The enzymatic electrosynthesis of formic acid from the reduction of carbon dioxide (CO₂) by using formate dehydrogenase (FDH) as a catalyst at the cathode in both its free and immobilized forms was studied in detail in a bioelectrochemical system (BES). The essential role of solubilizing CO₂ for its conversion was also studied by adding carbonic anhydrase (CA) to the FDH enzyme in both its free and immobilized forms. FDH alone in the free form showed large variation in the reduction current [(−6.2±3.9) A m⁻²], whereas the immobilized form showed less variation [(−3.8±0.5) A m⁻²] due to increased enzyme stability. The addition of CA with FDH increased the consumption of the current in both forms due to the fact that it allowed rapid dissolution of CO₂, which made it available for the catalytic reaction with FDH. Remarkably, stable consumption of the current was observed throughout the operation if both CA and FDH were immobilized onto the electrode [(−3.9±0.2) A m⁻²]. Product formation by the immobilized enzyme was also continued for three repetitive cycles, which revealed the longevity of the enzyme after immobilization. The recyclability of NADH (NAD=nicotinamide adenine dinucleotide) was also clearly evidenced on the derivative voltammetric signature. Extension of this study for continuous and long‐term operation may reveal more possibilities for the rapid capture and conversion of CO₂.

In this study, domestic wastewater was given a second life as dilution medium for concentrated organic waste streams, in particular artificial food waste. A two-step continuous process with first volatile fatty acid (VFA)/hydrogen production and second electricity production in microbial fuel cells (MFCs) was employed. For primary treatment, bioreactors were optimized to produce hydrogen and VFAs. Hydrolysis of the solids and formation of fermentation products and hydrogen was monitored. In the second step, MFCs were operated batch-wise using the effluent rich in VFAs specifically acetic acid from the continuous reactor of the first step. The combined system was able to reduce the chemical oxygen demand load by 90%. The concentration of VFAs was also monitored regularly in the MFCs and showed a decreasing trend over time. Further, the anode potential changed from -500 to 0 mV vs. Ag/AgCl when the VFAs (especially acetate) were depleted in the system. On feeding the system again with the effluent, the anode potential recovered back to -500 mV vs. Ag/AgCl. Thus, the overall aim of converting chemical energy into electrical energy was achieved with a columbic efficiency of 46% generating 65.33 mA/m² at a specific cell potential of 148 mV.