Electrochemical behaviour

Electronic conductivity is an important property for a cathode to display, as it is required for the transport of electrons to the triple phase boundary (TPB) in order to reduce O2 into 2O2-. DC conductivity of the 16ap phase was measured in order to obtain its electronic

conductivity. Ionic conductivity is usually assumed to have such a small contribution (1 - 2 orders of magnitude lower) under these conditions, the measured value can be assumed to be only electronic.217 Measurement was carried out as detailed in section 2.10.1.

DC conductivity was then measured using the standard four-probe method from 800 °C to 350 °C on cooling at 50 °C increments. The results from this measurement can be seen below

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in Figure 3.18. The 16ap phase displayed semi conducting behaviour, with a low electronic

conductivity comparable to the 10ap phase. At 800 °C the 16ap phase has a conductivity of

3.68 S.cm-1, similar to 2.33 S.cm-1 for the 10ap phase and around 10-2 smaller than LSCF of

333 S.cm-1.218 This low conductivity can be attributed to, as with the 10ap phase, a lack of

charge carriers. This is indicated by the Mössbauer and iodometric titration analysis in section 3.2.1, which both confirmed Fe3+ was the only form present. The activation energy Ea, calculated for conductivity between 500 - 800 °C is 0.111(4) eV, which is close to that of

Ca2Fe2O5, and is associated to the small polaron mechanism.219

Figure 3.18 Electronic conductivity of the 16ap Y2.24Ba2.28Ca3.48Fe7.44Cu0.56O21 as measured via DC conductivity, collected

upon cooling from 800°C to 500°C.

Although the electronic conductivity of the 16ap phase is quite low, it is not necessarily the

defining property for a cathode as long as it is not too low as to become the limiting factor to performance. BSCF is a prime example of a cathode with relatively low electronic conductivity (although higher than the 16ap) compared to other well-known cathodes such as

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despite this, it has the best known cathode performance at the intermediate temperature range.34 The defining measurement that is used to determine a cathodes performance is AC impedance of a symmetric cell, which enables the determination of the Area Specific Resistance (ASR), which takes all properties into account, such as electronic conductivity, ionic conductivity, and electrochemical activity of the cathode.220

The cathode performance of the 16ap phase was consequently investigated by AC impedance

of a symmetrical cell fabricated with 16ap cathode on each side of a GDC electrolyte

substrate, as detailed in section 2.10.2. Cathode inks were then prepared by ballmilling this powder with an organic binder for 18 hours, in the weight ratio 67:33 powder to binder. The electrode ink was applied to both surfaces of the electrolyte substrate by screen printing in one layer. The cell was dried at 100 °C for 1 hour between the application of each layer followed by a final heating in air at 950 °C for 1 hour to achieve good adherence of the electrodes to the electrolyte surface. This was used as the temperature for stability tests discussed in section 3.3.1. The contacts for the electrical measurement were gold wire and gauze fixed in position with gold paste, which were attached to the cell by heating to 800 °C for 1 hour.

AC impedance measurements were recorded over the frequency range 1 MHz to 0.01 Hz using a Solartron 1260 FRA with a modulation potential of 10 mV, over the temperature range of 500 °C to 800 °C in static air. The symmetrical cell was held for 90 minutes at each temperature to allow thermal equilibration and measurements were made using ZPlot v.2.9b (Scribner Associates) every 50°C. The area specific resistance (ASR) of the cathode was calculated by fitting the data using ZView.189 The data were fitted with an Inductor (I) and two Resistors (R) in series with associated Constant Phase Elements (CPE) in parallel with each resistor. The ASR was then calculated by normalizing the measured resistance for the electrode area and dividing by two to take into account the symmetry of the cell.

Figure 3.19 A below displays a Z'-Z'' plot of the data obtained for the 16ap cathode on a GDC

electrolyte substrate. The 16ap phase has an ASR of 0.27 Ω.cm2 at 700 °C and compares

favourably to well-known cathode LSCF (0.46 Ω.cm2)218 prepared in a similar way and the original 10ap published data (0.88 Ω.cm2).35 Table 3.4 shows the values of ASR and DC

conductivity for the 16ap phase between 500 and 800 °C. The ASR of the 16ap phase falls

below 0.15 Ω.cm2

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components of a fuel cell, the cathode, anode and electrolyte.221 It could therefore be suggested that the potential operating temperature of a cathode produced from the 16ap phase

would be 750 °C, within the commonly quoted 500 - 800 °C intermediate operating temperature range.38 The Ea of the ASR for the 16ap phase is 1.57(4) eV, slightly higher than

was reported for the 10ap phase (1.43 eV). This could be due to the lower frequency of Sq sites per polyhedra within the structure, which are thought to be the electrochemically active site.35 The error on the Ea of the ASR was calculated from a linear regression using the

computer software Excel. An R2 = 0.9963 obtained for data suggests that the limiting step for the performance of the 16ap phase as a cathode is dominated by one single process, or

processes of a similar Ea.

Figure 3.19 A) comparison of ASR versus temperature for the 16ap (red), 10ap35 (orange) and LSCF218 (blue) together with

their Ea B) Z'-Z'' plot showing cathode performance of the 16ap in a symmetrical cell on a GDC electrolyte at temperatures

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Temperature (°C) ASR (Ω.cm2) Conductivity (S.cm-1)

500 44.56 2.30 550 9.40 2.60 600 2.43 2.87 650 0.73 3.11 700 0.27