Effect of buffer charge on performance of air-cathodes used in microbial fuel cells

• Effect of buffer charge and pKa was examined on cathode performance.
• Cationic buffers enhanced current by as much as 95% at a pH below 8.
• There was no impact on current for buffers with a neutral or negative charge.
• The range of potential is expanded for an increase in net positive charge.

In microbial fuel cells (MFCs), buffers are typically used to improve performance by stabilizing the electrode pH and increasing the electrolyte conductivity, but the importance of the buffer net charge at current densities typical of MFCs on cathode performance has received little attention. Current production results in an electric field that drives positive ions towards the cathode, and negative ions to the anode. A series of biological buffers were selected with positive, negative, and neutral charges that had pKas ranging from 5 to 10.8. Cathodic current production using these different buffers in solutions with different pHs and conductivities was compared using linear sweep voltammetry (LSV). At lower pHs, buffers with positive charge increased cathodic current by as much as 95% within certain ranges (potential windows) of cathode potentials. No difference in cathodic current was shown in current for buffers with neutral or negative charge. The reason for this increase with the net positive charge buffers was likely due to a more stable electrode pH produced by electric field driving the positively charged ions towards the cathode. The potential window for the positively charged buffers was positively correlated to the concentration of cationic buffer in the electrolyte. At a pH higher than 9, no improvement in cathodic current was shown for buffers with positive charge, indicating at these higher pHs diffusion dominated buffer transport.