Transcript Document 7367345

Chapter 7
Electrode process of gas electrode
7.1 Hydrogen electrode
7.1.1 Experimental observation of hydrogen evolution
1) 1905: Tafel equation :
c = a + b lg j
B  V equation
j  current density
b  100 ~ 140 mV
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
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
electrocatalysis :
when b = 120 mV,  =a +0.12lgJ
when J  10 time .   0.12V
aPb =1.56 aPt = 0.1
at same negative polarization :
1.46
jPt
 10 0.12  1.48  1012
jPb
7.1.2 mechanism of hydrogen evolution
c
/V
adsorption of hydrogen:
d
at Pt electrode in HBr
b
ab :  is small , Q is large .
oxidation of Had
a
HBr
Q ( Ccm-2)
a . charging curve :
>> 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
7.1.3 possible mechanism of hydrogen evolution
(A) H + +M + e- = M Had
(B) 2 M Had
H2 + 2 M
(C) M Had +H + +e-
chemical desorption step iB
H2 +M electrochemical desorption iC
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
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
Langmuir adsorption isotherm :
if we assure
 F 
   exp 

 RT 
0
2.3RT
  const 
lg i
2 F
if adsorption is very strong   1
2.3RT
  const 
lg i
F
 =0.5 S =118 mV
no consideration of diffusion of H into metal lattice
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.
log( j0/A m-2)
2
Pt
Ir
0
Co
Ni
Fe
Cu
2
Cd
4
6
Re
Rh
Sn
Tl Zn
In
100
Bi
Ga
150
W
Mo
Nb Ti
Ta
200
250
M  H bond enthalpy/ kJ mol-1
300
350
CV of catalyst containing 30% Al in 0.5mol/LH2SO4
7.1.4 anodic oxidation of hydrogen
H2  2e   2H+ in fuel cell
micro  reversibility
(1) H2 (g)  H2 (dissolution)
Zn Pt
Au
i
(2) H2 (dissolution) 
Pb (3) H2 +2M  2MH ad

(4) H2 +2M e M Had +H+
(5) MHad  e M+H+(anodic)
MH +OH   e M+H2O (basic)
1) No diffusion polarization: i is independent on stirring
2) adsorption is r.d.s i  aH 2 reaction order is 1
3) diffusion is r.d.s i  aH 2 reaction order is 1
4) Electrochemical oxidation is r.d.s i  aH 2 reaction
order is 1
it was confirmed that diffusion is the r.d.s
7.2 oxygen electrode :
Zinc  air battery, Fuel cell
O2+ 4H+ +4e 
O2+2H2O+4e
2H2O 1.229V
4OH 0.40V
O2+ 2H+ +2e 
H2O2
H2O2 +2H+ +2e
2H2O
i0 over Pd. Pt .10-9 ~10-10A cm-2,can not attain equilibrium
much high overpotential
Oxidation of metal : >50 mechanisms
7.2.1 reduction of oxygen
1: O2 +2 H+ +2e  H2O2 (EC)
2: H2O2 +2 H+ +2e  2H2O (EC)
H2O2  1/2O2 +H2O (cat)
high overpotential
H2O2  H2O
O2 H2O2
+0.5
0
0.5
1.0
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
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
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
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
Cyclic voltammograms of the Pt electrode in
helium-deaerated () and O2 sat. (- - -) H2SO4
Pt-OH + Pt-O + H+ + e-  Pt-OH + Pt-OH
Pt-OH + Pt-OH + 2 H+ + 2 e-  2 Pt + 2 H2O
Charles C. Liang and Andre L. Juliard, J. Electroanal. Chem., 9 (1965) 390
Linear sweep voltammograms of the as-synthesized Pt/CDX975
catalysts in Ar- and O2-saturated 0.5 M H2SO4
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
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.
7.3 Direct methanol fuel cell
PtCH3OH, H2SO4O2, Pt
Anodic reaction:
CH3OH+H2O→CO2+6H++6e-
E=0.046V
Cathodic reaction:
6H++3/2O2+6e-→3H2O
E=1.23V
Cell reaction:
CH3OH+3/2O2=CO2+2H2O
Ecell=1.18V
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
Mechanism of oxidation and bifunctional theory
Pt+CH3OH  Pt(CH3OH)ads
(1)
Pt(CH3OH)ads  PtCOads + 4H+ + 4 e
(2)
M+H2O M(H2O)ads
(3a)
M(H2O)ads MOHads+ H++ e
(3b)
PtCOads + M (H2O)ads  Pt + M + CO2+ 2H++2e
(4a)
PtCOads+MOHads Pt + M + CO2+H++e
(4b)
Pt: for methanol oxidation, M: for water activation
2Pt+CH3OH→Pt-CH2OH+Pt-H
(1)
2Pt+PtCH2OH→Pt2CHOH+Pt-H
(2)
2Pt+Pt2CHOH→Pt3COH+Pt-H
(3)
Pt-H→Pt+H++e-
(4)
Pt3COH→ Pt2COH +H++Pt+ePt2COH →Pt2CO +Pt
(5)
Chapter 8
Electrode process of metal
8.1 deposition of metals
Mn+ + ne  M
1) For formation of single
metal:
2) For formation of alloy
RT aMn+
  
ln
nF
aM
RT
  
ln aM n+
nF
aMn+
RT
  
ln
nF
aM
3) For formation of sublayer of adatoms: UPD
facilitates reduction of metal ion
4) For reduction of complex
more overpotential
RT aMn+
  
ln
nF
aM
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
6) Effect of halid anion
Zn 2+ +2e-
Zn(Hg)
Electrolytes
KNO3
KCl
KBr
KI
103 k / cm s-1
3.5
4.0
8
70
Electrode
reaction
k without Clk with Cl-
Bi3+ = Bi(Hg) In3+ = In(Hg) Zn2+ = Zn(Hg)
3 10-4
1.6  10-4
35  10-4
>1
5  10-4
40  10-4
facilitates reduction of metal ion
Coordination effect, 1 effect, bridging effect
7) Effect of surfactants
retards reduction of metal ion
1 Effect
Adsorption: make potential shifts negatively for
0.5 V
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)
1) Homogeneous nucleation
2) Heterogeneous nucleation
3) Formation of crystal step
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
8.3 study on electrodepositon of metal
homogeneity of electroplating
electroplating at
different depths
Chapter 9 porous electrode
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
Schematic of the threephase interphase of a gasdiffusion electrode.
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
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 YoungLaplace equation
2 cos 
p 
r
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.
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
PTFE–CB and PTFE–MWCNT Composites
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) Highermagnification SEM of MWCNTs pressed into the gas-diffusion
layer
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