Heterostructured hollow cobalt-nickel core-shell hexacyanoferrate nanocubes for enhanced electrocatalytic oxidation of urea molecules in alkaline electrolyte

The hollow cobalt-nickel core-shell hexacyanoferrate (CoNiHCF) nanocube is prepared by forming a thin nickel hexacyanoferrate (NiHCF) shell on a hollow cobalt hexacyanoferrate (CoHCF) nanocube as a synergistic catalyst for urea oxidation reaction (UOR). The CoNiHCF reduces the peak-to-peak separation of the cyclic voltammo-grams for Ni2+/Ni3+ couple, which favors the reversible regeneration of active Ni3+ species. NiHCF, a kind of Prussian blue analogue, has greater UOR kinetics than nickel foam and CoHCF, due to its open framework structure with redox-active nickel and iron species. The hollow core-shell architecture affords numerous path-ways for fast electron and electrolyte transport, enhancing the adsorption and oxidation of urea molecules on the catalytic Ni3+ sites. Electrochemical impedance spectroscopy reveals superior UOR kinetics of CoNiHCF compared to the NiHCF catalyst, reflected by its low electrical resistance, charge-transfer resistance, and direct UOR resistance. The CoHCF core in the CoNiHCF provides a conductive host to immobilize the nickel-rich catalyst shell to promote the formation of high-valence species Ni3+ to boost the electrocatalytic activity of the CoNiHCF. The CoNiHCF catalyst exhibits a high oxidation current (175 mA cm-2 at 1.570 V), small onset potential (1.335 V), and remarkable durability, rendering it a highly promising catalyst material for UOR.