Facile preparation of N-doped mesocellular graphene foam from sludge flocs for highly efficient oxygen reduction reaction

The use of environmental waste products as materials for the production of energy is an extremely attractive prospect for both economic and social development. Sludge flocs (SFs) are environmental waste products that are difficult to handle. We used these SFs as a source of carbon and nitrogen for the preparation of N-doped mesocellular graphene foam (SF-NMGF) via a simple one-step pyrolysis method. The particular composition and structure of the SFs meant that the resultant SF-NMGF had a large Brunauer-Emmett-Teller surface area and consisted of a graphitic framework surrounded by ultrathin nanosheets. The material contained foam-like mesopores with a size centred at about 15 nm and the N was incorporated homogeneously with a high percentage (40.5 at%) of graphitic-N. As a result of these unique properties, the SF-NMGF had an excellent electrocatalytic activity with 4e when used as a metal-free catalyst for the oxygen reduction reaction (ORR). Specifically, the prepared SF-NMGF catalyst exhibited a high diffusion-limited current, superior durability and better immunity towards methanol crossover for the ORR in alkaline solution than a commercial 20 wt% Pt/C catalyst. The synthesis of the SF-NMGF can be scaled up at low cost, which will be beneficial for both sludge handling and the development of materials for the ORR.