Electrochemically embedded nickel oxide in a highly porous manganese oxide film with enhanced catalytic kinetics of the urea oxidation reaction

The use of well-dispersed nickel catalysts with atomic-scale sizes is an effective approach for maximizing catalytic activity and selectivity for the urea oxidation reaction (UOR). Herein, a porous ultrathin film of Birnessite-type manganese oxide is anodically deposited on a nickel substrate as a catalyst support. Some of the intercalated sodium ions are replaced by the dissolved nickel ions from the nickel substrate in the anodic process, forming manganese oxide with partially intercalated nickel ions (denoted MnO ₂ ). The amount of nickel species can be easily increased by cyclic voltammetry in the nickel sulfate electrolyte (denoted MnO ₂-Ni), without the relatively time-consuming process of conventional ion exchange. Most of the embedded nickel species are catalytic Ni ³⁺ ions, rendering MnO ₂ -Ni a favorable catalyst to boost the UOR. The cyclic voltammetry results reveal that Ni ³⁺ catalysts, when inserted into the layer-structured manganese oxide, can increase the electroactive surface area for the adsorption of urea and increase the charge-transfer rate constant for the Ni ³⁺/Ni ²⁺ redox couple, leading to an improved catalytic rate constant of the UOR. A highly porous manganese oxide matrix provides many transport pathways for facilitating UOR kinetics. Electrochemical impedance spectroscopy shows that MnO ₂-Ni has lower direct and indirect UOR impedances than MnO ₂ and bare Ni electrodes, leading to a greater oxidation current and lower onset potential.