*5.1. Single-Chambered MEC*

A research group put together single-chambered MECs to synthesize hydrogen and assess the process efficiency. The main design used a glass bottle with a total capacity of 50 mL, while the secondary configuration used vials made of borosilicate glass with a total capacity of 10 mL; the cells usually used a mixed culture and pure culture of *S. oneidensis*, respectively. The anode and cathode, measuring 3.5 × 4 cm<sup>2</sup> and 4 × 5 cm2, were kept 2 cm apart by plastic screws. The anode was type A carbon, and the cathode was type B carbon with platinum (0.5 mg·cm<sup>−</sup>2) as a catalyst separated by a J-cloth layer to prevent a short-circuit in both configurations [63,64]. Single-chambered MECs lack a membrane, as illustrated in Figure 4. When production rates are high, the microbial conversion of hydrogen to methane will be slow, with hydrogen being relatively insoluble in water. Energy losses of the membrane are lowered in membrane-less MECs, and the energy recovery process is high [65].

**Figure 4.** Schematic representation of a single-chambered microbial electrolysis cell.

5.1.1. An Up-Flow Single-Chambered MEC

Lee HS constructed an up-flow single-chamber MEC by inserting a cathode on top of the MEC and implemented out a program to monitor hydrogen and electron equivalents in batch trials to enhance the output of hydrogen gas. In a batch evaluation experiment lasting 32 h with a starting acetate concentration of 10 mM, the CE was 60% ± 1%, the H2 yield was 59% ± 2%, and methane production was insignificant [66].
