Defined and unknown roles of conductive nanoparticles for the enhancement of microbial current generation: A review

•Metallic and semiconductive nanoparticles (NPs) improve microbial electrocatalysis.•Exoelectrogen utilizes a variety of extracellular electron transfer (EET) mechanisms.•A long-distance electron transport pathway forms in exoelectrogen/NPs aggregation.•Biosynthesized NPs localized in outer membrane function as an EET pathway.•Importance of physiological interaction between bacteria and NPs is suggested. The ability of various bacteria to make use of solid substrates through extracellular electron transfer (EET) or extracellular electron uptake (EEU) has enabled the development of valuable biotechnologies such as microbial fuel cells (MFCs) and microbial electrosynthesis (MES). It is common practice to use metallic and semiconductive nanoparticles (NPs) for microbial current enhancement. However, the effect of NPs is highly variable between systems, and there is no clear guideline for effectively increasing the current generation. In the present review, the proposed mechanisms for enhancing current production in MFCs and MES are summarized, and the critical factors for NPs to enhance microbial current generation are discussed. Implications for microbially induced iron corrosion, where iron sulfide NPs are proposed to enhance the rate of EEU, photochemically driven MES, and several future research directions to further enhance microbial current generation, are also discussed.