# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 3 Electrocatalytic Nanomaterials for Promoting EET  
### Heteroatom-Doped carbons  
Carbon-based electrode materials are widely applied in microbial electrochemical systems due to their low cost, easy availability, and high biocompatibility. However, their electrocatalytic activity is generally limited by the low surface area, inappropriate pore structure, and inert surface chemistry, which severely limits the performance of microbial electrochemical systems. In recent years, non-metallic heteroatom (nitrogen, phosphorus, sulfur, boron, etc. )-doped carbon materials have attracted the attention of researchers. The catalytic ability, current density, cycling stability, and resistance to toxic side effects of carbon materials have been further improved by doping them with N, P, and S elements. This is mainly due to the fact that the geometry and electronegativity of non-metallic heteroatoms are different from those of carbon atoms. When doped into carbon atoms, these non-metallic heteroatoms could cause changes in the properties of surrounding carbon atoms in terms of electron density and spin, thus improving the electrocatalytic performance of carbon nanomaterials. This active effect has been demonstrated in a variety of carbon-based nanomaterials, including graphene [15], carbon nanotubes [53], MOFs [54], and biomass-derived carbon [55]. The element N is adjacent to C in the periodic table, and they have similar atomic radii, but their electronegativity is quite different. Therefore, the electronic structure of carbon nanomaterials will change significantly after N doping. Recently, N doping has been shown to be an effective electrode surface modification method that can enhance the EET process of EAMs, thereby improving the pollutant removal and power generation of MFCs. The preparation of N-doped carbon nanomaterials by pyrolysis of N-containing precursors (such as ammonia [56], polyaniline [53], polypyrrole [57], and natural biomass [55; 58]) alone or together with carbon materials is a common and very productive route to improve the performance of microbial electrochemical systems. For example, [PERSON] et al. [56] reported that an N-doped graphene aerogel electrode achieved a then-record power density of milliliter-scale MFCs because of the greatly reduced charge-transfer resistance and internal resistance of the N-doped electrode. Similarly, [PERSON] et al. [53] found that the N-doped MFC anode showed a much higher power density than the counterpart anode without N doping and speculated that the improvement was attributed to the enhanced adsorption of flavins and the biofilm adhesion on the N-doped surface through electrochemical analysis. Natural biomass is an important precursor for the preparation of N-doped carbons due to the presence of N-rich biomolecules. As a case, natural cassava straws were used as a carbon source to prepare activated N-doped porous carbon through a facile process of electrochemical activation; the resulting N-doped carbons, when used as an MFC anode,achieved a power density almost two times higher than the unmodified counterpart [55]. In addition, the N-doped biomass-derived carbon electrode was found to promote the acetate production from CO\({}_{2}\) and the coulomb efficiency in MES [59]. More interestingly, [PERSON] et al. [60] found that the N-doped carbon dots could be used to modify acetogenic bacteria to enhance their EET and the formation of electroactive biofilm, which subsequently improved the acetate production rate by 57-71% in CO\({}_{2}\)-fed MES. In general, the doped N mainly has three active forms: pyridinic, pyrrolic, and graphitic; the electrocatalytic activity of carbon nanomaterials varies according to the active N form [61; 62; 63]. For example, pyridinic N and graphitic N were reported to be the key active sites for activating hydrogen peroxide to generate hydroxyl radicals in electro-Fenton systems [63], while graphitic N was the main working N in nitroaromatic hydrogenation [62]. [PERSON] et al.