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SOLID
STATE
ELSEWER
IONICS
Solid State Ionics 86-88 (1996) 141551419
Automated
device for electrochemical measurements
to the system Pt/ solid electrolyte
and application
G. Fafilek”, K. Leeb, M.W. Breiter
Institut
fiir Technische Elektrochemie,
TU Wien, Getreidemarkt
9, A 1060 Wien, Austria
Abstract
at a prescribed set of constant temperatures was put together in our
curves in the potentiostatic or galvanostatic mode, to do cyclic
voltammetry or coulometry at constant potential or to measure the potential-time
dependence at a constant current. A special
holder for the solid electrolyte was constructed. Voltammetric measurements on yttrium stabilized zirconia and of the new
oxygen ion conductor BICUVOX.10 are presented as examples.
An automated device for voltammetric
measurements
institute. It allows one to take steady-state current-potential
Keywords:
Steady state measurements; Cyclic voltammetry; Zirconia; BICUVOX; Oxygen electrode; Solid electrolytes
1. Introduction
New methods
for data analysis like Nonlinear
(NLSF), Fast Fourier Transform or statistical evaluation of data require a large
amount of accurate data to obtain reliable and
significant results. NLSF has been successfully used
for the simulation of the frequency dependence of
impedance data by equivalent circuits. Automated
systems for impedance measurements
[l] were developed which allow one to acquire the necessary
amount of data for this type of processing in a
reasonable time. Although determining
the kinetic
parameters of the expected reaction mechanism with
NLSF from impedance data has become popular, a
similar development
is lacking for voltammetric
measurements in which the electrode potential versus
a reference electrode is controlled as a function of
Least
Squares
Fitting
*Corresponding author.
time by a potentiostat and the current is measured. A
device which allows the automation of voltammetric
measurements
at a prescribed set of constant temperatures was developed in our institute.
2. Description
of the setup
The Solartron 1286A Electrochemical
Interface
serves as a programmable
potentiostat. A PC/486compatible computer produces the desired potential
time profiles and controls the temperature (Fig. 1).
The furnace is connected to the dc power supply HP
6269B to avoid noise from the power line. Two
thermocouples
are used because of the difficulties
due to the long response time of the heat transfer
from the furnace to the sample. One thermocouple is
placed close to the heating element in the middle of
the hot zone and connected to a Eurothetm 815s.
The setpoint temperature is transferred to the control-
0167-2738/96/$15.00 Copyright 01996 Elsevier Science B.V. All rights reserved
PII SO167-2738(96)00323-2
1416
G. Fajilek et al. I Solid State tonics 86-88
(1996) 1415-1419
leads to SI 1286
REI
CE
CE RE2 WE
il
I
Fig. 1. Block diagram of the whole setup for high temperature
electrochemical
measurements.
ler from the computer through a serial interface. A
second thermocouple measures the temperature close
to the sample. The amplified and digitized voltage of
this thermocouple is acquired through a second serial
interface. The programmed
algorithm allows the
establishment of a chosen sample temperature within
the accuracy (if necessary) of 1 K in a reasonable
time period.
\Al205plate
Fig. 2. Sample holder for solid electrolytes.
3. Sample holder
The holder (Fig. 2) is designed for solid electrolytes to which three or four electrodes are attached. A similar sample holder is used in our group
for impedance measurements
on solid electrolytes
[l]. It consists of four stainless steel tubes and two
plates which are brazed together to form a rigid and
electrically
shielded construction.
Two additional
tubes are placed in the centre of the plates. Alumina
tubes inside these two and two of the outer tubes are
used for insulating the leads to the electrodes. On the
end of the holder which is placed in the hot zone of
the furnace an alumina plate is mounted. It supports
a platinum plate which is connected to one platinum
lead. On this plate the sample is placed with its
counter electrode (RE). Two conical platinum cylinders are used for contacting the working and the
reference electrodes on the top of the sample. One of
them is glued by a suitable cement onto one alumina
tube with one hole for a platinum wire. It is used as
connection
to the reference electrode (RE). The
second cylinder, glued on an alumina tube with two
holes, serves as a contact to the working electrode
(WE) and is connected to one lead for current and
one for voltage sensing. The tubes with the point
contacts are movable
and independently
spring
loaded in the cold part of the sample holder. A brass
head is mounted at this side of the holder. It holds
the BNC jacks, gas inlet and outlet as well as the
feedthrough for the thermocouple.
A quartz tube,
closed on one end, covers the holder and is fitted
with an o-ring to the brass head. Measurements can
be made under flow of gas, e.g. different mixtures of
oxygen and nitrogen in our case.
4. Programming
Different temperature ramps are possible in which
the start, stop and the temperature
step can be
programmed.
For each temperature the previously
chosen type of electrochemical
measurement
is
performed. Before a measurement
is started, the
potential difference E, between RE and WE at open
G. Fajlek et al. I Solid State Ionics 86-88
circuit condition is measured by the program. A
waiting period is applied until a stable value of
potential is obtained. The level of stability (mV/min)
can be set in the program. Three basic types of
measurements
are possible: potentiostatic
or galvanostatic
steady-state
measurements
and cyclic
voltammetry. Several types of ramps with start- and
stop-values and steps can be prescribed. To include
the possibility of time-controlled
measurements,
the
time dependence of the current after a potential step
or of the potential after a current step can be
measured. The maximum time resolution is determined by the fastest data acquisition rate of the
Solar&on 1286A which unfortunately
is only 15
readings/s. Therefore the highest reasonable value of
the ramp rate in the sweep mode is 200 mV/s. A
specified change of the measured value with time can
be used as criteria before proceeding to the next
point. After finishing the potentiostatic, galvanostatic
or sweep program the next temperature will be set.
5. Experimental
Yttrium (Frialit FZY) and calcium
(Degussit
ZR23) stabilized zirconia were used for reference
measurements
on the system Pt/ZrO,/Pt
for comparison with results in the literature and to find the
optimal electrode configuration with a small ohmic
potential drop. In addition, the results of cycling
voltammetry
and steady state measurements
were
compared with those of the cell Pt/BICUVOX/Pt.
BICUVOX [2] is a ternary oxide with good ionic
conductivity at lower temperatures. The solid electrolyte samples were cut into rectangular pieces and
polished with SIC paper up to grade 1200. After
a)
b)
Fig. 3. Electrode
positions,
(1996) 1415-1419
washing with distilled water in an ultrasonic bath,
the samples were dried in air at 160°C for several
hours. Electrodes
were produced
by sputtering
platinum (approximately
100 nm) or by applying
platinum paste without flux (Heraprint C3605). The
platinum paste had to be kept for 10 min in air at
800°C in the case of YSZ samples and at 700°C for
BICUVOX samples. Sputtered electrodes had to be
heated to 600°C before the measurements to accomplish recrystallisation.
The whole sample was flushed
by gas mixtures of oxygen/nitrogen
with the following 0, contents: lOO%, 20%, 1% and 0.1%. The area
of the working electrode and the counter electrode
was about 0.16 cm*, the area of the reference
electrode about 0.02 cm*. Different electrode configurations (Fig. 3) were used for the cyclic voltammetry on zirconia. Calculation
of the iR-free
values of polarization
is only possible when the
ohmic resistance of the electrolyte is independent of
time and current density [2]. The direct determination of the ohmic part of polarization between WE
and RE was made by a modified ac 4-probe technique. The principle of the 4-probe setup which was
used for this measurement is described in [3]. The
current was applied between CE and WE and the
voltage was measured between RE and WE (see Fig.
3(c)). Values for the impedance were taken at several
temperatures in a frequency range between 1 Hz and
100 kHz for the separation of the bulk resistance
from the electrode impedance. Measurements
with
the sampling iR-compensation
mode which is supported by the SI1286A were also made. The temperature range for electrochemical
measurements
was restricted by the accuracy and stability of the
RE. In all measurements, the reference electrode was
from the same type as the working and the counter
I CE
considered
1417
for voltammetric
measurements.
1418
G. Fajilek et al. I Solid State Ionics 86-88
electrode. Measurements were taken between 400°C
and 700°C. The potential sweep for cycling voltammetry was from 0 to + 0.4 volt down to - 0.4 V
and back to 0 with a rate of 50 mV/s. After setting
WE to 0 V against the RE for 10 s, the potential
sweep was applied two times. For steady-state
measurements the potential was varied between - 1
and + 0.5 volt in steps of 10 mV. After setting the
potential, a minimum waiting period time of 300 s
was used. When the current changed less than OS%/
min the next step was applied.
6. Measurements
and results
(1996) 1415-1419
result in cyclic voltammetry
in terms of clearly
defined peaks and a small shift of the peak potential
because of ohmic potential drop. This result is in full
agreement with the results of systematic measurements for different positions of RE and calculations
of Nagata et al. [4]. An analysis of the papers dealing
with CV on Pt/YSZ showed a similar dependence of
the CV-shape upon electrode configuration and morphology [5-71. Therefore position 3(c) was chosen
for all later measurements.
An example for the
dependence
of the CV-shape on the type of the
electrode and the ohmic potential drop is given in
Fig. 5. The superposition
of ohmic potential drop
leads to inaccurate results in peak position in cyclic
voltammetry.
6.1. Open circuit potential
6.3. Steady-state
Open circuit voltages E, were measured before a
voltammetric measurement was started. In the temperature range lower than 450°C an increase of the
open circuit potential between RE and WE was
observed (see Fig. 4). At higher temperatures the
voltage difference was small ( < 10 mV) and stable.
It was assumed that the reference electrode worked
correctly at temperatures higher than 400°C even
with an applied polarization. If the solid electrolyte
has a small electronic conductivity,
the RE is also
polarized because of the electronic current between
RE and WE.
6.2. Cyclic voltammetry
measurements
The results from steady-state measurements in the
sampled iR-mode [8] are shown in Fig. 6. The
stability of the measurement
was bad although
different values for the on/off switching time were
used. In addition the iR-value did not agree with the
results of the ac 4-probe measurements. At 600°C the
calculated value for the ohmic potential drop at the
highest current was only 42 mV (R,, = 17 0, I = 2.47
mA) in contrast to the measured value of 1024 mV.
This indicates that the time constants of the different
processes in the bulk and interface were small and
overlapped partially. The sampled iR-mode of the
instrument
was not adequate for a separation. In
The desirable position of the three electrodes is
shown in Fig. 3(c). This arrangement gave the best
1
0.06
i
0.00
Fig. 4. Dependence
potential M
of the open circuit voltages upon temperature.
Fig. 5. CV curves for sputtered platinum (solid line) and Pt-paste
(dashed line) electrodes. The dotted line is drawn with the
calculated S-free polarization (E - iR) with R = 3500 .f2 at 50°C
from the 4-probe data.
G. Fajlek
et al. I Solid State lonics 86-88
(1996) 14151419
1419
References
-1.00
j
’
-0.75
j
’
-0.50
’
-0.25
’
0.00
’
0.25
1
0.50
potentialM
Fig. 6. Steady-state curves for a BICUVOX sample with platinum
paste electrodes. Solid line: plot with overall polarization. Dotted
line: plot with iR-free polarization. Parameters for sampling mode:
on/off ratio = 20, off time = 100 ms.
addition, there are disturbing spikes of the electronic
switch in the shortest interruption period.
Acknowledgments
The support of this research by the Austrian Fonds
zur Fiirderung der wissenschaftlichen
Forschung is
gratefully acknowledged.
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PI 1286 Electrochemical