HER monitoring and kinetics at pH 0. Figure presents the operando XANES spectra of L-CoS for the HER in 0.5 M H2SO4. For pristine L-CoS, two important features are present in the rising absorption edge: (i) the quadrupolar transition from Co 1s to Co 3d orbitals induces a spectral signature at ca. 7710 eV and its peak intensity can be strongly affected by the variations of local Co geometry; (ii) a significant shoulder peak is present at ca. 7718 eV, assigned to the dipolar transitions from Co 1s to Co 3d-S 2p hybridized orbitals due to the prevailing Co-S bonds in L- CoS. Upon immersing the L-CoS into the electrolyte and applying the anodic potentials, the line shape of the rising absorption edge of L-CoS has features identical to pristine L-CoS, confirming an identical local electronic structure under the acidic HER conditions. The Co valence state was unraveled by the investigation of Co K-edge position shifts at different potentials (Figure ). The results demonstrate that in situ formed low-valent Co species dominate the HER kinetics of L-CoS under acidic conditions. Furthermore, the local coordination environments of Co centers in L-CoS were evaluated by operando EXAFS spectra (Figure ). Note that no obvious oscillation changes were found in the operando EXAFS spectra of L-CoS, corroborating that the Co-S bonds remain mostly unchanged during the acidic HER. Besides, the corresponding operando 2D FT-EXAFS contour plots (Figure ) of L-CoS only feature one intensity maximum at R + ΔR of 1.81 Å (Co-S pairs), and this signature exhibits changes in its profiles when the applied potentials were decreased. This implies that the adsorption/desorption of key HER intermediates mainly proceeds on the Co-S motifs. This conclusion is further supported by operando WTcontour plots analysis (Figure ). More specifically, similar scattering patterns, namely of Co-S bonds, are present in L-CoS during the HER. As the applied potentials decreased, only slight changes are found in the relative magnitude of the most intense maximum, indicating the key role of the S-Co-S motif in the HER. Fitting of the operando FT-EXAFS spectra of L-CoS (Figure , Figure , and Table ) indicates a slight decrease in the CNCo-S value when the catalysts are immersed into the electrolyte, which is mainly ascribed the electrolyte interaction effects and the removal of surface oxidized species. However, the CNCo-S value remains almost identical during the HER. However, the results show that the mean interatomic distance of Co-S firstly displays a change toward a shorter distance above -60 mV vs RHE, but thereafter, it turns to a longer distance a further decrease in the applied potential. A maximum interatomic distance of Co-S with 2.269 Å was obtained at -135 mV vs. RHE. After the removal of the applied potential, the reoxidation of Co further induces a shorter interatomic distance of Co-S, inferring that low-valent Co-S moieties play a key role in the OER of L-CoS in the HER (Figure and Figure ).