In situ resource utilization of lunar soil for highly efficient extraterrestrial fuel and oxygen supply

ABSTRACT Building up a lunar settlement is the ultimate aim of lunar exploitation. Yet, limited fuel and oxygen supplies restrict human survival on the Moon. Herein, we demonstrate the in situ resource utilization of lunar soil for extraterrestrial fuel and oxygen production, which may power up our solely natural satellite and supply respiratory gas. Specifically, the lunar soil is loaded with Cu species and employed for electrocatalytic CO2 conversion, demonstrating significant production of methane. In addition, the selected component in lunar soil (i.e. MgSiO3) loaded with Cu can reach a CH4 Faradaic efficiency of 72.05% with a CH4 production rate of 0.8 mL/min at 600 mA/cm2. Simultaneously, an O2 production rate of 2.3 mL/min can be achieved. Furthermore, we demonstrate that our developed process starting from catalyst preparation to electrocatalytic CO2 conversion is so accessible that it can be operated in an unmmaned manner via a robotic system. Such a highly efficient extraterrestrial fuel and oxygen production system is expected to push forward the development of mankind's civilization toward an extraterrestrial settlement.


INTRODUCTION
To achieve rational extraterrestrial exploitation and settlement on the Moon, the sustainable supply of fuels and oxygen is an indispensable issue [1,2]. The artificial production of hydrocarbon fuels (e.g. methane (CH 4 ) and ethylene (C 2 H 4 )) along with oxygen using carbon dioxide (CO 2 ) and water (H 2 O) as the feedstocks via a combination of photovoltaic and electrocatalysis is demonstrably feasible on Earth [3][4][5] and has been known as a potential strategy to be imitated at extraterrestrial sites. With the advancement in lunar exploration, it has been discovered that the lunar surface possesses CO 2 and H 2 O reserves [6,7], further supporting this proposal. For example, Schorghofer et al. discovered the presence of carbon dioxide cold traps on the Moon [8], which could provide a sufficient carbon source for human activity. In this regard, such a strategy is one of the most likely fuel and oxygen production technologies to be first implemented on the Moon. Yet, it appears that electrocatalytic CO 2 conversion can only be operated on a laboratory scale at extraterrestrial sites, which is an order of magnitude too small to accommodate the human energy requirements, due to the lack of efficient catalysts, stable electrolyser architectures, etc. Among them, catalysts play the dominant role in determining the efficiency and stability of electrocatalytic reactions. Therefore, this strategy can arguably be achieved with the exploration of appropriate catalysts from the Moon [9].
Recently, in situ resource utilization (ISRU) technology, which aims to enhance extraterrestrial exploration efficiency and reduce the amount of the resources that have to be transported from the Earth during extraterrestrial missions [10], has attracted wide attention from the scientific community for overcoming the limited transportation load of a C The Author(s) 2022. Published by Oxford University Press on behalf of China Science Publishing & Media Ltd. This is an Open Access article distributed under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited. spacecraft [11]. Such technology requires an ample understanding of the composition of the target extraterrestrial sites [10]. Up to the present, humans have performed 10 lunar sample return missions, including 6 Apollo, 3 Luna and 1 Chang'E (CE) mission [12], which enrich our knowledge of the composition of lunar regolith [13,14], endowing wide opportunities for expediting the development of ISRU. Typically, the composition of lunar regolith is rather simple with augite, plagioclase, olivine and ilmenite as the four main minerals in the returned lunar soils [15]. As such, it is conceivable that the effective use of these components for electrocatalyst production can greatly facilitate the exploitation of electrocatalytic fuel production on the Moon for sustainably supplying fuels.
Over and above that, labor force is valuable and limited at extraterrestrial sites. For this reason, alternatives to manned operation are highly sought after and the utilization of robotic systems for electrocatalytic CO 2 conversion can be a potential answer [16]. This solution raises a more stringent requirement for electrocatalytic system, whose entire unmanned process starting from catalyst preparation to electrocatalytic CO 2 conversion should be readily accessible. Herein, we first demonstrate the feasibility of ISRU of lunar soil obtained by the CE-5 return mission for electrocatalytic CO 2 conversion toward hydrocarbon fuel and oxygen production. We then analyse the active components in the lunar soil for the electrocatalytic reaction, establishing a highly accessible catalyst preparation procedure. To further show the high practicability of such a system on the Moon, we employ a robotic system to perform the unmanned catalyst preparation and electrolyser assembly toward electrocatalytic CO 2 conversion. Our work represents an important strategy for sustainably supplying fuels and oxygen toward reaching a human settlement on the Moon.

Electrocatalytic CO 2 conversion over lunar soil
In this work, we have obtained lunar soil from the CE-5 return mission (see Fig. 1a), the first sample of lunar regolith brought back to Earth since the Luna 24 mission in 1976 [17]. Given that we wish to employ this lunar soil for electrocatalytic CO 2 conversion, we first modify the lunar soil with the Cu species, which have been proven to be perfect active sites for CO 2 conversion [18,19]. As shown in Fig. 1b, the lunar soil demonstrates a bulk structure. The elemental mapping images (Fig. 1c) indicate that, apart from the Cu element, Al, Ca, Mg, Fe, Ti, Si and O elements can be determined on the Cu-loaded lunar soil (Cu/lunar soil). These elements are in good accordance with the composition of the most common pyroxene-augite [20], which is a silicate of calcium, magnesium, iron, titanium and aluminum. Upon Cu modification, we perform the electrocatalytic CO 2 conversion test in the flow cell (see Supplementary Fig. S1) with a gas diffusion electrode to overcome the CO 2 solubility limit in the aqueous solution. We begin the test by obtaining the linear sweep voltammetry (LSV) curves of the prepared samples in a flow cell with Ar and CO 2 feeding gases (Fig. 1d). The result shows that the current density in the flow cell with CO 2 feeding gas is significantly enhanced in low input potential compared to that with Ar feeding gas, suggesting the high tendency of the Cu/lunar soil toward CO 2 conversion. In addition, it has been discovered that the product selectivity of the Cu/lunar soil can be easily tuned by changing the Cu content and all the products produced through electrocatalytic CO 2 conversion using Cu/lunar soil are valuable fuels ( Fig. 1e and Supplementary Fig. S2), i.e. H 2 , CH 4 , CO and C 2 H 4 , confirming the high feasibility of lunar soil being employed for fuel production. Notably, all these products are gaseous fuels, allowing the facile collection and separation of the products from the electrocatalytic system during the reaction.
After demonstrating the high feasibility of our strategy for ISRU of lunar soil toward electrocatalytic CO 2 conversion for fuel production, we aim for identifying the main active component in lunar soil for optimizing the CH 4 production. Nevertheless, lunar soil is rather precious and limited on Earth. This limitation motivates us to employ augite on Earth (augite-E; see Supplementary  Fig. S3 for details), which has a similar composition and structure to lunar soil, for performing further investigation. To avoid the ambiguities associated with the imitation of lunar soil using augite-E, we first compare their compositions. As shown in Supplementary Figs S4 and S5, the Cu-loaded augite-E (Cu/augite-E) exhibits a bulk structure and its elemental mapping images demonstrate that the chemical composition of our obtained augite-E is similar to that of lunar soil [21]. To further evaluate the imitability of lunar soil by augite-E, we compare their X-ray diffraction (XRD) patterns. As revealed in Fig. 2a, augite-E has a similar crystal structure to lunar soil, allowing evaluation of the feasibility of lunar soil for ISRU using augite-E as an imitant. In addition, after loading of the Cu species on the augite-E, no significant change can be observed in the XRD pattern ( Supplementary  Fig. S6), suggesting that the Cu species do not alter the phase structures of the augite. Upon confirming the imitable structure of lunar soil by augite-E, we then employ the Cu/augite-E for electrocatalytic CO 2 conversion. Although the pristine augite-E demonstrates obvious H 2 production, no hydrocarbon products can be found in the system, manifesting its poor selectivity toward CO 2 conversion ( Supplementary Fig. S7). In sharp contrast, the Cu/augite-E demonstrates significant production of hydrocarbon fuels (Fig. 2b), further confirming the viability of the ISRU of lunar soil for CO 2 conversion catalyst preparation. Considering the fact that augite is composed of multiple silicates, we evaluate and compare the electrocatalytic CO 2 conversion performance of the main silicates in augite (i.e. aluminum silicate (Al 2 Si 2 O 7 , Supplementary Figs S8-S13), calcium silicate (CaSiO 3 , Supplementary Figs S14-S19) and mag-nesium silicate (MgSiO 3 , Supplementary Figs S20-S28) after loading with Cu species (Supplementary Table S1). Generally, 2% Cu/Al 2 Si 2 O 7 , 2% Cu/CaSiO 3 and 20% Cu/MgSiO 3 show the optimal electrocatalytic CO 2 -to-CH 4 performances (Supplementary Figs S12, S18 and S26). Among them, 20% Cu/MgSiO 3 exhibits the highest electrocatalytic performance toward CH 4 production (Fig. 2c), reaching a CH 4 Faradaic efficiency (FE) of 72.05% at a current density of 600 mA/cm 2 , which enables a CH 4 production rate of 0.8 mL/min (Supplementary Table S3). It is worth noting that the FE CH4 is 64.59% at 800 mA/cm 2 , corresponding to a maximum CH 4 partial current density of 516.7 mA/cm 2 . Such a result is obtained because 20% Cu/MgSiO 3 exhibits superior reaction kinetics ( Supplementary Fig. S29) and electron-transfer  rates ( Supplementary Figs S30 and S31) for electrochemical CO 2 conversion. As such, it can be confirmed that the MgSiO 3 is the main active component of augite in initiating the CH 4 production after loading with Cu species. Such a result suggests that the precise tuning of the composition of the lunar soil (i.e. increasing the ratio of the MgSiO 3 ) can result in enhanced production of hydrocarbon fuels during the electrocatalytic CO 2 conversion. What is more, the CH 4 FE obtained through electrocatalytic CO 2 conversion using our prepared samples is comparable with the performance of commonly applied electrocatalysts such as Cu-based metal-organic frameworks [22,23], Cu/CeO x [24] and Cu/Al 2 O 3 [25] (see Fig. 2d and Supplementary  Table S4), conclusively demonstrating that the ISRU of lunar soil for catalyst synthesis is highly feasible on the Moon. In addition, we also investigate oxygen evolution, which is another critical survival necessity for humans, in the system. Remarkably, superior oxygen production can be also obtained from the system, reaching a production rate of 2.3 mL/min with a current density of 600 mA/cm 2 ( Supplementary Fig. S32). Based on these results, it can be confirmed that MgSiO 3 is the main active CH 4 production component in lunar soil, which can be loaded with Cu species to achieve outstanding electrocatalytic CO 2 conversion performance.

Electrocatalytic CO 2 conversion mechanism
To have a full image of the electrocatalytic mechanism and performance of lunar soil, we perform in situ Raman spectroscopy characterizations (see Supplementary Fig. S33 for experimental set-up) for discerning the intermediates formed during the electrocatalytic CO 2 conversion using Cu/MgSiO 3 . As shown in Fig. 3a, under the open-circuit potential, two main peaks at 684 and 1023 cm −1 assigned to the silicates can be observed [26]. With the increase in the current density, these two peaks gradually decrease and a peak at 1071 cm −1 attributed to the adsorbed carbonate appears. In addition, a strong peak at 530 cm −1 attributed to the Cu-O-Si can be also found after supplying the current density to the system [27], suggesting the strong interaction between Cu species and MgSiO 3 during the reaction. Furthermore, a peak at 1832 cm −1 attributed to the * CO on the Cu appears, implying the reduction of Cu species into Cu nanoparticles during the reaction [28]. This peak gradually decreases with the increasing current density and a peak at 2066 cm −1 attributed to the adsorbed CO on top of terrace-like sites of Cu emerges, which is beneficial for the reduction reaction for CH 4 production [27]. Two minor peaks at 285 and 388 cm −1 assigned to the Cu-CO can be also observed [29], further confirming the roles of Cu in activating CO 2 and stabilizing * CO for the subsequent reduction reaction. A similar trend can be also observed by performing in situ Raman characterization for the electrocatalytic CO 2 conversion with the evolution of time (see Fig. 3b), corroborating the critical functions of Cu in such a reaction.

Unmanned and scalable fuel and oxygen production
Given that the ultimate aim of the strategy reported in this work is to build up a large-scale unmanned electrocatalytic fuel and oxygen production system, the participation of the robotic system in the electrocatalytic CO 2 conversion is highly desirable. To this end, we have developed a robotic system for electrocatalytic CO 2 conversion (Fig. 4a), which is enabled by the full accessibility of our developed process. Such a robotic system can collect and process the lunar soil (Fig. 4b), achieving the unmanned ISRU of lunar soil for catalyst production. As a following step, it can also perform the materials preparation for loading Cu on the lunar soil (Fig. 4c). Finally, the electrocatalytic system can be automatically set up by our developed robotic system, including catalyst ink preparation (Fig. 4d), electrode preparation (Fig. 4e) and flow-cell set-up (Fig. 4f). No significant differences can be found between the manned and unmanned electrocatalytic tests (Fig. 4g), implying the high feasibility of the robotic system for operating the electrocatalytic CO 2 conversion. Such an achievement again underscores that our proposed unmanned electrocatalytic CO 2 conversion strategy (Supplementary Movie 1) can be easily imitated on extraterrestrial sites. Another noteworthy perspective for such an unmanned system is the feasibility of in situ screening catalyst recipes based on lunar soil to achieve optimal performance in the future, given the varied compositions of lunar soil in different regions.

CONCLUSION
In short, we have comprehensively demonstrated the practicability of in situ lunar soil utilization for highly efficient extraterrestrial fuel and oxygen supply via electrocatalytic CO 2 conversion. After loading with Cu species, the selected silicate from lunar soil (i.e. MgSiO 3 ) can reach a methane production rate of 0.8 mL/min at a current density of 600 mA/cm 2 . Concurrently, oxygen can be produced with a production rate of 2.3 mL/min. With such a superior performance, this system is expected to supply sufficient fuel and oxygen during the extraterrestrial mission after scaling up.
More importantly, we have demonstrated the full accessibility of our catalyst preparation process and developed a robotic system for achieving unmanned electrocatalytic CO 2 conversion. No significant difference can be observed between the manned and unmanned systems, which further suggests the high possibility of imitating our proposed system in extraterrestrial sites and proves the feasibility of further optimizing catalyst recipes on the Moon. The study in this work can not only be regarded as a major step forward in ISRU of lunar soil, but also shines light on the fuel and oxygen production system for the sustainable supply of critical survival necessities on extraterrestrial sites. The findings on catalyst design in this work also provide important insights for the development of highly efficient electrocatalytic materials toward CO 2 conversion on Earth.

Electrochemical measurements
All electrochemical measurements were performed in a three-channel flow cell in aqueous 1 M KOH as shown in Supplementary Fig. S1. The electrochemical measurements were controlled by an electrochemical workstation (CHI 660e) equipped with a current amplifier (CHI 680c). To prepare the working electrode, 2.5 mg of catalyst was dispersed by sonication in the mixture of isopropanol (970 μL) and Nafion ionomer solution (5%, Sigma-Aldrich) (30 μL) for 30 min. Subsequently, 180 μL of the catalyst ink was dropped onto a gas diffusion layer (YLS 30T, Fuel Cell Store) as the cathode electrode (1 × 1 cm 2 ). A saturated Ag/AgCl electrode was used as the reference electrode. An anion exchange membrane (FAB-PK-130, Fuel Cell Store) was sandwiched between the anode (nickel foam) and cathode. 1 M KOH was circulated in the anolyte and catholyte chambers at a flow rate of 10 mL/min during CO 2 electrolysis. The high-purity CO 2 or argon (Ar) (Linde, 99.999%) gas flowed through the porous gas diffusion layer (GDL) to the catalyst layer in contact with the bulk electrolyte, forming the gas-electrolyte-catalyst triple-phase interface. The gas flow rate was set to 50 sccm via a mass flow controller (D08-1F, Sevenstar). An LSV experiment of 500% Cu/lunar soil was performed in CO 2 and Ar environments at a scan rate of 50 mV/s with 85% iR compensation. The cathodic gaseous products were analysed using gas chromatography (GC, 7890A and 7890B, Agilent) and the cathodic liquid products were analysed by 1 H nuclear magnetic resonance spectroscopy (Bruker AVANCE AVIII 400). The production rate of anodic product (i.e. O 2 ) was detected using an electronic soap film flowmeter (JCL-2010, Qingdao Juchuang Environmental Protection Group). All potentials were converted to the reversible hydrogen electrode in scale according to the following equation: The FE of the CO 2 electrolysis products was calculated using the following equation: where Q and Q total represent the charges transferring into the corresponding product and the total charge passed through the cathode during the electrolysis, respectively; F represents the Faraday constant (96485 C/mol); n is the mole amount of the corresponding product; and n e is the number of electrons transferred.

In situ Raman spectroscopy
In situ Raman spectra were recorded on a WITec Alpha 300R Raman spectrometer with a 633-nm laser as the excitation light source. The experiments were performed in a self-made flow cell as shown in Supplementary Fig. S33. CO 2 gas was introduced to the back of the GDL at a flow rate of 50 sccm controlled by a rotameter. The CO 2 electrolysis was controlled by chronopotentiometry. The current was increased gradually while in situ Raman spectra were recorded.