Metal-free nitrogen-doped carbon-based electrocatalysts for oxygen reduction reaction in microbial fuel cells: Advances, challenges, and future directions

One of the major obstacles to microbial fuel cell (MFC) development is the design of high-performance, durable, and cost-effective electrocatalysts for the oxygen reduction reaction (ORR) to improve system performance during the electrochemical process. Accordingly, metal-free nitrogen-doped carbon-based electrocatalysts, in different forms/families, have been brought into the spotlight as a promising alternative to address this challenge. In this critical review, we comprehensively focus on recent advances in the design of this type of electrocatalyst for application in MFCs. We discuss the main drawbacks in applying metal-free nitrogen-doped carbon-based electrocatalysts through different angles, from nano-scale challenges like the interaction of nitrogen species during the ORR process and identifying the main active sites in various nitrogen species, to macroscale issues such as different synthesizing methods during electrode preparation, MFC experiment conditions, and long-term operation, economic and cost assessment, just to name a few, that bridge lab-scale experiments to future real-world prototypes. Indeed, this review aims to open new windows for applying metal-free nitrogendoped carbon-based catalysts in MFCs by addressing the gaps between fundamental understanding of fabrication of this type of catalyst to applied engineering point of view for practical applications. Finally, by discussing the most important remaining challenges, we outline a conceptual framework for future researches.