山东大学：《物理化学》课程教学资源（讲义资料）7.12-14 Applied Electrochemistry-for students

Chapter 7 Electrochemistry 87.12 Basic principal and application of electrolysis

Chapter 7 Electrochemistry §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis Inert Oxidation of species in anode solution A nodic reaction Active dissolution Active Passivation and conversion anode Electrolysis reaction Anodization reduction of species in solution cathodic reaction reduction of oxide/conversion layer

Anodic reaction cathodic reaction Active dissolution Passivation and conversion Anodization Inert anode Active anode Oxidation of species in solution reduction of species in solution reduction of oxide/conversion layer Electrolysis reaction §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 1. Cathode reaction: discharge potential For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential 0.799V For evolution of gas, the overpotential is relatively large, PeAg*/Ag therefore, the overpotential should be taken into consideration 0.337V pe Cu2+/Cu Ag, Cu, H, and Pbzt will liberates at 0. 799 V;0.337 V: 0.000 0.000 V;-0126 V, respectively without consideration of P H*/H overpotential -0.126 Overpotential of hydrogen liberation on Cu is 0.6 V, on Pb P Pb+/pb is 1.56V

For evolution of gas, the overpotential is relatively large, therefore, the overpotential should be taken into consideration. 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 0.337 V ⊖ Cu2+/Cu -0.126 ⊖ Pb2+/Pb 0.799 V ⊖ Ag+ /Ag 0.000 ⊖ H+ /H2 For liberation of metal, the overpotential is usually very low, and the reversible potential can be used in stead of irreversible potential. 1. Cathode reaction: discharge potential §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 1. Cathode reaction--residual concentration Potential sweep and residual concentration a(Pb2+)=33×10-49 a(Cu2)=22×1016 a(Ag+)=15×108 0.799V 0.337V 0.126V 1.56V The liberation order and the residual concentration of the ions upon negative shift of potential of cathode

a(Ag+ ) = 1.510-8 0.799 V a(Cu2+) = 2.210-16 0.337 V a(Pb2+) = 3.310-49 -0.126 V -1.56 V The liberation order and the residual concentration of the ions upon negative shift of potential of cathode Potential sweep and residual concentration 1. Cathode reaction—residual concentration §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 1. Cathode reaction 2)Application 1)Separation of metal 2)Quantitative and qualitative analysis(polarography) 3)Electroplating of single metal and alloy 4)Electrolytic metallurgy(Al, Ti, Mn) 5)Electrorefining of metal(Cu) 6)Electrosynthesis(Aniline

2) Application 1) Separation of metal 2) Quantitative and qualitative analysis (polarography) 3) Electroplating of single metal and alloy 4) Electrolytic metallurgy (Al, Ti, Mn) 5) Electrorefining of metal (Cu) 6) Electrosynthesis (Aniline) 1. Cathode reaction §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 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-< CI< Br-<I Henri moissan 1906 Noble prize france 1852/09/28~1907/02/20 Investigation and isolation of the element fluorine

2. Anode reaction 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 1) Reaction on inert anode §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 2. Anode reaction (1)Active dissolution; 2)Reaction of active anode (2)Anodic passivation (3)Anodic oxidatio 2.0 FeOf(aq) 16 The reaction can be judged based on 1.2 Porbaix d Fe,O3 (I)Active dissolution (4) 04 At pH4 and low current density, active -0.8 Fe(s) dissolution occurs -1.2 681012 Fe→Fe2++2 Pourbaix diagram of iron-water system

(1) Active dissolution; (2) Anodic passivation (3) Anodic oxidation 2) Reaction of active anode Pourbaix diagram of iron-water system (1) Active dissolution: At pH=4 and low current density, active dissolution occurs. Fe → Fe2+ + 2e − The reaction can be judged based on Porbaix diagram 2. Anode reaction §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis (2)Anodic passivation 2. Anode reaction At pH= 12 and high potential, upon polarization, compact thin layer of Fe3O4 forms and passivation FeOf(aq) of iron takes place. Fe(aq) 3Fe+ 4H,0-8e-->fe,o+8H+ FegO 20 (3) Active dissolutio (4) 1.4 Trans-passivation Fe(s) passivation 8 20 Passivation curve of iron Potential /V

(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+ Passivation curve of iron Active dissolution passivation Trans-passivation 2. Anode reaction §7.12 Basic principal and application of electrolysis

87.12 Basic principal and application of electrolysis 2. Anode reaction Anodic oxidation of aluminum at (3)Anodic oxidation constant current density Initiation of pore lIWUIILU Porous aver aver top surface t/h Cross-section

Anodic oxidation of aluminum at constant current density (3) Anodic oxidation t / h E / V Barrier layer Porous layer Initiation of pores 2. Anode reaction top surface Cross-section §7.12 Basic principal and application of electrolysis

87.13 Corrosion and protection of metals

§7.13 Corrosion and protection of metals