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: