Recovering oxygen from simulated lunar regoliths collected by the Chinese Chang-e'5 moon mission.

[NNadir]

I came across this cool paper this evening about the recovery of oxygen from lunar samples recently returned by the recent Chinese space mission: Extracting Oxygen from Chang’e-5 Lunar Regolith Simulants Hao Shi, Peng Li, Zhengshan Yang, Kaiyuan Zheng, Kaifa Du, Lei Guo, Rui Yu, Peilin Wang, Huayi Yin, and Dihua Wang ACS Sustainable Chemistry & Engineering 2022 10 (41), 13661-13668.

The recovery depends on the application of the "FFC" process which I expect will ultimately change the world by reducing the cost of titanium (and other) metals. "FFC" refers to Fray, Farthing and Chen, cited thus in the paper: Chen, G. Z.; Fray, D. J.; Farthing, T. W. Direct Electrochemical Reduction of Titanium Dioxide to Titanium in Molten Calcium Chloride. Nature 2000, 407 (6802), 361– 364.

Regrettably I won't have much time to discuss this interesting paper - FFC papers often catch my eye, but using it to process lunar samples strikes me as interesting - but I can share some text, and a graphic.

The introductory text:

The exploration and understanding of the Moon have been a dream for a long time. On December 17th, 2020, Chang’e-5 brought back a new batch of lunar samples that were collected in a region of northern Oceanus Procellarum. (1−4) The sample collection area of Chang’e-5 was different from those of Apollo and Luna. Like the previous samples, the recently collected lunar regolith is composed of oxides, (5) which is a huge reservoir of oxygen and metals that are indispensable for future lunar exploration. (6) Thus, the in situ resource utilization (ISRU) of lunar minerals has attracted much attention, (7) and the key technology of ISRU is to extract oxygen and valuable materials using the local resources of the Moon and solar energy. Therefore, it is important to develop an effective way to obtain oxygen and metals from the Moon.

From lunar regolith, oxygen is typically obtained in three ways. The first way is chemical reduction, which is used to reduce oxides into metals and secondary oxides through external reagents: for example, using hydrogen (H2) to reduce ilmenite (FeTiO3) to generate Fe, TiO2, and H2O, which can be further electrolyzed to H2 and O2. (8) A carbothermal reduction process has also been extensively studied, and the reducing agents mainly include solid carbon, CO gas, methane, etc. (9−11) Thermodynamically, the efficiency of chemical reduction depends on the limited oxides such as FeTiO3 and Fe2O3 of the lunar regolith. The second way is to decompose lunar regolith by vacuum thermal dissociation. (12−14) High temperatures ( more than 2000 °C) and ultrahigh vacuum (less than 10–14 atm) are needed to drive the spontaneous thermodynamic decomposition of oxides to metals and oxygen. (15) The vacuum thermal decomposition has been well modeled a the practical demonstration has not yet been performed. The third way is an electrochemical process that uses electricity to split oxides into metals and oxygen. In water solutions, oxygen and metal extraction limited by the low electrochemical window of water and may consume the precious water resources of the Moon. Additionally, the reduction of Fe2O3 presents significant technical challenges in water solutions. (16) Room-temperature ionic liquids broaden the electrochemical window so that they can extract oxygen and many metals such as Al, Si, Ti, Cu, Zn, Cr, etc. (17) However, the extraction of metals in an ionic liquid from lunar regolith is different because of its complex composition. (18) Various oxides can be electrolyzed directly using a high-temperature electrolyzer in molten oxides or molten salts. (19−21) The molten oxide electrolysis is always performed at a temperature higher than 1500 °C, which poses significant challenges for finding suitable electrode materials, especially an affordable oxygen-evolution inert anode. (20,22−24) Lunar regolith simulant electrolysis in fluoride melts to produce oxygen has been reported at 950 °C. (25,26) This reduction process also faces the challenges of anode materials and the solubility limitation of lunar regolith. The FFC Cambridge process can electrolyze various oxides at a temperature below 1000 °C. (27−32) However, the chloride salt is so supercorrosive that a low-cost oxygen-evolution anode is still absent. (33,34) As a result, most molten CaCl2 electrolyzers have employed a carbon anode to generate CO2 but not the desired oxygen. (35) Thus, an electrolyzer that can convert lunar regolith to metals and oxygen with a cheap oxygen-evolution inert anode is urgently needed.

Herein, we adopted a molten CaCl2 electrolyzer to electrolyze the lunar regolith using a consumable carbon anode to produce metals and CO2. Then, the generated CO2 was electrochemically transformed to carbon and oxygen using a low-cost nickel alloy inert electrode in molten carbonate...

I'm not a big one for embracing fantasies of moon colonies and mines on the moon, but the chemistry proposed here, the FFC process and molten carbonate reduction of CO2 to elemental carbon are both processes that can go a long way to environmental sustainability, particularly in cases where the production of electricity is a side product designed to capture exergy from high temperature sources.

The electrochemical device for oxygen recovery:



The caption:

Figure 1. Thermodynamic analysis and dual-electrolyzer system. (a) Deposition potentials of the main oxides found in lunar regolith (in a molten CaCl2 system). (b) Standard deposition potential profiles of typical oxides (in a molten Li2CO3–Na2CO3–K2CO3 system). All thermodynamic data were calculated using HSC Chemistry 6.0. (c) Schematic diagram of the molten CaCl2 electrolyzer and the molten carbonate electrolyzer and the recycling and reuse of carbon between the two electrolyzers

The molten carbonate reduction has caught my eye before now. It is a potential way to reverse the combustion of coal, and in fact, produce very high purity carbon for use, carbon far more pure than the carbon in coal. Whether or not we wax romantic about such an idea, I note that future generations who might use this type of technology to clean up our mess, will have to reproduce more energy than we obtained when we burned the coal and dumped the waste directly into our favorite waste dump, the planetary atmosphere.

History will not forgive us, nor should it.

Cool paper though...