Using an anion exchange membrane for effective hydroxide ion transport enables high power densities in microbial fuel cells

High current densities have been obtained by bioanodes in electrochemical cells, but this performance has not previously been translated into higher power densities in microbial fuel cells (MFCs). The most critical factor for boosting power generation in MFCs was proven here to be OH^– ion transfer from the cathode to the bioanode by comparing performance of an MFC with an anion exchange membrane (AEM), in a near zero-gap configuration and no bulk catholyte, to that obtained using a cation exchange membrane (CEM) or non-ion selective ultrafiltration membrane (UFM). In the AEM-MFC hydroxide ion transport primarily balanced charge transfer between the electrodes, enabling 8.8 ± 0.5 W m⁻² (at 42 ± 1 A m⁻²) which is highest power density ever achieved using this anolyte (acetate in a 50 mM phosphate buffer medium). The power density with the AEM was 3 × higher than that obtained using a CEM (3.1 ± 0.1 W m⁻²) or UFM (2.4 ± 0.1 W m⁻²) as these other two membranes allowed cations (sodium and magnesium) to be transported to the cathode for balancing charge. The lack of cation transport into the cathode using the AEM without a bulk catholyte also avoided salt precipitation in the cathode, and thus enabling more stable power production over time. The large differences in power obtained using the AEM, compared to the CEM or UFM conclusively demonstrate the critical role of OH^– ion transport in MFCs.