EMA4303/5305 Electrochemical Engineering Lecture 05 Applications (1) Prof. Zhe Cheng Mechanical & Materials Engineering Florida International University
Corrosion Definition Electrochemical attack of metals due to interactions with oxidizing species in the environment. A complete corrosion reaction always involve two parts: oxidation half (cell) reaction and reduction half (cell) reaction Oxidation half-cell reaction Metal lose electrons and get oxidized Example: Zn Zn 2+ + 2e - E o (Zn/Zn 2+ )=-0.763V Reduction half-cell reaction depends on environment: If in an acidic environment without oxygen 2H + + 2e- H 2 E o (H 2 /H + )= 0.0V Overall reaction: Zn + 2H + = Zn 2+ + H 2 Cell potential: E o cell = E o cat E o an = 0.0 - (-0.763 ) V = 0.763 V If In an acidic environment but with oxygen O 2 + 4e - + 4H + 2H 2 O E o (H 2 O/O 2 )= 1.23 V Overall reaction: 2Zn + 4H + +O 2 = 2Zn 2+ + 2H 2 O Cell potential: E o cell = E o cat E o an = 1.23 - (-0.763) V = 1.993 V Both energetically favorable, which one dominates depend on environment EMA 5305 Electrochemical Engineering Zhe Cheng (2017) 5 Applications 2
3 Passivation (1) When the corrosion product is a dense solid that have very low solubility and does not readily permit passage of water, ions, and other active species (e.g., O 2 gas), passivation would occur. Example of Si passivation: For Si in neutral or acidic aqueous medium, the anodic half-cell reaction can be Si(s) + 2H 2 O = SiO 2(s) + 4H + E o (Si/SiO 2 )= -0.91 V In principle, the cathodic half-cell reaction can be either Without oxygen in acidic/neutral water: 2H + + 2e- H 2 E o (H 2 /H + )= 0.0V With oxygen in acidic/neutral water: O 2 + 4e - + 4H + 2H 2 O E o (H 2 O/O 2 )= 1.23 V For either one, the electrochemical cell potential will be highly positive, meaning there is a very large driving force for the reactions to proceed However, because the Si corrosion (oxidation) product of SiO 2 is very dense and impermeable to the typical corrosive species including H 2 O, H +, or O 2, despite the high cell potential, the full cell reactions do not really proceed at detectable rate in neutral or acidic condition
4 Passivation (2) Example of Si passivation: On the other hand, in basic medium, the anodic half-cell reaction can be: Si + 6OH - = SiO 3 2- + 3H 2 O + 4e - E o (Si/SiO 3 2- )= -1.697 V The cathodic half-cell reaction can be: O 2 + 2 H 2 O + 4 e - = 4OH E o (OH - /O 2 )= 0.401 V The overall reaction will be: Si + O 2 + 2OH - = SiO 2-3 + H 2 O E o cell = E o cat E o an = 0.401 - (-1.697) V = 2.098 V Such a reaction could happen in a strongly basic condition (sometimes with heat necessary) Similar cases for Al in neutral water or Fe in concentrated nitric acid
5 Passivation (2) Example of Si passivation: On the other hand, in basic medium, the anodic half-cell reaction can be: Si + 6OH - = SiO 3 2- + 3H 2 O + 4e - E o (Si/SiO 3 2- )= -1.697 V The cathodic half-cell reaction can be: O 2 + 2 H 2 O + 4 e - = 4OH E o (OH - /O 2 )= 0.401 V The overall reaction will be: Si + O 2 + 2OH - = SiO 2-3 + H 2 O E o cell = E o cat E o an = 0.401 - (-1.697) V = 2.098 V Such a reaction could happen in a strongly basic condition (sometimes with heat necessary) Similar cases for Al in neutral water or Fe in concentrated nitric acid
6 Pourbaix (E-pH) Diagram for Aluminum (1) Boundary lines (a) 2H + + 2e - = H 2 (b) 0.5O 2 + 2H + + 2e - = H 2 O (c) Al 3+ + 3e - = Al (d) 0.5Al 2 O 3 + 3H + + 3e - = Al + 1.5H 2 O (e) AlO 2- + 4H + + 3e - = Al + 2H 2 O (f) 2Al 3+ + 3H 2 O = Al 2 O 3 + 6H + (g) Al 2 O 3 + H 2 O = 2AlO 2- + 2H + Regions Immunity: low enough potential that it is protected thermdynamically Corrosion: in strong acid (ph<~4) Corrosion: in strong base (ph > ~8) Passivation: around neutral Limitations of Pourbaix diagrams No information about corrosion rate
Polarization Curve for Metal Corrosion Polarization curve for metal corrosion Change the potential of the metal (with respect to a reference electrode such as SHE or Ag/AgCl) and measure the change in current Example Polarization curve for a metal Me in an acidic solution without dissolved oxygen: Region 1: E < E(M z+ /M), Essentially no current, no corrosion Region 2: E(M z+ /M) < E < E F, Rapid increase in corrosion rate with more positive E Region 3: E F < E < E t, Rapid increase in current due to either break down of passivation film or alternative reaction (e.g., water electrolysis) EMA 5305 Electrochemical Engineering Zhe Cheng (2017) 5 Applications Lvov (2015), p. 178 7
8 Corrosion Potential & Current Density E corr & j corr Using Fe corrosion in dilute non-oxidizing acid solution without dissolved oxygen as an example, two separate polarization (I vs. E) curves: one for the redox of the metal (Fe) one for the redox of the other reducible species in environment: When the absolute value for the anodic current for the metal of interest (Fe in this case) equals the cathodic current for the environment species, a stable corrosion potential E corr and current density j corr are established Lvov (2015), p. 179
Evans Diagram for Corrosion Another way Use Fe corrosion in dilute acid without dissolved oxygen Only plot absolute value of current density, two sets of curves can be obtained: One for Fe redox One for H 2 redox Where they cross, gives E corr and j corr EMA 5305 Electrochemical Engineering Zhe Cheng (2017) 5 Applications Tan (2013), ch1 9
Corrosion Rate & Prevention Corrosion rate Rate of corrosion (CR) often has the unit of depth of metal corroded per unit time: CR = j corrm Me Me zf Corrosion prevention Replacing materials susceptible to corrosion by those that are not susceptible to environment attack Replace steel with plastics or precious metals Preventing the contact between active metal and its environment External coating via painting (Self) passivation: Al, Ti, or stainless steel Cathodic protection: To protect Fe, connect Fe with Zn (e.g., via Zn plating) as E o (Zn 2+ /Zn)=-0.762V is more negative than E o (Fe 2+ /Fe)=-0.447V, Fe will not corrode until Zn is consumed. This way, Zn is called sacrificial anode Adapted from Fig. 17.23, Callister & Rethwisch 8e. Galvanized Steel zinc Zn 2+ zinc 2e - 2e - steel e.g., zinc-coated nail
Batteries (1) Electrochemical systems that convert chemical energy in solids to electrical energy via electrochemical reactions Applications Power source for mobile device/systems: electronics, automobile, etc. Backup power source for critical systems: computer server, emergency lighting (Renewable) energy storage Classification by charging Primary battery Secondary or re-chargable battery Infinite possibilities Based on electrochemical series, there are infinite possibilities of constructing different batteries with different corresponding electrode reactions Practical requirements for batteries Fast electrode reactions Equilibrium cell potential >1V with working potential >~0.5V Good stability with little self-discharging or unwanted reactions with environment under typical packaging High power and energy density Low cost EMA 5305 Electrochemical Engineering Zhe Cheng (2017) 5 Applications Hamann (2007), p. 439-441 11
12 Batteries (2) Classification by chemistry/common Battery chemistry Lead-acid Alkaline battery Ni-Cd battery Nickel metal hydride (NiMH) battery Lithium ion battery Other batteries Na-S battery Li-S battery Li-O2 (or air) battery
13 Lead-Acid Battery Reactions involved (for discharge) Chemical electrical conversion Anodic (half-cell) reaction Pb + H 2 SO 4 = PbSO 4 + 2H + + 2e - E o (PbSO 4 /Pb)=0.359 vs. SHE Cathodic (half-cell) reaction PbO 2 + H 2 SO 4 + 2H + + 2e - = PbSO 4 + H 2 O E o (PbO 2 /PbSO 4 )=1.691 vs. SHE Overall cell reaction: Pb + PbO 2 + 2H 2 SO 4 = 2PbSO 4 + 2H 2 O E o cell = E o cat E o an = 1.691 (-0.359) = 2.05 V Some practical considerations Microstructure optimization: dense Pb & PbO 2 plate give very low current density Should NOT fully discharge, i.e., convert all Pb/PbO 2 to PbSO 4 as it is insulating! Hamann (2007), p. 441
14 Zn-MnO 2 Battery Reactions involved (for discharge) Chemical electrical conversion Anodic (half-cell) reaction Zn = Zn 2+ + 2e - Cathodic (half-cell) reaction 2MnO 2 + H 2 O + 2e - = 2MnOOH + 2OH - Electrolyte: Zn 2+ + 2NH 4 Cl + 2OH - = Zn(NH 3 ) 2 Cl 2 + H 2 O Overall cell reaction: Zn + 2MnO 2 + 2NH 4 Cl = MnOOH + Zn(NH 3 ) 2 Cl 2 Practical considerations MnO2 not very conductive and must be mixed with graphite powder and electrolyte to achieve decent rate capability
15 Lithium Ion Battery Reactions involved (for discharge) Chemical electrical conversion Anodic (half-cell) reaction Li x C 6 = Li x-y C 6 + yli + + ye - Cathodic (half-cell) reaction yli + + Li 1-y CoO 2 + ye - = LiCoO 2 Overall cell reaction: Li x C 6 + Li 1-y CoO 2 = Li x-y C 6 + LiCoO 2 E o cell = E o cat E o an 4.0 V Some practical considerations High energy density and power density comparing with other batteries High efficiency and no memory effect Flammable
16 Battery Parameters Open circuit voltage Specific capacity Amount of charge can be stored for an electrochemical cell. The theoretical value is given by: C th S = nf σ i x i M i n Number of electron transferred in the reaction as written F Faraday constant x i Coefficient for species i in the electrochemical cell reaction Molar mass for species i in the electrochemical cell reaction M i Specific energy Amount of energy that can be stored for an electrochemical cell. The theoretical value is E S th = C S th OCV = nf σ i x i M i OCV In practice, actual values are much lower due to non-active but necessary components, e.g., materials for electrolyte, current collector & packaging.
17 Complications in Battery Operation (1) Self-discharge As the negative electrode is more negative than some electrode reaction related to the electrolyte, it is possible that side reactions, such as hydrogen evolution (in acidic or neutral medium), might occur, which cause the metal electrode to discharge and lose capacity Battery discharge characteristics E E E Ideal discharge with negligible internal resistance j j Discharge with both activation and mass-transfer limitation j Discharge with almost no activation but mass-transfer limitation
Complications in Battery Operation (2) For secondary battery, charging voltage has to be larger than equilibrium cell potential To make charging sufficiently fast, the charging voltage may even be higher than the electrochemical window, which may induce side reactions, such as electrolysis of the electrolyte (e.g., water) Current yield or coulomb efficiency Energy yield Cycle number EMA 5305 Electrochemical Engineering Zhe Cheng (2017) 5 Applications Hamann (2007), p. 441 18