# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 3 Electrocatalytic Nanomaterials for Promoting EET
### Metal Oxides
The hybrids of Fe\({}_{3}\)O\({}_{4}\) nanoparticles and reduced graphene oxide have been shown to dramatically enhance microbial EET for electricity generation in MFCs due to the high affinity between Fe\({}_{3}\)O\({}_{4}\) and EAMs [97, 98]. In addition, the magnetic Fe\({}_{3}\)O\({}_{4}\) could significantly enhance the performance of MES, such as the production of acetate and methane [99, 100]. More interestingly, a highly conductive N-doped Fe\({}_{3}\)O\({}_{4}\) with a carbon dot shell (Fe\({}_{3}\)O\({}_{4}\)@CD) could directly hybridize the _G. sulfuruducns_ cells, in which the Fe\({}_{3}\)O\({}_{4}\)@CDs formed an interaction network with intracellular and extracellular conductive proteins for an enhanced EET [101]. As a result, the Fe\({}_{3}\)O\({}_{4}\)@CDs-fed cells achieved a 6.3 times higher current output compared to the control cells. Recently, the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanoarray-modified carbon electrode interface was reported by [PERSON] et al. to greatly promote the direct EET of _S. putrefaciens_ CN32 due to the possible function of the \(\alpha\)-Fe\({}_{2}\)O\({}_{3}\) nanoarray as an electron mediator [102]. For manganese oxides, they have been reported to improve both the electricity generation of the MFC anode and the chemical biosynthesis of the MES cathode. For example, the reduced graphene oxide/MnO\({}_{2}\) composite increased the  
Figure 4: Nanostructured TiO\({}_{2}\) with various morphologies were used to promote microbial EET in microbial electrochemical systems: (**a**) nanoparticles (with permission from [86]), (**b**) nanotubes (with permission from [88, 95]), and (**c**) nanosheets (with permission from [90]). MFC power density by 1.54 folds because of the increased growth of EAM biofilm and the decreased internal resistance [103], while the cauliflower-like polypyrrole/MnO\({}_{2}\) composite increased the power density and real capacitance by 3.58 and 4.84 folds, respectively [104]. A MnO\({}_{2}\)-coated felt cathode electrode obtained a 43% increase in the acetate production rate with 2-fold higher current consumption than the non-coated electrode in MES [105]. Similarly, the nanostructured MnO\({}_{2}\)/rGO nanohybrid-decorated carbon cloth enhanced the production of isobutyric acid and acetate by 2.09 and 2.91 times, respectively, compared to the bare carbon cloth [106]. Notably, an obviously positive shift of the onset potential was observed in the linear sweep voltamogram of the MnO\({}_{2}\)/rGO nanohybrid cathodes under the turnover condition, which indicated more favorable thermodynamics for the EAM cells acquiring electrons from the cathode. In addition, an in situ synthesized biohybrid of amorphous _Geobacter_-Mn\({}_{2}\)O\({}_{3}\) was shown to have high electrocatalytic activity for OER due to the synergistic effect, in which the _Geobacter_ cells contributed OER-active elements such as Fe and P, while amorphous Mn\({}_{2}\)O\({}_{3}\) provided a large electrochemically active surface area and excess catalytic sites for OER [107]. Moreover, other TMOs, including NiO [108; 109], CeO\({}_{2}\)[110], WO\({}_{3}\)[111], and SnO\({}_{2}\)[112], have successively been shown to significantly increase the microbial EET in microbial electrochemical systems, despite deviations in the specific enhancement mechanism. It should be noted that since most TMOs with semiconductive properties are relatively poor in electro-conductivity, they are often hybrid with highly conductive polymers (e.g., polyaniline [113] and polypyrrole [104]) and carbon nanomaterials (e.g., graphene [97; 98; 106], CNTs [112], and carbon dots [101]) when used as microbial electrodes. The above results demonstrated the electrocatalytic interaction between TMOs and EAMs for a high EET rate, which provides great potential for the design and fabrication of high-performance microbial electrodes based on the low-cost and readily available TMOs.