OER characterization at pH 14. The OER process emerges as the bottleneck for effective water electrolysis. Therefore, we further employed the as-prepared CoSe2 catalysts as anodic materials for the OER at pH 14. As demonstrated in Figure ,f and Figure ,f, to achieve a current density of 0.02 mA/cmECSA 2 or 10 mA/cmgeo. , a minimum overpotential of 286 mV or 266 mV is required for c-S-CoSe2, close to those of o-CoSe2 (287 mV or 269 mV), c/o-CoSe2 (296 mV or 275 mV), and c-CoSe2 (300 mV or 276 mV). The electrochemical durability results (Figure ) indicate that all the investigated CoSe2 catalysts retain their initial OER performance for 300 h with a slight potential increase. Besides, rotating-ring disk electrode (RRDE) experiments (Figure ) demonstrate a nearly 100% Faradaic efficiency for c-S-CoSe2, indicating that the OER predominantly occurs on the electrode surface. From the above results (Figure and Figure ), one can also notice that there were no significant OER performance differences in the as-synthesized CoSe2 catalysts. This is because under the OER conditions CoSe2 catalysts are expected to undergo restructuring into their corresponding oxides/hydroxides, which serve as a prerequisite to initiate the OER. However, the critical role of oxidized oxyanion components (i.e. SeOx -and SOx -) in the OER of the investigated catalysts remains to be fully elucidated. This requires operando time-resolved monitoring of the investigated catalysts under operational reaction conditions, which will be discussed in detail in the following section on operando characterizations.