Membrane separation processes for the benefit of the sulfur–iodine and hybrid sulfur thermochemical cycles

Thermochemical cycles have been proposed as processes for the manufacture of hydrogen from water in which the only other effluent is oxygen. In this paper, membrane-based technologies are described that have the promise of enabling the further development of thermochemical cycle processes. Membranes have been studied for the concentration of hydriodic acid (HI) and sulfuric acid using pervaporation. In this work, Nafion® and sulfonated poly(ether ether ketone) (SPEEK) membranes have effectively concentrated HI at temperatures as high as 134 °C (407 K) without any significant degradation of transport behavior. Additionally, sulfuric acid has been concentrated using Nafion® membranes at 100 °C (373 K). Measured fluxes of water and separation factors are commercially competitive and have been characterized with respect to acid concentration in the feed streams. Further, hydrogen permeability is discussed at 300 °C (573 K) with the goal of providing a method for the removal of the product gas from HI in the decomposition step, thus increasing the productivity of the equilibrium-limited reaction.