Hydrothermal growth of pompon-like manganese oxide microspheres with embedded nickel ions as single-atom catalysts for urea oxidation

Abstract ]Manganese oxides (δ-MnO2) with pompon-like microspheres and non-spherical shaped particles were hydrothermally grown on Ni foam substrates in the presence and absence of a structure-directing agent (ammonium fluoride), respectively. The intercalated K+ ions in the layer-structured MnO2 were exchanged by the dissolved nickel ions from Ni foam during hydrothermal synthesis, leading to the formation of MnO2 with embedded nickel ions (labeled as Ni foam@Ni-MnO2). The dominated oxidation state of nickel ions in MnO2 was found to be trivalent (Ni3+), making Ni foam@Ni-MnO2 a promising catalyst electrode for direct and indirect urea oxidation reactions (UORs). Ni foam@Ni-MnO2 exhibited large electroactive surface area and high oxidation state of nickel-ion catalyst for adsorption of urea molecule and the successive cleavage of urea. Pompon-like microspheres with ultrathin nanosheets offered abundant pore channels to facilitate the spread of urea molecules and products in the UOR process. Electrochemical impedance spectroscopy revealed that Ni foam@Ni-MnO2 with pompon-like microspheres has lower direct UOR impedance and indirect UOR impedance in the regeneration of active Ni3+ catalyst than Ni foam. Thus, Ni foam@Ni-MnO2 displayed greater current density and smaller onset potential than bare Ni foam in catalyzing UOR.