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

Jae Hyung Kim Seoul National University Hyunsung Jang Seoul National University Woong Choi Korea Institute of Science and Technology Hyewon Yun Seoul National University Eun Chong Lee Seoul National University Dongjin Kim Korea Institute of Science and Technology Ji Won Kim Clean Energy Research Center, Korea Institute of Science and Technology, Seoul 02792 Si Young Lee Seoul National University Yun Jeong Hwang (  yjhwang1@snu.ac.kr ) Seoul National University https://orcid.org/0000-0002-0980-1758


Introduction
To attain high CO selectivity in practical CO 2 capturing media, it is also necessary to nd active electrode materials and investigate its interaction with the electrolyte. Thus far, only metal catalysts have been utilized for the cCO 2 RR (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 CO 2 RR 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 cCO 2 RR. Nevertheless, SACs have never been examined for the cCO 2 RR.
Here, we demonstrate that the low cation sensitivity of the Ni-N/C catalyst allows the selective cCO 2 RR in universal CO 2 -capturing media, even in the exclusive presence of bulky cations. In CO 2 -captured MEA, the maximum CO faradaic e ciency (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 con guration, 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 CO 2 -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 CO 2 RR is less in uenced 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 rst report of a catalytic approach relieving the cation effect, which permits the universal electroconversion of captured CO 2 .

Results
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 modi cations to previous work. 34   was synthesized through the self-assembly of Zn 2+ 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 nal 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-eld 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 ne structure (EXAFS) analysis con rms the structural information seen in the microscopy images (Fig.   1c). The Ni K-edge k 3 -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), con rming the similar oxidation state (Ni 2+ ). 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 cCO 2 RR reactivity of catalysts. The catalytic properties of Ni-N/C and cAg for the cCO 2 RR were investigated in a CO 2absorbed 5 M MEA aqueous solution. The electrocatalytic conversion was performed in an H-cell after purging with Ar for 10 min to remove unabsorbed CO 2 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 cCO 2 RR in CO 2 -adsorbed pure MEA electrolyte without CO 2 ow (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 cCO 2 RR 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 H 2 selectivity in the cCO 2 RR (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 cCO 2 RR to CO. The changes in the oxidation state and structure of Ni-N/C during the cCO 2 RR were further investigated through an in situ XANES analysis. From the in situ XANES spectra, signi cant changes in the structure of Ni-N/C during the cCO 2 RR were 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 cCO 2 RR without electrolyte-modulating additives.
Practical applicability test using a zero-gap membrane electrode assembly. To evaluate the practical applicability of the cCO 2 RR in high current density, an electrochemical characterization was additionally performed with a zero-gap membrane electrode assembly for the cCO 2 RR in CO 2 -captured 5 M MEA. BPM was used to drive the regeneration reaction of carbamate (RNHCOO − + H + → RNH 2 + CO 2 ), to increase the local concentration of CO 2 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 cCO 2 RR in high current density. When an anion exchange membrane (AEM), the alkaline properties of which favor gas-fed CO 2 reduction reaction, is used, signi cantly lowered cCO 2 RR 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 CO 2 is essential for the cCO 2 RR 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 cCO 2 RR, which will be discussed in the following section.
The cCO 2 RR 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 CO 2 in MEA (Equation (2)) continuously moves to favor desorption of CO 2 . Hence, the number of carbamate anions and the desorption rate of CO 2 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 CO 2 adsorption, the CO F. E. was recovered to ~50% under Ar conditions. This result demonstrates stable Ni-N/C catalyst operation for the cCO 2 RR, and the reduction in CO selectivity is caused by the diminished amount of adsorbed CO 2 in the capturing medium. An important point to note here is the stability of the capturing media. To identify electrochemical degradation or modi cation of MEA, analysis of the MEA solution before and after the stability test was carried out by 1 H nuclear magnetic resonance (NMR) spectroscopy ( Supplementary Fig. 10). 38 No additional 1 H NMR peaks were observed after the stability test, indicating the stable operation of MEA during the cCO 2 RR without electrochemical modi cations. This result demonstrates that the cCO 2 RR is a promising catalytic system that could even resolve the degradation problems of capturing media in conventional CCUS processes.
Reaction pathway of the cCO 2 RR. Next, we designed experiments to understand the pathways of the cCO 2 RR 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) CO 2 is released from the MEA followed by electrocatalytic conversion by similar pathways to conventional CO 2 RR (Fig. 3a). A combination of these two scenarios is also possible. Sargent et al. argued for the direct reduction of the MEA-CO 2 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 CO 2 to be the reactant in temperature-controlled experiments. 27 In conventional CO 2 RR-to-CO, the reaction is initiated by electron injection into CO 2 , resulting in the unstable CO 2 − 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 CO 2 RR. In the cCO 2 RR, 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 CO 2 -absorbed 1-5 M MEA solutions, the electrochemical conversion rate of captured CO 2 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 signi cantly 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 CO 2 from carbamate rather than the carbamate itself.
To regulate the amount of CO 2 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 If the direct reduction of carbamate occurs in the cCO 2 RR, the mass transport limit in such low current density is di cult 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 CO 2 can improve j lim. , solidifying the proposition of liberated CO 2 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 H 2 being rapidly generated at high temperatures ( Supplementary Fig. 13). Therefore, for the effective electrochemical conversion of captured CO 2 , the optimal temperature should be found, which involves the trade-off between the amount of liberated CO 2 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 cCO 2 RR. The cCO 2 RR pathway appears to be comparable to the conventional CO 2 RR, but with a reduced concentration of CO 2 in the cCO 2 RR. Thus, the high performance of Ni-N/C must originate from the high tolerance of low CO 2 concentrations in the CO 2 RR because of its superior CO 2 RR intrinsic activity and high activation energy for the HER, as we previously reported with low-concentration CO 2 gas experiments. 33 Universal reactivity of Ni-N/C for the cCO 2 RR. Subsequently, we investigate how Ni-N/C exhibits high reactivity in the presence of bulky ammonium cations for the cCO 2 RR. In order to stabilize the CO 2 − intermediate in the CO 2 RR, 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 bene cial to the performance of the CO 2 RR. 42 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 cCO 2 RR (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 signi cant 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 cCO 2 RR 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 CO 2 and viscosity of the solvent, which can render the decoupling of the cation effect di cult.
Weak cation sensitivity of Ni-N/C for the CO 2 RR. To exclusively identify the difference in cation sensitivity of catalysts, we measured the activity and selectivity of the CO 2 RR in CO 2 -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 Na 2 CO 3 and ~75 % in 0.05 M Li 2 CO 3 (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 CO 2 − intermediate, thus

Conclusions
We have demonstrated that Ni-N/C is an effective catalyst for the direct electroconversion of captured CO 2 by amine-based absorbents. Ni-N/C showed 64.9 % of maximum CO F. E. at −50 mA cm −2 for the cCO 2 RR, making it the highest value ever reported in CO 2 -captured MEA solution without CO 2 ow 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 CO 2 being the reactant in the cCO 2 RR. We have also proposed that Ni-N/C is a universal catalyst to maintain high CO selectivity in various amine-based absorbent for the cCO 2 RR. By controlling the size of cations in carbonate electrolytes, we further demonstrate that the CO 2 RR 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 CO 2 capture and electroconversion systems and the preparation of catalysts for them. 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-ltered and washed several times with methanol. The resulting white powder was then dried overnight at 80 °C.

Methods
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 ionexchange reaction. Prior to the ion-exchange, ZIF-8 (0.100 g) was dispersed in hexane (10 mL) by sonication for 15 min. 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 G 2 (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 Scienti c) with a monochromated Al-Kα source. The X-ray absorption ne 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)  The capacitance, C, was calculated by Equation (4), Membrane electrode assembly electrolyzer experiment. The practicability test of the direct electrochemical conversion of captured-CO 2 was performed using a custom-designed membrane electrode assembly electrolyzer with a zero-gap con guration ( Supplementary Fig.  16). The anode was prepared by spraying 1. were used as the anolyte and catholyte, respectively. The electrolytes were circulated around a ow channel of bipolar plates using a peristaltic pump at a rotating speed of 16 rpm. The catalytic properties were evaluated by chronopotentiometry under Ar ow at a ow rate of 100 sccm, after the CO 2 -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 quanti ed at xed potentials or currents using gas chromatography (GC, Agilent 7890B). The gas ow 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 quanti ed using a thermal conductivity detector or a ame ionization detector equipped with a methanizer depending on the concentration of CO. The F.E. of gaseous products was calculated by Equation (5) Estimation of effective cation radius. The effective cation radius was estimated with reference to previous reports. 29,49,50 The effective cation radii of CO 2 -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 CO 2 -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 + ) was estimated using the relative effective radius between K + and M + in the previous report. 29 captured CO 2 . b, HAADF-STEM images of Ni-N/C. c, Ni K-edge k 3 -weighted EXAFS spectra of Ni-N/C and Ni foil. d,e, Ni 2p XPS spectra (d) and Ni K-edge XANES spectra (e) of Ni-N/C and NiPc. f,g, j CO (f) and CO F. E. (g) of Ni-N/C and cAg for the cCO 2 RR in CO 2adsorbed 5 M MEA solution.

Figure 2
Membrane electrode assembly test for the cCO 2 RR. a, Schematic illustration of CO 2 regeneration using BPM in the membrane electrode assembly. b, CO F. E. of Ni-N/C and cAg in chronopotentiometry using the membrane electrode assembly. The solid and dashed lines indicate the use of BPM and AEM, respectively.

Supplementary Files
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