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. Pr2NiO4+delta) 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 degrees C) and electrochemical tests in full cells. Proposed (Pr0.50Nd0.50)(2)Ni1-yCuyO4+delta 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 degrees C for 500 hours at 0.80 V with a minimal tradeoff between the activity (power density of 0.8-1.0 W cm(-2) at 850 degrees C) and performance stability. A preserved parent phase and suppressed performance degradation, when compared to Pr2NiO4+delta, make newly developed electrodes attractive candidates for the state-of-the-art solid oxide fuel cell applications. (C) The Author(s) 2017. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any medium, provided the original work is properly cited. All rights reserved.