Impact of the Surface Chemical Composition on the ORR Activity of Metal-Free Carbon-Based Electrodes and Their Performance in DMFC

Nine activated carbons (ACs) with hierarchical micro- and mesoporous textural structures and varied chemical compositions were evaluated as metal-free electrocatalysts for the Oxygen Reduction Reaction (ORR) under alkaline conditions. The base material was a commercial biomass-based carbon chemically activated with H3PO4, which possesses a hierarchical micro- and mesoporous structure. This material was modified by: oxidative treatment with HNO3 to increase the content of acidic oxygenated functional groups (OFGs); and by heat treatment in an inert atmosphere up to 800 °C to remove most of the acidic OFGs. Furthermore, the original and modified ACs were subjected to ammonization up to 400 or 800 °C to incorporate nitrogen. The results showed that there exists a synergistic effect among at least three critical factors that enhance the ORR performance of the materials: a high specific surface area (SSA); a high electrical conductivity (achieved by means of a well-developed carbon basal plane structure); and the presence of functional groups containing heteroatoms, mainly aromatic nitrogens. Notably, the ACs exhibited high tolerance to methanol crossover. Finally, as a proof-of-concept, a selected AC was tested in a single-cell Direct Methanol Fuel Cell (DMFC), yielding excellent performance. The results demonstrate the high potential of N-doped ACs as electrocatalysts, inexpensive and versatile materials that can replace costly Pt-based electrodes.