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Sep 27, 2017 - ABSTRACT: Water electrolysis is known as the most environmental friendly and renewable technology to generate hydrogen. To make it more...
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Cite This: ACS Sustainable Chem. Eng. 2017, 5, 10093-10098

Synthesis of Self-Supported Amorphous CoMoO4 Nanowire Array for Highly Efficient Hydrogen Evolution Reaction Jinxiu Zhao,† Xiang Ren,† Hongmin Ma,† Xu Sun,† Yong Zhang,† Tao Yan,‡ Qin Wei,† and Dan Wu*,† †

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Key Laboratory of Interfacial Reaction & Sensing Analysis in Universities of Shandong, School of Chemistry and Chemical Engineering, University of Jinan, 336 Nan Xin Zhuang West Road, Jinan 250022, China ‡ School of Resources and Environment, University of Jinan, 336 Nan Xin Zhuang West Road, Jinan 250022, China S Supporting Information *

ABSTRACT: Water electrolysis is known as the most environmental friendly and renewable technology to generate hydrogen. To make it more energy-efficient, development of a promising cathodic hydrogen evolution reaction electrocatalyst is important. In this communication, amorphous CoMoO4 nanowire array on Ti mesh (CoMoO4 NWA/Ti) was synthesized via a simple two-step hydrothermal method. As a three-dimensional hydrogen-evolving electrode, CoMoO4 NWA/Ti shows superior catalytic activity in 1.0 M KOH and demands overpotentials of only 81 and 243 mV to achieve current densities of 10 and 100 mA cm−2, respectively. Remarkably, it also has long-term electrochemical durability. KEYWORDS: CoMoO4, Amorphous, Nanowire array, Self-supported catalyst, Hydrogen evolution reaction

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INTRODUCTION Hydrogen as an ideal candidate can alleviate the shortage of fossil energy.1−4 However, pure hydrogen is almost nonexistent on the earth.5 Hydrogen produced from carbon feedstocks, such as natural gas reforming, accelerates fossil fuel depletion and CO2 emission, which makes it unsuitable.6−8 Electrochemical water splitting, in view of energy and environmental issues, is a promising approach to generate hydrogen but demands active electrocatalyst for cathodic hydrogen evolution reaction (HER).8−16 Currently, the state-of-the-art HER electrocatalyst is Pt; however, scarcity and high cost limit its application.17,18 Consequently, it is highly desired to develop earth-abundant and efficient HER electrocatalysts. Because of abundance and excellent performance, Co-based materials such as Co-NRCNTs,5 Co-NCNT,9 CoP,10 Co2P,11 CoP/CNT,14 Co@N−C,20 Ni0.33Co0.67S2 nanowire,21 CoOx@ CN,22 CoP/rGO-400,23 CoO/MoO x,24 etc. have been intensively studied as HER electrocatalysts.19 CoO/MoOx is converted from CoMoO4 through high-temperature hydrogenation treatment and needs overpotential of 163 mV at 10 mA cm−2 in 1.0 M KOH. Alkaline water splitting provides a strong candidate for commercialization toward cost-effective hydrogen production.25,26 In recent years, many researchers have noticed that the metal molybdates also have significant electrochemical properties, and CoMoO4 is one of the most important compounds.27,28 To the best of our knowledge, many studies have been mainly focused on the oxygen evolution reaction (OER) activity of CoMoO4. It is well© 2017 American Chemical Society

established that nanoarray structure directly grown on conductive material without adhesion has significant advantages in terms of exposing abundant active sites, reducing series resistance, promoting the diffusion of electrolyte and hydrogen product, and enhancing the catalysts mechanical stability.8,12,13,29−36 CoMoO4 nanowire array (CoMoO4 NWA) act as an efficient HER catalyst in alkaline conditions, which has not been reported before. On the basis of the above considerations, we synthesized selfsupported amorphous CoMoO4 nanowire array on Ti mesh (CoMoO4 NWA/Ti) via a simple two-step hydrothermal method. When used as a three-dimensional (3D) hydrogenevolving electrode, CoMoO4 NWA/Ti shows superior catalytic activity and long-term electrochemical durability in 1.0 M KOH. It demands overpotentials of 81 and 243 mV to reach current densities of 10 and 100 mA cm−2, respectively.

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EXPERIMENTAL SECTION

Materials. Co(NO3)2·6H2O, NH4F, urea, and Na2MoO4·2H2O were purchased from Tianjin Fuyu Chemical Reagent Co. Ltd., China. Pt/C (20 wt % Pt on Vulcan XC-72R) was purchased from Alfa Aesar (China) Chemicals Co., Ltd. Ti mesh was provided by Baoji Yunjie (China) Metal Production Co., Ltd., pretreated in HCl solution, and then cleaned by sonication in water and ethanol several times to Received: June 26, 2017 Revised: September 3, 2017 Published: September 27, 2017 10093

DOI: 10.1021/acssuschemeng.7b02093 ACS Sustainable Chem. Eng. 2017, 5, 10093−10098

Research Article

ACS Sustainable Chemistry & Engineering remove surface impurities. The water used throughout all experiments was purified through a Millipore system. Preparation of Co(OH)F NWA/Ti. Co(OH)F NWA/Ti was prepared as follows. In a typical synthesis,7 Co(NO3)2·6H2O (0.582 g), NH4F (0.186 g), and urea (0.60 g) were dissolved in 40 mL of water under vigorous stirring for 30 min. Then, the solution was transferred into a Teflon-lined stainless autoclave (40 mL), and a piece of cleaned Ti mesh (3 × 2 cm) was immersed into the solution. The autoclave was sealed and maintained at 120 °C for 6 h in an electric oven. After the autoclave cooled to room temperature, the resulting Co(OH)F NWA/Ti was taken out and washed with water thoroughly before being vacuum-dried. Preparation of CoMoO4 NWA/Ti. CoMoO4 NWA/Ti was prepared by hydrothermal reaction. Na2MoO4·2H2O (0.1 g) was dissolved in 35 mL of water under vigorous stirring for 30 min. Then, the solution was transferred into a Teflon-lined stainless autoclave (40 mL) and the as-prepared Co(OH)F NWA/Ti was immersed into the solution. The autoclave was sealed and maintained at 130 °C for 6 h in an electric oven. After being cooled slowly to room temperature, the CoMoO4 NWA/Ti was removed and washed with water thoroughly before being vacuum-dried. Preparation of Pt/C Electrode. To prepare Pt/C electrode, 50 mg of Pt/C and 20 μL of 5 wt % Nafion solution and 280 μL of ethanol were dispersed in 700 μL of water by 30 min sonication to finally form an ink. Then, 12.0 μL of catalyst ink was dropped on bare Ti (0.5 × 0.5 cm) with a catalyst loading of 2.4 mg cm−2. Physical Property Characterization. X-ray diffraction (XRD) data of the samples were collected on Bruker D8 ADVANCE Diffractiometer (λ = 1.5418 Å). Scanning electron microscopy (SEM) measurements were performed on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. Transmission electron microscopy (TEM) measurements were performed on a HITACHI H-8100 electron microscope (Hitachi, Tokyo, Japan) with an accelerating voltage of 200 kV. XPS spectra were collected with a Thermal ESCALAB 250 spectrometer using an Al Kα X-ray source (1486.6 eV photons). Electrochemical Measurements. Electrochemical measurements were performed with a CHI 760E electrochemical analyzer (Huachen, Shanghai) in a standard three-electrode system using CoMoO4 NWA/ Ti as working electrode, a graphite rod as counter electrode, and a Hg/ HgO as reference electrode. Except specifically explained, all measures were calibrated to RHE using the following equation: E(RHE) = E(Hg/HgO) + 0.9254 V. Polarization curves were obtained using linear sweep voltammetry (LSV) with a scan rate of 2 mV s−1, and no activation was used before recording the polarization curves. The longterm durability test was performed using potentiostatic electrolysis at fixed potentials. The ohmic resistance of the electrode was measured by AC impedance spectroscopy, and all currents presented were corrected against the ohmic potential drop. The electrochemical double layer capacitance (Cdl) was measured using a simple cyclic voltammetry (CV) method. The voltage window of cyclic voltammograms was 0.78−0.88 V vs RHE. The scan rates were 50, 100, 150, 200, 250, 300, and 350 mV s−1. Cdl was estimated by plotting the Δj = (ja − jb) at 0.82 V vs. RHE against the scan rate, where ja and jb are charging and discharging current, respectively.

Figure 1. (A) XRD patterns of bare Ti mesh, Co(OH)F NWA/Ti, and CoMoO4 NWA/Ti. SEM images of (B) Co(OH)F NWA/Ti and (C) CoMoO4 NWA/Ti (inset: high-magnification). (D) SEM image and EDX elemental mapping images of Co, Mo, and O in CoMoO4 NWA/Ti. TEM images of (E) Co(OH)F nanowire and (F) CoMoO4 nanowire. HRTEM image of (G) Co(OH)F and (H) CoMoO4 nanowires. (I) SAED pattern taken from CoMoO4 nanowire.

diffraction peaks of Ti at 35.1, 38.4, 40.2, 53.0, and 62.9°, which are indexed to the (100), (002), (101), (102), and (110) planes, respectively (JCPDS no. 44-1294). For Co(OH)F NWA/Ti, apart from the peaks of Ti, the additional diffraction peaks are derived from the Co(OH)F NWA phase with an orthorhombic structure (JCPDS no. 50-0827).10 In contrast, CoMoO4 NWA/Ti shows diffraction peaks of only Ti, indicating the formation of amorphous species. The SEM image of Co(OH)F NWA/Ti (Figure 1B) demonstrates that the entire surface of Ti mesh is completely covered with Co(OH)F nanowires. After anion-exchange reaction, the nanowire is retained but becomes thicker (Figure 1C). The cross-section SEM image for CoMoO4 NWA/Ti (Figure S5) indicates the nanowire array is about 5.0 μm in thickness. Energy-dispersive X-ray (EDX) spectrum provides an atomic ratio of 1:0.91:3.79 for Co/Mo/O in CoMoO4 (Figure S2). Figure 1D shows the SEM image of CoMoO4 NWA/Ti and the EDX elemental mapping images, confirming the uniform distribution of Co, Mo, and O. Moreover, the nanostructure can be further observed in the TEM and high-resolution TEM (HRTEM) images (Figures 1E−H). Figure 1E shows the TEM image of Co(OH)F nanowire, revealing that it is solid with a smooth surface. A comparison of the TEM image of CoMoO4 nanowire (Figure 1E) and Co(OH)F nanowire shows that the diameter of CoMoO4 nanowire is about twice that of Co(OH) F nanowire. The HRTEM image (Figure 1G) presents a wellresolved lattice fringe with an interplanar distance of 0.320 nm, corresponding to (002) plane of Co(OH)F. The HRTEM image (Figure 1G) and selected area electron diffraction (SAED) pattern (Figure 1I) taken from CoMoO4 nanowire

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RESULTS AND DISCUSSION The CoMoO4 NWA/Ti was fabricated by anion-exchange reaction of hydrothermally obtained Co(OH)F nanowire array on Ti mesh (Co(OH)F NWA/Ti). Figure S1 shows an optical photograph of bare Ti mesh, Co(OH)F NWA/Ti, and CoMoO4 NWA/Ti. After the surface of Ti mesh was coated with Co(OH)F nanowire array (Co(OH)F NWA), the color turned from gray to pink; the pink Co(OH)F NWA eventually became light purple CoMoO4 NWA through anion-exchange reaction. Figure 1A shows XRD patterns of bare Ti mesh, Co(OH)F NWA/Ti, and CoMoO4 NWA/Ti. Bare Ti mesh has 5 10094

DOI: 10.1021/acssuschemeng.7b02093 ACS Sustainable Chem. Eng. 2017, 5, 10093−10098

Research Article

ACS Sustainable Chemistry & Engineering suggest that it is amorphous in nature. All results confirm the successful preparation of self-supported amorphous CoMoO4 NWA/Ti from the crystalline Co(OH)F NWA/Ti precursor. The XPS spectrum further confirms the surface chemical state of CoMoO4. As shown in Figure 2A, the spectrum for

Figure 2. (A) XPS survey spectrum for CoMoO4 NWA. XPS spectra of CoMoO4 NWA in the (B) Co 2p, (C) Mo 3d, and (D) O 1s regions.

Figure 3. (A) LSV curves for CoMoO4 NWA/Ti, Co(OH)F NWA/ Ti, Pt/C on Ti mesh, and bare Ti mesh with a scan rate of 2 mV s−1 for HER. (B) Tafel plots for CoMoO4 NWA/Ti, Co(OH)F NWA/Ti, and Pt/C on Ti mesh. (C) Multicurrent step chronopotentiometric curve for CoMoO4 NWA/Ti. The current density started at −4 mA cm−2 and ended at −40 mA cm−2 with an increment of 4 mA cm−2 per 500 s. (D) LSV curves of initial and after 1000 cycle cyclic voltammetry scans recorded for CoMoO4 NWA/Ti and timedependent current density curve at a fixed overpotential of −184 mV vs RHE without iR-compensation (inset). All experiments were carried out in 1.0 M KOH.

CoMoO4 indicates that the product consists of Co, Mo, and O elements. In Figure 2B, the binding energies (BEs) of 780.3 and 796.4 eV correspond to Co 2p3/2 and Co 2p1/2, respectively, which are characteristic of Co2+ species.27,28 The BEs of Co 2p at 786.2 and 802.4 eV with two obvious shakeup satellites (sat.) correspond to Co2+. In Figure 2C, the BEs of 231.9 and 235.0 eV with a splitting width of 3.1 eV match to Mo 3d5/2 and Mo 3d3/2, respectively, confirming that Mo exists in form of its VI oxidation state, which is consistent with MoO42−.37,38 The O 1s spectrum (Figure 2D) shows only one peak at BE of 529.7 eV, which corresponds to the oxygen ions in oxides.38,39 XPS spectrum offers an atomic ratio of 1:0.92:4.14 for Co/Mo/O in CoMoO4, which is nearly consistent with the EDX results (Table S2). Typically, the binding energy belongs to the surface OH− at ∼532.5 eV;39,40 however, there is no peak in Figure 2D, indicating that Co(OH)F completely converted to CoMoO4, which reverifies the analysis of XRD. The XPS results also indicate that the product is CoMoO4. The HER electrocatalytic performance of CoMoO4 NWA/Ti (CoMoO4 loading: 2.4 mg cm−2) was examined using a typical three-electrode setup with a scan rate of 2 mV s−1 in 1.0 M KOH. For comparison, Co(OH)F NWA/Ti, commercial Pt/C on Ti mesh, and bare Ti mesh were evaluated with the same condition. Except when specifically explained, all experimental data are corrected with ohmic potential drop (iR) losses,41 which avoids the impact of electrolyte resistance and directly reveals the natural behavior of catalysts. Figure 3A shows the LSV polarization curves on reversible hydrogen electrode (RHE) scale. The bare Ti mesh has almost no HER catalytic activity, implying that the contribution of Ti mesh can be ignored. As expected, Pt/C on Ti mesh shows excellent HER activity with the need of an overpotential of 36 mV at 10 mA cm−2. To reach the same catalytic current density, an overpotential of 337 mV is needed for Co(OH)F NWA/Ti, whereas amorphous CoMoO4 NWA/Ti requires only a much

smaller overpotential of 81 mV. This overpotential is lower than those of reported non-noble-metal HER catalysts in 1.0 M KOH, including Co-NRCNTs (370 mV),5 Co-NCNT (180 mV),9 CoP/CC (209 mV),10 Co@N−C (210 mV),20 Ni0.33Co0.67S2 nanowire (88 mV),21 CoOx@CN (232 mV),22 CoP/rGO-400 (150 mV), 23 CoO/MoO x (163 mV), 24 CoMoO4 (353 mV),24 etc. A more detailed comparison is listed in Table S1. The catalytic performance of CoMoO4 NWA/Ti in 0.5 M H2SO4 and 0.1 M KOH was also studied (Figure S7). Figure 3B shows the Tafel plots for Pt/C on Ti mesh, Co(OH)F NWA/Ti and CoMoO4 NWA/Ti. It is also confirmed that the performance of CoMoO4 NWA/Ti is better than Co(OH)F NWA/Ti. The Tafel slope of 63 mV dec−1 for CoMoO4 NWA/Ti signifies that the HER occurs through a Volmer−Heyrovsky mechanism and that the Volmer process is the rate-determining step.24,42 Figure 3C displays a multicurrent step chronopotentiometric curve for amorphous CoMoO4 NWA/Ti in 1.0 M KOH with the current being increased from −4 to −40 mA cm−2 (4 mA cm−2 increment per 500s). The overpotential immediately levels off at −0.099 mV vs. RHE at the start current value and keep constant in the remaining 500 s. The other steps show similar results, indicating the excellent mass transportation, conductivity, and mechanical robustness.41 It is reported that amorphous materials had catalytic activity higher than that of their counterparts.24,43 We synthesized crystalline CoMoO4 NWA/ Ti by a reported method.44 The HER performance in 1 M KOH of CoMoO4 crystalline structure is provided in Figure S8. This finding also reveals that the amorphous structure can improve HER activity. Therefore, the excellent catalytic performance of CoMoO4 could be enhanced by the amorphous structure. Apart from the need for good catalytic activity, electrochemical durability is equally important for electrode evaluation. The durability of amorphous CoMoO4 NWA/Ti is evaluated using the polarization curves of before and after 1000 10095

DOI: 10.1021/acssuschemeng.7b02093 ACS Sustainable Chem. Eng. 2017, 5, 10093−10098

ACS Sustainable Chemistry & Engineering

Research Article

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CONCLUSIONS In conclusion, anion exchange reaction of crystalline Co(OH)F NWA/Ti in Na2MoO4 solution was certified as a good strategy for fabricating amorphous CoMoO4 NWA/Ti. When used as a 3D hydrogen-evolving electrode in 1.0 M KOH, such CoMoO4 NWA/Ti shows superior catalytic activity and long-term durability. It demands overpotentials of 81 and 243 mV to achieve current densities of 10 and 100 mA cm−2, respectively. It also has strong long-term durability for at least 20 h. This study not only supplies us with an attractive earth-abundant catalyst material for electrolytic hydrogen production under alkaline conditions but also presents a new direction to develop metal molybdate nanoarray applications.

cyclic voltammetry scans and time-dependent current density curve. The polarization curves with potential between −0.8 and −1.2 V vs. RHE at a scan rate of 100 mV s−1 are almost coincident with the initial one (Figure 3D). Electrolysis result at fixed overpotential of −184 mV further demonstrates that this catalyst is superior in long-term electrochemical durability, capable of maintaining its catalytic activity for at least 20 h (Figure 3D inset curve). To verify the morphology and composition of the CoMoO4 NWA/Ti after HER electrocatalysis, the SEM and XPS results were collected. Figure S3 shows the SEM image of CoMoO4 NWA/Ti after electrochemical tests, indicating its robust feature. The similarity of the Co 2p peaks (Figure S8) implies the valence of Co did not chang during HER electrochemical tests. The electrochemical surface area (ECSA) normally influences the catalytic activity of the catalysts. Generally, electrochemical double-layer capacitance (Cdl) is used to calculate ECSA.24,45 We further measured the Cdl of CoMoO4 NWA/Ti and Co(OH)F NWA/Ti. Figures 4A and B show CVs of

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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.7b02093. Optical photograph, EDX spectra, SEM images, LSV curves, XPS spectra, comparison of HER performances, and table of elemental weights and atomic ratios (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: 86-531-82767872; Fax: 86-531-82765969. ORCID

Yong Zhang: 0000-0002-5831-637X Qin Wei: 0000-0002-3034-8046 Dan Wu: 0000-0002-8732-5988 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Natural Science Foundation of China (Grants 21405059 and 21575047), Shandong Province (Grants ZR2016JL013 and ZR2014BL024), and the National Key Scientific Instrument and Equipment Development Project of China (Grant 21627809).

Figure 4. CVs of (A) CoMoO4 NWA/Ti and (B) Co(OH)F NWA/ Ti at various scan rates (50, 100, 150, 200, 250, 300, and 350 mV s−1). (C) Differences (Δj = (ja − jb) in current density at 0.82 V vs. RHE plotted against scan rate. (D) Nyquist plots of CoMoO4 NWA/Ti and Co(OH)F NWA/Ti obtained at OCP.

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CoMoO4 NWA/Ti and Co(OH)F NWA/Ti at the scan rates of 50, 100, 150, 200, 250, 300, and 350 mV s−1, respectively. It can be noted from Figure 4C that the Cdl of CoMoO4 NWA/Ti is 17 mF cm−2; however, that of Co(OH)F NWA/Ti is only 5 mF cm−2, demonstrating that CoMoO4 NWA/Ti has a surface area much higher than that of Co(OH)F NWA/Ti. Electrochemical impedance spectroscopy analysis was used to characterize the HER kinetics on the surface of the catalyst.24 Figure 4D shows the Nyquist plots of CoMoO4 NWA/Ti and Co(OH)F NWA/Ti at open circuit potential (OCP). One semicircle was observed in the Nyquist plots of both CoMoO4 NWA/Ti and Co(OH)F NWA/Ti. CoMoO4 NWA/Ti electrode has charge transfer resistance lower than that of Co(OH)F NWA/Ti. This suggests a much faster chargetransfer kinetics on the surface of the CoMoO4 NWA/Ti electrode.

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DOI: 10.1021/acssuschemeng.7b02093 ACS Sustainable Chem. Eng. 2017, 5, 10093−10098

Research Article

ACS Sustainable Chemistry & Engineering (45) Yan, X.; Tian, L.; Chen, X. Crystalline/amorphous Ni/NiO core/shell nanosheets as highly active electrocatalysts for hydrogen evolution reaction. J. Power Sources 2015, 300, 336−343.

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DOI: 10.1021/acssuschemeng.7b02093 ACS Sustainable Chem. Eng. 2017, 5, 10093−10098