kjm 3110 electrochemistry · electrode, we can get a consistent system of electrode potentials...
TRANSCRIPT
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KJM 3110 Electrochemistry
Chapter 6. Electrodes
With exercises
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• Till now, we have been introduced to electricity, chemistry and thermodynamics, electrochemical cells and their operating modes, electrolytic cells (electricity to chemical energy) and galvanic cells (chemical energy to electricity).
• We have realised that the textbook may have been written 10-20 years ago.
• The electrode is the most important and unique place in electrochemistry.
• Now, we will through several chapters look at electrodes and what goes on there in increasing detail and depth.
Status
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• The electrode of interest is called the working electrode (WE).
• We need as a minimum one other electrode to measure a voltage.
• This can then be called a reference electrode (RE) or counter electrode (CE).
• In electrochemistry we often use 3 electrodes:• WE
• RE
• CE
• Potentials of individual electrodes cannot be measured and are even hard to define.
• We can only measure differences: ΔE = EWE - ERE
Electrodes
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Reference electrodes
• By defining the potential of one electrode, we can get a consistent system of electrode potentials
• The Standard Hydrogen Electrode (SHE) is chosen as the standard reference electrode, to have a standard potential of 0 V.
• A more practical reference is the Ag/AgCl electrode
• It has a standard potential vs SHE of +0.22216 V.
• These reference electrodes are all suitable for aqueous cells only
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• Write the two electrode reactions of the SHE and the Ag/AgCl
• What does it mean that the Ag/AgCl cell is more positive?
• A cell has an electrode which measured vs a Ag/AgCl electrode has a voltage of 0.50 V. What is its potential vs SHE?• Rewrite Eq. 6.5 into a suitable form for this purpose
• Other reference electrodes are in use, e.g. the calomel electrode. Find out what it is and write its halfcell electrode reaction and find its standard potential vs SHE.
• What might be suitable reference electrodes in Li ion battery research? In SOFC research? In PCEC research?
Exercises
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Measurement of a WE potential
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E and ΔG: The book dwells at the Cu2+/Cu cell…
Alternatives?!
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Standard electrode potentials
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Play with G’s, not E’s…
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• For a cathodic (reduction) reaction
the half cell potential is
The Nernst equation
Reaction quotient Q
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• Where does the Nernst equation come from, in words and equations?
• What happens to the Nernst equation in the following cases:
• Standard state
• General state
• Equilibrium
• To repeat: What are the values of Q in the three cases?
Exercise
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Know the 2.303 and the 59 mV of the Nernst
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Simple element and ion(s) redox and dominance diagrams
Ered (V)
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Pourbaix diagrams for redox containing H+ or OH-
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• Horizontal lines: Only redox and potential
• Vertical lines: Only acid/base (chemistry), example
• Diagonal lines: Both
Pourbaix diagrams for redox containing H+ or OH-
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• Write the reactions for these two lines (without peeking in the book)
Exercise: Pourbaix diagram
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• Calculate the dependency of potential on pH for the phase stability line governed by
Exercise: Pourbaix diagram
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The very important O2 / H2O / H2 Pourbaix diagram
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• What are the lines?
• Write reactions for the stability lines and derive the pH dependencies
• Why are O3(g), H2O2, or OH* not represented?
• Sketch lines with 0.6 V overpotential required to run the half cell reactions at any appreciable speed
Exercises: O2 / H2O / H2 Pourbaix diagram
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• Active electrodes
vs
• Inert electrodes
• Often catalytic
• Noble metals
• Graphite
• Glassy carbon
• Oxides
• Enzymes
Working electrodes – materials and geometries
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• What do the electrodes on the previous slide have in common? (According to the text book)
• Discuss advantages and disadvantages to the different geometries
• How can we eliminate or reduce gradients in the activities in reactants and products?
• Name some active electrodes
• For each of the examples of classes of inert electrodes, name an example and why it may be interesting
Exercise
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• Measuring potentials at open circuit; thermodynamics
• Connected to activities and concentrations via the Nernst equation
• Measure vs suitable reference electrode
• Ion-specific electrode, or ion-selective electrode (ISE)
• Cu2+/ Cu and AgCl/Ag are examples of ISEs for Cu2+ and Cl- ions, respectively. But strong interference from e.g. Ag+ and Br- ions, respectively.
Potentiometry
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• Cu2+ / Cu
• AgCl / Ag
Simple ISEs
Cu
U
CuSO4(aq)
[Cu2+] = 1 M
Cu
[Cu2+] = ? M
Ag KCl(aq)
[Cl-] = 1 M
Ag
[Cl-] = ? M
U
AgCl
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• pO2 sensor
• “Lambda” sond
Solid electrolyte potentiometric sensors
Zr0.9Y0.1O1.95
(YSZ)
O2-
vO**
U
pO2 = 0.21 pO2 = ?
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• pH2 sensor
• Japanese TYK “NOTORP” for pH2 in molten aluminium based on proton conducting acceptor-doped CaZrO3
Solid electrolyte potentiometric sensors
CaZr0.9Y0.1O2.95
(CZY)
H+
U
pH2 = 1 atm pH2 = ?
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• Write the half cell reactions for the two electrodes in each of the cases in the two preceding slides for solid-state potentiometric sensors.
• Write the Nernst equation for the electrodes and for the total cells (sensors).
• What is the standard Gibbs energy change and standard cell voltage for the sensors?
• For the lambda probe in a combustion engine, make estimates of the pO2 it senses, and the probe voltage in the case that the fuel:O2 ratio is 1.01 (rich), 1.00 (balanced), and 0.99 (lean).
• What are the consequences of too high or too low fuel:air mixing ratios?
Exercises
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• We concentrate on one simple example; An ISE for F- ions.
• ISE is a AgCl/Ag electrode in KCl + KF solution with known [F-]inner
• The ISE membrane is selectively F- conductive
• Reference is a AgCl/Ag electrode in KCl, open to test solution with unknown [F-]outer
• The reference membrane is open, e.g. a liquid junction
• All constant when [F-]outer changes, except membrane (Donnan) potential difference
• Calibration necessary
Ion-selective electrodes (ISEs)
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• The pH electrode is an ISE where the membrane of the ISE is proton conducting.
• Glass exhibits a very small proton conductivity
• Thin glass membrane offers sufficient proton conduction• But we all know: The impedance of the pH electrode is still very high, giving
much noise and fluctuations
pH electrode
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• Activities:
• Concentrations and partial pressures:
• ISEs utilise potentials over ion selective membranes:
Ch. 6 Summary