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

2. Outline 1  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 2  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 3  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 4 Zn Ag PtH2H2 Ag

6. 5  Standard Hydrogen Electrode (SHE), or Normal Hydrogen Electrode (NHE) Reference electrodes  Saturated Calomel Electrode (SCE)  Silver-silver Chloride Electrode E 0 = 0 V ASSUMED E 0 = 0.241 V E 0 = 0.197 V

7. Oxidation and reduction processes 6 - - + + Potential Energy level of electrons Electrode Solution Vacant MO Occupied MO Electrode Solution e Electrode Solution Potential Energy level of electrons Vacant MO Occupied MO Electrode Solution e Representation of reduction process of a species in a solution Representation of oxidation process of a species in a solution

8. Potential difference across electrode/solution interphase 7 (v f ) e = (v b ) e Equilibrium state Let us assume Cu → Cu 2+ + 2e- occurs first At equilibrium across electrode/solution interphase Galvani potential difference across M/S

9. Measurement and calculation of the open-circuit potential 8  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/Cu 2+ ; Zn/Zn 2+ ; H 2 /H + )  We can’t calculate a thermodynamic potential for the Pt/H +,Br - electrode, because we can’t identify a redox couple. (no H 2 is introduced; deaerated)

10. Obtaining a current-potential curve (1) 9 1.5 Power supply i V Pt Ag AgBr Current 0.5 0 -0.5 Cathodic Anodic Onset of Br - oxidation on Pt Onset of H + reduction on Pt Pt/H +, Br - (1 M)/AgBr/Ag Cell potential 1.5 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) 10  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 11 least negative (or most positive E 0 ) be reduced first, oxidant least positive (or most negative E 0 ) be oxidized first, reductant

13. Faradaic and Nonfaradaic process 12  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 13  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) 14  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 15  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 16 series capacitance of C d and C SCE Representation of the cell in terms of linear circuit elements Hg/K +, Cl - /SCE

18. Potential step method for obtaining the capacitance 17 i E CdCd RsRs i E/R s 0.37E/R s t τ=R s C d E E t Applied (E) τ =time constant

19. Current step method for obtaining the capacitance 18 Constant current source CdCd RsRs i t Applied (i) E iR s t Slope= i/C d

20. Self readings and reviews of 1-2 19  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 20  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 21  When the voltage applied by the external power supply, E appl, is 0.64 V, i= 0.  When E appl is made larger (i.e., E appl >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. 電池 電解池

23. Relationship between the current and the reaction rate 22  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 23  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  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. i E i E

25. Pathway of a general electrode reaction 24  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 25  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: η mt, η ct, η rxn, etc.  The electrode reaction can then be represented by a resistance, R, composed of a series of resistances representing the various steps: R m, R ct, etc.

27. Applying different voltage to a Galvanic cell and the corresponding chemical reactions 26  When the voltage applied by the external power supply, E appl, is 0.64 V, i= 0.  When E appl is made larger (i.e., E appl >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. 電池 電解池

28. Distribution of the applied voltage 27  The extra applied voltage -0.80 V is distributed in two parts.  The potential of the Cd electrode, E cd, 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 28 Power supply i V Working electrode Reference electrode E appl Power supply Working electrode i V Counter electrode Reference electrode  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 iR s is small (say less than 1-2 mV), this two-electrode cell can be used to determine the i-E curve.  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.

30. Self readings and reviews of 1-3 29  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 30  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 31  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. 32 Illustration of the three types of mass transfer

34. Semiempirical treatment of steady-state mass transfer (1) 33  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. 34 Co*Co* CoCo C o (x=0) δoδo 0 x 1 2  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) 35  The largest rate of mass transfer of О occurs when C o (x=0) = 0. The value of the current under these conditions is called the limiting current.

37. Case (a) R is initially absent 36 i=0.5i l Rapid electron transfer kinetics

38. Case (b) O and R initially present 37 E eq

39. Semiempirical treatment of transient response (1) 38 Moles of O electrocatalyzed in diffusion layer The current flow causes a depletion of O The diffusion layer thickness is now a time-dependent quantity

40. 39 (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. (a) (b)

41. Comments on the results obtained by using the Semiempirical treatment 40  This approximate treatment predicts a diffusion layer that grows with t l/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)= δ o.  Even this simplified approach approximates reality quite closely; equation 1.4.34 differs only by a factor of 2/ Π 1/2 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 41  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. 42 Homework  Brush upon the contents of chapter 1  Problem 1.9

44. 43 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).