Dimensionless Analysis of a Physics-Based Model for Economical Hydroxide Exchange Membrane Carbon Capture
Authors: Colin Martinez Marco , Setzler Brian P , Yan Yushan 
Published: December 2024

Billions of tons of CO2 emissions are contributing to the detrimental decline of global climate. International agreements regarding the solution to lower emissions dictate that the abatement of CO2 emitting processes is no longer sufficient; negative emission technologies are needed. Technologies using solid and liquid sorbent to capture CO2 are growing at an impressive pace targeting global capture demands. However, capturing CO2 from ambient air poses significant challenges for existing technologies, primarily due to steep energy and capital costs. Moreover, these approaches often rely on energy sources that inadvertently contribute to CO2 emissions. The rapid development of green energy sources like hydrogen presents an opportunity for a new electrochemically-based CO2 capture method powered by hydrogen. Estimates using experimental data of the Hydroxide Exchange Membrane Carbon Capture (HEMCC) approach predict a fraction of the volume relative to traditional technologies. In this H2-powered process, hydroxides are electrochemically generated at the cathode via the oxygen reduction reaction, which chemically removes CO2 from the air. This produces (bi)carbonates that transport across the hydroxide exchange membrane and accumulate at the anode. As a result, the local pH environment at the anode—where hydrogen oxidation reaction occurs—lowers until CO2 generation becomes thermodynamically favorable, resulting in a concentrated CO2 stream.

Experimental validation of the HEMCC device has motivated further research; however, for commercialization, there is a need to increase electron efficiency (CO2/e-)—a proxy for lower hydrogen operating costs. Given the novel nature of this technology, our research aims to understand efficiency limitations to inform experiments. To this end, we have developed a steady-state, one-dimensional numerical model of the HEMCC system (Figure 1a). The model simulates four layers—anode, cathode, membrane, and interlayer—involving multiple parallel processes including ion transport, CO2 kinetics, ionic equilibrium, electrochemical kinetics, and gas transport. Using dimensionless diagnostic metrics provides a structured approach to elucidate key CO2 capture information about this complex model (>50 parameters). The study has determined that electron efficiency limitations can be improved through size optimization of electrodes and membranes as well as increasing ionic resistance (Figure 1b). Additional improvements were found by increasing CO2 transport or CO2 kinetics on the air side. This dimensionless approach has elucidated key variables controlling electron efficiency, enabling an intuition-based optimization for CO2 capture. Using this approach, we demonstrated a cost reduction of $23/tonCO2 compared to the baseline, assuming a H2 price aligning with the DOE H2 shot's target cost of $1/kgH2. However, for a more comprehensive analysis incorporating other operating and capital costs, future work should consider realistic system conditions. Given the potential for decentralization of the HEMCC, future work should leverage the computational efficiency of this numerical model and integrate yearly weather patterns to simulate real-time effects on system performance. In summary, future work should conduct a thorough technoeconomic analysis to assess the feasibility of this technology.

Figure 1