To gain in-depth insights into the intrinsic structure-activity relationships during water splitting, ex situ characterizations were first conducted for the catalysts before and after water electrolysis. Concerning the HER under acidic conditions, XANES spectra of L-CoS exhibit a similar spectral feature before and after the reaction, while a slight increase in the white line peak intensity (Figure Concerning the structural characterization of L-CoS after the alkaline HER (Figure ,d and Figure ), one can notice that a characteristic signature at ca. 7725 eV appears in the post-catalytic XANES spectrum, which is assigned to the strong contribution of the first shell Co-O. Meanwhile, a slightly positive energy shift in the rising absorption edge was observed in the post-catalytic samples, which corresponds to the generation of high-valent Co δ+ (δ > 0)species in L-CoS after the alkaline HER. Analysis of EXAFS and WT contour profiles show a decrease in the intensity signal of first shell Co-S backscattering after the HER, implying an underlying construction of partial O-coordinated Co centers in the post-catalytic L-CoS. Additionally, the post-catalytic L-CoS features a distinctive backscattering signal at R + ΔR of 2.82 Å in the FT-EXAFS spectra (Figure ), and the corresponding WT contour profile verifies that this newly formed scattering signature arises from the second shell Co-Co backscattering. From these results, one can conclude that the local electronic structure and coordination environments of Co centers in L-CoS undergo an irreversible structural evolution after the HER in 1 M KOH, inferring that the O incorporation into the structure plays a key role in the HER.