MECs

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. The  MEC (originally called a BEAMR) 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).

It is possible to have very high hydrogen production yields and energy efficiencies using MECs. For example, in our paper in the Proceedings of the National Academy of Science (PNAS) (Cheng and Logan, 2007, 104(47): 18871–18873), we have obtained hydrogen gas 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 as the fuel. At an applied voltage of 0.6 V, the overall energy efficiency of the process was 288% based solely on electricity applied (82% from the energy in acetic acid and electrical energy used). The gas production rate was 1.1 m3-H2 per m3 of reactor per day. Direct high-yield hydrogen gas production was further demonstrated using glucose, several other volatile acids (butyric, lactic, propionic, and valeric) and even cellulose at maximum stoichiometric yields of 54 to 91%, at overall energy efficiencies of 64-82%.

Scaling-up MECs has progressed further than that of MFCs, in part due to the simpler 2-phase reaction (protons in water, electrodes) compared to the 3-phase reaction for MFCs (oxygen in air, protons, electrodes).  Laboratory scale systems are typically 28 mL cube reactors, such as that shown in the schematic to the left, but we have built larger laboratory systems such as the 2.5 L reactor shown on the left. We also conducted field tests at the Napa Winery in California, and demonstrated effective treatment of winery wastewater using a 1000 L reactor with a net gain in energy relative to the electrical power input, although methane was produced in that system rather than H2 gas. We are currently working on new designs to more effectively convert cellulose fermentation endproducts to H2 gas at high production rates and recoveries, in a project with the National Renewable Energy Laboratory (NREL), in Golden, CO.