Ironies in microbial electrochemistry

Microbial electrochemistry is a cross-disciplinary field that has become a hot topic in environmental engineering and science. The microbial electrochemical cell (MxC) is a technology platform for removing biochemical oxygen demand (BOD) from organic wastes while generating valuable outputs. The unique feature of MxCs is the biofilm of anode-respiring bacteria (ARB) that live on the anode, oxidize organic molecules, and efficiently respire the electron to the anode by extracellular electron transport (EET). Valuable outputs depend on the reduction reaction at the cathode: They include electrical power, hydrogen gas, hydrogen peroxide, and acetate. Those wanting to create viable MxC technologies need to recognize four important ironies about what actually controls the performance of a MxC: (1) although the unique features of a MxC are the ARB and EET of electrons to the anode, the anode's current density is determined by the diffusion of protons out of the biofilm, which depends on a weak-base buffer; (2) the main energy loss in a MxC is at the cathode, and minimizing cathode overpotential depends on the transport of hydroxide away from the cathode with a weak-acid buffer; (3) selection and accumulation of the most efficient ARB in the biofilm anode is brought about by using a stressful negative anode potential, even though a high anode potential gives the fastest EET once the biofilm is established; and (4) efficient ARB have adaptable EET that takes advantage of changes of potential and pH within the biofilm.