chapter 7 electrode process of gas electrode. 7.1.1 experimental observation of hydrogen evolution...
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Chapter 7
Electrode process of gas electrode
7.1.1 Experimental observation of hydrogen evolution
1) 1905: Tafel equation :
c = a + b lg j j current density
B V equation b 100 ~ 140 mV
when j = 1A m-2 , c = a Vaccording to a :
high hydrogen overpotential metals: a = 1.0 ~ 1.5
Pb, Hg, Sn, Cd, Zn, Bi, Tl, Ga, Ca
7.1 Hydrogen electrode
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
Medium a = 0.5 ~ 0.7 V : Fe ,Co, Ni, Cu, W, Au
low a = 0.1 ~ 0.3V: Pt, Pd .
electrocatalysis :
when b = 120 mV, =a +0.12lgJ
when J 10 time . 0.12V
aPb =1.56 aPt = 0.1
at same negative polarization :
1.4612Pt 0.12
Pb
10 1.48 10j
j
7.1.2 mechanism of hydrogen evolution
adsorption of hydrogen:
a . charging curve :
ab : is small , Q is large .
>> Cdl , i = iec +ich iec thousands of microfaradgy cm-2
oxidation of Had
bc: C = Q/ ~ Cdl 36Fcm-2 no adsorption
cd : C = Q/ adsorption of oxygen oxidation of metal
at Pt electrode in HBr
ab
c d
Q ( Ccm-2)
/V
HBr
(A) H + +M + e- = M Had
(B) 2 M Had H2 + 2 M
(C) M Had +H + +e- H2 +M
chemical desorption step iB
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
7.1.3 possible mechanism of hydrogen evolution
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
0 expF
RT
2.3lg
2
RTconst i
F
if adsorption is very strong 1
2.3lg
RTconst i
F
=0.5 S =118 mV
no consideration of diffusion of H into metal lattice
on Hg, discharge of H+ is rds . 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.
100 150 200 250 300 3506
4
2
0
2
InZnTl
SnGa
BiCd Cu
FeNi
Co
Ta
Nb Ti
Mo W
Ir RhRePt
log(
j0 /A
m-2)
M H bond enthalpy/ kJ mol-1
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
Pb
AuPtZn
i
(1) H2 (g) H2 (dissolution)
(2) H2 (dissolution)
(3) H2 +2M 2MH ad
(4) H2 +2M e M Had +H+
(5) MHad 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 reaction order is 12Ha
it was confirmed that diffusion is the r.d.s
3) diffusion is r.d.s i reaction order is 1
2Ha
4) Electrochemical oxidation is r.d.s i reaction order is 1
2Ha
7.2 oxygen electrode :
Zinc air battery, Fuel cell
O2+ 4H+ +4e 2H2O 1.229V
O2+2H2O+4e 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)
high overpotential H2O2 1/2O2 +H2O (cat)
+0.5 0.5 1.00
O2 H2O2
H2O2 H2O
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
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
Essential criteria for choosing an electrocatalyst for oxygen reduction
Why Pt ?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)
J. J. Lingane, J. Electroanal. Chem., 2 (1961) 296
Oxygen reduction activity as a function of the oxygen binding energy
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
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 (rds)
OHad Oad +H+ +e
2 Oad O2
oxidation of metal :Pt, Au.
7.3 Direct methanol fuel cell
PtCH3OH, H2SO4O2, 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
Pt+CH3OH Pt(CH3OH)ads (1)
Pt(CH3OH)ads PtCOads + 4H+ + 4 e (2)
M+H2O M(H2O)ads (3a)
M(H2O)ads MOHads+ H++ e (3b)
PtCOads + M (H2O)ads Pt + M + CO2+ 2H++2e (4a)
PtCOads+MOHads Pt + M + CO2+H++e (4b)
Mechanism of oxidation and bifunctional theory
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+e-
Pt2COH →Pt2CO +Pt (5)
Chapter 8
Electrode process of metal
Mn+ + ne M
8.1 deposition of metals
n+M
M
lnaRT
nF a y
2) For formation of alloy
1) For formation of single metal:
n+Mln
RTa
nF y
n+M
M
lnaRT
nF a y
facilitates reduction of metal ion
3) For formation of sublayer of adatoms: UPD
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
4) For reduction of complexn+M
M
lnaRT
nF a y
more overpotential
Electrolytes Electrolytes KNOKNO33 KClKCl KBrKBr KIKI
101033 k / cm s k / cm s-1-1 3.53.5 4.04.0 88 7070
2+ -Zn +2e Zn(Hg)
6) Effect of halid anion
facilitates reduction of metal ion
Coordination effect, 1 effect, bridging effect
Electrode Electrode reactionreaction
BiBi3+3+ = Bi(Hg) = Bi(Hg) InIn3+3+ = In(Hg) = In(Hg) ZnZn2+ 2+ = = Zn(Hg)Zn(Hg)
k without Clk without Cl-- 33 10 10-4-4 1.6 1.6 10 10-4-4 35 35 10 10-4-4
k with Clk with Cl-- >1>1 5 5 10 10-4-4 40 40 10 10-4-4
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
Three phase electrode reaction
Gas diffusion electrode
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 three-phase interphase of a gas-diffusion 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
Sintered electrode
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
rp
cos2
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) Higher-magnification SEM of MWCNTs pressed into the gas-diffusion layer
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