Sludge-derived hydrochar as a potential electrocatalyst for improved CO2 reduction in microbial electrosynthesis†

Microbial electrosynthesis (MES) is a progressive technology that can sequester carbon dioxide (CO₂) to produce high-value multi-carbon organic compounds. However, the limited organic production rate is the primary bottleneck, limiting the real-life application of this technology. To overcome this challenge, the present investigation explores sludge-derived hydrochar as a cathode catalyst to enhance CO₂ bioreduction in MES. The hydrochar composite synthesized using anaerobic sludge (ANS) and alum sludge (ALS) exhibited excellent electrochemical properties with higher limiting current density and lower charge transfer resistance. Additionally, key structural properties, such as elevated specific surface area, abundant surface functional groups, and the presence of nitrogen in the form of pyridinic and graphitic nitrogen, are primarily responsible for enhancing the organic product synthesis in MES. Furthermore, the hydrochar composite catalyzed MES resulted in an acetate production of 41.14 ± 5.03 mM L⁻¹, which was nearly twice that of the uncatalyzed MES. Moreover, the current and carbon recovery efficiencies were found to be 52.44% and 45.44%, which were 1.47 and 2.44 times that of uncatalyzed MES. These results demonstrate the potential of sludge-derived hydrochar as a promising cathode electrocatalyst for enhancing CO₂ bioreduction in MES.