Electro-Optical Materials Laboratory Department of Chemical Engineering

1. Chapter 1 Introduction and Overview of Electrode Process Speaker: Ta-Jen Li (李達人) Electro-Optical Materials Laboratory Department of Chemical Engineering National Taiwan University 2011 / 07 / 11

2. Outline • 1.1 Introduction to electrochemical cells and reactions • 1.2 Nonfaradaic processes and the nature of the electrode- • solution interface • 1.3 Faradaic processes and factors affecting rates of electrode • reactions • 1.4 Introduction to mass-transfer controlled reactions

3. Introduction • Electrochemistry is the branch of chemistry concerned with the interrelation of • electrical and chemical effects. • The field of electrochemistry encompasses a huge array of different phenomena • (e.g., electrophoresis and corrosion), devices (electrochromic displays, electro • analytical sensors, batteries, and fuel cells), and technologies (the electroplating • of metals and the large-scale production of aluminum and chlorine). • The main emphasis in this text book is on the application of electrochemical • methods to the study of chemical systems. • In chapter 1, the terms and concepts employed in describing electrode reactions • are introduced. In addition, the approximate treatments of several different types • of electrode reactions to illustrate their main features.

4. Basic conceptions of electrochemical cells • Typical electrode materials include solid metals (Pt, Au), liquid metals (Hg), • carbon (graphite), and semiconductors (indium-tin oxide ITO). • In the electrolyte phase, charge is carried by the movement of ions. • The most frequently used electrolytes are liquid solutions containing ionic • species, such as, H+, Na+, Cl-, in either water or a non-aqueous solvent. • It is natural to think about events at a single interface, but we will find that one • can’t deal experimentally with such an isolated boundary. Instead, one must • study the properties of collections of interfaces called electrochemical cells. • (Single interfacial potential difference can not be measured) • A difference in electric potential can be measured between the electrodes in an • electrochemical cell. This is done with a high impedance voltmeter typically.

5. Typical electrochemical cells and their notations Ag Pt H2 Ag Zn

6. Reference electrodes • Standard Hydrogen Electrode (SHE), or Normal Hydrogen • Electrode (NHE) E0 = 0 V ASSUMED • Saturated Calomel Electrode (SCE) E0 = 0.241 V • Silver-silver Chloride Electrode E0 = 0.197 V

7. Oxidation and reduction processes Solution Electrode Electrode Solution e Vacant MO - Potential Energy level of electrons Occupied MO + Representation of reduction process of a species in a solution Electrode Solution Solution Electrode - Vacant MO Energy level of electrons Potential e + Occupied MO Representation of oxidation process of a species in a solution

8. Potential difference across electrode/solution interphase Let us assume Cu → Cu2+ + 2e- occurs first (vf)e = (vb)e Equilibrium state At equilibrium across electrode/solution interphase Galvani potential difference across M/S

9. Measurement and calculation of the open-circuit potential • A high impedance voltmeter (i.e., a voltmeter whose internal resistance is so • high that no appreciable current flows through it during a measurement) is • placed across the cell. This is called the open-circuit potential of the cell. • It is possible to calculate the open-circuit potential from thermodynamic data, • that is, from the standard potentials of the half-reactions involved at both • electrodes via the Nernst equation (chapter 2). • The key point is that a true equilibrium is established, because a pair of redox • forms linked by a given half-reaction (i.e., a redox couple) is present at each • electrode. (Cu/Cu2+; Zn/Zn2+; H2/H+) • We can’t calculate a thermodynamic potential for the Pt/H+,Br- electrode, • because we can’t identify a redox couple. (no H2 is introduced; deaerated)

10. Obtaining a current-potential curve (1) Cathodic Power supply Pt/H+, Br- (1 M)/AgBr/Ag Current Onset of H+ reduction on Pt i V 0 -0.5 1.5 0.5 1.5 Onset of Br- oxidation on Pt Ag Pt Anodic AgBr Cell potential 本書的習慣:(1) 陰極電流為正、(2) 負的電位在右邊

11. Obtaining a current-potential curve (2) • Potential of the Pt electrode is made more negative with respect to the Ag/AgBr • reference electrode. • Pt electrode: proton reduction; a cathodic current flows (working) • Ag/AgBr electrode: the oxidation of Ag in the presence of Br- form AgBr (reference) • Potential of the Pt electrode is made sufficiently positive with respect to the Ag/ • AgBr reference electrode. • Pt electrode: Br- oxidation; anodic current flows (working) • Ag/AgBr electrode: the reduction of AgBr to form Ag and Br- (reference) • The background limits are the potentials where the cathodic and anodic currents • start to flow at a working electrode. • The open-circuit potential is not well defined in the system under discussion. • The open-circuit potential lies somewhere between the background limits.

12. Potentials for possible reductions at a platinum and gold electrodes least positive (or most negative E0) be oxidized first, reductant least negative (or most positive E0) be reduced first, oxidant

13. Faradaic and Nonfaradaic process • Faradaic process • Charge transfer across the metal-solution interface • Oxidation or reduction occurs • Governed by Faraday’s law • Nonfaradaic process • No charge transfer across the metal-solution interface • Adsorption and desorption

14. Self readings and reviews of 1-1 • Self readings • Discussion on the i-E curves of Hg/H+, Br- (1 M/AgBr/Ag) (pages 7-9) • Reviews • Electrode materials, electrolytes, notation of electrochemical cells • Standard electrode potentials (reduction scale), reference electrodes • tendency of oxidation and reduction • Formation of potential, measurement of open-circuit potential

15. Ideal polarized electrode (IPE) • An electrode at which no charge transfer can occur across the metal-solution • interface, regardless of the potential imposed by an outside source of voltage, • is called an ideal polarizable electrode (IPE). • While no real electrode can behave as an IPE over the whole potential range • available in a solution, some electrode-solution systems can approach ideal • polarizability over limited potential ranges. • Mercury electrode in contact with a deaerated potassium chloride solution approaches • the behavior of an IPE over a potential range about 2 V wide.

16. Brief description of the electrical double layer • Inner Helmholtz plane (IHP) • locus of the electrical centers of the specifically • adsorbed ions • Outer Helmholtz plane (OHP) • locus of centers of the nonspecifically adsorbed • solvated ions • Diffuse layer • extends from the OHP into the bulk solution

17. A two-electrode cell and its approximated linear circuit Hg/K+, Cl-/SCE Representation of the cell in terms of linear circuit elements series capacitance of Cd and CSCE

18. Potential step method for obtaining the capacitance Rs Cd E i E/Rs E Applied (E) i E τ =time constant 0.37E/Rs t t τ=RsCd

19. Current step method for obtaining the capacitance Rs Cd Constant current source Applied (i) E Slope= i/Cd i iRs t t

20. Self readings and reviews of 1-2 • Self readings • Current step method for obtaining the capacitance (pages 16-18) • Reviews • Nonfaradaic process • Ideal polarizable electrode (large horizontal region, Hg electrode) • Structure of electrochemical double layer (EDL) • Methods for obtaining the capacitance

21. Galvanic and electrolytic cells • Galvanic cell • reaction occurs spontaneously • converting chemical energy to electrical • energy • primary cells, secondary cells, fuel cells • Electrolytic cell • imposition of an external voltage greater than • the open-circuit potential of the cell • converting electrical energy to chemical energy • electrolytic synthesis, electroplating

22. Applying different voltage to a Galvanic cell and the corresponding chemical reactions 電池 電解池 • When the voltage applied by the external power supply, Eappl, is 0.64 V, i= 0. • When Eappl is made larger (i.e., Eappl>0.64 V, such that the cadmium electrode • is made even more negative with respect to the SCE, the cell behaves as an • electrolytic cell.

23. Relationship between the current and the reaction rate • An electrode process is a heterogeneous reaction occurring only at the electrode- • electrolyte interface. • Since electrode reactions are heterogeneous, their reaction rates are described in • units of mol/s per unit area. (j is the current density)

24. Ideal polarizable electrode and ideal nonpolarizable electrode i i E E • Ideal nonpolarizable electrode is an • electrode whose potential does not • change upon passage of current. • Nonpolarizability is characterized by • a vertical region on an i-E curve. • An ideal polarized electrode shows a very large • change in potential upon the passage of a small • current. • Ideal polarizability is characterized by a • horizontal region of an i-E curve

25. Pathway of a general electrode reaction • When a steady-state current is obtained, the rates of all reaction steps in a series are the same. • The magnitude of this current is often limited by the inherent sluggishness of one or more • reactions called rate determining steps.

26. Processes in an electrode reaction represented as resistances • Each value of current density, j , is driven by a certain overpotential, which can • be viewed as a sum of terms associated with the different reaction steps: ηmt, ηct, • ηrxn, etc. • The electrode reaction can then be represented by a resistance, R, composed of a • series of resistances representing the various steps: Rm, Rct, etc.

27. Applying different voltage to a Galvanic cell and the corresponding chemical reactions 電池 電解池 • When the voltage applied by the external power supply, Eappl, is 0.64 V, i= 0. • When Eappl is made larger (i.e., Eappl>0.64 V, such that the cadmium electrode • is made even more negative with respect to the SCE, the cell behaves as an • electrolytic cell.

28. Distribution of the applied voltage • The extra applied voltage -0.80 V is distributed in two parts. • The potential of the Cd electrode, Ecd, must shift to a new value, e.g. -0.70 V vs. SCE. • The remainder of the applied voltage, -0.10 V, represents the ohmic drop required to drive • the ionic flow in the solution. • SCE is nonpolarizable at the extant current level and does not change its potential. • The ohmic potential drop in the solution should not be regarded as a form of • overpotential, because it is characteristic of the bulk solution and not of the • electrode reaction.

29. Two-electrode and three-electrode cells Power supply Power supply i i Working electrode Counter electrode Reference electrode Working electrode Eappl Reference electrode V V • Current is passed between the WE and CE. • Potential of the WE is monitored relative to • a separate RE. • A negligible current is drawn through the • RE due to the high impedance of voltmeter. • Used in most electrochemical experiments. • If the passage of current does not affect the • potential of the RE, the E of WE is given by • the equation shown in last page. • Under conditions when iRs is small (say less • than 1-2 mV), this two-electrode cell can be • used to determine the i-E curve.

30. Self readings and reviews of 1-3 • Self readings • Electrochemical experiments and variables affecting the rate of electrode • reaction (pages 19-21) • Reviews • Faradaic process • Two-types of electrochemical cells • Relationship between current and reaction rate (unit mol s-1 cm-2) • i-E curves of typical ideal polarizable and non-polarizable electrodes • polarization and overpotential • Two-electrode and three-electrode systems

31. Mass transfer controlled reactions • The simplest electrode reactions are those in which the rates of all associated chemical • reactions are very rapid compared to those of the mass-transfer processes. Under these • conditions, the chemical reactions can usually be treated in a particularly simple way. • (a) the homogeneous reactions can be regarded as being at equilibrium. • (b) the surface concentrations of species involved in the Faradaic process are related to • the electrode potential by an equation of the Nernst form.

32. Mass transfer to the electrode: The Nernst-Planck equation • Term 1: Diffusion • Movement of a species under the influence of a gradient of chemical potential (i.e., • a concentration gradient). • Term 2: Migration • Movement of a charged body under the influence of an electric field (a gradient of electrical potential). • Term3: Convection • Nature convection: due to density gradient • Forced convection: due to pressure gradient (stirring, electrode rotating (RDE))

33. Illustration of the three types of mass transfer

34. Semiempirical treatment of steady-state mass transfer (1) • Stirring is ineffective at the electrode surface (neglect convection term). • a stagnant layer of thickness δo exists at the electrode surface (Nernst diffusion • layer), with stirring maintaining the concentration of О at Co* beyond x = δo. • An excess of supporting electrolyte (neglect migration term). • The rate of mass transfer is proportional to the concentration gradient at the • electrode surface

35. Co* 1 Co 2 Co(x=0) 0 x δo • Concentration profiles (solid lines) and diffusion layer approximation (dashed lines). • x = 0 corresponds to the electrode surface and δo is the diffusion layer thickness.

36. Semiempirical treatment of steady-state mass transfer (2) • The largest rate of mass transfer of О occurs when Co(x=0) = 0. The value of • the current under these conditions is called the limiting current.

37. Case (a) R is initially absent Rapid electron transfer kinetics i=0.5il

38. Case (b) O and R initially present Eeq

39. Semiempirical treatment of transient response (1) The diffusion layer thickness is now a time-dependent quantity The current flow causes a depletion of O Moles of O electrocatalyzed in diffusion layer

40. (a) (b) (a) Growth of the diffusion-layer thickness with time (b) Current-time transient for a potential step to a stationary electrode (no convection) and to an electrode in stirred solution (with convection) where a steady-state current is attained.

41. Comments on the results obtained by using the Semiempirical treatment • This approximate treatment predicts a diffusion layer that grows with tl/2 and a • current that decays with t-l/2. • Without convection, the current continues to decay, but in a convective system, • it ultimately approaches the steady-state value characterized by δ(t)= δo. • Even this simplified approach approximates reality quite closely; equation 1.4.34 • differs only by a factor of 2/ Π1/2 from the rigorous description of current arising • from a nernstian system during a potential step (section 5.2.1).

42. Self readings and reviews of 1-4 • Self readings • i-E curve for a nernstian system where the reduced form is insoluble • (pages 19-21) • Reviews • Three types of mass transfer • Semiempirical treatment of steady-state mass transfer (3 cases) • Semiempirical treatment of transient response (vs. the results obtained • by using the methods described in chapter 5)

43. Homework • Brush upon the contents of chapter 1 • Problem 1.9

44. References • A. J. Bard and L. R. Faulkner, Electrochemical methods: Fundamentals and Applications, 2nd ed., John Wiley & Sons, Inc., New York (2001). • J. Wang, Analytical Electrochemistry, 3rd ed., John Wiley & Sons, Inc., New York (2006). • K. C. Pillai, C. C. Liu, Technologies of Chemical Sensors, NTUST, Taipei, Taiwan February 14-15 (2011).