To directly evidence the origins of the intrinsic catalytic activity and to precisely establish a full picture of the underlying reaction mechanisms, DFT simulations were conducted with the investigated CoSe2 catalysts. Figure represents the calculated projected density of states (PDOS) of the three catalysts. From the results, the Co d and Se/S p orbital states distributions spread over the Fermi level, reflecting the intrinsic metallic character of the CoSe2 materials. According to previous studies, the d-p band center proximity (denoted as Δεdp) is regarded as a key descriptor for elucidating the underlying HER kinetics. Specifically, a smaller Δεd-p enables facilitated electron mobility and optimization of the adsorption/desorption of H* intermediates on the active sites, thereby, offering a better HER activity. As shown in Figure , o-CoSe2 gives a Δεd-p value of 1.13, which is noticeably smaller than that of c-CoSe2 (1.19), pointing out a facilitated HER kinetics involved in o-CoSe2 compared to c-CoSe2. Partial S incorporation into CoSe2 further reduces the Δεd-p value to 0.50, ascribed to the newly induced Co d-S p orbital hybridization (Figure ). As a result of the optimized HER kinetics, the as-investigated c-S-CoSe2 delivers the best HER performance compared to those of o-CoSe2 and c-CoSe2 (Figure ). This is further corroborated by the calculated adsorption free energy of H* intermediates (ΔGH*) on the investigated three catalysts (Figure and). Notably, KSCN poisoning experiments clearly reveal that the acidic HER activity is independent of the KSCN additive, indicating that the adsorption/desorption of H* intermediates primarily occurs at the Se/S sites (Figure ). Based on this, our results (Figure ) show that the bestperforming HER c-S-CoSe2 catalyst features a minimum ΔGH* value of 0.13 eV, which is smaller than those of o-CoSe2 (0.19 eV) and c-CoSe2 (0.31 eV). Furthermore, we calculated the formation energy required for partial oxygen incorporation into the investigated catalysts to gain in-depth insights into the HER kinetics under alkaline conditions. The results (Figure ) indicate that for o-CoSe2 a minimum energy barrier of 2.69 eV needs to be overcome for surface oxygen incorporation, which is lower than that of c-CoSe2 (3.51 eV). This suggests that c-CoSe2 is more thermodynamically stable than o-CoSe2 towards the alkaline HER, in agreement with our above operando monitoring (Figure and). As discussed extensively in the previous operando monitoring section, the in situ generated low-valent Co species are identified as decisive factors to trigger the alkaline HER, therefore, we further calculated the ΔGH* value for the H* intermediates adsorbed on the Co sites in o-CoSe2 with rich Co exposure. The results (Figure ) reveal that a minimum ΔGH* value of 0.27 eV is determined in the o-CoSe2 with rich Co exposure, which is significantly higher than that of the Se sites (0.13 eV), indicating a sluggish HER kinetics proceeds at the Co sites at pH 14. In other words, the above analysis also offers a clear explanation for why o-CoSe2 catalysts feature poor electrochemical HER performance at pH 14 compared to pH 0 (Figure ).