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COMMUNICATION Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen et al. Reactivity differences of Sc3N@C2n (2n = 68 and 80). Synthesis of the first methanofullerene derivatives of Sc3N@D5h-C80
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Boosting electrochemical water oxidation through replacement of Oh Co sites in cobalt oxide spinel with manganese Prashanth W. Menezes,a Arindam Indra,a Vitaly Gutkinb and Matthias Driess*a
DOI: 10.1039/x0xx00000x www.rsc.org/
The strikingly high catalytic performance and stability of manganese substituted cobalt oxide spinel (MnxCo3-xO4) over pristine cobalt oxide spinel (Co3O4) for the alkaline electrochemical water oxidation is reported. The different role of cations could be uncovered along with the detection of drastic surface-structural changes during the catalysis using spectroscopic and microscopic methods. Employing renewable and clean energy sources as practical alternatives of fossil fuel-based technologies requires the development of new catalyst systems with higher efficiency and stability.1-3 Electrochemical energy conversion by splitting of water into molecular oxygen and hydrogen has been regarded as one of the most convenient techniques; however, the efficiency of the overall water splitting reactions largely depends on the oxygen evolution reaction (OER) as the bottle neck.4-6 Anodic water oxidation requires four electron and four proton transfer involving several high energy intermediates.7-9 Therefore, designing of the efficient catalyst systems for long-term water oxidation has been a key challenge in recent years. Lately, several groups have shown that transition-metal based catalysts, especially, based on manganese, cobalt and nickel can be utilized as alternatives for effective water oxidation instead of expensive and scarce precious metal-based RuO2 or IrO2 catalysts.1015 Over the years, manganese oxides have stimulated tremendous interest for water oxidation catalysis due to the natural presence of the CaMn4O5 cluster in the oxygen evolving complex of a Department of Chemistry, Metalorganics and Inorganic Materials, Technische Universität Berlin, Strasse des 17 Juni 135, D-10623 Berlin, Germany; E-mail: [email protected] b The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel † Electronic Supplementary Information (ESI) available: Complete experimental details and characterization of catalysts before and after electrochemical measurements. See DOI: 10.1039/b000000x/
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photosystem II involved in photosynthesis.6,9,15,16 Similarly, enormous efforts have been devoted to the design of suitable cobalt oxide based catalysts with higher efficiency.17-19 Although, pure spinel (A2+B23+O4 where A is in tetrahedral (Td) and B is in octahedral (Oh) coordination with six oxygen atoms) cobalt oxides are well investigated, the preferable substitution of cobalt by manganese in the lattices of Co3O4 has already garnered much attention lately in enhancing the OER activity.20-22 Recent studies show that the Oh centres of a spinel oxide are the most active sites, while the Td ones play a minor role.23,24 This encouraged us to (i) synthesize new materials where Oh metal sites are selectively and partially replaced by Mn (ii) probe them for electrochemical water oxidation catalysis at selected electrode potentials on different substrates in alkaline media, and (iii) in getting a deeper understanding of the water oxidation process by detecting intermediates, near-surface phenomena and elucidating structureactivity relationships of the active catalysts. Here we report the selective synthesis of partially manganese substituted cobalt oxide (MCO; MnxCo3-xO4, x = 0.3) and compared its strikingly high OER activity with those of pristine Co3O4 and Mn2O3. The MCO catalysts were synthesized in multi-gram scale using Mn oxalate, Co oxalate and CoMn-oxalate prepared in the microemulsion approach and their subsequent thermal treatment in synthetic air. The catalysts were electrophoretically deposited on fluorine doped tin oxide (FTO) and on nickel foam (NF) substrates. During alkaline electrochemical water oxidation, a boost in performance was attained for MCO due to the promotion effect of manganese with respect to pristine Co3O4 and Mn2O3 as well as the state of the art precious metal-based catalysts in alkaline media. The presented results prove experimentally the significant effect of Mn substitution in the Co3O4 lattice with a distinct activity in two different pH media on different electrode supports. Furthermore, the surface structural transformation of the MCO catalyst at the onset potential and at an elevated electrode potential of water oxidation was systematically determined by spectroscopic and microscopic
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methods, and thus a structure-activity relation was revealed. The impressively high catalytic activity and stability of MCO could directly be attributed to (i) the higher surface area, (ii) the selective dissolution of Mn from the Oh sites of near-surface of spinel oxide during water oxidation creating a defective and disordered structure with vacancies as well as (iii) the formation of a cobalt rich oxohydroxide-hydroxide shell on the spinel core. The transition-metal oxide catalysts discussed here were synthesized from the corresponding oxalate precursors (see Experimental Section). The powder X-ray diffraction pattern (PXRD) of the one dimensional nanochain manganese substituted cobalt oxalate precursor could be well matched with cobalt oxalate dihydrate with a slight shift in the peak positions due to incorporation of Mn in the crystal lattices (Figs. S1 and S2). The ratio of Co:Mn was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Table S1) and energy dispersive X-ray (EDX, Fig. S3) measurements and the amount of C, H was determined from elemental analysis (Table S2). Corresponding stretching and bending vibrations of the precursor were detected by infra-red (IR, Fig. S4 and Table S3). The nanorods morphology was also clearly visible from the scanning electron microscopy. (SEM, Fig. S5). The oxidation state of Co and Mn was determined to be +2 by X-ray photoelectron spectroscopy (XPS) and the detailed discussion is given in Fig. S6. Calcination of the oxalate precursor in synthetic air produced Mn substituted Co oxide, Mn0.3Co2.7O4 (MCO) and the PXRD pattern could also be matched with spinel Co3O4 with slight peak shifts due to Mn substitution into spinel lattices (Fig. S7 and S8). About 10% substitution of Co by Mn in MCO was observed which was determined from the ICP-AES and EDX analysis (Table S4 and Fig. S9). Complete removal of the oxalate and the formation of phase pure metal oxide were confirmed by IR spectrum (Fig. S10). SEM images indicated that after thermal treatment, the nanorods of the precursor were transformed into one dimensional nanochains (Fig. S11). Transmission electron microscopic (TEM) studies revealed that small nanoparticles of 10-20 nm interconnected to form the one dimensional nanochain (Fig. 1). The lattice spacing of 0.28 nm reflects the (220) plane of MCO and the selected area electron diffraction (SAED) pattern also shows the crystalline nature of the material. The oxidation state of Co and Mn in MCO was determined by XPS analyses (Fig. 2). The deconvoluted Mn2p (Fig. 2, left) showed two peaks at 642.2 eV for Mn2p3/2 and 653.7 eV for Mn 2p1/2 with a spin−orbit level energy spacing of 11.5 eV and is typical for Mn3+ based materials.25,26 This indicates that Mn is preferentially substituted in the Oh sites of spinel oxide. From the literature, it is well known that the Co(II) and Co(III) have similar 2p binding energies but can be differentiated by the Co2p1/2−2p3/2 spin−orbit level energy spacing, which is 16.0 eV for high-spin Co(II) and 15.0 eV for low-spin Co(III).27 The Co2p spectrum (Fig. 2, right) shows peaks at 781.1 eV for Co2p3/2 and 796.5 eV for Co2p1/2 with a difference in energy spacing of 15.4 eV indicating Co in +2/+3 oxidation state.28,29 Both Mn2p and Co2p XPS spectra obtained here are in accordance with the spinel type materials containing Mn and Co.25-29 The O1s spectra showed the corresponding metal-oxygen bond with surface oxygen species (Fig. S12).30,31 The other two
2 | J. Name., 2014, 00, 1-3
Journal Name DOI: 10.1039/C7CC03749J catalysts Co3O4 and Mn2O3 have been synthesized and characterized comprehensively. The detail characterizations are described in Fig. S13-S14. Co3O4 has the morphology of one dimensional nanochains whereas Mn2O3 crystallizes in a net like structure. High surface area of Mn2O3 (49 m2 g-1) has also been determined compared to Co3O4 (12 m2 g-1) and MCO (37 m2 g-1).
Fig. 1. The TEM (a, b), HRTEM (c) images with d-spacing 0.28 nm indicating (220) plane and SAED pattern (d) of MCO nanochains.
All three catalysts were deposited on fluorinated tin oxide (FTO) coated glass by electrophoretic deposition and electrochemical water oxidation was first carried out in 0.1 M KOH solution with a three-electrode set up (Fig. S15). The cyclic voltammogram (CV) showed redox peaks (I and II in Fig. S16) between 1.1 and 1.3 V which is ascribed to Co2+/Co3+, with a Co2+/Co3+ transition and the next pair of peaks (III and IV in Fig. S16) in the range of 1.3 to 1.5 V and are associated with Co3+/Co4+ redox transition.23,32 Interestingly, MCO unveiled improved water oxidation activity compared to Co3O4 and Mn2O3 (Fig. S15). This directly proves that the Mn substitution into the cobalt Oh lattices of Co3O4 could be the main reason for the significant enhancement in the catalytic activity. The overpotential required to attain the current density of 10 mAcm-2 for MCO was 390 mV compared to 430 mV for Co3O4 in 0.1 M KOH solution. The catalytic activity of Mn2O3 was poor and not comparable with the other two catalysts. Furthermore, the chronoamperometric (CA) studies were performed for 8 h that showed enhanced stability for MCO, however, about 30% decrease in current was observed for Co3O4 (Fig. S17).
Fig. 2. The Mn2p and Co2p XPS spectra of MCO. To demonstrate the superior activity of as-synthesized catalysts with other highly active transition metal based catalysts
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reported in the literature, we additionally conducted water oxidation experiments in 1 M KOH (Fig. 3). Only a overpotential 320 was resulted for MCO at current density of 10 mAcm-2 which is the one of the lowest among non-precious metal-based catalysts reported in literature (see Table S5) indicating significantly diminished OER activation energies,5,14,19,22,33 while 360 mV and 560 mV of overpotential was obtained for Co3O4 and Mn2O3. The OER activity of MCO is even better than the benchmark state of the art precious catalysts in alkaline media (Fig. 3 and Table S5).
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COMMUNICATION DOI: 10.1039/C7CC03749J electrophoretic deposition of MCO on FTO substrate (Fig. S19). Formation of a thin amorphous shell on the surface of the MCO particles was detected after CA measurements at the 1.53 V (onset potential, Fig. S20). An increase in the thickness of the amorphous shell was observed at the increased potential of 1.62 V (potential at 10 mAcm-2) (Fig. S21). This indeed suggests that, during water oxidation the amorphous shell starts growing and creates more active sites at the near-surface (Scheme 1). In the XPS study, Co2p spectra displayed a spin−orbit level energy spacing of 15.3 eV after applying an onset potential in comparison to the as-synthesised MCO with spacing 15.5 eV (Fig. S22).29,40 This directly indicates that there is an increase in the amount of Co(III) and probably in the surface structure at the onset potential. However, at an applied potential 1.63 V, a large amount of Co(III) was detected (spin−orbit level energy spacing 15.1 eV) from the higher energy shift of the peak that evidences major changes in the near-surface region.23,28 O1s spectra, for both, at onset and at an increased potential indicated higher fraction of –OH groups confirming, the surface of the material is hydroxylated (Fig. S23).28,40 Strikingly, no Mn could be detected at the surface of the films signifying selective dissolution of Mn from the Oh sites of spinel oxide to the electrolyte during water oxidation.
Fig. 3. Cyclic voltammetry (CV) of MCO, Co3O4, Mn2O3 synthesized by similar approach verses Mn2O3 and commercial noble catalysts in 1 M KOH solution with a scan rate of 20 mV/s on FTO substrates (loading ~ 1 mg).
In addition to this, recently, nickel foam (NF) have been used as the support to carry out effective water oxidation due to unique hierarchically structured porous configuration that allows large surface area and low electrical resistance. Therefore, our catalysts were further deposited on NF and explored for water oxidation (Fig. S18). Excellent overpotential of 370 mV was obtained for MCO at a higher current density of 100 mAcm-2. Similarly, 420 mV and 640 mV of overpotential at the same current density were resulted with Co3O4 and Mn2O3. The overpotentials obtained here are superior than most of the transition metal based catalysts (see Table S5) and can directly be compared to the recently reported high performance Ni and Co based phosphides on NF.30,34-36 To uncover the role of cations, to understand the near-surface structural changes during the water oxidation and to deduce a structure-activity relation, we characterized the MCO catalyst extensively before the catalysis as well as after 8 h CA measurements in 0.1 M KOH electrolyte. Recently, the potential dependent transformation of CoII to CoIII and to CoIV in cobalt oxide catalyst was demonstrated by Risch et al. and Bergmann et al.37,38 Similarly, for cobalt oxides, the theoretical studies have also predicted the change in structure at different electrode potentials.39 As the stability of the catalyst largely depends on the applied electrode potentials, we investigated the actual structure and the nature of the catalyst by applying two selected electrode potentials: (i) at onset and (ii) at an electrode potential after reaching to the current density of 10 mAcm-2. TEM and HRTEM studies revealed that the particle morphology and corresponding structure (could be seen from SAED) remained intact after the
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Scheme 1. The near-surface structural reorganization of MCO at the onset as well as at the elevated oxygen evolution potential.
Several studies on cobalt based spinel catalysts have already revealed that the Oh Co3+ sites are the real catalytically active sites in OER.23,24,38 From the extensive microscopic and spectroscopic analysis, the following elucidation for the superior activity of MCO in contrast to pristine cobalt oxide can be deduced. First of all, the substitution of Mn into cobalt of spinel oxide takes place selectively at the Oh (Co3+) lattices. Mn3+ in Oh sites provides more disorder in the structure of the spinel having eg1 configuration, i.e, uneven occupation in the antibonding orbital. This indeed enhances the overall surface area by four times creating more active species at the near-surface for catalysis.6 In addition, the role of unprecedented nanochains morphology of MCO with clean surfaces cannot be neglected. Further understanding on the nature of active species and the structure-activity relation was achieved from the TEM, HRTEM and XPS measurements which indicated loss of Mn (as seen from ICP-AES) transpires selectively from the Oh lattices during electrochemical water oxidation. At onset potential, only a partial structural reformation occurs that is limited to the outermost surface of the MCO catalyst by forming a thin amorphous layer (scheme 1). As electrode potential and oxygen evolution rate increases, the crystallinity further decreases and forms a more pronounced amorphous (Co rich) shell with disordered structure with cationic
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vacancies on the surface, which helps in enhancing the electrical conductivity, transport of –OH ions and felicitates bonding.32,41,42 This leads to defects and greater amount of hydroxylation (Co(OH)2CoOOH) that alters the coordination environment of Co of MCO. In addition to this, the Td Co2+ site at the near-surface also get oxidized to Oh Co3+ and accelerates the water oxidation reaction very efficiently and the obtained results here also substantiates the recent findings of Risch et al. and Bergmann et al. on cobalt oxide catalysts.37,38 In conclusion, a facile, easily scalable method for the synthesis of Mn substituted Co oxide nanochains were developed with the single-source precursor approach and investigated for the alkaline electrochemical water oxidation at the onset and elevated electrode potential. Mn substitution into the Oh lattice of Co-oxide substantially improved the catalytic activity as well as stability of the system. The amorphous shell on the surface of the crystalline MCO at the onset and at the increased electrode potential demonstrates that severe structural alterations occur creating a disordered structure with vacancies leading to the formation of an oxohydroxidehydroxide shell of cobalt on the spinel core due to the selective dissolution of Mn from the near-surface zone which indeed are catalytically active species. This work was financially supported by DFG (Cluster of Excellence UniCat, EXC 314/2). We thank Dr. Caren Göbel for the TEM measurements. Notes and references 1. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446-6473. 2. J. Luo, J.-H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N.-G. Park, S. D. Tilley, H. J. Fan and M. Gratzel, Science, 2014, 345, 15931596. 3. A. Indra, A. Acharjya, P. W. Menezes, C. Merschjann, D. Hollmann, M. Schwarze, M. Aktas, A. Friedrich, S. Lochbrunner, A. Thomas and M. Driess, Angew. Chem. Int. Ed., 2017, 56, 1653-1657. 4. Y. Yan, B. Y. Xia, B. Zhao and X. Wang, J. Mater. Chem. A, 2016, 4, 17587-17603. 5. J. H. Wang, W. Cui, Q. Liu, Z. C. Xing, A. M. Asiri and X. P. Sun, Adv. Mater., 2016, 28, 215-230. 6. A. Indra, P. W. Menezes, I. Zaharieva, E. Baktash, J. Pfrommer, M. Schwarze, H. Dau and M. Driess, Angew. Chem. Int. Ed., 2013, 52, 1320613210. 7. H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and P. Strasser, ChemCatChem, 2010, 2, 724-761. 8. J. Rossmeisl, Z. W. Qu, H. Zhu, G. J. Kroes and J. K. Norskov, J. Electroanal. Chem., 2007, 607, 83-89. 9. P. W. Menezes, A. Indra, P. Littlewood, M. Schwarze, C. Göbel, R. Schomäcker and M. Driess, ChemSusChem, 2014, 7, 2202-2211. 10. B. M. Hunter, H. B. Gray and A. M. Müller, Chem. Rev., 2016, 116, 14120-14136. 11. M. M. Najafpour, G. Renger, M. Holynska, A. N. Moghaddam, E. M. Aro, R. Carpentier, H. Nishihara, J. J. Eaton-Rye, J. R. Shen and S. I. Allakhverdiev, Chem. Rev., 2016, 116, 2886-2936. 12. M. Gong and H. J. Dai, Nano Res., 2015, 8, 23-39. 13. X. H. Deng and H. Tuysuz, ACS Catal., 2014, 4, 3701-3714. 14. S. Jung, C. C. L. McCrory, I. M. Ferrer, J. C. Peters and T. F. Jaramillo, J. Mater. Chem. A, 2016, 4, 3068-3076. 15. A. Indra, P. W. Menezes and M. Driess, ChemSusChem, 2015, 8, 776785.
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Journal Name DOI: 10.1039/C7CC03749J 16. H. Ooka, T. Takashima, A. Yamaguchi, T. Hayashi and R. Nakamura, Chem. Commun., 2017, DOI: 10.1039/c1037cc02204b. 17. M. W. Kanan and D. G. Nocera, Science, 2008, 321, 1072-1075. 18. F. Song and X. L. Hu, J. Am. Chem. Soc., 2014, 136, 16481-16484. 19. C. C. L. McCrory, S. Jung, I. M. Ferrer, S. M. Chatman, J. C. Peters and T. F. Jaramillo, J. Am. Chem. Soc., 2015, 137, 4347-4357. 20. F. Y. Cheng, J. A. Shen, B. Peng, Y. D. Pan, Z. L. Tao and J. Chen, Nat. Chem., 2011, 3, 79-84. 21. A. Q. Zhao, J. Masa, W. Xia, A. Maljusch, M. G. Willinger, G. Clavel, K. P. Xie, R. Schlögl, W. Schuhmann and M. Muhlert, J. Am. Chem. Soc., 2014, 136, 7551-7554. 22. C. Li, X. P. Han, F. Y. Cheng, Y. X. Hu, C. C. Chen and J. Chen, Nat. Commun., 2015, 6, 7345. 23. P. W. Menezes, A. Indra, A. Bergmann, P. Chernev, C. Walter, H. Dau, P. Strasser and M. Driess, J. Mater. Chem. A, 2016, 4, 10014-10022. 24. T. W. Kim, M. A. Woo, M. Regis and K.-S. Choi, J. Phys. Chem. Lett., 2014, 5, 2370-2374. 25. M. A. Stranick, Sur. Sci. Spectra, 1999, 6, 39. 26. M. Jahan, S. Tominaka and J. Henzie, Dalt. Trans., 2016, 45, 1849418501. 27. M. Oku and K. Hirokawa, J. Electron Spectros. Relat. Phenom., 1976, 8, 475-481. 28. P. W. Menezes, A. Indra, D. Gonzalez-Flores, N. R. Sahraie, I. Zaharieva, M. Schwarze, P. Strasser, H. Dau and M. Driess, ACS Catal., 2015, 5, 2017-2027. 29. A. Indra, P. W. Menezes, C. Das, C. Göbel, M. Tallarida, D. Schmeisser and M. Driess, J. Mater. Chem. A, 2017, 5, 5171-5177. 30. P. W. Menezes, A. Indra, C. Das, C. Walter, C. Göbel, V. Gutkin, D. Schmeisser and M. Driess, ACS Catal., 2017, 7, 103-109. 31. P. W. Menezes, A. Indra, O. Levy, K. Kailasam, V. Gutkin, J. Pfrommer and M. Driess, Chem. Commun., 2015, 51, 5005-5008. 32. X. Liu, Z. Chang, L. Luo, T. Xu, X. Lei, J. Liu and X. Sun, Chem. Mater., 2014, 26, 1889-1895. 33. X. M. Li, X. G. Hao, A. Abudula and G. Q. Guan, J. Mater. Chem. A, 2016, 4, 11973-12000. 34. J. Masa, S. Barwe, C. Andrpnescu, I. Sinev, A. Ruff, K. Jayaramulu, K. Elumeeva, B. Konkena, B. R. Cuenya and W. Schuhmann, ACS Energy Lett., 2016, 1, 1192-1198. 35. B. You, N. Jiang, M. L. Sheng, M. W. Bhushan and Y. J. Sun, ACS Catal., 2016, 6, 714-721. 36. X. G. Wang, W. Li, D. H. Xiong and L. F. Liu, J. Mater. Chem. A, 2016, 4, 5639-5646. 37. M. Risch, F. Ringleb, M. Kohlhoff, P. Bogdanoff, P. Chernev, I. Zaharieva and H. Dau, Energy Environ. Sci., 2015, 8, 661-674. 38. A. Bergmann, E. Martinez-Moreno, D. Teschner, P. Chernev, M. Gliech, J. F. de Araujo, T. Reier, H. Dau and P. Strasser, Nat. Commun., 2015, 6, 8625. 39. C. P. Plaisance and R. A. van Santen, J. Am. Chem. Soc., 2015, 137, 14660-14672. 40. A. Indra, P. W. Menezes, N. R. Sahraie, A. Bergmann, C. Das, M. Tallarida, D. Schmeisser, P. Strasser and M. Driess, J. Am. Chem. Soc., 2014, 136, 17530-17536. 41. J. G. McAlpin, Y. Surendranath, M. Dinca, T. A. Stich, S. A. Stoian, W. H. Casey, D. G. Nocera and R. D. Britt, J. Am. Chem. Soc., 2010, 132, 6882-6883. 42. B. S. Yeo and A. T. Bell, J. Am. Chem. Soc., 2011, 133, 5587-5593.
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TOC Boosting electrochemical water oxidation through replacement of Oh Co sites in cobalt oxide spinel with manganese
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Prashanth W. Menezes, Arindam Indra, Vitaly Gutkin, and Matthias Driess
Effect of substitution in the spinel lattice, structure-activity relation and nature of active species involved during oxygen evolution reaction were uncovered in the manganese cobalt oxide (MCO).
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This article can be cited before page numbers have been issued, to do this please use: M. Driess, P. M. Menezes, A. Indra and V. Gutkin, Chem. Commun., 2017, DOI: 10.1039/C7CC03749J. Volume 52 Number 1 4 January 2016 Pages 1–216
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This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the author guidelines.
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COMMUNICATION Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyen et al. Reactivity differences of Sc3N@C2n (2n = 68 and 80). Synthesis of the first methanofullerene derivatives of Sc3N@D5h-C80
Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the ethical guidelines, outlined in our author and reviewer resource centre, still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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Boosting electrochemical water oxidation through replacement of Oh Co sites in cobalt oxide spinel with manganese Prashanth W. Menezes,a Arindam Indra,a Vitaly Gutkinb and Matthias Driess*a
DOI: 10.1039/x0xx00000x www.rsc.org/
The strikingly high catalytic performance and stability of manganese substituted cobalt oxide spinel (MnxCo3-xO4) over pristine cobalt oxide spinel (Co3O4) for the alkaline electrochemical water oxidation is reported. The different role of cations could be uncovered along with the detection of drastic surface-structural changes during the catalysis using spectroscopic and microscopic methods. Employing renewable and clean energy sources as practical alternatives of fossil fuel-based technologies requires the development of new catalyst systems with higher efficiency and stability.1-3 Electrochemical energy conversion by splitting of water into molecular oxygen and hydrogen has been regarded as one of the most convenient techniques; however, the efficiency of the overall water splitting reactions largely depends on the oxygen evolution reaction (OER) as the bottle neck.4-6 Anodic water oxidation requires four electron and four proton transfer involving several high energy intermediates.7-9 Therefore, designing of the efficient catalyst systems for long-term water oxidation has been a key challenge in recent years. Lately, several groups have shown that transition-metal based catalysts, especially, based on manganese, cobalt and nickel can be utilized as alternatives for effective water oxidation instead of expensive and scarce precious metal-based RuO2 or IrO2 catalysts.1015 Over the years, manganese oxides have stimulated tremendous interest for water oxidation catalysis due to the natural presence of the CaMn4O5 cluster in the oxygen evolving complex of a Department of Chemistry, Metalorganics and Inorganic Materials, Technische Universität Berlin, Strasse des 17 Juni 135, D-10623 Berlin, Germany; E-mail: [email protected] b The Harvey M. Krueger Family Center for Nanoscience and Nanotechnology, The Hebrew University of Jerusalem, Edmond J. Safra Campus, Givat Ram, Jerusalem 91904, Israel † Electronic Supplementary Information (ESI) available: Complete experimental details and characterization of catalysts before and after electrochemical measurements. See DOI: 10.1039/b000000x/
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photosystem II involved in photosynthesis.6,9,15,16 Similarly, enormous efforts have been devoted to the design of suitable cobalt oxide based catalysts with higher efficiency.17-19 Although, pure spinel (A2+B23+O4 where A is in tetrahedral (Td) and B is in octahedral (Oh) coordination with six oxygen atoms) cobalt oxides are well investigated, the preferable substitution of cobalt by manganese in the lattices of Co3O4 has already garnered much attention lately in enhancing the OER activity.20-22 Recent studies show that the Oh centres of a spinel oxide are the most active sites, while the Td ones play a minor role.23,24 This encouraged us to (i) synthesize new materials where Oh metal sites are selectively and partially replaced by Mn (ii) probe them for electrochemical water oxidation catalysis at selected electrode potentials on different substrates in alkaline media, and (iii) in getting a deeper understanding of the water oxidation process by detecting intermediates, near-surface phenomena and elucidating structureactivity relationships of the active catalysts. Here we report the selective synthesis of partially manganese substituted cobalt oxide (MCO; MnxCo3-xO4, x = 0.3) and compared its strikingly high OER activity with those of pristine Co3O4 and Mn2O3. The MCO catalysts were synthesized in multi-gram scale using Mn oxalate, Co oxalate and CoMn-oxalate prepared in the microemulsion approach and their subsequent thermal treatment in synthetic air. The catalysts were electrophoretically deposited on fluorine doped tin oxide (FTO) and on nickel foam (NF) substrates. During alkaline electrochemical water oxidation, a boost in performance was attained for MCO due to the promotion effect of manganese with respect to pristine Co3O4 and Mn2O3 as well as the state of the art precious metal-based catalysts in alkaline media. The presented results prove experimentally the significant effect of Mn substitution in the Co3O4 lattice with a distinct activity in two different pH media on different electrode supports. Furthermore, the surface structural transformation of the MCO catalyst at the onset potential and at an elevated electrode potential of water oxidation was systematically determined by spectroscopic and microscopic
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methods, and thus a structure-activity relation was revealed. The impressively high catalytic activity and stability of MCO could directly be attributed to (i) the higher surface area, (ii) the selective dissolution of Mn from the Oh sites of near-surface of spinel oxide during water oxidation creating a defective and disordered structure with vacancies as well as (iii) the formation of a cobalt rich oxohydroxide-hydroxide shell on the spinel core. The transition-metal oxide catalysts discussed here were synthesized from the corresponding oxalate precursors (see Experimental Section). The powder X-ray diffraction pattern (PXRD) of the one dimensional nanochain manganese substituted cobalt oxalate precursor could be well matched with cobalt oxalate dihydrate with a slight shift in the peak positions due to incorporation of Mn in the crystal lattices (Figs. S1 and S2). The ratio of Co:Mn was determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES, Table S1) and energy dispersive X-ray (EDX, Fig. S3) measurements and the amount of C, H was determined from elemental analysis (Table S2). Corresponding stretching and bending vibrations of the precursor were detected by infra-red (IR, Fig. S4 and Table S3). The nanorods morphology was also clearly visible from the scanning electron microscopy. (SEM, Fig. S5). The oxidation state of Co and Mn was determined to be +2 by X-ray photoelectron spectroscopy (XPS) and the detailed discussion is given in Fig. S6. Calcination of the oxalate precursor in synthetic air produced Mn substituted Co oxide, Mn0.3Co2.7O4 (MCO) and the PXRD pattern could also be matched with spinel Co3O4 with slight peak shifts due to Mn substitution into spinel lattices (Fig. S7 and S8). About 10% substitution of Co by Mn in MCO was observed which was determined from the ICP-AES and EDX analysis (Table S4 and Fig. S9). Complete removal of the oxalate and the formation of phase pure metal oxide were confirmed by IR spectrum (Fig. S10). SEM images indicated that after thermal treatment, the nanorods of the precursor were transformed into one dimensional nanochains (Fig. S11). Transmission electron microscopic (TEM) studies revealed that small nanoparticles of 10-20 nm interconnected to form the one dimensional nanochain (Fig. 1). The lattice spacing of 0.28 nm reflects the (220) plane of MCO and the selected area electron diffraction (SAED) pattern also shows the crystalline nature of the material. The oxidation state of Co and Mn in MCO was determined by XPS analyses (Fig. 2). The deconvoluted Mn2p (Fig. 2, left) showed two peaks at 642.2 eV for Mn2p3/2 and 653.7 eV for Mn 2p1/2 with a spin−orbit level energy spacing of 11.5 eV and is typical for Mn3+ based materials.25,26 This indicates that Mn is preferentially substituted in the Oh sites of spinel oxide. From the literature, it is well known that the Co(II) and Co(III) have similar 2p binding energies but can be differentiated by the Co2p1/2−2p3/2 spin−orbit level energy spacing, which is 16.0 eV for high-spin Co(II) and 15.0 eV for low-spin Co(III).27 The Co2p spectrum (Fig. 2, right) shows peaks at 781.1 eV for Co2p3/2 and 796.5 eV for Co2p1/2 with a difference in energy spacing of 15.4 eV indicating Co in +2/+3 oxidation state.28,29 Both Mn2p and Co2p XPS spectra obtained here are in accordance with the spinel type materials containing Mn and Co.25-29 The O1s spectra showed the corresponding metal-oxygen bond with surface oxygen species (Fig. S12).30,31 The other two
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Journal Name DOI: 10.1039/C7CC03749J catalysts Co3O4 and Mn2O3 have been synthesized and characterized comprehensively. The detail characterizations are described in Fig. S13-S14. Co3O4 has the morphology of one dimensional nanochains whereas Mn2O3 crystallizes in a net like structure. High surface area of Mn2O3 (49 m2 g-1) has also been determined compared to Co3O4 (12 m2 g-1) and MCO (37 m2 g-1).
Fig. 1. The TEM (a, b), HRTEM (c) images with d-spacing 0.28 nm indicating (220) plane and SAED pattern (d) of MCO nanochains.
All three catalysts were deposited on fluorinated tin oxide (FTO) coated glass by electrophoretic deposition and electrochemical water oxidation was first carried out in 0.1 M KOH solution with a three-electrode set up (Fig. S15). The cyclic voltammogram (CV) showed redox peaks (I and II in Fig. S16) between 1.1 and 1.3 V which is ascribed to Co2+/Co3+, with a Co2+/Co3+ transition and the next pair of peaks (III and IV in Fig. S16) in the range of 1.3 to 1.5 V and are associated with Co3+/Co4+ redox transition.23,32 Interestingly, MCO unveiled improved water oxidation activity compared to Co3O4 and Mn2O3 (Fig. S15). This directly proves that the Mn substitution into the cobalt Oh lattices of Co3O4 could be the main reason for the significant enhancement in the catalytic activity. The overpotential required to attain the current density of 10 mAcm-2 for MCO was 390 mV compared to 430 mV for Co3O4 in 0.1 M KOH solution. The catalytic activity of Mn2O3 was poor and not comparable with the other two catalysts. Furthermore, the chronoamperometric (CA) studies were performed for 8 h that showed enhanced stability for MCO, however, about 30% decrease in current was observed for Co3O4 (Fig. S17).
Fig. 2. The Mn2p and Co2p XPS spectra of MCO. To demonstrate the superior activity of as-synthesized catalysts with other highly active transition metal based catalysts
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reported in the literature, we additionally conducted water oxidation experiments in 1 M KOH (Fig. 3). Only a overpotential 320 was resulted for MCO at current density of 10 mAcm-2 which is the one of the lowest among non-precious metal-based catalysts reported in literature (see Table S5) indicating significantly diminished OER activation energies,5,14,19,22,33 while 360 mV and 560 mV of overpotential was obtained for Co3O4 and Mn2O3. The OER activity of MCO is even better than the benchmark state of the art precious catalysts in alkaline media (Fig. 3 and Table S5).
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COMMUNICATION DOI: 10.1039/C7CC03749J electrophoretic deposition of MCO on FTO substrate (Fig. S19). Formation of a thin amorphous shell on the surface of the MCO particles was detected after CA measurements at the 1.53 V (onset potential, Fig. S20). An increase in the thickness of the amorphous shell was observed at the increased potential of 1.62 V (potential at 10 mAcm-2) (Fig. S21). This indeed suggests that, during water oxidation the amorphous shell starts growing and creates more active sites at the near-surface (Scheme 1). In the XPS study, Co2p spectra displayed a spin−orbit level energy spacing of 15.3 eV after applying an onset potential in comparison to the as-synthesised MCO with spacing 15.5 eV (Fig. S22).29,40 This directly indicates that there is an increase in the amount of Co(III) and probably in the surface structure at the onset potential. However, at an applied potential 1.63 V, a large amount of Co(III) was detected (spin−orbit level energy spacing 15.1 eV) from the higher energy shift of the peak that evidences major changes in the near-surface region.23,28 O1s spectra, for both, at onset and at an increased potential indicated higher fraction of –OH groups confirming, the surface of the material is hydroxylated (Fig. S23).28,40 Strikingly, no Mn could be detected at the surface of the films signifying selective dissolution of Mn from the Oh sites of spinel oxide to the electrolyte during water oxidation.
Fig. 3. Cyclic voltammetry (CV) of MCO, Co3O4, Mn2O3 synthesized by similar approach verses Mn2O3 and commercial noble catalysts in 1 M KOH solution with a scan rate of 20 mV/s on FTO substrates (loading ~ 1 mg).
In addition to this, recently, nickel foam (NF) have been used as the support to carry out effective water oxidation due to unique hierarchically structured porous configuration that allows large surface area and low electrical resistance. Therefore, our catalysts were further deposited on NF and explored for water oxidation (Fig. S18). Excellent overpotential of 370 mV was obtained for MCO at a higher current density of 100 mAcm-2. Similarly, 420 mV and 640 mV of overpotential at the same current density were resulted with Co3O4 and Mn2O3. The overpotentials obtained here are superior than most of the transition metal based catalysts (see Table S5) and can directly be compared to the recently reported high performance Ni and Co based phosphides on NF.30,34-36 To uncover the role of cations, to understand the near-surface structural changes during the water oxidation and to deduce a structure-activity relation, we characterized the MCO catalyst extensively before the catalysis as well as after 8 h CA measurements in 0.1 M KOH electrolyte. Recently, the potential dependent transformation of CoII to CoIII and to CoIV in cobalt oxide catalyst was demonstrated by Risch et al. and Bergmann et al.37,38 Similarly, for cobalt oxides, the theoretical studies have also predicted the change in structure at different electrode potentials.39 As the stability of the catalyst largely depends on the applied electrode potentials, we investigated the actual structure and the nature of the catalyst by applying two selected electrode potentials: (i) at onset and (ii) at an electrode potential after reaching to the current density of 10 mAcm-2. TEM and HRTEM studies revealed that the particle morphology and corresponding structure (could be seen from SAED) remained intact after the
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Scheme 1. The near-surface structural reorganization of MCO at the onset as well as at the elevated oxygen evolution potential.
Several studies on cobalt based spinel catalysts have already revealed that the Oh Co3+ sites are the real catalytically active sites in OER.23,24,38 From the extensive microscopic and spectroscopic analysis, the following elucidation for the superior activity of MCO in contrast to pristine cobalt oxide can be deduced. First of all, the substitution of Mn into cobalt of spinel oxide takes place selectively at the Oh (Co3+) lattices. Mn3+ in Oh sites provides more disorder in the structure of the spinel having eg1 configuration, i.e, uneven occupation in the antibonding orbital. This indeed enhances the overall surface area by four times creating more active species at the near-surface for catalysis.6 In addition, the role of unprecedented nanochains morphology of MCO with clean surfaces cannot be neglected. Further understanding on the nature of active species and the structure-activity relation was achieved from the TEM, HRTEM and XPS measurements which indicated loss of Mn (as seen from ICP-AES) transpires selectively from the Oh lattices during electrochemical water oxidation. At onset potential, only a partial structural reformation occurs that is limited to the outermost surface of the MCO catalyst by forming a thin amorphous layer (scheme 1). As electrode potential and oxygen evolution rate increases, the crystallinity further decreases and forms a more pronounced amorphous (Co rich) shell with disordered structure with cationic
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vacancies on the surface, which helps in enhancing the electrical conductivity, transport of –OH ions and felicitates bonding.32,41,42 This leads to defects and greater amount of hydroxylation (Co(OH)2CoOOH) that alters the coordination environment of Co of MCO. In addition to this, the Td Co2+ site at the near-surface also get oxidized to Oh Co3+ and accelerates the water oxidation reaction very efficiently and the obtained results here also substantiates the recent findings of Risch et al. and Bergmann et al. on cobalt oxide catalysts.37,38 In conclusion, a facile, easily scalable method for the synthesis of Mn substituted Co oxide nanochains were developed with the single-source precursor approach and investigated for the alkaline electrochemical water oxidation at the onset and elevated electrode potential. Mn substitution into the Oh lattice of Co-oxide substantially improved the catalytic activity as well as stability of the system. The amorphous shell on the surface of the crystalline MCO at the onset and at the increased electrode potential demonstrates that severe structural alterations occur creating a disordered structure with vacancies leading to the formation of an oxohydroxidehydroxide shell of cobalt on the spinel core due to the selective dissolution of Mn from the near-surface zone which indeed are catalytically active species. This work was financially supported by DFG (Cluster of Excellence UniCat, EXC 314/2). We thank Dr. Caren Göbel for the TEM measurements. Notes and references 1. M. G. Walter, E. L. Warren, J. R. McKone, S. W. Boettcher, Q. X. Mi, E. A. Santori and N. S. Lewis, Chem. Rev., 2010, 110, 6446-6473. 2. J. Luo, J.-H. Im, M. T. Mayer, M. Schreier, M. K. Nazeeruddin, N.-G. Park, S. D. Tilley, H. J. Fan and M. Gratzel, Science, 2014, 345, 15931596. 3. A. Indra, A. Acharjya, P. W. Menezes, C. Merschjann, D. Hollmann, M. Schwarze, M. Aktas, A. Friedrich, S. Lochbrunner, A. Thomas and M. Driess, Angew. Chem. Int. Ed., 2017, 56, 1653-1657. 4. Y. Yan, B. Y. Xia, B. Zhao and X. Wang, J. Mater. Chem. A, 2016, 4, 17587-17603. 5. J. H. Wang, W. Cui, Q. Liu, Z. C. Xing, A. M. Asiri and X. P. Sun, Adv. Mater., 2016, 28, 215-230. 6. A. Indra, P. W. Menezes, I. Zaharieva, E. Baktash, J. Pfrommer, M. Schwarze, H. Dau and M. Driess, Angew. Chem. Int. Ed., 2013, 52, 1320613210. 7. H. Dau, C. Limberg, T. Reier, M. Risch, S. Roggan and P. Strasser, ChemCatChem, 2010, 2, 724-761. 8. J. Rossmeisl, Z. W. Qu, H. Zhu, G. J. Kroes and J. K. Norskov, J. Electroanal. Chem., 2007, 607, 83-89. 9. P. W. Menezes, A. Indra, P. Littlewood, M. Schwarze, C. Göbel, R. Schomäcker and M. Driess, ChemSusChem, 2014, 7, 2202-2211. 10. B. M. Hunter, H. B. Gray and A. M. Müller, Chem. Rev., 2016, 116, 14120-14136. 11. M. M. Najafpour, G. Renger, M. Holynska, A. N. Moghaddam, E. M. Aro, R. Carpentier, H. Nishihara, J. J. Eaton-Rye, J. R. Shen and S. I. Allakhverdiev, Chem. Rev., 2016, 116, 2886-2936. 12. M. Gong and H. J. Dai, Nano Res., 2015, 8, 23-39. 13. X. H. Deng and H. Tuysuz, ACS Catal., 2014, 4, 3701-3714. 14. S. Jung, C. C. L. McCrory, I. M. Ferrer, J. C. Peters and T. F. Jaramillo, J. Mater. Chem. A, 2016, 4, 3068-3076. 15. A. Indra, P. W. Menezes and M. Driess, ChemSusChem, 2015, 8, 776785.
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TOC Boosting electrochemical water oxidation through replacement of Oh Co sites in cobalt oxide spinel with manganese
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Prashanth W. Menezes, Arindam Indra, Vitaly Gutkin, and Matthias Driess
Effect of substitution in the spinel lattice, structure-activity relation and nature of active species involved during oxygen evolution reaction were uncovered in the manganese cobalt oxide (MCO).
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