Reversibility of an electrochemical reaction

Files:
Chrono Potentiometry .EXP
Chrono Potentiometry 023.CRV.

A Chrono Potentiometry is used to study the reversibility of platinum oxide formation (with oxygen adsorption) and its reduction (with oxygen desorption) on a platinised platinum electrode.

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. Three fundamental mechanisms occur on a Pt electrode polarised in a strong acid media, H2SO4:

. 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/reduction and oxygen evolution (from + 400 mV to 1300 mV versus Calomel)

Two successive levels of constant currents are imposed at the Work electrode. The potential of the WORK versus REF is measured. The potential variation versus time exhibits waves which correspond to the transition time from one reaction (or reaction step) to another reaction (or reaction step). This transition time can be used in trace analysis (stripping analysis) or metal coating by electrodeposition [3] but also for battery studies (lifetime of the battery) but here it is used to investigate the reversibility of the electrochemical reactions [1] [2]. The transition time is used to estimate the charge in coulombs by integration of the imposed current versus time with the "Peak analysis" or the "integration" tool.

Solution H2SO4 0.5 M
WORK Stationary Platinised platinum
Plate ± 5X5 mm (CDC641T conductivity cell)
REF Calomel electrode (XR100)
AUX Platinum wire (XM100)
Cell: CP06
Temp. Ambient ( 25 °C)
Nitrogen: No bubbling

The experiment imposes 12 mA for a period of sufficient length to obtain a positive voltage equal to 1200 mV which corresponds to the limit in potential prior to the oxidation of the solvent itself. Once this potential is reached, the imposed current is -8 mA and the reduction process starts. A Pot. Tutorial CA method (chrono amperometry) starts the sequence to polarise the WORK at -300 mV versus REF in order to activate the surface. This potentiostatic pause is followed by a 10 min Open Circuit Potential method in order to start the experiment at rest.
You cannot run this method with a VoltaLab 10 (a measurement period of 20 ms or more must be set in this case).

Display: Type = Normal X = Time Y1 = Current Y2 =Potential

1) Anodic step at 12 mA.
The potential starts to be negative. This indicates that there are still some elements which can be oxidised. Then the potential increases abruptly around 200 mV. This first transition time corresponds to the beginning of the oxidation process under examination. Once the potential reaches the 1200 mV limit, the imposed current becomes negative.

2) Cathodic step at -8 mA.
The transition time which can be observed during the -8 mA step corresponds roughly to the transition from the reduction of the platinum oxides and the adsorbed oxygen to the reduction of the solvent itself (H+ reduction).
To help you to understand the potential evolution, display the Pot. Cyclic Voltammetry curve available in VM4 Demo Curves, file "No ODC 035.CRV".

Charge budget evaluation with Peak Analysis

Base line y = 0 integration
Point 1:1.922 sec. (Which corresponds to the "transition time" during the anodic step)
Point 2:8.297 sec. (Which corresponds to the "transition time" during the cathodic step)
Total:907.3 µC/cm² (Which corresponds to the "anodic " - "cathodic" charge transfer)
Positive:31.10 mC/cm² (Which corresponds to the "anodic " charge transfer)
Negative:-30.1 mC/cm² (Which corresponds to the "cathodic" charge transfer)

The charge transfer processes involved in the anodic area are quite reversible. The slight excess in positive charge can be interpreted as a beginning of oxygen evolution since oxygen gas will not remain attached to the interface.

References and notes

[1] Digby D. Macdonald "Transient techniques in Electrochemistry" pp 158-176 Plenum Press, 1977
[2] Ronald Woods "Chemisorption at Electrodes", p56 in Electroanalytical Chemistry, Vol 9, Edited by J.Bard. M. Dekker INC. , 1976
[3] The quantity of material which has been either removed by stripping or deposed can be obtained with the "Coulometric dissolution" tool which performs the relevant calculation using the Atomic mass, the valence of the reaction and the density of the material to estimate the thickness of a layer on the electrode surface.