Measuring the capacitance versus potential

This method can not be run with a VoltaLab 10 and a VoltaLab 50.

Files:
Mott-Schottky .EXP
Mott-Schottky 000_01S.CRV
Mott-Schottky 000_02S.CRV

The principle is to measure the impedance at fixed frequency during a potential scan to examine the adsorption/desorption processes. Platinum (Pt) is known as an "inert" metal with a strong catalytic activity for many electrochemical reactions. A lot of components can be adsorbed on Pt surfaces. For example the adsorption of hydrogen on pure platinum is one of the most described phenomena in electrochemistry literature. This experiment concerns three fundamental mechanisms which occur on a Pt electrode polarised in a strong acid media, H2SO4 [1] [3].

. Cathodic area: Hydrogen adsorption / desorption (from - 250 mV to +100 mV versus Calomel)
. Double layer area: no charge transfer/ adsorption (from +100 mV to +400 mV versus Calomel)
. Anodic area: Platinum "oxides" formation and reduction (from + 400 mV to 1300 mV versus Calomel)

The Pot. Fixed Freq. EIS (Capacitance) measurement provides information about charge-transfer and adsorption mechanisms. The experimental file includes impedance data and the result of capacitance calculation plus DC potential and DC current. One can say that the voltammetry is recorded simultaneously, along with the "Mott-Schottky" diagram.

SolutionH2SO4 0.5 M
WORK: Stationary Platinised platinum [2]
Plate ± 5X5 mm (CDC641T conductivity cell)
REF: Calomel electrode (XR100)
AUX: Platinum wire (XM100)
CP06 Measuring cell
Temp. Ambient (25 °C)
Nitrogen: No bubbling

A Potentiostatic pause (Chrono-amperometry) starts the sequence. It polarises the WORK at -250 mV /SCE in order to stabilise the WORK interface and to saturate the solution with H2 in the vicinity of the WORK interface. The impedance data are collected at 0.1Hz with an AC sine wave amplitude of 5 mV and the DC potential is scanned between -250mV and +1250 mV versus REF. The 2 curves files correspond respectively to the anodic direction potential scan and the cathodic direction potential scan.

Overlay the curves and use the "Axis" settings to differentiate from Y1 and Y2. Suggestions:
Mott-Schottky 000_01S.CRV = Potential scan in Anodic direction (Blue)
Mott-Schottky 000_02S.CRV = Potential scan in Cathodic direction (Red)

1) Display : Type = Normal X = Potential Y1 = Capacitance Y2 = -Zimaginary

Compare with Display : Type = Normal X = Potential Y1 = Capacitance Y2 = current

The results are very reproducible. If you display the capacitance or the imaginary part of the impedance and the current versus potential it becomes very clear that the information provided with a Mott-Schottky experiment (Capacitance) concerning the adsorption/desorption steps is much more informative than a simple voltammetry [1] [3].

[1] P.A. Christensen and A. Hamnett "Techniques and Mechanisms in Electrochemistry" p228 - Blackie A&P (Imprint of Chapman&Hall, Glasgow, 1994
[2] Platinised platinum amplifies the catalytic effect of Pt.
[3] Ronald Woods "Chemisorption at Electrodes" In "Electroanalytical Chemistry" Vol 9 pp 1-162 - M Dekker, 1976