7.3.1. Anode Material

The anode's performance is critical for bio-electrochemical systems that rely on bioelectrochemical reactions, which take place at the anode, and which can be replaced by MFCs, MECs, and MEC-ADs. It was established that anode activity is a restrictive component in the overall performance of the system [110]. As an electro-active bacterium (EAB) attaches itself to the bioanode surface, producing a biofilm, it provides the bioanode with energy. As a result of this oxidation by EAB, organic molecules are converted to CO2. Because of their remarkable ability to adhere to EAB, their huge surface area, and their abundance, carbon-based materials have become the most extensively used electrode material [111]. It has been shown that carbon compounds help to increase interfacial microbial colonization and, thus, the production of biofilms. Electrically conductive current collectors of metals are employed as electron acceptors, to overwhelm poor conductivity. Titanium wire is often utilized because of its corrosion resistance [40]. Additionally, the capability to simulate interfacial microbial colonies allows for improved current density by developing a beneficial microenvironment for electron transport that compensates for the decreased conductivity [112]. Graphite is affordable, plentiful, and conductive, and, because of this, it has become one of the most extensively used electrode materials [113]. Graphite electrodes have been implemented in many ways, including brush, granular, rods, felts, and foams [112]. Nevertheless, graphite's molecular structure and morphology are both planar in comparison to other carbon materials due to its low surface porosity required for bacterial adhesion. Surface area-increasing porous 3D carbon materials, such as carbon brushes, felts, meshes, and foams, have been the focus of recent research [114]. Carbon fiber (CF) electrodes have previously proven to be effective and are currently being used to achieve good outcomes [115]. Carbon nanotubes (CNTs) have incredible electrical, mechanical, biological, and thermal properties, making them ideal for real-time applications. Despite extensive study and application possibilities of carbon nanotubes, many issues such as biodegradability, biotoxicity, and biosafety remain difficult to address and should be addressed with caution prior to design and manufacturing [116]. Anodes of a mesh-like design (i.e., porous, woven, or multilobed) tend to generate greater current densities (more current flows) than flat or plate-shaped anodes because of improved mass transfer, surface area, and biofilm growth. Carbon fiber brushes give excellent test results; however, because of their very expensive cost, they are seldom employed in large-scale BES. Based on an independent study, which used recycled carbon fiber anodes and found that, in comparison to graphite felt anodes, the use of recycled carbon fiber electrodes produced better results while also being cheaper, Carlotta-Janes et al. found that it was possible to improve performance while cutting costs if recycled carbon fiber anodes were used. As there is a considerable portion of the anode in the current model without a biofilm, increasing the surface area of the anode is more likely to result in a greater increase in biofilm density and adherence on the anode. Reduction in anode size will be advantageous for commercial viability since the anode material constitutes about 70% of the whole system, which will need a 90% drop in cost to make it profitable [88]. Additionally, molybdenum anodes showed excellent overall durability, neither corroding nor lowering in current production for over 350 days. Another important consideration is the endurance of the electrode materials. Unfortunately, there are no data on electrode materials' long-term durability, and most experiments last about 1 year. Material dissipation in electrodes is often underestimated, which may lead to significant issues when determining which materials to utilize commercially. Stainless steel is also good since it has several characteristics which can be utilized [117]. In terms of conductivity and scale-up potential, stainless steel outperforms carbon anodes owing to lower capital expenditures, despite a relatively flat surface, which reduces its biocompatibility. Stainless steel has a high nickel concentration

and may efficiently catalyze the HER. Stainless-steel brush cathodes, for example, produced hydrogen at a rate of 1.7 <sup>m</sup>3·m<sup>−</sup>3·day−<sup>1</sup> and had a cathodic efficiency of 84%, comparable to Pt cathodes in single-chamber MECs. The high Ni content (8–11%) and the large specific surface area were also implicated for the rapid hydrogen generation (810 <sup>m</sup>2·m<sup>−</sup>3). Flame spray oxidation improves the biocompatibility of stainless steel by producing an iron oxide coating on the surface, which facilitates the adherence of iron-reducing bacteria and increases surface roughness without sacrificing corrosion resistance [118]. Because stainless steel has ye<sup>t</sup> to be tested on pilot systems, more research into its durability is needed. Cotterill et al. compared a 30 L tank to a 175 L tank to examine how tank capacity influences H2 production. H2 generation was fourfold greater in the small MEC in comparison to the bigger MEC, when the anode surface area was reduced from 1 m<sup>2</sup> to 0.06 m2. The larger MEC had a lesser performance, demonstrating a negative relationship between scale and gas output, implying that efficiency decreases as size increases [119]. As part of the commercialization process, a cost–benefit analysis of anode materials must be completed, which considers the material's availability, corrosion resistance, and capacity to scale up.
