Understanding the fundamental properties of PrOx based oxygen electrodes through ab-initio and electrochemical modelling for solid oxide cells application

Context and problematic: Solid Oxide Cells (SOCs) are reversible and efficient energy-conversion systems for the production of electricity and green hydrogen. Nowadays, they are considered as one of the key technological solutions for the transition to a renewable energy market. A SOC consists of a dense electrolyte sandwiched between two porous electrodes. To date, the large-scale commercialization of SOCs still requires the improvement of both their performances and lifetime. In this context, the main limitations in terms of efficiency and degradation of SOCs have been attributed to the conventional oxygen electrode in La_0.6Sr_0.4Co_0.2Fe_0.8O_3-δ material. To overcome this issue, it has recently been proposed to replace this material with an alternative oxygen electrode based on PrO_x. Indeed, this material has a very high electro-catalytic activity for the oxygen reduction and good transport properties. In this frame, the performance of cells incorporating this new electrode is very promising and might enable to reach the targets required for large-scale industrialization (i.e. -1.5 A/cm² at 1.3 V at 750°C with a degradation rate of 0.5%/kh). However, it has also been shown that PrO_x undergoes phase transitions depending on the cell operating conditions [1]. The impact of these phase transitions on the electrode basic properties and on its performance and durability are still unknown. Therefore, the purpose of the PhD is to gain an in-depth understanding of the physical properties for the different PrO_x phases in order to investigate their role in the electrode reaction mechanisms. The study will thus contribute to validate whether PrO_x based electrodes are good candidates for a new generation of SOCs and help to identify an optimized electrode microstructure and composition in terms of performance and durability.

Objectives and work plan:

Since the experimental evaluation of the PrO_x physical properties under operating conditions is very difficult, a numerical multi-scale modeling approach will be adopted. The proposed approach can be divided in two parts.

In the first part, density functional theory (DFT) calculations will be carried out to model the key properties of the different PrO_x phases at atomic scale. An ideal electrode material should present a high electronic and ionic conductivity (bulk properties) in combination with good catalytic properties for the oxygen exchange reaction (surface properties). The PhD candidate will carry out DFT calculations to investigate the conduction mechanisms and how they change according to the different phases. Special emphasis will be placed on the study of the charge balance during the formation of oxygen vacancies at interstitial and crystal sites, as there are still discrepancies in literature [2,3]. The energy of oxygen vacancy formation will be used into a thermodynamic model enabling the determination of oxygen under- or over-stoichiometry as a function of temperature and oxygen partial pressure. As well, the ionic diffusion between the different oxygen sites and the electronic conductivity, suggested to be governed by a polaron hopping mechanism between Pr of different oxidation state [4,5], will be determined using the Nudged elastic band method (NEB).  In addition, the reaction of the O₂ molecule with the PrO_x electrode will be studied. Therefore, the overall oxygen reduction reaction will be divided in a sequence of elementary reaction mechanisms (adsorption, dissociation, incorporation, diffusion of surface species). For each of this mechanism, the activation energy will be determined using NEB calculations. These activation energies will be used to identify the most likely reaction pathway for the oxygen exchange.   

In the second part, the determined bulk and surface properties will be used as input for an electrochemical elementary kinetic model built according to the proposed reaction mechanism. These macroscopic models are able to study the performance of the SOC under operation conditions. Output of these simulations are among other polarization curves and impedance spectra that can be directly compared to experimental data. Moreover, the elementary kinetic model will be used to conduct microstructural optimization for the identification of an improved PrO_x based electrode.  

[1] L. Yefsah, J. Laurencin, M. Hubert, D. Sanchez Ferreira, F. Charlot, K. Couturier, C. Celikbilek, E. Djurado, Electrochemical performance and stability of PrO1.833 as an oxygen electrode for solid oxide electrolysis cells, Solid State Ionics, 399 (2023) 116

[2] Hideaki Inaba and Keiji Naito, “Simultaneous Measurements of Oxygen Pressure, Composition, and Electrical Conductivity of Praseodymium Oxides: I. Pr7O12 and Pr9O16 Phases,” Journal of Solid State Chemistry 50, no. 1 (November 1983): 100–110

[3] Hideaki Inaba and Keiji Naito, “Simultaneous Measurements of Oxygen Pressure, Composition, and Electrical Conductivity in Praseodymium Oxides: II. Pr10O18 and PrO2−x Phases,” Journal of Solid State Chemistry 50, no. 1 (November 1983): 111–20,

[4] R. G. Biswas et al., “Electrical Transport Studies and Temperature-Programmed Oxygen Evolution of PrO1.83,” Journal of Materials Science 33, no. 12 (June 1, 1998): 3001–7,

[5] Hideaki Inaba and Keiji Naito, “Simultaneous Measurements of Oxygen Pressure, Composition, and Electrical Conductivity of Praseodymium Oxides: I. Pr7O12 and Pr9O16 Phases,” Journal of Solid State Chemistry 50, no. 1 (November 1983): 100–110

The work will be carried out at CEA-Liten, which is a leading research center on new energies.

The candidate must have a master in solid-state physics or solid-state chemistry, and a strong experience with modeling and numerical simulations.