Yat Li Department 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, K 3 Fe(CN) 6 A  B: B  D:Fe(CN) e - Fe(CN) 6 4- D  F: Diffusion layer is extended away from electrode surface F  H/I:Reduction of Fe(CN) 6 3- stop, current becomes zero again H/I  J: No current (no reducible or oxidizable species) Fe(CN) 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 H 2 O 2 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 Γ 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) I = vC The peak current is proportional to scan rate v, I cap : current v: scan rate C d : capacitance

Cyclic voltammograms_quantitative information 3. Number of electrons (n) For a reversible electrode reaction at 25 °C, the difference in peak potentials,  E p is expected to be  E p = │E pa - E pc │ = 90.6 / n 4. Surface coverage (Γ) I peak = n 2 F 2 vAΓ(4RT ) - When the number of electrons is known, the surface coverage can be calculated by the equation: