Activity and Stability of (Pr_1-xNd_x)₂NiO₄as Cathodes for Solid Oxide Fuel Cells: Part V. In Situ Studies of Phase Evolution

Abstract

This study is to complement an early report (the manuscript is attached for review purpose) on the role of interlayer on activity and performance stability in praseodymium nickelates. The aforementioned report showed a remarkable 48% increase in power density while switching from common GDC interlayer to a new interlayer chemistry (PGCO). Furthermore, a stable long-term performance was linked with suppressed reaction between the cathode and PGCO interlayer. In this article, we report in situ studies of the phase evolution. The high energy XRD studies at a synchrotron source showed fully suppressed phase transition in praseodymium nickelates with PGCO interlayer, while the electrodes on the GDC interlayer undergo substantial phase transformation. Furthermore, in operando and post-test XRD analyses shown fully suppressed structural changes in electrodes operated in full cells at 750 • C and 0.80 V for 500 hours. SEM-EDS analysis showed that the formation of PrO x at the cathode-interlayer interface may play a role in a decrease of mechanical integrity of the interfaces, due to thermal expansion mismatch, leading to a local stress between the two phases. Consequently, phase evolution at a narrow interface may propagate toward the electrode bulk, leading to structural changes and performance degradation. The interest for Pr 2 NiO 4 (PNO) electrode stems from the necessity to develop active and stable oxygen electrodes 1-6 for solid oxide fuel cells (SOFCs).7-9 PNO is known for its highly active nature, 7,8,10 originated from its superior oxygen ion diffusion, surface exchange coeffocient 2,7,9-11 and structural flexibility over a wide temperature region (from 500 to 900• C). 3,12 PNO electrode, however, does undergo the structural evolution to form a higher order phase (Pr 3 Ni 2 O 7 ) and Pr 6 O 11 (PrO x ). 8 The structural change has been a major concern because it possibly links with the performance degradation over a long-term operation.7,8 Conventional X-ray diffraction (XRD) has been extensively used to investigate the structural evolution in nickelates in the form of powders or planar electrodes. 8,10 This method has two major limitations due to its low flux and low resolution: (1) it might overlook the presence of additional phases in the system, which is especially true for praseodymium nickelates where XRD diffraction patterns of higher order phase(s) (e.x. Pr 3 Ni 2 O 7 ) may overlap with the parent PNO phase, making quantification challenging; 8 and (2) the quantification of phase evolution in electrochemically operated PNO electrode may show major structural change with almost 100% of the parent phase transition from the conventional XRD analysis, while the transmission electron microscopy (TEM) studies clearly show the regions of preserved PNO phase. 7On the other hand, the high energy and high flux XRD (obtained at a synchrotron source) allows us to detect and resolve the presence of secondary phase(s), and in combination with high resolution capability provides the most accurate measurement regardin...

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

“…We surmise the sintering temperature is too low for PNO, but to avoid oxidation, it is limited to <900 °C. Therefore, it is possible that the nickelate sintering temperature (mainly PNO) is too low for single phase formation [50,53]. Large Rpol for PNO, while Rohm is similar to LNO and NNO, indicates that the bulk of the electrode is limiting the cell performance [48,50,51].…”

“…In conventional composite electrodes the entire electrode bulk acts as a composite material (premixed ionic and electronic conductors), which leads to increase in the cell performance [52][53][54][56][57][58]. For instance, mixing of an ionic conductor (e.g SDC) with an electronic conductor (LSM) provides a shorter pathway for ionic transport within the electrode.…”

“…Nicollet et al[55] demonstrated that PrOx infiltration into a porous Gd-doped ceria cathode backbone leads to substantial peak power improvements, from 0.40 W cm -2 to 0.80 W cm -2 at 600 °C. Dogdibegovic et al[52][53][54] also showed that a thin film of PrOx on top of the Gd-doped ceria buffer layer between the electrolyte and cathode increased the cell performance by 30% at 0.80V in the temperature range between 650-850 °C. The mechanism was shown to be extension of the mixed conduction region for ORR arising from Pr-doping into the Gd-doped ceria buffer layer[54].…”

An electrochemically reversible lattice with redox active A-sites of double perovskite oxide nanosheets to reinforce oxygen electrocatalysis

“…1B and C). 33 The induction of oxygen vacancies by reduction and a large size mismatch between Ba 2+ and Pr 3+ ions invoke Asite cation ordering which results in the formation of a layered tetragonal structure consisting of repeating…”

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

“…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.…”