Constructing FeN4/graphitic nitrogen atomic interface for high-efficiency electrochemical CO2 reduction over a broad potential window
Atomically dispersed Fe-N-C catalyst has shown great performance in the CO 2 -to-CO conversion, yet the high CO selectivity is achieved only within a rather narrow potential range, which cannot well meet the requirements for the following CO dimerization or hydrogenation. Here, we developed a hydrogen-pyrolysis etching strategy to precisely manipulate the uncoordinated N dopants in the Fe-N-C catalyst. This strategy could preferentially eliminate pyridinic and pyrrolic N atoms while leaving graphitic N. The resulting catalyst gave a CO faradaic efficiency (FE CO ) above 90% over a broad window from À0.3 to À0.8 V (versus RHE) (in particular, FE CO > 97% at À0.6 V). In situ ATR-SEIRAS and first-principle calculations further revealed that this strategy can not only suppress the parasitic H 2 generation but also promote CO 2 activation and protonation with the assistance of co-adsorbed H 2 O on the ''atomic interface'' of ''FeN 4 /graphitic N.'' Recently, nitrogen-doped single-atom catalysts (N-SACs) have been favored by researchers because of their cost-effective atomic utilization and unique electronic structures. 9-13 Interestingly, N-SACs have shown great catalytic performances in the formation of *CO and are expected to potentially replace the conventional precious-metal-based CO 2 RR catalysts in the near future. Among various N-SACs, Fe-N-C has shown the lowest energy pathways, 14 high proton/electron transfer rates, 15 and the lowest overpotential of $80 mV for the CO 2 -to-CO conversion.
INTRODUCTION<!>RESULTS<!>Analysis of the Fe centers and the non-metallic elements<!>CO 2 RR performances<!>DFT calculations<!>DISCUSSION<!>EXPERIMENTAL PROCEDURES<!>Materials availability<!>Data and code availability
<p>The overdependence on petrochemical energy has resulted in an excessive amount of emitted greenhouse CO 2 gas in the atmosphere. Electrochemical reduction of CO 2 (CO 2 RR) to hydrocarbon fuels is an attractive strategy to overcome some problems in both environment and energy sectors. 1,2 Currently, Cu is the only metalbased catalyst capable of transforming CO 2 into high-valued hydrocarbons with a substantial conversion. 3 However, Cu suffers from high overpotentials, low product selectivity, and other issues related to mass transport. 2 Therefore, designing tandem catalysts to produce deep reduction products is an appealing alternative solution. 4,5 Accordingly, the conversion from CO 2 to valuable hydrocarbons can be divided into two major steps 6,7 : (1) reduction of CO 2 to the adsorbed CO (*CO); (2) further hydrogenation of the adsorbed *CO into *CHO, as an intermediate in the production of CH 4 or C 2 H 4 . The latter step commonly proceeds under high cathodic potentials, typically -0.8 V (versus RHE), which marks the onset production of CH 4 from CO 2 . 8 The bigger picture</p><p>The development of low-cost, high-performance electrocatalysts for electrochemical reduction reaction of CO 2 (CO 2 RR) and investigation of the related catalytic mechanism are of crucial importance to solve the problems of both greenhouse effect and energy shortage, yet such tasks remain rather challenging. In this work, we developed a selective etching strategy to remove the undesired H 2 -producing sites in Fe-N-C and put forward a new concept of metal/non-metal ''atomic interface'' as a robust catalyst for promoting CO 2 activation and protonation with the assistance of co-adsorbed H 2 O, which greatly broadens the potential range featuring high CO 2 -to-CO selectivity. This work not only demonstrates a rational design and precise structural modulation of nitrogen-doped single-atom catalysts but also enables us to design a new tandem catalyst of Fe-N-C for the subsequent synthesis of highvalued hydrocarbons.</p><p>Chem 7, 1297-1307, May 13, 2021 ª 2021 Elsevier Inc. 1297 ll However, owing to the competing hydrogen evolution reaction (HER) over Fe-N-C catalysts, a high faradaic efficiency for CO (FE CO ) can be achieved only within a very narrow potential range (about -0.4 $ -0.5 V versus RHE), [14][15][16][17] which is not compatible with the potential regime required for the formation of *CHO intermediates. Despite extensive efforts on suppressing HER, most studies have been focused only on the metal active centers. [14][15][16][17][18] Since many researchers have found that the metal-free N-doped carbon materials have a great potential for the electrochemical reactions, it is unreasonable to ignore the large amount of N atoms doped in the Fe-N-C matrix. [19][20][21][22][23] Therefore, regulating the N-doped species may help to expand the high-FE CO potential window of Fe-N-C and to improve designing new Fe-N-C materials for the subsequent tandem catalysts, aiming at the production of highly desirable hydrocarbons.</p><p>Herein, we report a selective etching strategy to control the local nitrogen chemical states of single FeN 4 sites by adjusting the pyrolysis conditions, which can achieve the precise regulation of N species. We succeeded to selectively eliminate the undesired ''protophilic sites'' in the N-doped carbon substrate by adding 5% H 2 into the inert gas atmosphere in high-temperature pyrolysis. This approach leaves only a dominant amount of graphitic N in the final resulting material, denoted as H 2 -FeN 4 /C. In addition, the density functional theory (DFT) calculations and experiments proved that the ''protophilic sites,'' including pyridinic and pyrrolic N atoms, are beneficial to HER. Electrochemical measurements confirmed that H 2 -FeN 4 /C exhibits a high selectivity with FE CO above 90% over a very broad potential range from À0.3 to À0.8 V (in particular, FE CO > 97% at -0.6 V). The performed experiments and DFT computations revealed that our selective etching strategy here could not only suppress the parasitic HER by removing the pyridinic N and pyrrolic N but also construct an ''atomic interface'', typically between the two atoms/sites with different electronic structures and chemical identities. 24 The generated ''atomic interface'' exists between the remaining graphitic N and FeN 4 , facilitating the synergistic adsorption of H 2 O and CO 2 to promote CO 2 activation and conversion into *COOH intermediate.</p><!><p>Catalyst synthesis and morphology characterization Ferrocene (Fe(Cp) 2 ), as the Fe precursor, was encapsulated into the selected zeolite imidazolate framework (ZIF8) through a well-known method to synthesize the FeN 4 /C composites. [25][26][27][28][29][30] Fe(Cp) 2 is a classic and inexpensive metallocene that does not contain N ligands itself and is a highly stable compound. 31 During the synthesis of Fe-N-C, the excess of ferrocene that is not captured by N would be directly volatilized to avoid aggregating into nanoparticles. This strategy eliminates the typical post-processing step of acid leaching to ensure that Fe species exist only in the form of single atoms in the final catalyst. As illustrated in Figure 1A, Fe(Cp) 2 has been accommodated into the molecular cages of ZIF8 to achieve Fe@ZIF8. After pyrolysis at 900 C under an H 2 /Ar (5:95) gaseous mixture for 3 h, the Fe@ZIF8 precursor was transformed into N and Fe co-doped porous carbon framework, denoted as H 2 -FeN 4 /C. The N 2 -FeN 4 /C sample was also prepared following the same procedure, but in a pure N 2 inert atmosphere (for details, please see Supplemental experimental procedures). The powder X-ray diffraction (PXRD) patterns of all samples showed a broad shoulder in the 2q range of 20 -30 , which can be assigned to the (002) plane of the amorphous graphitic carbon. Notably, no diffraction peaks characteristic of Fe particles were observed (Figure S1). Raman spectra (Figure S2) revealed that the content of introduced N defects in H 2 -FeN 4 /C was lower than that in N 2 -FeN 4 /C, as a result of the H 2 etching. 19 From the scanning electron microscopy (SEM) images (Figures 1B and S3), together with the transmission electron microscopy (TEM) images (Figures 1C and S3), it can be seen that the as-prepared samples retained their initial high surface area and porous dodecahedral shape after thermal treatments (Figures S3-S6). Also, H 2 -FeN 4 /C has the highest hydrophilicity among all the catalysts (Figures 1C, S3, and S5). High-angle annular dark-field scanning TEM and the corresponding energy-dispersive X-ray spectroscopy (HAADF-STEM-EDS) mapping (Figures 1D, 1E, and S3) indicated that Fe, C, and N were evenly dispersed throughout the entire dodecahedron without any metallic Fe particles. The aberration-corrected HAADF-STEM image (Figures 1F and S3) showed bright dots with a high density, as recognized by neural network deep-learning arithmetic recognition (Figures 1G and S3), and the bright dots correspond to the single Fe atoms.</p><!><p>Then, the electronic structures and coordination environments of the as-prepared samples were systematically investigated by X-ray absorption spectroscopy (XAS) and Fe valence-to-core (V2C) X-ray emission spectroscopy (XES). The Fe K-edge X-ray absorption near-edge structure (XANES) spectra revealed that the valence states of Fe in H 2 -FeN 4 /C and N 2 -FeN 4 /C were around Fe(II) and the former was lower, probably because of the reduction of H 2 (Figure 2A). In the XANES Fe Article L-edge spectra (Figure S7), the L 3 peaks of H 2 -FeN 4 /C and N 2 -FeN 4 /C are split by the ligand field and show the same characteristics, indicating that both samples have Fe centers with similar valence and spin states. [32][33][34] The Fourier-transformed (FT) k 2weighted extended X-ray absorption fine structure (FT-EXAFS) spectra (Figure 2B; Table S3 with phase correction; see Figure S8 for uncorrected data) show that H 2 -FeN 4 /C exhibits a major peak at $1.80 A ˚, which is consistent with the peak of Fe-N path of FePc (iron(II) phthalocyanine) and shorter than that of Fe-C path ($1.91A ˚) of Fe(Cp) 2 . The peak of N 2 -FeN 4 /C is located at $1.83 A ˚, which is closer to that of Fe-N. Furthermore, from wavelet transform (WT) of c(k) spectra (Figure 2C), we found that the centers of [c(k), c(R)] intensity maximum for H 2 -FeN 4 /C and N 2 -FeN 4 /C are closer to that of Fe-N bond. To more clearly identify the coordination atoms (especially the adjacent element, such as the C, N, and O), V2C XES was carried out (Figure 2D). 35 In the V2C region, two signals Kb 2,5 and Kb 00 were observed. In the Fe(Cp) 2 with Fe-C bonding, the Kb 00 feature peak is located at 7,103.0 eV, which can be assigned to C 4À 2s / Fe 1s V2C transition, while for FePc with Fe-N bonding, the peak is located at 7,098.9 eV, which can be assigned to N 3À 2s / Fe 1s V2C transition, and for Fe 2 O 3 with Fe-O bonding, the peak is located at 7,097.6 eV, which can be assigned to O 2À 2s / Fe 1s V2C transition. For both the H 2 -FeN 4 /C and N 2 -FeN 4 / C, the Kb 00 peaks are located at 7,099 eV, almost the same position for the peak of FePc (with well-identified Fe-N bonding). These data strongly indicate a Fe-N coordination environment rather than Fe-C or Fe-O. Moreover, the best FT-EXAFS fitting results of R space and k space showed that the Fe centers in both catalysts were bonded by four N atoms (Figures 2E and S9), confirming that Fe atoms in both catalysts exist as atomically dispersed FeN 4 atomic sites.</p><p>We further conducted a detailed study on different chemical states of the nonmetallic elements. Inductively coupled plasma optical emission spectrometry (ICP-OES) showed that the content of N was dozens of times as high as that of Fe in both catalysts (in H 2 -FeN 4 /C: N 4.303 wt %, Fe 0.459 wt %; in N 2 -FeN 4 /C: N 13.41 wt %, Fe 0.106 wt %) (Table S1). These findings confirmed that most N doping atoms were uncoordinated. High-resolution X-ray photoelectron spectroscopy (XPS) revealed different states of C and N. The C1s spectra (Figure S10) indicated that sp 2 -hybridized graphitic carbon atoms were dominant. 21 In the C K-edge XANES spectra (Figure S11), we also found that the intensity of the C-N-C peak decreased significantly in H 2 -FeN 4 /C, owing to the loss of N atoms by H 2 etching in the pyrolysis process. The spectral profile of N atoms can be deconvoluted into five peaks (Figures 2F  and 2G), corresponding to pyridinic N (398.5 eV), N-metal (399.5 eV), 36,37 pyrrolic N (400.1 eV), graphitic N (401.1 eV), and oxidized N (403.2 eV), respectively. 22,38 The profile showed that the pyridinic N and pyrrolic N had been substantially eliminated in H 2 -FeN 4 /C (pyridinic N = 0.28 wt %, pyrrolic N= 0.83 wt %) in contrast to that of N 2 -FeN 4 /C (pyridinic N = 7.36 wt %, pyrrolic N= 1.29 wt %). In the N K-edge XANES spectrum (Figure S12), we also found that the graphitic peak ($401.2 eV) was much higher than the pyridinic peak ($398 eV) in H 2 -FeN 4 /C. All the obtained results confirmed that the pyridinic N was the dominant N species in N 2 -FeN 4 /C; whereas, graphitic N was dominant in H 2 -FeN 4 /C, as a result of the H 2 etching in the pyrolysis process.</p><!><p>The electrochemical CO 2 RR performances were then evaluated for these catalysts in a three-electrode setup using a two-compartment electrochemical cell with CO 2 -saturated 0.1 M NaHCO 3 solution (pH = 6.8). The gas products were quantified by gas chromatography. As shown in the linear sweep voltammetry (LSV) curves (Figure 3A), the H 2 -FeN 4 /C displayed the highest current density over the entire potential range. At the potential with optimal performance (À0.6 V versus RHE), H 2 -FeN 4 /C exhibited a current density of 6.87 mA$cm À2 (normalized with respect to the geometric surface area), which is 13.5 times higher than that of N 2 -FeN 4 /C (0.51 mA$cm À2 ). In contrast to the reported state-of-the-art N-doped single-atom Fe catalysts (for which the FE CO would decline rapidly after the optimal potential of about 0.4-0.5 V versus RHE), 14,15 our H 2 -FeN 4 /C demonstrated a high selectivity for CO 2 RR with FE CO > 90% over a very broad potential range from -0.3 to -0.8 V (with FE CO above 97% at -0.6 V) (Figure 3B). This feature is well compatible with the potential regime needed for the formation of *CHO intermediate. 7 Besides, the H 2 -FeN 4 /C catalyst also showed a remarkable stability, and the CO partial current density and FE CO were nearly unchanged after electrolysis for 24 h at À0.6 V (versus RHE) (Figure 3C). Furthermore, we have designed a series of comparative catalysts to rule out the possible influences of Fe or Zn content that may lead to different CO 2 RR performances (Figures S13 and S14). In the overall two-electron CO 2 RR process, at the low overpotential region, the first electron-proton transfer step for the adsorbed CO 2 to *COOH intermediate is the rate-determining step 1 (Table S2). Among our studied catalysts, H 2 -FeN 4 /C revealed the lowest Tafel slope (86 mV dec À1 ). Even H 2 -N/C (the Fe-free catalyst) had a lower Tafel slope (161 mV dec À1 ) than N 2 -FeN 4 /C (270 mV dec À1 ) (Figure 3D). The Tafel data indicated À to form the adsorbed COOH (*CO 2 À + H + / *COOH). [39][40][41] In situ ATR-SEIRAS experiments and analysis In order to gain more in-depth understanding on the structure-activity relationship between the CO 2 RR performance and catalyst structure, we employed in situ attenuated total reflectance surface enhanced infrared absorption spectroscopy (ATR-SEIRAS) for further investigation. In situ ATR-SEIRAS is a powerful tool for detecting adsorbed substances or functional groups on the catalysts' surface [42][43][44] (Figure S15). In CO 2 -saturated 0.1 M NaHCO 3 solution, we used the infrared signal intensity measured at 0 V as the reference (R 0 ). Then we measured the infrared signal intensity (R) at different potentials from 0 to -1.0 V with a step interval of 0.1 V. In the in situ ATR-SEIRAS spectra obtained under different potentials (Abs = -lg(R/R0)), a negative peak indicates that a certain substance or functional group is consumed or has disappeared, and a positive peak indicates that a certain substance or functional group is produced or has increased.</p><p>In the in situ ATR-SEIRAS spectra of H 2 -FeN 4 /C (Figure 4B), two negative peaks at 2,343 and 1,620 cm -1 (which are assigned to adsorbed CO 2 and H 2 O, respectively) were observed from -0.2 V (the same potential as we detected CO generation over H 2 -FeN 4 /C). With the acceleration of the reaction, the CO 2 and H 2 O on the surface of H 2 -FeN 4 /C were simultaneously consumed from -0.2 to -1.0 V. In the case of N 2 -FeN 4 /C (Figure 4C), CO 2 consumption was not observed until -0.5 V (also the same potential as we started to detect CO production over N 2 -FeN 4 /C). Before  bines with H 2 O (the proton donor), 45 however, the next step of protonation (Figure 4A,</p><p>Step A2) cannot proceed until -0.5 V, so a large amount of *CO 3 H 2 (1,492 cm -1 ) is generated on the surface at relatively low potentials (-0.1 $ -0.5 V). This explained why H</p><!><p>In order to explore the structure-activity relationship between catalytic mechanism and catalyst structure, we constructed a series of structural models and conducted DFT calculations [46][47][48] for further investigation. First, the electrocatalytic activity of different types of excess N atoms (inset in Figure 5A) was explored for the undesired HER through DFT computations. The |DG H* | values of pyridinic N (0.55 eV) and pyrrolic N (0.49 eV) were much lower than that of graphitic N (1.024 eV). Although | DG H* | for the former two N species were higher than that of Pt ($0 eV), 49 nonetheless, pyridinic N and pyrrolic N can produce a substantial amount of H 2 under a certain potential (especially under a relatively large overpotential, the HER results are shown in   interfaces and their capabilities for the CO 2 activation in the presence of H 2 O. We concluded that the FeN 4 /graphitic N interface featured the lowest free energy of CO 2 activation among all the examined combinations (Figure 5B). In addition, Figure 5C clearly demonstrated a smaller bond angle of CO 2 in the case of FeN 4 /graphitic N interface, suggesting that the graphitic N can promote a bond between Fe and CO 2 , thereby accelerating further activation of CO 2 . At the same time, the co-adsorbed H 2 O plays an important role in promoting the subsequent protonation of *CO 2 À into *COOH.</p><p>The free-energy diagram of the CO 2 -to-CO conversion for the optimized geometric models (Figures 5B and S18) show that the ''FeN 4 /graphitic N'' interface required the lowest energies for the step from *CO 2 À to *COOH (0.1 eV). This observation clearly explained why H 2 -FeN 4 /C gave the highest FE CO . We also performed DFT calculations on the free energy in the HER process (Figures 5E and S19). We found that the order of HER performance was the same as the order of CO 2 RR (Figure 5E). The FeN 4 /graphitic N structure has the best performance on both HER and CO 2 RR. However, the Gibbs free energy change from CO 2 to *COOH is lower than that from H + to *H. Therefore, in the CO 2 -saturated NaHCO 3 electrolyte, CO 2 RR prevails in the competition against HER. Hence, both experiments and theoretical calculations showed that the H 2 etching strategy could improve the CO 2 RR activity in two aspects. On the one hand, etching away the excess pyridinic N and pyrrolic N can reduce the overall HER activity of the material itself; on the other hand, the constructed ''FeN 4 /Graphitic N'' atomic-level interface can promote CO 2 activation and protonation through the co-adsorption H 2 O on graphitic N.</p><!><p>In summary, we have developed a unique selective etching strategy to eliminate the undesired pyridinic and pyrrolic N atoms in Fe-N-C catalyst by H 2 pyrolysis to construct a FeN 4 /Graphitic N ''atomic interface''. This approach can not only suppress the competing HER but also promote the electron/proton transfer processes during CO 2 activation via the synergistic co-adsorption of CO 2 and H 2 O. Therefore, the H 2 -FeN 4 /C electrocatalyst shows a high CO 2 -to-CO conversion (>90%) over a very broad potential window (-0.3 $ -0.8 V). This work not only achieves a rational design and precise structural modulation of N-SACSs but also would enable the design of new tandem catalysts based on Fe-N-C for the subsequent synthesis of high-valued hydrocarbons.</p><!><p>Resource availability Lead contact Further information and requests for resources and materials should be directed to and will be fulfilled by the Lead Contact, Chen Chen (cchen@mail.tsinghua.edu.cn).</p><!><p>This study did not generate new unique reagents.</p><!><p>The data supporting the findings of this study are available in the article and Supplemental Information or from the corresponding authors upon reasonable request. We employed the AtomSegNet App, a software that has several deep neural network models trained to perform super-resolution processing and tracking of atomic features in HAADF-STEM images, to identify the metal centers (https://github.com/ xinhuolin/AtomSegNet).</p><p>Full details of experimental procedures can be found in the Supplemental information.</p>