First review of supported molten-salt membranes highlighting materials challenges, mechanistic development and manufacturing opportunities towards energy applications.

Dense dual-phase membranes consisting of a molten carbonate embedded in the interconnected pores of a ceramic framework are considered for potential application in CO₂ separation at elevated temperatures utilising ambipolar conduction of carbonate ions and other charge carriers in the molten or solid phase. We have developed an experimental setup for determining transport numbers of various species in the molten carbonate phase by means of electromotive force (emf) measurements. Transport numbers of native and foreign ions are characterized for the eutectic mixture of Li-Na-K carbonate, which is embedded in a porous alumina matrix with Pt or lithiated NiO electrodes. The voltage is measured under activity gradients of CO₂, O₂, or H₂O at 550°C. Electrochemical impedance spectra are collected during emf measurements. The true transport numbers of both native and foreign ions are calculated by taking into account the electrode polarization. The corrected transport numbers of native (carbonate and alkali metal) ions are close to unity at 550°C based on the emf under a <mml:math display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mrow><mml:mi>C</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:mrow></mml:msub></mml:math>pCO2 gradient. Minor transport of oxide ions and hydroxide ions in the molten carbonate is observed under a <mml:math display="inline"><mml:msub><mml:mi>p</mml:mi><mml:msub><mml:mi>O</mml:mi><mml:mn>2</mml:mn></mml:msub></mml:msub></mml:math>pO2 gradient and <mml:math display="inline"><mml:msub><mml:mi>p</mml:mi><mml:mrow><mml:msub><mml:mi>H</mml:mi><mml:mn>2</mml:mn></mml:msub><mml:mi>O</mml:mi></mml:mrow></mml:msub></mml:math>pH2O gradient, respectively. This offers an alternative understanding and operation of molten carbonate dual-phase membranes.