# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 3 Electrocatalytic Nanomaterials for Promoting EET
### Metal Oxides  
Transition metal oxides (TMOs) have gained significant attention in various electrocatalytic reactions [84], such as the oxygen reduction reaction, oxygen evolution reaction, hydrogen evolution reaction, and CO\({}_{2}\) reduction, due to their diverse redox chemistry and electronic properties. The electrochemical and electronic properties of TMOs can be finely tuned through compositional modifications and surface functionalization, allowing for the optimization of their electrocatalytic performance [84]. Without exception, due to their active electrocatalytic activity and high biocompatibility, TMOs (Table 1) have also been widely used as electrode materials for microbial electrochemical systems to enhance the microbial EET process. Among them, titanium dioxide (TiO\({}_{2}\)), with a suitable band structure for efficient charge separation and transfer, is one of the most attractive electrode materials to promote microbial EET. To date, nanoscale TiO\({}_{2}\) with various morphologies and structures (Figure 4), such as nanoparticles [85; 86], nanotubes [87; 88], nanorods [89], nanosheets [90; 91], and nanoclusters [92], have been widely used in microbial electrodes, where a TiO\({}_{2}\) functionalized interface could promote electroactive biofilm formation, substrate diffusion, and the EET rate. Intriguingly, [PERSON] et al. [91], when investigating the electron transfer kinetic from _S. Ioilica_ PY14 to an up-growing TiO\({}_{2}\) nanosheet-modified carbon paper, found that the change in the hydrophobic carbon paper to a super-hydrophilic TiO\({}_{2}\) nanosheet interface not only promoted bacterial adhesion but also altered the redox nature of outer-membrane Cyts. As a consequence, the mid-point potential of Cyts was shifted to a more negative potential, which indicated a higher thermodynamic driving force for the electron release from the bacterial cells. In addition, TiO\({}_{2}\), when with different morphologies, has been shown to stimulate the secretion of electron mediator flavins from the genus _Shevanella_, possibly by enhancing the related biosynthesis pathway, thereby promoting the micro  
Figure 3: The effect of _S. oneidensis_ MR-1 EET components on the location and size of biogenic Pd nanoparticles: (**a**) general diagram of EET and genotypic effect on Pd nanoparticle formation; (**b–g**) thin section transmission electron micrographs of _S. oneidensis_ MR-1 cells after Pd production under different conditions; and (**h,i**) nanoparticle size distributions (with permission from [81]). bial EET rate [17, 93]. Coincidentally, [PERSON] et al. found that TiO\({}_{2}\) nanoparticles could specifically induce the formation of conductive nanowires by stimulating the _pilA_ gene expression in _G. sulfuruducns_, which led to an approximately 5.1-fold increase in microbial electricity production [86]. Obviously, the above results substantially demonstrated the close interaction between highly electrocatalytic TiO\({}_{2}\) and the EET pathways of EMAs. The improvements in nanostructured TiO\({}_{2}\) in electrode biofilm growth and the microbial EET rate could be elaborately tuned by the regulation of its morphology, size, and porous structure [24]. [PERSON] et al. [94] found that N doping could modulate the electronic properties of TiO\({}_{2}\) nanosheets with smaller charge-transfer resistance, more positive flat-band potential, and larger transient-charge storage capacity, which facilitated the interfacial electron transfer from out-membrane Cyts to the TiO\({}_{2}\) nanosheets' modified carbon paper electrode. Since iron oxides and manganese oxides are natural electron correlates of typical electroactive microorganisms, they are also often adopted to modify microbial electrodes for the sake of promoting microbial EET. An \(\alpha\)-FeOOH nanow whiskers-decorated carbon paper significantly increased the EET of _S. lohica_ PV-4 cells, resulting in a 60% increase in current density [96].