Toward Understanding Structural Stability in Cu-Substituted (Pr1-xNdx)(2)NiO4+delta by in Situ and Operando Studies

Praseodymium nickelates are promising materials to be used as an oxygen electrode in solid oxide cells. Phase transformation in these materials, however, is a major challenge, which may cause durability issues in a fuel cell or electrolyzer. There is a need to develop a strategy that allows researchers to construct an in situ phase-stable and active cathode material. The aim of this study is to complement our previous publications on the electrochemical properties of the (Pr1-xNdx)(2)Ni1-yCuyO4+delta series, which has shown an increased performance stability by 2 orders of magnitude when compared to bare Pr2NiO4+delta. Systematic structural studies on a wide range of compositions were conducted in situ via long-term thermal annealing tests on powders. The structure-property relationship has been investigated using X-ray total scattering and atomic pair distribution function at a synchrotron source. In this work, we provide an attempt to identify a potential origin of structural stability and phase transition in praseodymium nickelates. A high temperature structure can be "frozen" preventing changes in M-O bond lengths, consequently leading to a stabilized structure. Results indicate that the two layers, associated with both Pr and Ni sites, are involved simultaneously in fully stabilizing the parent phase. Furthermore, it was found that Cu-doping on the Ni-site (in Pr2NiO4 structure) alone is not sufficient in stabilizing the parent phase. The structure and stability of the system were then investigated via density functional theory calculations, and computations of the density of states were undertaken for the Cu-doped and baseline compositions. It was found that p-d interactions, i.e., oxygen-metal orbital interactions, were enhanced as Cu was used to dope the PNNO, thus increasing the structural stability of the material.