Hierarchical 0D-2D Co/Mo Selenides as Superior Bifunctional Electrocatalysts for Overall Water Splitting.

Development of efficient electrocatalysts combining the features of low cost and high performance for both the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) still remains a critical challenge. Here, we proposed a facile strategy to construct in situ a novel hierarchical heterostructure composed of 0D-2D CoSe2/MoSe2 by the selenization of CoMoO4 nanosheets grafted on a carbon cloth (CC). In such integrated structure, CoSe2 nanoparticles dispersed well and tightly bonded with MoSe2 nanosheets, which can not only enhance kinetics due to the synergetic effects, thus promoting the electrocatalytic activity, but also effectively improve the structural stability. Benefiting from its unique architecture, the designed CoSe2/MoSe2 catalyst exhibits superior OER and HER performance. Specifically, a small overpotential of 280 mV is acquired at a current density of 10 mA·cm-2 for OER with a small Tafel slope of 86.8 mV·dec-1, and the overpotential is 90 mV at a current density of 10 mA·cm-2 for HER with a Tafel slope of 84.8 mV·dec-1 in 1 M KOH. Furthermore, the symmetrical electrolyzer assembled with the CoSe2/MoSe2 catalysts depicts a small cell voltage of 1.63 V at 10 mA·cm-2 toward overall water splitting.

Introduction

Hydrogen is a promising energy source that bon class="Chemical">asts a high power density and environmental friendliness; therefore, electrolysis of water is hotly pursued as a renewable, efficient, and pollution-free technique (Amiinu et al., 2017; Luo et al., 2018; Zhu et al., 2018). Electrocatalytic water splitting consists of the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), and electrocatalysts as the chemical reaction centers play a critical role in the water splitting electrolyzer. Although some noble metal oxide catalysts (RuO2 and IrO2) have high electrocatalytic performance for the OER and some noble metal catalysts (Pt and Ir) deliver good electrochemical property in the HER, the high cost and scarcity restrict their wide industrial application (Trasatti, 1972, 1984). Therefore, noble-metal-free catalysts with high stability and efficiency are crucial to large-scale hydrogen production from water splitting. Currently, the OER activity in alkaline solution is the bottleneck in overall water splitting due to the sluggish kinetics arising from the multiproton-coupled electron transfer steps (Jamesh and Sun, 2018). In practice, the HER catalyst in the electrolyzer should be compatible with the OER catalyst and functions in the same medium. Hence, development of suitable bifunctional noble-metal-free electrocatalyts with both high HER and OER performance in alkaline media is of great significance.

In recent years, transition metal dichalcogenides (TMDs) have attracted significant research interests owing to their earth-abundant reserves and accen class="Chemical">ptable activity for electrocatalytic HER (Xie et al., 2013; Zhang et al., 2017a; Xue et al., 2018; Wang et al., 2019). Particularly, layered MoSe2 has been considered as a promising HER electrocatalyst because of its unique structure features and high electrochemical activity (Shi et al., 2015; Chen et al., 2018a; Zhang et al., 2018). Theoretical research has demonstrated that the Gibbs free energy for H atom absorption on the edge of MoSe2 is lower than that of MoS2 due to the more metallic nature of MoSe2, revealing the higher HER performance (Tang et al., 2014; Lai et al., 2017; Yang et al., 2018). In addition, it also has been experimentally confirmed that the unsaturated Se edges in MoSe2 nanosheets are extremely active as the S edges in MoS2, which is responsible for the high HER activity (Jaramillo et al., 2007; Tang and Jiang, 2016). However, similar to MoS2, the HER activity of layered MoSe2 is largely limited by its poor conductivity and serious aggregation or restacking during the synthesis procedure (Mao et al., 2015; Qu et al., 2015), inhibiting the practical application of MoSe2 catalyst. Therefore, it is significant to improve the electrochemical activity of MoSe2-based catalyst. Recent works have shown that coupling MoSe2 with other transition metal selenides and constructing heterostrucurted materials could be an effective approach to further enhance the electrochemical performance of MoSe2. For instance, Wang et al. found that the MoSe2@Ni0.85Se nanowire delivered enhanced kinetics and performance for HER in alkaline conditions due to the high density of active edges of MoSe2 and the good conductivity of Ni0.85Se (Wang et al., 2017a). Zhang et al. synthesized 3D MoSe2/NiSe2 nanowires, which significantly enhanced HER activity with a low Tafel slope and overpotential in 0.5 M H2SO4, because the 3D structure affords more active sites (Zhang et al., 2017a). Liu et al. fabricated MoSe2-NiSe@carbon heteronanostructures and achieved glorious HER catalytic properties and excellent durability in both acidic and base conditions (Liu et al., 2018). In addition, the hierarchical mesoporous MoSe2@CoSe/N–C composite also exhibits outstanding HER activity (Chen et al., 2019b). Despite significant success, most of previous reports mainly focused on the improvement of HER performance, while the OER activity of MoSe2 catalyst in alkaline media has been ignored. Hence, the rational design and fabrication of MoSe2-based bifunctional electrocatalysts with satisfactory activity and stability toward overall water splitting in alkaline solution still remain a big challenge.

In this work, we developed a facile in situ phase separation strategy to n class="Chemical">construct a novel hierarchical heterostructure consisting of 0D−2D CoSe2/MoSe2 via the selenization of CoMoO4 nanosheets supported on a carbon cloth (CC) (Figure 1). Due to the in situ phase transformation, CoSe2 nanoparticles are uniformly anchored on MoSe2 nanosheets in the integrated structure, which can not only enhance reaction kinetics because of the synergetic effects, thus boosting the electrocatalytic activity, but also effectively suppress the aggregation/restacking of MoSe2 nanosheets, thereby improving the structural stability. Moreover, the hierarchical structure assembled by 0D−2D CoSe2/MoSe2 could provide abundant active sites for the electrochemical reactions. As a result, the designed CoSe2/MoSe2 architecture exhibits outstanding OER and HER performance in alkaline media. More specifically, a small overpotential of 280 mV is achieved at a current density of 10 mA·cm−2 for OER with a small Tafel slope of 86.8 mV·dec−1, and the overpotential is 90 mV at a current density of 10 mA·cm−2 for HER with a Tafel slope of 84.8 mV·dec−1 in 1 M KOH. Moreover, the symmetrical electrolyzer assembled with the CoSe2/MoSe2 catalysts delivers a small cell voltage of 1.63 V at 10 mA·cm−2 toward overall water splitting.

Figure 1

Schematic illustration of the fabrication of CoSe2/MoSe2.

Schematic illustration of the fabrication of CoSe2/n class="Chemical">MoSe2.

Experimental Section

Synthesis of CoMoO4 Nanosheet

Firstly, a pristine carbon cloth (CC) wn class="Chemical">as treated with nitric acid solution overnight, subsequently ultrasonicated in deionized (DI) water and dried in an oven at 80°C for 2 h. After that, 1 mmol cobalt acetate, 1 mmol ammonium molybdate, 2 mmol urea, and 5 mmol ammonium fluoride were dissolved in 30 mL DI water, followed by ultrasonication for 30 min. Then, the homogeneous solution was poured into a 50-mL Teflon-lined stainless autoclave with the CC kept at 150°C for 6 h. After cooling to room temperature, the CC was washed with DI water for several times and dried in a vacuum freeze-dryer overnight. Finally, the obtained sample was treated at 400°C for 2 h with a ramp rate of 5°C min−1 in an argon atmosphere.

Synthesis of CoSe2/MoSe2

The as-prepared n class="Chemical">CoMoO4 precursor was reacted with 0.5 g selenium powder at 450°C for 1 h under an Ar/H2 (90%/10%) atmosphere to form the CoSe2/MoSe2.

Synthesis of CoSe2

The CoSe2 wn class="Chemical">as prepared through two steps. Firstly, the treated CC was immersed in a 0.1 M Co(NO3)2 solution for the electrodeposition of Co (Yang et al., 2015). Then, the collected sample was reacted with 0.5 g selenium powder under an Ar/H2 (90%/10%) atmosphere at 450°C for 1 h.

Synthesis of MoSe2

Firstly, MoS2 wn class="Chemical">as prepared via hydrothermal reaction with the CC at 200°C for 12 h, followed by heating at 400°C for 2 h to form MoO3 (Wu et al., 2018). Then, the obtained MoO3 was reacted with 0.5 g selenium powder at 450°C for 1 h under an Ar/H2 (90%/10%) atmosphere.

Preparation of Pt/C

Four milligrams of 20% Pt/C and 20 μL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1) and then sonicated to form a uniform solution. Finally, the 1*1 cm2 CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

Preparation of RuO2

Four milligrams of RuO2 and 20 μL 5% Nafion solution were added into 1 mL solution of isopropanol and DI water (9:1), and then the sample was sonicated to form a uniform solution. Finally, the 1*1 cm2 CC was soaked in the homogeneous solution and dried in air at atmospheric temperature.

Characterization

The phase composition of the samples were characterized by X-ray diffraction (XRD, Bruker D8A A25), and the chemical states were determined through X-ray photoelectron spectroscopy (XPS, ESCALB 250Xi). The morphology and microstructure were recorded via field emission scanning electron microscopy (FE-SEM, FEI Nova NANOSEM 400) and high-resolution transmission electron microscopy (HR-TEM, JEM-2100 UHR STEM).

Electrochemical Measurements

All samples made use of a three-electrode system performed by a biologic VSP300 type electrochemical workstation (Biologic Science Instruments, France). The sample of CoSe2/n class="Chemical">MoSe2 was put on the electrode holder as the working electrode with a mass loading of 4 mg/cm2, the saturated calomel electrode (SCE) was the reference electrode, and a carbon rod served as the counter electrode. The electrolyte was 1 M KOH solution with saturated N2. Linear sweep voltammetry (LSV) was characterized by polarization curves of OER with a scanning rate of 5 mV s−1 from 0 to 0.8 V vs. SCE. Similarly, the polarization curves of HER were determined under the same condition from 0 to −0.8 V vs. SCE. The potentials were standardized by a reversible hydrogen electrode (RHE) as shown in the following: E (RHE) = E (SCE) + 0.059 × pH with instrument automatic 85% iR compensation. The electrochemically active surface area (ECSA) was calculated by cyclic voltammetry (CV) performed from −0.3 to −0.2 V vs. SCE with different scanning rates of 40, 60, 80, 100, and 120 mV s−1. Electrochemical impedance spectroscopy (EIS) measurements were conducted by biologic VMP3 (Biologic Science Instruments, France) from 100 KHz to 0.1 Hz. The overall water-splitting electrolyzer was performed with CoSe2/MoSe2 as electrodes and 1 M KOH as the electrolyte.

Results and Discussion

Figure 2A presents the FE-SEM image of the as-prepared n class="Chemical">CoMoO4 precursor, which presents uniform nanosheets (with a lateral size of 2 μm) perpendicularly grown on the CC substrate with high coverage. After a selenization process, the obtained CoSe2/MoSe2 sample well maintains the pristine morphology of the CoMoO4 precursor (Figure 2B). Moreover, the high-magnification SEM image further reveals that lots of nanoparticles are well dispersed on the surface of the nanosheet (Figure 2C), implying the structure and phase evolution during the selenization treatment. The elemental maps in (Figure 2D) show that Mo, Co, and Se are uniformly distributed throughout the nanosheets. In addition, the low-resolution TEM images in (Figures 2E,F) display that nanoparticles are uniformly distributed on the nanosheet during the thermal reduction procedure, forming the 0D/2D structure. Furthermore, the high-resolution TEM (Figure 2G) shows the lattice fringes of 0.26 nm and 0.65 nm corresponding to the (111) and (002) planes of CoSe2 and MoSe2, respectively (Qu et al., 2016; Liu et al., 2017), demonstrating the successful formation of the CoSe2/MoSe2 after the selenization reaction.

Figure 2

FE-SEM images of (A) CoMoO4 and (B,C) CoSe2/MoSe2; (D) elemental maps of CoSe2/MoSe2; (E,F) TEM of CoSe2/MoSe2; (G) HR-TEM images of CoSe2/MoSe2.

FE-SEM images of (A) CoMoO4 and (B,C) n class="Chemical">CoSe2/MoSe2; (D) elemental maps of CoSe2/MoSe2; (E,F) TEM of CoSe2/MoSe2; (G) HR-TEM images of CoSe2/MoSe2.

To investigate phase evolution during the selenization process, the crystal structure and phase composition of the obtained samples were characterized by X-ray diffraction (XRD) analysis (Figure 3A). The diffraction peaks of CoMoO4 precursor (in the black line) can be well indexed to the CoMoO4 phase (JCPDS No: 21-0868) (Wang et al., 2016). After the thermal reduction, some new diffraction peaks can be observed. The diffraction peaks at around 13.7 °, 27.6 °, 31.4 °, and 37.8 ° can be assigned to the MoSe2 phase (JCPDS No: 77-1715) (Qu et al., 2016), while the other peaks could be attributed to the phase of CoSe2 (JCPDS No:53-0449) (Liu et al., 2017). The XRD result clearly manifests the successful phase separation of the CoSe2 and MoSe2 from the CoMoO4 precursor via the selenization process.

Figure 3

(A) XRD patterns of CC, CoMoO4, and CoSe2/MoSe2; high-resolution XPS spectra of (B) Mo 3d, (C) Co 2p, and (D) Se 3d of CoSe2/MoSe2.

(A) XRD patterns of CC, CoMoO4, and n class="Chemical">CoSe2/MoSe2; high-resolution XPS spectra of (B) Mo 3d, (C) Co 2p, and (D) Se 3d of CoSe2/MoSe2.

X-ray photoelectron spectroscopy (XPS) men class="Chemical">asurement was carried out to analyze the composition and chemical state of as-prepared samples. (Figure 3B) illustrates the high-resolution Co 2p peaks at 778.8 eV (Co 2p3/2), 793.7 eV (Co 2p1/2), 780 eV (Co 2p3/2), and 796 eV (Co 2p1/2), corresponding to CoSe2 and cobalt–oxide bond, while those peaks at 784.1 eV and 801.5 eV are the satellite peaks (Mu et al., 2016; Wang et al., 2017b; Gao et al., 2018). Furthermore, the fine Mo 3d XPS spectrum (Figure 3C) shows the main peaks at 228.8 eV and 231.9 eV, which represent the Mo 3d5/2 and Mo 3d3/2 of MoSe2 (Wang et al., 2018a,b). Additionally, the peak located at 230 eV can be ascribed to the Se 3s of MoSe2 (Zhao et al., 2018). The Se 3d XPS spectrum (Figure 3D) displays the characteristic of CoSe2 and MoSe2 at 54.5 eV and 55.4 eV in agreement with the Se 3d5/2 and Se 3d3/2, respectively (Gao et al., 2018). Moreover, the peak at around 59.8 eV is confirmed to correspond to the selenium–oxygen bond (Kong et al., 2014). According to these results, the selenization process induced the phase separation from CoMoO4 into the nanoscale CoSe2 and MoSe2.

It is generally recognized that highly efficient electrocatalysts worked in n class="Chemical">alkaline solution is the bottleneck for large-scale application of overall water splitting. Linear sweep voltammetry (LSV) at a scanning rate of 5 mV s−1 was characterized by the electrocatalytic HER and OER capacities of the samples by a three-electrode system in 1 M KOH solution with saturated N2. By contrast, CoSe2, MoSe2 (Figure S1), RuO2, and Pt/C catalysts were performed in the same condition. The HER polarization curves and corresponding Tafel slops are depicted in (Figures 4A,B). The overpotential (η10) and Tafel slope of CoSe2/MoSe2 are 90 mV and 84.8 mV·dec−1, which are better than those of CoMoO4 (277 mV, 123.6 mV·dec−1), CoSe2 (205 mV, 195.2 mV·dec−1), and MoSe2 (199 mV, 152.4 mV·dec−1). CoSe2 has a metallic character, which can promote the dissociation of water and provide protons under alkaline conditions, thus improving the HER performance of MoSe2 (Kwak et al., 2016). In addition, the hierarchical nanosheet array assembled by the CoSe2/MoSe2 provides abundant active sites for the electrochemical reaction at the phase interface, which can further enhance the HER performance (Zhang et al., 2017a). Therefore, the CoSe2/MoSe2 catalyst exhibits improved HER performance benefiting from the synergistic effect. The catalyst of Pt/C illustrates an overpotential (η10) (59 mV) and Tafel slope (36.9 mV·dec−1) in 1 M KOH that are similar to those in other literatures (Chen et al., 2018b; Wan et al., 2018). Moreover, the overpotential of CoSe2/MoSe2 is superior to those of recently reported selenide catalysts such as NiSe NWs/Ni Foam (96 mV) (Tang et al., 2015), EG/cobalt selenide/NiFe–LDH (260 mV) (Hou et al., 2016), o-CoSe2/P (104 mV) (Zheng et al., 2018), CoSe2 NCs (520 mV) (Kwak et al., 2016), Co0.75Ni0.25Se/NF (106 mV) (Liu et al., 2019), 1T MoSe2/NiSe (120 mV) (Zhang et al., 2019), and SWCNTs/MoSe2 (219 mV) (Najafi et al., 2019) (Table S1).

Figure 4

(A) HER polarization curves of CoMoO4, CoSe2/MoSe2, CoSe2, MoSe2, and Pt/C; (B) Tafel slopes; (C) galvanostatic of CoSe2/MoSe2 for HER; (D) OER polarization curves of CoMoO4, CoSe2/MoSe2, CoSe2, MoSe2, and RuO2; (E) Tafel slopes in OER; (F) galvanostatic of CoSe2/MoSe2 for OER.

(A) HER polarization curves of CoMoO4, n class="Chemical">CoSe2/MoSe2, CoSe2, MoSe2, and Pt/C; (B) Tafel slopes; (C) galvanostatic of CoSe2/MoSe2 for HER; (D) OER polarization curves of CoMoO4, CoSe2/MoSe2, CoSe2, MoSe2, and RuO2; (E) Tafel slopes in OER; (F) galvanostatic of CoSe2/MoSe2 for OER.

The electrocatalytic OER properties are determined by LSV and polarization measurements n class="Chemical">as shown in (Figures 4D,E). The CoSe2/MoSe2 catalyst shows a lower overpotential (η10 of 280 mV) than those of the CoMoO4 (352 mV), CoSe2 (322 mV), MoSe2 (404 mV), and RuO2 (318 mV), respectively. More importantly, the OER performance of the designed CoSe2/MoSe2 sample exceeds those of recently reported selenide catalysts in OER, for instance, the Ag–CoSe2 (320 mV) (Zhao et al., 2017), CoSe2 NCs (430 mV) (Kwak et al., 2016), CoSe2/DETA (392 mV) (Guo et al., 2017), NiCo2Se4 holey nanosheets (295 mV) (Fang et al., 2017), NiSe–Ni0.85Se/CP (300 mV) (Chen et al., 2018a), SWCNTs/MoSe2 (295 mV) (Najafi et al., 2019), 1T/2H MoSe2 (397 mV) (Li et al., 2019), and CoSe2@MoSe2 (309 mV) (Chen et al., 2019c) (Table S2). Furthermore, the corresponding Tafel slope of CoSe2/MoSe2 is 86.8 mV·dec−1, which is smaller than those of the CoMoO4 (101.8 mV·dec−1), CoSe2 (124 mV·dec−1), MoSe2 (130 mV·dec−1), and RuO2 (93.4 mV·dec−1). The CoSe2/MoSe2 has lower overpotential and smaller Tafel, which can be attributed to its unique hierarchical heterostructure, facilitating electron transfer and accelerating OER kinetics. In this heterostructure, the transfer of electrons from CoSe2 phase to MoSe2 phase in the CoSe2/MoSe2 interface can result in electron-poor Co species and electron-rich Mo species (Liu et al., 2018). It is believed that the Se anion can affect the electron transfer between Co and Mo species, which is important for boosting catalytic ability (Yan et al., 2019). Besides, the formation of CoOOH is the primary cause to promote OER activity (Liu et al., 2015), and the increased 3d−4p repulsion between the center of the metal d band and the center of the p band of the Se site further promotes the rapid transfer of dioxygen molecules, thus improving OER performance (Li et al., 2017).

To understand the effects of the structure and composition of prepared catalyst on the electrochemical performance, several n class="Chemical">CoSe2/MoSe2 catalysts were collected at different selenization temperatures and the HER and OER performance were evaluated by LSV analysis (Figure S2). It can be seen that the CoSe2/MoSe2 sample obtained at 450°C (CoSe2/MoSe2-450) possesses better electrocatalytic properties than other counterparts, which can be ascribed to its superior structure. As shown in (Figure S3), with the selenization temperature increasing, the size of nanoparticles on the surface of nanosheets increased as well, indicating higher crystallinity. Generally, larger particle size will reduce the active surface of catalyst (Zhang et al., 2017b; Chen et al., 2019a). Therefore, when the selenization temperature elevated to 500°C (CoSe2/MoSe2-500), the catalytic performance slightly declined owing to its larger particle size and lower active surface. In addition, (Figure S4) displays the composition of the CoSe2/MoSe2 catalysts achieved at a different selenization temperature. As can be seen, when the selenization process proceeded at low temperature, the obtained CoSe2/MoSe2 catalyst has poor MoSe2 phase and low crystallinity, which are responsible for the poor electrochemical catalytic performance of the catalysts (CoSe2/MoSe2-350 and CoSe2/MoSe2-400). Therefore, the catalyst synthesized at 450°C shows the best performance, benefiting from the appropriate crystal structure and phase composition.

The electrochemically active surface area (ECSA) of as-prepared catalyst wn class="Chemical">as evaluated by the double-layer capacitance (C), which was measured by CV in a non-Faradaic reaction potential range (Deng et al., 2015). The C values of the CoSe2/MoSe2 (1.6 mF cm−2) is higher than those of CoSe2 (0.63 mF cm−2) and MoSe2 (0.8 mF cm−2), as shown in (Figure 5), suggesting more active sites of the CoSe2/MoSe2 catalyst. Furthermore, the smaller R value for the CoSe2/MoSe2 catalyst in the EIS measurement (Figure S5) implies the promoted charge transfer and boosted kinetics, which can be ascribed to the abundant interfaces and synergetic effect between the CoSe2 and MoSe2.

Figure 5

Electrochemical double-layer capacitance with the CV curves acquired at different scanning rates from 40, 60, 80, 100, and 120 mV s−1: (A) CoSe2/MoSe2, (B) CoSe2, and (C) MoSe2; (D) current densities (Δj = janode – jcathode, at 0.82 V) as a function of scanning rates of CoSe2/MoSe2, CoSe2, and MoSe2 with the corresponding slope being twice that of the Cdl values.

Electrochemical double-layer capacitance with the CV curves acquired at different scanning rates from 40, 60, 80, 100, and 120 mV s−1: (A) CoSe2/n class="Chemical">MoSe2, (B) CoSe2, and (C) MoSe2; (D) current densities (Δj = janode – jcathode, at 0.82 V) as a function of scanning rates of CoSe2/MoSe2, CoSe2, and MoSe2 with the corresponding slope being twice that of the Cdl values.

The structural stability is another significant parameter for catalysts in HER and OER. (Figures 4C,F) show a galvanostatic for CoSe2/n class="Chemical">MoSe2 catalyst in both the HER and OER processes. The morphology and composition of the catalyst after galvanostatic cycling are characterized by SEM and XPS. The CoSe2/MoSe2 could well inherit the pristine sheet-like structure, demonstrating good structural stability. In addition, the fine XPS spectra of the Co 2p, Mo 3d, Se 3d acquired from the sample of CoSe2/MoSe2 after galvanostatic measurement confirm the reservation of CoSe2 and MoSe2 (Figure S6), indicating phase stability during the electrochemical reactions.

To investigate its practical application of the obtained catalyst, an overall water splitting electrolyzer is n class="Chemical">assembled with CoSe2/MoSe2 as electrodes in 1 M KOH. It can decompose water at a low cell voltage of 1.63 V (current density at 10 mA·cm−2) (Figure 6A), and the efficiency is similar to those constituting of the noble-metal-based cathode and anode (RuO2 vs. Pt/C). Moreover, the overall water splitting performance of the CoSe2/MoSe2 is better than those of other recently reported non-noble metals at the same current density, such as (Ni,Co)0.85Se NSAs (1.65 V) (Xiao et al., 2018), a-CoSe/Ti mesh (1.65 V) (Liu et al., 2015), CoOx-CoSe (1.64 V) (Xu et al., 2016), Co0.85Se@NC (1.76 V) (Meng et al., 2017), CoB2/CoSe2 (1.73 V) (Guo et al., 2017), NiSe2/Ni (1.64 V) (Zhang et al., 2018), 1T/2H MoSe2/MXene (1.64 V) (Li et al., 2019), and Ni3Se2/CF (1.65 V) (Shi et al., 2015) (Table S3). Additionally, CoSe2/MoSe2 electrolyzer exhibits a slight increase in the potential after being cycled for 12 h in alkaline solution (Figure 6B).

Figure 6

(A) LSV curves of water splitting with CoSe2/MoSe2 as the anode and cathode; (B) galvanostatic testing of the CoSe2/MoSe2-based water splitting electrolyzer for 12 h at 10 mA cm−2.

Conclusion

In summary, a novel hierarchical 0D−2D Co/Mo n class="Chemical">selenide was developed by a facile in situ phase separation strategy. Benefiting from its unique structure and composition, the constructed CoSe2/MoSe2 catalyst exhibits small η10 of 280 mV and 90 mV and Tafel slopes of 86.8 mV·dec−1 and 84.8 mV·dec−1 for OER and HER, respectively. Furthermore, the electrolyzer comprising CoSe2/MoSe2 as the bifunctional catalyst shows a small water splitting cell voltage of 1.63 V at a current density of 10 mA·cm−2. This work provides insights into rational design and development of economical and valid bifunctional catalysts for overall water splitting.

Data Availability Statement

All datasets generated for this study are included in the article/Supplementary Material.

Author Contributions

LX implemented the experiment, analyzed the data and wrote the article. HS, XL, XZ, and BG participated in the formulation of the experimental scheme. YZ, KH, and PC revised the article.

Conflict of Interest

The authors declare that the research was n class="Chemical">conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.