Thermally activated carbon–nitrogen vacancies in double-shelled NiFe Prussian blue analogue nanocages for enhanced electrocatalytic oxygen evolution

Abstract

Prussian blue analogues (PBAs) with controllable frame structures and anchored defects represent promising electrocatalysts for oxygen evolution reaction (OER) in alkaline conditions. Herein, double-shelled NiFe PBAs nanocages have been synthesized through an ion-exchange reaction between K₃Fe(CN)₆ and single-shelled Ni(OH)₂ nanocages based on the regulation of nanoscale Kirkendall effect driven by the crystallinity difference between Ni(OH)₂ and NiFe PBAs. With further mild thermal activation, carbon–nitrogen (CN) vacancies could be formed in situ in large quantities in the double-shelled NiFe PBAs nanocages. Theoretical calculation and experimental results prove that the rationally designed double-shelled NiFe PBAs nanocages with an abundance of the exposed surface have a lower energy barrier for the departure of CN groups from NiFe PBAs lattices, and thus generate CN vacancies with a concentration as high as 18.5% via a mild thermal activation strategy, which far outweigh the content of CN vacancies in the solid NiFe PBAs nanocubes. The as-prepared NiFe PBAs with abundant CN vacancies have an impressive OER activity with a low overpotential of 267 mV at 20 mA cm⁻² and long-term stability with only ∼2.1% potential increase after 78 h. The opened double-shelled structure of NiFe PBAs nanocages offers highly exposed active sites and fast charge transfer kinetics. Moreover, post characterizations demonstrate the important role of CN vacancies in the suppression of Fe loss during the OER process and formation of Fe species as the active sites. This study provides an effective strategy to explore PBAs catalysts with desired nanostructures and defective multiplicity for enhanced OER catalysis.