The electrochemical reactions inside the porous electrodes of
batteries and fuel cells have been described by theorists, but never
measured directly.
Now, a team at MIT has figured out a way to measure
the fundamental charge transfer rate — finding some significant
surprises.
The study found that the Butler-Volmer (BV) equation, usually used to
describe reaction rates in electrodes, is inaccurate, especially at
higher voltage levels.
Instead, a different approach, called
Marcus-Hush-Chidsey charge-transfer theory, provides more realistic
results — revealing that the limiting step of these reactions is not
what had been thought.
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This illustration shows a battery
electrode made of lithium iron phosphate (left side of image) coated
with carbon, and in contact with an electrolyte material. As the battery
is discharged, lithium ions (shown in purple) jump across the coating
and insert themselves into the crystal structure, while electrons (shown
as circles with minus signs) in the carbon-coating tunnel into the
material and attach to iron ions (shown in red). (Phosphate groups are
left out of this diagram for clarity.) Illustration courtesy of Peng Bai and Martin Bazant
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