Effect of lanthanide (Ln=La, Nd, and Pr) doping on electrochemical performance of Ln2NiO4+δ−YSZ composite cathodes for solid oxide fuel cells

Abstract

In this study, we prepared three different Ruddlesden−Popper (RP)-phase nickelate catalysts, i.e., La2NiO4+δ (LNO), Nd2NiO4+δ (NNO), and Pr2NiO4+δ (PNO) via solid-state route. These materials were used to fabricate composite cathodes through their combination with yttria stabilized zirconia (YSZ), which is the most widely used electrolyte material for solid oxide fuel cells. X-ray diffraction (XRD) analysis revealed that after the initial sintering all three RP-based materials had good chemical compatibility with YSZ without any reactive impurities. The electrochemical impedance spectroscopy study showed that, among the three samples, the PNO−YSZ cathode exhibited the best oxygen reduction reaction (ORR) activity, with an area specific resistance (ASR) of 0.64 Ω cm2 at 800 °C, which was 16% and 41% lower compared to that of NNO-YSZ (0.76 Ω cm2), and LNO-YSZ (1.09 Ω cm2), respectively. During the initial long-term operation for 60 h at 800 °C, the measured ASR degradation rate of LNO−YSZ was 3.56 %h−1, while NNO−YSZ and PNO−YSZ showed better stability in comparison (2.23 and 1.87 %h−1, respectively). The post XRD analysis revealed the presence of a pyrochlore secondary phases (Ln2Zr2O7) in all three samples, which was considered a major degradation mechanism of Ln2NiO4+δ-YSZ composite cathodes. However, the amount of the impurity phase exhibited the same tendency as the degradation rate of different cathodes (LNO > NNO > PNO), demonstrating that Pr doping at the A-site of the RP-phase effectively suppressed the formation of a the pyrochlore impurity phase against YSZ. Thus, our results suggest that the ORR activity and durability of Ln2NiO4+δ−YSZ composite cathodes strongly depends on the A-site lanthanides.