Self-supported tungsten/tungsten dioxide nanowires array as an efficient electrocatalyst in the hydrogen evolution reaction

Abstract

In this study, a tungsten/tungsten dioxide (W/WO₂) nanowires array (NA) was constructed on a carbon paper (CP) (W/WO₂ NA@CP) through the thermal annealing of tungsten trioxide (WO₃) NA. W/WO₂ NA@CP was proven to be an efficient hydrogen evolution cathode with a strong durability in acidic solutions. W/WO₂ NA@CP needs an overpotential of 297 mV to drive a current density of 10 mA cm⁻² and 340 mV to drive a current density of 20 mA cm⁻². The catalytic activity of W/WO₂ NA@CP maintains for at least 50 h in potentiostatic electrolysis. In addition, W/WO₂ NA@CP shows nearly a 100% faradaic efficiency during hydrogen generation. The prominent catalytic activity of W/WO₂ NA@CP correlates with a large number of active sites for the hydrogen evolution reaction and fast electron transport from the CP to W/WO₂ nanowire.

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Introduction

Exploiting sustainable clean energy sources is vital and has become one of the greatest significances of the 21st century.¹ Hydrogen is the most abundant element in the universe and has garnered much attention as a potential fuel for polymer electrolyte membrane fuel cells to help solve the problems related to the depletion of fossil fuels and severe environmental problems. Fuel cell technology is promising and regarded as a renewable energy technology because of its environmental friendly nature (water and heat as by-products) and high thermal value as well as its higher efficiency compared to traditional internal combustion engine technology.² The hydrogen evolution reaction (HER) from water electrolysis, which can produce pure hydrogen on a large scale, is an uphill reaction that requires an efficient electrocatalyst to overcome its reaction kinetics.³ The best HER electrocatalysts are noble metals, such as Pt and Pt-based materials,⁴ but the disadvantage of Pt is that it is expensive and scarce. Therefore, massive research efforts have been devoted to developing a low cost and highly active electrode material for the HER, including non-precious metal oxides (MoO₂,⁵ WO₂,⁶ etc.), metal carbides (Mo₂C⁷), metal phosphides (CoP,⁸ FeP⁹), metal selenium (VSe₂ (ref. 10)) and so on.

In addition to the materials, broad attention has been recently paid to the morphology of the samples. Among various structures, one-dimensional (1D) nanostructures (such as nanorods, nanowires, nanotubes and amorphous nanowires) have drawn lots of attention.^11–13 Therefore, many synthetic methods for 1D nanostructures have been developed.^14–17 Recently, tungsten oxide has widely been applied to electrochromic devices, photocatalysts, gas sensors, second batteries and electrocatalysis because of its distinct optical and electronic properties.^18–22 Besides, tungsten oxide has also been identified as a candidate electrocatalyst for fuel cells with great promise,^23–27 because it can form a tungsten bronze compound (H_xWO₃), which can increase the conductivity. Rajeswari et al. reported the application of a WO₃ nanorods structure as an electrocatalyst in the HER,²⁶ which showed better HER properties in comparison with its bulk counterpart. In addition, Ganesan et al. produced WO₃ nanoparticles using chitosan biopolymer as a template, which could intercalate a larger amount of hydrogen than bulk WO₃.²⁷ There have been a great number of reports on the growth of tungsten oxide nanostructures, including WO₃ nanowires,^28–30 WO₃ nanorods^31,32 and WO₃ nanobelts.³³ However, there are limited examples concerning the synthesis of tungsten oxide nanocrystals on carbon paper (CP) as a HER electrocatalyst. Developing a facile route for the large-scale production of tungsten oxide with a high number of active sites and high purity is still a challenge.

Herein, a tungsten/tungsten oxide (W/WO₂) nanowire array (NA) was constructed on carbon paper (CP), and was proven to be highly active in the HER. W/WO₂ was grown directly on a conductive substrate (CP), resulting in a self-supporting structure, which has demonstrated superior performance over electrodes assembled randomly by a nanostructure of the HER catalyst.^34–37 This configuration offers the shortest path length of electron transport, and enables fast electron transport in the electrode. The W/WO₂ NA integrated on CP (W/WO₂ NA@CP) could work efficiently and stably in acidic solution. The overpotential required for a current density of 20 mA cm⁻² (η₂₀) was as small as 340 mV, and the Tafel slope was 74.5 mV dec⁻¹ in the acidic solution. The faradaic efficiency was nearly 100%. The long-term stability of the W/WO₂ NA@CP in hydrogen generation was confirmed by potentiostatic electrolysis. The study shows that the W/WO₂ NA@CP is a promising HER electrode. Furthermore, we demonstrated that the construction of a catalyst supported by carbon paper is an effective approach to markedly improve the catalytic activity of existing HER electrocatalysts.

Experimental

Preparation of W/WO₂ NA@CP

The precursor was prepared hydrothermally on the basis of a reported method.³⁸ Typically, 12.5 mmol of sodium tungstate was added evenly into ultrapure water (100 mL) under vigorous stirring. Then, the pH value of the solution was adjusted to 1.2 by dropping 3 M hydrochloric acid aqueous solution into the solution slowly, and the solution became a yellowish transparent solution. Subsequently, 35 mmol of oxalic acid was dissolved in the above mixed solution and diluted to 250 mL, and the H₂WO₄ solution was finally prepared. In the next step, the as-formed 20 mL H₂WO₄ precursor was transferred into a Teflon-lined stainless autoclave (35 mL volume), and then 1 g of ammonium sulfate was added to the solution and the solution stirred until it was clear. A piece of carbon paper (2 cm × 5 cm in size), which was ultrasonically cleaned by acetone, deionized water, and alcohol in sequence, was put into the autoclave, and the autoclave was sealed, and kept at 180 °C for 16 h. After the autoclave cooled down to room temperature, the carbon paper was brought out and cleaned with ultrapure water several times and then dried at 80 °C in air. The as-prepared sample was first calcined in N₂ at 500 °C for 1 h to form WO₃ NA@CP. For preparing W/WO₂ NA@CP, WO₃ NA/CP was put in a quartz tube and calcined at 700 °C for 30 min under a N₂/H₂ atmosphere.

Characterization

The morphology of the W/WO₂ NA@CP was measured by transmission electron microscopy (TEM, 300 kV, Tecnai G2 F30 S-TWIN, FEI) and scanning electron microscopy (SEM, S-4800, Hitachi). X-ray energy dispersive spectroscopy (EDS) spectra were recorded using an EDXA Instruments' TIA system equipped on the TEM. The structure of the W/WO₂ NA@CP was characterized by using an X-ray diffractometer, and the patterns were collected using a Bruker D8 Advance diffractometer with graphite-monochromated Cu Kα radiation (1.54178 Å).

Electrochemical measurements

All the electrochemical measurements were analyzed at room temperature (298 K) on a CHI 660E electrochemical workstation (CH Instruments, China). A typical three-electrode system was utilized, in which the W/WO₂ NA@CP performed as a working electrode, while a graphite rod (6 mm diameter) and a mercury/mercurous sulfate electrode (MSE) or mercury/mercury oxide electrode (MOE) were used as the counter electrode and reference electrode, respectively. Sulfuric acid solution (H₂SO₄, 0.5 M) or potassium hydroxide solution (KOH, 1 M) was used as an electrolyte. The MSE was used as a reference electrode in 0.5 M H₂SO₄ solution, and the MOE was used in KOH solution. Before the electrochemical measurements, the high purity H₂ (99.999%) was purged into the electrolyte solution for 30 min. In all the measurements, the MSE or MOE reference electrode was calibrated relative to the reversible hydrogen electrode (RHE). The RHE calibration was carried out in the highly pure hydrogen saturated electrolyte with Pt/C as the working electrode, according to the reported method.³⁹ The potentials were referenced to the RHE by adding a value of 0.698 V for the 0.5 M H₂SO₄ solution and 0.925 V for the 1 M KOH solution.

The electrocatalytic reduction of hydrogen on W/WO₂ CSNA@CP was evaluated in 0.5 M H₂SO₄ and 1 M KOH by linear sweep voltammetry (LSV) at a scan rate of 5 mV s⁻¹ in a rigorously stirred solution. The current densities were calculated with respect to the geometrical area of the electrodes. The measured LSV curves were corrected with the ohmic losses. The apparent Tafel slope was obtained from the iR-corrected LSV curves by using the equation η = b log(j) + a, where η is the iR-corrected potential, a is the Tafel constant, b is the Tafel slope and j is the current density. Electrochemical impedance spectroscopy (EIS) measurements were recorded in the frequency range of 10⁻² to 10⁶ Hz with 300 mV vs. RHE sinusoidal perturbations and 12 steps per decade in 0.5 M H₂SO₄ solution.

To determine the faradaic efficiency of the W/WO₂ NA@CP, the H₂ evolution was obtained by comparing the volume of generated gas and the quantity of charges passing the W/WO₂ NA@CP while a potentiostatic electrolysis measurement was carried out. The water displacement method was utilized to monitor the volume of H₂ during the potentiostatic electrolysis experiment, and the details were presented in our previous paper.⁴⁰

Results and discussion

The products were grown by hydrothermal reaction following a thermal reduction. The details of the synthesis can be found in the Experimental section.

The overall structural features of the samples prior to and after thermal reduction were revealed by X-ray diffraction (XRD) experiments. From the sample synthesized from the hydrothermal reaction, the peaks corresponding to monoclinic-phase WO₃ (JCPDS no. 71-2141) were distinct (Fig. S1, ESI†), allowing us to assign the sample to WO₃ NA@CP. After thermal reduction at 700 °C, a dramatic change in the apparent structure of the sample could be found. The sample turns from white-grey to black-blue after the thermal reduction (Fig. S2, ESI†). The XRD pattern of the reduced precursor is shown in Fig. 1. The pattern shows well-defined peaks beside those from CP, suggesting the good crystallinity of the products. The peaks correspond well with the monoclinic-phase WO₂ (JCPDS no. 32-1393) and cubic-phase W (JCPDS no. 1-1203), and no impurity phase could be found.

Fig. 1 XRD patterns for CP and W/WO₂ NA@CP.

SEM was utilized to investigate the morphology of samples. SEM images of W/WO₂ NA@CP, which was synthesized by the thermal reduction of WO₃ NA@CP, are shown in the panels (a) to (c) of Fig. 2. The low-magnification SEM image in Fig. 2a shows that the W/WO₂ NA grew homogeneously along the radical direction of the CP, and the CP was fully covered by the W/WO₂ NA. Fig. S3a (ESI†) shows a typical SEM image of the precursor obtained from hydrothermal growth. A closer examination reveals that the WO₃ NA has a typical tapered morphology (Fig. S3b, ESI†). Large-magnification SEM images (Fig. 2b and c) show that the surface of W/WO₂ NA is rougher than that of WO₃ NA (Fig. S3b, ESI†), and from the pictures, we can also discover that the length of WO₂ nanowires is ca. 1 μm, and the average diameter is 110 nm. The variation of the surface roughness is associated with the density difference between WO₃ and WO₂ and W. An energy dispersive spectrometer (EDS) was employed to evaluate the elemental information of a product. A typical EDS spectrum (Fig. 2d) shows that C, W and O can be found from the W/WO₂ NA@CP, in accordance with the composition suggested by the XRD pattern.

Fig. 2 (a)–(c) are the SEM images of W/WO₂ NA@CP. (d) EDX spectrum for W/WO₂ NA@CP. (e) TEM image of a W/WO₂ nanowire. (f) HRTEM image of a W/WO₂ nanowire.

A TEM image of W/WO₂ nanowires scratched from CP is shown in Fig. 2e. Additionally, a low-magnification TEM image of W/WO₂ nanowires array is shown in Fig. S4a (ESI),† in which a lot of nanowires can be found. The morphology of these nanorods is shown more clearly in Fig. S4b (ESI),† and the nanowires have a diameter of ca. 100 nm, in accordance with that suggested by SEM. A high-resolution TEM (HRTEM) image of the W/WO₂ nanowire in Fig. 2f shows well-resolved lattice fringes with an inter-plane distance of 0.34 nm, which corresponds to the (110) plane of WO₂. The dark-field scanning TEM (STEM) image and the corresponding EDS mapping of the W/WO₂ nanowire are shown in Fig. 3, and, furthermore, the image is recorded from a region enclosed by a rectangle in Fig. S4a (ESI).† The elemental-mapping images of O (Fig. 3b) and W (Fig. 3c) further confirm that the W and O are homogeneously distributed in the W/WO₂ nanowires.

Fig. 3 (a) Dark-field STEM image and corresponding elemental-mapping images of (b) O and (c) W.

The HER performance of the W/WO₂ NA@CP was evaluated by electrochemical polarization measurements. The measurements were carried out in a 0.5 M H₂SO₄ solution with a three-electrode configuration (see details in the Experimental section). Fig. 4a shows the polarization curves of the W/WO₂ NA@CP (loading amount: 2.2 mg cm⁻²), WO₃ NA@CP (loading amount: 2.6 mg cm⁻²), Pt/C (loading amount: 0.285 mg cm⁻²) and bare CP. It can be seen that the bare CP exhibits a negligible current flow in the potential range of −500 to 0 mV vs. the reversible hydrogen electrode (RHE), suggesting that the current flow of the W/WO₂ NA@CP in this potential range is associated with the W/WO₂ NA. The onset potential, which is defined as the potential corresponding to a current density of 1 mA cm⁻², was found to be ca. 120 mV vs. RHE for the W/WO₂ NA@CP. The η₂₀ was 340 mV for the W/WO₂ NA@CP, while the overpotential required for a current density of 10 mA cm⁻² (η₁₀) was 297 mV. For comparison, the HER performance of WO₃ NA@CP was also evaluated. It was shown that the HER performance of W/WO₂ NA@CP was much better than that of the WO₃ NA@CP. In addition, WO₃ NA@CP was subjected to different conditions of thermal reduction in order to exploit products with optimal activity in the HER. The reduction temperature was set to 600 °C, 650 °C, 700 °C and 750 °C, respectively, while the reduction time was varied from 20, 30 and 40 min. Different samples are herein denoted as R-AAA-BB, where AAA is the annealing temperature (i.e. 650, 700 and 750 °C) and BB is the annealing time (i.e. 20 and 40 min). Fig. S5 (ESI†) shows the polarization curves corresponding to different reduction recipes. Fig. S5a (ESI†) shows that the catalytic activity of W/WO₂ NA@CP first increases with the reduction temperature, while the performance declines when reduction temperature is as high as 750 °C. On the other hand, a reduction time of 30 min was found to produce a sample with the best HER performance (Fig. S5b, ESI†). When the sample was reduced at 700 °C for 20 min, we got a composite of W₁₉O₅₅, W₃O, WO₂, and in the XRD pattern (Fig. S6a, ESI†), the peak from WO₂ is much stronger than that from W₁₉O₅₅ and W₃O. Meanwhile, the reduction at 700 °C for 40 min results in a composite of W and WO₂, whereas the content of WO₂ is much smaller than that of a sample subjected to reduction at 700 °C for 30 min (Fig. S6b, ESI†). At lower temperatures (650 °C), the reduction of WO₃@CP results in a composite of W, W₃O, WO₂ and WO_2.9 in R-650-30, as suggested by the XRD pattern in Fig. S6c (ESI).† At higher temperatures (750 °C), WO₃ was totally reduced to W (R-75-30, Fig. S6d, ESI†). A comparison of the samples annealed at 700 °C for different times revealed the effect of WO₂ and W in the HER, whereby the content of WO₂ in this series of samples decreases with increasing the annealing time. It is shown in Fig. S5b (ESI†) that the performance of R-700-30 is better than that of R-700-20, implying that the performance of the sample will further decrease if the content of WO₂ further increases (i.e. towards a pure WO₂ NA@CP sample). On the other hand, the performance of R-700-30 is also better than that of R-700-40 and R-750-30, suggesting that the increase of the W content in the composite results in a decrease in the performance. The comparison demonstrates that the performance of W/WO₂ NA@CP would be superior to that of the pure WO₂ NA@CP and pure W NA@CP samples, while an optimal ratio of WO₂ and W will result in the best performance. It can be therefore concluded that the performance of W/WO₂ NA@CP correlates with the synergetic effect between W and WO₂. From this point of view, a pure W sample (R-750-30) and a sample with a content of WO₂ larger than that of R-700-30 (R-650-30) will also result in a decrease in the performance.

Fig. 4 Polarization curves of the different samples (a) in acidic solution (scan rate: 5 mV s⁻¹). Tafel plots corresponding to (b) the acidic solution. All the potentials were corrected with iR drop.

The parameters η₁₀ and η₂₀ are usually adopted as the key parameters for comparison of the catalytic activity of different HER catalysts. There are a few reports in the literature concerning the HER activity of tungsten oxides, and the HER performance of representative tungsten oxides are listed in Table S1 (ESI).† It is shown that the η₁₀ and η₂₀ values of W/WO₂ NA@CP are smaller than those of the reported tungsten oxides. In addition, although some reported tungsten oxide catalysts' performances are better than W/WO₂ NA@CP, the background current is bigger than that of W/WO₂ NA@CP.^41,42 It can thus be concluded that W/WO₂ NA@CP is one of the best tungsten oxide HER catalysts in acid solutions.

In addition, to probe into the HER performance in basic solutions, polarization curves of W/WO₂ NA@CP in a basic solution (KOH, 1 M) were measured (Fig. S7, ESI†). It is shown that the performance of W/WO₂ NA@CP seriously degrades with the measurement cycle. The degradation of the current density is attributed to the slow dissolution of WO₂ in basic solution. The Tafel slopes of W/WO₂ NA@CP were obtained by data fitting of the polarization curves. The Tafel plots are presented in Fig. 4b. In acidic solution, the Tafel slope is 74.5 mV dec⁻¹ for the W/WO₂ NA@CP, while that for WO₃ NA@CP is 151.2 mV dec⁻¹. According to the Tafel slope,⁹ the rate-determining step in the HER process might be a Volmer, Heyrovsky or Tafel reaction, with the corresponding characteristic Tafel slopes of 116, 38, or 29 mV dec⁻¹, respectively. In acidic solution, W/WO₂ NA@CP has a small Tafel slope of 74.5 mV dec⁻¹. This value lies between 38 and 116 mV dec⁻¹, suggesting that the HER process might proceed along a Volmer–Heyrovsky mechanism.⁴³

The stability of the W/WO₂ NA@CP was evaluated by assessing the time-dependent current under a static applied potential of 340 mV vs. RHE for 54 h in 0.5 M H₂SO₄ solution via a potentiostatic electrolysis experiment, as shown in Fig. 5a. It is shown that W/WO₂ NA@CP is a highly stable electrocatalyst for the HER in a potentiostatic electrolysis (54 h) in acidic solution. Two factors might induce the fluctuation of current density shown in Fig. 5a. The first one is the temperature variation during this time interval. For example, the temperature in the day is higher than that at night. Another factor might be the absorption and desorption of hydrogen gas bubbles on the surface of the electrode. The absorption of gas bubbles induces a dead area on the surface of a sample, which then cannot be accessed by the electrolyte. The presence of a dead area results in a decrease in the current density, while desorption of the gas bubbles induces the recovery of the current density. On the other hand, the polarization curve of W/WO₂ NA@CP was measured before and after the potentiostatic electrolysis experiment, and it was seen that the current density measured after the potentiostatic electrolysis shows a small difference from that measured prior to the potentiostatic electrolysis (Fig. 5b). The XRD pattern of W/WO₂ NA@CP subjected to long-term potentiostatic electrolysis is analogous to that of the unmeasured sample (Fig. S8a†), confirming the stability of W/WO₂ NA@CP in water electrolysis.

Fig. 5 (a) Time-dependent current density curve for W/WO₂ NA@CP under a static overpotential of 380 mV for 20 h. (b) Polarization curves of W/WO₂ NA@CP before and after the current stability test scans.

As shown in Fig. 6, the faradaic efficiency of the W/WO₂ NA@CP during H₂ evolution was detected by comparing the volume of generated gas and the quantity of charges passing the W/WO₂ NA@CP while a potentiostatic electrolysis measurement was being implemented. According to charges pass through the W/WO₂ NA@CP (∼0.35 cm²), in 686 s 1.25 mL of H₂ should be obtained. In the experiment, the volume of generated H₂ was monitored to be 1.23 mL, indicating that the faradaic yield of H₂ production was nearly 100% over the time-scale of the measurement (Fig. 6). To obtain an insight into the HER process and mechanism with W/WO₂ NA@CP and WO₃ NA@CP, electrochemical impedance spectroscopy (EIS) investigations were implemented. The EIS experiments were carried out at 300 mV vs. RHE, with the results shown in Nyquist plots in Fig. 7. The EIS spectra exhibit two semicircles. The semicircles at low frequencies correspond to the kinetics of the HER process on the surface of the catalyst. Here, the kinetics of the electrochemical reaction at an electrode's surface is usually assessed by the charge transfer resistance (R_ct), with a smaller R_ct value corresponding to a faster kinetics. R_ct can be deduced from the EIS spectra by data fitting, as in the present case using the equivalent circuit shown in Fig. S9.†

Fig. 6 Current efficiency for HER under potentiostatic electrolysis (applied potential: 508 mV vs. RHE).

Fig. 7 Nyquist plots of EIS spectra measured of the W/WO₂ NA@CP and precursor in acidic solution at V = 300 mV vs. RHE.

W/WO₂ NA@CP also has a smaller diameter semicircle in the low frequency range of the EIS spectrum, in comparison with WO₃ NA@CP. R_ct resulting from the data fitting exercise (see the details in Table S2, ESI†) is 149.4 Ω cm² and 14.69 Ω cm² for WO₃ NA@CP and W/WO₂ NA@CP, respectively. The R_ct of W/WO₂ NA@CP is smaller than that of WO₃ NA@CP, demonstrating the faster reaction rate of HER on the surface of the W/WO₂ NA@CP in comparison with on WO₃ NA@CP. The faster reaction rate on the surface of the W/WO₂ NA@CP affords its superior HER performance.

Conclusions

In summary, W/WO₂ NA@CP was herein synthesized, and exhibited efficient catalytic performance in acidic solution for hydrogen generation. The η₂₀ value is as small as 340 mV in acidic solution. A comparison of the theoretical and detected volume of hydrogen generated during potentiostatic electrolysis confirmed that the faradaic efficiency was nearly 100%. The results from potentiostatic electrolysis experiments suggested that W/WO₂ NA@CP can work stably in long-term hydrogen generation in acidic solution. The EIS measurements suggested that the charge transfer resistance of W/WO₂ NA@CP is relatively smaller. The method introduced herein offers a convenient approach for the design and exploitation of hierarchical materials exhibiting efficient catalytic activity for hydrogen generation from electrolysis.

Acknowledgements

This research was financially supported by the National Natural Science Foundation of China (61006049, 50925207, 51172100), the Ministry of Science and Technology of China (2011DFG52970), the Ministry of Education of China (IRT14R23), 111 Project (B13025), the Jiangsu Innovation Research Team, Jiangsu Province (2011-XCL-019 and 2013-479), and the Natural Science Foundation of Jiangsu (BK20131252).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra17194j

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