§7.12 Basic principal and application of electrolysis Chapter 7 Electrochemistry §7.12 Basic principal and application of electrolysis

Inert anode Oxidation of species in solution Anodic reaction Active dissolution Active anode Passivation and conversion Electrolysis reaction Anodization reduction of species in solution cathodic reaction reduction of oxide/conversion layer

1. Cathode reaction 1) Order of liberation Suppose a solution in an electrolytic cell containing Ag+, Cu2+, H+, and Pb2+ of 1 molarity. If the potential is initially very high and is gradually turned down, in which order will the metals be plated out onto the cathode? Ag+ + e  Ag: ⊖ Ag+/Ag = 0.799 V Cu2+ + 2e  Cu: ⊖ Cu2+/Cu = 0.337 V 2H+ + 2e  H2: ⊖ H+/H2 = 0.000 V Pb2+ + 2e Pb: ⊖ Pb2+/Pb = -0.126 V

Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb is 1.56 V For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential. 0.799 ⊖ Ag+/Ag 0.000 ⊖ H+/H2 For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration. 0.337 ⊖ Cu2+/Cu Ag+, Cu2+, H+, and Pb2+ will liberates at 0.799 V; 0.337 V; 0.000 V; -0.126 V, respectively without consideration of overpotential; Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb is 1.56 V -0.126 ⊖ Pb2+/Pb

Potential sweep: polarization curve a(Pb2+) = 3.310-49 -0.126 V -1.56 V a(Cu2+) = 2.210-16 0.337 V a(Ag+) = 1.510-8 0.799 V The liberation order and the residual concentration of the ions upon negative shift of potential of cathode

2) Application 1) Separation of metal 2) Quantitative and qualitative analysis 3) Electroplating of single metal and alloy 4) Electrolytic metallurgy 5) Electrorefining of metal 6) Electrosynthesis

F < Cl < Br < I 2. Anode reaction 1) Reaction on inert anode When inert material such as Platinum and graphite was used, the species in the solution discharge on the electrode in the order of liberation potential. F < Cl < Br < I Henri Moissan 1906 Noble Prize France 1852/09/28 ~ 1907/02/20 Investigation and isolation of the element fluorine

2) Reaction of active anode (1) Active dissolution; (2) Anodic passivation (3) Anodic oxidation Fe2+ Fe2O3 Fe pH  / V 2 4 6 8 10 12 14 Fe3O4 Fe3+ FeO22 We usually judge the reaction based on Porbaix diagram (1) Active dissolution: At pH=4 and low current density, active dissolution occurs. Fe  Fe2+ + 2e Pourbaix diagram of iron-water system

(2) Anodic passivation: Fe2+ Fe2O3 Fe pH  / V 2 4 6 8 10 12 14 Fe3O4 Fe3+ FeO22 (2) Anodic passivation: At pH= 12 and high potential, upon polarization, compact thin layer of Fe3O4 forms and passivation of iron takes place. 3Fe + 4H2O – 8e  Fe3O4 + 8 H+ Active dissolution passivation Trans-passivation Passivation curve of iron

Anodic oxidation of aluminum t / h E / V Barrier layer Porous layer Initiation of pores

SEM photograph of the AAM Cross-section top surface