Binary FeNi phosphides dispersed on N,P-doped carbon nanosheets for highly efficient overall water splitting and rechargeable Zn-air batteries

Journal Pre-proofs Binary FeNi phosphides dispersed on N,P-doped carbon nanosheets for highly efficient overall water splitting and rechargeable Zn-air batteries Jin-Tao Ren, Yan-Su Wang, Lei Chen, Li-Jiao Gao, Wen-Wen Tian, ZhongYong Yuan PII: DOI: Reference:

S1385-8947(20)30399-5 https://doi.org/10.1016/j.cej.2020.124408 CEJ 124408

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

9 November 2019 3 February 2020 9 February 2020

Please cite this article as: J-T. Ren, Y-S. Wang, L. Chen, L-J. Gao, W-W. Tian, Z-Y. Yuan, Binary FeNi phosphides dispersed on N,P-doped carbon nanosheets for highly efficient overall water splitting and rechargeable Zn-air batteries, Chemical Engineering Journal (2020), doi: https://doi.org/10.1016/j.cej.2020.124408

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Binary FeNi phosphides dispersed on N,P-doped carbon nanosheets for highly efficient overall water splitting and rechargeable Zn-air batteries Jin-Tao Ren, Yan-Su Wang, Lei Chen, Li-Jiao Gao, Wen-Wen Tian, Zhong-Yong Yuan* Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), National Institute for Advanced Materials, School of Materials Science and Engineering, Nankai University, Tianjin 300350, China *

Corresponding author. E-mail: [email protected]

1

Abstract Hybridization of non-precious metals with advanced carbon monolith provides a promising approach to substitute noble metal catalysts for various electrochemical reactions. The structural and electronic properties of non-precious metals significantly affect electrocatalytic performance, which depends on the physicochemical structure of carbon substrates and the types of metal components. Herein, binary FeNi phosphide nanoparticles coupled with N,P-modified carbon nanosheets were fabricated through an universal carbonization-phosphorization approach. Besides the structural advantage of 2D nanosheets, the electronic structure change of bimetallic FeNi phosphides via modulating Fe/Ni ratio is another reason responded for the exceptional activities with reduced overpotentials of this hybridized catalyst for both hydrogen evolution, oxygen evolution and oxygen reduction reactions. Furthermore, by employing the fabricated materials as electrode catalysts for overall water splitting and rechargeable Zn-air batteries with liquid and solid-state electrolytes, excellent performances are achieved in these energy conversion and storage devices, revealing the significant feasibility for practical applications. Keywords: metal phosphides; heteroatoms doping; porous carbon; trifunctional electrocatalysts; electrocatalysis.

1. Introduction The sustainable energy storage and conversion technologies, for instance, electrochemical water splitting and metal-air batteries, have attracting even-increasing 2

attention [1,2]. However, the sluggish reaction kinetics resulted from the multi-electron transfer steps for electrochemical reactions including hydrogen evolution reaction (HER), oxygen evolution reaction (OER), and oxygen reduction reaction (ORR), heavily impedes their energy efficiency and power utilization [3,4]. To date, the stateof-the-art electrocatalysts for HER, OER, and ORR are still Pt-based and Ir/Ru-based materials, however, the unfavorable cost and dramatically decreased catalytic efficiency terribly hamper the large-scale industrial development of the corresponding regenerative energy technologies [5,6]. In addition, single precious metals are unable to simultaneously provide sufficient activities for ORR, OER, and HER, whereas the different electrochemical processes occurred on one electrode for those energy technologies, for instance, both OER and ORR occur at the air cathode for rechargeable Zn-air battery, urgently need multifunctionality of electrocatalysts [7,8]. In this regard, developing the alternative catalysts with earth-abundant elements, as well as highly multifunctional activities beyond the traditional noble metals is urgently needed. Recently, transition metals (e.g., Fe, Co, and Ni) and their derivatives, for instance, alloys, oxides, nitrides, sulfides, selenides, carbides, hydroxides and phosphides, exhibit significant potential as promising alternatives to noble metal catalysts because of their abundant reserves, favorable-cost, and comparable electrocatalytic performance [9-12]. Furthermore, their catalytic activity can be further boosted to some extent by several strategies including elemental doping/alloying, tuning porosity and nanostructure engineering [13]. However, transition metal-based materials still suffer from the insufficient multifunctionality and the unfavorable durability under harsh 3

operation electrolytes. To address those problems, hybridization of transition metal and porous heteroatom-modified carbons into one architecture has been proposed in previous studies [14,15], which can not only inherit the advantages of individual components, but also obtain synergistic effect to boost the overall performance. Usually, the heteroatom-doped carbons and the metal-N-C materials always present high ORR activity [16,17], while, the transition metal phosphides have been confirmed as the highly efficient HER and OER electrocatalysts, although the active species of metal phosphides are the in situ formed surface phosphates or (oxy)hydroxides species during OER process [18,19]. The theoretical calculations and experimental results demonstrate that the O2 molecules can be activated by the Fe-P bond, and thus promoting the following ORR process [20,21]. Additionally, in contrast to single metal phosphides (e.g., FeP [22], Ni2P [23], and CoP [24]), coupling secondary metal to form binary metal phosphides (e.g., FeNiP [25], CoFeP [26], and CoMnP [27]) would tune the bond coordination and electronic structure, leading to the enhanced catalytic activity. It is indicated that the electronic environment of bimetallic nanoparticles evidently affect their electrochemical activities, which are related to the type of metals and their compositional ratio [28-30]. However, there still lack sufficient studies to investigate the binary metal nanoparticles and tune the ratio of the metals, not to mention bimetallic phosphide-carbon hybrid materials, though such materials have huge potential as multifunctional catalysts for HER, OER, and ORR. In this work, an one-pot carbonization-phosphorization strategy was developed for the fabrication of porous N,P-doped carbon nanosheets coupled with binary FeNi 4

phosphide nanoparticles. By using organophosphonic acid and polyvinylpyrrolidone as the precursors, the metal phosphide nanoparticles and N,P-doped carbon substrate were together obtained after pyrolysis. The morphology of the fabricated FeNiP/NPCS materials and the Fe/Ni ratio of binary FeNi phosphides were controlled to understand the structural and electronic properties on the electrochemical activities. The optimal FeNiP/NPCS catalyst pyrolyzed at 1000 ℃ with Fe/Ni ratio of 1 exhibits extraordinary activity and stability for HER, OER, and ORR. Furthermore, the synthesized materials were employed as multifunctional electrode catalysts for overall water splitting and rechargeable Zn-air batteries with liquid and solid-state electrolytes, showing high cell configuration efficiency. For instance, the two-electrode electolyzer delivers an overall current density of 10 mA cm-2 at 1.71 V with the faradaic efficiency of 97%, and maintains this potential beyond 12 h without evident decay. The assembled liquid Znair battery also exhibits large power density (163 mW cm-2), high round-trip efficiency (69%) and remarkable cycling stability (330 cycles).

2. Experimental Section 2.1. Synthesis For the synthesis of FeNiP/NPCS catalyst, the calculated graphitic carbon nitride (g-C3N4) (5 g), 1-hydroxyethylidene-1,1-diphosphonic acid (HEDP) (10 mmol), polyvinylpyrrolidone (PVP) (3 g) and transition metal nitrates ( 2 mmol) (Fe/Ni molar ratio of 1/1) were added into 50 mL deionized water under vigorous stirring. The obtained slurry was dried at 80 ℃, followed by annealing at 1000 ℃ for 2 h in N2 5

atmosphere. By adjusting the Fe/Ni molar ratios to 2/0, 1.5/0.5, 0.5/1.5, and 0/2, the samples of FeP/NPCS, Ni1.5Fe0.5P/NPCS, Ni0.5Fe1.5P/NPCS, and NiP/NPCS were obtained. For comparison, the FeNiP nanoparticles dispersed on carbon monolith (FeNiP/NPC) with featureless structure was also fabricated according to the similar procedures to that of FeNiP/NPCS without using graphitic carbon nitride. The bimetallic FeNi dispersed on N-doped carbon nanosheets (FeNi/NCS) was also fabricated through the similar procedures to that of FeNiP/NPCS in the absence of organophosphonic acid. The N,P-codoped carbon nanosheets (NPCS) was fabricated by pyrolyzing the reaction mixture without transition metal salts. 2.2. Physicochemical characterization Scanning electron microscopy (SEM, Jeol JSM-7500L) and transmission electron microscopy (TEM, Jeol JSM-2800) were taken to investigate the micro-/nanostructures of the obtained samples. A Bruker D8 Focus diffractometer was used to perform the powder X-ray diffraction (XRD) patterns. Raman spectra were scanned using a TEO SR-500I-A spectrometer with 532 nm excitation length. The chemical information of the catalysts was examined by X-ray photoelectron spectroscopy (XPS) on a Thermo Scientific ESCALAB 250Xi spectrometer. The Brunauer-Emmett-Teller (BET) specific surface areas were obtained from the N2 adsorption-desorption isotherms measured at 77 K using a Quantachrome Nova 2000e sorption analyzer. 2.3. Electrochemical measurement 6

All electrochemical measurements were tested on a Pine WaveDriver 20 electrochemical workstation with the typical three-electrode configuration using the graphitic rod as counter electrode, and the Ag/AgCl as the reference electrode. For the preparation of working electrode, 5 mg of catalyst was dispersed into 980 μL of deionized water/isopropanol (v:v=1:1) and 20 μL of Nafion solution (5 wt%) with sonication to obtain homogeneous catalyst ink. Then, 10 μL of catalyst ink was transferred onto the glass carbon disk (GCD) electrode with the loading mass of about 0.25 mg cm-2. The obtained electrode was used as the working electrode for the electrochemical testing under ambient temperature. All the presented current density was normalized to the geometric area of the working electrode. And the resultant potential (vs. Ag/AgCl) was calibrated with the reversible hydrogen electrode (RHE) by adding the value of (0.197 + 0.059 × pH). As to hydrogen evolution reaction, all the polarization curves were measured at a scan rate of 5 mA s-1 under 1600 rpm with iR compensation in 0.5 M H2SO4 and 1.0 M KOH. As to oxygen reduction reaction, the polarization curves were measured with the scan rate of 5 mV s-1 under 1600 rpm within O2-saturated 1.0 M KOH. All the present polarization curves were measured with iR compensation. As to oxygen reduction reaction, the linear sweep voltammetry (LSV) polarization curves were obtained at the scan rate of 5 mV s-1 under the rotation speed of 1600 rpm in O2-saturated 0.1 M KOH and 0.5 M H2SO4. The Tafel slope was calculated according to the Tafel equation of η = a + b·log(J/ 7

J0), where η, b, J and J0 indicate the overpotential, Tafel slope, current density, and exchange current density, respectively. The electron transfer number (n) per oxygen molecule was obtained from the Koutecky-Levich plots based on the equations of 1/J = 1/Jk + 1/JL = 1/Jk + 1/Bω1/2 and B = 0.2nF(DO2)2/3υ-1/6CO2, where J, Jk and JL are the measured current density, kinetic current and diffusion limiting current density, respectively. ω is the electrode rotating rate. n is the transferred electron number per oxygen molecule. F (96485 C mol-1) is the Faraday constant. DO2 (1.9 × 10-5 cm2 s-1) is the diffusion coefficient of O2 in 0.1 M KOH. υ (0.01 cm2 s-1) is the kinetic viscosity. CO2 (1.2 × 10-6 mol cm-3) is the bulk concentration of O2. The constant 0.2 is adopted when the rotation speed is expressed in rpm. Hydrogen peroxide yields (HO2-%) and the corresponding electron transfer number (n) were calculated from rotation ring-disk electrode (RRDE) measurements, wherein the disk electrode was scanned with the rate of 5 mV s-1, and the ring electrode potential was kept at 1.2 V (vs. RHE), by the equations of HO2−% = 200Ir/(IdN + Ir) and n = 4Id/(Id + Ir/N), where Id and Ir are the disk and ring currents, and the current collection efficiency (N) of the Pt ring is determined to be 0.37. The electrochemical double layer capacitance (Cdl) was determined by the cyclic voltammograms under the scan rate from 10 to 20 mV s-1 in a narrow non-Faradaic potential range. The plots of the capacitive currents at certain potential against the scan rates for electrocatalysts are the value of Cdl. The electrochemical impedance spectroscopy (EIS) tests were performed at holding potential in the frequency range 8

from 0.01 to 100000 Hz with the amplitude of 5 mV. The overall water splitting performance of the prepared catalyst was evaluated in a two-electrode configuration with the catalyst-coated Ni foam (1.0 mg cm-2) as both anode and cathode. The polarization curves were measured with the scan rate of 5 mV s-1 in 1.0 M KOH. The performance of overall water splitting cells with anode and cathode from Pt/C-IrO2 pair was also recorded as reference. The liquid Zn-air battery was assembled and tested on the homemade Zn-air cells. The commercial carbon paper with catalyst loading of 1.0 mg cm-2 was used as the air cathode. And the polished Zn plate was used as the anode. The 6.0 M KOH solution containing 0.2 M Zn(Ac)2 was employed as electrolyte. The all-solid-state Zn-air battery was assembled by the polished Zn plate as anode, and the carbon cloth coated with catalyst (1.0 mg cm-2) as cathode. The alkaline gel polymer as solid electrolyte was prepared as following procedures: 1.0 g polyvinyl alcohol (PVA) powder was dissolved into 10 mL H2O at 95 ℃ under stirring for 2 h. And then the 3 mL solution of 6.0 M KOH containing 0.2 M Zn(Ac)2 was transformed into the above transparent gel. After being stirring for another 30 min, the gel was freezed. The charge-discharge cycling stability for liquid and all-solid-state Zn-air batteries were tested on a LandCT2001A instrument.

3. Results and Discussion 3.1. Materials synthesis and characterization The FeNiP/NPCS catalytic materials were prepared through the scalable 9

carbonization-phosphorization

strategy

with

the

reaction

mixtures

of

organophosphonic acid HEDP, polyvinylpyrrolidone (PVP), iron and nickel nitrates, and g-C3N4 under N2 atmosphere. The total synthetic process is schematically illustrated in Scheme 1. To efficiently tailor the nanostructures and compositions of the fabricated hybrid materials, the thermal-decomposable g-C3N4 and phosphorusenriched HEDP were used in the synthesis. The organic compound PVP provides abundant N and C sources [16,31]. A thin layer of homogeneous precursor mixture of PVP, HEDP and metal salts is covered on g-C3N4 after drying in oven at 80 ℃. Along with the gradual decomposition of g-C3N4 under high-temperature pyrolysis, PVP, HEDP and metal salts are converted into heteroatoms-modified carbon and metal phosphide nanoparticles. The phosphonic groups of HEDP cross-link the metal cations to generate metal phosphides, and also provide additional P source to form P-doped carbons [24,32]. This synthetic strategy can readily tune the metal ratio of Fe/Ni in phosphides through the adjusting of feeding amount of metal nitrates. SEM (Fig. 1a-b) and TEM (Fig. 1c-e) images exhibit the FeNiP/NPCS catalyst treated at 1000 ℃ has ultra-thin nanosheet-like structure with densely spreading nanoparticles. High resolution TEM image (Fig. 1f) displays the interplane spacing of about 0.22 nm of the embedded nanoparticles, corresponding to the (111) plane of hexagonal Fe2P or Ni2P. The corresponding elemental mappings in Fig. 1g present the uniform distribution of N, P, Fe and Ni elements throughout the entire structure. And the high spatial correlation among Fe, Ni, and P further manifests the formation of bimetallic FeNiP materials. Likewise, the fabricated Fe2-xNixP/NPCS samples with 10

different Fe/Ni ratios show the similar nanosheet morphology with metal phosphide nanoparticles as investigated by SEM (Fig. S1) and TEM images (Fig. S2). Such interesting results demonstrate the universal nature of this carbonization strategy for the fabrication of metal phosphide-carbon hybrids with advanced nanosheet structure. In contrast, the control samples were prepared by employing the identical procedures to that of FeNiP/NPCS except the using of organophosphonic acid of HEDP. The FeNi nanoalloys embedded bamboo-like carbon nanotubes tangled with carbon nanosheets (FeNi/NCS) are clearly observed, as shown in Fig. S3. In addition, the aggregated FeNiP nanoparticles decorated carbon matrix (FeNiP/NPC) (Fig. S4) are obtained according to the similar procedures to that of FeNiP/NPCS except the presence of g-C3N4. On the one hand, the higher P content (30 at%) of HEDP ensures the generation of phosphorus from phosphonic acid to produce metal phosphides through in situ phosphorization under higher pyrolysis temperature [33]; on the other hand, the complete decomposition property of g-C3N4 under higher temperature (> 710 ℃) facilitates it to employ as self-sacrificial template to efficiently tailor the morphology of PVP-derived carbon materials [16,31]. These controlled results confirm that the reasonable assembly of different precursors is vital importance for the resultant products. The crystal structure of those fabricated samples was characterized by the XRD patterns. In Fig. 2a, for the pure FeP/NPCS and NiP/NPCS samples, the distinct diffraction peaks can be indexed to the standard powder diffraction files of Fe2P phase (JCPDS No. 51-0943) and Ni2P phase (JCPDS No. 03-0953), without any other 11

secondary phases. For the bimetallic samples of Fe0.5Ni1.5P/NPCS, FeNiP/NPCS and Fe1.5Ni0.5P/NPCS, they all share the similar diffraction characteristics as FeP/NPCS and NiP/NPCS, whereas their diffraction peaks exhibit positively shift or negatively shift when comparing with that of FeP/NPCS or NiP/NPCS, respectively. Such results may be caused by the introduction of secondary element to form bimetallic phosphides, leading to the enlarged crystal-plane distance [19,34]. Besides the typical diffraction peaks belonged to metal phosphides, the weak and broad peaks located at around 26o (2θ) are attributed to the (002) plane of graphitic carbon for those metal-carbon hybrid materials. The N2 adsoprtion-desorption isotherms (Fig. S5a) demonstrate the similar Brunauer−Emmett−Teller surface area (SBET) among those fabricated Fe2-xNixP/NPCS, for instance, FeP/NPCS (230 m2 g-1), Fe0.5Ni1.5P/NPCS (241 m2 g-1), FeNiP/NPCS (226 m2 g-1), Fe1.5Ni0.5P/NPCS (192 m2 g-1), and NiP/NPCS (216 m2 g-1). However, the metal-free NPCS exhibits a higher surface area of 468 m2 g-1 (Fig. S6). The decreased specific surface area of Fe2-xNixP/NPCS may be related to the aggregated metal phosphide nanoparticles in the fabricated porous carbon nanosheets [32,33]. The distinct hysteresis loops observed in the sorption isotherms demonstrate the typical porous nature for all samples, consistent with the corresponding pore size distribution curves (Fig. S5b), which is benefit for the exposure of active sites and improvement of electrolyte diffusion [35,36]. The carbonaceous skeleton information of those fabricated Fe2-xNixP/NPCS and NPCS was investigated by Raman spectroscopy. In Fig. 2b, the distinct characteristic 12

peaks located at about 1360 and 1590 cm-1 are ascribed to the D and G bands, respectively. The D band is associated with the defects or disorder in carbon frameworks. The bond-stretching vibration of sp2-hybridized carbon leads to the G band. The intensity ratio of D/G (ID/IG) is about 0.96 to 0.99 for those fabricated Fe2xNixP/NPCS, which demonstrates the high concentration of disorder or defect of obtained materials. And the broad 2D band centered at around 2700 cm-1 is related to the characteristic of graphitic carbon, which suggests the good conductivity of the fabricated carbon-based hybrid materials. XPS was performed to detect the electron and chemical state of FeNiP/NPCS. As shown in Fig. S7a, the XPS survey scan of FeNiP/NPCS demonstrates the presence of the target elements including C, N, O, P, Fe, and Ni. The C 1s spectrum in Fig. S7b exhibits the typical components of C-C/C=C, C–N/C-P, C–O/C=O, and π-π* at the binding energies of 284.6, 285.7, 287.2, 288.9 eV, respectively [37,38]. As displayed in Fig. S7c for O 1s spectrum, the peaks centered at 530.3 and 531.7 eV correspond to the bridging oxygen in Fe/Ni-O-P bonding and oxygen species in carbon frameworks, which are different to that of metal-free NPCS sample [39,40]. And the shoulder peak around 533.3 eV respects to the hydroxyl groups or adsorbed oxygen within the surface. For N 1s spectrum of FeNiP/NPCS (Fig. 2c), the fitted five peaks are ascribed to the typical pyridinic N (398.2 eV), Fe/Ni-N bonds (399.4 eV), pyrrole N (400.2 eV), graphitic N (401.0 eV), and oxidized N (402.5 eV) [41,42]. The pyridinic N, relating to the doped N at the edges of graphitic carbon, is always considered as the active sites for electrochemical process. And the graphitic N can improve the electronic 13

conductivity of carbon substrates. Specially, the Fe/Ni-N species inside graphitic carbon skeleton plays the vital effect on boosting the ORR activity [13,16]. However, the sample NPCS exhibits only the pyridinic N, pyrrole N, graphitic N, and oxidized N in its N 1s spectrum. For P 2p spectrum (Fig. 2d), besides the typical signals related to P-C species (133.1 eV) in carbon skeleton and P-O bonds (134.6 eV) corresponded to the oxidized surface phosphorus exposed to air, which are similar to those of NPCS sample, the double peaks at 129.4 and 130.3 eV correspond to the P 2p3/2 and P 2p1/2 pair of P-metal bonds for metal phosphides [43,44]. The significant P-C signal demonstrates the successful doping of P elements into carbon skeleton. The sharp peaks of Fe 2p (Fig. 2e) are detected at 707.2, and 719.9 eV, which match well with Fe-P bonds [41,45]. And the dominant peaks at 711.1 and 724.8 eV with their satellites of 713.9 and 730.9 eV correspond Fe 2p3/2 and Fe 2p1/2 of surface FeIII in iron phosphates, which is the superficial oxidation of metal phosphides upon exposing in air. Similar results are also observed for the high-resolution Ni 2p spectrum (Fig. 2f), wherein the Ni-P bonds (853.5 and 870.8 eV) and the Ni 2p3/2 and Ni 2p1/2 (855.0 and 873.1 eV) of NiII species with their satellite peaks (861.4 and 879.0 eV) are clearly presented [46]. The observed evident Fe-P and Ni-P bonds suggest the bimetallic FeNiP in the sample FeNiP/NPCS. Combining the abovementioned physicochemical measurements, the fabricated hybrid materials composing of bimetallic FeNiP nanoparticles, N,P dual-doped carbon skeletons, and Fe/Ni-N-C moieties embedded carbon nanosheets, are successfully obtained. Such multiple active components with hierarchically porous carbon 14

architecture are considered to be highly efficient for electrochemical reactions. 3.2. Electrochemical measurements The classical three-electrode cell configuration with the rotating disk electrode (RDE) was used to evaluate the electrocatalytic activity of the synthesized FeNiP/NPCS materials. The effect of the feeding amount of metallic salts on the electrochemical activities was first explored. From the corresponding polarization curves (Fig. S8), the sample of FeNiP/NPCS with 2 mmol feeding amount of metallic salts exhibits the highest electrochemical performance in terms of OER, ORR, and HER. For the samples with half (1 mmol) or double (4 mmol) feeding amount of metallic salts (named as FeNiP-0.5/NPCS or FeNiP-2/NPCS, respectively), the degraded electrochemical activities are both observed. So the dosage of metal sources has a significant effect on the electrochemical activity of the fabricated materials. Usually, the sufficient active sites of catalysts are the prominent issue for the outstanding electrochemical activities. Too low feeding amount of metallic salts would lead to the insufficient metal nanoparticles (Fig. S9a-c) as active sites for catalytic processes, as indirectly responded on the lowest electrochemical active surface area of FeNiP0.5/NPCS (Fig. S10) in comparison with FeNiP/NPCS and FeNiP-2/NPCS. However, too much metallic salts would obtain the aggregated nanoparticles and decreased specific surface area, as evidenced by the corresponding TEM observation (Fig. S9d-f) and N2 sorption isotherms (Fig. S11) for FeNiP-2/NPCS, and thus supplying smaller electrochemical active surface area (Fig. S10) in comparison with FeNiP/NPCS. Therefore, as corroborated by the experimental results on the corresponding 15

polarization curves, the feeding amount of 2 mmol metallic salts is optimized for fabricating the high-efficiency FeNiP/NPCS catalyst. The FeNiP/NPCS catalysts prepared at different annealing temperatures were also investigated to modulate the electrochemical activities. The detailed structural information about those different-temperature-annealed catalysts was analyzed by XRD patterns (Fig. S12). All samples exhibit the similar diffraction peaks, indicating the formation of bimetallic FeNi phosphides under various pyrolysis temperature. And the corresponding SEM images (Fig. S13) of them exhibit the identical hierarchically porous architecture with curly nanosheets. As revealed on their polarization curves (Fig. S14), the catalytic activities order of those catalysts follows 800 ℃ < 1100 ℃ ≈ 900 ℃ < 1000 ℃. Usually, it is not very easy to obtain ideal carbon substrates at low pyrolysis temperature (800 ℃), as illustrated by the larger charge transfer resistance (Rt) on Nyquist plots (Fig. S15) and lower specific surface area on N2 adsorption-desorption isotherms (Fig. S16), thus resulting in inferior catalytic performance of the 800 ℃annealed binary metal phosphide materials. However, the particle size of FeNiP nanoparticles increases evidently at high pyrolysis temperature (1100 ℃), associated with decreased specific surface area, as confirmed by the corresponding TEM images (Fig. S17) and N2 adsorption-desorption isotherms (Fig. S16). However, in comparison with FeNiP/NPCS treated at 1000 ℃, the electrochemical activities of both 800 ℃annealed and 1100 ℃-annealed FeNiP/NPCS samples are lower. The comparative experiment also demonstrates that although the micro-morphology of hybrid catalysts keep similar in the form of porous nanosheets, the bimetallic FeNi phosphides are the 16

main contributor to boost the electrochemical activity, and the particle size and physicochemical properties of carbon substrates also affect the overall performance of those fabricated hybrid materials [47]. Therefore, the samples discussed below are all synthesized with the optimal conditions of 2 mmol metallic salts and 1000 ℃ pyrolysis temperature. 3.2.1. HER performance The HER activity of FeNiP/NPCS was first investigated in acid condition (0.5 M H2SO4). The control samples including metal-free N,P dual-doped carbon nanosheets (NPCS), FeNi alloys decorated carbon nanosheets (FeNi/NCS) without P dopants, and carbon monolith supported FeNi phosphides (FeNiP/NPC) with monotonic morphology, were fabricated to discuss the component and structure advantages of the designed catalysts. Correspondingly, the commercial Pt/C (20 wt%) catalyst as benchmark was also measured. As expected, noble metal Pt/C catalyst exhibits the outstanding HER activity with nearly zero onset potential, as demonstrated in Fig. 3a. Comparing with the inferior HER performance of metal-free NPCS, the FeNi/NCS exhibits the enhanced activity, revealing the positive effect of metal components (FeNi alloys) on electrochemical hydrogen evolution. Furthermore, the FeNiP/NPC catalyst exhibits dramatically increased cathode current as compared with that of FeNi/NCS, which may originate from the catalytic nature of metal phosphides [48]. Benefiting from the favorably reversible binding of protons and metal centers, metal phosphide catalysts, for example, CoP-MNA [18], CoP@NPC [33], Cr-doped FeNi–P/NCN [49], and FePx/Fe–N–C/NPC [50], have been exhibiting great capability toward 17

electrochemical hydrogen evolution. In addition, with the deliberate modification of the carbon substrates with nanosheet architecture, the fabricated FeNiP/NPCS exhibits quite strong catalytic activity as compared with FeNiP/NCS, implying that the advanced construction of ultrafine FeNiP nanoparticles and porous carbon nanosheets increases the active sites of this fabricated hybrid materials, thereby triggering the catalytic activity. The overpotential related to the current density of 10 mA cm-2 is 126 mV for FeNiP/NPCS, which is smaller than that of other fabricated samples and the recently reported active catalysts (Table S1). The fitting of the Tafel plots (Fig. 3b) renders the Tafel slope of 64 mV dec-1 for FeNiP/NPCS, close to that of Pt/C catalyst (38 mV dec-1), lower than that of other samples including NPCS (202 mV dec-1), FeNiP/NPC (105 mV dec-1) and FeNi/NCS (133 mV dec-1), indicating the more favorable reaction kinetics on FeNiP/NPCS for hydrogen evolution. The stability test was conducted by the continuous CV cycling test and chronoamperometric response. The polarization curve of FeNiP/NPCS after 1000 CV cycles decays slightly, as shown in Fig. 3c, whereas the Pt/C catalyst declines evidently under the identical conditions, demonstrating the significant resistance to the accelerated degradation. Moreover, the chronoamperometry measurement at the constant overpotential of 200 mV is measured to evaluate the operation stability of catalysts. As shown in Fig. 3c, the fabricated FeNiP/NPCS delivers a current decay of 21.1 % over 12 h, which is slightly better than that of noble metal Pt/C catalyst (23.9 %). Both the ultrafine dispersed FeNiP nanoparticles and porous nanosheet structure are well maintained after HER stability tests, as measured by TEM images (Fig. S18). 18

In order to directly clarify the role of FeNiP species on hydrogen evolution process, the prepared FeNiP/NPCS was treated by hydrochloric acid aqueous solution (6.0 M) to remove the metallic species [51], and the obtained material was named as FeNiPfree/NPCS. The XRD pattern (Fig. S19) and TEM images (Fig. S20) of FeNiPfree/NPCS demonstrate that the FeNiP components are entirely removed, and the porous carbon substrates are preserved. As demonstrated on polarization curves (Fig. S21), significant activity loss is observed, and the overpotential at the current density of 10 mA cm-2 is increased from 126 mV for FeNiP/NPCS to 320 mV for acid-treated sample FeNiP-free/NPCS. This result further entitles the metal phosphides as the primary electroactive sites for hydrogen evolution process. Although the substrates of N,P-codoped porous carbon nanosheets are not directly involved in the hydrogen evolution reactions, the synergistic function of carbon substrate, including increased conductivity, improved dispersion of nanoparticles, suppressed aggregation, and enhanced stability, still plays important roles in boosting the electrocatalytic activity of the hybrid materials [52]. For comparison, the acid-treated sample FeNiP-free/NPCS was also measured for long-term operation. As revealed on chronoamperometric curves (Fig. S22), the FeNiP-free/NPCS exhibits a larger current attenuation as compared with that of FeNiP/NPCS, suggesting that the presence of metal phosphides may alleviate the active decay and improve the stability for long-term electrochemical operation. The effect of Fe/Ni ratio of those FeNiP-carbon catalysts (Fe2-xNixP/NPCS) on the electrochemcial activity was also evaluated. As to the five catalysts with different Fe/Ni ratio, from the XRD patterns (Fig. 2a) and TEM observation (Fig. S2), metal 19

phosphides nanoparticles with similar size distribution are uniformly dispersed on ultrathin carbon nanosheets for all samples. LSV polarization curves were used to evaluate the HER activities, as shown in Fig. S23a. Two indicators were used as the functional indicator of Fe content of Fe2-xNixP/NPCS catalysts for the HER activity. One is the overpotentials at the current density of 10 mA cm-2 (η10) and 100 mA cm-2 (η100), and another is the current density at the holding overpotential of 300 mV (J300). As plotting on the histogram (Fig. 3d), both of them reveal that the catalyst with Fe/Ni ratio of 1 exhibits the greatest HER activity among those five samples. Specifically, the current density of FeNiP/NPCS at the holding overpotential at 300 mV is even 2.4 and 1.96 times than that of FeP/NPCS and NiP/NPCS, respectively. In addition, the Tafel slopes of these bimetallic Fe2-xNixP/NPCS samples are centered to about 70 mV s-1 (Fig. S23b). In contrast, without the introduction of secondary metal, the Tafel slopes of pure FeP/NPCS and NiP/NPCS increase to 97 and 86 mV dec-1, revealing the poor intrinsic HER kinetics. Therefore, the FeNiP nanoparticles play important roles in catalyzing the hydrogen evolution process, and the suitable Fe/Ni ratio is directly related to its surface electrochemistry. Moreover, electrochemical impedance spectra (EIS) of these catalysts demonstrate that the FeNiP/NPCS sample has the lowest charge transfer resistance (Rt) value in comparison with other catalysts as shown in Nyquist plots (Fig. 3e), further indicating the enhanced reaction kinetics. To insight into the reason for the prominent activity of those fabricated electrocatalysts, electrochemical double layer capacitance (Cdl) was obtained to evaluate the electrocatalytic active surface area. It is widely accepted that 20

the Cdl is proportional to the electrochemical surface area of catalysts. The FeNiP/NPCS catalyst exhibits the largest Cdl of 15.7 mF cm-2 among all five Fe2-xNixP/NPCS samples (Fig. 3f), illustrating the larger electrochemically active surface area. This result demonstrates that the coexistence of Fe and Ni with the optimal Fe/Ni ratio of 1 to form bimetallic FeNiP within carbon substrate could synergistically boost the hydrogen evolution performance. Noticeably, the fabricated Fe2-xNixP/NPCS has a higher Cdl than that of metal-free sample of NPCS (8.5 mF cm-2, Fig. S25b), suggesting that the composite nanosheets with active metal phosphide nanoparticles dispersed on 2D carbon substrates supply the sufficient active sites, which contributes to the excellent activities of this fabricated catalyst. Hence, the proper Fe/Ni ratio of those Fe2-xNixP/NPCS can significantly modify the electronic property and surface composition, which determines the catalytic performance for the electrochemical reactions. The similar activity enhancement of the FeNiP/NPCS sample is also observed in alkaline solution (1.0 M KOH). As shown on the LSV curves (Fig. S26a), the noble metal Pt/C catalyst still exhibits the high activity for HER with the occurrence of cathodic current at nearly zero and a low Tafel slope of 51 mV dec-1 (Fig. S26b). As compared with pure FeP/NPCS and Ni/NPCS, the HER activity is obviously improved for bimetallic Fe2-xNixP/NPCS catalysts. The overpotential for the current density of 10 mA cm-2 is about 181 mV for the FeNiP/NPCS catalyst with the Tafel slope of 111 mV dec-1, which is lower than that of other samples, as plotted on the corresponding histogram (Fig. S26c). Such superior results of FeNiP/NPCS for HER in alkaline 21

condition are comparable or even superior to some active transition metal catalysts as listed in Table S2, manifesting the superiority of the fabricated materials for hydrogen evolution. It is observed that the FeNiP/NPCS electrocatalyst remains stable current density with negligible degradation after 1000 CV cycles in alkaline solution (Fig. S26d), demonstrating the excellent stability. From the experiment results, the formed metal phosphides on carbon substrates play the main contributor to the exceptional hydrogen evolution activities, and the metallic Fe/Ni ratio of Fe2-xNixP/NPCS can significantly modulate the reaction kinetics toward HER. On one hand, from the recently related research studies, the metal phosphides possess outstanding catalytic nature for HER [13,48]. As demonstrated on the XPS profiles (Fig. 2d-f), the binding energies of Fe-P/Ni-P bonds of FeNiP/NPCS exhibit a positive shift compared with that of metal Fe (706.7 eV)/Ni (852.6 eV), and the negatively shifted binding energy of P 2p for FeNiP/NPCS than that of elemental P (130.2 eV) is also observed. The partial negative charge of P sites with electron-rich status is favorable for the acceptance of proton and thus decreases the energy barrier to proton binding, which is beneficial for the subsequent hydrogen evolution [53]. Meanwhile, the electron-deficient Fe/Ni sites, which are hydride-acceptor centers, boost the electron transfer [54]. Therefore, the significant electron diffusion between Fe/Ni and P in the FeNiP/NPCS increases the electron transfer and decreases the energy barrier for the adsorption-desorption steps of reactant intermediates, and thereby leading to the enhanced electrocatalytic activity. On the other hand, theoretically, the strength of Gibbs free energy of hydrogen adsorption (ΔGH*) on electrocatalytic active 22

site is significantly related to the HER activity. According to the previous studies of various systems referred to hydrogen electrode from the experiment and theoretical results, the optimal HER electrocatalysts should have nearly zero of ΔGH* to ensure the fast electron/proton transfer step and the favorable hydrogen release process [55]. The H binding strength with ΔGH* significantly depends on the surface electron dispersion and strength of active sites, namely, Fe2-xNixP species in our catalyst systems. To get insight into the electron status of these Fe2-xNixP/NPCS samples, the high-resolution XPS measurement was thus measured. As shown in their XPS profiles (Fig. S27), the shift of Ni 2p and Fe 2p binding energies of bimetallic Fe2-xNixP/NPCS samples (e.g., Fe0.5Ni1.5P/NPCS, FeNiP/NPCS, and Fe1.5Ni0.5P/NPCS), comparing with that of single metallic counterparts (e.g., FeP/NPCS, and NiP/NPCS), is detected. Such evident shift of XPS binding energies demonstrates the modified electronic structure in the bimetallic phosphides upon the introducing of secondary metal, consistent with the reported catalyst systems, such as NiFeP@NPC [19], CoFeP nanoframes [44], CoNiP NCs [56], and CuCoP/NC [57]. Overall, the significant electronic structure modification is obtained for FeNiP/NPCS with the optimal Fe/Ni ratio of 1, which synergistically improve the related electrocatalytic activities. 3.2.2. OER performance To achieve the efficient overall water splitting, the anodic oxygen evolution activity of those fabricated catalysts is also investigated. The electrocatalytic OER performance of those fabricated metal phosphide samples was evaluated with identical equipment to that of HER tests in 1.0 M KOH aqueous electrolyte. All the polarization 23

curves with iR correction are displayed in Fig. 4a. The bimetallic Fe2-xNixP/NPCS samples exhibit higher catalytic activity than the single metal phosphides counterparts (e.g., FeP/NPCS, and NiP/NPCS) and metal-free NPCS. Note that the performance of FeNiP/NPCS is comparable to that of noble metal IrO2 catalyst, and greatly intensified OER current of FeNiP/NPCS is gradually overwhelming to that of IrO2 when the anodic potential beyond 1.59 V. The overpotentials related to the current densities of 10 mA cm-2 (η10) and 100 mA cm-2 (η100) are plotted in Fig. 4b. It is clearly observed that FeNiP/NPCS possesses the lowest overpotentials comparing with other Fe2xNixP/NPCS samples. And the FeNiP/NPCS catalyst shows the small overpotentials of 318 and 470 mV at the current density of 10 and 100 mA cm-2, respectively, which are close to those of IrO2 catalyst (307 and 390 mV), and lower to those of recently reported active catalysts fabricated from non-noble elements (Table S2). Fig. 4c presents the corresponding Tafel plots of the catalysts. The bimetallic Fe2xNixP/NPCS catalysts exhibit lower Tafel slopes (153, 95 and 103 mV dec-1 for Fe0.5Ni1.5P/NPCS, FeNiP/NPCS and Fe1.5Ni0.5P/NPCS, respectively) than that of the pure metal catalysts (210 and 148 mV dec-1 for FeP/NPCS and NiP/NPCS, respectively). The smaller Tafel slopes of bimetallic Fe2-xNixP/NPCS catalysts indicate more favorable reaction kinetics for oxygen evolution. Besides, the much smaller semicircular diameter of FeNiP/NPCS on the Nyquist plots (Fig. S28), in comparison to that of other samples, indicates the lower charge transfer resistance during oxygen evolution process. The long-term stability of FeNiP/NPCS catalyst, measured by the continuous CVs and chronoamperometric tests, is shown in Fig. 4d. 24

The FeNiP/NPCS electrode exhibits stable performance with negligible difference on the anodic current densities after 1000 potential cycles comparing with initial one. In addition, the chronoamperometry response with the holding current density of 10 mA cm-2 exhibits the slight variation of the required potential as least 12 h. The postcharacterization of the morphology and structure of the FeNiP/NPCS catalyst after stability tests demonstrates the nanoparticles dispersed nanosheets are still maintained, as proved by the TEM observation (Fig. S29). To further detect the active sites related to the outstanding electrocatalytic activity and provide the design principles for efficient catalysts, the high-resolution XPS analysis was further measured. For the electrode of FeNiP/NPCS after HER tests, the core level Fe 2p (Fig. 5a) and Ni 2p (Fig. 5b) spectra exhibit little difference compared with the original chemical state. Meanwhile, the pair of P 2p3/2 and P 2p1/2 in P 2p spectrum (Fig. 5c) also keep stable, illustrating the reduction process on surface species during HER. Such results indicate the well structure robustness of those supported metal phosphide nanoparticles for hydrogen production. For Fe 2p core level (Fig. 5d), the post-OER FeNiP/NPCS renders a positively shifted centered peak associated with enhanced shoulder in comparison to the original one, which indicates the transition of the original low state to higher oxidation state during the long-term oxygen evolution process. Similarly, the positively shifted binding energy of the fitted Ni 2p3/2 bonds and the significant satellite peak are also observed (Fig. 5e). In addition, comparing with the original ones, the integral areas related to Fe-P and Ni-P exhibit distinct decrease in the corresponding Fe 2p and Ni 2p spectra. The intense components at 530.9 and 533.7 25

eV are observed for O 1s spectrum (Fig. S31), ascribing to the metal oxides and surface hydroxyls/hydroxides. Simultaneously, along with the almost completely disappeared P 2p3/2 and P 2p1/2 moieties in the P 2p core level (Fig. 5f), the enlarged peak area located at 133.9 eV demonstrates the formation of more phosphate species. Therefore, surface restructuring behavior is available for FeNiP/NPCS during the electrochemical oxygen evolution, and the surface oxy/hydroxide overlayer is in situ formed on the metallic FeNiP nanoparticles, leading to the metal oxides-phosphides interface, which would boost the carrier transportation from the FeNiP moiety to FeNiOx part, and employ as the labile ligands that tailor the chelating and coordination state of metal centers during redox reactions and thus improve the electrocatalytic activities [58-60]. Following to the surface oxidation process, the surface phosphides would transfer into the phosphates or oxides. The formation of metal phosphates/oxides species on the surface of metal phosphides is beneficial to enhance OH- adsorption and reduce the Gibbs free energy of the reaction intermediates (e.g., O2, *OOH, *O, and *OH) on the electrocatalysts, and thereby contributing to enhanced OER activity [61-63]. As the multistep surface evolution reactions, especially for OER, the real active sites of the catalysts involved in electrochemical oxidation and reduction always contain several composites, and thus more advanced in situ characterizations and theoretical calculation are urgently demanded [38,41]. 3.2.3. ORR performance Considering the multiple-components including the N-coordinated Fe/Ni sites, N,P-codoped carbon and FeNiP nanoparticles, the ORR electrocatalytic performance 26

of the fabricated FeNiP/NPCS was also evaluated. The commercial Pt/C (20 wt%) and metal-free NPCS are also measured as reference. As shown on the CV curves (Fig. S32), the positive ORR peak of FeNiP/NPCS compared with other samples indicates its impressive oxygen reduction property. Fig. 6a shows the polarization curves. The half-wave potential (E1/2) of FeNiP/NPCS is 0.84 V, close to that of Pt/C catalysts, and superior to other samples, as clearly plotted on the histogram (Fig. 6b). In addition, the FeNiP/NPCS also exhibits a lower Tafel slope (91 mV dec-1) than other fabricated samples (Fig. 6c). The activity difference of those fabricated Fe2-xNixP/NPCS samples demonstrates that the optimal ratio of Fe/Ni is also vital for oxygen reduction performance. The ORR performance of the fabricated FeNiP/NPCS is comparable to or even superior to most of the reported active electrocatalysts in alkaline electrolyte, as listed in Table S3. In the previously reported studies, the N-coordinated metal (e.g., Fe, Co, and Ni) species, and N,P-doped carbon are both the essential active sites for ORR. The theoretical calculation and experiment results also demonstrated that the Fe-P bonds can effectively active oxygen molecules, and thus boost the oxygen reduction process [45]. Taking consideration of the robust resistant nature of N-coordinated metal species (e.g., isolated Fe/Ni single atoms with N), the FeNiP/NPCS was treated with 6.0 M HCl to remove FeNiP nanoparticles, leaving the metal-N species (labeled as FeNiPfree/NPCS). As demonstrated in Fig. 6a, comparing with the NPCS, acid-treated FeNiP-free/NPCS exhibits a better activity. Nevertheless, the poor ORR activity with 30 mV negatively shifted potential of E1/2 and decreased limiting current density of 27

FeNiP-free/NPCS in comparison with FeNiP/NPCS further demonstrate the ORR active moieties of metal phosphides. From these results, it is considered that the synergistic effect among different active components including N,P-doped carbon, metal-N species and FeNiP nanoparticles leads to the superior ORR activity of FeNiP/NPCS. According to the Koutecky–Levich (K-L) plots from the LSV curves under the varied rotation speeds (Fig. S33), the average electron transfer number (n) of FeNiP/NPCS are 3.79 – 3.98 within the potential region of 0.40 – 0.60 V, manifesting the four-electron dominated transfer pathway for oxygen reduction. The peroxide yield (HO2-%) for FeNiP/NPCS is below 10%, associated with the high n value beyond 3.80, from the calculation of RRDE measurements (Fig. 6d), which are both close to those of Pt/C catalyst, strongly demonstrating the high catalytic efficiency with favorable reaction pathway. As shown in Fig. 6e, an inconspicuous current drop (9.6 %) of the initial current density is observed for FeNiP/NPCS, lower than that of Pt/C catalyst (14.5 %), after continuous operation at the constant of 0.7 V for 36000 s. Furthermore, no obvious ORR current shift is observed on the FeNiP/NPCS electrode after adding methanol into electrolyte, whereas the Pt/C exhibits significant current difference. The methanol oxidation on the Pt active sites leads to the severe inactivation. It is thus demonstrated the high long-term durability and methanol tolerance of the FeNiP/NPCS in alkaline solution, which is vital for the actual technologies, for instance, methanol fuel cells. Furthermore, the FeNiP/NPCS sample shows a low potential difference (ΔE) of 0.80 V between the OER potential of 10 mA cm-2 and the ORR potential at E1/2 in 28

0.1 M KOH (Fig. 6f), which is lower than that of other Fe2-xNixP/NPCS (0.84 – 1.04 V), Pt/C (1.05 V) and IrO2 (0.99 V) catalysts, demonstrating the good reversible activity for ORR and OER. Such low potential difference of FeNiP/NPCS endows it with significant ability as air cathode for rechargeable metal-air batteries. The ORR performance of the fabricated FeNiP/NPCS and commercial Pt/C catalysts in acidic electrolyte (0.5 M H2SO4) was also evaluated. From the polarization curves (Fig. S34a), the half-wave potential (E1/2) of FeNiP/NPCS is 0.73 V, negatively shifted by 50 mV than that of Pt/C (0.78 V), demonstrating the comparable activity of FeNiP/NPCS. Meanwhile, the FeNiP/NPCS also shows high stability with a low decay of 22.7% of initial current on chronoamperometric curve (Fig. S34b), which is approaching to that of Pt/C catalyst, after a continuous operation over 24000 s. As observed from above experimental results, the FeNiP/NPCS exhibits high activity and durability, potentially employing as efficient catalyst for proton exchange membrane fuel cells. Thus, FeNiP/NPCS exhibits excellent trifunctional electrocatalytic properties of HER, OER and ORR, comparable to that of precious metal electrocatalysts, which attributes to the structural advantages and electronic modulation: a) the porous carbon substrates prevent the aggregation of metal phosphides and thus efficiently improve the number of active sites; b) the interconnected channels of the porous nanosheets improve the transport of electrons and reactant molecules; c) the carbon substrates with proper amount of heteroatom dopants effectively modulate the electronic properties and surface polarities, and thus ensure the good electrical conductivity and structure 29

robustness of the hybrid materials for increased electrocatalytic performance; d) the suitable Fe substitution to Ni of the fabricated bimetallic FeNi phosphides optimizes the electronic properties and surface composition of the final materials, further improving the catalytic performance. 3.2.4. Overall water splitting and Zn-air batteries performance Inspired by the impressive performance with high activity and stability for both HER, OER and ORR, it is feasible to construct the water splitting electrolyzer and rechargeable Zn-air batteries from the FeNiP/NPCS employed as the high-efficient multifunctional electrocatalysts. For the two-electrode construction, the catalyst coated Ni foam was used as both cathode and anode within 1.0 M KOH as electrolyte, as illustrated in Fig. S35a. The noble metal cells of IrO2//Pt/C exhibit a low voltage of 1.60 V at the current density of 10 mA cm-2 (Fig. S35b). The FeNiP/NPCS formed cells need the voltage of 1.71 V to deliver the 10 mA cm-2 current density (Fig. S35b), which is comparable to other reported non-noble electrocatalysts for water splitting (Table S4), indicating the high electrocatalytic output of the electrolyzer assembled from this fabricated metal phosphides-based materials. At the different constant current densities from 5 to 40 mA cm-2 as least 20 h (Fig. S35c), only slight voltage increase is observed for FeNiP/NPCS-based cells, firmly demonstrating the high durability for long-term electrochemcial process. Meanwhile, vast bubbles are continuously released from the anodic and cathodic electrodes (Fig. S35d). With the water-gas displacing instrument for HER and OER processes (Fig. S36), the calculated faradaic efficiency of FeNiP/NPCS-based cells is about 97% for overall water splitting (Fig. S37), revealing 30

the high efficiency of this developed catalysts for catalytic reactions. Such water splitting performance of FeNiP/NPCS catalyst endow it with significant potential as the advanced bifunctional catalysts for the electrochemical hydrogen production. The rechargeable liquid Zn-air batteries, as illustrated in Fig. 7a, were fabricated by using FeNiP/NPCS as air cathode, and the combination of Pt/C+IrO2 (mass ratio of 1) as reference for comparison. The battery formed with FeNiP/NPCS possesses a high open-circuit voltage of 1.51 V (Fig. 7b). Furthermore, the FeNiP/NPCS also exhibits excellent rechargeable performance with lower charge-discharge potential gaps, as revealed on the charging-discharging polarization curves (Fig. 7c), superior to that of Pt/C+IrO2 catalyzed batteries. The peak power density for the FeNiP/NPCS battery is up to 163 mW cm-2, which is 1.12 times of the noble cells of Pt/C+IrO2 (145 mW cm-2). This is evidently superior to the recently reported highly active materials as air cathode for Zn-air battery, for instance, NPMC-1000 [17], N-GCNT/FeCo-3 [47], Ni3Fe/N-C [64], and others in Table S5. When cycled at 10 mA cm-2 (Fig. 7d), the initial discharge and charge voltages for FeNiP/NPCS are 1.29 and 1.87 V, respectively, which are better than the Pt/C+IrO2 battery (1.28 and 1.88 V). The initial energy efficiency of the FeNiP/NPCS battery is as high as 69%. The applied voltages of FeNiP/NPCS battery keep stable even after the continuous cycling over 110 h (20 min per cycle). In contrast, the Pt/C+IrO2 battery exhibits unexpected performance decay with gradually increased potential gap along with the extension of cycling time, which can be ascribed to the unsatisfactory catalytic stability toward ORR and OER, and the gradually loss of activity of Pt/C and IrO2 catalysts. To manifest the reliable rechargeability of this 31

fabricated FeNiP/NPCS-based battery, enlarged cycling period (4 h per cycle) was worked at the current density of 10 mA cm-2. As profiled in Fig. S38, no significant voltage gap difference is observed after 45 cycles, revealing the extraordinary longterm cycling durability. In addition, determining from the discharging curves, the specific capacities of FeNiP/NPCS-catalyzed battery are calculated to be 602.7 and 565.8 mAh g-1 (Fig. S39), corresponding to the gravimetric energy densities of 783.5 and 712.9 mWh g-1zn normalized to the consumed Zn electrode, at the current densities of 10 and 20 mA cm-2, respectively. For practicable applications, three batteries containing FeNiP/NPCS contacted in series can power a light-emitting diodes (LED) plane with NK structure, as shown in Fig. S40. Moreover, when charged with a commercial silicon photovoltaic system for 10 min (corresponding schematic illustration in Fig. 7e), the discharge platform with the current density of 10 mA cm-2 of the FeNiP/NPCS battery stably operates for 1300 min without any evident variation. To illustrate the practical aspect of this fabricated catalyst FeNiP/NPCS, the allsolid-state Zn-air battery was also assembled using the polyvinyl alcohol (PVA) containing KOH as electrolyte, as schematic drawing in Fig. 7f. The solid-state battery with FeNiP/NPCS shows an open-circuit voltage of 1.34 V (Fig. 7f). And three fabricated batteries connected in series are able to provide the open-circuit voltage as high as 3.92 V. The fabricated battery with FeNiP/NPCS cathode can stably work at various discharging current densities (Fig. 7g). Moreover, the fabricated solid-state batteries can stably charge and discharge over 30 cycles (Fig. 7h), though the cycling stability of this solid-state Zn-air battery is inferior to that using liquid electrolyte, 32

which may be due to the low normalized ionic conductivity and poor water retention capability of PVA [65]. And three fabricated batteries in series can power the LED plane under the different bending angles (Fig. 7i), demonstrating its significant potential used for wearable-electronics fields.

4. Conclusions The trifunctional FeNiP/NPCS catalysts have been synthesized through the facile carbonization-phosphorization process. With the reasonably integrated metrics of binary FeNiP with optimal ratio of Fe/Ni, and N,P-doped porous carbon substrate with 2D nanosheet morphology, the fabricated FeNiP/NPCS catalyst exhibits significantly enhanced performance for HER, OER, and ORR. The overpotentials of 126 and 181 mV are required for FeNiP/NPCS catalyst to deliver the HER current density of 10 mA cm-2 in 0.5 M H2SO4 and 1.0 M KOH, respectively. The fabricated FeNiP/NPCS also exhibits high activity for OER and ORR, and only needs the potential difference (ΔE) of 0.80 V in 0.1 M KOH. The high stability of FeNiP/NPCS is also achieved for different elctrochemcial reactions. The fabricated overall water splitting cells from FeNiP/NPCS needs the voltage of 1.71 V to supply the benchmark current density of 10 mA cm-2. Moreover, working as the air cathode of FeNiP/NPCS, the fabricated Znair batteries with liquid and solid-state electrolytes exhibit appreciable reversibility and stability. Those water splitting electrolyzer and Zn-air barriers performance surpass most of active nanocatalysts and are even comparable to those of noble metal Pt/C and IrO2 catalysts. This work presents new investigation of the design principles governing 33

metal phosphides as multifunctional electrocatalysts and will motivate the development of further effective approaches for the design of highly efficient catalysts for energyrelated technologies.

Acknowledgement This work was supported by the National Natural Science Foundation of China (21421001, 21573115, and 21875118), the Natural Science Foundation of Tianjin (19JCZDJC37700) and the 111 project (B12015).

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44

Scheme 1. Schematic illustration of the synthesis of the FeNiP/NPCS.

45

Fig. 1. (a-b) SEM, (c-e) TEM and (f) HR-TEM images of FeNiP/NPCS. (g) Elemental mapping of N, P, Fe, Ni, and Fe+Ni+P.

46

Fig. 2. (a) XRD patterns and (b) Raman spectra of Fe2-xNixP/NPCS and NPCS. Highresolution XPS N 1s (c) and P 2p (d) spectra of FeNiP/NPCS and NPCS; and Fe 2p (e) and Ni 2p (f) spectra of FeNiP/NPCS.

47

Fig. 3. HER performance of the catalysts in 0.5 M H2SO4. (a) Polarization curves and (b) Tafel plots of Pt/C, FeNiP/NPCS, NPCS, FeNi/NCS, and FeNiP/NPC. (c) Chronoamperometry curves for Pt/C and FeNiP/NPCS. Inset of (c) represents the polarization curves of Pt/C and FeNiP/NPCS catalysts before and after 1000 CV cycles. (d) Comparison of overpotentials at 10 and 100 mA cm-2, and the current densities at the overpotential of 300 mV for Fe2-xNixP/NPCS. (e) Nyquist plots and (f) plots of the capacitive currents versus scan rates at 0.25 V (vs. RHE) for Fe2-xNixP/NPCS.

48

Fig. 4. OER performance of the catalysts in 1.0 M KOH. (a) Polarization curves of IrO2, Fe2-xNixP/NPCS and NPCS. (b) Comparison of overpotentials at 10 and 100 mA cm2.

(c) Tafel plots. (d) Chronoamperometry curves at the holding current density of 10

mA cm-2 for FeNiP/NPCS. Inset of (d) represents the polarization curves of FeNiP/NPCS catalyst before and after 1000 CV cycles.

49

Fig. 5. High-resolution XPS (a) Fe 2p, (b) Ni 2p and (c) P 2p spectra of the FeNiP/NPCS before and after HER test. High-resolution XPS (d) Fe 2p, (e) Ni 2p and (f) P 2p spectra of the FeNiP/NPCS before and after OER test.

50

Fig. 6. ORR performance of the catalysts in 0.1 M KOH. (a) Polarization curves of Pt/C, Fe2-xNixP/NPCS, NPCS, and FeNiP-free/NPCS. (b) Comparison of E1/2. (c) Tafel slopes. (d) Electron transfer number (n) and the peroxide yields (HO2-%) at different potentials. (e) Chronoamperometric curves of Pt/C and FeNiP/NPCS at 0.7 V (vs. RHE), and inset of (e) is the chronoamperometric responses of Pt/C and FeNiP/NPCS with the adding of methanol into electrolyte. (f) LSV curves of Fe2xNixP/NPCS, Pt/C, and IrO2 in the full OER-ORR potential range.

51

Fig. 7. (a) Schematic diagram of the liquid Zn-air battery. (b) Open-circuit voltage curve of the fabricated liquid Zn-air battery from FeNiP/NPCS air cathode, and inset in (b) is the open-circuit voltage of FeNiP/NPCS-based Zn-air battery. (c) Charge and discharge curves of those fabricated batteries and the corresponding power density plots. (d) Long-term galvanostatic charge-discharge curves of FeNiP/NPCS and Pt/C+IrO2based batteries. (e) Photovoltage charging and following discharging curves of the fabricated FeNiP/NPCS-based Zn-air battery, and inset in (e) is the schematic diagram of photovoltage charging and following discharge process. (f) Open-circuit voltage curve of the fabricated solid-state Zn-air battery from FeNiP/NPCS catalyst, and inset in (f) is the schematic diagram of the all-solid-state Zn-air battery, and the open-circuit voltage of three contacted FeNiP/NPCS-based solid-state Zn-air battery in series. (g) Discharge curves for solid-state FeNiP/NPCS-based battery at various discharging current densities. (h) Charge-discharge cycling plots of the assembled solid-state FeNiP/NPCS-based battery at the current density of 1 mA cm-2. (i) Photograph of the LED plane powered by three solid-state FeNiP/NPCS-based batteries with different bending angles in series. 52