Isolated FeN4 Sites for Efficient Electrocatalytic CO2 Reduction

Abstract The construction of isolated metal sites represents a promising approach for electrocatalyst design toward the efficient electrochemical conversion of carbon dioxide (CO2). Herein, Fe‐doped graphitic carbon nitride is rationally prepared by a simple adsorption method and is used as template to construct isolated FeN4 sites through a confined pyrolysis strategy, which avoids the agglomeration of metal atoms to particles during the synthesis process and thus provides abundant active sites for the CO2 reduction reaction. The isolated FeN4 sites lower the energy barrier for the key intermediate in the CO2 reduction process, leading to the enhanced selectivity for CO production with a faradaic efficiency of up to 93%.


DOI: 10.1002/advs.202001545
In contrast, heterogeneous catalysts could provide stable catalytic performance, while the diverse nature of active sites restricts the selectivity. [5] Thus the development of catalysts that combine the advantages of both heterogeneous and homogeneous catalysts is urgently imperative for the efficient conversion of CO 2 .
Single-atom-based catalysts provide a great potential to bridge the gap between heterogeneous and homogeneous catalysts. [6] Apart from the maximum atom efficiency, single-atom catalysts afford the isolated and well-defined active sites confined in the support of inorganic solid material, offering high selectivity and stability toward catalytic reaction. [7] Benefiting from the unique structure, the single-atom catalysts present comparable catalytic activity to that of homogeneous catalysts, meanwhile possessing high recyclability and stability arising from heterogenization. Among the single-atom catalysts, the isolated metal atoms coordinated with nitrogen (MN x ) in carbon substrates have shown excellent performance in electrocatalysis, [8] providing a promising way for electrocatalytic CO 2 reduction. However, construction of MN x sites is still a challenge since isolated active sites are easily agglomerated to particles during the synthesis process, leading to loss of catalytic performance. Recently, isolated NiN 4 sites have been successfully constructed via a confined pyrolysis strategy, in which Ni-doped graphitic carbon nitride (g-C 3 N 4 ) acts as the template and nitrogen source during the confined pyrolysis process. [9] Considering the synthesis of metal-doped g-C 3 N 4 by pyrolyzing the precursor of g-C 3 N 4 and metal salts is uncontrollable, a more simple and universal design of metal-doped g-C 3 N 4 , which is the pivotal step in the confined pyrolysis strategy, is still much desirable but remains challenging for the construction of MN x sites.
Herein, we report the construction of isolated FeN 4 sites in carbon substrates (denoted as FeN 4 /C) by the rational design of Fe-doped g-C 3 N 4 . A simple adsorption method was adopted for g-C 3 N 4 to trap Fe atoms, making it an excellent template for the formation of FeN 4 sites. Then the formation of a carbon layer on the surface of Fe-doped g-C 3 N 4 would provide a confined environment to suppress the agglomeration of Fe atoms to particles during the pyrolysis process, thus effectively constructing the isolated active sites. Benefiting from the unique structure and coordination environment, the isolated FeN 4 sites show high selectivity for the electrocatalytic conversion of CO 2 to CO, with the highest faradaic efficiency of 93% at −0.6 V versus RHE. The isolated configuration was revealed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure analysis. The theoretical calculation demonstrates that www.advancedsciencenews.com www.advancedscience.com isolated FeN 4 sites lower the energy barrier for the formation of COOH*, leading to the enhanced activity for CO production. We believe this study would pave a new avenue for rational design of highly efficient single atom catalysts with abundant active sites.
The morphology of the catalysts was confirmed by transmission electron microscopy (TEM). Figure 1a shows the TEM image of FeN 4 /C, which presents a sheet-like structure, similar to that of bare N-doped carbon (denoted as N/C) and g-C 3 N 4 , as seen in Figure S1 (Supporting Information). No particles are observed from the high-magnification TEM image for FeN 4 /C, as shown in Figure 1b. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image of FeN 4 /C in Figure 1c reveals that isolated bright spots corresponding to Fe atoms are homogeneously dispersed, demonstrating the single-atom form of Fe in the FeN 4 /C. Energy-dispersive X-ray spectroscopy (EDX) mapping analysis in Figure 1d indicates that Fe and N atoms distribute homogeneously in carbon substrate.
The Raman spectrum for FeN 4 /C and N/C in Figure S2 (Supporting Information) shows two distinct peaks at about 1570 and 1330 cm −1 , which could be assigned to graphitic sp 2 carbon (Gband) and disordered sp 3 carbon (D-band). [10] The ratio of the relative intensity of the D band to the G band presents no much difference for FeN 4 /C and N/C, with an I D /I G value of 1.38 and 1.31, respectively, indicating the similar extent of graphitization and disorder in the catalysts. The X-ray diffraction (XRD) patterns of FeN 4 /C and N/C in Figure S3 (Supporting Information) both show two typical broad peaks at about 24°and 44°, which could be ascribed to (0 0 2) and (1 0 1) lattice plane of graphite. [11] Apparently there is no new crystal phase present in the FeN 4 /C. While without the confinement of the carbon layer during pyrolysis, Fe 3 C and Fe particles would appear in the synthesized catalyst (denoted as Fe/C). As shown in the TEM images in Figure  S4 (Supporting Information), the catalyst shows a morphology of particles wrapped with outer carbon layer. XRD pattern of Fe/C in Figure S5 (Supporting Information) presents the distinct peaks corresponding to Fe 3 C (JCPDS Card No. 350772) and Fe (JCPDS Card No. 060696), demonstrating the dominant species in Fe/C are Fe 3 C and metallic Fe. Note that the further additional loading of Fe will also result in the formation of Fe nanoparticles in FeN 4 /C (denote the catalyst as Fe NPs/C). As seen in Figure S6a  Synchrotron-based X-ray absorption spectroscopy was further adopted to determine the precise local chemical configuration around isolated Fe sites. Figure 2a presents the Fe K-edge Xray absorption near-edge structure (XANES) curves of FeN 4 /C, in comparison to Fe NPs/C and Fe foil. It is clearly seen that the absorption-edge of FeN 4 /C shifts toward higher energy compared with Fe NPs/C and Fe foil, suggesting Fe in FeN 4 /C is in an oxidation state. [12] As a comparison, the absorption-edge of Fe NPs/C is located between the FeN 4 /C and Fe foil, which could be ascribed to the coexistence of oxidized Fe single atoms and metallic Fe particles. The Fe 2p X-ray photoelectron spectroscopy (XPS) of FeN 4 /C is shown in Figure S7 (Supporting Information), in which the characteristic Fe 2p 3/2 peak is located at about 710 eV, indicating the existence of Fe 2+ species in FeN 4 /C. [13] The Fourier-transformed (FT) k 3 -weighted extended X-ray absorption fine structure (EXAFS) spectra are shown in Figure 2b. For FeN 4 /C, its FT curve only displays a prominent peak at 1.5 Å, which is generally attributed to the Fe-N first coordination shell. [14] Beyond this distance, no obvious FT peaks are observed, especially at the distance of Fe-Fe interaction, indicating that the Fe atoms in FeN 4 /C are atomically dispersed. For Fe NPs/C, a distinct peak corresponding to Fe-Fe bond appears, demonstrating the existence of metallic Fe particles. The quantitative simulation for the EXAFS of FeN 4 /C was performed to obtain the precise chemical configuration around Fe atoms. Figure 2c shows that the experimental FT-EXAFS curve of the FeN 4 /C has been perfectly reproduced. The fitting results reveal isolated Fe atoms are coordinated with four N atoms at a distance of 1.97 Å. The FeN 4 structure has been also demonstrated as a more reasonable configuration after the high-temperature pyrolysis process. [8a] The relevant fitting parameters are given in Table S1 (Supporting Information). To further determine the chemical bond for FeN 4 sites, N 1s XPS was performed, as shown in Figure 2d. The N 1s spectra of N/C could be deconvolved into four peaks with binding energy at 398.4, 399.1, 401, and 404.14 eV, corresponding to pyridinic N, pyrrolic N, graphitic N, and N-oxide, respectively. [15] For FeN 4 /C, the peak assigned to pyridinic N clearly shifts to the higher energy side compared with that of N/C, indicating the pyridinic N bonds with the Fe atoms. [16] Thus the local structure of FeN 4 sites is revealed that the isolated Fe atoms coordinated with four pyridinic N atoms.
The electrocatalytic CO 2 reduction measurement demonstrates the introduction of isolated FeN 4 sites in carbon substrate greatly enhances the catalytic performance. The linear sweep voltammetry (LSV) curves (Figure 3a) show that FeN 4 /C gives a much higher catalytic current density than that of N/C and Fe/C, demonstrating the excellent activity of the isolated FeN 4 sites for CO 2 reduction. To evaluate the selectivity for CO 2 reduction, faradaic efficiency (FE) toward CO was measured and presented in Figure 3b. The results show that FeN 4 /C exhibited high www.advancedsciencenews.com www.advancedscience.com conversion efficiency to CO and greatly suppressed the competitive H 2 evolution reaction (no liquid product was detected, as shown in the 1 H NMR spectroscopy in Figure S8, Supporting Information), achieving a maximum FE of 93% for CO at −0.6 V. This performance could be comparable with the state-of-the-art catalysts for CO 2 reduction (Table S2, Supporting Information). In contrast, the N/C and Fe/C only give a maximum FE of 46% and 23%, respectively. The partial current density for CO production of the catalysts is presented in Figure 3c. It is clearly seen that FeN 4 /C shows a much higher catalytic current density of 2.5 mA cm −2 at −0.8 V, which is 35 times and 17 times of that for N/C and Fe/C, respectively. This agrees with the electrochemical impedance spectroscopy (EIS) measurement. As presented by the Nyquist plots in Figure 3d, FeN 4 /C has the much smaller charge transfer resistance than that of N/C and Fe/C, correlating to a faster charge-transfer process for the CO 2 reduction reaction. [17] Good stability of FeN 4 /C is also demonstrated in the 24 h stability test ( Figure S9, Supporting Information).
The influence of the Fe loading on the catalytic activity of FeN 4 /C is shown in Figure S10 (Supporting Information). As seen in the LSV curves, the initial increase of Fe loading leads to a better catalytic activity of CO 2 reduction, while a further increase of loading to 2.5 wt% (Fe NPs/C) will decrease the catalytic performance. Apparently, the appearance of Fe particles is detrimental to CO 2 reduction. The FE results in Figure S11 (Supporting Information) also show that the existence of Fe particles suppresses the selectivity for CO 2 reduction to CO. To compare the intrinsic activity of FeN 4 /C and Fe NPs/C, their electrochemical active surface area (ECSA) was determined by performing cyclic voltammetry test ( Figure S12a,b, Supporting Information) to get the double-layer capacitance ( Figure S12c, Supporting Information). [18] Fe NPs/C shows a lower ECSA compared with that of FeN 4 /C, indicating a decreased ability for affording the active sites. The intrinsic activity after normalization by the ECSA for the FeN 4 /C and Fe NPs/C is shown in Figure S12d (Supporting Information), in which FeN 4 /C presents an enhanced current density for CO production than that of Fe NPs/C. Based on the catalytic performance, we could see the unique structure and coordination environment endow the isolated FeN 4 sites with excellent catalytic performance for CO 2 conversion.
To ravel the high activity of FeN 4 /C for CO 2 conversion to CO, density functional theory (DFT) calculations were performed. Considering the formation of COOH* is the initial step for the reduction of CO 2 to CO, [19] we first explored the electronic structure of FeN 4 and N/C sites with adsorbed COOH*. Figure 4a,b present the charge density difference of FeN 4 /C and N/C with COOH* adsorption from the section (two-dimensional contour map along z-axis) and three-dimensional view. For FeN 4 sites, it could be seen that the depletion of electron density appears at Fe site and the electron density accumulation occurs on the C atom of adsorbed COOH*, indicating a charge transfer happens from the Fe site to C atom. While there is no obvious electron interaction between N/C and COOH*. This charge transfer for FeN 4 sites results in the effective binding strength for the Figure 4. a) Charge density difference of FeN 4 /C and N/C with COOH* adsorption from the section (two-dimensional contour map along the z-axis) and b) three-dimensional view. The red (yellow) and blue (turquoise) areas represent electron accumulation and depletion, respectively. c) Calculated free energy diagram for CO 2 reduction to CO. d) Difference in limiting potentials for CO 2 reduction and H 2 evolution. adsorbed intermediate, thereby effectively modulating the energy barrier in CO 2 reduction process. As shown in Figure 4c, for both N/C and FeN 4 /C, the formation of COOH* is the rate-limiting step for CO 2 reduction reaction (detailed energies of adsorbates can be found in Tables S3 and S4, Supporting Information). Benefiting from the charge transfer, the introduction of FeN 4 sites greatly decreases the barrier for the formation of COOH* compared with that of N/C, thus facilitating the subsequent reduction process, eventually resulting in the enhanced activity for CO production. Different from FeN 4 structure, Fe nanoparticle shows much strong adsorption for CO*, as shown in Figure S13 (Supporting Information), which restricts the further desorption of CO, and thus leads to relative low selectivity.
Considering that H 2 evolution is the competitive reaction, the difference between thermodynamic limiting potentials for CO 2 reduction and H 2 evolution (U L (CO 2 ) -U L (H 2 )) was also calculated as a reference for the selectivity in CO 2 reduction reaction. A more positive value would indicate a better selectivity for CO 2 conversion. [20] As shown in Figure 4d, the FeN 4 /C presents a more positive value than that of N/C, demonstrating a higher selectivity for CO 2 reduction. Based on the DFT analysis, FeN 4 sites endow the catalyst with a lowered energy barrier for CO 2 reduction, thus leading to the enhanced activity for CO production.
In conclusion, we have constructed isolated FeN 4 sites on carbon substrate through a confined pyrolysis strategy using Fe doped g-C 3 N 4 as a template. The isolated and well-defined FeN 4 sites endow the catalyst with the advantages of both heterogeneous and homogeneous catalysts, showing high activity toward electrocatalytic CO 2 reduction. Benefiting from the unique structure and coordination environment, the intermediate could be easily formed on the FeN 4 sites during CO 2 reduction process, resulting in the greatly improved selectivity for CO production. We anticipate our work will provide valuable guidance for the design of isolated active sites and would inject new vitality to the related electrocatalytic field.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.