Iron Active Center Coordination Reconstruction in Iron Carbide Modified on Porous Carbon for Superior Overall Water Splitting

Abstract In this work, a novel liquid nitrogen quenching strategy is engineered to fulfill iron active center coordination reconstruction within iron carbide (Fe3C) modified on biomass‐derived nitrogen‐doped porous carbon (NC) for initiating rapid hydrogen and oxygen evolution, where the chrysanthemum tea (elm seeds, corn leaves, and shaddock peel, etc.) is treated as biomass carbon source within Fe3C and NC. Moreover, the original thermodynamic stability is changed through the corresponding force generated by liquid nitrogen quenching and the phase transformation is induced with rich carbon vacancies with the increasing instantaneous temperature drop amplitude. Noteworthy, the optimizing intermediate absorption/desorption is achieved by new phases, Fe coordination, and carbon vacancies. The Fe3C/NC‐550 (550 refers to quenching temperature) demonstrates outstanding overpotential for hydrogen evolution reaction (26.3 mV at −10 mA cm−2) and oxygen evolution reaction (281.4 mV at 10 mA cm−2), favorable overall water splitting activity (1.57 V at 10 mA cm−2). Density functional theory (DFT) calculations further confirm that liquid nitrogen quenching treatment can enhance the intrinsic electrocatalytic activity efficiently by optimizing the adsorption free energy of reaction intermediates. Overall, the above results authenticate that liquid nitrogen quenching strategy open up new possibilities for obtaining highly active electrocatalysts for the new generation of green energy conversion systems.

. XRD spectrum of Fe3C/NC-550 and the sample after cycling test.
Material Characterizations: X-ray diffraction (XRD) measurements were implemented by a powder X-ray diffraction system (Rigaku, TTR-III) equipped with Cu Kα radiation (λ=0.15406nm) to identify structures of the obtained composites.The X-ray photoelectron spectroscopy (XPS) measurements were implemented by a Thermo ESCALAB 250Xi spectrometer with monochromatic Al Kα radiation (hγ=1486.6eV).All XPS spectra were characterized with respect to the C 1s peak at 284.6 eV.The structure of the as-fabricated products was investigated by scanning electron microscope (FE-SEM) (Hitachi, SU8000) and a transmission electron microscopy (TEM) (JEOL, JEM-2010, 200 kV).EPR tests were carried out in the X-band (9.45 GHz) with 5.00-G modulation amplitude and a magnetic field modulation of 100 kHz using a Bruker EPR spectrometer (A300-10-12, Bruker) at 77 K.
Nitrogen adsorption-desorption experiments were carried out at 77.35 K by means of an Autosorb-1 (Quantachrome Instruments) analyzer.The X-ray absorption fine structures (XAFSs), including X-ray absorption near-edge structures (XANESs) and extended X-ray absorption fine structures (EXAFSs) Data reduction, data analysis, and EXAFS fitting were performed with the Athena and Artemis software packages. [1,2] he energy calibration of the sample was conducted through a standard Ni foil, which as a reference was simultaneously To measure electrochemical double-layer capacitance (Cdl), the potentials were swept for a cycle using RDE at 1,600 rpm. at a range of no faradic processes six times at six different scan rates.The measured capacitive current densities at the average potential in the selected range were plotted as a function of the scan rates and the slope of the linear fit could be calculated as the Cdl.

Theoretical section:
We have employed the first-principles [3,4] to perform all density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) [5] formulation.We have chosen the projected augmented wave (PAW) potentials [6,7] to describe the ionic cores and take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 520 eV.Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV.The electronic energy was considered self-consistent when the energy change was smaller than 10 −4 eV.A geometry optimization was considered convergent when S5 the energy change was smaller than 0.05 eV Å −1 .In our structure, the U correction is used for Fe and Ni atoms.The vacuum spacing in a direction perpendicular to the plane of the structure is 20 Å for the surfaces.The Brillouin zone integration is performed using 2×2×1 Monkhorst-Pack k-point sampling for a structure.Finally, the adsorption energies (Eads) were calculated as Eads= Ead/sub-Ead-Esub, where Ead/sub, Ead, and Esub are the total energies of the optimized adsorbate/substrate system, the adsorbate in the structure, and the clean substrate, respectively.The free energy was calculated using the equation:

G=Eads+ZPE-TS
Where G, Eads, ZPE and TS are the free energy, total energy from DFT calculations, zero point energy and entropic contributions, respectively.that characterize the structural parameters obtained by EXAFS spectroscopy were estimated as N±20%; R±1%; σ 2 ±20%; ΔE0±20%.S0 2 was fixed as 0.78, which was determined by fitting the experimental data on Fe foil with fixed coordination numbers (in bold) according to the crystal structure, as well for FeO and Fe2O3 references.b Fitting range for Fe foil was selected to be 3.5 ≤ k ≤ 12.2 Å -1 (k 3 -weighted) and 1.3 ≤ R ≤ 3.0 Å, yielding the number of variable parameters being 4, out of a total of 9.12 independent data points.c Fitting range for FeO was selected to S16 be 3.1 ≤ k ≤ 11.4 Å -1 (k 3 -weighted) and 1.3 ≤ R ≤ 3.3 Å, yielding the number of variable parameters being 3, out of a total of 10.31 independent data points.d Fitting range for Fe2O3 was selected to be 1.8 ≤ k ≤ 10.6 Å -1 (k 3 -weighted) and 1.0 ≤ R ≤ 2.1 Å, yielding the number of variable parameters being 3, out of a total of 5.98 independent data points.e Fitting range for 764-1-Fe sample was selected to be 3.0 ≤ k ≤ 8.0 Å -1 (k 3 -weighted) and 1.0 ≤ R ≤ 3.0 Å, yielding the number of variable parameters being 2, out of a total of 6.19 independent data points; f Fitting range for 764-2-Fe sample was selected to be 3.5 ≤ k ≤ 9.0 Å -1 (k 3 -weighted) and 1.0 ≤ R ≤ 3.5 Å, yielding the number of variable parameters being 4, out of a total of 8.62 independent data points; g Fitting range for 764-3-Fe sample was selected to be 3.5 ≤ k ≤ 9.0 Å -1 (k 3 -weighted) and 1.0 ≤ R ≤ 3.8 Å, yielding the number of variable parameters being 3, out of a total of 9.58 independent data points.

Figure S12 .
Figure S12.LSV curves of as-obtained catalysts after normalizing the current by ECSA.• S18

Figure S17 .
Figure S17.The amount of H2 and O2 produced by the cathode and anode in 1 M KOH solution

Figure S20 .
Figure S20.The d-band center of Fe3C/NC, Fe3C/NC-350 and Fe3C/NC-550.••••••••••••••S26 measured.For EXAFS modeling, EXAFS of the Ni foil is fitted and the obtained amplitude reduction factor S02 value (0.795) was set Ni the EXAFS analysis to determine the coordination numbers (CNs) Ni the Ni-Ni scattering paths Ni sample.Data reduction, data analysis, and EXAFS fitting were performed with the Athena and Artemis software packages.The energy S4 calibration of the sample was conducted through a standard Fe foil, which as a reference was simultaneously measured.For EXAFS modeling, EXAFS of the Fe foil is fitted and the obtained amplitude reduction factor S02 value (0.760) was set Fe the EXAFS analysis to determine the coordination numbers (CNs) Fe the Fe-Fe scattering paths Fe sample.Electrochemical Measurements: All HER electrochemical performance tests were performed in 1 M KOH (pH=14) solution.The working electrode, reference electrode, and counter electrode are self-supporting electrode carrying electrocatalysts (1 cm×1 cm), saturated calomel electrode and graphite rods, respectively.Before the linear scanning voltammetry (LSV) test, the electrocatalyst needs to be activated.Cyclic voltammetry (CV) technology is used and the scan rate is 100 mV/s until a stable CV curve appears.The linear sweep voltammetry test has a sweep speed of 5 mV/s.

Figure S9 .
Figure S9.The wavelet transform (WT) EXAFS contour map of Fe foil and Fe2O3.

Figure S12 .
Figure S12.LSV curves of as-obtained catalysts after normalizing the current by ECSA.

Figure S15 .
Figure S15.XRD spectrum of Fe3C/NC-550 and the sample after cycling test.

Figure S17 .
Figure S17.The amount of H2 and O2 produced by cathode and anode in 1.0 M KOH solution (a).The process of overall water splitting (b).

Table S1 .
Fe K-edge EXAFS least-squares fitting parameters a for Fe foil, FeO, Fe2O3 standard, and Fe samples.
a N, coordination number; R, distance between absorber and backscatter atoms; σ 2 , Debye-Waller factor to account for both thermal and structural disorders; ΔE0, inner potential correction; R-factor (%) generally estimates the goodness of the fit.Error bounds (accuracies)