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  • 8/3/2019 Hydrous Oxide Modified Electrodes for Electrochemical Water Splitting TCD Chemistry Seminar January 2012

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    Hydrous oxide modified electrodesfor electrochemical water splitting

    e yonsTrinity Electrochemical Energy Conversion & Electrocatalysis Group

    School of ChemistryTrinity College

    Dublin 2

    IrelandSchool of ChemistryTrinity College Dublin19 January 2012

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    Plus a change, plus c'est la mmechose.Alphonse KARR, Les Gupes 1849.

    (The more things change, the more

    they remain the same. ...)

    There and back again.Bilbo Ba ins

    UCC 1980 Eirelec 2011, Adare

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    Lecture Outline Electrochemical fundamentals: electrolysis

    cells and fuel cells Hydrous oxide formation via CyclicPotential Multicycling (CPM)

    oxide modified electrodes Kinetics and mechanism of anodic oxygen

    reduction reaction at hydrous Fe and Nioxides

    Conclusions.

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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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    Water Electrolysis : electrochemical substance production

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    Overpotential lossesincreasenet electrical energy

    needed asinput to drivereactions atelectrodes.

    Self-driving electrolysis cell

    ( ) ,e cell C A E i E IR = + + +

    Thermodynamics(Nernst Potential) Kinetics:Cathode reaction

    Overpotential

    Kinetics: Anodic reaction overpotential

    Ohmic potential: Cell design

    Need to minimizeall overpotentiallosses to makeapplied potentialas close to Nernstpotential as possible.

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    Ballard PEM FuelCell

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    Overpotential losses reduce net voltage output.

    Self-driving Fuel Cell

    ( )ii E P =

    ( ) ,e cell C A E i E IR =

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    The bottom Line Water electrolysis device:

    Very sluggish Oxygen EvolutionReaction (OER) kinetics (v. highoverpotential) limit deviceoperational effectiveness (higherelectrical energy input required)

    Fuel Cell:

    Metal oxide materials exhibit usefulpotential as catalysts for OER and ORR inelectrochemical energy conversion devices.

    Major aim of Science Foundation Ireland(SFI) Principal Investigator ProgrammeGrant Number SFI/10/IN.1/I2969 is todevelop cheap and efficient oxide electrodematerials for use in water electrolysis andfuel cells.

    Very sluggish Oxygen ReductionReaction (ORR) kinetics limitvoltage output of device

    For both device types thecathodic Hydrogen EvolutionReaction (HER) or the anodicHydrogen Oxidation Reaction(HOR) are reasonablekinetically facile.

    SFI Programme has major focus on

    electrochemically generated hydroustransition metal oxides.

    Examine demanding multistep, multielectrontransfer reactions such as:

    Anodic oxygen evolution reaction (OER) Cathodic oxygen reduction reaction

    (ORR)

    Cathodic hydrogen evolution reaction(HER) Examination of kinetics and mechanism of

    ORR and HER at TM oxide electrodes inaqueous acid/base is very unexplored.

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    Transition Metal Oxides2 types:Compact anhydrous oxides ,e.g. rutile, perovskite, spinel.

    Oxygen present only as bridgingspecies between two metal cationsand ideal crystals constitutetightly packed giant molecules.Prepared via thermal techniques,

    Micro-dispersed hydrousoxides

    Oxygen is present not just as abridging species between metalions, but also as O -, OH and OH 2species in coordinated terminalgroup form.Hydrous oxides in contact withaqueous media contain large

    .

    trapped water plus electrolytespecies.Prepared via base precipitation,electrochemical techniques.Materials are prepared inkinetically most accessible ratherthan thermodynamically moststable form.Are often amorphous or only poorlycrystalline and prone torearrangement.

    L. D. Burke, M.E.G. Lyons,Modern Aspects Electrochemistry, 18 (1986)169-248.

    Geothite FeOOH

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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

    A0: OER

    Surface redox chemistry: Bright Fe electrode

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

    A4

    In situ RamanEQCM

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

    A3/C 2

    [Fe(OH)3.5 nH2O]0.5- (Na+)0.5 + e- Fe(OH)2n H2O + 0.5 Na+ + 1.5OH-

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

    C1

    C0: HERGreater fine structureobserved at low sweeprate.

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    e n

    t / A

    -1.0e-4

    0.0

    1.0e-4

    2.0e-4

    3.0e-4

    A 2A 3 A 4

    OER

    CV response Bright Fe electrode 1.0 M NaOH

    Potential / V vs. Hg/HgO

    -1.5 -1.0 -0.5 0.0 0.5 1.0

    C u r r

    -5.0e-4

    -4.0e-4

    -3.0e-4

    -2.0e-4

    C 1

    2

    HER

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    I / A

    0.0005

    0.0010

    0.0015

    0.0020

    u r r e n

    t / A

    0.000

    0.002

    0.004

    0.006

    0.008

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

    LPL

    UPL UPL

    LPL

    E / V (vs Hg/HgO)

    -1.2 -1.0 -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6 0.8-0.0015

    -0.0010

    -0.0005

    .

    Potential / V vs. Hg/HgO

    -1.5 -1.0 -0.5 0.0 0.5 1.0

    C

    -0.008

    -0.006

    -0.004

    -0.002

    (a) Cyclic voltammograms recorded in real timeduring the growth of the hydrous oxide layer on (a)Fe electrodes and (b) Ni electrodes in 1.0 M NaOH.

    CPM procedure optimized wrt upper potential limit, lower potential limit, sweep rate.

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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

    Hydrous oxide film regarded as a surfacebonded polynuclear species. Metal cationsin polymeric network held together bysequence of oxy and hydroxy bridges.Mixed conduction (electronic, ionic) behaviour

    C20.5 M NaOH

    Lyons, Burke, J. Electroanal. Chem., 170 (1984) 377-381Lyons, Burke, J. Electroanal. Chem., 198 (1986) 347-368Lyons, Brandon Phys. Chem. Chem. Phys., 11 (2009) 2203-2217Lyons, Doyle, Brandon, Phys. Chem. Chem. Phys., 2011DOI: 10.1039/c1cp22470k

    similar to that exhibited byPolymer Modified Electrodes.Can regard microdispersed hydrous oxide layer asopen porous mesh of interconnected surfaquometal oxy groups.

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    c m

    - 2 80

    100

    120

    Q=a(1-exp(-bN))Hydrous oxide growth kinetics

    / C 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

    N

    0 200 400 600

    Q / m

    C

    0

    20

    40

    60

    R=0.9947, R2 = 0.9895a = 103.94 6.05 mC/cm 2b = 0.0044 0.0006 cycle -1

    Number of Growth Cycles

    0 100 200 300 400 500

    C h a r g e

    0.000

    0.002

    0.004

    0.006

    0.008

    Lyons, Doyle, Brandon, PCCP 2011,

    CPM Methodology reproducible across space and time.

    R = 0.9935, R 2 = 0.9870a = 0.0136 0.0003 Cb = 0.0156 0.0011 cycle -1

    CPM methodology is scalable : oxide growth process does not dependon electrode size. Important for commercial viability.

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    C u r r e n

    t ( A )

    0.000

    0.002

    0.004

    0.006

    0.008

    0.010

    0 cycles75 cycles

    150 cycles

    Q / C

    0.0006

    0.0008

    0.0010

    0.0012

    0.0014

    0.0016

    Hydrous oxide growth via CPM : Multicycled Ni electrode, 1.0 M NaOH.

    Potential (V)

    -1.5 -1.0 -0.5 0.0 0.5 1.0-0.006

    -0.004

    -0.002

    N

    0 100 200 300 400 5000.0000

    0.0002

    0.0004

    Q versus NExperimental data95% Confidence Band95% Prediction Band

    R = 0.9869, R2 = 0.9739a = 0.00147.43x10 -5Cb=0.0450.0004 cycle -1

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    2 A

    2 A

    -Ni(OH) 2 -Ni(OH) 2

    -NiOOH

    Oxidation state2.0 - 2.2

    Oxidation state2.0 - 2.2

    Oxidation state

    2.7 - 3.0

    Oxidation state3.5 - 3.67

    o x

    i d a

    t i o n

    r e

    d u c

    t i o n

    Ageing

    Charge

    Charge

    Discharge

    Discharge

    Overcharge

    A2

    C2

    A2 C2

    pH > 14

    - NiOOH

    Uncycled Ni electrode 1M base, 50 mV/s ()/() ; ()/()

    2 A

    2C 2C

    2 2C

    Ni + OH-

    NiOH(ads.) + e-

    NiOH(ads.) + OH- Ni(OH)2 + e-A1

    Ni + 2.4 OH- [Ni(OH)2.4]0.4

    NiO + H2O + 2e - Ni + 2 OH-C1

    Ni(OH)2 NiO + H2O

    Peaks A1/C1 observed only if upper limitOf sweep does not exceed 0.20 V (Hg/HgO)

    Multicycled (N=300)Ni electrode, 1MBase, 50 mV/s

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

    M(IV)

    M(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 balancenegative charge on polymer chain.E uivalent circuit model: dual multi- rail

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

    M = Ir, Rh

    dE/d H = -3 2

    Rhodium oxide

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

    [Fe2(OH)6(OH2)3]2- + 3OH-[Fe2O3(OH)3(OH2)3]3- + 3H2O + 2e -

    Fe(II)

    Fe(III)

    Transmission Line as done for ECP films..

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

    D.P. Whelan J. Electroanal. Chem.,122 (1981) 403.

    = 2.303RT/F= -88.5 mV/decT = 298K

    Iron oxide

    Fe(III) Oxidized form yellow-green

    Fe(II) Reduced form transparent

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    i a l / V v s .

    H g

    / H g

    O

    -0.2

    0.0

    0.2

    Variation of hydrous Fe oxide peak A 3 potential with solution pH.

    Super-Nernstian shift

    Doyle, unpublished data Nov. 2011

    u r r e n

    t / A

    -0.001

    0.000

    0.001

    0.002

    O2

    evolutionA3

    pH

    8 9 10 11 12 13 14

    P e a

    k P o t e n

    -0.8

    -0.6

    -0.4

    Experiment 1 (120 cycles)Experiment 2 (120 cycles)Experiment 3 (120 cycles)Slope = 0.10 V/pH unit

    Potential / V vs. Hg/HgO

    -1.5 -1.0 -0.5 0.0 0.5 1.0

    C

    -0.004

    -0.003

    -0.002

    pH 14.0pH 11.5pH 9.0H2 evolution

    C2

    [Fe2(OH)6(OH2)3]n2- + 3nOH- [Fe2O3(OH)3(OH2)3]n3- +3nH2O + 2ne-A3/C2

    2.303( ) 0.059( ) /

    dE RT r q r q V pH

    dpH F = =

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    u r r e n

    t \ A

    0.002

    0.004

    0.006

    0.008pH 14.0pH 13.0pH 11.0pH 9.0

    H g

    / H g

    O )

    0.8

    1.0

    1.2

    1.4

    A2C2

    A2

    Variation of Ni oxide A 2,C2 and C2 peak potentials with solution pH.

    C2: dE/dpH = - 50 mV/dec

    C2

    : dE/dpH = - 74 mV/decA2: dE/dpH = - 97 mV/dec

    Ian Godwin, unpublished results, Dec 2011

    Potential / V

    -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5

    -0.004

    -0.002

    0.000

    pH

    8 10 12 14

    E p

    / V

    0.0

    0.2

    0.4

    .

    C2

    C2C2

    [Ni(OH)3.5(OH2)n]0.5- (Na+)0.5 +e-Ni(OH)2(OH)2)n + 0.5 Na+ + 1.5 OH-

    A2/C2

    (dE/dpH) ave = - 0.085 V

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    Bode Scheme of Squares: Ni Oxide Redox Chemistry

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    Au3+ + q OH- Au(OH)q(q-3)-

    ( )

    3

    3( ) q

    q Au OH

    q Au OH

    aK a a+

    =

    Au3+ + 3e- Au 30 ln3 Au RT

    E E aF += +

    Reduction of hydrous layer on AudE/dpH = - 0.090 V/pH unit

    Metal cations in hydrous film co-ordinate with q OH - ions

    ( 3)( )0 2.303ln

    3 3q

    q Au OH

    q qW

    a RT RT q E E pH

    F K K F

    = +

    ( 3)

    3

    ( ) qq Au OH

    q AuOH

    aa

    Ka

    +

    =

    Analysis of reduction potential of hydrous gold oxideWith change in solution pH.

    Au 2O3(OH) 3(OH 2)33-

    Au

    HO

    O -HO

    OH 2

    Au

    O-

    OH

    O-

    H2O

    H2O

    3-

    .0.059

    3 3V

    dpH F = =

    exp( ) 0.089dE dpH V

    { }0.06 3 0.090.09

    3 4.50.06

    q

    q

    =

    1.54.5( ) Au OH

    32 9( ) Au OH

    [Au2O3(OH)3(OH2)3]3- .

    Assume molecular complex present inHydrous Au oxide layer is:

    Oxidized form hydrous gold oxide

    or

    or

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    MO -

    OH2

    OH

    O -HOM

    OH2

    OH2

    OH

    O -M

    O

    OH 2

    OH

    OHOM

    OH2

    OH2

    OH

    OH

    H

    H

    +3nH 2O, 2ne-

    -3nOH -

    3-

    n

    2-

    n

    M2O3(OH) 3(OH 2)33-

    nM2(OH) 6(OH 2)3

    2-

    n

    M = Fe, Ni

    MII (OH)r (r-3)- + e- MIII (OH)q(q-2)- + (3/2) OH -

    r-q 1.5dE/dpH = - 0.090 V = - 0.06 (r-q) V

    [M2(OH)6(OH2)3]n2-

    + 3nOH-

    [M2O3(OH)3(OH2)3]n3- +3nH2O + 2ne-

    Redox switching in hydrous oxide layer involves a rapid topotactic reactioninvolving hydroxide ion ingress and solvent egressat the oxide/solution interface, and electron injection at themetal/oxide interface.Also involves motion of charge compensating cationsthrough film.

    Assume that oxidized form of hydrous metal oxide has same compositionas oxidized form of hydrous gold oxide.

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    Metal oxide wire pH sensor spinoff.

    Super-Nernstian pH shift signifiespossible development of more sensitiveand scalable metal oxide wire pHsensors.

    pH

    E Nernstian

    Super-Nernstian

    Enhanced sensitivity of sensorto given pH change.

    Sensor probe can be made very smallfor biomedical applications.

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    Kinetically limiting step in waterelectrolysis cells and PEM fuel cell.Multistep multi-electron transfer reactioninvolving adsorbed intermediates.Overall reaction (alkaline medium)

    O2 + 2H2O + 4e- 4OH-

    E 0

    = 0.303 V (vs. Hg/HgO)

    Krasilshchikov (1963)S + OH- SOHad + e-

    OH + OH- O- + H O

    Depending on RDS can explain a variety ofTafel slopes.Modification permits concept offormation/decomposition of higher oxide e.g.for Ni

    OH-

    OHad+ e-

    OHad+ OH- O-ad+ H2O2 -NiOOH + O-ad 2NiO2+ H2O + e-

    2NiO2 + H2O 2 -NiOOH + OadOad+ Oad O2

    Anodic Oxygen Evolution Reaction (OER)

    a a

    SO-ad SOad + e

    -

    2SO ad 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 alkaline

    solution.

    OER at oxidized metal and metal oxideelectrodes involves active participation ofoxide.Acid/base behaviour of oxide importantconsideration .Concept of active surface or surfaquo groupsimportant.

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    -0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

    T a

    f e l s

    l o p e

    / m

    V d e c -

    1

    35

    40

    45

    50

    55

    60

    65

    New Fe electrode'Aged' Fe electrode

    0.6 0.7 0.8 0.9 1.0 1.1

    L o g

    ( C u r r e n

    t / A )

    -5.5

    -5.0

    -4.5

    -4.0

    -3.5

    -3.0

    -2.5

    -2.0

    -1.5

    Uncycled30 cycles60 cycles120 cycles180 cycles

    240 cycles300 cycles

    Anodic OER, Fe aqueous alkaline solution.

    Tafel Plots

    arge cm

    Charge Q / C cm -20.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

    C u r r e n

    t D e n s

    i t y

    i / A c m

    - 2

    0.060

    0.065

    0.070

    0.075

    0.080

    0.085

    0.090

    0.85 V

    Potential / V vs. Hg/HgO

    Charge Q / C cm -20.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09

    P o

    t e n

    t i a l / V v s .

    H g

    / H g

    O

    0.695

    0.700

    0.705

    0.710

    0.715

    0.720

    0.725

    1 mA cm -2

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    Hydrous Iron Oxide Electrodes.OER Reaction Order Studies

    L o g

    ( i /

    A c m

    - 2 )

    -4

    -3

    -2

    -1

    0

    0.1 M0.5 M1.0 M2.0 M5.0 M

    60 mV dec -1

    N = 120 cyclesReaction order wrt OH - activityca. 0.9 (low TS region) and ca. 0.8

    (high TS region).

    Potential / V vs. Hg/HgO

    0.5 0.6 0.7 0.8 0.9 1.0 1.1-5

    120 mV dec -

    Log (a OH-)

    -1.5 -1.0 -0.5 0.0 0.5 1.0

    L o g

    ( C u r r e n

    t D e n s i

    t y i / A c m

    - 2 )

    -4.0

    -3.5

    -3.0

    -2.5

    -2.0

    -1.5

    -1.0

    -0.5

    60 mV dec -1 regionSlope = 0.87120 mV dec -1 regionSlope = 0.81

    Measure OER current densityat fixed overpotential fromanalysis of Tafel Plots as function ofOH- ion activity.

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    0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

    l o g

    ( I / A )

    -5

    -4

    -3

    -2

    -1

    0.1M0.25M0.75M1M1.5M2M2.5M3M4M5M

    I / A

    -0.002

    0.000

    0.002

    0.004

    0.006

    0.1M0.25M0.5M0.75M1M1.5M2M2.5M3M4M

    N = 120 cycles

    N = 120 cycles

    Low potential : TS = 60 mV/dec.High potential : TS = 120 mV/decNi in aqueous base: redox activity

    and OER behaviour.

    Tafel Plots for OER at Ni oxide layers grown via potential cycling(N = 120 cycles) in 1.0 M NaOH recorded as function of baseconcentration.

    E/V

    -0.6 -0.4 -0.2 0.0 0.2 0.4 0.6

    E=0.64V

    = 0.337VRegressionConfidence

    c = -3.0513507838m = 0.8526502073r = 0.96614858

    Ni oxide layer grown in 1.0 MNaOH. N = 120 cycles.Reaction order plot, low TafelSlope Region.mOH- = 0.85.

    E=0.71V

    = 0.407VRegressionConfidence

    c = -2.1450473735m = 0.8182632348r = 0.973540901

    Ni oxide layer grown in 1.0 MNaOH. N = 120 cycles.Reaction order plot, high Tafel

    Slope Region.mOH- = 0.82.

    Voltammetric response of hydrousNi oxide film as function of baseConcentration.

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    Charge (C)

    0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012

    T a

    f e l S l o p e

    ( V )

    0.055

    0.060

    0.065

    0.070

    0.075

    0.080

    0.085

    Slope: 0.055041Intercept 22.607r : 0.92

    0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1

    l o g

    ( I / A )

    -5.0

    -4.5

    -4.0

    -3.5

    -3.0

    -2.5

    -2.0

    -1.5

    -1.0

    Uncycled30 Cycles60 Cycles120 Cycles180 Cycles240 Cycles

    Effect of oxide charge capacity Qon OER catalytic efficiency: Ni inaqueous base.

    Variation of low overpotential Tafel Slope for OERat multicycled Ni oxide Electrode in 1.0 M NaOHas a function of oxide charge capacity Q (thickness).

    Ian Godwin, IJES 2012, in press

    Tafel Plot OER as function of hydrouslayer thickness (# cycles).Ni oxide electrode, 1M NaOH.

    Low overpotential Tafel Slopeincreases in a linear manner with increasinghydrous oxide charge capacity.

    =0.337V=0.347V=0.357V

    Oxygen evolution rate at fixed potential at oxide coated Ni in 1.0 MNaOH as function of redox charge storage capacity of hydrous layer.

    Maria OBrien, Lisa Russell, IJES 2012, in press

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    e / m

    V d e c - 1

    60

    70

    80

    90

    Charge (Q) / C cm -20.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14

    T a

    f e l S l o p

    30

    40

    50

    NickelIron'Aged' Iron

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    Assuming Langmuir adsorption conditions with step II rate determining:

    [Fe(VI)Om(OH)n(OH2)y]p- + OH- [Fe(VI)Om(OH)n(OH*)(OH2)y-1]p-

    + H2O + e -

    Kinetic parameters:

    Low overpotentials Tafel slope of 60 mV dec -1 decreasing to 40 mV dec -1 for very thickfilms (Fe). Tafel slope increases from ca. 55 mV dec -1 to ca. 80 mV dec -1 for very thick films(Ni).

    High overpotentials Tafel slope of 120 mV dec -1 (Multicycled Fe & Ni electrodes)

    Reaction order m OH- 1 for both Tafel regions (Multicycled Fe & Ni electrodes)

    Proposed OER Mechanism

    Net reaction flux is given by,

    Applying a quasi steady state approximation to SOH we get,

    Now assuming the heterogeneous electrochemical rate constants obey the Butler-Volmerequation:

    [Fe(VI)Om(OH)n(OH*)(OH2)y-1]p- [Fe(VI)Om(OH)n+1(OH2)y-1]p-

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    Assuming Langmuir adsorption conditions with step II rate determining:

    [Fe(VI)Om(OH)n(OH2)y]p- + OH- [Fe(VI)Om(OH)n(OH*)(OH2)y-1]p-

    + H2O + e -

    Kinetic parameters:

    Low overpotentials Tafel slope of 60 mV dec -1 decreasing to 40 mV dec -1 for very thickfilms.

    High overpotentials Tafel slope of 120 mV dec -1

    Reaction order m OH

    - 1 for both Tafel regions

    Proposed OER Mechanism

    At low overpotentials: k2

    0 > k-10

    [ ]0

    1 expS OH f k F RT ( )1

    2.303 2 120b RT F mVdec

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    Step I: RDS at high overpotentials, k 20 >> k-10

    Step II: RDS at low overpotentials, k 20

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    Tafel Plots OER Multicycled Hydrous Oxide coated Ni and Fe

    Electrodes (N = 120 cycles), 1.0 M Base, 298 K.

    d e n s

    i t y

    / A c m

    - 2 )

    -2

    -1

    0

    Nickel (120 growth cycles)

    Iron (120 growth cycles)

    Ni has reduced overpotential for OER onsetcompared with Fe.

    Overpotential ( )

    0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8

    L o g

    ( C u r r e n

    t

    -5

    -4

    -3

    [OH-] = 1.0 M

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    ( I / Q )

    -3

    -2

    -1

    0

    1

    Normalised (wrt oxide charge capacity)Tafel Plots OER at thermallyprepared and multicycledHydrous metal oxide electrodes, 1.0 M NaOH, 298 K.

    Overpotential ( ) vs. Hg/HgO

    0.0 0.2 0.4 0.6 0.8

    L o g

    -7

    -6

    -5

    -4RuO 2IrO2Iron (120 cycles)Nickel (120 cycles)

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    Tafel Plots OER Oxidized Ni and FeElectrodes (not multicycled), 1.0 M Base, 298 K.

    Lyons, Brandon Int. J. Electrochem. Sci., 3 (2008) 1463-1503.

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    Tafel Plots OER Oxidized Ni and FeElectrodes (not multicycled), 1.0 M Base, 298 K.

    Lyons, Brandon Int. J. Electrochem. Sci., 3 (2008) 1463-1503.

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    v s .

    H g

    / H g

    O

    0.9

    1.0

    1.1

    1.2

    1.3

    Exp 1Exp 2

    Slope = 92 mV/pH95% Confidence interval

    OER onset potential as function of solution pH, multicycled hydrousIron oxide coated electrode, N = 120 cycles.

    pH

    7 8 9 10 11 12 13 14 15

    P o

    t e n

    t i a l /

    0.5

    0.6

    0.7

    0.8

    Dr Richard Doyle, unpublished results, Dec. 2011

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    p o

    t e n

    t i a

    l / V

    0.9

    1.0

    1.1

    1.2

    1.3Intercept: 2.37Slope: -0.127r : 0.99

    OER onset potential as function of solution pH, multicycled hydrousNi oxide coated electrode, N = 120 cycles.

    0.127 / OERdE V decdpH

    =

    pH

    8 9 10 11 12 13 14 15

    O E R o n s e

    t

    0.5

    0.6

    0.7

    0.8

    Hydrous Ni oxide, N=120 cycles

    Ian Godwin, unpublished results, Dec. 2011

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    t i a l / V v s . H

    g / H g

    O

    -

    0.0

    0.5

    1.0

    1.5

    OER onset (92 mV/pH)

    A3 (86 mV/pH)

    OER onset potential variationexhibits similar trend wrt pHchange as variation of redox

    potential of main chargestorage peaks.Hence the overpotential foronset of oxygen evolutionon both Fe and Ni oxides

    OER onset potential depends on acid/base properties of hydrous oxide layer

    pH

    7 8 9 10 11 12 13 14 15

    P o t e n

    -1.5

    -1.0

    .

    C1 (57 mV/pH)

    is influenced by the acid/baseproperties of oxide.

    L.D.Burke, T.A.M. Twomey, J. Electroanal. Chem., 167 (1984) 285.

    Dr Richard Doyle, unpublished work Dec. 2011.

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    r r e n

    t / A

    0.0

    2.0e-4

    4.0e-4

    6.0e-4

    u r r e n

    t / A

    1.0e-5

    3.0e-5

    5.0e-5

    Ring

    RRDE Collector/Generator Experiment (known fraction of cathodically generated peroxide generated atDisc electrode anodically detected at ring electrode :Electrochemical oxygen reduction at multicycled (N = 60 cycles) hydrous ironOxy-hydroxide electrode, 1.0 M NaOH.

    Ering = 0.45 V

    Potential / V vs. Hg/HgO

    -1.0 -0.8 -0.6 -0.4 -0.2 0.0

    D i s k

    C

    -6.0e-4

    -4.0e-4

    -2.0e-4 R i n g

    C

    -5.0e-5

    -3.0e-5

    -1.0e-5

    500 rpm1000 rpm2000 rpm

    Disk

    Direct 4e - Pathway:O2 + 2H2O + 4e- 4 OH- E0 = 0.401 V (vs SHE)

    Peroxide Pathway:O2 + H2O + 2e- HO2- + OH- E0 = - 0.427 V (vs SHE)HO

    2- + H

    2O + 2e- 3OH- E0 = 0.942 V (vs SHE)

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    Concluding Comments Reproducible and scalable methodology developed for generation of hydrated Fe

    & Ni metal oxide thin films in aqueous base. Duplex layer model proposed for structure of oxide/solution interface region. Redox switching characteristics & electro-catalytric kinetics and mechanism

    with respect to anodic OER at Ni and Fe electrodes in aqueous base evaluatedand quantified. Novel anodic water splitting OER mechanism proposed involving surfaquo groups

    in hydrous oxide layer. OER onset potential depends on acid/base properties of.

    Hydrous oxide thin films exhibit super-Nernstian shifts in redox potential withrespect to changes in solution pH value. Implying commercial spinoff potentialfor new generation metal wire pH sensors for use in biomedical applications.

    Fe and Ni oxide materials are cheap and effective electrode materials for anodicwater splitting.

    Next stage is to examine application of these oxide materials to CathodicOxygen Reduction Reaction (ORR) and hydrogen evolution reaction (HER). Thelatter topics are currently unexplored at hydrous oxide materials.

    6 papers published in year 1 of project.

    The more things change the more they remain the same

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    The more things change the more they remain the same.

    L.D. Burke Physical Electrochemistry Group UCC 1980

    T i i El h i l E C i &

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    Trinity Electrochemical Energy Conversion &

    Electrocatalysis (TEECE) GroupCurrent Group Personnel:

    PI: Prof. Mike LyonsPDRF: Dr Richard DoylePG: Mr Ian GodwinUG Interns: Ms Maria OBrien, Ms Lisa Russell

    Group Alumni (Energy Conversion/storage):Dr Gareth KeeleyDr Michael Brandon

    Collaborators:Prof. Declan Burke, UCC (Passed awa December2011)

    Dr Michael Brandon, QUBDr Serge Rebouillat, DuPont GenevaDr Chris Bell, IC LondonDr Danny OHare, IC LondonProf. Richard Compton, PCL Oxford University

    TEECE Group funded by Science Foundation Ireland (SFI)Principal Investigator Programme .Grant Number SFI/10/IN.1/I2969.Title: Redox and catalytic properties of hydrated metal oxideelectrodes for use in energy conversion and storage devices,2011-2016.