Stability and Compatibility

Thermal stability tests were performed in air on the as-synthesised powder and on powder which was ballmilled in isopropanol for 20 hours. Ballmilling reduces the particle size and increases the surface area, which should increase the number of electrochemically active cathode sites for improved electrochemical performance. Stability tests were therefore carried out on samples that resembled the processed cathode as much as possible. Samples were then annealed in air at 750 °C for five days (to simulate intermediate SOFC operating temperatures) and 950 °C for five hours (cathode processing temperatures) and subsequent phase stability was characterised by PXRD, as seen in Figure 3.14. No new phase formation or decomposition of the 16ap phase was observed. According to Table 3.3, lattice parameters

do not change significantly.

To study chemical compatibility with the state-of-the-art electrolytes Ce0.9Gd0.1O1.95 (GDC, Fuel Cell Materials), Ce0.8Sm0.2O1.9 (SDC, Fuel Cell Materials) and La0.9Sr0.1Ga0.8Mg0.2O2.85 (LSGM, PI-KEM LTD), milled powder of the 16ap phase and electrolyte were mixed in a

weight ratio 1:1 by hand grinding, pressed into 5 mm pellets and annealed as for the stability test. The pellets were then ground into powders for characterization by PXRD, as seen in Figure 3.14. Again, as for the thermal stability test, there was no detectible degradation of the 16ap phase. No new reflections were observed within the PXRD patterns, suggesting new

phase formation through reactivity with the electrolyte did not occur. Lattice parameters, seen below in Table 3.3, also remained mostly constant with only small deviations, indicating a small amount of cation diffusion between electrolyte and 16ap was likely. The 16ap lattice

parameter deviations were most pronounced when combined with LSGM. It has been reported that for LSGM, rather than new phase formation cation diffusion between cathode and electrolyte are favoured, with transition metals Mn, Fe and Co in particular rapidly diffusing into LSGM.207, 208 It is likely that in this case, there is some exchange in cations between the 16ap phase and LSGM.

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Figure 3.14 X-ray diffraction patterns showing the stability of the synthesised 16ap material. The patterns show that the

material is both thermally stable and compatible with the two common electrolyte materials, Ce0.9Gd0.1O2−δ (GDC) and

La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM).

Under the same thermal treatment investigated for the 16ap phase, BSCF decomposes into an

insulating hexagonal perovskite140, 142, 143, 209-211 and a second phase with plate-like morphology that is rich in Co and poor in Sr compared to cubic Ba0.5Sr0.5Co0.8Fe0.2O3-δ.141, 142 BSCF also reacts with SDC under these conditions.212

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Phase(s) / Axis a (Å) b (Å) c (Å) 16ap as-made 5.4824(1)4824(1) 61.291(1) 5.5497(1)

Tests carried out at 950 °C for five hours

16ap 950 °C 5.4800(1) 61.279(3) 5.5457(1)

16ap with GDC 950 °C 5.4808(3) 61.311(8) 5.5478(3)

16ap with SDC 950 °C 5.4836(4) 61.35(1) 5.5490(4)

16ap with LSGM 950 °C 5.4772(3) 61.226(8) 5.5465(5)

Tests carried out at 750 °C for five days

16ap 750 °C 5.4814(1) 61.242(3) 5.5452(1)

16ap with GDC 750 °C 5.4840(3) 61.255(9) 5.5472(4)

16ap with SDC 750 °C 5.4826(4) 61.25(1) 5.5474(4)

16ap with LSGM 750 °C 5.4789(4) 61.193(8) 5.5440(7)

Table 3.3 Lattice parameters of 16ap phase before and after thermal stability and chemical compatibility tests at 950 °C for

five hours and 750 °C for five days with electrolytes GDC, SDC and LSGM.

Phase stability is essential for a functional material used within a device under operating conditions, therefore these first results were a promising start. However, to study the long term stability of the 16ap phase, samples were annealed under the quite extreme conditions of

a flowing atmosphere of 100 % CO2 at 750 °C for 24 hours. The stability was characterised by PXRD and it was observed that there was no degradation of the 16ap phase and no new

phase formation, as seen in Figure 3.15. This is another promising property exhibited by the 16ap, with the lattice parameters also remaining constant before and after the treatment. Many

other cathodes containing alkaline-earth elements have a low tolerance for CO2,137-139 particularly those that contain Ba, decomposing into carbonates in its presence.133-136 This then leads to the requirement of CO2 removal from air before it reaches the cathode, reducing the viability and flexibility of the fuel cell.37 The tolerance that the 16ap phase shows for such

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Figure 3.15 PXRD of as made 16ap and after aging in 100 % CO2 for 24 hours at 750 °C.

The TEC for each lattice parameter of the 16ap phase was determined by variable temperature

SXRD from 200 °C to 600 °C with increments of 50 °C. Each data set had the lattice parameters determined by Rietveld refinement, with each of the data points plotted below in Figure 3.16 A and B. The 16ap phase shows anisotropic expansion, with expansion along a

(11.3 × 10-6 K-1) and c (9.0 × 10-6 K-1) being similar, but along b, the stacking direction thermal expansion coefficient is roughly 1.5 times greater (16.9 × 10-6 K-1). This anisotropic behaviour is not unusual for layered materials and has been reported before, with examples including the 10ap35, YBa2Cu3O7213 and NaxCoO2.214

Over the measured temperature range there appeared to be no significant change to the structure, with SXRD patterns remaining remarkably similar throughout heating. Lattice parameters were the only significant change with temperature (see Figure 3.17). This change in lattice parameters occurs linearly with temperature, also indicating no structural change or significant oxygen evolution occurs, which was backed up by a TGA measurement that showed no significant weight change (see Figure 3.16 C). The volumetric expansion of

106 αv = 37.2 × 10-6

K-1 (see Figure 3.16 C) as calculated from lattice parameter evolution, is comparable to state-of-the-art electrolytes GDC (αv ≈ 36.5 × 10-6 K-1),215 SDC (αv ≈ 37.5 × 10-6

K-1),157 LSGM (αv ≈ 33 × 10-6 K-1).216 The close matching thermal expansion means that delamination of the 16ap as a cathode printed onto the electrolyte

would be unlikely during operation. This is a well-known problem for many other cathodes, especially Co rich phases such as LSC, BSCF and Sm0.5Sr0.5CoO3-δ (SSC), which have volumetric TECs in the region of 60 × 10-6 K-1.37

Figure 3.16 Plots of the expansion of lattice parameters versus temperature (error bars within points) together with TEC for lattice parameters A) a (black), c (red), B) b (blue) and C) Volume (olive). D) shows TGA experimental results of mass versus temperature.

The TGA experiment was undertaken using ~ 100 mg hand ground sample inside a Pt crucible. The measurement was undertaken using a TA Instruments Q600 thermal analyzer

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with a 100 ml per minute gas flow rate (10 % N2, 90 % atmospheric air), heating rate of 3 °C per minute from 50 °C to 800 °C, a dwell time of 1 hour at 800 °C and cycled twice.

Figure 3.17 VT-SXRD patterns of the 16ap, Y2.24Ba2.28Ca3.48Fe7.44Cu0.56O21-δ, taken at 50 °C increments over a range of

200 - 600 °C. Patterns show no significant change other than lattice parameter evolution.