Microbial fuel cells (MFCs) are a promising green approach for wastewater treatment with the simultaneous advantage of energy production. Among the various limiting factors, the cathodic limitation, with respect to performance and cost, is one of the main obstacles to the practical applications of MFCs. Despite the high performance of platinum and other metal-based cathodes, their practical use is limited by their high cost, low stability, and environmental toxicity. Oxygen is the most favorable electron acceptor in the case of MFCs, which reduces to water through a complicated oxygen reduction reaction (ORR). Carbon-based ORR catalysts possessing high surface area and good electrical conductivity improve the ORR kinetics by lowering the cathodic overpotential. Recently, a range of carbon-based materials have attracted attention for their exceptional ORR catalytic activity and high stability. Doping the carbon texture with a heteroatom improved their ORR activity remarkably through the favorable adsorption of oxygen and weaker molecular bonding. This review provides better insight into ORR catalysis for MFCs and the properties, performance, and applicability of various metal-free carbon-based electrocatalysts in MFCs to find the most appropriate cathodic catalyst for the practical applications. The approaches for improvement, key challenges, and future opportunities in this field are also explored.

Three-dimensional (3D) N-doped open-porous carbon foam was fabricated using the simple procedure of calcining a melamine sponge. The properties of the fabricated carbon foam and its performance in microbial fuel cells (MFCs) using Shewanella oneidensis MR-1 (S. oneidensis) were compared with those of commercial graphite felt. The MFC with the carbon foam anode produced approximately 2 times higher power density than the commercial graphite felt. The superior performance of the as-prepared carbon foam in MFC was attributed to the higher surface area (687.19 m² g⁻¹) and open-porous scaffold structure. Moreover, the appearance of the hydrophilic functional groups such as CN–C, N–CO on the surface of the as-prepared carbon foam facilitated extracellular electron transfer, resulting in a decrease in charge transfer resistance and an increase in biocompatibility. Owing to the excellent biocompatibility, a large amount of microbial biomass colonized both the surface and inside the carbon foam, which helped enhance the performance of the MFC.

In the present study, an electrochemical exfoliation technique was used to synthesize a series of zinc oxide/graphene (ZnO/Gr) nanocomposites using different concentrations of zinc nitrate in an electrolyte solution. The prepared ZnO/Gr nanocomposites were characterized using a range of analytical techniques and evaluated further for their photocatalytic activity over methyl orange (MO) and rhodamine B (RhB) under UV light. Scanning electron microscopy and transmission electron microscopy showed that the ZnO/Gr nanocomposite synthesized using 7 mmol of zinc nitrate exhibited a sheet-like morphology with a uniform decoration of ZnO nanoparticles over the graphene sheets, as well as the maximum photodegradation capacity for MO (>96%) and RhB (>89%) among the composites prepared. The loading of graphene was found to be crucial for deciding the enhancement in the photocatalytic activity, and resulted in a remarkable improvement in photocatalytic activity (∼7 times for MO and ∼2 times for RhB) over bare ZnO. Photoluminescence spectroscopic analysis revealed the improved separation of electron–hole pairs with the graphene loading, which is mainly responsible for the enhanced photocatalytic activity. The present work possesses the advantage of in situ synthesis of few layered less defective graphene/ZnO composites with enhanced photocatalytic activity.