Toward a Mechanistic Understanding and Optimization of Molten Alkali Metal Borates (A_xB_1–xO_1.5–x) for High-Temperature CO₂ Capture

Carbon capture, utilization, and storage technologies are needed to meet carbon emission reduction targets and prepare the energy industry for a carbon constrained world. Recent breakthroughs have identified the first liquid phase sorbents for CO2 capture at high temperatures. In this work, the material design space of the molten alkali metal borates (AxB1–xO1.5–x) is explored finding sodium and sodium-rich lithium–sodium borates with a mixing ratio, x, of around 0.75 to be optimal. A mechanistic understanding of the material is developed through exploration of the sodium borate phase diagram, the development of a kinetic equilibrium model, and estimation of effective diffusion coefficients. Interesting features of the sorbents, such as the proposed formation of dicarbonate ions and counter-intuitive trends in the diffusion coefficient, are identified and explained with implications for the design of future high temperature carbon capture facilities discussed.