ELECTROCHEMISTRY CHEM 4700 CHAPTER 4

1. ELECTROCHEMISTRYCHEM 4700CHAPTER 4 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 4 PRACTICAL CONSIDERATIONS

3. BASIC REQUIREMENTS OF CONTROLLED POTENTIAL EXPERIMENTS - Electrochemical cell with a three-electrode system Working Electrode (WE) Reference Electrode (RE) Counter/Auxiliary Electrode (CE/AE) - Potentiostat (Voltammetric Analyzer) - Plotter - Other components may be required depending on the type of experiment

4. ELECTROCHEMICAL CELL - Covered glass container of 5 – 50 mL volume - Contains three electrodes (WE, RE, CE) immersed in the sample solution - Electrodes are inserted through holes in the cell cover - N2 gas used as deoxygenated gas

5. ELECTROCHEMICAL CELL Working Electrode (WE) - Electrode at which the reaction of interest occurs (Pt, Au, Ag, C) Reference Electrode (RE) - Provides a stable and reproducible potential - Independent of the sample composition (Ag/AgCl, SCE) Counter/Auxiliary Electrode (CE/AE) - Current-carrying electrode made of inert conducting metal (Pt wire, Graphite rod)

6. ELECTROCHEMICAL CELL RE CE WE Opening N2 Teflon cap Glass container

7. SOLVENTS - Medium for electrochemical measurements - Contains a supporting electrolyte - Choice of solvent depends on the solubility and the redox activity of the analyte Solvent Properties - Electrical conductivity - Electrochemical activity - Chemical reactivity

8. SOLVENTS Additional Properties Considered - Should not react with analyte or product - Should not undergo electrochemical reactions over a wide range of potential Examples - Water (the most common) - Acetonitrile - Dimethylsulfoxide (DMSO) - Methanol - Dimethylformamide (DMF)

9. SUPPORTING ELECTROLYTE - Inert - Decrease the resistance of the solution - Eliminate electromigration effects - Maintain a constant ionic strength - Concentration range in usually 0.1 M – 1.0 M - Should be in large excess of analyte concentration

10. SUPPORTING ELECTROLYTE Examples For Aqueous Media - Inorganic salts (NaCl, KCl, KNO3) - Mineral acids (HCl, H2SO4) Examples For Organic Media - Tertaalkylammonium salts Buffer Systems - Used when pH control is essential - Phosphate, citrate, acetate

11. OXYGEN REMOVAL - Purging with an inert gas for about 10 minutes - Nitrogen gas is usually used - Purging is done just before voltammetric measurements Other Methods - Formation of peroxides followed by reduction of peroxides - Reduction by addition of sodium sulfite or ascorbic acid - Use of electrochemical or chemical scrubbers (zinc)

12. INSTRUMENTATION - Consists of two circuits - A polarizing circuit that applies the potential to the cell - A measuring circuit that monitors the cell current The Applied Potential (Eapp) Eapp = EWE – ERE – iR iR = ohmic potential drop

13. INSTRUMENTATION - RE is placed as close as possible to WE to minimize potential drop caused by the cell resistance (iR) - Flow cannot occur through RE hence the need for CE to complete the current path - Current flows through solution between WE and CE - Voltage is measured between WE and RE

14. REFERENCE ELECTRODES - Provides known and constant potential Examples - Saturated Calomel electrode (SCE) - Silver-silver chloride electrode (Ag/AgCl) - Mercury/Mercurous sulfate reference electrode - Alkaline/Mercurous oxide reference electrode

15. REFERENCE ELECTRODES Saturated Calomel electrode (SCE) - Saturated with KCl - Different KCl concentrations can be used (0.1 M KCl is least temperature sensitive but saturated KCl solution is easier to make and maintain) 1/2Hg2Cl2(s) + e- ↔ Hg(l) + Cl- E = + 0.241 V - The reference is not 0.000 V (SHE) but 0.241 V (SCE) - Stored in KCl solution when not in use

16. REFERENCE ELECTRODES Silver-Silver Chloride Electrode (Ag/AgCl) - Saturated with KCl AgCl(s) + e-↔ Ag(s) + Cl- E = + 0.197 V

17. WORKING ELECTRODES - Should possess high signal-to-noise ratio characteristic - Should be reproducible Selection Depends on Two Main Factors - The redox behavior of the target analyte - Background current over the potential region required Other Factors Include Potential window, electrical conductivity, surface reproducibility, mechanical properties, cost, availability, toxicity

18. WORKING ELECTRODES Chemically Inert Electrodes - Do not participate in the reaction Examples Carbon Gold Platinum ITO

19. WORKING ELECTRODES Reactive Electrodes - Participate in the reaction Examples Silver Copper Iron Zinc Mercury

20. MERCURY ELECTRODES - Extended cathodic potential window (due to its high hydrogen overvoltage) - Highly reproducible (minimized effect of impurities) - Readily renewable - Smooth surface Disadvantages - Limited anodic potential range (due to the oxidation of Hg) - Toxicity

21. MERCURY ELECTRODES Types of Mercury Electrodes Dropping Mercury Electrode (DME) - Used in polarography and electrocapillary studies Hanging Mercury Drop Electrode (HMDE) - For stripping analysis and cyclic voltammetry Mercury Film Electrode (MFE) - For stripping analysis and flow amperometry - Thin layer of Hg covering a conducting or inert support - Support is usually glassy carbon or iridium Solid Amalgam Electrode

22. SOLID ELECTRODES - Have extended anodic potential windows - Better for monitoring oxidizable compounds than Hg electrodes - Requires polishing to obtain reproducible results - May be stationary or rotating (planar disk) - Consists of a short cylindrical rod of the material tightly embedded in an insulating material

23. SOLID ELECTRODES Insulating Material - Teflon - Kel-F [polychlorotrifluoroethylene (PCTFE)] - Sealing between rod and insulating material is essential to avoid solution creeping and subsequent background response Disk solid electrodes are employed in flow analysis Examples Carbon, Platinum, Silver, Gold, Nickel, Copper

24. ROTATING DISK ELECTRODES (RDE) - Vertically mounted in a shaft of controllable speed - Rotated with a constant angular velocity (ω) about an axis perpendicular to the plain of disk surface - Thickness of diffusion layer is independent of diameter of disk - Provides efficient and reproducible mass transport - High sensitivity and precision

25. ROTATING DISK ELECTRODES (RDE) Teflon insulator Disk

26. ROTATING RING DISK ELECTRODES (RRDE) - Addition of a concentric ring separated by a small insulating gap - For elucidating various electrode mechanisms - For detection of short lived intermediate species - Species generated at the disk are detected at the ring

27. ROTATING RING DISK ELECTRODES (RRDE) Ring : Disk Current Ratio (N) N = - iR/iD - Fraction of species generated at the disk that are detected at the ring - Currents are in opposite directions hence the negative sign - Collection current (iR) is proportional to generation current (iD)

28. ROTATING RING DISK ELECTRODES (RRDE) Teflon insulator Ring Disk

29. CARBON ELECTRODES - Solid electrodes based on carbon - Broad potential window - Low background current - Rich surface chemistry - Low cost - Chemically inert - Suitable for sensing and detection applications However - Have slower electron transfer rates than metal electrodes

30. CARBON ELECTRODES Examples - Glassy carbon - Carbon paste - Carbon fiber - Carbon film - Screen-printed carbon strips - Graphite epoxy - Wax imprinted graphite - Kelgraf

31. CARBON ELECTRODES Glassy Carbon Electrodes - Vitreous (shiny and nonporous) - Good mechanical and electrical properties - Wide potential window - Solvent resistance (chemically inert) - Reproducible - Surface pretreatment (polishing) is essential A Special Type Reticulated Vitreous Carbon (RVC, sponge-like or network) for flow analysis and spectroelectrochemistry

32. CARBON ELECTRODES Carbon Paste Electrode - Graphite powder mixed with various water-immiscible nonconducting organic binders - Surface is easily renewable and modified - Low cost - Very low background current contributions - Exact behavior is not fully understood

33. CARBON ELECTRODES Carbon Paste Electrode Examples of Pasting Liquids - Mineral oil (Nujol) - Paraffin oil - Silicone grease - Bromonaphthalene - Decrease in pasting liquid increases electron transfer rates - Decrease in pasting liquid increases background current contributions

34. CARBON ELECTRODES Carbon Fiber Electrode - Made up of fibers of about 5 – 20 μm diameter - Fibers are mounted at the tip of a glass capillary with epoxy adhesive - Contamination of the carbon surface with epoxy should be avoided - Attractive for anodic measurements

35. CARBON ELECTRODES Carbon Fiber Electrode - For microenvironments and detection of neurotransmitter release Three Categories Low-modulus, Medium-modulus, and High-modulus High-modulus - Well-ordered graphite-like - Low porosity - Most suitable for electrochemical studies

36. CARBON ELECTRODES Diamond Electrodes - Boron doped diamond (BDD) film electrodes provide very low resistivity (< 0.01 Ω∙cm) - Wide potential window (~ 3 V) - Low and stable background currents - Negligible adsorption of organic compounds - Good reactivity requiring little or no pretreatment

37. CARBON ELECTRODES Diamond Electrodes - Low sensitivity to dissolved oxygen (no surface oxide formation) - Reproducible results - Extreme hardness - Small double-layer capacitance

38. CARBON ELECTRODES Diamond Electrodes - Provides good results under extreme conditions such as: very high anodic potential surfactant-rich media polarization in acidic media power ultrasound

39. METAL ELECTRODES - Platinum and gold are the most widely used - Copper, nickel, silver are other examples - Offer favorable electron transfer kinetics - Large anodic potential range - Low cathodic potential window (-0.2 to -0.5 V, depends on pH) - High background currents associated with the formation of surface oxide or adsorbed hydrogen layers

40. METAL ELECTRODES - The problem of surface oxide formation is less severe in nonaqueous media Comparison Between Pt and Au Electrodes - Gold electrodes are more inert - Pt is more prone to surface oxide film formation - Gold is preferred for stripping measurements of trace metals - Gold is preferred as substrate for self assembled monolayers (SAM)

41. METAL ELECTRODES Alloy Electrodes - Used for addressing adsorption or corrosion effects of one their components - Used for fuel cell applications - Corrosion and heat resistant Examples Platinum-Tin, Nickel-Ruthenium, Platinum-Ruthenium Ti-Zr-V-Cr-Ni, Tin-Lithium, Ruthenium-Cobalt

42. CHEMICALLY MODIFIED ECTRODES (CME) - Produced by placing a reagent on electrode surface to alter the surface - Basis of new analytical applications and different sensing devices - Accelerates electron transfer reactions - Enhanced selectivity and sensitivity - Differential accumulation - Stability of devices

43. CHEMICALLY MODIFIED ECTRODES (CME) - Protection from corrosion - Controlled and manipulated reactivity at interface - For fuel cells - Most common is polymer modified electrodes Examples - Nafion cation exchanger - Polyvinylferrocene - Polypyrrole - Clay

44. CHEMICALLY MODIFIED ECTRODES (CME) Self Assembled Monolayers (SAM) - Spontaneously adsorbed monolayers on electrode surface - SAM film is formed by immersing electrode in a solution containing the species of interest (usually overnight) - One end of species has special affinity for the electrode surface - SAM film is well organized and stable Applications Biosensors, electron transfer rate determination (e.g. of proteins)

45. CHEMICALLY MODIFIED ECTRODES (CME) Self Assembled Monolayers (SAM) Examples - Alkanethiols on gold surfaces - Alkyl siloxane (R2SiO) on metal oxide surfaces (SiO2) - Chlorosilane Factors Influencing Packing and Order Chain length, End group, Solvent, Immersion time, substrate morphology, coassembled monolayers (mixtures)

46. CHEMICALLY MODIFIED ECTRODES (CME) Carbon-Nanotube-Modified Electrodes (CNT) - Two types Single-Wall Carbon-Nanotubes (SWCNT) - Cylindrical nanostructure formed by rolling up a single graphite sheet into a tube Multi-Wall Carbon-Nanotubes (MWCNT) - Multiple rolled layers (concentric tubes) of SWCNT - Enhanced electrochemical activity - For amperometric biosensors

47. CHEMICALLY MODIFIED ECTRODES (CME) Sol-gel Encapsulation of Reactive Species - Encapsulation of species within sol-gel films - Formed by hydrolysis of alkoxide precursor [Si(OCH3)4] - Followed by condensation - A porous glass-like material forms - Rigid and stable porous network

48. CHEMICALLY MODIFIED ECTRODES (CME) Sol-gel Encapsulation of Reactive Species - Other composites have been formed by dispersing carbon or gold powders into sol-gel mixtures Encapsulation The condition of being enclosed (as in a capsule)

49. CHEMICALLY MODIFIED ECTRODES (CME) Electrochemically Modified Electrodes - Modification by attachment of electron transfer mediators on the electrode surface - Catalyzes slow electron transfer kinetics - The mediator facilitates the charge transfer between an analyte and the electrode - Current density is increased and overvoltage is lowered - Improved sensitivity and selectivity

50. CHEMICALLY MODIFIED ECTRODES (CME) Electrochemically Modified Electrodes - The electron transfer takes place between the electrode and the mediator (M) but not directly between the electrode and the analyte (A) Mox + ne- → Mred Mred + Aox → Mox + Ared - The active form of the mediator is electrochemically regenerated - The process is electron shuttling