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_O₃) and the evaluation of the nanocomposites ORR electrocatalytic performance. The synthesized Mn₃O₄ nanoparticles presented a spinel structure and a crystallite size between 30 and 38 nm. All the nanocomposites showed ORR electrocatalytic activity in alkaline medium, with Mn₃O₄@GF_O₃ nanocomposite presenting the least negative onset potential of E_onset = −0.14 V versus Ag/AgCl; higher diffusion-limiting current densities were achieved by Mn₃O₄@GF_O₃ and Mn₃O₄@GF_HNO₃ nanocomposites (j_{L; −0.6 V, 1600 rpm}  = −2.8 mA cm⁻²). Mechanistically, Mn₃O₄@GF_O₃ 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 20000 s, 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_O₃ nanocomposite is explained by (1) an adequate concentration of Mn₃O₄ nanoparticles onto GF_O₃ flakes, (2) the highest relative content of Mn species as Mn²⁺ ions and (3) predominance of quinone and epoxyl groups on GF_O₃ support, which appears to have a key role on the overall electrocatalytic activity of the Mn₃O₄@GF_ox nanocomposites.