Iron phthalocyanine with coordination induced electronic localization to boost oxygen reduction reaction

Iron phthalocyanine (FePc) is a promising non-precious catalyst for the oxygen reduction reaction (ORR). Unfortunately, FePc with plane-symmetric FeN4 site usually exhibits an unsatisfactory ORR activity due to its poor O2 adsorption and activation. Here, we report an axial Fe–O coordination induced electronic localization strategy to improve its O2 adsorption, activation and thus the ORR performance. Theoretical calculations indicate that the Fe–O coordination evokes the electronic localization among the axial direction of O–FeN4 sites to enhance O2 adsorption and activation. To realize this speculation, FePc is coordinated with an oxidized carbon. Synchrotron X-ray absorption and Mössbauer spectra validate Fe–O coordination between FePc and carbon. The obtained catalyst exhibits fast kinetics for O2 adsorption and activation with an ultralow Tafel slope of 27.5 mV dec−1 and a remarkable half-wave potential of 0.90 V. This work offers a new strategy to regulate catalytic sites for better performance.

S ince the oxygen reduction reaction (ORR) directly determines the energy efficiency of fuel cells and metal-air batteries, catalytic activation of oxygen (O 2 ) to accelerate the kinetics of ORR is crucial for these devices [1][2][3][4][5][6] . Though platinum (Pt)-based catalysts exhibit excellent O 2 adsorption and activation abilities in ORR, the high price and low reserve sternly restrict their large-scale applications [7][8][9][10] . Exploring non-Pt ORR catalysts with high efficiency is imperative for further development of fuel cells and metal-air batteries [11][12][13] . Among the reported non-Pt ORR catalysts, iron phthalocyanine (FePc) molecular catalyst has aroused much attentions due to its special FeN 4 active site and low reaction energy barrier during ORR processes [14][15][16] . However, FePc possesses a typical two dimensional and plane symmetric structure, which leads to the symmetric electron distribution in the well-defined FeN 4 -active sites and is not conducive to the O 2 adsorption and activation 17,18 . Therefore, breaking the symmetry of electronic density would be an effective strategy to enhance the O 2 adsorption/activation and then greatly improve the ORR activity of the FePc catalyst.
From the molecular structure, FePc with a tetracoordinate planar FeN 4 center offers extra coordination sites in the axial direction 19,20 , suggesting the symmetric electronic density can be modulated by suitable axial coordination [20][21][22][23] . Generally, the organic ligands with rich electron-donating functional groups, including oxygenic, nitrogenous, and sulfurous species, can be easily employed to coordinate with FePc [24][25][26] . However, organic ligands are not favored for electrocatalysis due to their poor conductivity. Modifying the surface of conductive carbon materials with oxygenic groups for stronger electron donation to FePc provides an alternative way to overcome the problem of conductivity, and realize axial coordination of O-FeN 4 sites 17,26,27 . The axial coordination of O-FeN 4 can break the electronic distribution symmetry of Fe, leading to better oxygen adsorption and activation abilities, and superior ORR activity than those of the FePc catalyst with symmetric FeN 4 sites.
In this work, we design a composite catalyst (FeAB-O) by coordination of the FePc molecule with oxygen-containing groups on an O 2 plasma-treated acetylene black (AB-O) matrix to achieve efficient O 2 adsorption and ORR. Theoretical calculations show that the axial O coordination (O-FeN 4 ) sites greatly break the electronic distribution symmetry of Fe and lead to electron localization on O. The obvious electronic localization on O-FeN 4 sites is beneficial for the axial O 2 adsorption and activation. X-ray adsorption experiments and the O 2 adsorption/ desorption tests confirm the axial O coordination and outstanding O 2 adsorption capacity of the FeAB-O catalyst, respectively. The ORR performance measurements show that the optimized FeAB-O catalyst has an ultralow Tafel slope of 27.5 mV dec −1 and a superior half-wave potential of 0.90 V vs. reversible hydrogen electrode (RHE), which is 30 and 50 mV higher than FePc supported onto acetylene black (AB) without axial O coordination (FePc/AB) and Pt/C, respectively. This work opens a new avenue to improve the ORR performance of metal phthalocyanine catalysts, and inspires electronic localization of active sites for regulating catalytic reaction activity.

Results
Theoretical calculations. Axial O coordination in FeAB-O and no O coordination in FePc/AB were clearly showed in the schematic diagrams (Fig. 1a), which leads to obvious differences on the electron localization functions (Fig. 1b). As expected, the FePc/AB shows a symmetric charge distribution. Instead, strong electronic localization on the axial O atom accompanying with axial asymmetrical electronic distribution of O-FeN 4 sites can be observed in FeAB-O. By analyzing the charge density differences and spin density (Supplementary Figs. 1 and 2), we found that the charges number and spin polarization of the symmetrical FeN 4 site did not significantly change, due to their weaker interaction. However, the axial O coordination accepts partial charges from the FeN 4 site to form the electron localization in FeAB-O [28][29][30] , which break the symmetry of electronic density near the FeN 4 site and change the spin polarization of FeN 4 site, obviously.
To study the interaction between catalysts and the adsorbed O 2 , the O 2 adsorption energy, charge density differences (between catalysts and adsorbed O 2 ), Bader charge analysis, projected density of states (PDOS), and spin density were performed (Fig. 1c To study the effect of electronic localization on the ORR processes, the free energies of ORR pathways on FeAB-O and FePc/AB were calculated (Fig. 1d). The free energy diagrams also show that the intermediate species adsorbed on FeAB-O is more stable than that on Fe/AB. Both the rate-determining steps on FeAB-O and FePc/AB are the oxygen adsorption/activation steps: Thus, the stable adsorption of reactant (oxygen) can facilitate the process of ORR. The corresponding overpotential of ORR on FeAB-O and FePc/AB are 0.70 and 0.80 V, respectively, suggesting the superior ORR performance of FeAB-O than that of FePc/AB. Therefore, the axial Fe-O coordination-induced electronic localization to improve the O 2 adsorption and activation can boost the ORR performance. In order to investigate the presence of axial O coordination, high-resolution X-ray photon spectroscopy (XPS) spectra and synchrotron X-ray absorption spectra were conducted ( Fig. 2b-d). To obtain more structure information, the 57 Fe Mössbauer spectra were collected at 300 K. As shown in Fig. 3a, the Mössbauer spectrum of FePc/AB has a doublet peaks (D1), which can be assigned to the square planar FeN 4 species 20  To prove the enhanced O 2 adsorption, the O 2 adsorption-desorption performances were measured (Fig. 3b-d and Supplementary Fig. 8). Obviously, FeAB-O exhibits stronger O 2 adsorption response than FePc/AB, suggesting better O 2 adsorption ability of FeAB-O than that of FePc/AB 42 . The O 2 temperature-programmed desorption (TPD) measurements were also performed to investigate the O 2 adsorption property. According to the thermogravimetry analysis (TGA) of FeAB-O and FePc/AB (Fig. 3c), the weight loss at 380 and 507°C can be ascribed to the decomposition of FePc and carbon, respectively. In Fig. 3d, the O 2 -desorption peaks located at 340°C can be assigned to the release of chemistry-adsorbed O 2 in the samples 43 . Interestingly, the O 2 -desorption peak of FeAB-O is higher than that of FePc/AB, indicating the robust O 2 adsorption ability of FeAB-O.
Evaluating catalyst performance for ORR. To identify the electrochemical ORR properties of catalysts, the cyclic voltammetry (CV) was measured in 0.1 M KOH. As presented in Fig. 4a, the CV curve of FeAB-O in N 2 -saturated electrolyte contains two pairs of peaks located at 0.8 and 0.3 V. The former is indexed to the reduction/oxidation peaks of Fe 3+ /Fe 2+ , the latter is signed to redox couple of Fe 2+ /Fe + 38 . With the increasing of dissolved O 2 molecule, obvious new reduction peaks located at about 0.9 V occur and increase, even beyond the reduction peak of Fe 3+ /Fe 2+ , suggesting the active site of variable O-FeN 4 . However, the FePc/ AB and the pristine FePc display was negligible in these two groups of peaks in Supplementary Fig. 9a, suggesting that the oxygenic carbon coordination with FePc via Fe-O is beneficial for electronic delocalization of Fe to form the active site. The CV curves of the control samples under N 2 and O 2 -saturated electrolyte were also conducted and shown in Supplementary  Fig. 9b-d. All the samples exhibit obvious oxygen reduction peaks and FeAB-O displays the most positive potential, indicating the optimal ORR performance of FeAB-O.
Next, the linear scan voltammetry (LSV) curves of FeAB-O and FePc/AB were conducted to further study their ORR properties (Fig. 4b). The theoretical calculations, electrochemical impedance spectroscopy (EIS), and resistance tests (Supplementary Fig. 10 Table 1). Notably, the FeAB-O has the most excellent Tafel slope of 27.5 mV dec −1 , which is lower than those of FePc/AB (37.5 mV dec −1 ) and Pt/C (71 mV dec −1 ), confirming the fastest kinetic process of FeAB-O for O 2 adsorption/activation and ORR.
The selectivity of ORR in FeAB-O was studied by rotating ring disk electrode (RRDE) measurements. Compared with Pt/C, the higher electron transfer number and lower H 2 O 2 yield can be observed in FeAB-O (Fig. 4e), indicating the ORR on FeAB-O is a typical four-electron reduction process, and the main product is H 2 O.
To explore the practical application of FeAB-O, long-term catalytic stability and methanol tolerance tests were performed at reduced potential of 0.4 V vs. RHE. As for the traditional Pt/C catalyst, Pt nanoparticles tend to aggregate after long-term ORR measurements, which leads to a decrease in activity and durability 44 . In this work, the ORR current density of FeAB-O maintained a level of 99.2% for over 10,000 s chronoamperometric I-t tests, exceeding that of Pt/C (93.5%) and FePc/AB (88.3%) ( Fig. 4f and Supplementary Fig. 12a). The outstanding durability of FeAB-O for ORR can be attributed to the high dispersity and stability of the O-FeN 4 sites 17,45 . No current oscillation was observed in FeAB-O when methanol is added ( Supplementary Fig. 12b), while a clear decline of current appears in Pt/C. A home-made aluminum-air battery was used to evaluate the practical performance of FeAB-O. The battery with FeAB-O as cathode catalyst shows higher open potential than that with Pt/C as cathode catalyst (Supplementary Fig. 12c). The corresponding discharge plots of long-term discharge were performed at current density of 50 mA cm −2 . As shown in Supplementary Fig. 12d, the FeAB-O exhibits superior performance than that of the commercialized Pt/C. These results demonstrated that FeAB-O has excellent potential for practical application.
To further confirm the importance of the axial O coordination of O-FeN 4 , FeAB with less axial O coordination was prepared by compositing of FePc with AB treated by O 2 -plasma for only 10 min in DMF solution. XRD and FT-IR characterization results ( Supplementary Fig. 13a, b) prove the presence of FePc and carbon in FeAB. The Fe2p XPS spectrum ( Supplementary  Fig. 13c) indicates the presence of axial O coordination in FeAB. The EXAFS results (Supplementary Fig. 13d) show the order of Fe coordination number is FeAB-O > FeAB > FePc/AB. These structural characterizations confirm the axial O coordination in FeAB is between FeAB-O and FePc/AB. As we expected, the electrochemical ORR performance of FeAB ( Supplementary  Fig. 14a, b) is also between FeAB-O and FePc/AB, confirming the axial O coordination induced the electronic localization, which improves the O 2 adsorption and then boosts ORR activity of catalysts.

Discussion
In summary, we proposed a coordination-induced electronic localization strategy to tune the O 2 adsorption ability and ORR performance of Characterizations. XRD data was collected by using a RIGAKU Rint-2000 X-ray diffractometer (graphite monochromatized Cu-Kα radiation with λ = 1.54184 Å). X-ray photoelectron spectroscopy (XPS) was measured by Thermo ESCALAB 250XI. FTIR measurements were performed by the Thermo iS50. The thermogravimetric experiments were conducted on TG 209 F3 Tarsus under the air atmosphere from the room temperature to 900°C with heating rate of 10°C min −1 . Scanning electron microscopy (SEM) was measured by a Quanta 200 field-emission SEM system. The transmission electron microscopy (TEM) images were achieved on Tecnai G2 F20. The 57 Fe Mossbauer spectra were achieved by using an MS-500 instrument (Germany, Wissel) in transmission geometry with constant acceleration mode at room temperature. The O 2 -TPD of the samples was measured using AutoChem II 2920 apparatus. The catalyst (100 mg) was pretreated at 150°C and purged with helium (He) for 2 h, and then cooled down to room temperature. And then, the catalyst was purged with 5% O 2 /He at 25°C for 2 h. Finally, the desorption profile of O 2 was recorded online under the atmosphere of He.
Electrochemical measurements. All of the electrochemical experiments were implemented with an electrochemical station of Auto Lab in a typical threeelectrode system. The Ag/AgCl (saturated KCl) electrode, carbon rod, and glassy carbon electrode (GCE) were used as the reference electrode, counter electrode, and working electrode, respectively. In this work, all electrode potentials were referenced to the reversible hydrogen elecrtrode (RHE) based on the following calculation equations: where E 0 AgCl (saturated KCl) = 0.197 V (25°C). The catalyst ink was prepared by ultrasonic dispersion of 4 mg of catalyst in a hybrid solution included 60 µL of Nafion (5 wt%), 470 µL of alcohol, and 470 µL of H 2 O. All of the catalysts were cast onto the RDE (0.19625 cm −2 ) and RRDE (0.2475 cm −2 ) with a loading amount of 0.2 mg cm −2 , and contrast sample of Pt/C was dropped RRDE with a loading amount of 0.1 mg cm −2 . Cyclic voltammograms (CV) measurements were performed with scan rate of 50 mV s −1 in the N 2 or O 2satureated 0.1 M KOH solution. The catalytic activity of samples was evaluated by using linear sweep voltammetry (LSV) at scan rate of 10 mV s −1 with different rotation rates. The electron transfer number (n) of catalysts was calculated through the Koutecky-Levich (K-L) equations: where J, J L , and J k represents the measured, diffusion-limiting, and the kinetic current density, individually. ω is the electrode-rotating angular velocity, F is the Faraday constant (96,485 C mol −1 ), D o is the diffusion coefficient of O 2 (1.9 × 10 −5 cm 2 s −1 in 0.1 M KOH), ν is kinetic viscosity (0.01 cm 2 s −1 ) of the electrolyte, and C o is the density of O 2 (1.2 × 10 −6 mol cm −3 ). Tafel slope was achieved from the Tafel equation: where E is the applied potential of LSV tests, a is a constant, b is the Tafel slope and J k is the kinetic current density. Moreover, the yields of peroxide species and the electron transfer number can be calculated from the LSV of RRDE measurement at 1600 rpm via as following equation: where I D and I R is the disk current and ring current, respectively. The N represents the current collection efficiency equaled to 0.37 of the RRDE in our experimental system.
Assembly of Al-air batteries. In a typical Al-air batteries, the polished aluminum plate is used as anode. The electrolyte is 6 M KOH contained 0.0005 M In(OH) 3 , 0.0075 M ZnO, and 0.01 M Na 2 SnO 3 ; the gas diffusion electrode with catalystloading amount of 2.0 mg cm −2 is employed as cathode in a home-made cell model. As a control, the commercial Pt/C (20 wt%) was also assembled in similar mode. The measurement of batteries was performed on the LAND testing system.
Computation methods. Our simulation study was calculated by using the Vienna ab initio simulation package (VASP) 46 . The PAW potentials describe the interaction of electron-ion 47 . The generalized gradient approximation of Perdew-Burke-Ernzerhof (PBE) was employed to describe the electron-electron exchange and correlation functional 48 . A plane wave cutoff energy of 400 eV was applied in our calculations. A rectangular supercell containing 180 carbon atoms are used as substrate. Spin-polarized calculations were employed for all systems. van der Waals (VDW) forces were corrected with the D2 method of Grimme 49 . The Gamma-point-only grid was used during the optimization. The convergence criterion was set 0.02 eV Å −1 for the force and 10 −5 eV per atom for energy. We used the correlation energy (U) of 4 eV and the exchange energy (J) of 1 eV for Fe 3d orbitals 50 . The Gibbs free energy can be expressed as where ΔE is the reaction energy calculated by the DFT methods, ΔZPE the changes in zero-point energies, and ΔS the entropy during the reaction, respectively.

Data availability
The data that support the findings of this study are available from the corresponding author on reasonable request.