## 3 Microalgae Microbial Fuel Cell (mMFC)
### Electrodic Materials and Membranes  
Electrodes are essential components of MFCs because they facilitate microbial attachment, growth, and biofilm formation, as well as serve as electron acceptors and donors at the end of the extracellular electron transfer process. Key features of electrodes include the following: (i) high biocompatibility, specific surface, and electroactive area to support more microorganisms and electron transfer; (ii) excellent electrical conductivity and low internal resistance to minimize power loss and enable rapid electron transfer; (iii) high mechanical strength and chemical stability to withstand complex conditions; and (iv) affordability and easy availability [141]. Carbonaceous materials are the most widely used electrode material in mMFCs (84% of the electrodes presented in Table 2). It is available in various forms such as cloth, paper, mesh, foam, graphite, felt, or reticulated vitreous carbon (RVC), and in configurations like planar, bars, packed, or brush [142; 143]. Carbon is frequently used as an anode material due to its chemical stability in microbial inoculum, high conductivity, high specific surface area, good biocompatibility, and relatively low cost [142; 143; 144; 146]. Larger  
Figure 5: Configurations of mMFC. (**A**) Single-chamber mMFC. (**B**) Dual-chamber mMFC. (**C**) Coupled mMFC. (**D**) Photosynthetic sedimentary MFC (PSMFC). surface areas can be achieved by using compact materials such as RVC or brush-type configurations. Metals like tungsten, titanium, and stainless steel can also be used as anode materials due to their high conductivity (compared to carbon) and low corrosivity (compared to other metals), but their cost is a limiting factor [144]. Research on developing composite materials for anodes is ongoing, with several studies using carbon-based electrodes combined with metals such as Fe, Al, Mn, or Ni [144]. Modifying the anode surface through ammonia, acid, or thermal treatment and electrochemical oxidation can also be attractive to increase bacterial adhesion and power density [147]. In mMFCs, oxygen serves as the electron acceptor at the cathode, making the ORR a limiting factor due to the low solubility of oxygen in electrolytes. This reaction occurs at the interface of three phases (liquid-gas and solid, water-oxygen-electrode), making the cathodic process the limiting element in an mMFC and affecting the MFC performance [145; 121]. Often, a catalyst is required to increase the rate of oxygen reduction. The most commonly used cathode material is carbon paper, doped with a Pt catalyst on one side at 0.1 mg cm\({}^{-2}\) to reduce costs. Additionally, noble-metal-free catalysts, such as pyrolyzed iron(II) phthalocyanine, have been proposed for use in MFC cathodes [146; 148]. Most MFC designs require the separation of anode and cathode compartments. mMFCs can use various types of separators, such as salt bridges, anion exchange membranes, and cation exchange membranes (CEMs), or even operate without a separator (membranes) [149; 150]. The cost of separators can be a significant issue, as they account for nearly 40% of the total MFC cost [141]. Naturally separated systems, like sediment MFCs or single-compartment mMFCs, are exceptions. Separators should be impermeable to chemicals like oxygen, ferricanide, other ions, or organic matter used as substrates, while being permeable to protons, and having long-term stability, low cost, and resistance to fouling [144]. The most commonly used proton exchange membrane (PEM) is Nafion, produced by Dupont Co. (Wilmington, DE, USA) [151]. There is no clear correlation between power density and microalgae species, mMFC configuration or electrode material (Figure 6). Notable variability within and between groups could be highlighted for microalgae species. _Chlorella_ spp. and _Scenedesmus_ spp. exhibit particularly broad ranges of power densities, suggesting that under certain conditions, they can achieve very high power outputs, while consortia display more moderate values with a few high outliers.