The current status of high temperature electrochemistry-based CO2 transport membranes and reactors for direct CO2 capture and conversion

The concept of direct CO2 capture and conversion has attracted significant interest from industries and academia in recent decades due to its potential to address the current grand challenge of global warming/climate change, rapid depletion of fossil fuels and realization of a future carbon neutral ecosystem. The incumbent benchmark technology for CO2 capture is the post-combustion flue-gas "amine washing", which is energy intensive and costly for large-scale commercial implementation. The CO2 conversion technologies, on the other hand, are still at their infancy with many technical challenges to overcome, but primarily being explored in laboratory-scale, low-temperature, solution-based and high-temperature, solid-oxide-based electrochemical cells with renewable electricity perceived as the energy input. In this article, we provide a comprehensive overview on an emergent class of high-temperature electrochemical CO2 transport membranes that can capture and convert CO2 into valuable chemicals in single catalytic reactor fashion. The review starts with the chemistry and transport theory of three basic types of membranes purposely designed for different CO2 feedstocks and downstream conversions. A range of key functional materials used in these membranes and their microstructural/electrochemical properties important to the CO2 transport are then thoroughly discussed in conjunction with the effects of surface modifications and operating conditions. Several types of combined CO2 capture and conversion catalytic reactors based on these membranes are also assessed with a focus on their working principles, system configurations and performance demonstrations. Finally, challenges and prospective of these electrochemical CO2 transport membranes and their associated conversion reactors are candidly discussed for future development.