Redox-state modulated ORR activity of Cd-based Prussian blue analog frameworks transformed via anion exchange with controlled redox-state from CdCO₃ cuboids

This work reports a facile solution based chemical transformation of thin film of CdCO₃ cuboids into Cd-based Prussian blue analogue structured thin film with controlled redox-state by manipulating the anion exchange reaction between the solid film and the aqueous solution of K₃[Fe(CN)₆] at 60 °C. The progress of the transformation reaction is monitored via electron microscopy, X-ray difractometry and X-ray photoelectron spectroscopy. Based on the scenario of the reaction progress revealed by these analysis, the CdCO₃ cuboids are first found to be transformed into the smaller cuboids of cadmium ferricyanide, which, on further reaction, are transformed into the cadmium ferrocyanide cuboids. Thus, by controlling the redox-state of the Fe-metal center on the frameworks, the electrocatalytic activity of the frameworks on oxygen reduction reaction (ORR) is studied. The electrocatalytically inert CdCO₃ cuboids after transformation to the frameworks with Fe³⁺-centers demonstrate an enhanced catalytic activity on ORR, while the Fe²⁺-centers of the frameworks are found to relegate the catalytic activity, thereby manifesting the redox-sate modulated ORR activity of the frameworks. To the best of the authors' knowledge, this is the first report of this kind on redox-state specific ORR activity.