the use of voltage probes in impedance spectroscopyimspe/lec/notes_fafilek.pdf · 2003-10-16 ·...
TRANSCRIPT
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!