Nanostructured copper oxide electrodeposited from copper(II) complexes as an active catalyst for electrocatalytic oxygen evolution reaction

    Nanostructured copper oxide electrodeposited from copper(II) complexes as an active catalyst for electrocatalytic oxygen evolution reaction Xiang Liu, Hongxing Jia, Zijun Sun, Haiyan Chen, Peng Xu, Pingwu Du PII: DOI: Reference:

S1388-2481(14)00170-2 doi: 10.1016/j.elecom.2014.05.029 ELECOM 5174

To appear in:

Electrochemistry Communications

Received date: Revised date: Accepted date:

10 May 2014 21 May 2014 22 May 2014

Please cite this article as: Xiang Liu, Hongxing Jia, Zijun Sun, Haiyan Chen, Peng Xu, Pingwu Du, Nanostructured copper oxide electrodeposited from copper(II) complexes as an active catalyst for electrocatalytic oxygen evolution reaction, Electrochemistry Communications (2014), doi: 10.1016/j.elecom.2014.05.029

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ACCEPTED MANUSCRIPT Nanostructured copper oxide electrodeposited from copper(II) complexes as an active catalyst for electrocatalytic oxygen

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evolution reaction

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Xiang Liu, Hongxing Jia, Zijun Sun, Haiyan Chen, Peng Xu, Pingwu Du* Department of Materials Science and Engineering, Department of Chemistry, CAS Key Laboratory of Materials for Energy Conversion, University of Science and

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Technology of China, Hefei, China 230026.

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*To whom correspondence should be addressed E-mail: [email protected] Tel./fax: +8655163606207.

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ACCEPTED MANUSCRIPT Abstract: In this report we show that nanostructured copper oxide thin films electrodeposited from copper(II) complexes can catalyze the oxygen evolution

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reaction. Cyclic voltammetry and bulk electrolysis with copper oxide film

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electrode in alkaline aqueous solutions showed significant catalytic currents. The catalyst film was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, energy-dispersive X-ray analysis and X-ray

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electrocatalyst for water oxidation.

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diffraction. The results identify that nanostructured copper oxide is an active

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Keywords: Transition metal; Electrocatalyst; Copper; Oxygen evolution

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ACCEPTED MANUSCRIPT 1. Introduction An efficient catalytic oxygen evolution reaction (OER, 2H2O → O2 + 4H+ + 4e-)

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is a requirement for one half of a water splitting system and still remains a great

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challenge in the conversion of electricity or solar energy into chemical fuels [1-3]. Synthetic complexes and metal oxides of ruthenium and iridium have been developed

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as water oxidation catalysts (WOCs) since the late 1970s [4-8]. However, owing to the low abundance and high cost of these noble metals, limitations may arise for the

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use of these catalysts in real applications. Therefore, efforts have also been made to find efficient and robust first-row transition metal-based WOCs, such as manganese

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[9], cobalt [3, 10], nickel [11], copper [12-15], and iron [16]. Among these elements,

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copper is a very cheap metal but has been less explored for catalytic water oxidation.

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Since 2013, Cu(II) complexes have been reported as novel homogeneous WOCs for oxygen evolution. Cu(II) complexes with a 2,2′-bipyridine [12] or a triglycylglycine

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macrocyclic ligand [15] showed good catalytic activity in alkaline solution. Simple inorganic copper ions can also catalyze water oxidation in a carbonate solution at pH = 10.8 [14].

In contrast to the above-mentioned homogeneous systems for oxygen evolution reaction in copper(II) solutions, herein we report a heterogeneous system for catalytic water oxidation based on a nanostructured copper oxide catalyst, which can be electrodeposited from molecular copper(II) 2-pyridylmethylamine complexes (Fig. 1a). To the best of our knowledge, these types of copper(II) complexes have not been

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ACCEPTED MANUSCRIPT previously reported for electrocatalytic water oxidation and the formation of copper

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oxide nanoparticles as an active electrocatalyst.

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2. Experimental 2.1 Materials and characterization.

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All chemicals were obtained from Aldrich and used without any further purification. All electrolyte solutions were prepared with deionized water (resistivity:

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18 MΩ·cm). The synthesis of 2-pyridylmethylamine ligands and copper complexes was carried out according to a reported method [17]. Scanning electron microscopy

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(SEM) images and energy-dispersive X-ray analysis (EDX) data were obtained with a SIRION200 Schottky field emission scanning electron microscope equipped with a

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Rontec EDX system. X-ray photoelectron spectroscopy (XPS) data and the valence states of metal elements were probed with an ESCALAB 250 instrument. Powder

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X-ray diffraction (XRD) was measured by D/max-TTR III via graphite monochromatized Cu Kα radiation of 1.54178 Å, operating at 40 kV and 200 mA. The scanning rate was 5˚ min-1 in 2θ. 2.2 Electrochemical methods. All electrochemical experiments were performed at room temperature with a CHI602E potentialstat (Shanghai chenhua Instrument Co., Ltd.) and an Ag/AgCl reference electrode (3 M KCl, 0.210 V vs. NHE). All the potentials are quoted versus NHE. Pt wire was used as the counter electrode, and a fluorine doped tin oxide (FTO) electrode was used as the working electrode. Bulk electrolysis was carried out at 4

ACCEPTED MANUSCRIPT variable potentials in a 0.1 M borate electrolyte. The concentrations of the copper(II) complexes 1-2 and CuCl2 were fixed at 0.68 mM. The Faradaic efficiency was

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determined using a fluorescence based oxygen sensor (Ocean Optics) for the

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quantitative detection of O2.

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3. Results and discussion

The cyclic voltammograms (CVs) of compound 1, [CuII(TPA)H2O](ClO4)2 (TPA

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= tris(2-pyridylmethyl)amine), were run in a 0.1 M borate solution using a FTO electrode (Fig. 1b). The onset of an oxidation wave was observed at about 1.2 V in a

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pH 9.2 solution, and gas bubbles were clearly observed on the FTO surface. The bubbles were further confirmed to be oxygen by both gas chromatography and a

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fluorescence based oxygen sensor, indicating that water oxidation reaction occurred during the anodic scan. The current densities for catalytic water oxidation are highly

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dependent on pH. Higher catalytic current intensities were observed when the pH was increased to 11.0. The black plot in Fig. 1b shows no appreciable catalytic wave in the absence of a copper(II) complex, indicating that the existence of 1 is essential for the catalytic reaction. Fig. 1c shows the CVs using a glassy carbon electrode, which presents an oxidative catalytic current peak at Ep,a= ~1.65 V (Ep,a is the oxidative peak potential), and in the reverse scan a current crossover appears (Fig. 1c, inset). The values of Ep,a have only slight differences (1.60-1.65 V) but the catalytic current densities increase from pH 7.0 to 11.0. As discussed in the earlier studies [12-14], the crossover feature may result from re-oxidation of an intermediate and all these 5

ACCEPTED MANUSCRIPT observations probably indicate the formation of a high oxidation state Cu intermediate(s) during water oxidation.

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Bulk electrolysis of 1 was performed at 1.41 V (Fig. 1d). The catalytic current

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densities increased with the pH, and a significant amount of oxygen gas bubbles were produced on the FTO. From pH 9.2 to 11.0, a gray catalyst film (Fig. 1e) appeared on

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the FTO electrode in one hour, and it became darker after a longer electrolysis time. Furthermore, when the film was used for bulk electrolysis in a borate solution

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containing no copper(II) complex, similar catalytic performance and gas bubbles were observed, indicating the high activity of the as-deposited film. To confirm the above

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observations, complex 2 was further examined for electrocatalysis. The CV data

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showed that complex 2 was also active for electrocatalytic water oxidation (Fig. 1f).

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Meanwhile, a similar heterogeneous film could be observed. To provide more insight into the advantage of using these copper complexes to generate an active catalyst film,

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we carried out experiments with a 0.68 mM CuCl2 solution and commercial CuO (5 µL from 10 mg/mL CuO-ethanol suspension dispersed on the FTO electrode) for electrocatalytic water oxidation in a 0.1 M borate solution at pH 9.2 (Fig. 1f). Both exhibited much lower catalytic currents and higher overpotentials than the catalyst films electrodeposited from a Cu(II) organic complex. Fig. 2a-c are the SEM images of the catalysts films deposited on the FTO surface at 1.41 V after a 9 C charge passed through the electrode at pH 9.2 (all films < 0.1 mg). Fig. 2a shows the film obtained from compound 1, which contains noticeable nanoparticles on the FTO with the size at ~30 nm. The inset is the cross-section image 6

ACCEPTED MANUSCRIPT of the film, illustrating its thickness is ~280 nm. These results clearly demonstrate that nanostructured materials were formed during the catalytic water oxidation reaction.

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Fig. 2b is the film obtained from compound 2, which shows the thickness of ~280 nm.

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Comparing this to the former two films, a more disordered layer with a film thickness of ~300 nm was obtained from the film obtained by using CuCl2 (Fig. 2c). The EDX data in Fig. 2d shows that the film obtained from compound 1 deposited at pH 9.2

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mainly contains Cu, O, Sn and Au. Cu and O resulted from the deposited catalyst film.

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In addition, Sn is from the FTO substrate, and Au was artificially sprayed on the film. X-ray diffraction study showed that only the substrate SnO2 peaks were observed on

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the electrode but no peaks from copper species, indicating the amorphous character of

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the electrodeposited catalyst film (Fig. 2d, inset).

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Subsequently, the catalyst films electrodeposited at pH 9.2 and 11.0 were analyzed by XPS, as shown in Fig. 3a-b. The survey data reveal that both catalyst

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films mainly contained Cu and O. The Cu 2p and O 1s peaks are attributed to the catalyst. In addition, the Sn 3d peak (red plot) is from the FTO substrate. Fig. 3b shows the high-resolution XPS spectrum of the Cu 2p region. In the Cu 2p spectrum, obvious shake-up satellite peaks are observed, which is the typical character of CuO [18]. The presence of CuO is also confirmed by the binding energies of Cu 2p3/2 and Cu 2p1/2 located at 933.9 and 953.9 eV, respectively [19]. The peak for oxygen at 532.0 eV can be attributed to the oxide on the surface of the films. Therefore, bulk electrolysis of a [CuII(TPA)H2O](ClO4)2 solution result in an active CuO based catalyst for water oxidation. 7

ACCEPTED MANUSCRIPT The Faradaic efficiency of the catalyst film was measured by a fluorescence based oxygen sensor (Fig. 3c). Bulk electrolysis was performed in a 0.1 M borate

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solution at pH 9.2, with a catalyst film electrode deposited at 1.41 V. After initiating

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electrolysis, oxygen bubbles were rapidly generated, accompanied by a rise in oxygen concentration in the headspace. The theoretical yield of oxygen was calculated by assuming that all of the current was caused by 4e- oxidation of water to produce

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oxygen. Comparing the experimental data with the theoretical data, a Faradaic yield

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of >90% for oxygen production was determined. The current density of the catalyst film was measured as a function of the overpotential (η = Vappl - iR - EpH, where η is

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the overpotential, Vappl is the applied potential, i is the stable current, R is the

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compensating resistance, and EpH is the thermodynamic potential for water oxidation

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at this pH (EpH = 1.23 V - 0.059pH V vs NHE)). Appreciable catalytic current was observed starting at η = 0.30 V, and a current density of ~1.0 mA/cm2 required η = 0.6

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V (Fig. 3d). The Tafel plot was fitted with a nearly linear relationship, and the slope was ~56 mV/decade. The results further indicate that the as-deposited copper oxide film is active for electrocatalytic water oxidation.

4. Conclusions In conclusion, molecular copper complexes 1-2 were studied as catalysts for electrocatalytic water oxidation reaction. In contrast to previous reports using copper(II) complexes as homogeneous catalysts, the present results clearly show that highly active CuO based catalyst films were deposited on conductive FTO substrates during bulk electrolysis in alkaline aqueous solutions. XRD spectra revealed the CuO 8

ACCEPTED MANUSCRIPT film was amorphous. The high Faradaic efficiency of the catalyst showed that the CuO based catalyst could efficiently convert water into oxygen under moderate

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overpotentials.

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Acknowledgements

This work was financially supported by NSFC (21271166), the Fundamental Research

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Funds for the Central Universities, the Program for New Century Excellent Talents in

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University (NCET) and the Thousand Young Talented Program. References

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[1] N.S. Lewis, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A. 103 (2006) 15729. [2] J.L. Dempsey, B.S. Brunschwig, J.R. Winkler, H.B. Gray, Acc. Chem. Res. 42 (2009) 1995.

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[3] P. Du, R. Eisenberg, Energy Environ. Sci. 5 (2012) 6012. [4] S.W. Gersten, G.J. Samuels, T.J. Meyer, J. Am. Chem. Soc. 104 (1982) 4029. (1988) 2795.

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[5] A. Harriman, I.J. Pickering, J.M. Thomas, P.A. Christensen, J. Chem. Soc., Faraday Trans. I 84 [6] J.F. Hull, D. Balcells, J.D. Blakemore, C.D. Incarvito, O. Eisenstein, G.W. Brudvig, R.H. Crabtree, J. Am. Chem. Soc. 131 (2009) 8730. [7] A. Savini, G. Bellachioma, G. Ciancaleoni, C. Zuccaccia, D. Zuccacciaa, A. Macchioni, Chem.

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[8] Z. Codolà, J.M.S. Cardoso, B. Royo, M. Costas, J. Lloret-Fillol1, Chem. -Eur. J. 19 (2013) 7203. [9] G.C. Dismukes, R. Brimblecombe, G.A.N. Felton, R.S. Pryadun, J.E. Sheats, L. Spiccia, G.F. Swiegers, Acc. Chem. Res. 42 (2009) 1935. [10] M.W. Kanan, D.G. Nocera, Science 321 (2008) 1072. [11] M. Dincă, Y. Surendranath, D.G. Nocera, Proc. Natl. Acad. Sci. U.S.A. 107 (2010) 10337. [12] S.M. Barnett, K.I. Goldberg, J.M. Mayer, Nat. Chem. 4 (2012) 498. [13] Z.F. Chen, P. Kang, M.T. Zhang, B.R. Stoner, T.J. Meyer, Energy Environ. Sci. 6 (2013) 813. [14] Z.F. Chen, T.J. Meyer, Angew. Chem. Int. Ed. 52 (2013) 700. [15] M.T. Zhang, Z.F. Chen, P. Kang, T.J. Meyer, J. Am. Chem. Soc. 135 (2013) 2048. [16] J.L. Fillol, Z. Codolà, I. Garcia-Bosch, L. Gómez, J.J. Pla, M. Costas, Nature Chem. 3 (2011) 807. [17] H. Nagao, N. Komeda, M. Mukaida, M. Suzuki, K. Tanaka, Inorg. Chem. 35 (1996) 6809. [18] J. Yu, Y. Hai, M. Jaroniec, J. Colloid Interface Sci. 35 (2011) 223. [19] S. Xu, J. Ng, A.J.o. Du, J. Liu, D.D. Sun, Inter. J. Hydrogen Energy 36 (2011) 6538.

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Fig. 1. (a) Molecular structures of copper(II) complexes. (b) CVs of 0.68 mM 1 in a

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0.1 M borate solution (pH 6.0-11.0) using FTO as the working electrode; (c) the same as (b) but using glassy carbon as the working electrode (inset: the CV obtained at pH 9.0); (d) Bulk electrolysis of 1 at 1.41 V under different pH. (e) Pictures of the FTO

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during O2 evolution, the catalyst film and bare FTO. (f) Bulk electrolysis of the

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catalyst film obtained from compound 1, 2, CuCl2 and commercial CuO in a 0.1 M

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borate solution at 1.41 V (inset: the CVs of 1, 2, CuCl2 and commercial CuO).

Fig. 2. (a-c) SEM images of the catalyst electrodeposited on the FTO at 1.41 V in a

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0.1 M borate solution at pH 9.2 containing 0.68 mM compound 1 (a), 2 (b) and CuCl2 (c). Inset: The cross-section of these films. (d) EDX spectra and XRD patterns (inset)

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of the catalyst film electrodeposited using compound 1.

Fig. 3. (a) XPS survey data of the catalyst obtained at pH 9.2 (black), and pH 11.0 (red). (b) Cu 2p region. (c) Theoretical and experimental oxygen by bulk electrolysis at 1.41 V using the catalyst film at pH 9.2. (d) Tafel plot of the CuO film in a 0.1 M borate buffer at pH 9.2.

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Graphical abstract

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ACCEPTED MANUSCRIPT Highlights

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Nanostructured copper oxide films have been electrodeposited from copper (II) complexes The films are used for electrocatalytic water oxidation with good catalytic activities The first observation of copper oxide nanoparticles as an active catalyst

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