The use of Voltage Probes in Impedance Spectroscopy

G. Fafilek

Inst. of Chemical Technology and AnalyticsUniversity of Technology Vienna,

Getreidemarkt 9/164EC, A-1060 Vienna, Austria

Why to be considered ?

systematic errors in impedance measurements (measuring artifacts!)

can be caused by the• cell• connections, setup (leads, electrodes, etc.)• meters and control circuits (potentiostat)• last but not least: by the sample itself!

topic will be:discussion of errors introduced by voltage probes like RE

Outline:

1. From 2-probe impedance measurements to measurements with reference electrodes

2. Derivation of artifacts in the 3-electrode configuration

and their minimization

3. Artifacts in 4-electrode configuration

4. an instrumental solution against artifacts

5. an exception: the sample itself causes the artifacts

Zx

Ix=Im

Ux

Zint

Zint x

xx

m

mmx I

ZIUI

UZZ int2⋅+==≠

m

mm

x

xx I

U ZI

U Z ===Um

The voltage drop at the interface Electrode | Ion-conductor increases the measured voltage

Ion

-con

duct

orE

lect

ron

-co

nduc

tor

m

mm I

UZ =UmZm

Ix=Im

Different impedance parts of the total impedance may be separatedbecause of different time constants

Separation Zint and Zxpossible !

-Im(Zm)

Re(Zm)

ZintZx

Rs

Ix=Im

Ux

ZCE

ZWE

Um

ac. measurements in the 3 electrode configuration

Ix

ZWEV

ZCE

A

ZRE

IV = 0~

U1

I1

U1

I1

U2

I2

Z

1

1

IUZ =

01

2

2 =

=I

m IUZ

2-pole 4-pole

load, burden

Ix

ZWEV

ZCE

A

ZRE

IV = 0

CCR

CWR

CWC

ZV

IV ≠ 0~

Derivation of the impedance response of a 3 terminal network

CRCERE CRRL =1

CERE

CERE

RRRRR

+=1

CR

WCWR

WE

WCCE

WE

WRRE

CCC

RCR

RCRC −+=2

[ ]2

2 CRCRRRRRRRWE

CRRECECEWEREWE ⋅++=

CR

WCWRWCWR C

CCCCC ++=1

RWE

RCE

RRE

CCR

CWR

CWC

S.Fletcher, Electrochemistry Communications, 3 (2001) 692

HL 11 >

3-terminal network

RWE

C1

C2 R2

R1

L1

Result of a measurement would be this

equivalent 2-terminal

network:

Important conclusions:

• Reduce the impedance of the counter electrode and of the reference electrode by

• large and reversible counter electrodes• well reversible reference electrodes and/or with ac. bypass• well conducting electrolyte solution

• Reduce the capacitive coupling between counter and reference electrode

Is it possible to avoid the coupling capacitances ?especially between CE and RE

as an example:

4-Probe conductivity measurements of an solid electrolyte

More voltage–probes ?

x

xx I

UZ =

Ux

Ix

Zx

Zint

Zint

V

Acurrent electrodes

voltage electrodes

Rs

Vout

samplesample

interfaceinterface

bulkbulkZi

Zi

Zi

Zi

Zb

Zx

Zb

Cs//Zin

Cs

Cs

Cg+Cin Cg Cg Cg

Cs

Ri

Um

Im

m

mmx I

UZZ =≠

Rv

Vout

Ri

Um

sample holder

INA1

INA2Im

Standard setup for a.c. 4-probe measurements

+

-

active shield

buffer

Cnoise

CsRs

ground

Ux Ux-Ud

Ud≈0

UnoiseV

signal line

Rs

Vout

Zi

Zi

Zi

Zi

Zb

Zx

Zb

Zin

Cg+CsIm

Um

virtual ground

Rs

Vout

sample holder

active shields

Ri

INA1

U/I

U1

U2

V1

V2

virtual ground

Um

Im

xiin ZZZfor ,>>

Active shielding to reduce stray and coupling capacities

G.Fafilek, M.W.Breiter, J.Electroanal.Chem. 430 (1997) 269

inix

inx

m

mm ZZZ

ZZI

UZ++

⋅==2

xm ZZ ≅

Zi

Zi

Zb

Zx

Zb

Um

ω↓⇒ Xc↑ ⇒ UC↑ ⇒ Um↓ ⇒ Zm↓

XCUx

UC

UC

CXC ω

1=

planeZ −

)Re(Z

)Im(Z−

Inductive loops0=ω

∞=ω

ω

Voltage divider effect

inductive properties !!

Zm

Input-resistance of the meter

or … ?

using very high input impedance amplifiers

→ no source of artifacts anymore

4-probe measurements on BICUVOX single crystalsComposition: BICUVOX.10 (Bi2Cu0.1V0.9O5.35)

Size: length = 5 mmwidth = 0.18 mmheight = 0.33 mm

Current electrodes: platinum paste for current collectors

Voltage electrodes: active shields, platinum tips, distance d = 3 mm, touching the side where the conduction planes reaches the surface.

100 101 102 103 104 1054x101

5x101

6x101

|Z| / Ω

f / Hz

-10

0

10

20

554 °C

-φ / deg

30 40 50 60-5

0

5

10

15

20

25

-IM(Z) / Ω

Re(Z) / Ω

Why inductive loops?

• well conducting solid electrolyte• low polarization resistance of the RE/SE interface • active shielding and high input impedance→ therefore no source of polarization at RE

Rel

Vbut

• small electronic conductivity→ internal load for the

RE/SE interface

Final conclusions:• Be aware of coupling effects between different signal lines

(crosstalk) → Inductance

• Be aware of interfacial effects at the voltage probes (RE) due to load by meter or leads (and still even if you are using a proper setup)→ Inductance

• Active shielding of RE leads can reduce both, crosstalk and load

• Avoid physical interpretation of artifacts!