Co3O4-MnO2-CNT Hybrids Synthesized by HNO3 Vapor Oxidation of Catalytically Grown CNTs as OER Electrocatalysts

An efficient two-step gas-phase method was developed for the synthesis of Co3O4-MnO2-CNT hybrids used as electrocatalysts in the oxygen evolution reaction (OER). Spinel Co-Mn oxide was used for the catalytic growth of multiwalled carbon nanotubes (CNTs) and the amount of metal species remaining in the CNTs was adjusted by varying the growth time. Gas-phase treatment in HNO3 vapor at 200 degrees C was performed to 1)open the CNTs, 2)oxidize encapsulated Co nanoparticles to Co3O4 as well as MnO nanoparticles to MnO2, and 3)to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5h under alkaline conditions, with longer exposure to HNO3 vapor up to 72h beneficial for improved electrocatalytic properties. The excellent OER performance can be assigned to the high oxidation states of the oxide nanoparticles, the strong electrical coupling between these oxides and the CNTs as well as favorable surface properties rendering the hybrids a promising alternative to noble metal based OER catalysts.