Facilely synthesized nitrogen-doped reduced graphene oxide functionalized and/or co-doped with metal ions as electrocatalyst for oxygen reduction reaction

Abstract

<h2>Abstract</h2> Due to fossil fuels depletion and environmental pollution, clean and sustainable energy technologies, e.g. fuel cells and metal-air batteries, have attracted extensive attention. To push further the research on these electrochemical devices, low-cost, durable and efficient electrocatalysts alternative to platinum are required, to boost the oxygen reduction reaction (ORR). A microwave-assisted method has been optimized, to obtain effective heterogeneous catalyst for ORR, starting from graphene oxide (GO), urea and a transition metal (e.g. Mn and Cu) precursor. We have proved that our synthetic method originates porphyrin-like structures containing pyrrole rings within the reduced GO (rGO) basal plane which coordinate the Mn²⁺. In the case of copper, however, Cu²⁺ forms an ionic tetra coordinated structure anchored at the rGO surface via residual oxygen containing functional groups. In both cases, metal complex acts as an ORR highly efficient catalytic reaction center and their identification were strongly supported by several characterization techniques, such as X-ray Photoelectron Spectroscopy (XPS), X-ray absorption spectroscopies (XAS) and Transmission Electron Microscopy (TEM), together with Density Functional Theory (DFT) simulations. All synthesized materials exhibit outstanding catalytic properties toward ORR, as evidenced by electron transfer numbers larger than 3.8 and peroxide percentages lower than 7%, similar to Pt/C reference electrode.