Cathode catalyst-assisted microbial electrosynthesis of acetate from carbon dioxide: promising material selection

As the core of cathode materials, sensitive metals play important roles in the optimization of acetate production from carbon dioxide (CO₂) in microbial electrochemical system (MES). In this work, iron (Fe), copper (Cu), and nickel (Ni) as sensitive metal cathode materials were evaluated for CO₂ conversion in MES. The MES with Fe-electrode as a promising electrode material demonstrated a superior CO₂ reduction performance with a maximum acetate accumulation of 417.9 ± 39.2 mg/L, which was 1.5 and 1.7 folds higher than that in the Ni-electrode and Cu-electrode groups, respectively. Furthermore, an outstanding electron recovery efficiency of 67.7 % was shown in the Fe-electrode group. The electron transfer between electrode-suspended sludge was systematically cross-evaluated by the electrochemical behavior and extracellular polymeric substances. The Fe-electrode group had the highest electron transfer rate with 0.194 s^-1 (k_app), which was 17.6 and 21.5 times higher than that of the Cu- and Ni-electrode groups, respectively. Fe-electrode was beneficial for reducing electrochemical impedance between the electrode and suspended sludge. Additionally, redox substances in extracellular polymeric substances of the Fe-electrode group were increased, implying more favorable electron transport dynamics. Simultaneously, enrichments of functional bacteria Acetoanerobium and increased key enzymes involved in the carbonyl pathway of the Fe-electrode group were observed, which also promoted CO₂ conversion in MES. This study provides a perspective on evaluating the promising sensitive metal electrode material for the process of CO₂ valorization in MES and offers a reference for the subsequent electrode modification.