V Magnetoelectrochemistry Body forces; Is there a concentration - - PowerPoint PPT Presentation

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

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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

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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

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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

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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

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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 ?

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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 peff

2

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

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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

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c = 0 c = c∞

E l e c t r

• d

e Solution

δd

concentration gradient is ~ linear over the diffusion layer δ

δh ~ 1 mm δd ~ 0.1 mm

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The effect of the magnetic field on the mass transport (copper deposition rate) is equivalent to gentle stirring

J = j0 + aB0.35

1 2 3 4 5 0.0 0.0 0.5 0.5 1.0 1.0 1.5 1.5 2.0 2.0 2.5 2.5 3.0 3.0 3.5 3.5

(b) (b)

0.1 M M CuSO CuSO

4,

, B B vertical, H H c/a c/a

• 40

40 mV mV

• 200

200 mV mV

• 550

550 mV mV Normalized shift shift in in j j

MEAS

B, B, Tesla

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Electrode Vortex at the electrode edge.

B = 0.3 T

Velocity profile

x y z

The Aogaki Cell

B j v v v v n = 1/3

J = j0 + aBn

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Magnetic field can shift the rest potential

• f 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 cm

• 2

)

Iron pH 1

Rest potential shift

E0(0) E0(B) jL(0) jL(B) E Anodic Cathodi c Ln| j| Ea Ec Cathodi c 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

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150 300 450 600 750

• 0.15
• 0.10
• 0.05

0.00 µ

0H = 0 T

µ

0H = 1.5 T

E

0 vs. SHE (V)

Time (s)

Corrosion of Fe in 1M KHO3 pH = 1 µ0H (T) Rate (nm s-1) 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

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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

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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 1 µm 500 nm 500 nm 60-70 60-70µm

Magnetic field gradient effects

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B ∇B Model oxygen reduction

• reaction. Borate buffer

pH 8.4

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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 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 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 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

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Electrode A Electrode C This is not a mass transport effect, as it is independent of electrode rotation speed

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Nitrobenzene - a model system for magnetoelectrochemistry

• Coloured paramagnetic reduced Nb species
• Original motivation to discount ‘concentration

gradient force’

• Extensive investigation, including impedance

spectroscopy

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• 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

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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?

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Outline

• Background
• TiO2:Fe



• Magnetic silicon



• Graphite



• Anthracene



• MgO:N



• Au nanoparticles



• A model — Charge-transfer ferromagnetism