Potassium carbonate acts as an absorber by reacting with CO2 to form potassium bicarbonate. It shares most of ammonia's advantages while being nonvolatile, enabling CO2 absorption at much higher temperatures. Its major disadvantage is a low absorption rate owing to poor CO2 mass transfer. Amine and amino acid promoters that form intermediates with CO2 to facilitate the generation of bicarbonate ions have been investigated. Grimekis et al. demonstrated that adding piperazine and MEA improved absorption rates and CO2 solubility at the same time, while MDEA and glycine significantly impacted CO2 solubility. Li et al. reported enhancements in both absorption and desorption by using glycine or lysine as promotors. Biocatalysts such as carbonic anhydrase were also found to be effective promotors, with absorption rates increased by up to eight times. Besides being studied as additives for other absorbents, amino acid salts have also attracted attention as CO2 absorbents themselves. They share the benefits of amines and carbonates and have high absorption capacity along with low toxicity and vapor pressure. Their major disadvantage is the easy formation of precipitates upon CO2 absorption due to their ionic nature, which creates complications in mass transfer and heat transfer. Reboiler duty, the energy required to recover the solvent after CO2 absorption, is an important benchmark for chemisorbents. A reboiler duty of 3.1 GJ/t (gigajoule/ton CO2) was reported for 40% MEA solution, whereas ammonia and K2CO3 solutions showed values as low as 2 GJ/t. For direct air capture (DAC), due to the low CO2 concentration in the air, the leading absorbents used are strong base solutions such as KOH or NaOH. Carbonate salts are formed in the process, which are then heated to release pure CO2. Alternatively, the carbonates can be treated with Ca(OH)2 to form CaCO3 precipitate to return KOH or NaOH to the process. The CO2 is thereby captured in the form of CaCO3 which can optionally be heated to obtain the gas, while regeneration of the caustic absorbent is readily accomplished in one step. The use of strong base absorbents means large amounts of energy is needed to separate CO2 from the absorbent, which exacerbates the high cost of DAC. Mahmoudkhani et al. achieved significant reduction in absorbent regeneration energy and temperature by using sodium trititanate in place of calcium hydroxide for causticization. More recently, Shu et al. reported an electrochemical process using an electrochemical cell having a pH gradient, allowing for reduction in energy consumption as well as simultaneous desorption and regeneration. Given the various limitations of the CO2 absorbents currently used, development of new materials or formulations is needed to achieve optimum performance. Given the wide range of possible materials, one focus of CO2 capture research has been the use of computer-aided molecular design to identify new structures or commercially available substances that have not been explored as CO2 absorbents. Salazar et al. studied 50 amines that were prescreened using solubility and boiling point data, selecting three that showed much lower theoretical reboiler duty than MEA for post-combustion capture. Papadopoulos et al. modeled the solubility and partial pressure of CO2 and identified both new as well as commercially available alternative amines that outperformed MEA in overall absorption/desorption cycles. Physical absorption relies on physical dissolution of CO2 as the driving force. The method is effective only at high pressure and lower temperature and thus is much more suitable for precombustion capture. However, physical absorption using noncorrosive solvents is much less demanding on equipment than chemical absorption, and has been practiced commercially. ref In addition, regeneration of physical solvents is much less energy intensive since the dissolved CO2 can be easily released through depressurization or moderate heating. Commonly studied good CO2 solvents include methanol, Selexol (polyethylene glycol dimethyl ether), N-methyl-2pyrrolidone (NMP) and propylene carbonate. The advantages and limitations of these solvents are summarized in Borhani's review.