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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