Sub-Ambient Performance of Potassium Sarcosinate for Direct Air Capture Applications: CO₂ Flux and Viscosity Measurements

Absorption based direct air capture (DAC) technologies have garnered significant interest in recent years due to their scalability, competitive regeneration energy requirements, and low susceptibility to degradation. One of the key advantages of DAC lies in its flexible siting options and the potential to utilize low-value land. However, most of the research in this field has been focused on ambient climate zones (T > 20 °C), overlooking sub-ambient (−30 °C < T < 20 °C) regions, which comprise approximately 70 % of the Earth's surface. To fully realize the potential of DAC, it is essential to understand how DAC solvents perform in these sub-ambient conditions before any large-scale deployment can be considered. Among DAC solvents of interest, potassium sarcosinate (K-SAR) has emerged as a promising candidate due to its high CO₂ capacity, fast uptake kinetics, compatibility with contactor packing materials, low volatility, good thermal and oxidative stability, and competitive regeneration energy requirements compared to current industry standards. This paper characterizes the CO₂ flux and viscosity of K-SAR at sub-ambient conditions and explores the potential of using ethylene glycol and triethylene glycol as additives to prevent solvent freezing in DAC applications. For 1 M K-SAR, the CO₂ flux ranges between 1.3 × 10^-5 and 8.0 × 10^-5 mol m⁻² s⁻¹ across a temperature range of −5 °C to 45 °C. Ethylene glycol is shown to effectively suppress the freezing point of K-SAR below −30 °C with volumetric loadings of the additive as low as 0.1. A reaction model was developed to predict the CO₂ flux for 1 M K-SAR at different temperatures, demonstrating good agreement between experimental and theoretical fluxes.