Confined Molybdenum Phosphide in P-Doped Porous Carbon as

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Confined Molybdenum Phosphide in P-Doped Porous Carbon as Efficient Electrocatalysts for Hydrogen Evolution Ji-Sen Li, Shuai Zhang, Jing-Quan Sha, Hao Wang, Ming-Zhu Liu, Ling-Xin Kong, and Guo-Dong Liu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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ACS Applied Materials & Interfaces

Confined Molybdenum Phosphide in P-Doped Porous Carbon as Efficient Electrocatalysts for Hydrogen Evolution Ji-Sen Li, †,* Shuai Zhang, † Jing-Quan Sha, † Hao Wang, † Ming-Zhu Liu, † Ling-Xin Kong, † and Guo-Dong Liu† †

Key Laboratory of Inorganic Chemistry in Universities of Shandong, Department of Chemistry

and Chemical Engineering, Jining University, Shandong 273155, China

ABSTRACT: Highly efficient electrocatalysts for hydrogen evolution reactions (HER) are crucial for electrochemical water splitting, where high-cost and low-abundance Pt-based materials are the benchmark catalysts for HER. Herein, we report the fabrication of MoP nanoparticles confined in P-doped porous carbon (MoP@PC) via a metal-organic frameworksassisted route for the first time. Remarkably, due to the synergistic effect of MoP nanocrystals, P-dopant, and porous carbon, the resulting MoP@PC composite exhibits superior HER catalytic activity with an onset overpotential of 97 mV, a Tafel slope of 59.3 mV dec-1, and good longterm durability, which compares to those of most reported MoP-based HER catalysts. Most importantly, the work opens a new route in the development of high-performance nonprecious HER electrocatalysts derived from MOFs.

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KEYWORDS: electrocatalysis, hydrogen evolution reaction, metal–organic frameworks, molybdenum phosphide, porous carbon INTRODUCTION Energy crisis and global warming issues caused by increasing depletion of fossil fuels have gained extensive attention. Hydrogen, as an ideal energy carrier, has thus become a hot research topic.1,

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To date, among various clean-energy technologies, water splitting for hydrogen

production is highly regarded as one of the most promising approaches.3-9 Generally speaking, Pt-based materials are regarded as the most promising catalysts for hydrogen evolution reaction (HER) due to the prominent catalytic activity.10,

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Nonetheless, they always suffer from the

disadvantages of limited supply on the earth and high cost, determining that they are not applied to large-scale commercialization.12, 13 Hence, developing high-performance and inexpensive nonnoble metal-based composites for HER is highly demanded, but it is also significantly more challenging for these materials. To this end, tremendous efforts have been dedicated to seeking non-precious metal HER catalysts. Among them, molybdenum phosphide (MoP) has been intensively investigated as electrocatalysts owing to its Pt-like catalytic performance.14-23 However, MoP nanoparticles would often suffer from aggregation and/or coalescence at high-temperature, leading to low specific surface areas and few active sites, which undoubtedly discount the electrocatalytic performance. Therefore, it is quite urgent to develop new strategies to fabricate high-efficiency MoP-based electrocatalysts for HER. More recently, metal-organic frameworks (MOFs)-derived carbon materials, as new stars, have been broadly applied for energy conversion and storage techniques.24-27 During heat treatment, carbon materials originating from the organic linkers of MOFs could be prepared.


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Simultaneously, metal ions could be transferred into metal oxide,28-30 carbide,31-34 nitride,35-36 and so on,37-44 which are in situ embedded into the resultant carbon matrix. Most importantly, because of the pore confinement effect of MOFs, the small size of nanoparticles could be obtained. Despite these great achievements in this field, regrettably, early transition-metal (such as Mo, W, and V) phosphides-contained carbons composites are difficult to synthesize utilizing single MOFs as precursors. Until now, to the best of our knowledge, the investigation into the fabrication and application of MoP encapsulated in porous carbon derived from MOFs for HER has been rarely reported. Herein, for the first time, we report a polyoxometalate (POM)-based MOFs-assisted approach for the preparation of MoP encapsulated in P-doped porous carbon (MoP@PC) through simple carbonization followed by phosphidation. Impressively, the MoP@PC composite exhibits the remarkable electrocatalytic activity for the HER as well as long-term stability due to the synergistic effects between MoP nanoparticles and P-doped porous carbon.

EXPERIMENTAL SECTION Materials:

Copper

acetate

monohydrate

(Cu(OAc)2·H2O),

phosphomolybdic

acid

(H3PMo12O40·nH2O), phosphotungstic acid (H3PW12O40·nH2O), L-glutamic acid (C5H9NO4), 1,3,5-benzenetricarboxylic acid (H3BTC, 98%), iron(Ⅲ) chloride hexahydrate (FeCl3·6H2O) and diammonium hydrogen phosphate ((NH4)2HPO4), which were purchased from Sinopharm Chemical Reagent, commercial Pt/C (Johnson Matthey, 20 wt%), and Nafion (Sigma-Aldrich, 5.0 wt%) were of analytical grade and utilized as received. The water as the solvent in the experiments was ultra-purified water (18.25 MΩ).


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Synthesis of NENU-5 and NENU-3. On the basis of the previously reported literature,30 NENU-5 was synthesized. Namely, 0.2 g of Cu(OAc)2·H2O, 0.1 g of phosphomolybdic acid hydrate and 0.07 g of L-glutamic acid were dissolved in 40 mL of ultra-purified water and stirred for 30 min under room condition. Subsequently, 40 mL of ethanol solution of H3BTC (0.14 g) was added into the above solution with continuous stirring. After 16 h, the green precipitate was centrifuged, rinsed with ethanol, and dried at vacuum at 70 oC for 12 h. For comparison, NENU-3 was prepared under the same condition, except that H3PW12O40 was used to replace H3PMo12O40. Synthesis of Cu-MoO2@PC, MoO2@PC, and WO2@PC. The NENU-5 was heated to 600 o

C for 6 h under Ar atmosphere at 2 oC min-1. The resulting sample was defined as Cu-

MoO2@PC. In order to remove Cu particles, the Cu-MoO2@PC was etched in 0.1 M FeCl3 solution for 6 h. The resulting sample was then thoroughly washed with water and dried at vacuum at 70 oC for 12 h (named as MoO2@PC). In addition, WO2@PC was prepared under the same condition, except that NENU-3 was used to replace NENU-5. Synthesis of MoP@PC, Cu3P-MoP@PC, and WP@PC. In a typical procedure, (NH4)2HPO4 (500 mg) and MoO2@PC (100 mg) were grinded to powders and placed in a porcelain boat, respectively. Subsequently, the boat was heatd at 850 oC for 2 h at 5 oC min-1 under H2/Ar (VH2 / VAr = 1:9) atmosphere. After naturally cooled to ambient temperature under H2/Ar environment, the obtained sample was denoted as MoP@PC. Additionally, the Cu3P-MoP@PC, and WP@PC were synthesized by the above procedures similar to that for MoP@PC, but using Cu-MoO2@PC and WO2@PC as raw materials, respectively.


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Instruments. The morphologies of these materials were investigated by transmission electron microscopy (JEOL-2100F), scanning electron microscope (JSM-7600F), energy-dispersive Xray spectroscopy (JSM-5160LV). X-Ray diffraction experiments were recorded on a D/max 2500/PC diffractometer with Cu Kα radiation (λ = 1.54060 Å, Japan). X-ray photon spectroscopy was measured using PHI 5000 Versa using Al kα radiation. Nitrogen sorption isotherms were carried out at 77 K on a Micromeritics ASAP 2050 system. The density functional theory model was adopted to calculate the pore size distribution. Electrochemical measurements. All electrochemical properties were evaluated in a conventional three-electrode system by a CHI 760D instrument (Chenhua, Shanghai, China) in H2SO4 (0.5 M) solution at room temperature. The working electrode is the sample modified glassy carbon electrode (GCE) with diameter of 3 mm, the reference electrode is a saturated calomel electrode (SCE), and the counter electrode is a graphite rode. The catalysts (4 mg) were dispersed in a water/Nafion solution (2 mL, Vw / VN = 9:1) by sonication. Subsequently, the resultant suspension (5 µL) was coated on the GCE surface, where the loading was about 0.14 mg cm-2. For comparison, commercial 20% Pt/C catalyst was also measured. The electrolyte were purged with high-purity N2 (99.999%) for 30 min to avoid low dissolved O2. All potentials were referenced to reversible hydrogen electrode (RHE) on the basis of the following calculation: ERHE = ESCE + 0.059 pH + EθSCE. In 0.5 M H2SO4, ERHE = 0.241 V + ESCE. Linear sweep voltammetry was assessed at 2 mV s-1. Electrochemical impedance spectroscopy was measured in the frequency range of 100 kHz - 100 mHz at the open-circuit voltage of 10 mV. Stability of the MoP@PC sample was studied by potential cycling 1000 cycles between -200 to 140 mV at 100 mV s-1 and chronoamperometry at 10 mA cm-2. The electrochemical double-layer


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capacitance was calculated by using cyclic voltammogram in a region from -59 to 41mV at different scan rates ranging from 20 to 200 mV s-1 RESULTS AND DISCUSSION Preparation and Characterization of MoP@PC Composite.

Scheme 1. Schematic illustration of the synthesis procedure for MoP@PC. The typical preparation procedure of the MoP@PC composite is schematic illustrated in Scheme 1. NENU-5 was firstly synthesized using a co-precipitation approach according to the previous report.30 The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of NENU-5, shown in Figure S1a-c, exhibit the polyhedral shape with a size of around 400 nm. From Figure S1d, it is observed that the PXRD pattern of assynthesized NENU-5 is in agreement with that of simulated NENU-5, which proves the good crystallinity and high phase purity. And then, the as-synthesized NENU-5 was utilized as the raw material and directly carbonized at 600 oC under a Ar flow (defined as Cu-MoO2@PC). A subsequent FeCl3 solution treatment was employed to remove the Cu species stemming from the Cu metal sites of NENU-5 (denoted as MoO2@PC). Finally, the MoO2@PC was transformed


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into MoP@PC under H2/Ar (10%/90% in volume rate), where (NH4)2HPO4 served as the phosphorus source.

Figure 1. (a) SEM (yellow arrows: pores), (b) TEM (inset: HRTEM image), (c) PXRD pattern, and (d) N2 sorption isotherm of MoP@PC. Inset in (d) shows the distribution of pore sizes. After the multistep treatments, the SEM image of MoP@PC, as demonstrated in Figure 1a, confirms that the composite with an average particle size of about 200 nm still remains the polyhedral morphology, whereas the surface of becomes much rougher, which may be associated with the decomposition and shrinkage during the multistep treatments.33 Meanwhile, the abundant nanopores can be clearly observed, which is beneficial to permeating and transporting the reaction electrolyte.33, 35, 45 The corresponding energy-dispersive X-ray spectroscopy (EDX) shows the hybrid mainly contains C, P, and Mo elements (Figure S2). As seen from the TEM image in Figure 1b, MoP@PC nanocrystals appear as rhombic, and the existence of porous architecture is also verified (yellow arrows). More importantly, it is found that MoP nanoparticles are confined in the porous carbon matrix, which can effectively protect


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nanoparticles from aggregation and oxidation. From the inset of Figure 1b, the high-resolution TEM (HRTEM) image shows distinctive lattice fringes with 0.21 nm, which is attributed to the (101) facet of MoP. As depicted in Figure 1c, the major obvious peaks at 27.9o, 32.0o, 43.1o, 57.1o, 64.9o, 67.6o, and 74.1o demonstrated in the powder X-ray diffraction (PXRD) pattern are assigned to the diffraction from (001), (100), (101), (110), (111), (102), and (201) planes of MoP (JCPDS, No, 65-6487, red lines), respectively. The small and weak peaks at 34.6o, 35.6o, 37.2o, 46.5o, 47.1o, 48.5o, and 50.2o belong to orthorhombic phase of Mo17O47 (JCPDS, No, 71-566, blue lines), which may be related to surface oxidation. Additionally, the broad and weak peak at 2θ value of 24.1o is indexed to porous carbon. The N2 sorption analysis reveals the large Brunauer-Emmett-Teller (BET) surface area (141.2 m2 g-1) of MoP@PC and the distribution of pore sizes is in the range of 8 to 25 nm diameter (mesopores) determined by density functional theory method (Figure 1d), which are in accord with the observation of SEM and TEM images.

Figure 2. (a) XPS survey spectrum and high resolution XPS spectra of (b) C 1s, (c) Mo 3d and (d) P 2p of MoP@PC, respectively.


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The surface valence state and material composition of MoP@PC was further characterized. The X-ray photoelectron spectroscopy (XPS) of MoP@PC shows the presence of C, P, Mo, and O elements in the hybrid (Figure 2a), and the corresponding element content was summarized in Table S1. As exhibited in Figure 2b, three C species occur in the high-resolution C1s spectrum, which correspond to C-C/C=C (284.6 eV), C-P (285.1 eV), and O-C=O (288.7 eV), respectively.23, 45 The high-resolution Mo 3d XPS can be deconvoluted into six peaks, assignable to Mo3+ (228.6/232.8 eV), Mo4+ (232.0/233.3 eV), and Mo6+ (233.8/236.6 eV) (Figure 2c), respectively.15, 20 Mo3+ can be attributed to MoP species, which are considered as the active sites towards HER.17 In parallel, Mo4+ and Mo6+ can originate from surface-oxidized Mo species,16 which is in accordance with the PXRD analysis. As shown in Figure 2d, the two peaks at 129.6 and 133.9 eV for the P 2p spectrum can be assigned to the P-Mo and P-C, respectively.19, 21 Also, the additional peak presented at 134.7 eV can be indicative of PO43- or P2O5 probably stemming from the surface oxidation.17, 21 For comparison, MoO2@PC and WP@PC were also synthesized under identical conditions without phosphidation and with PW12 instead of PMo12, respectively. The corresponding morphologies and structures of the resultant samples were also investigated by SEM, TEM, EDX, PXRD, N2 sorption, and XPS in detail, respectively, as illustrated in Figure S2-6. Hydrogen Evolution Reaction Performance of MoP@PC Composite.


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Figure 3. (a) Polarization curves of different samples. (b) Corresponding Tafel plots. (c) CV curves in an overpotential windows -59 to 41 mV of MoP@PC. Inset: scan rate dependence of the current densities of MoP@PC at -0.009 V (d) Polarization curves for MoP@PC initially and after 1000 cycles. Inset: Chronoamperometry i-t curve of MoP@PC at the current density of 10 mA cm-2. The electrocatalytic properties of different samples were examined in acidic media (0.5 M H2SO4) utilizing a three-electrode system. Hereafter, all electrode potentials are converted to reversible hydrogen electrode (RHE). As previously reported, 20% Pt-C exhitits a pronounced electrocatalytic HER performance with an onset overpotential of approximate 0 mV (Figure 3a).15, 19, 45 Nonetheless, it was found that the MoO2@PC is rather inactive, as proven by the large onset overpotential (600 mV). On the contrary, the MoP@PC hybrid displays a lower onset overpotential of 97 mV in comparison with that of WP@PC (173 mV). Simultaneously, to achieve 10 mA cm-2 current density for the HER, the MoP@PC only requires the overpotential of 258 mV, whereas that for WP@PC is 353 mV. As demonstrated in Figure 3b, the Tafel slope was further investigated to assess the electrocatalytic kinetic for the HER, which can be calculated from the corresponding polarization


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curve by Tafel equation (namely, η = b log (j) + a, where b is the Tafel slope). Unexpectedly, the value of the Tafel slope for MoP@PC is 59.3 mV dec-1, much lower than that of WP (75.9 mV dec-1) and MoO2@PC (77.3 mV dec-1). Especially, the Tafel slope falling within the region of 40-120 mV dec-1 indicates that the HER on the MoP@PC proceeds via a Volmer-Heyrovsky mechanism.19, 23 That is, the electrochemical desorption for the HER is the rate-limiting step. By extrapolating the corresponding Tafel plots to J axis, the exchange current density of MoP@PC is obtained to be 7.9*10-4 mA cm2, significantly larger than that of WP@PC (3.3*10-4 mA cm2), and MoO2@PC (4.3*10-10 mA cm2), further confirming the excellent H2 evolution efficiency of MoP@PC. Compared to MoO2@PC and WP@PC, the MoP@PC catalyst shows lower impedance, indicating an efficient electron transport at the MoP@PC/electrolyte interface (Figure S7). As per our best knowledge, the small Tafel slope is particularly impressive for MoP-based electrocatalysts toward HER, which is comparable to most of the values for MoPbased HER catalysts at present,15, 19, 23 as listed in Table S2. A series of cyclic voltammograms (CVs) was employed to investigate the electrochemically active surface area (ECSA) within -0.059 to 0.041 V. As shown in Figure 3c, the double-layer capacitance (Cdl) of MoP@PC is found to be 39.6 mF cm-2, which outperforms that of WP@PC (5.53 mF cm-2), and MoO2@PC (0.149 mF cm-2) (Figure S8), signifying that the unique structure could offer rich accessible surface area and boost the HER performance. To the best of our knowledge, such smaller Tafel slope, along with larger ECSA, has rarely been reported for MoP-based catalysts toward HER to date. Accordingly, these results powerfully prove that the MoP@PC composite is a highly active electrocatalyst for HER in acid media. As well known, stability is also a significant criterion for the practical application. Figure 3d exhibits the comparison of polarization curves of the MoP@PC operated before and after 1000


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cycles. Strikingly, it is found that both are almost similar performance, which underline the outstanding durability of MoP@PC in a long-term HER process. What's more, the stability of MoP was also investigated by time-dependent current density curve, as demonstrated in the inset of Figure 3d. Remarkably, negligible decay of catalytic performance is observed after 10 h testing, which elucidates the prominent stability of the as-prepared MoP@PC hybrid. The morphology and structure of MoP@PC after durability measurements were examined, which are identical with those of the initial sample (Figure S9). Taking into accout the above-mentioned results, we speculate that the existence of carbon layer can efficiently inhibit the detachment, corrosion, and oxidation of MoP nanoparticles. Such excellent electrocatalytic activity toward the HER could be attributable to the following reasons: (1) the pore confinement effect of MOFs contributes to the small size of MoP nanoparticles.22 Furthermore, MoP nanoparticles are confined in the carbon matrix stemming from the organic ligands of MOFs, which can provide abundance of exposed active sites;15, 35 (2) the carbon matrix can prevent the detachment or aggregation of MoP nanoparticles. Meanwhile, it can also reduce the interfacial resistance and afford good conductivity. More importantly, the porous structures are advantageous for the enlargement of the contact surface between electrolyte and electrode, and boost the charge and mass transfer for the HER;23, 45 (3) P doping can regulate the electronic structure of carbon atom, further enhance the conductivity of porous carbon.31, 34 Overall, the synergistic actions of MoP, porous carbon, and P dopant endow the MoP@PC composite with the superior HER performance and long-term stability.


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Figure 4. Electrocatalytic activity of MoP@PC with different PMo12 loading (0.05, 0.1, and 0.15 g) and Cu3P-MoP@PC. (a, c) Polarization curves. (b, d) Tafel plots. As control experiments, another two samples (denoted as [email protected] and [email protected]) with different PMo12 loading (0.05, and 1.5 g) were prepared under the same conditions. The electrocatalytic performances of the two catalysts for the HER were also examined (Figure 4a, b). Surprisingly, among the three catalysts, the MoP@PC shows the smallest onset potential and the lowest Tafel slope, which may be stemmed from the amount and distribution of catalytic sites. In other words, the [email protected] composite possesses few active sites; whereas, abundant MoP nanoparticles in [email protected] are apt to aggregate, further decreasing the density of highly active sites, which can be proven by the SEM and TEM images (Figure S10). As a result, the optimal PMo12 content is important to provide more effective active sites in this work. Subsequently, we probe the impact of the removal of Cu species on the catalytic activity toward the HER. From the SEM and TEM images of Cu3P-MoP@PC (Figure S11), it is found that many irregular Cu3P and MoP nanoparticles were obtained. The PXRD pattern illustrates that the nine peaks at 36.2o, 39.3o, 41.8o, 45.1o, 46.5o, 47.3o, 52.5o, 53.8o, and 59.2o correspond to the (112), (202), (211), (300), (113), (212), (220), (221), and (222) planes of Cu3P (JCPDS, No,


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2-1263), respectively. The additional peaks are ascribed to MoP (JCPDS, No, 65-6487). Compared to that of MoP@PC (141.2 m2 g-1), the BET surface area of Cu3P-MoP@PC is 58.6 m2 g-1, suggesting that the etching treatment in FeCl3 solution could effectively remove Cu nanoparticles, produce the large reactive surface area, and expose more active sites. Impressively, regardless of onset potential and Tafel slope, the MoP@PC elucidates superior electrocatalytic activity in comparison with Cu3P-MoP@PC (Figure 4c, d). That is, Cu species is inactive for the HER and the removal of Cu species plays key roles in promoting the electrocatalytic HER activity in this work. CONCLUSIONS In summary, we have rationally designed and developed novel hydrogen evolution electrocatalysts composed of MoP nanoparticles and P-doped porous carbons using POMOFs as precursors by facile carbonization and following phosphidation. The synergistic effect of MoP nanoparticles, P-dopant, and porous carbons endows the MoP@PC hybrid with remarkable electrocatalytic performance for the HER, which can be comparable, even superior to those of reported MoP-based catalysts so far. Most importantly, this work delivers a novel concept and provides a future opportunity for the design and fabrication of high-efficiency and low-cost alternatives to replace precious-metal catalysts toward HER. ASSOCIATED CONTENT Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Structure and morphology characterizations including SEM, TEM, EDX, XRD, N2 sorpotion, and XPS for NENU-5 and other control samples in Figures S1-S6, S9-11. EIS, CVs, LSVs, and Tafel plots for other control materials in Figures S7, 8, 12. Atomic percents of different catalysts by XPS measurement in Table S1 and literature comparison in Table S2.


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AUTHOR INFORMATION Corresponding Author E-mail: [email protected] ORCID Ji-Sen Li: 0000-0003-2578-422X Jing-Quan Sha: 0000-0002-5925-9565 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the China Postdoctoral Science Foundation (No. 2016M602178), the Natural Science Foundation of Shandong Province (No. ZR2014BQ037), the Project of Shandong Province Higher Educational Science and Technology Program (No. J16LC02), the Foundation of Basic Discipline Construction and Theoretical Research Center of Jining University (No. 2017JCXK005), and the Talent Team Culturing Plan for Leading Disciplines of University in Shandong Province.

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Confined Molybdenum Phosphide in P-Doped Porous Carbon as

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