Unveiling the Interaction of CH4 with Ni-doped Sr(Ti,Fe)O3-d (STFN) Electrodes Decorated with Exsolved Fe-Ni Nanoparticles ? An Operando AP-XPS Study on STFN Model Cells

ARCE MAURICIO; TOSCANI LUCIA; VIRGINIA PÉREZ-DIESTE; MARIANO SANTAYA; IGNACIO J. VILLAR-GARCÍA; MARCUS BÄR; JIMÉNEZ CATALINA; GAMBA NADIA; LILIANA MOGNI

Bostón

Congreso; 23rd International Conference on Solid State Ionics (SSI); 2022

Electrodes based on metallic nanoparticle (NP) exsolution are gaining increased attention due to its enhanced catalytic activity and remarkable stability in aggressive environments[1]. These materials are appealing for Reversible Solid Oxide Cell (RSOC) technologies, which have low environmental impact, operate at high temperature and can use carbon sources to efficiently generate electricity or fuels in the Solid Oxide Fuel Cell (SOFC) and Electrolyzer Cell (SOEC) modes, respectively. Ni-based electrodes are typically employed for this purpose, as they are inexpensive good hydrocarbon cracking catalysts, but coking is a major issue since it deactivates the electrode leading to degraded cell performance [2]. Recently, electrocatalysts based on Ni-Fe NP exsolutionfrom Ni-doped Sr(Ti0.3Fe0.7)O3-d perovskites (STFN) have been proposed as RSOCs electrodes due to its excellent stability, enhanced electrocatalytic activity and high C-tolerance[3, 4]. NP exsolution and reabsorption can be triggered by a combination of atmosphere and electrode polarization. Comprehensive understanding of the conditions where these processes take place, and to what extent regeneration is feasible is critical for optimized operation. The high tolerance towards C-containing gas feeds on this type of electrodes makes them suitable for RSOC, by oxidizing CH4 to generate electricity in SOFC mode and co-electrolysis of CO2 and H2O for power-to-gas conversion in SOEC mode. Nonetheless, it is still unclear if NPs promotethe redox reactions in regenerative operation, i.e. during cycling between SOFC/SOEC modes. Carbon gas feeds can interact with electrodes in many ways, such as C adsorption, migration, dissolution, and C-metal compound formation. Although widely studied on Ni-based catalysts, the complex nature of CH4 oxidation makes elucidation of reaction mechanisms and degradation processes very challenging. Temperature, atmosphere, and electrode polarization play a crucial role in the C-metal interaction, NP stability, degradation processes, and hence these parameters need to be adjusted for optimizing RSOC operation. Furthermore, this knowledge can help for designing smart regeneration strategies to recover the electrode, prolonging its lifetime. Surface-sensitive operando characterization by ambient pressure X-ray photoelectron spectroscopy (AP-XPS) is ideal for studying the metal-carbon interface in combination with electrochemical techniques, allowing us to get a close-to-real picture of STFN electrodes under operation in C-containing atmospheres.In this study, we will present a complete characterization of the surface chemistry of exsolved Ni-Fe NP from STFN under dry/wet CH4 atmospheres, focusing on the C-metallic NP interaction. Element depth distribution and surface segregation are studied by AP-XPS using different photon energies. Furthermore, we acknowledge the effect of temperature and polarization on model cells for CH4 oxidation under operating conditions by combining electrochemical techniques such as chronoamperometry and electrochemical impedance spectroscopy with AP-XPS, to understand the correlation between surface and electrochemical performance. These studies allowed us to gain atomistic information about electrode reaction mechanisms and degradation processes, with particular attention to coking and its effect on the electrocatalytic activity of the STFN electrode, and NP stability under working conditions. We could establish that between 500 and 700 °C (relevant to RSOC application) C-deposition is prevented in highly aggressive dry CH4, whereas coking occurs below this temperature and is enhanced at 400 °C. Despite this, C-deposition at 400 °C can be reversed upon oxidation above 500 °C in wet CH4 conditions.