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									      CHEM 146C_Experiment #8

Surface Electrochemistry: Adsorption of
Polyoxometalate on Graphite Electrodes

                            Yat Li
           Department of Chemistry & Biochemistry
             University of California, Santa Cruz

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

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

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-       Fe(CN)64-

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:   Fe(CN)64-         Fe(CN)63- + e-

J  K/A: Current decrease as the accumulated
         Fe(CN)64- used up

Record cyclic voltammograms of electrolyte solution with a clean graphite
working electrode as a function of scan rate

Record cyclic voltammograms of electrolyte solution with a graphite working
electrode modified with phosphomolybdic acid, as a function of scan rate

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

               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 )-

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