Phase instability in praseodymium nickelates is a major concern for the long-term operations of solid oxide fuel cells since it may lead to the performance degradation. In this work, praseodymium nickelates (ex. Pr 2 NiO 4+δ ) have been stabilized via substitution on both Pr-and Ni-sites. Systematic studies over a wide range of compositions were conducted via long-term thermal annealing studies (T ≤ 870 • C) and electrochemical tests in full cells. Proposed (Pr 0.50 Nd 0.50 ) 2 Ni 1-y Cu y O 4+δ compositions (y = 0. 05, 0.10, 0.20, and 0.30) showed the most promising results and serve as a comprehensive extension to our previous studies in this series of papers. A stable long-term performance was obtained for temperatures up to 790 • C for 500 hours at 0.80 V with a minimal tradeoff between the activity (power density of 0. Solid oxide fuel cells (SOFCs) attract growing interests as residential and industrial power sources primarily because of their high flexibility to a wide variety of fuels, including CO, hydrocarbons, and coal. In addition, SOFCs exhibit a high efficiency in combined heat and power systems (up to 80%) due to the direct conversion of chemical energy to electricity and heat, which is not limited by the Carnot cycle.1-3 Moreover, SOFC power systems are more affordable alternative to unreliable backup power systems that often tend to fail during startups and are only used in cases of main grid failures. 4 One of the key components in an SOFC is the cathode which catalyzes the oxygen reduction reaction (ORR). Cathodes with a low polarization resistance, stable structure, and stable performance over a wide temperature window are highly desired. A recent interest for Pr 2 NiO 4+δ (PNO) 5 stems from its superior properties over commonly used cathode materials 6 and the ability to accept various substituents at Pr and/or Ni sites, which allows for modifications of material's intrinsic properties.5,7 Such properties allow the PNO to be used on commonly utilized yttrium stabilized zirconia and gadolinium doped ceria electrolytes.PNO-based SOFCs, however, exhibit a performance degradation, 6,8 which may be linked to a rapid phase transition in PNO, 9 thus require means to stabilize its crystal structure. It has been a challenge to develop a compositional window which results in both stable structure and desirable catalytic activity. Our recent report 6 shows that A-site substitution with Nd in (Pr 1-x Nd x ) 2 NiO 4+δ cathodes is a promising solution to stabilize the cell performance. However, with a low Nd-content (≤ 50 mol%), a long-term operation leads to phase transition. In contrast, a high Nd content results in a stable phase during a 2,500-hour test, 10 but with a much lower performance (fourfold decrease). Therefore, further compositional modifications are required to simultaneously obtain high catalytic activity and stable crystal structure.An approach taken in this work was to substitute the Nisite in a promising (Pr 0.50 Nd 0.50 ) 2 NiO 4+δ (PNNO5050) electrode, * Electrochemical Society M...
