Insensitive cation effect on single-atom Ni catalyst allows selective electrochemical conversion of captured CO2 in universal media

doi:10.21203/rs.3.rs-1310811/v1

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Insensitive cation effect on single-atom Ni catalyst allows selective electrochemical conversion of captured CO2 in universal media

https://doi.org/10.21203/rs.3.rs-1310811/v1

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The direct electroconversion of captured CO2 is attracting attention as an alternative to the current energy-demanding CO2 separation processes. In conventional capturing media, the reaction inevitably takes place in the presence of bulky ammonium, leading to steric hindrance and low CO selectivity. Here, for the first time, we present a single atom Ni catalyst (Ni–N/C) exhibits superior activity for the electroconversion of captured CO2, without the need for additives. In a CO2-captured monoethanolamine-based electrolyte, Ni–N/C achieves a notably high CO selectivity of 64.9% at −50 mA cm−2 integrated with a membrane electrode assembly. We also propose that Ni–N/C demonstrates weak cation sensitivity to the CO2 reduction reaction, maintaining high CO production activity in various capturing solutions, while Ag shows a gradual decrease depending on the bulkiness of the amine. These trends provide insights into selective catalyst design for the electroconversion of captured CO2 in universal media.

Severe climate changes caused by anthropogenic carbon emissions have steered a global consensus toward carbon neutrality.^1–3 Accordingly, the development of efficient carbon capture, utilization, and storage (CCUS) technology has been recognized as one of the most urgent technologies to sustainable societies.^4,5CO₂ from exhaust gases can be selectively captured using a basic solvent to which CO₂ can be readily bound as carbamate or bicarbonate, and the captured CO₂ is separated either by thermal or pressure swings for utilization.^6,7 However, the regeneration processes are energy-intensive, imposing a huge penalty on the operating cost of the overall CCUS processes.^8,9 Moreover, during the high-temperature stripping operation, amine-based capturing media suffer from oxidative and thermal degradation.^10,11To overcome these drawbacks, the direct utilization of captured CO₂, which omits the CO₂ separation step, has been evaluated to enhance the feasibility of CCUS.^12–15

The direct electroconversion via the captured-CO₂ reduction reaction (cCO₂RR) has been proposed utilizing the CO₂ absorption media as the electrolyte (Fig. 1a).^16–27 However, inferior selectivity toward CO or formate has been exhibited compared with that of the conventional CO₂-fed electrochemical reduction reaction (CO₂RR).²⁸ The electrocatalytic activity is determined by the charge transfer at the electrode–electrolyte interface, which necessitates the thorough investigation of cCO₂RR in terms of the interactions with the absorbent medium to improve the conversion activity. Yet, it is still in the early stages of research, and the previous studies have focused either on the fundamental catalyst material itself or the electrolyzer configuration. The low CO production from electroreduction of the bicarbonate has recently been tackled by using a bipolar membranes (BPM) to develop a proton-rich environment in the vicinity of the cathode in a membrane electrode assembly. The acidified environment helps the chemical equilibrium shift in favor of CO₂ by the following reaction (Equation (1)):

This increases the local CO₂ concentration near the cathode and promotes CO production. Such a strategy, however, has been demonstrated in the bicarbonate electrolyte.

The industrial-standard CO₂ capturing medium is a 30 wt.% monoethanolamine (MEA) aqueous solution that has a higher CO₂ absorption capacity and kinetics.²⁴ Unfortunately, the selective cCO₂RR has been more challenging in MEA-based electrolytes, in which CO₂ forms bulky ions, e.g.,

The bulky cations can construct an electric double layer (EDL) at the cathode surface, which hinders the cCO₂RR.^26,29 It has been proposed that alkali metal salts, which have a smaller effective cation radius and allow better access to captured CO₂ on the catalyst surface, are required to achieve the CO production in the cCO₂RR.²⁶ However, in pure amine-based absorbent media with no metal salt additives, poor CO production has still been observed (Supplementary Table 1) and further research is required to understand this activity. Furthermore, in the practical CO₂ capture process, a wide range of amine-based solvent formulations has been exploited to enhance the CO₂ absorption or regeneration rate.³⁰ Therefore, understanding universal performance in diverse capturing media is important in bringing the immediate applications of the cCO₂RR to industry.

To attain high CO selectivity in practical CO₂ capturing media, it is also necessary to find active electrode materials and investigate its interaction with the electrolyte. Thus far, only metal catalysts have been utilized for the cCO₂RR (Supplementary Table 1). In the last decade, single atom catalysts (SACs), where metal atoms are fully dispersed on supports, have been explosively studied and an unprecedented catalytic reactivity has been exhibited for various catalytic reactions.³¹ Single atom Ni catalysts embedded in N-doped carbon (Ni–N/C) have shown particularly impressive CO₂RR activity and selectivity toward CO compared to metal catalysts in suppressing the hydrogen evolution reaction (HER).^32,33 Moreover, the dominant carbon matrix of Ni–N/C would manifest the differing potential of zero charge (PZC) with a metal electrode. Such differences can affect the formation of the EDL, which would be effective in modulating the electrode–electrolyte interface for improving the performance of the cCO₂RR. Nevertheless, SACs have never been examined for the cCO₂RR.

Here, we demonstrate that the low cation sensitivity of the Ni–N/C catalyst allows the selective cCO₂RR in universal CO₂-capturing media, even in the exclusive presence of bulky cations. In CO₂-captured MEA, the maximum CO faradaic efficiency (F. E.) of Ni–N/C (63.2%) is superior to that of commercial Ag nanoparticles (cAg, 30.1%), and the maximum CO partial current density (j_CO) of Ni–N/C (−3.5 mA cm⁻²) is three times higher than that of cAg (−0.9 mA cm⁻²) in the H-cell. In the zero-gap membrane electrode assembly configuration, the CO F. E. of Ni–N/C (64.9%) is 2.5 times higher than that of cAg (25.5%) at −50 mA cm⁻², which is the highest performance ever reported in CO₂-captured MEA solution without additives. We further report that Ni–N/C can maintain the CO F. E. at as much as 50%, even in bulkier amine solutions, in contrast to cAg, where the CO F. E. gradually falls to 0% with increasing amine bulkiness. Through a cation control experiment, we identify that the reactivity of Ni–N/C for the CO₂RR is less influenced by the size of the cation compared with that of cAg. We propose that a high surface charge density induced by the positive-shifted PZC of Ni–N/C allows for a marginal cation effect, which enables the maintenance of high CO selectivity in bulky amine-based electrolytes, in addition to the excellent suppression of HER of Ni-N/C. To the best of our knowledge, this is the first report of a catalytic approach relieving the cation effect, which permits the universal electroconversion of captured CO₂.

Preparation and characterization of Ni–N/C. The Ni–N/C catalyst was prepared by the thermal conversion of zeolitic imidazolate framework (ZIF)-8, with some modifications to previous work.³⁴ ZIF-8 was synthesized through the self-assembly of Zn²⁺ and imidazolate with a particle size of ~100 nm (Supplementary Fig. 1); Zn was replaced with Ni by ion-exchange. The resulting light-green powder was then carbonized at 1000 °C to produce a final product, Ni–N/C. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of Ni–N/C displayed rather contracted morphologies compared to ZIF-8 after high-temperature treatment (Supplementary Fig. 2).³⁵ From high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images of Ni–N/C (Fig. 1b and Supplementary Figs. 2c and d), bright dots can be observed on the support, indicating the generation of atomically dispersed Ni sites embedded on the N-doped carbon matrix without any agglomerated metal particles (Supplementary Fig. 3). Extended X-ray absorption fine structure (EXAFS) analysis confirms the structural information seen in the microscopy images (Fig. 1c). The Ni K-edge k³-weighted extended EXAFS spectrum of Ni foil shows a major scattering peak at ~2.0 Å at reduced distances, which describes the Ni–Ni bond of metallic Ni. The EXAFS spectrum of Ni–N/C exhibits a Ni–N scattering peak at ~1.5 Å at reduced distances without the presence of an Ni–Ni scattering peak, from which the isolation of Ni atoms bound to the N-doped carbon support can be concluded.

The Ni 2p X-ray photoelectron spectroscopy (XPS) of Ni–N/C shows a peak around 855 eV (Fig. 1d), corresponding to Ni(II) phthalocyanine (NiPc), and indicates the ionic character of Ni in Ni–N/C. X-ray absorption near edge structure (XANES) spectra of Ni–N/C and NiPc also show similar edge positions (Fig. 1e), confirming the similar oxidation state (Ni²⁺). In addition, the pre- and post-edge peaks in XANES spectra display distorted Ni–N/C coordination environments. The peak labeled A is usually attributed to square-planar structures such as NiPc, absent in Ni–N/C.^32,36 The higher intensity of peak B compared to that of peak C in the XANES spectrum of Ni–N/C can be explained by the distortion of D_4h symmetry, owing to the displacement of the metal centre,³⁷ and is potentially induced by high-temperature pyrolysis.

Electrochemical cCO₂RR reactivity of catalysts. The catalytic properties of Ni–N/C and cAg for the cCO₂RR were investigated in a CO₂-absorbed 5 M MEA aqueous solution. The electrocatalytic conversion was performed in an H-cell after purging with Ar for 10 min to remove unabsorbed CO₂ in the MEA solution. Ni–N/C was found to exhibit superior performance compared to cAg both in the j_CO and product selectivity. Ni–N/C has a maximum j_CO of −3.5 mA cm⁻², three times larger than cAg (−0.9 mA cm⁻², Fig. 1f). Furthermore, it shows a maximum CO F. E. (63.2%) of approximately double that of cAg (30.1%) with suppressed HER selectivity (Fig. 1g and Supplementary Fig. 4). Notably, the CO F. E. of Ni–N/C is the highest value ever reported for the cCO₂RR in CO₂-adsorbed pure MEA electrolyte without CO₂ flow (Supplementary Table 1). In addition, the on-set potential of Ni–N/C (−0.45 V vs. RHE) is lower than that of cAg (−0.60 V vs. RHE), which demonstrates the excellent catalytic activity of Ni–N/C for the cCO₂RR to CO.

To examine the effect of metal centers on SACs, a number of different metal-based SACs were prepared. Mn-, Fe-, Co-, and Cu-based SACs can be obtained by controlling metal precursors during the ion-exchange step (Supplementary Fig. 5). In contrast with Ni, other metal SACs show almost zero j_CO with high H₂ selectivity in the cCO₂RR(Supplementary Fig. 6). On N-doped carbon (N/C) prepared without ion-exchange, the HER only takes place in high overpotential regions. These results demonstrate that the Ni center plays a key role in the selective cCO₂RR to CO. The changes in the oxidation state and structure of Ni–N/C during the cCO₂RR were further investigated through an in situ XANES analysis. From the in situ XANES spectra, significant changes in the structure of Ni–N/C during the cCO₂RRwere not observed (Supplementary Fig. 7), illustrating the catalytic importance of the distorted coordination structure of single atom Ni catalyst, which allows for excellent catalytic performance in the cCO₂RR without electrolyte-modulating additives.

Practical applicability test using a zero-gap membrane electrode assembly. To evaluate the practical applicability of the cCO₂RR in high current density, an electrochemical characterization was additionally performed with a zero-gap membrane electrode assembly for the cCO₂RR in CO₂-captured 5 M MEA. BPM was used to drive the regeneration reaction of carbamate (RNHCOO⁻ + H⁺ → RNH₂ + CO₂), to increase the local concentration of CO₂ near the cathode and thus enhance conversion activity (Fig. 2a). At −50 mA cm⁻², the CO F. E. of Ni–N/C was 64.9% (Fig. 2b and Supplementary Fig. 8), which is approximately 2.5 times higher than that of cAg (25.5%), showing a similar trend to H-cell experiments and demonstrating the superior catalytic performance of Ni–N/C for the cCO₂RR in high current density. When an anion exchange membrane (AEM), the alkaline properties of which favor gas-fed CO₂ reduction reaction, is used, significantly lowered cCO₂RR selectivity has been observed for both Ni–N/C and cAg (Fig. 2b and Supplementary Fig. 8). This could be because AEM is inferior to proton transfer toward the cathode side. From these results, we propose that building a high concentration of liberated CO₂ is essential for the cCO₂RR at high current density of membrane electrode assembly, consistent with previous reports in bicarbonate solutions.¹⁸ The activity difference in the usage of BPM with AEM provides information on the initial stage of the reaction pathway of cCO₂RR, which will be discussed in the following section.

The cCO₂RR was then performed in 5 M MEA using Ni–N/C in the membrane electrode assembly with chronopotentiometry at −50 mA cm⁻² for 10 h. Since the reaction was carried out under Ar, the chemical equilibrium of adsorption/desorption of CO₂ in MEA (Equation (2)) continuously moves to favor desorption of CO₂. Hence, the number of carbamate anions and the desorption rate of CO₂ were gradually decreased over time, which caused a progressive drop in the CO F. E. after 10 h (Supplementary Fig. 9). Nevertheless, after the MEA solution was refreshed by CO₂ adsorption, the CO F. E. was recovered to ~50% under Ar conditions. This result demonstrates stable Ni–N/C catalyst operation for the cCO₂RR, and the reduction in CO selectivity is caused by the diminished amount of adsorbed CO₂ in the capturing medium. An important point to note here is the stability of the capturing media. To identify electrochemical degradation or modification of MEA, analysis of the MEA solution before and after the stability test was carried out by ¹H nuclear magnetic resonance (NMR) spectroscopy (Supplementary Fig. 10).³⁸ No additional ¹H NMR peaks were observed after the stability test, indicating the stable operation of MEA during the cCO₂RR without electrochemical modifications. This result demonstrates that the cCO₂RR is a promising catalytic system that could even resolve the degradation problems of capturing media in conventional CCUS processes.

Reaction pathway of the cCO₂RR. Next, we designed experiments to understand the pathways of the cCO₂RR and investigate the effects of different catalysts. First, two scenarios can be considered at the initial stage: i) the active site of the catalyst directly attacks the carbamate anion, or ii) CO₂ is released from the MEA followed by electrocatalytic conversion by similar pathways to conventional CO₂RR (Fig. 3a). A combination of these two scenarios is also possible. Sargent et al. argued for the direct reduction of the MEA–CO₂ adduct (carbamate) through the detection of the ethanolammonium cation on the electrode surface using in situ surface-enhanced Raman spectroscopy,²⁶ whereas Goetheer et al. considered the liberated CO₂ to be the reactant in temperature-controlled experiments.²⁷ In conventional CO₂RR-to-CO, the reaction is initiated by electron injection into CO₂, resulting in the unstable CO₂⁻ intermediate, which is typically regarded as the rate-determining step (RDS).^39,40 The stabilization of this intermediate on the catalyst surface is the key to facilitating the CO₂RR. In the cCO₂RR, if the direct reduction of carbamate takes place in the initial stage of the reaction as in scenario (i) and the electron transfer step to carbamate is the RDS of the reaction, the CO production reaction rate with respect to carbamate concentration could help identify the initial stage. Thus, in CO₂-absorbed 1–5 M MEA solutions, the electrochemical conversion rate of captured CO₂ was measured with both Ni–N/C and cAg. The carbamate concentration of the respective MEA solution was determined by quantitative analysis of NMR data (Supplementary Fig. 11) using internal standards. The j_CO for both catalysts is not significantly affected by the concentration of carbamate (Supplementary Fig. 12). Hence, the reaction is considered zeroth-order with respect to carbamate (Fig. 3b), i.e., the reaction rate is independent of the concentration of carbamate for both catalysts. From these results, we speculated that the reactant was in fact the released CO₂ from carbamate rather than the carbamate itself.

To regulate the amount of CO₂ released in the H-cell, the reaction temperature was controlled. Since the regeneration reaction is endothermic,⁴¹ the rate of the reaction can be accelerated by increasing the reaction temperature. As the reaction temperature is increased from 5 °C to 40 °C, the limiting current density (j_lim.) increases for both catalysts (Ni–N/C: −0.52 mA cm⁻² (5 °C) → −1.85 mA cm⁻² (RT) → −3.5 mA cm⁻² (40 °C); cAg: 0 (5 °C) → −0.39 mA cm⁻² (RT) → −0.91 mA cm⁻² (40 °C), Figs. 3c and d). If the direct reduction of carbamate occurs in the cCO₂RR, the mass transport limit in such low current density is difficult to detect. Furthermore, the amount of carbamate in the electrolyte is reduced as the reaction temperature is increased, and thus even if the j_lim. originates from the mass diffusion of carbamate, the j_lim. has to be decreased. On the other hand, the increased amount of liberated CO₂ can improve j_lim., solidifying the proposition of liberated CO₂ being the reactant. The sharp increase in reaction rate by the BPM being used in the membrane electrode assembly experiment is consistent with this scenario. It should be noted that the decrease in j_lim. at 60 °C may originate from the drop in CO selectivity due to H₂ being rapidly generated at high temperatures (Supplementary Fig. 13). Therefore, for the effective electrochemical conversion of captured CO₂, the optimal temperature should be found, which involves the trade-off between the amount of liberated CO₂ and the reaction rate of HER.

From the experimental results discussed, it can be concluded that the superior performance of Ni–N/C cannot be attributed to the different initial reactions of cAg and Ni–N/C for the cCO₂RR. The cCO₂RR pathway appears to be comparable to the conventional CO₂RR, but with a reduced concentration of CO₂in the cCO₂RR. Thus, the high performance of Ni–N/C must originate from the high tolerance of low CO₂ concentrations in the CO₂RR because of its superior CO₂RR intrinsic activity and high activation energy for the HER, as we previously reported with low-concentration CO₂ gas experiments.³³

Universal reactivity of Ni–N/C for the cCO₂RR. Subsequently, we investigate how Ni–N/C exhibits high reactivity in the presence of bulky ammonium cations for the cCO₂RR. In order to stabilize the CO₂⁻ intermediate in the CO₂RR, cations with high surface charge density on the cathode are essential.²⁹ Thus, a smaller effective size of alkali metal cation, which creates a higher surface charge density on the cathode, is beneficial to the performance of the CO₂RR.^42–47 However, amine-based capturing media inevitably generate bulky ammonium cations in the adsorption of CO₂, which hampers the stabilization of the CO₂⁻ intermediate. However, the effect of bulky cations in the cCO₂RR has not yet been investigated. To understand the effects of bulky cations, we carried out the cCO₂RR in systems wherein the bulkiness of the capturing molecule can be controlled. The electrocatalytic performances were examined in six different CO₂ capturing media (1 M of potassium bicarbonate, MEA, 3-amino-1-propanol, 2-(methylamino)ethanol, 2-amino-2-methyl-1-propanol, and diethanolamine), (Figs. 4a–4f and Supplementary Fig. 14). The effect of the properties of absorbents on activity trends is also discussed below, with the aim of gaining insights into the activity for the cCO₂RR (Figs. 4g and h).

Both j_CO and CO F. E. were found to decrease with increasing bulkiness of capturing media for both Ni–N/C and cAg. One noticeable difference between Ni–N/C and cAg was activity sensitivity depending on cation type. The bulkiness of the cation has a significant impact on cAg, showing a dramatic loss in j_CO as the effective size of the cation increases. The j_CO of cAg fell as low as zero in diethanolamine, a secondary amine (Fig. 4h). In contrast, the j_CO of Ni–N/C is less affected by the effective cation size (Fig. 4g), and the CO F. E. of Ni–N/C was maintained at ~ 50%, irrespective of the electrolyte type (Figs. 4a–4f). From these results, we hypothesized that the marginal cation effect is manifested on Ni–N/C, and the weak cation sensitivity allows the selective cCO₂RR in universal absorbent media. We note, however, that the different types of amine electrolytes not only change the size of the cation but also alter the amount of desorbed CO₂ and viscosity of the solvent, which can render the decoupling of the cation effect difficult.

Weak cation sensitivity of Ni–N/C for the CO₂RR. To exclusively identify the difference in cation sensitivity of catalysts, we measured the activity and selectivity of the CO₂RR in CO₂-saturated 0.05 M carbonate electrolytes containing different alkali metal cations (Li⁺, Na⁺, K⁺, and Cs⁺). Similar to the amine solutions, the j_CO of Ni–N/C and cAg decreased as the effective cation radius increased from Cs⁺ to Li⁺ (Figs. 5a and b), but the cation sensitivity was clearly different in Ni–N/C compared to cAg. The maximum CO F. E. of Ni–N/C (~100 %) was maintained irrespective of the electrolyte used, whereas that of cAg was reduced to ~85 % in 0.05 M Na₂CO₃ and ~75 % in 0.05 M Li₂CO₃(Figs. 5c and d). The normalized j_CO by j_CO in the presence of Cs⁺ is higher in Ni–N/C than that in cAg, and the slope of the normalized value with respect to the effective cation radius of Ni–N/C is less steep than that of cAg (Fig. 5e). These results demonstrate that the cation sensitivity can be varied depending on the type of catalyst and that Ni–N/C shows weaker cation sensitivity compared with cAg.

The accumulated cation density at the EDL can be determined by the size of the cation, the applied potential, and the PZC of the electrode.⁴⁸ The weak cation sensitivity of Ni–N/C may arise from its relatively high PZC. In the presence of any cation, Ni–N/C exhibits a ~0.6 V higher PZC compared to cAg (Supplementary Fig. 15). This positive-shifted PZC can create an increased surface charge density for the same applied potential in the reduction reaction, which in turn may be prone to stabilizing the CO₂⁻ intermediate, thus mitigating the cation effect. The positive-shifted PZC of Ni–N/C, therefore, allows for the effective accumulation of cations to stabilize the CO₂⁻ intermediate, even in the presence of large cations such as Li⁺ or ammonium, without additives. The marginal cation effect on Ni–N/C permits the selective cCO₂RR in universal capturing media.

We have demonstrated that Ni–N/C is an effective catalyst for the direct electroconversion of captured CO₂ by amine-based absorbents. Ni–N/C showed 64.9 % of maximum CO F. E. at −50 mA cm⁻²for the cCO₂RR, making it the highest value ever reported in CO₂-captured MEA solution without CO₂ flow and additives. To understand the initial stage of the reaction in the capture media, the CO productivity was investigated by controlling the carbamate concentration, reaction temperature, and type of membrane in the membrane electrode assembly, which provided evidence for liberated CO₂ being the reactant in the cCO₂RR. We have also proposed that Ni–N/C is a universal catalyst to maintain high CO selectivity in various amine-based absorbent for the cCO₂RR. By controlling the size of cations in carbonate electrolytes, we further demonstrate that the CO₂RR performance of Ni–N/C is less affected by cation size compared to that of cAg. The weak cation sensitivity allows for the maintenance of a CO F. E of Ni–N/C of up to 50%, even in bulkier capturing media. This distinguishes it from commercial Ag, where it was found that the CO F.E. gradually decreased to 0% with increasing amine bulkiness. In addition to high suppressing ability of HER, the weak cation effect of Ni–N/C may be attributed to the high PZC of Ni–N/C, which can maintain a high surface charge density, irrespective of the size and bulkiness of cations. We envisage that these results will provide important catalytic insights into integrated CO₂ capture and electroconversion systems and the preparation of catalysts for them.

Chemicals. Zinc nitrate hexahydrate (Zn(NO₃)₂·6H₂O, ≥99.0%), 2-methylimdazole (MeIM, 99%), manganese nitrate hydrate (Mn(NO₃)₂·xH₂O, 99.99%), iron chloride (FeCl₂, 98%), copper nitrate trihydrate (Cu(NO₃)₂·3H₂O, 99-104%), 5 wt.% Nafion, ethanolamine (≥99.0%), diethanolamine (≥99.0%), potassium bicarbonate (≥99.7%), 3-amino-1-propanol (99%), 2-(methylamino)ethanol (≥98%), and 2-amino-2-methyl-1-propanol (≥95%) were purchased from Sigma–Aldrich. Methanol (99.5%), n-hexane (95%), ethanol (94.5%) and 2-propanol (99.8%) were purchased from Samchun Chemicals (Korea). Nickel nitrate hexahydrate (Ni(NO₃)₂·6H₂O, crystalline/certified) was purchased from Fisher Chemicals. Cobalt nitrate hexahydrate (Co(NO₃)·6H₂O, 99-102%), carbon paper (TGP-H-120 20%WP, Toray), commercial Ag (20-40 nm, 99.9%), and IrO₂ (Premion®, 99.99%) were purchased from Alfa Aesar. AEM (Sustanion X37-50 Grade RT) for the membrane electrode assembly was purchased by Dioxide Materials. BPM (Fumasep FBM-PK, FuMA-Tech), PtTi mesh, and gas diffusion layers (GDL, Sigracet 39 BB, SGL Carbon) were purchased from Fuel Cell Store. All chemicals were utilized without further purification.

Synthesis of ZIF-8. ZIF-8 was synthesized by a previously-reported method, with some modifications.³² After Zn(NO₃)·6H₂O (2.93 g) and MeIM (6.49 g) were each separately dissolved completely in methanol (100 mL), the solutions were mixed by magnetic stirring at room temperature (RT) for 24 h to induce the self-assembly between metal nodes and organic linkers. The resulting suspension was vacuum-filtered and washed several times with methanol. The resulting white powder was then dried overnight at 80 °C.

Synthesis of ZIF-8-derived M–N/C catalysts. To produce ZIF-8-derived single atom Ni–N/C catalysts, Zn was replaced by Ni in an ion-exchange reaction. Prior to the ion-exchange, ZIF-8 (0.100 g) was dispersed in hexane (10 mL) by sonication for 15 min. Ni(NO₃)·6H₂O solution (0.6 M, 25 μL) was added dropwise to the ZIF-8-dispersed hexane suspension under sonication. The ion-exchange reaction was carried out under vigorous stirring (1000 rpm) at RT for 3 h. The ion-exchanged ZIF-8 was separated from hexane by centrifugation (8000 rpm for 10 min). The resulting powder was dried overnight at 80 °C. The dried powder was carbonized at 1000 °C at a ramping rate of 4.9 °C min⁻¹ for 2 h under Ar flow. To prepare single atom Mn, Fe, Co, or Cu catalysts, Ni(NO₃)·6H₂O is replaced by Mn(NO₃)₂·xH₂O, FeCl₂, Co(NO₃)₂·6H₂O, and Cu(NO₃)₂·3H₂O, respectively, in the ion-exchange step.

Material characterizations. SEM images were captured using an Inspect F instrument (FEI). The TEM, HAADF-STEM, and energy dispersive spectroscopy images were obtained by TECNAI F20 G² (FEI), Titan 80-300 (FEI) equipped with a double-sided spherical aberration (Cs) corrector, and Talos F200X (FEI), respectively. XPS spectra were obtained using a Nexsa (Thermo Fisher Scientific) with a monochromated Al-Kα source. The X-ray absorption fine structure (XAFS) was analyzed at the 1D KIST–PAL beamline of the Pohang Light Source (PLS-II) in a 3.0 GeV storage ring with a ring current of 250 mA. The X-ray beam was monochromated by a Si(111) double crystal. The XAFS spectra were collected in transmission and fluorescence modes. An electrochemical in situ XAFS analysis was performed using the modified H-cell. The catalyst-loaded (1 mg cm⁻¹) carbon paper strip was attached to the window of the H-cell and sealed with Kapton tape. An Ag/AgCl (3 M KCl filling solution) reference electrode and Pt plate counter electrode were used to form the three-electrode system. The XAFS spectra were obtained in CO₂-adsorbed MEA (5 M) solution after applying a fixed potential for 10 min from the fluorescence mode with Ar gas purging. The post-treatment of the obtained spectra was performed by Demeter software.

Electrochemical characterizations. Electrochemical characterizations were performed using an IviumStat electrochemical analyzer in a gas-tight H-cell using a three-electrode system. To prepare the working electrodes, catalysts were deposited onto carbon paper (area: 0.5 cm²) using 500 μL of catalaminyst ink spray. The catalyst ink was prepared by dispersing catalysts (3 mg) in a mixture of 2-propanol (3 mL) and 5 wt.% Nafion solution (30 μL) under sonication for 30 min. Ag/AgCl and Pt plate electrodes were used for the reference and counter electrodes, respectively. To separate the catholyte and anolyte, an AEM (Selemion AMV) was used. KHCO₃ (0.5 M) was used as an anolyte. As catholytes, Ar (99.999%)-saturated monoethanolamine (1–5 M), KHCO₃ (1 M), 3-amino-1-propanol (1 M), 2-(methylamino)ethanol (1 M), 2-amino-2-methyl-1-propanol (1 M), and diethanolamine (1 M) were used after CO₂-adsorption by CO₂ (99.99%) flow for at least 1 h. The activity and selectivity of catalysts for the CO₂RR were measured by chronoamperometry, by applying fixed potentials under Ar flow, at a flow rate of 100 sccm. Prior to applying each potential, CO₂-capture and Ar-saturation steps were sequentially carried out for 10 min each. iR-compensation was conducted for each potential by determining a series resistance from electrochemical impedance spectroscopy (EIS). Potential conversion into the reversible hydrogen electrode (RHE) scale was performed using Equation (3):

All electrochemical characterizations were carried out in a water bath set to 40 °C, except in temperature-controlled experiments. Cation sensitivity was investigated in CO₂-saturated 0.05 M M₂CO₃ electrolytes (M: Li, Na, K, and Cs) at RT. Differential capacitance was measured to obtain the PZC of catalysts in 0.05 M M₂CO₃ electrolyte with a frequency of 100 mHz and a 10 mV potential amplitude. The capacitance, C, was calculated by Equation (4),

Membrane electrode assembly electrolyzer experiment. The practicability test of the direct electrochemical conversion of captured-CO₂ was performed using a custom-designed membrane electrode assembly electrolyzer with a zero-gap configuration (Supplementary Fig. 16). The anode was prepared by spraying 1.2 mL of IrO₂ ink on quadruply stacked sheets of PtTi mesh. The IrO₂ ink was prepared by dispersing IrO₂ (0.300 g) in a mixture of 2-propanol (10 mL) and 5 wt.% Nafion solution (1.2 mL) under sonication for 30 min. The cathode was assembled by spraying a catalyst ink on a GDL (area: 5 cm²). The catalyst ink was prepared by mixing catalyst (9 mg), 2-propanol (1.5 mL), and 5 wt.% Nafion solution (90 μL) under sonication for 30 min. The cathode catalyst layer, membrane, and anode were stacked with a zero-gap. For the membrane, either BPM or AEM were applied. KHCO₃ (0.5 M) and MEA (5 M) aqueous electrolytes were used as the anolyte and catholyte, respectively. The electrolytes were circulated around a flow channel of bipolar plates using a peristaltic pump at a rotating speed of 16 rpm. The catalytic properties were evaluated by chronopotentiometry under Ar flow at a flow rate of 100 sccm, after the CO₂-capture and Ar-saturation steps were sequentially carried out over 10 min. The temperature of the membrane electrode assembly electrolyzer was set to 40 °C by temperature controllers connected to the bipolar plates.

Gaseous product analysis. The gaseous products were quantified at fixed potentials or currents using gas chromatography (GC, Agilent 7890B). The gas flow outlet in the H-cell or the membrane electrode assembly electrolyzer was directly connected to the GC inlet. The gaseous products were injected using a six-port valve system. Ar (99.999%) was used as the carrier gas, and a Carboxen 1000 packed column (Supelco) was used. The number of gaseous products was quantified using a thermal conductivity detector or a flame ionization detector equipped with a methanizer depending on the concentration of CO. The F.E. of gaseous products was calculated by Equation (5):

Quantitative analysis of ¹H NMR. The concentration of carbamate was quantified by ¹H NMR (Varian 500, Agilent). To prepare a solution for NMR measurement, the MEA solutions (1–5 M) were diluted by mixing 5 μL of MEA solution with 495 μL of deionized water. Subsequently, 450 μL of the diluted solutions was added to 50 μL of D₂O in which 50 mmol of dimethyl sulfoxide (DMSO) was dissolved. The concentration of carbamate was determined by comparison with the integrated peak area between the carbamate and the DMSO internal standard.

Estimation of effective cation radius. The effective cation radius was estimated with reference to previous reports.^29,49,50 The effective cation radii of CO₂-adsorbed MEA, 3-amino-1-propanol, 2-amino-2-methyl-1-propanol, and diethanolamine were estimated from the reported effective cation radii of ethylammonium, isopropylammonium, isobutylammonium, diethylammonium, respectively. The effective cation radius of CO₂-adsorbed 2-(methylamino)ethanol was estimated by halving the sum of the effective cation radii of dimethylammonium and diethylammonium.⁴⁹ The effective cation radius of K⁺ was estimated by considering the relative effective radius between ethyl ammonium and K⁺ in the previous report.⁵⁰ The effective cation radius of other alkali metal cations (Li⁺, Na⁺, and Cs⁺)wasestimated using the relative effective radius between K⁺ and M⁺ in the previous report.²⁹

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Acknowledgements

This work was supported by the Institute for Basic Science (IBS-R006-D1) and also by Creative-Pioneering Researchers Program through Seoul National University. We also acknowledge the valuable discussion with Jong Suk Yoo (University of Seoul) to understand the cation sensitivity of Ni-N/C.

Author contributions

Y.J.H. supervised the project. Y.J.H. and J.H.K. conceived and designed the experiments and co-wrote the manuscript. J.H.K. conducted synthesis, physicochemical characterisation, and electrochemical analysis with assistance from H.J., W.C., H. Y., E.C.L., D.K., J.K., and S.Y.L.

Competing interests

The authors declare no competing interests.

Additional information

Correspondence and requests for materials should be addressed to Yun Jeong Hwang.

There is NO Competing Interest.

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Insensitive cation effect on single-atom Ni catalyst allows selective electrochemical conversion of captured CO2 in universal media

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