Modulating the electronic structure of atomically dispersed Fe–Pt dual-site catalysts for efficient oxygen reduction reactions

Atomically dispersed catalysts, with a high atomic dispersion of active sites, are efficient electrocatalysts. However, their unique catalytic sites make it challenging to improve their catalytic activity further. In this study, an atomically dispersed Fe–Pt dual-site catalyst (FePtNC) has been designed as a high-activity catalyst by modulating the electronic structure between adjacent metal sites. The FePtNC catalyst showed significantly better catalytic activity than the corresponding single-atom catalysts and metal-alloy nanocatalysts, with a half-wave potential of 0.90 V for the oxygen reduction reaction. Moreover, metal–air battery systems fabricated with the FePtNC catalyst showed peak power density values of 90.33 mW cm−2 (Al–air) and 191.83 mW cm−2 (Zn–air). By combining experiments and theoretical simulations, we demonstrate that the enhanced catalytic activity of the FePtNC catalyst can be attributed to the electronic modulation effect between adjacent metal sites. Thus, this study presents an efficient strategy for the rational design and optimization of atomically dispersed catalysts.


Synthesis of Catalysts
The FePtNC catalysts were synthesized by the following method. Firstly, 0.13 g glucose and 2.5 g dicyandiamide were dissolved in 10 mL deionized water with 80°C water bath heating. After vigorous stirring for 1 hour, the metal salt mixture (5 mg FeCl 3 ·6H 2 O and 1 mg H 2 PtCl 6 ·6H 2 O were dissolved in 2 mL deionized water) was slowly dropped into the transparent solution, and then continue stirring for 12 h.
Secondly, the bottle containing the above solution was quickly placed in a liquid nitrogen environment for 0.5 hours, and then freeze-dried. Finally, ground by mortar, the powder was heated at 550 ℃ for 1 h, then heated at 900 ℃ for 2 h under argon atmosphere, and cooled to room temperature naturally. The FeNC catalysts were synthesized by the same steps except without the addition of H 2 PtCl 6 ·6H 2 O. The PtNC catalysts were synthesized by the same steps except that FeCl 3 ·6H 2 O was not added.
The NC catalysts were synthesized with the same steps except that the metal salt was not added.
The FePtNPs catalysts were synthesized by the following method. Firstly, the prepared NC was dissolved in 10 mL of deionized water, and then the metal salt mixture (5 mg FeCl 3 ·6H 2 O and 1 mg H 2 PtCl 6 ·6H 2 O were dissolved in 2 ml of deionized water) was added to the solution. After vigorous stirring for 12 h, the solution was dried at 60 ℃.
Finally, ground by mortar, the powder was heated at 600 ℃ for 2 h under hydrogen atmosphere (10 % H 2 , 90 % Ar), and then cooled to room temperature naturally.

Materials Characterizations
The TEM, AC-HAADF STEM and EDS mapping were acquired by using spherical aberration corrected Titan Cubed Themis G2 300 TEM (300 KV) and FEI Tecnai F30 TEM (300 KV). The powder XRD was performed on an X-ray diffractometer (Rigaku, Ultima-IV) with a Cu Kα radiation source (scanning speed, 10°min -1 ). The XPS spectra were obtained on an X-ray photoelectron spectrometer (Thermo Scientific ESCALAB Xi+); The metal content of catalysts was measured by ICP-OES (SPECTRO SPECTROBLUE FMX36); SEM images were obtained on a field emission scanning electron microscopy (Zeiss GeminiSEM 500); The BET specific surface area is calculated by nitrogen adsorption and desorption isotherms (Micromeritics, TriStar II 3020); The Raman spectra were collected in a laser confocal Raman microscopy system (Nanophoton Corporation), with 532 nm laser as excitation source; XAFS measurements were performed at the XAFS Beamline in the Australian Synchrotron (ANSTO) in Melbourne, Australia. A Ge 100 element detector was used to collect the fluorescence signal, and the energy was calibrated using Fe and Pt foil. The beam size was about 1 mm 2 . The XAFS data were processed using Athena and Artemis of Demeter software packages. 1

Electrochemical Measurements
The electrochemical ORR measurements were performed through an electrochemical workstation (CHI 660E) with a typical three-electrode system. A rotating disk electrode (RDE) loaded with catalyst ink was used as the working electrode, graphite rod as the counter electrode and Ag/AgCl as the reference electrode, respectively. The catalyst ink was prepared by mixing 5 mg catalyst, 50 μL Nafion solution (5%) and 950 μL The electron transfer number and ORR reaction kinetics were calculated based on the Koutecký-Levich equation: In these equations, is the diffusion-limiting current density and is the kinetic current density. The is the angular velocity (rad s -1 ). is the transferred electron

Zn-air Battery Measurements
Zn-air battery was assembled using an Zn foil as the negative electrode material, the carbon cloth loaded with the catalyst (1 mg cm -2 ) as the positive electrode material and 6 M KOH/0.2 M Zn(OAc) 2 as the electrolyte, respectively. The preparation process of the catalyst-loaded carbon cloth is that the catalyst is dissolved in 450 μL deionized water, 450 μL ethanol and 100 μL Nafion solution (5%), and then ultrasonically mixed evenly. Finally, the prepared catalyst ink is slowly added to the surface of the carbon cloth and dried naturally. The performance measurements of the battery were carried out in CHI 660E and LAND systems, respectively.

Al-air Battery Measurements
Al-air battery was assembled with a similar process to the Al-air battery, except using Al foil as the negative electrode material, and 4 M NaOH/0.05 mM Na 2 SnO 3 ·3H 2 O as the electrolyte. 6                                  R is the distance between the central atom and surrounding coordination atoms; CN is the coordination number; * indicates that this item is the known coordination number; σ 2 is the Debye-Waller factor (described the attenuation due to thermal motion).