Editorial: Microbial Fuel Cells and Microbial Electrochemistry: Into the Next Century!

Abstract

In the year 1911 the first publication appeared that reported on electrochemical effects caused by the activity of microorganisms. This study is considered as the hour or birth of microbial fuel cells (MFCs). MFCs are the archetype microbial bioelectrochemical system (BES), which produce electricity from microbially catalyzed anodic oxidation processes. The greatest potential of MFCs lies in the use of wastewater as fuel, which allows combining wastewater treatment and energy recovery. Since that first study more than hundred years of development have passed. Yet, it has only been during the last ten years that microbial fuel cell research started to develop an impressive momentum (see Figure 1). Thus, since the year 2002 the number of publications has been increasing almost exponentially. New types of microbial BESs have been proposed, including microbial electrolysis cells (Figure 2) and microbial desalination cells. The research field has developed from a scientific peculiarity into an increasingly differentiated, highly dynamic, and productive field that can be denominated as microbial electrochemistry. The growing community now also led to the foundation of the International Society for Microbial Electrochemical Technologies, ISMET, a society with the goal to link researchers and engineers in the area of bioelectrochemical systems.1 Number of publications from 1980–2011 containing the phrase "microbial fuel cell" (SciFinderScholar, May 2012). The type of bioelectrochemical cell depends on the cathode reaction: (a) oxygen reduction in MFCs, (b) hydrogen evolution, and (c) CO2 reduction in microbial electrolysis cells. This special issue of ChemSusChem is dedicated to provide a current perspective on the field of microbial electrochemistry—especially on MFCs. The issue contains invited papers from leading groups in a wide range of aspects ranging from fundamental biological and electrochemical understanding, material research, and system engineering to questions referring to potential applications and economic feasibility. This selection can be only a fragmentary view of the rapidly growing research field, and many excellent groups could not be considered due to the limited space of such an issue. The study of the electron transfer mechanisms within microbial biofilms is certainly one of the most exciting and challenging aspects of BES research. How is it possible that electroactive bacteria such as Geobacter sulfurreducens transfer electrons along distances up to several hundred micrometers long? Here, recent years have seen very interesting developments and debates. Malvankar and Lovley discuss biofilm conductivity based on the groups' recent findings on the electron transfer through microbial nanowires, whereas Bond et al. propose an alternative mechanism based on the electron transfer via a network of extracellular cytochromes. Their model is denominated as superexchange electron transfer. Evidence for this electron transfer path is presented in an experimental, cyclic voltammetric study (Strycharz-Glaven and Tender), in which Geobacter sulfurreducens biofilms were investigated at different stages of biofilm growth. The study illustrates that it is not just the extracellular electron transfer that determines the electrochemical biofilm performance, but also the electron transport from within cells to their outside. This aspect is further elaborated in a spectroelectrochemical study exploiting the cytochromes' electrochromism, that is, the change of their VIS absorption properties upon redox switching. Liu and Bond conclude that despite of recent progress and new theoretical considerations all models of electron transfer through the G. sulfurreducens biofilms remain speculative. This statement certainly calls for the development of new experimental methods for the investigation of electroactive microbial biofilms. In this respect, a promising biological technique is real-time spatial gene expression analysis (Franks et a