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 V according 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 a Pb =1.56 a Pt = 0.1 at same negative polarization :

7.1.2 mechanism of hydrogen evolution adsorption of hydrogen: a. charging curve : ab :  is small,  Q is large. >> C dl, i = i ec +i ch i ec thousands of microfaradgy cm -2 oxidation of H ad bc: C = Q/  ~ C dl 36  F  cm -2 no adsorption cd : C = Q/  adsorption of oxygen  oxidation of metal at Pt electrode in HBr a b c d Q ( C  cm -2 )  /V HBr

(A) H + +M + e - = M  H ad (B) 2 M  H ad H M (C) M  H ad +H + +e - H 2 +M chemical desorption step i B electrochemical desorption i C 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 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 if adsorption is very strong   1  =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 66 44 22 0 2 In Zn Tl Sn Ga Bi Cd Cu Fe Ni Co Ta Nb Ti Mo W Ir Rh Re Pt log( j 0 /A  m -2 ) M  H bond enthalpy/ kJ  mol -1

CV of catalyst containing 30% Al in 0.5mol/LH 2 SO 4

7.1.4 anodic oxidation of hydrogen H 2  2e   2H + in fuel cell micro  reversibility  Pb Au Pt Zn i (1) H 2 (g)  H 2 (dissolution) (2) H 2 (dissolution)  (3) H 2 +2M  2M  H ad (4) H 2 +2M  e   M  H ad +H + (5) M  H ad  e   M+H + (anodic) MH +OH   e   M+H 2 O (basic)

1) No diffusion polarization: i is independent on stirring 2) adsorption is r.d.s i  reaction order is 1 it was confirmed that diffusion is the r.d.s 3) diffusion is r.d.s i  reaction order is 1 4) Electrochemical oxidation is r.d.s i  reaction order is 1

7.2 oxygen electrode : Zinc  air battery, Fuel cell O 2 + 4H + +4e  2H 2 O 1.229V O 2 +2H 2 O+4e  4OH  0.40V O 2 + 2H + +2e  H 2 O 2 H 2 O 2 +2H + +2e  2H 2 O i 0 over Pd. Pt ~ A  cm -2,can not attain equilibrium much high overpotential Oxidation of metal : >50 mechanisms

7.2.1 reduction of oxygen 1: O 2 +2 H + +2e   H 2 O 2 (EC) 2: H 2 O 2 +2 H + +2e   2H 2 O (EC) high overpotential H 2 O 2  1/2O 2 +H 2 O (cat) +0.5  0.5  O2 H2O2O2 H2O2 H 2 O 2  H 2 O

Reaction pathways for oxygen reduction reaction Path A – direct pathway, involves four-electron reduction O H e -  2 H 2 O ; E o = V vs NHE Path B – indirect pathway, involves two-electron reduction followed by further two-electron reduction O H e -  H 2 O 2 ; E o = V vs NHE H 2 O H e -  2 H 2 O ; E o = 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 H 2 O 2  Good conductivity  Low cost Essential criteria for choosing an electrocatalyst for oxygen reduction

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/cm 2 ) 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 O Pt  Pt 2 O 2 Pt 2 O 2 + H + + e -  Pt 2 -O 2 H Pt 2 -O 2 H  Pt-OH + Pt-O Pt-OH + Pt-O + H + + e -  Pt-OH + Pt-OH Pt-OH + Pt-OH + 2 H e -  2 Pt + 2 H 2 O Cyclic voltammograms of the Pt electrode in helium-deaerated (  ) and O 2 sat. (- - -) H 2 SO 4 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 O 2 -saturated 0.5 M H 2 SO 4

Proposed mechanism for oxygen reduction on Pt alloys  Increase of 5d vacancies led to an increased 2  electron donation from O 2 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 O 2 T. Toda, H. Igarashi, H. Uchida and M. Watanabe, J. Electrochem. Soc., 146 (1999) 3750

7.2.2 evolution of oxygen H 2 O ad  OH ad +H + +e  (r  d  s) OH ad  O ad +H + +e  2 O ad  O 2  oxidation of metal :Pt, Au.

7.3 Direct methanol fuel cell Pt  CH 3 OH, H 2 SO 4  O 2, Pt Anodic reaction: CH 3 OH+H 2 O→CO 2 +6H + +6e - E=0.046V Cathodic reaction: 6H + +3/2O 2 +6e - →3H 2 O E=1.23V Cell reaction: CH 3 OH+3/2O 2 =CO 2 +2H 2 O E cell =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-MO x M = Au, Co, Cu, Fe, Mo, Ni, Sn or W

Pt+CH 3 OH  Pt  (CH 3 OH) ads (1) Pt  (CH 3 OH) ads  Pt  CO ads + 4H e  (2) M+H 2 O  M  (H 2 O) ads (3a) M  (H 2 O) ads  M  OH ads + H + + e  (3b) Pt  CO ads + M  (H 2 O) ads  Pt + M + CO 2 + 2H + +2e  (4a) Pt  CO ads +M  OH ads  Pt + M + CO 2 +H + +e  (4b) Mechanism of oxidation and bifunctional theory Pt: for methanol oxidation, M: for water activation

2Pt+CH 3 OH→Pt-CH 2 OH+Pt-H (1) 2Pt+PtCH 2 OH→Pt 2 CHOH+Pt-H (2) 2Pt+Pt 2 CHOH→Pt 3 COH+Pt-H (3) Pt-H→Pt+H + +e - (4) Pt 3 COH→ Pt 2 COH +H + +Pt+e - Pt 2 COH →Pt 2 CO +Pt (5)

Chapter 8 Electrode process of metal

M n+ + ne   M 8.1 deposition of metals 2) For formation of alloy 1) For formation of single metal: 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 complex more overpotential

Electrolytes KNO 3 KClKBrKI 10 3 k / cm s ) Effect of halid anion facilitates reduction of metal ion Coordination effect,  1 effect, bridging effect Electrode reaction Bi 3+ = Bi(Hg) In 3+ = In(Hg) Zn 2+ = Zn(Hg) k without Cl - 3    k with Cl - >1 5   10 -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

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