Nitrogen-doped hydrochar-derived bifunctional cathodes from residual carbon sources for enhanced electro-Fenton degradation under circumneutral pH conditions

This study investigates the potential of hydrochar derived from residual carbon sources (orange peel residues, sewage sludge, and refinery sludge) as sustainable precursors for bifunctional cathodes in electro-Fenton (EF) systems, addressing key operational limitations of conventional electro-Fenton systems, particularly the need for acidic pH and external iron dosing. The hydrochars were chemically activated with potassium hydroxide (KOH) in the presence of urea or melamine at temperatures of 700, 850, and 1000 ºC. KOH activation markedly enhanced porosity and surface area, while nitrogen doping introduced electroactive pyridinic and graphitic species that promote oxygen reduction reactions and hydroxyl radical generation. Electro-Fenton degradation of phenol, used as a model pollutant, demonstrated the superior performance of KOH-activated urea-derived hydrochars compared to melamine-derived and non-activated counterparts. Among all tested materials, the urea-derived hydrochar activated at 850 °C exhibited the highest activity, achieving 90% phenol removal and 80% mineralization within 30 min under homogeneous EF conditions (pH 3, 2 mM Fe 2+ ). Remarkably, this material also exhibited outstanding performance under heterogeneous EF conditions (circumneutral pH, without external iron addition), achieving 80% phenol removal and 70% mineralization. This behaviour highlights the ability of the system to operate efficiently under circumneutral pH without external iron addition, significantly simplifying the process and reducing risks associated with chemical handling and secondary iron contamination. Overall, this study demonstrates a practical strategy to simplify electro-Fenton processes and enhance their applicability under safer and more sustainable operating conditions.