Porous NiFe-Oxide Nanocubes as Bifunctional Electrocatalysts for Efficient Water-Splitting

Electrocatalytic water-splitting, a combination of oxygen and hydrogen evolution reactions (OER and HER), is highly attractive in clean energy technologies, especially for high-purity hydrogen production, whereas developing stable, earth-abundant, bifunctional catalysts has continued to pose major challenges. Herein, a mesoporous NiFe-oxide nanocube (NiFe-NC) system is developed from a NiFe Prussian blue analog metal–organic framework as an efficient bifunctional catalyst for overall water-splitting. The NiFe-NCs with ～200 nm side length have a Ni/Fe molar ratio of 3:2 and is a composite of NiO and α/γ-Fe₂O₃. The NCs demonstrate overpotentials of 271 and 197 mV for OER and HER, respectively, in 1 M KOH at 10 mA cm^–2, which outperform those of 339 and 347 mV for the spherical NiFe-oxide nanoparticles having a similar composition. The electrolyzer constructed using NiFe-NCs requires an impressive cell voltage of 1.67 V to deliver a current density of 10 mA cm^–2. Along with a mesoporous structure with a broad pore size distribution, the NiFe-NCs demonstrate the qualities of a desired corrosion-resistant water-splitting catalyst with long-term stability. The exposure of active sites at the edges and vertices of the NCs was validated to play a crucial role in their overall catalytic performance.