Valorizing Coffee Waste into N-, P-, O-Heteroatom-Doped Carbons for Metal-Free Electrocatalysis

Spent coffee grounds were valorized as a renewable feedstock to prepare N-, P-, and O-codoped activated carbons via pyrolysis, targeting the development of metal-free electrocatalysts for the oxygen reduction reaction (ORR). Hexachlorocyclotriphosphazene (HCCP) was used as a doping agent, and the influence of different porosity activators such as NH4HCO3, KOH, and oxalic acid was studied. This work employs NH4HCO3 as a benign, metal-free activator and secondary nitrogen source compared against conventional porosity agents. Among the mild activators, NH4HCO3 simultaneously provided nitrogen and released NH3 and CO2, while oxalic acid generated a clean CO2 stream, both contributing to mesoporous structure formation. XPS confirmed the successful incorporation of N and P into the carbon matrix, with a high fraction of graphitic nitrogen enhancing the conductivity and catalytic activity. Raman spectroscopy, TEM, and XRD analyses revealed predominantly sp2-hybridized, turbostratic carbon, with higher structural order at 900 °C compared to 700 °C. The combined effect of pyrolysis temperature, HCCP, and NH4HCO3 produced mesoporous, nitrogen-rich graphitic carbon structures with promising ORR performance, demonstrating a green strategy that maximizes atom economy, minimizes energy input and waste, and transforms low-value biomass into high-performance metal-free electrocatalysts.