Hydrogen production from inexhaustible supplies of fresh and salt water using microbial reverse-electrodialysis electrolysis cells

Abstract

There is a tremendous source of entropic energy available from the salinity difference between river water and seawater, but this energy has yet to be efficiently captured and stored. Here we demonstrate that H 2 can be produced in a single process by capturing the salinity driven energy along with organic matter degradation using exoelectrogenic bacteria. Only five pairs of seawater and river water cells were sandwiched between an anode, containing exoelectrogenic bacteria, and a cathode, forming a microbial reverse-electrodialysis electrolysis cell. Exoelectrogens added an electrical potential from acetate oxidation and reduced the anode overpotential, while the reverse electrodialysis stack contributed 0.5-0.6 V at a salinity ratio (seawater:river water) of 50. The H 2 production rate increased from 0.8 to 1.6 m 3 -H 2 ∕m 3 -anolyte/day for seawater and river water flow rates ranging from 0.1 to 0.8 mL∕ min. H 2 recovery, the ratio of electrons used for H 2 evolution to electrons released by substrate oxidation, ranged from 72% to 86%. Energy efficiencies, calculated from changes in salinities and the loss of organic matter, were 58% to 64%. By using a relatively small reverse electrodialysis stack (11 membranes), only ∼1% of the produced energy was needed for pumping water. Although Pt was used on the cathode in these tests, additional tests with a nonprecious metal catalyst (MoS 2 ) demonstrated H 2 production at a rate of 0.8 m 3 ∕m 3 ∕d and an energy efficiency of 51%. These results show that pure H 2 gas can efficiently be produced from virtually limitless supplies of seawater and river water, and biodegradable organic matter.electrohydrogenesis | microbial electrolysis cell | microbial fuel cell | renewable energy | sustainable energy E xoelectrogenic bacteria oxidize organic matter and can transfer electrons to electrically conductive materials such as graphite or metal, making it possible to convert waste organic matter into useful energy. In microbial fuel cells (MFCs), exoelectrogens on the anode, coupled with oxygen reduction at the cathode, can generate a potential as large as ∼0.8 V (open circuit; pH 7; 0.2 atm O 2 ), although less voltage (∼0.23 to 0.5 V) is generated in practice (1). Exoelectrogens can also be used to drive electrochemical H 2 production in a microbial electrolysis cell (MEC) (2, 3). However, the potential generated by substrate oxidation (−0.30 V vs. Standard Hydrogen Electrode; 1 g∕L acetate; pH 7) is not sufficient to drive H 2 evolution (−0.41 V vs. Standard Hydrogen Electrode at pH 7) (1). Thus, additional energy (∼0.11 V in theory) is needed to overcome this thermodynamic threshold, and an external voltage of >0.4 V is typically applied to MECs (4). This additional energy could be provided by a renewable source of energy, such as solar (5), wind, or waste organic matter (6). However, no method has yet been developed to directly achieve H 2 production in one process without an external voltage supply.Reverse electrodialysis (RED) holds great promise as a method for generat...

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

“…15,19 Total hydrogen generation ranged from 27 mL H 2 (Y = 2.8 mol H 2 /mol acetate, SR = 200) to 30 mL H 2 (Y = 3.4 mol H 2 /mol acetate, SR Infinite) over each fed-batch cycle (part B of Figure 1), which is similar to the reported value with NaCl salts in MRECs. 3 There was no significant linear relationship between gas volume and salinity difference between the HC and LC streams (R 2 = 0.18, p = 0.57). The Coulombic efficiencies ranged from 61 to 72% at different SRs (part C of Figure 1), with complete recovery of current as hydrogen gas (r cat = 100%).…”

“…The HC solution was pumped into the HC cell from the cathode chamber and flowed serially through 5 HC cells in the RED stack. 3,14 Similarly, the LC solution flowed through all the 5 LC cells, but in the opposite direction from the anode to cathode side, unless otherwise noted ( Figure S1 of the Supporting Information). The anode and cathode chambers were operated in fed-batch mode at room temperature and were refilled when the current decreased to less than 0.5 mA forming one complete cycle of operation.…”

“…The performance of the MREC was evaluated as previously described in terms of: Coulombic efficiency (η CE , %) based on total coulombs recovered compared to the mass of substrate consumed; cathodic hydrogen recovery (r cat , %), the moles of hydrogen actually recovered at the cathode compared to the moles that theoretically could have been produced from the current; volumetric hydrogen production rate (Q, m 3 H 2 / m 3 /d) normalized to the anolyte volume; and hydrogen yield (Y, mol H 2 /mol acetate) based on the hydrogen produced and acetate consumed. 3 The volumetric current density (I vol , A/m 3 ) was an average of the maximum current production over a 1 h period divided by the anolyte volume. Cathode overpotential was calculated by subtracting the theoretical cathode potential calculated by the Nernst equation from the measured cathode potential.…”

“…This combined MEC and RED process, called a microbial reverseelectrodialysis 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.…”

“…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.…”