Oxygen reduction reaction catalyzed by ε-MnO2: Influence of the crystalline structure on the reaction mechanism

Highlights: ► Activity of ε-MnO2/C for the ORR in alkaline media is reported for the first time. ► ε-MnO2/C has a very high catalytic activity for the ORR, higher than α-MnO2/C. ► Kinetic of proton insertion into structure of MnO2 influences the activity for ORR. ► Structural defects on MnO2 may be the origin of peroxide formation from the ORR. - Abstract: The oxygen reduction reaction (ORR) was studied in KOH electrolyte on carbon supported epsilon-manganese dioxide (ε-MnO2/C). The ε-MnO2/C catalyst was prepared via thermal decomposition of manganese nitrate and carbon powder (Vulcan XC-72) mixtures. X-ray powder diffraction (XRD) measurements were performed in order to determine the crystalline structure of the resulting composite, while energy dispersive X-ray analysis (EDX) was used to evaluate the chemical composition of the synthesized material. The electrochemical studies were conducted using cyclic voltammetry (CV) and quasi-steady state polarization measurements carried out with an ultra thin layer rotating ring/disk electrode (RRDE) configuration. The electrocatalytic results obtained for 20% (w/w) Pt/C (E-TEK Inc., USA) and α-MnO2/C for the ORR, considered as one of the most active manganese oxide based catalyst for the ORR in alkaline media, were included for comparison. The RRDE results revealed that the ORR on the MnO2 catalysts proceeds preferentially through the complete 4e− reduction pathway via a 2 plus 2e− reduction process involving hydrogen peroxide as an intermediate. A benchmark close to the performance of 20% (w/w) Pt/C (E-TEK Inc., USA) was observed for the ε-MnO2/C material in the kinetic control region, superior to the performance of α-MnO2/C, but a higher amount of HO2− was obtained when ε-MnO2/C was used as catalyst. The higher production of hydrogen peroxide on ε-MnO2/C was related to the presence of structural defects, typical of this oxide, while the better catalytic performance in the kinetic control region compared to α-MnO2/C was related with the higher electrochemical activity for the proton insertion kinetics, which is a structure sensitive process.