1. No category

Electrochemistry
Fundamentals
1. Energy Bands
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Energy bands
An atom
Energy
Vacuum
level
A small
molecule
E=0
Empty
states
Filled
states
Bulk
material
A Large
Molecule
LUMO
Fermi
level
HOMO
LUMO: lowest unoccupied molecular orbital
HOMO: highest occupied molecular orbital
qChemistry is controlled by the electron transition
around the filled/empty states.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Energy bands of various bulk materials
Conduction
band (CB)
Small band gap
between
filled (VB) and
empty states
No band gap
or overlapped
between
filled (VB) and
empty (CB)
states
Large band gap
between
filled (VB) and
empty (CB)
states
Valence
band (VB)
Core
bands
Metal
Semiconductor
Insulator
qElectronic devices study is focused on semiconductors.
qElectrochemistry study is focused on metals.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Energy bands of various bulk materials at 0 K
Partially filled
Completely filled
Completely empty
Ec
Ec
Eg
Eg
Ec
Ev
Ev
Eg
Ec
Ev
Ev
Semiconductor
Insulator
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Metals
Fermi level of semiconductors and metals
q Fermi-Dirac distribution function
1
f (E ) =
1 + e ( E - EF ) kT
Ei : Intrinsic semi.’s EF
Intrinsic
f(E): Probability that a state at energy E is
occupied by an e- at temp. T
1 – f(E) : Probability that a state at energy E is
empty at temp. T
q Fermi level (EF)
v
An energy level which has a 50% probability
of being occupied at any given time.
n-type
Φ: work
function
p-type
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Fermi level of metals
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
What if you have two metals in contact?
http://jas.eng.buffalo.edu/education/pn/pnformation_B/index.html
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Ions in solution
qElectrolyte
vA
substance that ionizes when dissolved in suitable
ionizing solvents such as water.
v Ex: salt in water
Ø
NaCl(s) → Na+(aq) + Cl−(aq)
qIon’s electronic structure
v HOMO,
LUMO, HOMO-LUMO gap
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Metal electrode in an electrolyte solution
q Just like a pn junction,
there are junction
potentials whenever two
dissimilar materials come
into contact.
q Charge is transferred to
equilibrate Fermi levels,
producing a charge
separation and a contact
potential difference.
q This is the key interface
for electrochemistry for
bio sensing.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Chemical potential and electrochemical potential
q Chemical potential (µ)
v
A potential energy that can be absorbed or released during a chemical
reaction.
q Fermi level (EF)
v
In semiconductor physics, the chemical potential of a system of
electrons at a temperature of 0 K is known as the Fermi level.
q Electrochemical potential (ECP, ̅ )
v
v
ECP represents how easy or difficult it is to add more of that species to
that location. A species will move from areas with higher ECP to areas
with lower ECP; in equilibrium, the ECP will be constant everywhere for
each species.
For example, if a glass of water has a lot of dissolved sugar on one side
and none on the other side, each sugar molecule will randomly diffuse
around the water, until there is equal concentration of sugar
everywhere. We say that the sugar molecules have a "chemical
potential," which is higher in the high-concentration areas, and the
molecules move to lower their chemical potential.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Electrochemistry
Fundamentals
2. Electrochemical Cells
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Electrochemical cells
qElectrochemistry is a study about energy interchange
(by electron transfer between materials) between
electrical domain and chemical domain.
Chemical
potential
energy
Galvanic cell
(voltaic cell)
Electrical
potential
energy
Electrolytic cell
qGalvanic cell (voltaic cell)
vA
spontaneous reaction generates electrical energy
qElectrolytic cell
v Absorbs
energy from an electrical source to drive a nonspontaneous reaction
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Redox reaction
qRedox reaction
v Oxidation:
loss of 1 or more e- (becoming more +)
v Reduction: gain of 1 or more e- (becoming more -)
Ø
Oil rig (oxidation involves loss, reduction involves gain)
v Anode:
oxidation occurs at the anode
v Cathode: reduction occurs at the cathode
© McGraw-Hill
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Electrochemical half cell
qHalf cell
vA
piece of metal (e.g., electrode) is placed in an
electrolyte solution. Then, there is a charge
separation across the boundary between the metal
and the solution.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Half-cell potential
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Half-cell potential
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Reference electrode
qStandard hydrogen electrode (SHE)
v Hydrogen
electrode is perfect as the
reference electrode as it has E0 = 0 V.
However, it is not practical due to the
involvement of flowing hydrogen gas
(potential for accidental explosion).
q Ag/AgCl electrode
+ e- = Ag + Cl- (E0 = +0.22 V)
v Inexpensive, not toxic
v Most common
v AgCl
q Calomel electrode
v Hg/HgCl
v Toxic
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Electrochemical cell
qElectrochemical cell
v Two
half-cells are
connected by a salt
bridge or a filter paper
membrane. Then, two
electrodes are
connected externally.
v Salt bridge: Packed
with a viscous, aqueous
solution of NaCl, KCl or
KNO3 in gel-ish agar.
The viscosity prevents
mixing with the
electrolytes, but
permitting exchange of
charges.
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Zn-Cu galvanic cell
Potato/lemon battery:
Cu wire on one side,
galvanized nail
(covered with Zn) on
the other
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Nernst equation
qNernst equation
RT
E =E ln Q
nF
o
n: number of moles
F: Faraday’s constant (96485 C/mol.)
R: universal gas constant (8.314 J/K-mol)
T: absolute temperature
Q: reaction quotient
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee
Voltaic cell vs. electrolytic cell
Advanced Bio Convergence Micro/Nano SoC, Prof. JB Lee