Enhancing the performance of an osmotic microbial fuel cell through self-buffering with reverse-fluxed sodium bicarbonate

Osmotic microbial fuel cells (OsMFCs) combine the merits of microbial fuel cell (MFC) and forward osmosis (FO) for simultaneous contaminant removal, electricity generation, and high-quality water extraction. As an FO based technology, reverse solute flux (RSF) is one of the key challenges for its operation. Herein, RSF was converted into a positive effect on the system performance by using NaHCO₃ solution as a draw solution (DS)/catholyte. It was found that reverse-fluxed NaHCO₃ helped buffer the anolyte pH and thus enhance electricity generation, compared to the OsMFC using the NaCl DS/catholyte. At the same concentration, the NaHCO₃ DS/catholyte achieved a higher Coulomb production of 1349.2 ± 80.3 C and higher anolyte pH of 6.48 ± 0.19 than those of the NaCl DS/catholyte. At the same conductivity, the NaHCO₃ DS/catholyte exhibited better electricity generation performance with a comparable recovered water volume of 417.7 ± 13.7 mL to that of the NaCl DS/catholyte. As the NaHCO₃ concentration increased from 0.1 M to 0.75 M, the OsMFC electricity generation was enhanced due to the increased RSF from 19.2 ± 2.3 to 210.8 ± 17.5 mmol m⁻²h⁻¹. In the anode, 92.0 ± 0.8% to 97.1 ± 0.9% of reverse-fluxed NaHCO₃ was used to neutralize protons. These results have demonstrated a new strategy that uses the bicarbonate migration driven by both a concentration gradient and electricity generation to successfully raise the alkalinity of the anolyte towards enhancing electricity generation.