Fuel cell(type of galvanic cell, spontaneous)
Fuel cell
Direct combustion, so we separate fuel cell, oxidation form reduction.
Work has direction, and organized formed of energy.
Heat = not useful b/c randomized energy.
Oxygen fuel cell
Deeply drop is due to pH gradient rapid rather than activation over potential, we cant have active
overmentioned while cell is not working. It needs to activate electron exchange at electrode.
Another evidence, Rest potential (1.2V~ 1.3V) does not change, it is losing due to over potential, larger
loss in terms of resistance. pH gradient with paper never totally neutralizes. The rate, Migration ion
totally proportional to concentration different to the gradient. And gradient strongly diminish, So we
see it is not activation overpotential.
In chlorine cell,
Chlorine pH due to side reaction, and side reaction keep happening so It is ok. Due to very limited side
reaction slowly. So proton neutralize oH, they can slowly recover
Some of polarization overpotential, contribute to only resistance. Concentration overpotential, high
“Surfactant “ why increase
Attractive to water on ions, pull out in bulk,
it does not allow to close near surface. They are
not only pulled by hydration sphere and pull each other at higher concen.
ion-ion interaction quite
significantly. Ion-ion interaction are stranger at high concentration and ion close to each other. Thinner
layer water only screen very small fraction of those interaction. H2o has dipole moment, Dipole moment
can screen those ion interaction when water between get charges.
Infinite dilution, very large water between ionic charges. So they do not see each other due to water
molecule shield ion ion interaction.
Concentration ion is high in bulk, and depleted on surface.
Surface tension = amount work need to expand surface by unit surface area., so if we have strong
interaction, strong attractive force between water molecule,, water moleuce
Surface tension increase.
So lower concentration ion near the surface.
And high in bulk.
Empty layer thickness (where surface tension valuable, concentration exchange)
Half layer thicknee?
Empty layer thinckness is just estimate. Indirectly.
1. Constant pressure, and temp.
2. Ideal is for NACl is 1 to 4 M, but we did .1M ~
How do we calculate empty laer thickness
Gibs isotherm. Listen again around 35:00
When metal ion in solution
1) Incraseing the charge on the cation, increase the metal ion’s ability. Highly charged metal ions
produce more acidic solution than low charges.
2) Smaller the cation, the more acidic are its solutions.
SMALL ION WITH LARGE CHARGE, Al ion potential getting higher, H2O molecule stick harder to ion,
so some property charges because of that, H2o attach Al3+ so strongly they can form very organize
configuration. Ex) Hydrated aluminum structure.
Hydrate Al structure, Hydrated aluminum can be deprotonated so it can be acidic.
Hydration sphere can be more than one layer. If ion small radius & large charge.
What else can affect friction
a) Viscosity(temperature, intermolecular forces dependence), higher viscosity
= decrease
b) Direct relate to ion mobility.
If higher intermolecular forces = increase viscosity. Sides of molecular also, polymer = large viscosity.
= lots of friction
K > Na > Li
But Li drag more water in solution so experience much larger friction, * effective hydrated radius
much larger tan its radius along *
Cl ion has a greater mobility than NA+ so. Cl diffuses faster than Na+ so region rich in Cl-
Surface tension
If we want to increase surface , we have to bring molecular such as bulk molecules to surface. But it
requires work. To break intermolecular interaction for each surface.
Stretching surface = breaking intermolecular forces
Ex ) soap tail less attractive to H2O , head more attractive. It needs little amount to change.
If ion moves to surface, it loss hydration sphere, = much unfavor.
Empty layer thickness :
it not allow to go to surface b/c too much energy, instead H2O move to
surface, avg, ions, polarize, strongly attractive in bulk solution.
So small ion with large ion potential = large change surface tension
b/c strong interaction with H2O bulk concentration & avoid surface b/c they cant get rid of hydration
sphere, so drawing in so effet emplty layer for surfactant.
Surfactant : increase surface tension, due to small charge ion, more charge = more increase puling
each other in higher concentration. Ion-ion interaction = stronger due to ion-ion closer. Water molecule
have dipole moment.
ex) Cr3+, Al3+ have large change, Li potential difference we expect,
If it is related to work of cell itself, it would reversible .
Our case if not reversible, activation energy should be same. Same direction.
So we look other losses. Formation of pH gradient. Once we have done electrolysis, we have very strong
pH gradient in beginning. It came from electrolysis. It generated from excess hydroxide by removing
proton. Form water Other side, chlorine, water constant. Verly little. Due to it is slow once it dissolved.
Standard voltage for Hydrogen is 0V, in case we have to have 1.3~ something. But we have
2V due to we had basic solution other side.(special consideration).oxygen electrode different from
chlorine electrode(side reaction it goes same direction.), In oxygen cell, it produce. Hydronium ion,
either oxygen consuming oxygen, we change pH electrode actively due to chemical reaction.
pH gradient is more sensitive in Oxygen cell, due to mobility, conductivity. One side, we have
hydroxide, and hydronium ion.
Strong gradient is faster. They will neutralize between two electrode. Chemical reaction
consume some of acidic pH on oxygen side. and very unstable pH gradient.
Our cell is not reversible due to constant change pH gradient.
Low current density.
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