yat li department of chemistry & biochemistry university of california, santa cruz chem...
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Yat LiDepartment of Chemistry & Biochemistry
University of California, Santa Cruz
CHEM 146C_Experiment #8
Surface Electrochemistry: Adsorption of Polyoxometalate on Graphite Electrodes
Objective
In this laboratory experiment, we will learn:
1. The basic concept of electrochemistry and cyclic voltammetry
2. How to study the electrochemical behavior of a surface-adsorbed redox species
Electrochemistry
Electrochemistry encompasses a group of qualitative and quantitative analytical methods based on the electrical properties of a solution of the analyte when it is made part of the electrochemical cell.
• stiochiometry and rate of interfacial charge transfer • the rate of mass transfer • the extent of adsorption or chemisorptions• the rates and equilibrium constants for chemical reaction
Electrochemical cell
1. Three electrode configuration
• Working electrode: usually graphite; potential is varied linearly with time
• Reference electrode: e.g. Ag/AgCl; potential remains constant throughout the experiment
• Counter electrode: usually platinum coil, simply conducts electricity from the signal source through the solution to the working electrode
2. Supporting electrolyte: non-reactive electrolyte, conducts electricity
3. Analyte: e.g. redox species
Cyclic voltammetry_excitation signal
In voltammetry, a variable potential excitation signal is impressed on a working electrode in an electrochemical cell.
Cyclic voltammetry: potential will be cycled between two potentials
Triangular waveform
Same scan rate and region
Cyclic voltammograms
For example, K3Fe(CN)6
A B:
B D: Fe(CN)63- + e- Fe(CN)6
4-
D F: Diffusion layer is extended away from electrode surface
F H/I: Reduction of Fe(CN)63- stop, current
becomes zero again
H/I J:
No current (no reducible or oxidizable species)
Fe(CN)63- + e- Fe(CN)6
4-
J K/A: Current decrease as the accumulated Fe(CN)6
4- used up
Procedure_1
Record cyclic voltammograms of electrolyte solution with a clean graphite working electrode as a function of scan rate
Procedure_2
Record cyclic voltammograms of electrolyte solution with a graphite working electrode modified with phosphomolybdic acid, as a function of scan rate
Procedure_3
Record cyclic voltammograms of electrolyte solution with a graphite working electrode modified with phosphomolybdic acid as function of H2O2 concentration
Cyclic voltammograms_quantitative information
1. Number of charge (Q)
The integrated area under each wave represents the charge Q associated with the reduction or oxidation of the adsorbed layer
Q = n F A Γn: number of electronsF: Faraday constant A: the electrode surface areaΓ: the surface coverage in moles of adsorbed molecules per surface area
2. Capacitance (C)
I = vC
The peak current is proportional to scan rate v,
Icap: currentv: scan rateCd: capacitance
Cyclic voltammograms_quantitative information
3. Number of electrons (n)
For a reversible electrode reaction at 25 °C, the difference in peak potentials, Ep is expected to be
Ep = │Epa - Epc│ = 90.6 / n
4. Surface coverage (Γ)
Ipeak = n2F2vAΓ(4RT )-
When the number of electrons is known, the surface coverage can be calculated by the equation:
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