Synthesis of Self-Supported Amorphous CoMoO4 Nanowire Array for

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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 ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b02093 • Publication Date (Web): 27 Sep 2017 Downloaded from http://pubs.acs.org on September 28, 2017

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ACS Sustainable Chemistry & Engineering

Synthesis of Self-supported Amorphous CoMoO4 Nanowire Array for Highly Efficient Hydrogen Evolution Reaction Jinxiu Zhaoa, Xiang Rena, Hongmin Maa, Xu Suna, Yong Zhanga, Tao Yanb, Qin Weia, and Dan Wua,* a

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 b School of Resources and Environment, University of Jinan, 336 Nan Xin Zhuang West Road, Jinan 250022, P.R. China * Dan Wu is the corresponding authors E-mail address: [email protected]; Tele: 86-531-82767872; Fax: 86-531-82765969 ABSTRACT: Water electrolysis is known as the most environmental friendly and renewable technology to generate hydrogen. In order to make it more energy-efficient, developing promising cathodic hydrogen evolution reaction (HER) 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 only demands overpotentials of 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 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 condition, which, however, has not been reported before.

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, like nature gas reforming, would accelerate fossil fuel depletion and CO2 emission, which is not worth the candle.6–8 Electrochemical water splitting, in view of energy and environmental issues, is a promising approach to generate hydrogen, but it 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 earthabundant and efficient HER electrocatalysts.

Based on the above considerations, we have synthesized self-supported amorphous CoMoO4 nanowire array on Ti mesh (CoMoO4 NWA/Ti) via a simple two-step hydrothermal method. When used as a three-dimensional (3D) hydrogen-evolving 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.

Because of abundance and excellent performance, Co-based materials have been intensively studied as HER electrocatalysts19, such as Co-NRCNTs,5 CoNCNT,9 CoP,10 Co2P,11 CoP/CNT,14 Co@N-C,20 Ni0.33Co0.67S2 nanowire,21 CoOx@CN,22 CoP/rGO400,23 CoO/MoOx24 etc. 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 costeffective 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 compound.27,28 To the best of our knowledge, many researches have been mainly focused on the OER activity of CoMoO4. It is well-established that nanoarray structure directly grown on conductive material without adhesion has significant advantages in

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. and pretreated in HCl solution and then cleaned by sonication in water and ethanol for several times to 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 synthesis7, Co(NO3)2·6H2O (0.582 g), NH4F (0.186 g) and urea (0.60 g) were dissolved in 40 mL water under vigorous stirring for 30 min. Then the solution was transferred into a Teflon-

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RESULTS AND DISCUSSION

lined stainless autoclave (40 mL) and a piece of cleaned Ti mesh (3 cm × 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 down at room temperature, the resulting Co(OH)F NWA/Ti was taken out and washed with water thoroughly before 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 water under vigorous stirring for 30 min. Then the solution was transferred into a Teflon-lined stainless autoclave (40 mL) and the asprepared 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 cooled down slowly at room temperature, the CoMoO4 NWA/Ti was taken out and washed with water thoroughly before vacuum dried. Preparation of Pt/C electrode. To prepare Pt/C electrode, 50 mg Pt/C and 20 µL 5 wt% Nafion solution and 280 µL ethanol were dispersed in 700 µL water by 30 min sonication to form an ink finally. Then 12.0 µL catalyst ink was droped in bare Ti (0.5 cm × 0.5 cm) with a catalyst loading of 2.4 mg cm–2 Physical Property Characterization. XRD data of the samples were collected on Bruker D8 ADVANCE Diffractiometer (λ=1.5418 Å). SEM measurements were performed on a XL30 ESEM FEG scanning electron microscope at an accelerating voltage of 20 kV. TEM measurements were performed on a HITACHI H-8100 electron microscopy (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).

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 nanowire and (H) CoMoO4 nanowire. (I) SAED pattern taken from CoMoO4 nanowire.

The CoMoO4 NWA/Ti was fabricated by anionexchange reaction of hydrothermally obtained Co(OH)F nanowire array on Ti mesh (Co(OH)F NWA/Ti). Figure S1 shows a optical photograph of bare Ti mesh, Co(OH)F NWA/Ti, and CoMoO4 NWA/Ti. After the surface of Ti mesh being 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.

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 with a scan rate of 2 mV s–1 and no activation was used before recording the polarization curves. The long-term 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 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 ∆j = (ja – jb) at 0.82 V vs. RHE against the scan rate, where ja and jb are charging and discharging current, respectively.

Figure 1A shows X-ray diffraction (XRD) patterns of bare Ti mesh, Co(OH)F NWA/Ti and CoMoO4 NWA/Ti. Bare Ti mesh has five 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 only shows diffraction peaks of Ti, indicating the formation of amorphous species. The scanning electron microscope (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-

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consistent with MoO42–.37,38 The O 1s spectrum (Fig. 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 consist with the EDX results (Table S2). Typically, the binding energy is belonged 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 re-verifies the analysis of XRD. The XPS results also indicates that the product is CoMoO4.

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 Afterelements. Moreover, the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images (Figure 1E-H) can further observe the nanostructure. Figure 1E shows the TEM image of Co(OH)F nanowire, revealing that it is solid with a smooth surface. Compare the TEM image of CoMoO4 nanowire (Figure 1E) and Co(OH)F nanowire, we can see that the diameter of CoMoO4 nanowire is about twice that of Co(OH)F nanowire. The HRTEM image (Figure 1G) presents a well-resolved 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 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.

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) Multi-current steps 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 scannings recorde for CoMoO4 NWA/Ti and time-dependent 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.

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 are evaluated with the same condition. Except 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 nature behavior of catalysts. Figure 3A shows the linear sweep voltammetry (LSV) polarization curves versus 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 337mV is needed for Co(OH)F NWA/Ti, whereas amorphous CoMoO4 NWA/Ti only requires a much smaller overpotential of 81 mV. This overpotential is lower than

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.

X-ray photoelectron spectroscopy (XPS) spectrum can further confirm the surface chemical state of CoMoO4. As shown in Figure 2A, the spectrum for 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 are corresponding to Co 2p3/2 and Co 2p1/2, respectively, which are the characteristic of Co2+ species.27,28 The BEs of Co 2p at 786.2 and 802.4 eV with two obvious shake-up satellites (Sat.) correspond to Co2+. In Fig. 2C, the BEs of 231.9 and 235.0 eV with a splitting width of 3.1 eV are matched to Mo 3d5/2 and Mo 3d3/2, respectively, confirming that Mo exists in form of its VI oxidation state, which is



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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/MoOx (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 were also performed (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 the Volmer process is the rate-determining step.24,42 Figure 3C displays a multi-current steps 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 that the excellent mass transportation, conductivity and mechanical robustness.41 It is reported that amorphous materials had higher catalytic activity than their counterparts.24,43 We have synthesized crystalline CoMoO4 NWA/Ti by a reported method.44 The HER performance in 1 M KOH of CoMoO4 crystalline structure has provided in Figure S8. This finding also reveals that amorphous structure can improve the HER activity. Therefore, the excellent catalytic performance of CoMoO4 could be enhanced by the amorphous structure. Apart from the need of good catalytic activity, electrochemical durability is equally important for electrode evaluation. The durability of amorphous CoMoO4 NWA/Ti is evaluated by the polarization curves of before and after 1000 cycle cyclic voltammetry scannings 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 test, indicating its robust feature. The similarity of the Co 2p peaks (Figure S8) implies the valence of Co has not changed during HER electrochemical test.

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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 Fig. 4C that the Cdl of CoMoO4 NWA/Ti is 17 mF cm–2, however, the Co(OH)F NWA/Ti is only 5 mF cm–2, demonstrating CoMoO4 NWA/Ti has a much higher surface area than Co(OH)F NWA/Ti. Electrochemical impedance spectroscopy analysis is used to characterize the HER kinetics on the surface of the catalyst.24 Fig. 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 lower charge transfer resistance than Co(OH)F NWA/Ti. This suggests a much faster chargetransfer kinetics on the surface of the CoMoO4 NWA/Ti electrode.

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 (∆j = (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.

CONCLUSIONS In conclusion, anion exchange reaction of crystalline Co(OH)F NWA/Ti in Na2MoO4 solution has been certified as a nice 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 an attractive earth-abundant catalyst material for electrolytic hydrogen production under alkaline condition, but presents a new direction to develop metal molybdates nanoarrays applications.

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. Fig. 4A and B show cyclic voltammograms (CVs) of CoMoO4 NWA/Ti

ASSOCIATED CONTENT Supporting Information

Figure S1 ~ Figure S9; Table 1 and Table 2.


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AUTHOR INFORMATION Corresponding Author

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*E-mail: [email protected] Notes (14)

The authors declare no competing financial interest.

ACKNOWLEDGMENT This study was supported by the Natural Science Foundation of China (No. 21405059, 21575047), Shandong Province (No. ZR2016JL013, ZR2014BL024), and the National Key Scientific Instrument and Equipment Development Project of China (No.21627809).

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As a 3D hydrogen-evolving electrode, CoMoO4 NWA/Ti shows superior catalytic activity and strong long-term durability for at least 20 h in 1.0 M KOH.


Synthesis of Self-Supported Amorphous CoMoO4 Nanowire Array for

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