Facile route to achieve MoSe₂-Ni₃Se₂ on nickel foam as efficient dual functional electrocatalysts for overall water splitting

Abstract

Since the catalytic activity of present nickel-based synthetic selenide is still to be improved, MoSe₂-Ni₃Se₂ was synthesized on nickel foam (NF) (MoSe₂-Ni₃Se₂/NF) by introducing a molybdenum source. After the molybdenum source was introduced, the surface of the catalyst changed from a single-phase structure to a multi-phase structure. The catalyst surface with enriched active sites and the synergistic effect of MoSe₂ and Ni₃Se₂ together enhance the hydrogen evolution reactions (HER), the oxygen evolution reactions (OER), and electrocatalytic total water splitting activity of the catalyst. The overpotential of the MoSe₂-Ni₃Se₂/NF electrocatalyst is only 259 mV and 395 mV at a current density of 100 mA/cm² for HER and OER, respectively. MoSe₂-Ni₃Se₂/NF with a two-electrode system attains a current density of 10 mA/cm² at 1.60 V. In addition, the overpotential of HER and OER of MoSe₂-Ni₃Se₂/NF within 80000 s and the decomposition voltage of electrocatalytic total water decomposition hardly changed, showing an extremely strong stability. The improvement of MoSe₂-Ni₃Se₂/NF catalytic activity is attributed to the establishment of the multi-phase structure and the optimized inoculation of the multi-component and multi-interface.

Key words： three-dimensional molybdenum nanoma- terials; hydrogen evolution reaction; oxygen evolution reaction; overall water splitting

1 Introduction

The rapid economic development based on fossil fuels greatly intensifies environmental pollution and energy crises, prompting the exploration of sustainable clean energy [1–4]. As an energy source, hydrogen has the advantages of high energy density, environmental friendliness, and renewability, thus has the potential to replace fossil fuels [5–7]. An effective strategy for hydrogen production is overall water splitting and it is one of the energy conversion methods with a great application potential. Renewable energy such as solar energy can drive overall water splitting [8–13]. Hydrogen production by overall water splitting is resulted from the hydrogen and oxygen evolution reactions (HER, OER), reacting at the cathode and the anode, respectively. The number which shows 1.23 V in decomposition voltage water is theoretical. However, due to the sluggish kinetics and the impact of electrolyte chemistry, the actual decomposition voltage is much larger than 1.23 V [14–16]. Currently, commercialized hydrogen evolution catalysts, such as Pt, and oxygen evolution catalysts, such as RuO₂ with superior performances are low-reserve in nature and expensive, which greatly limits their applications [17,18]. Therefore, research on the low-cost catalyst with a better activity and stability in the improvement of overall water splitting technology is indispensable.

Up to the present, many types of low-cost electrocatalysts with superior performances and good catalytic stabilities have been developed, including molybdenum [19–21], iron [22–24], cobalt [25–28], nickel [29–34], and vanadium-based [35,36] compounds, of which, more attention have been paid to nickel-based compounds in storage and conversion of energy owing to their high abundance in nature, easy access, and low price. The density functional theory results illustrate that Se doping is beneficial for decreasing the kinetic barrier and the free energy of HER. Nickel-based selenide with specific phases has been reported to have a good electrocatalytic activity, but it still needs great improvement in order to meet the practical requirement [37,38]. A variety of strategies have been adopted to develop the electrochemical water splitting activity of nickel-based selenide, including heteroatom doping [39], phase change [40], and combination with conductive substrates [41,42], such as carbon nanotubes, conductive graphene, etc.

In this work, by using nickel foam (NF) as the substrate, MoSe₂-Ni₃Se₂/NF multiphase composite material was prepared by utilizing a one-pot hydrothermal method. As a result, the activity of the modified electrocatalyst was increased significantly for HER and OER. The MoSe₂-Ni₃Se₂/NF electrocatalyst showed an unexceptionable activity with an overpotential of only 259 mV and 395 mV for HER and OER at a current density of 100 mA/cm², respectively. In addition, when assembled in a two-electrode system, the MoSe₂-Ni₃Se₂/NF electrocatalyst could reach a current density of 10 mA/cm² with an overpotential of 1.60 V for overall water splitting.

2 Experimental

2.1 Materials

The selenium powder was purchase from Aladdin. The ammonium heptamolybdate tetrahydrate (AHT), commercial Pt/C (20% (mass ratio)), and RuO₂ were purchased from Tianjin Kaida Chemical Plant, Shanghai Hesen Electric Co., Ltd., and Sigma-Aldrich, respectively. The acetone, hydrazine hydrate solution, and ethanol were purchased from Beijing Chemical Plant. The foam nickel (NF) was purchased from Guangdong Candlelight New Energy Technology Co., Ltd. All experiments were conducted using deionized water (deionized water, 18.25 MΩ·cm).

2.2 Synthesis of MoSe₂-Ni₃Se₂/NF, Ni₃Se₂/NF, Pt/C-NF, and RuO₂-NF

To remove the oxide layer, the NF substrate was soaked in an acetone solution for 24 h. Then, it was taken out, ultrasonically cleaned in ethanol and DI water for 30 min, and dried it in an oven at 50°C overnight. The selenium powder (0.1575 g) was dispersed in 10 mL of hydrazine hydrate solution and stirred for 5 h until the solution turns brown-black. Meanwhile, the AHT (0.1765 g) was dispersed in 50 mL of DI water and stirred for 30 min until the particles were completely dissolved. Under stirring, the selenium solution was slowly added into the AHT solution drop by drop, and then stirred for 30 min. The reaction mixture was shifted to a 100 mL autoclave reactor with a piece of NF (2 cm× 5 cm). Then, the autoclave was put into an oven and sintered at 200°C for 20 h. After that, the reactor was moved out and cooled down naturally. The resultant NF-supported catalyst was ultrasonically washed with DI water and ethanol, and dried at 50°C overnight. The product which was obtained by the experience was recorded as MoSe₂-Ni₃Se₂/NF. For comparison, the Ni₃Se₂/NF was prepared following the same steps, but without introducing AHT. The Pt/C or RuO₂ loaded samples were prepared by weighing 4 mg of Pt/C and RuO₂ in two sorts of sample tubes, containing 190 μL of isopropanol, 50 μL of naphthol, 380 μL of deionized water, and 380 μL of ethanol. The mixture was kept ultrasonicating for 30 min. Finally, 200 μL of the above ink was evenly smeared on the top layer of NF (1 cm × 1 cm). The catalyst loading rate on the sample is approximately 0.8 mg/cm².

2.3 Physical characterization

X-ray diffraction (XRD) analysis was performed on Siemens D5005 diffractometer with Cu-Kα source (λ = 1.5418 Å) at 40 kV and 30 mA, and a scan rate of 5 (° )/min. Scanning electron microscope (SEM) and (high resolution-) transmission electron microscopy (TEM (100 kV), HR-TEM (200 kV)) pictures were obtained by using the JEOL SM-6360LV field emission scanning electron microscope and the JEOL JSM-2100F driving device electron microscope, respectively. In addition, a field emission transmission electron microscope (Tecnai G2 F-30 S-TWIN) was used to conduct the elemental analysis. X-ray photoelectron spectroscopy (XPS) was performed through the ESCALAB 250 spectrometer by a mono-chromatic Al Kα X-ray source (1486.6 eV).

2.4 Electrochemical characterization

All electrochemical experiments were conducted via the CHI660E electrochemical workstation, using the 1.0 mol/L aqueous KOH electrolyte solution. A three-electrode cell with the catalyst-supported NF (0.5 cm × 0.5 cm) was used as the working electrode, while the Hg/HgO electrode and carbon rod were used as the reference and counter electrodes, respectively. Before experiencing the electrocatalytic performance, the electrolyte was filled with nitrogen or oxygen for 30 min. The reference potential of HER and OER was modified by E_RHE = E_Hg/HgO + 0.059 pH+ 0.099 V. The polarization curve (LSV) was recorded at a scan rate of 5 mV/s, and each electrode was iR (85%) corrected to eliminate the effect of the internal obstruction of the solution. The cyclic voltammetry (CV) was performed at a sweep speed of 20, 40, 60, 80, and 100 mV/s. The electrochemical impedance spectroscopy (EIS) was evaluated in the frequency range of 100 kHz to 0.1 Hz. The stability tests were performed by chronopotentiometry at an invariable current of 10 mA/cm² for HER, OER, and total water splitting (80000 s). The electrocatalytic water splitting experiment was conducted in a two-electrode battery, with MoSe₂-Ni₃Se₂/NF (0.5 cm × 0.5 cm) as the cathode and anode.

The Faraday efficiency (FE) is calculated by

(1)FE(\%)\mathrm{}=n_{\mathrm{Exp}}/n_{\mathrm{Theo}},

where n_Exp means the total number of moles of the collected H₂ and O₂ gases and n_Theo = 3Q/4F (Q is the charge passing through the electrodes and F is Faraday constant).

3 Results and discussion

MoSe₂-Ni₃Se₂/NF synthesized by the method is shown in Fig. 1, while the XRD patterns of Ni₃Se₂/NF and MoSe₂-Ni₃Se₂/NF are demonstrated in Fig. 2. The characteristic peaks of the nickel matrix correspond to three strong diffraction peaks at 44.58°, 51.96°, and 76.43° [43]. The main diffraction peaks at 21.09°, 29.65°, 36.78°, 37.25°, 42.76°, 47.80°, 48.38°, 52.75°, 53.60°, 61.67°, 62.63°, 69.96°, 70.61° correspond to the characteristic peaks of Ni₃Se₂ (JCPDS No.19-0841) [42]. Upon addition of AHT, the crystalline phase of MoSe₂ appears with the main diffraction peaks at 30.98°, 34.01°, 38.67°, 46.65°, 55.11°, and 56.26°, corresponding to the characteristic peaks of MoSe₂ (JCPDS No.29-0914) [44].

Fig.1 Synthesis process of MoSe₂-Ni₃Se₂/NF.

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Figures 3(a) and 3(b) illustrate the smooth surface of NF after pretreatment. Upon addition of the selenium powder, the obtained Ni₃Se₂/NF gives a uniform single-phase structure (Figs. 3(c) and 3(d)). However, if the selenium powder and ammonium molybdate tetrahydrate are simultaneously introduced, the NF surface generate a multiphase structure (Figs. 3(e) and 3(f)). The formation of the MoSe₂-Ni₃Se₂/NF composite structure leads to an increase in the specific surface area of the electrocatalyst, which supplies more active sites and accelerates the electron transport.

Fig.2 XRD patterns of NF, Ni₃Se₂/NF, MoSe₂-Ni₃Se₂/NF.

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Fig.3 SEM images at different resolutions.

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The morphology of the MoSe₂-Ni₃Se₂/NF is further investigated via TEM which suggests that the composite has a non-uniform nanosheet structure (Fig. 4(a)). In the corresponding HRTEM image, the (002) crystal plane of Ni₃Se₂ can get lattice fringes with a pitch of about 0.28 nm [45], and the (012) crystal plane of MoSe₂ can get lattice fringes with a pitch of about 0.71 nm [44]. In addition, the element map in Fig. 4(c) manifests that Ni, Se, and Mo are uniformly distributed on the surface of MoSe₂-Ni₃Se₂/NF. The energy dispersive spectroscopy spectra of the sample and the corresponding images are given in Electronic Supplementary Material (Fig. S1). The content of Mo, Ni, and Se are 6.48%, 75.29%, 18.23% respectively, which reveals the even distribution of Ni, Se and Mo on the surface of MoSe₂-Ni₃Se₂/NF.

Fig.4 TEM images at different resolutions.

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Fig.5 XPS total spectrum.

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The surface chemical state and electronic properties of MoSe₂-Ni₃Se₂/NF electrocatalysts were evaluated by XPS. Figure 5(a) exhibits that the main elements of the MoSe₂-Ni₃Se₂/NF electrocatalyst are Ni, Mo, and Se (Fig. 5(a)) [46]. In Fig. 5(b), after deconvolution of Ni 2p, the six peaks at 852.34, 855.60, 861.25, 869.30, 873.30, and 879.34 eV correspond to Ni⁰, Ni²⁺ 2p_3/2, satellite peak, Ni⁰, Ni²⁺ 2p_1/2, and satellite peak, respectively [21]. The peaks at 228.60 eV and 231.67 eV correspond to 3d_5/2 and 3d_3/2 of Mo⁴⁺, respectively. The peaks at 232.09 eV and 235.07 eV correspond to Mo⁶⁺. The presence of the peak of Mo⁶⁺ on the surface of the electrocatalyst may be due to the slight oxidation of air (Fig. 5(c)) [46]. The peaks at 54.05 eV and 55.15 eV correspond to Se 3d_5/2 and 3d_3/2, respectively. In addition, a strong peak at 58.72 eV corresponding to SeO_x is observed, which may be caused by the air exposure [21] (Fig. 5(d)).

Fig.6 Electrochemical characterization of HER.

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Figure 6(a) displays the electrocatalytic activity of the samples in the 1.0 mol/L KOH solution. When voltage is applied to the cathode, the LSV curve of MoSe₂-Ni₃Se₂/NF indicates that the current density is increasing sharply. The overpotential at 10 and 100 mA/cm² is only 106 and 259 mV, respectively. For comparison, the LSV curve of the Pt/C-NF exhibits a lowest overpotential (η₁₀ = 45 mV and η₁₀₀ = 169 mV). On the contrary, the commercial NF and Ni₃Se₂/NF demonstrate an overpotential of 285 and 136 mV at 10 mA/cm², respectively, which confirms the advantage of introducing AHT. Another important basis to evaluate the catalytic performance of an electrocatalyst is the Tafel slope. As the HER catalytic activity of a sample improves, the Tafel slope decreases [47–49]. At present, the HER electron reflection model has been generally mentioned as

(2)\mathrm{Volmer\; reaction}:{\mathrm{H}}_{\mathrm{2}}\mathrm{O}(\mathrm{aq})\mathrm{}+{\mathrm{e}}^{–}+\mathrm{}*\mathrm{}\to \mathrm{H}*\mathrm{}+{\mathrm{OH}}^{–}(\mathrm{aq}),

(3)\mathrm{Heyrovsky\; reaction}:\mathrm{H}*\mathrm{}+{\mathrm{H}}_{\mathrm{2}}\mathrm{O}(\mathrm{aq})\mathrm{}+{\mathrm{e}}^{–}\to {\mathrm{H}}_{\mathrm{2}}+{\mathrm{OH}}^{–}(\mathrm{aq})\mathrm{}+\mathrm{}*,

(4)\mathrm{Tafel\; reaction}:\mathrm{H}*\mathrm{}+\mathrm{H}*\mathrm{}\to {\mathrm{H}}_{\mathrm{2}}+\mathrm{2}*.

The Tafel slopes of commercial NF, Ni₃Se₂/NF, MoSe₂-Ni₃Se₂/NF and commercial Pt/C on NF are 149.5, 120.3, 98.4, and 60.4 mV/dec (Fig. 6(b)), revealing the excellent HER performance of MoSe₂-Ni₃Se₂/NF. The electrocatalytic active surface area of the electrocatalyst was evaluated by measuring the electric double layer capacitance of the sample. As the electric double layer capacitance value increases, the number of active groups on the catalyst surface increases, too [50,51]. The C_dl value of MoSe₂-Ni₃Se₂/NF is 38.8 mF/cm² (Fig. S2(a, b)), while the C_dl value of Ni₃Se₂/NF is only 7.9 mF/cm² (Figs. S2(c), S2(d)) [52]. The EIS of the sample was further measured to analyze the HER kinetic process of the electrocatalyst. The R_ct value is inversely proportional to the HER kinetic rate and electron transfer rate [53,54]. As shown in Fig. 6(c), the R_ct value of MoSe₂-Ni₃Se₂/NF is much smaller than that of Ni₃Se₂/NF, which may be attributed to the improved electron transfer efficiency of MoSe₂-Ni₃Se₂/NF with a multiphase surface structure, in contrast to the single-phase structure of Ni₃Se₂/NF. `

In the long term stability test, the overpotential of the MoSe₂-Ni₃Se₂/NF composite material hardly changes within 80000 s (Fig. 6(d)), which, therefore, demonstrates the satisfactory durability of the MoSe₂-Ni₃Se₂/NF composite.

The OER activity of MoSe₂-Ni₃Se₂/NF, Ni₃Se₂/NF and commercial RuO₂-NF and NF was evaluated in the 1.0 mol/L KOH solution. The overpotential of MoSe₂-Ni₃Se₂/NF at 100 mA/cm² is 395 mV, comparable to the activity of commercial RuO₂-NF with an overpotential of 341 mV. However, the overpotential of Ni₃Se₂/NF and NF at 100 mA/cm² is 455 and 500 mV, respectively (Fig. 7(a)). The OER activity of the catalyst was also evaluated by Tafel plot. The OER reaction process is more complicated. The currently recognized reaction process is shown as

(5)*\mathrm{}+{\mathrm{OH}}^{–}\to \mathrm{OH}*\mathrm{}+{\mathrm{e}}^{–},

(6)\mathrm{OH}*\mathrm{}+{\mathrm{OH}}^{–}\to {\mathrm{H}}_{\mathrm{2}}\mathrm{O}(\mathrm{l})\mathrm{}+\mathrm{O}*\mathrm{}+{\mathrm{e}}^{–},

(7)\mathrm{O}*\mathrm{}+{\mathrm{OH}}^{–}\to \mathrm{OOH}*\mathrm{}+{\mathrm{e}}^{–},

(8)\mathrm{OOH}*\mathrm{}+{\mathrm{OH}}^{–}\to {\mathrm{H}}_{\mathrm{2}}\mathrm{O}(\mathrm{l})\mathrm{}+{\mathrm{O}}_{\mathrm{2}}*\mathrm{}+{\mathrm{e}}^{–},

(9){\mathrm{O}}_{\mathrm{2}}*\to *+{\mathrm{O}}_{\mathrm{2}}.

The Tafel slopes of MoSe₂-Ni₃Se₂/NF, Ni₃Se₂/NF, and RuO₂-NF and NF are 62.3, 87.6, 42.4, and 102.6 mV, respectively (Fig. 7(b)). In Fig. 7(c), the EIS test reveals that MoSe₂-Ni₃Se₂/NF has a lower R_ct value than Ni₃Se₂/NF.

In addition, the stability of MoSe₂-Ni₃Se₂/NF is also implemented by chronoamperometry, which almost does not change in the overpotential of MoSe₂-Ni₃Se₂/NF at 10 mA/cm² within 80000 s (Fig. 7(d)). Thus, the MoSe₂-Ni₃Se₂/NF has a satisfactory catalytic durability under alkaline conditions.

Fig.7 Electrochemical characterization of OER.

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There are two reasons for improving the performance of HER and OER. The first reason is that introduction of the molybdenum source leads to the surface reconstruction of the catalyst (transformation from a single-phase structure to a multi-phase structure), which supply more active sites and speeds up the electron transfer efficiency. The second reason is that with the introduction of molybdenum source, a multi-component multi-interface electrocatalyst is formed, which may lead to an increase in the electrocatalytic activity.

Based on the excellent HER and OER activities of MoSe₂-Ni₃Se₂/NF, two electrode cells are assembled for total water splitting. The result shows that the required full water decomposition voltage is only 1.60 V at 10 mA/cm² (Fig. 8(a)). In contrast, the electrocatalytic water splitting voltage of Pt/C-NF|RuO₂-NF and Ni₃Se₂/NF is 1.53 and 1.66 V at a current density of 10 mA/cm², respectively.

Fig.8 Electrochemical characterization of overall water splitting.

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Fig.9 Durability tests at 10 mA/cm² of MoSe₂-Ni₃Se₂/NF for total water splitting in lake water.

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In addition, the durability of MoSe₂-Ni₃Se₂/NF for electrocatalytic water splitting was also evaluated by chronoamperometry at 10 mA/cm². After 80000 s, the overall water splitting voltage of MoSe₂-Ni₃Se₂/NF hardly changed, indicating its good potential in practical application (Fig. 8(b)). Meanwhile, under the condition of 10 mA/cm², the total water decomposition activity of MoSe₂-Ni₃Se₂/NF in lake water (our school lake water) was tested. At 10 mA/cm², the overpotential of the sample is 1.62 V (Fig. (9)), close to the overpotential in deionized water, indicating that it has a great practical application value. Through testing and calculation, the Faraday efficiency of HER and OER of the electrocatalyst is provided in Electronic Supplementary Material; the calculation results show that the Faraday efficiency of the catalyst is up to 93% (Fig. S3). After the durability test of the electrocatalyst for overall water splitting, the XRD, SEM, XPS, and EDS of MoSe₂-Ni₃Se₂/NF were tested. The results show that the structure and the surface morphology of the electrocatalyst roughly maintain the original characteristics (Figs. S4–S7).

4 Conclusions

The MoSe₂-Ni₃Se₂/NF heterogeneous composite was fabricated by utilizing a one-pot hydrothermal method. The MoSe₂-Ni₃Se₂/NF heterogeneous composite demonstrated an excellent electrocatalytic activity and a long-term stability for HER, OER, and total water splitting. When the sample was used as a bifunctional electrocatalyst to drive the electrocatalytic total water splitting, an overpotential of only 1.60 V is required at 10 mA/cm². Meanwhile, the catalyst demonstrated a satisfactory durability and an almost unchanged overpotential after a durability test of 80000 s. The increase in the catalytic activity of the electrocatalyst can be attributed to the fact that the introduction of the molybdenum source has caused the surface of the electrocatalyst to be reconstructed to form a heterogeneous structure, which increases the active sites on the electrocatalyst surface and accelerates the electron transport speed, thereby improving the electrocatalytic activity. In addition, with the introduction of the molybdenum source, a multi-component multi-interface electrocatalyst is formed, which may lead to an increase in the electrocatalytic activity. It is hoped that this work may provide a direction for the surface regulation of nanomaterials and the development of efficient dual-functional electrocatalysts.

Acknowledgments

This work was supported by the Scientific Research Projects of Jilin Provincial Department of Education (Grant No. JJKH20211285KJ) and Jilin Provincial Science and Technology Development Foundation (Grant No. 20200201090JC).

Electronic Supplementary Material

Supplementary material is available in the online version of this article at https://doi.org/10.1007/s11708-022-0813-0 and is accessible for authorized users.

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