Mechanical energy-induced CO2 conversion using liquid metals

doi:10.21203/rs.3.rs-112257/v1

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Mechanical energy-induced CO₂ conversion using liquid metals

https://doi.org/10.21203/rs.3.rs-112257/v1

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We report a green carbon capture and conversion technology offering scalability and economic viability for mitigating CO₂ emissions. The technology uses suspensions of gallium liquid metal to reduce CO₂ into carbonaceous solid products and O₂ at near room temperature. The nonpolar nature of the liquid gallium interface allows the solid products to instantaneously exfoliate, hence keeping active sites accessible. The solid co-contributor of silver-gallium rods ensures a cyclic sustainable process. The overall process relies on mechanical energy as the input, which drives nano dimensional triboelectrochemical reactions. By altering the secondary solvent and changing the reactor height, the dissolution and conversion efficiency can be tuned. The optimum reactor height is only 27 cm, when gallium/silver fluoride mix at 7:1 mass ratio is employed as the reaction material. At CO₂ input of ~8 sccm, 92% efficiency was obtained at the record low input energy of 228.5 kW∙h for the capture and conversion of a tonne of CO₂. The potential impact of this green technology is remarkable, likely benefiting a variety of industries and offering an economical solution for CO₂ capture and conversion.

Environmental Chemistry

Nanoscience

Green Carbon Capture

Carbon Conversion Technology

Gallium Liquid Metal

Instantaneous Exfoliation

Nano Dimensional Triboelectrochemical Reactions

Innovative technologies are urgently demanded for capturing and converting CO₂ into value-added species, at low input energy, to mitigate the negative effects of this greenhouse gas and support a sustainable carbon cycle^{1, 2}. Activating CO₂ into CO₂^•‒ radicals or other intermediates is a crucial step for CO₂ conversion, while the inertness of CO₂ molecules imposes a significant challenge³. To this end, external energy is required, and catalytic systems are commonly engaged to lower the energy barrier for CO₂ converting. The choices of both the applied energy and functional materials, and how they are pooled together for CO₂ conversion govern the efficiency balances. To date, light and electricity have been extensively used as the triggering energies together with many well-known catalytic systems^4-8. However, in general, the existing electro- or photo- empowered CO₂ reduction systems suffer from sluggish reaction rates and high energy consumption. Additionally, there has also been no magical definitive catalyst, that could synergistically operate with these energy types to carry out the conversion at low costs and high efficiencies.

Mechanical energy is easy to generate and often a waste source of energy. Except for organic synthesis and polymerisation^{9, 10}, mechanical energy has received little attention in catalytic systems and has not yet been utilised for CO₂ reduction. In conductive particles suspensions, the introduction of mechanical stimuli can both increase the temperature¹¹ and generate triboelectrification, as a result of the frictional contact and modulation of gaseous content solubility, respectively^12-14.

Solid metallic catalysts have exhibited excellent performance for electrocatalytic or photocatalytic CO₂ reduction^15-17. However, the active sites of solid metallic catalysts can deteriorate under intense mechanical stimuli and/or can be deactivated when carbonaceous materials adhere onto the catalytic sites during CO₂ reduction. In principle, the use of liquid metals, and specific solid structures, can solve these problems. Gallium (Ga)-based liquid metals have shown intriguing properties for catalysis, including tunability by the incorporation of other elements, and remarkable resistance to coking^18-21 and also excellent mechanical tolerance. Additionally, it is known that Ga (0) can be oxidized to Ga (I) in the presence of organic materials²². In this work, we explore whether the same effect can also apply to CO₂ and whether we can find a process to reduce Ga(I) back to Ga (0) in a closed cycle stimulated by mechanical agitation. We envision that by employing liquid metal mixes of Ga and a compound of Ag, a closed cyclic catalytic system can be developed. This system can then covert CO₂ into value-added species in the reversible Ga-Ga⁺ cycle, which will be fully explored.

Assessment of CO₂ conversion for various scenarios

We exploited a suspension of Ga and Ag (I) salt mixes as the precursors of the co-catalysts and ultrasound was initially employed to stimulate the CO₂ reduction. Dimethylformamide (DMF), with good stability during mechanical agitation and high CO₂ solubility of 0.14 M at 40 °C (to assure the liquid state of Ga), was chosen as the solvent²³. We observed that during the reaction, the CO₂ molecules near the interface of the suspended particles were reduced to form carbonaceous sheets in a process that will be detailed later.

The best outcomes were obtained when Ga and AgF were mixed in a DMF solution which also contained 0.10 M HCl to remove the native oxide on the surface of Ga. Ga and AgF were sonicated together (Fig. 1a - using a probe sonicator for 30 min) to generate sub-micron Ga droplets of 230 nm median diameters and Ag_0.72Ga_0.28 rods of micron/sub-micron lengths and median diameters of 160 nm (Fig. 1b and Supplementary Fig. 1). The characterisation outcomes revealing the elemental composition will be presented in a later section.

In the reactor, CO₂ was bubbled into and dissolved in DMF through a diffuser (Fig. 1c). The dissolved CO₂ is reduced to solid carbonaceous materials at the interface of the Ga droplets. The mechanically enforced CO₂ conversion can be scaled up using a variety of mechanical sources that produce frictional contact. To examine this prospect, CO₂ conversion by an overhead mixer was also performed and validated (Fig. 1d).

Due to the ultra-smooth nature of the liquid metal droplets, the produced carbonaceous materials on the surface are in the form of sheets^{18, 24}. These low dimensional sheets, on the non-polarized liquid metal surface, are exfoliated during mechanical stimulation (Fig. 1e)^{18, 24}. Most importantly, the carbon sheets migrate to the top of the reactor and can be isolated due to the density difference with reference to that of metallic components (Fig. 1c,d).

The qualitative and quantitative analyses of the production of carbon, when the Ga/AgF (7.0 to 1.0 mass ratio) suspension in DMF is utilised in a 20 mL reactor, are presented in Fig. 1f and Supplementary Fig. 2. The 7.0 to 1.0 mass ratio and the reaction temperature were chosen according to previously optimised data for C-C bond formation reactions²². Additionally, the performance of the system formed by direct alloying of Ga with silver (50 to 1.0 or 20 to 1.0 mass ratio of Ga/Ag, Supplementary Fig. 3e,f) and different silver salts (Fig. 1g-k), including AgCl, AgBr, AgI, AgOTf, AgNO₃ (also 7.0 to 1.0 mass ratio) were compared. The homogeneous mixture (20 μL) was drop-casted onto a glass substrate and dried for Raman analysis, with the whole drop-cast region included during the Raman spectroscopy measurement (Supplementary Fig. 3a-d). The changes in the intensity of the carbon D and G bands at 1350 and 1600 cm^-1, respectively, were employed to obtain the overall picture. Thermal gravimetric analyses (TGA) and gas chromatography (GC) were also conducted for comparative quantitative assessment of the solid carbon and gaseous products (Supplementary Table 1).

For the Ga/AgF system that exhibited the best performance, the production of carbon was observed in < 1 hour of reaction (Fig. 1f) and increased continuously over time according to TGA. The TGA showed that 4.95 mg of carbonaceous materials were produced per hour in a 20 mL reactor at a CO₂ flow rate of ~10 sccm (Supplementary Fig. 2). In comparison, the AgCl, AgBr, AgI, and AgOTf mixes also presented CO₂ conversion capability, but they were not as efficient as the AgF system (Fig. 1g-j, for brevity only Raman spectra are shown and not TGA). With no emerging D and G bands after 5 hours of reaction (Fig. 1k and Supplementary Fig. 3e,f), Ga/AgNO₃ and Ga-Ag alloys were found to be ineffective for CO₂ reduction. The reason for these different conversion activities will be discussed later.

To verify that the CO₂ reduction relies on the synergism of Ga and AgF, experiments were conducted by employing Ga and AgF separately (Supplementary Fig. 3g,h), both of which resulted in undetectable carbon production. We note that our attempts with other types of salts (e.g. KCl and NaCl) (Supplementary Fig. 3i,j) and magnetic stirring (less powerful in comparison to ultrasonication and overhead stirring) (Supplementary Fig. 3k), showed no carbon formation, indicating the crucial role of the chosen silver salt (AgF) and a threshold for mechanical energy input into the reaction process. Controlled N₂ bubbling also did not show any formation of carbonaceous products (Supplementary Fig. 3l).

We explored the minimum co-catalyst mass required in the system to maintain the efficiency of CO₂ conversion. Diluting the catalyst by 10 times offered nearly the same conversion efficiency, still achieving an equivalent production of 4.75 mg of carbonaceous materials per hour at ~10 sccm CO₂ bubbling rate (Fig. 1l and Supplementary Fig. 2), whereas the output was dramatically reduced for dilutions of 50 or 100 times (Supplementary Fig. 3m,n. TGA profiles are not shown for brevity).

The amount of CO₂ dissolved in solution also significantly influenced the efficiency of CO₂ conversion. Ethanolamine (ETA) is a suitable choice for increasing this amount since CO₂ solubility is 5.6 M in pure ETA²⁵ in comparison to 0.14 M in DMF at 40 °C. With the addition of 10 vol% ETA in DMF (referred to as DMF+ETA hereafter), CO₂ was continuously reduced with a higher efficiency, producing 7.95 mg of carbonaceous materials per hour in the same reactor at ~10 sccm CO₂ bubbling rate (Fig. 1m and Supplementary Fig. 2). Interestingly, 22.2 cm³ CO was also produced in one hour (Supplementary Fig. 4). In contrast, when DMSO or H₂O were used, the efficiency was very low and carbon products could not be quantified by TGA, owing to their limited CO₂ solubility (Supplementary Fig. 3o,p).

Carbonaceous materials characterisation

Carbonaceous materials produced from CO₂ were isolated for further characterisation. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) analysis (Fig. 2a and Supplementary Fig. 5a) of the isolated carbonaceous materials reveal that the solid product consists of carbon and oxygen before any secondary washing, with trace quantities of the metallic species that can be easily removed. Fourier transform infrared (FTIR) spectroscopy (Supplementary Fig. 5b,c) further confirms that the carbonaceous materials are primarily comprised of C=C and C-O bonds¹⁸. Based on X-ray photoelectron spectroscopy (XPS) analysis, the C1s region of the carbonaceous materials shows characteristic peaks of sp² carbon and C-O bonding, at 284.2 and 286.1 eV, respectively (Supplementary Fig. 5d)²⁶. The presence of C-O bonds is validated from the O1s XPS region of the sample (Supplementary Fig. 5e). Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) images demonstrate that part of the carbonaceous material is akin to that of slightly crystalline graphene oxide (Fig. 2b), and a certain proportion of the product exists in the amorphous state (Supplementary Fig. 5f).

Efficiency and energy requirement for CO₂ conversion

The CO₂ conversion efficiencies under different configurations were determined using TGA and GC measurements as summarised in Supplementary Table 1 (see Supplementary Information). The conversion efficiency is defined as (captured and reduced CO₂/total input CO₂)´100%, which are all obtained from the optimum mix of Ga/AgF (to be explained further in the next section).

In the small-volume reactor of 20 mL and height of 4.5 cm, which was used as the characterisation unit in the previous sections, the conversion efficiencies are 1.5% and 6.2%, for DMF and DMF+ETA cases, respectively (Fig. 2c). To demonstrate the scalability, we increased the dimensions of the reactor (Fig. 2d). When the height of the reactor was increased to 40 cm for only DMF solvent (volume of 500 mL), 27% of the input CO₂ at the flow rate of ~8.6 sccm could be continuously captured and converted (Fig. 2c). Thus, when the total height of the reactor would be 148 cm, the CO₂ conversion could reach the full capacity.

The height of the reactor for near-full conversion could be significantly decreased when DMF+ETA was used as the solvent as this combination could significantly increase the CO₂ solubility. The conversion efficiency reached the unprecedented value of 92% (at the flow of ~8.0 sccm CO₂) in this case for a reactor as small as 27 cm in height and 330 mL in volume (Fig. 2c and see Supplementary Fig. 6 for the photo of the set-up). The amount of produced O₂ and carbonaceous materials (highly oxidised carbon), under different conditions, are presented in Supplementary Fig. 7 and Supplementary Table 1.

According to these measurements, the total energies required for converting 1.0 tonne of CO₂, for DMF and DMF+ETA cases, are obtained as 699.5 kW∙h and 228.5 kW∙h, respectively (Supplementary Information). An overall estimation based on the current price of electricity suggests that the operational cost of CO₂ capture and conversion using DMF+ETA is lower than any other state-of-the-art technologies^27-31.

Analysis of the suspensions

To elucidate how the functional materials are formed during the probe sonication stage, the reactions between Ga and silver salts were investigated by characterising the sonication products. Sonicating Ga with AgF (as an exemplar) forms an intermetallic phase Ag_0.72Ga_0.28 (Fig. 3a) and GaF₃. The existence of metallic Ag (as Ag_0.72Ga_0.28) is confirmed by the Ag3d XPS peaks at 367.8 and 373.8 eV (Fig. 3b)³². The metallic fluorides can be verified by the F1s XPS peak at 684.3 eV (Fig. 3c)³³.

The compositions and morphologies of the materials were investigated and correlated with the CO₂ reduction performance. As illustrated in Fig. 3a, XRD patterns of Ga mixed with silver salts, which lead to CO₂ conversion (i.e., AgF, AgCl, AgBr, AgI and AgOTf), show the presence of Ag_0.72Ga_0.28 crystalline peaks. The control experiments show that Ag₂Ga particles (generated from the sonication of Ga-Ag alloy, Supplementary Fig. 8) and Ag particle inclusions (using Ga/AgNO₃ as the precursors, Fig. 3a,d) are inactive materials for CO₂ conversion. These results suggest that the formation of Ag_0.72Ga_0.28 is a prerequisite for CO₂ reduction. Interestingly, the Ag_0.72Ga_0.28 crystals, generated from different silver salts, show distinct morphologies (Fig. 3e-i and Supplementary Fig. 9) of particles (Fig. 3e-g) or rods (Fig. 3i for AgF) or a combination of both (Fig. 3h for AgCl) together with the Ga spheres. The presence of only rod-shaped morphology (Fig. 3i for AgF) is found to associate with an enhanced CO₂ catalytic capability. The Ga/AgF system, which generated the highest efficiency for CO₂ conversion, showed the rod-shaped Ag_0.72Ga_0.28, while Ag_0.72Ga_0.28 with non-rod morphology from other silver salts (or limited rod morphology for AgCl) exhibited limited catalytic abilities. The high-resolution TEM images, SAED pattern (Fig. 3j,k) and the TEM-based EDS mapping (Fig. 3l-n) further confirm the existence of the Ag_0.72Ga_0.28 rods and their growth direction along the [201] plane (Fig. 3k). As shown in Fig. 3o-q, the native oxide layer on the surface of the Ga droplets can be observed when dried for analysis. Furthermore, there were no obvious changes to the Ag_0.72Ga_0.28 structures after 5 hours of reaction according to both XRD (Supplementary Fig. 9x) and SEM (Supplementary Fig. 10), indicating that the Ag_0.72Ga_0.28 rods were resilient towards mechanical agitation.

The concentration of gallium and silver ions in solution during the reaction was measured by inductively coupled plasma mass spectrometry (ICP-MS) (Supplementary Fig. 11). The ion concentrations fluctuated without showing any increasing or decreasing trend, indicating that the catalysts are not consumed, and that the system is stoichiometrically stable.

Reaction mechanism

Based on the configuration of the catalytic process, we can consider a scenario in which the contact of the Ga/DMF interface is altered by the interfacial formation of CO₂ bubbles. CO₂ bubbles are formed as the Ga/DMF interface becomes warmer due to localised friction¹¹. As such, the interfacial solubility of CO₂ in DMF decreases. The formation of bubbles induces a significant increase in the transient, capacitive, open circuit voltage through triboelectrification between the separated Ga conductive liquid metal and the DMF dielectric as previously described^{12, 13}. The formation of a closed loop, by the Ag_0.72Ga_0.28 rods, thereafter, initiates the CO₂ conversion. The presence of the triboelectric effect can be validated by a proof-of-concept macro sized experiment, as depicted in Supplementary Fig. 12. The details of triboelectric voltage calculations are presented in Supplementary Information.

The CO₂ reduction in our system is completed through a reversible Ga-Ga⁺ cycle. Cyclic voltammetry was conducted to provide an insight into the catalytic mechanism of the Ag_0.72Ga_0.28 rods via the suggested route. The results showed that, for the working electrode containing Ga droplets and Ag_0.72Ga_0.28 rods as the co-contributors, Ga is oxidized to Ga⁺ at 0.18 V and then reduced to elemental gallium at -0.31 V (Fig. 4a)³⁴. Considering that the triboelectric process generates time-dependent voltages of several volts, the carbonaceous sheets are rapidly produced on the surface of liquid metals as we have previously demonstrated³⁵. We also note that Ga⁺ reduction was not observed when either Ga droplets (Inset in Fig. 4a) or Ga droplets with non-rod morphology Ag_0.72Ga_0.28 were used as the working electrode (Supplementary Fig. 13), showing the importance of the long rods for the cyclic reaction.

To investigate the reaction mechanism, we performed a detailed assessment of the reaction intermediates and the by-products generated during CO₂ reduction. The overall reaction process in DMF is described by chemical reaction equations (1-6). The equations are separated into ‘liquid metal components’ reactions (equations (1-4)) and ‘solid components’ reactions (equations (5,6)). For the description of the liquid metal component reactions, a series of characterisations were conducted. Nuclear magnetic resonance (NMR) analysis showed that the solvent DMF was not involved in the reaction (Supplementary Fig. 14a). The CO₂ reduction is realised via the voltage provided by the nano triboelectrochemical process on the surface of Ga liquid droplets that turn Ga into Ga⁺, while CO₂ is activated into the CO₂^•‒ radical (equation (1)). The existence of the CO₂^•‒ radical during the reaction is demonstrated by electron paramagnetic resonance (EPR), which uses 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a radical trapping agent to form DMPO-CO₂^•‒ adduct for spectroscopic analysis (Fig. 4b, see Methods)³⁶. The CO₂ to CO₂^•‒ process is followed by the generation of the intermediates CO and O^2- radicals (equation (2))³⁷. The former is further converted to carbonaceous materials on the liquid metal surface (equation (3) which is the optimum case and it can be altered according to the C to O ratio in the obtained carbonaceous solid). The equations are as follows:

The description of the ‘solid components’ reactions is as follows. According to the cyclic voltammetry results, the oxidized Ga⁺ can be reduced to elemental Ga by receiving an electron from the Ag_0.72Ga_0.28 and the Ag_0.72Ga_0.28 turns into Ag_0.72Ga_0.28⁺ (equation (5)). The catalytic cycle is closed by the electrons provided from the O^2- to O₂ (equation (4)) to reduce Ag_0.72Ga_0.28⁺ back to Ag_0.72Ga_0.28 (equation (6)), where the existence of O₂ can be confirmed through GC (Supplementary Fig. 4 and Supplementary Table 1).

The catalytic cycle of CO₂ reduction at the interface of Ga droplets is illustrated in Fig. 4c. This catalytic mechanism also aligns with the DMF+ETA case, and the extra by-products are produced due to the presence of ETA that promotes the process towards CO production according to equation (5) (Supplementary Fig. 4 and Supplementary Fig. 14b)³⁸.

Since the reaction is activated by the triboelectric potential, other forms of mechanical stimuli can also be applied, and the system can be readily scaled up. As a demonstration, we further validate our strategy by coupling an overhead stirrer to a 50 mL reactor. We found that CO₂ conversion continuously takes place in a stable manner when the stirring speed exceeds a threshold of 200 rpm (at room temperature, Supplementary Fig. 15a) and the conversion efficiency increases along with the increase of the stirring speed (equivalently, the mechanical energy input) (Supplementary Table 1 and Supplementary Fig. 15b,c).

In conclusion, we demonstrated a promising mechanical energy-induced CO₂ conversion method which operates solely with mechanical stimuli. The catalytic process was found to be mechanically enabled by co-contributors containing liquid metal Ga droplets and resilient intermetallic Ag_0.72Ga_0.28 structures. We demonstrated the crucial roles of the composition and morphology of the functional materials, as well as the importance of precursor selection for triboelectrochemical CO₂ conversion. The obtained CO₂ capture and conversion efficiency was 92% for a reactor with a height as small as 27 cm at the input CO₂ flow rate of ~8 sccm.

The prospects of further exploring the proposed system are extensive. This methodology can be extended for large-scale systems and long-term operation by employing different forms of mechanical energies. This methodology with its remarkable capabilities provides promising prospects for future practical deployment of CO₂ conversion technologies. Altogether, the novel concept provided by this new system offers a platform to fundamentally impact the future of carbon capture technologies and relevant industries.

Acknowledgments

This work was supported by the Australian Research Council (ARC) Laureate Fellowship grant (FL180100053), the ARC Center of Excellence FLEET (CE170100039) and Chevron. The authors would like to thank Dr. Yin Yao, Dr. Sean Lim of the Electron Microscope Unit within the Mark Wainwright Analytical Centre at UNSW Sydney, and Dr. Donald Thomas of the Spectroscopy Laboratory and the Nuclear Magnetic Resonance Facility for their technical assistance.

Author contributions

J.T., D.E., and K.K.-Z. initiated the concept and designed the experiments. J.T. conducted the experiments and carried out the characterisations with the help of J.T., M.M., M.B., J.S., M.A.R., J.H., D.J.L, and R.J. The following individuals contributed to the data analyses, scientific discussions and authorship of the paper: J.T., J.T., T.D., A.P.O., R.B.K., D.E., M.D.D., K.K.-Z. All authors revised the manuscript and provided helpful comments.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to K.K.-Z.

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Materials. Gallium (Ga, ingot, purity: 99.99%) was purchased from Rotometals, USA, and silver powder (purity: 99.9%) was obtained from Sigma-Aldrich. All the salts, including AgF, AgCl, AgBr, AgI, AgOTf, AgNO₃, KCl, NaCl and NaHCO₂, were obtained from Sigma-Aldrich with a purity of 99.5%. The solvents DMF (purity: 99.8%, boiling point: 153 °C), ETA (purity: ~ 98%, boiling point: 170 °C) and HCl (33 wt% in water) were acquired from Chem-Supply Pty Ltd. HNO₃ (acidimetric: ≥ 65.0%), DMPO (purity: 99.9%) and H₂O₂ (30 wt% in water) were purchased from Sigma-Aldrich. Milli-Q water was used throughout the experiments for sample preparation and reaction.

Preparation of functional materials. (1) Sliver salts as precursors: During a typical co-contributor preparation process (using AgF as an example), Ga (7.0 g) was first added into a glass vial which is pre-filled with 5.0 mL DMF solution, followed by adding HCl solution to give a final 0.10 M to remove the surface oxide layer of Ga. AgF (1.0 g) was then added to the mixture as the precursor.

(2) Ag (150 nm particle size) as precursors: For the preparation of Ag-Ga alloy, the silver powder was added to Ga (7.0 g) in concentrations of 2.0 wt% and 5.0 wt%, respectively. The mixtures were ground using a mortar and pestle inside a N₂-filled glove box to minimize oxidation of the liquid metal¹⁸. The grinding process, typically lasts 40 min, was stop until the sample showed a smooth and reflective appearance.

(3) Probe sonication procedures: The mixture from step (1) or step (2) was sonicated with a probe sonicator (VC 750, Sonics & Materials) under the protection of N₂. The sonication amplitude was set to 55%, corresponding to an ultrasonic power input of ~410 W. The sonicator was paused for 1 s after each 9 s sonication and the total sonication time was 30 min.

CO₂ conversion experiment. (1) Bath sonication as the energy source: After the preparation of functional material, CO₂ was bubbled into the reactor through a diffuser and the flow rate of CO₂ gas (10 sccm) was controlled using a gas mass flow controller (MKS, GE50A). Bath sonication (FREQ 50Hz, Unisonics) was employed as the mechanical energy source to trigger CO₂ conversion. The temperature of the reaction solution was kept around 40 °C during the 5 hours reaction.

(2) Overhead stirrer as the energy source: When CO₂ was bubbled into the reactor through a diffuser at the same rate of 10 sccm, an overhead stirrer (DLS Digital Overhead Stirrer, 120 W) was utilized as the mechanical energy source. Different rotation speeds, including 200, 300, 400, 500 and 1000 rpm, were applied for initiating the CO₂ conversion. The experiment was performed at room temperature for 24 hours.

Sample characterisations. Raman spectra were collected via a Raman spectrometer (Via Raman microscope, Renishaw) utilizing the 532 nm laser source. XPS was performed on a Thermo Scientific K-alpha X-ray spectrometer. The carbon product was studied using micro-FTIR spectroscopy, on a PerkinElmer Spectrum 100 FTIR Spectrometer which is coupled to a Spotlight 400 FTIR Imaging System with stage controller. The morphology and structure of materials were imaged by SEM (JEOL JSM-IT-500 HR). The TEM and SAED characterisations were performed on a Phillips CM200 TEM system. Both the SEM and the TEM systems are coupled with an EDS detector for elemental and compositional analysis. The crystalline phases of the samples were characterised by XRD (Philips X’Pert Pro MPD, λ = 1.54 Å, Cu-Kα radiation). The TGA for carbonaceous material quantification was performed on a Thermogravimetric Analyzer TGA Q5000 IR. ICP-MS was performed on NexION 2000 B ICP Mass Spectrometer to determine the concentration of gallium and silver ions. EPR experiments for the detection of the CO₂^•‒ radicals were conducted on a Bruker EMX X-Band ESR Spectrometer (Bohr). NMR experiments were performed to investigate the liquid species in the solution, which was performed by using Bruker Avance III 600 MHz Cryo NMR (Ernst).

Preparation and analysis of TGA samples. The carbonaceous materials for the TGA experiments were separated by centrifugation. After a specific reaction time (T, h), the homogeneous mixture from the reactor (2.0 mL) was added into a centrifuge tube followed by centrifuging at a speed of 10000 rpm for 10 min. During this process, the suspended solid materials were separated into different layers. Most of the metallic catalysts deposit at the bottom of the tube due to their high density, in comparison to the carbonaceous materials which remains suspended in the top layer. The centrifugation process was repeated three times and each time the carbon-containing top layer was collected. The collected materials (sample volume V_TGA = 2.0 mL solution during certain reaction time T (h)) were then dried (110 °C) to remove DMF from the samples before the TGA experiments. During the TGA experiments, the heating rate was set to 10 °C/min and the upper temperature limit was fixed at 800 °C.

ICP-MS samples: During the process of CO₂ reduction, 1.0 mL reaction mixture was taken as a sample every hour and then we centrifuged the sample for 15 min at 10000 rpm until all the suspended materials deposit at the bottom of the tube. Then, we took 0.20 mL clear solution from the top of the sample and heated it on the hot plate until it was completely dried. Thereafter, 0.20 mL of nitric acid was added to dissolve the residue for ICP-MS.

Cyclic voltammetry experiments: Cyclic voltammetry experiments were conducted to investigate the roles of the Ag_0.72Ga_0.28 rods during the CO₂ reduction. Ga droplets and Ag_0.72Ga_0.28 rods were obtained from the Ga and AgF precursors. Then, we painted the Ga/Ag_0.72Ga_0.28 (Ag_0.72Ga_0.28 in the shape of a rod) on fluorine doped tin oxide (FTO) and baked it until the material dried and immobilised on the FTO as the working electrode. A calomel reference electrode and a gold counter electrode were used to set up a three-electrode configuration. DMF+ETA solution containing 0.10 M HCl was utilized as the electrolyte to keep the condition consistent to the reaction situation. As a comparison, Ga droplets and Ga/Ag_0.72Ga_0.28 (Ag_0.72Ga_0.28 with non-rod morphology, using Ga/AgI) were painted on FTO as the working electrode respectively and all other parameters were kept identical.

EPR experiments: To provide mechanistic insights, EPR experiments were conducted to confirm the existence of the activated CO₂. DMPO is known as a standard CO₂^•‒ radical captor and the combination of DMPO with the CO₂^•‒ radical shows characteristic signals^36,39. During the experiments, 20 mg DMPO was added and dissolved into 5.0 mL Milli-Q water and then 1.0 mL DMPO solution was further added into the reaction system. After bubbling CO₂ into the solution in the presence of bath sonication for 30 min, 1.0 mL reaction solution was centrifuged to remove all the suspended materials for EPR measurement. As a control, 1.0 mL DMPO solution was added into the reaction system in the process of bath sonication without bubbling CO₂.

To verify that the EPR signal obtained from our reaction system was due to the formation of CO₂^•‒, an independent reaction involving CO₂^•‒ formation is further conducted. It is well established that the CO₂^•‒ radical can be generated from NaHCO₂ and H₂O₂ under the irradiation of ultraviolet light³⁶. NaHCO₂ and 30% H₂O₂ were dissolved in 5.0 mL Milli-Q water in the concentration of 100 mM and 100 μM, respectively, with the DMPO concentration being 50 mM. After irradiation with ultraviolet light for 10 min, we mixed 1.0 mL photolytic solution with 20 mL DMF solution for EPR analysis to keep the condition consistent with the conditions of our CO₂ conversion reaction. By comparison, the EPR spectra from the reaction system and photolytic solution were in good agreement, thereby validating the existence of CO₂^•‒ radical during our CO₂ conversion reaction (Fig. 4c).

Data availability

The datasets generated during the current study are available from the corresponding authors upon reasonable request.

References

Barbieriková, Z., Dvoranová, D., Brezová, V. Photoinduced transformation of glycerol in titania suspensions. (An EPR spin trapping study of radical intermediates). Catal. Today 313, 106-113 (2018).

There is NO Competing Interest.

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Mechanical energy-induced CO₂ conversion using liquid metals

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