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Producing hydrogen gas is possible at very high yields by electrohydrogenesis, in a reactors that have various names including: a "bioelectrochemically assisted microbial reactor" or BEAMR; biocatalyzed electrolysis cells (BECs); and microbial electrolysis cells (MECs). These names are based on the idea is that fuel cells produce electricity, and electrolysis cells produce hydrogen.
The MEC is based on modifying a microbial fuel cell (MFC) in two ways: adding a small amount of voltage (>0.2 V) to that produced by bacteria at the anode; and not using any oxygen at the cathode. The addition of the voltage makes it possible to produce pure hydrogen gas at the cathode. This MEC/BEAMR system is therefore operated as a completely anaerobic reactor. The voltage needed to be added can be produced using power from an MFC or by using hydrogen gas produced by the MEC in a conventional hydrogen fuel cell. The idea behind this system is that the protons and electrons produced by the bacteria can be recombined at the cathode as hydrogen gas-- a process called the hydrogen evolution reaction (HER). Theoretically we need 0.41 V to make H2 from acetate, and the bacteria produce ~0.2 to 0.3 V. Thus, we only need to add about 0.2 V or more to make hydrogen gas in the MEC/BEAMR. This voltage is much less than that needed for water electrolysis, which is about 1.8 V in practice. It takes a lot of energy to split water, but "splitting" up organic matter by the bacteria is a thermodynamically favorable reaction when oxygen is used at the cathode. In the MEC process, no oxygen is present and the reaction is not spontaneous for hydrogen production unless a small boost of voltage is added to that produced by the bacteria. Thus, the MEC process is more of an "organic matter electrolysis" procedure (versus water electrolysis).
We recently demonstrated very high hydrogen production yields and energy efficiencies in a paper published in the Proceedings of the National Academy of Science. We report there: "By improving the materials and reactor architecture, hydrogen gas was produced at yields of 2.01-3.95 mol/mol (50-99% of the theoretical maximum) at applied voltages of 0.2 to 0.8 V using acetic acid, a typical dead-end product of glucose or cellulose fermentation. At an applied voltage of 0.6 V, the overall energy efficiency of the process was 288% based solely on electricity applied, and 82% when the heat of combustion of acetic acid was included in the energy balance, at a gas production rate of 1.1 m3-H2 per m3 of reactor per day. Direct high-yield hydrogen gas production was further demonstrated using glucose, several volatile acids (acetic, butyric, lactic, propionic, and valeric) and cellulose at maximum stoichiometric yields of 54 to 91%, and overall energy efficiencies of 64-82%." (Cheng and Logan, 2007, PNAS, 104(47): 18871–18873). For a copy of this paper, go to
www.pnas.org, or a direct link is:
http://www.pnas.org/cgi/reprint/0706379104v1 This paper can be freely downloaded.