Impact of the surface chemical composition on the ORR activity of metal-free activated carbon-based electrocatalysts and their performance in DMFCs

Nine activated carbons (ACs) with hierarchical micro- and mesoporous textural architectures and varied chemical compositions were evaluated as metal-free electrocatalysts for the Oxygen Reduction Reaction (ORR) in an alkaline medium. A commercial sample, chemically activated with H 3 PO 4 , served as the starting material. It was modified either via oxidative treatment with HNO 3 to increase the content of acidic oxygen-containing functional groups (OFGs), or via heat treatment in an inert atmosphere up to 800 °C to remove the existing acidic OFGs. The pristine and modified ACs were further subjected to N-doping via ammonization up to 400 or 800 °C. The electrochemical results revealed a synergistic effect among three critical factors that enhance ORR activity: a high micropore volume (primarily narrow micropores); a high degree of aromaticity; and the presence of heteroatoms (especially aromatic nitrogen species). In this context, the highest ORR activities were observed for the materials ammonized up to 800 °C. To the best of our knowledge, this work represents the first systematic study to correlate the synthesis conditions of metal-free AC-based electrocatalysts with their physicochemical properties and, subsequently, to link these properties to the materials' electrochemical performance. The prepared materials exhibited exceptional tolerance to methanol crossover and superior durability compared to a commercial Pt/C reference electrocatalyst. Remarkably, a selected sample was successfully employed as the cathode of a single-cell Direct Methanol Fuel Cell (DMFC). These results demonstrate that N-doped ACs, which are inexpensive and versatile materials, can effectively replace costly Pt-based electrodes as ORR electrocatalysts.