A new setup for characterization of solid material oxygen exchange and conductivity in a broad oxygen partial pressure range and at elevated temperatures is presented. The development target of this setup is directed towards the detection of ultra-low amounts of exchanged oxygen. For this, electrochemical cells made of yttria-stabilized zirconia (YSZ) were optimized and applied in a flow-through arrangement. The design and process measures enable a lower limit of detection below 100 pmol of exchanged oxygen. Furthermore, the system characteristics concerning oxygen dispersion, titration efficiency and electrode kinetics are described.
Oxygen solid electrolyte coulometry (OSEC) performed with electrochemical
cells made of yttria-stabilized zirconia is a long-standing method used for
gas sensing, material characterization as well as biological or medical
measurements of oxygen exchange (Fouletier et al., 1975; Teske et al., 1986;
Uhlmann et al., 1999; Sahibzada et al., 2000; Vashook et al., 2012;
Stöber et al., 2018). The method is based on the measurement of charges
or currents for batch-like or flow-through titration of oxygen by solid
electrolyte cells based on Faraday's law. A typical operation temperature of
such cells is 750
The experimental setup used in this work consists of the parts illustrated in Fig. 1.
Schematic representation of the titration setup consisting of an oxygen titration cell (cell one), a sample furnace with a holder for four-point measurements, followed by a second oxygen titration cell (cell two).
A gas mixing station (not shown) for adjusting the measuring gas flow and the
incoming oxygen partial pressure
This constant polarization is accompanied by high-resolution measurement of
the electrolysis current. The used titration cells contain additional
downstream Nernst cells (NC1 and NC2, not shown in Fig. 1) for the
potentiometric monitoring of
Schematic representation of the four-point contacting of the sample for AC and DC conductivity measurements.
The electrical connection with the sample was established by pressed platinum contacts as schematically given in Fig. 2. These contacts are pressed onto the sample by two different high-temperature springs in a reversible manner to ensure rapid and reliable electrical connection for each sample to be investigated in the above-mentioned temperature range. The four-point contacting is provided for AC and DC conductivity measurements at samples to be investigated as the main focus of the setup. Those measurements are not presented in this paper, which is directed towards the characterization of important setup parameters. As described in detail in the next chapter, the characterization of the lower limit of detection was carried out by polarizing an YSZ sample using an Interface 1000 potentiostat (Gamry Instruments, Warminster, USA).
The control unit for the setup is described in detail by Schelter et al. (2013). It monitors the potentials of downstream NC1/2, sample temperature and titration current at cell two.
All measurements were conducted with
To determine the oxygen partial pressure range, in which the coulometric
oxygen titration can be applied, the polarizations of cell one
To demonstrate the lower limit of detection for exchanged oxygen, a sample of
8 mol-% YSZ with the dimensions of
An example of the oxygen dispersion in the non-heated sample furnace and the titration efficiency of cell two is shown in Fig. 3.
Plot of the current pulse in cell one (black) and the titration
currents in cell two (blue, red). Furnace temperature 25
The black curve indicates a pulse of the pumping current applied at cell one,
while the titration currents measured at the flow rates 50 and
100 mL min
The results in Fig. 3 show that nearly 100 % of the oxygen introduced by pumping can be measured at cell two up to very large amounts of introduced oxygen.
The titration kinetics in cell two was characterized by measuring the amount
of oxygen slipping through cell two at
Amount of oxygen, slipped through cell two during titration as a
measure for the electrode kinetics, depending on the amount of introduced
oxygen at different flow rates; cell temperatures 750
In addition, the fraction of slipped oxygen in relation to the total amount
of pumped oxygen is displayed for three selected measuring points. As
expected, the amount of slipping oxygen increases with the amount of
introduced oxygen and with the flow rate. The largest oxygen pulse applied at
100 mL min
The results verify that relatively high amounts of oxygen can be titrated
with appropriate precision if the flow rate does not considerably exceed
50 mL min
The extent of slip-free coulometric titration at increasing basic oxygen
partial pressure in the measuring gas is illustrated in Fig. 5 for pulses of
78 nmol
Titration yield for an oxygen pulse of 500
The yield of the titration was found to be 100 % for the experiments
conducted with a
This result gives a first insight into the upper limit of detection of this setup for high-precision coulometry at higher oxygen partial pressure.
The experiments for characterizing the setup at small amounts of pumped
oxygen were accomplished by pumping very small amounts of oxygen into the
system at the sample position by using the YSZ sample described in Sect. 2.2.
While adjusting
During the pulse the amount of oxygen uptake is much smaller than that of oxygen release since the surrounding gas atmosphere contains nearly no oxygen. Therefore, the sample releases net oxygen during the pulse and takes it up again after pulse end. The released oxygen then flows downstream and can be detected in cell two. This measurement can also be used as a highly sensitive calibration method for this kind of OSEC setup.
Oxygen titration peak measured with cell two of the setup given in
Fig. 1 at
Oxygen titration peak measured with cell two of the setup given in
Fig. 1 at
In Fig. 6 an example of the resulting titration current pulse at cell two is
shown. Here a pulse of
To characterize the lower limit of detection of this setup, the polarization
time of the sample was stepwise decreased. In Fig. 7 the titration current is
plotted for a polarization of 25 mV for 10 s. This caused a charge transfer
of 80
These results demonstrate the capabilities of this setup to detect ultra-small amounts of oxygen down to values around 100 pmol, which are exchanged between the sample and the measuring gas.
The key parameters upper and lower limits of detection and titration kinetics
of a setup for oxygen exchange measurements at solid materials were
investigated. The setup was tested at temperatures between room temperature
and 650
The results prove that the newly developed arrangement with its highly
improved overall tightness enables the precise measurement of exchanged
oxygen or oxidizable gases from the picomol up to the higher nanomol range.
During the pulse-like oxygen introduction experiments considerable deviations
of the titrated oxygen from the total oxygen to be measured were found at
pulse currents above 2 mA for the flow rate 100 mL min
The underlying measurement data are not publicly available and can be requested from the authors if required.
AH, as the main author, planned and manufactured important parts of the setup, planned and performed the experiments, carried out the data evaluation and drafted the manuscript. JY participated in building the setup and in performing experiments. JZ designed the experimental setup and supervised the experiments. VV provided fundamental ideas for the experimental setup. WO and MM discussed the experimental results and contributed to the manuscript.
The authors declare that they have no conflict of interest.
This article is part of the special issue “Sensors and Measurement Systems 2018”. It is a result of the “Sensoren und Messsysteme 2018, 19. ITG-/GMA-Fachtagung”, Nürnberg, Germany, from 26 June 2018 to 27 June 2018.
Parts of the work are funded by the German Federal Ministry of Education and Research, project nos. 03EK3029C and 03SF0542D, and by Sächsische Aufbaubank, grant 100243804/3235. The authors want to thank Matthias Schelter for his valuable contributions to this work. Edited by: Peter A. Lieberzeit Reviewed by: two anonymous referees