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DOI: 10.1039/C5CC00242G
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Received 00th January 2012, Accepted 00th January 2012
Electrochemical Driven Water Oxidation by Molecular Catalysts in situ Polymerized on the Surface of Graphite Carbon Electrode Lei Wanga, Ke Fana, Quentin Daniela, Lele Duana, Fusheng Lia, Bertrand Philippeb, Håkan Rensmob and Licheng Suna,c*
DOI: 10.1039/x0xx00000x www.rsc.org/
A simple strategy to immobilize highly efficient ruthenium based molecular water-oxidation catalysts on the basal-plane pyrolytic graphite electrode (BPG) by polymerization has been demonstrated. The electrode 1@BPG has obtained a high initial turnover frequency (TOF) of 10.47 s-1 at ~700 mV overpotential, and a high turnover number (TON) up to 31600 in 1 h electrolysis.
Water oxidation is widely considered as the bottleneck of water splitting in artificial photosynthesis.1 After the pioneering study on molecular water oxidation catalysts (WOCs) by Meyer and coworkers,2 great efforts have been made on developing highly efficient WOCs,3-9 including the [Ru(bda)L2] types of WOCs designed by our group.5 The family of [Ru(bda)L2] WOCs have shown extremely high catalytic activities (TOF over 1000 s-1)5, 27 for homogeneous water oxidation; however it is achieved by using the strong chemical oxidant (NH4)2Ce(NO3)6 (CeIV) under acidic conditions. The harsh conditions dramatically reduce the lifetimes of these catalysts. In order to fairly evaluate the catalyst performance under the experimental conditions closer to reality and also to move from the half reaction to total water splitting, immobilization of a molecular catalyst onto an electrode surface to fabricate an electrochemical-driven water oxidation device is a good way to go.10 Approaches of electrode functionalization with inorganic or organometallic catalysts for water splitting11-16 can be summarized by the following methods: first, through bonding of a catalyst with a functional group, such as carboxylic and phosphoric acid;12, 14 second, by π-π stacking interaction between a catalyst and an electrode surface via introducing an aromatic
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group;11, 13 the third, attaching a catalyst on an electrode by electro-polymerization.17-22 Recently, Li et al. immobilized a pyrene-functioned [Ru(bda)L2] catalyst onto a carbon nanotube electrode by the method of π- π stacking.13 The corresponding electrode prepared by this method showed a TOF of 0.3 s-1 at ~600 mV overpotential. The reason for this relatively low TOF is probably due to the behavior of the catalyst on the surface of electrode is unfavorable for the radical coupling pathway; this crucial pathway for high catalytic efficiency in homogeneous system was well studied by our group.5 Therefore, in order to build a water oxidation device with higher efficiency, we need to introduce a new immobilization method which might lead the [Ru(bda)L2] catalysts go through the radical coupling pathway.
Figure 1. Chemical structures of Ru–bda (H2bda: 2,2 ′ −bipyridine−6,6′−dicarboxylic acid) complexes 1 and 2 with pyrrole–appended bridges, and the scheme of polymerized monomers. Based on these considerations, we introduce here a electrochemical water oxidation device by in situ polymerization
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(Figure S6) distinctly show redox peaks at 0.85 V, 1.07 V and 0.86 V, 1.06 V respectively attributable to E (RuIV/III) and E (RuV/IV). In addition, all of these peak potentials of the immobilized catalysts have been listed in Table S1 together with that of RuO2. X-ray Photoelectron Spectroscopy (XPS) were carried out to further prove the successful immobilization, and also exclude the formation of the RuO2 (Figure S5). The data indicates that no obvious RuO2 was formed in our systems. As can be seen in Figure S8, better linearity was obtained from the plots of current vs. scan rate(ν) than that of current vs. square root of scan rate, indicating that the Ru catalysts have been embedded on the electrode surface. In addition, the energy dispersive X-ray spectroscopy (EDS) shows the surface loading of the Ru catalysts (Figure S9). All of the above evident clearly confirmed that both catalysts 1 and 2 were attached on the surface of BPG. 8 pristine BPG 1 @BPG 2 @BPG
6 4 2 0 0.3
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E (V vs.NHE)
Figure. 2 CV curves based on different electrode: 1@BPG (red line), 2@BPG (blue line) and pristine BPG (black dash line). Conditions: pH 7.2 phosphate buffer (0.1 M, IS = 0.1), scan rate: 100 mV s−1. The surface coverage of catalyst for both 1@BPG and 2@BPG were estimated according to the literature method and the followed equation 1.26 (1) -1
Where F (Faraday’s constant) = 96485 C mol , n (number of electrons transfer) = 1, R (ideal gas constant) = 8.314 JK–1mol–1, T = 298 K, A = electrode surface area (cm2), Γ = surface catalyst loading (mol cm–1), current (A), scan rate (υ, V/s). The slope can be obtained from the peak current plots vs υ (Figure. S8), which is supposed to be a linear relationship for catalyst-modified electrodes since the RuII–RuIII process is considered as a Nernstian reaction. Average surface catalyst concentrations of 4.2 × 10–10 and 6.8 × 10–10 mol cm–2 were respectively determined for 1@ BPG and 2@ BPG. Taking 1@BPG as an example, we applied a potential-step chrono-amperometric method (range from 1.19 to 1.59 V) to evaluate the electrocatalytic activity. As can be seen in Figure 3, the background current at high potential may due to the oxidation of the graphite working electrode, this may cause certain surface area changing and further affect the catalytic current, however the influence is very tiny. By subtracting the background of pristine BPG, the obtained catalytic current density at steadystate for oxidizing water plotted vs overpotential (ƞ) indicates a Tafel behavior (Figure 3 inset). The unusual big slope of 245 mV
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of [Ru(bda)L2] catalyst on the surface of graphite carbon electrode. The structures of catalysts are shown in Figure 1. A pyridine ligand covalently linked to a pyrrole with different carbon chains is coordinated to the [Ru(bda)(4iodopyridine)DMSO] complex, forming the final catalysts 1 and 2. This strategy may provide the following advantages: (a) the flexible linker of tunable length in the catalyst allows the high possibility of a radical coupling pathway; (b) the pyrrole anchoring group of the catalyst for polymerization is replaceable by other functional groups; (c) the axial ligand of the catalyst is changeable to improve the catalytic efficiency; (d) this method can be applied not only to graphite carbon electrode but also to other carbon and metal based alternative electrodes.23, 24 Complexes 1 and 2 were prepared and fully characterized by NMR and mass spectrometry (see SI). Two pyrrole-appended pyridine ligands were synthetized in two steps according to the literature (SI). The synthesis of the unsymmetric Ru–bda complexes has been reported elsewhere.25 We here introduced a more efficient synthetic method: firstly the intermediate [Ru(bda)(DMSO)2] was prepared by reacting cis-Ru(DMSO)4Cl2 with H2bda in methanol in the presence of triethylamine; 1 equivalent of pyrrole-appended pyridine ligand was then added to the intermediate solution in the presence of DMSO; and another equivalent 4-iodopyridine was added at end to complete the ligand exchange reaction. This one-pot two-step reaction obtained a tremendously higher yield (rising from 10% to 60%) of the products than the method reported in our previous work. The water oxidation activities of the prepared catalysts were evaluated in homogeneous systems by the reported menthod.25 As shown in Figure S2, complex 1 (TON: 5360 ± 289) shows better performance than 2 (TON: 4420 ± 236). The kinetic study proves that the water oxidation is in second order regarding to catalyst 1 (Figure S3), indicating a radical coupling reaction mechanism as reported in our previous work.5 This result means that the relatively long bridge might still keep the opportunity for the catalyst to perform the same radical coupling pathway for water oxidation even after the immobilization on electrode surface. A basal plane pyrolytic graphite electrode (BPG) was used as the substrate for the catalyst loading. The catalyst loading was conducted by a well-known pyrrole-oxidized electropolymerization method.18,21 20 times of successive cyclic voltammograms in a relatively high concentration (10-2 M) of catalyst in organic solvent were performed to immobilize the catalyst onto the electrode surface (Figure S4). The modified electrodes 1@BPG and 2@BPG, respectively by catalyst 1 and 2, were then rinsed with absolute ethanol to remove physically adsorbed complexes. Figure 2 exhibits CVs of a buffer electrolyte based on these functionalized electrodes and a pristine BPG for comparison. As expected, no redox signal appeared for the pristine BPG. In contrast, both 1@BPG and 2@BPG clearly show reversible redox couples for E (RuIII/II) at 0.69 V and 0.67 V respectively in CV curves (here all of the potentials are vs. NHE), and onset potential for water oxidation around 1.1 V. By comparison with the CVs of complexes 1 and 2 in homogeneous solutions (Figure S6), there was no remarkable shift of the E (RuIII/II) after immobilization, but a dramatically increased current density indicating the high surface catalyst concentration. In addition, differential pulse voltammetries (DPVs) of 1@BPG and 2@BPG
Journal Name DOI: 10.1039/C5CC00242G
Current density (mA/cm2)
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(2)
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To confirm the O2 production, controlled-potential bulk electrolysis was carried out in a pH 7.2 phosphate buffer solution. Figure 4 shows the water-electrolysis performance of both electrode 1@BPG and 2@BPG within one hour period at 1.54 V vs. NHE. The sufficient catalytic charge was passed through the BPG electrode for generation of O2 gas which was confirmed by GC (Figure S11). The total TONs of ~31600 and 20500 were determined respectively for 1@BPG and 2@BPG from the GC data. The corresponding Faraday efficiency is 90 ± 6 % for both electrodes. Although high TONs were obtained, the current density decreases with time, which is probably caused by the dissociation of the catalysts from the surface based on the CV after electrolysis (Figure S12).
4.0
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log I (A/cm-2)
2.0 -2.8 1.8 -3.0 -3.2 1.6 -3.4 1.4 -3.6 1.2 -3.8 1.0 -4.0 0.4 0.8 0.6 0.4 0.2 TOF =-1 0.64 s 0.0 -0.2 -0.4 0.3 0.4
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Figure 4. Potentiostatic water electrolysis on 1@BPG (red line) and 2@BPG (blue line) in pH 7.2 buffer (0.1 M, IS = 0.1).
Figure. 3 Upper: Chronoamperometric current density measured for 1@BPG under pH 7.2 buffered solutions. Below: Tafel plots of TOF and current density (inset) as a function of ƞ for 1@BPG.
5 1@BPG 1 + polypyrrole @BPG
4 )
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Where F is Faraday constant, Q is the integrated charge pass through the working electrode (background subtracted). As shown in Figure 3b, an initial TOF = 0.64 s–1 was obtained at ƞ = 0.37 V, which rose up to 10.47 s–1 at ƞ = 0.77 V.
The Pourbaix diagram of 1@BPG was recorded in aqueous solutions (Figure S10). In comparison with our previous work,5 an identical proton coupling electron transfer process was found under low pH range (under the pKa value of RuII–OH2, 4.5). The difference is, however, the proton was removed at pH 7 in the RuII–OH2 to RuIII–OH step. Nevertheless, we believe that the formed RuV ＝ O is still possible to go through the radical coupling pathway. The detailed mechanism was proposed on the basis of the combined experimental data, as shown in Scheme 1.5,
3 2 1 0 0
2000
4000
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Scheme 1. The proposed pathways of 1@ BPG at pH 7.
electrochemical
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O2-generation
Figure 5. Potentiostatic water electrolysis based on 1@BPG (blue line) and 1 + polypyrrole@BPG (red line) at 1.54 V vs NHE in pH 7.2 phosphate buffer (0.1 M, IS = 0.1). The unsatisfactory durability is one of the pressing issues of water splitting device. Meyer group recently applied an atomic
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may due to the low catalyst loading. Assuming that the Faraday efficiency is 90% (see the result from electrolysis), the TOFs of 1@BPG for water oxidation under different overpotentials can be estimated by the following equation 2.16
ChemComm
layer deposition (ALD) method to have largely improved the device stability.28 Following a similar idea, we have here introduced a protection layer of polypyrrole to prevent the catalyst from dissociation. This protection layer was simply achieved by further electro-polymerization on the catalyst modified electrode in a pyrrole solution. As shown in Figure 5, under this protection, the stability in the late phase was improved significantly as expected. CVs of the electrode show tiny changes before and after electrolysis (Figure S13), indicating a good stability of the immobilized catalyst on the surface. A possible reason for the lower initial current density is the coverage of the Ru catalyst by polypyrrole which blocks the access of water to the catalyst. This problem needs further verification and as predicated can be solved by introducing materials with a high porousity, and the relative work is undergoing. In summary, we have successfully immobilized our molecular water oxidation catalysts in situ on the conductive carbon surfaces via polymerization. This design by enriching the catalysts on the electrode surface allows certain possibility for the radical coupling process in water oxidation and also greatly improves the catalytic efficiency. The length of the linkage doesn’t have to be very long by comparing 1@BPG and 2@BPG. The TON over 30000 of 1@BPG was obtained by 1 hour bulk electrolysis at 700 mV overpotential. To the best of our knowledge, TOF of 10.47 s–1 (ƞ = 0.77 V) is a very high number ever reported on the electrode surface. This high efficiency provides an opportunity for light driven water splitting with a high quantum yield. In principle this immobilization method is generally applicable for tunable catalysts structures and more importantly won’t affect the existing catalytic property of the catalyst. Application of this method in the light-driven water oxidation setup is undergoing in our group. Such work will encouragingly provide candidates for solar–driven water– splitting devices and help to accelerate the transition to future sustainable energy systems. We thank the Swedish Research Council, K & A Wallenberg Foundation, Swedish Energy Agency, China Scholarship Council (CSC), National Natural Science Foundation of China (21120102036) and the National Basic Research Program of China (2014CB239402) for financial support of this work.
Notes and references a
Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden, E–mail: [email protected] b Department of Physics and Astronomy, Uppsala University, Box 516, 751 20, Uppsala, Sweden c State Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, P. R. China 1. M. Hambourger, G. F. Moore, D. M. Kramer, D. Gust, A. L. Moore and T. A. Moore, Chem. Soc. Rev., 2009, 38, 25-35. 2. S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Soc., 1982, 104, 4029-4030. 3. J. D. Blakemore, N. D. Schley, D. Balcells, J. F. Hull, G. W. Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig and R. H. Crabtree, J. Am. Chem. Soc., 2010, 132, 16017-16029.
4 | J. Name., 2012, 00, 1-3
Journal Name DOI: 10.1039/C5CC00242G 4. J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2008, 130, 16462-16463. 5. L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet and L. Sun, Nat Chem, 2012, 4, 418-423. 6. L. Wang, L. Duan, B. Stewart, M. Pu, J. Liu, T. Privalov and L. Sun, J. Am. Chem. Soc., 2012, 134, 18868-18880. 7. M. D. Kärkäs, T. Åkermark, H. Chen, J. Sun and B. Åkermark, Angew. Chem., Int. Ed., 2013, 52, 4189-4193. 8. R. Zong and R. P. Thummel, J. Am. Chem. Soc., 2005, 127, 1280212803. 9. D. J. Wasylenko, C. Ganesamoorthy, M. A. Henderson, B. D. Koivisto, H. D. Osthoff and C. P. Berlinguette, J. Am. Chem. Soc., 2010, 132, 16094-16106. 10. J. R. McKone, N. S. Lewis and H. B. Gray, Chemistry of Materials, 2013, 26, 407-414. 11. J. D. Blakemore, A. Gupta, J. J. Warren, B. S. Brunschwig and H. B. Gray, J. Am. Chem. Soc., 2013, 135, 18288-18291. 12. Z. Chen, J. J. Concepcion, J. W. Jurss and T. J. Meyer, J. Am. Chem. Soc., 2009, 131, 15580-15581. 13. F. Li, B. Zhang, X. Li, Y. Jiang, L. Chen, Y. Li and L. Sun, Angew. Chem., Int. Ed., 2011, 50, 12276-12279. 14. K. S. Joya, N. K. Subbaiyan, F. D'Souza and H. J. M. de Groot, Angew. Chem., Int. Ed., 2012, 51, 9601-9605. 15. F. M. Toma, A. Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato, S. Rapino, B. R. Gonzalez, H. Amenitsch, T. Da Ros, L. Casalis, A. Goldoni, M. Marcaccio, G. Scorrano, G. Scoles, F. Paolucci, M. Prato and M. Bonchio, Nat Chem, 2010, 2, 826-831. 16. L. Tong, M. Gothelid and L. Sun, Chem. Commun., 2012, 48, 1002510027. 17. Z. Fang, S. Keinan, L. Alibabaei, H. Luo, A. Ito and T. J. Meyer, Angew. Chem., Int. Ed., 2014, 53, 4872-4876. 18. T. R. O'Toole, B. P. Sullivan, M. R. M. Bruce, L. D. Margerum, R. W. Murray and T. J. Meyer, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1989, 259, 217-239. 19. P. Denisevich, H. D. Abruna, C. R. Leidner, T. J. Meyer and R. W. Murray, Inorg. Chem., 1982, 21, 2153-2161. 20. D. L. Ashford, A. M. Lapides, A. K. Vannucci, K. Hanson, D. A. Torelli, D. P. Harrison, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2014, 136, 6578-6581. 21. J. Mola, E. Mas-Marza, X. Sala, I. Romero, M. Rodríguez, C. Viñas, T. Parella and A. Llobet, Angew. Chem., Int. Ed., 2008, 47, 5830-5832. 22. A. Krawicz, J. Yang, E. Anzenberg, J. Yano, I. D. Sharp and G. F. Moore, J. Am. Chem. Soc., 2013, 135, 11861-11868. 23. I. Villarreal, E. Morales, T. F. Otero and J. L. Acosta, Synthetic Metals, 2001, 123, 487-492. 24. M. Zhou and J. Heinze, J Phys Chem B, 1999, 103, 8451-8457. 25. Y. Gao, X. Ding, J. Liu, L. Wang, Z. Lu, L. Li and L. Sun, J. Am. Chem. Soc., 2013, 135, 4219-4222. 26. A. J. F. Bard, L. R., Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley: New York, 2001. 27. L. Wang, L. Duan, Y. Wang, M. S. G. Ahlquist and L. Sun, Chem. Commun., 2014, 50, 12947-12950. 28. A. K. Vannucci, L. Alibabaei, M. D. Losego, J. J. Concepcion, B. Kalanyan, G. N. Parsons and T. J. Meyer, Proc. Natl. Acad.Sci. U. S. A., 2013, 110, 20918-20922.
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Insert Table of Contents artwork here
A simple strategy to immobilize highly efficient ruthenium based molecular water-oxidation catalysts on the basal-plane pyrolytic graphite electrode (BPG) by electro-polymerization has been demonstrated with a high initial turnover frequency (TOF) of 10.47 s-1 at ~700 mV overpotential, and a high turnover number (TON) up to 31600 in 1 h electrolysis.
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This article can be cited before page numbers have been issued, to do this please use: L. Sun, L. Wang, K. Fan, Q. Daniel, F. Li, H. Rensmo, B. Philippe and L. Duan, Chem. Commun., 2015, DOI:
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 Information for Authors. 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 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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Journal Name
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Cite this: DOI: 10.1039/x0xx00000x
Received 00th January 2012, Accepted 00th January 2012
Electrochemical Driven Water Oxidation by Molecular Catalysts in situ Polymerized on the Surface of Graphite Carbon Electrode Lei Wanga, Ke Fana, Quentin Daniela, Lele Duana, Fusheng Lia, Bertrand Philippeb, Håkan Rensmob and Licheng Suna,c*
DOI: 10.1039/x0xx00000x www.rsc.org/
A simple strategy to immobilize highly efficient ruthenium based molecular water-oxidation catalysts on the basal-plane pyrolytic graphite electrode (BPG) by polymerization has been demonstrated. The electrode 1@BPG has obtained a high initial turnover frequency (TOF) of 10.47 s-1 at ~700 mV overpotential, and a high turnover number (TON) up to 31600 in 1 h electrolysis.
Water oxidation is widely considered as the bottleneck of water splitting in artificial photosynthesis.1 After the pioneering study on molecular water oxidation catalysts (WOCs) by Meyer and coworkers,2 great efforts have been made on developing highly efficient WOCs,3-9 including the [Ru(bda)L2] types of WOCs designed by our group.5 The family of [Ru(bda)L2] WOCs have shown extremely high catalytic activities (TOF over 1000 s-1)5, 27 for homogeneous water oxidation; however it is achieved by using the strong chemical oxidant (NH4)2Ce(NO3)6 (CeIV) under acidic conditions. The harsh conditions dramatically reduce the lifetimes of these catalysts. In order to fairly evaluate the catalyst performance under the experimental conditions closer to reality and also to move from the half reaction to total water splitting, immobilization of a molecular catalyst onto an electrode surface to fabricate an electrochemical-driven water oxidation device is a good way to go.10 Approaches of electrode functionalization with inorganic or organometallic catalysts for water splitting11-16 can be summarized by the following methods: first, through bonding of a catalyst with a functional group, such as carboxylic and phosphoric acid;12, 14 second, by π-π stacking interaction between a catalyst and an electrode surface via introducing an aromatic
This journal is © The Royal Society of Chemistry 2012
group;11, 13 the third, attaching a catalyst on an electrode by electro-polymerization.17-22 Recently, Li et al. immobilized a pyrene-functioned [Ru(bda)L2] catalyst onto a carbon nanotube electrode by the method of π- π stacking.13 The corresponding electrode prepared by this method showed a TOF of 0.3 s-1 at ~600 mV overpotential. The reason for this relatively low TOF is probably due to the behavior of the catalyst on the surface of electrode is unfavorable for the radical coupling pathway; this crucial pathway for high catalytic efficiency in homogeneous system was well studied by our group.5 Therefore, in order to build a water oxidation device with higher efficiency, we need to introduce a new immobilization method which might lead the [Ru(bda)L2] catalysts go through the radical coupling pathway.
Figure 1. Chemical structures of Ru–bda (H2bda: 2,2 ′ −bipyridine−6,6′−dicarboxylic acid) complexes 1 and 2 with pyrrole–appended bridges, and the scheme of polymerized monomers. Based on these considerations, we introduce here a electrochemical water oxidation device by in situ polymerization
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(Figure S6) distinctly show redox peaks at 0.85 V, 1.07 V and 0.86 V, 1.06 V respectively attributable to E (RuIV/III) and E (RuV/IV). In addition, all of these peak potentials of the immobilized catalysts have been listed in Table S1 together with that of RuO2. X-ray Photoelectron Spectroscopy (XPS) were carried out to further prove the successful immobilization, and also exclude the formation of the RuO2 (Figure S5). The data indicates that no obvious RuO2 was formed in our systems. As can be seen in Figure S8, better linearity was obtained from the plots of current vs. scan rate(ν) than that of current vs. square root of scan rate, indicating that the Ru catalysts have been embedded on the electrode surface. In addition, the energy dispersive X-ray spectroscopy (EDS) shows the surface loading of the Ru catalysts (Figure S9). All of the above evident clearly confirmed that both catalysts 1 and 2 were attached on the surface of BPG. 8 pristine BPG 1 @BPG 2 @BPG
6 4 2 0 0.3
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0.9
1.2
1.5
1.8
E (V vs.NHE)
Figure. 2 CV curves based on different electrode: 1@BPG (red line), 2@BPG (blue line) and pristine BPG (black dash line). Conditions: pH 7.2 phosphate buffer (0.1 M, IS = 0.1), scan rate: 100 mV s−1. The surface coverage of catalyst for both 1@BPG and 2@BPG were estimated according to the literature method and the followed equation 1.26 (1) -1
Where F (Faraday’s constant) = 96485 C mol , n (number of electrons transfer) = 1, R (ideal gas constant) = 8.314 JK–1mol–1, T = 298 K, A = electrode surface area (cm2), Γ = surface catalyst loading (mol cm–1), current (A), scan rate (υ, V/s). The slope can be obtained from the peak current plots vs υ (Figure. S8), which is supposed to be a linear relationship for catalyst-modified electrodes since the RuII–RuIII process is considered as a Nernstian reaction. Average surface catalyst concentrations of 4.2 × 10–10 and 6.8 × 10–10 mol cm–2 were respectively determined for 1@ BPG and 2@ BPG. Taking 1@BPG as an example, we applied a potential-step chrono-amperometric method (range from 1.19 to 1.59 V) to evaluate the electrocatalytic activity. As can be seen in Figure 3, the background current at high potential may due to the oxidation of the graphite working electrode, this may cause certain surface area changing and further affect the catalytic current, however the influence is very tiny. By subtracting the background of pristine BPG, the obtained catalytic current density at steadystate for oxidizing water plotted vs overpotential (ƞ) indicates a Tafel behavior (Figure 3 inset). The unusual big slope of 245 mV
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of [Ru(bda)L2] catalyst on the surface of graphite carbon electrode. The structures of catalysts are shown in Figure 1. A pyridine ligand covalently linked to a pyrrole with different carbon chains is coordinated to the [Ru(bda)(4iodopyridine)DMSO] complex, forming the final catalysts 1 and 2. This strategy may provide the following advantages: (a) the flexible linker of tunable length in the catalyst allows the high possibility of a radical coupling pathway; (b) the pyrrole anchoring group of the catalyst for polymerization is replaceable by other functional groups; (c) the axial ligand of the catalyst is changeable to improve the catalytic efficiency; (d) this method can be applied not only to graphite carbon electrode but also to other carbon and metal based alternative electrodes.23, 24 Complexes 1 and 2 were prepared and fully characterized by NMR and mass spectrometry (see SI). Two pyrrole-appended pyridine ligands were synthetized in two steps according to the literature (SI). The synthesis of the unsymmetric Ru–bda complexes has been reported elsewhere.25 We here introduced a more efficient synthetic method: firstly the intermediate [Ru(bda)(DMSO)2] was prepared by reacting cis-Ru(DMSO)4Cl2 with H2bda in methanol in the presence of triethylamine; 1 equivalent of pyrrole-appended pyridine ligand was then added to the intermediate solution in the presence of DMSO; and another equivalent 4-iodopyridine was added at end to complete the ligand exchange reaction. This one-pot two-step reaction obtained a tremendously higher yield (rising from 10% to 60%) of the products than the method reported in our previous work. The water oxidation activities of the prepared catalysts were evaluated in homogeneous systems by the reported menthod.25 As shown in Figure S2, complex 1 (TON: 5360 ± 289) shows better performance than 2 (TON: 4420 ± 236). The kinetic study proves that the water oxidation is in second order regarding to catalyst 1 (Figure S3), indicating a radical coupling reaction mechanism as reported in our previous work.5 This result means that the relatively long bridge might still keep the opportunity for the catalyst to perform the same radical coupling pathway for water oxidation even after the immobilization on electrode surface. A basal plane pyrolytic graphite electrode (BPG) was used as the substrate for the catalyst loading. The catalyst loading was conducted by a well-known pyrrole-oxidized electropolymerization method.18,21 20 times of successive cyclic voltammograms in a relatively high concentration (10-2 M) of catalyst in organic solvent were performed to immobilize the catalyst onto the electrode surface (Figure S4). The modified electrodes 1@BPG and 2@BPG, respectively by catalyst 1 and 2, were then rinsed with absolute ethanol to remove physically adsorbed complexes. Figure 2 exhibits CVs of a buffer electrolyte based on these functionalized electrodes and a pristine BPG for comparison. As expected, no redox signal appeared for the pristine BPG. In contrast, both 1@BPG and 2@BPG clearly show reversible redox couples for E (RuIII/II) at 0.69 V and 0.67 V respectively in CV curves (here all of the potentials are vs. NHE), and onset potential for water oxidation around 1.1 V. By comparison with the CVs of complexes 1 and 2 in homogeneous solutions (Figure S6), there was no remarkable shift of the E (RuIII/II) after immobilization, but a dramatically increased current density indicating the high surface catalyst concentration. In addition, differential pulse voltammetries (DPVs) of 1@BPG and 2@BPG
Journal Name DOI: 10.1039/C5CC00242G
Current density (mA/cm2)
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(2)
1.59 V
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To confirm the O2 production, controlled-potential bulk electrolysis was carried out in a pH 7.2 phosphate buffer solution. Figure 4 shows the water-electrolysis performance of both electrode 1@BPG and 2@BPG within one hour period at 1.54 V vs. NHE. The sufficient catalytic charge was passed through the BPG electrode for generation of O2 gas which was confirmed by GC (Figure S11). The total TONs of ~31600 and 20500 were determined respectively for 1@BPG and 2@BPG from the GC data. The corresponding Faraday efficiency is 90 ± 6 % for both electrodes. Although high TONs were obtained, the current density decreases with time, which is probably caused by the dissociation of the catalysts from the surface based on the CV after electrolysis (Figure S12).
4.0
45
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log I (A/cm-2)
2.0 -2.8 1.8 -3.0 -3.2 1.6 -3.4 1.4 -3.6 1.2 -3.8 1.0 -4.0 0.4 0.8 0.6 0.4 0.2 TOF =-1 0.64 s 0.0 -0.2 -0.4 0.3 0.4
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Figure 4. Potentiostatic water electrolysis on 1@BPG (red line) and 2@BPG (blue line) in pH 7.2 buffer (0.1 M, IS = 0.1).
Figure. 3 Upper: Chronoamperometric current density measured for 1@BPG under pH 7.2 buffered solutions. Below: Tafel plots of TOF and current density (inset) as a function of ƞ for 1@BPG.
5 1@BPG 1 + polypyrrole @BPG
4 )
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Where F is Faraday constant, Q is the integrated charge pass through the working electrode (background subtracted). As shown in Figure 3b, an initial TOF = 0.64 s–1 was obtained at ƞ = 0.37 V, which rose up to 10.47 s–1 at ƞ = 0.77 V.
The Pourbaix diagram of 1@BPG was recorded in aqueous solutions (Figure S10). In comparison with our previous work,5 an identical proton coupling electron transfer process was found under low pH range (under the pKa value of RuII–OH2, 4.5). The difference is, however, the proton was removed at pH 7 in the RuII–OH2 to RuIII–OH step. Nevertheless, we believe that the formed RuV ＝ O is still possible to go through the radical coupling pathway. The detailed mechanism was proposed on the basis of the combined experimental data, as shown in Scheme 1.5,
3 2 1 0 0
2000
4000
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Scheme 1. The proposed pathways of 1@ BPG at pH 7.
electrochemical
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O2-generation
Figure 5. Potentiostatic water electrolysis based on 1@BPG (blue line) and 1 + polypyrrole@BPG (red line) at 1.54 V vs NHE in pH 7.2 phosphate buffer (0.1 M, IS = 0.1). The unsatisfactory durability is one of the pressing issues of water splitting device. Meyer group recently applied an atomic
J. Name., 2012, 00, 1-3 | 3
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may due to the low catalyst loading. Assuming that the Faraday efficiency is 90% (see the result from electrolysis), the TOFs of 1@BPG for water oxidation under different overpotentials can be estimated by the following equation 2.16
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layer deposition (ALD) method to have largely improved the device stability.28 Following a similar idea, we have here introduced a protection layer of polypyrrole to prevent the catalyst from dissociation. This protection layer was simply achieved by further electro-polymerization on the catalyst modified electrode in a pyrrole solution. As shown in Figure 5, under this protection, the stability in the late phase was improved significantly as expected. CVs of the electrode show tiny changes before and after electrolysis (Figure S13), indicating a good stability of the immobilized catalyst on the surface. A possible reason for the lower initial current density is the coverage of the Ru catalyst by polypyrrole which blocks the access of water to the catalyst. This problem needs further verification and as predicated can be solved by introducing materials with a high porousity, and the relative work is undergoing. In summary, we have successfully immobilized our molecular water oxidation catalysts in situ on the conductive carbon surfaces via polymerization. This design by enriching the catalysts on the electrode surface allows certain possibility for the radical coupling process in water oxidation and also greatly improves the catalytic efficiency. The length of the linkage doesn’t have to be very long by comparing 1@BPG and 2@BPG. The TON over 30000 of 1@BPG was obtained by 1 hour bulk electrolysis at 700 mV overpotential. To the best of our knowledge, TOF of 10.47 s–1 (ƞ = 0.77 V) is a very high number ever reported on the electrode surface. This high efficiency provides an opportunity for light driven water splitting with a high quantum yield. In principle this immobilization method is generally applicable for tunable catalysts structures and more importantly won’t affect the existing catalytic property of the catalyst. Application of this method in the light-driven water oxidation setup is undergoing in our group. Such work will encouragingly provide candidates for solar–driven water– splitting devices and help to accelerate the transition to future sustainable energy systems. We thank the Swedish Research Council, K & A Wallenberg Foundation, Swedish Energy Agency, China Scholarship Council (CSC), National Natural Science Foundation of China (21120102036) and the National Basic Research Program of China (2014CB239402) for financial support of this work.
Notes and references a
Department of Chemistry, KTH Royal Institute of Technology, 10044 Stockholm, Sweden, E–mail: [email protected] b Department of Physics and Astronomy, Uppsala University, Box 516, 751 20, Uppsala, Sweden c State Key Laboratory of Fine Chemicals, DUT–KTH Joint Education and Research Center on Molecular Devices, Dalian University of Technology (DUT), Dalian 116024, P. R. China 1. M. Hambourger, G. F. Moore, D. M. Kramer, D. Gust, A. L. Moore and T. A. Moore, Chem. Soc. Rev., 2009, 38, 25-35. 2. S. W. Gersten, G. J. Samuels and T. J. Meyer, J. Am. Chem. Soc., 1982, 104, 4029-4030. 3. J. D. Blakemore, N. D. Schley, D. Balcells, J. F. Hull, G. W. Olack, C. D. Incarvito, O. Eisenstein, G. W. Brudvig and R. H. Crabtree, J. Am. Chem. Soc., 2010, 132, 16017-16029.
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Journal Name DOI: 10.1039/C5CC00242G 4. J. J. Concepcion, J. W. Jurss, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2008, 130, 16462-16463. 5. L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov, A. Llobet and L. Sun, Nat Chem, 2012, 4, 418-423. 6. L. Wang, L. Duan, B. Stewart, M. Pu, J. Liu, T. Privalov and L. Sun, J. Am. Chem. Soc., 2012, 134, 18868-18880. 7. M. D. Kärkäs, T. Åkermark, H. Chen, J. Sun and B. Åkermark, Angew. Chem., Int. Ed., 2013, 52, 4189-4193. 8. R. Zong and R. P. Thummel, J. Am. Chem. Soc., 2005, 127, 1280212803. 9. D. J. Wasylenko, C. Ganesamoorthy, M. A. Henderson, B. D. Koivisto, H. D. Osthoff and C. P. Berlinguette, J. Am. Chem. Soc., 2010, 132, 16094-16106. 10. J. R. McKone, N. S. Lewis and H. B. Gray, Chemistry of Materials, 2013, 26, 407-414. 11. J. D. Blakemore, A. Gupta, J. J. Warren, B. S. Brunschwig and H. B. Gray, J. Am. Chem. Soc., 2013, 135, 18288-18291. 12. Z. Chen, J. J. Concepcion, J. W. Jurss and T. J. Meyer, J. Am. Chem. Soc., 2009, 131, 15580-15581. 13. F. Li, B. Zhang, X. Li, Y. Jiang, L. Chen, Y. Li and L. Sun, Angew. Chem., Int. Ed., 2011, 50, 12276-12279. 14. K. S. Joya, N. K. Subbaiyan, F. D'Souza and H. J. M. de Groot, Angew. Chem., Int. Ed., 2012, 51, 9601-9605. 15. F. M. Toma, A. Sartorel, M. Iurlo, M. Carraro, P. Parisse, C. Maccato, S. Rapino, B. R. Gonzalez, H. Amenitsch, T. Da Ros, L. Casalis, A. Goldoni, M. Marcaccio, G. Scorrano, G. Scoles, F. Paolucci, M. Prato and M. Bonchio, Nat Chem, 2010, 2, 826-831. 16. L. Tong, M. Gothelid and L. Sun, Chem. Commun., 2012, 48, 1002510027. 17. Z. Fang, S. Keinan, L. Alibabaei, H. Luo, A. Ito and T. J. Meyer, Angew. Chem., Int. Ed., 2014, 53, 4872-4876. 18. T. R. O'Toole, B. P. Sullivan, M. R. M. Bruce, L. D. Margerum, R. W. Murray and T. J. Meyer, Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, 1989, 259, 217-239. 19. P. Denisevich, H. D. Abruna, C. R. Leidner, T. J. Meyer and R. W. Murray, Inorg. Chem., 1982, 21, 2153-2161. 20. D. L. Ashford, A. M. Lapides, A. K. Vannucci, K. Hanson, D. A. Torelli, D. P. Harrison, J. L. Templeton and T. J. Meyer, J. Am. Chem. Soc., 2014, 136, 6578-6581. 21. J. Mola, E. Mas-Marza, X. Sala, I. Romero, M. Rodríguez, C. Viñas, T. Parella and A. Llobet, Angew. Chem., Int. Ed., 2008, 47, 5830-5832. 22. A. Krawicz, J. Yang, E. Anzenberg, J. Yano, I. D. Sharp and G. F. Moore, J. Am. Chem. Soc., 2013, 135, 11861-11868. 23. I. Villarreal, E. Morales, T. F. Otero and J. L. Acosta, Synthetic Metals, 2001, 123, 487-492. 24. M. Zhou and J. Heinze, J Phys Chem B, 1999, 103, 8451-8457. 25. Y. Gao, X. Ding, J. Liu, L. Wang, Z. Lu, L. Li and L. Sun, J. Am. Chem. Soc., 2013, 135, 4219-4222. 26. A. J. F. Bard, L. R., Electrochemical Methods: Fundamentals and Applications, 2nd ed., John Wiley: New York, 2001. 27. L. Wang, L. Duan, Y. Wang, M. S. G. Ahlquist and L. Sun, Chem. Commun., 2014, 50, 12947-12950. 28. A. K. Vannucci, L. Alibabaei, M. D. Losego, J. J. Concepcion, B. Kalanyan, G. N. Parsons and T. J. Meyer, Proc. Natl. Acad.Sci. U. S. A., 2013, 110, 20918-20922.
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A simple strategy to immobilize highly efficient ruthenium based molecular water-oxidation catalysts on the basal-plane pyrolytic graphite electrode (BPG) by electro-polymerization has been demonstrated with a high initial turnover frequency (TOF) of 10.47 s-1 at ~700 mV overpotential, and a high turnover number (TON) up to 31600 in 1 h electrolysis.
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