104, 20142–22014 (2007). state by chemical titration. For Ln ¼ Pr, Gd and Sm, the samples were dissolved in 3. Dau, H. et al. 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B 74, 054406 (2006). yield mode at the spherical grating monochromator beamline at the Canadian Light 24. Jørgensen, J.-E. & Keller, L. Magnetic ordering in HoBaCo O . Phys. Rev. B 2 5.5 Source. The spectra were ﬁrst normalized by I being measured with a gold mesh 77, 024427 (2008). before the specimen and then normalized by subtraction of a constant before the edge, and then division by a suitable constant after the edge. XAS at the Co K-edge 25. Suard, E., Fauth, F., Caignaert, V., Mirebeau, I. & Baldinozzi, G. Charge was collected in ﬂuorescence yield mode at the National Synchrotron Lightsource. ordering in the layered Co-based perovskite HoBaCo O . Phys. Rev. B 61, 2 5 The energy scale was calibrated by simultaneously measuring BSCF with known R11871–R11874 (2000). 6 NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications & 2013 Macmillan Publishers Limited. All rights reserved. NATURE COMMUNICATIONS | DOI: 10.1038/ncomms3439 ARTICLE 26. 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K., Bligaard, T., Rossmeisl, J. & Christensen, C. H. Towards the program under award DE-FG02-05ER15728 and by the Ofﬁce of Naval Research (ONR) computational design of solid catalysts. Nat. Chem. 1, 37–46 (2009). under contract numbers N00014-12-1-0096 (MIT) and N00014-12-WX20818 30. Nørskov, J. K. et al. Origin of the overpotential for oxygen reduction at a fuel- (NSWCCD). The National Synchrotron Light Source is supported by the U.S. cell cathode. J. Phys. Chem. B 108, 17886–17892 (2004). Department of Energy, Division of Material Sciences and Division of Chemical Sciences 31. Garcı´a-Mota, M. et al. Importance of correlation in determining electrocatalytic under contract DE-AC02-98CH10886. The Ofﬁce of Naval Research and contributions oxygen evolution activity of cobalt oxides. J. Phys. Chem. C. 116, 21077–21082 from Participating Research Team members support Beamline X11. Canadian Light (2012). Source is supported by the NSERC, NRC, CIHR of Canada, the Province of 32. Goodenough, J. B. Theory of the role of covalence in the perovskite-type Saskatchewan, WEDC and the University of Saskatchewan. manganites [La,M(II)]MnO . Phys. Rev. 100, 564–573 (1955). 33. Rondinelli, J. M., May, S. J. & Freeland, J. W. Control of octahedral connectivity in perovskite oxide heterostructures: an emerging route to multifunctional materials discovery. MRS Bull. 37, 261–270 (2012). Author contributions 34. Taranco´n, A., Burriel, M., Santiso, J., Skinner, S. J. & Kilner, J. A. Advances Y.S.-H. and A.G. designed the experiments. A.G. made the synthesis, structural and in layered oxide cathodes for intermediate temperature solid oxide fuel cells. chemical analysis. K.J.M. and A.G. performed the electrochemical measurements. M.R. J. Mater. Chem. 20, 3799–3813 (2010). and J.Z. conducted XAS measurements. M.R. normalized and analysed the XAS spectra. 35. Suntivich, J., Gasteiger, H. A., Yabuuchi, N. & Shao-Horn, Y. Electrocatalytic C.E.C. performed TEM analysis. Y.-L.L. carried out the DFT calculation. A.G. and Y.- measurement methodology of oxide catalysts using a thin-ﬁlm rotating disk S.H. wrote the manuscript and all authors edited the manuscript. electrode. J. Electrochem. Soc. 157, B1263–B1268 (2010). 36. Dau, H., Liebisch, P. & Haumann, M. X-ray absorption spectroscopy to analyze nuclear geometry and electronic structure of biological metal centers – potential Additional information and questions examined with special focus on the tetra-nuclear manganese complex of oxygenic photosynthesis. Anal. Bioanal. Chem. 376, 562–583 Supplementary Information accompanies this paper at http://www.nature.com/ (2003). naturecommunications 37. Lee, Y. L., Kleis, J., Rossmeisl, J. & Morgan, D. Ab initio energetics of Competing ﬁnancial interests: The authors declare no competing ﬁnancial interests. LaBO (001) (B¼ Mn, Fe, Co and Ni) for solid oxide fuel cell cathodes. Phys. Rev. B 80, 224101 (2009). Reprints and permission information is available online at http://npg.nature.com/ 38. Kresse, G. & Hafner, J. Ab initio molecular-dynamics for liquid metals. Phys. reprintsandpermissions/ Rev. B 47, 558–561 (1993). 39. Kresse, G. & Furthermu¨ller, J. Efﬁcient iterative schemes for ab initio total- How to cite this article: Grimaud, A. et al. Double perovskites as a family of highly energy calculations using a plane-wave basis set. Phys. Rev. B 54, 11169–11186 active catalysts for oxygen evolution in alkaline solution. Nat. Commun. 4:2439 (1996). doi: 10.1038/ncomms3439 (2013). NATURE COMMUNICATIONS | 4:2439 | DOI: 10.1038/ncomms3439 | www.nature.com/naturecommunications 7 & 2013 Macmillan Publishers Limited. All rights reserved.

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Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

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Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

Double perovskites as a family of highly active catalysts for oxygen evolution in alkaline solution

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