US researchers say they have demonstrated how cells fuelled by bacteria can be “self-powered” and produce a limitless supply of hydrogen using a microbial electrolysis cell (MEC). The MECs use something called “reverse electrodialysis” (RED), which refers to the energy gathered from the difference in salinity between saltwater and freshwater, in combination with bacteria that are able to produce electricity as they break down organic matter.
Until now an external source of electricity was required in order to power the process. The claimed breakthrough here is that they do not need an electrical power source anymore, all they do is add some fresh water and some salt water and some membranes, and the electrical potential that is there can provide that power. The scene is now set to produce the hydrogen.
In their paper, Prof Logan and colleague Younggy Kim explained how an envisioned RED system would use alternating stacks of membranes that harvest this energy; the movement of charged atoms move from the saltwater to freshwater creates a small voltage that can be put to work. Prof Logan explained “If you think about desalinating water, it takes energy. If you have a freshwater and saltwater interface, that can add energy. We realised that just a little bit of that energy could make this process go on its own.”
He said that the technology was still in its infancy so the current cost of operating the new technology is too high to be used commercially. They liken this to the development of solar power, as it has taken many years to lower the cost of PV cells after first proving the concept. He hopes that as this technology is refined and upscaled the cost can be brought down.
The next step is to develop larger-scale cells with the hope that this type of integrated system has significant potential to treat wastewater and simultaneously produce [hydrogen] gas without any consumption of external electricity.
Prof Logan added that a working example of a similar microbial fuel cell was currently on display at London’s Science Museum, as part of the Water Wars exhibition
Younggy Kim and Bruce E. Logan, Penn State university
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 H2 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 H2 production rate increased from 0.8 to 1.6 m3-H2/m3-anolyte/day for seawater and river water flow rates ranging from 0.1 to 0.8 mL/ min. H2 recovery, the ratio of electrons used for H2 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 (MoS2) demonstrated H2 production at a rate of 0.8 m3/m3/d and an energy efficiency of 51%. These results show that pure H2 gas can efficiently be produced from virtually limitless supplies of seawater and river water, and biodegradable organic matter.
Full paper available from PNAS (Proceedings of the National Academy of Sciences of USA)