v magnetoelectrochemistry - trinity college dublinstamenov/manse/manse midterm... · manse midterm...

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


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


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


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


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


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 ?


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


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


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


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


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


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


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


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


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


B

∇B

Model oxygen reduction reaction. Borate buffer pH 8.4


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


Electrode A

Electrode C

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


Nitrobenzene - a model system for magnetoelectrochemistry

  Coloured paramagnetic reduced Nb species   Original motivation to discount ‘concentration

gradient force’   Extensive investigation, including impedance

spectroscopy


  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


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?


Outline

  Background

  TiO2:Fe

  Magnetic silicon

  Graphite

  Anthracene

  MgO:N

  Au nanoparticles

  A model — Charge-transfer ferromagnetism

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