Utilization of Microbe-Derived Electricity for Practical Application

Abstract

T finding that microorganisms can convert chemical energy to electricity via respiration has inspired intensive research interests and rapid progress of microbial fuel cell (MFC) and its various derivative technologies, which are collectively called as bioelectrochemical systems (BESs). BESs offer an opportunity to directly recover electric energy from wastewater or for other applications, such as environmental remediation and chemical synthesis. Today, while efforts in the former direction of BES are continuing, its expanded applications beyond electricity recovery are gradually becoming a new focus of research. We believe that these beneficial opportunities to apply BES technology, alternative to direct large-scale power generation, should be pursued more aggressively. Currently, there is still very limited success in scaling-up and long-term operation of BES despite of the intensive studies over the past decade, which has leads to extensive concerns about the practical feasibility of this technology. Can MFC ultimately become an energy producer as we originally expected? How far are BESs from a real-world application? These are critical questions to be answered for guiding the research efforts for BES development, especially at the point when many of the BES processes have now become close to the threshold from laboratory bench to technological implementation. It should be recognized that the practical feasibility of a BES could be closely associated with its application niches. Hence, it is interesting to know which application niche, if any, would be more practically achievable. Herein, we offer a preliminary comparison on the practicability of BES for power generation and nonelectricity-recovery applications based on a cost balance analysis. For simplification, hydrogen-producing microbial electrolysis cells (MECs) is taken as an example of the latter niches. To be a practically viable technology, BES must have acceptable costs. A recent study shows that, to meet an economically balanced operation, an MFC requires the internal resistance <40 mΩ/m (relative to anode area, hereafter the same) at a current density of 25 A/m, whereas an MEC only needs to have an internal resistance <80 mΩ/m and reaches 20 A/m current. Clearly, an economic balance of MECs is theoretically more easily achievable than MFCs. Now let us have a look at their actually achievable levels. For the most reported liter-scale MFCs the internal resistance is generally above 100 mΩ/m and the current density below 5 A/m (Figure 1). By contrast,