Hierarchical microspheres constructed by hexagonal NiCo(OH)2 nanosheets with rich Ni3+ species and carboxylic groups for efficient urea oxidation reaction.

Metal organic frameworks (MOFs) derived metal hydroxides have been widely studied for urea oxidation reaction (UOR). However, few studies have paid attention to the effect of the surface-absorbed ligand molecules on the catalytic activity. Herein, hierarchical microspheres constructed by hexagonal NiCo(OH) 2 nanosheets with rich Ni3+ species and surface carboxylic groups are synthesized by using the NiCo-based MOFs as the sacrificial templates. The X-ray photoelectron spectroscopy and in - situ Raman characterizations confirm the presence of the surface carboxylic groups. The introduction of the surface carboxylic groups is revealed for the first time to greatly enhance the UOR catalytic activity. Owing to the unique structure (two-dimensional nanosheets in three-dimensional microspheres) and the numerous Ni3+ species and surface carboxylic groups, the as-prepared hexagonal NiCo(OH) 2 nanosheets exhibit outstanding electrochemical performance, requiring only 1.368 V vs. reversible hydrogen electrode to reach a current density of 100 mA cm−2. • Hexagonal NiCo(OH) 2 nanosheets were fabricated from metal organic frameworks. • The NiCo(OH) 2 contains numerous Ni3+ species and surface carboxylate groups. • The quantity of Ni3+ species and carboxylic groups is controlled by the Ni/Co ratio. • The surface carboxylate groups can greatly promote the urea oxidation reaction. [ABSTRACT FROM AUTHOR]