Characterization and Modeling of the Performance and Durability of Praseodymium Oxide Infiltrated Oxygen Electrodes

Solid oxide cells (SOCs) are electrochemical devices working at high temperatures. They present many advantages such as a high-energy efficiency and a good reversibility in fuel cell and electrolysis modes, making them a key technology for future energy conversion and storage. Despite these advantages, their industrialization is still hindered by insufficient performance and durability. In this context, it is known that the conventional material of the oxygen electrode limits the SOCs performances. It also suffers from a chemical decomposition upon operation. Exploration of new materials and microstructures with promising electro catalytic and oxygen transport properties is a way to mitigate these issues. Praseodymium Oxide (PrO x ) is one such material, which has recently attracted attention. Many recent studies have shown excellent initial performances which have been achieved using a PrO x catalyst infiltrated in porous (about 30% of porosity) ceria-gadolinium oxide (CGO) backbone (1). However, there are no studies yet, which investigate the combined role of reaction mechanisms and microstructures on performance. Additionally, the long-term stability of PrO x infiltrated electrode under electrolysis and fuel cell operation at high temperature has not been analyzed yet. Therefore, an approach combining electrochemical testing, post-test characterizations and reaction kinetic modeling has been adopted, with the objective to better understand the initial performance of the PrO x infiltrated oxygen electrode and its long-term stability. A hydrogen electrode-supported cell has been considered for this study. It is composed of an Y 0.16 Zr 0.84 O 1.92 (8YSZ) dense electrolyte (ca. 8 μm) and a Ni-8YSZ fuel electrode functional layer (ca. 25 μm) supported on a thick Ni-3YSZ substrate (ca. 280 μm). The oxygen electrode is a bilayer (ca. 50-60µm), which includes a functional layer of porous Ce 0.8 Gd 0.2 O 2-δ infiltrated with PrO x and current collecting layer of LaNi 0.5 Fe 0.5 O 3 also infiltrated with PrO x . A CGO barrier layer (ca. 3 μm) is added between the electrolyte and the oxygen electrode to limit the reactivity. Electrochemical Testing Electrochemical tests have been performed in an in-house test bench, which is already described in ( 2 ). The polarization curves were acquired at different temperatures (700, 750 and 800 °C) and for various inlet flow rates at the hydrogen electrode (6, 12, and 18 NmL∙min -1 ∙cm -2 ). The initial cell performances are shown in Figure 1a. In electrolysis mode, a current density of around -1 A.cm -2 is reached at the thermo-neutral voltage for 750°C and a steam conversion (SC) of 64%. Additionally, electrochemical impedance spectra (EIS) was measured at the open circuit voltage (OCV) and 750°C under various oxygen partial pressures at the oxygen electrode (10/90 H2/H2O, total inlet flow rate of 12 NmL∙min -1 ∙cm -2 at the hydrogen electrode). The raw impedance spectra were collected using a potentiostat/galvanostat (Autolab PGSTAT-302N) equipped with a Frequency Response Analyzer module (FRA2) and a 20 A current booster and then post-treated with the Zview software (ver. 2.70, Scribner Associates) based on equivalent electric circuit model (EECM). The EECM proposed by ( 3 ), consists of a resistance that represents the ohmic resistance (Rs) in series with an inductor element (L) that takes into account the inductance of the wires. Four resistance-constant phase element (R-CPE) were used to fit accurately the different EIS contributions at high frequency (HF), medium (or intermediate) frequency (MF) and low frequency (LF). The fit of the raw data by the EECM and the simulated spectra after removing the inductance is shown in Figure 1a. The limiting processes involved in each frequency region were identified using sensitivity analysis and confirmed using literature. The HF contribution mainly corresponds to rate determining steps of the reactions and charge transfers in the Hydrogen electrode, while the MF contribution corresponds to the gas diffusion impedance in the Hydrogen electrode side fully convoluted with contributions from the reactions at the oxygen electrode. The LF contribution was attributed to the gas conversion impedance ( 4 , 5 ). Two long-term durability tests were performed to test the long-term stability and performance degradation of PrO x infiltrated electrodes under operation. These experiments were carried out at 750°C for 3600h applying a current density of -1A.cm -2 with 64% Steam conversion (SC) in SOEC mode and +1A.cm -2 with 64% Fuel Utilization (FU) in SOFC mode. A gas mixture with 10/90 and 90/10 vol. % H 2 /H 2 O at the Hydrogen electrode was fed for the SOEC and SOFC operations respectively. The oxygen electrode was fed with air for both modes of operation. The total inlet gas fluxes at the oxygen and hydrogen electrodes were 36 NmL·min −1 ·cm −2 and 12 NmL·min −1 ·cm −2 , respectively. For both these tests, the evolution of the cell voltage with the operating time is shown in Figure 1b. A rather fast voltage degradation was observed at the beginning of the test over a few hundred hours. After this initial period, the degradation slowed down towards a constant rate. The stabilized voltage degradation rates for the cells under SOEC operation and SOFC operation were 0.65 %/kh and 1.08 %/kh respectively. EIS at OCV were acquired periodically after every 500 hours of aging under the same operating conditions. The EECM model described previously was applied on the spectra to quantitatively analyze the evolution of the different contributions. For the cell aged under SOEC operation, an increase in the series resistance accompanied with an increase in the resistance at HF was observed, while the LF and MF contributions were quite stable. For the SOFC operation, an increase was observed in the HF resistance only, while the Rs, MF and LF contributions were stable. The increase of the series resistance and the HF contribution in electrolysis mode can be attributed to the Ni migration in the Hydrogen electrode ( 6 ). As mentioned previously, processes of the oxygen electrode were observed in the MF region. Since, there was no visible increase in the polarization resistance in the MF, these analyses point towards the excellent performance stability of the PrO x infiltrated oxygen electrode under operation in both the SOEC and SOFC modes. For the post-test characterizations, the durability test under current have been complemented with thermal ageing at OCV at 750°C and 850°C for 500 hours and 1000 hours. Post-test characterizations Scanning electron microscopy (SEM), X-ray diffraction, high-resolution secondary ion mass spectrometry (NanoSIMS) and synchrotron near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) have been used to finely characterize the potential microstructural and chemical evolution in the cell components. At the Hydrogen electrode, migration of Ni particles away from the electrolyte and coarsening were observed by SEM in the SOEC-aged samples. Whereas, only coarsening of Ni particles was observed in the SOFC-aged samples. These observations were in-line with the previous observations made in our laboratory ( 6 ) and confirm the EIS evolution. Indeed, the Ni coarsening explains the increase in the HF contributio