Fluoride- and hydroxide-containing CO2-selective membranes for improving H2 utilization of solid oxide fuel cells

Abstract

Solid oxide fuel cells (SOFCs) have been commercialized due to their ability to provide a reliable energy supply at a high efficiency. However, because of the incomplete oxidation on the anode side, there is still a considerable amount of unused fuel (e.g., H2 and CO) present in the anode exhaust. A CO2-selective membrane was proposed to remove CO2 in the anode exhaust and recycle the retained H2 and CO back to the SOFC. The challenge lies in maintaining a high CO2/H2 selectivity (>100) and a good CO2 permeance (>100 GPU) at an operating temperature of 120 °C. Earlier studies have yielded facilitated transport membranes that employed tetramethylammonium hydroxide (TMAOH) as the mobile carrier for CO2. However, a better stability of the membrane at a higher temperature of 130 °C was desired. In this study, fluoride- and hydroxide-containing species, including tetramethylammonium fluoride (TMAF), cesium fluoride (CsF), potassium fluoride (KF), and N,N-dimethylpiperidinium hydroxide (DMPOH), were examined for their potential of replacing TMAOH to facilitate CO2 transport. The thermal stability of the quaternaryammonium-containing compounds was characterized by TGA and NMR, and the catalyzing activities of the fluoride-containing species were compared using NMR. Based on the characterization results, TMAF was chosen as the substitute for TMAOH, and the membrane synthesized with the optimal composition was tested for transport performances at various temperatures. In particular, a CO2 permeance of 108 GPU and a CO2/H2 selectivity of 106 were obtained at 120 °C. The TMAF-containing membrane also demonstrated an improved stability than the TMAOH-containing membrane at 130 °C.