# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 4 Conclusion and Perspectives  
With the concept that microbial electrochemical systems are collaborative systems of microbial catalysis and electrocatalysis, this tutorial review presents a summary of electrocatalytic nanomaterials used for promoting the microbial EET process and a discussion on the underlying effect mechanisms. A series of nanostructured catalysts with highly electrocatalytic activity, including heteroatom-doped nanocarbons, precious metals, and transition metal compounds (TMOs, TMSs, TMCs, and TMNs), have been demonstrated to show great merits in boosting both the outward and inward EET through diverse enhancement mechanisms. The functionalization of microbial electrodes with nanomaterials generally offers advantages in reducing interfacial electron exchange resistance, promoting EAM cell adhesion and biofilm growth, and providing more active sites for electrochemical redox reactions. More impressively, these electrocatalytic nanomaterials showed unique features that improve the microbial EET kinetics by tuning the redox potential of outer-membrane Cyrs and electron shuttles and heightening the amounts of available EET components by stimulating the related biosynthetic metabolism. These pieces of evidence not only further demonstrate the inherent bridge between the biocatalysis of EAMs and electrocatalysis of electrodes but also show a promising avenue for the realization of high-performance microbial electrochemical systems through rational electrode design and fabrication. Of course, in order to achieve the above goals at an early date, we believe that more efforts need to be made in the following areas in the future. Firstly, the application of electrocatalytic nanomaterials is still mainly concentrated in the MFC anodes with outward EET and relatively little in the MES cathodes with inward EET. However, given the deterioratingclimate and depleting resources, MESs that convert CO\({}_{2}\) to highly valuable chemicals using electricity as electron donors should be more competitive in future practical applications. Therefore, more efforts need to be devoted to the design and use of electrocatalytic nanomaterials to strengthen the performance of MES cathodes and to comprehensively investigate the underlying mechanisms. Meanwhile, the construction of an artificial photosynthesis system by the direct hybridization of EAMs with nanocatalysts with semiconductor properties is emerging and deserves rapid development. Secondly, electrocatalytic nanomaterials can directly impact the microbial EET components and even the intracellular metabolism. Thus, the biological modification of EAMs by means of systematic and synthetic biology is a new and promising direction to realize the adaptive connection between biocatalysis and electrocatalysis. Thirdly, the microbial EET pathway at the bio-abiotic interface is a highly complex process, and the specific mechanisms, especially inward EET, are not well understood. This is a key obstacle to the rational design of electrocatalytic materials and the modification of biocatalytic cells. In addition, the phylogenetic diversity of EAMs further complicates the EET pathways. Therefore, it is not only necessary to use various multidisciplinary techniques to clarify the EET mechanism of specific EAM strains deeply but also to use artificial intelligence technologies for big data analyses and simulation, which is expected to accelerate the development and commercialization of microbial electrochemical systems. Conceptualization, X.W. and X.L. ; validation, X.W. and Q.Z. ; formal analysis, X.W. ; investigation, X.W. ; writing--original draft preparation, X.W. ; writing--review and editing, X.L. and Q.Z. ; visualization and funding acquisition, X.W. All authors have read and agreed to the published version of the manuscript. This research was funded by the Science and Technology Research project of the Jiangxi Provincial Department of Education, grant number 191558. The research data presented in this study are available upon request from the corresponding authors.