Materials. CO2 (Beijing Beiwen Gas Chemical Industry Co., Ltd., research grade) had a purity of 99.999% and used as received. Iron(III) sulfate hydrate (A. R. grade), Tris(hydroxymethyl)aminomethane (99.5%) and dopamine hydrochloride were purchased from Innochem, Pluronic F127 (Mn ~ 8400) was supplied by Alddin. Acetonitrile (AcN, A. R. grade), acetone (A. R. grade) were provided by Sinopharm Chemical Reagent Co., Ltd, China. Toray Carbon Paper (CP, TGP-H-60, 19×19 cm) and Nafion N-117 membrane (0.180 mm thick, ≥ 0.90 meg/g exchange capacity) were purchased from Alfa Aesar China Co., Ltd. Polytetrafluoroethylene (PTFE, 60 wt% aqueous solution) was purchased from Sigma-Aldrich Co. LLC. 1-Butyl-3-methylimidazolium hexafluorophosphate ([Bmim]PF6, purity > 99%) was obtained from the Centre of Green Chemistry and Catalysis, Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences.
Preparation of Fe-based catalysts xFe2O3[email protected]
The polymerization of dopamine (DA) was carried out according to the previous literature47. To prepare xFe2O3[email protected], 0.4 g of dopamine hydrochloride (DA), 0.16 g of F127, desired amount of Fe2(SO4)3, 16 mL of deionized water and 8 mL of EtOH were added into a glass bottle and stirred to form a clear solution at 30°C. One hour later, 8 mL of Tris(hydroxymethyl)aminomethane (Tris) solution (0.16 g of Tris in 8 mL of deionized water) was added under stirring. The reaction solution was stirred overnight. The product was collected by centrifugation, washed with water for twice and ethanol three times to remove inorganic salts and surfactants, and dried at 60°C under vacuum to obtain microsphere powder. The obtained nanoparticles were heated in a tubular furnace at 300°C for 3 h and 600°C for 2 h under N2 atmosphere. The heating rate was 1°C min-1. The corresponding products were named as xFe2O3[email protected], where the subscript x corresponds to the mole fraction of Fe in the catalysts after carbonization.
The corresponding amount of Fe2(SO4)3 added for preparation xFe2O3[email protected] with different Fe content are as follows: 0.05Fe2O3[email protected]: 1.05 g Fe2(SO4)3, 0.11Fe2O3[email protected]: 1.58 g Fe2(SO4)3, 0.17Fe2O3[email protected]: 2.1 g Fe2(SO4)3, 0.2Fe2O3[email protected]: 2.63 g Fe2(SO4)3, 0.3Fe2O3[email protected]: 3.68 g Fe2(SO4)3.
When the amount of Fe2(SO4)3 in the system was higher than 3.68g (the molar ratio of Fe3+ and DA is 8.4:1), the loading of Fe in the catalysts was almost unchanged due to the saturated coordination. Therefore, the mole fraction of 0.3 for Fe atom was the highest content of Fe atom in the as-prepared xFe2O3[email protected] nanoparticles.
Preparation of the control sample without CN shell Fe2O3-CNF
The obtained 0.3Fe2O3[email protected] was heated in a muffle furnace in the air at 600°C for 6 h to obtain Fe2O3-CNF. The heating rate was 1°C min− 1.
Material characterisations. PXRD data were collected on the X-ray diffractometer (Model D/MAX2500, Rigaka) with Cu-Kα radiation at a scan speed of 5 °/min. The morphologies of catalysts were characterized by scanning electron microscope (SEM) (TECNAI 20PHILIPS electron microscope) and transmission electron microscope (TEM) (JEOL-2100F). The energy dispersive X-ray spectroscopy (STEM-EDX) elemental mapping analysis was obtained by JEOL-2100F. The BET surface area and porosity properties of the materials were determined by N2 adsorption-desorption isotherms using a Micromeritics ASAP 2020M system. The Fe loadings in the catalysts were determined by ICP-AES method (VISTA-MPX). X-ray photoelectron spectroscopy (XPS) analysis was performed on the Thermo Scientific ESCALab 250Xi using a 200 W monochromated Al Kα radiation.
57Fe Mössbauer spectroscopy. The 57Fe Mössbauer spectra were recorded on an SEE Co W304 Mössbauer spectrometer at room temperature, using a 57Co/Rh source in transmission geometry which was equipped with a helium cryostat (Advanced Research Systems, Inc., 4 K). The data were fitted by using the MossWinn 4.0 software.
Electrocatalytic CO2 reduction. The electrolysis experiments were performed at ambient temperature in a typical H-type cell. The Ag/Ag+ (0.01 M AgNO3 in 0.1 M TBAP-MeCN) was used as the reference electrode and the Pt gauze was used as counter electrode. The cathode and anode compartments were separated through a Nafion 117 proton exchange membrane. [Bmim]PF6/MeCN/H2O (W/W/W = 30/65/5) solution was used as cathode electrolyte. 0.5 M of H2SO4 aqueous solution was used as anodic electrolyte. CO2 was bubbled into the catholyte (25 mL/min) with continuous stirring for 30 min before electrolysis. After that, potentiostatic electrochemical reduction of CO2 was carried out with CO2 bubbling.
DFT calculations. The present spin-polarized first principle DFT calculations were performed by Vienna Ab initio Simulation Package(VASP)48 with the projector augmented wave (PAW) method49. The exchange-functional was treated using the generalized gradient approximation (GGA) of Perdew-Burke-Ernzerhof (PBE)50 functional. The energy cutoff for the plane wave basis expansion was set to 450 eV and the force on each atom less than 0.02 eV/Å was set for convergence criterion of geometry relaxation. Four-layer Fe2O3 (400) was employed with a 15 Å vacuum along z direction in order to avoid the interaction between periodic structures. The Brillouin zone integration is performed using 2×2×1 k-point sampling through all the computational process. The self-consistent calculations apply a convergence energy threshold of 10− 5 eV. The free energies of the CO2 reduction steps (CRR) were calculated by the Eq. 51: ΔG = Δ𝐸𝐷𝐹𝑇+Δ𝐸𝑍𝑃𝐸−TΔS, where ΔEDFT is the DFT electronic energy difference of each step, ΔEZPE and ΔS are the correction of zero-point energy and the variation of entropy, respectively, which are obtained by vibration analysis, T is the temperature (T = 298 K).