A grain or two of salt may be all that microbial electrolysis cells need to produce hydrogen from wastewater or organic byproducts without adding carbon dioxide to the atmosphere or using grid electricity, according to Penn State (State College, Pa.) engineers.
“This system could produce hydrogen anyplace that there is wastewater near seawater,” said Bruce E. Logan, Kappe Professor of Environmental Engineering. “It uses no grid electricity and is completely carbon-neutral. It is an inexhaustible source of energy.”
The King Abdullah University of Science and Technology (Saudi Arabia) supported this work.
Microbial electrolysis cells that produce hydrogen are the basis of this recent work, but previously, to produce hydrogen, the fuel cells required some electrical input. Now, Logan, working with postdoctoral fellow Younggy Kim, is using the ionic difference between river water and seawater to add the extra energy needed to produce hydrogen.
The key to these microbial electrolysis cells is reverse-electrodialysis, or RED. A RED stack consists of alternating ion-exchange membranes — positive and negative — with each RED layer contributing additively to the electrical output, according to a Penn State press release.
For RED technology alone to hydrolyze water — split it into hydrogen and oxygen gases — requires 1.8 V, which in practice would require about 25 pairs of membranes and increase pumping resistance, the press release states.
Microbial fuel cells, on the other hand, require only 0.414 V to produce hydrogen. Previous research has shown microbial electrolysis cells, by themselves, produce about 0.3 V of electricity from exoelectrogenic bacteria, which consume organic material and produce an electric current. So, by adding 11 ion-exchange membranes — five RED layers, which produce about 0.5 V — to microbial cells, the combined system can produce hydrogen gas, the press release states.
“Biodegradable liquids and cellulose waste are abundant, and with no energy in and hydrogen out, we can get rid of wastewater and byproducts,” Logan said in the press release.
The results of the research, published in the Sept. 19 issue of the Proceedings of the U.S. National Academy of Sciences, “show that pure hydrogen gas can efficiently be produced from virtually limitless supplies of seawater and river water and biodegradable organic matter.”
Logan’s cells were between 58% and 64% efficient and produced between 0.8 and 1.6 m3 of hydrogen for every cubic meter of liquid passed through the cell each day, the press release states. The researchers estimated that only about 1% of the energy produced in the cell was needed to pump water through the system.
“This could be an inexhaustible source of energy,” Logan said.
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