Gold paste and Au mesh were applied on electrode surfaces as current collector. For cathode with Ni, Ni pieces were used (but proven to be oxidized when using steam without H2) rather than Au as current collector in order to avoid unwanted
reaction between Ni and Au. The specimens with gold paste were fired at 900oC for 2
hours.
2.3.2 Electrochemical tests
Two-probe and four-probe tests were done for whole-cell impedance and I-V measurements.
2.3.2.1 Two-probe tests
Initial thick cells were measured by two-probe tests. The jig for the two-probe electrolysis measurement is illustrated in figure 2.10. It can provide the cell with
controlled atmosphere at both electrode surfaces at temperatures up to 950oC. The jig
is a custom-made alumina cylinder with holes for gas flow and electrode access. The counter electrode was made from single-bore alumina tube with a Pt-wire electrical contact. The working electrode was a two-bore alumina tube with Pt wire electrical contact and Pt/Rh thermocouple wire which measures the real temperature inside the jig. The prepared pellets were then sealed in the jig with gold rings and with pressures on top to help sealing. Gas inlet for the anode was O2, and for the cathode was
3%steam/Ar/4%H2 or 3%steam/Ar. Steam was produced by sending pure Ar/5% H2 or
pure Ar though a steam generator. The steam generator was a gas bubbler in water surrounded by a heating mantle set at temperature for required amount of steam. The electrodes were connected to a Solartron SI 1255 Frequency Response Analyzer for impedance measurements.
Figure 2.10. a. Two-probe test setup for polarizations measurements; b. Schematic drawing of the four probe test.
SOECs with 2mm YSZ electrolyte and six different cathode structures were tested: NiO/YSZ (35% YSZ by weight), LSCM, LSCM/CGO (50% LSCM by weight), LSCM/YSZ (75% LSCM by weight), LSCM/YSZ (50% LSCM by weight), and LSCM/NiO(50% LSCM by weight). Cathodes with NiO were pre-reduced to Ni by checking OCV stabilization before test.
2.3.2.2 Four-probe tests
The four-probe test is a test that could eliminate the stray resistance from the system, eg from thin leads. An SOEC was fixed in an electrolysis measurement jig by attaching the pellet to the alumina tube in the jig using Ceramabond 552-VFG (Aremco Products, INC.), which could prevent leakage and separate two atmospheres for the two electrodes. The method for sealing cells with Ceramabond, which avoids compressive stresses, could also prevent cracking of thin cells that do not have high mechanical strength. Cells were tested by two-electrode four-wire measurement. Two wires on the same side should be insulated to each other. The cells worked with
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controlled flowing mixture gas (either 3%steam/Ar/4%H2 or 3%steam/Ar) for the
cathode surface and with the anode side open to air. Steam in the mixture gas for cathode side was generated by a gas bubbler in water sitting in a heating mantle which provides different water temperatures for required amounts of steam compositions.
Counter electrode (Anode)
Working electrode (Cathode)
CE RE1
WE RE2
cathode anode
Figure 2.11. a. Four-probe, 2-electrode test setup for polarization measurements; b. Schematic drawing of the four probe test.
Measurements of impedance, current-voltage curve and potential step were done by a Solartron SI 1255 Frequency Response Analyzer at different temperatures and in
two different cathode atmospheres (3%steam/Ar/4%H2 or 3%steam/Ar). The potential
step was set to sweep the potential between -1.0V and -1.5V / -2.0V across cell in three discrete potential steps and to keep each step for 960 seconds. Each step was long enough for cell stabilization. Gas chromatography was applied to analyze the composition of outlet gas from testing jig while constant voltage was applied.
2.3.2.3 AC Impedance
AC impedance is a powerful method for characterization of many electrochemical properties of materials and the interfaces between electrolyte and electrodes.
AC impedance.
A monochromatic signal V(t) = Vm sin(wt), involving the single frequency f= w/2π, is applied to a cell and the resulting steady state current i(t) = Im sin(wt + θ) measured. The complex resistance can be illustrated by Figure 2.12.
Figure 2.12. The impedance Z plotted as a planar vector using rectangular and
polar coordinates17
The total impedance, which is termed as Z, can be expressed by: Z = ∣Z∣cos(θ) + ∣Z∣sin(θ)
Re(Z) ≡ Z’ =∣Z∣cos(θ) Im(Z) ≡ Z’’=∣Z∣sin(θ)
Where Re(Z) and Im(Z) are the real impedance and imaginary impedance. θ is the phase difference between the voltage and the current; it is zero for purely resistive behavior14
Equivalent circuits.
The real circuit may be very complicated and we can approximate the real system to an equivalent circuit to determine the different contributions.
The impedance of a typical electrochemical system could consist of the resistance of electrolyte (RΩ), the capacitance of the interface (Cd), and Faraday Impedance (Zf) which is caused by the charge move and species transfer during the oxidation and reduction. When it works under alternating current (AC), Warburg
Impedance (Zw), which is due to the resistance and capacitance caused by periodical
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equivalent circuit can be shown by Figure 2.13.
Figure 2.13. Equivalent circuit. Where RΩis the resistance of electrolyte, Cd is the
capacitance of the interface, Zf is the Faraday Impedance, Rs is the resistance in
Zf; Cd is the capacitance in Zf, Zw is the Warburg Impedance, RCT is the
resistance of charge transfer18.
Suppose that the electrochemical reaction is only under control of charge movement and species transfer, the total impedance can be illustrated in the following (Figure 2.14):
Figure 2.14. Complex impedance spectroscopy picture. Where RΩis the
resistance of electrolyte, Cd is the capacitance of the interface, Zf is the Faraday
Warburg Impedance, RΩ is the resistance of charge transfer18.