Thickness-Dependent Plasma Activation in Binder-Free Multilayer Ni-Co-O Anodes for Alkaline Oxygen Evolution

Abstract

Binder-free Ni-Co-O anodes were fabricated by ultrasonic spray-coating multilayer catalyst films directly onto Ni substrates, followed by nitrogen plasma treatment to enhance performance in alkaline water electrolysis. By systematically varying coating thickness, we were able to elucidate the relationship between plasma-induced surface reconstruction and electrode architecture. Plasma exposure, with an effective modification depth of approximately 1 µm, promoted near-surface cobalt enrichment and improved wettability while preserving the bulk crystal structure, as confirmed by X-ray photoelectron spectroscopy, energy-dispersive X-ray spectroscopy, and grazing-incidence X-ray diffraction. Despite this only partial activation of thicker coatings, their enhanced porosity and micro-scale roughness facilitated superior electrolyte penetration and gas release, leading to higher catalytic efficiency. Consequently, the thickest plasma-treated electrode exhibits an overpotential of 335 mV at 100 mA cm^-2 in 1 M KOH, outperforming the Ni substrate reference (380 mV), while maintaining robust mechanical integrity without delamination. These findings demonstrate that catalytic performance depends on the interplay between finite plasma activation depth and multilayer thickness, highlighting the need to tailor coating thickness to balance surface modification with effective electrolyte penetration and gas transport. Nitrogen plasma treatment thus provides a scalable strategy for engineering mechanically robust and efficient binder-free Ni-Co-O anodes for alkaline OER.