Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b03752 • Publication Date (Web): 29 Jan 2018 Downloaded from http://pubs.acs.org on January 30, 2018

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ACS Sustainable Chemistry & Engineering

Co-doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction Electrocatalyst with Enhanced Activity Xiaoli Xiong,1 Chao You,1 Zhiang Liu,3 Abdullah M. Asiri,4 and Xuping Sun2,* 1

College of Chemistry and Materials Science, Sichuan Normal University, Chengdu 610068, Sichuan, China, 2College of Chemistry, Sichuan University, Chengdu 610064, China, 3College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China, and 4Chemistry Department, Faculty of Science & Center of Excellence for Advanced Materials Research, King Abdulaziz University, P.O. Box 80203, Jeddah 21589, Saudi Arabia Corresponding Author: [email protected] (X.S.) ABSTRACT: It is highly desired to enhance the catalytic activity of oxygen evolution reaction (OER) electrocatalysts made of earth-abundant elements. In this Letter, we report that the OER activity of CuO nanoarray can be largely enhanced by Co doping. In 1.0 M KOH, the Co-CuO nanoarray on copper foam requires current density of 50 and 100 mA cm-2 at overpotential 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 overpotential of 300 mV. KWORDS: Co doping, CuO, nanoarry, oxygen evolution reaction, electrocatalyst

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. Oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) are generally deemed to two important half reactions for water splitting. However, the kinetics of 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 an alternative OER electrocatalysts based on non-noble metals. Recent years, transition metal and their derivatives have attracted great attention as electrode materials for OER.15,16 Various different transition metal and their alloys have been well studied,17,18 and Cu has appeared as an interesting transition metal with high abundance, rich redox properties and nontoxicity.19 Much research effort has been put to develop 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 materials20,26,33,34 and their OER performances are still needed to be improved.34 Doping is a widely applied technolo-

gy 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 Co-doped CuO nanoarry on copper foam (Co-CuO NA/CF) via cation exchange reaction as a 3D OER catalyst. In basic solution, the Co-CuO NA/CF shows efficientt activity with current density of 50 and 100 mA cm-2 at overpotential of 299 and 330 mV, respectively, superior to other Cu-derived catalyst in activity. The Co-CuO NA/CF is also exhibits outstanding long-term electrochemical stability.

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/CF and Co-CuO NA/CF. For CuONA/CF, the characteristic peaks at 35.5° and 38.7° are attributed to CuO (JCPDS No. 45-0937)39 and the diffraction peaks at 43.30°, 50.43° and 74.13°are assigned to the CF substrate (JCPDS No. 04-0836).40 The resulting Co-CuO still presents characteristic 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 CuO nanowire array. Clearly, the Co-CuO NA/CF still retain its nanoarray structure (Figure 1c). Figure 1d shows thetransmission 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

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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 uniformly distribution of Cu, Co and O elements in the entire Co-CuO NA/CF nanoarray (Figure 1f and Figure S1). These results prove that the formation of Codoped CuO nanoarray after cation exchange reaction.

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shows the XPS spectra in the Cu 2p and Co 2p 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 And the peak at 954.8 eV corresponds to the BE of Cu 2p1/2 with a Sat. 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 Co 2p two peaks of 780.2 eV and 769.7 eV are consistent with the BEs of Co 2p3/2 and Co 2p1/2, respectively, confirming the presence of Co element with the form of Co2+.43 The BEs at 785.5 eV and 802.1 eV with two Sat. 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 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, the 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.

Scheme 1. Schematic illustration of the preparation of Co-CuO NA/CF nanoarray .

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

Figure 1. (a) Typical XRD pattern for CuO NA/CF and Co-CuO NA/CF. (b) SEM images for CuO NA/CF and (c) SEM images for Co-CuO 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.

Figure S2 presents the XPS survey spectrum for Co-CuO NA/CF, also confirming the presence of Co element. Figure 2

To evaluate the catalytic performance toward OER, CoCuO NA/CF (Co-CuO loading: 3.2 mg cm-2) was investigated as a working electrode using an electrochemical workstation in 1.0 M KOH aqueous solution. Bare CF, CuO nanoarray on CF (CuO NA/CF) and RuO2 on CF (RuO2/CF) were also measured under the same conditions for comparison. All experimental data were corrected with ohmic potential drop (iR) losses arising from solution resistance.37 (E = E0 – I Rs, where E is initial potential before corrected, I is measured current and Rs is the resistance of the solution) and all potentials were reported on a reversible hydrogen electrode (RHE) scale except specifically specified. Figure 3a presents the linear sweep voltammetry (LSV) curves. RuO2/CF exhibits excellent OER activity with overpotential of 269 mV to reach 50 mV cm-2. Although CuO NA/CF is also active for the OER, Co-CuO NA/CF shows much superior catalytic activity with -2 the achievement of 50 and 100 mA cm (based on geometric area) at overpotentials of 299 and 330 mV, respectively, comparing favourably to the behaviours of many reported Cu and Co-based OER catalysts in alkaline conditions (Table S1). As shown in Figure 3b, the Tafel slope for Co-CuO NA/CF, CuO NA/CF and RuO2/CF are 134, 196, and 117 mVdec-1, respectively. The Co-CuO NA/CF exhibit a lower Tafel slope of 134 mVdec-1, only 17 mV dec-1 more than that of RuO2/CF, implying more rapid OER rate on Co-CuO NA/CF electrode.


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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 multi-step chronopotentiometric curve for CoCuO NA/CF in 1.0 M KOH. The applied current was increased stepwise from 50 mA cm-2 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 maintains constant for the rest 500 s. Similar results can be obtained from other steps, implying the good mass transportation, electroconductibility of the CoCuO NA/CF electrode.45,46 The durability of electrode is also 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 long-time electrochemical durability of the Co-CuO NA/CF by bulk water electrolysis and found that it retained its catalytic activity for at least 15 h (Figure 3e). After OER test, both XRD and TEM analyses suggest no changes for Co-CuO (Figure S4). To determine the Faradaic efficiency (FE) of CoCuO NA/CF for water oxidation, the gas was confirmed by gas chromatography and quantified with a calibrated pressure sensor. Figure 3f displays that the content of O2 produced increases with electrolysis process and the FE is nearly 100 %.

layer capacitance (Figure 4a and 4b). As shown in Figure 4c, the 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 CoCuO 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) suggests that Co-CuO NA/ CF has a much smaller semicircle radius than that of CuO NA/ CF, indicating that a much lower Rct and thus a higher charge-transfer rate and more rapid catalytic kinetics.48,49

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) The capacitive currents at 1.09 V as a function of scan rate for CuO NA/CF and Co-CuO NA/CF. (d) Nyquist plots for CuO NA/CF and Co-CuO NA/CF.

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 oxidation peak current versus scan rate from CVs and the slope is 0.58. Figure S7c 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 overpotenti al of 300 mV.

CONCLUSION

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 1st 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) The content of gas measured vs. time for OER of Co-CuO NA/CF.

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 double-

In summary, Co doping has been proposed as an effective strategy to enhance the OER activity of CuO. In 1.0 M KOH, such Co-doped CuO nanoarray only demands overpotential 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.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: xx.xxxx/xxxxxxxxx-



xxx.xxxxxxx. 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; Table S1.

AUTHOR INFORMATION Corresponding Author *

E-mail: sunxp_scu@hotmail. com

Notes The authors declare no competing financial interest.

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

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Co-doped CuO nanoarray on copper foam (Co-CuO NA/CF) acts as a superior and durable water oxidation electrocatalyst needing overpotential of 299 mV to reach 50 mA cm-2 in 1.0 M KOH.


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Co-Doped CuO Nanoarray: An Efficient Oxygen Evolution Reaction

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