Chemistry Reference
In-Depth Information
Compact
layer
Diffuse layer =
Stern layer
-
+
-
+
-
-
-
+
-
+
-
Solution
Electrode
IHP OHP
Ions which can
be specifically
adsorbed
Hydrated
ions
+
FIGURE 6.1
Guy-Chapman-Stern.model.of.the.double.layer..IHP,.inner.Helmholtz.plane.and.OHP,.outer.
Helmholtz.plane.
where
i
C
.is.the.capacitive.current
E
.is.the.applied.potential
R
s
.is.the.resistance.of.the.solution
t
.is.the.time
C
d
.is.the.capacity.of.the.double.layer
Typical.values.for.the.capacitance.of.electrode.surfaces.are.10.to.50.μF.cm
−2
,.but.they.may.dif-
fer. signiicantly. for. CPEs. particularly. when. modifying. the. interface. structure.. For. a. capacitance.
C
d
.of.20.μF.and.a.resistance.
R
s
.of.1.Ω,.the.time.constant.
τ
.(=
R
s
.·.
C
d
).is.around.20.μs..Within.3
τ
.the.
capacitive.current.drops.by.95%.of.its.value..The.thickness.of.the.diffuse.double.layer.depends.on.
the.type.of.electrolyte.and.its.concentration:.according.to.the.Gouy-Chapman-Stern.model.for.a.1:1.
electrolyte.it.is.around.10.nm.for.a.total.ion.concentration.of.10
−3
.mol.dm
−3
,.100.nm.for.10
−5
.mol.dm
−3
,.
and.1.μm.for.10
−7
.mol.dm
−3
..When.applying.a.linear.voltage.ramp.rather.than.a.constant.potential,.
i
C
.
drops.to.a.constant.value.of.
υ
·.
C
d
,.where.
υ
.is.the.scan.rate.(V.·.s
−1
).
6.1.2 M
ass
T
ransporT
With.an.electric.ield.applied.to.a.solution,.there.are.basically.three.circumstances.that.may.provoke.
the.movement.of.charged.particles.in.this.ield..The.displacement.of.particles.is.in.general.called.
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