Elucidating the Performance Limits for Electrochemical CO2 Separation Using Exergy Loss Analysis and Zero-Dimensional Modeling

Abstract

The use of renewable electricity to power electrochemical CO2 removal and concentration from point sources, air, and seawater is receiving considerable interest as one strategy in our portfolio of options for mitigating climate change. Two common embodiments of this idea involve CO2 separation via electrochemically reversible binding of CO2 to a nucleophilic redox mediator, or a pH swing/gradient in aqueous solution. For either embodiment to be feasible, the energetic cost of regenerating the sorbent should be low at practical separation throughputs. Consequently, there have been a number of efforts to understand how the thermodynamic minimum work input for a given separation cycle varies under different conditions using modeling. In this talk, we demonstrate that the thermodynamic minimum work input for electrochemical CO2 separation is the sum of exergy losses incurred from differences between the partial pressure of CO2 within the CO2 source/exit streams and the partial pressure of CO2 in the sorbent. This framework rationalizes minimum work inputs for pH-swing and redox-mediator-based CO2 separation cycles, and motivates the measurement or estimation of the aforementioned CO2 partial pressures in experimental studies. Applying a zero-dimensional model recently developed in our group to pH-swing-driven CO2 separation, we will then discuss how the total energetic cost of CO2 separation can be simulated under a variety of scenarios relevant to practical CO2 concentration from flue gas.