(68h) Molten Salts Mediated High-Temperature Carbon Capture

Successive renewing of world record high every year in the atmospheric carbon dioxide concentration evokes us the critical urgency of substantial actions to lower the global CO₂ emissions. The establishment of versatile CO₂ capture systems from anthropogenic carbon sources is the key challenge to ensure the rigid feasibility of sustainable carbon neutral society. In particular, the mitigation of intensive emission sources from the energy-related sector, which includes power (electricity and heat) generation, transportation, and heavy industry manufacture will shape the future pathway toward the net-negative scenarios. Herein, we demonstrate the recent progress of molten salts mediated high temperature carbon capture technology, which was first initiated in the Prof. T. Alan Hatton’s group at MIT and has been studied actively worldwide to the present as a new class of approach for low-cost carbon capture from industrial sources.

Molten salts are the fluidic ionic melts consisting of the electrically charged species, showing high thermal stability, high liquid fluidity in wide temperature range, low-vapor pressure, high electrical conductivity and catalytic effects. The ionic melts have been used in various industrial applications, such as thermal energy storage media for concentrated solar power plants, heat transfer coolants at nuclear reactor, high temperature electrolytes for fuel cells or batteries, and the catalytic fluids for pyrochemical processes like coal gasification. A series of recent studies have further revealed that the liquid melts, in particular the salts including oxide ions (O^2-), which are also called as “molten ionic oxides”, play the key role to expand the practical capability of high temperature carbon capture technology. First, molten alkali metal nitrates/nitrites coated on the surface of MgO show the outstanding catalytic effects to accelerate the CO₂ absorption by MgO at intermediate temperatures around 300 ºC.^[1] The peculiar catalytic effects of molten nitrates/nitrites can be explained by the occurrence of inter-dissolutions of gaseous CO₂ and solidus MgO in the molten salts including O^2- and NO₃^2- under the reaction with CO₂, which is followed by the rapid nucleation of the carbonate crystals with fast product layer diffusion of ionic species through the generated carbonates. It was also revealed that the surface coating of molten nitrates/nitrites has the marked effects to minimize the morphological deterioration during the cyclic operations of CO₂ capture and desorption.^[2] The outstanding catalytic and granular stabilizing effects of molten nitrate/nitrite salts are not limited to MgO, but also appeared on different basic metal oxides and compounds, such as CaO, Li₃BO₃, and MgO-Li₂O-B₂O₃.^[3] The remarkable properties of molten salts are further demonstrated by their superior performances as the liquid fluidic high-temperature CO₂ absorbents. We have designed a new class of liquid absorbents for CO₂, as well as for the different industrially concerned acidic gases, such as NO_x or SO_x, by using molten alkali-metal borates.^[4-6] The borates-based liquid absorbents discovered in our studies show the rapid uptake of high amount of CO₂ at medium to high temperature around 500-700 ºC with the excellent cyclic regenerability without deterioration. Techno-economic studies on the conceptional CO₂ capture process using these borates melts revealed that the cost of CO₂ avoided can be lowered markedly than the case using conventional amine process.^[7] Furthermore, it was also elucidated that the high temperature melts absorbing high concentration of CO₂ could work as superior electrolytes for high-efficiency electrochemical CO₂ conversion to various value-added chemicals.^[8] The applications as reactive fluidic media for continuous CO₂ separation in carbon-free H₂ production processes in sorption-enhanced steam methane reforming (SE-SMR) is also an important potential technology to be brought by the borate melts.

In this presentation, we will discuss the details for the outstanding capabilities of molten salts to accelerate the developments of advanced carbon capture systems. The new scientific insights and technological breakthroughs enlightened by Prof. T. Alan Hatton for high temperature carbon capture using molten salts will open the solid pathway to sustainable future society under global net-zero emissions.

References

[1] T. Harada, F. Simeon, E.Z. Hamad, and T. Alan Hatton, Chem. Mater. 2015, 28, 1943-1949.

[2] T. Harada and T. Alan Hatton, Chem. Mater. 2015, 27, 8153-8161

[3] T. Harada and T. Alan Hatton, J. Mater. Chem. A, 2017, 5, 22224-22233

[4] T. Harada, C. Halliday, A. Jamal, and T. Alan Hatton, J. Mater. Chem. A, 2019, 7, 21827-21834

[5] Shiyi Zang and Takuya Harada, ACS Sustainable Chemistry & Engineering 2025, 13, 12, 4768–4777

[6] David Unnervik and Takuya Harada, Advanced Sustainable Systems, 2025, ASAP (https://doi.org/10.1002/adsu.202400969)

[7] C. Halliday and T. Alan Hatton, Applied Energy, 2020, 280, 116016

[8] L. Bromberg, M. P. Nitzsche, and T. Alan Hatton, Nanoscale, 2022, 14, 13141-13154