Eirelec 2011- Oxygen Evolution at Oxidized Metal and Metal Oxide Electrodes - Iron in Base

8/6/2019 Eirelec 2011- Oxygen Evolution at Oxidized Metal and Metal Oxide Electrodes - Iron in Base

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Mike LyonsSchool of Chemistry

Trinity College Dublin 2

[email protected]

Eirelec 11: Electrochemistry-The Future?Dun Raven Arms, Adare, Limerick, 17 May 2011

Plus a change, plus c'est la mme chose.Alphonse KARR, Les Gupes 1849.(The more things change, the more they remain the

same. ...)
mailto:[email protected]:[email protected]


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Hydrogen Economy Transition Metal Oxides: compact and hydrous. Preparation of Hydrous Oxide modified Fe

electrodes via Repetive Potential Cycling Method

(RPCM) Duplex layer model of oxide/solution interface. Acid/base behaviour of hydrous oxide film :

interconnected anionic surfaquo groups.

Redox switching behaviour of hydrous oxide film. OER kinetics and mechanism of oxide modified Fe

electrodes in aqueous base. Concluding comments.


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The Hydrogen Economy:Hydrogen as an energy carrier.

G.W. Crabtree, M.S. Dresselhaus, M.V.

Buchanan, The hydrogen Economy PhysicsToday, Dec.2004, pp.39-45.U. Bossel, Does a hydrogen economy makesense? Proc. IEEE, 94 (10)(2006), pp.1826-1836.

P.P. Edwards, V.L. Kuznetsov, W.I.F. David,N. Brandon. Energy Policy 36(2008) 4356-4362.


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2 types: Compact anhydrous oxides,

e.g.

rutile, perovskite, spinel. Oxygen present only as bridging

species between two metalcations and ideal crystalsconstitute tightly packed giant

molecules. Prepared via thermal techniques,e.g decomposition of unstablesalt

Micro-dispersed hydrousoxide

s Oxygen is present not just as a

bridging species between metalions, but also as O-, OH and OH2species in coordinated terminalgroup form.

Hydrous oxides in contact withaqueous media contain largequantities of loosely bound andtrapped water plus electrolytespecies.

Prepared via base precipitation,electrochemical techniques.

Materials are prepared inkinetically most accessiblerather than thermodynamicallymost stable form.

Are often amorphous or onlypoorly crystalline and prone torearrangement.

L. D. Burke, M.E.G. Lyons,

Modern Aspects Electrochemistry, 18 (1986)169-248.

Geothite

FeOOH


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Cathodic Hydrogen Evolution Reaction (HER)

Acid: 2H3O+ + 2e- H2(g) +2 H2OBase: 2H2O + 2e- H2(g) + 2 OH-

Simplest (therefore most studied) representative electro-catalytic reaction.Multistep process involving adsorbed intermediates.

Classical analysis assumes HEROccurs on oxide free metal surface.

Volmer (V) : hydrogen adsorption or discharge step.H3O+ + M + e- MHads + H2O

H2O + M + e- MHads + OH-

Heyrovsky (H): Electrochemical Desorption step.MHads + H3O+ + e- H2(g) + M + H2OMHads + H2O + e- H2(g) + M + OH-

Tafel (T) : Chemical Desorption step.MHads + MHads H2(g) + 2M

2 main mechanisms : Volmer-Heyrovsky (VH) Volmer-Tafel (VT).


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Kinetically limiting step in waterelectrolysis cells and PEM fuel cell.

Multistep multi-electron transfer reaction

involving adsorbed intermediates. Overall reaction (alkaline medium)

O2 + 2H2O + 4e- 4OH-

E0 = 0.303 V (vs. Hg/HgO)

Krasilshchikov (1963)

S + OH-

SOHad + e-

SOHad + OH- SO-ad + H2O

SO-ad SOad + e-

2SOad 2S + O2

Bockris Electrochemical Oxide (1956)

S + OH-

SOHad + e-

SOHad + OH- SO + H2O + e

-

SO + SO 2S + O2

Krasilshchikov/modification thereof, ispathway most often proposed for OER onmetal / metal-oxide electrodes in alkalinesolution.

Depending on RDS can explain a variety ofTafel slopes.

Modification permits concept of

formation/decomposition of higher oxide e.g.for Ni

OH- OHad+ e-

OHad+ OH- O-ad+ H2O

2 -NiOOH + O-ad 2NiO2+ H2O + e-

2NiO2 + H2O 2 -NiOOH + Oad

Oad+ Oad O2

OER at oxidized metal and metal oxideelectrodes involves active participation ofoxide.

Acid/base behaviour of oxide importantconsideration .

Concept of active surface or surfaquo groupsimportant.

Anodic Oxygen Evolution Reaction (OER)


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L.D. Burke, E.J.M. OSullivan J. Electroanal. Chem., 93 (1978) 11.

Rhodium redox chemistry: alkaline solution

L.D. Burke, E.J.M. OSullivan, J. Electroanal. Chem., 117(1981) 155.

Microdispersed hydrous oxide formedvia potential cycling technique (similar

to poly(aniline) deposition to form PME).

Duplex layer model: oxide/solution

interface.


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OSullivan & Burke,

J. Electrochem. Soc.,137 (1990) 466.

OER activity & mechanism(shift in OER potential anddecrease in Tafel slope)

depends on charge capacity(thickness) of hydrousoxide layer.

Increasing oxidethickness

OER atmulticycledhydrous rhodiumoxide modified

electrodes.


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Fe + OH- FeOH(ads.) + 2e-

FeH(ads.) Fe + H+ + e-

A1

FeOH(ads.) + OH- Fe(OH)2 + e-

FeOH(ads.) + OH- FeO + H2O + e-

A2

[Fe2(OH)63H2O]2- + 3OH- [Fe2(OH)9]3- +3H2O + 2e-

A3/C2

[Fe(OH)3.5

nH2O]0.5-(Na+)

0.5+ e- Fe(OH)

2n H

2O + 0.5Na+ + 1.5OH-

FeO.FeOOH + H2O + 3e- Fe + FeO22- + H2O + OH-

C1

A0: OER

C0: HER

Surface redox chemistry: Bright Fe electrode

3Fe(OH)2 + 2OH- Fe3O4 + 4H2O + 2e

-

3FeO + 2OH- Fe3O4 + H2O + 2e-

A4

In situ Raman

EQCMRRDE

Greater fine structureobserved at low sweeprate.


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Hydrous Oxide Growth via Cyclic Potential Multicycling (CPM)Procedure of Fe electrode in aqueous alkaline solution.

Layer growth parameters: Upper, lower potential

sweep limits. Solution temperature. Solution pH. Potential sweep rate. Base concentration. N

A3

C2 0.5 M NaOH

Lyons, Burke, J. Electroanal. Chem., 170 (1984) 377-381


Lyons, Brandon Phys. Chem. Chem. Phys., 11 (2009) 2203-2217


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N

0 200 400 600

Q/mC

cm-

2

0

20

40

60

80

100

120

Q=a(1-exp(-bN))

R=0.9947, R2 = 0.9895a = 103.94 6.05 mC/cm2

b = 0.0044 0.0006 cycle-1

Hydrous oxide growth kinetics

Number of Growth Cycles

0 100 200 300 400 500

Charge/C

0.000

0.002

0.004

0.006

0.008

0.010

0.012

0.014

0.016

Murphy Ph.D Thesis UCC 1981

Fe wire electrode, 1.0 M NaOH Inlaid Fe foil electrode, 0.5 M NaOH

Doyle, unpublished work, TCD 2011

RPS Methodology reproducible across space and time.

R = 0.9935, R2 = 0.9870a = 0.0136 0.0003 Cb = 0.0156 0.0011 cycle-1


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Hydrous oxide film regarded as a surfacebonded polynuclear species. Metal cations

in polymeric network held together bysequence of oxy and hydroxy bridges.Mixed conduction (electronic, ionic) behavioursimilar to that exhibited byPolymer Modified Electrodes.Can regard microdispersed hydrous oxide layer asopen porous mesh of interconnected surfaquometal oxy groups.


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Elow er

/ V (vs Hg/HgO)

-1.7 -1.6 -1.5 -1.4 -1.3 -1.2 -1.1 -1.0 -0.9

Q

/mC

0

10

20

30

40

50

60

70

80

Upper Limit E = 0.324 V

Effect of lower potential limit ELL of growth sweep on development of hydrousoxide charge capacity.

Q (proportional to oxidelayer thickness) identified asanodic chargecapacity measured between-1.226 and 0.324 V, at33 mV/s after repetitivetriangularsweep at 3.3 V/s for 5 min

between fixed UPL = 0.324Vand variable LPL as indicated.

ELL, optimum


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Eupper

/ V (vs Hg/HgO)

-1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8 1.0 1.2

Q

/mC

0

10

20

30

40

50

60

70

80

Lower limit E = - 1.426 V

Effect of upper limit EUL of growth sweep on development of hydrousoxide charge capacity.

Q (proportional to oxidelayer thickness) identified asanodic chargecapacity measured between-1.226 and 0.324 V, at33 mV/s after repetitivetriangularsweep at 3.3 V/s for 5 min

between fixed LPL = -1.426Vand variable UPL as indicated.

EUL, optimum


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16/42


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Hydration process promoted by increasingadsorption of OH- ions as pH increases.Hence adsorbed OH- species repel each otherand attract hydrated positive counter ions intooxide matrix hence encouraging hydroxylation

processes.


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[Fe2(OH)6(OH2)3]2-

2Fe(OH)2 + 3H2O

2Fe(OH)2 2FeO + 2H2O

Increased instability of hydrouslayer and more effectivepassivation as solutiontemperature increases.


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A4

A3

C2

EP

A2

C1

A1: not determined. Peak ill-defined in pH rangeStudied.A2: Regular Nernstian shift. -0.06 V/dec (wrt SCE)-2.303RT/F V/dec; zero shift wrt RHE.A3: Super-Nernstian shift. 0.087 V/dec (wrt SCE) or-0.028 V/dec (wrt RHE), i.e. - 3/2(2.303RT/F) V/decC

2

: Super-Nernstian shift. -0.092 V/dec (wrt SCE) or 0.033V/dec (wrt RHE), i.e. -3/2 (2.303RT/F) V/dec.A4: Regular Nernstian shift. . -0.06 V/dec (wrt SCE)-2.303RT/F V/dec; zero shift wrt RHE.C1: + 0.044 V/dec (wrt RHE) or + 0.044 0.059 = -0.015 V/decwrt SCE.EP passivation peak shows regular Nernstian behaviour.

A1

Voltammetric response as function of solution pH


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[Fe2(OH)6(OH2)3]2- + 3OH-

[Fe2O3(OH)3(OH2)3]3- + 3H2O + 2e-

[M2O3 (OH)3 (OH2)3]n 3- + 3nOH- [MO2(OH)2(OH2)2]2n2- + 3nH2O + 2ne-

M(IV)

M(III)

Fe(II)

Fe(III)

Redox switching involves topotactic chargestorage reactions in open hydrous oxide layer whichBehaves as ion exchange membrane.Hydrated counter/co-ions (M+, H+, OH- assumedpresent in pores and channels of film to balance

negative charge on polymer chain.Equivalent circuit model: dual/multi- railTransmission Line as done for ECP films..

Super-Nernstian Redox Potentialvs pH shift related to hydrolysiseffects in hydrous layer yieldinganionic oxide structures.

L.D. Burke, M.E.G. Lyons, E.J.M.OSullivan,D.P. Whelan J. Electroanal. Chem.,122 (1981) 403.

M = Ir, Rh

dE/dpH = -3r/2

r= 2.303RT/Fr= -88.5 mV/decT = 298K

Rhodium oxide

Iron oxide

Redox switching chemistry: hydrous oxideLayer, Mixed conduction mechanism:ion/electron transfer.

Fe(III) Oxidized form yellow-green

Fe(II) Reduced form transparent


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A

B A

B A

B A

B

Elec

trode

ne-

Polymer layer Solution

Charge ejection/injection:

potential gradient drivenCharge ejection/injection:

potential gradient driven

Charge propagation:

concentration gradient drivenCharge propagation:

concentration gradient driven

DE ckD 2exE

Mixed conductivity:Electron hopping coupledwith counter ion transport.

Microdispersed hydrousOxide has many physicalcharacteristics similar tothose of electroactivepolymer material.


22/42

22

Migration/diffusion model for electron hopping.

xc

ab

x

b

xD

t

bE

x

f

t

b

t

a E

xc

ab

x

bDf EE

D-term M- term

Eqn. Of continuity Steady state D/M flux

A

B A

B A

B A

B

E

lectrode

ne-

Polymer layer Solution

Charge ejection/injection:

potential gradient drivenCharge ejection/injection:

potential gradient driven

Charge propagation:

concentration gradient drivenCharge propagation:concentration gradient driven

DE

xxx fkexpkk xx 1f

2

2

2

2

Exc

ba

xx

b

c

ba

x

bD

t

b

RT

FE

Potential gradient

between sites

Nernst-Planck

equation.


23/42

macroscopic charge propagation through polymer

can be represented in terms of a diffusion-migration process rate of electron hopping quantified via electron exchangerate constant kex or electron hopping diffusion coefficient DE

local potential gradients between redox sites producea migration term in description of electron hopping flux

DE predicted to vary linearly with redox site concentration

electron hopping can be further described using theMarcus theory of ET

mechanism of electron hopping dependent on degree of localmobility of redox groups ; physical diffusion of redox groups alsomay be important

electron hopping diffusion coefficients can be evaluated usingtransient or steady state electrochemical techniques

Features of Redox Conduction.


24/42

Scan Rate / V s-1

0.0 0.1 0.2 0.3 0.4 0.5

Charge/C

0.000

0.005

0.010

0.015

0.020

1 cycle

30 cycles

120 cycles

Variation of integrated voltammetric charge withSweep rate. Multicycled hydrous oxide coated Feelectrodes, 0.5 M NaOH

More deeply buried Fesites accessed by ECdriving force at longer

Timescales.

Finite rate of electron

self exchange kinetics.


25/42

Thin films, 30 cycles Thick films 240 cycles

Laviron Analysis

DCT,ox = 1.88 x10-10 cm2s-1

DCT,red =3.77 x10-10 cm2s-1

c = r/M ; n = 2Fe(II):[Fe2(OH)6(OH2)3]

2-

M = 267 g mol-1

r (Fe(OH)2=3.4 gcm-3

Fe(III):[Fe2O3(OH)3(OH2)3]

3-

M = 265 g mol-1

r(Fe2O3.nH2O) 3 gcm-3

Macroscopic average value.Rate controlling transportprocess during redox switchingprobably ion diffusion.Need EQCM analysis forfurther confirmation.


26/42

26

Z

Y

X

Polymerstrand

Polymer strand/pore solution

interface

PoreelectrolyteSupport

electrode

Bulk electrolytesolution

Z

Y

X

Polymerstrand

Polymer strand/pore solution

interface

PoreelectrolyteSupport

electrode

Bulk electrolytesolution

General dual rail transmission linemodel is used to reflect the coupledprocesses of electronic and ionic

transport within a conductingPolymer/metal oxide thin film.

Dual Rail Transmission Line Modelfor Redox Switching in ECP/metal oxide films.

Two main approaches to TL analysis: Assume ECP film is highly porous matrix.

Use porous electrode models(de Levie, Bisquert, Paasch) to analyseelectrochemical response.

Polymer film assumed homogeneous

(Vorotyntsev, Buck, Albery).Model diffusive (ambipolar) transport ofPolarons and counterions in terms ofcharacteristic resistances andNernst-Planck equations. Porosityaccounted for by associated distributedcapacitance.

Circuit Details.

.

Bisquert et al. J. Electroanal. Chem.

508 (2001) 59-69

The TL circuit consists of distributedimpedances corresponding to chargetransport along the polymerstrand (Z), ion transport withinthe pore electrolyte (X) and animpedance representing the interfacebetween the strand and the pore (Y).At the simplest level Z correspondsto the electronic resistance RE,

X to the ionic resistance RI, andY to a distributed capacitance CS .


27/42

Doyle, 2011, unpublished work

Hydrous Fe oxideLayer,30 cycles

Un-cycled Fe electrode

(vs Hg/HgO)

Layer growth parameters: Upper, lower potentialsweep limits.

Solution temperature Solution pH Potential sweep rate. Base concentration.

Hydrous Oxide Growth via potential multicyclingprocedure in aqueous alkaline solution.



Lyons, Brandon Phys. Chem. Chem. Phys., 11 (2009) 2203-2217

Tafel slope= 50 mV/dec

Tafel Slope= 40 mV/dec

Tafel slope uncycled electrode = 50 mV/decTafel slope for HO coated electrode ca. 40-45 mV/decindependent of HO thickness.

Note: experiments performed in 0.5 MNaOH on aged Fe electrodes.


28/42

Potential / V vs. Hg/HgO

0.6 0.7 0.8 0.9 1.0 1.1

Log(Current/A)

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

bare

30 cycles

60 cycles

120 cycles

180 cycles240 cycles

300 cycles

Effect of potential cycling on Tafel Plot behaviour (iRu corrected)recorded for hydrous oxide coated Fe electrodes in 0.5 M NaOH.

Potential span of Low Tafel Sloperegion less for multi-cycled oxidecoated electrodes than foruncycled electrode.Tafel slope at low potentialindependent of hydrous oxidethickness once multicycling protocolhas begun (unlike rhodium oxide).Tafel slope (iR corrected) forHydrous oxide coated Fe electrode

less than that recorded foruncoated electrode.

Q /C

0.004 0.006 0.008 0.010 0.012 0.014 0.016 0.018 0.020 0.022

b/mVdec

-1

38

40

42

44

46

48

50

Tafel Slopes vary from ca. 50 mV/decfor un-cycled Fe electrode to ca. 40 mV/decfor Fe electrodes coated with thickHydrous oxide layers (0.1 M NaOH).


29/42

Hydrous Oxide: OER Kinetic parametersLow overpotentialsb = 60 mV dec-1 = 2.303(RT/F)mOH- = 3/2High overpotentials

b = 120 mV dec-1 = 2.303(2RT/F)mOH- = 1.0

Dual Tafel Slope behaviour not due tochange in RDS but arises due to potentialdependence of total fractional coverage Sof electrosorbed reaction intermediates.

P d E 1 30V f 15 f ll d b 1 l b 1 1 5


30/42

Aged passive oxideCoated Fe electrode

Aged electrode, severe cathodicpre-treatment: OER Kinetic parameters

Low overpotentialsb = 60 mV dec-1 = 2.303(RT/F)mOH- = 3/2High overpotentialsb = 120 mV dec-1 = 2.303(2RT/F)mOH- = 1.0

Pre-reduction at E = - 1.30V for t = 15 min, followed by 1 cycle between 1.175And 0.625 V in 1.0 M NaOH at 40 mV/s.


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S + OH- SOH + e- (AI)SOH + OH- SO - + H2O (AII)SO - + OH- SO2H- + e- (AIII)SO2H- + OH- S + H2O + O2 + 2e- (AIV)

S = electrocatalytically active ironsurfaquo group = stabilized Fe(VI) moeity

QSSA, Langmuir Adsorption conditions:

OHOHS

akRTFk

RTFaakFk

i2

01

2

2

0

1

)1(exp

exp4

RTFkkaaFki OHS exp4 010122

Step (AII) RDSLow overpotential

RTFaaFki OHS exp40

1

Step (AI)RDSHigh overpotential

Simple Kinetic analysis (Lyons 1984) predictsCorrect Tafel slopes over entire range, but predicts a reaction order mOH-

of 2 at low (instead of 3/2) and 1 at high .

OER Kinetic Analysis:Model ALyons 1984

Dual Tafel Slope behaviour attributedto change in RDS as potential increases.

OH OH (BI)


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S + OH- S-OH + e- (BI)S-OH + OH- S-H2O2 + e- (BII)S-H2O2 + OH- S-OH2- + H2O (BIII)S-H2O2 + S-HO2- O2 + H2O + OH- (BIV)

S-H2O2 = physisorbed hydrogen peroxide

Assume intermediate surface coverage of S-OH species.Pseudo equilibrium condition & Temkin adsorption isotherm(interaction parameter gj) assumed.

OER Kinetic Analysis:Model BLyons & Brandon 2009

Assume at low step (BII) is rate limiting.

SS

S RT

F

RT

gg

akf

OSHSOH

SOHOH

exp

1

exp220

2

2/1110

8.00

S

SS

RT

F

RT

gakf SOHSOHOH

exp

1exp02

22OSHSOHgg

Assume step (BI) is in pseudo-equilibrium.

FKaRTg

RT

FKa

RT

Fa

k

k

RT

g

RT

g

RT

F

RT

gkf

RT

F

RT

gakf

ff

OHSOH

OHOHSOHSOH

SOH

SOH

SOHSOH

SOHSOHOH

S

SS

S

S

ln

expexpexpexp1

1exp

1exp

expexp1

0

1

0

1

0

11

0

11

11

SRT

FaKkff OHSOH

1exp21022

2/32ln

ln

303.21

303.2log

2/1

2/1

S

S

OH

OH

a

a

fm

FRT

FRT

fb

OH

At high surface coverage conditions

change to where SOH = S

1. Step (BII)still rate determining.Langmuir adsorption pertains. Also gSOH 0.

SRT

Fakff OH

exp022

0.1

2303.2

OHm

F

RTb

OER Ki ti A d (EC l i ti t ) d d F


33/42

Overpotential / V0.2 0.3 0.4 0.5 0.6 0.7

LogCurrentDensityi/Acm

-2

-6

-5

-4

-3

-2

-1

0

FreshAged

b = 39.5 mVdec-1

b = 44 mVdec-1

OER Kinetics: Aged (EC polarization measurements) pre-reduced Feelectrodes (not multicycled).

Overpotential / V (Hg/HgO, 1 M)0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7

LogCurrentDensityi/Acm

-2

-8

-7

-6

-5

-4

-3

-2

-1

0

1st forward

reverse2nd forward

Slope of 40 mVdec-1

Potential E / V (Hg/HgO, 1 M)

-1.4 -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8

CurrentDensityi/Acm-2

-0.002

-0.001

0.000

0.001

LESS AGEDMORE AGED

Peak A4 becomes more enhanced on aging.Compact anhydrous oxide chemistryDominates interfacial EC behaviour.Is associated with increase in low overpotentialTafel slope from ca. 40 mV to ca. 45-47 mV.

Pre-treatment: cathodic polarization at E = - 1.10VIn 1.0 M NaOH, t = 5 min, followed by singleCycle at 40 mV/s between limits 1.175 to 0.625 V.

OER Ki ti A d d d F


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Cyclic voltammograms (1.0 M NaOH, scan rate = 40mV s-1) characterising an Fe electrode prior to its 1st,5th and 16th utilisation in OER polarisationexperiments.

OER steady state polarisation curves for a pre-reducedFe electrode in various NaOH solutions. The tracedenoted as fresher 1.0 M was recorded for the sameelectrode in an earlier experiment, before satisfactoryreproducibility with respect to Tafel slope had becomeestablished.Inset Reaction order plots constructed from thereproducible polarisation data at a potential of E= 0.7V.

[Fe(VI)Om(OH)n]p- + OH-

[Fe(VI)Om+1(OH)n-1]p- + H2O + e

-

[Fe(VI)Om+1(OH)n-1]p- + OH-

[Fe(VI)Om OOH(OH)n-1]p- + e- RDS

[Fe(VI)Om OOH(OH)n-1]p- + OH-

[Fe(VI)Om OO(OH)n-1]p- + H2O + e-[Fe(VI)Om OO(OH)n-1]

p-+ OH-

[Fe(VI)Om(OH)n]p- + O2 + e

-

p = 2m+n-6

Effect of oxide on OER Kinetics at oxidized aged Fe explainedin terms of Conway-Meyer dual barrier model.

b 2.3034RT/5FmOH 1

Physisorbed peroxide intermediate model assumed.

OER Kinetics: Aged pre-reduced Feelectrodes (not multicycled).

Low overpotential


35/42

Conway-Meyer Dual Barrier Model

VM

VF

VS

Barrier Oxide Film

Compact EDL

Solution

F

S

si

FS

Feffi

SF

SF

a

eff

i

m

is

mm

F

RT

F

RT

ib

RT

Fakf

S

SF

F

is

,,

)(303.2303.2

log

exp)(

S

SS

J.J. MacDonald, B.E. Conway,Proc. Roy. Soc., Ser.A, 269 (1962) 419-440.

R.E. Meyer, J. Electrochem. Soc.,107 (1960) 847-853.

Reaction order in absence of barrier

Effective reaction order

Effective symmetryFactor S = (F/(F+S))S

Potential dependent field assisted chargeTransfer across a barrier oxide film(process F) in series with interfacialcharge transfer reaction (process S).Effective Symmetry factor (and henceTafel slope) and reaction order issome fraction F() =F/(F + S)of the true values.

SiSF

FSieffi

Seff

S

SF

FS

mmFm

bFb

F

,,,

1

S

S OH S OH (BI)


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S + OH- S-OH + e- (BI)S-OH + OH- S-H2O2 + e- (BII)S-H2O2 + OH- S-OH2- + H2O (BIII)S-H2O2 + S-HO2- O2 + H2O + OH- (BIV)

Assume step (BII) is rate determining (low values), and thatdual barrier model pertains.Analysis of barrier free situation suggests that bS = 40 mV/dec(2.303(3RT/2F) and mOH = 2.

SS

RT

FKaf SF

F

OH

1exp

2

12

)(

/047.05

4

303.21

303.21303.2log

2/1

,

2/1

S

SF

SF

OH

SF

FOH

SF

FeffOH

SF

SFa

eff

mm

decVF

RT

F

RT

F

RT

ib

21

/235.04

303.2303.2303.2

exp

2/1,1

,

2/1

S

SS

SFOH

SF

SF

F

m

OH

SF

F

effOH

S

F

SF

SF

SFeff

OH

mm

decVF

RTb

F

RT

F

RTb

RT

FKaf

Low overpotentials

Assume step (BI) is rate determining (high values), and that dual barrier model pertains.Analysis of barrier free situation suggests bS = 0.120V /dec (2.303(2RT/F) and mOH = 1.0.

Aged pre-reduced FeelectrodesOER Kinetic analysisIncorporating DBM.

High overpotentials

Not easy to check this prediction at high .


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Bockris, Otagawa, J. Phys. Chem.,87 (1983 )2960

Bockris, Otagawa, J. Electrochem. Soc., 131 (1984) 290.

Hydrous OxideLayer

Compact OxideLayer

Lyons BrandonPhysisorbed peroxideOER model incorporatingDual Barrier concept.

Interlinked surfaquogroups

OER Kinetics: Oxidized metal electrodes: Passive oxide films


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Electrode Experiment, b, mOH- Dual

barrier

b, mOH

for

analysis

Isotherm

L or T

Path-

wayNi no-pre-treat 2RT/3F order not

measurableNo As listed left L, 0 C

Ni pre-reduced 2RT/3F, 1 No As listed left L, 0 C

Ni pre-oxidised 2RT/3F, 1 No As listed left L, 0 C

Co no-pre-treat 2RT/3F(5M)4RT/5F(1M) low [OH-

] ----- ----- -----Co pre-reduced 4RT/5F, 1 Yes 2RT/3F, 2 L, 0 E

Co aged low RT/F, 3/2 No As listed left T, rI

>> rII

E

Co aged high 2RT/F, 1 No As listed left L, 1 E

Fe no-pre-treat 2RT/3F(1M)4RT/5F(5M) high [OH-] ----- ----- -----

Fe pre-reduced(fresh)

2RT/3F order notmeasurable

No ----- ----- -----

Fe pre-reduced(aged)

4RT/5F, 1 Yes 2RT/3F, 2 L, 0 E

Fe aged low RT/F, 3/2 No As listed left T, rI

>> rII

E

Fe aged high 2RT/F, 1 No As listed left L,

1 E

OER Kinetics: Oxidized metal electrodes: Passive oxide films

M. Brandon, Ph.D Thesis University of Dublin, 2008


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Hydrous microdispersed oxides readily prepared via repetitive cyclic potential sweep method

of parent metal (Fe, Co, Ni, Mn, Rh, Ir, Au, Pt, Pd) in aqueous alkaline solution. Method similarto electropolymerization of ECP films.

Charge storage/charge percolation properties of hydrous oxide depend on electrochemicaland environmental variables such as: lower and upper potential sweep limits, potential sweeprate, base concentration, solution temperature, solution pH.

Acid base behaviour of anodically formed transition metal oxides important when consideringmechanism of both redox switching & oxygen evolution

Hydrous oxide material is catalytically active wrt OER.

Hydrous oxides more difficult to reduce than less hydrated compact materials. General mechanistic ideas : hydrous oxide akin to electroactive polymer . Hydrous oxide is

mixed ionic and electronic conductor. Hydrous oxide consists of an open structure derivedfrom extended linkages of active surfaquo groups which are involved in anodic OER process.

Full understanding of OER mechanism on passive oxide coated Fe and hydrous oxide coated Feelectrodes in aqueous alkaline solution now obtained via SS Tafel Plot, open circuit potentialdecay, complex impedance spectroscopy and reaction order measurements.

Main ideas developed in:

M.E.G. Lyons, M.P. Brandon, Phys. Chem. Chem. Phys., 11 (2009) 2203-2217.

M.E.G. Lyons, M.P. Brandon, J. Electroanal. Chem., 631 (2009) 62.

M.E.G. Lyons, M.P. Brandon, J. Electroanal. Chem., 641 (2010) 119.

M.E.G. Lyons, M.P. Brandon, Int. J. Electrochem. Sci., 3 (2008) 1463-1503.

M.E.G. Lyons, L.D. Burke, J. Electroanal. Chem., 198 (1986) 347-368.

M.E.G. Lyons, L.D. Burke, J. Electroanal. Chem., 170 (1984) 377-381


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Dr Richard Doyle, TCD

Dr Michael Brandon, TCDProf. Declan Burke UCC


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Acknowledgements: Funding Science Foundation Ireland (SFI) Principal

Investigator Programme Grant Number SFI/10/IN.1/I2969.

DuPont (Geneva)

Enterprise Ireland

IRCSET

Trinity College Dublin

EU ALFA Programme.MEDIS

EU/LAALFA
http://www.tcd.ie/

Documents

Eirelec 2011- Oxygen Evolution at Oxidized Metal and Metal Oxide Electrodes - Iron in Base