Electron-accepting (electrotrophic) biocathodes were produced by first enriching graphite fiber brush electrodes as the anodes in sediment-type microbial fuel cells (sMFCs) using two different marine sediments and then electrically inverting the anodes to function as cathodes in two-chamber bioelectrochemical systems (BESs). Electron consumption occurred at set potentials of ؊439 mV and ؊539 mV (versus the potential of a standard hydrogen electrode) but not at ؊339 mV in minimal media lacking organic sources of energy. Results at these different potentials were consistent with separate linear sweep voltammetry (LSV) scans that indicated enhanced activity (current consumption) below only ca. ؊400 mV. MFC bioanodes not originally acclimated at a set potential produced electron-accepting (electrotrophic) biocathodes, but bioanodes operated at a set potential (؉11 mV) did not. CO 2 was removed from cathode headspace, indicating that the electrotrophic biocathodes were autotrophic. Hydrogen gas generation, followed by loss of hydrogen gas and methane production in one sample, suggested hydrogenotrophic methanogenesis. There was abundant microbial growth in the biocathode chamber, as evidenced by an increase in turbidity and the presence of microorganisms on the cathode surface. Clone library analysis of 16S rRNA genes indicated prominent sequences most similar to those of Eubacterium limosum (Butyribacterium methylotrophicum), Desulfovibrio sp. A2, Rhodococcus opacus, and Gemmata obscuriglobus. Transfer of the suspension to sterile cathodes made of graphite plates, carbon rods, or carbon brushes in new BESs resulted in enhanced current after 4 days, demonstrating growth by these microbial communities on a variety of cathode substrates. This report provides a simple and effective method for enriching autotrophic electrotrophs by the use of sMFCs without the need for set potentials, followed by the use of potentials more negative than ؊400 mV.

Hydrogen gas can be electrochemically produced in microbial reverse-electrodialysis electrolysis cells (MRECs) using current derived from organic matter and salinity-gradient energy such as river water and seawater solutions. Here, it is shown that ammonium bicarbonate salts, which can be regenerated using low-temperature waste heat, can also produce sufficient voltage for hydrogen gas generation in an MREC. The maximum hydrogen production rate was 1.6 m 3 H 2 /m 3 ·d, with a hydrogen yield of 3.4 mol H 2 / mol acetate at a salinity ratio of infinite. Energy recovery was 10% based on total energy applied with an energy efficiency of 22% based on the consumed energy in the reactor. The cathode overpotential was dependent on the catholyte (sodium bicarbonate) concentration, but not the salinity ratio, indicating high catholyte conductivity was essential for maximizing hydrogen production rates. The direction of the HC and LC flows (co-or countercurrent) did not affect performance in terms of hydrogen gas volume, production rates, or stack voltages. These results show that the MREC can be successfully operated using ammonium bicarbonate salts that can be regenerated using conventional distillation technologies and waste heat making the MREC a useful method for hydrogen gas production from wastes. ■ INTRODUCTIONHydrogen gas can be electrochemically produced at the cathode in a microbial electrolysis cell (MEC) from current generated using microorganisms at the anode by adding a voltage (>0.11 V using acetate) that is theoretically much less than that needed to split water (>1.2 V).1 In practice, the applied voltages are much higher and typically 0.4 to 1 V, substantially lowering the possible overall energy recovery.2 A renewable source of the electrical power is needed for applying this added voltage to make the MEC a green and sustainable method of hydrogen production.It was recently shown that salinity-gradient energy could be harnessed as the source of voltage needed to enable hydrogen gas production.3 Reverse electodialysis (RED) is a method for converting salinity differences between seawater and river water into electrical power. The RED stack consists of a series of alternating anion exchange membranes (AEMs) and cation exchange membranes (CEMs) that dictate the direction of the flow of positive or negative ions from the high salinity solution creating a method to convert an electrochemical potential into electrical current. In a RED system, seawater and river water are pumped between the membranes in a stack that can contain ∼20 or more membrane pairs (∼0.1 to 0.2 V per membrane pair) to generate sufficient potential to split water. 4,5 However, by incorporating a RED stack of only ∼5 membrane pairs between the electrodes in an MEC, it is possible to both avoid the need to split water and also to eliminate the need for an external power source for hydrogen gas production. This combined MEC and RED process, called a microbial reverseelectrodialysis electrolysis cell (MREC), was recently shown to pr...

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