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

INTERFACIAL PROPERTIES CHAPTER 2 ELECTRODE REACTIONS & INTERFACIAL PROPERTIES

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

CYCLIC VOLTAMMETRY - 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 Eo 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 - 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 - 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 (ipa) - Cathodic peak current (ipc) - Anodic peak potential (Epa) - Cathodic peak potential (Epc)

CYCLIC VOLTAMMETRY Reversible Systems - Peak current for a reversible couple is given by the Randles-Sevcik equation (at 25 oC) n = number of electrons A = electrode area (cm2) C = concentration (mol/cm3) D = diffusion coefficient (cm2/s) ν = potential scan rate (V/s)

CYCLIC VOLTAMMETRY Reversible Systems ip is proportional to C - Implies electrode reaction is controlled by mass transport ip/ic ≈ 1 for simple reversible couple - For a redox couple

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

CYCLIC VOLTAMMETRY Reversible Systems - The half peak potential - E1/2 is called the polarographic half-wave potential Multielectron Reversible Systems - The CV consists of several distinct peaks if the Eo values for the individual steps are well separated (reduction of fullerenes)

CYCLIC VOLTAMMETRY 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 Irreversible Systems α = transfer coefficient na = number of electrons involved in a charge transfer step ko = standard heterogeneous rate constant (cm/s) - ip is proportional to C but lower depending on the value of α For α = 0.5 ip,reversible/ip,irreversible = 1.27 - That is irreversible peak current is ~ 80% of reversible ip

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

CYCLIC VOLTAMMETRY 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 ipa/ipc < 1 - All of R can be converted to Z for very fast chemical reactions

CYCLIC VOLTAMMETRY 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 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 Applications E = redox step and C = chemical step CE - Slow chemical reaction precedes the electron transfer step Z → O + ne- ↔ R ipa/ipc > 1 (approaches 1 as scan rate decreases) ipa is affected by the chemical step ipc is not proportional to ν1/2

CYCLIC VOLTAMMETRY Applications E = redox step and C = chemical step ECE - Chemical step interposed between redox steps O1 + ne- ↔ R1 → O2 + ne- → R2 - The two redox couples are observed separately - The system behaves as EE mechanism for very fast chemical reactions - Electrochemical oxidation of aniline

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

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

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

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

SPECTROELECTROCHEMISTRY - 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 ipa = anodic peak current ipc = cathodic peak current Modulated Absorbance Am = -log(I/Io)

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

ELECTROCHEMILUMINESCENCE (ECL) - 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)32+ Nitro compounds Polycyclic hydrocarbons Luminol

SCANNING PROBE MICROSCOPY - 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 TUNNELING MICROSCOPY (STM) - 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

ATOMIC FORCE MICROSCOPY (AFM) - 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

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

ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM) - 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) - The frequency change (∆f) relates to the mass change (∆m) according to the Sauerbrey equation n = overtone number fo = base resonant frequency of the crystal (prior to mass change) A = area (cm2) μ = shear modulus of quartz (2.95 x 1011 gcm-1s-1) ρ = density of quartz (2.65 g/cm3)

ELECTROCHEMICAL QUARTZ CRYSTAL MICROBALANCE (EQCM) - 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

IMPEDANCE SPECTROSCOPY - 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 - 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 - 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