Controlling CO2 flux in a CO2-permeable membrane with a H2O driving force

Abstract

Gas separation membranes hold significant promise for carbon capture and storage (CCS) as they offer high modularity in combination with technical simplicity. It is routinely expected that a difference in the partial pressure of CO₂ (i.e., a CO₂ driving force) across a CO₂-permeable membrane dictates CO₂ flux. Here, however, we show that in a molten-salt membrane fabricated using molten hydroxides, a H₂O driving force in the opposite direction to CO₂ permeation exerts control. We demonstrate this by using the opposing H₂O driving force to operate the membrane in ways that challenge the conventional understanding of CO₂-permeable membranes. For example, increasing the CO₂ flux whilst decreasing the CO₂ driving force. Throughout, we employ a model membrane support to facilitate recovery (and subsequent characterisation) of the molten salt, showing that membranes fabricated using molten hydroxides transform into majority molten carbonate membranes during CO₂ separation. The carbonate : hydroxide ratio is shown to be a function of time, temperature, and gas-phase composition, and high carbonate : hydroxide ratios are correlated with high CO₂ fluxes. Overall, our work demonstrates that molten-salt membranes evolve, and that a H₂O driving force can be used to control CO₂ flux.