In situ hydrogenation of molybdenum oxide nanowires for enhanced supercapacitors

Abstract

In situ hydrogenation of orthorhombic molybdenum trioxide (α-MoO₃) nanowires has been achieved on a large scale by introducing alcohol during the hydrothermal synthesis for electrochemical energy storage supercapacitor devices. The hydrogenated molybdenum trioxide (H_xMoO₃) nanowires yield a specific capacitance of 168 F g⁻¹ at 0.5 A g⁻¹ and maintain 108 F g⁻¹ at 10 A g⁻¹, which is 36-fold higher than the capacitance obtained from pristine MoO₃ nanowires at the same conditions. The electrochemical devices made with H_xMoO₃ nanowires exhibit excellent cycling stability by retaining 97% of their capacitance after 3000 cycles due to an enhanced electronic conductivity and increased density of hydroxyl groups on the surface of the MoO₃ nanowires.

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1. Introduction

Electrochemical energy storage devices, in particular supercapacitors based on layered transition metal oxides nanostructures, have attracted considerable interest due to their higher specific capacitance than conventional carbon based materials and better electrochemical stability than conducting polymers.^1–4 Among various layered metal oxide nanostructures, molybdenum trioxide (MoO₃) with a two-dimensional layered structure of linked MoO₆ octahedra has gained considerable attention in supercapacitor applications as it can accommodate up to 1.5 Li/M.¹ Furthermore, it offers a broad range of oxidation states which offer a broad range of redox reactions and van der Waals gaps between octahedron Mo–O sheets, used for the intercalation of foreign atoms which results in an enhanced pseudo capacitance.^5–7 Despite the theoretical expectation that electrodes made from MoO₃ exhibit low specific capacitance, poor cycling stability and rate capability due to their inherent poor electronic and ionic conductivity their performance in practical applications is limited.⁸ Many researchers have tried to improve the electronic conductivity of MoO₃ based electrodes by exploring hybrid composite structures of MoO₃ with polymers, MWCNTs or with graphite and its derivatives.^5,6,9–11 Although MoO₃ nanocomposites with MWCNTs exhibit a pronounced increase in specific capacitances, the incorporation of MWCNTs does not change the intrinsic properties of MoO₃.^6,9 Furthermore, the introduction of MWCNTs increases the fabrication cost and hence restricts the composite's potential development. Therefore, it is highly desirable to fundamentally improve the electrical conductivity, pseudocapacitance, and rate capability with a high specific capacitance while maintaining a low fabrication cost of MoO₃. In order to address these issues, we have hypothesized that in situ hydrogenation of MoO₃ nanowires during synthesis could address these limitations which consequently improve the specific capacitance and stability of MoO₃ nanowires. We believed that the in situ hydrogenation of MoO₃ results in the enhancement of the donor densities through the controlled introduction of oxygen vacancies which significantly lower the work function and increase the conductivity of MoO₃ nanowires. Therefore it is expected that hydrogenation of MoO₃ nanowires may be one of the best potential ways to solve the reaction kinetics and to increase the capacitance by increasing the ionic and electronic conductivity of MoO₃ nanowires which leads to high capacity supercapacitors and is very promising for the next generation of high-performance electrochemical electrodes.

2. Experimental methods

Molybdenum dioxide (MoO₂) (99%), H₂O₂, HCl (38%), and ethanol were purchased from Sigma-Aldrich and used as received without further purification. MoO₃ nanowires were synthesized by dissolving 10 mmol of MoO₂ in 20 mL of H₂O₂ and stirred with a magnetic stirrer for 30 min and the pH of the solution was adjusted to 1–2 by the addition of dilute HCl with constant stirring.⁹ The resulting yellowish solution was transferred to a Teflon-lined stainless autoclave (25 mL capacity) at 180 °C for 10 h and then cooled to ambient temperature. The resultant white precipitates were filtered, washed several times with distilled water and ethanol, and dried on a hot plate at 120 °C for 12 h to completely eliminate the water and ethanol. For the synthesis of H_xMoO₃ (0 < x < 0.35) during the hydrothermal growth, a controlled amount of ethanol was introduced into the above yellowish solution and the solution was transferred to the Teflon-lined stainless autoclave (25 mL capacity) at 180 °C for 10 h and then cooled to ambient temperature. The color of the resulting H_xMoO₃ nanowires changed from white to blue and dark blue because of increasing hydrogen insertion which is attributed to the in situ hydrogenation of MoO₃ nanowires.

3. Material and electrode characterization

The morphological and structural properties were examined by field emission scanning electron microscopy (FE-SEM, JEOL JSM-7401F) and X-ray diffraction (XRD, D8 FOCUS 2200V, Bruker AXS, Cu Kα radiation (λ) 1.5418 Å). X-ray photoelectron spectroscopy (XPS; Perkin-Elmer PHI 660) was used to examine the effect of hydrogenation on the chemical composition and oxidation state of the MoO₃ nanowires. All electrochemical measurements were carried out in a three-electrode assembly consisting of a Ag/AgCl, Pt foil and glassy carbon electrode (GCE) as a reference, counter and working electrode, respectively, using a Biologic VMP3 potentiostat/galvanostat. For the working electrode (glassy carbon), MoO₃ and H_xMoO₃ nanowires were prepared by dispersing 10 mg of nanowires in 500 μL of ethanol by ultrasonication for 20 minutes. From this, 50 μL dispersion was placed on a glassy carbon electrode. The solvent was slowly evaporated by placing the electrode in an oven at 80 °C. A Nafion solution (5 μL) was coated on the electrode as a binder and allowed to dry at room temperature. Cyclic voltammetry was performed between 0 and 1 V vs. Ag/AgCl for various scan rates ranging from 5 to 100 mV s⁻¹. Galvanostatic charge–discharge studies were performed by a Won A Tech Potentiostat/galvanostatic instrument (WPG, 100 South Korea).

4. Results and discussion

The morphology of the MoO₃ and H_xMoO₃ (0 < x < 0.35) nanowires was examined with field emission scanning electron microscopy (FE-SEM) and the results are presented in Fig 1. The FE-SEM images in Fig. 1(a and b) show that MoO₃ and H_xMoO₃ nanowires have no difference in morphology, however, the diameter of the H_xMoO₃ nanowires was found to be lower than that of the pure MoO₃ nanowires (∼100 nm for H_xMoO₃ whereas for pure MoO₃ it is ∼150 nm) which clearly indicates the increased surface area of H_xMoO₃ nanowires providing more active sites for electrochemical reactions. Moreover, both MoO₃ and H_xMoO₃ nanowires have an average length of 5 μm.

Fig. 1 (a) FE-SEM image of the MoO₃ nanowires synthesized at 180 °C for 10 hours, (b) FE-SEM image of the H_xMoO₃ nanowires synthesized at 180 °C for 10 hours.

The crystallographic information of MoO₃ and H_xMoO₃ was examined by X-ray diffraction and the XRD pattern is shown in Fig. 2a. The XRD patterns of the MoO₃ and H_xMoO₃ nanowires are in good agreement with the standard peaks for the orthorhombic phase of MoO₃ (JCPDS card 89-7112). The XRD results confirm that the in situ doping of hydrogen in MoO₃ nanowires does not destroy the pristine crystal structure. However, in contrast to pure MoO₃, the in situ hydrogen doping produced a slight shift (as shown in the inset of the main peak in Fig 2a) of the peaks towards a lower diffraction angle due to the expansion of the lattice unit cell of MoO₃ which is in good agreement with previously reported results.^12,13 The effect of in situ hydrogenation on the chemical composition and oxidation state of MoO₃ nanowires was further analyzed using X-ray photoelectron spectroscopy (XPS). The high-resolution Mo 3d XPS core level XPS spectra of pristine and hydrogenated MoO₃ nanowires are shown in Fig. 2b. The two broad peaks centered at ≈233 and ≈236 eV correspond well with the characteristic Mo 3d_5/2 and Mo 3d_3/2 peaks of Mo⁶⁺ in both MoO₃ and H_xMoO₃ (0 < x < 0.35) nanowires as shown in Fig. 2(c and d).^14–16 Compared with the XPS spectrum of Mo 3d of MoO₃ nanowires, a small negative shift (∼0.2 eV) can be observed in the XPS spectrum of hydrogenated MoO₃ nanowires as shown in Fig. 2d, demonstrating that the treatment with hydrogen caused a change of surface bonding of MoO₃ nanowires. This decrease in binding energy of the Mo 3d electron is mainly due to the extra electron screening in the metallic cations and confirms that there are some Mo⁵⁺ ions in the H_xMoO₃ nanowires.^17,18 We believed that the additional electrons in Mo⁵⁺ relative to Mo⁶⁺ are not strictly localized and H_xMoO₃ nanowires exhibit some metallic characteristics, probably due to these additional non-localized electrons which are essential to enhance the electrochemical performance of MoO₃ nanowires without adding any conductive material like CNTs or carbon black. Furthermore, the deconvolution of O 1s peaks of MoO₃ and H_xMoO₃ shows two contributions of oxygen O²⁻ at a lower binding energy ≈531 eV (Mo–O–Mo) and surface adsorbed oxygen O⁻ at a higher bonding energy ≈532 eV (Mo–OH). The Mo–OH peak intensity of the H_xMoO₃ nanowires is substantially higher than that of the pristine MoO₃ nanowires, indicating that the MoO₃ surface is functionalized by hydroxyl groups after hydrogenation which are suitable to enhance the pseudocapacitance in MoO₃. To investigate the effect of hydrogenation on the electrical properties of MoO₃, I–V curves were measured using a four probe station which includes a Keithley 2612 Source Meter. The electrical measurements reveal that MoO₃ and H_xMoO₃ nanowires have electric resistances of 20.80 MΩ and 1.25 MΩ, respectively. This increase in conductance of H_xMoO₃ compared to MoO₃ is mainly because hydrogenation increases the free electrons within the oxide's structure as inferred from the XPS data. To evaluate the electrochemical properties of MoO₃ and H_xMoO₃ nanowires, electrochemical measurements were conducted in a three-electrode electrochemical cell with Pt wire and Ag/AgCl (satd KCl) electrodes as counter and reference electrodes in a 1 M H₂SO₄ electrolyte solution. The selection of this electrolyte is attributed to the fact that H⁺ in H₂SO₄ has a much smaller radius (0.012 Å) as compared to Na⁺ (0.95 Å) in NaOH and K⁺ (1.3 Å) in KOH which causes the faster transfer of electrolyte ions, and more and easier H⁺ insertion into the MoO₃ and H_xMoO₃ nanowires. To investigate the effect of hydrogenation on the electrical properties of MoO₃, electrochemical impedance measurements were conducted on the pristine MoO₃ and H_xMoO₃ samples. Fig. 3a shows the Nyquist plots of the MoO₃ and H_xMoO₃ nanowire electrodes. EIS indicated that the semicircle in the plot became shorter after the in situ hydrogenation of MoO₃ nanowires proving the good electrical properties of the H_xMoO₃ that resulted from the hydrogenation.^12,13,19 The electrical conductivity indicated that hydrogen played an important role in the conduction and had better electrochemical and electrical properties, which enable it to serve as an effective electrode material for energy storage devices. The EIS data were fitted by using an equivalent circuit showing the components of the whole impedance, bulk solution resistance (R_s), charge-transfer resistance (R_ct), pseudocapacitive element (C_p) and a constant-phase element (CPE) accounting for the double-layer capacitance, as shown in the inset of Fig. 3a. In the high frequency area, the intersection of the curve at the real part Z₀ indicates R_s of the electrochemical system, and the semicircle (which corresponds to C_p and R_ct) displays the charge-transfer process at the working electrode–electrolyte interface. The R_ct values were 100 and 345 Ω for MoO₃ and H_xMoO₃ nanowires, respectively. As the redox reaction of MoO₃ and H_xMoO₃ electrodes mainly occurs on the surface of the electrode, the capacity primarily depends on the interfacial charge-transfer resistance. The lower charge-transfer resistance of H_xMoO₃ nanowires as compared to MoO₃ nanowires at the working electrode–electrolyte interface improves the electron transfer and the occurrence of the redox reaction leading to enhanced electrochemical properties.

Fig. 2 (a) XRD patterns of α-MoO₃ and H_xMoO₃ nanowires synthesized at 180 °C for 10 hours (inset in (a) depicts the magnification of the (201) peak from the XRD spectrum), (b) XPS spectra of α-MoO₃ and H_xMoO₃ nanowires, (c) XPS spectra of the Mo 3d electron in MoO₃ nanowires and (d) XPS spectra of the Mo 3d electron in H_xMoO₃ nanowires.

Fig. 3 (a) Impedance analysis of the MoO₃ and H_xMoO₃ nanowires in a 1 M aqueous solution of H₂SO₄ at room temperature. The inset shows the equivalent circuit diagram from the EIS analysis, (b) cyclic voltammograms (CVs) of the MoO₃ and H_xMoO₃ nanowire electrodes at a scan rate of 20 mV s⁻¹ in a 1 M aqueous solution of H₂SO₄, (c) CV of the H_xMoO₃ nanowire electrode at various scan rates from 5 to 100 mV s⁻¹ in a 1 M aqueous solution of H₂SO₄ and (d) specific capacitance variation of the MoO₃ and H_xMoO₃ nanowires with various thicknesses ranging from 5 nm to 20 nm electrodes at different scan rates.

The electrochemical redox process in MoO₃ and H_xMoO₃ nanowires was examined through cyclic voltammetry using 1.0 M H₂SO₄ as an electrolyte, in the voltage range of 0–1.0 V for various scan rates ranging from 5 to 100 mV s⁻¹. Fig. 3b shows the cyclic voltammograms (CVs) of the MoO₃ and H_xMoO₃ nanowire electrodes at a scan rate of 20 mV s⁻¹. The H_xMoO₃ nanowire electrodes exhibited an elliptical curve and a much more enhanced capacitive current density as compared to the pristine MoO₃ nanowires, indicating a faradaic reaction at the interface of the electrodes with the electrolyte ions, which is a typical behavior of pseudocapacitors. Fig. 3b shows three couples of redox peaks at 0.23 V–0.5 V (vs. Ag/AgCl), indicating three redox transformations at different oxidation states of Mo species including the Mo^IV, Mo^V and Mo^VI species.^20–23 The rate performance of H_xMoO₃ nanowires was determined by studying the CV at different scanning rates as shown in Fig. 3c. The elliptical curve shapes of these CVs at different scan rates, increasing from 5 to 100 mV s⁻¹, remain unchanged indicating a good capacitive behavior and high-rate capability of H_xMoO₃ nanowires. The variation of the specific capacitance with increasing scan rates for the nanowires is shown in Fig. 3b and it was observed that the specific capacitance of H_xMoO₃ is significantly higher than the pristine MoO₃ nanowires. The H_xMoO₃ nanowire electrodes achieve specific capacitance of 120 F g⁻¹ at a scan rate of 100 mV s⁻¹, which is a 20-fold enhancement compared to the pristine MoO₃ nanowires (6 F g⁻¹) and also higher than the values recently reported for MoO₃ nanowires and their composites.^6,8

This enhancement in the electrochemical performance of H_xMoO₃ nanowires is mainly due to the increase in their electronic conductivity and density of the hydroxyl groups on the surface of the MoO₃ nanowires, and thereby enhances the pseudocapacitance. It is imperative to note that the decrease in the specific capacitance of H_xMoO₃ nanowires at the higher scan rate of 100 mV s⁻¹ is only 19%, indicating that hydrogenated nanowires have excellent capacitive characteristics and are a promising material for supercapacitor applications. In contrast, the pristine MoO₃ nanowires retain only 39.8% of the initial capacitance, as the rate capability of the electrode material is related to the electrode conductivity and the rate of ion diffusion in the electrode. In the current study, MoO₃ and H_xMoO₃ samples have the same nanowire morphologies, so they should have a similar ion diffusion rate. Therefore, the improved rate capacitance in the H_xMoO₃ nanowire sample (Fig. 3d) should be due to the enhanced electrical conductivity of the nanowire electrodes. The long-term electrochemical stability of H_xMoO₃ was examined by CV at a scan rate of 20 mV s⁻¹ for 3000 cycles and the charge–discharge performance at a constant current of 0.5 A g⁻¹, the corresponding results are presented in Fig. 4a and c respectively. The capacity decay was only 3%, even after 3000 cycles, indicating the excellent stability of the electrode material in energy storage applications. The electrochemical performance of MoO₃ and H_xMoO₃ nanowires was further studied by galvanostatic charge–discharge measurements. Fig. 4b shows the charge–discharge curves of MoO₃ and H_xMoO₃ nanowire electrodes collected at a current density of 0.5 A g⁻¹. Both MoO₃ and H_xMoO₃ nanowires showed a non-linear charge–discharge curve, indicating that the nanowires are pseudocapacitive with specific capacitances of 23 F g⁻¹ and 168 F g⁻¹ respectively. Additionally, a significant reduction in the IR drop can be observed in the charge–discharge curve of the H_xMoO₃ electrodes as compared to the MoO₃ nanowires which further confirms the improvement of conductivity of nanowires by the in situ hydrogenation of MoO₃. The influence of the current density on the specific capacitance of H_xMoO₃ nanowires is shown in Fig. 4d. The specific capacitance is high at low current densities, mainly due to the low ohmic drop, which enables the full access of the inner active sites or pores of the electrode. The decrease in capacitance values with an increasing current density is mainly because an effective utilization of the pseudocapacitive material for the ion insertion is limited to the outer surface of the electrodes.^24,25 However, it is interesting to note that H_xMoO₃ nanowires, at higher current density of 10 A g⁻¹, demonstrated a specific capacitance of 108 F g⁻¹, a 36-fold higher value compared to 3 F g⁻¹ for pristine MoO₃ nanowires. This enhancement in the electrochemical performance of hydrogenated nanowires was mainly due to the increase in ionic conductivity of H_xMoO₃ compared to pristine MoO₃ nanowires which allows easy diffusion of the mobile ions into the stacked layers of MoO₃ that are held together by weak van der Waals forces.

Fig. 4 (a) Cyclic voltammetry of the first and after 3000 cycles of H_xMoO₃ nanowire electrodes with a scan rate of 20 mV s⁻¹ in a 1 M aqueous solution of H₂SO₄, (b) galvanostatic charge–discharge curve of MoO₃ and H_xMoO₃ nanowires at a charge–discharge current density of 0.5 A g⁻¹, (c) cycling performance of H_xMoO₃ nanowire electrodes at a constant current of 0.5 A g⁻¹ (the inset shows the charge–discharge curves of the last few cycles at a constant current of 0.5 A g⁻¹), and (d) specific capacitance variation of H_xMoO₃ nanowires at different current densities.

5. Conclusion

In conclusion, in situ hydrogenation of MoO₃ nanowires was achieved through a simple, low cost hydrothermal method and this hydrogenation significantly improves the specific capacitance, rate capability and cycling stability of MoO₃ nanowires as electrode materials for supercapacitors. The enhancement in electrochemical performance can be attributed to the combined contribution from the increased density of the surface hydroxyl group, and the improved ionic and electronic conductivity of MoO₃ nanowires.

Acknowledgements

The authors would like to extend their sincere appreciation to the Deanship of Scientific Research at the King Saudi University for its funding of this research through the Research Group Project no RGP-VPP-312.

Notes and references

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