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This experiment can be run with a VoltaLab 50 or a VoltaLab 80.
Files:
Pot Universal DP with Pt in H2SO4.EXP
Pot Universal DP raw data [fig e(i)]R.CRV
Pot Universal DP raw data [fig e(ii)]R.CRV
Pot Universal DP raw data [fig e(iii)]P.CRV
Pot Universal DP CA extracted from raw data [fig e(iv)]R.CRV
Pot scan at fixed frequency [fig g]S.CRV
Abstract
The interest of a new electrochemical method,
the 'Universal Differential Pulse" method, is investigated. The
principle of the method is briefly described. The experimental curves
and results obtained on platinum in sulphuric acid are compared
with traditional experiments such as Linear Voltammetry (Cyclic
Voltammetry), Chrono Amperometry and Electrochemical impedance measurement
with potential scan at fixed frequency. The 'Universal Differential
Pulse" method can provide simultaneously information about the "resistive"
and the "capacitive" behaviour of the electrochemical interface.
Since it is one single experiment performed at medium scan rates,
the electroactive interface can be expected to be homogeneous within
the experiment.
The Universal Differential Pulse method
principle
The principle is to define a combination of up
to 8 levels of imposed potential versus time. Each step can last
255 times the acquisition period time (sampling rate). This sampling
rate can be selected from 0.5 ms to 999s.
[fig.a]
Universal DP RECURRENT mode
Up to 8 independent pulse levels can
be imposed with the Universal DP method.
The differential current measurement
is taken between positions represented with
the triangles (inverted solid minus outline) and quoted versus the
potential imposed at the position represented by the triangle. These
differential values are displayed in real time versus the potential.
[fig.b]
Universal DP VOLTAMMETRY mode
In this example 4 levels are used to define
a voltammetry with a superimposed pulse signal.
The Raw data file records
the current during the whole experiment with a sampling rate equal
to the acquisition period time, down to 1 ms.
[fig.c] Universal
DP NORMAL mode
Between each pulse, which consists of
three levels in this example, the applied potential is reset to
an initial level to “regenerate” the interface
The post run processing enables
the user to recalculate any differential curve from the raw data,
adjusting the "sampling window".
Experimental
The mechanisms which occur on a Pt electrode
polarised in a strong acid media such as sulphuric acid are described
in detail elsewhere [1] [2].
Results
[Fig.e] Raw
data from a Universal DP Voltammetry mode experiment
| [fig.e (I)] |
Raw data presented as (i,E)
=f(t). Overall scan rate = 5 mV/s. Data acquisition rate = 10
ms. The raw data curve is presented with an overlay of two individual
linear voltammetries one LV at point 195 and one LV at point
200. |
| [fig.e (II)] |
Zoom which exhibits the imposed
multipulse signal and the CA. The pulse signal is located at
the end of the step. The imposed Vg signal displays the pulses. |
| [fig.e (III)] |
The automatic differential pulse
voltammetry is to be compared with the impedance data [fig.g]. |
| [fig.e (IV)] |
One single CA extracted from
the Raw data file. This extraction uses a filter to minimise
the number of points without loosing information. |
The "lower envelope" of [fig.e (l)] is close
to the anodic scan on the cyclic voltammetry represented [fig.a].
As an evidence Linear Voltammetries [fig.e (l)] and Chrono Amperometry
[fig.e (lll)] can be extracted from these raw data. Furthermore,
differential data can be extracted from the raw data file using
the “Differential extraction” post run processing tool.
Comparison of the Impedance data with the
differential data
The Universal Raw data file contains information
relative to both capacitive effect (differential measurement and
individual chrono amperometries) and resistive effects with actual
charge transfer (individual linear voltammetries). A brief comparison
with the results obtained from an electrochemical impedance measurement
(potential scan at fixed frequency) illustrate these capabilities.
[fig. g] Electrochemical
Impedance (Potential scan at fixed frequency)
Frequency = 0.1Hz - scan rate 47 mV/min
AC sine wave amplitude = 5 mV and
DC potential versus SCE
[fig.e (III)]
Universal DP (Differential data)
Scan rate = 5 mV/s - DC potential versus
SCE
Current = level 190 - di = Level 200- Level 195
The impedance data [fig.g] and the differential
data [fig.e (lll)] are similar in the cathodic region since the
behaviour of the interface is essentially capacitive. In the anodic
region, since the platinum oxide formation starts, it is not possible
to compare directly either the modulus of the impedance or the real
or imaginary part with the differential pulse measurement. The scan
rate is 10 times greater for the Universal method and the information
relative to the DC behaviour is far more detailed (due to the raw
data file) than the information on DC current provided by the Pot.
Fixed Freq. EIS (Capacitance) experiment. The information relative
to the impedance of the interface is more complete with impedance
measurement since the Real and the Imaginary parts are recorded
at each potential. Information about the diminution of the capacitive
contribution is provided by the examination of the individual chrono
amperometry [fig. e (ll) & (lll)]
Conclusion
VoltaLab "all-in-one instrument" which already
performs electrochemical impedance measurements and voltammetry
introduces a new method named the Universal differential pulse method.
It is a new tool to investigate the properties of electrochemical
interfaces and the validity of the mixed information contained in
the experimental files can be cross checked with both more traditional
voltammetric and electrochemical impedance measurements.
References and notes
[1] P.A.
Christensen and A. Hamnett "Techniques and Mechanisms in Electrochemistry"
p228 - Blackie A&P (Imprint of Chapman&Hall, Glasgow, 1994
[2] Ronald
Woods "Chemisorption at Electrodes" In "Electroanalytical Chemistry"
Vol 9 pp 1-162 - M Dekker, 1976
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