Hydrous Oxide Modified Electrodes for Electrohemical Water Splitting Tarragona 24 July 2012
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Transcript of Hydrous Oxide Modified Electrodes for Electrohemical Water Splitting Tarragona 24 July 2012
Hydrous oxide modified electrodes for electrochemical water splitting
Mike Lyons Trinity Electrochemical Energy Conversion & Electrocatalysis (TEECE) Group,
School of Chemistry & CRANN Trinity College Dublin
Ireland [email protected]
Seminar, 24th July 2012,
Institute of Chemical Research of Catalonia (ICIQ),
Campus Universitari de Tarragona.
Trinity College School of Chemistry
Lecture Outline
• Electrochemical fundamentals: electrolysis cells and fuel cells
• Hydrous oxide formation via Cyclic Potential Multicycling (CPM)
• Redox Switching behaviour of hydrous Fe and Ni oxide modified electrodes
• Potential of Metal Oxide electrode as pH sensor
• Kinetics and mechanism of anodic oxygen evolution reaction at – hydrous oxide coated Fe & Ni electrodes (subjected to CPM)
– Electro-precipitated Nickel oxy-hydroxide thin films deposited on Au support electrodes
– Comparision with other metal oxide systems
• Conclusions.
Electrode Electrolyte
Electronically conducting phase : metal, semiconductor, conducting polymer material etc.
Ionically conducting medium : electrolyte solution, molten salt, solid electrolyte, polymeric electrolyte, etc.
Conduction occurs via migration of electrons . Solid state Physics : energy band theory. Material transport occurs
via migration, diffusion and convection
HIET
Electrochemical Science underpinned by interaction of electrons with matter and electrochemical technologies based on consequences of Heterogeneous interfacial electron transfer (HIET).
Electrochemical Science : the essentials
The Hydrogen Economy: Hydrogen as an energy carrier.
G.W. Crabtree, M.S. Dresselhaus, M.V. Buchanan, ‘The hydrogen Economy’ Physics Today, Dec.2004, pp.39-45. U. Bossel, ‘Does a hydrogen economy make sense?’ 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.
,e cell C AE i E IR
Overpotential losses increase net electrical energy needed as input to drive reactions at electrodes.
Electrolysis cell: electrochemical substance producer
Thermodynamics (Nernst Potential)
Kinetics: Cathode reaction Overpotential
Kinetics: Anodic reaction overpotential
Ohmic potential: Cell design
Need to minimize all overpotential losses to make applied potential as close to Nernst potential as possible.
Electrical energy produces chemicals.
Ballard PEM Fuel Cell
Fuel Cell : Electrochemical Energy Producer
,e cell C AE i E IR
Overpotential losses reduce net voltage output.
The bottom line • Water electrolysis device:
– Very sluggish Oxygen Evolution Reaction (OER) kinetics (v. high overpotential) limit device operational effectiveness (higher electrical energy input required)
• Fuel Cell: – Very sluggish Oxygen Reduction
Reaction (ORR) kinetics limit voltage output of device
• For both device types the cathodic Hydrogen Evolution Reaction (HER) or the anodic Hydrogen Oxidation Reaction (HOR) are reasonable kinetically facile.
• Optimizing the kinetics of the oxygen electrode and understanding the mechanism of the oxygen electrode reaction presents a grand challenge and has direct implications for photo-electrochemical splitting of water.
• Metal oxide materials exhibit useful potential as catalysts for OER and ORR in electrochemical energy conversion devices.
• Major aim of Science Foundation Ireland (SFI) Principal Investigator Programme Grant Number SFI/10/IN.1/I2969 is to develop cheap and efficient oxide electrode materials for use in water electrolysis and fuel cells.
• SFI Programme has major focus on:
• Electrochemically generated hydrous transition metal oxides (TMO)
• Electro-precipitated TMO
• Thermally prepared TMO
• Nanostructured TMO.
• Examine demanding multistep, multielectron transfer reactions such as:
• Anodic oxygen evolution reaction (OER)
• Cathodic oxygen reduction reaction (ORR)
• Cathodic hydrogen evolution reaction (HER)
Transition Metal Oxides 2 types:
Compact anhydrous oxides, e.g. rutile, perovskite, spinel. Oxygen present only as bridging
species between two metal cations and ideal crystals constitute tightly packed giant molecules.
Prepared via thermal techniques, e.g decomposition of unstable salt
Micro-dispersed hydrous oxides Oxygen is present not just as a
bridging species between metal ions, but also as O-, OH and OH2 species in coordinated terminal group form.
Hydrous oxides in contact with aqueous media contain large quantities of loosely bound and trapped water plus electrolyte species.
Prepared via base precipitation, electrochemical techniques.
Materials are prepared in kinetically most accessible rather than thermodynamically most stable form.
Are often amorphous or only poorly crystalline and prone to rearrangement.
L. D. Burke, M.E.G. Lyons, Modern Aspects Electrochemistry, 18 (1986)169-248.
Geothite FeOOH
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)6·3H2O]2- + 3OH- → [Fe2(OH)9]3- +3H2O + 2e-
A3/C2
[Fe(OH)3.5 •nH2O]0.5- •(Na+)0.5 + e- Fe(OH)2•n H2O + 0.5 Na+ + 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 EQCM RRDE
Greater fine structure observed at low sweep rate.
2A
2C 2C
-Ni(OH)2-Ni(OH)2
-NiOOH
Oxidation state 2.0 - 2.2
Oxidation state 2.0 - 2.2
Oxidation state 2.7 - 3.0
Oxidation state 3.5 - 3.67
oxid
ati
on
red
uct
ion
Ageing
Charge
Charge
Discharge
Discharge
Overcharge
A2’’
C2’’
A2’ C2’
pH > 14
- NiOOH
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 limit Of sweep does not exceed – 0.20 V (Hg/HgO)
Uncycled Ni electrode 1M base, 50 mV/s
Multicycled (N=300) Ni electrode, 1M Base, 50 mV/s
II/III ; II/III
2A
2A
2C 2C
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
Hydrous oxide film regarded as a surface bonded polynuclear species. Metal cations in polymeric network held together by sequence of oxy and hydroxy bridges. Mixed conduction (electronic, ionic) behaviour similar to that exhibited by Polymer Modified Electrodes. Can regard microdispersed hydrous oxide layer as open porous mesh of interconnected surfaquo metal oxy groups.
Number of Growth Cycles
0 100 200 300 400 500
Cha
rge
/ C
0.000
0.002
0.004
0.006
0.008
0.010
0.012
0.014
0.016
R = 0.9935, R2 = 0.9870 a = 0.0136 ± 0.0003 C b = 0.0156 ± 0.0011 cycle-1
Q=a(1-exp(-bN))
CPM methodology is scalable. Hydrous oxide growth process does not depend on electrode size. Important for commercial viability.
Potential (V)
-1.5 -1.0 -0.5 0.0 0.5 1.0
Curr
ent
(A)
-0.006
-0.004
-0.002
0.000
0.002
0.004
0.006
0.008
0.010
0 cycles
75 cycles
150 cycles
R = 0.9869, R2 = 0.9739 a = 0.0014±7.43x10-5C b=0.045±0.0004 cycle-1
Hydrous oxide growth via CPM : Multicycled Ni electrode, 1.0 M NaOH.
N
0 100 200 300 400 500
Q/C
0.0000
0.0002
0.0004
0.0006
0.0008
0.0010
0.0012
0.0014
0.0016
Q versus N
Experimental data
95% Confidence Band
95% Prediction Band
Ni(II) Ni(III)
Ni(0) Ni(II)
Ni(III) Ni(II)
Q=a(1-exp(-bN))
Typical voltammetric response recorded for a hydrous Ni(OH)2 thin film growth on Ni support electrode grown in aqueous 1.0 M NaOH for N = 30 cycles at 150 mV/s between limits -1.45-0.65 V vs Hg/HgO.
[Fe2(OH)6(OH2)3]2- + 3OH-
[Fe2O3(OH)3(OH2)3]3- + 3H2O + 2e-
Fe(II)
Fe(III)
Redox switching involves topotactic charge storage reactions in open hydrous oxide layer which Behaves as ion exchange membrane. Hydrated counter/co-ions (M+, H+, OH- assumed present in pores and channels of film to balance negative charge on polymer chain. Equivalent circuit model: dual/multi- rail Transmission Line as done for ECP films..
Super-Nernstian Redox Potential vs pH shift related to hydrolysis effects in hydrous layer yielding anionic oxide structures.
L.D. Burke, M.E.G. Lyons, E.J.M.O’Sullivan, D.P. Whelan J. Electroanal. Chem., 122 (1981) 403.
Redox switching chemistry: hydrous oxide Layer, Mixed conduction mechanism: ion/electron transfer.
Fe(III) Oxidized form yellow-green
Fe(II) Reduced form transparent
Potential / V vs. Hg/HgO
-1.5 -1.0 -0.5 0.0 0.5 1.0
Curr
ent / A
-0.004
-0.003
-0.002
-0.001
0.000
0.001
0.002
pH 14.0
pH 11.5
pH 9.0
O2 evolutionA3
H2 evolution
C2
C1
pH
8 9 10 11 12 13 14
Pe
ak
Po
tentia
l / V
vs.
Hg/H
gO
-0.8
-0.6
-0.4
-0.2
0.0
0.2
Experiment 1 (120 cycles)
Experiment 2 (120 cycles)
Experiment 3 (120 cycles)
Slope = 0.10 V/pH unit
A3/C2
2.303( ) 0.059( ) /
dE RTr q r q V pH
dpH F
Variation of Ni oxide A2,C2 and C2’ peak potentials with solution pH.
Potential / V
-1.5 -1.0 -0.5 0.0 0.5 1.0 1.5
Cu
rre
nt \ A
-0.004
-0.002
0.000
0.002
0.004
0.006
0.008
pH 14.0
pH 13.0
pH 11.0
pH 9.0
pH
8 10 12 14
Ep/V
(Hg/H
gO
)
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
A2 C2
C2’
C2’
A2
C2
C2’: dE/dpH = - 50 mV/dec
C2: dE/dpH = - 74 mV/dec A2: dE/dpH = - 97 mV/dec
[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
Both anions, electrons and cations are transported during redox switching.
M
O-
OH2
OH
O-HO
M
OH2
OH2
OH
O-
M
O
OH2
OH
OHO
M
OH2
OH2
OH
OH
H
H
+3nH2O, 2ne-
-3nOH-
3-
n
2-
n
M2O3(OH)3(OH2)3
3-
nM2(OH)6(OH2)3
2-
n
[M2(OH)6(OH2)3]n2- + 3nOH- →
[M2O3(OH)3(OH2)3]n3- +3nH2O + 2ne-
M = Fe, Ni
Redox switching in hydrous oxide layer involves a rapid topotactic reaction involving hydroxide ion ingress and solvent egress at the oxide/solution interface, and electron injection at the metal/oxide interface. Also involves motion of charge compensating cations through film.
MII (OH)r(r-3)- + e- MIII (OH)q
(q-2)- + (3/2) OH-
r-q ≈ 1.5 dE/dpH = - 0.090 V = - 0.06 (r-q) V
Assume that oxidized form of hydrous metal oxide has same composition as oxidized form of hydrous gold oxide.
OH- H+
H2O
OH-
H+ H2O
Oxidation
Reduction
Electrode Film
e-
e-
charge
2 2discharge( )Ni OH OH NiOOH H O e
Electro-precipitated Nickel Oxyhydroxide : Redox Switching
The redox switching reaction (associated with the A2/C2 voltammetric peaks) reflects the change in oxidation state of the film as a result of a potential perturbation. Redox centres immediately adjacent to the support electrode are directly affected by the electrode potential, whereas charge is further propagated along the oxy-iron polymer strands in the hydrous layer via a sequence of electron self exchange reactions between neighbouring oxy-metal sites. This process is envisaged to be analogous to redox conduction exhibited by electroactive polymer films. In the simplest terms this electron “hopping” may be modelled in terms of a diffusive process, and so the charge percolation rate may be quantified in terms of a charge transport diffusion coefficient, DCT. Diffusive time constant tD can be extracted via Cyclic Voltammetry.
Redox switching involves both Electron and ion transport. Have Ni(II)/Ni(III) redox Transition in ploymeric Microdispersed hydrous Oxide layer. Oxide film is mixed conductor. Can be modeled as dual rail electrical transmission line.
[Ni2(OH)6(OH2)3]n2- + 3nOH- →
[Ni2O3(OH)3(OH2)3]n3- +3nH2O + 2ne-
Dual rail transmission line model for porous thin film mixed electronic/ionic conductor. Note that c1 and c2 corresponds to the specific conductivity of the solid oxide phase and electrolyte solution respectively and z represents the specific polarization/charge transfer element at the solid/solution interface.
(a) A.J. Terezo, J. Bisquert, E.C. Pereira, G.Garcia-Belmonte, Separation of transport, charge storage and reaction processes of porous electrocatalytic IrO2 and IrO2/Nb2O5 electrodes. J. Electroanal. Chem., 508 (2001) 59-69. (b) J. Bisquert, G. Garcia-Belmonte, F. Fabregat Santiago, N.S. Ferriols, M. Yamashita, E.C. Periera, Application of a distributed impedance model in the analysis of conducting polymer films. Electrochem. Commun., 2 (2000) 601-605.
22
A
B A
B A
B A
B
Ele
ctro
de
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 2
exE
Anion X- injection
Mixed conductivity: Electron hopping coupled with counterion transport.
log
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
Ep /
V
-0.2
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0 cycles
30 cycles
60 cycles
90 cycles
120 cycles
180 cycles
240 cycles
360 cycles
-3.5 -3.0 -2.5 -2.0 -1.5 -1.0 -0.5 0.0 0.5
0.0
0.2
0.4
0.6
0.8
Variation of voltammetric peak potential with log sweep rate for a series of multicycled Ni electrodes in 1.0 M NaOH at 298 K. Attention is focused on the main anodic and cathodic charge storage peaks corresponding to the Ni(II)/Ni(III) redox transition located at potentials prior to active oxygen evolution.
Quasi-reversible
Reversible
V s-1
0.0 0.5 1.0 1.5 2.0
Q (
C)
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0 cycles
30 cycles
60 cycles
90 cycles
120 cycles
180 cycles
240 cycles
360 cycles
Variation of voltammetric redox charge capacity Q with analytical sweep rate for a series of Ni oxide hydrous films grown under potential cycling conditions for various number of cycles. Attention is focused on the main anodic charge storage peak corresponding to the Ni(II)/Ni(III) redox transition located at potentials prior to active oxygen evolution.
V s-1
0.0 0.5 1.0 1.5 2.0
p1
/2/ V
0.0
0.1
0.2
0.3
0.4
0.5
0.6
0 cycles
30 cycles
60 cycles
90 cycles
120 cycles
180 cycles
240 cycles
360 cycles
Variation of peak width at half peak height with sweep rate for a series of multicycled nickel electrodes in 1.0 M NaOH.
/2
2 3.53ln 3 2 2p
RT RTE
nF nF
Ideal Reversible behaviour
( / V s-1
)-1/2
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6
I p /
A
0.000
0.005
0.010
0.015
0.020
0.025
0.030
0.035
0 cycles
30 cycles
60 cycles
90 cycles
120 cycles
180 cycles
240 cycles
360 cycles
Typical Randles-Sevcik plots recorded for a series of multicycled Ni electrodes in 1.0 M NaOH as a function of number of oxide growth cycles.
1/2
1/2
20.4463P
nF Di nFA
RT L
Randles-Sevcik equation
2D
D
Lt
Can estimate diffusive time constant tD from linear region of plot. RS data is non-linear however. RS analysis assumes semi-infinite Diffusion in layer. Finite/bounded diffusion more appropriate.
Quantifying redox switching in hydrous multicycled nickel oxyhydroxide thin films.
W1/2
0 2 4 6
0.0
0.1
0.2
0.3
0.4
0.5
6.9 pW i
1.3 pW i
2
2
6.9
0.179
RT D
nF L
D
L
21.3
RT D
nF L
20.034
D
L
1/2
0.446 tanhPi RT
nFA L nFD
0.56 0.05W W
1/2 1/2tanh 0D D D P
D
G i
t t tt
3/2
1/2
1/20.446
0.56
0.05
nFA
RT
nF
RT
nF
RT
2
D D Lt
22
D
nFL LW
DRT X
Aoki Equation Finite diffusion in thin film
Solve Aoki equation numerically via Bisection Algorithm To get value of diffusive time constant.
Diffusive time constant.
No of Cycles
50 100 150 200 250 300 350 400
D/L
2 /
s-1
0.0
0.2
0.4
0.6
0.8
1.0
1.2
Randles-Sevcik Analysis
Aoki Analysis ( V s-1
)
Variation of diffusive frequency characterizing redox switching in multicycled hydrous oxide coated Ni electrodes in 1.0 M NaOH with number of oxide growth cycles.
Diffusive frequency : Hydrous Nickel oxyhydroxide thin films.
Potential / mV vs. SCE
-1000 -500 0 500 1000 1500
Curr
ent
/ A
-4.0e-5
-2.0e-5
0.0
2.0e-5
4.0e-5
6.0e-5
8.0e-5
1.0e-4
Electro-precipitated Nickel Oxy-hydroxide films : Au support.
Electro-deposition solution: aqueous solution of 0.1 M NiSO4, 0.1 M sodium acetate, 1 mM KOH . Electro-deposition method: Cyclic Potential Multicycling (CPM) ;Gold substrate, 30 cycles between -0.9 V & 1.2 V vs. SCE at 50 mV/s.
N
Au electrode Pt electrode
GC electrode
Cyclic voltammogram recorded for a glassy carbon electrode modified with a thin nickel hydroxide film (30 growth cycles) in 1.0 M NaOH. Sweep rate, 40 mV/s. Potentials are quoted with respect to Hg/HgO reference electrode (1 M OH-).
Cyclic voltammogram recorded for a Pt electrode modified with a thin nickel hydroxide film (30 growth cycles) in 1.0 M NaOH. Sweep rate, 40 mV/s. Potentials are quoted with respect to Hg/HgO reference electrode (1 M OH-).
Au electrode
Cyclic voltammogram recorded for a polycrystalline gold electrode modified with a thin nickel hydroxide film (30 growth cycles) in 1.0 M NaOH. Sweep rate, 40 mV/s. Potentials are quoted with respect to Hg/HgO reference electrode (1 M OH-)
GC electrode
Pt electrode
Unmodified Au electrode
Au electrode
-Ni(OH)2 + OH- - NiOOH + H2O + e- E0 = 0.37 V, Ep = 74 mV
-Ni(OH)2 + OH- - NiOOH + H2O + e- E0 = 0.496 V, Ep = 113 mV
Bode Square Scheme
Electro-precipitated Ni(OH)2
Potential / V vs. Hg/HgO
0 200 400 600 800
Curr
ent
/ A
-0.0003
-0.0002
-0.0001
0.0000
0.0001
0.0002
0.0003
0.0004
0.0005
10 mV/s
20 mV/s
50 mV/s
Effect of growth scan rate: Each film was deposited from the same solution , and using the same number of growth cycles (30 cycles).
Typical voltammetric response recorded for an electroprecipitated nickel oxide film deposited on a polycrystalline gold substrate subjected to slow multicycling (sweep rate 10 mV/s) between 0.1 and 0.7 V (vs Hg/HgO) in 5.0 M NaOH.
Initial Cycle
Final CycleN = 85
Typical voltammetric response recorded for an electroprecipitated nickel oxide film deposited on a polycrystalline gold substrate subjected to slow multicycling (sweep rate 10 mV/s) between 0.1 and 0.7 V (vs Hg/HgO) in 5.0 M NaOH. The initial and final response profiles are presented.
Temporal effects on Nickel oxyhydroxide layer redox chemistry.
1 mV/s 2 mV/s 5mV/s 10 mV/s
20 mV/s 100 mV/s
300 mV/s
50 mV/s 100 mV/s 200 mV/s
400 mV/s 500 mV/s 750 mV/s
1000 mV/s 1500 mV/s 2000 mV/s
Sweep rate : CV response
CV in 1M NaOH at ν= 40mV s-1 : Ni deposited on Au electrode at -1.5V vs Ag/AgCl for 300s in 0.1 M Ni(NO3)2/0.075M KNO3
CV in 1M NaOH at ν= 40mV s-1. but after additional multicycling for 30 cycles between -1.45 V to 0.65 V vs Hg/HgO @ 150 mV s-1
Deposition from Ni(NO3)2 : NO3
- + 7H20 + 8e- NH3+ 10 OH- (extra OH- generated from ammonia/ammonium ion equilibrium, NH3 + H2O NH4+ + OH-) Production of OH- increases the surface pH of the electrode resulting in precipitation of Ni(OH)2 Ni2+ + 2 OH- Ni(OH)2
Fine details of voltammetric response in aqueous base depends on pretreatment history. Note sharp onset of redox switching: Ni(II)/Ni(III).
Super-Nernstian pH shift signifies possible development of more sensitive and scalable metal oxide wire pH sensors.
pH
E Nernstian
Super-Nernstian
Enhanced sensitivity of sensor to given pH change. Sensor probe can be made very small for biomedical applications.
Metal oxide wire pH sensor spinoff.
Open circuit potentials of electroprecipitated Ni on Au electrode versus log(time). Data from this graph used to generated correlation graph. Faster film degradation in acidic pH. Electrolyte: 1M NaOH Reference: Hg/HgO for pH 5 or greater. SCE for pH 4 and lower. Timescale: 60,000 seconds.
Potential Response to pH
pH
0 2 4 6 8 10 12 14
Po
ten
tia
l vs (
Hg/H
gO
) /V
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1,000 seconds
15,000 seconds
200 seconds
Plot 3 Regr
Open Circuit Potential versus pH for the electro-precipitated Ni oxyhydroxide films on Au working electrode. Multiple times are shown and taken into account for linear regression. Some scattering in pH 3 and lower data points. Slope used to calculate pH for correlation graph.
Variation of OCP with changes in solution pH.
Calculated pH vs Measured pH
Measured pH
0 2 4 6 8 10 12 14 16
Ca
lcu
late
d p
H
0
2
4
6
8
10
12
14
Measured pH vs Calculated pH
Plot 1 Regr
Intercept = -0.74Slope = 1.01r ² = 0.99
1.01 0.74calc measpH pH
Comparing pH calculated from variation of OCP vs pH solution plot (Super Nernstian) with pH measured via Glass electrode.
Typical electroprecipitation CV for Ni on Au working electrode. Rate: 20mV/s Limits: (-0.9 to 1.2)V Reference: SCE Electrolyte: 1M NaOH, 30 cycles
Typical CV performed after 30 cycle electrodeposition of nickel oxyhydroxide on Au, (A) before & (B) after titration experiment. Sweep Rate: 40mV/s Limits: (-0.4 to 0.7)V Reference: Hg/HgO Electrolyte: 1M NaOH
pH, Potential versus Volume of Sulphuric Acid Added
Volume Sulphuric Acid Added / cm^3
0 10 20 30 40 50
pH
0
2
4
6
8
10
12
14
16
Pot
entia
l vs
SC
E /V
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Volume acid added /cm^3 vs pH
Volume acid added /cm^3 vs OCP /V
A
Strong Acid (1.0 M H2SO4) vs Strong base (1.0 M NaOH) Titration.
B
‘Well behaved’ titration
Au
ph, Potential versus Volume of Sulphuric Acid Added
Volume of 1M Sulphuric Acid Added /cm^3
0 10 20 30 40 50
pH
0
2
4
6
8
10
12
14
Pote
ntial vs S
CE
/V
0.2
0.4
0.6
0.8
Volume of 1M Sulphuric Acid Added vs pH
Volume of 1M Sulphuric Acid Added vs OCP /V
‘Misbehaved’ titrations
Strong Acid (1.0 M H2SO4) vs Strong base (1.0 M NaOH) Titration.
pH, Potential versus Acid Added
Volume of Sulphuric Acid Added /cm^3
0 10 20 30 40 50
pH
0
2
4
6
8
10
12
14
16
Pote
ntial vs S
CE
/V
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Volume Added /cm^3 vs pH
Volume Added /cm^3 vs OCP /V
Table 2. Kinetic parameters derived for 5 most common OER mechanisms [169].
Rate-determining step
Langmuir Temkin
𝝏𝑬
𝝏 𝐥𝐧𝒊
𝝏 𝐥𝐧 𝒊
𝝏 𝐥𝐧𝑪𝑶𝑯− 𝑬, 𝜻𝒄
𝝏𝑬
𝝏 𝐥𝐧𝒊
vb θ→0 θ→1 θ→0 θ→1 NA
d A
e NA
d A
e condition
f
(I) Bockris’s Oxide Path 1. M + OH
− → MOH + e
−
2. 2MOH → MO + M + H2O 3. 2MO → 2M + O2
4 2 1
2RT/F RT/2F
RT/4F
∞
∞
1 2 4
0
0
2RT/F RT/F
0.5 1
rOH ~ rO rOH >> rO K2 ~ 1 K2 << 1
RT/2F RT/4F
RT/F RT/3F
2 2
1 1
(II) Bockris’s Electrochemical Path 1. M + OH
− → MOH + e
−
2. MOH + OH− → MO + H2O + e
−
3. 2MO → 2M + O2
2 2 1
2RT/F 2RT/3F
RT/4F
2RT/F
∞
1 2 4
1
0
2RT/F RT/F
1
1.5
rOH ~ rO rOH >> rO K2 ~ 1 K2 << 1
RT/2F RT/4F
RT/F RT/3F
2 4
1 3
(III) Krasil’shchikov’s Path 1. M + OH
− → MOH + e
−
2. MOH + OH− → MO
− + H2O
3. MO
− → MO + e−
4. 2MO → 2M + O2
2 2 2 1
2RT/F RT/F
2RT/3F
RT/4F
∞
2RT/F
∞
1 2 2 4
1
0
0
∞ 2RT/F 2RT/F 2RT/F
1
1.5 0 1
rOH ~ rO− rOH >> rO− K2 ~ 1 K2 << 1 K3 ~ 1 K3 << 1
RT/2F RT/4F
RT/F RT/3F
2 2
1 1
(IV) O’Grady’s Path 1. M
z + OH
− → M
zOH + e
−
2. MzOH → M
z+1OH + e
−
3. 2Mz+1
OH + 2OH− → M
z + H2O + O2
2 2 1
2RT/F 2RT/3F
RT/4F
2RT/F
∞
1 1 4
0
2
2RT/F RT/F
0
0.5
r1 ~ r2
g
r1 >> r2 K2 ~ 1 K2 << 1
RT/2F RT/4F
RT/F RT/3F
4 4
3 3
(V) Kobussen’s Path 1. M + OH
− → MOH + e
−
2. MOH + OH− → MO + H2O + e
−
3. MO + OH
− → MO2H
−
4. MO2H− + OH
− → MO2
− + H2O + e
−
5. MO2
− → M + O2 + e
−
1 1 1
1 1
2RT/F 2RT/3F
RT/2F
2RT/5F
2RT/7F
2RT/3F
∞
2RT/F
∞
1 2 3
4 4
1
1
1
0
2RT/F RT/F
∞ RT/F
2RT/F 2RT/F
1
1.5 1 2
1 2
rOH ~ rO rOH >> rO K2
~ 1
K2 << 1
K3 ~ 1 K3 << 1 K4 ~ 1 K4 << 1
RT/F RT/2F
2RT/F 2RT/3F
1 1
0.5 1.5
a Symmetry factors, i.e. β, γ, and δ, in all steps, were taken as ½.
b Stoichiometric number.
c ζ is the potential difference between the
outer Helmholtz plane and the bulk of the solution. d Nonactivated desorption of O2.
e Activated desorption of O2.
f r is a coefficient
determining the variation of heat of adsorption of a particular species with coverage. Unless stated, r values for each species were
taken as equal. Ki is the equilibrium constant of the ith step. g r1 and r2 refer to r for M
zOH and r for M
z+1OH, respectively.
Kinetically limiting step in water electrolysis cells and PEM fuel cell.
Multistep multi-electron transfer reaction involving adsorbed intermediates.
Depending on RDS can explain a variety of Tafel slopes.
Key mechanistic parameters are : Tafel Slope, reaction order wrt hydroxide ion activity.
Overall reaction (alkaline medium)
O2 + 2H2O + 4e- 4OH-
E0 = 0.303 V (vs. Hg/HgO) OER at oxidized metal and
metal oxide electrodes involves active participation of oxide.
Acid/base behaviour of oxide important consideration .
Concept of active surface or surfaquo groups important.
Anodic Oxygen Evolution Reaction (OER)
S. Rebouillat, M.E.G. Lyons, M.P. Brandon, R.L. Doyle, Int. J. Electrochem. Sci., 6 (2011) 5830-5917.
Charge Q / C cm-2
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
Tafe
l slo
pe /
mV
dec
-1
35
40
45
50
55
60
65
New Fe electrode
'Aged' Fe electrode
Charge Q / C cm-2
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
Curr
ent D
ensity i / A
cm
-2
0.060
0.065
0.070
0.075
0.080
0.085
0.090
0.85 V
Potential / V vs. Hg/HgO
0.6 0.7 0.8 0.9 1.0 1.1
Log
(C
urr
ent
/ A
)
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
Uncycled
30 cycles
60 cycles
120 cycles
180 cycles
240 cycles
300 cycles
Charge Q / C cm-2
0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09
Pote
ntial /
V v
s.
Hg/H
gO
0.695
0.700
0.705
0.710
0.715
0.720
0.725
1 mA cm-2
Anodic OER, Fe aqueous alkaline solution.
Tafel Plots
Hydrous Iron Oxide Electrodes. OER Reaction Order Studies
Potential / V vs. Hg/HgO
0.5 0.6 0.7 0.8 0.9 1.0 1.1
Log (
i /
A c
m-2
)
-5
-4
-3
-2
-1
0
0.1 M
0.5 M
1.0 M
2.0 M
5.0 M
60 mV dec-1
120 mV dec-1
Log (aOH-
)
-1.5 -1.0 -0.5 0.0 0.5 1.0
Log (
Curr
ent
Density i / A
cm
-2)
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
60 mV dec-1
region
Slope = 0.87
120 mV dec-1
region
Slope = 0.81
N = 120 cycles Reaction order wrt OH- activity ca. 0.9 (low TS region) and ca. 0.8 (high TS region).
Measure OER current density at fixed overpotential from analysis of Tafel Plots as function of OH- ion activity.
E/V
0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
log(I
/A)
-5
-4
-3
-2
-1
Tafel Plots for OER at Ni oxide layers grown via potential cycling (N = 120 cycles) in 1.0 M NaOH recorded as function of base concentration.
0.1M
0.25M
0.75M
1M
1.5M
2M
2.5M
3M
4M
5M
E/V
-0.6 -0.4 -0.2 0.0 0.2 0.4 0.6
I/A
-0.002
0.000
0.002
0.004
0.006
0.1M
0.25M
0.5M
0.75M
1M
1.5M
2M
2.5M
3M
4M
N = 120 cycles
N = 120 cycles
Low potential : TS = 60 mV/dec. High potential : TS = 120 mV/dec
E=0.64V
= 0.337V
Regression
Confidence
c = -3.0513507838m = 0.8526502073r ² = 0.96614858
Ni oxide layer grown in 1.0 M NaOH. N = 120 cycles. Reaction order plot, low Tafel Slope Region. mOH
- = 0.85.
E=0.71V
= 0.407VRegression
Confidence
c = -2.1450473735m = 0.8182632348r ² = 0.973540901
Ni oxide layer grown in 1.0 M NaOH. N = 120 cycles. Reaction order plot, high Tafel Slope Region. mOH
- = 0.82.
Voltammetric response of hydrous Ni oxide film as function of base Concentration.
Ni in aqueous base: redox activity and OER behaviour.
Charge (C)
0.0000 0.0002 0.0004 0.0006 0.0008 0.0010 0.0012
Ta
fel S
lop
e (
V)
0.055
0.060
0.065
0.070
0.075
0.080
0.085
Slope: 0.055041Intercept 22.607r ²: 0.92
Variation of low overpotential Tafel Slope for OER at multicycled Ni oxide Electrode in 1.0 M NaOH as a function of oxide charge capacity Q (thickness).
Tafel Plot OER as function of hydrous layer thickness (# cycles). Ni oxide electrode, 1M NaOH.
E/V
0.3 0.4 0.5 0.6 0.7 0.8 0.9 1.0 1.1
log
(I/A
)
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
Uncycled 30 Cycles 60 Cycles 120 Cycles 180 Cycles 240 Cycles
Low overpotential Tafel Slope increases in a linear manner with increasing hydrous oxide charge capacity.
Effect of oxide charge capacity Q on OER catalytic efficiency: Ni in aqueous base.
=0.337V
=0.347V
=0.357V
Oxygen evolution rate at fixed potential at oxide coated Ni in 1.0 M NaOH as function of redox charge storage capacity of hydrous layer.
(a) M.O’Brien, L. Russell, I. Godwin, R.L. Doyle, and M.E.G. Lyons, Redox switching and oxygen evolution at hydrous nickel oxide in aqueous alkaline solution, Electrochemical Society Transactions, Spring Meeting Seattle , USA, 2012, In press.
(b) M.E.G. Lyons, L. Russell, M. O’Brien, R.L. Doyle, I. Godwin, M.P. Brandon, Redox switching and oxygen evolution at hydrous oxyhydroxide modified nickel electrodes in aqueous alkaline solution: Effect of hydrous oxide thickness and base concentration. Int. J. Electrochem. Sci., 7 (2012) 2710-2763.
Z' /
0 100 200 300 400 500 600
Z''
/
0
50
100
150
200
250
300
0.72 V
0.76 V
0.80 V
0.84 V
Circuit fit
Log (Freq / Hz)
-1 0 1 2 3 4 5
Lo
g (
Z /
)
1
2
3
0.72 V
0.76 V
0.80 V
0.84 V
Circuit fit
Log (Freq / Hz)
-1 0 1 2 3 4 5
Phase a
ngle
/ d
eg
0
10
20
30
40
50
60
0.72 V
0.76 V
0.80 V
0.84 V
Circuit fit
E / V R / Cdl / mF cm-2
() Rfar / Cfilm / mF
cm-2 () Rfilm /
0.70 5.1500 551 (0.78) 858.5 42.7 (0.90) 48.5
0.72 5.1340 560 (0.80) 516.1 45.4 (0.89) 35.6
0.74 5.0510 554 (0.83) 281.0 54.6 (0.86) 25.8
0.76 4.9600 567 (0.86) 143.4 66.8 (0.84) 18.2
0.78 4.8750 576 (0.87) 71.9 77.0 (0.82) 12.8
0.80 4.6430 536 (0.90) 36.8 112.0 (0.78) 9.6
0.82 4.1190 464 (0.93) 19.5 258.0 (0.69) 8.0
0.84 4.3430 419 (0.93) 12.0 97.7 (0.77) 5.3
0.86 4.5170 382 (0.94) 7.8 39.0 (0.84) 3.9
EIS Measurements : Hydrous iron oxide film
E/V(vs Hg/HgO)
0.68 0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86 0.88
Rfilm
/
0
10
20
30
40
50
60
EIS data
Metal oxide film becomes more conducting with increasing potential.
R.L. Doyle 2012, sub
•At OER overpotentials where simple Tafel behaviour prevails: Differentiating gives: Noting that di/dη = di/dE = 1/Rfar: •EIS Tafel slope 66 mV dec-1
•Good agreement with steady-state polarisation Tafel slope of 60 mV dec-1.
Potential / V vs. Hg/HgO
0.68 0.72 0.76 0.80 0.84 0.88
Log (
1/R
far)
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
0.0
66 mV dec-1
Potential / V vs. Hg/HgO
0.65 0.70 0.75 0.80 0.85 0.90 0.95 1.00 1.05
Log (
Curr
ent
Density i / A
cm
-2)
-4.0
-3.5
-3.0
-2.5
-2.0
-1.5
-1.0
-0.5
60 mV dec-1
120 mV dec-1
EIS Tafel Analysis
E/V (Hg/HgO)
0.68 0.70 0.72 0.74 0.76 0.78 0.80 0.82 0.84 0.86 0.88
Rfa
rad
aic/
0
200
400
600
800
1000
EIS data
Tafel Plots OER Multicycled Hydrous Oxide coated Ni and Fe Electrodes (N = 120 cycles), 1.0 M Base, 298 K.
Ni has reduced overpotential for OER onset compared with Fe.
Overpotential (
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Lo
g (
Curr
ent
de
nsity /
A c
m-2
)
-5
-4
-3
-2
-1
0
Nickel (120 growth cycles)
Iron (120 growth cycles)
[OH-] = 1.0 M
Overpotential / mV
200 300 400 500 600 700
Log
(C
urr
ent
/ A
)
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
-2.5
-2.0
0.1 M
0.5 M
1.0 M
5.0 M
Tafel Plots OER, alkaline solutions. Electro-precipitated Nickel oxy-hydroxide thin films deposited via CPM on Au supports.
OER onset overpotential decreases with increasing base concentration.
cOH- / M
0 1 2 3 4 5 6
b/
mV
dec
-1
30
40
50
60
70
80
90
Run 1
Run 2
Run 3
N = 30 cycles SR = 50 mV s-1 Plating medium: 0.1 M NiSO4, 0.1 M NaAc.3H2O, 0.001 M KOH CPM : - 900, + 1200 mV (vs SCE), 50 mVs-1.
Low potential Tafel slope for OER at electro-precipitated nickel oxyhydroxide film grown on Au support via CPM, as function of OH- concentration.
Limiting low potential TS = 47 mV/ dec
cOH- /M
0 1 2 3 4 5 6
b/
mV
dec
-1
100
110
120
130
140
150
Run 1
Run 2
Run 3
High potential Tafel slope for OER at electro-precipitated nickel oxyhydroxide film grown on Au support via CPM, as function of OH- concentration.
Log (aOH-)
-1.5 -1.0 -0.5 0.0 0.5 1.0
Log (
Curr
ent / A
)
-6.5
-6.0
-5.5
-5.0
-4.5
-4.0
-3.5
-3.0
Run 1
Run 2
Run 3
Slope = 1.12
Reaction order plot. Electroprecipitated Nickel oxy-hydroxide thin films.
1.12OH
m
E/mV(vs Hg/HgO)
/mV mOH-
Run 1 mOH
- Run 2
mOH-
Run 3
620 317 1.29 1.35 1.18
640 337 1.31 1.26 1.24
680 377 1.29 1.52 1.21
700 397 1.38 1.39 1.13
Run 1: QCV = 8.23 x 10-4C Run 2: QCV = 7.06 x 10-4C Run 3: QCV = 9.51 x 10-4C
Reaction order for OER : electro-precipitated nickel oxide film, Au support. Aqueous alkaline solution.
2 2SOH OH SOH H O
SOH SOH e
2SOH OH SO H O
SO SO e
SO OH SOOH e
2 2SOOH OH SO H O e
2 2SO OH O SOH
OER Mechanism at Fe & Ni oxyhydroxide modified electrodes
S = surfaquo group attached to oxide lattice via bridging oxygen ligands.
Hy
dro
us L
ay
er
H2O
O2 e-
OH-
e-
e-
OH-
OH-
OH-
OH-
H2O
e-, H2O
Low η RDS: T.S.=60 mV/dec
mOH- = 1
High η RDS (θ→1):T.S.=120 mV/dec
mOH- = 1
SOH2
SOH-
SOH
SO-SOOH
SO
SO2
OER mechanism involving surfaquo groups in the hydrous oxy-metal hydroxide layer.
Octahedrally co-ordinated Ni(III)
Fast initial deprotonation
Metal oxide
Metal oxo
Low RDS TS = 40 mV/dec mOH
- = 1
Metal peroxide Low RDS TS = 40 mV/dec mOH
- = 2
DFT calculations being initiated to generate quantitative Energy Landscape for OER Mechanism.
Mechanism valid for M = Fe & Ni. Explains Tafel Slope & reaction order data.
Overpotential V
0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
Lo
g (
I/Q
)
-10
-8
-6
-4
-2
0
2
4
Electrodep Ni(OH)2 on Pt
Electrodep Ni(OH)2 on GC
Electrodep Ni(OH)2 on Au
Electrodep aged Ni(OH) on Au
Hydrous Nickel Oxide 120 cycles
Hydrous Iron Oxide 120 cycles
Thermally prepared NiO on Ni Substrate
Thermally prepared RuO2 Ni substrate
Thermally prepared RuO2 on Ti substrate
Thermally prepared Rh2O3 on Ni substrate
Thermally prepared Rh2O3 on Ti substrate
An atlas of electrochemical reactivity for anodic OER
Electro-precipitated -Ni(OH)2 : The ‘right’ type of oxide for OER
Current DSA ‘state of art’
We have succeeded in preparing very effective catalytic electrodes for anodic OER using very cheap Materials (nickel oxide).
All currents (OER rate) scaled to voltammetric charge which is proportional to real surface area.
Tafel Plots IR Corrected.
Concluding Comments
• Reproducible and scalable methodology developed for generation of hydrated Fe & Ni metal oxide thin films and electroprecipitated nickel oxyhydroxide films on Au, GC and Pt supports in aqueous base.
• Duplex layer model proposed for structure of oxide/solution interface region.
• Hydrous oxide thin films exhibit super-Nernstian shifts in redox potential with respect to changes in solution pH value. Implying commercial spinoff potential for new generation metal wire pH sensors for use in biomedical applications.
• Dynamics of redox switching in hydrous oxide layer quantified via Aoki Model.
• Electro-catalytic kinetics and mechanism with respect to anodic OER at Ni and Fe electrodes in aqueous base evaluated and 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 layer.
• Currently developing molecular scale model for detailed OER pathway in terms of interlinked surfaquo group model.
• Fe and Ni oxide materials are cheap and effective electrode materials for anodic water splitting.
• Next stage is to examine application of these oxide materials to Cathodic Oxygen Reduction Reaction (ORR) and hydrogen evolution reaction (HER). The latter topics are currently unexplored at hydrous oxide materials.
• Extending analysis to metal oxide films prepared via thermal decomposition of precursor & via sol/gel routes.
Main ideas developed in:
M.E.G. Lyons, L.D. Burke, J. Electroanal. Chem., 170 (1984) 377-381.
M.E.G. Lyons, L.D. Burke, J. Electroanal. Chem., 198 (1986) 347-368.
M.E.G. Lyons, M.P. Brandon, International Journal of Electrochemical Science, 3(12) (2008) 1386-1424.
M.E.G. Lyons, M.P. Brandon, International Journal of Electrochemical Science, 3(12) (2008) 1425-1462.
M.E.G. Lyons, M.P. Brandon, Int. J. Electrochem. Sci., 3(12) (2008) 1463-1503.
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) 62-70.
M.E.G. Lyons, M.P. Brandon, J. Electroanal. Chem., 641 (2010) 119-130.
Michael E.G. Lyons, Richard L. Doyle, International Journal of Electrochemical Science,6, 2011, pp.5710-5730.
Michael E.G. Lyons, Serge Rebouillat, Michael P. Brandon, Richard L. Doyle, International Journal of Electrochemical Science, 6, 2011, 5830-5917.
Michael E.G. Lyons, Richard L. Doyle, Michael P. Brandon, Physical Chemistry Chemical Physics, 13, 2011, pp.21530-21551.
Michael E.G. Lyons, Stephane Floquet, Physical Chemistry Chemical Physics, 13, 2011, pp. 5314- 5335.
Michael E.G. Lyons, Lisa Russell, Maria O Brien, Richard L Doyle, Michael P Brandon, International Journal of Electrochemical Science, 7, 2012, 2710-2763.
M.O’Brien, L. Russell, I. Godwin, R.L. Doyle, M.E.G. Lyons, Transactions Electrochemical Society, Spring Meeting 2012, Seattle, USA, in press.
R.L. Doyle, M.E.G. Lyons, Transactions Electrochemical Society, Spring Meeting 2012, Seattle, USA, in press.
Trinity Electrochemical Energy Conversion & Electrocatalysis (TEECE) Group
Current Group Personnel: PI: Prof. Mike Lyons
PDRF: Dr Richard Doyle
PG: Mr Ian Godwin
UG PCAM Interns: Ms Maria O’Brien, Ms Lisa Russell,
Patrick O’Brien
Visiting Researcher: Ms Anja Cakala (TU Wien)
Group Alumni (Energy Conversion/storage): Dr Gareth Keeley
Dr Michael Brandon
Collaborators: Prof. Declan Burke, UCC (Passed away December
2011)
Dr Michael Brandon, QUB
Dr Serge Rebouillat, DuPont Geneva
Dr Chris Bell, IC London
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 oxide electrodes for use in energy conversion and storage devices, 2011-2016.