Hydrogen Generation in Microbial Reverse-Electrodialysis Electrolysis Cells Using a Heat-Regenerated Salt Solution

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 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...

Optimization of membrane stack configuration for efficient hydrogen production in microbial reverse-electrodialysis electrolysis cells coupled with thermolytic solutions

“…The use of these natural waters also requires substantial and energy intensive pre-treatment to minimize membrane fouling . To avoid these limitations, thermolytic solutions (Elimelech and Phillip, 2011) such as ammonium bicarbonate (NH 4 HCO 3 ), have been proposed as the source of the salinity gradient energy for RED stacks Luo et al, 2012;Nam et al, 2012). With a thermolytic solution the ionic species can be volatilized and captured into HC solutions at temperatures below that needed to boil water.…”

Hydrogen production from continuous flow, microbial reverse-electrodialysis electrolysis cells treating fermentation wastewater

“…Both AEMs and CEMs were standard ion exchange membranes (PC-SA and PC-SK, PCA GmbH) provided with the electrodialysis cell. HC (1.4 M ammonium bicarbonate) and LC (distilled water) solutions (10 L each) flowed in parallel through the stack and were recycled at 300 mL/min in a closed loop (Nam et al, 2012). The LC solution entered into the channel next to the anode chamber to help reduce ammonia crossover into the anode chamber, and HC entering next to the cathode chamber.…”

“…Higher hydrogen gas production rates could possibly be obtained by using higher MREC influent COD concentrations, as this should increase COD removal rate and stabilize anode potentials. The fermentation wastewater examined here was diluted to a concentration to be similar to those previously used in MECs (Lalaurette et al, 2009;Nam et al, 2012;Nam et al, , 2014Ullery and Logan, 2014), so the use of higher influent COD concentrations could easily be achieved. The effect of organic loading on the MREC performance, in terms of anode potentials and hydrogen gas production rates, should therefore be examined in future studies.…”

“…Thermolytic salt solutions, such as ammonium bicarbonate Elimelech and http://dx.doi.org/10.1016/j.biortech.2015.05.088 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved. Phillip, 2011;Luo et al, 2012;Nam et al, 2012), can be used in the stack to provide salinity gradient energy. These thermolytic solutions can be used to regenerate HC and LC solutions in closedloop systems using low grade waste heat and conventional distillation systems.…”

“…An ammonia bicarbonate HC solution was used in the stack in order to examine hydrogen gas production using a thermolytic solution that could be regenerated using waste heat. The RED stack had thin channels, and therefore improved power production, relative to those previously examined for hydrogen gas production in MRECs (Luo et al, 2013;Nam et al, 2012). Tests were initially conducted on stack performance using acetate, and a synthetic dark fermentation effluent, to better control the impact of feed solutions on system performance.…”