HER monitoring and kinetics at pH 14. The kinetics of local electronic structures and coordination environments of Co centers in L-CoS toward the alkaline HER were also investigated with operando XAS characterizations (Figure and Figures ). Figure represents the operando Co K-edge XANES spectra of L-CoS recorded from 50 to -300 mV vs. RHE at pH 14. Different from our operando monitoring of L-CoS at pH 0 (Figure ), the characteristic fingerprints of Co 1s to Co 3d and to Co 3d -S 2p transitions undergo an obvious profile change during the cathodic polarization (Figure and Figure ). In detail, their peak intensities decrease drastically after immersing the L-CoS into the electrolyte, indicating that a local structural change occurred at the initial S-Co-S-Co-S configurations. During the cathodic polarization from 0 to -250 mV, the local geometry of Co centers remains quite unchanged, as confirmed by only slight intensity fluctuation observed for the characteristic electronic transition features (Figure ). With a further decrease of the applied potential below -250 mV vs. RHE, a distinct increase in the signal intensity is evident (Figure ), especially for the fingerprint of Co 1s to Co 3d transitions. However, this signal reverts to a lower intensity after the removal of the applied potentials, suggesting that the local geometric evolution of Co centers dominates the HER of L-CoS. As discussed before, the fingerprint of Co 1s to Co 3d transitions in L-CoS shows a strong correlation with the local Co coordination geometries. In general, an increase in the peak intensity of this signal reflects the increment of [CoS6] octahedral distortion or the generation of additional metallic Co-Co pairs, for details cf. next section. Besides, for the catalysts immersed into the electrolyte, two extra characteristic fingerprints are observed at 7730 and 7746 eV in the XANES spectra of L-CoS (Figure ), which are attributed to dipolar transitions from Co 1s to Co 3d-O 2p hybridized orbitals and Co 1s to Co 4p orbitals, correspondingly. The presence of these characteristic features can be ascribed to the formation of Co-O-Co moieties in the structure. With a decrease in the applied potentials from 50 to -250 mV, these signatures undergo peak profile changes and their corresponding peak positions shift systematically to lower energy, verifying that a local structural rearrangement occurred in the Co-O-Co moieties. For the potentials in the range of -250 to -300 mV vs. RHE, no significant spectral features can be identified as the dipolar electronic transitions from the Co-O-Co moieties. However, after removing the applied potentials, distinct spectral features of the Co-O-Co moieties appear again and their corresponding peak intensity displays an increased tendency as a function of recording time (Figure ). Combination of all these operando XANES analyses firmly points out that L-CoS undergoes a multistep evolution in the geometric environment of its Co centers during the alkaline HER, and that the in situ generated low-valent Co species with strong metallic properties account for the observed HER activity.