Carbothermal-Reduction-Assisted Phosphidation of Cobalt Affords

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Carbothermal-Reduction-Assisted Phosphidation of Cobalt Affords Mesoporous Nitrogen-Doped Carbon-Embedded CoP Nanoelectrocatalysts for the Oxygen Reduction Reaction Mopidevi Manikanta Kumar and C. Retna Raj* Functional Materials and Electrochemistry Laboratory, Department of Chemistry, Indian Institute of Technology (IIT) Kharagpur, Kharagpur 721302, India

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S Supporting Information *

ABSTRACT: We demonstrate a new facile single-step synthesis of mesoporous nitrogen-doped carbon-embedded cobalt phosphide (NC-CoP) nanoparticles using a single-source precursor of the cobalt(II) bis(terpyridine)-based complex ([Co(pyterpy)2](PF6)2) by carbothermal reduction. Phosphidation of cobalt is achieved with the hexafluorophosphate counteranion of the precursor complex for the first time. Polypyridyl complexes such as [Co(bpy)3](PF6)2 and [Co(terpy)2](PF6)2 do not yield the desired CoP. The atomic percent of carbon in the precursor complex controls the growth of CoP. The as-synthesized NC-CoP has a large surface area of 120 m2/g and shows excellent electrocatalytic activity toward the oxygen reduction reaction (ORR). It is highly durable and favors the four-electron pathway for the reduction of oxygen to water at low overpotential. The synergistic effect between NC and CoP facilitates the electron-transfer kinetics for ORR. KEYWORDS: cobalt phosphide, carbothermal reduction, nitrogen-doped carbon, electrocatalysis, oxygen reduction reaction is essential because of the highly flammable and corrosive nature of the solvent. On the other hand, the solid-state thermal synthetic methods involve the use of red/white phosphorus or NaH2PO2/NH4H2PO2/NH4H2PO4 as the source of phosphorus at medium-to-elevated temperatures.10,16,17 Herein, we demonstrate a new solid-state approach for the synthesis of nitrogen-doped carbon-supported cobalt phosphide (NC-CoP) nanoparticles using a single-source precursor and its electrocatalytic performance toward the oxygen reduction reaction (ORR). Our new synthetic approach involves thermal annealing of bis[4′-(4‴-pyridyl)-2,2′:6′,2″terpyridine]cobalt(II) hexafluorophosphate ([Co(pyterpy)2](PF6)2) in the absence of traditional phosphidating agents.

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he synthesis of nonprecious electrocatalysts such as transition-metal oxides, nitrides, carbides, phosphides, etc., for electrochemical energy conversion and storage applications received significant attention.1,2 Among them, the transition-metal phosphides (TMPs) are very promising for electrochemical water splitting, photovoltaics, and battery applications.3−7 Several strategies have been developed for the synthesis of mono- and bimetallic TMPs.3−10 The typical procedure for the synthesis of TMPs involves the phosphidation of transition metals using suitable phosphidating reagents at elevated temperature and an inert atmosphere.9,10 Traditionally, bulk TMPs were obtained by thermal annealing of the corresponding metal and red phosphorus at temperatures of >900 °C in sealed tubes under vacuum for many days.10−12 Trioctylphosphine, trioctylphosphine oxide, and triphenylphosphine were used as phosphidating agents in the solvothermal synthetic methods.13−15 High-boiling organic solvents are required for the solution-phase synthetic methods because the decomposition temperatures of these phosphidating agents are very high. An oxygen-free reaction environment © XXXX American Chemical Society

Received: November 6, 2018 Accepted: January 30, 2019 Published: January 30, 2019 A

DOI: 10.1021/acsanm.8b01994 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

Letter

ACS Applied Nano Materials Scheme 1. Synthesis of NC-CoP

Figure 1. XRD (a), XPS surface survey scan (b), and high-resolution Co 2p (c) and P 2p (d) profiles of as-synthesized NC-CoP.

intense peak at 288.3 eV for OCO.23 The peak at 285.2 eV corresponds to C−N, suggesting nitrogen doping onto the carbon network.24 The N 1s signal is deconvoluted into three peaks at 402.3, 400.4, and 398.4 eV, corresponding to the pyridinic N-oxide, pyrrolic, and pyridinic nitrogen (Figure S5b).24,25 The percentage of total nitrogen content was calculated to be 4.10%. The pyridine N-oxide content is very small (8.02%) with respect to the other two nitrogen atoms (pyridinic, 36.84%; pyrrolic, 55.13%). Electron microscopic analysis shows that the CoP particles have quasi-spherical shape, with sizes ranging from 80 to 90 nm, and are embedded inside the NC (Figure 2). The highresolution transmission electron microscopy (HRTEM) image (Figure 2c) shows lattice fringe spacing of 0.18 and 0.24 nm, corresponding to the (211) and (111) planes, respectively, of CoP. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) image and corresponding elemental mapping analysis evidence the presence of the CoP particle and NC (Figure 2d). The surface area of NCCoP was obtained from nitrogen adsorption−desorption analysis. A type IV isotherm with H3 hysteresis loop characteristics of materials with slit-shaped pores26 was observed (Figure S6). The Brunauer−Emmett−Teller specific surface area was calculated to be 120.38 m2/g. The Barrett−

NC-CoP was obtained by thermal annealing of the [Co(pyterpy)2](PF6)2·H2O complex (Scheme 1). The ligand pyterpy and cobalt complex were synthesized and characterized according to literature procedures (Figures S1−S4). The as-synthesized complex was then thermally annealed in an argon atmosphere at an optimized temperature of 900 °C for 1 h without using any additional phosphidating agent. The black product was collected and subjected to further characterization. The X-ray diffraction (XRD) profile of NC-CoP (Figure 1a) shows a characteristic signature corresponding to the orthorhombic phase of CoP (JCPDF 29-0497) along with a broad peak between 20° and 30° for nitrogen-doped carbon (NC). The chemical nature of NC-CoP was examined with Xray photoelectron spectroscopy (XPS) analysis. The survey scan XPS profile (Figure 1b) shows signatures for phosphorus, carbon, nitrogen, and cobalt. The high-resolution Co 2p profile (Figure 1c) shows two peaks at 778.1 eV (2p3/2) and 796.1 eV (2p 1/2 ) and the satellite peak at 786.7 eV (2p 1/2 ) corresponding to the CoP species. The peaks at 798.1 and 782.2 eV and the shakeup satellite at 803.2 eV are ascribed to the oxidized form of cobalt,18−20 which originates from aerial surface oxidation. The P 2p peaks are indexed to P1/2, P3/2, and P−O (Figure 1d).21,22 The high-resolution C 1s profile (Figure S5a) shows a strong signature for CC at 284.3 eV and a lowB

DOI: 10.1021/acsanm.8b01994 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 2. Field-emission scanning electron microscopic (a), TEM (b), HRTEM (c), and HAADF-STEM (d) images of NC-CoP. The STEM elemental mapping showing the distribution of cobalt, carbon, phosphorus, and nitrogen in a single NC-CoP particle is shown in the right lower panel of the figure. The inset in part b is the magnified TEM image of the NC-CoP particle.

from the hexafluorophosphate anion of the complex phosphidates the cobalt species produced during the carbothermal reduction. It is interesting to highlight here that the growth of CoP depends on the atomic percent of carbon in the precursor complex. Thermal annealing of cobalt polypyridyl complexes such as [Co(bpy)3](PF6)2 and [Co(terpy)2](PF6)2 does not yield the expected CoP nanoparticles. Both complexes yield the corresponding pyrophosphate instead of phosphide (Figure 3). Although phosphidation of cobalt was achieved, the desired cobalt phosphide was not obtained with these two complexes.

Joyner−Halenda analysis shows that NC-CoP is mesoporous with a narrow pore diameter of 3.8 nm (inset in Figure S6). The Raman spectral profile of NC-CoP has characteristic D and G bands at 1350 and 1593 cm−1, respectively (Figure S7). The undoped graphitic carbon is known to have a G band at 1580 cm−1, and nitrogen doping shifts the G band to high frequency.27 Such a slight shift in the G band can be attributed to nitrogen doping. The intensity ratio (ID/IG) of the D and G bands is 0.99, suggesting disorder due to nitrogen doping in the carbon matrix. In order to understand the possible pathway for thermal decomposition of the precursor complex and growth of CoP, thermogravimetric analysis (TGA) was performed. The precursor cobalt complex was subjected to TGA under a dynamic argon atmosphere with a heating rate suitably controlled at 10 °C min−1 from ambient temperature to 1000 °C. As shown in Figure S6b, the initial weight loss of 64.5% is associated with the loss of a water molecule and two tridentate ligands in the temperature range of 50−400 °C. The corresponding derivative thermogravimetry (DTG) profile evidences the changes at 415, 470, and 900 °C associated with decomposition of the precursor complex and the formation of NC-CoP. The differential thermal analysis (DTA) profile shows an endothermic peak at ∼470 °C (Figure S8). The change in the temperature exceeding 400 °C is ascribed to the carbothermal-reduction-assisted phosphidation of an in situ generated cobalt species to form CoP. The hexafluorophosphate anion thermally decomposes into gaseous PF5 at temperatures of >400 °C.28,29 The metal hexafluorophosphates thermally decompose into PF5(g) in the temperature range of 195−600 °C.29,30 In our case, in situ generated PF5(g) (corresponding to an endothermic peak at ∼470 °C)

Figure 3. XRD profiles of the product obtained by thermal annealing of (a) [Co(bpy)3](PF6)2, (b) [Co(terpy)2](PF6)2, and (c) [Co(phen)3](PF6)2. C

DOI: 10.1021/acsanm.8b01994 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

Figure 4. Cyclic (a) and hydrodynamic (b) voltammograms illustrating the ORR activity of NC-CoP in 0.1 M KOH. (c) Plot of % HO2̅ generated and ‘n’ involved in ORR. Polarization curves in (d) illustrate the durability of NC-CoP toward ORR.

transfer and imparts high durability.31−33 In our case, the active catalyst is embedded inside NC, and it is anticipated to have high ORR activity. Prior to evaluation of the catalytic activity, the capacitive performance of NC-CoP was calculated from the nonfaradaic current obtained in the potential range of −0.2 to +0.2 V at different sweep rates in 0.1 M KOH (Figure S10). NC-CoP has a specific capacitance of 36.2 mF cm−2, suggesting that the material is highly accessible for the electrolyte. Figure 4 illustrates the electrocatalytic performance of NC-CoP toward the ORR. Well-defined cyclic and hydrodynamic voltammetric responses for the reduction of oxygen were obtained. The sharp cyclic voltammetric peak implies a facile electron transfer for the reduction of oxygen (Figure 4a). The diffusion-limited current density scales linearly while increasing the rotation of the electrode. Interestingly, NC-CoP has onset and limiting current densities of 0.82 V and 5.2 mA cm−2, which are close to those of the traditional 20% platinum/carbon catalyst (Figure S11). The polarization curves were further analyzed by Koutecky−Levich (K−L) analysis to obtain the number of electrons transferred (n) and reaction pathway (Figure S12). The slope of the K−L plot remains the same in the potential ranging from 0.65 to 0.10 V, suggesting that the number of electrons transferred is almost the same at all of the potentials. The ORR kinetics was further examined with rotating-ring-disk-electrode (RRDE) analysis (Figure S13). RRDE analysis reveals that NC-CoP favors the four-electron pathway for the reduction of oxygen with an average electron-transfer number of 3.8. The HO2̅ generated during the ORR was estimated from the RRDE profile and was ∼10.6% in the whole potential range examined in this investigation (Figure 4c). The kinetics of the ORR was further examined with Tafel analysis, and the Tafel plot (Figure S14) shows a slope of 54 mV dec−1, which is less than that of the traditional platinum catalyst, suggesting facile electrontransfer kinetics. To understand the catalytic effect of NC, a control experiment with NC thermally derived from the pyterpy ligand was performed (synthesized at conditions

On the other hand, thermal annealing of [Co(phen)3](PF6)2 yields a mixture of CoP and Co2P (Figure 3), suggesting that the carbon content of the complex has a vital role in the growth of cobalt phosphide. In order to quantitatively estimate the atomic percent of carbon in the precursor complexes, energy-dispersive X-ray elemental analysis was performed. The atomic percents of carbon in [Co(bpy)3](PF6)2 and [Co(terpy)2](PF6)2 were found to be less than those of the other two complexes [Co(phen)3](PF6)2 and [Co(pyterpy)2](PF6)2 (Figure S9). The bpy and terpy complexes have closely similar atomic percents of carbon (∼70%), whereas the phen and pyterpy complexes have high atomic percents of carbon (phen, 74%; pyterpy, 79.1%). In the cases of the [Co(bpy)3](PF6)2 and [Co(terpy)2](PF6)2 complexes, the amount of carbon available is not sufficient for carbothermal reduction to produce the desired cobalt phosphide, although it could yield the pyrophosphate. However, the other two complexes could yield the desired phosphide materials because of the presence of the required amount of carbon. The synthesis of an inexpensive electrocatalyst for the ORR is of significant interest for the development of electrochemical energy conversion and storage devices like metal−air battery, fuel cell, etc. The activity of the cathode catalyst largely controls the overall performance of these devices. Traditionally, platinum is used as the cathode catalyst in a polymer electrolyte membrane fuel cell. The cost of platinum, lack of durability, etc., are some of the serious concerns with the platinum-based catalysts. The development of inexpensive non-platinum-metal-based catalysts is the current need. Although several efforts are being made, it is challenging to achieve high activity and long durability with these catalysts for practical energy conversion and storage applications.2 It is wellknown that the catalyst support has large control over the overall ORR performance of the catalyst.31 For instance, encapsulation/incorporation of the active catalyst with the catalyst support such as heteroatom-doped carbon or reduced graphene oxide or carbon nanotube favors facile electron D

DOI: 10.1021/acsanm.8b01994 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX

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ACS Applied Nano Materials

and solid-state approaches, our method does not require any additional phosphidating agent. The as-synthesized NC-CoP has a high electrocatalytic activity toward the ORR. Further works for the synthesis other TMPs using this approach are in progress.

identical with those of the as-synthesized NC-CoP). The electron-transfer kinetics for the ORR on NC is rather poor, as evidenced by the sluggish voltammetric response with a low onset potential of 0.69 V, which is 130 mV less positive than NC-CoP. Moreover, the limiting current density on NC is significantly low (2.5 mA cm−2) compared to NC-CoP (Figure S15). This implies that the catalytic effect of NC-CoP originates from the synergistic effect between NC and CoP. Doping of the catalyst support materials with electronegative nitrogen significantly influences the electroneutrality of the C− C bond and polarizes the carbon atom.34 Such polarization eventually improves the facile adsorption of oxygen and enhances the electron-transfer kinetics. Furthermore, it is generally observed and established that the chemical nature of nitrogen in the carbon framework has an influence on the electrocatalytic properties. For instance, the presence of graphitic and pyridinic nitrogen in the carbon framework significantly improves the ORR performance of the catalyst, although the actual mechanism is not well understood.2 In our case, although no graphitic nitrogen was found, NC-CoP has >36% of pyridinic nitrogen, and it contributes to the ORR activity of NC-CoP. The ORR durability of NC-CoP was evaluated by cycling the potential of the electrode repeatedly in the potential range between 0.9 and 0.2 V for 2000 cycles in an oxygen-saturated electrolyte. Only a ∼6.2% decrease in the limiting current density and a ∼8 mV negative shift in the half-wave potential were noticed after 2000 cycles (Figure 4d). In order to understand the change in the catalyst surface structure, the XRD profile of the catalyst after durability test was examined. The post-XRD analysis shows that NC-CoP retains its structural integrity even after the durability test, although a small decrease in the diffraction intensity was noticed (Figure S16). The diffraction patterns remain unaltered, confirming the durability of NC-CoP. Postmortem TEM analysis (Figures S17 and S18) further evidences that the durability test does not change the crystal structure of the catalyst. The HRTEM image shows fringe spacing corresponding to the (111) and (211) planes of CoP. However, a significant change in the morphology of the catalyst was noticed. The quasi-spherical NC-CoP nanostructure transformed to flakelike and nanorodlike nanostructures of 30−40 nm. The potential cyclinginduced morphological change did not significantly affect the electrocatalytic performance of the catalyst because the original crystal phase is preserved. It is worth highlighting here that the durability of NC-CoP is superior to that of the traditional 20% platinum/carbon catalyst (Figure S19). A significant decrease in the limiting current and a negative shift in the half-wave potential have been observed with the traditional platinum/ carbon catalyst. The ORR activity of NC-CoP is comparable to the activity of other TMPs and non-platinum electrocatalysts (Table S1). In conclusion, a new facile approach for the synthesis of cobalt phosphide based on carbothermal-reduction-assisted phosphidation is demonstrated for the first time. The transition-metal complexes with suitable ligands and phosphorus-containing counteranions with a sufficient amount of carbon can be ideal precursors for the single-step synthesis of TMPs without using the traditional phosphidating reagent. This carbothermal approach can be extended for the synthesis of other TMPs. The main advantage of this method is the use of a single-source precursor without using any additional phosphidating agent. Unlike the conventional solvothermal

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.8b01994. Materials and methods, synthesis, and characterization of metal complex precursors, ligands, and NC-CoP, NMR spectrum of the ligand, CHN analysis data for the complex, UV−visible and Fourier tranform infrared spectra, high-resolution C 1s and N 1S XPS, TGA, DTG, DTA, Raman spectral profiles, adsorption isotherm, K−L plot, RRDE profile, post-ORR XRD, and post-ORR TEM profile for the NC-CoP catalyst (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

C. Retna Raj: 0000-0002-7956-0507 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Science and Engineering Research Board, New Delhi, India (Grant EMR/ 2016/002271). M.M.K. acknowledges IIT Kharagpur for a research fellowship. An IYC grant from the Department of Science and Technology for a field-emission electron microscope to the Department of Chemistry, IIT Kharagpur, is acknowledged. We thank Prof. Chacko Jacob of Materials Science Centre and Micro/Nano-Robotics and Fabrication Facility Lab, IIT Kharagpur for EDX analysis.

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DOI: 10.1021/acsanm.8b01994 ACS Appl. Nano Mater. XXXX, XXX, XXX−XXX