Relaciones Entre Grupos Construidos

During long-term phase durability and performance studies121, 169 our group

observed that phase transformation in electrodes after electrochemical operation did not

equate to those thermally annealed in symmetric cells.Since the phase transformation in

operating praseodymium nickelate electrodes is rarely reported, this discrepancy has not been addressed in the open literature. Instead, the conventional approach to evaluate phase evolution in praseodymium nickelates includes thermal annealing studies on powders and electrodes in symmetric cells. After annealing, the resulting materials are cooled down and studied via XRD. However, this method is based on thermodynamics of phase transformation, and may not necessarily represent phase evolution behavior in operating cells. During thermal annealing the material eventually reaches thermal equilibrium, and the phase evolution is governed by the thermodynamic variables. On the other hand, a SOFC does not operate in equilibrium regime due to exchange of matter and energy with the surroundings. Consequently, the thermodynamics may no longer dominate and the role of electrochemical potential on the phase transformation requires further consideration.

Figure 1.23 shows quantified169 _{phase evolution in PNNOseries on thermally}

annealed electrodes for 500 hours at 750 °C, and electrochemically operated electrodes in full cells at 0.80 V. Two sets of full cells show reproducible phase transformation. However, it is imperative to note that phase transformation in thermally annealed electrodes is substantially lower than in operated electrodes. This finding brings to a question as to what extent do operating conditions on PNNO materials influence the phase

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Figure 1.23 Phase evolution in thermally annealed (750 °C) and electrochemically

operated (Pr1-xNdx)2NiO4 electrodes after 500 hours at 750 °C and 0.80 V.

Electrochemical operation generates difference in electrochemical potential across the fuel cell, which can be considered as analogous to temperature gradients or pressure differences that drive phase transformations in other system. Therefore, better understanding of such behavior in solid oxide fuel cells is of fundamental importance. During thermal annealing, the changes on the electrode surface and in the bulk are only affected by fluctuations in the thermodynamic parameters (e.g. temperature, pressure, molar number, and the chemical potential). However, an operating SOFC is a non- equilibrium systems subjected to continuous and non-linear flux across the material. During electrochemical reaction, the electrode undergoes a transfer of matter and energy with the surroundings and is also subjected to external thermodynamic forces. Furthermore, the presence of electrochemical potential, as a dominating driving force in operating cells,

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cannot be overlooked. An attempt to further obtain understanding of this phenomena has been addressed in detail in chapters 10 and 11 of this thesis.

1.6 ELECTROCHEMICAL PERFORMANCE IN RP NICKELATES AND

CUPRATES

Unlike the abundant conduction and structural studies of RP nickelates, discussed in sections 1.3 and 1.4, the performance evaluation reports are rather limited. This may stem from the complexity of measurements of SOFCs and difficulty in obtaining reproducible measurements, which would allow statistically significant comparison between different cathode materials. In this section, an attempt was made to combine the

relevant results from the literature reports and further expand on our findings. Figure 1.24a

shows the distribution of the literature reports on nickelates from 1990 to 2017. Majority of the studies (~92%) were focused on materials properties (e.g. phase purity, phase transformation, conductivity). However, the performance reports account for less than 8% of total research in the field of nickelates, with 4% of reports involving research on symmetric cells. Since the common electrolyte thickness in electrolyte supported cells is between 100-250 μm, the ohmic resistance of the cell is high, allowing only relatively low current density to be injected into the cell, which is rather unusual in SOFC stacks. Furthermore, the two electrodes in symmetric cells will undergo two different processes under electrochemical potential, the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER), which brings to a question if the contribution from the two electrodes can be considered equal during quantification studies. During prolonged operation or at elevated currents the OER electrode undergoes delamination due to the pressure buildup at the electrolyte/electrode interface. This occurrence does not allow

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systematic studies since both electrodes are changing, hence it is challenging to isolate the contributions of individual electrodes towards area specific resistance (ASR) measurements.

As a result, the majority of the performance reports (~85%) present only the initial cell performance without much consideration towards performance stability and cell durability, Figure 1.24b. It is imperative to note that these reports only address the cell and electrode ASR measured within a few hours from the cell startup, without the actual power density measurements. Unfortunately, such approach does not provide a full representation

Figure 1.24 Representation of nickelate reports between 1990-2017. (a) Percentage of

reports on nickelate material studies (conductivity, diffusion, surface exchange), cathode

materials on symmetric cells, and performance in symmetric and full cells. (b) Percentage

of reports presenting only initial studies, performance stability between 25-100 hours, and long term operation over 500 hours.

of the materials activity and durability in SOFCs. On the contrary, it only provides the very initial cathode resistance and activity, which from industrial perspective, is of significantly less interest when compared to stable long-term operation in full cells. It is well know that the targeted SOFC operation of ~40,000 hours requires stable operation with less than 4%/1,000 hour degradation. A very few performance reports (~10%) actually address the power density within at least 25 hours of operation, while only 5% of the performance

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reports address the prolonged electrochemical operation beyond 500 hours. The majority of performance stability reports originate from our research group with the exception of a handful of studies from other researchers.