1

FEM simulation of the scanning electrochemical potential microscopy (SECPM)

Fayçal. R. Hamou

Max-Planck Institut für Eisenforschung GmbH

Interface Chemistry and Surface Engineering Department

Atomistic Modeling Group (AMG) IMPRS-SurMat

P.U. Biedermann, M. Rohwerder, A.T. Blumeneau

Presented at the COMSOL Conference 2008 Hannover

2

Electrochemical Scanning Probe

Microscope Systems

Electrochemical Atomic Force Microscope

(ECAFM)

Electrochemical Scanning Tunneling Microscope

(ECSTM)

Scanning Electrochemical Potential Microscope

(SECPM)

STM imaging of electrode surfaces in solution under electrochemical control.

nanometer-resolution AFM imaging of electrode surfaces in solution

Scanning Electrochemical Microscope (SECM)

Imaging the surface using the Faradaic current from electrolysis of solution species

ESPM

3

Scanning Electrochemical Potential Microscopy (SECPM)

•in-situ imaging or potential mapping of the electrode surface with nanometer-scale resolution

Applications: electroplating, corrosion, and battery research and development.

•Measure the profile of Electrochemical Potential at the metal/electrolyte interface

4

Potential imaging

5

EDL Potential Profiling

6

Potential Profiling:

SECPM measurement

• Low concentration

• Polar liquids

• Totally Inert metallic probe

Interpretation of the

SECPM measurement

Estimation of the parameters:

•Debye length

•Concentration profile at the interface.

Theoretical model : Gouy-Chapmann-Stern

Fitting the results

7

C. Hurth, C. Li, and Allen J. Bard, J. Phys. Chem. C 2007, 111, 4620-4627

Recent experimental results

• The Gouy-Chpmann-Stern failed to describe the experiment results

• Non-Boltzmannian distribution of the ions

• The tip perturbs the electric double layer (EDL interaction)

• Theoretical approach is needed to interpret the SECPM measurement

M. Rohwerder and J.W. Yan MPIE

8

Motivation

SECPM measurement

FEM

SECPM Simulation

EDL Simulation

probe Simulation

• EDL interaction

• Tip geometry effect

• Coating effect

• Theoretical Model for fitting

9

Axial symmetric Model

Electric Field

Electric potential

V

Working electrode

z

r

z

Revolving the geometry

Extrusion of the solutionThe full 3D model

v

E

E

Tip

10

( ) 01112

1 =∇∇−∇− VCRTzFDCD

( ) 02222

2 =∇∇−∇− VCRTzFDCD

( )o iε ε V ρ NC zq−∇ ∇ = =∑

Solving the PD equations for C1 C2 V and Vt

Finite element method0 't tV Gauss equationε ε ρ∇ =

1 1 2 2( , ) ( , )Q N z qC x y N z qC x y= +∫Surrounding the metallic tip

ED

LP

robe

SE

CP

M

Nernst-Planck Equation in the steady state

Multiphysics Modelling

Symmetric electrolyte 1:1

Physics

11

Protruding tip Non-Protruding tip

Tip geometry effect

12

Tip geometry effect: Electric potential distribution

Protruding tip Non-Protruding tip

13

Cation Anion

Tip geometry effect: Ions distribution

14

Tip geometry effect: Ions distribution

Cation Anion

15

Tip geometry effect: electric potential

16

Tip geometry effect: ions distribution

17

ε =2.3ε =4.5

Coating effect

Cation distribution

18

Coating effect: electric potential

19

Coating effect: ions distribution

20

Outlook: Ion size effect, steric effect

The Gouy-Chapmann-Stern Model not valid for SECPM fitting or simulation

Steric effect

SECPM experiment: applied electric potential >200 mV

Modified Poisson Boltzmann

2

20

2sinh

1 2 sinh

b

zqVNzqC kTV

zqVkT

εε υ

⎛ ⎞⎜ ⎟⎝ ⎠∇ =⎛ ⎞+ ⎜ ⎟⎝ ⎠

M. S. Kilic, M.Z Bazant and A. Ajdari: Phys. Rev. E, 75, 021502 (2007)

21

Steric effect

Potential Cation concentration

Near future: SECPM Simulation with the modified Poisson-Boltzmann Model

22

Thanks to:

• IMPRS-SurMat program for financial support

• Dr. Andreas Erbe form MPIE

• YOU for listening