Activity and Stability of (Pr_1-xNd_x)₂NiO_4+δas Cathodes for Oxide Fuel Cells: Part VI. The Role of Cu Dopant on the Structure and Electrochemical Properties

Abstract

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...

High performance metal-supported solid oxide fuel cells with infiltrated electrodes

“…Ea,LNO=1.3 eV vs. Ea,NNO=1.15 eV) [50]. The purity of the nickelate phase in ceramic electrodes can be closely correlated to the cell performance (the higher the phase purity the higher the performance) [48][49][50][51]. We surmise the sintering temperature is too low for PNO, but to avoid oxidation, it is limited to <900 °C.…”

LaxPr4−xNi3O10−δ: Mixed A-Site Cation Higher-Order Ruddlesden-Popper Phase Materials as Intermediate-Temperature Solid Oxide Fuel Cell Cathodes

“…Initial studies of the Pr 2 NiO 4+δ and Nd 2 NiO 4+δ phases produced electrode performances with slightly higher ASRs; Ferchaud et al reported an ASR of 2.5 Ω cm 2 for PNO at 600 • C [22]. A 50:50 composition, PrNdNiO 4+δ (PNNO), reported by Dogdibegovic et al, presented an ASR of 0.7 Ω cm 2 at 800 • C [23]. A recent attempt at improving the performance of these materials by Bassat et al yielded the remarkable performance of 0.03 Ω cm 2 at 700 • C for a PNO cathode [24].…”

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

“…The structural refinement on PNNO-Cu y series, reported in our previous work, showed an increase in the cell volume with increasing molar fraction of Cu, mainly driven by a 0.90% increase in the c lattice parameter. In addition, the fractional position coordinates on heavy metal cations (Pr, Nd) indicated an elongation in the z direction and overall compression in the y direction, while the orthorhombic Bmab symmetry was retained.…”

“…Supercells containing 56 atoms were constructed, and the self-consistent electronic structure calculations were performed for Pr 2 NiO 4 , Pr 2 Ni 0.875 Cu 0.125 O 4 , PrNdNiO 4 , and PrNdNi 0.875 Cu 0.125 O 4 cathode materials. The calculations were carried out using the density-functional theory (DFT) method as implemented in the Vienna ab initio simulation package VASP . Projector augmented wave (PAW) pseudopotentials were used .…”

“…Our recent work showed that Cu doping on the Ni-site in (Pr 0.50 Nd 0.50 ) 2 NiO 4+δ (PNNO) electrodes resulted in stable long-term operation (nearly zero degradation), when compared to 6.4%/1,000 h degradation in PNO cells . The aim of this study is to complement our previous publications on the electrochemical properties of the (Pr 1– x Nd x ) 2 Ni 1– y Cu y O 4+δ series, − by establishing the relationship between performance stability and structural behavior in (Pr 0.5 Nd 0.5 ) 2 Ni 1– y Cu y O 4+δ (PNNO-Cu y ), where y = 0.05, 0.10, and 0.20. As a result of manipulations on the Ni-site, changes in the oxygen octahedron layer were expected, but their role in the changes in the rock-salt layer was unknown.…”