Electrocatalytic activity of new Mn3O4@oxidized graphene flakes nanocomposites toward oxygen reduction reaction

The demand for cost-efficient and non-precious metal-based electrocatalysts toward oxygen reduction reaction (ORR) is crucial in the field of electrochemical energy conversion/storage technologies. Herein, we report a facile one-step co-precipitation route for the in situ synthesis of Mn3O4 nanoparticles onto graphene flakes with different types of selective oxidations (denominated as GF_HNO3, GF_KMnO4 and GF_O-3) and the evaluation of the nanocomposites ORR electrocatalytic performance. The synthesized Mn3O4 nanoparticles presented a spinel structure and a crystallite size between 30 and 38nm. All the nanocomposites showed ORR electrocatalytic activity in alkaline medium, with Mn3O4@GF_O-3 nanocomposite presenting the least negative onset potential of E-onset=-0.14V versus Ag/AgCl; higher diffusion-limiting current densities were achieved by Mn3O4@GF_O-3 and Mn3O4@GF_HNO3 nanocomposites (j(L; -0.6V, 1600rpm)=-2.8mAcm(-2)). Mechanistically, Mn3O4@GF_O-3 nanocomposite stood out with a n(O2) value very close to 4, suggesting the dominance of the one-step 4-electron transfer mechanism. All the nanocomposites showed a robust electrocatalytic performance over 20000s, with current retention values in the range of 87.0-90.3%, and excellent tolerance to methanol, surpassing one of the great limitations of Pt/C electrocatalyst. Globally, the best ORR electrocatalytic performance of the Mn3O4@GF_O-3 nanocomposite is explained by (1) an adequate concentration of Mn3O4 nanoparticles onto GF_O-3 flakes, (2) the highest relative content of Mn species as Mn2+ ions and (3) predominance of quinone and epoxyl groups on GF_O-3 support, which appears to have a key role on the overall electrocatalytic activity of the Mn3O4@GF_ox nanocomposites.