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A General Strategy for the Synthesis of Transition-Metal Phosphide/ N-doped Carbon Frameworks for Hydrogen and Oxygen Evolution Zonghua Pu, Chengtian Zhang, Ibrahim Saana Amiinu, Wenqiang Li, Lin Wu, and Shichun Mu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 28 Apr 2017 Downloaded from http://pubs.acs.org on May 1, 2017

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ACS Applied Materials & Interfaces

A General Strategy for the Synthesis of Transition-Metal Phosphide/N-doped Carbon Frameworks for Hydrogen and Oxygen Evolution Zonghua Pu,† Chengtian Zhang,† Ibrahim Saana Amiinu,† Wenqiang Li,† Lin Wu,† and Shichun Mu†,* †

State Key Laboratory of Advanced Technology for Materials Synthesis and Processing, Wuhan University of Technology, Wuhan 430070, P. R. China

ABSTRACT: Transition metal phosphides (TMPs) have been identified as promising non-precious metal electrocatalyst for hydrogen evolution reaction (HER) and other energy conversion reactions. Herein, we reported a general strategy for synthesis of a series of TMPs (Fe2P, FeP, Co2P, CoP, Ni2P and Ni12P5) nanoparticles (NPs) with different metal-phases embedded in a N-doped carbon (NC) matrix using metal salt, ammonium dihydrogen phosphate and melamine as precursor with varying molar ratios and thermolysis temperatures. The resultant TMPs can serve as highly active and durable bifunctional electrocatalyst toward HER and oxygen evolution reaction (OER). In particular, the Ni2P@NC phase only requires an overpotential of ~138 mV to derive HER in 0.5 M H2SO4, and ~320 mV for OER in 1.0 M KOH at the current density of 10 mA cm-2. Due to the encapsulation of NC that can effectively prevent corrosion of embedded TMP NPs, Ni2P@NC exhibits almost unfading catalytic performance even after 10 h under both acidic and alkaline solutions. This synthesis strategy provides a new avenue to exploring TMPs as highly active and stable electrocatalyst for the HER, OER and other electrochemical applications. Keywords: transition metal phosphides, N-doped carbon, hydrogen evolution reaction, oxygen evolution reaction, water splitting Introduction Hydrogen generation through efficient and sustainable electrochemical water splitting is an important component of various emerging clean-energy technologies including solardriven fuel generation systems, fuel cells and electrolyzers.1-3 However, the two half-cell reactions of water electrolysis, which are hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), require efficient electrocatalysts to enhance the sluggish kinetics and improve energy conversion efficiency. Presently, most active electrocatalysts for HER and OER are still based on noble metals such as Pt and IrO2/RuO2. Therefore, it is highly imperative to develop earth-abundant electrocatalysts with both high activity and low cost.4-10 In recent years, transition metal phosphides (TMPs) have been widely investigated as promising family of highly active and earth-rich electrocatalysts for HER and OER. For instance, nanostructured Ni2P and CoP, as well as amorphous MoP and WP have been synthesized by Schaak’s groups through heating metal salt precursors and tri-n-octylphosphine (TOP) in organic solvents.11-14 Almost at the same time, Sun’s group reported a series of TMPs nanostructures including FeP, CoP, CoP2, Ni2P and NiP2,15-20 which have been synthesized using in situ generated PH3 from hypophosphites (NaH2PO2) phosphorization transition metal nanoparticles (NPs) or oxide/hydroxide-nanostructures. On the one hand, using TOP as P source makes the reaction highly corrosive and flammable due to its high decomposition temperature.11 On the other hand, using the NaH2PO2 as P source is subject to poisoning and auto-ignition of PH3, making the experiment extremely dangerous.21 In addition, there are also many other methods to prepare TMPs, such as electrodeposition,22,23 pyrolysis of Pcontaining porous ionic polymers,24 anion exchange pathway,25 reducing metal orthophosphates26,27 and

solvothermal reaction using red phosphorus as precursor.28 However, the potential scalability of these synthetic methods still remains underexploited.29 In this study, a number of TMPs NPs with different phases encapsulated in N-doped carbon (NC) including Fe2P@NC, FeP@NC, Co2P@NC, CoP@NC, Ni2P@NC and Ni12P5@NC have been synthesized through a general strategy by varying molar ratios of the precursors and calcination temperatures. This synthesis method minimizes some of the issues traditionally presented in TMPs. First, it avoids handling and storing highly dangerous reagents such as PH3 gas and white phosphorus. Second, it enhances the stability due to NC encapsulation which could effectively prevent TMPs from corrosion even under extreme conditions. Third, it shows high promise for scalablility. The resultant materials exhibit excellent HER and OER activities with outstanding durability. The overpotentials at the benchmark current density of 10 mA cm-2 compare favourably with the most active HER and OER electrocatalysts reported to date. Experimental Section Materials: Fe(NO3)3·6H2O, Co(NO3)2·6H2O, Ni(NO3)2·6H2O, NH4H2PO4, and ethanol were purchased from Aladdin Reagent. KOH and H2SO4 were purchased from Beijing Chemical Works. Melamine was purchased from Xinglong Chemical Corp. Pt/C (20 wt%) and Nafion (5 wt%) were purchased from Sigma-Aldrich. All the reagents in the experiment were analytical grade and used without further treatments. The ultrapure water used throughout all the experiments was purified through a Millipore system. Preparation of Co2P@NC, CoP@NC, Ni2P@NC, Ni12P5@NC, Fe2P@NC and FeP@NC: In a typical proce-

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dure, a certain amount of metal salts, NH4H2PO4 and melamine were grounded to form homogeneous powder. Notably, 4.0 g melamine was used in all experiments. The solid mixture was then annealed at different temperature (700 or 800 °C) for 2 h under Ar atmosphere at a heating rate of 5 °C min-1. After cooled to room temperature, the black powder was collected, washed by centrifugation with ethanol and water several times to remove any residues, and finally dried in vacuum at 60 °C for 24 h. Remarkably, Table S1 lists the specific conditions for each sample. Preparation of Ni2P and NC: Briefly, 1.68 mmol Ni(NO3)2·6H2O and 7 mmol NH4H2PO4 were grounded to form the homogeneous powder. The powder was reduced in H2/Ar (5 wt%) at 700 °C for 2 h, and then naturally cooled to room temperature. A black Ni2P was obtained. NC was fabricated from activated carbon (0.5 g) and melamine (2.0 g) at 700 °C for 2 h by calcination under Ar atmosphere. Preparation of working electrodes: 5.0 mg catalyst powder was dispersed in 0.5 mL 0.5% Nafion solution by ultrasonic for 1 h. Then 7 µl solution was cast on a glassy carbon electrode (GCE: diameter = 3 mm) with a catalyst loading about 1.0 mg cm-2. Electrochemical measurements: All electrochemical measurements were performed on a CHI 660E electrochemical workstation (CH Instruments, Inc., Shanghai) in a standard three-electrode mode with two-compartment cell. The acidic (0.5 M H2SO4) electrochemical measurements were performed using a saturated calomel electrode (SCE) as reference electrode. The electrochemical measurements in alkaline solution (1.0 M KOH) were performed using Hg/HgO as reference electrode. The carbon rod was used as counter electrode in all measurements. Polarization curves were obtained at a scan rate of 5 mV s-1. In all measurements, the reference electrode was calibrated with respect to reversible hydrogen electrode (RHE). All Polarization data were iR-corrected. Material characterization: Transmission electron microscopy (TEM) was conducted on a JEM-2100F electronic microscope. X-ray diffraction (XRD) patterns were collected on a Rigaku X-ray diffractometer equipped with a Cu Kα radiation source. Scanning electron microscopy (SEM) was conducted on a XL30 ESEM FEG electronic microscope. Xray photoelectron spectroscopy (XPS) was obtained on an ESCALABMK II X-ray photoelectron spectrometer. Raman shifts were recorded on a LabRAMAramis Raman spectrometer instrument using the Ar ion laser with an excitation wavelength of 633 nm. Brunauer–Emmett–Teller (BET) SSA was analyzed using a Quantachrome NOVA 1000 system at liquid N2 temperature. Results and Discussion The preparation procedure is shown in Scheme 1 in which precursors of metal salt, NH4H2PO4 and melamine were mixed homogeneously in an agate mortar, and then placed in a quartz tube, followed by heating under Ar gas flow at elevated temperatures (700 or 800 °C) for 2 h at 5 °C min-1. After cooled to room temperature, a series of black samples of Co2P@NC, CoP@NC, Ni2P@NC, Ni12P5@NC, Fe2P@NC and FeP@NC were achieved by repeating the procedure accordingly. The specific conditions for each sample are listed in Table S1.

Scheme 1. Schematic illustration of the reaction step. Ni2P@NC is selected for detailed studies due to its best HER and OER catalytic activities. Figure 1a shows the TEM images of the obtained Ni2P@NC where a large amount of small Ni2P NPs are distributed within the carbon matrix. High resolution TEM (HRTEM) image (Figure 1b) further reveals that Ni2P NPs are embedded within carbon nanolayers. Additionally, the observed lattice fringes of Ni2P NPs is ~0.221 nm, corresponding to the hexagonal Ni2P (111) crystal planes.30-32 The scanning TEM (STEM) and energy dispersive X-ray (EDX) elemental mapping images (Figure S1) further confirm the uniform distribution of Ni, P, N and C elements. The XRD pattern of Ni2P@NC, Figure 1c, further indicates successful formation of Ni2P. Furthermore, the formation of NC layers can be verified by Raman spectroscopy (Figure 1d), which displays D- and G-bands located at ~1353 and ~1592 cm-1, respectively. The intensity ratio of ID/IG = (0.97) < 1 attests that the carbon support is partially graphitic.31 All of these results indicate that Ni2P@NC has been successfully prepared. Additionally, as shown in Figure S2, the SEM image and XRD pattern show that Ni2P can be obtained through H2 reduction of the nickel salt and ammonium dihydrogen phosphate mixture.

Figure 1. (a) TEM and (b) HRTEM images of Ni2P@NC. (c) XRD pattern of Ni2P@NC. (d) Raman spectrum of Ni2P@NC.

The XPS analysis detects that the catalyst surface is composed Ni, P, N, C and O (Figure S3). The presence of O signal is unavoidable due to absorbed water, O2, or gradual oxidation at the surface upon exposure to air.33,34 As shown in Figure 2a, three subpeaks at 862.8, 857.4 and 853.9 eV are found in the Ni 2p3/2 window, respectively. The subpeak at 853.9 eV is larger than that of metallic Ni (852.8 eV) demonstrating that the corresponding Ni species have a slightly positive charge. For the P 2p spectrum (Figure 2b), two subpeaks at 130.2 and 129.3 eV emerge, which can be attributed to binging energy (BE) of P 2p1/2 and P 2p3/2.35,36 The N 1s XPS spectrum (Figure 2c) shows three types of nitrogen envi2 ronments, corresponding to the pyridinic-N (398.5 eV), pyr-


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ACS Applied Materials & Interfaces rolic-N (400 eV) and quaternary-N (400.8 eV).37 The highresolution C 1s spectrum (Figure 2d) is deconvoluted into three different subpeaks at 284.6, 285.9 and 289.3 eV, which correspond to C-C, C-N/C=N and O-C=O.38,39 All of these observations indicate successful preparation of Ni2P NP@NC.

slope of 30 mV dec-1 in 0.5 M H2SO4 (Figure 4b). Under the same conditions, the Tafel slope for Ni2P and Ni2P@NC are 61 and 57 mV dec-1, respectively, suggesting a VolmerHeyrovsky mechanism.11

Figure 2. XPS spectra in the (a) Ni 2p, (b) P 2p, (c) N 1s and (d) C 1s regions for Ni2P@NC.

To further investigate the versatility of our method, a series of TMPs NPs embedded in NC with different phases including Co2P@NC, CoP@NC, Ni12P5@NC, Fe2P@NC and FeP@NC were fabricated by using the similar method except for the precursor and annealing temperature (The stoichiometric details for each sample are listed in Table S1). First, the XRD patterns (Figure 3a, 3d, 3g, 3j and 3m) reveal that the crystalline structures are match well with the standard card for all the composite materials. XPS and Raman spectra (Figure S4-S8) analyses also indicate the successful formation of Ni12P5@NC, Co2P@NC, CoP@NC, FeP@NC and Fe2P@NC hybrid structures. Finaly, TEM and HRTEM images further confirm that these hybrid structures are composed of TMPs embedded carbon matrixes (Figure 3b-3c, 3e-3f, 3h-3i, 3k-3l and 3n-3o). The HER performance of Ni2P@NC was evaluated in strongly acidic solution (0.5 M H2SO4). NC, Ni2P and Pt/C were also tested as control samples. The catalyst loading for each sample was about 1.0 mg cm-2. Figure 4a shows the HER polarization curves in which Pt/C displays the highest HER activity while the bare GCE exhibits negligible activity. Ni2P@NC shows very good HER activity in acidic solutions and requires overpotentials of 96, 138, 156 and 239 mV to achieve the cathodic current densities of 2, 10, 20 and 100 mA cm-2, respectively. These values are comparable to the most active phosphide based electrocatalysts reported to date, including Ni2P hollow NPs (117 mV@10 mA cm-2),11 Ni12P5 (137 mV@10 mA cm-2),35 Ni2P NPs (140 mV@20 mA cm2 40 ), CoP nanotubes (129 mV@10 mA cm-2),41 Co1.6Ni0.4P (165 mV@10 mA cm-2),42 CoP/CNTs (165 mV@10 mA cm-2),43 Cu3P NWs/Cu (143 mV@20 mA cm-2),44 FeP2/C NPs (220 mV@10 mA cm-2),43 WP2 nanorods (148 mV@10 mA cm-2),46 WP NAs/CC (130 mV@10 mA cm-2),47 and some other nickel phosphide-based catalysts are listed in Table S2. Both NC and Ni2P exhibit lower HER performance than Ni2P@NC. These results suggest that the superior HER activity of Ni2P@NC could be attributed to the synergistic/cooperative effects between NC and Ni2P. In addition, the Pt/C exhibits a Tafel

Figure 3. (a) XRD pattern, (b)TEM and (c) HRTEM images of Ni12P5@NC. (d) XRD pattern (e) TEM image and (f) HRTEM image of CoP@NC. (g) XRD pattern (h) TEM and (i) HRTEM image of CoP@NC. (j) XRD pattern (k) TEM image and (l) HRTEM image of FeP@NC. (m) XRD pattern, (n) TEM image and (o) HRTEM image of Fe2P@NC.

Stability is another significant factor in evaluating the performance of a HER electrocatalyst. Therefore, the Ni2P@NC was further studied by cyclic voltammetric (CV) test and galvanostatic measurements. As shown in Figure 4c, after 1000 CV cycles, the linear sweep voltammetrys (LSVs) display negligible current density loss. In addition, galvanostatic measurements (Figure 4d) of the Ni2P@NC catalyst at a current density of 10 mA cm-2 reveal no significant decrease in activity after 10 h of sustained hydrogen evolution. After stability measurements, the TEM images and XRD pattern suggest that Ni2P @NC still maintain its original morphology and crystalline structure (Figure S9). In addition, the similar high-resolution Ni, P, N and C XPS spectra of the prepared and post-HER Ni2P @NC (Figure S10) samples also confirmed the retention of the electrocatalysts in terms of composition. These results demonstrate that Ni2P@NC is indeed a stable HER electrocatalyst in 0.5 M H2SO4 solutions. Similarly, the HER performance of other synthesized TMPs were also investigated in 0.5 M H2SO4 solutions. As shown3 in Figure 4e, Ni12P5@NC, FeP@NC, CoP@NC, Co2P@NC



and Fe2P@NC catalysts need overpotentials of 140, 145, 175, 186 and 200 mV to reach a current density of 10 mA cm-2, respectively. The typically Tafel plots and corresponding Tafel slopes of all the samples are displayed in Figure 4f. The slope for Ni12P5@NC, FeP@NC, CoP@NC, Co2P@NC and Fe2P@NC is 62, 66, 68, 71 and 97 mV dec-1, respectively. Additionally, previous studies from Sun and Schaak’s group reveals that phosphides of cobalt and iron show the highest HER activities under acidic condition,8,11,12,15 but the Ni2P@NC catalyst shows the highest HER activity in our experimental. This may be attributed to the small particles size as evidenced by the TEM and size distribution (Figure S11).

nickel phosphide-based electrocatalysts (Table S3). In addition, the Ni12P5@NC, CoP@NC, Co2P@NC, FeP@NC and Fe2P@NC samples require overpotentials of ~330, ~348, ~383, ~409 and ~415 mV, respectively, to reach a current density of 10 mA cm-2 (Figure 5c). Tafel analysis suggests that the Tafel slope is 50 mV dec-1 for Ni2P@NC (Figure 5b), which is smaller than that of Ni2P (178 mV dec-1), implying the favorable OER kinetics over Ni2P@NC. Furthermore, the durability of the Ni2P@NC for OER is also tested by continuously cycling. As shown in Figure 5e, after 1000 CV cycles, only a slight positive shift of the overpotential occurs, suggesting outstanding durability of Ni2P@NC for OER. In addition, chronoamperometry measurement of Ni2P@NC was performed under an overpotential of 330 mV (Figure 5f). After 12 h test, the current density shows only slight degradation, also suggesting the excellent durability of the Ni2P@NC catalyst. After OER electrolysis, TEM images (Figure S12a-12b) indicate that the Ni2P@NC catalyst still maintains its original morphology. In addition, XRD pattern shows that the Ni2P@NC retained its initial crystalline structure (Figure S12c). However, the high resolution Ni 2p XPS spectrum of the post-OER Ni2P@NC revealed an intensity decrease of the peak at 853.9 eV (assignable to Niδ+ in Ni2P@NC) and an increased peak at 855.8 eV (corresponding oxidized Ni species), suggesting the partial oxidation of Ni2P@NC (Figure S13a). Furthermore, this oxidation phenomenon was also revealed by the intensity increase ascribed to oxidized phosphorus species in its high resolution P 2p XPS spectrum (Figure S13b).Such observation agrees well with previous reported Ni-based OER electrocatalyst, such as Ni2P nanowires and Ni5P4/Ni plate. 48,49 And the mechanism of the Ni2P@NC for OER may consistent with the reported mechanism. Ni cations on the surface is partially oxidized into NiOOH and form Ni2P/NiOOH core– shell structures as the actual surface-active sites. 57,58

Figure 4. (a) Polarization curves for the HER in 0.5 M H2SO4 for Ni2P@NC, Ni2P, NC, bare GCE along with the commercial Pt/C catalyst for comparison. (b) corresponding to Tafel slops for Ni2P@NC, Ni2P and Pt/C. (c) Polarization curves for Ni2P@NC initial and after 1000 CV scanning between +0.28 and -0.32 V vs. RHE. (d) Galvanostatic data (10 h) for the Ni2P@NC held at a constant current density of 10 mA cm-2 for the HER (without iR-correction). (e) Polarization curves for the HER in 0.5 M H2SO4 for Ni12P5@NC, Co2P@NC, CoP@NC, Fe2P@NC and FeP@NC, (f) corresponding to Tafel slops. Furthermore, as shown in Figure 5a, the OER catalytic activity of Ni2P@NC was tested in alkaline solution (1.0 M KOH). To reach the current densities of 10 and 20 mA cm-2, Ni2P@NC needs overpotentials of ~320 and ~350 mV, respectively. Likewise, Ni2P@NC shows much higher OER current density than other compared samples. For example, the current densities at 1.6 V are 40, 17 and 5 mA cm-2 for Ni2P@NC, Ni2P and NC, respectively. Additionally, at the current density of 10 mA cm-2, Ni2P@NC also compares well with the activity of most reported TMPs including Co-P films (335 mV),23 Ni2P nanowires (440 mV),48 Ni5P4/Ni plate (290 mV),49 Co-P/N-doped carbon matrices,(354 mV),50 CoP NPs/CC (340 mV),51 CoP nanorod/CC (320 mV),51 CoPh/G (292 mV),52 CoP NS (361 mV),53 CoP/C (360 mV),54 CoP/CNT (330 mV),55 FeP@NPCs (300 mV)56 and other

Figure 5. (a) Polarization curves for the OER in 1.0 M KOH for Ni2P@NC, Ni2P, NC, bare GCE along with the RuO2 4 catalyst for comparison. (b) corresponding to Tafel slops for


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ACS Applied Materials & Interfaces Ni2P@NC, Ni2P and RuO2. (c) Polarization curves for the OER in 1.0 M KOH for Ni12P5@NC, Co2P@NC, CoP@NC, Fe2P@NC and FeP@NC, (d) corresponding to Tafel slops. (e) Polarization curves for Ni2P@NC initial and after 1000 CV scanning between 1.07 and 1.87 V vs. RHE. (f) Timedependent current density curve for Ni2P@NC under static overpotential of 330 mV for 10 h (without iR-correction). In addition, Ni2P@NC was tested as a bifunctional electrocatalyst for overall water splitting, which is much closer to the real application. Therefore, a water-splitting electrolyzer (1.0 M KOH) employing Ni2P@NC as both cathode and anode catalyst was constructed. As shown in Figure S14, the Ni2P@NC||Ni2P@NC couple offers the current densities of 10, 20 mA cm–2 at a cell voltages of only 1.67 and 1.77 V, respectively, dramatically lower than those of recently reported no-noble metal bifunctional catalysts including Ni5P4 films (1.70 V@10mA cm-2),49 Co–P/rGO (1.70 V@10 mA cm2 59 ), and C3N4–CNT-CF (1.8 V@10mA cm-2) 60. These results demonstrate that the Ni2P@NC||Ni2P@NC system can potentially used for full water splitting applications. To get a further insight into the high catalytic activity of the Ni2P@NC. First, the double layer capacitance (Cdl)representing the electrochemically active surface area (ECSA) are investigated. The measured Cdl is 16.5 mF cm-2 for Ni2P@NC, which is larger than these of Fe2P@NC (0.74 mF cm-2), FeP@NC (9.65 mF cm-2), Co2P@NC (0.89 mF cm-2), CoP@NC (0.94 mF cm-2), and Ni12P5@NC (1.36 mF cm-2) (Figure S15). Second, as shown in Figure S16, the BET specific surface area of Ni2P@NC (56.89 m2 g-1) is larger than these of Fe2P@NC (15.38 m2 g-1), FeP@NC (15.55 m2 g-1), Co2P@NC (32.27 m2 g-1), CoP@NC (38.83 m2 g-1), and Ni12P5@NC (24.59 m2 g-1), which leads to more active sites.61 Third, the Nyquist plots (Figure S17) further indicate that the Ni2P@NC shows the smallest semicircle diameter at the same overpotential of 120 mV, implying that Ni2P@NC possesses a faster charge-transfer capacity during the HER process, which may be another reason for the high electrocatalytic performance.62,63

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (51372186, 51672204), and the Fundamental Research Funds for the Central Universities (No.2016-YB-001).

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Conclusion In conclusion, a number of transition metal phosphides with different metal-phases embedded in N-doped carbon matrixes have been synthesized via a facile solid-state reaction. Among TMPs samples, Ni2P @NC exhibits the highest HER and OER activity. In addition, Ni2P@NC displays a robust durability under both acidic and alkaline solutions. This approach is quite general and could allow low-cost preparation of TMPs embedded within nitrogen-doped carbon as attractive material for broader electrocatalytic applications.

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ASSOCIATED CONTENT Supporting Information Tables; TEM and SEM images; XRD patterns; XPS, and Raman spectra; Size distribution; CVs and LSV curves; Nyquist plots; BET. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *

E-mail: [email protected]

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