Flower-like manganese oxide with intercalated nickel ions (Ni3+) as a catalytic electrode material for urea oxidation

The Na+-intercalated manganese oxide nanoflowers and Na+-free manganese oxide nanoparticles were anodically electrodeposited on carbon fibers from manganese (II) acetate solutions with and without sodium sulfate, respectively. After intercalation of nickel ions, the Na+-intercalated manganese oxide remained its original form of nanoflowers (denoted as MnO2/Na/Ni), while the Na+-free manganese oxide underwent severe deformation to form a dense film with aggregated nanoparticles (denoted as MnO2/Ni). It was evident that the preintercalated Na+ ions were replaced with Ni3+ ions which were inserted in the interstitial space of the tunnel-structured manganese oxide. Cyclic voltammetry revealed that the well-dispersed Ni3+ species in MnO2/Na/Ni could behave as a catalytic center for facilitating the urea oxidation reaction (UOR) in an alkaline electrolyte, resulting in superior catalytic capability than the MnO2/Ni and bulk Ni(OH)(2). Electrochemical impedance spectroscopy demonstrated that an impressive electrocatalytic activity of MnO2/Na/Ni electrode was largely ascribed to its lower charge-transfer resistance of the Ni3+/Ni2+ redox reaction in indirect UOR process and smaller direct UOR resistance on Ni3+ catalyst than the MnO2/Ni electrode with aggregated catalyst layer.