Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction

Letter pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. 2018, 6, 2883−2887

Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction Electrocatalyst with Enhanced Activity Xiaoli Xiong,† Chao You,† Zhiang Liu,§ Abdullah M. Asiri,∥ and Xuping Sun*,‡ †

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China College of Chemistry, Sichuan University, Chengdu 610064, China § College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China ∥ Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: It is highly desired to enhance the catalytic activity of oxygen evolution reaction (OER) electrocatalysts made of earthabundant elements. In this Letter, we report that the OER activity of a CuO nanoarray can be largely enhanced by Co doping. In 1.0 M KOH, the Co-CuO nanoarray on copper foam requires a current densities of 50 and 100 mA cm−2 at overpotentials of only 299 and 330 mV, respectively. It also shows superior long-term durability over 15 h with a turnover frequency of 0.056 mol O2 s−1 at an overpotential of 300 mV.

KEYWORDS: Co doping, CuO, Nanoarry, Oxygen evolution reaction, Electrocatalyst

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and nontoxicity.19 Much research effort has been put into developing Cu-based materials for OER electrocatalysis, including hydroxides, oxides, and phosphides.20−32 The performance of Cu-based OER catalysts, however, is still not comparable to Ni/Co-based catalyst materials,20,26,33,34 and their OER performances still need to be improved.34 Doping is a widely applied technology to modify the electronic properties of transition metal ions, which can achieve hybrid materials with desirable properties and functionalities, and element doping can adjust the valence states because of synergetic effects.15,35,36 It is established that Co is an effective dopant to enhance the OER performance of metallic materials.37,38 The study on the modulation of OER activity of CuO by using Co as promoter, however, has never been reported before. In this Letter, we demonstrate the development of a Codoped CuO nanoarry on copper foam (Co-CuO NA/CF) via a cation exchange reaction as a 3D OER catalyst. In basic solution, Co-CuO NA/CF shows efficient activity with current densities of 50 and 100 mA cm−2 at overpotentials of 299 and 330 mV, respectively, superior to other Cu-derived catalysts in activity. Co-CuO NA/CF also exhibits outstanding long-term electrochemical stability.

INTRODUCTION The increasingly serious environmental concerns and impending global energy crisis caused by the depletion of fossil fuels have stimulated intensive search for clean and renewable energy sources.1 Hydrogen (H2), as a clean, zero carbon emission, and renewable energy carrier, plays a key role in a future sustainable energy system as a carrier of sustainable energy to replace hydrocarbons.2−6 Electrochemical water splitting is regarded as a promising route to large-scale production of high-purity hydrogen. The oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are generally deemed as two important half reactions for water splitting. However, the kinetics of the oxygen evolution reaction (OER) are sluggish and considered to be a hindrance for water splitting which greatly limits the efficiency of water splitting.7−9 Therefore, it is highly needed to lower the substantial OER overpotential and thus enhance the energy conversion efficiency.10,11 Although noble metal-based catalysts, such as iridium (Ir) and ruthenium (Ru) oxides, show high OER activity,12,13 they suffer from high price and limited supply, limiting their widespread industrial application.14 Therefore, it is attractive to design alternative OER electrocatalysts based on non-noble metals. In recent years, transition metals and their derivatives have attracted great attention as electrode materials for OER.15,16 Various different transition metals and their alloys have been well studied,17,18 and Cu has appeared as an interesting transition metal with high abundance, rich redox properties, © 2018 American Chemical Society

Received: October 16, 2017 Revised: January 1, 2018 Published: January 29, 2018 2883

DOI: 10.1021/acssuschemeng.7b03752 ACS Sustainable Chem. Eng. 2018, 6, 2883−2887

Letter

ACS Sustainable Chemistry & Engineering

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RESULTS AND DISCUSSION The fabrication process of Co-CuO NA/CF is presented in Scheme 1. Figure 1a presents the XRD patterns of CuO NA/

peaks of CuO but with a little partially offset. The scanning electron microscope (SEM) images of CuO NA/CF (Figure 1b) demonstrate the full coverage of CF with a CuO nanowire array. Clearly, Co-CuO NA/CF still retains its nanoarray structure (Figure 1c). Figure 1d shows the transmission electron microscopy (TEM) image for one single Co-CuO nanowire. The high-resolution TEM (HRTEM) image taken from Co-CuO reveals well-resolved lattice fringes with an interplanar distance of 0.23 nm corresponding to the (111) plane of CuO (Figure 1e). The coressponding energydispersive X-ray (EDX) elemental mapping image further demonstrates a uniform distribution of Cu, Co, and O elements in the entire Co-CuO NA/CF nanoarray (Figure 1f and Figure S1). These results prove the formation of a Co-doped CuO nanoarray after a cation exchange reaction. Figure S2 presents the XPS survey spectrum for Co-CuO NA/CF, also confirming the presence of the Co element. Figure 2 shows the XPS spectra in the Cu 2p and Co 2p

Scheme 1. Schematic Illustration of Preparation of Co-CuO NA/CF Nanoarray

Figure 2. XPS survey for Co-CuO NA/CF in (a) Cu 2p and (b) Co 2p regions.

regions. As shown in Figure 2a, the peak at 934.7 eV corresponds to the binding energy (BE) of Cu 2p3/2, along with two satellite peaks located at 941.5 and 943.1 eV.41,42 The peak at 954.8 eV corresponds to the BE of Cu 2p1/2 with a satellite peak at 962.8 eV. These observations suggest the existence of Cu with high oxidation states (Cu2+).41,42 Note that other peaks can be attributed to Cu(0) of CF.42 Figure 2b shows that the Co 2p two peaks of 780.2 and 769.7 eV are consistent with the BEs of Co 2p3/2 and Co 2p1/2, respectively, confirming the presence of the Co element with the form of Co2+.43 The BEs at 785.5 and 802.1 eV with two satellite peaks also correspond to Co2+.44 In addition, the XPS spectra for CuO and CoO are shown in Figure S3. The Cu 2p region of CuO reveals similar BEs but a little offset on some peaks compared with Co-CuO, indicating that part of the Cu element may be replaced by Co in the CuO crystal. This also implies that the interaction arises between Co and CuO.37,38 In order to estimate the amount of Cu and Co in the material, Co-CuO was completely rinsed in diluted hydrochloric acid (0.1 M), and the obtained solution was analyzed by inductively coupled plasma optical emission spectroscopy (ICP-OES). It suggests that the amounts of Cu and Co are about 2.93 and 0.267 mg cm−2, respectively, corresponding to a Cu/Co atomic ratio of 10.1:1. These results suggest that the Co doping in CuO is successful. To evaluate the catalytic performance toward OER, Co-CuO NA/CF (Co-CuO loading: 3.2 mg cm−2) was investigated as a working electrode using an electrochemical workstation in a 1.0 M KOH aqueous solution. Bare CF, a CuO nanoarray on CF (CuO NA/CF), and RuO2 on CF (RuO2/CF) were also measured under the same conditions for comparison. All

Figure 1. (a) Typical XRD pattern for CuO NA/CF and Co-CuO NA/CF. (b) SEM images for CuO NA/CF. (c) SEM images for CoCuO NA/CF. (d) TEM image of one single Co-CuO nanowire. (e) HRTEM image taken from Co-CuO. (f) SEM and EDX elemental mapping images for Co-CuO/CF.

CF and Co-CuO NA/CF. For CuONA/CF, the characteristic peaks at 35.5° and 38.7° are attributed to CuO (JCPDS No. 450937),39 and the diffraction peaks at 43.30°, 50.43°, and 74.13°are assigned to the CF substrate (JCPDS No. 040836).40 The resulting Co-CuO still presents characteristic 2884


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ibility of the Co-CuO NA/CF electrode.45,46 The durability of the electrode is also a major concern for OER. Their stability was probed by cyclic voltammetric scans from +1.2 to +1.8 V vs RHE at a scan rate of 50 mV s−1. In Figure 3d, we observed that the LSV curve after 1000 cyclic voltammetric scans was almost the same as the initial one in 1.0 M KOH, revealing the high stability of Co-CuO NA/CF. We also investigated the longtime electrochemical durability of Co-CuO NA/CF by bulk water electrolysis and found that it retained its catalytic activity for at least 15 h (Figure 3e). After the OER test, both XRD and TEM analyses suggest no changes for Co-CuO (Figure S4). To determine the Faradaic efficiency (FE) of Co-CuO NA/CF for water oxidation, the gas was confirmed by gas chromatography and quantified with a calibrated pressure sensor. Figure 3f shows that the content of O2 produced increases with the electrolysis process, and the FE is nearly 100%. We further estimated their relative electrochemically active surface areas for Co-CuO NA/CF and CuO NA/CF electrodes using cyclic voltammgrams (CVs) by extracting the doublelayer capacitance (Figure 4a and b). As shown in Figure 4c, the

experimental data were corrected with ohmic potential drop (iR) losses arising from solution resistance.37 (E = E0 − I Rs, where E is the initial potential before corrected, I is the measured current, and Rs is the resistance of the solution), and all potentials were reported on a reversible hydrogen electrode (RHE) scale except as specifically specified. Figure 3a presents

Figure 3. (a) LSV curves of RuO2/CF, CuO NA/CF, and Co-CuO NA/CF for OER. (b) Tafel plots of RuO2/CF and Co-CuO NA/CF for OER. (c) Chronopotentiometric curves of Co-CuO NA/CF. (d) LSV curves of Co-CuO NA/CF in first and 1000th potential cycles. (e) Chronopotentiometric durability test (without iR correction) of Co-CuO NA/CF with a stationary current density of 50 mA cm−2. (f) Content of gas measured vs time for OER of Co-CuO NA/CF. Figure 4. (a) CVs of (a) CuO NA/CF and (b) Co-CuO NA/CF at scan rates of 5, 10, 20, 40, 80, 120, 160, and 200 mV s−1. (c) Capacitive currents at 1.09 V as a function of scan rate for CuO NA/CF and CoCuO NA/CF. (d) Nyquist plots for CuO NA/CF and Co-CuO NA/ CF.

the linear sweep voltammetry (LSV) curves. RuO2/CF exhibits excellent OER activity with an overpotential of 269 mV to reach 50 mV cm−2. Although CuO NA/CF is also active for OER, Co-CuO NA/CF shows much superior catalytic activity with the achievement of 50 and 100 mA cm−2 (based on geometric area) at overpotentials of 299 and 330 mV, respectively, comparing favorably to the behaviors of many reported Cu- and Co-based OER catalysts in alkaline conditions (Table S1). As shown in Figure 3b, the Tafel slopes for Co-CuO NA/CF, CuO NA/CF, and RuO2/CF are 134, 196, and 117 mVdec−1, respectively. Co-CuO NA/CF exhibits a lower Tafel slope of 134 mVdec−1, only 17 mV dec−1 more than that of RuO2/CF, implying a more rapid OER rate on a Co-CuO NA/CF electrode. The above proves that Co is a valid doping agent which can adjust the valence states to enhance the OER performance; this may be derived from the unique nanoarray structure and the synergetic effects between Co and Cu oxides.15,35,36 Figure 3c presents the multistep chronopotentiometric curve for Co-CuO NA/CF in 1.0 M KOH. The applied current was increased stepwise from 50 to 500 mA cm−2 and remains 500 s for each increment of 50 mA cm−2. The initial potential is 1.52 V and then stays constant for the remaining 500 s. Similar results can be obtained from other steps, implying good mass transportation and electroconduct-

capacitance of Co-CuO NA/CF (74.98 mF cm−2) is larger than that of CuO NA/CF (35.78 mF cm−2), reflecting that Co-CuO NA/CF has a larger ECSA and thus offering more active sites for OER electrocatalysis.47 The Brunauer−Emmett−Teller (BET) surface areas of Co-CuO and CuO were further determined as 210.323 and 106.715 m2 g−1, respectively (Figure S5), and Figure S6 shows the normalized LSV curve for Co-CuO NA/CF. Electrochemical impedance spectroscopy data (Figure 4d) suggest that Co-CuO NA/CF has a much smaller semicircle radius than that of CuO NA/CF, indicating a much lower Rct and thus a higher charge-transfer rate and more rapid catalytic kinetics.48,49 The OER activity of a catalyst can also be expressed in terms of turnover frequency (TOF), which is calculated as the number of O2 molecules formed per active sites at a constant overpotential.49,50 The CVs of Co-CuO NA/CF were collected at different scan rates from 20 to 50 mV s−1 (Figure S7a). Figure S7b shows the linear slope of the oxidation peak current versus scan rate from CVs, and the slope is 0.58. Figure S7c 2885


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(8) Suntivich, J.; May, K. J.; Gasteiger, H. A.; Goodenough, J. B.; Shao-Horn, Y. A Perovskite Oxide Optimized for Oxygen Evolution Catalysis from Molecular Orbital Principles. Science 2011, 334, 1383− 1385. (9) Yin, Q.; Tan, J. M.; Besson, C.; Geletii, Y. V.; Musaev, D. G.; Kuznetsov, A. E.; Hill, C. L.; Luo, Z.; Hardcastle, K. I. A Fast Soluble Carbon-Free Molecular Water Oxidation Catalyst Based on Abundant Metals. Science 2010, 328, 342−345. (10) Zhou, W.; Wu, X. J.; Cao, X.; Huang, X.; Tan, C.; Tian, J.; Wang, J.; Liu, H.; Zhang, H. Ni3S2 Nanorods/Ni Foam Composite Electrode with Low Overpotential for Electrocatalytic Oxygen Evolution. Energy Environ. Sci. 2013, 6, 2921−2924. (11) Zhao, Y.; Nakamura, R.; Kamiya, K.; Nakanishi, S.; Hashimoto, K. Nitrogen-Doped Carbon Nanomaterials as Non-Metal Electrocatalysts for Water Oxidation. Nat. Commun. 2013, 4, 2390. (12) Beni, G.; Schiavone, L. M.; Shay, J. L.; Dautremont-Smith, W. C.; Schneider, B. S. Electrocatalytic Oxygen Evolution on Reactively Sputtered Electrochromic Iridium Oxide Films. Nature 1979, 282, 281−283. (13) Lee, Y.; Suntivich, J.; May, K. J.; Perry, E. E.; Shao-Horn, Y. J. Synthesis and Activities of Rutile IrO2 and RuO2 Nanoparticles for Oxygen Evolution in Acid and Alkaline Solutions. J. Phys. Chem. Lett. 2012, 3, 399−404. (14) Xie, L.; Tang, C.; Wang, K.; Du, G.; Asiri, A. M.; Sun, X. Cu(OH)2@CoCO3(OH)2•nH2O Core-Shell Heterostructure Nanowire Array: An Efficient 3D Anodic Catalyst for Oxygen Evolution and Methanol Electrooxidation. Small 2017, 13, 1602755. (15) Jin, H.; Mao, S.; Zhan, G.; Xu, F.; Bao, X.; Wang, Y. Fe Incorporated a-Co(OH)2 Nanosheets with Remarkably Improved Activity Towards the Oxygen Evolution Reaction. J. Mater. Chem. A 2017, 5, 1078−1084. (16) Jin, H.; Wang, J.; Su, D.; Wei, Z.; Pang, Z.; Wang, Y. In situ Cobalt−Cobalt Oxide/N-Doped Carbon Hybrids As Superior Bifunctional Electrocatalysts for Hydrogen and Oxygen Evolution. J. Am. Chem. Soc. 2015, 137, 2688−2694. (17) Schäfer, H.; Sadaf, S.; Walder, L.; Kuepper, K.; Dinklage, S.; Wollschläger, J.; Schneider, L.; Steinhart, M.; Hardege, J.; Daum, D. Stainless Steel Made to Rust: A Robust Water-Splitting Catalyst with Benchmark Characteristics. Energy Environ. Sci. 2015, 8, 2685−2697. (18) Schäfer, H.; Beladi-Mousavi, S. M.; Walder, L.; Wollschläger, J.; Kuschel, O.; Ichilmann, S.; Sadaf, S.; Steinhart, M.; Küpper, K.; Schneider, L. Surface Oxidation of Stainless Steel: Oxygen Evolution Electrocatalysts with High Catalytic Activity. ACS Catal. 2015, 5, 2671−2680. (19) Gawande, M. B.; Goswami, A.; Felpin, F. X.; Asefa, T.; Huang, X.; Silva, R.; Varma, R. S.; Zou, X.; Zboril, R. Cu and Cu-Based Nanoparticles: Synthesis and Applications in Catalysis. Chem. Rev. 2016, 116, 3722−3811. (20) Hou, C.; Chen, Q.; Wang, C.; Liang, F.; Lin, Z.; Fu, W.; Chen, Y. Self-Supported Cedarlike Semimetallic Cu3P Nanoarrays as a 3D High-Performance Janus Electrode for Both Oxygen and Hydrogen Evolution under Basic Conditions. ACS Appl. Mater. Interfaces 2016, 8, 23037−23048. (21) Wang, L.; Ge, X.; Li, Y.; Liu, J.; Huang, L.; Feng, L.; Wang, Y. Nickel Enhanced the Catalytic Activities of Amorphous Copper for the Oxygen Evolution Reaction. J. Mater. Chem. A 2017, 5, 4331−4334. (22) Ma, S.; Luo, X.; Kropf, A. J.; Wen, J.; Wang, X.; Lee, S.; Myers, D. J.; Miller, D.; Wu, T.; Lu, J.; Amine, K. Insight into the Catalytic Mechanism of Bimetallic Platinum-copper Core-shell Nanostructures for Nonaqueous Oxygen Evolution Reactions. Nano Lett. 2016, 16, 781−785. (23) Liu, Y.; Li, Q.; Si, R.; Li, G.; Li, W.; Liu, D.; Zou, X.; et al. Coupling Sub-Nanometric Copper Clusters with Quasi Amorphous Cobalt Sulfide Yields Efficient and Robust Electrocatalysts for Water Splitting Reaction. Adv. Mater. 2017, 29, 1606200. (24) Chen, H.; Gao, Y.; Sun, L. Highly Active Three-Dimensional NiFe/Cu2O Nanowires/Cu Foam Electrode for Water Oxidation. ChemSusChem 2017, 10, 1475−1481.

presents the plot of TOF as a function of overpotential for CoCuO NA/CF. So, we obtained a high TOF for Co-CuO NA/ CF as 0.056 mol O2 s−1 at an overpotential of 300 mV.

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CONCLUSION In summary, Co doping has been proposed as an effective strategy to enhance the OER activity of CuO. In 1.0 M KOH, such a Co-doped CuO nanoarray only demands overpotentials of 299 and 330 mV to achieve current densities of 50 and 100 mA cm−2, respectively, with outstanding durability. This finding offers us a high-efficiency, low-cost, and easily constructed 3D catalyst electrode for the OER application under alkaline conditions and also opens a new door to enhance the OER activity of CuO catalysts for applications.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03752. Experimental section, EDX elemental mapping images, XPS spectra, LSV curve, XRD and TEM images, N2 adsorption/desorption isotherms, plot of CVs and plot of TOF, and Table S1. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaoli Xiong: 0000-0002-5407-4350 Abdullah M. Asiri: 0000-0001-7905-3209 Xuping Sun: 0000-0001-5034-1135 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (No. 21575137). REFERENCES

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Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction

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