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.
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 al). It allows to non-destructively evaluate gene expression in anode biofilms in real-time and to provide additional insight into the physiology of current production. New experimental methods may require the development of adapted electrochemical cells, for example, for rapid screening and in situ measurements. For such purposes, microfluidic devices proposed by Li and co-workers may represent a valuable development.
There is a multitude of sophisticated spectroscopic, electrochemical, and biological techniques available to investigate and analyze electroactive biofilms at different hierarchical levels—whole biofilm, cell level, and sub-cell (molecular) level (Harnisch and Rabaey). This multitude of techniques, however, requires the establishment of a standardization framework to allow comparability and complementarity. This need for standardization is also a major emphasis of the Minireview by Logan, in which the critical information that should be provided, techniques that should be used, and protocols that should be followed to produce reproducible and comparable experimental results and studies is discussed.
Having been neglected for a long time, MFC cathodes have recently received considerable research attention. The abandonment of experimental, unsustainable cathodes (e.g., based on ferricyanide), the replacement of platinum as expensive oxygen reduction catalyst, and the search for low-cost alternatives are major tasks to be solved to bring MFCs closer to application. Especially the physical conditions in MFCs, which are significantly different compared to chemical fuel cells, represent a major challenge. Popat and co-workers provide a thorough analysis of the cathodic limitations in MFCs, especially highlighting the impact of pH gradients and OH− accumulation. The oxygen reduction reaction (ORR) limitations are discussed by Erable et al., and the authors further describe the progress in the use of microorganisms as ORR electrocatalysts. There are different mechanisms involved in the microbially catalyzed ORR. Some mechanisms seem specific to the oxygen reduction, others are similar to anodic microbial electrocatalysis, for example, the involvement of endogenous redox mediation in ORR (Liu et al.).
But it is not only the oxygen reduction that is important as a cathode reaction. For microbial electrolysis cells, the hydrogen evolution represents the archetype reaction. For this reaction, chemical electrocatalysts such as platinum, nickel, or tungsten carbide have been proposed, but also microorganisms are able to catalyze the reaction. An example for such microbially catalyzed hydrogen evolution reaction is presented by Aulenta and co-workers, one of the first pure culture studies on microbially catalyzed electrochemical hydrogen evolution.
For many bioelectrochemical reactions, there is a choice between microbial and enzymatic catalysis. This leads to the differentiation between enzymatic and microbial fuel cells. The differences and similarities, the advantages, and the disadvantages are described by Lapinsonnière and co-workers. Their Minireview shows that future developments may even lead to a merging of both worlds by the development of hybrid systems that join their respective advantages.
A future success of microbial bioelectrochemical systems inevitably depends on a further increase of the performance of the bioelectrochemical cells and their components at decreasing material costs. Wei et al. propose the use of lignite semicoke as an inexpensive electrode precursor that can be transformed into an efficient anode material by means of a simple carbonization procedure. Such a carbonization step can also be used to transform natural biomass structures into efficient electrode materials and biofilm habitats. This is illustrated by Chen et al. in a study that exploits the porous structure of kenaf (Hibiscus cannabinus, a crop plant) stems as raw materials to produce high-performance MFC anodes.
But where lie the practical applications for microbial bioelectrochemical systems, and how can these systems become economically successful? Sleutels and co-workers highlight these questions by addressing the factors essential for the practical application of BESs and comparing benefits (value of products and cleaning of wastewater) with costs (capital and operational costs). Based on this, the maximum internal resistance and current density that is required to make MFCs or hydrogen producing microbial electrolysis cells (MECs) cost effective is analyzed. But how can this knowledge be transformed into an applicable system? A crucial point is the engineering of scalable electrochemical cells. Here, Kim et al. suggest the use of modularized, multiple cells. They have analyzed the performance of such cell systems at different levels of electric and hydraulic connection and propose strategies to minimize potential drop phenomena that occur in such multiple-cell systems.
In addition to the actual wastewater treatment aspect that surely represents a major driving force in the development of MFCs, there are further applications that may be able to exploit the advantages of these fuel cells. One of these applications is the energy supply of autonomous robots by means of MFCs (Ieropoulos et al.). More down-to-earth, literally, are plant-MFCs, in which the MFC anode, buried in the soil, operates on plant root exudates as fuel. Besides the aspect of electricity generation, plant-MFCs have demonstrated the reduction of methane emissions, for example, from rice fields. The recent progress in plant-MFCs is reviewed by Deng et al.
Microbial electrochemistry now goes into the second century of its development. BES research, characterized by its inherent multidisciplinarity and spanning from fundamental research to system engineering, remains highly exciting. Further, novel concepts and developments are very likely to come up, and soon microbial bioelectrochemical systems such as MFCs will have to demonstrate and prove their transition from lab bench to application.