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Fundamentals of Electrochemistry
Electrochemistry
Electrochemistry – Chemical change accompanied by the
exchange or movement of electrons. It is the study of
electron-transfer reactions.
Ox + ne- Red
Oxidation: Loss of electrons Fe Fe2+ + 2 e-
Reduction: Gain of electrons 2H+ + 2e- H2
Electrochemistry is Everywhere !!
• Automobiles
– Batteries, Fuel Cells,
Supercapactitors, Oxygen
Sensors
• Biochemistry
– Photosynthesis,
Respiration, Fermentation
• Corrosion
– Rust, Passivation,
Cathodic Protection
• Displays
– Electrochromic Windows,
Conducting Polymers
• Electroplating
– Jewelry, Microelectronics
• Fuel Cells
– Cars, Aerospace
…
Nearly every Industry and Institute needs Electrochemistry !
Divide and Conquer
Electrochemists think of Half Reactions
– Separate Oxidation and Reduction
– Each can be studied independently
– May happen in different places!
• Fuel Cell or Battery
Fe + 2 H+
Fe
2 H+ + 2e-
Fe+2 + H2
Fe+2 + 2e-
H2
Redox Reactions
• Oxidation is always accompanied by Reduction
Oxidation ( Anode )
Fe Fe2+ + 2e–
Reduction ( Cathode )
Hydrogen ion: 2H+ + 2e– H2
Water: 2H2O + 2e– H2 + 2OH–
Oxygen: O2 + 2H2O + 2e– 4OH– (neutral)
O2 + 4H+ + 4e– H2 + 2OH– (acid)
Electrochemical Quantities
• Current (i): Electron flow as a result of a redox reaction
– Current measures the RATE of the reaction (electrons per
second)
– Anodic (oxidation) and cathodic (reduction) currents have
different polarity (signs).
– Unit: ampere ( 1 A = 6 x 1018 electrons/s )
• Charge (Q): Total number of electrons
– Charge measures the AMOUNT of the reaction.
– Unit: coulomb (1 C = 6 x 1018 electrons = 1A for 1s )
Unit: faraday ( 1 F = 1 mole of e- = 6 x 1023 e-
= 96 487 C )
Electrochemical Quantities
• Potential (E):
– Voltage measures the ENERGY of the reaction
– Voltage is the equivalent of wavelength in optical
spectroscopy. Potential is the driving force for the redox
reaction.
– Unit: volt ( 1 V = 1 joule / coulomb )
– The potential is related to the thermodynamics of the
system:
DG = -n F E (negative DG is spontaneous)
joule/mole = (# e-) (coulombs/mole) (joule/coulomb)
Electrochemical Quantities
• Potential (E)
– You can only measure a DIFFERENCE in potential/voltage
• Between What and What?
• What is the reference point or “Ground State”
– Need a Reference Electrode to provide that reference point!!
• Zero Volts is NOT “Nothing”
– Oxidation or reduction can happen at any pot’l !
• +, –, or zero
– It depends on the Reference Electrode used!
• Use of a Reference Electrode allows the electrochemist focus on either
Half-Reaction
– Reaction at Working Electrode!
Quick Review !
• Current – Rate of Reaction
• Charge – Amount of Material
• Voltage – Energy (Difference) of Reaction
• Reference Electrode – Ground State/Reference
• Oxidation and Reduction – Always Paired!
• Half-Reactions
• Oxidation – Anode
• Reduction – Cathode
? Working Electrode / Counter Electrode ?
Which is the Anode? Which is the Cathode?
• Focus Our Interest on the Working Electrode
Potential and Current Conventions
+ Anodic, “Oxidation”
Current
Working
Electrode
Potential
– Cathodic, “Reduction”
More Oxidizing,”Noble”More Reducing, “Active”
Current polarity depends upon potentiostat manufacturer,
geography, application!
EOpen Circuit, imeas = 0
+_
Range of Potential and Current
• With the exception of Lithium batteries, 90% of electrochemical
experiments take place within +/- 2 volts. Including batteries, the
potential rarely exceeds +/- 10 volts for a single cell. (Except for
titanium!)
• Current can vary from tens or hundreds of amps to femtoamps (10-15
amps). That’s 15-17 orders of magnitude! Modern potentiostats are
capable of auto-ranging the current over 9-10 decades of current.
(Important for Corrosion)
Actual current at a Working Electrode depends on the current density,
area, and nature of the experiment.
Electrochemical Techniques
In electrochemistry, there are three variables: potential (E), current (i),
and time.
• Potentiometric: Measure E (pH Meter) at i=0
• Zero Resistance Ammeter (ZRA): Measure i between two connected
electrodes.
Apply an excitation, measure a response.
• Potentiostatic: Control E, Measure i (i vs E, i vs t)
• Galvanostatic: Control i, Measure E (E vs i, E vs t)
Model of the Electrode-Solution Interface or Double
Layer*
*”Electrochemical Methods”, Bard and Faulkner
In electrochemistry,
everything of interest
takes place at the
interface!
Electrode Processes
• Faradaic Process: Current flow as a result
of electron transfer (charge transfer, redox
reaction).
• Non-Faradaic Process: Current flow as a
result of the capacitive nature of an
electrode. This is termed the double-layer
capacitance, Cdl.
Capacitance of an Electrode
• A capacitor is a circuit element composed of two conductors separated by a
dielectric material.
• C (Farads) = q(Coulombs)/E (Volts)
• A charged electrode in contact with an electrolyte behaves like a capacitor
because of the structure of the electrical double layer at the electrode-
solution interface.
• When the applied potential on the electrode is changed, a charging current
will flow. This current is small but measurable.
• A typical double-layer capacitance of an electrode is 10-40 F/cm2.
• The double-layer capacitance, Cdl, is a factor in every electrochemical
experiment.
Potentiostatic Experiments
• A potentiostat controls the potential between the Working
Electrode and the Reference Electrode while it measures the
current between the Working Electrode and the Counter
Electode.
• Most electrochemical experiments are potentiostatic. Because of
the relationship between the potential and the thermodynamics of
the system, potentiostatic experiments are easier to understand.
• Change the potential in some systematic way and measure the
current response. The applied potential will force a redox reaction to
occur.
The Potentiostatic Experiment
• Working Electrode: Electrode Being Studied.
• Reference Electrode: Saturated Calomel (SCE) or
Silver-Silver Chloride (Ag/AgCl).
• Counter Electrode: Should be Conductive and Inert
(Graphite or Platinum).
• Solution May/May Not be Stirred, Deaerated (O2).
• Temperature Control Should be Considered
Encouraged!
Potentiostatic Experiment
Time
Potential,V
A potentiostatic experiment is performed at a constant potential while
measuring the current. By common usage, it has also come to mean
an experiment in which the potential is stepped into a region where a
faradaic reaction occurs.
Analog Potential Scan Experiment
Time
Potential,V
Analog
Scan Rate
mV/sec
In many electrochemical experiments, the potential is
“scanned” in a linear fashion. Scan rates vary from 0.01
mV/s to 10,000 V/s. Manual (non-computerized) instruments
use an analog “ramp”.
Digital Potentiodynamic Experiment
Time
Potential,VComputerized Instruments Generate a Potential Waveform
Called a “Staircase” that is Described by the Scan Rate:
Scan Rate (mV/sec) = Step Height/Step Time
Step Height, mV
Step Time, sec
Electrolyte Resistance
• The goal of the potentiostat is to control the potential of the
Working Electrode vs. a Reference Electrode.
• There is always a potential difference (a potential drop) due to
current flow through the resistance in the bulk of the solution.
• The resistance of the electrolyte between the Working and
Reference Electrodes causes an error in the Applied Potential.
• Eactual = Eapplied - EiR
• Placing the Reference Electrode as close as possible to the
surface of the Working Electrode helps, but does not solve the
problem.
iR Compensation
• To correct for the error in the applied potential, the
potentiostat must compensate for the iR drop in the
solution.
• The most common techniques for iR compensation are
current interrupt (for slow experiments) and positive
feedback (for fast experiments).
Current Interrupt iR Compensation
• The current is briefly (40-200 sec) turned off after
each data point.
• After interruption, the potential due to iR drop
disappears.
• The actual potential at the electrode remains
constant within the time scale of the interruption
because of the capacitive nature of the interface.
• To correct, the measured iR error is “added” to the
applied potential.
Current Interrupt Timing
E T T
E1
E2
Cell Off
Time
E3
Interrupt Time = 40 – 200 sec
DE = Potential Error Due to iR Drop
Requirements for Successful
Current Interrupt
• Need minimum double layer capacitance
of 20-100 Farads
• Ru must be less than 1000 ohms
• Ratio of Ru to Rp must be less than 10
• Scan Rate/Data Acquisition Rate Must be
Slow
Positive Feedback iR
Compensation
• Somehow Measure R
• Add a Signal iR to the Applied Signal
– Multiply I Signal by R with Analog Circuitry
– Add Product to Applied Voltage
• Suitable for Rapidly Changing Currents
– Rapidly Changing Voltages
– Fast Scans
– Voltage Steps
Positive Feedback- Measure
R• Pulse
– 10 - 50 mV Pulse
– Analyze Current Waveform ( RC Decay )
– BAS, PE
• EIS !
– High Frequency Limit
– Solution Resistance Only
– EIS Capability in Every PCI4 / FAS2 / Reference
600
• If Overcompensate
– Oscillate!
Galvanostatic Experiments
• A galvanostat controls the current between
the Working and Counter Electrodes
• If desired, the potential of the Working
Electrode may be measured vs. a
Reference Electrode
• Not as Popular as Potentiostatic
– Hard to Know What Process Consumes the
Current
Zero Resistance Ammeter
• A ZRA electronically connects two electrodes and
measures the current flow between them. The potential
difference of the couple is measured versus a Reference
Electrode.
• The two most important experiments using a ZRA are
electrochemical noise and galvanic corrosion.

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Fundamentals of Electrochemistry

  • 2. Electrochemistry Electrochemistry – Chemical change accompanied by the exchange or movement of electrons. It is the study of electron-transfer reactions. Ox + ne- Red Oxidation: Loss of electrons Fe Fe2+ + 2 e- Reduction: Gain of electrons 2H+ + 2e- H2
  • 3. Electrochemistry is Everywhere !! • Automobiles – Batteries, Fuel Cells, Supercapactitors, Oxygen Sensors • Biochemistry – Photosynthesis, Respiration, Fermentation • Corrosion – Rust, Passivation, Cathodic Protection • Displays – Electrochromic Windows, Conducting Polymers • Electroplating – Jewelry, Microelectronics • Fuel Cells – Cars, Aerospace … Nearly every Industry and Institute needs Electrochemistry !
  • 4. Divide and Conquer Electrochemists think of Half Reactions – Separate Oxidation and Reduction – Each can be studied independently – May happen in different places! • Fuel Cell or Battery Fe + 2 H+ Fe 2 H+ + 2e- Fe+2 + H2 Fe+2 + 2e- H2
  • 5. Redox Reactions • Oxidation is always accompanied by Reduction Oxidation ( Anode ) Fe Fe2+ + 2e– Reduction ( Cathode ) Hydrogen ion: 2H+ + 2e– H2 Water: 2H2O + 2e– H2 + 2OH– Oxygen: O2 + 2H2O + 2e– 4OH– (neutral) O2 + 4H+ + 4e– H2 + 2OH– (acid)
  • 6. Electrochemical Quantities • Current (i): Electron flow as a result of a redox reaction – Current measures the RATE of the reaction (electrons per second) – Anodic (oxidation) and cathodic (reduction) currents have different polarity (signs). – Unit: ampere ( 1 A = 6 x 1018 electrons/s ) • Charge (Q): Total number of electrons – Charge measures the AMOUNT of the reaction. – Unit: coulomb (1 C = 6 x 1018 electrons = 1A for 1s ) Unit: faraday ( 1 F = 1 mole of e- = 6 x 1023 e- = 96 487 C )
  • 7. Electrochemical Quantities • Potential (E): – Voltage measures the ENERGY of the reaction – Voltage is the equivalent of wavelength in optical spectroscopy. Potential is the driving force for the redox reaction. – Unit: volt ( 1 V = 1 joule / coulomb ) – The potential is related to the thermodynamics of the system: DG = -n F E (negative DG is spontaneous) joule/mole = (# e-) (coulombs/mole) (joule/coulomb)
  • 8. Electrochemical Quantities • Potential (E) – You can only measure a DIFFERENCE in potential/voltage • Between What and What? • What is the reference point or “Ground State” – Need a Reference Electrode to provide that reference point!! • Zero Volts is NOT “Nothing” – Oxidation or reduction can happen at any pot’l ! • +, –, or zero – It depends on the Reference Electrode used! • Use of a Reference Electrode allows the electrochemist focus on either Half-Reaction – Reaction at Working Electrode!
  • 9. Quick Review ! • Current – Rate of Reaction • Charge – Amount of Material • Voltage – Energy (Difference) of Reaction • Reference Electrode – Ground State/Reference • Oxidation and Reduction – Always Paired! • Half-Reactions • Oxidation – Anode • Reduction – Cathode ? Working Electrode / Counter Electrode ? Which is the Anode? Which is the Cathode? • Focus Our Interest on the Working Electrode
  • 10. Potential and Current Conventions + Anodic, “Oxidation” Current Working Electrode Potential – Cathodic, “Reduction” More Oxidizing,”Noble”More Reducing, “Active” Current polarity depends upon potentiostat manufacturer, geography, application! EOpen Circuit, imeas = 0 +_
  • 11. Range of Potential and Current • With the exception of Lithium batteries, 90% of electrochemical experiments take place within +/- 2 volts. Including batteries, the potential rarely exceeds +/- 10 volts for a single cell. (Except for titanium!) • Current can vary from tens or hundreds of amps to femtoamps (10-15 amps). That’s 15-17 orders of magnitude! Modern potentiostats are capable of auto-ranging the current over 9-10 decades of current. (Important for Corrosion) Actual current at a Working Electrode depends on the current density, area, and nature of the experiment.
  • 12. Electrochemical Techniques In electrochemistry, there are three variables: potential (E), current (i), and time. • Potentiometric: Measure E (pH Meter) at i=0 • Zero Resistance Ammeter (ZRA): Measure i between two connected electrodes. Apply an excitation, measure a response. • Potentiostatic: Control E, Measure i (i vs E, i vs t) • Galvanostatic: Control i, Measure E (E vs i, E vs t)
  • 13. Model of the Electrode-Solution Interface or Double Layer* *”Electrochemical Methods”, Bard and Faulkner In electrochemistry, everything of interest takes place at the interface!
  • 14. Electrode Processes • Faradaic Process: Current flow as a result of electron transfer (charge transfer, redox reaction). • Non-Faradaic Process: Current flow as a result of the capacitive nature of an electrode. This is termed the double-layer capacitance, Cdl.
  • 15. Capacitance of an Electrode • A capacitor is a circuit element composed of two conductors separated by a dielectric material. • C (Farads) = q(Coulombs)/E (Volts) • A charged electrode in contact with an electrolyte behaves like a capacitor because of the structure of the electrical double layer at the electrode- solution interface. • When the applied potential on the electrode is changed, a charging current will flow. This current is small but measurable. • A typical double-layer capacitance of an electrode is 10-40 F/cm2. • The double-layer capacitance, Cdl, is a factor in every electrochemical experiment.
  • 16. Potentiostatic Experiments • A potentiostat controls the potential between the Working Electrode and the Reference Electrode while it measures the current between the Working Electrode and the Counter Electode. • Most electrochemical experiments are potentiostatic. Because of the relationship between the potential and the thermodynamics of the system, potentiostatic experiments are easier to understand. • Change the potential in some systematic way and measure the current response. The applied potential will force a redox reaction to occur.
  • 17. The Potentiostatic Experiment • Working Electrode: Electrode Being Studied. • Reference Electrode: Saturated Calomel (SCE) or Silver-Silver Chloride (Ag/AgCl). • Counter Electrode: Should be Conductive and Inert (Graphite or Platinum). • Solution May/May Not be Stirred, Deaerated (O2). • Temperature Control Should be Considered Encouraged!
  • 18. Potentiostatic Experiment Time Potential,V A potentiostatic experiment is performed at a constant potential while measuring the current. By common usage, it has also come to mean an experiment in which the potential is stepped into a region where a faradaic reaction occurs.
  • 19. Analog Potential Scan Experiment Time Potential,V Analog Scan Rate mV/sec In many electrochemical experiments, the potential is “scanned” in a linear fashion. Scan rates vary from 0.01 mV/s to 10,000 V/s. Manual (non-computerized) instruments use an analog “ramp”.
  • 20. Digital Potentiodynamic Experiment Time Potential,VComputerized Instruments Generate a Potential Waveform Called a “Staircase” that is Described by the Scan Rate: Scan Rate (mV/sec) = Step Height/Step Time Step Height, mV Step Time, sec
  • 21. Electrolyte Resistance • The goal of the potentiostat is to control the potential of the Working Electrode vs. a Reference Electrode. • There is always a potential difference (a potential drop) due to current flow through the resistance in the bulk of the solution. • The resistance of the electrolyte between the Working and Reference Electrodes causes an error in the Applied Potential. • Eactual = Eapplied - EiR • Placing the Reference Electrode as close as possible to the surface of the Working Electrode helps, but does not solve the problem.
  • 22. iR Compensation • To correct for the error in the applied potential, the potentiostat must compensate for the iR drop in the solution. • The most common techniques for iR compensation are current interrupt (for slow experiments) and positive feedback (for fast experiments).
  • 23. Current Interrupt iR Compensation • The current is briefly (40-200 sec) turned off after each data point. • After interruption, the potential due to iR drop disappears. • The actual potential at the electrode remains constant within the time scale of the interruption because of the capacitive nature of the interface. • To correct, the measured iR error is “added” to the applied potential.
  • 24. Current Interrupt Timing E T T E1 E2 Cell Off Time E3 Interrupt Time = 40 – 200 sec DE = Potential Error Due to iR Drop
  • 25. Requirements for Successful Current Interrupt • Need minimum double layer capacitance of 20-100 Farads • Ru must be less than 1000 ohms • Ratio of Ru to Rp must be less than 10 • Scan Rate/Data Acquisition Rate Must be Slow
  • 26. Positive Feedback iR Compensation • Somehow Measure R • Add a Signal iR to the Applied Signal – Multiply I Signal by R with Analog Circuitry – Add Product to Applied Voltage • Suitable for Rapidly Changing Currents – Rapidly Changing Voltages – Fast Scans – Voltage Steps
  • 27. Positive Feedback- Measure R• Pulse – 10 - 50 mV Pulse – Analyze Current Waveform ( RC Decay ) – BAS, PE • EIS ! – High Frequency Limit – Solution Resistance Only – EIS Capability in Every PCI4 / FAS2 / Reference 600 • If Overcompensate – Oscillate!
  • 28. Galvanostatic Experiments • A galvanostat controls the current between the Working and Counter Electrodes • If desired, the potential of the Working Electrode may be measured vs. a Reference Electrode • Not as Popular as Potentiostatic – Hard to Know What Process Consumes the Current
  • 29. Zero Resistance Ammeter • A ZRA electronically connects two electrodes and measures the current flow between them. The potential difference of the couple is measured versus a Reference Electrode. • The two most important experiments using a ZRA are electrochemical noise and galvanic corrosion.