FeP Nanocrystals Embedded in N-Doped Carbon Nanosheets for

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FeP Nanocrystals Embedded in N-Doped Carbon Nanosheets for Efficient Electrocatalytic Hydrogen Generation over a Broad pH Range Yang Yu, Zhuo Peng, Muhammad Asif, Haitao Wang, Wei Wang, Zexing Wu, Zhengyun Wang, Xiaoyu Qiu, Hua Tan, and Hongfang Liu ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/acssuschemeng.8b01746 • Publication Date (Web): 16 Jul 2018 Downloaded from http://pubs.acs.org on July 17, 2018

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ACS Sustainable Chemistry & Engineering

(Ministry of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering

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ACS Sustainable Chemistry & Engineering 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

FeP Nanocrystals Embedded in N-Doped Carbon Nanosheets for Efficient Electrocatalytic Hydrogen Generation over a Broad pH Range Yang Yu,† Zhuo Peng,† Muhammad Asif,† Haitao Wang,† Wei Wang,† Zexing Wu,‡ Zhengyun Wang,† Xiaoyu Qiu,† Hua Tan,† and Hongfang Liu*,† †

Key laboratory of Material Chemistry for Energy Conversion and Storage (Ministry

of Education), Hubei Key Laboratory of Material Chemistry and Service Failure, School of Chemistry and Chemical Engineering, Huazhong University of Science and Technology, No.1037 Luoyu Road, Wuhan 430074, P. R. China ‡

State Key Laboratory Base of Eco-chemical Engineering, College of Chemistry and

Molecular Engineering, Qingdao University of Science and Technology, No.53 Zhengzhou Road, Qingdao 266042, P. R. China

Corresponding Author *E-mail: [email protected].

KEYWORDS: Sol-gel, N-doped carbon nanosheets, Hydrogen generation, General, Excellent catalytic performance

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ABSTRACT: Recently, great attentions have been triggered to exploit stable and highly efficient non-precious metal-based catalysts applied to energy conversion techniques by electrocatalytic reaction. Transition metal phosphides (TMPs) are promising and intriguing candidates for hydrogen evolution reaction (HER) as well as other energy conversion techonologies. Here we present a facile sol-gel route aimed at synthesizing

FeP

nanocrystals

embedded

in

N-doped

carbon

nanosheets

(FeP/NCNSs). The obtained nanocomposite exhibits excellent hydrogen generation activity to achieve 10 mA cm-2 at relatively low overpotential (114 mV) in acidic electrolyte (0.5 M H2SO4). In the respect of structural integrity, the N-doped carbon layer could protect the FeP nanocrystals from acidic degradation, thus endowing the hybrid with superior durability to the bare FeP nanoparticles during long-term operation. Furthermore, the as-synthesized electrocatalyst also exhibits high catalytic activity and remarkable cyclability in alkaline and even neutral electrolytes. More importantly, other TMPs nanocrystals encapsulated in N-doped carbon nanosheets like Ni2P/NCNSs can be similarly fabricated through this synthetic strategy, which also presents excellent catalytic performance and comfortable cycling durability towards HER. Therefore, this research provides a feasible and further generic route for designing and constructing of transition metal phosphides/carbonaceous matrices nanohybrid to enhance the performance towards HER and even other energy conversion reactions when implemented as electrode materials.

INTRODUCTION



Hydrogen (H2) has always been considered as a potential aspirant to supersede the conventional fossil fuels (such as coal, petroleum and natural gas, etc.) due to its abundant resources, sustainability, and high energy density as well as zero emissions.1, 2 Among various emerging clean-energy techniques, water electrolysis is one of the most efficient and non-polluting approach for the production of highly pure hydrogen.3-7 At present, in spite of platinum-based materials as the outstanding catalysts toward electrochemical hydrogen generation, their rarity as well as accompanying inevitable high expense seriously hampered their extensive applications.8,

9

Therefore, great efforts have been dedicated to explore stable,

competent and inexpensive HER electrocatalysts on the basis of non-precious metals (such as iron, cobalt, nickel, copper, molybdenum and tungsten) and their derivatives (chalcogenides, nitrides, carbides, borides and phosphides).10-20 In recent years, earth-enriched 3d transition metal phosphides (TMPs, metal = iron, cobalt, nickel and copper) have captured extensive attentions owing to their excellent mechanical strength, electronic conductivity and chemical stability, which are superior to those of other transition metal compounds.21-31 It has been well-documented that phosphorus (P) element in TMP plays an important role in HER process during which electronegative P atoms are capable of grabbing the electrons from metal centers, and meanwhile P atoms can be considered as base to attract and trap protons. Obviously, TMPs with higher phosphorus content could expose more reactive sites so as to enhance HER activity while improving the corrosion resistance and further stability for HER.3, 32, 33 Nevertheless, with the P content increasing, the 3 ACS Paragon Plus Environment

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electron delocalization of metal atoms will be greatly restrained, thus resulting in decline of electronic conductivity as well as HER activity.4, 34 So it is of immense significance to make a compromise between intrinsic electrochemical activity and electron transfer capacity. In this regard, construction of TMPs possessing abundant active sites and their hybridization with conductive matrices in nanoscale will be a feasible strategy for enhancing HER performance.35-37

It is found that the carbon shell of metal/carbon composite enables to the shell protect active materials nanoparticles (NPs) from acidic degradation, agglomeration and coalescence with adjacent NPs.38-41 Motivated by the phenomena, many researches have been focused on appealing nanoarchitectures of metals or their compounds nanoparticles encapsulated in porous and thin carbon layers when compared to the naked species, which have been manifested to be efficient for accelerating charge and mass transport process.42-45 For instance, Yang et al. reported a hybrid of cobalt monophosphide nanoparticles encased in thin carbon layer with nitrogen-doped (CoP@NC) as an excellent HER catalyst.26 Density functional theory (DFT) theoretical calculations proved that cobalt phosphide core together with nitrogen-doped carbon shuck mainly synergistically contributed to such outstanding HER performance, and numerous active sites are located at the carbon atoms adjoining to the doped nitrogen atoms in carbon layer. Pu et al. adopted a phytic acid-assisted pyrolysis method to fabricate a series of nanocomposites of metal phosphides nanoparticles (MP NPs) encapsulated in N, P-codoped carbon (MP NPs@NPC, M = Fe, Co, Ni), showing superior HER capability in acidic, basic and 4 ACS Paragon Plus Environment


even neutral media.35 Wang et al. demonstrated the nanoarchitecture of well-dispersed Cu3P crystals embedded into hierarchical porous nitrogen and phosphorous-codoped carbon matrices by utilizing a unique Cu-based metal organic framework as self-sacrificial template, which possesses high electrocatalytic activity towards HER especially in acidic medium.43 In this study, we present a facile sol-gel method with subsequent pyrolysis and phosphorization strategy for fabricating the hybrid of FeP nanocrystals embedded in N-doped carbon nanosheets (FeP/NCNSs). This rational design and construction of nanocrystals encapsulated in nitrogen-doped carbon nanosheets showed excellent electrocatalytic activity and stable long-term performance over a wide pH range, and further demonstrated their highly promising potential for HER applications.

EXPERIMENTAL SECTION

Synthesis of Fe-based precursor, Fe2O3 NPs and N-doped carbon nanosheets (NCNSs) In a typical sol-gel synthesis with modifications,46-48 1.2 g Fe(NO3)3·9H2O was thoroughly dissolved in 150 mL absolute ethanol via vigorously stirring (denoted as solution A). 10.0 g urea and 1.0 g citric acid were also totally dissolved in 50 mL deionized (DI) water with mild agitation (denoted as solution B). Then, solution B was slowly poured into solution A to form an orange sol. The sol was consecutively stirred at 75 °C water bath until the thorough evaporation of mixed solvent to form a saffron gel. Then it was transferred into a drying cabinet and kept at 100 °C overnight. 5 ACS Paragon Plus Environment

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The obtained xerogel was firstly annealed at 350 °C for 4 hours and then treated at a higher temperature of 650 °C (5 °C min−1) in a highly pure N2 flow. To obtain Fe2O3 NPs, the as-prepared Fe-based precursor was annealed at 450 °C for 6 hours in a muffle furnace (the ramp rate is 2 °C min-1) to remove the carbon species thoroughly. N-doped carbon nanosheets (NCNSs) were fabricated through the same steps as FeP/NCNSs without adding Fe salt.

Synthesis of FeP/NCNSs and FeP NPs 50 mg obtained Fe-based precursor and 1.5 g NaH2PO2 were put into two separate quartz boats. The NaH2PO2 is located at the upstream side of the tube furnace and heated to 300 °C kept for 2 h in highly pure N2 atmosphere with a slow heating rate of 2 °C min-1. Finally, the resulting solid was centrifuged with water and ethanol several times and dried. The FeP NPs were prepared with similar process of Fe2O3 NPs precursor

Synthesis of Ni2P/NCNSs Likewise, using nickel nitrate instead of ferric nitrate via similar steps mentioned above, Ni2P/NCNSs was also obtained.

Preparation of the working electrodes In order to get the homogeneous catalyst ink, 5 mg of the obtained nanohybrid was dispersed in 980 µL ethanol along with 20 µL ~5 wt% of Nafion® 117 solution, and the mixture was treated with ultrasonication for more than 1 hours. Then, extracting 9 6 ACS Paragon Plus Environment


µL of above suspension to dropwise cast on the surface of glass carbon (GC) electrode with a catalyst loading mass of ~0.6 mg cm−2.

Material characterization The phase composition for as-synthesized samples were characterized using a Bruker D8 Advance X-ray diffractometer, with Cu Kα radiation (λ = 1.5418 Å) at room temperature. The macroscopic morphology, micro/submicro-structure and surface elemental and chemical states for the products were characterized by scanning electron microscopy (SEM: FEI, Nova NanoSEM 450) and transmission electron microscopy (TEM: Talos-f200s) and X-ray photoelectron spectroscopy (XPS: VG inc., MultiLab 2000). ICP-AES (Perkin-Elmer Optima 4300DV) and elemental analysis (Vario EL cube CHNSO elemental analyzer) were used to determine the composition and elemental ratio for the obtained materials. Nitrogen isothermal adsorption/desorption measurements and pore size distributions were analyzed via a Micromeritic TriStar® II 3020 analyzer at a ultralow temperature of 77 K. Raman shifts were recorded on a LabRAM Aramis Raman spectrometry instrument with an argon ion laser of which the excitation wavelength is ~633 nm. FT-IR spectrum was conducted using a Nicolet FTIR Is 10 spectrometer.

Electrochemical measurements All measurements were tested using an electrochemical workstation (CHI 760e) with typical three-electrode systems in a sealed two-compartment electrolyzer. Graphite rod, saturated calomel electrode and Hg/HgO electrode were used as the 7 ACS Paragon Plus Environment

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auxiliary electrode, reference electrode in acidic (0.5 M H2SO4 solution with a pH of about 0.36) and neutral media (1.0 M PBS, the pH of ~7.4) and reference electrode in alkaline medium (1.0 M KOH solution of which the pH value is ~13.63), respectively. All linear sweep voltammetry (LSV) curves (polarization curves) were obtained at 5 mV s−1 and with iR correction. Electrochemical impedance spectroscopy (EIS) for HER were carried out in the frequency range of 100 kHz–0.1 Hz with an AC amplitude of 10 mV. In addition, cyclic voltammetry (CV) was conducted at non-Faradaic region to evaluate the electrochemical double layer capacitance (Cdl) for the as-fabricated electrocalysts. Faradaic efficiencies measurements were conducted in the abovementioned sealed two-compartment cell via the drainage gas-collecting method at a certain current density in three different electrolytes, respectively. The catalyst ink was drop-cast onto a piece of carbon fiber paper of which the size is ~1 cm × 2 cm, and occupied the area of 1 cm2. RESULTS AND DISCUSSION



Figure 1. Synthetic protocol for FeP/NCNSs hybrid. Figure 1 displays the fabrication process of FeP/NCNSs. The formation for Fe species/NCNSs composites can be described as a self-template and bottom-up process. The alcoholic solution A of ferric nitrate and the aqueous solution B of urea and citric acid were homogeneously mixed in a beaker under agitation to obtain a yellow sol (Figure S1a) and then an orange gel (Figure S1b) formed when the mixed solvent volatilized. After drying to remove the excess solvent, the brown xerogel (Figure S1c) was annealed at 350 °C to generate the C3N4 monolith consisting of layered construction, which could be verified through corresponding XRD pattern, FESEM image and Fourier transform infrared (FT-IR) spectroscopy (Figure S2a-c). With the template pyrolysis at 650 °C, the g-C3N4 framework was thermally exfoliated into crumpled carbon nanosheets in which the in situ generated Fe species were confined. In the meanwhile, the residual N species were uniformly distributed 9 ACS Paragon Plus Environment

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into the hybrid (Figure S2d-f). After the phosphorization by thermal decomposition of NaH2PO2 under inert atmosphere, the Fe species was then in situ converted into FeP, thus achieving FeP/NCNSs heterostructures.

Figure 2. (a) XRD pattern of as-prepared FeP/NCNSs hybrid, (b-d) FESEM and TEM images, (e and inset) HRTEM images and (f) HAADF-STEM image and corresponding mapping of Fe, P, C, N, and O elements for FeP/NCNSs. XRD pattern (Figure 2a) of FeP/NCNSs could be assigned to FeP (JCPDS No. 65-2595), consistent with the space group Pnma. The broad peak at ~26 ° corresponds to (002) plane of the graphitic carbon structure. FESEM and TEM images (Figure 2b-d) exhibit the wrinkled sheets-like morphology with lots of tiny nanocrystals decorated.

High resolution TEM (HRTEM) manifests the FeP nanocrystals

possessing the average size between 5-10 nm with a little agglomeration, are well dispersed on the thin carbon layer (Figure 2e). For comparison, FeP NPs without any 10 ACS Paragon Plus Environment


carbon species using Fe2O3 NPs as the precursor and pure NCNSs are also prepared, respectively (Figure S3a, S3c and S3e). Bare FeP NPs shows an interconnected and aggregated structure which inherited from bare Fe2O3 NPs while pure NCNSs presents a porous and smooth sheet-like morphology (Figure S3b, S3d and S3f). And the inset of Figure 2e displays an obvious d-spacing of ~0.273 nm, in accordance with orthorhombic FeP (011) planes. STEM image and corresponding element mappings verify the nanocrystals consisting of Fe element and P element are well wrapped in N-doped thin carbon flakes, as well as all the involved elements are uniformly distributed in the composite (Figure 2f). The corresponding EDS reveals the atomic ratio for Fe to P in FeP/NCNSs is approximately 1:1 (Figure S4), which accords with the result of ICP-AES (Table S1).

Figure 3. (a) Raman spectra of FeP/NCNSs and pure NCNSs, (b) N2 adsoprtion/desorption isotherms of NCNSs, FeP/NCNSs and FeP NPs, XPS of (c) Fe 2p, (d) P 2p, (e) C 1s and (f) N 1s performed on FeP/NCNSs. 11 ACS Paragon Plus Environment

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To further characterize the other non-metallic component, elemental analysis (Table S2) demonstrated that the carbon and nitrogen contents are ~36.8 wt% and ~3.33 wt%, respectively. According to Raman spectra (Figure 3a), the characteristic bands should be ascribed to the D band (centered at ~1350 cm-1, which represents the degree of disorder and defects) and G band (located at ~1590 cm-1, which represents the degree of graphitization) of carbon nanosheets, respectively. It is examined that the carbon in FeP/NCNSs was partially graphitized. Furthermore, the higher intensity ratio (ID/IG = 1.22) for FeP/NCNSs to that of pure NCNSs (ID/IG = 0.93) indicated the more defects of FeP/NCNSs than that of pure NCNSs, which may be due to the interaction between carbon nanosheets and FeP nanocrystals. Based on the characteristics of type IV isotherms, it is apparently manifested that FeP/NCNSs, FeP NPs and pure NCNSs have mesoporous structures, which was consistent with the results of pore size distribution (Figure 3b and Figure S5). The Brunauer–Emmett–Teller (BET) surface area for FeP/NCNSs, FeP NPs and pure NCNSs is 110.0, 24.8, and 240.2 m2 g−1, respectively. Moreover, FeP/NCNSs possess much larger surface area than FeP NPs, thus demonstrating that the introduction of carbonaceous matrices contributes to the surface area of electrocatalyst and that FeP/NCNSs possesses more accessible active sites than bare FeP NPs. Further the XPS characterizations are employed to elucidate the detailed elemental composition and chemical states of FeP/NCNSs. The contents of all related elements are presented in Table S3. As depicted in Fig. 3c, Fe 2p spectrum shows two peaks of 12 ACS Paragon Plus Environment


707.3 and 720.7 eV, which corresponds to Fe 2p3/2 and Fe 2p1/2 in FeP, respectively. Moreover, the broad peaks at 711.7 and 723.6 eV are associated with oxidized Fe species in FeP because of the unavoidable surface oxidation.49 Two peaks located at 129.3 and 130.2 eV can be assigned to P 2p3/2 and P 2p1/2 in the deconvoluted P 2p spectrum (Figure 3d), in which the other peak (~133.2 eV) corresponds to the P–O oxidized species of phosphide.50 From Figure 3e, four peaks centered at 284.5, 285.9, 286.4, and 289.2 eV emerge in the high resolution spectrum for C 1s, which could be vested in C–C, C–O/C–N, C=O/C=N, and O–C=O, respectively.51 Figure 3f presents three peaks (398.6, 400.3 and 401.2 eV) of the N 1s spectrum, corresponding to pyridinic N (~47.5 at%), pyrrolic N (~34.5 at%) and quaternary N (~18.0 at%), respectively.52 As known that pyridinic N and quaternary N predominantly contribute to activation for adjacent carbon atoms during electrochemical HER process, it could be preliminarily inferred that the relatively high contents of pyridinic N and quaternary N in FeP/NCNSs will be beneficial to the HER process.37, 53


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Figure 4. (a) LSV curves for commercial 20% Pt/C, FeP/NCNSs, FeP NPs, NCNSs and blank GCE in 0.5 M H2SO4. (b) Tafel plots for commercial 20% Pt/C, FeP/NCNSs hybrid, bare FeP NPs. (c) LSV curves for FeP/NCNSs and FeP NPs initially and after 5000 CV cycling from 0.05 to -0.35 V at 100 mV s-1 (inset: the chronoamperometric curves of FeP/NCNSs and FeP NPs under 122 mV and 202 mV (without iR compensation), respectively). (d and inset) Plot of capacitive densities vs. various scanning rates from 20 to 180 mV s-1 and TOFs for FeP/NCNSs and FeP NPs. (e-f) Polarization curves and corresponding Tafel plots for FeP/NCNSs in three different electrolytes.

The electrocatalytic performances of as fabricated samples for HER was evaluated in 0.5 M H2SO4 with the commercial 20% Pt/C as a reference. As depicted in Figure 4a, commercial 20% Pt/C exhibits outstanding electrocatalytic hydrogen generation activity with the onset overpotential (ηonset, the overpotential at 1 mA cm−2) of nearly 0 mV, while NCNSs possesses poor and even negligible catalytic activity with a large ηonset over 450 mV. FeP NPs require overpotentials (η) of 114, 196 and 290 mV to deliver 1, 10 and 40 mA cm−2. In contrast, FeP/NCNSs showed superior HER activity (overpotentials of 56, 114 and 160 mV at 1, 10 and 40 mA cm−2, respectively) to FeP NPs, which are remarkably comparable or even better than most of the recently developed TMPs towards HER (Table S5). LSV curves for FeP/NCNSs with different mass loadings in acidic medium are shown in Figure S6a. The HER activity for FeP/NCNSs is optimal with a catalyst loading of 0.6 mg cm-2. The corresponding plot of η10 versus FeP/NCNSs mass loading presents the variation tendency of catalytic 14 ACS Paragon Plus Environment


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activity (Figure S6b), while some literatures have reported similar variation tendency for electrocatalytic activity as for the mass loading (Table S4). In detail, the η10 in acidic medium for FeP/NCNSs is 145, 129, 114, and 118 mV corresponding to a catalyst loading mass of 0.2, 0.4, 0.6, and 0.8 mg cm-2, respectively.

The Tafel slope fitted from LSV curve through the Tafel equation (η = a + b log j) as the essential property for electrocatalysts describes the controlling step during hydrogen evolution process. For the entire HER process, the reaction steps contribute to generate H2 as a result of the discharge step (H+(aq) +e- → Hads, Volmer reaction) coupled with electrochemical desorption step (Hads + H+(aq) + e- → H2 (g), Heyrovsky reaction) or chemical desorption step (Hads + Hads → H2

(g),

Tafel reaction) and the

course of reaction are usually referred to as Volmer–Heyrovsky process or Volmer– Tafel process.54 In Figure 4b, the Tafel slope of commercial 20% Pt/C is as low as 29 mV dec−1, well coincided with previous literatures.55,

56

The FeP/NPs possess an

inferior Tafel slope of 84 mV dec−1 to FeP NPs (64mV dec−1), which demonstrates that the hydrogen generation on FeP/NCNSs should follow the Volmer−Heyrovsky mechanism.57, 58 From the Tafel plot from static mode of operations and the plot of overpotential as a function of log ǀjǀ (Figure S7), it can be seen that the η10 and corresponding Tafel slope is 114 mV and ~63 mV dec-1, respectively, which is in accord with the results from the extraction of LSV curve for FeP/NCNSs. Moreover, exchange current density (j0) as another essential parameter is usually determined to reflect the inherent electrochemical reaction rate. Obviously, the FeP/NCNSs displays


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a j0 of 0.132 mA cm−2, which is about four times larger than that of FeP NPs (0.034 mA cm− 2) (Figure S8). Besides the catalytic activity, cyclability should be also an essential index to assess the continuous operational capability for an electrocatalyst. According to Figure 4c and Figure S9, it can be noted that only a minute current decayed after 5000 CV cycles for FeP/NCNSs, while both bare FeP NPs and commercial 20% Pt/C exhibited a relatively larger current loss in comparison with FeP/NCNSs. In order to further confirm the long-term stability, FeP/NCNSs was examined via galvanostatic electrolysis in acidic medium under 122 mV without iR correction. During the long-term stability measurements, no obvious degradation is observed after 20 h (Fig. 4c inset), indicating the excellent durability of this nanocomposite. In addition, FeP NPs display an inferior stability to FeP/NCNSs, with a current loss of about 25% over 11 h. Moreover, XRD pattern, FESEM and TEM images for FeP/NCNSs after HER stability test could further elucidate the stability of the nanoarchitecture (Figure S10). Therefore, it is concluded that the FeP/NCNSs is unequivocally an efficient and durable electrocatalyst for HER.

To have an insight into the reason of the superb activity of electrocatalyst, the electrochemically active surface areas (ECSAs) of the as synthesized samples are assessed by electrochemical double-layer capacitance (EDLC, Cdl) measurements using CVs at various scan rates (Figure 4d, Figure S11 and Figure S12). Compared with FeP NPs (Cdl = 5.87 mF cm-2) and pure NCNSs (Cdl = 6.15 mF cm-2), 16 ACS Paragon Plus Environment


FeP/NCNSs delivers a higher Cdl of 26.7 mF cm-2. Correspondingly, the high ECSA of the FeP/NCNSs would be attributed to enhanced ion transfer capacity between the electrolyte ions and the electrochemically accessible active sites. Furthermore, the EIS measurements of FeP/NCNSs and bare FeP NPs are also carried out at 10 mA cm-2. From Figure S13, the observed semicircle stands for the charge transfer resistance (Rct) when electrochemical reaction occurs at the electrode–electrolyte interface. The bare FeP NPs show a higher Rct (199 Ω) than FeP/NCNSs (16 Ω), revealing that the N-doped carbon matrices may boost the electron transfer of hybrid as well as further improve electrocatalytic activity. Besides, the hydrogen evolution rates for both FeP/NCNSs and bare FeP NPs in acidic medium are very close to the theoretical value, showing nearly 100% Faradaic efficiency (as shown in Fig. S14). The turnover frequencies (TOFs) for both FeP/NCNSs and FeP NPs are calculated under 150 mV. In consideration of approximately 100% Faradaic efficiency for both FeP/NCNSs and FeP NPs, the TOF of FeP/NCNSs is 0.18 s-1, which is twice of FeP NPs (0.09 s-1), illustrating superior catalytic activity for the hybrid (Figure 4d inset). The work capabilities of the as prepared hybrid has been tested in wide pH environment towards HER (Figure 4e, f). While using 1.0 M KOH, FeP/NCNSs exhibits a relatively small η10 (205 mV) and corresponding Tafel slope (70 mV dec−1). In the neutral solution (1.0 M PBS), the hybrid possesses η10 (409 mV) and Tafel slope (92 mV dec−1). The hydrogen evolution property for FeP/NCNSs tested in alkaline and neutral media is comparable to those of recently developed materials (Table S6 and Table S7). Additionally, FeP/NCNSs also exhibit excellent durability 17 ACS Paragon Plus Environment

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and Faradaic efficiency comparable to the theoretically calculated value under basic and neutral conditions (Figure S15 and S16). Overall results further vindicate the excellent electrochemical activity and durable stability for FeP/NCNSs in all pH value.

For the sake of confirming the general applicability of our strategy, Ni2P/NCNSs have also been fabricated via a similar synthetic protocol (Figure S17). The morphology exhibits the hierarchical heterostructure of Ni2P nanocrystals embedded in N-doped carbon nanosheets. Unexpectedly, it is apparently observed that lots of carbon nanotubes are formed on the surface of carbon nanosheets, which may be owing to the catalysis of Ni species during annealing process.59-61 Additionally, this nanocomposite also displays good HER performance in 0.5 M H2SO4. The Ni2P/NCNSs exhibits a η10 (176 mV) with little decay after 5000 CV scanning (Figure S18a-b) and good stability over 84% in acidic solution (Figure S18c), thus evidencing the structural integrity of this nanoarchitechture. All the above mentioned results together indicate that this modular strategy can not only be applied for synthesizing hierarchical structures of other transition metal phosphides, but also endows the hybrid with high activity towards HER. In view of the aforementioned aspects, the excellent HER performance of the FeP/NCNSs catalyst could be ascribed to four factors as follows: First, the intrinsic property endows FeP with superior electrocatalytic activity towards HER; Second, in comparison with the agglomerated and even coalescent FeP nanoparticles, NCNSs as 18 ACS Paragon Plus Environment


the support enable uniform dispersion of FeP and physicochemical protection for the catalytic species against degradation to take full advantage of the active catalytic sites. Third, nitrogen atoms, especially pyridinic and quaternary N, could promote to interact with protons and endow the carbonaceous composites with more highly active reactive sites. Last but not least, coupling of nanosized FeP with N-doped carbon nanosheets could provide an enlarged specific surface area and is beneficial for electron transport as well as decrease the internal resistance for the catalyst, thus further boosting the HER activity. CONCLUSIONS

In summary, the hybrid of FeP nanocrystals encased in N-doped carbon nanosheets has been successfully fabricated through a facile sol-gel route. Confining FeP in N-doped carbon nanosheets endows the electrocatalyst with good structural integrity, superior charge transfer capability and sufficient active sites for boosting electrocatalytic process. The nanocomposite exhibits excellent HER performances with a small overpotential, a low Tafel slope and long-time stability in acidic electrolyte. Even though working in basic and neutral media, it still presents good activity and stability towards HER. Such high catalytic activity should be owing to the synergistic effect between FeP nanocrystals and carbon sheets with N doped. More importantly, this facile self-template and bottom-up synthesis can be extended to the design and construction of other TMP nanocrystals integrated with N-doped carbon matrices, which is of great significance for HER electrocatalyst application. 19 ACS Paragon Plus Environment

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Supporting Information. Optical photograph of the precursor sol, gel and xerogel; Physical characterizations of the precursor treated at 350 °C and 650 °C; XRD pattern and SEM image of pure NCNSs, Fe2O3 NPs and FeP NPs; SEM, EDS and Pore size distribution of FeP/NCNSs, FeP NPs and pure NCNSs; Exchange current density of FeP/NCNSs and FeP NPs; XRD pattern, SEM, TEM and HRTEM images for FeP/NCNSs after durability test; CVs of FeP/NCNSs and FeP NPs with different scanning rates; Comparison of EIS between FeP/NCNSs and FeP NPs; E-t curves of FeP/NCNSs in acidic, alkaline and neutral electrolytes at a constant current density; Physical characterizations and electrocatalytic performances of Ni2P/NCNSs; ICP-AES and elemental analysis of FeP/NCNSs; Comparisons of HER performances in acidic, basic and neutral media for FeP/NCNSs with other electrocatalysts. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected].

Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by National Natural Science Foundation of China (Project No. U1662114). The Foundation of Hubei Key Laboratory of Material Chemistry and 20 ACS Paragon Plus Environment


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Table of Contents (TOC) Graphic and Synopsis

The uniqueness of the FeP nanocrystals/N-doped carbon nanosheets offers universal and sustainable HER electrocatalysts in wide pH range.


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FeP Nanocrystals Embedded in N-Doped Carbon Nanosheets for

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