Simple and Facile Fabrication of Anion-Vacancy-Induced MoO_3−X Catalysts for Enhanced Hydrogen Evolution Activity

Abstract

Advanced catalysts for clean hydrogen generation and storage offer an attractive possibility for developing a sustainable and ecofriendly future energy system. Transition metal oxides (TMO) are appealing candidates to be largely considered as electrode catalysts. However, for practical applications, there are still challenges—the intrinsic catalytic properties of TMOs should be further improved and TMOs should be synthesized by practical routes for cost-effective and scalable production of catalysts. Therefore, finding promising ways to fabricate highly active TMOs with outstanding electrochemical hydrogen evolution performance is required. Here, we present a direct and facile synthetic approach to successfully provide highly efficient MoO_3−X catalysts with electrochemically active oxygen vacancies through a one-step thermal activation process on a Mo metal mesh. Variations in the oxidation states of molybdenum oxides can significantly increase the active sites of the catalysts and improve the electrochemical activity, making these oxide compounds suitable for hydrogen evolution reaction (HER). Compared to the bare Mo mesh and fully oxidized Mo (MoO₃) electrodes, the fabricated MoO_3−X electrode exhibits better electrochemical performance in terms of overpotentials and Tafel slope, as well as the electrochemical 1000 cycling stability, confirming the improved HER performance of MoO_3−X. This provides new insight into the simple procedure suitable for the large-production supply.

1. Introduction

The development of new materials for efficient energy generation is of paramount importance in order to meet ever-increasing energy demands and to provide environmentally friendly energy sources. It is expected that hydrogen gas and corresponding energy applications from water-splitting reactions should be capable of providing both high energy capacity and renewable (ecofriendly) resources [1,2,3]. Further research on catalytic electrode materials for an efficient hydrogen evolution reaction (HER) are needed to maximize its catalytic performance. Currently, earth abundant transition metal-based oxides (TMOs) have emerged as promising alternative electrocatalysts for noble metal replacement based on theories and experiments on their metal active sites, electrical structures, and surface properties. Those electrode materials have demonstrated their catalytic efficiency and high catalyst stability during HER regardless of electrolyte pH [4,5,6]. There are still a lot of challenges involved in using these materials as industrial electrode candidates due to their relatively low catalyst performance compared to noble metals, e.g., platinum, and their synthesis methods, which still lack scalability. Enhancing the manufacturing scalability of TMOs and finding their simple fabrication process into high intrinsic catalytic performance are issues that need to be addressed.

To this end, numerous studies are being solicited by introducing and engineering anion defects into the crystal structure of TMOs [7,8,9]. It has been reported that anion deficiency in TMOs can provide favorable electrochemical kinetics as a result of the reduction of transition metal orbitals, which are regarded as the electrochemical active sites to promote gas evolution reactions [10,11,12]. However, it has been noted that, in hydrogen electrode applications, TMOs with anion vacancies cannot maintain their original structure during the adsorption of electrolyte ions while generating hydrogen gas. In other words, failure to accomplish the structural stability of TMO materials will cause a drastic decrease in electrochemical cyclability. Nevertheless, it has been demonstrated that certain structured and layered materials with vacancies can successfully exploit the favorable electrochemical kinetics during the adsorption and desorption of ions [13,14,15]. For example, molybdenum trioxide (MoO₃) is one of the promising TMO candidates with layered structures. Among many oxide materials, Molybdenum oxide (MoO₃) is an environmentally benign and low-cost chemical compound, which shows HER performance. In particular, its orthorhombic phase with the layered morphology are electrochemically stable due to its unique structure [16,17]. Its catalytic activity and electrochemical properties can be systematically managed through the formation of anion vacancy with rigid structural stability. Specifically, anion deficient MoO₃ (MoO_3−X) can maintain its original layered characteristics consisting of distorted MoO₆ octahedron sites that allow electrolyte ions to favorably intercalate and react with Mo sites [18,19]. Therefore, MoO_3−X can be successfully used as the electrode material in lithium-ion batteries and supercapacitors, as well as HER systems. However, synthesizing MoO_3−X electrodes in a productive method remains a challenge due to its multistep synthetic process [20,21]. Especially, to induce oxygen vacancies, further hydrogen treatment under high vacuum is required, which cannot be the mass-productive method by using highly explosive H₂ gas. Moreover, instead of using Mo precursors, direct synthesis on molybdenum substrate can be crucial to fabricate the electrode in a productive and scalable level. This is because the Mo sources originating from Mo substrates (meshes) and oxygen atoms can be reacted and converted into MoO_3−X electrode when the oxygen sources are limited compared to the Mo sources.

Herein, we synthesize the molybdenum oxide electrode materials with a controlled level of oxygen vacancies through a simple and facile thermal activation process for HER electrocatalysts, exhibiting the enhanced electrochemical performance in an alkaline media. Synthesizing catalytic materials directly on its metal substrate, such as the Mo mesh, can be beneficial for efficiently utilizing the corresponding transition metal compounds instead of using active material ink combined with carbon compounds. It has been previously reported that MoO₃ can be obtained through the oxidation of a transition metal during annealing at a high temperature in an air atmosphere without additional precursors. Following the annealing process, placing the as-synthesized MoO₃ under a reducing atmosphere condition, such as H₂ gas, induced MoO_3−X [22,23,24]. Here, we successfully and simply produce MoO_3−X electrodes with the one-step oxidation process under argon on the Mo mesh. The remarkable material and morphological properties of MoO_3−X ensure that the catalytic electrode exhibits a relatively low overpotential with a Tafel slope of 69 mV dec⁻¹ compared to that of MoO₃ with a Tafel slope of 91 mV dec⁻¹. Along with its enhanced HER performance, the MoO_3−X electrodes show negligible performance degradation even with 1000 cycles of the linear sweep voltammetry (LSV) measurements. This confirms the electrochemical properties of MoO_3−X synthesized by our simple and facile one-step oxidation process are comparable to the reported MoO₃ electrodes.

2. Results

Promising electrocatalyst candidates with thermodynamically certain energy in water electrolysis should be recognized as ideal candidates with the favorable absorption of hydrogen ions and the desorption of H₂ gas in terms of free energy of hydrogen absorption ($∆G_H^{°}\approx$ 0) [25]. In this regard, among noble metals, Pt is used as a conventional reference catalyst for HER due to its calculated $∆G_H^{°}$ close to zero, and it shows high electrical conductivity and outstanding electrochemical performance. Pt/C is an electrocatalyst composed of platinum supported on carbon materials, which can synergistically employ its large surface area and a large number of corresponding active sites. Therefore, the electrochemical activity, electrode conductivity, and large surface area of target materials are critical as an important indicator to access $∆G_H^{°}$ and electrochemical performance in HER [6].

The MoO₃ electrodes with anion vacancy and nanostructured morphology were simply synthesized by the thermal activation process. Firstly, oxygen vacancies in MoO₃ can improve the electrical conductivity of the electrode, which is reported in many other articles [26]. Second, the nanostructured morphology of the electrode can provide the large surface area. Figure 1 represents MoO_3−X synthesized by controlling the temperature of the activation process without the addition of a conventional reducing gas source, such as hydrogen. A cleaned Mo mesh was introduced at the middle of the thermal annealing quartz tube for thermal treatment under an ambient flow rate of Ar at different temperatures for MoO_3-x and MoO₃. The MoO_3−X electrode was fabricated at low temperature condition (~450 °C), and the MoO₃ electrode was synthesized at high temperature condition (~600 °C). Moreover, the enhanced HER performance is reported due to the increased surface area of MoO_3−X since its nanostructure can enhance the activity of TMOs in catalytic reactivity. Combining all catalytic features, the as-prepared MoO_3−X electrodes can utilize the aforementioned advantages to improve the catalytic activity.

To assess the morphological properties of MoO₃ and MoO_3−X, various scanning electron microscopy (SEM) analysis techniques were carried out. Figure 2b,c,e,f shows the SEM images of MoO_3−X and MoO₃, respectively. As can be seen from low-magnification SEM images, the molybdenum trioxide materials were grown and uniformly distributed on the Mo mesh for both the samples. With the higher magnification, the MoO_3−X electrode shows the small nanosized grains with a diameter size of 200–400 nm. In contrast, the larger microsized grains were found in the MoO₃ electrode. Based on the SEM results, it can be confirmed that MoO_3−X, with a smaller particle size than MoO₃, will provide the large specific surface area and a large number of the corresponding electrochemically active sites. Finally, Figure 2a,d represents the schematic illustration of both MoO_3−X and MoO₃ electrodes, respectively. We have used the lack of oxygen conditions to induce the oxygen vacancy in the MoO_3−X electrodes. After the low-temperature heat treatment, even though the temperature is high enough to overcome the activation energy of oxidization, there are not enough oxygen sources to react with the Mo substrate. Therefore, oxygen vacancies were generated. During the oxidation process, the adsorption of oxygen on the surface of metal is usually not a rate-limiting step. However, due to the lack of oxygen and the abundance of metal sources, it becomes a rate-limiting step, and the oxygen vacancies of MoO₃ occur [27].

In Figure 3a, the color change of MoO₃ proves the presence of oxygen vacancies. The MoO_3−X electrode has a blue color on the surface of the Mo mesh, while the as-synthesized MoO₃ electrode possesses a bright gray and black color after the excess thermal activation process. Similar to the cases with other oxygen-deficient materials such as WO_3−x, it has been noted that the blue color of the TMO is induced from characteristic outer d-shell electron density (the delocalization of the electron density) [28,29,30]. In the MoO_3−X articles, the presence of oxygen vacancy has been signified through the color change [22]. The blue color of the as-prepared MoO_3−X electrode might be the indirect evidence to prove the presence of oxygen vacancy. To further understand the crystal phases and oxidation states, the MoO₃ and MoO_3−X electrodes were examined by Raman and X-ray photoelectron spectroscopy (XPS) as shown in Figure 3b–d. From the Raman spectroscopy in Figure 3b, three sharp peaks at 280 cm⁻¹, 663 cm⁻¹, and 818 cm⁻¹ are identified and well assigned to the orthorhombic MoO₃. Moreover, two relatively distinct peaks at 490 cm⁻¹ and 890 cm⁻¹ correspond to the stretching mode of MoO_3−X [10,31]. The surface electronic and oxidation states of the as-prepared samples were characterized by XPS. Figure 3c,d compares the high resolution XPS spectra of the Mo 3d of MoO_3−X and MoO₃, respectively. The Mo⁶⁺ 3d doublet is clearly observed at about 235.9 eV (Mo⁶⁺ 3d_3/2) and at 232.7 eV (Mo⁶⁺ 3d_5/2) with a d spin-orbit splitting of 3.13 eV. Moreover, there are two additional doublets at 234.7 eV and 231.5 eV, indicating the oxidation states of Mo⁵⁺, which cannot be found in the XPS spectra of MoO₃, as shown in Figure 3d [14,15,16]. The various material characterization results based on the SEM, XPS, and Raman spectroscopy demonstrate that the existence of anion deficiency and nanostructured surface morphologies on the electrodes will be beneficial for the overall electrochemical activity. The oxygen vacancy and reduced oxidation states may improve the electrical conductivity of the electrode because of the increased carrier concentration. Moreover, the incorporation of oxygen vacancies can provide more surface active sites and enhance mass transport kinetics [14,21].

The electrocatalytic activity of the as-synthesized MoO_3−X and MoO₃ electrodes was examined in an alkaline aqueous solution (0.1 M KOH). In all experiments, the three-electrode system with Pt and Ag/AgCl was used as the counter electrode and reference electrode, respectively, while the Mo mesh with MoO₃ and MoO_3−X was used as the working electrode. In addition, the 20 wt% Pt/C was used as the reference electrocatalyst to objectively figure out the performance of the as-synthesized electrodes. Linear sweep voltammetry (LSV) was performed to measure the electrochemical performance of the electrocatalysts (Figure 4b–d), and to compare the effects of the anion deficiency of the as-synthesized MoO_3−X electrodes. Schematically, HER begins through the transfer of electrolyte ions, which are being drawn by the applied potentials on working electrodes (Figure 4a). The polarization curves in Figure 4b show the generated electrons as the estimated current density (mA cm⁻²), that is obtained by the adsorbed protons. The electrode reaches at a current density of 10 mA cm⁻² at a potential of 187 mV for the MoO_3-x electrode, while the overpotential of MoO₃ at a current density of 10 mA cm⁻² is at 261 mV. This overpotential value of MoO_3−X is closer to that of the Pt/C catalyst system (56 mV at a current density of 10 mA cm⁻²) compared to MoO₃, indicating the enhanced catalytic activity of MoO_3−X. The LSV curves also demonstrate that the significant effects of the existence of anion deficiency also result in the overpotential reduction at a current density of 50 mA cm⁻². The comparative Tafel plots and calculated Tafel slopes are derived from the polarization curves. The oxidation state of Mo is strongly related to how well Mo can carry out a series of the corresponding hydrogen reduction and adsorption processes (HER). The performance of the MoO_3−X electrodes is improved, but still follows the Heyrovsky mechanism [32,33]. In Figure 4c, the commercial Pt/C sample shows a Tafel slope of 57 mV dec⁻¹. The MoO₃ and MoO_3−X electrodes show Tafel slopes of 91 mV dec⁻¹ and 69 mV dec⁻¹, respectively, indicating enhancement of HER activity using the MoO_3−X electrode. Specifically, the effects of anion deficiency have shown the decrease of the Tafel slope by 22 mV dec⁻¹. In addition to the activity and Tafel slope of the samples, cycling stability is another critical concern for TMO electrocatalysts with anion vacancies. The cycled LSV was conducted for the stability test of MoO_3−X and an HER performance test under the same conditions. The cycled LSV was performed from +0.2 V versus reversible hydrogen electrode (RHE) to −0.4 V versus RHE for 1000 cycles with a slight initial increase, but negligible performance deterioration (15 mV and 12 mV increase at current density of 10 mA cm⁻² and 50 mA cm⁻², respectively) as shown in Figure 4d, confirming the high electrochemical and structural stability of MoO_3−X in HER.

3. Materials and Methods

3.1. Preparations of Materials

The structured MoO_3−X electrodes were synthesized by the simple thermal oxidation process on the Mo mesh. First, the Mo mesh (Fisher Scientific, Hampton, NH, USA) was ultrasonically cleaned in ethanol, iso-propanol, and deionized water, and blown dry with nitrogen. The Mo mesh was introduced at the middle of the quartz tube, which was placed in the tube furnace. The temperature of the furnace was raised to the reaction temperature of 450–600 °C at a ramp rate of 5 °C/min under an ambient flow rate of Ar at 30 standard cubic centimeters per minute (sccm), and the sample was thermally oxidized for 2 h in an argon atmosphere.

3.2. Material Characterization

The morphologies of the as-prepared electrodes were investigated by scanning electron microscopy (SEM) using Hitachi S-4300 (Hitachi, Oxford, UK). Raman spectroscopy was conducted using Jobin Yvon LabRam Aramis Raman spectroscopy (Oxford, UK) with a 532 nm laser with a power of ∼20 μW and a spot size of 1.09 μm² under ambient pressure and room temperature. X-ray photoelectron spectroscopy (XPS, Oxford, UK) was performed on a Thermo scientific K-Alpha instrument with an Al Kα X-ray source. The spectrometer was adjusted to align a binding energy of 284.5 eV for the C 1s line.

3.3. Electrochemical Measurements

HER catalytic activity tests were carried out in a standard three-electrode setup controlled by a Metrohm Autolab (Metrohm, Oxford, UK, 2019). A working electrode (Mo oxides with an Mo mesh), an Ag/AgCl (in 1 M KCl, aq) reference electrode, a graphite rod counter electrode, and a 0.1 M KOH solution as the electrolyte were used. The reference electrode was calibrated against and converted to a reversible hydrogen electrode (RHE).

4. Conclusions

The direct and facile synthesis method for the fabrication of the MoO_3−X electrode has been validated in this study. The MoO_3−X electrode was obtained directly through a thermal activation process on the Mo mesh, which is used as the working electrode with high electrical conductivity and large surface area, as well as the Mo precursor-free synthesis. By applying the activation procedures without the additional Mo metal precursors, the MoO₃ and nonstoichiometric MoO_3−X electrodes with oxygen vacancies were fabricated on the surface of the Mo mesh. The MoO_3−X electrodes showed superior performance in HER. The MoO_3−X electrodes require a potential of 187 mV versus RHE to obtain a current density of 10 mA cm⁻², which is 74 mV less than the MoO₃ electrode and also represents the negligible electrocatalytic degradation during 1000 cycles of LSV. The verification of the effective one-step synthetic method by simply controlling the oxygen levels in TMOs could contribute to their practical applications in the hydrogen catalytic field.