Operando FT-EXAFS spectra characterizations were further conducted to directly evidence the kinetics of the local Co coordination environments of L-CoS during the alkaline HER. As shown in Figure , upon immersing into the electrolyte, the FT-EXAFS spectrum of L-CoS features a prominent second shell scattering at 2.48 Å, associated with the Co-Co1 backscattering. [Note: The second shell "Co-Co" path from the Co-O-Co motif shows two different radial distances of 2.48 and 2.84 Å, and they are referred to as "Co-Co1" and "Co-Co2", correspondingly. ] Notably, the above analysis of operando XANES (Figure ) spectra highlights the crucial role of O incorporation in tuning the local Co geometry of L-CoS during the HER. On this basis, the observed Co-Co1 backscattering features can be thus attributed to a local geometric change from the S-Co-S-Co-S to S-Co-O-Co1-S motifs (Figure and). During the cathodic polarization from 50 to -140 mV vs. RHE, the intensity of the Co-Co1 scattering changes toward weakened signals (Figure ). Meanwhile, significant second shell Co-Co2 scattering (at 2.84 Å -1 ) is first observed at -100 mV vs. RHE, but thereafter, it vanishes upon decreasing the applied potential to -250 mV vs. RHE. To gain further insights into these observed structural evolutions, fitting of the operando FT-EXAFS spectra was further carried out (Figure and Table ). The results (Figure and discussion and details in Figure ) demonstrate that the in situ formed S-Co-O-Co1-S motif undergoes a structural optimization into the S-Co-O-Co2-S motif in the potential range of -100 to -140 mV vs. RHE. Then, a further decrease in the applied potentials (lower than -250 mV vs. RHE) promotes a structural conversion of the newly formed S-Co-O-Co2-S into the S-Co-Co-S motifs. In the range of -250 to -300 mV vs. RHE, it is noteworthy that the HER proceeds with a significant increase in the CNCo-Co (S-Co-Co-S motif), while the CNCo-S remains quite unchanged. This implies that the in situ reconstructed low-valent S-Co-Co-S configurations are prerequisites to manage the HER of L-CoS under alkaline conditions. This conclusion is also accordant with operando WT-EXAFS spectra analysis (discussion and details in Figure ), where during the HER the main scattering features are dominated by the backscattering of the first coordination shell of Co-S/Co, and therefore, the in situ generated S-Co-Co-S moieties act as the true catalytically active species to trigger the alkaline HER of L-CoS.