fundamentals of...
TRANSCRIPT
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Fundamentals of Electrochemistry
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1. Distinctive Concepts in Electrochemistry 2. Thermodynamics and Kinetics
in Electrochemistry
3. Electrode potentials
4. Ions in solution: electrolytes
5. Electrochemical kinetics
6. Electrochemical cells
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Heterogeneous charge transfer reaction; Electron tunneling at electrode/solution interface
Cu2+ + 2e → Cu0 (electrochemical reduction)
Fe2+ → Fe3+ + e (electrochemical oxidation)
Anode (산화전극): electrochemical oxidation Cathode (환원전극): electrochemical reduction
Electrochemical reactions
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Heterogeneous charge transfer (전하전달)
Cu2+ + 2e → Cu
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1. Electrode potential is an expression of electron energy 2. Many things can happen at once 3. Electrochemical system is not homogeneous 4. Current is an expression of reaction rate 5. E and I can not be controlled simultaneously 6. Current passes through a “closed loop”
Distinctive Concepts in Electrochemistry
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Electrode potential is an expression of electron energy
Electrode potential (전극전위)↔ Electron energy (전극 내 전자의 에너지)
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Electrode potential is tunable by potentiostat 일정전위기
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Electrode potential is an expression of electron energy
FeCp2+
FeCp2+ + e → FeCp2
LUMO; Lowest un-occupied molecular orbital HOMO; highest occupied molecular orbital
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Many things can happen at once
Mass transfer: (물질전달) Diffusion (확산) Migration (이동) Convection (대류)
Electrochemical reactions: Mass transfer + Charge transfer
Rate-determining step
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Electrochemical system is not homogeneous
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Electrochemical system is not homogeneous
General chemical reactions: Homogeneous Electrochemical reactions: only within diffusion layer
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Current is an expression of reaction rate
PSV
FeCp2+
FeCp2
R’
O’
ee
e e
Ref CtrWk
전류 (i) ∝ 반응속도
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How to control reaction rate (current) ?
E and I can not be controlled simultaneously !!
과전압
Charge transfer limited
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Current vs. rate-limiting step
Charge transfer limited
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Current passes through a “closed loop” (닫힌고리)
- electrode + electrode
ic: cathodic current
ia : anodic current
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Cathode area >> Anode area, but ic = |ia |, what happens?
Current passes through a “closed loop”
1) Cathode 2) Anode 3) Electrolyte 4) Circuit
Crevice corrosion (틈새 부식)
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“closed loop” in redox-flow battery
Vanadium redox-flow cell
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Shutdown mechanism in lithium ion batteries
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Anode Dye Electrolyte Cathode
R O
Ecell
Do/D+
Mediator
D*
eLoad
e
e
e
e
hv
“closed loop” in dye-sensitized solar cells
염료감응 태양전지
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Thermodynamics and Kinetics in Electrochemistry
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Thermodynamics vs. Kinetics
FeCp2+ Faradaic current
Non-faradaic current
Thermodynamics: 가능성
Kinetics: 속도(전류)
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Thermodynamics vs. Kinetics
Equivalent circuit (등가회로)
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Ideally polarized electrodes (이상 분극전극)
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Ideally polarized electrodes (이상 분극전극)
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1) Always double-layer charging current with E change 2) Ideally polarized electrodes: Rct → ∞ 3) Ideally non-polarizable electrodes: Rct → 0 (이상 비분극 전극) 4) EDLC; electric double-layer capacitors (전기이중층 커패시터)
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Electrode potentials 1. Standard electrode potential
2. Equilibrium potential
3. Formal potential
4. Mixed potential
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1. Electrical potential is defined by difference (차이) 2. Electrical potential : charge separation at
interfaces (계면에서 전하 분리) 3. Infinitesimally small (극소량) charge separation 4. Potential difference must be defined at
equilibrium or steady-state (평형 또는 일정 상태)
Electrode Potentials (전극전위)
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Electrode Solution
Electron Cation
r
Infinitesimally small charge separation
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Potential difference at interfaces
Potential difference must be defined at equilibrium (평형) or steady-state (일정 상태)
Ag+ + e (in Ag) → Ag
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Chemical Potential (화학전위)?
Escaping tendency (이탈 경향)
∂∂
=i
i nGαµ
jnPT ,,
iα
(a)
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Electrochemical Potential (전기화학전위)?
αααφµµ ⋅+= Fziii
iα
αφ ( ions)
Additional energy
(b)
Q x V = energy
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Conditions for Equilibrium
Potential-determining equilibrium (전위결정 평형식)
Ag+ + e (in Ag) = Ag
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Potential difference at equilibrium state
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Inert electrode/redox pair or gas
PtFe3+
Fe2+
X-
(a)
PtH2
2H+
X-
(b)
모든 반쪽전지 (half-cell)는 1개의 전위결정 평형식을 가짐
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Standard electrode potential (표준 전극전위)
HCl
H2
Pt electrode
Fe2+ Fe3+
Pt wire
Salt bridge
V
NHE (SHE)를 기준으로 반쪽전지의 전압(V) 측정
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Standard electrode potential (E0)
V
PtH2
H+
Fe2+
Fe3+
X- X-
Pt
H2/H+Fe2+/Fe3+
Vljφ∆
Pt
Pt
1φ∆
2φ∆
Potential 차이는 상(phase)과 상의 계면에서 !!
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Nernst equation
전위결정 평형식 (potential-determining equilibrium)에서 이온들의 활동도(activity)만으로 표시
Fe3+ + e = Fe2+
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Standard electrode potential (E0)
Standard electrode potential (E0) Half-cell에 관여된 potential-determining equilibrium에서 모든 이온의 activity = 1인 경우 Eeq
평형상태의 값이다 (thermodynamic parameter) 모든 반쪽전지는 고유한 E0을 갖는다 NHE (normal hydrogen electrode)를 기준으로 한 250C에서 값
Fe3+ + e = Fe2+
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Standard electrode potential (E0)
Li+ + e = Li E0 = - 3.04 V (vs. NHE)
2H+ + 2e = H2 E0 = 0.0 V (vs. NHE)
Cl2 + 2e = 2Cl- E0 = 1.36 V (vs. NHE)
← More negative More positive →
← 산화경향 증가 환원경향 증가 →
금속의 이온화 경향:
Li> K> Ca> Na> Mg> Al> Zn> Fe> Ni> Sn> Pb> Cu> Au
← ← ← ← E0 more negative
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Practical reference electrodes(기준전극)
E (vs. NHE)
0.2412 V
0.197 V
0.0 V
Hg/Hg2Cl2(s), KCl(sat’d)
Ag/AgCl(s), KCl(sat’d)
Pt/H2/H+ (NHE)
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Formal potential
E0 ≈ E0’
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Mixed potentials (혼성전위 )
Zn
Zn2+
( =1)
Zn2+
AnodeCathodeX-
2H+
H2(g)
e ea
For a Galvanic cell 1) Cathode 2) Anode 3) Electrolyte 4) Circuit
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Mixed potentials; Zn corrosion
2H++ 2e H2(g)ic
ia
0.0 -0.76
Mixed potential
E (-) (vs. NHE)
Zn Zn2++ 2e
icia
(ic = |ia|)
Zn의 용해(부식)과 수소의 발생
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Mixed potentials; Zn corrosion
2H++2e H2(g)ic
ia
0.0 -0.76 E (-) (vs. NHE)
Zn Zn2++2e
pH
Mixed potentialDecrease in ic and ia
pH dependence
Eeq = Constant - 0.059 V x pH
(at 298K)
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H2 and O2 evolution as a function of pH
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Alkaline battery
Negative cover
Seal
Steel can
Cathode (MnO2)
Separator
Anode (Zn)
Currentcollector
Electrolyte: concentrated KOH Negative electrode; Zn powder Positive electrode: MnO2 (EMD) Mercury-free cells
Addition of mercury (Hg)
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Metal - log I0 (A/cm2)
Metal - log I0 (A/cm2)
Ru 2.1 Fe 6.0
Pd 2.3 Cu 6.7
Rh 2.8 W 7.0
Pt 3.6 Cr 7.4
Co 5.2 Zn 10.5
Ni 5.2 Cd 11.0
Ag 5.4 Pb 12.2
Au 5.5 Hg 12.5
Exchange current density for H2 evolution 1.0 M H2SO4
- Large overpotential for H2 evolution - Large Rct
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Ion-ion interactions Ion-solvent interactions
Ions in solution : electrolyte
1) Activity coefficient; Debye-Hückel theory
2) Migration of ions
3) Molar conductivity
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(Salt Conc.)1/2 (Salt Conc.)1/2
log
0.0
Λ±γ
Λ̊
Activity coefficient and transport property
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Debye-Hückel theory
Ionic atmosphere
Ionic concentration ↑; Ionic strength ↑; Debye length ↓; More stabilized; γj↓
δ−
δ+ = δ−
δ−δ−
δ−
I1
∝Debye length
+
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Activity coefficient vs ionic strength
0.0
log
DHL
EDHL
Real
R-S
I
γj
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Li+ Na+ K+
Crystal radius a /Å 0.60 0.95 1.33
Hydrated radius b/Å 2.4 1.8 1.3
Hydration number 2-22 2-13 1-6
Li+, Na+, K+ 이온의 결정학적 반경, 수화된 반경, 수화 수
a From x-ray diffraction analysis, b Using Stokes’ law
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Fe3+
H
H
OH
H
O
H
H
OH
H
OO
HH
OHH O
HH
OHH
OHH
OHH
OHHO
HH
OHH
OH
H
OH
H
OHH
O
HH
O
HH
O
HH
O
HH
O
HH
O
HH
O
HH
O
HH
Bulk solution
Innerhydration
shellOuterHydration
shell
Ion-solvent interaction
Ion-solvent interactions
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Ionic conductivity
A A
l
V
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Molar conductivity
NiSO4 (weak electrolyte)
Ideal
KCl (strong electrolyte)
Λ
Λ
˚
(Salt Conc.)1/2
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Relaxation effect and electrophoretic effect
Eexternal
Erelaxation
Ionic atmosphere
Decrease in mobility
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1. Charge transfer kinetics
Butler-Volmer equation
Tafel plot
2. Mass transfer kinetics
Electrochemical kinetics
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Activated complex theory
Reaction coordinate
Free
ene
rgy
∆G‡
∆G‡
Reactant Product
Activated complex
b
f
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Activated complex theory
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Activation energy by potential control
Reaction coordinate
Free
ene
rgy
nF(E - Eo’) αnF(E - Eo’)
(1-α)nF(E - Eo’)
O + ne R
∆G‡ ∆G‡
∆G‡
oc oa
oc
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Activated complex theory
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Butler-Volmer equation
ηc
ic
Eeq = - 0.11 V
E (vs. SCE)
= 2.2 ia
= 2.2 µA
µA
ic
ia
io
io
inet
Charge transfer rate
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Linear relationship: i vs. E
inet
ic
ia
ic
ia
E (-)
Slope
(−η)
nF i0= RT
= 1Rct
=
inet =
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Tafel plot
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Tafel plot
log │inet│
- 0.10.10.2η (V)
log i0
Slope (1-α)nF=
2.3RTSlope αnF
=2.3RT
- 0.2
η = a + b log inet
Tafel equation
Tafel behavior
Tafel region
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Electrochemical cells 1. Electrolytic cells
2. Galvanic cells
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Electrolytic cells (전해셀)
Brine (NaCl/H2O) electrolysis (소금물 전기분해 (또는 전해))
RT aO Eeq = E0 + — ln —— nF aR
Nernst equation
E0 : Standard electrode potential (표준전극전위) at 298 K NHE: Normal hydrogen electrode
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Electrolytic cells
1) Cathode 2) Anode 3) Electrolyte 4) Power supply
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Electrolytic cells; charge transfer limited
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Electrolytic cells; charge transfer limited
Eappl: 전해를 위해 두 전극 사이 걸어주어야 하는 전압
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ic
ia
E
ηa
1.36 V
Electrode A
ηa
Electrode B
( - )
Electrolytic cells
Electrode selection Rsolution
Electrode gap Separator (분리막)
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Galvanic cells (갈바니 셀)
2H2 + O2 → 2H2O (∆G
0
<< 0 ; spontaneous, 자발적)
Electrochemical pathway (전기화학 경로) Anode: 2H2 → 4H+ + 4e Cathode: O2 + 4H+ + 4e → 2H2O Overall: 2H2 + O2 → 2H2O (∆G < 0 ; spontaneous)
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Galvanic cells
Fuel cell (연료전지)
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Galvanic cells
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Galvanic cells
Ewk: 갈바니 셀의 작동 전압
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Secondary batteries (이차전지)
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Secondary batteries (이차전지)
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Cathode vs. anode
Cathode: emit electrons Anode: accept electrons
Cathode: Cu2+ + 2e → Cu0
(emit electrons)
Anode: Fe2+ → Fe3+ + e
(accept electrons)
Cathode ray tube
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Cathode vs. anode
Hollow Cathode Lamp
Ar → Ar+ + electrons electrons; to anode (accept electrons) Ar+ ; attack cathode (accept positive charges = emit negative charges (electrons)