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Article

Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production

by
Pengzuo Chen
*,
Weixia Huang
,
Kaixun Li
,
Dongmei Feng
and
Yun Tong
*
Department of Chemistry, Key Laboratory of Surface & Interface Science of Polymer Materials of Zhejiang Province, Zhejiang Sci-Tech University, Hangzhou 310018, China
*
Authors to whom correspondence should be addressed.
Catalysts 2022, 12(8), 895; https://doi.org/10.3390/catal12080895
Submission received: 14 July 2022 / Revised: 9 August 2022 / Accepted: 12 August 2022 / Published: 15 August 2022
(This article belongs to the Special Issue Advanced Earth-Abundant Catalysts for Energy Related Electrochemistry)
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Abstract

:
The development of highly active non-precious metal electrocatalysts is crucial for advancing the practical application of hydrogen evolution reaction (HER). Doping engineering is one of the important strategies to optimize the electrocatalytic activity of electrocatalysts. Herein, we put forward a simple strategy to optimize the catalytic activity of MoO3 material by incorporating the Cu atoms into the interlayer (denoted as Cu-MoO3). The prepared Cu-MoO3 nanosheet has a larger surface area, higher conductivity, and strong electron interactions, which contributes to optimal reaction kinetics of the HER process. As a result, the Cu-MoO3 nanosheet only needs a small overpotential of 106 mV to reach the geometric current density of 10 mA cm−2. In addition, it also delivers a low Tafel slope of 83 mV dec−1, as well as high stability and Faraday efficiency. Notably, when using the Cu-MoO3 as a cathode to construct the water electrolyzer, it only needs 1.55 V to reach the 10 mA cm−2, indicating its promising application in hydrogen generation. This work provides a novel type of design strategy for a highly active electrocatalyst for an energy conversion system.

Graphical Abstract

1. Introduction

The continuous use of fossil fuels leads to serious environmental pollution and energy crises, forcing people to seek a new type of energy system. Hydrogen, as an environmentally friendly and energy-dense fuel, is considered one of the best candidates to replace fossil fuels [1,2,3]. High-purity hydrogen can be prepared by electrocatalytic water-splitting technology, which is an important way to promote the large-scale production of hydrogen [4,5,6,7]. However, the slow kinetic process leads to low catalytic efficiency of both hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), resulting in high cost [8,9,10,11]. The application of catalysts can effectively improve the HER efficiency, but currently, the best materials are still concentrated in noble metal systems [12,13,14]. The high cost and low reserve greatly limit the wide application of precious metal materials. Therefore, it is very important to develop non-precious metal electrocatalysts that can replace noble metals for hydrogen production.
In this regard, molybdenum oxide (MoO3) is widely used for electrocatalysis because of its high activity, environmental friendliness, low cost, and high durability [15,16,17]. However, the simple MoO3 has certain disadvantages as an HER electrocatalyst due to the strong Mo-H bond during the HER process, which largely limits the Hads desorption step and hinders the reaction kinetics of hydrogen production [18,19]. Thus, the development of new regulation strategies to optimize the adsorption-free energy of Mo metal sites for hydrogen reaction intermediates is the key to improving the catalytic activity of MoO3 material. Recently, a series of surface-interface chemical regulation strategies have been widely used to design novel electrode materials, including interface structure design, defect engineering, and metal cation or anion doping [20,21,22,23,24,25]. Among them, doping engineering is one of the more effective and commonly used methods. For example, the doping of electron-rich dopants into the structure of MoO3 can decrease the density of states, thereby weakening the Mo-H bonding during the HER process [26,27]. In addition, metal cation doping can introduce heterogeneous metal atoms as synergistic catalytic centers to accelerate the kinetics of HER catalytic reaction based on optimizing the electronic structure of MoO3 material [28,29]. Although doping engineering exhibits significant advantages, there are no reports on improving the catalytic activity of MoO3 by introducing atomic doping.
Herein, a controllable Cu atom-doped MoO3 nanosheet is successfully designed by hydrothermal reaction and the disproportionation of Cu+ under reflux conditions. The incorporation of Cu atoms into the MoO3 interlayer can improve the surface area and conductivity of MoO3 material, resulting in a faster electron transfer process as well as more catalytic active centers for HER. Benefiting from those advantages, the prepared Cu-MoO3 nanosheet exhibits superior catalytic performance for HER in an alkaline medium. In addition, the constructed Cu-MoO3//FeOOH-NF electrolyzer also displays a low voltage of 1.55 V at a current density of 10 mA cm−2, along with high stability. This work provides a feasible idea for the design of novel and efficient non-precious metal electrocatalysts.

2. Results and Discussion

In this work, the Cu-doped MoO3 nanosheet is synthesized by a two-step process. The MoO3 nanosheet is firstly prepared by hydrothermal reaction, followed by the disproportionation of Cu+ under reflux conditions. The powder X-ray diffraction (XRD) pattern is measured to analyze the phase of the Cu-doped MoO3 nanosheet. As shown in Figure 1a, the XRD pattern of Cu-MoO3 exhibits typical diffraction peaks, which can be ascribed to the phase of the MoO3 sample. These diffraction peaks are consistent with the pure MoO3 nanosheet, as shown in Figure S1. No diffraction peaks are observed for Cu-based nanoparticles, indicating the successfully doping of Cu atoms into the framework of the MoO3 nanosheet. This strategy is likely to be the intercalation of copper atoms into the layers of MoO3, benefiting from the layered structure of MoO3 (Figure 1b).
Transmission electron microscopy (TEM) is utilized to study the morphology of as-obtained products. As shown in Figure S2, the TEM images of pristine MoO3 display the typical nanosheet morphology with a diameter of several hundred nanometers to micrometers. After the introduction of Cu atoms, the morphology of the nanosheet maintains well (Figure 2a–c). This result indicates that Cu doping does not cause significant changes in morphology. In addition, the high-resolution TEM (HRTEM) image is collected to study the microstructure of the Cu-MoO3 nanosheet. In Figure 2d, the lattice fringe of 0.375 nm is consistent with the (110) plane of MoO3. Furthermore, the energy dispersive X-ray spectroscopy elemental mapping images confirm the composition information of the Cu-MoO3 nanosheet. As shown in Figure 3, the elements of Cu, Mo, and O are uniformly distributed throughout the whole MoO3 nanosheet, indicating the successful doping of Cu atoms into the final samples.
The compositions and valence states of each element in the Cu-MoO3 nanosheet are demonstrated by X-ray photoelectron spectroscopy (XPS). As shown in Figure 4a, the XPS survey of the Cu-MoO3 nanosheet proves the composition of Mo (23.3%), Cu (8.4%) and O (68.3%) elements, which are consistent with the above elemental mapping images. In a contrast, the XPS survey of pristine MoO3 nanosheet shows the contents of Mo and O elements without Cu signal (Figure S3). In addition, the high-resolution Mo 3d spectrum is shown in Figure 4b. The peaks located at the binding energy of 233.4 eV and 236.5 eV can be attributed to the Mo 3d 2p5/2 and Mo 2p3/2, which is the characteristic peaks of Mo6+ [26]. In addition, the two peaks located at the binding energy of 232.8 eV and 235.9 eV belong to the characteristic peaks of Mo4+ 2p5/2 and Mo4+ 2p3/2 [18]. The appearance of Mo4+ indicates the electron injection from Cu atoms into the MoO3 nanosheet, increasing the intrinsic conductivity of MoO3 material [30,31,32]. Moreover, the O 1s spectrum is shown in Figure 4c, the peak located at 531.0 eV corresponds to the metal-oxygen bond, while the peak at 532.3 eV is ascribed to the adsorbed water [15]. In Figure 4d, the Cu 2p spectrum of the Cu-MoO3 nanosheet shows the two typical peaks of 932.7 eV and 952.6 eV, which is consistent with the low valence state of Cu species [33]. The detailed oxidation state of Cu species can be analyzed by using Cu LMM. As shown in Figure S5, one typical peak located at 570.2 eV can be ascribed to the oxidation peak of Cu1+. The above results confirm the successful introduction of Cu species into the MoO3 nanosheet, which provides an ideal platform to study the catalytic performance of this kind of hybrid electrocatalyst.
The electrocatalytic performance of obtained samples and commercial Cu foam are evaluated in 1 M KOH solution. As shown in Figure 5a, the polarization curve of commercial Cu foam exhibits poor HER performance. The current density is only 2.6 mA cm−2 at the overpotential of 200 mV. The improvement of catalytic activity is observed after the deposition of the MoO3 sample. The MoO3 catalyst exhibits an overpotential of 200 mV at 10 mA cm−2, which is better than Cu foam. After the further introduction of Cu species, the HER catalytic activity realized greatly improved. The overpotential only needs 106 mV to reach the geometric current density of 10 mA cm−2, suggesting the promotion role of Cu doping for HER activity. Notably, the current density increases to 32 mA cm−2 at the overpotential of 200 mV, which is almost three and 10 times higher than that of MoO3 and Cu foam.
Moreover, the Tafel slope of those catalysts is further calculated to study the HER kinetics. As shown in Figure 5b, the obtained Cu-MoO3 catalyst shows a small Tafel slope of 83 mV dec−1, which is smaller than MoO3 nanosheet (108 mV dec−1) and Cu foam (480 mV dec−1). This result indicates the Cu-MoO3 catalyst possesses the faster reaction kinetics of HER to realize better catalytic activity. After systematic comparison (Figure 5c), the modification of Cu atoms induced the electron transfer, leading to the excellent catalytic activity of Cu-MoO3 for HER. Stability is another important indicator to study catalysts. As shown in Figure 5d, the CV curves before and after 3000th cycling cycles confirm the similar shape of Cu-MoO3, suggesting its high stability for HER in an alkaline medium. In addition, the chronoamperometric test of Cu-MoO3 nanosheet delivers slight decay of current density over 100 h (Figure S4), indicating outstanding stability during a long-term HER process.
The electrochemical impedance spectroscopy (EIS) is measured to investigate the reaction kinetics of those catalysts for the HER process. As shown in Figure 6a, the curve of the Cu-MoO3 nanosheet shows a smaller diameter of the semicircle (7.1 Ω) compared to the MoO3 material (11.2 Ω), confirming the optimized reaction kinetics after the introduction of Mo atoms. The electrochemical surface area (ECSA) is calculated by comparing the prepared catalysts’ electrochemical double layer capacitances (Cdl) with that of blank Cu foam. The fitting data are shown in Figure 6b. The Cu-MoO3 nanosheet shows the highest Cdl value of 9.8 mF cm−2, which is higher than MoO3 (7.3 mF cm−2) and Cu foam (1.8 mF cm−2). This result indicates the Cu-MoO3 nanosheet has the largest surface active area, which can offer more catalytic actives during the HER process. To study the intrinsic catalytic activity, the HER performance of those samples is normalized by the Cdl values and the results are shown in Figure 6c,d. The Cu-MoO3 nanosheet still exhibits the better catalytic performance than pristine MoO3. This result indicates that the active specific surface area is not the only factor to improve the catalytic activity, as the electronic interaction between Cu and MoO3 is another important factor [34,35,36,37].
To analyze the effect of Cu doping on improving the HER catalytic performance, the MoO3 nanosheet with various contents of Cu is synthesized by regulating the usage of the Cu source. As shown in Figure 7a and Figure S6, the contents of Cu are measured by the ICP method, which are 3.2%, 7.8% and 12.0% for Cu1-MoO3, Cu2-MoO3, and Cu3-MoO3 samples, respectively. Moreover, the HER catalytic activity of all prepared samples is tested in 1 M KOH solution. The catalytic activity increase from pristine MoO3 sample to Cu2-MoO3 nanosheet, and then decrease to Cu3-MoO3 sample. We speculate that in Cu-doped MoO3 electrocatalysts, the electronic conductivity and catalytically active sites are two competing relationships. When the doping amount of Cu is low, the electronic conductivity of MoO3 material is not significantly improved, so the improvement of catalytic activity is insufficient. When the amount of Cu doping is too high, more Cu sites cover the surface of the MoO3 catalyst to hinder the Mo catalytic active centers, resulting in a decrease in catalytic performance for HER. Therefore, those results indicate the optimal doping amount of Cu is about 7.8% in this system. In addition, the Cu2-MoO3 nanosheet is also used as a working electrode to analyze the Faraday efficiency of hydrogen production. As shown in Figure 7b, the Cu2-MoO3 nanosheet exhibits high Faraday efficiency over 97% during a long time of HER test, indicating the intrinsic high catalytic performance of Cu doped MoO3 electrocatalyst (Figure S7).
Given the high HER catalytic activity of Cu-MoO3 nanosheet, a simple water electrolyzer is assembled by utilizing Cu-MoO3 and FeOOH-NF as both cathode and anode. The polarization curve of this Cu-MoO3//FeOOH-NF water electrolyzer is shown in Figure 8a. A small voltage of 1.55 V can be realized at the current density of 10 mA cm−2. In addition, the current density of the Cu-MoO3//FeOOH-NF electrolyzer is high as 75 mA cm−2 at the voltage of 1.7 V. Finally, the Cu-MoO3//FeOOH-NF electrolyzer also shows superior operation stability, which can maintain stable over 30 h at the test voltage of 1.6 V (Figure 8b). The result confirms the promising potential application of Cu-MoO3 electrocatalyst in hydrogen generation.

3. Materials and Methods

3.1. Materials

Molybdenum metal powder (Mo), hydrogen peroxide (H2O2), and Potassium hydroxide (KOH) are purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.). Tetrakis(acetonitrile) copper(I) hexafluorophosphate is bought from Sigma. Copper foam is obtained from SaiBo electrochemical material network. All chemicals are used without any further purification.

3.2. The Synthesis of Pristine MoO3 Nanosheet

Typically, molybdenum metal powder (2 mmol) is put into a Teflon vessel that contains 30 mL ethanol. Next, 3 mL H2O2 solution is added to the above solution and magnetically stirred for about 1 h. Then, a transparent yellow solution is prepared. The Teflon vessel is then sealed by a stainless steel autoclave and put into the oven at 160 °C for 12 h. After cooling to room temperature, the samples are collected by centrifugation. Next, the samples are fully rinsed with ethanol several times and dried in a vacuum oven at 80 °C.

3.3. The Synthesis of Cu-Doped MoO3 Nanosheet

In a typical process, 30 mg MoO3 nanosheet is first dispersed in 10 mL acetone solution by sonication treatment. Next, the uniform solution is heated to 52 °C under constant stirring. Then, different amounts of tetrakis(acetonitrile) copper(I) hexafluorophosphate are added to above solution. The reaction is kept for 20 min under stirring. The final product can be obtained by centrifugation and washed with acetone several times. The usage of Cu sources is 0.1, 0.15 and 0.2 mmol to prepare the Cu1-MoO3, Cu2-MoO3, and Cu3-MoO3 samples, respectively.

3.4. Structural Characterization

The X-ray diffraction (XRD) pattern is used to study the phase of prepared samples by using a Philips X’Pert Pro Super diffractometer with Cu-Kα radiation. The transmission electron microscopy (TEM) is conducted by a HT-7700 field-emission electron microscope with an acceleration voltage of 120 kV. The high-resolution TEM (HRTEM) and elemental mapping images are collected on a JEOL JEM-ARF200F TEM/STEM. The X-ray photoelectron spectroscopy (XPS) is carried out on a PHI5000 Versa Probe II XPS system. The diameter of the analyzed area is 10 μm and monochromatic X-rays are generated by an Al Kα source (1486.7 eV).

3.5. Electrochemical Measurements

A three-electrode configuration is utilized to study the electrochemical performance by using the workstation of CHI 660E. The working electrodes are obtained by drop-coating strategy. In a typical process, 5 mg samples mixed with 35 μL Nafion solution are added into 1 mL isopropyl alcohol/H2O mixed solution and dispersed for 30 min by sonication. After the formation of a homogeneous ink, a certain volume of ink is dropped onto a clean copper foam substrate. After drying, the loading mass of catalysts is calculated to 1.5 mg cm−2, and the working electrode is directly used for the electrochemical test. In addition, the 1 M KOH solution is used as an electrolyte, a saturated calomel electrode is used as the reference electrode, while the graphite rod is used as the counter electrode. The polarization curves are recorded by testing the linear sweep voltammetry (LSV) curves at 5 mV s−1. The electrochemical impedance spectroscopy (EIS) is conducted at the frequency range of 100 kHz to 0.1 kHz. All presented potentials are converted to a reversible hydrogen electrode (RHE) by the Nernst equation.

4. Conclusions

In summary, a simple strategy is designed to improve the catalytic activity of MoO3 material by the controllable modification of Cu atoms. The Cu-doped MoO3 nanosheet is successfully prepared by hydrothermal reaction and the followed disproportionation of Cu+. The introduction of Cu atoms not only improves the surface active area, but also transfers more electrons into the MoO3 nanosheet, leading to more catalytic active sites and faster electron transfer ability during the HER process. As a result, the optimized Cu-MoO3 sample shows the superior catalytic activity for HER in alkaline medium. The overpotential only needs 106 mV to reach the geometric current density of 10 mA cm−2, with a small Tafel slope of 83 mV dec−1. In addition, the assembled Cu-MoO3//FeOOH-NF electrolyzer delivers the low voltage of 1.55 V at current density of 10 mA cm−2. Our work opens a new way to exploit advanced low-cost electrocatalysts for large-scale hydrogen production.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/catal12080895/s1, The supporting information includes XRD pattern, TEM images, XPS spectra, stability test and other data. Figure S1: The XRD pattern of pristine MoO3 nanosheet; Figure S2: The TEM images of pristine MoO3 nanosheet; Figure S3: The XPS survey of pristine MoO3 nanosheet; Figure S4: The a) Mo 3d and b) O 1s spectra of pristine MoO3 nanosheet; Figure S5: The Cu LMM spectrum of Cu-MoO3 nanosheet; Figure S6: The content of Cu in these prepared MoO3 nanosheet; Figure S7: The sta-bility test of Cu-MoO3 sample for HER in alkaline medium.

Author Contributions

Writing—original draft, W.H.; Writing—review and editing, P.C., and Y.T.; Validation, K.L.; methodology, D.F., Visualization, W.H.; Supervision and funding acquisition, P.C., and Y.T. This manuscript was written by the contributions of all authors. All authors have read and agreed to the published version of the manuscript.

Funding

This work was financially supported by the National Natural Science Foundation of China (21905251). The Zhejiang Provincial Natural Science Foundation of China (LQ22B030008) and the Science Foundation of Zhejiang Sci-Tech University (ZSTU) under Grant No. 21062337-Y.

Data Availability Statement

Data available upon request.

Acknowledgments

The authors would like to thank the shiyanjia lab (www.shiyanjia.com) for the measurements of SEM images.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 1. (a) The XRD pattern of Cu doped MoO3 nanosheet and (b) the crystal structure of MoO3 sample.
Figure 1. (a) The XRD pattern of Cu doped MoO3 nanosheet and (b) the crystal structure of MoO3 sample.
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Figure 2. (a–c) The TEM images and (d) HRTEM image of Cu doped MoO3 nanosheet.
Figure 2. (a–c) The TEM images and (d) HRTEM image of Cu doped MoO3 nanosheet.
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Figure 3. The elemental mapping images of Cu doped MoO3 nanosheet.
Figure 3. The elemental mapping images of Cu doped MoO3 nanosheet.
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Figure 4. (a) XPS survey and high-resolution spectra of (b) Mo 3d, (c) O 1s and (d) Cu 2p of Cu doped MoO3 nanosheet.
Figure 4. (a) XPS survey and high-resolution spectra of (b) Mo 3d, (c) O 1s and (d) Cu 2p of Cu doped MoO3 nanosheet.
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Figure 5. (a) The polarization curve, (b) Tafel slopes, (c) comparison of performance and (d) stability of as-prepared samples in 1 M KOH solution.
Figure 5. (a) The polarization curve, (b) Tafel slopes, (c) comparison of performance and (d) stability of as-prepared samples in 1 M KOH solution.
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Figure 6. (a) The EIS spectra, inset: image of equivalent circuit model, (b) double-layer capacitance plots of Cu foam, MoO3 and Cu-MoO3, (c) Cdl normalized performance and (d) the performance comparison of MoO3 and Cu-MoO3 samples.
Figure 6. (a) The EIS spectra, inset: image of equivalent circuit model, (b) double-layer capacitance plots of Cu foam, MoO3 and Cu-MoO3, (c) Cdl normalized performance and (d) the performance comparison of MoO3 and Cu-MoO3 samples.
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Figure 7. (a) The plots of HER catalytic performance of the MoO3 nanosheet with different Cu doping. (b) The Faraday efficiency of Cu2-MoO3 sample at 10 mA cm−2 for HER.
Figure 7. (a) The plots of HER catalytic performance of the MoO3 nanosheet with different Cu doping. (b) The Faraday efficiency of Cu2-MoO3 sample at 10 mA cm−2 for HER.
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Figure 8. (a) The polarization curve of Cu-MoO3//FeOOH-NF water electrolyzer. (b) The stability test of Cu-MoO3//FeOOH-NF water electrolyzer at 1.6 V in an alkaline medium.
Figure 8. (a) The polarization curve of Cu-MoO3//FeOOH-NF water electrolyzer. (b) The stability test of Cu-MoO3//FeOOH-NF water electrolyzer at 1.6 V in an alkaline medium.
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Chen, P.; Huang, W.; Li, K.; Feng, D.; Tong, Y. Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production. Catalysts 2022, 12, 895. https://doi.org/10.3390/catal12080895

AMA Style

Chen P, Huang W, Li K, Feng D, Tong Y. Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production. Catalysts. 2022; 12(8):895. https://doi.org/10.3390/catal12080895

Chicago/Turabian Style

Chen, Pengzuo, Weixia Huang, Kaixun Li, Dongmei Feng, and Yun Tong. 2022. "Copper Incorporated Molybdenum Trioxide Nanosheet Realizing High-Efficient Performance for Hydrogen Production" Catalysts 12, no. 8: 895. https://doi.org/10.3390/catal12080895

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