Electrode process of gas electrode

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Chapter 7 Electrode process of gas electrode
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7.1 Hydrogen electrode
7.1.1 Experimental observation of hydrogen
evolution
1) 1905 Tafel equation
?c a b lg j j ? current
density
b ? 100 140 mV
B ? V equation
when j 1A? m-2 , ?c a V
according to a
high hydrogen overpotential metals a 1.0 1.5
Pb, Hg, Sn, Cd, Zn, Bi,
Tl, Ga, Ca
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Medium a 0.5 0.7 V Fe ,Co, Ni, Cu, W,
Au
low a 0.1 0.3V Pt, Pd .
2) application
lead-acid storage battery Pb, Pb ?Sb , Pb ?Ca,
Pb ?Ca. Sn.
dry battery Hg ,Ga
corrosion protection plating with Sn, Zn, Pb
porous electrode Pb , foamed nickel
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electrocatalysis
when b 120 mV, ? a 0.12lgJ when J ? 10
time . ? ? 0.12V
aPb 1.56 aPt 0.1
at same negative polarization
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7.1.2 mechanism of hydrogen evolution
adsorption of hydrogen
at Pt electrode in HBr
ab ?? is small , ?Q is large .
oxidation of Had
a . charging curve
gtgt Cdl , i iec ich iec thousands of
microfaradgy cm-2
bc C Q/ ? Cdl 36?F?cm-2 no
adsorption
cd C Q/ ? adsorption of oxygen ? oxidation
of metal
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7.1.3 possible mechanism of hydrogen evolution
(A) H M e- M? Had
chemical desorption step iB
(B) 2 M? Had H2 2 M
electrochemical desorption iC
(C) M? Had H e- H2 M
cases
(1) A ? B . A fast , B slow, combination
mechanism
(2) A ? B . A slow, B fast, slow discharge
mechanism
(3) A ? C. A fast , C slow, Electrochemical
desorption mechanism
(4) A ? C. A slow, C fast, slow discharge
mechanism
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for (1) Hg, Pb, Cd . discharge of H is
r.d.s followed by electrochemical desorption
For (2) Ni, W, Cd. proton discharge followed
by r.d.s electrochemical desorption
For (3) Pt, Pd, Rh. Proton discharge followed
by r.d.s chemical desorption
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Langmuir adsorption isotherm
if we assure
if adsorption is very strong ? ? 1
? 0.5 S 118 mV
no consideration of diffusion of H into metal
lattice
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on Hg, discharge of H is r?d?s . Slow ?
discharge mechanism
It was believed that discharge of H on Pb, Cd,
Zn, Sn, Bi, Ga, Ag, Au, Cu followed the same
mechanism as on Pt.
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CV of catalyst containing 30 Al in 0.5mol/LH2SO4
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7.1.4 anodic oxidation of hydrogen
micro ? reversibility
H2 ? 2e ? ? 2H in fuel cell
(1) H2 (g) ? H2 (dissolution)
(2) H2 (dissolution) ?
(3) H2 2M ? 2M?H ad
(4) H2 2M ?e? ?M? Had H
(5) M?Had ? e ??MH(anodic)
MH OH ? ? e ??MH2O (basic)
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1) No diffusion polarization i is independent on
stirring
it was confirmed that diffusion is the r.d.s
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7.2 oxygen electrode
Zinc ? air battery, Fuel cell
O2 2H 2e ? H2O2
O2 4H 4e ? 2H2O 1.229V
H2O2 2H 2e? 2H2O
O22H2O4e? 4OH? 0.40V
i0 over Pd. Pt .10-9 10-10A ?cm-2,can not attain
equilibrium
much high overpotential
Oxidation of metal gt50 mechanisms
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7.2.1 reduction of oxygen
1 O2 2 H 2e? ? H2O2 (EC)
2 H2O2 2 H 2e? ? 2H2O (EC)
high overpotential
H2O2 ? 1/2O2 H2O (cat)
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Reaction pathways for oxygen reduction reaction
Path A direct pathway, involves four-electron
reduction O2 4 H 4 e- ? 2
H2O Eo 1.229 V vs NHE Path B indirect
pathway, involves two-electron reduction followed
by further two-electron reduction
O2 2 H 2 e- ? H2O2 Eo 0.695 V
vs NHE H2O2 2 H 2 e- ? 2
H2O Eo 1.77 V vs NHE
Halina S. Wroblowa, Yen-Chi-Pan and Gerardo
Razumney, J. Electroanal. Chem., 69 (1979) 195
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Essential criteria for choosing an
electrocatalyst for oxygen reduction

• Reversible
• Structural stability during oxygen adsorption
  and reduction
• Stability in electrolyte medium and also in
  suitable potential window
• Ability to decompose H2O2
• Good conductivity
• Low cost

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Why Pt ?

• ? High work function ( 4.6 eV )
• ? Ability to catalyze the reduction of oxygen
• ? Good resistance to corrosion and dissolution
• ? High exchange current density (10-8 mA/cm2)

Oxygen reduction activity as a function of the
oxygen binding energy
J. J. Lingane, J. Electroanal. Chem., 2 (1961) 296
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Difficulties
? Slow ORR due to the formation of OH species
at 0.8 V vs NHE
O2 2 Pt ? Pt2O2 Pt2O2 H e- ?
Pt2-O2H Pt2-O2H ? Pt-OH Pt-O Pt-OH Pt-O
H e- ? Pt-OH Pt-OH Pt-OH Pt-OH 2 H
2 e- ? 2 Pt 2 H2O
Cyclic voltammograms of the Pt electrode in
helium-deaerated (?) and O2 sat. (- - -) H2SO4
Charles C. Liang and Andre L. Juliard, J.
Electroanal. Chem., 9 (1965) 390
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Linear sweep voltammograms of the as-synthesized
Pt/CDX975 catalysts in Ar- and O2-saturated 0.5 M
H2SO4
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Proposed mechanism for oxygen reduction on Pt
alloys
? Increase of 5d vacancies led to an increased 2?
electron donation from O2 to surface Pt and
weaken the O-O bond ? As a result, scission of
the bond must occur instantaneously as electrons
are back donated from 5d orbitals of Pt to 2?
orbitals of the adsorbed O2
T. Toda, H. Igarashi, H. Uchida and M. Watanabe,
J. Electrochem. Soc., 146 (1999) 3750
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7.2.2 evolution of oxygen
H2Oad ? OH ad H e? (r?d?s)
OHad ? Oad H e?
2 Oad ? O2 ?
oxidation of metal Pt, Au.
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7.3 Direct methanol fuel cell
Pt?CH3OH, H2SO4?O2, Pt
Anodic reaction CH3OHH2O?CO26H6e-
E0.046V Cathodic reaction
6H3/2O26e-?3H2O E1.23V Cell
reaction CH3OH3/2O2CO22H2O
Ecell1.18V
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Progress of electrocatalysts
Single metal platinum, black platinum,
platinum on supports graphite, carbon
black, active carbon, carbon nanotube, PAni
Binary catalyst Pt-M M Ru,
Sn, W, Mo, Re, Ni, Au, Rh, Sr, etc.
Ternary catalysts Pt-Ru-M, Pt-Ru-MOx
M Au, Co, Cu, Fe, Mo, Ni, Sn or W
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Mechanism of oxidation and bifunctional theory
PtCH3OH ? Pt?(CH3OH)ads
(1) Pt?(CH3OH)ads ? Pt?COads
4H 4 e? (2) MH2O
?M?(H2O)ads
(3a) M?(H2O)ads ?M?OHads H e?

(3b) Pt?COads M? (H2O)ads ? Pt M CO2
2H2e? (4a) Pt?COadsM?OHads? Pt M
CO2He? (4b)
Pt for methanol oxidation, M for water
activation
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2PtCH3OH?Pt-CH2OHPt-H
(1) 2PtPtCH2OH?Pt2CHOHPt-H
(2) 2PtPt2CHOH?Pt3COHPt-H
(3) Pt-H?PtHe-
(4) Pt3COH? Pt2COH HPte-
Pt2COH ?Pt2CO Pt
(5)
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Chapter 8 Electrode process
of metal
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8.1 deposition of metals
Mn ne? ? M
1) For formation of single metal
2) For formation of alloy
3) For formation of sublayer of adatoms UPD
facilitates reduction of metal ion
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4) For reduction of complex
more overpotential
5) For deposition for nonaqueous solution
overcome decomposition of water and competing
reaction of H. The liberation order may
change. Electrodeposition of Li, Na, Mg, Ln, Ac
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6) Effect of halid anion
Electrolytes KNO3 KCl KBr KI
103 k / cm s-1 3.5 4.0 8 70
Electrode reaction Bi3 Bi(Hg) In3 In(Hg) Zn2 Zn(Hg)
k without Cl- 3? 10-4 1.6 ? 10-4 35 ? 10-4
k with Cl- gt1 5 ? 10-4 40 ? 10-4
facilitates reduction of metal ion Coordination
effect, ?1 effect, bridging effect
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7) Effect of surfactants
retards reduction of metal ion ?1 Effect
Adsorption make potential shifts negatively for
0.5 V
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8.2 electro-crystallization

1. Reduction of metal ion forms adatom
2. Adatom move to crystallization site

Current fluctuation during deposition of Ag on
Ag(100)
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1) Homogeneous nucleation 2) Heterogeneous
nucleation 3) Formation of crystal step
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8.3 under-potential deposition, UPD
Deposition of metal on other metal surface before
reaching its normal liberation potential.
monolayer, sub-monolyaer
UPD of Pb from
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8.3 study on electrodepositon of metal
homogeneity of electroplating
electroplating at different depths
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Chapter 9 porous electrode
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Gas diffusion electrode
Three phase electrode reaction
Gas diffusion electrodes (GDE) are electrodes
with a conjunction of a solid, liquid and gaseous
interface, and an electrical conducting catalyst
supporting an electrochemical reaction between
the liquid and the gaseous phase
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Schematic of the three-phase interphase of a
gas-diffusion electrode.
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Sintered electrode
1. top layer of fine-grained material 2. layer
from different groups 3. gas distribution layer
of coarse-grained material
the catalyst is fixed in a porous foil, so that
the liquid and the gas can interact. Besides the
wetting characteristics, the gas diffusion
electrode has to offer an optimal electric
conductivity, in order to enable an electron
transport with low ohmic resistance
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An important prerequisite for the gas diffusion
electrodes is that both the liquid and the
gaseous phase coexist in the pore system of the
electrodes which can be demonstrated with the
Young-Laplace equation
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Bonded electrode
gas distribution layer with only a small gas
pressure, the electrolyte is displaced from this
pore system. A small flow resistance ensures
that the gas can freely propagate along the
electrode. At a slightly higher gas pressure the
electrolyte in the pore system is suppressed of
the work layer.
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Since about 1970, PTFE's are used to produce an
electrode having both hydrophilic and hydrophobic
properties. This means that, in places with a
high proportion of PTFE, no electrolyte can
penetrate the pore system and vice versa. In that
case the catalyst itself should be
non-hydrophobic
PTFECB and PTFEMWCNT Composites
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Cross-section SEM images of a gas-diffusion
electrode at different magnifications. (A) Cross
section of GDE with (2) GDL (CB with 35 wt PTFE)
and (3) MWCNT catalytic layer (3.5 wt PTFE) with
(1) nickel mesh as the current collector. (B)
Higher-magnification SEM of MWCNTs pressed into
the gas-diffusion layer
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