Cu3Mo2O9 Nanosheet Array as a High-Efficiency Oxygen Evolution

Jan 22, 2018 - College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China. ∥ Chemistry Department, Faculty of Science...
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Cu3Mo2O9 Nanosheet Array as a High-Efficiency Oxygen Evolution Electrode in Alkaline Solution Ying Gou,†,‡ Lin Yang,‡ Zhiang Liu,§ Abdullah M. Asiri,∥ Jianming Hu,*,† and Xuping Sun*,‡ †

College of Physics and Electronic Engineering, Chongqing Normal University, Chongqing 400047, China College of Chemistry, Sichuan University, Chengdu 610064, Sichuan, 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: Developing highly active and sustained oxygen evolution reaction (OER) catalysts for energy-saving electrolytic hydrogen generation is highly attractive. In this work, we report the development of a Cu3Mo2O9 nanosheet array on nickel foam (Cu3Mo2O9/NF) as a highly active OER catalyst. Such Cu3Mo2O9/NF shows high catalytic activity in 1.0 M KOH, passing 50 mA cm−2 at an overpotential of 325 mV. It also shows superior long-term durability more than 24 h with a turnover frequency (TOF) of 0.09 mol O2 s−1 at overpotential of 400 mV.

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INTRODUCTION The worsening global energy crisis and environment pollution has stimulated a mass of research in exploring green and sustainable energy production.1−4 Hydrogen has been recognized as the most environmentally friendly replaceable energy with renewable and clean features.5 Electricity-driven water splitting is an effective pathway for providing hydrogen fuels.6 However, the oxygen evolution reaction (OER) is kinetically slow and greatly restricted the water splitting procedure.7−9 Efficient OER catalysts need to reach a high current density at low overpotential to increase the reaction rate. Currently, precious-metal-based materials including IrO2 and RuO2 are the most-efficient OER electrocatalysts, but their mass uses are narrow due to their expense and low earth abundance.10 Thus, it is necessary to exploit inexpensive, rich, and efficient OER catalysts. Copper oxide (CuO) has attracted considerable attention in electrochemical fields due to its environmentally benign nature, nontoxicity, and low cost.11,12 As an OER catalyst, CuO has low electrical conductivity, and its activity is not remarkable.13 Recently, mixed metal oxides have attracted considerable attention as compared to single metal oxides because of their rich redox reactions and high electrical conductivity.14 Some studies indicate that charge transfer through hopping processes between cations of different valence states demands relatively smaller activation energies.15 Mo is particularly attractive among the transition metals, with high stability and resistance to corrosion. As an example, Li et al. reported NiFeMo © XXXX American Chemical Society

nanosheets needing an overpotential of 280 mV to attain 10 mA cm−2, outperforming the regular NiFe nanosheets.16 For electrochemical measurements, however, a polymer binder is usually required such as nafion or PTFE prior, which could block active sites and impede diffusion to reduce electrochemical performance of the catalyst.17 Catalyst nanoarrays directly grown on current collectors have intrinsic advantages, such as a large surface area, high electroconductibility, and strong long-term stability.18−20 Thus, we expect that Cu−Mo oxides may offer a high charge transfer rate, good reaction kinetics, strong long-term stability, and superb OER performance, which, however, has not been addressed before. In this study, we show our recent discovery that the Cu3Mo2O9 nanosheet array was supported on nickel foam (Cu3Mo2O9/NF) as an OER catalyst. Such Cu3Mo2O9/NF shows efficient activity with a current density of 50 mA cm−2 at overpotentials of 325 mV in 1.0 M KOH. It also exhibits outstanding long-time electrochemical stability over 24 h with a turnover frequency (TOF) of 0.09 mol O2 s−1 at an overpotential of 400 mV.

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EXPERIMENTAL SECTION

Materials. Cupric nitrate (Cu(NO3)2, 99.99%) and sodium molybdate dihydrate (Na2MoO4·2H2O, 99.995%) were purchased from Beijing Chemical Works. Rhodium(III) chloride hydrate (RuCl3· Received: October 15, 2017

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DOI: 10.1021/acs.inorgchem.7b02641 Inorg. Chem. XXXX, XXX, XXX−XXX

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surface active sites concentration (mol cm−2), and R and T are the ideal gas constant and the absolute temperature, respectively.22 The TOF value was calculated according to equation

3H2O, 99.99%) and Nafion (5 wt %) were provided by Sigma-Aldrich. NF was provided by Hangxu Filters Flag Store, Hengshui, Hebei. All the reagents were used without further purification. Preparation of Cu3Mo2O9/NF. Before synthesis, NF was treated by ultrasonication in 3.0 M HCl solution for 15 min and cleaned with deionized water. Cu(NO3)2 (2.25 mmol) and Na2MoO4·2H2O (2.25 mmol) were mixed in 95 mL of distilled water and then transferred to a 100 mL polytetrafluoro-ethylene autoclave. Afterward, washed NF substrate (3 × 3 cm) was immersed in the reaction solution. The autoclave was maintained at 160 °C for 15 h in a furnace and reached ambient temperature. The NF was washed thoroughly with distilled water and subsequently dried overnight in a vacuum oven. Finally, the precursor was treated at 450 °C for 2 h at a ramping rate of 1 °C/min in an argon atmosphere. The as-made catalyst was denoted as Cu3Mo2O9/NF. For comparison, we also synthesized Cu3Mo2O9/NF10h and Cu3Mo2O9/NF-20h catalysts with the same conditions but at 10 and 20 h, respectively. Preparation of RuO2/NF. RuO2 was synthesized based on previous work.21 Briefly, 1.0 mL of NaOH solution (1.0 M) and 2.61 g of RuCl3·3H2O were mixed in 100 mL of distilled water. The solution was stirred at 100 °C for 45 min and then centrifuged for 10 min and filtered. The precipitates were rinsed with distilled water several times and dried in a vacuum oven at 80 °C for 12 h. Finally, the product was calcined in air at 350 °C for 1 h. The previously prepared RuO2 powder (0.01 g) was dispersed into a solution of Nafion, ethanol, and water with a volume ratio of 10:250:250 via sonication and deposited onto NF (loading: 2.65 mg cm−2). Materials Characterization. The powder X-ray diffraction (XRD) data were recorded on a LabX XRD-6100 X-ray diffraction instrument with Cu Kα irradiation (λ = 0.154 nm, 40 kV, 30 mA). The morphology of the material was observed by JSM-6701F scanning electron microscopy (SEM). JEM-2100F transmission electron microscopy (TEM) images were taken by using an electron acceleration energy of 200 kV. X-ray photoelectron spectroscopy (XPS) studies were conducted using an ESCALABMK II X-ray photoelectron spectrometer with a Mg Kα X-ray resource. Electrochemical Tests. Electrochemical measurements were carried out using a CHI 760E electrochemical conventional machine(Chenhua, Shanghai, China). The measurements were carried out in 1 M KOH aqueous solution using a conventional three-electrode configuration. Cu3Mo2O9/NF was used as a working electrode, Hg/ HgO electrode as a reference electrode, and a graphite plate as an auxiliary electrode. The temperature of the solution was kept at 25 °C for all the measurements via the adjustment of air conditioning and heating support, which ensured the variation of diffusion coefficient below 1%. All of the potentials were converted to reversible hydrogen electrode (RHE) scale using the formula E(RHE) = E (Hg/HgO) + (0.098 + 0.0591 pH) V. The ohmic potential drop (iR) losses arising from solution resistance were corrected to all experimental results unless otherwise noted, using the following formula:

TOF = JA /4Fm Here, J is the current density at defined overpotential, A is the geometric area of the testing electric pole, F is the Faradaic constant (96485 C mol−1), and m is the number of active materials (in mol).23

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RESULTS AND DISCUSSION Figure 1a presents the XRD pattern of Cu3Mo2O9 scraped from NF. The diffraction peaks at 12.1°, 25.9°, 27.1°, 35.8°, and

Figure 1. (a) Typical XRD pattern for Cu3Mo2O9 with reference to the standard diffractions. (b) SEM images for Cu3Mo2O9/NF. (c) HRTEM image of Cu3Mo2O9, the inset shows the SAED pattern. (d) SEM image for Cu3Mo2O9/NF and the corresponding elemental maps of Cu, Mo, O.

49.7° correspond to the (020), (002), (140), (042), and (411) planes of Cu3Mo2O9 (JCPDS No. 24-0055), respectively. The SEM images (Figure 1b) of Cu3Mo2O9/NF show that the NF is uniformly covered by the Cu3Mo2O9 nanosheet array. Figure S1a also displays the SEM image of Cu3Mo2O9 powder, which shows its sheet structure. Cross-section analyses suggest that the height of Cu3Mo2O9/NF-10h, Cu3Mo2O9/NF, and Cu3Mo2O9/NF-20h is about 3.26, 5.73, and 6.35 μm, respectively (Figure S1b, c, and d). The high-resolution TEM (HRTEM) image (Figure 1c) reveals the legible lattice fringes with a spacing of 0.163 nm, corresponding to the (112) plane of the Cu3Mo2O9 phase. The selected area electron diffraction (SAED) pattern (Figure 1c inset) exhibits separate points, corresponding to the (112), (230), (342), and (430) planes for the Cu3Mo2O9 phase. The corresponding energy-dispersive Xray (EDX) spectrum (Figure S2) verified the presence of Cu, Mo, and O elements. As shown in Figure 1d, the SEM image and the corresponding EDX elemental mapping image demonstrate a uniform distribution of Cu, Mo, and O elements in the entire nanosheet array. The X-ray photoelectron spectroscopy (XPS) technique was also carried out to study the chemical states of each element in Cu3Mo2O9 nanosheets. As shown in Figure 2a, the survey XPS spectrum displays that the sample contains Cu, Mo, and O elements. In the Cu 2p spectrum (Figure 2b), the two peaks of

E(iR ‐corrected) = E(RHE) − iR Here, i is the current and R is the uncompensated ohmic electrolyte resistance. Electrochemical impedance spectroscopy (EIS) tests were conducted under the open circuit potential (OCV). The uncompensated resistances were ∼3.8, ∼4.4, ∼2.6, and ∼3.2 Ω for Cu3Mo2O9/ NF-10h, Cu3Mo2O9 powder/NF, Cu3Mo2O9/NF, and Cu3Mo2O9/ NF-20h in O2-saturated 1.0 M KOH, respectively. TOF Calculations. For TOF calculations, first calculated is the concentration of surface active sites associated with the redox species, and the linear relationship between the oxidation peak current and scan rate is extracted from the electrochemical CV scans. The slope can be calculated based on the following equation:

Slope = n2F 2A Γ0/4RT Here, n is the number of transferred electrons, F is the Faradaic constant, A is the geometric area of the testing electric pole, Γ0 is the B

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Figure 4. CVs for (a) Cu3Mo2O9 powder/NF and (b) Cu3Mo2O9/NF at the diverse sweep rates from 20 to 300 mV s−1 in the nonfaradaic region. (c) The capacitive current density at 0.9254 V vs RHE with respect to the scan rates for Cu3Mo2O9 powder/NF and Cu3Mo2O9/ NF. (d) Nyquist plots of Cu3Mo2O9 powder/NF and Cu3Mo2O9/NF.

Figure 2. (a) XPS full survey spectrum for Cu3Mo2O9. High resolution (b) Cu 2p, (c) Mo 3d, and (d) O 1s spectra.

954.7 and 934.9 eV belonged to the Cu 2p1/2 and Cu 2p3/2 energy level, respectively, suggesting the existence of Cu2+ bound to oxygen. Two shakeup satellite peaks (identified as “Sat.”) also belonged to the Cu2+.24 In the Mo 3d region (Figure 3c), the two peaks of 235.3 and 232.5 eV belonged to

of Cu3Mo2O9/NF-10h, Cu3Mo2O9/NF, and Cu3Mo2O9/NF20h. The oxidation peaks for all samples at around 1.4−1.5 V are ascribed to the change of CuII to CuIII.28,29 Importantly, the Cu3Mo2O9/NF exhibits superior performance with an overpotential of 325 mV at 50 mA cm−2, lower than that of Cu3Mo2O9/NF-10h (442 mV) and Cu3Mo2O9/NF-20h (383 mV). We thus select the Cu3Mo2O9/NF as the typical sample for further analysis. As observed in Figure 3a, bare NF has poor OER performance, while RuO2/NF exhibits remarkable OER catalytic activity with a low overpotential of 263 mV at 50 mA cm−2. The Cu3Mo2O9 powder/NF electrode requires an overpotential of 385 mV to reach 50 mA cm−2, 60 mV larger than Cu3Mo2O9/NF. Note that the catalytic activity of Cu3Mo2O9/NF outperforms other nonprecious materials in alkaline media, including CoMn-LDH (η10mAcm−2 = 350 mV),30 NiCo-LDH (η10 mA cm−2 = 367 mV),31 Ni−Co2-O (η10mAcm−2 = 362 mV),32 CoP-MNA (η10mAcm−2 = 390 mV),33 and Co@ Co3O4@NMCC-rGO (η10mAcm−2 = 340 mV).34 Table S1 gives a more detailed comparison. Considering that carbon cloth (CC) is commonly used as an inert substrate with a flexible feature, we also supported our Cu3Mo2O9 nanosheet array on CC (Cu3Mo2O9/CC, Cu3Mo2O9 loading: 0.67 mg cm−2) and tested its OER activity in 1.0 M KOH, as presented in Figure S4a; 430 mV overpotential is required to attain 50 mA cm−2. Mass-normalized polarization curves show negligible difference between Cu3Mo2O9/CC and Cu3Mo2O9/NF (Figure S4b), demonstrating the superior OER performance of Cu3Mo2O9. Figure 3b shows the Tafel plots for Cu3Mo2O9 powder/NF, Cu3Mo2O9/NF, and RuO2/NF. These Tafel plots are fitted to equation: η = b log j + a (where η is the overpotential, j is the current density, and b is the Tafel slope). As observed, the Cu3Mo2O9/NF exhibits a lower Tafel slope of 146 mV dec−1 than Cu3Mo2O9 powder/NF (168 mV dec−1), confirming good reaction dynamics toward the OER on Cu3Mo2O9/NF. To better understand the superior OER activity of Cu3Mo2O9/NF, we also estimated the electrochemical active surface areas (ECSAs) of bare NF, Cu3Mo2O9 powder/NF, and Cu3Mo2O9/NF via determining the double-layer capacitance

Figure 3. (a) LSV curves of bare NF, Cu3Mo2O9 powder/NF, Cu3Mo2O9/NF, and RuO2/NF. (b) Tafel plots for Cu3Mo2O9 powder/NF, Cu3Mo2O9/NF, and RuO2/NF.

the Mo 3d3/2 and Mo 3d5/2 energy levels, respectively, indicating the presence of Mo6+.25 Figure 4d shows the O 1s spectrum, and a well-resolved peak at 530.2 eV is related to the bridging oxygen atoms.26 The above-stated results suggest the successful preparation of Cu3Mo2O9. To evaluate the catalytic performance toward OER, Cu3Mo2O9/NF (Cu3Mo2O9 loading: 2.65 mg cm−2) was investigated as a working electrode using a conventional three electrode configuration with a slow sweep rate of 5 mV s−1 in 1.0 M aqueous KOH. Similar measurements for bare NF and RuO2 on NF (RuO2 loading: 2.65 mg cm−2) were also performed. To demonstrate the benefit of in situ catalyst growth for binder-free catalyst electrodes, we also made another electrode by immobilizing Cu3Mo2O9 powder scratched down from NF using Nafion as a polymer binder (Cu3Mo2O9 powder/NF). All experimental data are ohmic potential drop (iR) corrected potentials to counteract for the resistance of the electrolyte unless otherwise stated, and potentials were corrected to reversible hydrogen electrode (RHE) scale.27 Figure S3 exhibits the linear sweep voltammetry (LSV) curves C

DOI: 10.1021/acs.inorgchem.7b02641 Inorg. Chem. XXXX, XXX, XXX−XXX

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10 to 80 mV s−1 (Figure 6a). As shown in Figure 6b, the linear dependence between the corresponding oxidation peak current

(Cdl) from the cyclic voltammograms (CVs; Figure S4a,b and 5a). The Cdl values of bare NF, Cu3Mo2O9 powder/NF, and Cu3Mo2O9/NF were calculated to be 0.45, 0.627, and 6.43 mF cm−2 (Figure S4c and 5b), respectively, indicating that Cu3Mo2O9/NF has a much higher surface area to increase the electrolyte interaction and thus exposes more active sites to improved OER performance.35 We further collected the electrochemical impedance spectroscopy (EIS) data. The semicircular radius of Cu3Mo2O9/NF is smaller than that of Cu3Mo2O9 powder/NF (Figure 4d), indicating a much lower charge-transfer resistance (Rct). This result effectively illustrates that Cu3Mo2O9/NF is more beneficial to the electron transfer, which can significantly assist overcoming the reaction barrier during the OER in 1 M KOH.36 Such observations suggest that the polymer binder not only blocks the active sites but increases the series resistance. Figure 5a displays the multistep chronopotentiometric curve for Cu3Mo2O9/NF in 1.0 M KOH. The applied current was

Figure 6. (a) CVs for Cu3Mo2O9/NF in the Faradaic capacitance current range at the diverse sweep rates between 10 and 80 mV s−1. (b) Corresponding oxidation peak current with respect to the scan rate for Cu3Mo2O9/NF (inset: plot of TOF as a function of overpotential for Cu3Mo2O9/NF).

and the sweep rate can be observed (see Experimental Section). Then, TOF values for Cu3Mo2O9/NF were determined to be 0.02 and 0.09 mol O2 s−1 at overpotentials of 300 and 400 mV, respectively (inset of Figure 4b). Furthermore, the TOF value of Cu3Mo2O9/NF is also larger than most reported nonprecious OER catalysts under the same conditions, like CoO nanoparticles (0.02 mol O2 s−1 at 507 mV),41 Ni0.25Co0.75Ox (0.01 mol O2 s−1 at 300 mV),42 and NiCo LDH (0.01 mol O2 s−1 at 300 mV).43

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CONCLUSION In summary, we have successfully synthesized Cu3Mo2O9 nanosheet arrays directly grown on NF as an OER catalyst. The Cu3Mo2O9/NF exhibited efficient OER catalytic properties with a small overpotential of 325 mV at 50 mA cm−2 in 1.0 M KOH and outstanding durability for over 24 h. This finding offers us a high-efficiency, low-cost ,and easily constructed 3D catalyst electrode for the OER application under alkaline conditions.

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Figure 5. (a) Chronopotentiometric curves of Cu3Mo2O9/NF (without iR correction). (b) LSV curves of Cu3Mo2O9/NF in first and 500th potential cycles. (c) Chronopotentiometric durability test (without iR correction) of Cu3Mo2O9/NF with 50 mA cm−2. (d) SEM image for Cu3Mo2O9/NF after long-term durability test.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b02641. SEM images, EDX spectrum, LSV curves, CVs, Table S1 (PDF)

increased stepwise from 40 to 400 mA cm−2 (40 mA cm−2 per 500 s). At the start of 40 mA cm−2, the potential rapidly stabilizes at 1.55 V and then remains the same for 500 s. All other current values also have similar results, indicating a lower mass transport resistance and high electroconductibility of the Cu3Mo2O9/NF electrode.37,38 The durability of the electrode is also major concern for OER. In Figure 5b, we observe that the LSV curve after 500 cyclic voltammetric scans was almost the same as the initial one in 1.0 M KOH. We also investigated the long-time electrochemical durability of the Cu3Mo2O9/NF by bulk water electrolysis and found that it retained its catalytic activity with 50 mA cm−2 over 24 h (Figure 5c). Notably, Cu3Mo2O9/NF still preserves its nanosheet morphology after the long-term durability test (Figure 5d). The OER activity of a catalyst can also be expressed in terms of TOF, which is calculated as the number of O2 molecules formed per active site at a constant overpotential.39,40 The TOF value of Cu3Mo2O9/NF was calculated by CV technology to quantify the surface concentration of active sites. The CVs of the Cu3Mo2O9/NF were observed at diverse sweep rates from

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

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) and the Natural Science Foundation Project of Chongqing Science and Technology Commission (No. cstc2015jcyjA10033). We also appreciate D

DOI: 10.1021/acs.inorgchem.7b02641 Inorg. Chem. XXXX, XXX, XXX−XXX

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Hui Wang from the Analytical & Testing Center of Sichuan University for her help with SEM characterization.

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DOI: 10.1021/acs.inorgchem.7b02641 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.7b02641 Inorg. Chem. XXXX, XXX, XXX−XXX