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

Abstract

Manganese oxides are promising electrocatalysts for the oxygen evolution reaction due to their versatile redox properties. Manganese oxide (MnO_x) 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 H₂ or NH₃, and to strongly oxidizing conditions using HNO₃ vapor, which enabled us to flexibly tune the oxidation state of Mn from 2+ in MnO to 4+ in MnO₂. 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_x/NCNTs samples compared with the MnO_x/OCNTs samples. Furthermore, the reduction of MnO₂ to MnO occurred in one-step on NCNTs, whereas Mn₂O₃ intermediate states were observed for OCNTs. STEM and TEM images revealed a smaller and uniform dispersion of the MnO_x 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 MnO₂ generated by HNO₃ vapor treatment, is more active than Mn in lower oxidation states, using the potential at 10 mA cm⁻² and the Tafel slopes as the performance metrics.