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 Ln₂NiO_4+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). Pr₂NiO_4+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 Pr₂NiO_4+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 Pr₂NiO_4+δ into the third term of the R.P. series (Pr_n+1Ni_nO_3n+1), Pr₄Ni₃O_10+δ, the perovskite PrNiO_3-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 Pr₄Ni₃O_10+δ and PrNiO_3-δwere thoroughly studied.

PrNiO_3-δ is chemically very stable at operating temperatures up to one month while the electrochemical properties remain limited. The symmetrical half-cell with PrNiO_3-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 Pr₂NiO_4+δ [3]. On the other hand, Pr₄Ni₃O_10+δ is a very promising material. In addition, Pr₄Ni₃O_10+δ is highly chemically stable during long term up to 1 month under air at 600, 700 and 800 °C, contrarily to Pr₂NiO_4+δ [4]. Moreover, a single cell Pr₄Ni₃O_10+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 Pr₄Ni₃O_10+δ and Pr₂NiO_4+δ are similar, but Pr₄Ni₃O_10+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.

References

[1] V. Vibhu, A. Rougier, C. Nicollet, A. Flura, J.-C. Grenier, J.-M. Bassat, Solid State Ionics, 278(2015) 32-37.

[2] V. Vibhu, J.-M. Bassat, A. Flura, C. Nicollet, J.-C. Grenier, A. Rougier, ECS Transaction, 68(2015) 825-835.

[3] V. Vibhu, J.-M. Bassat, A. Flura, C. Nicollet, N. Penin, J.-C. Grenier, A. Rougier, to be submitted in Journal of Solid State Electrochemistry, 2017.

[4] V. Vibhu, A. Rougier, C. Nicollet, A. Flura, S. Fourcade, N. Penin, J.-C. Grenier, J.-M. Bassat, Journal of Power Sources, 317 (2016) 184-193.