Deciphering the Influence of Morphology and Crystal Structure on Alkaline Hydrogen Evolution Activity in Polymorphic Cobalt Diselenide

Polymorphic cobalt diselenide (CoSe2) has emerged as a highly promising catalyst for the alkaline hydrogen evolution reaction (HER) due to its favorable electrocatalytic activity and stability. Previous studies have emphasized the unique advantages of different crystal structures (orthorhombic and cubic) of CoSe2 in the HER. This prompted us to investigate whether the HER catalytic activity of polymorphic CoSe2 phases is contingent on their crystal structure and morphology. Specifically, we fabricated two distinct phases of CoSe2, namely, orthorhombic and cubic, each characterized by unique morphologies on carbon cloth (CC) and referred to as o-CoSe2/CC and c-CoSe2/CC. The o-CoSe2/CC exhibits Cadamba-like flower structures, while c-CoSe2/CC has a densely packed and vertically aligned nanoneedle-like morphology. Interestingly, our findings indicate that both catalysts demonstrated analogous HER performance under identical reaction conditions. For instance, o-CoSe2/CC achieved an overpotential of 178 ± 3 mV (@10 mA cm–2) with a Tafel slope of 111 ± 5 mV dec–1, closely mirroring the performance of c-CoSe2/CC (187 ± 4 mV, 112.8 ± 3 mV dec–1). This equivalence in the performance can be attributed to several factors, including their similar electrochemically active surface areas, equivalent numbers of active sites, comparable reaction kinetics, and analogous charge-transfer rates. This research work decisively demonstrates that in polymorphic materials, structural variations and morphological differences may have minimal influence on dictating the HER activity, while the number and kind of active sites are the dominant factors in regulating the HER activity.