Carbon dioxide electrochemical reduction by copper nanoparticles/ionic liquid-based catalytic inks

The development of copper nanoparticle (CuNP)-based catalysts for the electrochemical reduction of carbon dioxide (ECO₂-R) offers a promising approach to enhance its transformation into other industrially significant compounds. This study reports ECO₂-R at −1.3 V vs RHE using CuNPs and catalytic inks composed of CuNPs and ionic liquids (ILs), observing significant differences in the selectivity of each catalyst. Specifically, CuNPs alone show a preference for producing ethylene and aqueous products, such as formic acid, ethanol, and formaldehyde. In contrast, the addition of ILs to the catalytic system redirects selectivity toward gaseous products, with methane being the main product. These findings highlight the potential to optimize catalyst composition to tailor the selectivity of CO₂ conversion processes. ILs modify the catalytic environment and influence reaction pathways, enabling the selection of specific products.

1 Introduction

The atmospheric carbon dioxide (CO₂) level has drawn the attention of the scientific community in recent years (Jones et al., 2017; Häder and Barnes, 2019; Alma et al., 2020; Dusenge et al., 2020; Leisner, 2020; Lv et al., 2020; Wu et al., 2020). There is a direct relationship between increasing CO₂ concentration and global temperatures (Shukla et al., 2017). Recent measurements of the CO₂ concentration in the atmosphere show the highest increase in the last 800 years (World Meteorological Organization (WMO), 2019; Yan et al., 2019), reaching values of approximately 426 ppm (National Oceanic and Atmospheric Administration (NOAA), n.d.). This estimates that the CO₂ level has quadrupled since 1960. The excessive elevation of atmospheric CO₂ levels is a global predicament that fosters the occurrence of the greenhouse effect, thereby contributing to the phenomenon of global warming. Despite the severe damage to both the environment and human beings caused by climate change, it is predicted that fossil fuels will remain an important source of energy in the future. Additionally, human activities have released carbon sequestered by plants millions of years ago into the atmosphere. Therefore, it is crucial to develop carbon dioxide capture technologies to mitigate atmospheric CO₂ levels. Carbon dioxide capture is an effective way to reduce the concentration of this gas in the atmosphere, enabling its transformation into other industrially relevant species and, thus, closing the anthropogenic carbon cycle (Dreyse et al., 2015; Calfumán et al., 2017; Landaeta et al., 2018; 2020; Honores et al., 2019; Guzmán et al., 2020; Nguyen et al., 2020; Mardones-Herrera et al., 2022). One of the methods currently being explored for CO₂ conversion is electrochemical reduction using various catalysts (Handoko et al., 2018; Asadi et al., 2019; Jo et al., 2019; Low et al., 2019; Yue et al., 2019; Zhang et al., 2019; Zhou et al., 2019). However, due to the high formation enthalpy of CO₂ at standard temperature and pressure (298 K and ∆Hf = −393.5 kJ/mol), identifying an appropriate electrocatalyst that can operate efficiently and selectively at low overpotentials while remaining economically viable presents a significant challenge in terms of sustainability and energy efficiency.

Copper is a transition metal that uniquely transforms carbon dioxide into species with two or more carbon atoms in their structure. The product distribution is influenced by both the local pH on the electrode surface and the crystallographic orientation of the copper nanoparticles (CuNPs). It is crucial to highlight that the key intermediate in hydrocarbon formation is the CO species, as widely recognized in the literature (Rutkowska et al., 2020; Wang et al., 2020). In addition, it is necessary to distinguish between the different pathways for different types of products to be obtained. Research using density functional theory (DFT) proposes that this distinction involves obtaining products with different numbers of carbon atoms in their structure (C1 and C2) (Kortlever et al., 2015). Another point to consider is the selectivity of copper structures toward carbon dioxide. The Cu(111) crystallographic orientation is selective for methane, while Cu(100) is selective for ethylene, and Cu(110) produces oxygenated products. For the C1 pathway, the CO intermediate species is first reduced to a formyl-type species (*CHO) or *COH, which is further reduced to methane. This mechanism assumes an early breaking of the second C-O bond, but Durand et al. (2011) proposed that a late breaking of this bond may be thermodynamically more favorable than the previously mentioned pathway. Conversely, if high overpotentials are used, it is possible to induce the dimerization of the CO intermediate species, resulting in the formation of ethylene. However, it should be noted that most of the ethylene produced occurs when the current density reaches values above 10 mA cm⁻² and takes place in both the Cu(100) and Cu(111) crystallographic orientations (Montoya et al., 2013).

On the other hand, the key to the C2 pathway at low overpotentials is the dimerization of the CO intermediate species mediated by an electron transfer to form the *C₂O₂⁻ intermediate. It should be noted that this process is the rate-determining step in the carbon dioxide reduction reaction via the C2 pathway, followed by a subsequent protonation of the negatively charged dimer (Calle-Vallejo and Koper, 2013; Montoya et al., 2015). DFT studies on these processes²⁸ showed that the intermediate produced by dimerization is more stable in square orientations formed by four atoms, explaining the selectivity toward ethylene formation by the Cu(100) facet at low potentials.

The formation of other reaction products, such as oxygenated products, is not entirely clear as the faradaic efficiencies associated with these are very low, and the reaction products can only be detected by very sensitive methods, such as nuclear magnetic resonance. However, the literature acknowledges that formate production follows a separate pathway from that of methane and ethylene formation as formate cannot be further reduced to other products (Cook et al., 1989; Hori et al., 1989).

Another critical aspect of the electrochemical reduction of carbon dioxide (ECO₂-R) mechanism is the local pH on the electrode surface. Increased buffering capacity results in higher selectivity for methane (Chen et al., 2015) due to decreased local pH, consistent with the results published by Koper et al., which indicates that methane production is directly related to proton activity (Kortlever et al., 2015). Conversely, low buffering capacity, or an alkaline pH environment, favors the dimerization of the CO intermediate to form ethylene. Alternatively, nanoparticulated materials show better activity toward CO₂ reduction due to an increased active surface area. Thus, copper nanoparticles are used as catalysts for CO₂ electrochemical reduction (Li X. et al., 2022; Li et al., 2022 G.; Castro-Castillo et al., 2022; Mardones-Herrera et al., 2022; Yang et al., 2023). However, copper nanoparticles face a significant drawback as they tend to rapidly oxidize (Isaacs et al., 2018) upon exposure to atmospheric oxygen and agglomerate. This limitation poses a significant challenge to their long-term use and must be addressed before they can be effectively used in various applications.

One approach to preventing the short-term oxidation of CuNPs is using them in conductive ink formulations. Copper-based inks comprise a colloidal dispersion of conductive nanoparticles (specifically, copper nanoparticles) in a dispersant medium serving as the continuous phase, a dispersant agent that imparts stability to the suspension by preventing agglomeration, and a binder that enhances adhesion of the ink to the underlying substrate (Tam et al., 2016). Moreover, conductive inks are widely used in oxygen reduction and oxygen evolution reactions (Polani et al., 2018; Costa Bassetto et al., 2019; Venezia et al., 2019), flexible electronics (Chen et al., 2021; Tian et al., 2021), solar cells (Li et al., 2021), and fuel cell applications (Gatard et al., 2021), but they are rarely used in electrochemical carbon dioxide reduction. The incorporation of copper nanoparticles into this system is anticipated to produce a novel and long-lasting material, preserving the electrocatalytic properties of the copper nanoparticles over an extended period. Nevertheless, conductive inks have different species in their composition, especially polymers, where their presence probably interferes with the electron transfer reaction between CuNPs and CO₂. To attempt to rectify this issue, some ionic liquids (ILs) have been incorporated into the ink composition due to their capability to increase the conductivity of flexible electronic devices (Chun et al., 2013), and they have been used for oxygen reduction reaction (ORRs) with this purpose (Snyder et al., 2010), so it is possible to apply this property for the systems of this work.

Imidazolium cation-based ILs were selected based on their extensively investigated interaction with CO₂ (Anthony et al., 2002; 2005; Aki et al., 2004; Muldoon et al., 2007). The role of anions in ILs is the main factor in CO₂ absorption (Zeng et al., 2017). Anthony et al. (2005) studied the effect of different cations—imidazolium, ammonium, pyrrolidinium, and phosphonium—with the same anion and found that solubility is almost the same; on the other hand, Aki et al. (2004) and Muldoon et al. (2007) showed that CO₂ solubility in imidazolium-based ILs exhibited an increase in the following order of anions: [NO₃⁻] < [dca] < [BF₄] < [PF₆] < [OTf] < [TFSI] < [methide] (tris-(trifluoromethyl sulfonyl)methide) at 25°C. This study also demonstrates that the solubility of CO₂ occurs due to the interaction with the anion of the IL, but the mechanism by which this occurs is still unknown. Furthermore, it has been observed that ILs that feature fluorinated anions exhibit a greater affinity for CO₂ absorption than those with non-fluorinated anions. Additionally, the solubility of CO₂ within ILs is directly proportional to the number of fluorine substituents present in the anionic component (Baltus et al., 2004; Zhang et al., 2008; Jung et al., 2012; Pereiro et al., 2013). All these solubility properties enable them to be used as an electrolyte in the ECO₂-R processes (Barrosse-Antle and Compton, 2009; Rosen et al., 2011; Zhou et al., 2014; Lau et al., 2016; Feng et al., 2018; Ma et al., 2022), and they act as an electrocatalyst in those ECO₂-R processes (Quezada et al., 2014b; 2014a; Honores et al., 2017), decreasing the energy barriers of reactions by building complexes with the intermediates generated during the reaction (Maniam and Paul, 2021). In contrast, the concept of surface-confined ionic liquid layer (SCILL) provides a way to regulate catalytic activity and selectivity (Zhang and Etzold, 2021). Recently, this concept was applied in electrocatalysis, and it was shown to be a new and growing field where key applications have emerged in energy storage and conversion processes, such as hydrogen evolution (Zhang C. et al., 2023), oxygen evolution (Avid et al., 2022), and electrochemical reduction of carbon dioxide (Parada et al., 2022). ILs in SCILL systems can function in different ways: they can modify the concentration and/or local mass transport of reactants and/or intermediates at active sites due to differences in solubility and/or diffusion coefficients of these species compared to conventional solvents (Zhang C. et al., 2023). In addition, they can function as ligands on the surface by interacting directly with active sites, causing changes in the electronic structures of the surface. Additionally, ILs can selectively hinder unwanted surface sites by competing with them for adsorption, resulting in changes in product distribution. Furthermore, the steric properties of ILs can help stabilize copper nanoparticles (He and Alexandridis, 2015). Therefore, in addition to all the aforementioned characteristics, ILs are also valuable as components in catalytic inks.

The primary objective of the present study is to investigate the electrochemical reduction of carbon dioxide using CuNPs and catalytic inks composed of CuNPs and various ILs. The study aims to synthesize and characterize these materials, evaluate their electrocatalytic performance for CO₂ reduction, and compare the selectivity and efficiency of different IL formulations in directing the product distribution toward desired compounds, such as methane and ethylene. Additionally, the present work analyzes the interaction mechanisms between CuNPs, ILs, and CO₂ to understand how ILs influence the catalytic environment and reaction pathways. By achieving these objectives, the study seeks to optimize the composition of CuNP-based catalytic inks to enhance their selectivity and efficiency for industrially significant CO₂ conversion processes.

2 Materials and methods

2.1 Materials

Anhydrous ethylene glycol (EG) (99.8%), copper sulfate pentahydrate (CuSO₄∙5H₂O), polyvinylpyrrolidone (PVP), M_w∼40,000, sodium hypophosphite (NaH₂PO₂∙H₂O), and phenol were purchased from Sigma-Aldrich; 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide, 1-butyl-3-methylimidazolium tetrafluoroborate, and 1-butyl-3-methylimidazolium trifluoromethanesulfonate ILs (Table 1) were purchased from IoLiTec and dried in vacuum at 70°C for 8 h; potassium bicarbonate (KHCO₃), ethanol, methanol, dimethyl sulfoxide (DMSO), 2-propanol, and deuterium oxide (D₂O) were purchased from Merck; argon, CO₂, and nitrogen were purchased from Indura; and the Nafion^® 117 proton exchange membrane was purchased from Fuel Cell Store. Milli-Q water was obtained using a Heal Force Water Purification System. All chemicals except ILs were used without further purification and treatment.

Table 1

Table 1. ILs evaluated in this study.

2.2 Synthesis of copper nanoparticles, ink formulation, and electrode preparation

A previously reported procedure was used to prepare copper nanoparticles (Lee et al., 2008). First, 5.55 g of PVP and 19.45 mmol of sodium hypophosphite were dissolved in 15 mL of a 3:1 EG:methanol solution (SA). Then, 5.68 mmol of copper sulfate was dissolved in 5 mL of a 3:1 EG:methanol solution (SB). SB was introduced in a three-necked flask, and both solutions were saturated with argon for 30 min. SB was heated up to 90°C; then, SA was added drop by drop to the stirring system, and after approximately 10 min, the solution turned from green to henna, indicating the formation of copper nanoparticles. The reaction was quenched with chilled and nitrogen-saturated Milli-Q water. To separate the copper nanoparticles from the reaction system, the resulting suspension was centrifuged for 15 min at 10,000 rpm. Afterward, the copper nanoparticles were washed with chilled and nitrogen-saturated water and deaired ethanol. Finally, the copper nanoparticles were dried in a vacuum environment for 3 h at 110°C.

Catalysts were prepared by adding 20 mg of CuNPs to 1 g of isopropanol or 1 g of solution of PVP/IL/isopropanol, as shown in Table 2. Each catalyst was sonicated for 30 min before use. PVP was used to provide surface adhesion and avoid the agglomeration of CuNPs.

Table 2

Table 2. Nomenclature and composition of each catalyst.

The electrodes used in electrochemical measurements were prepared by adding 5 μL of suspension (CuNPs or CuNP-IL ink) three times on a 3-mm-diameter glassy carbon disk, and for ECO₂-R, 20 μL of suspension was deposited three times on a 1-cm²-surface area glassy carbon plate by spin coating at 100 rpm. For CuNP-IL inks, before use, the electrode was dried for 5 min at 60°C. On the other hand, each glassy carbon with a deposit of PVP/IL and CuNPs/PVP/IL ink is named as mentioned in Table 2.

When the catalysts are on a glassy carbon surface, the abbreviation "GC” is prefixed, for example, “GC/CuNPs.”

2.3 Characterization of CuNPs and CuNP-IL inks

Copper nanoparticle morphology was determined by field emission scanning electron microscopy (FE-SEM) using an EDS detector. The crystallographic characterization was determined using an Bruker D8 Advance X-ray Diffractometer, Cu Kα, λ = 1.5406 Å, 40 KV, and 40 mA in the range of 5 < 2Ө < 80. The identification and crystallographic analysis were performed using CRYSFIRE suite software, and the crystallite size was determined using DIFFRAC.EVA 5.2 software. FT-IR spectroscopy was performed using an IR-ATR Spectrometer Thermo IS-10. UV–visible spectroscopy was performed using a Shimadzu MultiSpec-1510 Spectrophotometer.

Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a 760-B CH Instrument with a glassy carbon disk electrode (0.071 cm²) as the working electrode. CV analyses were carried out at 50 mV/s using an Ag/AgCl 3M reference electrode, with potentials converted to reversible hydrogen electrode (RHE) for clarity. EIS measurements were carried out at open-circuit potential (OCP) in the presence of argon and CO₂, with an amplitude of 10 mV and a frequency range of 10,000–0.001 Hz. ECO₂-R was carried out in an electrochemical cell (Figure 1) with a proton exchange membrane (Nafion^® 117) at −1.3 V vs RHE for 3 h, using a 1-cm² glassy carbon plate and a multi-potentiostat (Ivium Technologies). Each electrochemical measurement was carried out in 0.1 M KHCO₃ as the supporting electrolyte, purged with argon or CO₂ for 30 min, with an Ag/AgCl 3M reference electrode and a platinum wire as the counter electrode.

Figure 1

Figure 1. Electrochemical experimental setup.

2.4 Determination of products

Except for formaldehyde, water-soluble products were determined by nuclear magnetic resonance ¹H-NMR (400 MHz, Bruker) using a method previously reported by Kuhl et al. (2012), where calibration curves were fabricated with analytes of interest in a suitable range of concentration, with internal standards of phenol (50 mM) and DMSO (10 mM) in a 0.1 M KHCO₃ electrolyte. The water peak was suppressed in every collected spectrum. All samples were prepared using 700 μL of electrolyte with ECO₂-R products and 35 μL of the internal standard of phenol and DMSO in D₂O.

Formaldehyde was determined by UV–visible spectroscopy (Shimadzu MultiSpec-1510 with deuterium and tungsten lamps) through the chromotropic acid method. A calibration curve in the range 0.5–5 ppm was prepared using the following procedure: 500 uL of solution standard, 500 μL of chromotropic acid at 1%, and 4 mL of concentrated H₂SO₄ were mixed and heated up to 80ºC–100°C. Then, 2 mL Milli-Q water was added, and 20 min later, the UV–visible measurements were performed at 575 nm.

Gas-phase products were quantified by gas chromatography (DANI Master GC) using a FID detector measuring hydrocarbons such as methane, ethane, ethylene, and propylene, and a µTCD detector was used to quantify the presence of hydrogen. For calibration curve construction, different volumes (10, 20, 30, 40, and 50 µL) of standard using a 50-µL Hamilton syringe were introduced to the GC. For methane quantification, DCG Refinery Gas Standard #1, CGA 170, mole, 5.2 L, 70 psi was used, and for hydrogen, Linde ultra-pure (99.999%) hydrogen was used as the standard. For electrolysis product quantification, 50 µL of the gas sample was measured. Calibration curves for methane and hydrogen are given in Supplementary Figure S1, S2.

3 Results and discussion

3.1 Characterization of CuNPs and CuNP–IL inks

The FE-SEM images of the (a) synthesized CuNPs; (b) CuNP–TFSI ink; (c) CuNP–OTf ink; and (d) CuNP–BF₄ ink are shown in Figure 2A. There is a granular morphology with some degree of agglomeration, which has a predominant size between 50 and 60 nm (Supplementary Figure S3A). Figures 2B,C, D display FE-SEM images for the catalytic inks, where the morphology of the nanoparticles has not changed and the size distribution is similar to CuNPs (Supplementary Figure S1B–D). Instead, it is possible to observe that when CuNPs are present in the ink formulation, the resulting image appears blurry. This is attributed to the addition of PVP and ILs, which, due to their high density and viscosity, impede the achievement of sharp images (Lu et al., 2019).

Figure 2

Figure 2. FE-SEM images of (A) CuNPs; (B) CuNP–TFSI ink; (C) CuNP–OTf ink; and (D) CuNP–BF4 ink.

Energy-dispersive X-ray (EDX) spectroscopic images for CuNP–TFSI, CuNP–OTf, and CuNP–BF₄ inks, respectively, are displayed in Supplementary Figures S4–S7. For CuNP–TFSI and CuNP–OTf inks, fluorine and sulfur were observed due to the presence of TFSI and OTf anions, and for CuNP-BF₄ ink, fluorine from the BF₄ anion was detected. Furthermore, it is possible to note the presence of nitrogen in all ink spectra, which can be from the IL cation and PVP. Alternatively, Supplementary Figures S8–S10 display FE-SEM and EDX spectroscopy mapping images, revealing a uniform distribution of all components on the surface of the glassy carbon plate.

The X-ray diffraction pattern of the CuNPs shows that the main crystallographic orientation is Cu(111) (Figure 3) and presents characteristic peaks of cubic copper that were confirmed by index card JCPDS-ICDD, number 85–1,326, and is consistent with the previous report for this synthesis (Lee et al., 2008). The sample was indexed (R. Shirley, 2020) as a cubic cell in an Fm-3 space group with calculated cell parameter a = 3.6153 Å. The crystallite size was calculated using DIFFRAC.EVA 5.2 software, using the X-ray broadening of the (1 1 1), (2 0 0), and (2 2 0) diffraction peaks by the well-known Scherrer equation, D = 0.9 λ/B cos θ, where D is the particle size in nm, λ is the wavelength of the X-ray (0.15405 nm for Cu–Ka), B is the corrected full-width at half-maximum, and θ is the diffraction angle. The calculated sizes were 43.3, 31.9, and 33.8 nm using CRYSFIRE suite software, which are of same magnitude and in good agreement order with the SEM analysis, and there is no detectable copper oxide. Calculations of the cell parameters and particle size details are given in Supplementary Figures S11, S12.

Figure 3

Figure 3. X-ray diffraction patterns of copper nanoparticles.

The UV–visible spectra of fresh (a) and 6-month-old (b) CuNPs and inks stored under atmospheric conditions are shown in Figure 4. The maximum absorption wavelength values are given in Table 3. For the fresh samples and aged inks, the UV–visible spectra exhibit a surface plasmon resonance characteristic of copper nanoparticles (Chan et al., 2007). However, a change in the CuNPs is notable as the maximum absorption shifted from 603 to 618 nm, and the shape of the spectrum also changed. In contrast, the ink spectra experienced minor shifts, with no significant changes in their overall shape. These results suggest that the ink formulation provides stabilization and protection to the CuNPs from oxidation and agglomeration (Tam et al., 2016).

Figure 4

Figure 4. UV–visible spectroscopy for (A) fresh and (B) 6-month-old CuNPs and inks in deaired Milli-Q water.

Table 3

Table 3. Maximum absorption wavelength for CuNPs and catalytic inks.

3.2 Surface characterization through FT-IR spectroscopy

The FT-IR spectra of CuNP–BF₄ ink (Figure 5) and other catalysts are reported (Supplementary Figure S13). The FT-IR spectra for ILs (Johnston and Shriver, 1993; Schrader, 1996; Heimer et al., 2006; Holomb et al., 2008; Jeon et al., 2008; Noack et al., 2010; Cha et al., 2014; Xu et al., 2017; Huang et al., 2018; Kausteklis et al., 2018; 2019) and PVP(Haaf et al., 1985; Lu et al., 2015; Shahi et al., 2017) are in good agreement with those in the literature, and in both images, two dotted lines were positioned at 3,521 cm^-1 and 1,644 cm^-1, which correspond to polymeric OH⁻ and carbonyl bond from the PVP FT-IR spectra, respectively. Regarding the spectra obtained for the inks, some shifts were observed when compared with the spectra of the individual species, of which two stand out: the bathochromic shift in the vibration associated with the terminal polymeric OH⁻ and the hypsochromic shift in the band corresponding to the carbonyl bond from the PVP compound in each case. For the vibration associated with the OH⁻ bond, a difference of 89 cm^-1 was obtained for CuNP–TFSI ink and CuNP–BF₄ ink, and for CuNP–OTf ink, the shift corresponds to 48 cm^-1, while for the shifts in the carbonyl bond band, a difference of 31 cm^-1 was obtained for CuNP–TFSI ink and 26 cm^-1 for CuNP–BF₄ and CuNP–OTf inks.

Figure 5

Figure 5. FT-IR spectra of CuNP–BF4 ink. The dotted lines correspond to the 3,521 and 1,644 cm^-1 positions.

The obtained values suggest that there may be an interaction between the CuNPs and the atoms associated with these bonds. It could also be expected that the ILs cause these changes in the bands, so the presence of each of these interactions in each case may also imply a difference in the surface environment.

3.3 Electrochemical measurements

3.3.1 Cyclic voltammetry

Cyclic voltammetry for GC/CuNPs and inks is shown in Figure 6 and S14–S16. In each one, there is a black cyclic voltammogram, which corresponds to the capacitive response of the blank electrode: the glassy carbon in the case of GC/CuNPs and GC/IL-PVP in the case of inks. In Supplementary Figure S14A, it is possible to observe two cathodic peaks at 0.33 and 0.5 V vs RHE and two anodic peaks at 0.76 and 1.0 V vs RHE in argon-saturated 0.1 KHCO₃ solution, corresponding to the reduction and oxidation processes of copper from GC/CuNPs, respectively. Meanwhile, cyclic voltammetry measurements for both GC/CuNP–OTf (Supplementary Figure S11A) and GC/CuNP–BF₄ (Figure 6A) inks showed only one oxidation and reduction process, and in GC/CuNP–TFSI ink, there are two oxidation peaks and one reduction peak. For CO₂-purged systems, there are also significant differences. In GC/CuNPs (Supplementary Figure S16B), there is only one peak for oxidation and another for reduction. Regarding copper-based inks, there is only one peak each for reduction and oxidation. Furthermore, there are important changes in the current intensity in each peak, for instance, CuNPs in Ar show a maximum value of −6 µA for the reduction peak and 8 µA for the oxidation peak. However, in CO₂, there is an increase to −210 and 240 µA. For CuNP-IL inks, lower current intensities were obtained than those of CuNPs alone. For CuNP–OTf and CuNP–TFSI inks, current intensities are lower than −15 and 15 µA for each peak in the Ar and CO₂ atmosphere, respectively, but for CuNP–BF₄, for both peaks in Ar and CO₂, current intensities of over −65 and 60 µA were observed (Figure 6). This phenomenon may be attributed to the GC/CuNP–BF₄ ink having a larger electroactive area, thus exhibiting higher conductivity. Nevertheless, GC/CuNP–TFSI ink exhibits a resistive behavior in the measurement in the CO₂ atmosphere compared to the measurement in argon and the other inks studied, suggesting that this modified electrode may be poorly conductive or possess a smaller electroactive surface area, or it could lead to hydrogen evolution reaction (HER) (Wang et al., 2021). On the other hand, CuNPs alone exhibit a higher intensity current when purged with CO₂. This can be attributed to the addition of new components, i.e., ILs, which work as chemical-trapping agents for CO₂ (Zhang et al., 2020). These differences are very significant because they suggest a change around the surface of GC/CuNP-IL (Castro-Castillo et al., 2022) inks compared to GC/CuNPs.

Figure 6

Figure 6. Cyclic voltammetry for GC/CuNP–BF₄ ink in 0.1 M KHCO₃ purged with argon (A) and CO₂ (B) at a scan rate of 50 mVs^-1. The blank measurements were performed with a modified glassy carbon electrode using the GC/BF₄ ink.

3.3.2 Linear sweep voltammetry

Linear sweep voltammetry (LSV) studies were performed in 0.1 M KHCO₃ in the presence of argon and carbon dioxide (Figures 7A,B) at a sweep rate of 5 mV∙s^-1, using the OCP as the initial potential (Table 4). The onset potential for each is given in Table 5. In each case, there is a decrease in onset potential from Ar to CO₂-purged cells, which indicates that the required energy for ECO₂-R is less than that for HER. The three inks show similar behavior regarding the onset potential, but there is a difference in the current intensity; this behavior is consistent with the results from cyclic voltammetry, but in this case, the GC/CuNP–BF₄ ink is the catalyst with the higher current intensity. On the other hand, GC/CuNP-TFSI ink shows the lower current intensity for ECO₂-R of all the studied inks, and it is in good agreement with the literature due to the TFSI anion behavior (Cadena et al., 2004; Wang et al., 2021).

Figure 7

Figure 7. LSV for GC/CuNPs (A) and GC/CuNP–OTf ink, GC/CuNP–BF₄ ink, and GC/CuNP–TFSI ink (B) in the presence of argon and CO₂ with 0.1 M KHCO₃ as the supporting electrolyte at a scan rate of 5 mVs^-1.

Table 4

Table 4. OCP values for each studied electrode in the presence of argon.

Table 5

Table 5. Onset potential vs RHE obtained from LSV measurements for each electrode.

3.3.3 Electrochemical reduction of carbon dioxide

ECO₂-R was performed at −1.3 V vs RHE in 0.1 M KHCO₃ as the supporting electrolyte for 3 h. The product distribution results are given in Figure 8 and Supplementary Table S1. The Faradaic efficiency (FE) was calculated using Supplementary Equation 1 (Chen et al., 2015).

Figure 8

Figure 8. Faradaic efficiency for ECO₂-R for each reaction product at −1.3 V vs RHE.

There were significant differences in product distribution between GC/CuNPs and each ink studied. For GC/CuNPs, the main product was ethylene, accounting for 41.9%. Nevertheless, lower FE for this product was obtained for the inks, with values of 12.9% for GC/CuNP–TFSI ink, 26.1% for CuNP–BF₄ ink, and 12.4% for GC/CuNP–OTf ink. It is reported in the literature that for the Cu(111) crystallographic orientation, methane should be the major product (Gattrell et al., 2006; Wang et al., 2023). However, methane formation is also dependent on the local pH, i.e., the activity of protons. In the case of GC/CuNPs, at this potential, the reaction leads to a basic local pH in the vicinity of the electrode, which favors the dimerization of the formyl intermediate, ultimately leading to the formation of ethylene⁹⁹. In contrast, inks showed a major FE for methane compared to GC/CuNPs. GC/CuNP–TFSI, GC/CuNP–BF₄, and GC/CuNP–OTf inks showed methane as the main product with a 50.1%, 63.9%, and 38% FE, respectively, and for GC/CuNPs, the FE was only 9%. In the case of inks, an acidic environment provided by the ILs directs the reaction via the C1 pathway to produce methane (Cao et al., 2023). No carbon monoxide was detected in any of the studied electrolysis. Hydrogen was obtained in all electrolysis performed: for GC/CuNPs, an FE of 15% was obtained, while for GC/CuNP-TFSI ink, it was 30.1%. For GC/CuNP–OTf and CuNP–BF₄ inks, the FE for hydrogen was 7.2% and 5.4%, respectively.

3.3.4 Electrochemical impedance spectroscopy

Data were adjusted to an equivalent circuit shown in Figure 9 based on a previous reference (Martínez-Rojas et al., 2020). χ² values are 0.001 and 0.002, so the adjustment is representative in each analyzed system. The R1 value is the resistance of the solution, CPE1 is the double-layer capacitance, and R2 is the charge transfer resistance of the process. CPE2 is a capacitive element from the produced charges on the electrode surface, which increases with the addition of charged species such as ILs. CPE3 and R3 correspond to the double-layer components that describe the morphological irregularities of the electrode surface. When examining the data given in Table 6, it is possible to observe that the resistance of the solution (R1) shows the same behavior in all the studied systems, so the system is stable in each measurement. The charge transfer resistance (R2) for GC/CuNPs shows an increase from argon to carbon dioxide presence, but for GC/CuNP-BF₄ and GC/CuNP-OTf inks, the opposite effect occurs, i.e., R2 is higher in the presence of CO₂ than in argon, indicating a better performance in ECO₂-R.

Figure 9

Figure 9. Equivalent circuit and diagram for studied systems.

Table 6

Table 6. Equivalent circuit elements from Figure 9 values for CuNPs and each studied ink.

On the other hand, the CPE3 component exhibits an increase in the GC/CuNP CO₂ compared to GC/CuNP Ar; meanwhile, for the other experiments, CPE3 retains the same magnitude order. This indicates that the stability in the ink systems is higher than that in GC/CuNPs, and the double layer decreases in function with the increase in stability.

The Nyquist plot for GC/CuNPs in the presence of Ar (red) and CO₂ (black) is displayed in Figure 10A. For the Ar-purged experiment, there is a capacitive response in the whole frequency range, and in the presence of CO₂, it is possible to observe the formation of a semicircle, indicating that CO₂ interacts with the CuNPs. In this case, the interaction occurs through adsorption, as it is at the OCP. In the case of the Bode plot (Figure 10B), a local accumulation of charge (Calfumán et al., 2017) takes place due to the formation of products, indicating the possible formation of an active site for GC/CuNPs in the presence and absence of CO₂, related to the relaxation time constants (Martínez-Rojas et al., 2020). In argon-purged measurements, there are two active sites, one at high frequencies and the other at low frequencies (2.2 ms and 85 s, respectively, [Table 7]), where different types of charge transfer can occur. On the other hand, in the presence of CO₂, a coupling of active sites can be observed, resulting in a shift toward intermediate frequencies. Relaxation time calculations were made using equation S2. These differences between both experiments can be attributed to the morphological irregularities, the different crystallographic orientations in the CuNPs (Kas et al., 2015; Castro-Castillo et al., 2022; Mardones-Herrera et al., 2022), and their respective interactions with CO₂. In addition, a 45° phase angle was observed at low frequencies in the presence of CO₂, which means a diffusional behavior of carbon dioxide to the electrode surface.

Figure 10

Figure 10. Nyquist diagram (A,C,E,G) and Bode plot (B,D,F,H) for GC/CuNPs, GC/CuNP–OTf, GC/CuNP–BF₄, and GC/CuNP–TFSI, respectively, in the presence of argon (red) and CO₂ (black). Dotted lines correspond to Log|Z| vs Log f, and square and triangle lines correspond to phase angle vs. Log (F)

Table 7

Table 7. Relaxation time constants of each studied electrode.

The Nyquist and Bode plots for GC/CuNP–OTf and GC/CuNP–BF₄ inks show the same behavior (Figures 10C–F). A capacitive behavior is observed in the presence of argon in the Nyquist plot in both cases (Figures 10C,E), and for the CO₂-purged experiment, an increase in the capacitive response was observed, which means a decrease in the resistance to the charge transfer, indicating that the ink causes an increase in the interaction of CO₂ with the surface of the electrode. Bode plots (Figures 10D,F) exhibit the same behavior; the processes occur in the same regions, so the active sites are better defined than in CG/CuNPs, and there is a relevant decrease in the capacitance compared with CG/CuNPs in the presence of CO₂. These differences in active sites between the GC/CuNPs and inks could explain the product distribution in each case, where ethylene was the main product for GC/CuNPs and methane was the main product for GC/CuNP–BF₄ and GC/CuNP–OTf inks due to the formation of the active sites shown in each Bode plot.

On the other hand, the GC/CuNP–TFSI ink Nyquist plot (Figure 10G) shows a different behavior. The Nyquist plot shows a decrease in the capacitive response, which is in good agreement with the results obtained from cyclic voltammetry measurements. Furthermore, the Bode plot in the absence of CO₂ shows a wider frequency range than the other inks; thus, it is possible that there are more active sites for HER. This can be related to high faradaic efficiency for HER compared to the other experiments and can be due to the capacity of the [BMIm]TFSI to dissolve CO₂, which depends on the fluorinating degree of the anion (Cadena et al., 2004) and the nature of the TFSI anion that promotes HER (Wang et al., 2021).

However, the Bode plot for GC/CuNP–TFSI ink (Figure 10H) is very similar to that of the other inks, so the active sites should show the same behavior, but there is a main selectivity for methane and hydrogen (Figure 8); in contrast, in the other inks, there is a major selectivity for other C1 and C2 products. This can be related to two properties of this ionic liquid: the promotion of HER mentioned before, which explains the higher FE for the hydrogen obtained, and the other property is the hydrophobic nature of the [BMIm]TFSI IL. The hydrophobic surface promotes the increase in CO₂ and H⁺ concentration in the electrode surface (Zhang Y. et al., 2023), which means that the ink should lead the C1 pathway to produce methane (Kortlever et al., 2015).

Based on the results obtained from the analysis of relaxation time constants, it can be inferred that there are different active sites for both CuNPs and inks, both in the presence of argon and carbon dioxide. On the other hand, the finding of these active sites and the product distribution obtained for each catalyst demonstrate that the inks have a SCILL behavior (Zhang and Etzold, 2021) due to the ILs promoting the suppression of certain products. For instance, in GC/CuNP-TFSI ink, the FE for ethylene is one of the lowest, and the FE for hydrogen is the highest. This is in good agreement with the SCILL study reported by Zhang et al. (2020), where IL molecules interact with one or more intermediates, leading to the production of the obtained species.

3.3.5 Proposed reaction mechanism

The following mechanism (Figure 11) is proposed based on published works (Nie et al., 2013; Kas et al., 2015). Gaseous CO₂ is adsorbed onto the surface of copper nanoparticles, which is the first necessary step for the reduction reaction to occur. Once adsorbed, the CO₂ molecule receives an electron, forming a radical anionic intermediate CO₂⁻, stabilized by [BMIm]BF₄ (Ganesh, 2020; Li et al., 2023). The CO₂⁻ intermediate can receive another proton (H⁺) and electron, converting into CO adsorbed on the copper surface.

Figure 11

Figure 11. Proposed reaction mechanism for ECO₂-R.

The adsorbed CO can proceed through different pathways based on the catalyst configuration and reaction conditions. Differences in the intermediate steps of adsorption of CO₂ in the proposed mechanism are due to the interaction of the different crystallographic orientations of CuNPs (Kortlever et al., 2015). It is reported that Cu(111) leads to methane, Cu(100) to ethylene, and Cu(110) to oxygenated products. On the other hand, different ionic liquids stabilize intermediates in specific ways, influencing product formation. For example, BF₄⁻ stabilizes intermediates, favoring the production of methane (CH₄) and ethylene (C₂H₄).

3.3.6 Electrochemical stability of CuNPs and CuNP-IL inks

For the CuNPs, the initial current is more negative than −10 mA and stabilizes at approximately −20 mA after approximately 1 h, indicating significant catalytic activity and stable production of CO₂ reduction products over time. Copper nanoparticles can reduce CO₂ to various hydrocarbons, such as ethylene and methane, depending on the crystallographic orientation and local pH conditions (Kas et al., 2014).

For the GC/CuNP–BF₄ ink (Figure 12), the initial current is also more negative than −10 mA but stabilizes at approximately −10 mA, which is less negative compared to CuNPs without an ionic liquid. The presence of [BMIm]BF₄ appears to reduce the catalytic activity of the CuNPs. According to the literature, BF₄⁻ stabilizes certain intermediates that favor methane and ethylene production, reducing the overall observed current. It has been reported that [BMIm]BF₄ produces significant amounts of methane and ethylene during the electrochemical reduction of CO₂ (Lau et al., 2016). For the GC/CuNP-OTf ink, the initial current mirrors that of other systems but stabilizes at approximately −15 mA. GC/CuNP–OTf ink seems to have an intermediate stabilizing effect on the catalytic activity of the CuNPs, likely through interactions with CO₂ reduction intermediates. This suggests that GC/CuNP–OTf ink may promote the formation of products like CO and formate, maintaining the electrode surface to be more active (Feng et al., 2018). In the case of GC/CuNP–TFSI ink, the initial current starts similar to GC/CuNP–BF₄ ink but stabilizes at approximately −10 mA. GC/CuNP–TFSI ink may act similarly to GC/CuNP-BF₄ ink by stabilizing certain intermediates and reducing the overall catalytic activity while favoring specific product formation. [BMIm]TFSI is known for its thermal and electrochemical stability (Feng et al., 2018). These results suggest that the ionic liquids [BMIm]BF₄, [BMIm]OTf, and [BMIm]TFSI significantly impact catalytic activity and product selectivity. We successfully achieved the electrochemical reduction of CO₂ using copper nanoparticles. The choice of ionic liquid influences the stabilization of reactive intermediates and the overall process efficiency, allowing for the tailoring of different hydrocarbon products as needed.

Figure 12

Figure 12. Electrolysis of GC/CuNPs (black), GC/CuNP–BF₄ ink (red), GC/CuNP–OTf (blue), and GC/CuNP–TFSI ink (green) at a controlled potential of −1.3 V vs RHE, saturated in CO₂ (pH 6.8). The current (I) is recorded as a function of time (t) over 7 h to determine the stability.

On the other hand, the performance of 6-month-old inks was similar to that of fresh ink obtained (Figure 13). Methane remains the main product of each ink, and the FE increased in the case of CuNP–OTf ink. Ethylene FE has similar value ranges, decreasing in the case of 6-month-old CuNP–BF4 ink. The FEs of formic acid, ethanol, and acetone are also similar across all the compared catalysts. These results suggest that despite the aging of the CuNP–IL inks, some products maintain stable efficiency. Despite some differences found, it can be inferred that the inks remain stable over a period of 6 months.

Figure 13

Figure 13. Faradaic efficiency for ECO₂-R for each reaction product at −1.3 V vs RHE with 6-month-old CuNP–IL inks.

4 Conclusion

In this study, copper nanoparticles were synthesized using the wet chemical reduction method, resulting in a granular morphology with Cu(111) as the predominant crystallographic orientation. No copper oxides were detected, and copper-based inks exhibited a morphology and size distribution similar to CuNPs. Electrochemical reduction of carbon dioxide at −1.3 V vs RHE was performed using GC/CuNPs and GC/CuNP–IL-based inks. Significant differences in product distribution were observed, with ethylene and formic acid being the primary products for GC/CuNPs, while methane was obtained in varying proportions for GC/CuNP-IL inks despite the expected production of ethylene under the reaction conditions. Hydrogen was consistently generated in each electrolysis, particularly in the GC/CuNPs and GC/CuNP-TFSI ink systems. EIS analyses supported these findings, showing different active sites in Bode plots that could influence the concentration or local mass transport of reactants or intermediates in each ink system studied. The observed behavior of a surface-confined ionic liquid layer (SCILL) in the electrodes was attributed to changes in product distribution resulting from the addition of different ILs. Specifically, while the C2 mechanism was favored in GC/CuNPs, leading to ethylene formation, the C1 pathway predominated in GC/CuNP–TFSI, GC/CuNP–BF₄, and GC/CuNP–OTf inks, resulting in methane formation. Furthermore, GC/CuNP–TFSI inks exhibited high faradaic efficiency for hydrogen production, consistent with the nature of the TFSI anion.

Data availability statement

The raw data supporting the conclusion of this article will be made available by the authors, without undue reservation.

Author contributions

VG: writing–original draft, conceptualization, and methodology. EM: formal analysis, methodology, and writing–original draft. NS: methodology, writing–original draft, and formal analysis. FA: writing–review and editing. FM-R: formal analysis and writing–review and editing. DR: formal analysis and writing–original draft. JH: writing–review and editing and conceptualization. MI: writing–original draft, writing–review and editing, and funding acquisition.

Funding

The author(s) declare that financial support was received for the research, authorship, and/or publication of this article. This research work was financially supported by Funding Program 045–2019-FONDECYT-BM-INC.INV. (PROCIENCIA) assigned to the project “Manufactura Avanzada Incorporada En Dispositivos Recargables Ion-Litio Empleando Nuevos Materiales Basados En Liquidos Ionicos.”

Acknowledgments

The authors acknowledge FONDECYT Regular Projects 1181126, 1211518, and 1221179, FONDEQUIP EQM150020, EQM190016 and EQM150101, Millennium Institute of Green ammonia ICN 2021_023, and the ANID doctoral fellowship 21170164.

Conflict of interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Publisher’s note

All claims expressed in this article are solely those of the authors and do not necessarily represent those of their affiliated organizations, or those of the publisher, the editors, and the reviewers. Any product that may be evaluated in this article, or claim that may be made by its manufacturer, is not guaranteed or endorsed by the publisher.

Supplementary material

The Supplementary Material for this article can be found online at: https://www.frontiersin.org/articles/10.3389/fenvc.2024.1447014/full#supplementary-material

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