# Comparative Study on Hydrogen Evolution Reaction of Various Cobalt-Selenide-Based Electrocatalysts

Authors: Ha Huu Do; Quyet Van Le; Jin Hyuk Cho; Sang Hyun Ahn; Soo Young Kim
Journal: International Journal of Energy Research
 (2023)

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## Abstract

The growth of stable and efficient catalysts is vital for the electrochemical hydrogen evolution reaction (HER). Metal–organic frameworks (MOFs) have been recognized as ideal templates for fabricating efficient nanomaterial-based electrocatalysts for the HER. In this study, nitrogen-containing Co-MOF (ZIF-67), Co-MOF-74, and cobalt chloride salt were selenized to create various cobalt-selenide-based materials, i.e., cobalt selenide@nitrogen-doped carbon (CoSe2@NC), CoSe2@C, and CoSe2, respectively. The core–shell structure of CoSe2@NC originated from ZIF-67 exhibited better HER catalytic activity than those of CoSe2@C and CoSe2. CoSe2@NC exhibited a low overvoltage of 184 mV at 10 mA cm−2 and a small Tafel slope of 58.4 mV dec−1. In addition, this catalyst exhibited excellent durability while maintaining its performance after 12 h of testing. The high catalytic activity is ascribed to the integrated effect of the core–shell architecture, N-doped carbon, and large surface area, making protected active sites, high conductivity, and exposed active sites possible. The results demonstrate the efficiency of using MOFs as precursors for cobalt selenide fabrication and provide a potential synthetic strategy for noble-metal-free electrocatalysts for hydrogen production.


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## Body

## 1. Introduction

The overuse of fossil fuels is undoubtedly taking a heavy toll on the environment owing to carbon emissions and chemical waste. Therefore, finding sustainable and environmentally friendly energy sources is important [1, 2]. The electrochemical hydrogen evolution reaction (HER) is a noncarbon pathway that produces hydrogen to replace fossil fuels [3–6]. However, the high-energy requirement for water splitting impedes hydrogen formation at a high flow rate. Traditionally, catalysts have been applied for water electrolysis to reduce the energy barrier and accelerate reaction kinetics [7–9]. Pt-based materials are promising catalysts for HER [10–12]. For example, Chen et al. prepared a ternary Pt-Ni-Co electrocatalyst with a very small overvoltage [13]. Shen et al. created trimetallic Pt-Cu-Ni nanograins embedded on carbon fiber as an outstanding cathode, which exhibits an excellent HER performance in both acidic and alkaline electrolytes [14]. However, they are expensive and unstable under reaction conditions. Thus, the development of nonprecious electrocatalysts with excellent activity and outstanding durability is vital for industrial applications [15–17].Transition-metal selenides, such as MoSe2, WSe2, CoSe2, and NiSe2 [18–22], are Pt-free electrocatalysts with high performance for water dissociation, as proved in theoretical and experimental studies. Among these materials, CoSe2 exhibits a remarkable performance and high stability in various solutions. However, the agglomeration of CoSe2 nanograins, which usually occurs during the synthesis process, decreases the catalytic activity [23]. As a result, different approaches have been developed to increase the HER catalytic performance of CoSe2. For example, Kong et al. deposited CoSe2 nanograins on carbon cloth as a binder-free and stable electrode for accelerating HER activity [24]. Dai et al. fabricated a hollow CoSe2 structure by means of the Kirkendall effect, which exhibited a low overpotential [25]. Another strategy is to create CoSe2 nanoparticles from cobalt-based metal–organic frameworks (MOFs), which have the advantages of a large surface area, good distribution of metal nodes, and alterable chemical components [26]. For example, Zhou et al. created CoSe2 anchored on carbon nanotubes from a Co-based MOF for accelerating the HER [27]. Although several studies have employed MOFs as sacrificial templates to prepare CoSe2, no study has compared the HER performances of various CoSe2-based materials synthesized from multiple cobalt sources. Besides, Co-based MOF is usually pyrolyzed to form Co metal at relatively high temperatures, followed by selenization technique [28]. This process consumes a lot of energy, and the inheriting of MOF morphology is not well done. For example, Lu et al. pyrolyzed ZIF-67 into Co metal before converting it into CoSe2 nanograin implanted in an N-doped carbon (NC) skeleton for overall water dissociation [29].In this study, three precursors (ZIF-67, Co-MOF-74, and CoCl2.6H2O) were used to create CoSe2@NC, CoSe2@C, and CoSe2, respectively, through one-step selenization at low temperature. The CoSe2@NC polyhedrons inherited the regular morphological structure of ZIF-67, which had a larger surface area than CoSe2@C and CoSe2. In addition, nitrogen-doped carbon layers protect the CoSe2 nanograins from electrolyte influences, maintaining their durability after 12 h of operation. As a result, the HER activity of CoSe2@NC is higher than that of CoSe2@C and CoSe2, which is assigned to the integrated strategy of the core–shell architecture and nitrogen-doped carbon, as well as the high surface area. The results of this study prove that using MOFs as precursors is efficient for preparing cobalt selenide electrocatalysts for hydrogen evolution.


## 2. Experimental

### 2.1. Chemical and Materials

Co(NO3)2.6H2O, CoCl2.6H2O, 2,5-dihydroxyterephthalic acid (H4DHBDC), 2-methylimidazole, N, N-dimethylformamide, and 5% Nafion solution were supplied by Sigma-Aldrich. Ethanol and methanol were purchased from Alfa Aesar. Deionized (DI) water was obtained from a Millipore Milli-Q machine.


### 2.2. Fabrication of ZIF-67 and Co-MOF-74

#### 2.2.1. Fabrication of ZIF-67

0.718 g of Co(NO3)2.6H2O was stirred in 50 mL of methanol to obtain solution X. Also, 1.622 g of 2-methylimidazole was stirred in 50 mL of methanol to obtain solution Y. Solution X was poured into solution Y under magnetic stirring for 20 h at 25°C. Purple crystals were centrifuged four times with CH3OH and dried under vacuum at 60°C for 12 h.


#### 2.2.2. Fabrication of Co-MOF-74

First, 0.1 mmol of Co(NO3)2.6H2O and 0.05 mmol of H4DHBDC acid were added to a mixed solvent containing 3 mL of N, N-dimethylformamide, 3 mL of ethanol, and 3 mL of DI water. The reaction mixture was then stirred for 30 min and transferred to a glass vial (10 mL) before being placed in a furnace at 100°C. After 24 h, the glass vial is cooled to room temperature, and dark-purple crystals were obtained by centrifugation. These crystals were washed six times with methanol and dried under vacuum at 250°C. The final sample was preserved in vacuum condition.


### 2.3. Fabrication of CoSe2@NC, CoSe2@C, and CoSe2

Here, 0.1 g each of cobalt-containing sources ZIF-67, Co-MOF-74, and CoCl2.6H2O were mixed well with 0.1 g of selenium powder in a crucible and then annealed for 4 h at 350°C to create CoSe2@NC, CoSe2@C, and CoSe2, respectively.


### 2.4. Material Characterization

The morphological structure of the as-synthesized products was confirmed using scanning electron microscopy (SEM, Carl Zeiss), transmission electron microscopy (TEM, JEOL), and high-resolution TEM. X-ray diffraction (XRD) patterns were recorded using a Bruker D8-Advance device with Cu Kα radiation. Raman spectra of the composites were collected on a LabRAM-HR Evolution with a 532 nm laser. The elemental composition was determined using a K-alpha X-ray photoelectron spectrometry (XPS) system.


### 2.5. Electrochemical Measurements

The working electrodes were fabricated by coating 5 L of homogeneous suspension onto glassy carbon electrodes (radius of 1.5 mm) with a loading of 0.30 mg cm−2. This suspension was created by sonication of a mixture of 8 mg of material, 0.9 mL of DI water, 1 mL of ethanol, and 0.1 mL of Nafion. The electrochemical HER properties were assessed using a device (Ivium 55630) with a three-electrode system (Pt mesh as the counter electrode and saturated calomel electrode as the reference and working electrode) and 0.5-M H2SO4 solution. Polarization plots were reported with IR compensation at a scan rate of 2 mV s−1. Electrochemical impedance spectroscopy was implemented in the frequency mode from 105 to 0.1 Hz. Cyclic voltammograms (CVs) utilized for electrochemical double-layer capacitance (Cdl) determination were obtained in the nonfaradaic potential region at 25, 50, 75, 100, 125, and 150 mV s−1. The durabilities of the electrocatalysts were compared using chronoamperometric responses (12 h) at specific potentials. The reported voltages were changed to the reversible hydrogen electrode (RHE): ERHE=ESCE+E0SCE+0.059×pH.


## 2.1. Chemical and Materials

Co(NO3)2.6H2O, CoCl2.6H2O, 2,5-dihydroxyterephthalic acid (H4DHBDC), 2-methylimidazole, N, N-dimethylformamide, and 5% Nafion solution were supplied by Sigma-Aldrich. Ethanol and methanol were purchased from Alfa Aesar. Deionized (DI) water was obtained from a Millipore Milli-Q machine.


## 2.2. Fabrication of ZIF-67 and Co-MOF-74

### 2.2.1. Fabrication of ZIF-67

0.718 g of Co(NO3)2.6H2O was stirred in 50 mL of methanol to obtain solution X. Also, 1.622 g of 2-methylimidazole was stirred in 50 mL of methanol to obtain solution Y. Solution X was poured into solution Y under magnetic stirring for 20 h at 25°C. Purple crystals were centrifuged four times with CH3OH and dried under vacuum at 60°C for 12 h.


### 2.2.2. Fabrication of Co-MOF-74

First, 0.1 mmol of Co(NO3)2.6H2O and 0.05 mmol of H4DHBDC acid were added to a mixed solvent containing 3 mL of N, N-dimethylformamide, 3 mL of ethanol, and 3 mL of DI water. The reaction mixture was then stirred for 30 min and transferred to a glass vial (10 mL) before being placed in a furnace at 100°C. After 24 h, the glass vial is cooled to room temperature, and dark-purple crystals were obtained by centrifugation. These crystals were washed six times with methanol and dried under vacuum at 250°C. The final sample was preserved in vacuum condition.


## 2.2.1. Fabrication of ZIF-67

0.718 g of Co(NO3)2.6H2O was stirred in 50 mL of methanol to obtain solution X. Also, 1.622 g of 2-methylimidazole was stirred in 50 mL of methanol to obtain solution Y. Solution X was poured into solution Y under magnetic stirring for 20 h at 25°C. Purple crystals were centrifuged four times with CH3OH and dried under vacuum at 60°C for 12 h.


## 2.2.2. Fabrication of Co-MOF-74

First, 0.1 mmol of Co(NO3)2.6H2O and 0.05 mmol of H4DHBDC acid were added to a mixed solvent containing 3 mL of N, N-dimethylformamide, 3 mL of ethanol, and 3 mL of DI water. The reaction mixture was then stirred for 30 min and transferred to a glass vial (10 mL) before being placed in a furnace at 100°C. After 24 h, the glass vial is cooled to room temperature, and dark-purple crystals were obtained by centrifugation. These crystals were washed six times with methanol and dried under vacuum at 250°C. The final sample was preserved in vacuum condition.


## 2.3. Fabrication of CoSe2@NC, CoSe2@C, and CoSe2

Here, 0.1 g each of cobalt-containing sources ZIF-67, Co-MOF-74, and CoCl2.6H2O were mixed well with 0.1 g of selenium powder in a crucible and then annealed for 4 h at 350°C to create CoSe2@NC, CoSe2@C, and CoSe2, respectively.


## 2.4. Material Characterization

The morphological structure of the as-synthesized products was confirmed using scanning electron microscopy (SEM, Carl Zeiss), transmission electron microscopy (TEM, JEOL), and high-resolution TEM. X-ray diffraction (XRD) patterns were recorded using a Bruker D8-Advance device with Cu Kα radiation. Raman spectra of the composites were collected on a LabRAM-HR Evolution with a 532 nm laser. The elemental composition was determined using a K-alpha X-ray photoelectron spectrometry (XPS) system.


## 2.5. Electrochemical Measurements

The working electrodes were fabricated by coating 5 L of homogeneous suspension onto glassy carbon electrodes (radius of 1.5 mm) with a loading of 0.30 mg cm−2. This suspension was created by sonication of a mixture of 8 mg of material, 0.9 mL of DI water, 1 mL of ethanol, and 0.1 mL of Nafion. The electrochemical HER properties were assessed using a device (Ivium 55630) with a three-electrode system (Pt mesh as the counter electrode and saturated calomel electrode as the reference and working electrode) and 0.5-M H2SO4 solution. Polarization plots were reported with IR compensation at a scan rate of 2 mV s−1. Electrochemical impedance spectroscopy was implemented in the frequency mode from 105 to 0.1 Hz. Cyclic voltammograms (CVs) utilized for electrochemical double-layer capacitance (Cdl) determination were obtained in the nonfaradaic potential region at 25, 50, 75, 100, 125, and 150 mV s−1. The durabilities of the electrocatalysts were compared using chronoamperometric responses (12 h) at specific potentials. The reported voltages were changed to the reversible hydrogen electrode (RHE): ERHE=ESCE+E0SCE+0.059×pH.


## 3. Results and Discussion

A graphical illustration of the fabrication process is shown in Figure1. In the synthesis, the CoSe2, CoSe2@C, and CoSe2@NC electrocatalysts were created from CoCl2.6H2O, Co-MOF-74, and ZIF-67 precursors, respectively, through a selenization process. In the ZIF-67 precursor, CoSe2 nanoparticles were anchored on the N-doped carbon matrix, leading to good dispersion of the active sites. Co-MOF-74 also has CoSe2 nanograins embedded on carbon frameworks, whereas CoSe2 prepared from cobalt chloride does not contain a carbon framework, which could cause the aggregation of nanoparticles. The structural properties of the Co-MOF-74 and ZIF-67 precursors were analyzed using XRD, as shown in Figure S1. The typical peaks are well matched with those reported previously, revealing that Co-MOF-74 and ZIF-67 were successfully prepared [30–34]. The XRD patterns of all samples (Figure 2(a)) can be attributed to orthorhombic CoSe2. The intensive peaks at 28.97°, 30.78°, 34.52°, 35.96°, 47.72°, 50.23°, 53.48°, 56.95°, and 63.29° were indexed in the (011), (101), (111), (120), (211), (002), (031), (131), and (122) planes, respectively [35, 36]. Figure 2(b) shows the Raman spectra of the various cobalt selenide materials. CoSe2@NC and CoSe2@C showed peaks at 1345.5 and 1578.3 cm−1, attributed to the D and G bands of the carbon moieties. In addition, the vibrational frequencies of 174 and 667 cm−1 were indexed to the Ag and A1g stretching modes of CoSe2, respectively [27, 35]. More importantly, a high ID/IG ratio (1.02) implies that the codoping of Co and N creates rich defects of carbon in CoSe2@NC. They could introduce more active centers of Co-Nx moieties, leading to accelerated HER activity.Figure 1
Graphic illustration of the synthesis of CoSe2, CoSe2@C, and CoSe2@NC.Figure 2
(a) Powder XRD patterns of CoSe2 (black line), CoSe2@C (red line), and CoSe2@NC (blue line) and (b) Raman spectra of CoSe2 (black line), CoSe2@C (red line), and CoSe2@NC (blue line).
(a)(b)SEM was utilized to investigate the morphological architectures of various CoSe2 materials. Figures 3(a) and 3(b) show the microsphere structure of CoSe2 with a size of 100 nm, whereas CoSe2@C has a rod-like architecture (Figures 3(c) and 3(d)), identical to Co-MOF-74 (Figure S2a). This phenomenon occurs with CoSe2@NC, which has a polyhedron morphology and a uniform size of 300 nm (Figures 3(e) and 3(f)), inheriting the ZIF-67 structure (Figure S2b). However, the surface of CoSe2@NC is rough and slightly reduced in size. The excellent inheritance of the morphology and porosity of the MOF precursors helped CoSe2@NC (355.2 m2 g−1) and CoSe2@C (129.3 m2 g−1) provide higher BET surface areas than that of CoSe2 (38.7 m2 g−1), as shown in Figure S3. This had beneficial effects on CoSe2@NC in electron/mass transfer [37–39]. TEM and HR-TEM analyses were performed to analyze the morphology of the material further. As shown in Figures 4(a) and 4(b), the CoSe2 nanograins were implanted in N-doped carbon layers to produce a core–shell architecture of CoSe2@NC. In addition, the d-spacing of 0.306 nm was assigned to the (011) plane of CoSe2 (Figure 4(c)). Scanning transmission electron microscopy and elemental mapping show a good distribution of Co, Se, N, and C, as shown in Figure 4(d).Figure 3
SEM images of (a, b) CoSe2, (c, d) CoSe2@C, and (e, f) CoSe2@NC.
(a)(b)(c)(d)(e)(f)Figure 4
(a) TEM images of CoSe2@NC, (b) HR-TEM of CoSe2@NC, and (c) scanning TEM image with elemental mapping of CoSe2@NC.
(a)(b)(c)XPS was utilized to analyze the oxidation state and elemental component of CoSe2@NC. The high-resolution XPS spectra of Co 2p in Figure 5(a) show the binding energies of Co2+2p3/2 and Co2+2p1/2 at 780.6 and 796.9 eV, respectively [23]. Regarding the Se 3d spectra (Figure 5(b)), the binding energies at 55.7 and 54.8 eV are indexed to Se 3d3/2 and Se 3d5/2, respectively, indicating the presence of Se22− in CoSe2@NC [40]. In addition, a broad peak at 59.2 eV is indexed to SeOx [41]. The N 1s spectra in Figure 5(c) exhibit peaks at 400.8, 399.5, and 398.4 eV, corresponding to pyrrolic, Co-Nx, and pyridinic [42, 43]. N-doped carbon materials can enhance conductivity, leading to accelerated electrochemical catalytic activity [44–48]. A high-magnification XPS spectrum of C 1s is shown in Figure 5(d). Peaks at 286.3, 285.5, and 284.3 eV are attributed to N-C, -N=C, and C-C [49].Figure 5
X-ray photoelectron spectra of CoSe2@NC: (a) Co 2p, (b) Se 3d, and (c) N 1s; (d) C 1 s.
(a)(b)(c)(d)To prove that the use of MOF precursors to fabricate cobalt selenides is efficient in enhancing the HER catalytic activity, the electrochemical properties of CoSe2, CoSe2@C, and CoSe2@NC were analyzed using a three-electrode system. Figure 6(a) depicts the current–voltage plots of various materials at the same scan rate. The CoSe2@NC sample displayed the lowest overpotential (184 mV), achieving a current density of 10 mA cm−2, indicating better HER catalytic activity than CoSe2@C (220 mV) and CoSe2 (289 mV). The HER performance of CoSe2@NC can be comparable with the other CoSe2-based electrocatalysts (Table S1). Furthermore, CoSe2@NC gives a low Tafel slope of 58.4 mV dec−1, which are smaller values than those of CoSe2@C (71.1 mV dec−1) and CoSe2 (93.6 mV dec−1) (Figure 6(b)). These outcomes indicate that the HER mechanism of CoSe2-based electrocatalysts follows a Volmer–Heyrovesky reaction. Electrochemical impedance spectroscopy was analyzed on CoSe2, CoSe2@C, and CoSe2@NC at a voltage of −0.2 V to confirm the HER kinetics at the electrode–solution interface [50, 51]. As displayed in Figure 6(c), the electron transfer resistance (Rct) of CoSe2@NC (24.5 Ω) is lower than those of CoSe2@C (77.2 Ω) and CoSe2 (149.2 Ω), revealing that N-doped carbon can improve the electron transfer in the HER kinetics of CoSe2@NC. In particular, N-doping creates Co-Nx phases and N-C species, which are favorable for the adsorption of protons to create intermediate Hads and produce H2 molecules on catalyst surfaces, thus accelerating reaction kinetics [52–54]. Also, the N-doped carbon skeleton facilitates the well-distribution of CoSe2 nanograins and protects them in reaction conditions [28, 55, 56]. This could maximize accessible active centers and enhance the stability of the electrocatalyst. Moreover, the electrochemical surface area (ECSA) was predicted using the double-layer capacitance (Cdl) originated from the CV measurements (Figure S4). As displayed in Figure 6(d), the Cdl quantity of CoSe2@NC is 3.24 mF cm−2, which is larger than those of CoSe2@C (1.87 mF cm−2) and CoSe2 (1.04 mF cm−2), implying a larger ECSA and more active centers for hydrogen evolution (Table S2).Figure 6
HER performance: (a) polarization plots of CoSe2, CoSe2@C, CoSe2@NC, and Pt/C; (b) Tafel slopes of catalysts; (c) Nyquist plots (recorded with a voltage set at −0.2 V vs. RHE); (d) extracted double-layer capacitances from the CV of different catalysts.
(a)(b)(c)(d)To compare the intrinsic activities of the electrocatalysts, the turnover frequency (TOF) was determined at the HER overpotential. TOF of cobalt selenide-based electrocatalysts is determined according to a reported formula [57, 58].
(1)TOF=jAnFm,where j is the current density at a voltage of −200 mV (versus RHE) (A cm-2), A is the geometrical surface area of the working electrode (cm2), n is the number of electrons transferred to form a molecule of the product (for H2, it is 2), F is Faraday constant (C mol-1), and m is the number of moles of catalyst coating on the working electrode. As depicted in Figure 7(a), the TOF of CoSe2@NC exhibits a higher TOF of 0.0895 s-1, compared to those of CoSe2@C (0.0197 s-1) and CoSe2 (0.0045 s-1). This implies that CoSe2@NC exhibits a higher performance of the active centers in the electrocatalytic process. Furthermore, the chronoamperometric responses indicated that the as-synthesized CoSe2@NC had excellent stability (Figure 7(b)). CoSe2@NC had approximately 41% of the initial current density, whereas a 76% loss in current density was observed for the CoSe2 catalyst. Also, the decreased current density is attributed to the interference of the hydrogen bubble on the surface of materials. Also, the crystal architecture and morphology of CoSe2@NC did not change, which were verified by XRD and SEM after 12 h of testing (Figure S5). The results prove the efficiency of the core–shell structure in improving the stability of the catalysts. Considering the above evaluations, the high HER properties of result CoSe2@NC can be elucidated as follows: (1) the integrating effect of core-shell structure and high surface area of CoSe2@NC allow protecting CoSe2 nanograins, highly exposed active sites, and fast electron transport; (2) N-doping illustrates rich Co-Nx and N-C moieties, good conductivity, and more adsorbed protons, thus accelerating HER performance.Figure 7
(a) TOFs of CoSe2, CoSe2@C, and CoSe2@NC at an overpotential of 200 mV and (b) chronoamperometric plots of CoSe2, CoSe2@C, and CoSe2@NC at specific potentials for 12 h.
(a)(b)


## 4. Conclusion

The HER catalytic activities of cobalt selenides CoSe2@NC, CoSe2@C, and CoSe2, synthesized using different precursors, were compared. ZIF-67 was used as an N-containing cobalt source to produce the core–shell architecture of CoSe2@NC. The HER performance of CoSe2@NC was higher than those of CoSe2@C and CoSe2, which were generated from Co-MOF-74 and CoCl2.6H2O, respectively. In particular, CoSe2@NC only required a low overpotential of 184 mV to achieve 10 mA cm−2, whereas the values were 220 and 289 mV for CoSe2@C and CoSe2, respectively. This outcome was assigned to the integrated strategy of the core–shell architecture, rich Co-Nx active centers, and high surface area. In addition, the NC layers protected the CoSe2 nanograins from electrolyte influences, maintaining their durability after 12 h of operation. The results imply that using MOFs as sacrificial templates can efficiently prepare cobalt-selenide-based electrode materials for hydrogen evolution.



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*Source: 6016603-2023-02-03.xml*