Engineering of crystal phase regulation or introduction of heteroatom emerges as an effective strategy to facilitate the underlying catalytic reaction kinetics, thereby optimizing the electrocatalytic performances of many reported catalysts. To evaluate the electrochemical water splitting performance, the as-prepared CoSe2 catalysts were assembled as both cathodic and anodic electrodes for the HER and OER in different media. Note that the recorded linear sweep voltammetry curves were normalized by the electrochemical active surface area (ECSA) with a 90% iR drop correction to ensure a more accurate assessment of the intrinsic catalytic activities (Figures ). HER characterization at pH 0 and 14. The electrochemical HER performance of the as-prepared CoSe2 catalysts was evaluated at both pH 0 and 14. Under acidic conditions (Figure and Figure ), the as-prepared c-S-CoSe2 displays the best HER performance with an overpotential of -94 mV (-88 mV) at the current density of -0.5 mA/cmECSA 2 (-10 mA/cmgeo. ), a Tafel slope value of 64.02 mV/dec, and a charge transfer resistance of 8.7 Ω, which outperformed those of o-CoSe2 (-164 mV/-149 mV, 78.19 mV/dec, and 72.4 Ω ), c/o-CoSe2 (-182 mV/-159 mV, 78.19 mV/dec, and 84.4 Ω ), and c-CoSe2 (-200 mV/-217 mV, 78.19 mV/dec, and 355.1 Ω). Moreover, the electrochemical stability results corroborate that the investigated catalysts maintain their initial HER activities after 300 h without any obvious changes (Figure ). In sharp contrast, under alkaline conditions (Figure and Figure ), the best HER performance can be still achieved in c-S-CoSe2, while the other CoSe2 catalysts exhibit a disparate activity order compared to acidic conditions: c-CoSe2 > c/o-CoSe2 > o-CoSe2. These results altogether unravel that crystal phase engineering or heteroatom substitution plays a pivotal role in optimizing the HER kinetics of CoSe2 catalysts in different media, which calls for operando unraveling the true chemical nature of active species and sites during the reaction processes.