Sustainable nitrogen-doped carbons from coking wastewater and spent coffee grounds for electrocatalytic nitrate to ammonia conversion

It is widely recognized that the electrocatalytic activity of carbon materials could be enhanced through nitrogen doping with nitrogen-containing precursors. Coking wastewater, known for its high concentrations of toxic nitrogen compounds, presents an intriguing possibility as a nitrogen-containing precursor for nitrogen-doped carbons (NCs), which was explored in this study. We prepared NCs by directly carbonizing spent coffee grounds soaked in coking wastewater, and investigated their efficacy as electrocatalysts for the nitrate reduction reaction. Results demonstrated that the NC, carbonized at a temperature of 800℃, exhibited a porous and rough morphology, achieving a maximum increase of 334.06% in pyridinic nitrogen, identified as a crucial active site for nitrate electrocatalytic reduction. Additionally, the NC delivered high catalytic activity for nitrate conversion (99.24%) and ammonia selectivity (80.52%), maintaining stable activity over 5 cycles. In-situ infrared spectra revealed cooperative interactions between pyridinic nitrogen and its adjacent carbon atoms, boosting the electrocatalytic conversion of nitrate to ammonia. By coupling the electrocatalytic recycling of waste nitrate with the valorization of coking wastewater, we successfully developed a sustainable approach to producing nitrogen-doped carbon electrocatalysts. This distinctive ‘waste-to-treasure’ perspective provides new opportunities for coking wastewater treatment and can be further utilized for converting nitrate to valuable ammonia products. •N-doped carbon materials derived from pyrolysis of waste coffee grounds dipped by coking wastewater.•Enriched pyridinic N moieties in the prepared N-doped carbon materials.•Enhanced nitrate-to-ammonia activity on N-doped carbon electrocatalysts prepared at 800℃.•Synergistic catalysis between pyridinic N and its adjacent carbon atoms.•High-value utilization of coking wastewater and waste coffee grounds.