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MANSE Midterm Review
V Magnetoelectrochemistry
Body forces; Is there a ‘concentration gradient’ force ? Lorentz force effects Hydrogen evolution Magnetic field gradient effects Nitrobenzene - a model for magnetoelectrochemistry Planned work
MANSE Midterm Review
Staff, Invited Talks, Publications
• Lorena Monzon postdoc from January 2009 (previous postdoc Nandu Chaure)
• Peter Dunne postgrad • Zhu Diao postgrad • Giovanni Zangari (U.Virginia) Sabbatical visitor Summer 2007 • Damaris Fernandez (U. Santiago) visiting postgrad • Gasparo Varvaro (CNR Rome) visiting postdoc
Collaborators. Fernando Rhen (Tyndall, U. Limerick) Ryoichi Aogaki (Samihara Inst, Japan)
Talks: Asia Magnetics Society, Pusan 200 ICEPM, Dresden 2009
MANSE Midterm Review
Publications: —Magnetic-field-induced rest potential shift of metallic electrodes in nitric acid solution, M. F. M. Rhen, P. Dunne and J. M. D. Coey, Magnetohydrodynamics 42 395-401 (2006) — Magnetic field induced modulation of anodic area: the rest potential analysis of Zn and Fe. F. M. F. Rhen and J. M. D. Coey, Journal of Physical Chemsitry C 111 3412-3416 (2007) — Inhomogeneous electrodeposition of copper in a magnetic field, Damaris Fernandez and JMD Coey, Electrochemistry Communications, 11 (2009) in press — Design and application of a magnetic field gradient electrode, N. B. Chaure, M. F. M. Rhen and J. M. D. Coey. Electrochemical Communications 9 155-158 (2007) — Enhanced oxygen reduction at composite electrodes producing a large magnetic gradient, NB Chaure and JMD Coey, Journal of the Electrochemical Society, 156 F39-47 (2009); also in Virtual Journal of Nanoscale Science and Technology (Jan 19 2009) — The magnetic concentration gradient force – is it real? J.M.D. Coey, F.M.F. Rhen, P. Dunne and S. McMurray, Journal of Solid State Electrochemistry 11 711-717 (2007) — Levitation in paramagnetic liquids, P. Dunne, J. Hilton and J. M. D. Coey, Journal of Magnetism and Magnetic Materials 316 273-6 (2007) — Magnetic stabilization and vorticity in paramagnetic liquid tubes, J. M. D. Coey, R Aogaki, F Byrne and P Stamenov, Proceedings of the National Academy of Science (submitted) — Magnetic field effect on hydrogen evolution, Z Diao, G. Zangari and J. M. D. Coey, Electrochemical Communications 11 (2009) in press
MANSE Midterm Review
Simple electrochemical cell
Potentiostat
Magnetic field perpendicular to the surface
Magnetic field parallel to the surface
Working electrode Counterelectrode
Reference electrode
j
Cyclic voltametry I(V)
Chronoamperometry I(t)
Rotating disc electrode I(ω)
Impedance spectroscopy I(f)
Noise spectroscopy V(t)
Hydrodynamic modeling
- Potentiostatic mode - fixed V - Galvanostatic mode - fixed I
I = I(V, t, ω, f, B),
B B
Introduction
MANSE Midterm Review
Force driving diffusion RT∇c 1010 N m-3
Lorentz force j x B 103
Field gradient force (μ0/2)cχ∇H2 103
Driving force for natural convection Δρg 102
Viscous drag ρν∇2v 102
Magnetic damping σv x B x B 10
Amperian force ~μ0 j2l 10-4
c is the molar concentration, χ is the molar susceptibility
Body force densities
MANSE Midterm Review
E = - (μ0/2)cχH2
The force density acting on a non-uniformly magnetized material is most easily calculated from the Coulomb model: f = µ0qmH0 ⇒ F = -µ0(∇. M)H0 qm is magnetic ‘charge’
but B = µ0(H + M) and ∇.B = 0 0 = ∇.(H + M) ∇. H = - ∇. M F = µ0(∇. H)H0
but the applied field H0is uniform, so the force is zero when the demagnetizing field is Hm = -NM = NχH is negligible.
F = - ∇E = (μ0/2)cχ∇H2 + (μ0/2) χH2∇c
Is there a concentration gradient force ?
MANSE Midterm Review
Susceptibility of ionic solutions is the sum of the contributions of the ions and that of the water; χwater = -9.0 10-6
χ = χwater + cχmol
Electrolyte susceptibility
Susceptibility of ions at 295 K
Ion Configuration S peff2 χmol χ
(m3 mol-1) (1- molar)
Ti3+ V2+ Cu2+ 3d1, 3d9 1/2 3 15.7 10-9 6.7 10-6
V3+, Ni2+ 3d2, 3d8 1 8 41.9 32.9
Cr3+, Co2+ 3d3, 3d7 3/2 15 78.6 69.6
Mn3+, Fe2+ 3d4, 3d8 2 24 125.7 116.7
Mn2+, Fe3+ 3d5 5/2 35 183.3 174.3
χ Is at most ~ 10-4 Hence the demagnetizing field is negligible Ferrofluid
Hm/H0 ≈ 0.1
B
MANSE Midterm Review
The Lorentz force F = j x B is responsible for most of the observed magnetic field effects in electrochemistry — the magnetohydrodynamic (MHD) effect.
Lorentz force effects
A current density of 1 mA mm-2 in a field of 1 tesla gives a body force of 103 N m-3. The Lorentz force can be expected to significantly modify the pattern of convection and flow in electrochemical cells.
Electrodeposition of Cu
MANSE Midterm Review
c = 0
c = c∞
E l e c t r o d e
Solution
δd
concentration gradient is ~ linear over the diffusion layer δ
δh ~ 1 mm δd ~ 0.1 mm
MANSE Midterm Review
The effect of the magnetic field on the mass transport (copper deposition rate) is equivalent to gentle stirring
J = j0 + aB0.35
0 1 2 3 4 50.0
0.5
1.0
1.5
2.0
2.5
3.0
3.5(b) 0.1 M CuSO4, B vertical, H c/a
- 40 mV - 200 mV - 550 mV
Norm
aliz
ed s
hift
in j
MEA
S
B, Tesla
MANSE Midterm Review
Electrode
Vortex at the electrode edge.
B = 0.3 T
Velocity profile
x yz
The Aogaki Cell B
j
v v v v
n = 1/3
J = j0 + aBn
MANSE Midterm Review
Magnetic field can shift the rest potential of magnetic and nonmagnetic electrodes The effect is related to corrosion
-0.4 -0.3 -0.2 -0.1 0.0
1E-4
1E-3
0.01
0.1
E vs. SHE (V)
1.5 T 0 T|j|
(A c
m-2)
Iron pH 1
Rest potential shift
E0(0) E0(B)
jL(0)
jL(B)
E
Anodic
Cathodic Ln| j|
Ea Ec
Cathodic
B
At the rest potential, there are compensating cathodic and anodic currents. When the cathodic current is mass-transport limited, the primary mechanism is a small-scale stirring produced by the Lorentz force; ‘Micro MHD effect’.
Evans diagram
MANSE Midterm Review
0 150 300 450 600 750-0.15
-0.10
-0.05
0.00
µ0H = 0 T
µ0H = 1.5 T
E0 v
s. S
HE
(V)
Time (s)
Corrosion of Fe in 1M KHO3 pH = 1 µ0H (T) Rate (nm s-1) 0 17.0
1.5 29.2
Magnetic field can inhibit the corrosion of copper or silver in acid
The corrosion of both copper and silver in nitric acid involves a catalyst HNO2
and formation of a passive oxide layer. Magnetic field (or electrode rotation) helps to remove the HNO2 catalyst from the vicinity of the electrode, thereby reducing corrosion. The driving force is the Lorentz force, producing the micro MHD effect. The lengthscale of the local electrochemical cells, for micro-MHD effect, is ~ 10 microns.
Corrosion
MANSE Midterm Review
t
V
1/f2 1/f2noise characteristic of a coalescece penomemon
Field reduces average bubble size by half - 45 to 24 microns; twist off effect
Overpotential for hydrogen generation reduced by 10 %
B
Galvanostatic - 10 mA
Hydrogen
MANSE Midterm Review
F = (1/µ0)cχ∇B2
Field gradient force
With suitably designed field gradient electrodes it is possible to create very large magnetic field gradients, and exert force densities of up to 106 N m-2, which can have important effects in confining reagent species at the electrode surface. Free alumina membrane template
Alumina membrane template with back metallic contact
Pt
Electrodeposited alloy into the membrane
1 µm 500 nm
60-70µm
Magnetic field gradient effects
MANSE Midterm Review
MANSE Midterm Review
B
∇B
Model oxygen reduction reaction. Borate buffer pH 8.4
MANSE Midterm Review
Data from chronoamperometry experiments.
Current density (A m-2) (Chronoamperometry)
Air-saturated borate bath Oxygenated borate bath
Electrode
Cathode
Rotat-
ion
rate
(rpm)
0.0 T 0.4 T Enhanc
ement
1.0 T Enhanc
ement
0.0 T 0.4 T Enhanc
ement
1.0T Enhanc
ement
A
0
500
1000
2000
3000
1.2
5.2
7.5
9.0
11.0
1.7
7.5
10.0
14.0
17.0
41.0
44.0
34.0
55.0
55.0
(47)
6.0
12.0
16.0
22.0
27.0
330.0
130.0
113.0
144.0
145.0
(135)
4.0
12.5
17.0
22.0
26.0
20.0
40.0
50.0
62.0
68.0
400.0
220.0
194.0
180.0
160.0
(189)
27.0
50.0
70.0
87.0
100.0
575.0
300.0
310.0
295.0
284.0
(297)
B
0
500
1000
2000
3000
2.4
4.0
4.5
5.6
6.1
3.3
5.0
5.5
6.8
7.3
37.0
25.0
22.0
21.0
20.0
(22)
3.4
5.0
5.6
6.9
7.5
41.0
25.0
24.0
23.0
23.0
(24)
2.6
4.8
6.0
7.6
8.7
3.7
5.5
7.5
11.0
12.0
26.0
15.0
25.0
44.0
38.0
(31)
3.0
5.8
8.0
11.8
13.0
16.0
21.0
33.0
55.0
50.0
(40)
C
0
500
1000
2000
3000
0.2
1.2
2.2
2.8
3.6
0.2
1.5
2.3
3.1
3.7
0.0
7.0
5.0
10.0
3.0
(6)
0.2
1.6
2.3
3.1
3.8
0.0
14.0
5.0
10.0
5.0
(9)
1.5
5.0
6.7
8.7
11.0
1.7
5.2
7.0
9.0
11.5
13.0
4.0
5.0
4.0
4.0
(4)
1.8
5.2
7.5
9.5
11.5
20.0
4.0
12.0
10.0
5.0
(8)
Pt/cobalt nanowire eletrode
Pt/cobalt film electrode
Pt electode
Cobalt nanowires in an applied field produce much enhanced B∇B close to the Pt electrode surface.
More than 10 x current enhancement
MANSE Midterm Review
Electrode A
Electrode C
This is not a mass transport effect, as it is independent of electrode rotation speed
MANSE Midterm Review
Nitrobenzene - a model system for magnetoelectrochemistry
Coloured paramagnetic reduced Nb species Original motivation to discount ‘concentration
gradient force’ Extensive investigation, including impedance
spectroscopy
MANSE Midterm Review
Static magnetic fields have variou unexpected effects on electrochemical processes
Many of these effects can be traced to magnetohydrodynamic effects driven by the Lorentz force j x B, on a whole-cell or micron scale.
There are interesting opportunities for manipulating paramagnetic species in solution, O2, free radicals ….. using nanostructured field gradient electrodes.
Possible benefits for energy-related applications; electrolysis of water, oxygen electrode in fuel cells,….
Conclusions
MANSE Midterm Review
Future work
Bipolar nanowire array Self-assembly of organic cables Further work with field gradient electrode - hydrolysis Electrodeposition in ionic liquids Can Maxwell stress influence electrode reactions of
paramagnetic species?
MANSE Midterm Review
Outline
Background
TiO2:Fe
Magnetic silicon
Graphite
Anthracene
MgO:N
Au nanoparticles
A model — Charge-transfer ferromagnetism