# Electrocatalytic Nanomaterials Improve Microbial Extracellular Electron Transfer: A Review
## 1 Introduction  
The extracellular electron transfer (EET) feature of electroactive microorganisms (EAMs) [1, 2] is a widespread natural process in biogeochemical cycles, with significant application potential in various fields such as wastewater treatment, environmental bioremediation, clean electricity generation, biosensing, high-energy fuel conversion, and high-value chemical synthesis [3, 4]. EET-based microbial electrocatalysis involves the biocatalytic process of microorganisms and the electrocatalytic process of electrodes [5], both of which play key roles and have tremendous synergistic effects on the fast kinetics of electrodes, making the bioelectrocatalytic process much more complex than either the biocatalytic or electrocatalytic one [6]. Over the past decades, the combination of microbial catalysis and electrocatalysis has led to the construction of microbial electrochemical systems, utilizing microorganisms as catalysts for matter and energy conversion in various applications [7], such as microbial fuel cells (MFCs) for generating electrical energy from wastewater [8], microbial electrolytic cells (MECs) for biological hydrogen production [9], and microbial electrosynthesis (MES) for converting greenhouse gas CO\({}_{2}\) to valuable chemicals [10]. However, the practical application of microbial electrochemical systems is limited by low efficiency, yield, and electron transfer efficiency, hindering their widespread commercial viability. Among them, the weak interfacial electron transfer rate between electroactive microorganisms and electrodes is considered to be the most fundamental bottleneck. Traditional electrode materials (e.g., graphite rod, graphite plate, carbon cloth, and stainless steel) have limited interaction with microorganisms and electron transfer processesdue to their low specific area and infertile structure. The decoration and functionalization of these conventional electrode materials certainly enhance the performance of microbial electrochemical systems. Many studies have shown that the surface modification of traditional electrodes through various physical or chemical methods (e.g., ammonia treatment, thermal treatment, acidification treatment, and electrochemical modification) could greatly improve their surface chemical properties so as to enhance the performance to varying degrees [11; 12]. Notably, nanomaterials with unique properties such as surface effect, volume effect, quantum size, and macroscopic quantum tunneling effect have garnered significant attention in recent years and have been widely used in various fields and industries, including microbial electrochemical systems [13]. With the theme of \"nanomaterial\"' AND \"microbial electrochem\" OR microbial electrocatalysis OR extracellular electron transfer\", 422 pieces of research literature, published from 2010 to mid-2024, were retrieved from the Web of Science core database. A variety of nanoscale carbon materials (such as graphene [14; 15] and carbon nanotubes [16]) have been widely used to improve the conductivity and biocompatibility of microbial electrodes, thereby promoting the growth of electrode biofilms and the electron exchange at the biotic-abiotic interface. Nanosized metal compounds (such as titanium dioxide [17] and molybdenum carbide [18; 19]) can greatly improve the surface properties of microbial electrodes with increased interface reactivity. Moreover, some bio-friendly nanomaterials (such as palladium [20] and ferrous sulfide [21]) have been directly used to functionalize microbial cells to improve their EET ability. There is no doubt that nanomaterials have made important progress in promoting the extracellular electron transfer of microorganisms and thus improving the performance of microbial electrochemical systems, and they are still advancing.