Single cell electrochemical characterization

DC polarization curves, namely, I-V curves as well as electrochemical impedance spectroscopy (EIS herein) were recorded for electrochemical characterization. For some cells, the cell potential as a function of operation time was measured at a constant current density (i.e. galvanostatic mode), in order to investigate the stability of cathode materials.

I-V curve– I-V curves describe the trends of the cell voltage with the increase of current density applied in SOEC, in other words, the polarization behaviour upon the current flow through the cell. On the I-V curve, one can get the open circuit voltage (OCV), the resistance of the cell (the slope of the I-V curve) and the limiting current density if there is one. Fig. 2.10 shows the typical I-V curve of a SOEC operated at 900oC in CO2-CO mixture. The I-V curve was recorded both in SOEC and SOFC operation. The positive current was in SOFC mode and negative current represented SOEC operation. I-V curves were measured in various atmospheres, for instance, CO2-CO mixture, CO2-N2

mixtures, H2O-H2 mixtures and so on. Factors affecting I-V behaviour including fuel

Chapter 2: Methods and techniques -0.8 -0.6 -0.4 -0.2 0.0 0.2 0.4 0.5 1.0 1.5 2.0 2.5 V o lta g e /V

Current density / Acm-2

SOFC

SOEC

OCV

Fig. 2.10Typical I-V curve of a SOEC operated at 900oC in CO2-CO mixture (the tested cell was consisted of a 2mm YSZ electrolyte, a Ni/YSZ cermet cathode and a LSM/ScSZ anode, and the cathode atmosphere was CO2-CO 90-10 mixture. The fairly linear I-V curve indicated the predominant contribution from ohmic resistance of thick

YSZ electrolyte to cell performance)

Fig. 2.11The impedance Z plotted as a planar vector using rectangular and polar coordinates[16]

Impedance spectroscopy– Electrochemical impedance spectroscopy (EIS), also known as Ac-impedance spectroscopy, has been widely used in studies of electrochemical

Chapter 2: Methods and techniques

systems and is regarded as a powerful technique to characterize electrical properties of electrode materials and their interfaces with electrolyte.

The general approach for EIS characterization is to apply a single-frequency voltage or current as stimuli to electrodes/interfaces and measure the resulting variations in

magnitude and phase of the cell voltage/current. For example, a monochromatic signal ν

(t) = Vm sin(ωt), involving the single frequency ν ≡ ω/2π, is applied to a cell and the resulting steady state current i(t) = Im sin(ωt + θ), where θ is the phase difference between voltage and current, is measured. Using Fourier transformation, the relation between system properties and response to periodic voltage/current excitation can be simplified as an impedance Z (ω) = Z’+ jZ’’ which can be plotted as a planar vector quantity with either rectangular or polar coordinates, as shown in Fig. 2.11. The two rectangular coordinate values can be expressed as

Re (Z) ≡ Z’ = |Z| cos(θ) and Im (Z) ≡ Z’’ = |Z| sin(θ)

Where Re (Z) and Z’ stands for the real part of impedance, with Im (Z) and Z’’ the

imaginary part of impedance, and θ is the phase angle.

Shown in Fig. 2.12 and Fig. 2.13 are the complex plot, i.e. Nyquist plot of EIS. For an idealized fuel cell, the Nyquist plot is characterized by a semicircular arc, with its intersection at high frequency reflecting electrolyte resistance and the difference between intersections at high frequency and low frequency the polarization resistance Rp. The EIS of a real electrochemical system is complicated, as displayed in Fig. 2.13, and is more commonly featured with several depressed semicircular arcs and with overlapped semicircular arcs which make it difficult to separate and analyse the contributions.

Chapter 2: Methods and techniques

Fig. 2.12An idealized Nyquist plot of the EIS of a fuel cell[17]

Fig. 2.13Nyquist plot of EIS of a real electrochemical system[18]

While the impedance data may be complicated for interpretation of

properties/mechanism of the electrochemical system being studied, a data-fitted equivalent circuit model can be established to determine different contributions and suggest valuable chemical processes or mechanisms for the electrochemical system. Such equivalent circuit can produce the same response as the electrochemical system does and should be as simple as possible.

Chapter 2: Methods and techniques

Fig. 2.14a. Typical equivalent circuit of an electrochemical cell; b. subdivision elements of Zf[18]

Fig. 2.14 displays a typical equivalent circuit of an electrochemical cell. Rel, Cd, and Zf respectively represent the electrolyte resistance, a pure capacitor associated with the double layer of the electrode/electrolyte interface, and the Faradaic impedance. The impedance Zf can be divided in a resistance Rs in series with a pseudo-capacitance Cs and a charge transfer resistance Rct and a Warburg impedance Zw. The above only reflects the simplest electrode process happening at the electrode/electrolyte interface. When the electrode process is a complicated one with more steps involved more complex circuits have to be built.

In this study, impedance measurements were performed using three-probe configuration with ZAHNER IM6e Electrochemical Workstation. The frequency in the EIS testing

ranged from 105 to 0.1 or 0.015Hz, and AC voltage amplitude of 10mV or 20mV was

used. The EIS were recorded at OCV as well as at different potential being applied to SOEC. For some SOECs, impedance data were fitted using equivalent circuit model for analysis (see section 4.4).

Stability measurement- Stability is one of the critical properties considered for the selection of materials applied in solid oxide cells. Stability measurement was thus carried out to study the performance degradation of SOEC cathode material in long-

Chapter 2: Methods and techniques

With respect to CO2 electrolysis, stability test were performed in CO2-CO 70-30

mixture at 900oC, and LSCM/GDC cathode SOEC potential was recorded as a function

of operation time at a constant current (-0.25Acm-2) being applied to SOEC (section

4.6). As for H2O-CO2co-electrolysis, stability measurement was conducted on Pd-GDC

co-impregnated LSCM cathode SOEC operated in various H2O-H2-CO2-CO atmospheres, and cell potential at current density of -0.15 Acm-2 was recorded along extended operation time (section 7.3.4).