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WO3 Nanoarray: An Efficient Electrochemical Oxygen Evolution Catalyst Electrode Operating in Alkaline Solution Xuqiang Ji,†,‡ Min Ma,† Ruixiang Ge,† Xiang Ren,† Hui Wang,*,§ Jingquan Liu,‡ Zhiang Liu,⊥ Abdullah M. Asiri,∥ and Xuping Sun*,† †

College of Chemistry and §Analysis and Test Center, Sichuan University, Chengdu 610064, China College of Materials Science and Engineering, Qingdao University, Qingdao 266071, China ⊥ College of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, China ∥ Chemistry Department, King Abdulaziz University, Jeddah 21589, Saudi Arabia

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‡

S Supporting Information *

In this Communication, a WO3 nanoarray on carbon cloth (WO3/CC) is prepared and investigated as a high-performance and long-lasting electrocatalyst for alkaline water oxidation (compared with other metal substrates like macroporous Ni foam and Ti mesh, CC has better flexibility and thus is much better for device integration). In a 1.0 M KOH electrolyte, WO3/ CC displays high OER activity, and the current density reaches 10 mA cm−2 at an overpotential of 280 mV, outperforming most reported nonprecious metal oxide catalysts. Notably, there is almost no performance attenuation after 100 h of catalysis at a specified current density of 20 mA cm−2. Figure 1a shows the X-ray diffraction (XRD) patterns for blank CC and WO3/CC (see the Supporting Information for

ABSTRACT: It is fascinating to design and synthesize high-efficiency and noble-metal-free alkaline oxygen evolution reaction (OER) electrocatalysts. In this Communication, we describe the one-step hydrothermal synthesis of a WO3 nanoarray directly grown on conductive carbon cloth (WO3/CC) for efficient water oxidation in 1.0 M KOH. As a monolithically integrated array catalyst, WO3/CC exhibits superior OER activity demanding overpotential as low as 280 mV to afford a benchmarking catalytic current density of 10 mA cm−2. It is worth noting that WO3/CC also possesses strong electrochemical durability with 95% Faradaic yields.

T

he global energy shortage and environment deterioration induced by petroleum consumption alarmingly remind us to explore sustainable and clean energy alternatives.1−3 Pure hydrogen is considered to be an environmentally friendly clean chemical fuel energy to power stationary equipment and transportation.4,5 Electrochemical water splitting is a subtle technology for large-scale hydrogen preparation6,7 but is intrinsically restricted by the anodic oxygen evolution reaction (OER) involving multiple steps of proton-coupled electron transfer to form a covalent O−O bond.8 Thus, highly efficient electrocatalysts for OER are needed to achieve rapid water oxidation kinetics. Ruthenium and iridium oxides show outstanding catalytic activity for OER but with great restriction of their scarcity and high cost,9 which stimulates an urgent demand to fabricate efficient OER electrocatalysts composed by earthabundant elements. So far, many great efforts have been devoted to synthesizing OER electrocatalysts based on transition metals, including Ni,10−13 Co,14−16 Fe,17,18 Mn,19−21 etc. As an interesting nonnoble metal, W is rarely reported for electrochemical water oxidation and recently WO2 particles have been proposed as an active OER electrocatalyst, requiring an overpotential of 300 mV to attain 10 mA cm−2 in 1.0 M KOH.22 As another class of tungsten oxide, WO3 has been widely reported as a photoanode for photoelectrochemical water oxidation but with a limited current density in neutral media,23−27 but its use for alkaline water oxidation electrocatalysis has not been explored before. © 2017 American Chemical Society

Figure 1. (a) XRD patterns for WO3/CC and CC. (b) XPS survey spectrum of a WO3 naonowire. High-resolution XPS spectra of the (c) W 4f and (d) O 1s core levels in WO3.

preparative details). WO3/CC shows diffraction peaks at 13.98°, 23.38°, 24.5°, 26.62°, 28.02°,33.64°, 36.84°, 37.64°, 39.72°, 42.7°, 43.4°,49.76°, 55.94°, and 63.42° indexed to the (100), (002), (110), (111), (200), (112), (202), (210), (211), (300), (113), (220), (204), and (402) planes of the WO3 phase (JCPDS 85-2459),28 respectively. The X-ray photoelectron Received: October 5, 2017 Published: December 4, 2017 14743

DOI: 10.1021/acs.inorgchem.7b02552 Inorg. Chem. 2017, 56, 14743−14746

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electrochemical impedance spectra (Figure S2). The potentials reported in this work were calibrated to a reversible hydrogen electrode. Figure 3a shows the linear-sweep-voltammetry (LSV)

spectroscopy (XPS) survey spectrum (Figure 1b) evidences the presence of W and O.29 Parts c and d of Figure 1 further show XPS spectra of the W 4f and O 1s core levels in WO3. In Figure 1c, the binding energies at 36.3 and 38.4 eV are ascribed to W 4f7/2 and W 4f5/2, respectively, suggesting the high oxidation state of the W in the sample (6+).30,31 As shown in Figure 1d, the fitted peak at 531.1 eV in the O 1s spectrum is assigned to the W−O bond in WO3 and the peak at 532.4 eV is attributed to the surface hydroxides. These above analyses all confirm the successful synthesis of WO3.32 The scanning electron microscopy (SEM) images of WO3/ CC (Figure 2a,b) indicate that the entire surface of CC is fully

Figure 3. (a) LSV polarization curves of RuO2/CC, WO3/CC, and blank CC in 1.0 M KOH for OER. (b) Corresponding Tafel plots of RuO2/CC and WO3/CC. (c) LSV polarization curves for WO3/CC before and after 1000 consecutive cycles. (d) Curve of the current density versus time for WO3/CC at 0.55 V versus Hg/HgO without iR compensation.

polarization curves. Blank CC shows very poor water oxidation activity, while RuO2/CC is highly efficient for OER electrocatalysis with an overpotential of 235 mV to afford 10 mA cm−2. Note that WO3/CC also shows outstanding electrocatalytic activity and demands an overpotential of 280 mV to drive 10 mA cm−2. Such superior activity is comparable to the performance of most metal-oxide-based OER electrocatalysts in alkali (Table S1). WO3/CC shows activity superior to that of our recent catalysts including NiCo2O4/CC,34 Co3O4/CC,35 and NiO/ CC.36 The initial LSV curve without iR correction (Figure S3) suggests that WO3/CC attains 10 mA cm−2 at an overpotential of 290 mV. Such a WO3/CC demands overpotentials of 370 and 246 mV to reach 10 mA cm−2 in 0.1 M and 30 wt % KOH, respectively (Figure S4). From the cyclic voltammograms (Figure S5a), the doublelayer capacitance (CDL) for WO3/CC was determined as 13.4 mF cm−2 (Figure S5b). The electrochemically active surface area (ECSA) was calculated to be 33.5 cm−2 according to the equation ECSA = CDL/0.4.37 The LSV curve normalized by ECSA (Figure S5c) shows that this catalyst electrode needs an overpotential of 300 mV to drive 10 mA cm−2 in 1.0 M KOH. Given that WO3 is a photoanode material, we also conducted the LSV test for WO3/ CC kept in the dark and found that this electrode shows identical catalytic currents in the dark and under natural light irradiation (Figure S6). These results suggest the intrinsic high OER activity of WO3/CC in alkaline media. Figure 3b shows the Tafel plots of RuO2/CC and WO3/CC. As expected, RuO2/CC has a low Tafel slope of 58 mV dec−1. Also, the Tafel slope for WO3/CC is 82 mV dec−1. In our present study, WO3 was immersed in 1.0 M KOH, and the corresponding XPS spectral analysis proved the existence of W5+ (Figure S7). Such W5+ species can be further oxidized to W6+ during a positive scan to drive subsequent water oxidation. Stability is also highly critical for the catalyst. We probed its stability by cyclic voltammetry, and a negligible current loss was

Figure 2. (a and b) SEM images of WO3/CC. (c and d) TEM images of the WO3 nanowire. The inset is the SAED pattern of the WO3 nanowire. EDX elemental mapping images of WO3/CC for the (e) W and (f) O elements. The inset in part e is the EDX image of WO3.

covered with a WO3 nanowire array. Cross-sectional SEM analysis further shows that the thickness of the WO3 nanoarray is around 2 μm (Figure S1). The high-resolution transmission electron microscopy (HRTEM) image taken from the WO3 nanowire (Figure 2c) unfolds well-resolved lattice fringes with a clearly identified lattice fringe space of 0.236 nm corresponding to the (210) plane of the WO3 phase (Figure 2d). The clear selected-area electron diffraction (SAED) pattern (Figure 2c, inset) implies that the nanowire has good crystallization. The corresponding energy-dispersive X-ray (EDX) elemental mapping images of WO3/CC (Figure 2e,f) demonstrate that the W and O elements are uniformly distributed in the WO3 product, and EDX spectroscopy (Figure 2e, inset), also proves the existence of the W and O elements. The electrocatalytic OER performance of WO3/CC was investigated in alkali with a scan rate of 5 mV s−1 employing a traditional three-electrode configuration. Blank CC and RuO2 deposited on CC (RuO2/CC) were also examined under the same test conditions. The influence of solution resistance was also taken into consideration, further correcting all experimental data with a corresponding ohmic potential drop (iR),33 and the resistances for WO3/CC and RuO2/CC were obtained from the 14744

DOI: 10.1021/acs.inorgchem.7b02552 Inorg. Chem. 2017, 56, 14743−14746

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based 3D catalyst electrode for efficient water oxidation under alkaline conditions but also broadens the horizon and enriches knowledge for the utilization of tungsten oxide.

observed after 1000 consecutive cycles with a scan rate of 100 mV s−1, indicating excellent stability of WO3/CC (Figure 3c). This 3D catalyst electrode also possesses strong electrochemical durability, with preservation of good catalytic activity for at least 100 h (Figure 3d). The multistep chronopotentiometric measurement for WO3/CC was carried out by changing the anodic current density from 20 to 110 mA cm−2, and 10 mA cm−2 was increased per 500 s. The potential rapidly mounted into 0.56 V versus Hg/HgO for the initial current density and remained unchanged for the remaining 500 s. Also, similar responses are presented for all other current densities, suggesting the distinctive mass transportation, good conductivity, and strong mechanical robustness of the WO3/CC electrode (Figure S8). After bulk OER electrolysis, WO3/CC still maintained its initial nanoarray morphology, further demonstrating its robust nature (Figure S9). An amorphous layer was formed on the WO3 nanowire (Figure S10), which can be attributed to the hydration step of WO3 dissolution during water oxidation.38 A further stability test of WO3/CC was conducted in 30 wt % KOH, and rapid catalyst deactivation is observed (Figure S11). SEM images for the catalyst electrode before and after the stability test in 30 wt % KOH demonstrates the disappearance of the nanowire (Figure S12), indicating the complete dissolution of WO3. Obviously, a high-concentration alkaline solution can accelerate a reversible dissolution reaction on WO3, resulting in low catalytic efficiency and poor stability. Therefore, a suitable alkaline condition (1.0 M KOH) is preferred in WO3 electrocatalysis.39 The quantity of theoretically calculated oxygen divided by practically evolved oxygen suggests 95% Faradaic efficiency (FE), as shown in Figure S13. The turnover frequency (TOF) was determined by the formulas TOF = jA/4Fm and slope = n2F2m/4RT.40 In order to obtain the slope value, a series of cyclic voltammograms were collected within a potential range from 0.2 to 0.4 V versus Hg/ HgO (Figure 4a). A linear plot related to the oxidation currents

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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/acs.inorgchem.7b02552. Experimental section, SEM and TEM images, Nyquist plots, LSV curves, cyclic voltammograms, plot of capacitive currents versus scan rates, plot of current density versus time, FE data, plot of TOF versus overpotential, and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.W.). *E-mail: [email protected] (X.S.). ORCID

Jingquan Liu: 0000-0001-6178-8661 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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Figure 4. (a) Cyclic voltammograms of WO3/CC at scan rates of 10, 20, 30, 40, and 50 mV s−1 in 1.0 M KOH. (b) Linear curve of the oxidation peak current as a function of the scan rate for WO3/CC.

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DOI: 10.1021/acs.inorgchem.7b02552 Inorg. Chem. 2017, 56, 14743−14746