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Microbial Electrolysis Cells (MEC)

Bioenergy can be produced from different biological alternatives: (i) solid biomass that can be directly used as fuel, (ii) liquid biofuels such as ethanol, biodiesel, etc., (iii) biogas, (iv) biohydrogen and (v) bioelectrochemical systems (BES). BES involve direct production of electricity or chemical compounds (e.g. hydrogen) by means of electrochemically active organisms. These systems are considered promising due to their high efficiency and versatility. The best BES known so far is the microbial fuel cell (MFC), in which electric energy is obtained from the exergonic conversion of chemical energy, through a process known as electrogenesis. Organic matter oxidation at the anode is carried out by microorganisms that liberate protons towards the cathode; electrons are transferred to the latter through an external circuit for completing oxygen reduction and the production of clean water. More recently, a similar process has been developed, known as electrohydrogenesis. In this process, oxygen is not provided to the cathodic compartment and, since the overall reaction is endergonic, a low potential ( 0.2 V) needs to be applied to the circuit, since the metabolic production of hydrogen using acetate as substrate is not thermodynamically possible. As a result of these modifications, hydrogen gas is produced at the cathode. This process is carried out in microbial electrolysis cells (MEC). In the case of the project Défi H12, only the anodic reaction is catalyzed by microorganisms and the cathodic reactions (reduction) remain abiotic. Compared to classical water electrolysis technologies, MEC work at lower potentials (5 to 10 times), reducing the energy costs proportionally.

Classical water electrolysis guarantees hydrogen production at the cathode and the oxidation of water at the cathode, by the following reactions (in acid media):

Anode 2 H2O -> ½O2 + 2H+ + 2e- EO2/H2O = 1,23 - 0,059pH /ESH (1)

Cathode 2H+ + 2e- -> H2 EH+/H2 = 0,0 – 0,059pH /ESH (2)

Between anode and cathode there is needed a potential difference above the value of thermodynamical equilibrium (1.23 V, standard conditions).

Since the project Défi H12 proposes to convert in hydrogen the organic acids produced in fermentation with a minimal energy supply, representative reactions would be those involving the oxidation of acetic acid:

Anode CH3COOH + 2H2O -> 2CO2 + 8e- + 8H+ ECO2/CH3COOH = 0,187 - 0,066pH /ESH (3)

Cathode 8H+ + 8e- -> 4H2 EH+/H2 = 0,0 – 0,059pH /ESH (4)

Thus, compared to classical water electrolysis, microbial electrolysis needs much lower potentials that can be as low as 0.2 V. Consequently, for the same amount of hydrogen produced, this novel type of electrolysis would allow to reduce the energy costs about 10 times.