Electrochemical kinetics 87.11 Polarization of electrode
§7.11 Polarization of electrode Electrochemical kinetics
87.11 Polarization of electrode What are we going to learn? Ideal vs real electrochemical processes Concepts: overvoltage, polarization, overpotential; concentration/diffusion polarization eectrochemical polarization Method: overpotential measurement; Rules: (1) polarization direction (2)Tatel equation Model: exchange equilibrium Theoretical treatment: Master equation, Butler-Volmer equation Applications: electrolysis, battery, corrosion protection
What are we going to learn? Ideal vs. real electrochemical processes. Concepts: overvoltage, polarization, overpotential; concentration/diffusion polarization, electrochemical polarization Method: overpotential measurement; Rules: (1) polarization direction; (2) Tafel equation; Model: exchange equilibrium Theoretical treatment: Master equation, Butler-Volmer equation Applications: electrolysis, battery, corrosion protection §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.0 Introduction Surface region Electrode process em. IXn ransier Fast equilibrium and rate-determining step Desorption/ adsorption Diffusion--concentration polarization EC rxn I Bulk solution Electrochemical reaction -electrochemical R polarization Desorption/adsorption R RS Chem. rxn Mass R transfer
7.11.0 Introduction Electrode process: Fast equilibrium and rate-determining step Diffusion—concentration polarization; Electrochemical reaction – electrochemical polarization §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.0 Introduction Electrochemical kinetic parameters Reaction rate: current density nFr/mol s nFkc A Activation energy: overpotential k T 4G+anFAo anFd k eXI p( h )=ko exp RT RT
7.11.0 Introduction Electrochemical kinetic parameters: 1 / mol s n i nFr nFkc j A A A − = = = , 0 0 exp( ) exp( ) k TB G nF c nF k k h RT RT + = − = − Reaction rate: current density Activation energy: overpotential §7.11 Polarization of electrode
87.11 Polarization of electrode 7.l1.1. Decomposition voltage and overvoltage 1.229 0.401 0.000 H,O 170V 1.229v -0.828 1.0 2.0E 02468101214pH Electrolysis of water Reversible decomposition Effective decomposition voltage voltage
7.11.1. Decomposition voltage and overvoltage Electrolysis of water Reversible decomposition voltage Effective decomposition voltage §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.1. Decomposition voltage and overvoltage Decomposition voltage the minimum potential difference which 170v must be applied between electrodes before 1.229V decomposition occurs and a current flows Onset potential 0.0---1.0 2.0E/V The reversible electromotive force of the cell (Theoretical decomposition voltage)is 1. 229 V. The effective decomposition voltage is 1.70 V. A discrepancy of ca. 0.5V, which is named as overvoltage exist
The reversible electromotive force of the cell (Theoretical decomposition voltage) is 1.229 V. The effective decomposition voltage is 1.70 V. A discrepancy of ca. 0.5 V, which is named as overvoltage, exist. Decomposition voltage: the minimum potential difference which must be applied between electrodes before decomposition occurs and a current flows. 1.70 V 1.229 V 0.0 1.0 2.0 E / V I / A Onset potential 7.11.1. Decomposition voltage and overvoltage §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11. 2 Thermodynamics of irreversible cell For reversible cell: Wre= nFEre For irreversible cell: w?=nFE For electrolytic cell For galvanic cell E0 Ere >E; Ae= ere-E>0 △E=(q1i9i)-(9,r-.r) △E=(9,rq,r)(q,r-qmr (Pa ir -P. re)+(. re -. ir (p. re-p ir)+(pa. ir-pare) (Pa, ir"Pa, re)=na (Pe, re-Pe, ir )=ne (o re - ir)=n (a ir-a re)=n △E=7+T △E=m+ma cre air are +7a cIr cre aIr
7.11. 2 Thermodynamics of irreversible cell For reversible cell: Wre = nFEre For irreversible cell: Wir = nFEir For electrolytic cell: Ere 0 E = (a, ir-c, ir) - (a, re - c, re) = (a, ir - a, re) + (c, re - c, ir) (a, ir − a, re ) = a E = c + a (c, re − c, ir ) = c c,ir c,re c = − a,ir a,re a = + For galvanic cell: Ere > Eir; E = Ere − Eir > 0 E = (c, re− a, re)−( c, ir − a, ir) = (c, re−c, ir) + (a, ir−a,re) E = c + a (c, re − c, ir ) = c (a, ir − a, re ) = a c,ir c,re c = − a,ir a,re a = + §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.2 Thermodynamics of irreversible cell Galvanic cell Electrolytic cell c. re re a,ir a, re a,Ir a, re Under irreversible conditions, electrode potential differs from its reversible value this phenomenon is defined as polarization The discrepancy between reversible potential and irreversible potential is termed as overpotential(n). By definition, overpotential always has positive value
Galvanic cell Electrolytic cell c, ir = c, re − c a, ir = a, re + a c, ir = c, re − c a, ir = a, re + a Under irreversible conditions, electrode potential differs from its reversible value, this phenomenon is defined as polarization. The discrepancy between reversible potential and irreversible potential is termed as overpotential (). By definition, overpotential always has positive value. 7.11.2 Thermodynamics of irreversible cell §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.2 Thermodynamics of irreversible cell Galvanic cell Electrolytic cell q,i=卯,re-ne re Pa ir=Pa re t na a. re 几m T ne a E a Pc ir ir Pa Pa C.re g,re驴 The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed. Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell
The irreversible potential and the irreversible electromotive force of cell depend on the current density imposed. Polarization cause decrease in electromotive force of galvanic cell and increase in decomposition voltage of electrolytic cell. Galvanic cell Electrolytic cell c, ir = c, re − c a, ir = a, re + a c, ir = c, re − c a, ir = a, re + a 7.11.2 Thermodynamics of irreversible cell §7.11 Polarization of electrode
87.11 Polarization of electrode 7.11.3 Origin of overpotential I)Resistance overpotential (n 2)Concentration overpotential(nd n- nR t np t na 3)Activation overpotential (n) 1)Resistance overpotential (nR) Electrode, electrode/solution interface, solution and separator all have resistance TRIR Elimination: How can we lower the inner resistance of a cell?
7.11.3 Origin of overpotential 1) Resistance overpotential ( R ) 2) Concentration overpotential ( C ) 3) Activation overpotential (a ) 1) Resistance overpotential ( R ) Electrode, electrode/solution interface, solution and separator all have resistance. Elimination: How can we lower the inner resistance of a cell? R = I R = R + D + A §7.11 Polarization of electrode
