Interfacial Engineering of a Phase-Controlled Heterojunction for High-Efficiency HER, OER, and ORR Trifunctional Electrocatalysis.

The development of low-cost and high-performance electrocatalysts for simultaneously boosting the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR) is highly crucial but still challenging. Herein, a facile one-step solid-phase polymerization and confined pyrolysis strategy is developed for scalable synthesis of a Fe x P/Fe3C-based (x = 1, 2) heterojunction with controllable iron phosphide crystal phases. By effective heterojunction interface regulation, the strong synergic effect between FeP/Fe3C and N- and P-codoped carbon (NPC) modified the electronic structure, resulting in an excellent electrocatalytic performance for the HER, OER, and ORR synchronously. Typically, the FeP/Fe3C@NPC catalyst exhibits efficient HER activity with a low overpotential of 10 mA cm-2 for the HER (97 mV) and OER (440 mV) and a high half-wave potential of 0.87 V for the ORR, as well as excellent stability in alkaline media. Theoretical calculations demonstrated that Fe3C can promote the activation of water molecules, while FeP is beneficial to the removal of H2 and the FeP/Fe3C heterojunction can facilitate both Volmer and Heyrovsky steps in the HER process simultaneously. Moreover, FeP has a stronger inhibitory effect on OH adsorption, revealing that the FeP/Fe3C heterojunction also shows a better promoting effect for both the OER and ORR, respectively.

Introduction

The rising global demand for energy and the increasingly prominent issue of climate change are urgent problems humans must solve; thus, developing clean and sustainable energy is essential to addressing the major challenges of demand and climate change.[1−3] Fuel cells, metal–air batteries and electrochemical water splitting are regarded as ideal strategies for renewable energy conversion and storage sources.[4−8] However, high energy barriers and sluggish kinetics impede the current industrial application of the electrochemical hydrogen evolution reaction (HER) and oxygen reduction and evolution reactions (ORR and OER). A noble metal such as Pt is the most efficient catalyst for the HER and ORR and the oxides of Ir and Ru show the best electrocatalytic performance for the OER.[9−15] Unfortunately, they have low abundance, high cost, and poor stability.[16] In addition, noble-metal catalysts are usually single-functional and cannot catalyze multiple reactions with one electrode. Generally, different cathodic and anodic reactions need different catalysts and the optimal operating conditions for these catalysts are different (e.g., pH, electrolyte, and potential window), resulting in unavoidable complications, compromise of the overall system performance, and an increased cost of the system construction. Therefore, electrocatalysts composed of earth-abundant elements which feature low cost and are highly efficient, robust, and multifunctional is imperative for future scale-up production.[17−19]

Recently, transition-metal phosphides (TMPs) have been reported to be efficient electrocatalysts for water electrolysis.[20−22] Their high activity and stability, natural abundance, and low cost make them promising alternatives to noble metals. Transition-metal carbides (TMCs) such as iron carbides also show promising performance in the ORR.[23,24] The formation of heterogeneous structures between different substances can combine different properties, resulting in a multifunctional electrocatalyst.[25−31] The previous research demonstrated that interfacial engineering of electrocatalysts was proved to be efficient in improving electrocatalytic performance.[32] Therefore, the HER, OER, and ORR performance may be simultaneously enhanced by the construction of a TMP/TMC heterojunction. However, the controllable construction of TMP/TMC heterostructures catalysts with abundant interfaces remains challenging. TMPs exist with many kinds of crystal phases, and the different crystal phases tend to expose different crystalline surfaces and the crystal face coefficients cannot be easily matched, leading to a difficulty in controlling the crystal phase during the construction of a heterojunction, which leads to a complex product composition. Thus, a clear understanding of the synergistic catalytic effect between TMP/TMC heterojunctions becomes more challenging, thus hindering the development of heterojunction catalysts.

Herein, we have developed a facile one-step solid phase polymerization and confined pyrolysis approach for the synthesis of an FeP/Fe3C-based (x = 1, 2) heterojunction with controllable iron phosphide crystal phases (Fe2P and FeP) as a trifunctional electrocatalyst for the HER, OER, and ORR. The as-constructed heterojunction catalysts are comprised of highly dispersed FeP/Fe3C nanoparticles encapsulated in N- and P-codoped carbon (referred to as FeP/Fe3C@NPC and Fe2P/Fe3C@NPC). On comparison of Fe2P/Fe3C@NPC and Fe3C@NPC, the interfaces of FeP and Fe3C facilitate the charge transfer, thus leading to an enhanced efficiency toward electrocatalysis. By an effective heterojunction interface regulation, the synergy between the FeP/Fe3C heterojunction interface and NPC modified the electronic structure, thus improving the HER, OER, and ORR electrocatalytic performance synchronously, realizing the effect of “1 + 1 > 2”. Typically, the FeP/Fe3C@NPC catalyst exhibits outstanding HER activity with a low overpotential at 10 mA cm–2 for the HER (97 mV) and OER (440 mV) and a high half-wave potential of 0.87 V for the ORR, as well as excellent stability in alkaline media. Furthermore, an as-assembled Zn–air battery using the FeP/Fe3C@NPC catalyst exhibits a high power density of 148 mW cm–2 and excellent cyclic stability, which was superior to those of Pt/C + Ir/C catalysts, suggesting the promising potential of the FeP/Fe3C@NPC catalyst in the energy storage and electrocatalysis fields.

Results and Discussion

Synthesis and Structure Characterization of FeP/Fe3C@NPC Catalyst

The FeP/Fe3C@NPC and Fe2P/Fe3C@NPC catalysts were synthesized by a facile one-step solid-phase polymerization and confined pyrolysis approach (Figure a). First, FePPc was synthesized by a solid-phase polymerization reaction in a muffle furnace. Then, through the pyrolysis of FePPc with different mounts of sodium phytate, the phosphatization and carbonization processes proceeded, forming the FeP/Fe3C-based (x = 1, 2) heterojunction. By controlling the mole ratio of sodium phytate and FeCl3, heterostructured FeP/Fe3C@NPC and Fe2P/Fe3C@NPC can be obtained successfully. This synthesis method is simple and easily controllable; it has no metal loss, no solvent is involved, and the reaction can be carried out on a large scale. Additionally, Fe3C@NC was also prepared without the addition of sodium phytate for comparison.

Figure 1

(a) Synthesis scheme of the FeP/Fe3C-based (x = 1, 2) heterojunction with controllable iron phosphide crystal phases. (b) TEM, (c) HRTEM, (d) SAED, (e) HAADF-STEM, (f) HAADF-STEM-EDS mapping, (g) N2 adsorption–desorption isotherm, and (h, i) pore size distribution curves of the as-synthesized FeP/Fe3C@NPC catalyst.

The crystal phases of the as-synthesized catalysts were analyzed by X-ray diffraction (XRD). As shown in Figure S1, all of the XRD patterns show a peak at 25.9°, corresponding to the (002) crystal plane of graphene.[33] In comparison to other samples, the intensities of the (002) peaks for FeP/Fe3C@NPC and Fe2P/Fe3C@NPC are much broader, indicating the low graphitization degrees in the samples, attributed to the combination of the heterostructured FeP/Fe3C or Fe2P/Fe3C leading to a more disordered atomic arrangement.[34] The XRD patterns proved the heterostructure composition of FeP/Fe3C@NPC and Fe2P/Fe3C@NPC. The peaks at 35.2, 39.5, 42.8, 43.6, 44.8, 45.8, 49.1, 51.8, and 54.1° can be attributed to the crystal faces (200), (002), (211), (102), (031), (121), (221), (122), and (202), respectively, in Fe3C (PDF 00-35-0772). The peaks at 32.7, 34.6, 35.6, 46.4, 46.9, 48.3, and 59.5° can be assigned to the (011), (200), (120), (121), (220), (211) and (002) crystal planes of FeP (PDF 00–39–0809), and the peaks at the 40.7°, 44.2°, 47.4°, 52.9° and 54.7° belong to the (111), (201), (210), (002) and (211) crystal planes of Fe2P, respectively (PDF 00-51-0943). The different phase compositions of FeP/Fe3C@NPC and Fe2P/Fe3C@NPC demonstrated that sodium phytate acts as the P source and its feeding amount affects the phosphating degree.

Transmission electron microscope (TEM) images displayed the ultrathin flake structure of the carbon support (Figure S2), and the FeP/Fe3C (Figure b), Fe2P/Fe3C (Figure S3), and Fe3C (Figure S4) nanoparticles were distributed on the carbon uniformly. A high-resolution TEM (HRTEM) image (Figure c) shows a clear crystal boundary (red line) between FeP and Fe3C. The fringe spacings of 0.1876, 0.235, and 0.1873 nm can be well assigned to the (022), (210), and (131) crystal planes of Fe3C, respectively, and the fringe spacings of 0.193, 0.244, and 0.266 nm can be well assigned to the (220), (111), and (022) crystal planes of FeP, respectively. The diffraction rings in the selected area electron diffraction (SAED) (Figure d) can be assigned to the (022) and (220) crystal planes of FeP and the (131) and (210) crystal planes of Fe3C. From the high-angle annular dark-field scanning transmission electron microscope (HAADF-STEM) image (Figure e), the FeP/Fe3C nanoparticles could be seen to be encapsulated by a carbon layer, which can protect the FeP/Fe3C nanoparticles from the corrosion of electrolytes in the electrocatalytic process, and this thin carbon layer can also promote the transportation of electrons and ions, accelerating the kinetic process. HAADF-STEM and energy dispersive X-ray spectroscopy (EDS) elemental mapping images (Figure f) confirmed the existence of the Fe, C, N, and P elements. C and N were distributed in the whole sample uniformly, P was distributed in the whole sample and the nanoparticle part, while Fe was only distributed on the nanoparticle part, proving that the FeP/Fe3C nanoparticles were anchored by the N- and P-doped carbon.

The N2 adsorption–desorption isotherms (Figure g) and Barrett–Joyner–Halenda (BJH) pore size distribution curves (Figure h,i) were used to further demonstrate the porous structure of FeP/Fe3C@NPC. The type IV isotherms with an obvious H3 hysteresis effect illustrate the mesoporous and microporous characteristics of the catalysts. FeP/Fe3C@NPC has a larger Brunauer–Emmett–Teller (BET) surface area (245 m2 g–1), and the average pore size is 1.1 nm.

Raman spectra were used to characterize the structure of N- and P-doped carbon (Figure S5). The Raman shift at about 1358 cm–1 can be assigned to the D band, reflecting the defects in the sp2 carbon network, and 1580 cm–1 is assigned to the G band, representing the structural intensity.[33] The FeP/Fe3C@NPC showed a larger ID/IG value (1.04) in comparison to those of Fe2P/Fe3C@NPC (1.02), Fe3C@NPC (1.00) and NPC (0.98), suggesting increased defects or disorders, which can favor electron transfer in the electrocatalytic process, improving the catalytic performance.

The chemical states of the surface elements were characterized by X-ray photoelectron spectroscopy (XPS). As shown in Figure S6, in the Fe 2p spectrum, the peaks located at 710.7 and 722.9 eV can be attributed to the Fe species in Fe–P or Fe–N, while the peaks at 714.1 and 725.65 eV correspond to the higher-valence Fe species and the peaks at 716.4 and 728.2 eV are satellite peaks.[35] For the N 1s spectrum, the peaks at 399.3, 398.3, 401.4, and 400.6 eV can be attributed to the Fe–N bonds, pyridinic N, graphitic N, and pyrrolic N, respectively. The oxide N also can be detected by the peak at 403.1 eV.[36] The peaks located at 130.1, 131.6, and 134.4 eV in the P 2p survey can be assigned to P–Fe bonds, P–C bonds, and P–O bonds,[37] respectively.

The electronic structures and local chemical configurations of the samples were further studied by synchrotron-radiation-based X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS). The XANES of the Fe K-edge indicated that the absorption edges of FeP/Fe3C@NPC and Fe3C@NPC were located between the Fe foil and FeO, revealing that the valence of Fe in FeP/Fe3C@NPC and Fe3C@NPC is between 0 and +2 (Figure a). Furthermore, we also observed a shift toward higher energy from Fe3C@NPC to FeP/Fe3C@NPC, which indicated that the oxidation state of Fe was increased.[33] The first derivatives of the Fe K-edge XANES (Figure b) also corroborated this result. The absorption edge energies (E0) are defined as the values of the first maxima in the first-derivative spectra.[38] The E0 values of Fe in Fe foil, FeO, Fe2O3, FePc, Fe3C@NPC, and FeP/Fe3C@NPC are 7112, 7122, 7126, 7123, 7118.8, 7121 eV, respectively. The average oxidation states were analyzed from the Fe K-edge XANES to precisely determine the valence state of Fe (inset of Figure b). It can be seen that the average oxidation states of Fe in Fe3C@NPC and FeP/Fe3C@NPC are +1.3 and +1.8, respectively, in contrast to those for Fe foil (0), FeO (+2), and Fe2O3 (+3). From the Fourier-transformed (FT) k3-weighted EXAFS curve of FeP/Fe3C@NPC, by comparison with Fe foil, FeO, Fe2O3, FePc, and Fe3C@NPC, two primary peaks in R space of 1.3 and 1.8 Å can be observed (Figure c), which can be attributed to the Fe–C and Fe–P coordination shells of FeP/Fe3C@NPC. In addition, the wavelet transform (WT) contour plots were obtained to further demonstrate the Fe–C and Fe–P patterns. From the Fe WT contour plots (Figure d), the Fe–C and Fe–P coordination of FeP/Fe3C@NPC can be observed at 4 and 7 Å–1, respectively, in contrast to those for Fe3C@NPC, Fe2O3, FeO, FePc, and Fe foil.

Figure 2

(a) XANES and (b) the first derivatives of the Fe K-edge (the inset in (b) shows the fitted average oxidation states of Fe from XANES spectra). (c) k3-weighted Fourier transform (FT) spectra of Fe at R spaces (d) WT contour plots.

Electrocatalytic Performance

We first tested the HER and OER electrocatalytic performance in 1 M KOH solution with a three-electrode system. From the linear sweep voltammetry (LSV) curves for the HER (Figure a), we found that the Fe3C@NPC showed even worse electroactivity, suggesting that Fe3C may not be a suitable catalyst for the alkaline HER. However, the activity was enhanced when heterostructures between Fe3C and iron phosphide (FeP or Fe2P) were formed. FeP/Fe3C@NPC showed better electroactivity than Fe2P/Fe3C@NPC. FeP/Fe3C@NPC shows a smaller overpotential at the current density at 10 mA cm–2 (η10 = 97 mV) in comparison to that for Fe2P/Fe3C@NPC (η10 = 134 mV). The better HER activity of FeP/Fe3C@NPC in comparison to Fe2P/Fe3C@NPC is due to the phosphating degree in FeP being higher than that in Fe2P, which could provide more P atoms to capture protons.[39] The Tafel curves obtained from the LSV curves demonstrated the smallest Tafel slope for FeP/Fe3C@NPC (89.3 mV dec–1) among the synthesized samples (Figure b), demonstrating that this reaction occurs by a Volmer–Heyrovsky mechanism. The smallest semicircle radius in the electrochemical impedance spectroscopy (EIS) curves for FeP/Fe3C@NPC corresponds to the smallest Rct value (Figure S7), indicating that FeP/Fe3C@NPC has the best conductivity and the most favorable kinetics. The electrochemical double-layer capacitance (Cdl) of the FeP/Fe3C@NPC catalyst is 21.3 mF cm–2 (Figure S8), which is higher than those of Fe2P/Fe3C@NPC (15.6 mF cm–2) and Fe3C@NPC (0.95 mF cm–2), indicating the higher electrocatalytically active surface area of FeP/Fe3C@NPC catalyst for HER. the The LSV curves for the OER (Figure d) suggested that FeP/Fe3C@NPC also has the lowest overpotential at the same current density (η10 = 440 mV) and the smallest Tafel slope (108.6 mV dec–1) among all the samples (Figure e). Similarly, FeP/Fe3C@NPC also has the smallest Rct value (Figure S9), demonstrating the faster kinetics in the OER process. Moreover, the Cdl value of FeP/Fe3C@NPC catalyst is 8.9 mF·cm–2 (Figure S10), which is higher than those of Fe2P/Fe3C@NPC (5.4 mF cm–2) and Fe3C@NPC (0.8 mF cm–2), further indicating the higher electrocatalytically active surface area of FeP/Fe3C@NPC catalyst for the OER. The chronoamperometric curves proved that the electrocatalytic activity can be maintained for at least 24 h for the HER and OER (Figure c), demonstrating the good stability. This hierarchical nanostructure, in which the FeP/Fe3C heterostructure was anchored by the N- and P-codoped carbon, increased the active site dispersion and prevented its aggregation in the electrocatalytic process, synchronously improving the electrocatalytic activity and stability. In addition, the formation of the heterogeneous interface optimizes the electronic structure of the catalyst, resulting in the increased oxidation state of Fe, which benefits the electrocatalytic process.[40]

Figure 3

(a, d, f) LSV, (b, e, h) Tafel, and (c) i–t curves of the as-synthesized catalysts. The inset in (f) shows an activity comparison of Jk and E1/2 for the ORR. (g) LSV curves of the FeP/Fe3C@NPC catalyst at 1600 rpm for the ORR (the inset shows the K-L plots at different rotation rates). (i) LSV curves of the initial FeP/Fe3C@NPC catalyst and the catalyst after 10000 cycles (the inset shows the chronoamperometric responses of 20% Pt/C and FeP/Fe3C@NPC with 3 M CH3OH added to the electrolyte), (j) Charge–discharge polarization curve and power density plot. (k) Charge–discharge cycling performance of the FeP/Fe3C@NPC catalyst.

The electrocatalytic ORR activities of the FeP/Fe3C@NPC, Fe2P/Fe3C@NPC, Fe3C@NPC and Pt/C catalysts were measured at room temperature by the rotating ring-disk electrode (RRDE) method. Figure f shows the LSV curves of the as-synthesized catalysts in O2-saturated 0.1 M KOH solution. In comparison with the Fe2P/Fe3C@NPC and Fe3C@NPC catalysts, the FeP/Fe3C@NPC catalyst has a higher ORR activity with a half-potential (E1/2) of 0.87 V and a diffusion-limited current density (J) of 4.8 mA cm–2, which are comparable to those of Pt/C (E1/2 = 0.88 V, J = 4.5 mA cm–2). The calculated kinetic current density (Jk) of the FeP/Fe3C@NPC catalyst at 0.8 V is 10.7 mA cm–2 (inset of Figure f and Figure S11), higher than those of the Fe2P/Fe3C@NPC (5 mA cm–2) and Fe3C@NPC (2.4 mA cm–2) catalysts, further revealing the high ORR activity of the FeP/Fe3C@NPC catalyst. The Tafel plots (Figure h) also suggested that the FeP/Fe3C@NPC catalyst delivered a small slope of 82.1 mV dec–1 in 0.1 M KOH, respectively. From the Koutecky–Levich (K-L) plots at different rotation rates (Figure g), we also calculated the average electron transfer number, which is about 4 in 0.1 M KOH within the potential range of 0.375–0.5 V (inset of Figure g). Moreover, the FeP/Fe3C@NPC catalyst also exhibited long-term stability even after more than 10000 cycles in RRDE tests (Figure i). Nearly no E1/2 activity loss was found in 0.1 M KOH. Furthermore, there was no significant ORR current decay on the FeP/Fe3C@NPC catalyst after addition of CH3OH to the 0.1 M KOH electrolyte (inset of Figure i), revealing the high tolerance of FeP/Fe3C@NPC catalyst to CH3OH crossover. However, the Pt/C catalyst has a severe current drop with a loss rate of about 20% after the injection of CH3OH, showing that the Pt/C catalyst can be severely deactivated by the effect of methanol oxidation. The results further clearly demonstrate the high ORR activity and durability of the FeP/Fe3C@NPC catalyst in alkaline solution, which can be attributed to the strong synergy of the FeP and Fe3C heterojunction.

The high OER and ORR activity of the FeP/Fe3C@NPC catalyst in alkaline media indicated that the FeP/Fe3C@NPC catalyst can potentially be applied in a Zn–air battery. The FeP/Fe3C@NPC catalyst exhibits a lower charge–discharge voltage gap (1.15 V at 50 mA cm–2) and greater power density (148 mW·cm–2) in comparison to that of a Pt/C + Ir/C based battery (1.59 V at 50 mA cm–2 and 46 mW cm–2; Figure j), reflecting the enhanced rechargeability of the Zn–air battery. Additionally, the battery also exhibits excellent stability without an obvious voltage change after 180 cycles (60 h; Figure k).

DFT Calculation

In order to prove the synergy between the Fe3C FeP, density functional theory (DFT) calculations were conducted. We constructed two models, including FeP/Fe3C and Fe2P/Fe3C heterojunctions. For the HER reaction, the adsorption energy of H and OH (ΔEH*+OH*) was calculated to describe the decomposition process of H2O molecules on the electrode surface and the Gibbs free energy of H adsorption (ΔGH*) was calculated to describe the subsequent adsorption and desorption process of the hydrogen intermediate. It is generally accepted that the lower the energy of the final state during the decomposition of water molecules (i.e., the stronger the adsorption of H and OH), the easier the water molecule decomposition (i.e., facilitating a Volmer step), and the closer the ΔGH* is to 0 eV leads to a balance between the adsorption of protons and the desorption of H2 (i.e., facilitating a Heyrovsky step) in conjunction with a bifunctional mechanism. First, we compared the ΔEH*+OH* values at different sites in the Fe2P/Fe3C (Figure a) and FeP/Fe3C (Figure b) heterojunctions. For both of the heterojunctions, H* and OH* exhibit a strong adsorption on the Fe3C side away from the interface (−1.94 eV for FeP/Fe3C, −1.44 eV for Fe2P/Fe3C), while the adsorptions of H* and OH* on the Fe2P or FeP side away from the interface are relatively weak (−1.04 and −0.60 eV, respectively). In contrast, for Fe2P/Fe3C (−1.34 eV) or FeP/Fe3C (−1.63 eV) at the heterojunction interface, the adsorption of H and OH can be enhanced, indicating that the Fe3C component of the Fe2P/Fe3C and FeP/Fe3C interfaces is beneficial for water molecule activation. Moreover, the adsorption energy of H* and OH* on the FeP/Fe3C heterojunction interface is higher than that on the Fe2P/Fe3C heterojunction interface, which indicated that the water molecules are easily activated at the FeP/Fe3C heterojunction interface.

Figure 4

ΔEH*+OH* values at different sites in the (a) Fe2P/Fe3C and (b) FeP/Fe3C heterojunctions (the insets show the corresponding adsorption sites). ΔGH* values at different sites (sites 1–4) in the (c, e) Fe2P/Fe3C and (d, f) FeP/Fe3C heterojunctions.

Then, we compared the ΔGH* values at different sites (site 1–4) in the Fe2P/Fe3C (Figure c,e) and FeP/Fe3C (Figure d,f) heterojunctions. The calculation results show that, for both of the heterojunctions, the desorption of H2 processes on the Fe2P or FeP side away from the interface are inhibited. Interestingly, for FeP/Fe3C, the FeP component can greatly promote the desorption of H2, and the adsorption of protons and the desorption of H intermediates can achieve a very good balance at the FeP/Fe3C interface (the ΔGH* value being very close to 0 eV). In contrast, for Fe2P/Fe3C, the Fe2P component shows a strong adsorption of H*, and the interface sites in Fe2P/Fe3C cannot promote the removal of H2. Therefore, although Fe3C can promote the activation of water molecules, the subsequent removal of H protons inhibits the rate-determining step (RDS) of the HER for Fe3C. In comparison with Fe2P, the FeP interface can promote the removal of H2, and the FeP/Fe3C heterojunction can facilitate both the Volmer and Heyrovsky steps, confirming that the HER activity of the FeP/Fe3C heterojunction is better than those of Fe3C and the Fe2P/Fe3C heterojunction.

For OER and ORR processes, the adsorption and desorption of OH intermediates are the important factors affecting the entire RDS and the adsorption of OH cannot be too strong or too weak. For both Fe2P/Fe3C and FeP/Fe3C, the ΔEH*+OH* value is 1 order of magnitude smaller than the of ΔEH* value at the same sites on Fe3C away from the interface. This indicates that Fe3C exhibits strong adsorption of OH, which is not conducive for the OER and ORR processes. Because FeP has a stronger inhibitory effect on OH adsorption in comparison to Fe2P, the FeP/Fe3C heterojunction has a better promoting effect for both the OER and ORR.

Conclusion

In conclusion, we have developed a simple one-step solid-phase polymerization and confined pyrolysis method for the synthesis of a FeP/Fe3C-based (x = 1, 2) heterojunction with controllable iron phosphide crystal phases (Fe2P and FeP) as trifunctional electrocatalysts for the HER, OER, and ORR. By effective heterojunction interface regulation, the synergy between the FeP/Fe3C heterojunction interface and NPC modified the electronic structure, thus improving the HER, OER, and ORR electrocatalytic performance synchronously. The FeP/Fe3C@NPC catalyst exhibits outstanding HER activity with a low overpotential at 10 mA cm–2 for the HER (97 mV) and OER (440 mV) and a high half-wave potential of 0.87 V for the ORR, as well as excellent stability in alkaline media. Furthermore, the as-assembled Zn–air battery using the FeP/Fe3C@NPC catalyst exhibits a high power density of 148 mW cm–2 with a low charge–discharge voltage gap of 1.15 V (at 50 mA cm–2) and excellent cyclic stability, which were superior to those of Pt/C + Ir/C catalysts, suggesting the promising potential of the FeP/Fe3C@NPC catalyst in the energy storage and electrocatalysis fields. DFT calculations demonstrated that Fe3C can promote the activation of water molecules, while FeP is beneficial to the removal of H2 and the FeP/Fe3C heterojunction can facilitate both the Volmer and Heyrovsky steps in the HER process simultaneously. Moreover, FeP has a stronger inhibitory effect on OH adsorption, revealing that the FeP/Fe3C heterojunction also shows a better promoting effect for both the OER and ORR. This work provides a straightforward method to prepare heterostructured catalysts for electrocatalytic applications, and the results can shed some light on the design of multifactional electrocatalysts.

Methods

Synthesis of FePPc

FePPc was synthesized by the same method as in ref (41).

Synthesis of PPc

PPc can be prepared by the same method as for FePPc except without the addition of FeCl3.

Synthesis of FeP/Fe3C@NPC

For the synthesis of FeP/Fe3C@NPC, FePPc powder (0.2 g) was mixed with sodium phytate (0.4 g) and ground fully. Then the mixture was placed in a tube furnace and heated to 900 °C for 3 h at a heating rate of 2 °C min–1 under flowing Ar gas and then naturally cooled to room temperature. The as-obtained samples were directly used without any post-treatment.

Synthesis of Fe2P/Fe3C@NPC

For the synthesis of Fe2P/Fe3C@NPC, FePPc powder (0.2 g) was mixed with sodium phytate (0.2 g) and ground fully. Then the mixture was placed in a tube furnace and heated to 900 °C for 3 h at a heating rate of 2 °C min–1 under flowing Ar gas and then naturally cooled to room temperature. The as-obtained samples were directly used without any post-treatment.

Synthesis of Fe3C@NC

FePPc powder (0.2 g) was placed in a tube furnace and heated to 900 °C for 3 h at a heating rate of 2 °C min–1 under flowing Ar gas and then naturally cooled to room temperature. The as-obtained samples were directly used without any post-treatment.

Synthesis of NPC

For the synthesis of NPC, PPc powder (0.2 g) was mixed with sodium phytate (0.4 g) and ground fully. Then the mixture was placed in a tube furnace and heated to 900 °C for 3 h at a heating rate of 2 °C min–1 under flowing Ar gas and then naturally cooled to room temperature. The as-obtained samples were directly used without any post-treatment.

Materials Characterization

Microstructure Characterization

The surface area of the samples was calculated by the Brunauer–Emmett–Teller (BET) equation. The Barrett–Joyner–Halenda (BJH) method was used to estimate the mesoporous and microporous distribution curves. Raman measurements were obtained using a Horiba HR-800 spectrometer.

Powder X-ray Diffraction (XRD)

An X-ray powder diffractometer (Rigaku D/max 2500Pc) was used to determine the crystal phase structure of the sample with a monochromated Cu target Kα radiation source (X-ray wavelength λ = 1.5418 Å).

N2 Adsorption–Desorption Experiments

The samples were degassed at 300 °C before the measurement. Then a Quantachrome SI-MP instrument was used to perform the N2 adsorption–desorption experiments at 77 K.

X-ray Photoelectron Spectroscopy (XPS)

XPS was carried out with a ULVAC PHI Quantera microscope. The binding energies (BEs) were calibrated by setting the measured BE of C 1s to 284.8 eV.

Transmission Electron Microscopy (TEM)

TEM was carried out with a Hitachi 7700 instrument working at 100 kV. A field emission electron microscope (JEOL JEM-2100F) working at 200 kV was used to collect the high-resolution TEM (HRTEM) and high-angle annular dark-field scanning TEM (HAADF-STEM) images and the corresponding energy-dispersive X-ray spectroscopic (EDX) mapping.

X-ray Absorption Fine Structure (XAFS)

The XAFS spectra were obtained at the 1W1B station at the BSRF (Beijing Synchrotron Radiation Facility, People’s Republic of China) operating at 2.5 GeV with a maximum current of 250 mA. XAFS measurements at the Fe K-edge were performed in fluorescence mode using a Lytle detector. All samples were pelletized as disks of 13 mm diameter with 1 mm thickness using graphite powder as a binder.

The acquired EXAFS data were processed according to the standard procedures using the ATHENA module implemented in the IFEFFIT software packages, as in ref (41).

Electrocatalytic Measurements

A CHI 760E electrochemical workstation (CH Instruments, Inc., Shanghai) was used to carry out the electrochemical measurements with a standard three-electrode setup, as in ref (33). The electron transfer number (n) and kinetic current density (Jk) can be calculated from the Koutecky–Levich equationwhere J is the measured current density, Jk and Jl are the kinetic and limiting current densities, respectively, ω is the angular velocity of the disk, n is the electron transfer number, F is the Faraday constant (96485 C mol–1), C0 is the bulk concentration of O2 (1.2 × 10–6 mol cm–3), D0 is the diffusion coefficient of O2 in 0.1 M KOH (1.9 × 10–5 cm2 s–1), and V is the kinematic viscosity of the electrolyte (0.01 cm2 s–1).[42]

The stability test of the Fe–N4 SAs/NPC catalyst was performed in an O2-saturated 0.1 M KOH electrolyte at room temperature by applying potential cycling at a sweep rate of 50 mV s–1 for 10000 cycles.

The OER and HER measurements[33] were performed in O2- or N2-saturated 1 M KOH solutions, respectively. All of the measured potentials in this work were determined with iR compensation and were converted to the reverse hydrogen electrode (RHE) by the equation

The rechargeable Zn–air battery test was conducted with a homemade battery, where an as-synthesized FeP/Fe3C@NPC-catalyst-coated carbon paper and Zn plate were used as the air electrode and anode, respectively. The catalyst loading was 0.5 mg cm–2 and a 6 M KOH solution containing 0.2 M ZnCl2 was used as the electrolyte. The cycling test was conducted by one discharge step (2 mA cm–2 for 10 min) followed by a charge step for the same current and time.

Computational Details

Theoretical calculations were performed using the Vienna ab initio simulation package (VASP) based on density functional theory.[43] Interactions between core and valence electrons were described by the projector augmented wave (PAW) pseudopotentials.[44] The generalized gradient approximation (GGA) in the scheme proposed proposed by Perdew, Burke, and Ernzerhof (PBE) was adopted to express the electron exchange correlation with a cutoff energy of 400 eV, while the van der Waals effect and hydrogen-bonding interactions were accounted for by DFT-D3.[45] All atoms were fully relaxed in z dimensions until all residual forces were below 0.02 eV Å–1, and the convergence of energy was set to 1 × 10–5 eV.