The theoretical potential for hydrogen production in neutral pH
(pH 7) is 0.61 V, VCat vs. Ag/AgCl [164]. Exoelectrogens generate
an anode potential of approximately Van = 0.5 V. Therefore the
minimum applied potential (Vapp = Van VCat) is 0.11 V [164]. For
acetate, the actual applied voltage is >0.3 V due to electrode
overpotentials and ohmic resistance [164].
The design of MEC systems initially used similar components as
used in PEM fuel cells [164]. However, flat electrode designs
limited the surface area for the exoelectrogens and the membranes
increased the ohmic resistances so alternative designs were
developed. The most recent design uses a graphite brush for the
exoelectrogen substrate (anode) and no membrane separator
[162]. This design succeeded to decrease the applied voltage from
1.0 V using a gas diffusion membrane and 0.5 V with a Nafion
membrane to 0.4 V in the membraneless design. The efficiency is a
function of the lower heating value of the hydrogen divided by the
lower heating value of the organic material plus the electrical
energy provided