Spin engineering of single-site metal catalysts

Single-site metal atoms (SMAs) on supports are attracting extensive interest as new catalytic systems because of maximized atom utilization and superior performance. However, rational design of configuration-optimized SMAs with high activity from the perspectives of fundamental electron spin is highly challenging. Herein, N-coordinated Fe single atoms are successfully distributed over axial carbon micropores to form dangling-FeN4 centers (d-FeN4). This unique d-FeN4 demonstrates much higher intrinsic activity toward oxygen reduction reaction (ORR) in HClO4 than FeN4 without micropore underneath and commercial Pt/C. Both theoretical calculation and electronic structure characterization imply that d-FeN4 endows central Fe with medium spin (t2g4 eg1), which provides a spin channel for electron transition compared with FeN4 with low spin. This leads to the facile formation of the singlet state of oxygen-containing species from triplet oxygen during the ORR, thus showing faster kinetics than FeN4. This work provides an in-depth understanding of spin tuning on SMAs for advanced energy catalysis.


INTRODUCTION
The sluggish kinetics of the oxygen reduction reaction (ORR) is a notorious bottleneck for the scalable application of metal-air batteries and fuel cells, which may arise from multiple intermediate steps and high dissociation energy of O=O double bond. 1,2 Although platinum (Pt)-based catalysts exhibit excellent performance toward ORR, the limited reserve and high price of Pt have stimulated the exploration of non-precious electrocatalysts. [3][4][5] Among various non-precious ORR electrocatalysts, single-site Fe coordinated with nitrogen atoms (e.g., FeN 4 ) has attracted much attention because the unoccupied 3d orbitals in central Fe can reduce the bond strength between active site and oxygen-containing intermediates. 6 However, traditional FeN 4 with symmetric configuration generally suffers from unfavorable O 2 adsorption and dissociation, leading to unsatisfactory activity, especially in acidic solution. Therefore, various strategies have been investigated by combining experimental with theoretical to improve the ORR performance of FeN 4 sites. One versatile strategy of doping selective atoms (e.g., sulfur) or implanting secondary metal-N x can downshift the d-band center of Fe by withdrawing electrons. 7,8 Another strategy is regulating the geometric structures either in the Fe-N x basal plane or in the perpendicular direction. With regard to basal plane configuration, a typical pore-forming route was designed to form edge-sited FeN 4 moieties, which breaks the symmetry of electron distribution to boost electron transfer. [9][10][11] As for the axial direction of Fe-N x plane, Chen et al. 12 constructed an axial Fe-O coordination to evoke electronic localization among the axial direction of O-FeN 4 sites, thus improving O 2 adsorption and activation. Li et al. 13 prepared a N-FeN 4 C 10 moiety and revealed that the fifth coordinating N atom renders a larger 3d electron density of high-spin Fe 2+ located out of the N 4 plane. Furthermore, a local contraction strain by off-plane ripples could trigger the positive shift of 3d orbitals of central Fe in FeN 4 , facilitating O 2 adsorption. 14 The aforementioned studies broaden the family and promote the development of FeN 4 electrocatalysts. However, the exploration of new configuration and in-depth understanding of FeN 4 from the perspective of fundamental electron spin-orbit coupling are still desirable.
From the viewpoint of orbital physics, these strategies of changing the in-plane or off-plane configurations can be unified by engineering local spin states. 15 13,17 As the ground spin state of O 2 is a spin triplet with two unpaired electrons ([O=O[), whereas the ground spin state of OH À /H 2 O reactants is a singlet without unpaired electrons, the reaction kinetics of ORR with a spin transition is theoretically slow. 17 To accelerate the electron transfer with spin transition, the additional energy or internal structure design is highly required. 18 For spin-aligned electrocatalysts, such a spin-selective transition process would promote a spin polarization in the oxygen-redox process to facilitate the conversion between triplet oxygen and singlet OH À /H 2 O. 19 Therefore, it is attractive to design new FeN 4 configuration with appropriate spin state to promote the spin-selective electron transfer and obtain the lower energy barrier ($1 eV) from oxygen with the triplet ground state to its next excited state. 20 Herein, we successfully prepared a novel SMA electrocatalyst with dangling FeN 4 sites (d-FeN 4 ) and superior activity for acidic ORR, in which FeN 4 sites were uniformly dispersed over micropores. Compared with FeN 4 without micropores underneath, theoretical calculations reveal that the Fe in d-

RESULTS AND DISCUSSION Synthesis and microstructure of d-FeN 4
The adsorption-pyrolysis process of d-FeN 4 is illustrated in Figure 1A. Carbon materials with abundant micropores ( Figure S1) were used as the carriers to capture FePc molecules on the basis of the adsorption effect. This micropore-rich substrate acts as a site-selective capture for FePc molecules, which is certified by the thermodynamic calculation in Figure 1B. The adsorption energy at the pore sites is about À0.98 eV, indicating a favorable adsorption with 0.94 eV at the plane sites. Under pyrolysis conditions, the adsorbed FePc molecules could convert to the d-FeN 4 configuration in which the resulting single-site Fe atoms are freely distributed over carbon micropores. The variation trend of micropore content definitely confirms the aforementioned formation process. Because a great number of micropores are blocked with FePc-derived FeN 4 species, the specific surface area (SSA) decreases remarkably. Figure 1C clearly displays the sharp change of SSA from 1,334 m 2 /g MC to 523.5 m 2 /g d-FeN 4 . In contrast, the control sample MC-700 prepared by pyrolysis of MC at 700 C without FePc molecules possesses a comparable SSA of 1,338 m 2 /g to MC (1,334 m 2 /g) ( Figure S2). This strongly suggests that the adsorption and conversion of FePc molecules on the micropore dominate the decrease of SSA. Meanwhile, an obvious drop can be seen at low pressure, suggesting the disappearance of a mass of micropores. It is also remarkable that pore volume less than 2 nm occurs ll The Innovation 3(4): 100268, July 12, 2022 obviously diving, as marked by the arrows in Figure 1D and Table S2, which is attributed to the patched FeN 4 species. In detail, the cumulative volume less than 2 nm of MC is 0.533 mL/g, much larger than 0.174 mL/g of d-FeN 4 , showing a decreasing volume about 67.4% because of d-FeN 4 covering on the micropore. On the other hand, Raman spectra also confirm the change of microstructure ( Figure S3). The intensity ratio of the D band (1,350 cm À1 ) to G band (1,580 cm À1 ) decreases from 1.08 for MC to 0.91 for d-FeN 4 , as summarized in Figure 1E, which demonstrates that MC possesses more intrinsic defects than d-FeN 4 . With regard to the control sample, the value for MC-700 is 1.07, similar to MC. This suggests that the lower ratio for d-FeN 4 originates from the adsorption and conversion of FePc above the pore.
To further analyze the single-atom d-FeN 4 , high-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) was used. The HAADF-STEM image reveals the atomic Fe in d-FeN 4 sample because of the separate bright dots marked with red cycles ( Figure 1F). Electron energy loss spectroscopy (EELS) at the white dotted line-marked region in Figure 1F demonstrates the coexistence of N and Fe as two peaks at 400 and 708 eV ( Figure 1G). In addition, no Fe peaks can be observed in the XRD spectrum ( Figure S4). This is consistent with the observation by HAADF-STEM, indicating that there is no agglomeration of Fe. The signal at g = 3.03 in the EPR spectrum is the fingerprint of the atomic FeN 4 moieties ( Figure S5). 21 All above results suggest that the atomically dispersed  Table S3). 22 By tuning the FePc content, the loading amount of d-FeN 4 can also be regulated, of which the maximum is about 1.07 wt %, as confirmed by XPS ( Figure S7).

Spin state of electrons in d-FeN 4
The local structure of d-FeN 4 was further unveiled by X-ray absorption nearedge structure (XANES) and extended X-ray absorption fine structure. The absorption edges of d-FeN 4  high-resolution Fe 2p XPS spectra displayed in Figure S8, the peaks of Fe 2p spectrum can be ascribed to Fe 3+ and Fe 2+ through deconvolution. The relative content of Fe 3+ increases from 55% of FeN 4 to 66% of d-FeN 4 . The calculated valence state of Fe in d-FeN 4 is +2.7, which is higher than +2.1 of Fe in FeN 4 but smaller than +3 of Fe 2 O 3 ( Figure S9). The Fe EXAFS reveal that signal of Fe-N coordination shifts positively, indicating a lengthened Fe-N bond ( Figure 2B). By calculating the first derivative, the "shoulder" of XANES between 7,110 and 7,140 eV originating from the band structure becomes more obvious. 23 As shown in Figure S10, the curve of d-FeN 4 shows a large value around 7,125-7,130 eV, indicating that d-FeN 4 has more high-energy electrons than FeN 4 without axis micropore. The same conclusion can be drawn by XAS of Fe L-edge. The higher intensity from 710 to 714 eV certifies higher spin of electrons in d-FeN 4 than FeN 4 ( Figure 2C). 24 The zero-field cooling temperature-dependent magnetic susceptibility was further measured to disclose the spin configuration of d-FeN 4 and FeN 4 . As displayed in Figure 2D, the calculated effective magnetic moment of d-FeN 4 and FeN 4 is 3.83 m eff and 2.08 m eff , respectively. According to a previous report (m eff = ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi n Ã ðn + 2Þ p ), 25

Electrocatalytic ORR performance
The positive effect of medium-spin electrons in Fe during ORR was studied in 0.1 M HClO 4 . Linear sweep voltammetry (LSV) was first performed by using a rotating disk electrode to evaluate the electrocatalytic activity as shown in Figure 3A. The as-synthesized d-FeN 4 shows a high half potential of 0.83 V, which is very close to 0.85 V of commercial Pt/C and far superior to traditional FeN 4 . Besides, the equal onset potential (0.93 V) for both d-FeN 4 and Pt/C indicates a keen response at the d-FeN 4 sites during ORR process. The d-FeN 4 shows a similar Tafel slope of 73 mV dec À1 as Pt/C (64 mV dec À1 ), which is far smaller than that of FeN 4 (135 mV dec À1 ) ( Figure 3B). It suggests that pore-induced spin flipping could markedly accelerate the dynamics process. After taking the loading of active sites into account, the TOF value indicates that the d-FeN 4 behaves the highest intrinsic activity (Figure 3C). The rotating ring-disk electrode test in Figure S12 demonstrates that d-FeN 4 behaves an equal excellent selectivity compared with Pt/C with an almost 4e À process and low H 2 O 2 yields ( Figure 3D). More important, the current density of d-FeN 4 remains 91.1% after adding methanol, much higher than 76.3% of Pt/C, displaying a superior methanol tolerance ( Figure 3E). On the other hand, d-FeN 4 shows the long-term stability with 89% of initial current over 10,000 s, much better than 72.6% of Pt/C ( Figure 3F).

Electronic interaction between O 2 with FeN 4
Spin transfer of electrons is helpful for ORR, as all the intermediates (*OOH, *O, and *OH) are singlet state. To understand the electronic interaction of oxygen with FeN 4 and d-FeN 4 , the electronic structures of were first analyzed using density functional theory calculations (Figure 4). The structure models of FeN 4 and d-FeN 4 are shown in Figures 4A and 4C, respectively. Upon the structure relaxation, Fe atom in the d-FeN 4 model dips obviously because of the presence of vertical micropore (Table S1). This d-FeN 4 over micropores leads to a longer Fe-N bond than traditional FeN 4 , resulting in obvious electron loss from Fe ( Figure S13). The more critical effects during the process of adsorbing oxygen were expounded by the electron localization functions and projected electronic densities of states (pDOS). As shown in Figures 4B and 4E, a stronger electron localization at the marked area with 0.17 can be observed around Fe atom in d-FeN 4 . In contrast, traditional FeN 4 displays only 0.08 at the same position, indicating that the micropore could modify the localization of electrons in d-FeN 4 and thus result in a stronger interaction between oxygen and active site. 12 Then, the binding features between two sites and oxygen are demonstrated through pDOS as shown in Figures 4C and 4F. The energy levels of Fe b-spin d orbitals in d-FeN 4 and O b-spin 2p orbital match better than in FeN 4 . 28 Such disparity can be clearly illustrated through integrating the matched region (Figure 4G). The integral value of d-FeN 4 is nearly 3-fold that of FeN 4 (0.34 versus 0.12). This shows that d-FeN 4 could adsorb oxygen in thermodynamics more easily than FeN 4 ( Figure 4G). The CO-TPD curves in Figure S14 supports such conclusion experimentally. The peak located at 400 K can be attributed to the physical adsorption and the peak at a higher temperature is assigned to chemical adsorption. The chemical adsorption peak of d-FeN 4 emerges at 549 K, much higher than 515.5 K for FeN 4 , illustrating strong binding between d-FeN 4 and O. In addition, the spin density of Fe in d-FeN 4 sites shows a higher spin state than that of Fe in FeN 4 , suggesting that there is a spin channel around Fe in the d-FeN 4 site, while no channel with equal strength exists in FeN 4 . 17 Thermodynamically rate-determining step of ORR on d-FeN 4 DFT calculations were further used to gain deep insight into the improved ORR activity. The difference of O 2 configurations after adsorbed at FeN 4 and d-FeN 4 was studied. As shown in Figures 5A, 5B, and S15, the bond length of O 2 at d-FeN 4 is 1.48 Å, much longer than that at FeN 4 (1.28 Å), implying that the O=O bond at the d-FeN 4 site is activated more easily. Second, the distalis O atom at d-FeN 4 behaves a more negative Bader charge of À0.22 |e| than À0.12 |e| at FeN 4 , which is beneficial for capturing H proton. 29 Both O=O bond length and Bader charge illustrate that O 2 adsorbed at d-FeN 4 could occur more easily hydrogenation and subsequent steps. The free-energy evolution further verifies the smooth ORR process ( Figures 5C and 5D)

CONCLUSIONS
A specific dangling FeN 4 configuration (d -FeN 4 ), in which single-site Fe atoms were dispersed over carbon micropores, was successfully synthesized by adsorption of FePc on micropores and subsequent pyrolysis, which shows outstanding ORR activity in acidic solution. Both DFT calculation and electronic structure characterization show that this unique d-FeN 4 endows the central Fe atom with the medium spin state (t 2g 4 e g 1 ) and provides a spin channel for the electron transition, leading to the facile formation of singlet state of oxygen-containing species (e.g., *OOH, *OH, and H 2 O) from triplet O 2 during the ORR. The electrochemical experiments imply that the as-synthesized d-FeN 4 shows a comparable ORR activity and a much superior stability to commercial Pt/C in HClO 4 electrolyte, much better than traditional FeN 4 without underneath pore regulation. The theoretical calculation implied that the change of Gibbs free energy (DG *OOH ) from O 2 to *OOH intermediate is only 0.15 eV at the d-FeN 4 site, much lower than 0.42 eV at FeN 4 . This further demonstrates high electrocatalytic activity and fast reaction kinetics for d-FeN 4 ascribed to optimized electron spin state induced by micropores underneath. The present strategy of catalyst design and synthesis could be also extended to other singleatom catalytic systems with great potentials in various energy conversion and storage devices. Structure characterization X-ray diffraction (XRD) was performed to determine the crystallographic structure with a Cu Ka (l = 0.15405 nm) radiation source. Raman spectrometry was tested with a laser wavelength of 532 nm. X-ray photoelectron spectroscopy (XPS) was performed using an ESCALAB 250 X-ray photoelectron spectrometer with the Al Ka (hʋ = 1486.6 eV) radiation source. The morphology and microstructure of the samples were observed using scanning electron microscopy (SEM; JEOL JSM-5610) and transmission electron microscopy (TEM; JEM-2100 F). Electron paramagnetic resonance (EPR) was performed using Bruker EMX spectrometer. Calculation of the spin number was based on a reference powder of Cu(II) TPP with 1 spin per molecule. The zero-field cooling (ZFC) temperature-dependent magnetic susceptibility was obtained using a vibrating sample magnetometer (VSM) with MPMS XL-7. The specific surface areas were obtained using the multipoint Brunauer-Emmett-Teller method. Pore size distribution curves, pore volume, and pore diameter were calculated by the desorption branch of the isotherms using the Barrett-Joyner-Halenda (BJH) method.
X-ray absorption fine structure (XAFS) spectra at the Fe K-edge was obtained at the Advanced Photon Source (APS), beamline BM9. The samples were measured in X-ray fluorescence mode. The data were processed using the ATHENA program for background subtraction, normalization, and energy calibration. 32 The extended XAFS (EXAFS) was analyzed using the IFEFFIT package, 33 and EXAFS fitting was performed with FEFF6L. 34  The data were normalized to the incident photon flux.

Electrochemical measurements
Electrochemical measurements including cyclic voltammetry (CV), rotating disk electrode (RDE), rotating ring-disk electrode (RRDE), and chronoamperometric (CA) measurements were carried on an electrochemical workstation (AFMSRCE; Pine Research Instrumentation). All measurements were performed in the electrolyte of 0.1 M HClO 4 solution using a threeelectrode system. A graphite rod and saturated calomel electrode (SCE) were used as the counter electrode and reference electrode, respectively. A rotating disk electrode (0.19625 cm 2 geometric surface area of glassy carbon) or rotating ring-disk electrode (0.2475 cm 2 geometric surface area of glassy carbon and 0.1866 cm 2 of Pt ring) coated with catalyst was used as the working electrode. Experimentally, the catalyst ink was prepared by adding 5 mg sample into 500 mL deionized water, 500 mL ethanol, and 20 mL Nafion (5 wt %) solution and then the solution was sonicated for 1 h. Then, 10 mL ink was dropped on the surface of the electrode and dried at 50 C, and another 10 mL ink was added and dried to form a thin film. Commercial 40 wt % Pt/C was prepared in the same way for comparison. The potentials versus SCE were converted to a reversible hydrogen electrode (RHE) scale.
The conversion was calculated on the basis of the following equation: Before collecting the data, the electrode was activated using the CV measurement for 100 cycles with a scan rate of 100 mV s À1 . RRDE measurements were taken to evaluate the electron transfer number (n) and peroxide yields during ORR catalytic process. The equations are as follows: A B C D Figure 5. where I r is the ring current, I d is the disk current, and N is the current collection efficiency of the Pt ring electrode. The kinetic current density (J K ) was calculated using the equation where J L is the limiting diffusion current density, and J is the current density at a specific potential in the mixed-kinetic region.

DFT calculations
All density functional theory (DFT) calculations were performed using the Vienna Ab Initio Simulation Package (VASP). 35,36 The generalized gradient approximation method with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation function was used to manage the electron exchange and correlation energy. 37 The plane wave basis (kinetic energy cut-off value 450 eV) was used to describe the valence electrons. A mesh of 1 3 1 3 1 was used for the k-point sampling obtained from the gamma center. The model is 22.14 3 22.14 3 36.8 Å 3 , and the thickness of the vacuum layer is 20 Å. The atomic positions were fully optimized until the energy and forces converged to 1 3 10 À5 eV and À0.0257 eV Å À1 , respectively. The calculations of pathways during ORR were conducted following the method used by Kulkarni  where U is the electrode applied potential, e is the transferred charge, and n is the number of proton-electron transferred pairs.