Embracing Phase Transitions to Rationalize the High Oxygen Evolution Reaction Activity of Nickel Selenides in Alkaline Electrolytes

The transformation of electrodeposited NiSe nanocatalysts was studied under oxygen evolution reaction (OER) conditions in alkaline electrolytes by a combination of operando X-ray absorption (XAS) and operando electrochemical impedance spectroscopy (EIS). The upper layers of nickel selenides transform into nickel oxide/oxyhydroxide species, which exhibit higher catalytic OER activity than the as-synthesized nickel oxide/oxyhydroxide, as reported previously. The coupled operando XAS and EIS experiments reveal that the Ni3+/Ni4+ transition occurs at lower potentials than the OER, involving a transient phase transformation at the foot of the nickel oxidation peak. Based on these findings and using insights from complementary operando (Raman spectroscopy, scanning flow cell inductively coupled plasma mass spectrometry) and ex situ measurements (XAS and X-ray photoelectron spectroscopy), a catalytic cycle is proposed within the framework of frustrated phase transitions linking the molecular transformation of substrate and products (H2O/O2) to structural and electronic changes of the solid catalyst (β-Ni(OH)2/β-NiOOH). Specifically, the Ni–O distance of the formed β-NiOOH surface species on top of the NiSe layers elongates, facilitating rapid local transitions between different catalyst phases and oxidation states, which are essential for rapid completion of the catalytic cycle under operando conditions.