Transcript Controlled potential microelectrode techniques—potential

Controlled potential microelectrode
techniques—potential sweep methods
(1) Potential sweep methods: linear sweep
voltammetry (LSV) and cyclic voltametry (CV).
(2) Cyclic voltammetry is a very popular technique for
initial electrochemical studies of new systems and
has proven very useful in obtaining information
about fairly complicated electrode reactions.
(3) Signal
Response
Linear sweep voltammetry
Signal
Resulting i-E curve
A typical LSV response curve for the
reduction
• At a potential well positive of E0´, only nonfaradaic
currents flow for awhile.
• When the potential reaches the vicinity of E0´, the
reduction begins and current starts to flow.
• As the potential continues to grow more negative, the
surface concentration of the reactant must drop, hence
the flux to the surface and the current increase.
• As the potential moves past , the surface concentration
drops to near zero and mass transfer of reactant to the
surface reaches a maximum rate.
• Then it declines as the depletion effect sets in.
Cyclic voltammetry
Cyclic potential sweep
(initial potential and
switching potential)
Resulting cyclic
voltammogram
Sweep voltammogram depends on a
number of factors including:
• Scan rate
• Pathway of a general electrode reaction
• Reaction rate of the rate-determining
steps)
• Chemical reactivity of the electroactive
species
Scan rate
• In LSV, the potential is scanned from a
lower limit to an upper limit
• Unit of scan rate(υ): V/s or mV/s
• Effects of scan rate on charging current:
Ei
ic  Cd  [(  Cd ) exp(t / RsCd )
Rs
Factors affecting electrode reaction rate
In general, the electrode reaction rate is governed by
rates of processes such as:
(1) Mass transfer (e.g., from the bulk solution to the
electrode surface).
(2) Electron transfer at the electrode surface.
(3)Chemical reactions preceding or following the
electron transfer.
(4)Other surface reactions.
◆ The magnitude of this current is often limited by
the inherent sluggishness of one or more
reactions called rate-determining steps.
Scan rate
If the scan rate is altered the current
response also changes.
Rate-determining steps
RT 1 1
1

i(   )
nF i0 il ,c il ,a
  i(Rct  Rmt ,c  Rmt ,a )
• Here we see very clearly that when i0 is much greater than the
limiting currents, Rct<<Rmt,c + Rmt,a and the overpotential, even
near Eeq, is a concentration overpotential. On the other hand, if
i0 is much less than the limiting currents, then Rmt,c +
Rmt,a<<Rct, and the overpotential near Eeq is due to activation
of charge transfer.
Peak current and scan rate
i p  0.4463nFAC D
*
1/ 2
 RT  1/ 2


 nF 
• At 25℃, ip is
ip  (2.69 10 )n AC D 
5
3/ 2
*
1/ 2 1/ 2
Nernstian (reversible) systems
• Peak current is linear with square root of
scan rate
• No effects of scan rate on peak potential
• Reductive peak current is equal to
oxidative peak current
• Value of peak potential difference is 58 mV/n
•
RT
E p  E p / 2  2.2
nF
Totally irreversible systems
ip  (2.99 10 )n( n) AC D 
5
1/ 2
*
1/ 2 1/ 2
1/ 2
1/ 2

D 
RT
  n F  
0`
Ep  E 
0.780  ln  0   ln 
 
 n F 
 RT  
 k 
Voltammogram and Rate constant
The figure below shows a series of
voltammograms recorded at a single
voltage sweep rate for different
values of the reduction rate constant
(kred)
Voltammogram and reverbilitity
The figure below shows the voltammograms for a
quasi-reversible reaction for different values of
the reduction and oxidation rate constants.
Reversal techniques for the reduction
• If Eλ is at least 35/n mV past the cathodic peak,
the reversal peaks all have the same general
shapes.
• If the cathodic sweep is stopped and the
current is allowed to decay to zero, the
resulting anodic i-E curve is identical in shape
to the cathodic one, but is plotted in the
opposite direction on both the I and E axes.
Multicomponent systems (1)
• For a two-component system this
technique allows establishing the baseline
for the second wave by halting the scan
somewhere before the foot of the second
wave and recording the i-t curve, and then
repeating the experiment.
• The second run is made at the same rate
and continues beyond the second peak.
Multicomponent systems (2)
• For a two-component system, an alternate
experimental approach involves stopping
the sweep beyond Ep and allowing the
current to decay to a small value (the
concentration gradient of O is essentially
zero near the electrode).
• Then one continues the scan and
measures ip′ from the potential axis as a
baseline.
Multistep charge transfers
• For the stepwise reduction of a substance O, the
situation is similar but more complicated.
• In general the nature of the i-E curve depends on
△E0= E02-E01.
• When △ E0 <-100 mV, two separate waves are
observed. When △ E0 is between 0 and -100 mV, the
individual waves are merged into a broad wave.
When △ E0 =0, a single peak with a peak current
intermediate between those of those of single-step 1e
and 2e reactions is found. For △ E0 ≥180 mV, a
single wave characteristic of a direct 2e reduction is
observed.
Electrode reactions with coupled
homogeneous chemical reactions
• If E represents an electron transfer at the
electrode surface, and C represents a
homogeneous chemical reaction.
• Classification of reactions: CE reaction,
EC reaction, Catalytic (EC′) reaction, ECE
reaction.
Notes
• kf: heterogeneous rate constant for
oxidation
• kb: heterogeous rate constant for reduction
• K: equilibrium constant
• λ: dimensionless homogeneous kinetic
parameter, specific to mechanism
• DP: diffusion zone, KP: pure kinetic region,
Following reaction-EC
• Note that at small values of λ,essentially
reversible behavior is found. For large
values of λ (in the KP region), no current is
observed on scan reversal and the shape
of the curve is similar to that of a totally
irreversible charge transfer.
• In the KP region, Ep is given by
RT
RT
E p  E1/ 2 
0.780 
ln 
nF
2nF
knF
( 
)
RT
The figure below shows a cyclic voltammogram
recorded for the EC reaction when the chemical rate
constant kEC is extremely large.
EC' mechanism
2-hydroxyacridinone
• Electrochemical oxidation of 2-hydroxyacridinone
was studied by cyclic voltammetry (CV), spectroelectrochemical methods and controlled potential
electrolysis. The photochemical oxidation was also
investigated.
Z. Mazerska, S. Zamponi,
R. Marassi, P. Sowiński, J.
Konopa. J. Electroanal.
Chem. 521 (2002) 144–
154
Voltammograms
• Voltammograms were obtained at a glassycarbon electrode (area: 0.7 cm2). A conventional
three-electrode electrochemical cell containing a
platinum counter electrode (CE) and a saturated
calomel reference electrode (SCE) was
employed. All samples were deoxygenated by
passing Ar for 10 min. The electrodes were
cleaned between runs by polishing with Al2O3
suspension (0.05 μM).
Voltammograms
• On the first positive sweep one oxidation peak,
Ia, appeared and three significantly lower
peaks, Ic, IIc and IIIc, were formed in the
reverse scan. On the second positive sweep
new oxidation bands, IIIa and IIa, were
observed, which seem to form couples with the
reduction peaks, IIIc and IIc, respectively. The
cyclic voltammograms recorded under various
pH conditions are presented.
Photochemical synthesis
• The 1 mM solution of 2-hydroxyacridinone
in the quartz flask was exposed to the light
emitted with the UV lamp and was stirred
intensively during the respective period of
time.
• It is demonstrated, by comparison with the
voltammogram of the substrate, that
photochemical product p2 was the species
responsible for the IIIc–IIIa couple.
Adsorbed intermediates in
electrode processes
• Only adsorbed O and R electro-activenernstian reaction:
2
2
n F
*
Ip 
 A
4 RT
• Only adsorbed O electroactive-irreversible
reaction:
0
RT
RTk
Ep  E 
ln(
)
 n F  n F
0`
Electrochemical behavior of riboflavin
immobilized on different matrices
A.C. Pereira, A.S.
Santos, L. T. Kubota. J.
Colloid Interface
Science 265 (2003)
351–358.
Effects of Scan rate on voltammograms
Effects of Scan rate on voltammograms
Cyclic voltammograms of the eletrostaticallyassembled iron
porphyrin ITO modified electrode in an aqueous solution
containing o.1 mol/L trifluoromethanesulphonate lithium
Structural representation of meso-tetra(4-pyridyl)
porphynato iron(III)
Cyclic voltammograms of the NADH solutions using (A) a
bare glassy carbon electrode and (B) an electrode modified
with tetraruthenated cobalt porphyrin
Structural representation of the tetraruthenated
cobalt porphyrin complex
Cyclic voltammograms of the tetraruthenated cobalt
porphyrin complex (A) and (B) the corresponding films
Multiclyclic voltammogram of [Ru(tpp)(bpy)2] (tpp=
5,10,15,20-tetraphenylporphyrin) at scan rate of 0.2 V/s
in 0.1 mol/L TBAP-dichrolomethane
Cyclic voltammograms of the poly-[Ru(tpp)(bpy)2] (tpp=
5,10,15,20-tetraphenylporphyrin) deposited on the platium
electrode in 0.1 mol/L TBAP-dichrolomethane, scan rate of
(a) 100, (b) 80, (c) 60, (d) 40, (e) 20 mV/s