enormous amount of energy dwells where rivers meet the ocean.
Harnessing this energy—released when fresh and salty water mix—requires
expensive porous membranes that can control the flow of ions.
Researchers now report that membranes made of 2-D metal carbide or metal
nitride materials, known as MXenes, could work as well as conventional
membranes yet should be easier and cheaper to make at large scales (ACS
Nano 2019, DOI: 10.1021/acsnano.9b02579).
Harvesting electricity from salinity gradients
has the potential to meet 20% of the world’s energy needs, says Peng
Wang, an environmental scientist and engineer at King Abdullah
University of Science and Technology who led the new work. One such
pilot-scale technology is reverse electrodialysis (RED), which uses
membranes to separate positively and negatively charged ions, creating
an electric potential that generates electricity. But it remains
impractical at large scales, partly because it relies on expensive
semipermeable membranes with tiny pores that are easily clogged by
impurities and bacteria.
A promising alternative
membrane for RED is a film made of stacked ultrathin sheets with
maze-like channels through which ions can navigate. Tailoring the
surface charge of the individual sheets selects whether anions or
cations pass through, so charges can be separated to generate
first developed by Drexel University researchers for battery
electrodes, are perfect materials for such layered membranes. They are
strong and flexible in addition to being very easy and inexpensive to
make. Plus, the materials’ surface charge and the size of the channels
in a MXene membrane can be tailored for specific ions.
Wang, Husam N. Alshareef, and
their colleagues made a 50 mm wide, 3 µm thick layered membrane out of a
MXene, negatively charged titanium carbide. They tested it by putting a
1 mM potassium chloride solution on one side of the membrane and a 1 M
solution on the other side. Potassium cations can only cross from the
saltier to the less salty side. This crossing generated a power output
density of 21 W/m2 at room temperature and 54 W/m2 at 58 °C. These densities are comparable to today’s polymeric reverse electrodialysis membranes.
Others have obtained higher
power densities with experimental single-layer membranes that have
nanopores or with single nanotubes, says David A. Vermaas, a chemical
engineer at TU Delft, but MXenes are “a much more practical material for
scaling up.” State-of-the-art polymeric membranes cost about $50 per
square meter, Vermaas says. Wang estimates that the MXene membrane could
come in at less than half that.
The team needs to ensure the
membranes are chemically and mechanically durable and resistant to
fouling over long-term operation. The researchers also need to make and
test much larger membranes, which should be an advantage of this
approach. “Large-scale MXene membranes could be easily fabricated by
utilizing methods such as spray deposition or roll-to-roll coating,”
Christopher A. Gorski, a civil
and environmental engineer at Pennsylvania State University, says that
the membrane should be tested at industrially relevant, practical
conditions. The salinity gradient and pH in the study were too high to
represent real waters, which typically have pHs of 5–8, he says.
Nevertheless, he adds, given the expense of existing ion-selective
membranes, “it’s definitely worthwhile to try to identify new
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