English

• The increasing threat of the intense global energy demands and pressing environmental concerns have activated extensive research on clean energy conversion and storage systems with high efficiency, low-cost and environmental benignity [1-6]. Hydrogen is considered to be one of the most ideal energy sources due to its good combustion performance, ideal heating value, and wide utilization forms [7, 8]. Water splitting is regarded as one of the most effective methods to generate H₂ [1, 9-14].

  Water splitting involves two half reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). OER, as one of the half reactions of water splitting, is challenging both thermodynamically (2H₂O → O₂ + 4H⁺ + 4e^-, ΔE = 1.23 V vs. normal hydrogen electrode) and kinetically (significant rearrangement of atoms) [13]. Therefore, the design and development of highly active and stable catalysts for OER is urgently needed for energy conversion. As a result, chemists and material scientists have paid tremendous efforts in this area. In the past decades, RuO₂ and IrO₂ are considered to be the best OER catalysts in both acidic and alkaline environments [15-19]. Despite their high activity, the large scale use of these noble-metal-based catalysts is limited by their low natural abundance and high cost [20]. As an alternative strategy, cheap first-row transition-metal-based materials and complexes have been largely studied as electrocatalysts for OER [21-31].

  For many decades, porphyrin derivatives have attracted great interests and have been widely studied in many research areas due to their unique structures and properties [21, 22, 32-37]. Metal porphyrins and corroles are also a class of promising electrocatalysts because of their robust property in both acidic and basic solutions. However, most molecular catalysts are used in homogeneous catalysis, which not only has low utilizations but also has great limitations in practical applications. There are two main reasons: one is that only a small fraction of the catalyst in the diffusion layer will work in the catalytic process; the other is that products separation and catalyst regeneration are difficult. To address these issues, several strategies have been developed to make molecular catalysts heterogeneous. An efficient approach is covalent or noncovalent grafting of molecular catalysts on conductive materials, such as graphenes, carbon nanotubes, metal oxides [38-50]. Although considerable efforts have been devoted to grafting molecular catalysts on conductive materials, there is still a problem that the amount of loaded catalysts may be too small. Electrodeposition is widely used to produce metal-based material catalysts, which has proven to be an effective strategy for the preparation of highly active sites and highly efficient water oxidation catalysts [51]. Inspired by these results, electropolymerization of molecular catalysts with specific functional groups on carbon materials has attracted increasing attentions for OER [51], oxygen reduction reaction (ORR) [52], and carbon dioxide reduction reaction (CO₂RR) [53]. Herein, we report the electropolymerization of Co porphyrins and corroles on conductive carbon cloth (CC) electrode for OER.

  Four monomeric cobalt porphyrins and corroles with different functional groups were designed and used for polymerization. Metal-free porphyrins, tetrakis(p-N-pyrrolylphenyl)porphyrin (1-P) [54], tetrakis(thien-3-yl)-porphyrin (2-P) [55], 5, 10, 15-tris(4-aminophenyl)corrole (1-C) [52, 56] and 5, 10, 15-tris(3-thienyl)corrole (2-C) [57, 58] were synthesized by using literature methods, and their purity was verified by mass spectrometry (Figs. S3, S5, S8, S10 in Supporting information). The reaction of porphyrin and corrole ligands with Co salts afforded Co-1-P, Co-2-P, Co-1-C and Co-2-C, respectively (Fig. 1). The purity of the bulk samples of Co-1-P, Co-2-P, Co-1-C and Co-2-C was confirmed by mass spectrometry (Figs. S4, S6, S9, S11 in Supporting information). The cyclic voltammograms (CVs) of these Co complexes were recorded in dry acetonitrile solution containing 0.1 mol/L Bu₄NPF₆. In general, Co-1-P, Co-2-P and Co-1-C showed two redox events, while Co-2-C showed three redox events (Figs. S12–S15 in Supporting information). This result also proved the purity of these four complexes.

  Figure 1

  

  Figure 1.  Structure diagram of (a) Co-1-P, (b) Co-2-P, (c) Co-1-C, (d) Co-2-C.

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  Electropolymerization was conducted by using a CC electrode as the working electrode. The CC electrode was dipped into an acetonitrile solution containing 0.1 mol/L Bu₄NPF₆ and a specific Co complex (1 mmol/L) with the use of a carbon rod as the auxiliary electrode and Ag/AgNO₃ as the reference electrode [59, 60]. After 60 cycles scanning between 0 V and 1.7 V (vs. ferrocene) under N₂, the working electrode was gently rinsed with dichloromethane and ethanol, and then was finally dried at room temperature under dark.

  After electropolymerization, the resulted hybrids were analyzed by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) element mapping, showing the absence of irregular particles of metal porphyrins and corroles (Fig. 2 and Fig. S16 in Supporting information). As shown in Fig. 2, comparing to bare CC electrodes (Fig. 2a), Co-1-P@CC exhibited significant complex loading (Fig. 2b). The SEM image of Co-1-P@CC (Fig. 2c) and the corresponding EDX elemental mapping images of C (Fig. 2d), N (Fig. 2e) and Co (Fig. 2f) clearly confirmed the electrodeposition and uniform distribution of Co porphyrins on the surfaces of CC electrode. Moreover, the XPS analysis of Co-1-P@CC showed signals attributed to Co 2p (796.8 and 780.6 eV), which is identical to that of Co-1-P molecular complex, further confirming the presence of Co-1-P on CC electrode (Fig. S17 in Supporting information). For Co-2-P, Co-1-P and Co-2-C, XPS (Figs. S18–20 in Supporting information) and EDX elemental mapping (Figs. S21–23 in Supporting information) results clearly showed signals of Co, N and C, further confirming the presence of these Co porphyrins and corroles in their electrodeposited films. Moreover, Co-1-P, Co-2-P, Co-1-C and Co-2-C were analyzed by Raman. Comparing to the simply adsorbed monomeric Co porphyrin and Co corrole on CC electrodes, electropolymerized films displayed the decrease of the I_D/I_G ratio (Figs. S24–27 in Supporting information). This result indicates that polymerization reaction occurred between the molecules.

  Figure 2

  

  Figure 2.  (a) SEM images of blank CC. (b, c) SEM images of Co-1-P@CC. (d-f) Corresponding elemental mapping of Co-1-P@CC.

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  A three-electrode system was selected to test the OER property of electropolymerized samples in 0.1 mol/L KOH solutions. As shown in Fig. 3a, the blank CC exhibited very poor catalytic current of OER. The linear sweep voltammetry (LSV) of Co-1-P@CC displayed a significant catalytic current with the onset potential of 1.61 V versus RHE (reversible hydrogen electrode, all potentials recorded in aqueous solutions are referenced to RHE unless otherwise noted). This value corresponds to an onset overpotential of 380 mV, which is smaller than those of Co-2-P@CC (409 mV), Co-1-C@CC (430 mV) and Co-2-C@CC (405 mV). Particularly, Co-1-P@CC revealed the highest current density under the same applied potentials (i.e., 1.75 V). Encouraged by the above results, we also studied the effects of electroplating times on catalytic performance. As shown in Fig. 3b, the catalytic current increased with the increase of the electrodeposition CV cycle numbers. However, Co-1-P is easily fallen away from the CC electrode after more than 60-cycle CV. Thus, CC electrodes electroplymerized through 60-cycle CV were used for the following measurements. In addition, we normalized the activity with the amount of Co contents on CC electrode. The Co contents of four samples were determined by the inductively coupled plasma mass spectrometry (ICP-MS, Table S1 in Supporting information). As shown in Fig. 3c, Co-1-P@CC still displayed the highest catalytic performance. Tafel slope of Co-1-P@CC (70.8 mV/dec) is smaller than that of Co-2-P@CC (85.5 mV/dec), Co-1-C@CC (96.9 mV/dec) and Co-2-C@CC (156.3 mV/dec), indicating good kinetics during OER (Fig. 3d).

  Figure 3

  

  Figure 3.  (a) LSVs of blank CC, Co-1-P@CC, Co-2-P@CC, Co-1-C@CC and Co-2-C@CC in 0.1 mol/L KOH solution with a scan rate of 10 mV/s. (b) LSVs of Co-1-P@CC electrodes prepared from different CV cycle numbers. (c) Normalized OER activities by Co content. (d) Tafel slopes of blank CC, Co-1-P@CC, Co-2-P@CC, Co-1-C@CC and Co-2-C@CC.

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  Controlled potential electrolysis (CPE) for OER in 0.1 mol/L KOH solutions was performed to examine the catalyst stability. The currents of Co-1-P@CC were constant in 10 h electrolysis (Fig. 4a). The corrected electric charges accumulated during electrolysis have a linear dependence on the time (Fig. 4b), which is a further line of evidence of the robustness of Co-1-P@CC. After CPE, the electrode was analyzed, showing almost identical LSV scans of Co-1-P@CC before and after electrolysis (Fig. 4c). These results are supportive of the catalyst stability for water oxidation. During electrolysis, O₂ gas bubbles were generated on the electrode surface. By using a gas-tight electrochemical cell, the amount of O₂ formed on the headspace can be determined by gas chromatography, which gave a Faradaic efficiency > 95% (Fig. 4d).

  Figure 4

  

  Figure 4.  (a) Controlled potential electrolysis of Co-1-P@CC at 1.65 V in 0.1 mol/L KOH solutions. (b) The electric charge curve of Co-1-P@CC during 10 h CPE. (c) LSV scans of Co-1-P@CC before and after electrolysis. (d) Gas chromatography detection of evolved O₂ during electrolysis with Co-1-P@CC at 1.65 V and the theoretical amount of O₂ produced.

  DownLoad: Full-Size Img PowerPoint

  In summary, a simple electropolymerization method has been developed to immobilize molecules of Co porphyrins and corroles on conductive carbon substrates. The resulted catalyst, Co-1-P@CC, display an onset overpotential of 380 mV for OER and a Tafel slope of 70.8 mV/dec in 0.1 mol/L KOH solutions and show high stability over 10 h CPE. In addition, the electrocatalysts is tightly attached to the electrode substrate to form a monolithic electrode without using any binder polymer, increasing stability during the catalytic process and reducing overpotential due to ohmic resistance. This work provides a new strategy by loading active species onto a solid surface for water splitting reactions, which is valuable to be explored in other electrocatalytic processes.

  Declaration of competing interest

  There are no conflicts to declare.

  Acknowledgments

  We are grateful for support from National Natural Science Foundation of China (Nos. 21773146 and 21902099), China Postdoctoral Science Foundation (No. 2018M631120), Shaanxi Province Postdoctoral Science Foundation (No. 2018BSHEDZZ107), Fundamental Research Funds for the Central Universities (Nos. GK202103045 and GK202103050), Research funds of Shaanxi Normal University, and the open fund of State Key Laboratory of Structural Chemistry.

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2021.04.048.

  
  
• 1. [1]

     W. Zhang, W. Lai, R. Cao, Chem. Rev. 117(2017) 3717-3797. doi: 10.1021/acs.chemrev.6b00299

  2. [2]

     H. Lei, H. Fang, Y. Han, et al., ACS Catal. 5(2015) 5145-5153. doi: 10.1021/acscatal.5b00666

  3. [3]

     B. Wang, X. Cui, J. Huang, R. Cao, Q. Zhang, Chin. Chem. Lett. 29(2018) 1757-1767. doi: 10.1016/j.cclet.2018.11.021

  4. [4]

     H. Lei, X. Li, J. Meng, et al., ACS Catal. 9(2019) 4320-4344. doi: 10.1021/acscatal.9b00310

  5. [5]

     X.P. Zhang, A. Chandra, Y.M. Lee, et al., Chem. Soc. Rev. 50(2021) 4804-4811. doi: 10.1039/D0CS01456G

  6. [6]

     Y. Liu, Y. Han, Z. Zhang, et al., Chem. Sci. 10(2019) 2613-2622. doi: 10.1039/C8SC04529A

  7. [7]

     R. Blankenship, D. Tiede, J. Barber, et al., Science 332(2011) 805-809. doi: 10.1126/science.1200165

  8. [8]

     D. Gust, T. Moore, A. Moore, Acc. Chem. Res. 42(2009) 1890-1898. doi: 10.1021/ar900209b

  9. [9]

     Y. Tong, H. Liu, M. Dai, L. Xiao, X. Wu, Chin. Chem. Lett. 31(2020) 2295-2299. doi: 10.1016/j.cclet.2020.03.029

  10. [10]

      G. Xu, H. Lei, G. Zhou, et al., Chem. Commun 55(2019) 12647-12650. doi: 10.1039/C9CC06916J

  11. [11]

      M.G. Walter, E.L. Warren, J.R. McKone, et al., Chem. Rev. 110(2010) 6446-6473. doi: 10.1021/cr1002326

  12. [12]

      X. Gao, Y. Chen, T. Sun, et al., Small 15(2019) e1904579. doi: 10.1002/smll.201904579

  13. [13]

      H. Lei, A. Han, F. Li, et al., Phys. Chem. Chem. Phys. 16(2014) 1883-1893. doi: 10.1039/C3CP54361G

  14. [14]

      X. Guo, N. Wang, X. Li, et al., Angew. Chem. Int. Ed. 59(2020) 8941-8946. doi: 10.1002/anie.202002311

  15. [15]

      H.J. Choi, N. Ashok Kumar, J.B. Baek, Nanoscale 7(2015) 6991-6998. doi: 10.1039/C4NR06831A

  16. [16]

      Z. Chen, D. Higgins, A. Yu, L. Zhang, J. Zhang, Energy Environ. Sci. 4(2011) 3167-3192. doi: 10.1039/c0ee00558d

  17. [17]

      Y. Nie, L. Li, Z. Wei, Chem. Soc. Rev. 44(2015) 2168-2201. doi: 10.1039/C4CS00484A

  18. [18]

      Y. Deng, L. Yang, Y. Wang, et al., Chin. Chem. Lett. 32(2021) 511-515. doi: 10.1016/j.cclet.2020.03.076

  19. [19]

      C. Wang, L. Jin, H. Shang, et al., Chin. Chem. Lett. 32(2021) 2108-2116. doi: 10.1016/j.cclet.2020.11.051

  20. [20]

      H. Qin, Y. Wang, B. Wang, et al., J. Energy Chem. 53(2021) 77-81. doi: 10.1016/j.jechem.2020.05.015

  21. [21]

      T. Nakazono, A.R. Parent, K. Sakai, Chem. Commun. 49(2013) 6325-6327. doi: 10.1039/c3cc43031f

  22. [22]

      D. Wang, J.T. Groves, Proc. Natl. Acad. Sci. U. S. A. 110(2013) 15579-15584. doi: 10.1073/pnas.1315383110

  23. [23]

      J.D. Blakemore, R.H. Crabtree, G.W. Brudvig, Chem. Rev. 115(2015) 12974-13005. doi: 10.1021/acs.chemrev.5b00122

  24. [24]

      D.K. Dogutan, Jr. McGuire R., D.G. Nocera, J. Am. Chem. Soc. 133(2011) 9178-9180. doi: 10.1021/ja202138m

  25. [25]

      D.G. Hetterscheid, J.N. Reek, Angew. Chem. Int. Ed. 51(2012) 9740-9747. doi: 10.1002/anie.201202948

  26. [26]

      M.D. Kärkäs, O. Verho, E.V. Johnston, B. Åkermar, Chem. Rev. 114(2014) 11863-12001. doi: 10.1021/cr400572f

  27. [27]

      M.M. Najafpour, G. Renger, M. Holynska, et al., Chem. Rev. 116(2016) 2886-2936. doi: 10.1021/acs.chemrev.5b00340

  28. [28]

      W. Schofberger, F. Faschinger, S. Chattopadhyay, et al., Angew. Chem. Int. Ed. 55(2016) 2350-2355. doi: 10.1002/anie.201508404

  29. [29]

      W. Sinha, A. Mizrahi, A. Mahammed, B. Tumanskii, Z. Gross, Inorg. Chem. 57(2018) 478-485. doi: 10.1021/acs.inorgchem.7b02696

  30. [30]

      B. Wurster, D. Grumelli, D. Hötger, R. Gutzler, K. Kern, J. Am. Chem. Soc. 138(2016) 3623-3626. doi: 10.1021/jacs.5b10484

  31. [31]

      L. Xu, H. Lei, Z. Zhang, et al., Phys. Chem. Chem. Phys. 19(2017) 9755-9761. doi: 10.1039/C6CP08495H

  32. [32]

      Y. Han, Y. Wu, W. Lai, R. Cao, Inorg. Chem. 54(2015) 5604-5613. doi: 10.1021/acs.inorgchem.5b00924

  33. [33]

      Z. Liang, H. Guo, G. Zhou, et al., Angew. Chem. Int. Ed. 60(2021) 8472-8476. doi: 10.1002/anie.202016024

  34. [34]

      L. Xie, X.P. Zhang, B. Zhao, et al., Angew. Chem. Int. Ed. 60(2021) 7576-7581. doi: 10.1002/anie.202015478

  35. [35]

      Z. Liang, H. Wang, H. Zheng, W. Zhang, R. Cao, Chem. Soc. Rev. 50(2021) 2540-2581. doi: 10.1039/D0CS01482F

  36. [36]

      L. Xie, X. Li, B. Wang, et al., Angew. Chem. Int. Ed. 58(2019) 18883-18887. doi: 10.1002/anie.201911441

  37. [37]

      L. Xie, J. Tian, Y. Ouyang, et al., Angew. Chem. Int. Ed. 59(2020) 15844-15848. doi: 10.1002/anie.202003836

  38. [38]

      X. Li, H. Lei, J. Liu, et al., Angew. Chem. Int. Ed. 57(2018) 15070-15075. doi: 10.1002/anie.201807996

  39. [39]

      Z. Chen, J.J. Concepcion, H. Luo, et al., J. Am. Chem. Soc. 132(2010) 17670-17673. doi: 10.1021/ja107347n

  40. [40]

      E.L. Demeter, S.L. Hilburg, N.R. Washburn, T.J. Collins, J.R. Kitchin, J. Am. Chem. Soc. 136(2014) 5603-5606. doi: 10.1021/ja5015986

  41. [41]

      S. Gentil, D. Serre, C. Philouze, et al., Angew. Chem. Int. Ed. 55(2016) 2517-2520. doi: 10.1002/anie.201509593

  42. [42]

      I. Hijazi, T. Bourgeteau, R. Cornut, et al., J. Am. Chem. Soc. 136(2014) 6348-6354. doi: 10.1021/ja500984k

  43. [43]

      M. Jahan, Q. Bao, K.P. Loh, J. Am. Chem. Soc. 134(2012) 6707-6713. doi: 10.1021/ja211433h

  44. [44]

      P. Kang, S. Zhang, T.J. Meyer, M. Brookhart, Angew. Chem. Int. Ed. 53(2014) 8709-8713. doi: 10.1002/anie.201310722

  45. [45]

      A. Maurin, M. Robert, J. Am. Chem. Soc. 138(2016) 2492-2495. doi: 10.1021/jacs.5b12652

  46. [46]

      S. Kim, D. Jang, J. Lim, et al., ChemSusChem 10(2017) 3473-3481. doi: 10.1002/cssc.201701038

  47. [47]

      M. Tavakkoli, M. Nosek, J. Sainio, et al., ACS Catal. 7(2017) 8033-8041. doi: 10.1021/acscatal.7b02878

  48. [48]

      P.D. Tran, A. Le Goff, J. Heidkamp, et al., Angew. Chem. Int. Ed. 50(2011) 1371-1374. doi: 10.1002/anie.201005427

  49. [49]

      P.J. Wei, G.Q. Yu, Y. Naruta, J.G. Liu, Angew. Chem. Int. Ed. 53(2014) 6659-6663. doi: 10.1002/anie.201403133

  50. [50]

      J. Wang, L. Xu, T. Wang, et al., Adv. Energy Mater. (2021) 2003575.

  51. [51]

      D.L. Ashford, A.M. Lapides, A.K. Vannucci, et al., J. Am. Chem. Soc. 136(2014) 6578-6581. doi: 10.1021/ja502464s

  52. [52]

      A. Friedman, L. Landau, S. Gonen, Z. Gross, L. Elbaz, ACS Catal. 8(2018) 5024-5031. doi: 10.1021/acscatal.8b00876

  53. [53]

      X.M. Hu, Z. Salmi, M. Lillethorup, et al., Chem. Commun. 52(2016) 5864-5867. doi: 10.1039/C6CC00982D

  54. [54]

      A. Bettelheinv, B.A. White, S.A. Raybuck, R.W. Murray, Inorg. Chem. 26(1987) 1009-1017. doi: 10.1021/ic00254a011

  55. [55]

      W. Chen, J. Akhigbe, C.B. Ckner, C.M. Li, Y. Lei, J. Phys. Chem. C 114(2010) 8633-8638. doi: 10.1021/jp101011f

  56. [56]

      M.G. Walter, C.C. Wamser, J. Phys. Chem. C 114(2010) 7563-7574. doi: 10.1021/jp910016h

  57. [57]

      N. Maiti, J. Lee, S.J. Kwon, et al., Polyhedron 25(2006) 1519-1530. doi: 10.1016/j.poly.2005.10.016

  58. [58]

      N. Maiti, J. Lee, Y. Do, H.S. Shin, D.G. Churchill, J. Chem. Crystallogr. 35(2005) 949-955. doi: 10.1007/s10870-005-5178-9

  59. [59]

      A.N. Marianov, Y. Jiang, ACS Sustainable Chem. Eng. 7(2019) 3838-3848. doi: 10.1021/acssuschemeng.8b04735

  60. [60]

      A.N. Marianov, Y. Jiang, Appl. Catal. B 244(2019) 881-888. doi: 10.1016/j.apcatb.2018.11.084

• 

  Figure 1  Structure diagram of (a) Co-1-P, (b) Co-2-P, (c) Co-1-C, (d) Co-2-C.

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  Figure 2  (a) SEM images of blank CC. (b, c) SEM images of Co-1-P@CC. (d-f) Corresponding elemental mapping of Co-1-P@CC.

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  Figure 3  (a) LSVs of blank CC, Co-1-P@CC, Co-2-P@CC, Co-1-C@CC and Co-2-C@CC in 0.1 mol/L KOH solution with a scan rate of 10 mV/s. (b) LSVs of Co-1-P@CC electrodes prepared from different CV cycle numbers. (c) Normalized OER activities by Co content. (d) Tafel slopes of blank CC, Co-1-P@CC, Co-2-P@CC, Co-1-C@CC and Co-2-C@CC.

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  Figure 4  (a) Controlled potential electrolysis of Co-1-P@CC at 1.65 V in 0.1 mol/L KOH solutions. (b) The electric charge curve of Co-1-P@CC during 10 h CPE. (c) LSV scans of Co-1-P@CC before and after electrolysis. (d) Gas chromatography detection of evolved O₂ during electrolysis with Co-1-P@CC at 1.65 V and the theoretical amount of O₂ produced.

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