Electrochemical reactions are normally initiated in solution by metal electrodes such as platinum, which are expensive and limited in supply. Researchers at Case Western Reserve University recently demonstrated that an atmospheric-pressure microplasma can act as a gaseous, metal-free electrode to mediate electron-transfer reactions in aqueous solutions. |
Engineers
at Case Western Reserve University have made an electrochemical cell
that uses a plasma for an electrode, instead of solid pieces of metal.
The
technology may open new pathways for battery and fuel cell design and
manufacturing, making hydrogen fuel and synthesizing nanomaterials and
polymers.
A description of the research is now published in an online edition of the Journal of the American Chemical Society.
“Plasmas
formed at ambient conditions are normally sparks which are
uncontrolled, unstable, and destructive,” says Mohan Sankaran, a chemical
engineering professor and senior author of the paper. “We’ve developed a
plasma source that is stable at atmospheric pressure and room
temperature which allows us to study and control the transfer of
electrons across the interface of a plasma and an electrolyte solution.”
Sankaran
worked with former students Carolyn Richmonds and Brandon Bartling,
current students Megan Witzke and Seung Whan Lee and fellow chemical
engineering professors Jesse Wainright and Chung-Chiun Liu.
The group used a traditional set up with their nontraditional electrode.
They
filled an electrochemical cell, essentially two glass jars joined with a
glass tube, with an electrolyte solution of potassium ferricyanide and
potassium chloride.
For
the cathode, argon gas was pumped through a stainless steel tube that
was placed a short distance above the solution. A microplasma formed
between the tube and the surface.
The anode was a piece of silver/silver chloride.
When a current was passed through the plasma, electrons reduced ferricyanide to ferrocyanide.
Monitoring
with ultraviolet-visible spectrophotometry showed the solution was
reduced at a relatively constant rate and that each ferrycyanide
molecule was reduced to one ferrocyanide molecule.
As the current was raised, the rate of reduction increased. And testing at both electrodes showed no current was lost.
The researchers, however, found two drawbacks.
Only
about one in 20 electrons transferred from the plasma was involved in
the reduction reaction. They speculate the lost electrons were
converting hydrogen in the water to hydrogen molecules, or that other
reactions they were unable to monitor were taking place. They are
setting up new tests to find out.
Additionally,
the power needed to form the plasma and induce the electrochemical
reactions was substantially higher than that required to induce the
reaction with metal cathodes.
The
researchers know their first model may not be as efficient as what most
industries need, but the technology has potential to be used in a
number of ways.
Working
with Sankaran, Seung has scanned a plasma over a thin film to reduce
metal cations to crystalline metal nanoparticles in a pattern.
“The
goal is to produce nanostructures at the same small scale as can be
done now with lithography in a vacuum, but in an open room,” Seung says.
They
are investigating whether the plasma electrode can replace traditional
electrodes where they’ve come up short, from converting hydrogen in
water to hydrogen gas on a large scale to reducing carbon dioxide to
useful fuels and commodity chemicals such as ethanol.
The
researchers are fine-tuning the process and testing for optimal
combinations of electrode design and chemical reactions for different
uses.
“This is a basic idea,” Sankaran says. “We don’t know where it will go.”
Electron-Transfer Reactions at the Plasma–Liquid Interface
