Nitrogen-rich biochar electrodes from urban green waste for microbial CO2 electroreduction in bioelectrochemical systems

Bioelectrochemical systems (BES), including microbial electrosynthesis (MES), represent a sustainable route for carbon recycling through CO2 electroreduction driven by electroactive microorganisms. However, their performance is often limited by sluggish cathodic reactions and the high cost of efficient electrodes. Nitrogen-rich biochar obtained from biomass provides a low-cost, conductive, and porous matrix with abundant active sites, making it suitable for enhancing electron transfer and catalytic activity. In this study, two biomass precursors with distinct nitrogen contents-hazelnut shells (HZS, low N) and urban green waste (UGW, high N)-were screened for biochar electrode production. Physicochemical and electrochemical analyses identified UGW as the most promising feedstock. The pyrolysis and activation processes were optimized by tuning temperature, residence time, and activation strategies (KOH and CO2 flow) to maximize nitrogen retention up to 2.90 wt% and porosity in the 450-613 m2g-1 range. The resulting UGW-derived biochar exhibited partially graphitized, nitrogen-enriched structures with high activity for oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), and CO2 electroreduction under near-neutral conditions. When used as cathodes in MES cells, these materials promoted enhanced CO2 fixation and supported microbial communities dominated by Clostridiaceae and Eubacteriaceae, achieving average current densities of around 0.20 mA cm-2 over 21-day chronoamperometric tests, consistently higher than those of the biochar-free control. These results highlight UGW-derived nitrogen-rich biochars as sustainable cathode materials enabling efficient CO2 electroreduction and MES for circular carbon utilization.