Oxidative coupling of methane (OCM) can be performed electrocatalytically by utilizing solid oxide cells, which provide a readily controlled oxygen supply through dense electrolytes. La0.7Sr0.2Ni0.2Fe0.8O3(LSNF) perovskite is an effective anode for OCM. Its surface characteristics and electrocatalytic activity can be improved by reduction and the resultant exsolution of bimetallic NiFe nanoparticles from its bulk. X‐ray diffraction (XRD) and environmental transmission electron microscopy proved that the evolution of hetero‐phases under reducing environment resulted in bimetallic NiFe nanoparticles being formed on the surface. A 36 % improvement in C2+hydrocarbon production rate was achieved due to the reduction of LSNF with the exsolved NiFe nanoparticles. Co‐feeding of H2O enhanced selective conversion of CH4resulting in the production rate of C2+hydrocarbons being increased by 56 %. Analysis of impedance spectra and in‐situ DRIFTS under a CH4+H2O atmosphere provided an understanding for the enhancement on the electrocatalytic OCM.
Doped perovskite metal oxide catalysts of the form A(BxM1-x)O3-δhave been instrumental in the development of solid oxide electrolyzers/fuel cells. In addition, this material class has also been demonstrated to be effective as a heterogeneous catalyst. Co-doped barium niobate perovskites have shown remarkable stability in highly acidic CO2sensing measurements/environments (1). However, the reason for their chemical stability is not well understood. Doping with transition metal cations for B site cations often leads to exsolution under reducing conditions. Many perovskites used for the oxidative coupling of methane (OCM) or the electrochemical oxidative coupling of methane (E-OCM) either lack long term stability, or catalytic activity within these highly reducing methane environments. The Mg and Fe co-doped barium niobate BaMg0.33Nb0.67-xFexO3-δshown activity in E-OCM reactors over long periods (2) (>100 hrs) with no iron metal exsolution observed by diffraction or STEM EDX measurements. In contrast, iron decorated BaMg0.33Nb0.67O3showed little C2 conversion activity.
more » « less- NSF-PAR ID:
- 10487371
- Publisher / Repository:
- IOP Science
- Date Published:
- Journal Name:
- ECS Transactions
- Volume:
- 111
- Issue:
- 6
- ISSN:
- 1938-5862
- Page Range / eLocation ID:
- 587 to 593
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Abstract The instability of the surface chemistry in transition metal oxide perovskites is the main factor hindering the long-term durability of oxygen electrodes in solid oxide electrochemical cells. The instability of surface chemistry is mainly due to the segregation of A-site dopants from the lattice to the surface. Here we report that cathodic potential can remarkably improve the stability in oxygen reduction reaction and electrochemical activity, by decomposing the near-surface region of the perovskite phase in a porous electrode made of La1-xSrxCo1-xFexO3mixed with Sm0.2Ce0.8O1.9. Our approach combines X-ray photoelectron spectroscopy and secondary ion mass spectrometry for surface and sub-surface analysis. Formation of Ruddlesden-Popper phase is accompanied by suppression of the A-site dopant segregation, and exsolution of catalytically active Co particles onto the surface. These findings reveal the chemical and structural elements that maintain an active surface for oxygen reduction, and the cathodic potential is one way to generate these desirable chemistries.
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Electron doping in perovskites is an effective approach to design and tailor the structure and property of materials. In A 2 BB′O 6−δ -type double perovskites, B-site cation order can be tunable by A-site modification, potentially leading to significant effect on the oxygen nonstoichiometry of the compounds. La 3+ -doped Sr 2 FeMoO 6−δ (Sr 2−x La x FeMoO 6−δ , SLFM with 0 ≤ x ≤ 1) double perovskites have been designed and characterized systematically in this study as anode materials for solid oxide fuel cells. Rietveld refinement of powder X-ray diffraction reveals a crystalline symmetry transition of SLFM from tetragonal to orthorhombic with the increase of La content, driven by the extra electron onto the antibonding orbitals of e g and t 2g of Fe/Mo cations. An increase in Fe/Mo anti-site defect accompanies this phase transition. Solid oxide fuel cells incorporating the Sr 1.8 La 0.2 FeMoO 6−δ (SLFM2) anode demonstrate impressive power outputs and stable performance under direct CH 4 operation because of its altered electronic structure, desired oxygen vacancy concentration and enhanced reducibility. Density functional theory plus U correction calculations provide an insight into how La doping affects the Fe/Mo anti-site defects and consequently the oxygen transport dynamics.more » « less
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1149/11106.0587ecst
