Thermal optimization of manganese dioxide nanorods with enhanced ORR activity for alkaline membrane fuel cell

Abstract

AbstractIn this study, pristine MnO2 catalyst was synthesized by hydrothermal technique and annealed at 400 and 600°C (MnO2@400°C and MnO2@600°C) transforming it into nanorods in a nitrogen atmosphere. The heat treatment process ameliorates the catalytic activity of the MnO2 catalyst by inducing oxygen vacancies. The catalyst is characterized by scanning electron microscope, and X‐ray diffraction, Fourier transforms infrared spectroscopy (FTIR), Electron paramagnetic resonance (EPR), and X‐ray Photoelectron Spectroscopy, whereas the oxygen reduction reaction (ORR) activity examined through rotating disc electrode and fuel cell test station. To elucidate the effect of sintering temperature on the MnO2 nanorods, the high angle annular dark‐field (HAADF) imaging was carried out on the samples. MnO2@400°C showed preeminent robustness to withstand austere alkaline environment during the experiment and possessed high electrocatalytic activity for the ORR with current density and onset potential values of 6.4 mA cm–2 and 0.80 V VS (RHE), respectively. The single‐cell test experiment of alkaline fuel cell delivered peak power density 81 mW/cm at 50°C. It is believed that this idiosyncratic behavior of MnO2 nanorods is due to the preferential growth on (211) and (310) indices and coexistence of optimized Mn4+/Mn3+ oxidation states as well as the creation of optimum oxygen vacancies on MnO2 nanorods. Moreover, the stretching of MnO2 nanorods occurs with increasing temperature and thus increasing the surface area to volume ratio. Thus thermal treatment shows that MnO2 is highly sensitive toward temperature variation and optimum ORR results can be obtained at adequate temperatures.