Transcript Electrochemistry - University of California, Santa Cruz

CHEM 146C_Experiment #8
Surface Electrochemistry: Adsorption of
Polyoxometalate on Graphite Electrodes
Yat Li
Department of Chemistry & Biochemistry
University of California, Santa Cruz
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
Same scan rate and region
Triangular waveform
Cyclic voltammograms
For example, K3Fe(CN)6
A  B:
No current (no reducible or
oxidizable species)
B  D:
Fe(CN)63- + e-
D  F:
Diffusion layer is extended away
from electrode surface
Fe(CN)64-
F  H/I: Reduction of Fe(CN)63- stop, current
becomes zero again
H/I  J:
Fe(CN)64-
Fe(CN)63- + e-
J  K/A: Current decrease as the accumulated
Fe(CN)64- 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=nFAΓ
n: number of electrons
F: Faraday constant
A: the electrode surface area
Γ: the surface coverage in moles of adsorbed molecules per surface area
2. Capacitance (C)
The peak current is proportional to
scan rate v,
I = vC
Icap: current
v: scan rate
Cd: capacitance
Cyclic voltammograms_quantitative information
3. Number of electrons (n)
For a reversible electrode reaction at 25 °C, the difference in peak
potentials, DEp is expected to be
DEp = │Epa - Epc│ = 90.6 / n
4. Surface coverage (Γ)
When the number of electrons is known, the surface coverage can be
calculated by the equation:
Ipeak = n2F2vAΓ(4RT )-