# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 3 Electrocatalytic Nanomaterials for Promoting EET
### Metal Carbides  
Transition metal carbides (TMCs), such as Mo\({}_{2}\)C and WC, are also widely used in diverse electrocatalytic reactions, including microbial electrocatalysis (Table 1), due to their Pt-like electric and catalytic characteristics. As early as 2010, [PERSON] et al. found that the MFC power density delivered by the Mo\({}_{2}\)C anodic catalyst could be compared to that obtained by the Pt anodic catalyst [136]. A possible contribution of nano Mo\({}_{2}\)C catalyst to microbial electricity production was its electrocatalytic activity towards the oxidation of hydrogen, a common electron-rich metabolite of microorganisms [126]. In addition, [PERSON] et al. proved that the nanoporous Mo\({}_{2}\)C catalyst, grown on carbon fibers, showed significant electrocatalytic activity towards endogenous electron shuttle flavins with an obvious negative shift of formal redox potential, which indicated a more favorable thermodynamic driving force for anodic indirect EET [19]. To further improve the EET at the _S. oneidensis_ MR-1/anode interface, they introduced the highly conductive rGO to overcome the insufficient conductivity of Mo\({}_{2}\)C nanoparticles, which produced a record-high MFC power density [127]. Considering that pure Mo\({}_{2}\)C nanoparticles are difficult to synthesize and easy to aggregate, [PERSON] et al. prepared a highly dispersed polydopamine-modified Mo\({}_{2}\)C/MoO\({}_{2}\) nanoparticle as an anode electrocatalyst by using polydopamine that contained amine and phenol groups as the soft template, which achieved an obvious increase in MFC power density compared to the unmodified Mo\({}_{2}\)C/MoO\({}_{2}\) catalyst [137]. Recently, nanostructured Mo\({}_{2}\)C has also been shown to increase the electrocatalytic activity of microbial cathodes greatly. For example, a three-dimensional macroporous carbon co-decorated with polydopamine and Mo\({}_{2}\)C (Mo\({}_{2}\)C/N-doped LS), when used as an MES cathode, achieved an increased acetate production rate by 2.5 times compared with carbon felt, resulting in a maximum acetate concentration of 6.08 g L\({}^{-1}\) and a 64% coulomb efficiency [59]. Recently, the Mo\({}_{2}\)C-modified carbon cloth cathode was found to greatly promote the electric-driven fumarate reduction by _S. oneidensis_ MR-1 due to the increased interfacial EET [18].  
In addition to Mo\({}_{2}\)C, WC also showed electrocatalytic activity towards microbial metabolites (e.g., hydrogen and formate), thus enabling a high MFC power and current  
Figure 5: The FeS single-cell electron collector on individual _S. oneidensis_ MR-1 cell: (**a**) schematic illustration of collector assembly; (**b**) scanning electron micrograph; (**c**) transmission electron micrograph; (**d**,**e**) EDS mapping; (**f**) fluorescence microscopy image; (**g**) current output under different conditions; and (**h**) proposed electron transfer pathway (with permission from [117]).  
density when used as an anodic catalyst [138]. The carbon cloth decorated with WC nanoparticles was found to not only promote the attachment of dense biofilm but also enrich the exoelectrogens of _Geobacter_, _Geothrix_, and _Pseudomonas_, which led to an enhanced MFC power density, chemical oxygen demand removal rate, and coulombic efficiency by 2.14, 1.22, and 1.71 folds, respectively, compared with the naked carbon cloth anode [128]. A two-dimensional Ti\({}_{3}\)C\({}_{2}\) MXene anode was also found to greatly improve the MFC output voltage and power density, which benefited from its multilayer structure, high surface area, good conductivity for fast microbial growth, mass transfer, and more active sites for biological electrocatalytic redox reactions [129]. The above reports substantially demonstrated the significant benefits of the electrocatalytic activity of TMCs in boosting microbial EET in microbial electrochemical systems.