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ELECTROCHEMISTRY CHEM 4700 CHAPTER 2 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university.

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Presentation on theme: "ELECTROCHEMISTRY CHEM 4700 CHAPTER 2 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university."— Presentation transcript:

1 ELECTROCHEMISTRY CHEM 4700 CHAPTER 2 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2 CHAPTER 2 ELECTRODE REACTIONS & INTERFACIAL PROPERTIES

3 - Involves linear scanning of potential of a stationary electrode using a triangular waveform - Solution is unstirred - The most widely used technique for quantitative analysis of redox reactions Provides information on - the thermodynamics of redox processes - the kinetics of heterogeneous electron transfer reactions - the kinetics of coupled reactions CYCLIC VOLTAMMETRY

4 - The current resulting from an applied potential is measured during a potential sweep - Current-potential plot results and is known as cyclic voltammogram (CV) CYCLIC VOLTAMMETRY

5 CYCLIC VOLTAMMOGRAM (CV) Triangular waveform (left) and CV (right) of ferricyanide

6 - Assume only O is present initially - A negative potential sweep results in the reduction of O to R (starting from a value where no reduction of O initially occurs) - As potential approaches E o for the redox process, a cathodic current is observed until a peak is reached - The direction of potential sweep is reversed after going beyond the region where reduction is observed - This region is at least 90/n mV beyond the peak CYCLIC VOLTAMMETRY

7 - R molecules generated and near the electrode surface are reoxidized to O during the reverse (positive) scan - Results in an anodic peak current - The characteristic peak is a result of the formation of a diffusion layer near the electrode surface - The forward and reverse currents have the same shape CYCLIC VOLTAMMETRY

8 - Increase in peak current corresponds to achievement of diffusion control - Decrease in current (beyond the peak) does not depend on the applied potential but on t -1/2 Characteristic Parameters - Anodic peak current (i pa ) - Cathodic peak current (i pc ) - Anodic peak potential (E pa ) - Cathodic peak potential (E pc ) CYCLIC VOLTAMMETRY

9 Reversible Systems - Peak current for a reversible couple is given by the Randles-Sevcik equation (at 25 o C) n = number of electrons A = electrode area (cm 2 ) C = concentration (mol/cm 3 ) D = diffusion coefficient (cm 2 /s) ν = potential scan rate (V/s) CYCLIC VOLTAMMETRY

10 Reversible Systems i p is proportional to C i p is proportional to ν 1/2 - Implies electrode reaction is controlled by mass transport i p /i c ≈ 1 for simple reversible couple - For a redox couple CYCLIC VOLTAMMETRY

11 Reversible Systems - The separation between peak potentials - Used to determine the number of electrons transferred - For a fast one electron transfer ∆E p = 59 mV - E pa and E pc are independent of the scan rate CYCLIC VOLTAMMETRY

12 Reversible Systems - The half peak potential - E 1/2 is called the polarographic half-wave potential Multielectron Reversible Systems - The CV consists of several distinct peaks if the E o values for the individual steps are well separated (reduction of fullerenes) CYCLIC VOLTAMMETRY

13 Irreversible Systems - Systems with sluggish electron transfer - Individual peaks are reduced in size and are widely separated - Characterized by shift of the peak potential with scan rate CYCLIC VOLTAMMETRY

14 Irreversible Systems α = transfer coefficient n a = number of electrons involved in a charge transfer step k o = standard heterogeneous rate constant (cm/s) - i p is proportional to C but lower depending on the value of α For α = 0.5 i p,reversible /i p,irreversible = That is irreversible peak current is ~ 80% of reversible i p CYCLIC VOLTAMMETRY

15 Quasi-reversible Systems - Current is controlled by both charge transfer and mass transport - Voltammograms are more drawn out - Exhibit larger separation in peak potentials compared to reversible systems - Shape depends on heterogeneous rate constant and scan rate - Exhibits irreversible behavior at very fast scan rates CYCLIC VOLTAMMETRY

16 Applications 1. Study of Reaction Mechanisms E = redox step and C = chemical step E - Only redox step O + ne - ↔ R CYCLIC VOLTAMMETRY

17 Applications E = redox step and C = chemical step EC - Redox step followed by chemical step O + ne - ↔ R + A → Z - R reacts chemically to produce Z - Z is electroinactive - Reverse peak is smaller since R is chemically removed i pa /i pc < 1 - All of R can be converted to Z for very fast chemical reactions CYCLIC VOLTAMMETRY

18 Applications E = redox step and C = chemical step EC - Redox step followed by chemical step O + ne - ↔ R + A → Z Examples - Ligand exchange reactions as in iron porphyrin complexes - Oxidation of chlorpromazine to produce a radical cation and subsequent reaction with water to produce sulfoxide CYCLIC VOLTAMMETRY

19 Applications E = redox step and C = chemical step EC - Catalytic regeneration of O during a chemical step O + ne - ↔ R + A ↔ O - Peak ratio is unity Example - Oxidation of dopamine in the presence of ascorbic acid CYCLIC VOLTAMMETRY

20 Applications E = redox step and C = chemical step CE - Slow chemical reaction precedes the electron transfer step Z → O + ne - ↔ R i pa /i pc > 1 (approaches 1 as scan rate decreases) i pa is affected by the chemical step i pc is not proportional to ν 1/2 CYCLIC VOLTAMMETRY

21 Applications E = redox step and C = chemical step ECE - Chemical step interposed between redox steps O 1 + ne - ↔ R 1 → O 2 + ne - → R 2 - The two redox couples are observed separately - The system behaves as EE mechanism for very fast chemical reactions - Electrochemical oxidation of aniline CYCLIC VOLTAMMETRY

22 Applications 2. Study of Adsorption Processes - For studying the interfacial behavior of electroactive compounds Symmetric CV ∆E p = 0 - Observed for surface-confined nonreacting species - Ideal Nernstian behavior CYCLIC VOLTAMMETRY

23 Applications Symmetric CV - Peak current is directly proportional to surface coverage (Γ) and scan rate (ν) Holds for relatively - slow scan rates - slow electron transfer - no intermolecular attractions within the adsorbed layer CYCLIC VOLTAMMETRY

24 Applications Symmetric CV CYCLIC VOLTAMMETRY volts current ∆E p,1/2 Q (area under peak)

25 Applications Symmetric CV - The surface coverage can be determined from the area under the peak (Q) Q = quantity of charge consumed CYCLIC VOLTAMMETRY or

26 Applications 3. Quantitative Determination - Based on the measurement of peak current CYCLIC VOLTAMMETRY

27 - Simultaneous measurement of spectral and electrochemical signals - Coupling of optical and electrochemical methods - Employs optically transparent electrode (OTE) that allows light to pass through the surface and adjacent solution Examples Indium tin oxide (ITO), platinum, gold, silver, nickel deposited on optically transparent glass or quartz substrate SPECTROELECTROCHEMISTRY

28 i pa = anodic peak current i pc = cathodic peak current Modulated Absorbance A m = -log(I/I o ) SPECTROELECTROCHEMISTRY

29 Applications - Useful for elucidation of reaction kinetics and mechanisms (for probing adsorption and desorption processes) - Thin layer SE methods for measuring E o and n (Nernst equation) - Infrared SE methods for providing structural information - UV-Vis spectroscopic procedures - Vibration spectroscopic investigations - Luminescence reflectance and scattering studies SPECTROELECTROCHEMISTRY

30 - Technique for studying electrogenerated radicals that emit light - Involves electrochemical generation of light-emitting excited-state species - Usually carried out in deoxygenated nonaqueous media Examples of Species Ru(bpy) 3 2+ Nitro compounds Polycyclic hydrocarbons Luminol ELECTROCHEMILUMINESCENCE (ECL)

31 - Used to acquire high resolution data of surface properties - Achieved by sensing the interactions between a probe tip and the target surface as the tip scans across the surface Examples - Scanning Tunneling Microscopy (STM) - Atomic Force Microscopy (AFM) - Scanning Electrochemical Microscopy (SECM) SCANNING PROBE MICROSCOPY

32 - Direct imaging of surfaces on the atomic scale - Very sharp atomic tip moves over the sample surface with a ceramic piezoelectric translator - The basic operation is the electron tunneling between the metal tip and the sample surface - Tunneling current is measured as potential is applied between the tip and the sample - Measured current at each point is based on sample-tip separation SCANNING TUNNELING MICROSCOPY (STM)

33 - High resolution imaging of the topography of surfaces (surface structure) - Allows for nanoscopic surface features while the electrode is under potential control - Measures the force between the probe and the sample - The probe has a sharp tip made from silicon or silicon nitride attached to a force-sensitive cantilever - Useful for exploring both insulating and conducting regions ATOMIC FORCE MICROSCOPY (AFM)

34 - Faradaic currents at a microelectrode tip are measured while the tip is moved close to the substrate surface immersed in a solution containing an electroactive species - The tip currents are a function of the conductivity and chemical nature of the substrate as well as the tip-substrate distance Images obtained give information on - electrochemical activity - chemical activity - surface topography SCANNING ELECTROCHEMICAL MICROSCOPY (SECM)

35 - Cannot be used for obtaining atomic resolution Used to investigate - Ionic flux through the skin or membranes - Localized biological activity (biosensors) - Heterogeneous reaction kinetics SCANNING ELECTROCHEMICAL MICROSCOPY (SECM)

36 - For elucidating interfacial reactions based on simultaneous measurement of electrochemical parameters and mass changes at the electrode surface - Uses a quartz crystal wafer sandwiched between two electrodes which induces electric field - The electric field produces a mechanical oscillation in the bulk of the wafer ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM)

37 - The frequency change (∆f) relates to the mass change (∆m) according to the Sauerbrey equation ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM) n = overtone number f o = base resonant frequency of the crystal (prior to mass change) A = area (cm 2 ) μ = shear modulus of quartz (2.95 x gcm -1 s -1 ) ρ = density of quartz (2.65 g/cm 3 )

38 - Decrease in mass corresponds to increase in frequency Useful for probing - processes that occur uniformly across the surface - deposition or dissolution of surface layers - ion-exchange reactions at polymer films - study of polymeric films - Cannot be used for molecular level characterization of surfaces ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM)

39 - For probing the features of chemically-modified electrodes - For understanding electrochemical reactions - For electron transfer kinetics and diffusional characteristic studies Impedance - Complex resistance encountered when a current flows through a circuit made of combinations of resistors, capacitors, or inductors IMPEDANCE SPECTROSCOPY

40 - Plots of faradaic impedance spectrum is known as Nyquist plot Consists of - a semicircle portion at high frequencies (corresponds to the electron-transfer-limited process) and - a straight line portion at low frequencies (coreesponds to the diffusion-limited process) IMPEDANCE SPECTROSCOPY

41 - The impedance spectrum has only the linear portion for very fast electron transfer processes - Very slow electron transfer processes are characterized by a large semicircle region - Diameter of the semicircle equals the electron transfer resistance IMPEDANCE SPECTROSCOPY


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