Temperature-Induced Structure Transformation from Co0.85Se to Orthorhombic Phase CoSe2 Realizing Enhanced Hydrogen Evolution Catalysis.

Transition-metal chalcogenides (TMC) have been widely studied as active electrocatalysts toward the hydrogen evolution reaction due to their suitable d-electron configuration and relatively high electrical conductivity. Herein, we develop a feasible method to synthesize an orthorhombic phase of CoSe2 (o-CoSe2) from the regeneration of Co0.85Se, where the temperature plays a key role in controlling the structure transformation. To the best of our knowledge, this is the first report about this synthetic route for o-CoSe2. The resulting o-CoSe2 catalysts exhibit enhanced hydrogen evolution reaction performance with an overpotential of 220 mV to reach 10 mA cm-2 in 1.0 M KOH. Density functional theory calculations further reveal that the change in the Gibbs free energy of hydrogen, water adsorption energy, and the downshifted d-band center make o-CoSe2 more suitable for accelerating the HER process.

Introduction

On account of the global energy crisis and environmental issues, hydrogen has been extensively considered to hold great promise for the development of alternative and sustainable energy sources to replace fossil fuels.[1−4] Water electrolysis is one attractive technology to produce hydrogen (H2) from renewable sources.[5−7] To achieve the large-scale and efficient production of H2, it is necessary and vital to develop high-performance electrocatalysts toward the hydrogen evolution reaction (HER).[8−10] Up to now, noble metals (such as Pt and Ru) have been deemed to be the state of the art electrocatalysts for HER, but their high cost and scarcity have severely hindered the widespread industrial application.[11−13] Thus, it is desirable to seek low-cost non-noble-metal catalysts to boost the HER process efficiently.

As one of the attractive alternatives, transition-metal chalcogenides (TMC) have been widely investigated as highly active electrocatalysts in many studies due to their suitable d-electron configuration and relatively high electrical conductivity.[14−18] Among them, cobalt selenides can be considered to be a special case due to their unique synthesis method of structure transformation engineering. For instance, there are two phases of CoSe2, one is a cubic phase (c-CoSe2) and the other is an orthorhombic phase (o-CoSe2).[19,20] It has been confirmed in Xie’s report that the realization of a structure transformation from o-CoSe2 to c-CoSe2 can enhance alkaline HER performance.[21] The obtained c-CoSe2/C needs an overpotential of only 190 mV to achieve a current density of 10 mA cm–2, while a higher overpotential of 270 mV is needed for o-CoSe2/C. Density functional theory (DFT) calculations reveal that both the change in Gibbs free energy (ΔGH*) of c-CoSe2 and the water adsorption energy are optimized, which can strengthen the electrocatalytic activity of the catalyst for HER. Structure transformation engineering has also been achieved by controlling the level of P-doping, and the o-CoSe2 induced by doping has shown HER activity superior to that of the original c-CoSe2.[22] The P-doped o-CoSe2 catalyst possesses an optimized electronic structure and local coordination environment after the structure transformation, which leads to a substantial energetic benefit for the HER. Furthermore, a structure transformation has been realized from o-CoSe2 to m-Co3Se4 (monoclinic phase Co3Se4), attributed to the addition of Cu(II) ions. The resulting Cu-14-Co3Se4/GC catalyst with a trace amount of Cu shows enhanced HER, OER, and ORR activities.[23] Therefore, an in-depth investigation into the structural change and the corresponding electrocatalytic performance of cobalt selenides might be beneficial to design highly efficient catalysts toward HER. Furthermore, the OER activity of o-CoSe2 in some reports has been improved.[24−26] There has been very little research work on o-CoSe2 as an electrocatalyst for the alkaline HER which will impede its further application in water splitting.

Herein, we demonstrate the structural transformation from flowerlike Co0.85Se nanosheets (denoted Co0.85Se NSS) to o-CoSe2 nanosheets (denoted o-CoSe2 NSS) with an improved alkaline HER performance through a simple heat treatment strategy. DFT calculations suggest that the change from Co0.85Se NSS to o-CoSe2 NSS can modulate the intrinsic electronic structure, which can benefit the kinetics of the HER process. As a result, the prepared o-CoSe2 NSS exhibits a better electrocatalytic activity in 1.0 M KOH with an overpotential of 220 mV instead of the 286 mV needed by Co0.85Se NNS to reach 10 mA cm–2. This work provides a broader perspective to develop CoSe2 materials with high activity by a simple and controllable structure transformation method.

Experimental Section

Synthesis of ZIF-67 Precursor

In a typical process for the synthesis of ZIF-67 (reported in a previous article[24]), 5.85 g of Co(NO3)2·6H2O and 6.16 g of 2-methylimidazole (2-MeIm) were dissolved in 150 mL portions of methanol with continuous stirring until they fully dissolved to become two transparent solutions, denoted A and B. Then solution B was quickly poured into solution A and the mixed solution was stirred for 24 h at room temperature. Finally, a purple product was obtained by filtration with ethanol for several times and dried at 60 °C.

Synthesis of Co0.85Se NNS and o-CoSe2 NNS

Co0.85Se NNS

Co0.85Se NNS was obtained by a selenization process using a hydrothermal method. Typically, 50 mg of ZIF-67 powder was poured into 10 mL deionized water and mixed with ultrasound for 10 min. After that, the mixed solution was added to 15 mL deionized water containing sodium selenite (Na2SeO3). Moreover, 2 mL hydrazine hydrate (N2H4·H2O) was added in the above solution with 10 min of stirring. Then the mixed solution was transferred into a 50 mL Teflon-linked steel autoclave and kept at 200 °C for 20 h. The obtained black product was washed several times with deionized water and ethanol, respectively, and then dried at 60 °C.

o-CoSe2 NNS

o-CoSe2 NNS was synthesized by a simple heat treatment. The above Co0.85Se NNS product was placed in a tube furnace and annealed at 300 °C at a heating rate of 2 °C min–1 for 2 h under an Ar atmosphere.

Material Characterization

The composition and crystal structure of the obtained products were characterized by X-ray diffraction (XRD, Rigaku Smartlab3 instrument with Cu Kα radiation). The microstructure was investigated by transmission electron microscopy (TEM, JEOL, JEM-2200FS) and scanning electron microscopy (SEM, SUPRA 55, Zeiss). X-ray photoelectron spectroscopy (XPS) was carried out with a PHI 5000 Versa Probe III instrument with an Al Kα X-ray source.

Electrochemical Measurements

The electrochemical measurements were carried out on an electrochemical workstation (CHI 660E, CH Instruments Inc., Shanghai, China) in a typical three-electrode cell. The as-synthesized products served as the working electrodes, and a Hg/HgO electrode and a graphite rod were used as the reference electrode and the counter electrode, respectively. A 1 M KOH aqueous solution was used as the electrolyte. Polarization curves of HER were measured at a scan rate of 2 mV s–1 without iR compensation. Electrochemical impedance spectra (EIS) tests were performed at an overpotential of 275 mV from 100 kHz to 10 mHz. The electrochemical stability was determined by a chronoamperometric test. To evaluate the electrochemical double-layer capacitance (Cdl), the cyclic voltammetry (CV) curves were obtained at different scan rates (20–180 mV s–1) in the voltage ranges of −0.1 to 0 V without a Faradaic potential region. The electrochemical active surface area (ECSA) was directly proportional to Cdl. All potential measurements were calibrated against the reversible hydrogen electrode (RHE).

Theoretical Calculations

We used a first-principles method[28,29] to obtain the spin-polarization density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew–Burke–Ernzerhof (PBE) formulation.[30] The projector augmented wave (PAW) potentials[31,32] were employed to describe the ionic cores and take valence electrons into account using a plane wave basis set with a cutoff energy of 450 eV. Partial occupancies of the Kohn–Sham orbitals were obtained using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was taken as being self-consistent, while the energy change was smaller than 10–6 eV. A geometry optimization was supposed to be convergent when the energy change was smaller than 0.05 eV Å–1. The adsorption energies (Eads) were calculated using the formula Eads = Ead/sub – Ead – Esub, where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the gas phase, and the clean substrate, respectively. The free energies of elemental reaction steps were calculated by the computational hydrogen electrode model developed by Nørskov et al. The different free energies (ΔG) were calculated using the formula ΔG = ΔE + ΔEZPE – TΔS, where ΔE, ΔEZPE, and ΔS are the binding energy, the zero-point energy, and the entropy change, respectively.

Results and Discussion

Flowerlike o-CoSe2 NSS was prepared by the regeneration of flowerlike Co0.85Se NSS through a heat treatment under an Ar atmosphere. Figure shows the detailed synthesis process and its application in the alkaline hydrogen evolution reaction. First, ZIF-67 polyhedra were produced by a common synthetic method using Co2+ and 2-methylimidazole in an aqueous solution, as shown in Figures S1 and S2.[27] After that, a hydrothermal method was applied to synthesize Co0.85Se NSS by selenizing ZIF-67 polyhedra using Na2SeO3 as the selenium source. As shown in Figure a–c, the flower-like Co0.85Se material is approximately 1 μm in size and is composed of many nanosheets. Finally, o-CoSe2 NSS was derived from Co0.85Se NSS,maintaining the original nanosheet structure well through calcination at 300 °C under an Ar atmosphere and there was no obvious change in size, as shown in Figure d–f. Furthermore, the electrocatalytic performances of Co0.85Se NSS and o-CoSe2 NSS were measured in the alkaline hydrogen evolution reaction. To the best of our knowledge, this is a new and facile strategy for the synthesis of o-CoSe2 in comparison with the typical selenization methods.[33−35]

Figure 1

Schematic illustration of o-CoSe2 NSS derived from Co0.85Se NSS for the alkaline HER.

Figure 2

SEM images of (a–c) Co0.85Se NSS and (d–f) o-CoSe2 NSS.

XRD measurements were performed to characterize the crystalline structure of o-CoSe2 and Co0.85Se for a confirmation of the structure transformation. As shown in Figure S3, before the calcination, the characteristic diffraction peaks at 33.1, 44.6, and 50.4° can be well indexed to the (101), (102), and (110) planes for Co0.85Se (JCPDS No. 52-1008; crystal structure shown in Figure ) without any impurity peaks. The characteristic diffraction peaks observed in Figure a which are located at 30.7, 34.5, 35.9, and 47.7° correspond to the (011), (101), (111), (120) and (211) planes for the pure orthorhombic phase of CoSe2 (o-CoSe2, JCPDS No. 53-0445; crystal structure shown in Figure ) after the calcination. This strongly suggests that o-CoSe2 NNS is successfully transformed from Co0.85Se NNS after a calcination and the different ratios of Co and Se can probably be attributed to Se vacancies. The TEM image of o-CoSe2 (Figure b) shows that o-CoSe2 inherits the nanosheet structure of Co0.85Se (Figure S4a,b) well. Moreover, a high-resolution TEM (HRTEM) image of Co0.85Se (Figure S3c) only shows a set of lattice fringes corresponding to the (110) plane with an interplanar spacing of 0.18 nm, while the HRTEM image of o-CoSe2 in Figure c exhibits two sets of obvious lattice fringes with interplanar spacings of 0.30 and 0.38 nm, which can be well ascribed to its (011) and (110) planes, respectively. The corresponding energy dispersive spectrometry (EDS) element mapping images (Figure d and Figure S3d) of o-CoSe2 and Co0.85Se clearly show the uniform distribution of Co and Se, respectively. On the basis of the above analysis, o-CoSe2 NSS can be well transformed from the preprepared Co0.85Se NSS at a temperature of 300 °C, while the original nanosheet structure is maintained.

Figure 3

(a) XRD patterns of o-CoSe2 NNS and Co0.85Se NNS, (b, c) TEM and HRTE images of o-CoSe2 NNS (d) TEM image and the corresponding EDS element mappings of o-CoSe2, (e, f) high-resolution XPS spectra of Co 3p and Se 3d of o-CoSe2 NNS, respectively.

XPS was applied to investigate the elemental compositions and surface chemical states of o-CoSe2 and Co0.85Se (Figure e,f and Figure S3e,f). As shown in Figure e, the high-resolution Co 2p spectrum consists of two pairs of spin–orbital peaks of Co3+ (778.9 eV for 2p3/2 and 794.6 eV for 2p1/2) and Co2+ (780.7 eV for 2p3/2 and 796.6 eV for 2p1/2), accompanied by two broad satellite peaks of Co 2P3/2 and Co 2p1/2.[26,36] Moreover, the peaks of an Se 3d spectrum in Figure f at 54.2 and 55.6 eV can be attributed to Se 3d5/2 and 3d3/2.[26,37] The remaining two peaks at 58.8 and 60.7 eV can be assigned to Co 3p and SeO, respectively.[26,36] The characterizations and analysis of XPS further demonstrate the successful preparation of o-CoSe2. In addition, an XPS analysis of Co included in Co0.85Se was also performed, as shown in Figure S3e. The two sets of peaks of Co 2p corresponded to Co3+ (780.6 eV for 2p3/2 and 792.7 eV for 2p1/2) and Co2+ (779.8 eV for 2p3/2 and 795.7 eV for 2p1/2), respectively.[38,39] Then, the two broad peaks at 784.1 and 801.4 eV are attributed to satellite peaks similar to earlier reports.[38,39] The peaks of the Se 3d spectrum in Figure S3f at 54.1 and 55.1 eV can be attributed to Se 3d5/2 and 3d3/2, respectively.[40] The last two peaks at 58.7 and 60.4 eV are ascribed to Co 3p and SeO, respectively.[40]

The electrochemical performance of Co0.85Se and o-CoSe2 NNS was determined to evaluate the electrocatalytic HER activities using a standard three-electrode system in an aqueous alkaline solution (1.0 M KOH, pH 14). Figure a shows the linear sweep voltammetry (LSV) polarization curves of Co0.85Se and o-CoSe2 NNS, which are the typical characteristics to measure catalytic HER performance. A smaller overpotential of 220 mV for o-CoSe2 in comparison to that of 286 mV for Co0.85Se is required to reach the same current density of 10 mA cm–2. The obtained corresponding Tafel slope calculated after linear fitting is 107 mV dec–1 for o-CoSe2, which is obviously smaller than that of 133 mV dec–1 for Co0.85Se (Figure b). The small Tafel slope is beneficial for practical applications as the overpotential increases slightly to achieve a greater current density. Hence, o-CoSe2 has been confirmed to possess an enhanced alkaline HER activity in comparison with Co0.85Se. EIS was used to characterize the electrocatalytic kinetics of interfacial charge transfer, and the charge transfer resistance (Rct) was obtained from Nyquist plots, as shown in Figure c. A smaller Rct value suggests an improved charge transfer ability along with superior electrode kinetics of o-CoSe2 samples, which will induce faster electron transfer, further promoting the electrochemical HER activity. CV curves shown in Figure S5a,b were obtained at different scan rates to measure the double-layer capacitance Cdl, which is proportional to the ECSA. Obviously, the value of Cdl for o-CoSe2 NNS (13.4 mF cm–2) is larger than that of 2.5 mF cm–2 for Co0.85Se NNS (Figure S5c). Clearly more active sites are exposed after the transformation of o-CoSe2 from Co0.85Se, which contribute to the superior HER activity. Continuous HER tests of o-CoSe2 and Co0.85Se NSS at a static overpotential were conducted (Figure d), which shows that the current density hardly changes after 20 h in an alkaline medium. In addition, the LSV curves of o-CoSe2 and Co0.85Se NSS before and after the stability tests also exhibit negligible changes (Figure S5d). Both of these results illustrate that the o-CoSe2 NNS sample has excellent stability for the alkaline HER.

Figure 4

Electrochemical performance of Co0.85Se NNS and o-CoSe2 NNS in 1 M KOH for the HER: (a) polarization curves; (b) Tafel plots; (c) Nyquist plots; (d) stability tests at a constant overpotential.

The enhanced HER performance of o-CoSe2 NNS was further understood by correlating the intrinsic electronic structure and catalytic activity on the basis of density functional theory (DFT) calculations. Crucially, the effect of the o-CoSe2 sample on the free energy of hydrogen (reaction intermediate) adsorption and the water adsorption energy was studied, and their schematic models are displayed in Figures S6 and S7. ΔGH* (the value of hydrogen Gibbs free energy) is the most important characteristic to evaluate the HER activity of the catalyst,[41,42] as the optimal HER activity of an electrocatalyst should have a ΔGH value of around zero. It can be seen that the ΔGH* of Co sites in o-CoSe2 (0.117 eV) is much closer to the thermoneutral value (0) and smaller than that in Co0.85Se (0.285 eV) from Figure a, suggesting the higher activity of o-CoSe2, which is in good agreement with experimental HER results. In addition, the adsorption energy of an H2O molecule (ΔGH) on the catalyst is the other important parameter to evaluate HER performance in alkaline solution (Figure b). ΔGH values for Co0.85Se and o-CoSe2 are 0.584 and 0.321 eV, respectively; the lower ΔGH value of o-CoSe2 indicates that H2O is more easily adsorbed and activated on the surface of o-CoSe2 NNS to facilitate the HER process. Moreover, the density of states (DOS) of Co0.85Se and o-CoSe2 (Figure c,d) has also been calculated. The value of the d-band center of o-CoSe2 is −2.154 eV, which is downshifted from the Fermi level in comparison with Co0.85Se (−1.804 eV) after the regeneration. According to the d-band theory, a downshift of the d-band center will induce a weakening adsorption energy of H and accelerate the desorption of hydrogen on the catalyst surface, which is conducive to an improvement in HER activity.[43−48] In consideration of the theoretical investigations, o-CoSe2 NNS derived from Co0.85Se NNS tends to exhibit the expected result with a better HER activity, which is consistent with the electrocatalytic performance detailed above.

Figure 5

Theoretical calculations of o-CoSe2 and Co0.85Se: (a) free energy diagram for HER on Co sites; (b) calculated water adsorption energy on the surface of the catalysts; (c) the DOS of Co0.85Se; (d) the DOS of o-CoSe2.

Conclusions

In summary, the temperature-induced structural transformation from Co0.85Se NNS to o-CoSe2 NNS has been reported. Such materials inherit the original nanosheet structure well after calcination. Importantly, o-CoSe2 NNS possesses an enhanced HER performance with an overpotential of 220 mV at 10 mA cm–2. As expected, DFT studies also reveal that the d-band center downshift of o-CoSe2 can promote an improvement in HER activity, which is consistent with the electrocatalytic test results. It is believed that our new findings in this work can facilitate the design and development of o-CoSe2 as a highly efficient catalyst for the HER.