HER monitoring and kinetics at pH 0. We first performed operando Co K-edge XAS characterizations for o-CoSe2 to monitor the chemical nature of Co centers during the acidic HER (Figure and and). From the operando Co K-edge XANES monitoring (Figure ), it is found that the spectral features at ca. 7725 eV, associated with the dipolar transitions from Co 1s to 4p hybridized orbitals, undergo a remarkable profile change along with the cathodic polarizations. In detail (Figure Operando Raman spectra characterizations were conducted to shed light on the surface kinetics of o-CoSe2 during the acidic HER. As demonstrated in Figure , the characteristic Se-Se librational vibrations were preserved well during the acidic HER. Besides, five newly formed Raman signatures appeared at 428, 592, 896, 983, and 1052 cm -1 can be also observed during the cathodic polarizations, originating from the adsorption of H2SO4 electrolytes on the electrode surface. Identification of the nature of adsorbed SeOx -species on o-CoSe2 under acidic conditions is challenging, as their vibration fingerprints overlapped with those of v(HSO4 -) (Figure ). To gain in-depth insights into the surface kinetics of o-CoSe2 during the acidic HER, inductively coupled plasma mass spectrometry (ICP-MS) analysis was performed for the electrolyte before and after the reaction. The results (Table ) showed a slight increase in the concentrations of Co and Se elements in the electrolyte after the HER, reflecting the dissolution of surface oxidized species into the electrolyte. Based on the above, one can conclude that during the acidic HER the investigated o-CoSe2 electrode experiences surface corrosion with the dissolution of surface oxidized oxyanion into the electrolytes, and thereafter the resulting electrode retains its structural integrity well for catalyzing the acidic HER (Figure ). HER monitoring and kinetics at pH 14. The origins of the intrinsic HER activities and the underlying reaction kinetics of o-CoSe under alkaline conditions were also investigated via operando XAS and Raman spectra characterizations (Figure and). Figure presents the operando time-resolved Co K-edge XANES spectra of o-CoSe2 during the HER at pH 14. One can notice that the characteristic spectral features of Co 1s to 4p hybridized orbitals display a disparate profile change during the alkaline HER compared to the observations during the acidic HER. In detail, from immersing the catalysts into the electrolyte to the alkaline HER, their peak intensities first undergo a significant enhancement along with an increased Co valence state (Figure and Figure ). Then, these key features decreased drastically as the applied potential decreased from 50 to -100 mV vs. RHE, meanwhile, the Co valence states decreased from +1.11 to +0.90. For the applied potentials below -100 mV, the spectral features of Co 1s to 4p hybridized orbitals remain almost identical, but they revert to higher signal intensities after removing the applied potentials. In comparison, the Co valence states exhibit only a slight decrease from +0.90 to +0.87 for the applied potentials in the range -100 to -250 mV vs. RHE; a further decrease of the applied potentials to -300 mV vs. RHE leads to a distinct decrease in the Co valence states to +0.81. Notably, after the removal of the applied potentials, a significant increase in the Co valence states with a maximum value of +1.19 is observed. All these results suggest that o-CoSe2 undergoes dynamic changes in the local coordination geometries of the catalytically active centers during the alkaline HER, in which the in situ generated low-valent Co species play a critical role in managing the underlying HER kinetics. ), it is found that the CNCo-Se value exhibits barely changes during the alkaline HER. In sharp contrast, the CNCo-O value shows a slightly increasing tendency upon immersion of the catalysts into the electrolyte, but thereafter it undergoes a continuous decreasing tendency, ultimately approaching zero under the cathodic polarizations. After the removal of the applied potentials, a significant increase in the CNCo-O value can be observed. These observations are also accordant with the above operando analysis on the Co valence states. On this basis, we propose that the investigated o-CoSe2 catalysts undergo the following structural rearrangement and the reaction kinetics during the alkaline HER (Figure ): (i) o-CoSe2 experience surface oxidation along with a surface interaction with the electrolytes during the electrochemical pre-activation process, as supported by the increased Co valence states and CNCo-O values; (ii) investigations of the operando time-resolved Co Kedge XAS data suggest that the HER proceeds along with an electrochemically driven structural rearrangement from the reconstructed oxidized species absorbed on the surface into the low-valent metallic Co species, which act as the true catalytically active species; (iii) after the removal of the applied potentials, these active metallic Co species can further interact with the electrolytes and revert into oxide-related species as the HER resting species. Such a surface interaction with the oxidized species further converts into active metallic Co species can be also evident from the operando Raman spectra analyses. As demonstrated in Figure , with the cathodic polarizations, the characteristic Se-Se librational and SeOx -signatures feature distinct profile changes towards weakened signal intensities. At the same time, a newly formed Raman signature at ca. 680 cm -1 , associating with the Co-O vibrations, was first observed when the applied potentials were below 100 mV vs. RHE. A further decrease of the applied potentials results in the disappearance of the aforementioned Co-O vibrations but generates newly formed Co-Se signatures. This points out that the initial reconstructed surface oxidized species on o-CoSe2 undergoes a further restructuring into active Co species during the HER. It is also noteworthy that during the alkaline HER o-CoSe2 features another significant Raman signature at ca. 252 cm -1 , which exhibits profile changes toward a higher signal intensity as the applied potential decreases. Previous studies indicated that the aforementioned vibrations feature a similar Raman fingerprint as amorphous Se species. Besides, after the removal of the applied potentials, a distinct v(SeO4 2-) Raman signal can be detected from the electrolytes. This observation is also consistent with the ICP-MS analysis (Table ), in which a significant content of Se ions can be observed in the electrolyte after the HER. With the above operando XAS and Raman results, one can conclude that o-CoSe2 undergoes a surface selenium-oxygen exchange reaction preceding the alkaline HER, and these in-situ generated oxidized species can further convert into catalytically active metallic Co species, accompanied by the leaching of oxyanion components into the electrolyte.