## 6 The Design and Application of MESs
### MFCs for Power Production  
Traditionally, MFCs are used to generate power from the oxidation of organic matter by bacteria and the transfer of electrons from the anode to the cathode [105]. The classic H-style two-chamber  
Figure 3: Schematic representations of different MES architectures. (**a**) Two-chamber microbial fuel cell, (**b**) single-chamber microbial fuel cell, (**c**) benthic or sediment microbial fuel cell, (**d**) microbial electrolysis cell, (**e**) stacked microbial fuel cell, (**f**) MEC synthesizing organic compounds, (**g**) desalination cell, (**h**) representative microbial solar cell. Images adapted from references [1; 2; 15].  
MFC (2 CMFC) is often the MFC design used in laboratory research and is similar to a standard electrochemical cell [15]. The basic configuration is of an anodic chamber separated from the cathodic chamber by a PEM. In this design, the microbes pass electrons obtained from the oxidation of the electron donor to the anode [106]. The electrons flow from the anode to the cathode, where they react with the protons that have passed into the cathodic chamber via the PEM and the electron acceptor (oxygen) to form water. To improve the power output of 2 CMFCs, SCMFCs with air cathodes were developed which reduce the internal resistance of the MFC and the cost [106; 107]. In these systems, the cathodic chamber is removed and the cathode is exposed directly to the air, with or without a membrane [106; 107]. The membrane prevents oxygen diffusion into the anodic chamber as well as preventing water leakage to the cathode [106; 108]. A further attempt to increase current output and substrate oxidation came with stacked MFCs, which evolved out of SCMFCs [1; 109; 110]. MFCs can be stacked either in series or parallel. When connected in series, the voltages of each MFC are added together and a common current flows through all [110]. However, if the MFCs are connected in parallel, the voltage is averaged and the currents are added. This flexibility allows the user to construct stacks that produce any desired current or voltage [110]. The standard set-up consists of the anode separated from the cathode by a PEM and separated from the next cell by a rubber gasket. Once the desired number of cells are added, the whole stack is bolted together [15; 110]. One consideration of stacked systems is that the stack must be continuously fed with substrate as opposed to batch feeding [110; 111]. This is to prevent fuel starvation in one of the cells of the stack. If this occurs and the microbial community dies, the overall voltage produced declines. So far, stacked systems show the most potential for miniaturisation and commercial up-scaling due to their ability to be connected in either series or parallel and their increased voltage production [111; 112; 113]. The efficiency of current generation in stacked MFC systems has also been successfully improved through the electrical reconfiguration of stacks between parallel and series to optimise voltage range [114]. Recent examples of stacked MFC systems have reached peak power densities of 50.9 \(\pm\) 1.7 W/m\({}^{3}\) and COD removal of 97%, while stacked MEC systems for the refining of bioethanol have reached 91% efficiency of methane generation from waste products [115; 116].