## 1 Introduction  
In recent years, microbial electrochemical systems (MESs) have shown increasing promise in a number of biotechnological applications, including waste treatment, chemical generation, and the production of renewable energy [1, 2, 3, 4]. Microbial electrolysis cells (MECs) are a form of MESs that can utilize electrons within a cathode chamber in order to yield specific chemical compounds through the activity of specific electrochemically active microbes [3]. Such MECs have been used to produce a range of chemical products, including alcohol, butanol, butyric acid, and succinic acid. In one study, researchers were able to achieve a 34% increase in _Clostridium tyrobutyricum_ BAS7-mediated butyric acid production by applying a \(-\)1.5 V cathodic potential to their MEC system while using neutral red (NR) as an electron carrier [5]. Methyl viologen has been utilized in such MEC systems as well, being used to promote the reduction of acetate at a \(-0.55\) V applied cathodic potential, leading to a 6-fold acceleration of ethanol production [6]. When an MEC was subjected to a constant \(-1.8\) V potential during a 60 h fermentation reaction, this was sufficient to yield \(18\) g/L succinic acid [7]. While the increase in the product concentration was obvious, the electron utilization was inefficient due to the complex culture conditions and potentiostatic method. This meant that the extracellular electronic supplement using the potentiostatic method far exceeded the intracellular electronic requirements. As such, there is an urgent need to develop reliable methods suited to controlling MECs. Redox reactions are essential mediators of a wide range of essential biological processes [8]. The redox potential of a system is determined based upon factors including the pH equilibrium constant, concentrations of dissolved oxygen, and the amounts of particular compounds dissolved in a given medium [9]. ORP measurements have been an increasing subject of focus. [PERSON] et al., for example, found succinic acid yields from _Escherichia coli_ LL016 at a \(-400\) mV ORP level to be 39% higher than at a \(-200\) mV ORP level [10]. Similarly, in a mixed-acid anaerobic _Corynebacterium crentum_ fermentation system, a culture potential of \(-300\) mV led to a \(0.31\) g/g succinic acid yield, which was 72% higher than when this ORP value was instead \(-40\) mV [11]. For ORP-controlled fermentation reactions, appropriately lower ORP levels lead to higher intracellular NADH levels and/or NADH/NAD\({}^{+}\) ratios [10]. Intracellular NADH and NAD\({}^{+}\) are essential for many biological reactions, and as such, the use of ORP-controlled fermentation reactions that modulate this NADH/NAD\({}^{+}\) ratio and yield higher NADH concentrations lead to increased enzymatic activity and ATP synthesis, along with reductive metabolite production [11; 12]. In MECs, the intracellular NADH levels have also been shown to be increased upon the application of an appropriate potential to the cathode [13]. In these MECs, it is possible for electrons to be transferred to a given mediator and then to microbes, thereby compensating for any shortage in reducing power [5]. As such, in MECs, these higher intracellular NADH levels are linked to increased enzymatic activity and ATP synthesis, along with reductive metabolite production [7]. As such, modulating the ORP represents an attractive means of controlling MEC processes. The redox potential of the extracellular environment plays a pivotal role in the regulation of cellular metabolism, as it influences metabolic flux, intracellular enzymatic activities, and the balance of redox cofactors, such as the NADH/NAD\({}^{+}\) ratio, which are essential for various metabolic pathways [14; 15; 16]. As such, many different approaches have been employed to regulate the extracellular redox potential. For example, [PERSON] et al.