Single phase (Pr 1-x Nd x ) 2 NiO 4 cathode powders (x = 0, 0.25, 0.50, 0.75, and 1.0) were synthesized via a glycine-nitrate combustion and high temperature calcination. Anode supported cells were used to investigate the cathode property. A reproducible performance, within 9% for each cathode composition, was observed providing a wealth of data for quantitative studies. Area specific resistance analysis and i-V measurements between 650 and 850 • C showed a decrease in the cell performance with increasing Nd content. Impedance spectrum analysis suggests that the decline in performance results from an increase in electrode polarization. While Pr 2 NiO 4 cells showed significant performance degradation of 6.40%/1,000 hours, the degradation rate for (Pr 0.75 Nd 0.25 ) 2 NiO 4 cells was reduced by an order of magnitude (0.56%/1,000 hours) with a 7% lower power output. Likewise, the cathodes with a higher Nd content showed further improvement in performance stability with a marginal degradation rate of 0.06%/1,000 hours. During the past few years, praseodymium nickelate (Pr 2 NiO 4+δ , PNO) has attracted increasing attention as the cathode for solid oxide fuel cells (SOFCs) because of its unique properties over a wide temperature range, 1-5 including: (1) a high oxygen diffusion coefficient (e.g.(3) a low polarization resistance in a single cell, (4) a large oxygen overstoichiometry (δ as much as 0.22 at room temperature), 6 and (5) the ability to accept various substituents with similar ionic radii and valance at Pr and/or Ni sites.2 Moreover, its coefficient of thermal expansion (13.2 × 10 −6 K −1 ) 4,7 is compatible to doped ceria (13 × 10 −6 K −1 ) 7 and yttria-stabilized zirconia (11 × 10 −6 K −1 ) 7 electrolytes. The electrochemical performance of PNO, however, did show an appreciative degradation 4 which likely resulted from its readily occurring phase transitions at operating conditions. 8 PNO partially decomposes to praseodymium oxide (PrO x ) and a higher order layered structure (Pr 3 Ni 2 O 7 ) during operation, 8 which presents a concern because of possible structural collapse and longterm performance degradation. 4,8 Current approaches to stabilize the PNO phase focus on substituting A and/or B-site ions.9,10 Both La 2 NiO 4 and Nd 2 NiO 4 (NNO) are more stable than PNO, 11 but have lower power density.12 This work reports an attempt to stabilize the long-term cell performance in praseodymium nickelate. (Pr 1-x Nd x ) 2 NiO 4 (PNNO, x = 0, 0.25, 0.50, 0.75, and 1.0) cathodes were synthesized and electrochemically evaluated in anode supported cells. Nd was chosen as an A-site substituent due to its similar physical properties to Pr and its ability to suppress the formation of PrO x and higher order phases. The i-V and electrochemical impedance spectroscopy (EIS) measurements were performed between 650-850• C, while the performance stability was measured at 750• C and 0.8 V for 500 hours. ExperimentalStarting materials, Pr(NO 3 ) 3 , Nd(NO 3 ) 3 , and Ni(NO 3 ) 2 (99.9% REO, Alfa Aesar, Haverhill, MA) were...
This study is to complement an early article (Dogdibegovic et al., J. Electrochem. Soc., 163(13), F1344 (2016)) on the electrochemical activity and performance stability of (Pr 1-x Nd x ) 2 NiO 4+δ (PNNO) electrodes. Here, we report the crystal structure, electrical properties, and microstructures of PNNO series as the cathodes for solid oxide fuel cells. Rietveld refinements on powders (x = 0, 0.25, 0.50, 0.75, and 1) show that the unit cell volume decreases with an increase in x, primarily due to a decrease in the c lattice parameter. Larger cell volume (∼1.50%) and higher total electrical conduction (40%) in Pr 2 NiO 4+δ are in favor with its mixed conducting properties during operation, but Pr 2 NiO 4+δ cathode exhibits a severe phase evolution. Substitution of Pr with Nd shows the suppression of phase evolution in both thermally annealed powders and electrodes. An increase in Nd content leads to a full preservation of the parent phase in both (Pr 0.25 Nd 0.75 ) 2 NiO 4+δ and Nd 2 NiO 4 after 2,500 hour annealing at elevated temperatures. Reaction with GDC buffer layer was also suppressed with the presence of Nd, which was shown by a reduction of Pr and Ni elemental diffusion into GDC bulk. STEM analysis confirms multiple phases present in an operated Pr 2 NiO 4+δ electrode, while suppressed phase transition was observed in electrodes with high Nd content.
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