High purity CH₄ production from CO₂via cascade electro-thermocatalysis using metal nanoclusters with high CO₂ binding affinity

Abstract

Electrochemical CO₂ reduction reaction (CO₂RR) has emerged as a promising strategy to convert CO₂ into value-added chemicals and fuels. While methane is especially desirable owing to its extensive use as a fuel, existing infrastructure, and large global market, the direct electroreduction of CO₂ to CH₄ is hindered by challenges such as low product purity and high overpotentials. In this study, an efficient cascade electrolysis and thermocatalysis system for the high-purity production of CH₄ from CO₂ has been demonstrated. Electrochemical syngas production was carried out using CO₂RR-active electrocatalysts, including Au₂₅ and Ag₁₄ nanoclusters (NCs). While both NCs exhibited high CO₂-to-CO activity in alkaline media, Ag₁₄ NCs enabled syngas production with a varying ratio (H₂/CO) by adjusting the CO₂ flow rate, achieving near-theoretical single-pass conversion efficiency (SPCE) of over 45% (theoretical limit = 50%). Electrokinetic analysis revealed that the strong CO₂ binding affinity and exceptional CO selectivity of Ag₁₄ NCs contribute to superior syngas tunability and carbon conversion efficiency. Electrochemically generated syngas (H₂/CO = 3) at 800 mA cm⁻² was directly fed into a thermocatalysis reactor, producing CH₄ with a purity exceeding 85%.

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Broader context

Converting CO₂ into deep-reduced chemicals and fuels is a promising strategy to address environmental issues and achieve a carbon-neutral society. Currently, electrochemical and thermocatalytic methods are extensively studied to develop efficient conversion processes, along with other approaches. However, electrochemical CO₂ conversion suffers from low product purity, while significant challenges remain in the energy-intensive CO production step via the reverse water–gas shift (RWGS) reaction in thermocatalytic conversion. We report an efficient cascade electro-thermocatalysis process that electrochemically converts CO₂ into syngas, followed by subsequent thermal CO methanation. This cascade system not only bypasses the energy-intensive RWGS reaction but also ensures high CH₄ purity in the product gas stream. Additionally, we present key design principles for electrocatalysts in electrochemical syngas production with near-theoretical single-pass conversion efficiency, enabled by the high CO₂ binding affinity of nanocluster electrocatalysts. This unique property allows tunable CO₂-free syngas production with various syngas ratios (H₂/CO) by adjusting the initial CO₂ flow rate. The electrochemically produced CO₂-free syngas is then converted into CH₄ with a purity exceeding 85% at an industrially relevant current density of 800 mA cm⁻². These findings establish the viability of the cascade electro-thermocatalysis system for high-purity chemical production with high carbon conversion efficiency.

Introduction

Electrochemical CO₂ reduction reaction (CO₂RR) has garnered significant interest as a promising strategy for converting CO₂ into valuable chemicals and fuels, while also serving to store surplus renewable energy.^1–3 Among several products derived from CO₂, CO production has garnered substantial research attention owing to its wide-ranging applications as a component of syngas (a mixture of CO and H₂) in numerous industrial processes, including methanation, alcohol production, and Fischer–Tropsch synthesis.^4–6 Furthermore, electrochemically produced CO (or syngas) can effectively mitigate sulfur and nitrogen contamination—issues commonly encountered in traditional fossil fuel-based methods such as natural gas reforming and coal gasification, which are also associated with significant pollution and high energy consumption.^7–10 Various Au- and Ag-based nanoparticles have been developed as selective CO₂RR catalysts for CO production.^11–13 However, these catalysts typically exhibit polydispersity in size, shape, and surface structure, resulting in suboptimal activity and selectivity.¹¹

Over the past decade, atomically precise metal nanoclusters (NCs) have emerged as a promising frontier in electrochemical CO₂RR.^14–16 Since Kauffman et al.'s pioneering report in 2012,¹⁷ a variety of Au-, Ag-, and alloy-based NCs have been developed as CO₂RR electrocatalysts by tailoring their structures and compositions.^18–24 Notably, metal NCs have enabled atomic-level insights and driven significant advancements in electrocatalyst design. Using Au₂₅(SR)₁₈, Au₃₈(SR)₂₄, and Au₁₄₄(SR)₆₀ NCs (SR = thiolate) as model catalysts, we have demonstrated that these NCs undergo electrochemical activation via partial ligand loss, generating dethiolated Au sites that serve as active centers for CO₂-to-CO conversion.²⁵ Furthermore, the efficiency of CO₂-to-CO electroreduction can be enhanced by transplanting highly active Au sites into catalytically less-active Ag₂₅(SR)₁₈ and inactive Ni₄(SR)₈ NCs.^26,27 Additionally, the cation-relaying effect has been demonstrated to boost CO production on Au₂₅(SR)₁₈ NCs by employing anionic terminal groups on the protecting ligands.²⁸

While significant progress has been made in developing selective CO₂RR catalysts for CO and formate production,²⁹ much less has been achieved toward developing catalysts for deep-reduced products such as CH₃OH, CH₄, and C₂H₄. Cu-based electrocatalysts have been widely employed for converting CO₂ into various deep-reduced products, owing to their appropriate adsorption energy for the *CO intermediate.^30–32 Ni-based electrocatalysts have also shown potential for hydrocarbon production.^33,34 However, low product purity remains a major challenge in the electrochemical CO₂RR process for deep-reduced products. For example, widely investigated deep-reduced products such as CH₃OH,^35,36 CH₄,^37–42 and C₂H₄,^43,44 typically exhibit purities below 30% due to the presence of undesired byproducts and unreacted CO₂. These impurities in the product stream necessitate additional separation steps, further emphasizing the challenge of achieving high-purity chemical production.

Thermocatalytic conversion of CO₂ is another promising strategy for producing deep-reduced products. This approach typically involves CO₂-to-CO conversion via the reverse water–gas shift (RWGS) reaction, followed by subsequent CO hydrogenation steps.^4–6 However, RWGS requires high temperatures due to its endothermic nature and also relies on external hydrogen sources,⁴⁵ significantly reducing the overall energy efficiency of the process. Cascade catalysis, which integrates electrochemical CO₂RR with thermochemical or biochemical reactions,^46–49 offers an effective alternative for producing deep-reduced products by bypassing the energy-intensive RWGS process. When combined with hydrocarbon production technologies such as CO methanation and Fischer–Tropsch synthesis, electrochemical CO₂RR could serve as a viable synthetic route for various deep-reduced chemicals and fuels.

Herein, we introduce an efficient cascade electro-thermocatalysis system for CH₄ production from CO₂. Electrochemical syngas was generated using CO₂RR-active electrocatalysts under varying CO₂ flow rates, including 1-hexanethiolate-protected Au₂₅ [Au₂₅(SC₆H₁₃)₁₈], 3,3-dimethyl-1-butynyl-protected Ag₁₄ [ClAg₁₄(C≡C–tBu)₁₂], and Ag nanoparticles (Ag NPs). Au₂₅(SC₆H₁₃)₁₈ and ClAg₁₄(C≡C–tBu)₁₂ are hereafter abbreviated as Au₂₅ and Ag₁₄, respectively, throughout this paper. Among these, Ag₁₄ NCs exhibited the highest carbon conversion efficiency, outperforming Au₂₅ NCs and Ag NPs. Electrokinetic analysis indicated that the superior single-pass conversion efficiency (SPCE) of Ag₁₄ NCs originates from their strong CO₂ binding affinity and exceptional CO selectivity. Ag₁₄-based electrolyzer was seamlessly integrated into a thermocatalysis reactor, achieving CH₄ production with a purity exceeding 85% at current densities ranging from 200–800 mA cm⁻². This integrated system effectively enhances product selectivity and SPCE, addressing key challenges in electrochemical CO₂-to-CH₄ conversion.

Results and discussion

Electrochemical production of CH₄ from CO₂ remains highly challenging owing to low selectivity and poor SPCE. On the other hand, thermal methanation of syngas is a well-established process capable of producing high-purity CH₄ when an appropriate syngas composition (H₂/CO = 3) is supplied.⁵⁰ In this study, we explore a cascade electro-thermocatalysis system consisting of a CO₂-to-syngas electrolyzer integrated with a thermal syngas methanation reactor. As illustrated in Scheme 1, CO₂ and water are first electrochemically converted into a mixture of CO and H₂ with a predetermined composition, which is then directly injected into the thermocatalysis reactor to produce high-purity CH₄. Notably, this system can be readily extended to produce several chemicals and fuels, such as lower olefins, gasoline, diesel, and more, by utilizing appropriate thermocatalysts and tailored syngas ratios.^4–6

Scheme 1 Schematic of a cascade electro-thermocatalysis system for producing high-purity CH₄. The CO₂-electrolyzer generates CO₂-free syngas (CO + H₂), which is subsequently converted to CH₄ in the thermocatalysis reactor.

Electrochemical syngas production with tailored ratios has been achieved by employing a combination of CO₂RR and hydrogen evolution reaction (HER) catalysts that selectively produce CO and H₂ (eqn (1) and (2)).^51–54 However, this syngas production is typically conducted in excess CO₂, needing additional CO₂ separation from syngas product before it can be injected into the thermocatalysis reactor.⁵¹ Furthermore, the OH⁻ byproduct of HER may react with CO₂ to generate carbonates (eqn (3)), further reducing SPCE.

CO₂ + H₂O + 2e⁻ → CO + 2OH⁻ (1)

2H₂O + 2e⁻ → H₂ + 2OH⁻ (2)

CO₂ + 2OH⁻ → CO₃²⁻ + H₂O (3)

To minimize the fraction of unreacted CO₂ after CO₂RR, we explored the possibility of controlling the syngas ratio by adjusting the CO₂ flow rate. When CO₂RR is conducted using a highly efficient CO₂RR catalyst at a low CO₂ flow rate, CO₂-to-CO conversion would predominantly occur, with all supplied CO₂ being consumed before HER initiates. Thus, syngas ratio can be effectively controlled by adjusting CO₂ flow rate. Additionally, this approach is expected to significantly reduce carbonate production from HER byproduct.

It has been demonstrated that ligand-protected Au and Ag NCs exhibit high electrocatalytic activity for CO₂-to-CO conversion.^18–24,55 In this study, Au₂₅ and Ag₁₄ NCs were employed as CO₂RR catalysts owing to their exceptional CO selectivity and stability. The Au₂₅ and Ag₁₄ NCs were synthesized according to established protocols in the literature.^25,55 As shown in UV-Visible absorption spectra (Fig. S1, ESI†), synthesized NCs exhibit characteristic absorption peaks at 670 and 450 nm for Au₂₅ NCs and 280 nm for Ag₁₄ NCs. The homogeneity of the NCs, with average core diameters of 1.3 nm for Au₂₅ and 1.2 nm for Ag₁₄ NCs, was clearly observed in the transmission electron microscopy (TEM) images (Fig. S1, ESI†). These NCs were further characterized by electrospray ionization (ESI) mass spectrometry. As shown in Fig. 1a and b, intense single peaks for both NCs at m/z 7034 Da (orange line) and m/z 2520 Da (gray line) correspond to the [Au₂₅(SC₆H₁₃)₁₈]⁻ and [ClAg₁₄(C₆H₉)₁₂]⁺ ions, respectively. The combined absorption and mass spectrometry analyses firmly establish the successful synthesis of the NCs, confirming their molecular purity.

Fig. 1 Crystal structures (redrawn from ref. 56 and 57) and ESI mass spectra of (a) Au₂₅ and (b) Ag₁₄ NCs. All carbon atoms are omitted in panel (a) and displayed in wireframe form in panel (b) for clarity. Insets in panel (a) and (b) compare experimental data (lines) with the simulated isotope patterns (blue bars). The mass spectra of Au₂₅ and Ag₁₄ NCs were obtained in negative and positive ionization modes, respectively. (c) j_CO and (d) corresponding CO selectivities measured for Au₂₅/GDE-, Ag₁₄/GDE-, and Ag NP/GDE-equipped zero-gap CO₂ electrolyzers as functions of cell potential.

CO₂RR activities of the synthesized Au₂₅ and Ag₁₄ NCs were evaluated in a zero-gap electrolyzer at a CO₂ flow rate of 50 mL min⁻¹ (Fig. S2, ESI†). For comparison, commercial Ag NPs (Dioxide Materials) were also studied. NCs were immobilized in a gas diffusion electrode (GDE), which consisted of a microporous layer (MPL) and a gas diffusion layer (GDL). Fig. 1c presents CO₂RR activities of NC/GDE and Ag NP/GDE in the zero-gap cell at varying cell potentials (E_cell). Both NCs demonstrated higher CO₂RR activity than Ag NPs across the potential range. Additionally, current density for CO production (j_CO) on both NCs showed an exponential increase with increasing cell potential. Conversely, the Ag NPs exhibited a sharp decline in j_CO and CO selectivity at potentials above 2.5 V (Fig. 1d).

As illustrated in Fig. 1c, Au₂₅ NCs exhibited the lowest cell potentials compared with other catalysts, with CO selectivity maintained above 97% within the potential range of 1.6–2.1 V. However, it dropped below 90% at higher cell potentials. Conversely, the Ag₁₄ NCs achieved CO selectivity, exceeding 98% across the potential range. In a previous CO₂RR study, the exceptional CO selectivity of the Ag₁₄ NCs was attributed to their unique adsorption properties, which facilitated enhanced CO₂-to-CO conversion and dramatically suppressed HER activity.⁵⁵

There have been numerous attempts to achieve controlled syngas production with specific H₂/CO ratios by varying the applied potential or utilizing a combination of CO₂RR and HER catalysts.^51–54 In this study, we first investigated the controlled production of syngas using a combination of Au₂₅ NCs and commercial Pt/C, which served as the CO₂RR-active and HER-active catalysts, respectively. Fig. 2a presents the results of electrolysis performed with the mixed catalysts at a current density of 200 mA cm⁻². As shown in the figure, CO fraction in the syngas can be precisely adjusted between 25 and 90% by varying the mixing ratio of the two catalysts. However, this approach led to over 60% of unreacted CO₂ in the product gas, which requires separation before injection into the thermocatalysis reactor.

Fig. 2 Fractions of the product gas generated from mixed catalysts composed of Au₂₅ NCs and Pt/C in the zero-gap electrolyzer at 200 mA cm⁻² with a CO₂ flow rate of 30 mL min⁻¹. By employing different ratios of Au₂₅ NCs to Pt/C in the mixed catalysts, various syngas compositions were achieved. (b) Fractions of the product gas generated from Au₂₅ NCs at 200 mA cm⁻² as a function of CO₂ flow rate, and (c) the corresponding SPCE. (d) j_CO of three electrocatalysts as a function of the CO₂ partial pressure at E_cell = 1.8 V (sphere). Total flow rate of the feed gas was 200 mL min⁻¹, and partial pressure of CO₂ was regulated using Ar gas. The binding affinity constants of the catalysts are described in the inset by fitting j_CO–P_CO2 plots with eqn (11) (solid line).

To address the issue of unreacted CO₂ remaining after syngas production, we explored the possibility of controlling CO fraction in syngas by adjusting the CO₂ flow rate. Highly efficient CO₂RR catalysts are essential to ensure that CO₂-to-CO conversion predominantly occurred, with all supplied CO₂ being consumed before HER initiates. Fig. S3 (ESI†) illustrates the CO and H₂ selectivities and the residual CO₂ fraction in the product gas at varying CO₂ flow rates. As shown in the figure, CO₂RR predominantly occurs over HER on the Au₂₅ and Ag₁₄ NCs, achieving CO selectivity greater than 90% at high flow rates (30–200 mL min⁻¹). At these flow rates, only a portion of the supplied CO₂ was converted into CO, leaving residual CO₂ to constitute over 60% of the product gas. CO selectivity gradually decreased, while H₂ production increased as the CO₂ flow rate was reduced below 15 mL min⁻¹ (Fig. 2b). These results indicate that CO₂-to-CO conversion still occurs preferentially over HER even at low CO₂ flow rates, enabling control over syngas composition (i.e., H₂/CO ratio) by varying the CO₂ flow rate. Notably, residual CO₂ fraction dramatically decreased to approximately zero at CO₂ flow rates below 15 mL min⁻¹.

Assuming the theoretical SPCE of 50% in alkaline CO₂-to-CO electroreduction,⁵⁸ the maximum j_CO and CO selectivity can be calculated based on the CO₂ flow rate (see the Supplementary Notes and Table S1 in the ESI†). For instance, CO selectivities of 25, 50, and 75% are expected at CO₂ flow rates of 3.75, 7.5, and 11.25 mL min⁻¹, respectively, at a total current density of 200 mA cm⁻². In Fig. S3 (ESI†), Au₂₅ and Ag₁₄ NCs achieve CO selectivities of 22 and 23%, respectively, which are close to the theoretical limit of 25% at a CO₂ flow rate of 3.75 mL min⁻¹. Actual SPCE values, calculated from these CO selectivities, were 45 and 46%, respectively, approaching the theoretical limit of 50% and significantly exceeding that of Ag NPs, which exhibited an SPCE of 27%. SPCE values obtained from the three catalysts at CO₂ flow rates ranging from 3.75–15 mL min⁻¹ are presented in Fig. 2c. As depicted, Au₂₅ and Ag₁₄ NCs maintained SPCE values exceeding 43% across all flow rates. This result demonstrates that undesired CO₂ consumption due to HER can be effectively mitigated by employing highly efficient CO₂RR catalysts under controlled CO₂ supply conditions.

To understand the origin of the high SPCE observed for NCs during syngas production, an electrokinetic study was conducted in a kinetically controlled regime. A CO₂-fed flow electrolyzer was employed to monitor the cathode reaction (Fig. S4a, ESI†). As shown in Fig. S4 (ESI†), the Tafel slopes for Au₂₅ and Ag₁₄ NCs, obtained from a plot of log(j_CO) versus potential, were determined to be 40.7 and 42.0 mV dec⁻¹, respectively. Ag NPs exhibited a significantly higher Tafel slope of 131 mV dec⁻¹. This result strongly indicates that the mechanism of CO₂-to-CO electroreduction on the NCs is distinctly different from that of Ag NPs. CO₂-to-CO conversion under alkaline media can be described by the following elemental steps, where M* denotes the active site:

M* + CO₂ + e⁻ → M*–CO₂⁻ (4)

M*–CO₂⁻ + H₂O → M*–COOH + OH⁻ (5)

M*–COOH + e⁻ → M*–COOH⁻ (6)

M*–COOH⁻ → M*–CO + OH⁻ (7)

M*–CO → M* + CO (8)

Tafel slopes of 120 and 40 mV dec⁻¹ indicate that the reaction is governed by the first [eqn (4)] and second [eqn (6)] electron transfer steps, respectively.^11,12,59,60 Since the proton transfer step could also play a role in the rate-determining step (RDS) of CO₂RR, a kinetic isotope effect (KIE) study using H/D was performed on the NC catalysts. Fig. S5 (ESI†) presents the j_CO values obtained from Au₂₅ and Ag₁₄ NCs as a function of applied potential in H₂O- and D₂O-based 1.0 M KOH solutions. Both NCs display approximately identical j_CO curves regardless of electrolyte condition, suggesting that the proton transfer step is not involved in the RDS for these NCs. Collectively, significantly low Tafel slopes observed for the NCs imply that the first electron transfer step is greatly facilitated on these NCs, while it constitutes the RDS for CO₂RR on Ag NPs.

To gain further mechanistic insights into CO₂RR on these NCs, we investigated the reaction order with respect to the CO₂ concentration. Fig. 2d shows the j_CO values measured at a cell potential of 1.8 V (iR-corrected) as a function of the partial pressure of CO₂ (P_CO2) ranging from 0.1–1.0 atm. As shown in the figure, the j_COversus P_CO2 plots for Au₂₅ and Ag₁₄ NCs exhibit concave curves, while the plot for Ag NPs is approximately linear. Concave curves observed for Au₂₅ and Ag₁₄ NCs are particularly notable, as they suggest exceptional CO₂RR activities even at low P_CO2. This finding has practical implications for the direct conversion of diluted CO₂ without needing a concentration step.⁶¹

Concave curves can be analyzed using a modified Michaelis–Menten kinetic model.^25,61 In this model, the five elemental steps (eqn (4)–(8)) for CO₂-to-CO electroreduction are reduced to a two-step process. This process consists of a CO₂ binding step, forming an intermediate complex (M*–CO₂⁻), followed by a catalytic step that generates the CO product (eqn (9) and (10)):

When the concentration of active sites is much lower than that of CO₂, a steady-state condition is rapidly established. In this state, the concentration of the M*–CO₂⁻ complex remains approximately constant over the timescale of product formation. The catalysis rate (j_CO) is then given by:

where represents the CO₂ binding affinity constant of the active site, k^o_cat is the standard rate constant, β is the symmetry factor, η is the overpotential, and other symbols are as commonly defined (see the Supplementary Notes for further details, ESI†).

Fitting the j_CO–P_CO2 plots in Fig. 2d with eqn (11) reveals that the P_CO2 dependence of j_CO is governed by K, the CO₂ binding affinity of the catalyst. Specifically, the j_CO–P_CO2 plot becomes concave when the catalyst exhibits a high CO₂ binding affinity (i.e., K ≫ 1). Conversely, the plot appears linear when the interaction between the catalyst and CO₂ is weak (i.e., K ≪ 1). The high K values of 5.74 and 4.00 atm⁻¹, determined for Au₂₅ and Ag₁₄, respectively, explicitly indicate strong interactions between CO₂ and the catalyst, which underlie the exceptional CO₂RR activity and high SPCE. By contrast, the K value for Ag NPs is only 0.22 atm⁻¹, which accounts for the low SPCE observed in Fig. 2c.

In recent CO₂RR studies, Seong et al. demonstrated that Au₂₅ and Ag₁₄ NCs undergo electrochemical activation by losing some ligands, exposing de-ligated metal sites that serve as active sites.^25,55 Density functional theory (DFT) studies on Au₂₅ and Ag₁₄ NCs further revealed that the upshifted energy of the d-state in the Au and Ag active sites provides an optimal binding strength for CO₂ intermediates, leading to exceptional catalytic activity in the CO₂-to-CO conversion. CO₂ binding affinities determined for Au₂₅ and Ag₁₄ NCs demonstrate that they are sufficiently high to form stable intermediates, essential for the CO₂-to-CO conversion process. The high SPCE observed for Au₂₅ and Ag₁₄ NCs can be attributed to the unique CO₂-binding properties of the NCs, which enable high CO₂RR activity even under CO₂-deficient conditions.

To further assess syngas production under industrially relevant current densities, electrolysis was performed at current densities exceeding 200 mA cm⁻². Target CO selectivity was set at 25%, and the CO₂ flow rate was adjusted to achieve the predetermined j_CO at each current density. Fig. 3a and b illustrate the CO selectivity and corresponding SPCE values for three catalysts across a current density range of 200–800 mA cm⁻². As depicted in the figures, the CO selectivity of Au₂₅ decreased significantly, from 22 to 15%, as the current density increased from 200 to 800 mA cm⁻² (i.e., with increasing overpotential). In contrast, the CO selectivity of Ag₁₄ remained steady, ranging from 21 to 23%, irrespective of the current density. For Ag NPs, CO selectivity was 13% at 200 mA cm⁻² and dropped below 7% at current densities over 400 mA cm⁻². Calculated SPCE values exhibited a similar trend. The SPCE of Au₂₅ declined substantially, from 45 to 30%, as the current density increased, whereas the SPCE of Ag₁₄ remained relatively constant, ranging from 42 to 47%, near the theoretical limit for CO₂-to-CO conversion in alkaline media. Meanwhile, the SPCE of Ag NPs decreased from 28% at 200 mA cm⁻² to below 20% when the current density surpassed 400 mA cm⁻².

Fig. 3 (a) CO selectivities (b) SPCE obtained from Au₂₅ and Ag₁₄ NCs, and Ag NPs at a total current density range of 200–800 mA cm⁻². Fractions of CO₂ utilized during syngas production on (c) Au₂₅ and (d) Ag₁₄ NCs. CO + CO₃²⁻ (CO₂RR) represents the fraction of CO₂ consumed for CO production and CO₃²⁻ formation, whereas CO₃²⁻ (HER) denotes the CO₂ captured in CO₃²⁻ formation from the HER byproduct.

To understand the origin of the SPCE decrease, we analyzed the fraction of CO₂ utilized during electrolysis. Considering the theoretical limit of SPCE (50%) for CO₂-to-CO conversion in alkaline media, the SPCE of 45% observed for Au₂₅ at 200 mA cm⁻² indicates that most of the available CO₂ is consumed in the CO₂RR to produce CO. As the current density increased, SPCE decreased significantly, suggesting a decline in the fraction of CO₂ undergoing CO₂RR. Interestingly, unreacted CO₂ remained negligible across all current densities (Fig. 3c).

As shown in eqn (2), HER also generates OH⁻ ions, which can react with CO₂ to form carbonate ions. Therefore, CO₂ loss due to HER must be considered. Fig. 3c demonstrates that CO₂ capture into carbonate ions increased substantially for Au₂₅ as HER activity intensified at higher current densities. Fractional analysis revealed that, for Ag NPs, there is not only carbonate formation from HER but also unreacted CO₂ (Fig. S6, ESI†). Conversely, carbonate formation due to HER remained low (Fig. 3d) for Ag₁₄, resulting in a consistently high SPCE of 42–47% across the current density range. This analysis underscores the importance of achieving superior CO selectivity over HER to maintain high SPCE for syngas production at elevated current densities.

The exceptional stability of Ag₁₄ in sustaining CO selectivity and SPCE at elevated current densities establishes it as a promising candidate for industrial syngas production. To further evaluate its performance, we examined the long-term stability of syngas production at 400 mA cm⁻² in a zero-gap electrolyzer with a flowing 1.0 M KOH electrolyte. CO₂ flow rate was adjusted to 8.7 mL min⁻¹ to achieve a syngas ratio (H₂/CO) of 3. As shown in Fig. 4a, Ag₁₄ NCs demonstrated excellent electrocatalytic stability, maintaining a cell potential of 2.7 V. CO selectivity was sustained at 25% over 24 h of operation, with the average SPCE value remaining consistent at 45 ± 2% (Fig. 4b). It is well-documented that alkaline CO₂RR in zero-gap electrolyzer faces challenges related to salt precipitation, which limits long-term operation at high current densities.⁶² However, the syngas production approach proposed in this study utilizes an equivalent amount of CO₂ to achieve the desired ratio, effectively preventing salt precipitation over 24 h of operation (Fig. S7a, ESI†). Conversely, supplying 200 mL min⁻¹ of CO₂ at the same current density led to massive salt precipitation within 1 h (Fig. S7b, ESI†). These results demonstrate that highly pure syngas with the desired H₂/CO ratio can be efficiently produced by employing highly CO-selective Ag₁₄ NCs and controlling the CO₂ supply.

Fig. 4 (a) Gas fractions and cell potentials, and (b) corresponding SPCE values recorded during electrochemical syngas production on Ag₁₄ NCs for 24 h at 400 mA cm⁻². A 1.0 M KOH (3.0 mL min⁻¹) and CO₂ gas (8.7 mL min⁻¹) were supplied to the anode and cathode sides of the zero-gap electrolyzer, respectively.

High SPCE syngas production on Ag₁₄ motivated us to explore CH₄ production by directly integrating the zero-gap electrolyzer into a thermocatalysis process, as illustrated in Fig. 5a. Ni-based catalysts are widely used for CH₄ production owing to their high conversion efficiency and selectivity at low temperatures of 250–400 °C.^50,63 Therefore, we employed a Ni-based Si/Al/Mg zeolite (referred to as Ni-zeolite hereafter) as the thermocatalyst. The catalyst was synthesized via the co-precipitation method following established literature protocols (see ESI† for details).⁶⁴ Fig. S8a (ESI†) presents high-resolution TEM results and energy dispersive X-ray spectroscopy (EDS) map images of the Ni-zeolite catalyst. As shown in the figure, Ni nanoparticles are uniformly deposited on the Si/Al/Mg-based zeolite support. Quantitative analysis revealed that the Ni-zeolite catalyst comprises 52 wt% Ni/NiO, 34 wt% Al₂O₃, 6 wt% SiO₂, and 7 wt% MgO. X-ray diffraction (XRD) patterns of the catalyst in Fig. S8b (ESI†) exhibit characteristic peaks for metallic Ni and NiO, confirming that the synthesized Ni nanoparticles possess a mixed valence state. Additionally, polymorphic SiO₂ chabazite was prominently observed in the XRD patterns.⁶⁵

Fig. 5 (a) A schematic of the syngas-to-CH₄ conversion in a thermal reactor with an inner diameter of 11.2 mm and a length of 35 cm, packed with Ni-zeolite catalyst particles. The desired syngas ratio (H₂/CO = 3) was achieved by adjusting CO₂ flow rates at each current density, and the thermocatalysis reactor temperature was maintained at 300 °C throughout the experiment. (b) Fractions of the product gas obtained from the cascade electro-thermocatalysis system. (c) Comparison of CH₄ purity in the product gas from the cascade system with data from direct electrochemical CO₂-to-CH₄ conversion reported in prior studies.^37–42 (d) Cell potentials of the electrolyzer and (e) carbon conversion efficiency obtained from the terminal gas stream of the cascade system. (f) Long-term stability of methane production using the cascade system. Product gas selectivity and cell potential were recorded for 10 h at 400 mA cm⁻². The asterisk mark at the 2 h mark indicates when the connection between the electrolyzer and thermal reactor was temporarily disconnected and reconnected to assess back-pressure issues.

As a syngas ratio of H₂/CO = 3 is required to produce highly pure CH₄, the CO₂ flow rates were adjusted to achieve 25% CO selectivity at the applied current densities. Fig. S9 (ESI†) shows the product gas selectivities at various current densities obtained from the Ag₁₄/GDE-equipped electrolyzer. As shown in the figure, the product gas consistently contained 25.5 and 74.5% CO and H₂, respectively, across all current densities, indicating that the desired syngas ratio was successfully achieved by regulating CO₂ flow rates. Notably, the residual CO₂ in the product gas was less than 1% at all current densities. The produced syngas was then directly injected into the thermocatalysis reactor, where the reactor bed temperature was set to 300 °C. This temperature demonstrated the highest conversion efficiency among the temperatures investigated (Fig. S10, ESI†).

Fig. 5b presents the product gas selectivities of the cascade electro-thermocatalysis system across a current range of 200–800 mA cm⁻². Results clearly show that the electrochemically produced syngas was directly converted into high-purity CH₄, achieving a CH₄ concentration exceeding 85%. Notably, almost all CO in the syngas was successfully converted to CH₄. The presence of approximately 1% residual CO₂ is attributed to the thermodynamic equilibrium of the syngas-to-CH₄ conversion at 1 atm.⁵⁰ However, the product gas contained slightly more H₂ than theoretically expected (<5%). Based on the product concentration, the initial CO selectivity was calculated to be 24.5%, which is 1% lower than the syngas production results shown in Fig. S9 (ESI†). This discrepancy is attributed to the back pressure from the methanation reactor, which caused additional CO₂ loss and a subsequent reduction in CO selectivity in the electrolyzer due to CO₂ dissolution into the electrolyte. We believe that managing the pressure balance between the electrolyzer and the thermal reactor in large-scale experiments could mitigate this issue, thereby improving CH₄ purity further.

Despite advancements in selective CO₂-to-CH₄ conversion electrocatalysts, achieving high product gas purity remains challenging owing to the excess amount of unreacted CO₂.^37–42Fig. 5c and Table S2 (ESI†) compare the CH₄ purity of the product gas achieved using the cascade electro-thermocatalysis system with that of other electrochemical systems. As shown in the figure, the CH₄ purity and current density achieved by the cascade system are significantly higher than those obtained through direct CO₂-to-CH₄ electrocatalysis.^37–42 These results strongly suggest that coupling electrochemical syngas production with a thermal methanation process is a highly feasible approach for producing high-purity CH₄.

Furthermore, the Ag₁₄-based cascade electro-thermocatalysis system demonstrated outstanding syngas production performance across all current densities. At total current densities of 200 and 800 mA cm⁻², the cell potential reached 2.5 and 3.1 V, respectively, which are significantly lower than those reported for other catalysts used in CH₄ production (Fig. S11, ESI†). Given that CO₂-to-CH₄ electroreduction typically requires high overpotentials,^{37–42,66,67} the production of CO at significantly lower overpotentials offers a distinct advantage of the cascade system (Table S2, ESI†). Additionally, since methanation is an exothermic reaction, the energy input required for the thermocatalysis process during operation would be minimal, and the waste heat generated could be effectively utilized for supplementary processes, such as power generation.⁶⁸

Based on the CO and CH₄ selectivities obtained from the electrolyzer and thermal reactor, carbon conversion efficiency of the entire cascade system was calculated. Combined with the near-theoretical SPCE from electrolysis and the exceptional CO conversion efficiency during methanation, overall carbon conversion efficiency reached 45% at 200 mA cm⁻² and slightly decreased to 41% at 800 mA cm⁻² (Fig. 5e) owing to enhanced HER at higher current densities. Notably, the cascade system surpasses the theoretical limit of CO₂-to-CH₄ electroreduction in alkaline media (20%), where the remaining 80% of CO₂ is typically captured as carbonate ions. Notably, the cascade electro-thermocatalysis system can be readily adapted for multi-carbon product generation, which often faces challenges of low carbon conversion efficiency owing to poor product selectivity and extensive carbonate formation.

Finally, long-term stability of the cascade system was evaluated by monitoring E_cell and product selectivity at a current density of 400 mA cm⁻². As presented in Fig. 5f, the cascade system exhibited reasonable stability, maintaining an E_cell of 2.7 V during 10 h of operation. The gradual decline in CH₄ selectivity is thought to be due to the back pressure of the methanation reactor, which caused a reduction in CO selectivity. In fact, the CH₄ selectivity was recovered to 87% after disconnecting and reconnecting the electrolyzer and thermal reactor at a reaction time of 2 h, confirming the back-pressure issue in the system. Nevertheless, CH₄ purity was maintained above 78%, with negligible residual CO₂ during the 10-h operation. This surpasses the CH₄ purity achieved through direct CO₂-to-CH₄ electrocatalysis. These results demonstrate that the cascade electro-thermocatalysis system offers highly selective and stable CH₄ production with superior carbon conversion efficiency.

Conclusions

The efficient cascade electro-thermocatalysis system for high-purity CH₄ production from CO₂ was successfully demonstrated. Electrochemical syngas production was conducted using CO₂RR-active electrocatalysts, specifically Au₂₅ and Ag₁₄ NCs, and Ag NPs, by regulating CO₂ flow rates. Au₂₅ and Ag₁₄ NCs exhibited near-theoretical SPCE during syngas production at low CO₂ flow rates, whereas Ag NPs displayed low SPCE with unreacted CO₂ present in the product gas. Electrokinetic analyses performed on these catalysts revealed that the high CO₂RR activities of Au₂₅ and Ag₁₄ NCs under CO₂-deficient conditions originated from their high CO₂ binding affinities, which facilitate the otherwise sluggish first electron transfer step. Syngas production at higher current densities exceeding 200 mA cm⁻² revealed that, unlike Au₂₅, CO selectivity for Ag₁₄ remained high across the current density range of 200–800 mA cm⁻², demonstrating that exceptional CO selectivity is required to achieve high SPCE at elevated current densities. The Ag₁₄-equipped electrolyzer, integrated with a Ni-based thermocatalysis reactor, achieved CH₄ production with a purity exceeding 85% across all current densities. These results highlight the potential of integrating electrochemical and thermocatalytic processes for high-purity methane production and open avenues for value-added hydrocarbon production.

Author contributions

S. M. H., J. K., and D. L. designed the project. S. M. H., M. P., and S. K. conducted catalyst synthesis and electrochemical experiments. S. M. H., S. K., and C. J. conducted thermocatalysis experiments. J. K. and D. L. supervised the project. S. M. H. and D. L. wrote and revised the manuscript. All authors discussed the results and provided comments on the manuscript at all stages.

Data availability

The data supporting this article have been included as part of the ESI.†

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF) grants (no. NRF-2022R1A2C3003610, RS-2024-00359914) and the Carbon-to-X Project (Project no. 2020M3H7A1096388) through the NRF funded by the Ministry of Science and ICT, Republic of Korea. This work was supported in part by the Yonsei University Research Fund (Post Doc. Researcher Supporting Program) of 2024 (project no.: 2024-12-0026).

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental methods, supplementary notes, photographic images, electrochemical data, thermal methanation data, TEM and corresponding EDS images, UV-Vis absorption spectra, cell configurations (Fig. S1–S11), and supplementary tables (Tables S1 and S2). See DOI: https://doi.org/10.1039/d5ey00094g

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