Electrochemical behavior of rapidly synthesized amorphous 3d-VIIIB-metal aerogels as bifunctional electrocatalysts for water splitting

Abstract

Oxygen vacancies (O_V) in metal oxide electrocatalysts are broadly regarded as pivotal factors that improve electrochemical catalytic efficiency in water splitting. However, the intrinsic properties of catalysts also play an important role in the overall catalytic performance. Here, we present a facile synthesis strategy and investigate the interplay between O_V content and intrinsic properties in bifunctional electrocatalysts for the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Despite being derived from the same class of metal(II) chloride precursors, the three aerogels exhibit distinctly different amorphous porous architectures: sphere-like particles (Fe), flower-like hierarchical nanosheets (Co), and densely packed ultrasmall spheres (Ni). Among the three, O_V-rich Co aerogels stand out due to the synergistic combination of the highest O_V percentage (89.4%), the largest specific surface area (134.5 m² g⁻¹), and favorable intrinsic electronic properties of the Co center, which together maximize active site density and facilitate efficient charge transfer for both HER and OER intermediates. Consequently, Co aerogels deliver outstanding bifunctional performance with overpotentials of only 383 mV (OER) and 365 mV (HER) at 100 mA cm⁻² in alkaline electrolyte, along with long-term durability over 70 h. Importantly, despite possessing substantial O_V content, Fe and Ni aerogels exhibit negligible or poor catalytic activity in neutral electrolyte, demonstrating that O_V content alone is insufficient, and intrinsic material properties are equally decisive in governing electrocatalytic performance. This study thus provides mechanistic insights into the relative contributions of O_V and intrinsic properties in water splitting electrocatalysis, alongside a scalable synthesis route for high-performance Co-based aerogel electrocatalysts.