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

Abstract

Abstract An efficient two‐step gas‐phase method was developed for the synthesis of Co 3 O 4 –MnO 2 –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 HNO 3 vapor at 200 °C was performed to 1) open the CNTs, 2) oxidize encapsulated Co nanoparticles to Co 3 O 4 as well as MnO nanoparticles to MnO 2 , and 3) to create oxygen functional groups on carbon. The hybrid demonstrated excellent OER activity and stability up to 37.5 h under alkaline conditions, with longer exposure to HNO 3 vapor up to 72 h 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.