La2-XPrxNiO4+δ-Based Efficient SOFC Cathodes: Effect of Microstructure, Composition and Architecture
Authors: Sharma Rakesh Kumar , Khamidy Nur Istiqomah , Bassat Jean marc , Djurado Elisabeth 
Published: February 2020

Solid oxide fuel cells (SOFCs) operating at intermediate temperature (500 – 700 °C) have attracted much attention due to high energy conversion efficiency, fuel adaptability and low emissions [1]. Both composition and microstructural /architectural design of the cathode film play an important role in obtaining the optimal electrochemical performances in this temperature range [2-9]. Recently, Ln2NiO4+δ (Ln = La, Pr, Nd) rare-earth nickelates have displayed promising oxygen electrode performance because of their interesting structure, transport and catalytic performance. Among nickelates, Pr2NiO4+δ presents excellent electrochemical properties at intermediate temperatures while it is characterized by a rather poor chemical stability in comparison to La2NiO4+δ. The preparation of efficient oxygen electrode materials which present very low degradation rates is still a challenge.

The present study is focused on designing Pr doped lanthanum nickelates, La2-xPrxNiO4+δ with 0 ≤ x ≤ 2 with the aim of finding the best compromise between chemical stability and optimized electrochemical performance.

First, single and double layered La2NiO4+δ electrodes with controlled microstructure were deposited on Ce0.9Gd0.1O2-δ (CGO) electrolyte using electrostatic spray deposition (ESD) or/and screen-printing (SP). The microstructure of La2NiO4+δ films has been optimized by varying some key process parameters. A coral-like microstructure of La2NiO4+δ films was successfully obtained resulting from nozzle to substrate distance, flow rate, substrate temperature deposition time equal to 50 mm, 1.5 mL/h, 350 °C and 3 h, respectively [6]. Electrochemical properties of La2NiO4+δ deposited symmetrically on CGO were measured by impedance spectroscopy. They are found to be strongly dependent on the cathode microstructure and architecture. For example, a double layer La2NiO4+δ cathode, consisted of 3-D coral nanocrystalline film (average particle size ~ 150 nm) with a continuous nanometric dense sub-layer (100 nm) at the electrode/electrolyte interface, prepared by ESD and topped by a La2NiO4+δ current collector prepared by SP, shows the best performance leading to the polarization resistance (Rpol) down to 0.42 Ω cm² at 600 °C (Fig 1) [6].

In a second part, this double layer design was extended toLa2-xPrxNiO4+δ (with x = 0, 0.5, 1 and 2) cathodes with a view to taking advantage of the complimentary properties of the two extreme compositions Pr2NiO4+δ and La2NiO4+δ, i.e. higher electronic conductivity of Pr2NiO4+δ superior stability of La2NiO4+δ. The chemical stability and polarization resistance (Rpol) were found to decrease when the Pr content was increased. Polarization resistance (Rpol) equal to 0.42, 0.12 and 0.08 Ω cm2 at 600 °C has been obtained for La2NiO4+δ, LaPrNiO4+δ and Pr2NiO4+δ, respectively. Among the complete La2-xPrxNiO4+δ solid solution, LaPrNiO4+δ shows the best compromise between electrochemical properties (the lowest Rpol value available in the literature for this composition, 0.12 W cm2 at 600 °C) and chemical stability in air (up to 30 days at 700 °C) [9].

Finally, the role of the electrode/electrolyte interface has been investigated on the polarization resistance of La2NiO4+δ and Pr2NiO4+δ cathodes. For this purpose, the thin dense nickelate sub-layer was replaced by a thin porous (~3-4 mm) composite layer based on a mixture of CGO (deposited by SP) and La2NiO4+δ or Pr2NiO4+δ (deposited by ESD). As a consequence, further reduction in Rpol down to 0.16 (Fig 1) and 0.04 Ω cm2 was successfully obtained for La2NiO4+δ and Pr2NiO4+δ, respectively [8]. This composite LnNO-CGO sub-layer is playing an important role in enhancing the electrochemical properties of these cathodes leading to the lowest Rpol values available in the literature for these compositions, to the best of our knowledge.

This talk will end with our latest results incorporating a LnNO-CGO composite sub-layer to the double layer LaPrNiO4+δ electrode, the most promising composition in terms of electrochemical properties and chemical stability.

References:

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[9] R.K. Sharma, S.K. Cheah, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Mater. Chem. A. (2017) DOI: 10.1039/C6TA08011A

Figure 1