Electrochemical conversion of CO 2 using LMs

Synthesis of different weight fractions of metallic cerium (0.5, 1.0 and 3.0 wt%) into liquid galinstan was performed using a mechanical alloying approach (see Methods). Cerium containing LM was created, since cerium oxides are known to reduce CO 2 to CO via the Ce3+–Ce4+ cycle4,5. Cerium’s solubility in liquid gallium and its alloys is expected to be between 0.1 and 0.5 wt%, while Ce 2 O 3 is expected to dominate the LM surface, as a 2D layer, under ambient atmospheric conditions due to the high reactivity of cerium when compared to the constituents of galinstan, and the known oxidation mechanism of metallic cerium that leads to the initial formation Ce 2 O 3 at the metal–air interface15,21,22.

The electrochemical reduction of CO 2 using LMCe catalysts and pure LM (control) was conducted in a dimethylformamide (DMF)-based electrolyte, due to the high solubility of CO 2 in the solvent6. Linear sweep voltammetry (LSV) was carried out utilising either CO 2 or N 2 (control) saturated electrolytes (Fig. 1a).

Fig. 1 Characteristics of CO 2 reduction by the LMCe electrocatalyst. a linear sweep voltammogram (LSV) of galinstan with different concentrations of Ce measured in 0.1 M tetrabutylammonium hexafluorophosphate (TBAPF 6 ) and 2 M H 2 O in dimethylformamide (DMF) in N 2 and CO 2 saturated electrolyte. Inset shows a magnified view. b Chrono-amperometry results of liquid galinstan and solid gallium containing 3% Ce measured at −3 V vs. Ag/Ag+ in CO 2 saturated electrolyte. Inset shows chrono-amperometry result of a liquid galinstan alloy containing 3 wt% Ce (LMCe3%) at −2 V vs. Ag/Ag+ in CO 2 saturated electrolyte. Please note the Faradaic efficiency for the various products at −2 and −3 V vs. Ag/Ag+ which is shown in c. c Faradaic efficiencies of LMCe3% for the production of CO, H 2 , and solid carbonaceous material at corresponding potentials measured in CO 2 saturated electrolytes. The Faradaic efficiency for the carbonaceous material was determined via a deduction process. Please refer to the Methods section for further details Full size image

The cerium-containing alloys were able to support significant current densities and featured very low onset potentials (up to −310 mV vs. CO 2 /C) in the presence of CO 2 . The control experiment conducted in N 2 atmosphere yielded negligible current densities (Fig. 1a). Consecutive cycles of saturating the electrolyte with N 2 and CO 2 were conducted (Supplementary Fig. 1) and a significant current density was only observed when the electrochemical tests were conducted in CO 2 saturated electrolytes, demonstrating that the observed electrochemical processes are the result of the presence of the dissolved CO 2 . The experiments were found to be repeatable and close to identical current densities are observed in multiple subsequent cycles. The low current densities for N 2 saturated electrolytes also indicate that the hydrogen evolution reaction, which is a competitive process to CO 2 reduction, exhibits a relatively high overpotential on the LMCe electrode.

A further control experiment entailed conducting a typical CO 2 reduction reaction in a different solvent, while also running a N 2 saturated and CO 2 free electrolyte-based control experiment (Supplementary Fig. 2). When acetonitrile was used, similar behaviour was observed as in the DMF-based experiment, indicating that the solvents are not likely taking part in the reaction.

In agreement with previous work, pristine galinstan was found to be a rather catalytically inactive electrode16. However, the activity of the alloy increased upon the addition of elemental cerium to the metallic melt. The observed current density correlates with increasing cerium content, and the onset potential for the most active LMCe3% electrode was found to be effectively −310 mV vs. CO 2 /C (Fig. 1a—inset and Supplementary Fig. 3). During the experiment, gas evolution was observed at higher applied potentials, indicating gaseous products. The inactivity of the cerium-free LM electrode in CO 2 saturated electrolytes highlights the importance of cerium for the catalytic process.

Characterisation of carbonaceous materials

When CO 2 was present in the electrolyte and a cerium-containing alloy was used, carbonaceous material could be produced which formed black floating debris in the electrolyte after prolonged electrolysis (Supplementary Fig. 4). The product was collected and purified for further analysis. Transmission electron microscopy (TEM, Fig. 2b and Supplementary Fig. 5) and scanning electron microscopy (SEM, Supplementary Fig. 6) analysis of these particulates revealed the appearance of small agglomerated flat sheets. High-resolution TEM (HRTEM) imaging and selected area electron diffraction (SAED) studies revealed an amorphous structure, indicating interatomic distances (0.34 nm) consistent with amorphous carbon (Fig. 2b)23. Atomic force microscopy (AFM, Supplementary Fig. 7) analysis of the produced carbonaceous nano-flakes found a typical thickness of 3 nm. Fourier transform infra-red (FTIR) spectroscopy (Fig. 2a and Supplementary Fig. 8) in combination with Raman spectroscopy (Fig. 2a) confirmed that the solid product is indeed predominantly composed of carbonaceous materials19. Similarly, the Raman spectrum reveals intense, broad features at 1332 and 1601 cm−1, which are characteristic of amorphous carbon sheets23. Furthermore, energy-dispersive X-ray (EDX) analysis revealed that the material is predominantly composed of carbon and oxygen, with insignificant quantities of the metal species present (Fig. 2b—bottom inset and Supplementary Fig. 9).

Fig. 2 Characterisation of carbonaceous materials. a Fourier transform infra-red (FTIR) spectrum of the isolated carbonaceous materials, featuring intense FTIR absorption lines at 832 and 1475 cm−1 which are characteristic of C=C bonds. b High-resolution transmission electron microscopy (HRTEM) image of isolated layered carbonaceous materials (scale bar, 5 nm), with selected area electron diffraction (SAED) image (inset, scale bar 5 1/nm) and elemental composition determined by EDS (inset). c Raman spectroscopic measurement of carbonaceous materials on a liquid galinstan alloy containing 3 wt% Ce (LMCe3%) surface after electrochemical reduction in CO 2 and N 2 saturated electrolytes measured at 0 and −1.5 V vs. Ag/Ag+. Inset: magnified view of the Raman peaks at 409 and 465 cm−1. d Operando Raman spectra of the LMCe3% surface during electrocatalysis at indicated potentials Full size image

Elemental analysis of the products using X-ray photoelectron spectroscopy (XPS) was in agreement with the EDX results with the produced carbonaceous materials being mainly composed of carbon (84.49 at.%) while containing 14.99 at.% of oxygen (Supplementary Table 1). Small quantities of Sn are present and are likely associated with residual LM that was not successfully removed during the workup procedure. Detailed analysis of the C1s region of the XPS spectrum (Supplementary Fig. 10) revealed that the carbonaceous materials predominantly contains C–C and C=C bonds, while containing a significant fraction of covalently bound oxygen. FTIR analysis (Fig. 2a) also revealed the presence of C–H and C–O–H moieties. As such the obtained product is best described as amorphous carbonaceous nanosheets with a typical thickness of 3 nm.

Overall, the absence of a current response in the N 2 control experiment, together with the isolated carbonaceous products indicate that the electrochemical process on the LMCe3% electrode was capable of converting gaseous CO 2 into solid amorphous carbonaceous nanosheets at a low onset potential of only −310 mV vs. CO 2 /C, which is remarkable when considering the stability of the CO 2 molecule. The control experiments (Supplementary Figs. 1–3), combined with a careful experimental design, allowed excluding the catalyst material, as well as the electrolyte as potential sources of the carbonaceous materials. The developed process occurred at room temperature, while previously developed electrocatalysts were only found to convert CO 2 into solid products, such as carbon nanotubes, at very high temperatures (above 600 °C)24,25. A comparison of the onset potential and over potential for various CO 2 reduction reactions in non-aqueous solutions (leading to gaseous and liquid products) is presented in Supplementary Table 2.

Characterisation of the catalytic process

A detailed analysis of the electrochemical processes that occurred at the LMCe3% electrode was conducted and the Faradaic efficiencies for different products at various potentials were determined (Fig. 1c). Gas chromatography was employed to analyse the gaseous products. The Faradaic efficiency for the carbonaceous product was determined via a deduction process due to the challenges associated with the gravimetric analysis of small quantities of products that are generated during electrolysis (see discussion in the Methods section). As such, the determined efficiency is an upper estimate. However, electrochemical measurements in nitrogen saturated electrolytes (Fig. 1a) suggest that any parasitic processes (e.g. surface oxide reduction), and side reactions that may occur, are limited in magnitude and would have a small effect on the estimated Faradaic efficiency. The measurements revealed that solid carbonaceous materials were the dominant product at low potentials (faradaic efficiencies ~75% over the potential range −1.8 to −2.0 V vs. Ag/Ag+), while carbon monoxide becomes dominant at higher negative potentials.

The production of CO at more negative potentials likely occurs due to a separate process. The low potential region of the Tafel plot (Supplementary Fig. 11) reveals a distinct slow-moving process that occurs for reduction of CO 2 to carbonaceous materials. Moderate quantities of hydrogen were produced as a side product. Nuclear magnetic resonance (NMR) spectroscopy was conducted on the electrolyte and revealed that small organic molecules were not produced (Supplementary Fig. 12). The presence of two parallel catalytic processes, which produce carbonaceous carbon in one instance and gaseous products in the second instance, renders the determination of an over-potential for the exclusive production of carbonaceous material producing reaction difficult. Therefore, the onset potential for the carbonaceous material producing process has been utilised herein.

The developed LMCe catalyst was observed to be stable during continued electrolysis experiments in either the higher potential region, where gas products are dominant (Fig. 1b), or the low potential region, where solid materials were produced (Fig.1b—inset). For comparison, an alloy containing 97% gallium and 3% cerium was synthesised, which remained solid at room temperature. Although, the solid electrode initially exhibited similar catalytic activity during CO 2 electrolysis (Fig. 1b), the performance rapidly declined due to coking, highlighting that the liquid state of the electrode was crucial for continuous operation. The extraordinary stability of the liquid electrode may be associated with the lack of van der Waals adhesion on the liquid surface12,14. This observation leads to the conclusion that the processes, which result in carbonaceous products associated with deactivation via coking on solid catalysts can be exploited for continuously converting CO 2 into solid products on LM electrodes.

Operando Raman spectroscopy was conducted to elucidate the operating mechanism of the catalyst. Figure 2d shows the Raman spectrum of the LMCe surface in the CO 2 saturated electrolyte without any applied potential. Here the peak at 409 cm−1 is of particular significance since, it is characteristic for Ce 2 O 3 26, confirming that the surface of the LM contains significant amounts of Ce3+ ions. This is in excellent agreement with XPS measurements of the LMCe surface (Supplementary Fig. 13). The observation of Ce 2 O 3 at the LM/air interface is consistent with oxidation studies on metallic cerium, which initially oxidised to form Ce 2 O 3 , which then partially converts to CeO 2 after prolonged exposure to air (days)27.

Upon the application of reductive potential, additional peaks arise at 465, 1332 and 1601 cm−1 attributed to the formation of CeO 2 and amorphous carbon species, respectively23,28. When a N 2 saturated electrolyte was utilised, no new Raman peak emerged, confirming that the spectral changes were due to the CO 2 reduction reaction (see also Supplementary Discussions for further details).

The presence of solid carbon species that arose due to an electrochemical reduction process and the emergence of CeO 2 , which resulted from the oxidation of Ce 2 O 3 to CeO 2 , revealed critical insights into the catalytic mechanism.

The surface of the LMCe catalyst was initially dominated by Ce 2 O 3 at room temperature. When a sufficiently negative electrochemical potential was applied, a portion of the surface Ce 2 O 3 reduced to elemental Ce. Electrochemical studies on the LM electrode revealed that the onset of the Ce3+ reduction to Ce0 occurs at –1.2 V vs. Ag/Ag+ (Supplementary Fig. 14), which coincides with the onset potential of the electrocatalytic reaction at the LMCe catalyst. During electrocatalysis the zero-valent cerium atoms, which were produced, are capable of reacting with CO 2 in a four-electron process, leading to the formation of CeO 2 and carbonaceous products.

Due to the applied reductive potential, the CeO 2 was continuously reduced back to elemental Ce which drove the catalytic process. This correlates with the principle of the incipient hydrous oxide adatom mediator (IHOAM) model of electrocatalysis29. The process can be described by the chemical reactions 1–5. Reactions 1–4 are proposed to occur at the working electrode (Fig. 3), with reaction 5 describing the oxygen evolution reaction at the counter electrode.

$$2\,{\mathrm{Ce}}_{({\mathrm{Galinstan}})} + 1^{1/2}{\mathrm{O}}_{2({\mathrm{air}})} \to 2\,{\mathrm{Ce}}_2{\mathrm{O}}_3$$ (1)

$$2\,{\mathrm{Ce}}_2{\mathrm{O}}_3 + 3\,{\mathrm{H}}_2{\mathrm{O}} + 6\,{\mathrm{e}}^ - \to 2\,{\mathrm{Ce}}^{(0)} + 6\,{\mathrm{OH}}^ -$$ (2)

$${\mathrm{Ce}}^{(0)} + {\mathrm{CO}}_2 \to {\mathrm{CeO}}_2 + {\mathrm{C}}$$ (3)

$${\mathrm{CeO}}_2 + 2\,{\mathrm{H}}_2{\mathrm{O}} + 4\,{\mathrm{e}}^ - \to {\mathrm{Ce}} + 4\,{\mathrm{OH}}^ -$$ (4)

$$4\,{\mathrm{OH}}^ - \to {\mathrm{O}}_2 + 2\,{\mathrm{H}}_2{\mathrm{O}} + 4\,{\mathrm{e}}^ -$$ (5)

Fig. 3 Schematic of the catalytic process. The proposed process is based on operando Raman measurements, it includes pre-catalytic reactions and the catalytic cycle for the CO 2 reduction to amorphous carbon sheets. The picture is created by authors Full size image

Cerium has a solubility limit between 0.1 and 0.5 wt% in liquid gallium22. Considering that 3 wt% Ce in LM leads to the best performing catalyst despite exceeding the solubility limit, further analysis was required. HRTEM analysis of LMCe droplets (Fig. 4) revealed the formation of a 2D cerium oxide layer with a thickness of ~1.7 nm on the LM surface. Furthermore, it is also seen that the excess Ce is present in the form of metallic nanoparticles, which are embedded within the LM. Analysis of the interatomic spacings in HRTEM images utilising fast Fourier transform (FFT) revealed that the crystalline solid inside the LM is elemental Ce30. The formation of Ce nanoparticles is notable due to the pyrophoric nature of the element. Their emergence is enabled because of the oxygen free environment within the LM. The presence of these solid inclusions facilitates the catalytic process by serving as a Ce source near the interface (Fig. 4).

Fig. 4 Characteristics of cerium oxide nanoparticles. a TEM image of a LMCe3% nanodroplet featuring encapsulated solid elemental cerium nanoparticles and an atomically thin layer of cerium oxide (scale bar, 10 nm). b FFT image of the crystalline section (scale bar 5 1/nm). c HRTEM image, the lattice parameters were indexed to elemental cerium (scale bar 2 nm)30 Full size image

Electrode fabrication from carbonaceous materials

The isolated solid carbonaceous materials featured a highly porous superstructure as a result of an agglomerated plate-like morphology (Fig. 2b, Supplementary Figs. 5 and 6). Consequently, the collected carbonaceous product was fabricated into a two-electrode capacitor to show an example for the application of the by-products. The maximum capacitance of 250 F g−1 was recorded at 10 mV s−1, which is comparable to some of the best performing carbon-based supercapacitors in aqueous electrolytes31. These observations place the developed synthesis route among the most competitive techniques for producing high performance electrode materials using low-cost precursors under ambient condition (Fig. 5).

Fig. 5 Supercapacitor behaviour of carbonaceous materials collected from CO 2 conversion. Cyclic voltammograms of a double layer capacitor fabricated from synthesised carbonaceous materials in H 2 SO 4 (1 M) electrolyte. Calculated specific capacitance of the capacitor at various scan rates (inset) Full size image

In the case of a future large-scale adoption of the developed process in the form of a negative emission technology, a portion of the produced carbonaceous materials may find application as electrode materials for energy storage applications; while any produced CO may be utilised as a feedstock for further industrial processes.