Nitrogen-doped porous carbon derived from digested sludge for electrochemical reduction of carbon dioxide to formate

Carbon-based materials have been applied as cost-effective electrocatalysts to reduce carbon dioxide (CO₂) into valuable chemicals. Here, an environment-friendly method is proposed to obtain nitrogen-doped porous carbons (NPCs) from digested sludge, which is an abundant waste product from sewage treatment plants. The materials were used as a metal-free electrocatalyst for electrochemical reduction of CO₂ to formate. The synthesized material (NPC-600) had a mesoporous and microporous structure with a specific surface area of 246.21 m² g^-1 and pore volume of 0.494 cm³ g^-1. Active sites based on nitrogen atoms accounted for 2.98 atom% of the content and included pyrrolic-, pyridinic-, and graphitic-N, which is useful for CO₂ adsorption and electron transfer in electrochemical reduction. The Faradaic efficiency for formate production from CO₂ in the presence of NPC-600 was 68% at the potential of -1.5 V vs. SCE. Tafel analysis indicated that the pathway of CO₂ conversion involved the reduction of CO₂ to CO₂*^- intermediate, which was then converted to HCOO*^- and finally formate.