A grain of salt or two 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 engineers.
"This system could
produce hydrogen anyplace that there is wastewater near sea water,"
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."
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 difference between
river water and seawater to add the extra energy needed to produce
Their results, published in the Sept. 19 issue of the
Proceedings of the 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
Logan's cells were between 58 and 64 percent efficient and
produced between 0.8 to 1.6 cubic meters of hydrogen for every
cubic meter of liquid through the cell each day. The researchers
estimated that only about 1 percent of the energy produced in the
cell was needed to pump water through the system.
The key to these microbial electrolysis cells is
reverse-electrodialysis or RED that extracts energy from the ionic
differences between salt water and fresh water. A RED stack
consists of alternating ion exchange membranes -- positive and
negative -- with each RED contributing additively to the electrical
"People have proposed making electricity out of RED stacks,"
said Logan. "But you need so many membrane pairs and are trying to
drive an unfavorable reaction."
For RED technology to hydrolyze water -- split it into hydrogen
and oxygen -- requires 1.8 volts, which would in practice require
about 25 pairs of membranes and increase pumping resistance.
However, combining RED technology with exoelectrogenic bacteria --
bacteria that consume organic material and produce an electric
current -- reduced the number of RED stacks to five membrane
Previous work with microbial electrolysis cells showed that they
could, by themselves, produce about 0.3 volts of electricity, but
not the 0.414 volts needed to generate hydrogen in these fuel
cells. Adding less than 0.2 volts of outside electricity released
the hydrogen. Now, by incorporating 11 membranes -- five membrane
pairs that produce about 0.5 volts -- the cells produce
"The added voltage that we need is a lot less than the 1.8 volts
necessary to hydrolyze water," said Logan. "Biodegradable liquids
and cellulose waste are abundant and with no energy in and hydrogen
out we can get rid of wastewater and by-products. This could be an
inexhaustible source of energy."
Logan and Kim's research used platinum as a catalyst on the
cathode, but subsequent experimentation showed that a non-precious
metal catalyst, molybdenum sulfide, had 51 percent energy
Source: The Pennsylvania State University
Image: Bruce Logan/Penn State