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 (H 2 ) from renewable sources. 5−7 To achieve the large-scale and efficient production of H 2 , it is necessary and vital to develop highperformance 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-noblemetal 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 delectron 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 CoSe 2 , one is a cubic phase (c-CoSe 2 ) and the other is an orthorhombic phase (o-CoSe 2 ). 19,20 It has been confirmed in Xie's report that the realization of a structure transformation from o-CoSe 2 to c-CoSe 2 can enhance alkaline HER performance. 21 The obtained c-CoSe 2 /C needs an over-potential 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-CoSe 2 /C. Density functional theory (DFT) calculations reveal that both the change in Gibbs free energy (ΔG H* ) of c-CoSe 2 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-CoSe 2 induced by doping has shown HER activity superior to that of the original c-CoSe 2 . 22 The P-doped o-CoSe 2 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-CoSe 2 to m-Co 3 Se 4 (monoclinic phase Co 3 Se 4 ), attributed to the addition of Cu(II) ions. The resulting Cu-14-Co 3 Se 4 /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-CoSe 2 in some reports has been improved. 24−26 There has been very little research work on o-CoSe 2 as an electrocatalyst for the alkaline HER which will impede its further application in water splitting.
Herein, we demonstrate the structural transformation from flowerlike Co 0.85 Se nanosheets (denoted Co 0.85 Se NSS) to o-CoSe 2 nanosheets (denoted o-CoSe 2 NSS) with an improved alkaline HER performance through a simple heat treatment strategy. DFT calculations suggest that the change from Co 0.85 Se NSS to o-CoSe 2 NSS can modulate the intrinsic electronic structure, which can benefit the kinetics of the HER process. As a result, the prepared o-CoSe 2 NSS exhibits a better electrocatalytic activity in 1.0 M KOH with an overpotential of 220 mV instead of the 286 mV needed by Co 0.85 Se NNS to reach 10 mA cm −2 . This work provides a broader perspective to develop CoSe 2 materials with high activity by a simple and controllable structure transformation method.

EXPERIMENTAL SECTION
2.1. Synthesis of ZIF-67 Precursor. In a typical process for the synthesis of ZIF-67 (reported in a previous article 24  2.3. 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.
2.4. 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 (C dl ), 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 C dl . All potential measurements were calibrated against the reversible hydrogen electrode (RHE).
2.5. 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 (E ads ) were calculated using the formula E ads = E ad/sub − E ad − E sub , where E ad/sub , E ad , and E sub 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 + ΔE ZPE − TΔS, where ΔE, ΔE ZPE , and ΔS are the binding energy, the zero-point energy, and the entropy change, respectively.

RESULTS AND DISCUSSION
Flowerlike o-CoSe 2 NSS was prepared by the regeneration of flowerlike Co 0.85 Se NSS through a heat treatment under an Ar atmosphere. Figure 1 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 Co 2+ and 2-methylimidazole in an aqueous solution, as shown in Figures S1 and S2. 27 After that, a   Figure 3e, the highresolution Co 2p spectrum consists of two pairs of spin− orbital peaks of Co 3+ (778.9 eV for 2p 3/2 and 794.6 eV for 2p 1/2 ) and Co 2+ (780.7 eV for 2p 3/2 and 796.6 eV for 2p 1/2 ), accompanied by two broad satellite peaks of Co 2P 3/2 and Co 2p 1/2 . 26,36 Moreover, the peaks of an Se 3d spectrum in Figure  3f at 54.2 and 55.6 eV can be attributed to Se 3d 5/2 and 3d 3/2 . 26,37 The remaining two peaks at 58.8 and 60.7 eV can be assigned to Co 3p and SeO x , respectively. 26,36 The characterizations and analysis of XPS further demonstrate the successful preparation of o-CoSe 2 . In addition, an XPS analysis of Co included in Co 0.85 Se was also performed, as shown in Figure  S3e. The two sets of peaks of Co 2p corresponded to Co 3+ (780.6 eV for 2p 3/2 and 792.7 eV for 2p 1/2 ) and Co 2+ (779.8 eV for 2p 3/2 and 795.7 eV for 2p 1/2 ), 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 3d 5/2 and 3d 3/2 , respectively. 40 The last two peaks at 58.7 and 60.4 eV are ascribed to Co 3p and SeO x , respectively. 40 The electrochemical performance of Co 0.85 Se and o-CoSe 2 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 4a shows the linear sweep voltammetry (LSV) polarization curves of Co 0.85 Se and o-CoSe 2 NNS, which are the typical characteristics to measure catalytic HER performance. A smaller overpotential of 220 mV for o-CoSe 2 in comparison to that of 286 mV for Co 0.85 Se 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-CoSe 2 , which is obviously smaller than that of 133 mV dec −1 for Co 0.85 Se (Figure 4b). The small Tafel slope is beneficial for practical applications as the overpotential increases slightly to achieve a greater current density. Hence, o-CoSe 2 has been confirmed to possess an enhanced alkaline HER activity in comparison with Co 0.85 Se. EIS was used to characterize the electrocatalytic kinetics of interfacial charge transfer, and the charge transfer resistance (R ct ) was obtained from Nyquist plots, as shown in Figure 4c. A smaller R ct value suggests an improved charge transfer ability along with superior electrode kinetics of o-CoSe 2 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 C dl , which is proportional to the ECSA. Obviously, the value of C dl for o-CoSe 2 NNS (13.4 mF cm −2 ) is larger than that of 2.5 mF cm −2 for Co 0.85 Se NNS ( Figure S5c). Clearly more active sites are exposed after the transformation of o-CoSe 2 from Co 0.85 Se, which contribute to the superior HER activity. Continuous HER tests of o-CoSe 2 and Co 0.85 Se NSS at a static overpotential were conducted (Figure 4d), which shows that the current density hardly changes after 20 h in an alkaline medium. In addition, the LSV curves of o-CoSe 2 and Co 0.85 Se NSS before and after the stability tests also exhibit negligible changes ( Figure S5d). Both of these results illustrate that the o-CoSe 2 NNS sample has excellent stability for the alkaline HER.
The  (Figure 5c,d) has also been calculated. The value of the d-band center of o-CoSe 2 is −2.154 eV, which is downshifted from the Fermi level in comparison with Co 0.85 Se (−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-CoSe 2 NNS derived from Co 0.85 Se NNS tends to exhibit the expected result with a better HER activity, which is consistent with the electrocatalytic performance detailed above.

CONCLUSIONS
In summary, the temperature-induced structural transformation from Co 0.85 Se NNS to o-CoSe 2 NNS has been reported. Such materials inherit the original nanosheet structure well after calcination. Importantly, o-CoSe 2 NNS possesses an enhanced HER performance with an overpotential of 220 mV