Comparison of Pr-Based Cathodes for IT-SOFCs in the Ruddlesden-Popper Family

Abstract

The present study is focused on alternative oxygen electrodes for Intermediate Temperature Solid Oxide Fuel Cells (IT-SOFCs). Among the K2NiF4 type compounds, the nickelates Ln2NiO4+δ (Ln = La, Pr or Nd) have already shown their suitability as oxygen electrode due to their mixed ionic and electronic conductivity (i.e. MIEC properties). Pr2NiO4+δ showed the best electrochemical properties [1] at intermediate temperature among the three nickelates: R p = 0.15 Ω∙cm² at 600 °C for instance, for screen printed electrode. Interestingly, the ageing of the Pr2NiO4+δ//CGO//8YSZ symmetrical half cells during 1 month under air shows no significant change in the R p value at i dc = 0 condition, despite the decomposition of Pr2NiO4+δ into the third term of the R.P. series (Prn+1NinO3n+1), Pr4Ni3O10+δ, the perovskite PrNiO3-δ and Pr6O11. Such decomposition was observed after one month at different SOFC operating temperatures, namely 600, 700 and 800 °C under air [2]. Aiming at further understanding the relationship between the electrochemical behavior and the phase stability, the physico-chemical and electrochemical properties of Pr4Ni3O10+δ and PrNiO3-δwere thoroughly studied. PrNiO3-δ is chemically very stable at operating temperatures up to one month while the electrochemical properties remain limited. The symmetrical half-cell with PrNiO3-δ electrode shows relatively high R p value, for instance R p = 0.9 Ω∙cm² at 600 °C, which is almost six times higher than the one of Pr2NiO4+δ [3]. On the other hand, Pr4Ni3O10+δ is a very promising material. In addition, Pr4Ni3O10+δ is highly chemically stable during long term up to 1 month under air at 600, 700 and 800 °C, contrarily to Pr2NiO4+δ [4]. Moreover, a single cell Pr4Ni3O10+δ//CGO//8YSZ//Ni-YSZ produces a very high power density of 1.60 W·cm-2 at 800 °C and 0.68 W·cm-2 at 700 °C. Hence, the electrochemical behaviors of Pr4Ni3O10+δ and Pr2NiO4+δ are similar, but Pr4Ni3O10+δis a more promising oxygen electrode since it is highly chemically stable. Herein, the comparison of Pr-based cathodes, and more especially their chemical stability, conductivity and electrochemical properties will be presented and discussed in details.