Deep Electronic State Regulation through Unidirectional Cascade Electron Transfer Induced by Dual Junction Boosting Electrocatalysis Performance

Abstract Unidirectional cascade electron transfer induced by multi‐junctions is essential for deep electronic state regulation of the catalytic active sites, while this advanced concept has rarely been investigated in the field of electrocatalysis. In the present work, a dual junction heterostructure (FePc/L‐R/CN) is designed by anchoring iron phthalocyanine (FePc)/MXene (L‐Ti3C2‐R, R═OH or F) heterojunction on g‐C3N4 nanosheet substrates for electrocatalysis. The unidirectional cascade electron transfer (g‐C3N4 → L‐Ti3C2‐R → FePc) induced by the dual junction of FePc/L‐Ti3C2‐R and L‐Ti3C2‐R/g‐C3N4 makes the Fe center electron‐rich and therefore facilitates the adsorption of O2 in the oxygen reduction reaction (ORR). Moreover, the electron transfer between FePc and MXene is facilitated by the axial Fe─O coordination interaction of Fe with the OH in alkalized MXene nanosheets (L‐Ti3C2‐OH). As a result, FePc/L‐OH/CN exhibits an impressive ORR activity with a half‐wave potential (E 1/2) of 0.92 V, which is superior over the catalysts with a single junction and the state‐of‐the‐art Pt/C (E 1/2 = 0.85 V). This work provides a broad idea for deep regulation of electronic state by the unidirectional cascade multi‐step charge transfer and can be extended to other proton‐coupled electron transfer processes.

followed by centrifugation at 3500 r/min for 60 min to collect the dark green upper liquid containing the L-Ti3C2Tx nanosheets.A certain amount of L-Ti3C2Tx nanosheets solution was added to the NaOH solution, then stirred for 8 h under an N2 atmosphere, centrifuged at 10 000 rpm, washed with water to pH=7, and finally freeze-dried to obtain L-Ti3C2-OH nanosheets.For comparison, L-Ti3C2-F was prepared by the same preparation method.The difference is that the NaOH solution was replaced with NaF.

Synthesis of g-C3N4 nanosheets.
A certain amount of melamine was put into a crucible with a lid and calcined at 550 ℃ for 4 h in a tube furnace with a heating rate of 2 ℃/min.The yellow solid was obtained after natural cooling to room temperature.They were ground into powder and sieved through a standard 100-mesh sieve.Then, the solid was calcined at 550 ℃ for 2 h in a muffle stove with a heating rate of 5 ℃/min giving g-C3N4 nanosheets.

Synthesis of FePc/L-R.
FePc (20 mg) and L-Ti3C2-R (20 mg) were dispersed in the 10 mL DMF solution under ultrasonic conditions, respectively.Next, FePc solution was poured into L-Ti3C2-R solution and the mixture was sonicated for 30 min.It was further stirred for 24 h to deepen the combination of FePc and L-Ti3C2-R.
The solid was collected by suction filtration and washed with DMF and absolute ethanol.Then, the FePc/L-R was obtained by freeze-drying.

Synthesis of FePc/L-R/CN.
g-C3N4 (100 mg) was dispersed to 30 mL ethanol under ultrasonic conditions at room temperature, and FePc/L-R was added to the above solution under vigorous stirring.The mixture was then heated to 70 ºC and continuously stirred for 24 h.The solid was collected by filtration and washed several times with ethanol.Finally, the green FePc/L-R/CN was obtained by freeze-drying.The prepared sample was finally annealed at 120 ℃ for 1 h in an N2 atmosphere to strengthen the interaction between substances.

Characterization.
Brunauer-Emmett-Teller (BET) tests were carried out using an ASAP-2460 gas adsorption analyzer at 77 K (Micromeritics Instrument Co., USA).Inductively coupled plasma mass spectrometry (ICP-MS) analysis was accomplished using Thermo ICAP Qc equipment (Thermo Fisher Scientific Co., USA) to obtain the loading mass of FePc.X-ray diffraction (XRD) was carried out to characterize the composition and crystal structures of the prepared materials.The Raman spectra were collected from a LabRam HR Evolution (Horiba) Raman spectrometer using a 532 nm laser.Fourier transform infrared (FT-IR) transmittance spectra were recorded using a Nicolet iS20 (Thermo Fisher Scientific Co., USA) IR spectrometer.X-ray photoelectron spectroscopy (XPS) was recorded on an ESCALAB 250 Xi spectrometer (Thermo Fisher Scientific Co., USA) with an Al Kα X-ray radiation.The Mössbauer spectra ( 57 Fe) were achieved by using an MS-500 instrument (Germany, Wissel) in transmission geometry with constant acceleration mode at room temperature.X-band Electron paramagnetic resonance (EPR) spectra were obtained by using a Bruker EMXplus spectrometer.Temperature magnetic susceptibility (M-T) curves were obtained by using a Quantum Design Squid-VSM physical property measurement system in the temperature region of 7 to 300 K to obtain the .

Electrochemical Measurements.
The electrochemical performance testing was carried out on the CHI 760E electrochemical analyzer using the classic three-electrode system.In the alkaline electrolyte, the glass carbon electrode (GCE) was used as the working electrode, and the Hg/HgCl electrode and the Pt electrode were considered as the reference electrode and the counter electrode, respectively.All of the measured potentials were converted into the reversible hydrogen electrode (RHE) scale.The sample (4 mg) was homogeneously dispersed in 1 mL of the mixed liquor of Nafion (5 wt.%) and ethanol, and then the ink was formed after 1 h of ultrasonic treatment.10 μL ink was dropped on the surface of the polished glassy carbon electrode and dried at 25 °C.In the ORR experiment, the O2 saturation was ensured by leading into a constant O2 in the KOH solution.
Electrochemical data processing: Rotating ring-disk electrode (RRDE) measurements were performed at a scan rate of 5 mV s −1 at 1600 rpm, and the ring electrode voltage was kept at 1.3 V (vs.RHE).
The number of electrons transferred (n) and kinetic current density (JK) during ORR were calculated according to Koutecky-Levich Equation: where J is the measured current, JL is the diffusion-limited current densities, JK is the kinetic-limited current densities, ω is the electrode rotation rate, n is the electron transfer number, B is the reciprocal of the slope, F is the Faraday constant (96485 C mol −1 ), C0 is the concentration of dissolved oxygen in the electrolyte (1.2×10 −6 mol cm −3 ), D0 is the diffusion coefficient of dissolved oxygen (1.9×10 −5 cm 2 s −1 ), υ is the kinematic viscosity of the electrolyte (0.01 cm 2 s −1 ).
The Hydrogen peroxide yield (H2O2%) and the electron transfer number (n) were determined by the following equations: where ID is disk current, IR is ring current, and N is the current collection efficiency of the Pt ring, which was provided as 0.37 by the manufacturer.

Zn-Air battery tests.
The gas diffusion electrode with a catalyst loading amount of 2.0 mg cm −2 is employed as a cathode in a homemade Zn-air batteries model.The aqueous solution of 6.0 M KOH (98%) and 0.2 M Zn(OAc)2 was used as an electrolyte.The measurement of batteries was performed on the LAND testing system.

Computational details.
All calculations were carried out with the CP2K package (version 7.1) in the framework of the density functional theory (DFT), based on the hybrid Gaussian and plan-wave schemes. [1,2]Molecular orbitals of the valence electrons were expanded into DZVP-MOLOPT-SR-GTH basis sets, while atomic core electrons are described through Goedecker-Teter-Hutter (GTH) pseudopotentials. [3,4]A plane-wave density cutoff of 800 Ry was adopted.The long-range van der Waals interaction is described by the DFT-D3 (BJ) approach. [5]All the structures were fully relaxed by CP2K with the BFGS scheme, and the force convergence criterion was set to 4.5 * 10 −4 hartree/bhor.Due to the large numbers of atoms in the models, an orbital transformation (OT) scheme with only a gamma k-point was adopted during the calculations.
The data processing was conducted using the VASPKIT code and VESTA package.MXene nanosheets were prepared by the multi-step process (Figure S1a).In brief, the layered MXene  In XRD patterns, no diffraction peak corresponding to the (104) crystal plane of the Al layer in ML-Ti3C2Tx indicates that Al has been removed after etching. [6]After surface functionalization, the (002) peaks corresponding to the multilayer structure are shifted to a lower angle, which was caused by the increase of the (002) crystal plane spacing of L-Ti3C2Tx.In XPS spectra, no obvious F-terminating groups were detected for L-Ti3C2-OH and the signals of Ti-OH and Ti-O for L-Ti3C2-OH are much higher than these of L-Ti3C2-F, indicating successful replacement of surface-terminating groups. [7]n the Raman spectra, characteristic Raman bands were observed for L-Ti3C2Tx (136, 268, 390, and 591 cm −1 ).For L-Ti3C2-OH, the peak intensity attributed to the Ti-O bond (136 cm −1 ) is significantly higher than that of L-Ti3C2Tx, illustrating the surface-terminating groups were successfully replaced by -OH, while the peak intensity of L-Ti3C2-F is lower than that of L-Ti3C2Tx, further indicating the abundant surface-terminating groups were successfully replaced by -F. [8]In Fourier transform infrared (FT-IR) spectroscopy, L-Ti3C2Tx and L-Ti3C2-R have identical stretching vibrations at about 3425 cm −1 , which are attributed to the -OH bond of H2O.
The lower HOMO and LUMO energy levels of the C-F bond indicate that valence electrons are strongly bound to the nucleus, and the strong electronegativity of fluorine causes it attractive for the lone pair.

Table S2.
Comparison of ORR performance between FePc/L-OH/CN and state-of-the-art Fe-N-C

(
ML-Ti3C2Tx, Tx = F or OH) nanostructures were first prepared by HF etching of Ti3AlC2 to remove the Al layer.Then the MXene nanosheets (L-Ti3C2Tx) were obtained by ultrasonic exfoliation, and finally, the surface functionalized MXene nanosheets (L-Ti3C2-R, R = OH or F) were obtained by alkalization or fluorination.

Figure S4 .
Figure S4.High-resolution core-level XPS spectra of a) F 1s and b) O 1s of L-Ti3C2-R.

Figure S10 .
Figure S10.XPS spectra of a) Ti 2p and b) N 1s of L-OH/CN and FePc/L-OH/CN.

Figure S21 .
Figure S21.Relationship between scan rate and current density at 1.05 V derived from CV

Figure S23 .
Figure S23.LSV curves and K-L curves of FePc/L-OH/CN at different speeds in 0.1 M KOH solution.

Figure S25 .
Figure S25.I-t curves of Pt/C and FePc/L-OH/CN for the methanol-tolerance test.

Figure S26 .
Figure S26.The DFT calculations of the electronic structure of a) FePc/L-OH and b) FePc/L-OH/CN

Figure S27 .
Figure S27.a) Schematic illustration of a typical ZAB.b) Open circuit potential.c) Polarization curves

Figure S28 .
Figure S28.Long-term galvanostatic charge-discharge test of FePc/L-OH/CN-based ZAB at a current

Figure S29 .
Figure S29.The LED is powered by the FePc/L-OH/CN-based ZAB.
b R, the distance to the neighboring atom; c σ 2 , the Mean Square Relative Displacement (MSRD); d ΔE0, inner potential correction; R factor indicates the goodness of the fit.S0 2 was fixed to 0.884, according to the experimental EXAFS fit of Fe foil by fixing CN as the known crystallographic value.* This value was fixed during EXAFS fitting, based on the known structure of Fe.

Table S1 .
EXAFS fitting parameters at the Fe K-edge for various samples(Ѕ0 2 =0.884) a CN, coordination number;