Electrocatalytic activity of new Mn.sub.3O.sub.4@oxidized graphene flakes nanocomposites toward oxygen reduction reaction

• Published in: Journal of materials science Vol. 54; no. 12; pp. 8919 - 8940
• Main Authors: Araújo, Mariana P, Nunes, Marta, Rocha, Inês M, Pereira, M. F. R, Freire, Cristina
• Format: Journal Article
• Language: English
• Published: Springer 01.06.2019
• Subjects: Electrochemistry; Graphene; Graphite; Precipitation (Meteorology); Spinel group
• ISSN: 0022-2461; 1573-4803
• DOI: 10.1007/s10853-019-03508-6

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 Mn.sub.3O.sub.4 nanoparticles onto graphene flakes with different types of selective oxidations (denominated as GF_HNO.sub.3, GF_KMnO.sub.4 and GF_O.sub.3) and the evaluation of the nanocomposites ORR electrocatalytic performance. The synthesized Mn.sub.3O.sub.4 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 Mn.sub.3O.sub.4@GF_O.sub.3 nanocomposite presenting the least negative onset potential of E.sub.onset = -0.14 V versus Ag/AgCl; higher diffusion-limiting current densities were achieved by Mn.sub.3O.sub.4@GF_O.sub.3 and Mn.sub.3O.sub.4@GF_HNO.sub.3 nanocomposites (j.sub.L; -0.6 V, 1600 rpm = -2.8 mA cm.sup.-2). Mechanistically, Mn.sub.3O.sub.4@GF_O.sub.3 nanocomposite stood out with a n.sub.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 Mn.sub.3O.sub.4@GF_O.sub.3 nanocomposite is explained by (1) an adequate concentration of Mn.sub.3O.sub.4 nanoparticles onto GF_O.sub.3 flakes, (2) the highest relative content of Mn species as Mn.sup.2+ ions and (3) predominance of quinone and epoxyl groups on GF_O.sub.3 support, which appears to have a key role on the overall electrocatalytic activity of the Mn.sub.3O.sub.4@GF_ox nanocomposites.