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8/3/2019 1 Intro and Phase Potential
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heterogeneous electron transfer:Idealized
actual:
P= physisorbed impurity
e-
e-
Eapplied
counter electrode
Ox + n e- Red
Ox
Red
e-
k o
k o
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Special Features of Electrochemistry
• analytical sensitivity (1 pamp = 10-17 moles/second)
• can make microscopic probes ( < 1 um)
• wide range of time scales ( ~50 nsec up)
• large, tunable driving force
• direct interconversion of chemical and electrical energy(not a heat engine, not Carnot limited)
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2 um
Ewing, Strein, Lau, Acc. Chem. Res. 1992, 25, 440
carbon microelectrodes
single-cell voltammogram,matching that of noradrenaline
amperometric response fromsingle adrenal chromaffin cell
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Special Features of Electrochemistry
• analytical sensitivity (1 pamp = 10-17 moles/second)
• can make microscopic probes ( < 1 um)
• wide range of time scales ( ~50 nsec up)
• large, tunable driving force(Cl2, Na, F2 production, fast reactions, electrosynthesis)
• direct interconversion of chemical and electrical energy(not a heat engine, not Carnot limited)
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air H2
H2O(exhaust)
e- load
polymer electrolyte membrane, catalysis, support,(all of the important stuff)
gas flow channels
recirculated fuel
(~ 200 amps, 0.5 volt)
heat, ~ 85 oC
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Fuel cell powered bus, Ballard Power Systems, 1997
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Ballard Fuel Cell + electric motor, 275 hp (205 kW)
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Who cares about electrode kinetics?
• electrosynthesis(Al, Mg, NaOH, Cl2, etc)
• electrochemical energy
conversion (fuel cells,batteries, photocells)
• electroanalysis (glucose,
O2, K+,
pH, etc.)
• corrosion(almost everything)
≈ $ 40 billion/yr of U.S. GDP
≈ $ 80-200 billion/yr in U.S.A.
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The Big Issues:
1. Thermodynamics of electrochemical cells
(how does an electron affect free energy, etc.?)
2. Kinetics of electron transfer
(what happens when an electron is a reagent?)
3. Mass transport
(things have to get to the electrode to react)
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-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
Ener gyr elativetovacuum,
eV
f(E)
Fermi distribution function
Cu, Ef = -4.65 V
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-0.1 0.1 0.3 0.5 0.7 0.9 1.1
Ener gyr elativetovacuum,
eV
f(E)
Fermi distribution function
Pt, Ef = -5.70 V
Cu and Pt, both 298 K
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µ K +
= µ K +
o+ RT ln a + Fφ
K +
K in+
K out+
φin
φout
K + permeable membrane
electrostatic potentialK + activity
µ K +
a
a
