Ion transport in microbial fuel cells: Key roles, theory and critical review

Microbial fuel cells (MFCs) offer the possibility to convert the chemical energy contained in low-cost organic matter directly into electrical energy. Nevertheless, in the current state of the art, microbial electrocatalysis imposes the use of electrolytes of low ionic conductivity, at around neutral pH and with complex chemical compositions, which are far from being ideal electrolytes for an electrochemical process. In this context, ion transport through the electrolyte is a key step, which strongly conditions the electrode kinetics and the global cell performance. The fundamentals of ion transport in electrolytes are recalled and discussed in the light of MFC constraints. The concept of transport number is emphasized in order to provide an easy-to-use theoretical framework for analysing the pivotal roles of ion transport through the electrolyte. Numerical illustrations show how the concept of transport number can be used to predict MFC behaviour on the basis of the electrolyte composition. The tricky problem of pH balance is discussed, the interest of separator-less MFCs is emphasized, and the concept of “microbial separator” is proposed as a worthwhile future research direction. The role of separators in driving ion transport is then reviewed following a comprehensive classification, from the most compact, non-porous membranes to the most porous separators. For each group, basic are first recall to guide the critical analysis of the experimental data. This analysis brings to light a few research directions that may be reoriented and some critical issues that need in-depth investigation in the future. The suitability of cation-/anion-exchange membranes is discussed in comparison to that of porous separators. Finally, the large discrepancy observed on data obtained for the same separator type suggests that, in the future, specific analytical set-ups should be developed.