# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 3 Electrocatalytic Nanomaterials for Promoting EET
### Precious Metals  
The application of precious metals, such as platinum (Pt), gold (Au), silver (Ag), and palladium (Pd), in microbial electrodes is an area of interest due to their unique properties, including high electrical conductivity, corrosion resistance, and catalytic activity [67]. In the early stages of research, noble metal nanoparticles were mainly used to modify carbon-based electrodes to improve their electrochemical interaction with EAMs. Both the Pt nanoparticle-decorated graphene [68] and the Pd nanoparticle-coated carbon cloth [69] significantly enhanced the electrochemical dynamics of the microbial anodes, resulting in an increase in MFC power density. However, recent studies have found that directly functionalizing EAM cells with these electrocatalyically active noble metal nanostructures, which can be biosynthesized in situ by the EAM cells, is a more effective and facile route to improve their EET ability [70; 71]. It was reported that Au nanoparticles could be precipitated in situ into a _G. sulfuruducens_ biofilm through the biological reduction of the Au(III) precursor in a continuous-flow three-electrode MFC, which increased the conductivity of biofilm and the electricity generation [20]. Recently, [PERSON] et al. developed reduced graphene oxide-silver nanoparticle (rGO/Ag) scaffolds for the anode of _Shewanella_ MFC and found that the rGO/Ag scaffolds could release silver ions to facilitate _Shewanella_ attachment to them and produce the _Shewanella_-Ag hybrids [72]. Particularly, the resulting hybrid species showed the presence of abundant Ag nanoparticles, both inside and throughout the bacterial membrane, which could potentially act as metallic shortcuts to bypass the sluggish electron transfer process mediated by the biological redox centers and make direct contact with the external electrodes to extract the charge more efficiently, thereby contributing to the record-breaking power density of _Shewanella_ MFC.  
Biologically produced Pd nanoparticles are the most common in bacteria-associated catalytic systems, mainly due to their outstanding catalytic activity and ease of biosynthesis [73]. Back in the early 2010s, the biogenic Pd nanoparticles bound to the cell membrane of _Desulfovibrio desulfuricans_ have been demonstrated to participate in EET through the interaction with periplasmic hydrogenases and Cyts, as well as take part in electrocatalysis on an electrode surface [74]. Importantly, a variety of bacteria (e.g., _Enterobacter cloace_ SgZ-5T [75], _Citrobacter freundii_ JH [76], _Bacillus megaterium_ Y-4 [77], _G. sulfuruducens_[78], _S. Ioilica_ PV-4 [79], and _Escherichia coli_[80]) have been reported to be able to, when in situ, synthesize Pd nanoparticles to improve their electron transport processes for different applications, including the electrochemical hydrogen/oxygen evolution reaction and pollutant degradation. Moreover, the phenotype of biosynthesized Pd nanoparticles could be tailored through by controlling the biological reduction pathways [81], reaction conditions [82] and organic electron donors [83], which provides more opportunities for their practical applications. Particularly, as illustrated in Figure 3, the biosynthetic location and size of the Pd nanoparticles could be modulated through controlling the expression and availability of two key EET-involving components of _S. oneidensis_ MR-1--the outer-membrane Cyts (MtrC) and electron shuttles (flavins) [81]--which indicated the possibility of customizing biogenic Pd nanoparticles to, in turn, promote a microbial EET in situ.