Metal phosphide catalysts anchored on metal-caged graphitic carbon towards efficient and durable hydrogen evolution electrocatalysis

Author’s Accepted Manuscript Metal Phosphide Catalysts Anchored on Metalcaged Graphitic Carbon towards Efficient and Durable Hydrogen Evolution Electrocatalysis Xiaomei Wang, Weiguang Ma, Zhiqiang Xu, Hong Wang, Wenjun Fan, Xu Zong, Can Li www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(18)30239-8 https://doi.org/10.1016/j.nanoen.2018.04.011 NANOEN2641

To appear in: Nano Energy Received date: 4 January 2018 Revised date: 2 April 2018 Accepted date: 3 April 2018 Cite this article as: Xiaomei Wang, Weiguang Ma, Zhiqiang Xu, Hong Wang, Wenjun Fan, Xu Zong and Can Li, Metal Phosphide Catalysts Anchored on Metal-caged Graphitic Carbon towards Efficient and Durable Hydrogen Evolution Electrocatalysis, Nano Energy, https://doi.org/10.1016/j.nanoen.2018.04.011 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Metal Phosphide Catalysts Anchored on Metal-caged Graphitic Carbon towards Efficient and Durable Hydrogen Evolution Electrocatalysis Xiaomei Wang a, b, Weiguang Ma a, Zhiqiang Xu a, b, Hong Wang a, b, Wenjun Fan a, c, Xu Zong,* a and Can Li* a a

State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy

of Sciences; Dalian National Laboratory for Clean Energy, The Collaborative Innovation Center of Chemistry for Energy Materials (iChEM), Zhongshan Road 457, Dalian 116023, China b c

University of Chinese Academy of Sciences, Beijing 100049, China. The Key Laboratory of Fuel Cell Technology of Guangdong Province & The Key

Laboratory of New Energy Technology of Guangdong University, School of Chemistry and Chemical Engineering, South China University of Technology, Guangzhou 510641, Guangdong, China * Email: [email protected]; [email protected] Phone: (86)-411-84379070; (86)-411-84379698 Address: State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Science, Dalian, Liaoning Province 116023, China

Abstract Developing cost-effective and stable electrocatalysts towards hydrogen evolution reaction (HER) is important for the conversion of renewable energy via electrocatalytic water splitting reaction. Herein, we present a simple and general strategy for the preparation of a series of metal (M: Fe, Co and Ni) phosphide catalysts that were anchored on metal-caged carbon matrix. In the first step, 1

metal-grafted carbon precursors (M/M@NC) were rationally synthesized through metal-catalyzed growth process. Following phosphorization treatment transformed the metal precursors to metal phosphides (MP/M@NC) with the inheritance of the strong catalyst-support connection, which leads to efficient charge transport and prevents the detachment of the catalysts from the support. Benefitting from several structural advantages including high dispersability of active sites, good conductivity and strong interfacial interaction, the as-prepared MP/M@NC catalysts show high performance for HER in acidic solution. In particular, the FeP/Fe@NC exhibits extremely low overpotentials of -49 and -130 mV for achieving current densities of -10 and -100 mA cm-2, respectively, and remarkable long-term durability, which highlights the importance of rational catalyst/support design towards efficient and stable electrocatalysis. Keywords: Hydrogen evolution reaction; electrocatalytic water splitting; metal phosphide; metal-caged carbon matrix; catalyst-support interaction 1. Introduction Producing hydrogen (H2) from water splitting via electrocatalysis has received great attention in the past decade due to its potential to convert and store renewable energy [1, 2]. Platinum group metals (PGMs) have demonstrated to be the most active and stable catalysts for the hydrogen evolution reaction (HER) [3, 4]. However, the scarcity and high cost of PGMs seriously hinder their large-scale application. As a consequence, exploring cost-effective and especially stable catalysts as alternatives to PGMs for HER is highly desirable. Recent research have identified a series of 2

attractive materials consisted of earth-abundant elements including Ni-Mo alloys [5, 6], metal sulfides [7-9], carbides [10, 11], nitrides [12, 13], and phosphides [14-18] as potential candidates for HER, and several strategies such as nanostructural modification [19, 20] and composition tuning [21, 22] have been well applied to improve the catalytic activity of the HER catalysts. Moreover, strategies such as carbon encapsulation [11, 23-27], free-standing design [28, 29], and chemical tuning [30] were reported to improve the stability of a given catalyst. To prepare efficient HER catalysts, carbon materials are generally used as the support due to their high conductivity, large surface area, and excellent chemical stability [31]. These attractive features of carbon materials can enable efficient charge transfer and good dispersion of the catalysts, and therefore leading to good catalytic performance. For example, various HER catalysts such as FeP [18, 32], MoWP [15], CoP [33], and MoS2 [34] have been prepared on carbon support and demonstrated good catalytic performance. However, it is worth noting that these catalysts or their precursors are generally deposited on pre-formed carbon materials with impregnation or hydrothermal methods. Therefore, the interface between the catalysts and carbon support is supposed to be formed with an ex-situ manner, which leads to relatively weak connection and inefficient charge transport between the two components. We hypothesize that if we can strengthen the connection by forming the interface with an in-situ manner, more efficient charge transport between the two components and therefore higher activity could be obtained. Moreover, the strong connection between the catalysts and the support could significantly prevent the detachment of the 3

catalysts from the support and thus improve the stability of the catalysts.

It is well-known that metals such as Fe, Co, and Ni can catalyze the formation of carbon materials such as graphene and carbon nanotubes [23, 27, 35, 36]. Meanwhile, the metal catalysts can be grafted on the carbon matrix with an in-situ manner, resulting in strong connection between the two components. In principle, these metal-grafted carbon materials could be an ideal platform for further processing to achieve attractive functionality. Herein，we show that iron phosphide catalysts anchored on Fe-caged carbon matrix (FeP/Fe@NC) were prepared by phosphorization of Fe-grafted carbon precursors (Fe/Fe@NC) that were in-situ produced through Fe-catalyzed growth process. The as-prepared FeP/Fe@NC catalyst demonstrates excellent activity as well as outstanding stability towards HER. Furthermore, this two-step strategy can be successfully extended to the synthesis of other metal phosphide catalysts such as CoP/Co@NC and Ni2P/Ni@NC, testifying the universality of the present approach. 2. Material and methods 2.1 Chemical reagents

Dicyandiamide (C2H4N4) was obtained from Alfa Aesar. Ammonium ferric citrate (C6H11FeNO7) was provided by J&K chemical Ltd. Sodium hypophosphite hydrate (NaH2PO2∙H2O), cobalt (II) acetate tetrahydrate (C4H6CoO4∙4H2O) were received from Sinopharm Chemical Reagent Co., Ltd. Nickel acetate tetrahydrate (Ni(CH3COO)2∙4H2O) was obtained from Tianjin Kemiou Chemical Reagent Co., Ltd. Nafion (5 wt%) was purchased from Dupont Corp. (USA). Commercial 20 wt% Pt/C 4

electrocatalyst was purchased from Johnson Matthey (UK). All of the chemical reagents were analytical grade and used without further purification unless otherwise stated. All aqueous solutions were prepared with 18.2 MΩ∙cm deionized water from a Millipore deionized water system. 2.2 Preparation of catalysts

Fe/Fe@NC: Fe/Fe@NC precursors were synthesized with dicyandiamide (C2H4N4) and ammonium ferric citrate (C6H11FeNO7) [36]. In a typical experiment, a certain mass ratio (X: 4, 6, 8, and 10) of C2H4N4 to C6H11FeNO7 were dissolved in 75 mL of deionized water, and then the solution was stirred and dried to remove water. The obtained reddish mixture was placed in a quartz tube and heated to the target temperature at a rate of 10 oC min-1, then kept at the target temperature for 2 h in flowing Ar atmosphere. The target temperature (T) was set as 600, 625, 650, 665, 680 and 700 oC, respectively. The resulting samples were denoted as Fe/Fe@NC-T-X. FeP/Fe@NC: The Fe/Fe@NC-T-X precursors and sodium hypophosphite hydrate (NaH2PO2∙H2O), the molar ratio of Fe to P is 1:5, were mixed and grinded to fine powders by using a mortar. Then, the mixture was calcined at 500 oC for 2 h in Ar atmosphere. The obtained samples were washed 6 times with deionized water and dried at 80 oC overnight. The resulting samples were denoted as FeP/Fe@NC-T-X. Fe@NC: As-prepared Fe/Fe@NC-650-8 precursor was leached by 0.5 M H2SO4 for 8 h, then washed 6 times with deionized water and dried at 80 oC overnight. Fe@NC-P: Fe@NC was subjected to phosphorization treatment with the same procedure as for the Fe/Fe@NC precursors and then called as Fe@NC-P. 5

Bulk FeP: The bulk FeP was prepared with the same approach as for the synthesis of FeP/Fe@NC in the absence of dicyandiamide. FeP-Fe@NC: The as-prepared Fe/Fe@NC-650-8 was leached by H2SO4, the Fe content of H2SO4 was determined by ICP-AES and then the equivalent mole of Fe was impregnated on the pre-formed Fe@NC. The obtained mixture was subjected to phosphorization treatment and washed as mentioned above. CoP/Co@NC and Ni2P/Ni@NC: CoP/Co@NC and Ni2P/Ni@NC, with cobalt (II) acetate tetrahydrate and nickel acetate tetrahydrate as sources of cobalt and nickel, were prepared in the optimized conditions of FeP/Fe@NC with a little bit modification of the pyrolysis temperature of their precursors (650, 680 and 700 oC). 2.3 Material characterizations

X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution TEM (HRTEM), X-ray photoelectron spectroscopy (XPS), inductively coupled plasma atomic emission spectrometry (ICP-AES) were employed to analyze the samples. The phase analyses of samples were performed by XRD (Rigaku D/Max-2500/PC powder diffractometer) with Cu-Kα radiation. The morphologies of samples were investigated by field emission SEM (FE-SEM, Quanta 200 FEG), TEM (HITACHI HT7700) and HRTEM (FEI Tecnai G2 F30). The XPS spectra were carried out on a Thermo Esclab 250Xi photoelectron spectroscopy with a monochromatic Al Kα X-ray radiation as the X-ray source for excitation. The ICP-AES measurements were performed on a SHIMADZU ICPS-8100. The specific surface area and pore diameter distribution were measured 6

on a NOVA 4200e adsorption analyzer by N2 adsorption at −196 °C using the BET method. The content angle measurements were performed on a DSA100 contact angle meter. 2.4 Electrode preparation

To prepare the working electrode, 2 mg of catalyst powders were ultrasonically dispersed in a mixture solution of 150 μL H2O and 50 μL 1% nafion (dissolved in ethanol) for about 0.5 h. Then, 10 μL of as-prepared ink was dropped onto a commercial glassy carbon (GC) electrode (Wuhan Gaoss Union Co. Ltd., 3 mm in diameter, 0.071 cm2). Finally, the electrode was dried at ambient temperature, and the catalyst loading is 1.4 mg cm-2. To finish the post electrolysis characterizations, 500 μL of suspension was dropped onto a 1 cm2 Ti foam substrate in batches, and the electrode was dried by infrared heating subsequently. 2.5 Electrochemical measurements

The electrochemical measurements of catalysts were carried out in a three-electrode configuration by using a CHI 660E electrochemical workstation in 0.5 M H2SO4. The drop-coated glassy carbon (GC) or Ti foam electrode was used as the working electrode, saturated calomel electrode (SCE, saturated KCl) was used as the reference electrode, and Pt foil or carbon plate was used as the counter electrode (CE). It was reported that Pt may dissolve from the Pt CE and then redeposit on the working electrode to distract the quantification of the HER activity [37]. In our work, the polarization curves obtained were similar when using Pt foil or carbon plate as the CE (Fig. S7)，therefore precluding the influence of this factor in short-term measurement 7

[38]. However, in order to avoid the possibility of Pt contamination during long-term electrolysis, all stability tests were performed using a carbon plate as the CE. The linear sweep voltammetry (LSV) curves of as-prepared electrodes were tested at a scan rate of 50 mV s-1. Chronopotentiometric measurements (CP) test was performed at a current density of -10 mA cm-2. Cyclic voltammetry (CV) scanning was carried out from -0.12 to 0.28 V at a scan rate of 100 mV s-1. The electrochemical impedance spectroscopy (EIS) measurements were conducted using the three-electrode configuration, and the frequency range was 0.1 Hz to 100 kHz, the amplitude was 10 mV at a fixed overpotential of -200 mV in 0.5 M H2SO4. The faradaic efficiency (FE) of HER catalyzed by FeP/Fe@NC was obtained in an H-type electrochemical reactor with a nafion separator. The sample line of the reactor was connected to online gas chromatography (Agilent 7890 GC, TCD, Argon carrier) to real-time detect the generated hydrogen. Before measurement, the reaction system was thoroughly degassed with ultrapure argon to expel the air. Then, constant current of -3 mA was applied to the electrode and the concentration of hydrogen was analysed with gas chromatography. Calibration was carried out using a similar setup but with two cleaned Pt foils as working and counter electrode, respectively. All potentials measured were calibrated to vs. RHE using the following equation: E (RHE)

=E (SCE) + 0.241 V + 0.059 × pH. All the electrochemical data were collected

with 90% iR compensation unless otherwise stated. 3. Results and discussion

8

Scheme 1 Schematic procedure of preparing FeP/Fe@NC catalysts.

The FeP/Fe@NC catalysts were prepared with a simple two-step approach as shown in Scheme 1. In the first step, a mixture of dicyandiamide and ammonium ferric citrate was annealed in argon gas to obtain the Fe/Fe@NC precursor. In the second step, the Fe/Fe@NC precursor was converted to FeP/Fe@NC by phosphorization with NaH2PO2 as the reagent. As the physicochemical features of the Fe/Fe@NC precursors are important to the subsequent phosphorization treatment and the performance of the FeP/Fe@NC catalysts, we first optimized the synthesis of the precursors by varying the pyrolysis temperature (T) and the mass ratio (X) of dicyandiamide to ammonium ferric citrate. The resulting samples were denoted as Fe/Fe@NC-T-X. Fig. 1a shows the X-ray diffraction (XRD) patterns of the Fe/Fe@NC-T-8 precursors. At a pyrolysis temperature of 600 oC, only diffraction peaks indexed to C3N4 and Fe were observed. With the increase of the pyrolysis temperature, the diffraction peaks indexed to graphitic C3N4 disappeared with the simultaneous evolution of those indexed to graphitic carbon, indicating the gradual catalytic conversion of C3N4 to graphitic carbon by metallic Fe [39]. Moreover, the diffraction peaks assigned to Fe became sharper, demonstrating the gradual increase of the size of Fe particles. The morphologies of the Fe/Fe@NC-T-8 precursors were then 9

investigated by transmission electron microscopy (TEM). We found that the Fe nanoparticles (NPs) were well dispersed in the carbon-based matrix of all the samples even the particle size is not uniform (Fig. 1b-g). The particle size of Fe NPs gradually increased from ca. 5-15 nm to ca. 30-50 nm with the increase of pyrolysis temperature from 600 to 700 oC. As for the carbon-based matrix, the morphological change is negligible at pyrolysis temperatures lower than 665 oC. However, the carbon-based matrix changed from nanoplates to carbon nanotubes at pyrolysis temperatures above 680 oC. Therefore, the pyrolysis temperature was found to have great influence on the physiochemical nature of the precursors. With the increase of the pyrolysis temperature, the graphitization of the carbon-based matrix will be improved [39], which will increase the conductivity of carbon-based materials and therefore is beneficial to the charge transfer and electrochemical performance. However, the increase of the pyrolysis temperature will simultaneously increase the size of the Fe NPs and thus reduce the density of the active sites, which is disadvantageous to the electrochemical performance. Therefore, balancing these two factors by optimizing the synthesis temperature is important to obtain good electrochemical performance. As the mass ratio of dicyandiamide to ammonium ferric citrate can also exert influence on the above two factors and therefore the electrocatalytic performance of the catalyst, precursors with different dicyandiamide to ammonium ferric citrate ratios were also synthesized (Fig. S1 and S2). To get further information on the structural feature of the Fe/Fe@NC precursors, the Fe/Fe@NC-650-8 precursor was investigated with high resolution TEM (HRTEM) 10

as a typical example. As shown in Fig. 1h, a lattice fringe spacing of 0.202 nm, corresponding to the (110) lattice plane of metallic Fe, was observed. Moreover, Fe NPs were found to exist both on the surface and inside of the carbon matrix. Fe/Fe@NC-650-8 was then treated with H2SO4 to obtain Fe@NC. It was found that although Fe/Fe@NC-650-8 and Fe@NC exhibited similar morphology, the intensity of the diffraction peaks assigned to Fe decreased dramatically for Fe@NC when compared with Fe/Fe@NC-650-8 (Fig. S3). Therefore, it is reasonable to conclude that Fe NPs presented on the surface can be removed upon acid treatment, while those encapsulated in the carbon matrix can be preserved. This unique structural feature of Fe/Fe@NC will facilitate the further functionalization of the Fe NPs on the surface while keep those inside the carbon matrix intact during processing. The Fe/Fe@NC precursors were then subjected to phosphorization treatment after the clear identification of their physicochemical features. XRD analysis showed that diffraction peaks indexed to FeP were evolved due to the conversion of the surface Fe NPs upon phosphorization (Fig. 2a, Fig. S4). Diffraction peaks assigned to Fe were also observed, which corresponds to the Fe NPs buried in the carbon matrix. TEM analysis showed that the FeP/Fe@NC catalysts inherited the original morphologies of the corresponding Fe/Fe@NC precursors except for the slightly increased size of FeP NPs compared with that of Fe. This indicates that the efficient stabilization of uncaged Fe NPs through strong metal-carbon connection in the precursor will lead to a good dispersion of the resulting FeP NPs (Fig. 2b-g and Fig. S5). The HRTEM image (Fig. 2h) of FeP/Fe@NC-650-8 exhibited lattice fringes with interspace of 11

Fig. 1 (a) XRD patterns of Fe/Fe@NC precursors prepared at different pyrolysis temperatures with mass ratio of 8 (Fe/Fe@NC-T-8), TEM images of (b) Fe/Fe@NC-600-8, (c) Fe/Fe@NC-625-8, (d) Fe/Fe@NC-650-8, (e) Fe/Fe@NC-665-8, (f) Fe/Fe@NC-680-8, and (g) Fe/Fe@NC-700-8, and (h) HRTEM image of Fe/Fe@NC-650-8.

0.202 nm and 0.272 nm, corresponding to the (110) lattice plane of metallic Fe and the (011) lattice plane of FeP, respectively. Notably, the Fe NPs were found to be encapsulated into the carbon nanoshells while the FeP NPs were presented on the surface of the carbon matrix. X-ray photoelectron spectroscopy (XPS) analysis was then carried out to elucidate the compositions of FeP/Fe@NC-650-8. The survey spectrum (Fig. S6) manifested the presence of C, N, Fe and P element in the FeP/Fe@NC-650-8 catalyst. In the Fe 12

Fig. 2 (a) XRD patterns of the FeP/Fe@NC catalysts prepared by phosphorization of the Fe/Fe@NC-T-8 precursors, TEM images of (b) FeP/Fe@NC-600-8, (c) FeP/Fe@NC-625-8, (d)

FeP/Fe@NC-650-8,

(e)

FeP/Fe@NC-665-8,

(f)

FeP/Fe@NC-680-8,

and

(g)

FeP/Fe@NC-700-8, and (h) HRTEM image of FeP/Fe@NC-650-8.

2p spectrum (Fig. 3a), four peaks were observed. The binding energy (BE) values at 707.12 and 711.82 eV were attributed to the Fe 2p3/2, and the two peaks located at 720.06 and 725.77 eV can be assigned to the Fe 2p1/2. The P 2p spectrum (Fig. 3b) showed two peaks at 130.22 and 129.29 eV reflecting the BE of P 2p1/2 and P 2p3/2, respectively, and along with one peak at 133.54 eV. The peaks at 707.12, 720.06 and 129.29 eV were close to the BE for Fe and P in FeP, while peaks at 711.82, 725.77 13

and 133.54 eV can be attributed to oxidized Fe and P species arising from superficial oxidation of FeP when exposed to air [40-43]. These analyses confirm that FeP/Fe@NC catalysts were successfully synthesized by the phosphorization of the Fe/Fe@NC precursors.

Fig. 3 High resolution XPS spectra for the (a) Fe 2p, and (b) P 2p regions of FeP/Fe@NC-650-8.

The HER activities of the FeP/Fe@NC catalysts were evaluated by linear sweep voltammetry (LSV) using a three-electrode configuration in 0.5 M H2SO4. The influence of the synthesis temperature of the Fe/Fe@NC-T-8 precursors on the HER activities of their corresponding catalysts was first investigated. As shown in Fig. 4a, the FeP/Fe@NC-600-8 and FeP/Fe@NC-625-8 exhibited overpotentials of -150 and -134 mV, respectively, to achieve a current density of -10 mA cm-2. A significant increase in the HER activity was observed when the synthesis temperature was increased to 650 oC. However, further increase in the synthesis temperature to 700 oC led to a reduced activity. As we stated above, the conductivity of the carbon-based support will be increased with increased pyrolysis temperature due to the conversion of the semiconducting g-C3N4 to graphitic carbon [44]. This explains why FeP/Fe@NC-650-8

exhibited

better

activity

than

FeP/Fe@NC-600-8

and 14

FeP/Fe@NC-625-8. However, the amount of active sites for HER will be reduced with increased pyrolysis temperature due to the gradually increased particle size of Fe and FeP NPs and decreased specific surface area of the catalysts (Fig. S8 and Table S1), which explains why FeP/Fe@NC-700-8 showed inferior activity than FeP/Fe@NC-650-8. Balancing the influence of the synthesis temperature on the conductivity and the amount of available active sites, 650 oC was found to be the optimum synthesis temperature for the precursor and used in the following studies.

Fig. 4 Linear sweep voltammetry curves of (a) FeP/Fe@NC-T-8 and (b) FeP/Fe@NC-650-X catalysts and commercial 20 wt% Pt/C in 0.5 M H2SO4, (c) chronopotentiometric curve of FeP/Fe@NC-650-8 at a current density of -10 mA cm-2, and the insert is cyclic voltammetry test at a scan rate of 100 mV s-1 between -0.12 and 0.28 V (Carbon plate was used as the counter electrode, and the data were collected without iR compensation ), (d) the faradaic efficiency for HER on FeP/Fe@NC-650-8. The fluctuation of the faradaic efficiency is caused by the aggregation and release of the H2 bubbles on the surface of the catalyst.

We then investigated the influence of the mass ratio of dicyandiamide to ammonium ferric citrate on the HER activities of the FeP/Fe@NC catalyst by setting 15

the synthesis temperature at 650 oC. We find that when the mass ratio is 8 (Fig. 4b), the resulting FeP/Fe@NC catalyst exhibited the optimum activity with overpotentials of only -49 and -130 mV to achieve current densities of -10 and -100 mA cm−2, respectively. This activity is only slightly lower than that of commercial 20 wt% Pt/C, which exhibited overpotentials of -32 and -92 mV to achieve current densities of -10 and -100 mA cm-2, respectively. For the FeP/Fe@NC catalyst, the physiochemical feature of the precursor is found to be important to the final performance of the catalyst. During the formation of the precursor, the dicyandiamide is the source for the carbon matrix and ammonium ferric citrate is the source of Fe catalysts for catalyzing carbon formation and also the precursor for FeP. Therefore, the ratios of dicyandiamide to ammonium ferric citrate can also influence the conductivity, surface area, and the ratios of Fe/C of the precursor. We suppose that at a ratio of 8, an optimum balance is obtained for the precursor and therefore the best performance is achieved at this condition. In principle, this is similar with the influence of the pyrolysis temperature on the HER activity.

As stability is an important criterion for evaluating the performance of a catalyst, the stability of the FeP/Fe@NC catalyst was also investigated. Chronopotentiometry (CP) test performed at a current density of −10 mA cm-2 indicated that the FeP/Fe@NC catalyst maintained a steady HER activity over a period of 90 h (Fig. 4c). Moreover, the HER polarization curve of the FeP/Fe@NC catalyst exhibited negligible change after 5000 cycles of cyclic voltammetry (CV) test (Fig. 4c insert). The Faradaic efficiency (FE) for HER on the FeP/Fe@NC catalyst was calculated to 16

be nearly 100% (Fig. 4d). Therefore, the FeP/Fe@NC catalyst showed excellent electrochemical stability for HER. The morphology of the FeP/Fe@NC catalyst was also investigated with SEM, TEM and HRTEM after the stability test (Fig. S9). No noticeable morphological change was observed when compared with the original sample, again confirming the good stability of the catalyst during the test. Therefore, the FeP/Fe@NC catalyst exhibited excellent activity as well as stability for HER in acidic solution. To understand the origins of the outstanding HER performance of the FeP/Fe@NC catalyst, three reference samples were synthesized. Fe@NC-P was obtained by phosphorization of Fe@NC. Bulk FeP was prepared with the same process as for the synthesis of FeP/Fe@NC while in the absence of dicyandiamide. FeP ex-situ deposited on pre-formed Fe@NC support (FeP-Fe@NC) was synthesized by phosphorization of Fe-impregnated Fe@NC precursor (see the material and methods). XRD analysis of Fe@NC-P showed that the diffraction peaks can only be indexed to the Fe and C, which indicated that the Fe NPs buried in the carbon matrix of Fe@NC were preserved during phosphorization treatment. For the bulk FeP and FeP-Fe@NC, diffraction peaks assigned to FeP were observed (Fig. 5a). TEM image of Fe@NC-P showed that its morphology was similar to that of the Fe@NC (Fig. 5c). For the bulk FeP, large aggregated FeP particles were observed, and for the FeP-Fe@NC, FeP particles were unevenly deposited on the Fe@NC support rather than homogeneously distributed (Fig. 5d-e). This demonstrated that the pre-synthesis of the Fe NPs on the carbon support will allow for more effective control on the formation of the resulting 17

FeP/Fe@NC catalysts. The bulk FeP and FeP-Fe@NC samples were then investigated with XPS. The similar peak features of Fe 2p spectra as for the FeP/Fe@NC catalyst were observed, confirming the formation of FeP in the bulk FeP and FeP-Fe@NC samples (Fig. 5b), which is in good agreement with the XRD results. However, a slightly positive shift of Fe 2p peak positions in FeP-Fe@NC and FeP/Fe@NC was observed when compared to those of bulk FeP. This can be attributed to the electron transfer from FeP to the carbon matrix due to their interaction [45, 46]. Moreover, the more positive shift of Fe 2p peak positions in FeP/Fe@NC catalyst when compared with that of FeP-Fe@NC indicates a stronger interfacial interaction between FeP and carbon matrix of FeP/Fe@NC, which will facilitate more efficient electron transport.

Fig. 5 (a) XRD patterns of Fe@NC-P, bulk FeP, and FeP-Fe@NC samples, (b) High resolution XPS spectra for the Fe 2p obtained from bulk FeP, FeP-Fe@NC and FeP/Fe@NC samples, and TEM images of (c) Fe@NC-P, (d) bulk FeP, (e) FeP-Fe@NC. (The Fe@NC-P sample was obtained by phosphorization of Fe@NC. Bulk FeP was prepared through the same process as for the synthesis of FeP/Fe@NC in the absence of dicyandiamide. FeP ex-situ deposited on pre-formed Fe@NC support (FeP-Fe@NC) was synthesized by 18

phosphorization of Fe-impregnated Fe@NC precursor.)

The HER performances of Fe@NC-P, bulk FeP, FeP-Fe@NC together with FeP/Fe@NC were then investigated. As shown in Fig. 6a, FeP/Fe@NC and Fe@NC-P exhibited overpotentials of -49 and -398 mV, respectively, to achieve a current density of -10 mA cm−2. This demonstrates that FeP NPs on the surface of the carbon matrix are the active sites for HER. For bulk FeP, it exhibited a much higher overpotential of -195 mV when compared with FeP-Fe@NC (-97 mV) and FeP/Fe@NC (-49 mV), indicating the beneficial role of the carbon matrix for promoting the conductivity and dispersability of FeP catalyst and the modulation of the electronic properties due to their interaction. Furthermore, FeP/Fe@NC exhibited even higher HER catalytic activity than FeP-Fe@NC. This can be well explained by the stronger interaction between FeP and the carbon support of FeP/Fe@NC, which will lead to more efficient interfacial charge transport and prominent modulation on the electronic structure of the FeP catalyst as indicated by the XPS analysis. This conclusion can be further verified by the catalytic and charge transport kinetics, as well as intrinsic activity analyses. As shown in Fig. 6b, among all the investigated FeP-based samples, FeP/Fe@NC exhibited the smallest Tafel slope (67 mV dec-1) compared to FeP-Fe@NC (88 mV dec-1 ) and bulk FeP (121 mV dec-1), indicating more favorable HER catalytic kinetics [47]. Moreover, the electrochemical impedance spectroscopy (EIS) analysis (Fig. 6c) indicated that the charge-transfer resistance at the catalyst/electrolyte interface decreased in the order of bulk FeP > FeP-Fe@NC > FeP/Fe@NC, indicating that FeP/Fe@NC catalyst possessed the optimum charge 19

transport kinetics [48].

In addition, the electrochemically active surface area (ECSA) of the samples were measured using a simple CV method [49]. The results in Fig. S10 suggested that the increase in the catalytically active sites is in good agreement with the increased catalytic performance. Moreover, normalization of the active site activity (ASA) by taking into account the active site concentration at overpotential of 120 mV was shown in Fig. 6d [50, 51]. Due to the unknown capacitive behavior (Cs) of the FeP catalyst especially on Fe@NC substrate, we carried out activity normalization of ASA*Cs as reported [51]. The FeP/Fe@NC catalyst exhibited an ASA*Cs of 2.93 mA cm-2, which is much higher than FeP-Fe@NC and bulk FeP with ASA*Cs values of 1.81 and 1.26 mA cm-2, respectively. These results clearly demonstrate that the strong interfacial interaction at the FeP and Fe@NC interface is beneficial to more favourable catalytic kinetics, efficient charge transport as well as higher intrinsic activity, and thus excellent catalytic performance for HER.

Fig. 6 (a) Linear sweep voltammetry curves of the catalysts for HER in 0.5 M H2SO4, (b) the 20

Tafel plots of the catalysts, (c) the Nyquist plots of the catalysts, and (d) normalization of the active site activity (ASA) by taking into account the active site concentration at overpotential of 120 mV. Due to the unknown capacitive behavior (Cs) of the FeP catalyst especially on Fe@NC substrate, we carried out activity normalization of ASA*Cs.

We then investigated the origin of remarkable stability of FeP/Fe@NC catalyst. As shown in Fig.7, compared to FeP/Fe@NC which exhibited quite high stability, both FeP-Fe@NC and bulk FeP degraded substantially with prolonged reaction and accelerated degradation measurements. The fluctuation of the potentials in Fig. 7a is caused by the growth and release of the as-formed H2 bubbles and the strongest fluctuation of the curve of FeP/Fe@NC can be ascribed to the weakest hydrophilicity (Fig. S11) [52-54]. Elemental analysis (Table S2) indicated that the Fe element eluted from FeP during the HER test can be substantially inhibited by anchoring the FeP catalysts on the carbon support and this trend can be intensified by strengthening the connection between FeP and the carbon support. Moreover, the high resolution XPS analysis of Fe 2p before and after stability tests (Fig. S12) showed that the intensity of the peaks at 711.82 and 725.77 eV, which correspond to the oxide Fe species, increased substantially for bulk FeP and FeP-Fe@NC while negligibly for FeP/Fe@NC. Therefore, FeP/Fe@NC exhibited better anti-oxidation ability during the test. Based upon the above analysis, we can conclude that the excellent stability of the FeP/Fe@NC catalyst is due to the effective immobilization of the FeP NPs on the metal-caged carbon support through strong connection, which can retard the detachment and the oxidation of FeP NPs.

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Fig. 7 (a) Chronopotentiometric curves of FeP/Fe@NC, FeP-Fe@NC, and bulk FeP at a current density of -10 mA cm-2 and (b) cyclic voltammetry of catalysts at a scan rate of 100 mV s-1 between -0.12 and 0.28 V in 0.5 M H2SO4 (Carbon plate was used as the counter electrode, and the data were collected without iR compensation).

After establishing the synthesis protocol for FeP/Fe@NC, we extended the same process to obtain CoP/Co@NC and Ni2P/Ni@NC catalysts due to the similar roles of Co and Ni in catalysing the growth of carbon materials (Fig. S13, Fig. S14). Different from Fe, Co and Ni catalysed the growth of carbon nanotubes from dicyandiamide at temperatures of 650 to 700 oC (Fig. S15, Fig. S16). After phosphorization, the as-prepared catalysts, such as CoP/Co@NC-680-8 and Ni2P/Ni@NC-680-8, exhibited similar morphologies with their corresponding precursors (Fig. S17, Fig. S18). When acted as the HER catalysts, the CoP/Co@NC-680-8 and Ni2P/Ni@NC-680-8 exhibited good electrocatalytic activities with overpotentials of -99 and -144 mV to achieve current densities of -10 mA cm-2, respectively, and remarkable long-term durability (Fig. 8). This indicates that the present approach can also be well applied to the synthesis of series of phosphide catalysts with strong catalyst-support interactions.

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Fig. 8 Linear sweep voltammetry curves of (a) CoP/Co@NC and (c) Ni2P/Ni@NC catalysts in 0.5 M H2SO4, and chronopotentiometric curves of (b) CoP/Co@NC-680-8 and (d) Ni2P/Ni@NC-680-8 at a current density of -10 mA cm-2 (Carbon plate was used as the counter electrode, and the data were collected without iR compensation).

4. Conclusions In summary, the FeP/Fe@NC, acted as an outstanding HER catalyst in acidic solution, have been successfully fabricated by a simple two-step approach. It was found that phosphorization of Fe/Fe@NC precursors produced through Fe-catalyzed growth process is beneficial to strengthen the interfacial connection between the FeP NPs and Fe@NC support. By rationally tuning the synthesis parameters, FeP catalysts were prepared on highly-conductive Fe@NC support. Benefiting from high conductivity, good dispersibility and efficient charge transport, the as-prepared FeP/Fe@NC showed excellent electrocatalytic activity with extremely low overpotentials of -49 and -130 mV to achieve current densities of -10 and -100 mA cm−2, respectively. Meanwhile, it also showed remarkable long-term stability with maintaining its activity for both at least 90 hours and 5000 cycles of accelerated degradation 23

measurements. It was confirmed that the remarkable stability of FeP/Fe@NC is attributed to the strong interfacial connection of FeP and Fe@NC, which favoured the structural integrity, and suppressed the detachment and the oxidation of the catalysts during reactions. Furthermore, as a promising versatile strategy, such catalyst/support design could be extended to the synthesis of a range of efficient and stable catalysts including CoP, Ni2P, as well as metal nitrides, metal sulfides, and carbides for electrochemical energy conversions. Acknowledgements This work was financially supported by the National Key R&D Program of China (No. 2017YFA0204804), National Natural Science Foundation of China (No. 21573219), the Strategic Priority Research Program of Chinese Academy of Sciences (No.

XDB17000000),

the

China

Postdoctoral

Science

Foundation

(No.

2016M590237), and CNPC-DICP Joint Research Center. X. Z. acknowledges the support from Young Thousand Talents Program of China.

Declarations of interest None.

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Xiaomei Wang received her B.S. degree in chemistry from Lanzhou University in 2014. Now, she is a Ph.D. candidate at Dalian Institute of Chemical Physics, Chinese Academy of Sciences under the supervision of Prof. Can Li and Prof. Xu Zong. Her current research focuses on the development of highly efficient electrocatalysts for water splitting and energy conversion.

Weiguang Ma received his Ph.D. degree from Changchun Institute of Applied Chemistry, Chinese Academy of Sciences in 2015. Currently, he is a postdoctoral fellow at Dalian Institute of Chemical Physics, Chinese Academy of Sciences under the supervision of Prof. Can Li. His research interest is photoelectrochemical and electrocatalytic splitting of hydrogen sulfide and water.

Zhiqiang Xu received his B.S. degree in 2014 from Jilin University. He currently is a Ph.D. student at Dalian Institute of Chemical Physics under the supervision of Prof. Can Li and Prof. Xu Zong. His research interests involve the development of highly efficient catalysts for water splitting and energy conversion.

Hong Wang received his B.S. degree in materials chemistry from Harbin Engineering University in 2015. Currently, he is pursuing his Ph.D. degree with the supervision of Prof. Can Li and Prof. Xu Zong at Dalian Institute of Chemical Physics, Chinese Academy of Sciences. His research interests include the development of novel photocatalysts for solar conversion and storage.

Wenjun Fan received his B.S. degree and Master degree from Hebei University of Science and Technology and Kunming University of Science and Technology in 2010 and 2014, respectively. Currently, he is a Ph.D. candidate of South China University of Technology under the supervision of Prof. Shijun Liao and Prof. Can Li. His research focuses on catalyst design for fuel cells and solar energy conversion.

Xu Zong received his Ph.D. degree in Physical Chemistry from Dalian Institute of Chemical Physics (DICP), Chinese Academy of Sciences in 2010. He then worked as a postdoctoral research fellow in the University of Queensland from 2010-2014. In 2014, he returned to DICP as the recipient of Hundred Talent Program of DICP and was promoted to professor in 2015 as the recipient of One Thousand Youth Talents Program of China. His research interests are design and construction of coupled system for solar-to-chemical conversion and electrocatalysts for water splitting. He has published more than 50 peer-reviewed papers with over 3000 citations.

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Can Li received his Ph.D. degree in Physical Chemistry from Dalian Institute of Chemical Physics, Chinese Academy of Sciences, in 1989, and he joined the same institute and was promoted to full professor in 1993. He was the President of the International Association of Catalysis Societies (2008–2012). Currently, he is the director of the Dalian National Laboratory for Clean Energy. His research interests include (1) UV Raman spectroscopy and ultrafast spectroscopy; (2) environmental catalysis and green catalysis; (3) heterogeneous asymmetric catalysis; and (4) solar energy utilization.

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Graphical abstract

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Reasearch highlights  FeP catalysts anchored on Fe-caged graphitic carbon (FeP/Fe@NC) were synthesized by phosphorization of Fe-grafted carbon precursors (Fe/Fe@NC) that were in-situ produced through Fe-catalyzed growth process.  The as-prepared catalysts demonstrate outstanding electrocatalytic performance for hydrogen evolution reaction (HER) in acidic media, which is comparable to that the state-of-art non-noble metal catalysts.  The synthetic protocol is beneficial to strengthening the interfacial interaction between the FeP NPs and Fe@NC support, which contributes to the outstanding activity and stability for HER.  Similar strategy can be well applied to the synthesis of a range of other catalysts including CoP/Co@NC, Ni2P/Ni@NC.

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