Hydrogen Generation in Microbial Reverse-Electrodialysis Electrolysis Cells Using a Heat-Regenerated Salt Solution Joo-Youn Nam, Roland D. Cusick, Younggy Kim, and Bruce E. Logan* Department of Civil and Environmental Engineering, The Pennsylvania State University, University Park, Pennsylvania 16802, United States *S Supporting Information ABSTRACT: 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 counter- current) 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. ■ INTRODUCTION Hydrogen 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 reverse- electrodialysis electrolysis cell (MREC), was recently shown to produce hydrogen gas from acetate using high (0.6 M) and lower concentrations (0.006 to 0.012 M) of NaCl solutions. 3 One limitation of the MREC for hydrogen gas production is that this process requires sources of organic matter and seawater in close proximity making the process useful only in coastal and not inland regions. This is possible for many large cities in the USA that are located on the coast with wastewater treatment plants that discharge into the ocean, as the wastewater can first serve as a source of organic matter, and then as the low salinity solution in the RED stack. Another potential limitation of this process is the use of seawater, which can result in biofouling of the membranes unless water is treated as it is in reverse osmosis desalination systems. 6 The need for close access to seawater and biofouling of the water in the stack could be avoided by using recycled sources of clean salt solutions. Different salts such as ammonium bicarbonate (NH 4 HCO 3 ), magnesium sulfate, sodium sulfate, sodium chloride, potassium sulfate, potassium nitrate, potas- sium chloride have all been used in forward osmosis (FO) systems using osmosis pressure gradient for desalination. 7−9 Ammonium bicarbonate is unique among these salts as it can Received: January 18, 2012 Revised: March 19, 2012 Accepted: March 30, 2012 Published: March 30, 2012 Article pubs.acs.org/est © 2012 American Chemical Society 5240 dx.doi.org/10.1021/es300228m | Environ. Sci. Technol. 2012, 46, 5240−5246