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, Ln₂NiO_4+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, Pr₂NiO_4+δ presents excellent electrochemical properties at intermediate temperatures while it is characterized by a rather poor chemical stability in comparison to La₂NiO_4+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, La_2-xPr_xNiO_4+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 La₂NiO_4+δ electrodes with controlled microstructure were deposited on Ce_0.9Gd_0.1O_2-δ (CGO) electrolyte using electrostatic spray deposition (ESD) or/and screen-printing (SP). The microstructure of La₂NiO_4+δ films has been optimized by varying some key process parameters. A coral-like microstructure of La₂NiO_4+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 La₂NiO_4+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 La₂NiO_4+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 La₂NiO_4+δ current collector prepared by SP, shows the best performance leading to the polarization resistance (R_pol) down to 0.42 Ω cm² at 600 °C (Fig 1) [6].

In a second part, this double layer design was extended to La_2-xPr_xNiO_4+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 Pr₂NiO_4+δ and La₂NiO_4+δ, i.e. higher electronic conductivity of Pr₂NiO_4+δ superior stability of La₂NiO_4+δ. The chemical stability and polarization resistance (R_pol) were found to decrease when the Pr content was increased. Polarization resistance (R_pol) equal to 0.42, 0.12 and 0.08 Ω cm² at 600 °C has been obtained for La₂NiO_4+δ, LaPrNiO_4+δ and Pr₂NiO_4+δ, respectively. Among the complete La_2-xPr_xNiO_4+δ solid solution, LaPrNiO_4+δ shows the best compromise between electrochemical properties (the lowest R_pol 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 La₂NiO_4+δ and Pr₂NiO_4+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 La₂NiO_4+δ or Pr₂NiO_4+δ (deposited by ESD). As a consequence, further reduction in R_pol down to 0.16 (Fig 1) and 0.04 Ω cm² was successfully obtained for La₂NiO_4+δ and Pr₂NiO_4+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 R_pol 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 LaPrNiO_4+δ electrode, the most promising composition in terms of electrochemical properties and chemical stability.

References:

[1] J.R. Wilson, A.T. Duong, M. Gameiro, H.Y. Chen, K. Thornton, D.R. Mumm, S.A. Barnett, Electrochemistry Communications. 11 (2009) 1052-1056.

[2] D. Marinha, L. Dessemond, J.S. Cronin, J.R. Wilson, S.A. Barnett, E. Djurado, Chem. Mater. 23 (2011) 5340-5348.

[3] D. Marinha, J. Hayd, L. Dessemond, E. Ivers-Tiffée, E. Djurado, J. Power Sources. 196 (2011) 5084–5090

[4] D. Marinha, L. Dessemond, E. Djurado, J. Power Sources. 197 (2012) 80-87.

[5] R.K. Sharma, M. Burriel, E. Djurado, J. Mater. Chem. A. 3 (2015) 23833-23843.

[6] R.K. Sharma, M. Burriel, L. Dessemond, V. Martin, J.M. Bassat, E. Djurado, J. Power Sources. 316 (2016) 17-28.

[7] R.K. Sharma, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Power Sources. 325 (2016) 337-345.

[8] R.K. Sharma, M. Burriel, L. Dessemond, J.M. Bassat, E. Djurado, J. Mater. Chem. A. 4 (2016) 12451-12462.

[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