Eirelec 2011- Oxygen Evolution at Oxidized Metal and Metal Oxide Electrodes - Iron in Base
Transcript of 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
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, Burke, J. Electroanal. Chem., 198 (1986) 347-368
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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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.
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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.
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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.
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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.
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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.
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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 .
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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, Burke, J. Electroanal. Chem., 170 (1984) 377-381
Lyons, Burke, J. Electroanal. Chem., 198 (1986) 347-368
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.
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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).
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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
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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
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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
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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/