Ceramic/Metal-Supported, Tubular, Molten Carbonate Membranes for High-Temperature CO2 Separations

Abstract

Dual-phase membranes, consisting of molten alkali metal carbonates infiltrated into an oxygen ion or mixed-conducting substrate, are a promising technology for high-temperature CO2 separations and other energy conversion applications, such as membrane reactors. In this paper, we report a new fabrication technique for triple-phase membranes capable of high-temperature CO2 transport using an oxygen ion-conducting ceramic combined with an electronic-conducting metal layer. An oxygen ion-conducting, 4YSZ porous substrate tube was coated with a thin layer of metal (Pd/Ag or Ni) using electroless plating to prepare a mixed-conducting (oxygen ion and electronic) support for the triple-phase membranes. The porous section was then filled with a molten carbonate (MC) salt consisting of a eutectic mixture of Li/K or Na/K carbonates. This enabled both the mixed oxygen and carbonate ion conductor (MOCC) and mixed electron and carbonate ion conductor (MECC) CO2 transport mechanisms. The highest CO2 permeance (flux/driving force) of 1.07 × 10–7 mol m–2 s–1 Pa–1 (a flux of 0.8 cm3(STP) cm–2 min–1) was measured on a 4YSZ support coated with a Pd/Ag layer and infiltrated with the eutectic Li/K carbonate salt; it was measured at 725 °C with a 50/50 CO2/O2 feed gas mixture at a transmembrane pressure of 110 kPa. The apparent activation energy for CO2 transport was calculated to be 57 kJ mol–1 between 450 and 600 °C, suggesting that the rate-limiting step was the diffusion of the carbonate ion in the molten carbonate; however, the activation energy was observed to increase to 74 kJ mol–1 (650–750 °C), which was attributed to the influence of oxygen anion transport at higher temperatures.