Assessing Perovskite Electrode Materials Stability for Electrochemical Oxidative Coupling of Methane

Abstract

Identification of large shale gas reserves has reduced the price of methane significantly. Interest in converting methane to value-added commodity chemicals has increased as a result of these findings [1]. Direct conversion of methane to ethylene using heterogeneous methods such as oxidative and non-oxidative coupling of methane (OCM and NOCM) utilize metal oxide catalysts with varying success. However, a catalyst with commercial viability remains elusive. Recently, iron-doped Sr2Fe1.5+0.075Mo0.5O6-δ electrodes (SFMO-075) have been shown to convert methane to ethylene more efficiently in an electrochemical OCM set up (EC-OCM) [2]. However, one of the main concerns with perovskite electrodes is the stability and durability of these materials. We have developed two different perovskites comprising Ba, Mg, Ca, Nb and Fe and evaluated them for EC-OCM. However, the chemical and electrochemical stability of these materials is largely unexplored. Specifically, during EC-OCM, these materials are exposed to CH4, CO2 and H2O. Each metal oxide’s chemical stability needs to be evaluated under these gases at concentrations relevant to EC-OCM. Further, pure methane supplied at the anode presents a highly-reducing atmosphere, necessitating a study on the redox stability of these materials under very low oxygen partial pressures. We shall report our results on the chemical stability of three different perovskite materials (including SFMO) in EC-OCM relevant conditions. We evaluate these perovskite powders before and after gas exposure using TGA, XPS, XRD and FT-IR measurements. While SFMO was found to be unstable in these conditions with a crystal structure collapse, the other two perovskites show promising stability. Their thermodynamic stability in these conditions along with the feasibility of various reaction mechanisms (analyzed through HSC Chemistry) will be discussed. Our study demonstrates important characteristics of catalysts for EC-OCM and may lend to the design of innovative perovskites capable of converting methane to ethylene electrochemically. References: 1) P. Schwach, X. Pan, and X. Bao, “Direct Conversion of Methane to Value-Added Chemicals over Heterogeneous Catalysts: Challenges and Prospects,” Chem. Rev., 117, 8497, 2017. 2) C. Zhu, S. Hou, X. Hu, J. Lu, F. Chen, and K. Xie, “Electrochemical conversion of methane to ethylene in a solid oxide electrolyzer,” Nat. Commun., 10, 1173, 2019.