Zhen He

Microbial fuel cells (MFCs) have been conceived and intensively studied as a promising technology to achieve sustainable wastewater treatment. However, doubts and debates arose in recent years regarding the technical and economic viability of this technology on a larger scale and in a real-world applications. Hence, it is time to think about and examine how to recalibrate this technology's role in a future paradigm of sustainable wastewater treatment. In the past years, many good ideas/approaches have been proposed and investigated for MFC application, but information is scattered. Various review papers were published on MFC configuration, substrates, electrode materials, separators and microbiology but there is lack of critical thinking and systematic analysis of MFC application niche in wastewater treatment. To systematically formulate a strategy of (potentially) practical MFC application and provide information to guide MFC development, this perspective has critically examined and discussed the problems and challenges for developing MFC technology, and identified a possible application niche whereby MFCs can be rationally incorporated into the treatment process. We propose integration of MFCs with other treatment technologies to form an MFC-centered treatment scheme based on thoroughly analyzing the challenges and opportunities, and discuss future efforts to be made for realizing sustainable wastewater treatment.

The upflow microbial fuel cell (UMFC) was developed to generate electricity while simultaneously treating wastewater. During a five-month period of feeding a sucrose solution as the electron donor, the UMFC continuously generated electricity with a maximum power density of 170 mW/m2. To achieve this power density, the artificial electron-mediator hexacyanoferrate was required in the cathode chamber. The power density increased with increasing chemical oxygen demand (COD) loading rates up to 2.0 g COD/ L/day after which no further increases in power density were observed, indicating the presence of limiting factors. The overarching limiting factor for the UMFC in this study was the internal resistance, which was estimated as 84 omega at the maximum power density, and restricted the power output by causing a significant decrease in operating potential. Low Coulombic efficiencies varying from 0.7 to 8.1% implied that the electron-transfer bacteria were incapable of converting all of the available organics into electricity, so the excessive substrate created niches for the growth of methanogens. We found that the soluble COD (SCOD) removal efficiencies remained over 90% throughout the operational period, mainly because of methanogenic activity, which accounted for 35 to 58% of the SCOD removed at a loading rate of 1.0 g COD/L/ day. Additionally, transport limitation due to insufficient substrate diffusion was shown by cyclic voltammetry (CV).

Electrochemical impedance spectroscopy (EIS) is a powerful nondestructive technique that can act as a beneficial addition to the current techniques for studying microbial fuel cells (MFCs). Its application in MFC research should be further explored in the analysis of the internal resistance of MFCs, electrode materials, catalyst coatings on electrodes, biofilm development and electrochemical reactions on the anodes and the cathodes of MFCs.

Abstract This review addresses the development and experimental progress of biocathodes in microbial fuel cells (MFCs). Conventional MFCs consist of biological anodes and abiotic cathodes. The abiotic cathode usually requires a catalyst or an electron mediator to achieve high electron transfer, increasing the cost and lowering the operational sustainability. Such disadvantages can be overcome by biocathodes, which use microorganisms to assist cathodic reactions. Biocathodes are feasible in potentiostat‐poised half cells, but only very few studies have investigated them in complete MFCs. The classification of biocathodes is based on which terminal electron acceptor is available. For aerobic biocathodes with oxygen as the terminal electron acceptor, electron mediators, such as iron and manganese, are first reduced by the cathode (abiotically) and then reoxidized by bacteria. Anaerobic biocathodes directly reduce terminal electron acceptors, such as nitrate and sulfate, by accepting electrons from a cathode electrode through microbial metabolism. Biocathodes are promising in MFCs, and we anticipate a successful application after several breakthroughs are made.

A cost-effective route for the preparation of Fe3C-based core-shell structured catalysts for oxygen reduction reactions was developed. The novel catalysts generated a much higher power density (i.e., three times higher at Rex of 1 Ω) than the Pt/C in microbial fuel cells. Furthermore, the N-Fe/Fe3[email protected] features an ultralow cost and excellent long-term stability suitable for mass production. Detailed facts of importance to specialist readers are published as ”Supporting Information”. Such documents are peer-reviewed, but not copy-edited or typeset. They are made available as submitted by the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.