Chemical Stability and CO2 Electrolysis Performance of La0.3Sr0.7Fe0.7Cr0.3O3- δ in CO2/CO Environments

Abstract

LMFCr (M = Sr, Ca) has emerged as a highly promising class of perovskite oxide materials for use as symmetrical electrodes in reversible solid oxide fuel cells (RSOFCs).1 While there are numerous studies on the efficacy of these materials as symmetrical electrodes for SOFCs,1-5 a detailed study on their chemical stability and electrochemical performance as fuel electrodes in CO2/CO environments has not yet been reported. This work aims to address the stability of the Sr analogue of LMFCr, namely, La0.3Sr0.7Fe0.7CrO3-δ (LSFCr), in low pO2 environments consisting of CO2/CO gas mixtures (10-11-10-18 atm) to establish the range of pO2 over which the electrode material is structurally and electrochemically stable. This is relevant under CO2 reduction conditions as the stability limit will define the allowable CO2 utilization in these cells in practice.The chemical and phase stability was studied on LSFCr powders and pellets via XRD, TGA, and SEM in different pO2 atmospheres ranging from pure CO2, CO (balance N2), and 90-70% CO2:10-30% CO containing mixtures. Powdered LSFCr remains structurally stable in 20-100% CO2 (balance N2, pO2 = 10-11 – 10-12 atm) from room temperature to 800 oC without any decomposition. On the other extreme, a reversible decomposition occurs at 800 oC in 30% CO (balance N2, pO2 ~ 10-26 atm) resulting in the formation of a Ruddlesden-Popper phase, Fe nanoparticles (NPs), and some carbon fibers, originating from the Fe NPs. However, this decomposition can be reversed by heat treatment in air at 800 oC. While no evidence for coke formation is obtained in 90-70% CO2:10-30% CO (pO2 = 10-17 – 10-18 atm) mixtures at 800 oC, in 70CO2:30CO, minor impurities of SrCO3 and Fe NPs are observed, with the latter potentially being beneficial to the electrochemical activity of the perovskite.Electrochemical characterization was carried out on symmetrical cells prepared by printing ~25 µm single phase LSFCr electrode layers, with an area of ca. 0.5 cm2, on both sides of a 1” diameter SDC buffered YSZ electrolyte, followed by painting on Au paste as the current collectors. The cells were mounted in an open flanges cell testing setup (Fiaxell, Switzerland) and supplied with 50 mL/min air at the oxygen electrode and 50 mL/min CO2:CO mixtures at the fuel electrode. The performance of full cells was determined in various CO2:CO ratios (Pure CO2, 90:10, and 70:30) at 800 oC via electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and chronoamperometry. Consistent with prior work, symmetrical 2-electrode full cells showed very good electrochemical performance at 800 oC, both in the SOFC and SOEC modes. Furthermore, LSFCr full cells exhibited excellent short-term stability in 15-min potentiostatic stability tests at 1.2-1.5 V, while medium-term potentiostatic tests at 1.1 and 1.2 V showed that LSFCr exhibits exceptional stability during CO2 electrolysis in all gas atmospheres over several 100 hours, with the oxygen electrode facilitating the oxygen evolution reaction (OER) at the same time. These results suggest that LSFCr has the potential to be a very efficient and stable electrode material for symmetrical SOC applications. References (1) Chen, M.; Paulson, S.; Thangadurai, V.; Birss, V. Journal of Power Sources 2013, 236, 68-79.(2) Chen, M.; Paulson, S.; Thangadurai, V.; Birss, V. ECS Transactions 2012, 45, 343-348.(3) Chen, M.; Paulson, S.; Kan, W. H.; Thangadurai, V.; Birss, V. Journal of Materials Chemistry A 2015, 3, 22614-22626.(4) Molero-Sanchez, B.; Addo, P.; Buyukaksoy, A.; Paulson, S.; Birss, V. Faraday Discussions 2015, 182, 159-175.(5) Addo, P. K.; Molero-Sanchez, B.; Chen, M.; Paulson, S.; Birss, V. Fuel Cells 2015, 15, 689-696.