Tuning the oxidation state of manganese oxide nanoparticles on oxygen- and nitrogen-functionalized carbon nanotubes for the electrocatalytic oxygen evolution reaction.

Manganese oxides are promising electrocatalysts for the oxygen evolution reaction due to their versatile redox properties. Manganese oxide (MnO) nanoparticles were synthesized on oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) by calcination in air of Mn-impregnated CNTs with a loading of 10 wt% Mn. The calcined samples were exposed to reducing conditions by thermal treatment in H or NH, and to strongly oxidizing conditions using HNO vapor, which enabled us to flexibly tune the oxidation state of Mn from 2+ in MnO to 4+ in MnO. The samples were characterized by X-ray photoelectron spectroscopy, X-ray diffraction, transmission electron microscopy and temperature-programmed reduction. The oxidation state of Mn was more easily changed in the MnO/NCNTs samples compared with the MnO/OCNTs samples. Furthermore, the reduction of MnO to MnO occurred in one-step on NCNTs, whereas MnO intermediate states were observed for OCNTs. STEM and TEM images revealed a smaller and uniform dispersion of the MnO nanoparticles on NCNTs as compared to OCNTs. Electrocatalytic oxygen evolution tests in 0.1 M KOH showed that Mn in high oxidation states, specifically 4+ as in MnO generated by HNO vapor treatment, is more active than Mn in lower oxidation states, using the potential at 10 mA cm and the Tafel slopes as the performance metrics.