On the use of cyclic voltammetry for the study of anodic electron transfer in microbial fuel cells

Although the mechanisms of bioelectrocatalytic substrate oxidation processes in microbial fuel cells and, especially, of anodic electron transfer are of utmost importance for the performance of microbial fuel cells, little is known, so far, of the nature of the underlying mechanisms. For this reason, research activities in this field have considerably intensified over the past years. Different concepts and mechanisms for electron transfer from the biocatalyst to the fuel cell anode have been proposed. Thus, it can be distinguished between direct electron transfer (DET) and mediated electron transfer (MET) mechanisms. Examples of DET are, the electron transfer via membrane bound cytochromes (e.g., from Geobacter sp., Rhodoferax ferrireducens) or via conductive bacterial pili (‘‘nanowires’’), as recently proposed for Shewanella oneidensis MR-1. MET, on the other hand, has been reported to occur either by primary metabolites (e.g., hydrogen, formate) or by secondary metabolites, such as phenazine derivates or quinones or flavines, as has just been discovered for Shewanella oneidensis MR-1. Many of these proposed transfer mechanisms are of a putative nature and are controversially discussed. Often, the involved redox species are barely identified or understood. Some of the greatest challenges in their study are (i) the complexity of the microbial metabolism, (ii) the often extremely low concentrations of the involved redox species and (iii) the complex (electro)chemical nature of bacterial cultures and even of microbial cell membranes, which may contain several redox active species that do not necessarily contribute to the bioelectrocatalytic current flow. Cyclic voltammetry is a standard tool in electrochemistry and has regularly been exploited to study and to characterize the electron transfer interactions between microorganisms or microbial biofilms and microbial fuel cell anodes. In most of these publications the microorganisms have been studied either in their bioelectrocatalytically active state or in the inactive state. In this communication, we demonstrate that by applying cyclic voltammetry at different stages of microbial growth and metabolic activity, valuable information on the anodic electron transfer processes in microbial fuel cells can be gained. We also show that due to the complexity of the underlying processes, the simplified assumptions and models of anodic electron transfer are difficult to prove.