Engineering Internal and External Low-Coordination Atoms in Nickel–Organic Framework Nanoarrays to Promote the Electrochemical Oxygen Evolution Reaction

Monometallic nickel–organic frameworks based on a carboxylated ligand [2,6-naphthalenedicarboxylic acid (Ni-NDC)] have abundant and uniformly distributed single-atom Ni sites, enabling superior oxygen evolution reaction (OER) activity. In theory, most of the Ni atoms inside Ni-NDC microcrystals are coordinatively saturated except for the surface. Therefore, there are no accessible low-coordination atoms (LCAs) as electrocatalytic sites for the OER. One effective way is to expose more LCAs by preparing self-supporting Ni-NDC nanoarrays (Ni-NDC NAs) with hierarchical secondary structural units. Another effective method is to create more internal LCAs by removing partial ligands or coordination atoms attached to the Ni atoms. Herein, by combining the two strategies, we engineered LCAs in the interior and exterior of Ni-NDC to synergistically accelerate the OER. In brief, ultrathick “brick-like” Ni-NDC NAs were first prepared with dissolution and coordination effects of NDC on self-sacrificial templates of “agaric-like” nickel hydroxide nanoarrays [Ni(OH)2 NAs]. Subsequently, dual-coordinated NDC was partially replaced by monocoordinated 2-naphthoic acid (NA). The Ni-NDC NAs were further tailed into ultrathin “liner leaf-like” nanoneedle arrays (LCAs-Ni-NDC NAs). As a consequence, the LCAs-Ni-NDC NAs have more internal and external LCAs, which can deliver an OER performance that is superior to that of Ni-NDC NAs.