Spinel Mn-Co oxide in N-doped carbon nanotubes as bifunctional electrocatalyst
Martin Muhler, Ruhr-Universität Bochum, Bochum, GermanyWolfgang Schuhmann, Analytical Chemistry and Center of Electrochemistry / Ruhr-University Bochum, Bochum, Germany
Decentralized supply of energy from renewable sources strongly relies on the efficient conversion and storage of energy in different forms, e.g., in form of H2 by electrolysis of water and recovery of the energy during the reverse process in fuel cells, and in rechargeable metal air batteries. While the reversibility of the electrochemical reactions involving H2 is efficient, the main challenge lies with improving the efficiency of the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER).
Recently, transition metal-based catalysts such as Mn and Co oxides have attracted enormous interest as low-cost alternatives to noble-metal catalysts capable of catalyzing both the ORR and the OER. However, these oxides have high electrical resistance, which fortunately can be mitigated by the use of conductive additives such as graphene and carbon nanotubes (CNTs). In particular, N-doped carbon materials have been widely used not only to promote electron transfer but also to serve as complementary sites for the ORR, thereby enhancing the ability of the materials to electrocatalyze both the ORR and the OER.
In this work, we demonstrate the synthesis of spinel Mn-Co oxide nanoparticles partially embedded in N-doped CNTs (NCNTs), which were used as bifunctional electrocatalysts for both ORR and OER under alkaline conditions. Thermal oxidative treatment of catalytically grown NCNTs, in which residual Co and Mn oxide nanoparticles are buried, simultaneously ruptures the NCNTs and oxidizes the Co and Mn oxide nanoparticles forming spinel Mn-Co oxides nanoparticles partially embedded in the NCNTs. Due to a cooperative effect from the nitrogen groups in NCNT and the spinel Mn-Co oxide particles, the capability of the resulting catalysts to electrolyze both the ORR and the OER becomes tremendously enhanced producing exceptionally active bifunctional catalysts for reversible oxygen electrodes.