Electrocatalyst with Dynamic Formation of the Dual-Active Site from the Dual Pathway Observed by <i>In Situ</i> Raman Spectroscopy

Abstract

Understanding the catalysis mechanism of the sluggish oxygen evolution reaction (OER) involved in water splitting is of vital importance for the development of clean hydrogen energy. Earth-abundant transition-metal (oxy)hydroxide with low cost and high performance is one of the most promising OER catalysts. These catalysts often dynamically and heterogeneously transform from inactive pre-catalysts into active phases under operation conditions, and thus, the operando/in situ method is needed for the direct observation. Herein, using in situ Raman spectroscopy and density functional theory simulation, we correlate the OER activity with the dynamic crystal- and electronic-structure reconstruction of nano-sheet cobalt hydroxide. A complicated dual-transformation path is observed as the applied voltage is gradually increased; the pristine single-phase α-Co(OH)2 catalyst transforms into the hydrous Co(OH)2 phase through hydroxide intercalation, then to mixed β/γ-CoOOH phases through dehydration and dehydrogenation, and finally to OER-active γ-CoOOHx and β-CoOOHy. Moreover, the observed spectral and Tafel behaviors at different scan rates manifest the rate-dependent formation of the dual-active-phase, demonstrating the correlation between the OER ability and thermodynamics of structural reconstruction, which is critical in the fabrication of high-activity catalysts.