# Novel Microfluidic Septum to Optimize Energy Recovery in Single-Chamber Microbial Fuel Cells
## 1 Introduction  
Concerns over climate change and energy security are pushing the demand for sustainable and reliable alternatives to fossil fuels. In this context, bio-electrochemical systems (BESs) represent a family of promising technologies for a wide range of applications, which include energy conversion into hydrogen by the so-called Microbial Electrolysis Cells (MECs) [1; 2; 3], microbial fuel cells (MFCs) for electrical power generation [4; 5; 6] and Microbial Electrosynthesis Cells for the valorization of CO\({}_{2}\) into new added-value products [7; 8; 9]. As an added benefit, BES operation can valorize the chemical energy entrapped in organic molecules present in wastewater, facilitating its remediation [5; 10]. Among BESs, microbial fuel cells have drawn particular attention due to their ability to convert organic matter into electrical energy, leveraging the catalytic activity of the electroactive biofilm present at the anode electrode [4; 6; 11; 12]. Notably, numerous studies are trying to address the issues currently associated with MFCs, among which are conversion efficiency, scalability, architecture, and cost-effectiveness [4; 13; 14; 15; 16; 17; 18; 19]. Over the years, several MFC architectures have been investigated to enhance energy conversion and power production, with the possibility to be easily up-scalable [19]. MFC architectures can be classified into two main categories: two-compartment MFCs and single-compartment MFCs. The first category is characterized by an anodic and a cathodic chamber, usually separated by an ion-transporting membrane. The minimal design is represented by single-chamber MFCs (SCMFCs), which have shown significant potential in reducing the internal resistance of devices, simplifying fabrication, and reducing costs [20; 21]. As discussed in the literature [22], the absence of an ion exchange membrane between the anodic and cathodic chambers ensures several advantages: it reduces mass transport limitation for species that are crucial for the metabolic activity of the anodic biofilm and entails a lower fabrication and maintenance complexity with respect to cells featuring membranes. At the same time, this may also represent a drawback as oxygen can more easily diffuse from the open-air cathode towards the bio-anode, negatively affecting its operation due to the proliferation of competing, non-electroactive microbes [4; 22; 23]. To this end, there is evidence in the literature of two main strategies to improve the overall membrane-less SCMFC. These are based on the reduction in oxygen cross-over in the proximity of anode electrodes, or the identification of the ideal distance between anode and cathode electrodes [24; 25]. Another important feature, which directly affects the interaction between electrolyte and biofilm on the anode electrodes, was addressed by fluid dynamics studies [26; 27; 28; 29; 30]. [PERSON] et al. [31; 32] demonstrated that asymmetric a-SCMFCs, characterized by misaligned inlets and outlets, ensure a higher drift area, with an electrolyte flow characterized by a drop-like shape. The microfluidic-based design of a-SMFCs demonstrated an increase in the electrode surface area reached by the new electrolyte with each replacement. This led to confirmation that a drop-like shape configuration was suitable for the enhancement of the carbon-energy source distribution inside the devices, thus leading to an improvement in the overall a-SCMFCs' performance. Analyzing the literature on single-chamber MFCs [24; 25], an anode-to-cathode distance of 2 cm can represent an appropriate trade-off between maintaining good electrical performances while ensuring, in proximity of anodes' surfaces, a low amount of oxygen being dissolved in the electrolyte. This spacing can be achieved by the inclusion of a separating material, which also allows oxygen cross-over from the open-air cathode to be limited.