Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for

Apr 24, 2017 - A single-step heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to ...
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Large-Scale Synthesis of Carbon-Shell-Coated FeP Nanoparticles for Robust Hydrogen Evolution Reaction Electrocatalyst Dong Young Chung,†,‡,∥ Samuel Woojoo Jun,†,‡,∥ Gabin Yoon,†,§,∥ Hyunjoong Kim,†,‡ Ji Mun Yoo,†,‡ Kug-Seung Lee,⊥ Taehyun Kim,†,‡ Heejong Shin,†,‡ Arun Kumar Sinha,†,‡ Soon Gu Kwon,*,†,‡ Kisuk Kang,*,†,§ Taeghwan Hyeon,*,†,‡ and Yung-Eun Sung*,†,‡ †

Center for Nanoparticle Research, Institute for Basic Science (IBS), Seoul 08826, South Korea School of Chemical and Biological Engineering, and Institute of Chemical Processes, Seoul National University, Seoul 08826, South Korea § Department of Materials Science and Engineering, Research Institute of Advanced Materials (RIAM), Seoul National University, Seoul 08826, South Korea ⊥ Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea ‡

S Supporting Information *

ABSTRACT: A highly active and stable non-Pt electrocatalyst for hydrogen production has been pursued for a long time as an inexpensive alternative to Pt-based catalysts. Herein, we report a simple and effective approach to prepare high-performance iron phosphide (FeP) nanoparticle electrocatalysts using iron oxide nanoparticles as a precursor. A singlestep heating procedure of polydopamine-coated iron oxide nanoparticles leads to both carbonization of polydopamine coating to the carbon shell and phosphidation of iron oxide to FeP, simultaneously. Carbon-shell-coated FeP nanoparticles show a low overpotential of 71 mV at 10 mA cm−2, which is comparable to that of a commercial Pt catalyst, and remarkable long-term durability under acidic conditions for up to 10 000 cycles with negligible activity loss. The effect of carbon shell protection was investigated both theoretically and experimentally. A density functional theory reveals that deterioration of catalytic activity of FeP is caused by surface oxidation. Extended X-ray absorption fine structure analysis combined with electrochemical test shows that carbon shell coating prevents FeP nanoparticles from oxidation, making them highly stable under hydrogen evolution reaction operation conditions. Furthermore, we demonstrate that our synthetic method is suitable for mass production, which is highly desirable for large-scale hydrogen production.

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Fe−P,13,14 and FexCo1−xP21 have been highlighted as attractive candidates of HER electrocatalysts. Especially, iron phosphide is highly advantageous for the large-scale production of H2 because iron is the most earth abundant and cheapest metallic element.22,23 However, due to its low stability, this material is not appropriate for long-term operation of HER. Regarding the durability problem, a number of recent studies showed that carbon encapsulation of nanoparticles (NPs) can considerably improve their long-term stability in harsh reaction conditions for electrochemical reactions.24−30 For example, the Bao group reported that CoNi alloy NPs coated with graphene show good long-term durability (up to 1000 cycles).31 Our IBS Center previously reported that encapsulation of FePt alloy NPs with a carbon shell resulted in exceptional thermal stability at annealing temperature as high as 700 °C.32 Being inspired by those examples, we conjectured that carbon shell formation can be a general strategy of providing physicochemical protection

INTRODUCTION Hydrogen economy is expected to be a promising solution to the problems of environmental pollution and fossil fuel depletion in our age. With its high energy density and environmentally friendly nature, hydrogen can be utilized as an ideal clean energy source to take on the role of petroleum in the current energy system, provided that an energetically and economically efficient hydrogen production process is available. With this regard, electrochemical water splitting has been extensively studied as a next-generation energy conversion device for hydrogen production. Pt-based catalysts exhibit unrivaled catalytic performance in terms of low overpotential and low Tafel slope for the hydrogen evolution reaction (HER) of electrochemical water splitting. However, high cost and scarcity of Pt impede its general use in the global energy system. As a result, developing alternate non-Pt electrocatalysts that satisfy both good catalytic performance and reasonable cost has pivotal importance in the roadmap of hydrogen economy. Among various non-Pt electrocatalysts including metal sulfides,1−5 selenides,6,7 carbides,8−11 and phosphides,12−15 transition metal phosphides such as Ni−P,12,16 Co−P,17−20 © 2017 American Chemical Society

Received: February 13, 2017 Published: April 24, 2017 6669

DOI: 10.1021/jacs.7b01530 J. Am. Chem. Soc. 2017, 139, 6669−6674

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Figure 1. (a) Schematic representation of carbon-shell-coated FeP NP preparation. (b−e) TEM images of as-synthesized iron oxide NPs (b), carbon-shell-coated FeP NPs (c,d), and FeP NPs prepared without carbon shell (e).

nanometer to a few nanometers by changing the polydopamine treatment time (Figure 1d and Figure S3 in Supporting Information).32 For electrochemical performance, the thickness of the carbon shell was optimized less than 1 nm, which is thick enough to provide physicochemical protection but not to interfere with the catalytic reaction at the surface of the NPs. Carbonization of polydopamine coating into the carbon shell is further confirmed by detection of nitrogen-doped carbon from X-ray photoelectron spectroscopy (XPS, Figure S4 in Supporting Information). When the polydopamine coating step is omitted from the procedure, aggregation of NPs occurs during the heat treatment (Figure 1e). This shows that an in situ formed carbon shell prevents the NPs from morphological change during the phosphidation. FeP NPs as small as 4 nm are also prepared by the same coating and heat treatment procedures, although their electrochemical property is not as good as 7.6 nm ones (Figure S5 in Supporting Information). Electrochemical Performance for Hydrogen Evolution Reaction. We tested the electrochemical catalytic activity of carbon-supported FeP NPs (FeP/C) with and without a carbon shell for HER in 0.5 M H2SO4 (aq), and the results are compared with that of a commercial Pt/C electrocatalyst (Figure 2). The current was normalized by the glassy carbon geometric area (0.2475 cm2). Overpotential at the current density of 10 mA cm−2 is measured to be 71 and 73 mV for FeP/C with and without a carbon shell, respectively. In the inset of Figure 2, a Tafel slope of FeP/C with a carbon shell is 52 mV decade−1, whereas a commercial Pt/C shows 30 mV decade−1, which is close to the known values of Pt in acidic media.39−41 Interestingly, both overpotential and the Tafel slope of our FeP/C are comparable to the best HER performance of the state-of-the-art non-noble metal catalyst (Table S1 in Supporting Information).38 To investigate the active site of FeP/C with a carbon shell, we conducted a couple of control experiments. First, a nanoparticle-free sample was prepared by polydopamine coating and phosphidation of the carbon support without iron oxide NPs (“C with shell”). Its polarization curve in Figure 2 confirms that the carbon shell has negligible activity. Second, carbon-supported iron oxide NPs with a carbon shell was prepared through the heat treatment without a phosphorus source (“FeOx/C with shell”). The poor

for various nanocatalysts, including metal phosphides, to achieve high stability. Herein, we report a simple and straightforward approach to prepare carbon-shell-protected FeP NPs that have both high catalytic activity and long-term durability for HER. In the preparation, polydopamine-coated iron oxide NPs are thermally treated with a phosphorus source. Through a single-step heating procedure, carbonization of dopamine coating and phosphidation of iron oxide NPs take place simultaneously, producing carbon-shell-coated FeP NPs. From an electrochemical study of our carbon-shell-coated FeP NPs, overpotential and Tafel slope are measured as 71 mV at 10 mA cm−2 and 52 mV decade−1, respectively. In a long-term durability test, the NPs show no appreciable activity loss up to 10 000 cycles. Extended X-ray absorption fine structure (EXAFS) measurement and density functional theory (DFT) calculation reveal that oxidation resistance of carbon-shellcoated NPs is the origin of the long-term durability. In addition, we confirm that our method is suitable for large-scale synthesis, which is important for industrial hydrogen production using non-Pt electrocatalysts.

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RESULTS AND DISCUSSION Synthesis of Carbon-Shell-Protected FeP Nanoparticles. The synthetic protocol of carbon-shell-coated FeP NPs is illustrated in Figure 1a. First, 7.6 nm sized iron oxide NPs are synthesized by following the previously reported method (Figure 1b).33 The NPs are deposited on the carbon support (Figure S1 in Supporting Information) and treated with dopamine solution.32,34,35 Polydopamine-coated iron oxide NPs are annealed at 400 °C under Ar in the presence of NaH2PO2 powder as the phosphorus source. During the heat treatment, polydopamine coating is converted to carbon shell and iron oxide NPs to FeP NPs, respectively.13,36,37 X-ray diffraction (XRD) analysis confirms the full phosphidation of iron oxide (Fe3O4) into FeP (Figure S2 in Supporting Information). Transmission electron microscopy (TEM) analysis proves that the NPs are encapsulated with a thin carbon shell, and their size and shape are nearly unchanged after the phosphidation process (Figure 1c). The thickness of the carbon shell is controllable in the range from sub6670

DOI: 10.1021/jacs.7b01530 J. Am. Chem. Soc. 2017, 139, 6669−6674

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On the other hand, without a carbon shell, overpotential is increased up to ∼100 mV during the same number of cycles (Figure 3b). Outstanding long-term durability was also corroborated by chronopotentiometry data (Figure S6 in Supporting Information). At a current density of 10 mA cm−2, overpotential of FeP/C with a carbon shell is maintained at 70 mV after 44 h with small variance over time in the range of 67−72 mV. Without carbon shell protection, however, FeP/ C is rapidly deactivated with increased overpotential of 93 mV in 25 h. Structural Analysis of Highly Stable Carbon-ShellCoated FeP Nanoparticles. To study the origin of the longterm durability of carbon-shell-coated FeP/C, EXAFS analysis was performed on Fe K-edge of the samples. Before electrochemical reactions, signals from both carbon-shellcoated and uncoated FeP NPs show similar features. After 5000 potential cycles, the first coordination shell signal at R ∼ 1.7 Å from the uncoated sample is slightly shifted to the left (Figure 3c). Referring to standard EXAFS data of Fe2O3 and Fe3O4, we attribute the shift to the increasing contribution of Fe−O over Fe−P. Consistently, electron energy loss spectroscopy (EELS) line scan analysis reveals that surface oxidation takes place for the uncoated FeP NPs during HER (Figure S7 in Supporting Information). On the other hand, signals from the carbon-shell-coated ones show little difference before and after the cycling (Figure 3d). These results confirm the protective role of the carbon shell that prevents surface oxidation of FeP NPs, which in turn suppresses the loss of electrocatalytic activity. The relationship between the catalytic activity and the surface oxidation of FeP NPs is further investigated by DFT calculation. In modeling HER catalysts, hydrogen adsorption free energy ΔGH has been widely accepted as a descriptor for the catalytic activity.15,44,45 If hydrogen bonding on the surface of the catalyst is too strong (ΔGH ≪ 0), sluggish desorption of

Figure 2. Polarization curves for FeP/C with and without a carbon shell and commercial Pt/C electrocatalysts. The corresponding Tafel plots are shown in the inset. Data from control experiments with the samples with no NPs (“C with shell”) and iron oxide NPs instead of FeP (“FeOx with shell”) are shown together. Electrochemical performance for HER is measured in 0.5 M H2SO4 solution with a rotating ring disk electrode at a rate of 2000 rpm.

activity of this sample excludes the possibility of performance enhancement by the Fe−N−C effect at the interface of the NPs and N-doped carbon shell.41−43 These results reveal that the catalytic activity of FeP/C with a carbon shell is attributed to the FeP surface, and the carbon shell provides only physicochemical protection for the durability of FeP NPs. The long-term durability test was conducted by potential cycling in the range from −0.25 to +0.1 V with the scan rate of 50 mV s−1. As shown in Figure 3a, FeP/C with a carbon shell shows no appreciable activity loss after 5000 potential cycles.

Figure 3. Long-term durability test of FeP/C electrocatalysts. (a) Polarization curves for 5000 cycle tests of FeP NPs with (left) and without (right) a carbon shell. (b) Plots of overpotential vs potential cycle for the data in panel a. (c,d) EXAFS analysis of FeP NPs without (c) and with (d) the carbon shell. 6671

DOI: 10.1021/jacs.7b01530 J. Am. Chem. Soc. 2017, 139, 6669−6674

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Figure 4. (a) DFT calculation result for hydrogen bonding energy of FeP and FeP-O as a function of hydrogen coverage. (b) Polarization curves for FeP/C with and without a carbon shell before and after surface oxidation under 1.5 V for 20 min. (c) Schematic illustrations of FeP NPs with and without carbon shell coating. (d) Polarization curves for 10 000 cycle tests of FeP/C with the carbon shell obtained by large-scale synthesis. The inset shows a picture of the product and its weight. The mass ratio of FeP is 20%.

20 min), which is a severe surface oxidative condition,46 to test the effect of the carbon shell in preventing surface oxidation. After this treatment, the HER activity of FeP NPs without a carbon shell dramatically decreased (Figure 4b). On the contrary, FeP NPs with carbon shell retained their activity. Our result is consistent with a recent report that the emergence of (oxy)phosphates after potential exposure up to 1.4 V leads to the decrease of HER activity.47 Combining the evidence, the high durability of carbon-shell-coated FeP NPs under HER condition is attributed to high oxidation resistance of a metal phosphide surface protected by the carbon shell (Figure 4c). Large-scale preparation of carbon-shell-coated FeP/C is demonstrated in Figure 4d and Figure S9 in Supporting Information. As mentioned above, one important motivation for the study of a non-Pt electrocatalyst is the cost-efficient mass production of hydrogen. Our method is easily scaled up to tens of grams of carbon-shell-coated FeP/C from a single batch reaction, which is only limited by the size of the reactor. The product from large-scale preparation shows the excellent durability with negligible activity loss after 10 000 potential cycles (Figure 4d, potential cycling in the range from −0.25 to +0.1 V with the scan rate of 50 mV s−1), suggesting the successful feasibility of large-scale electrolysis applications. Multiple sampling tests confirm that scale-up of the preparation method does not affect the catalytic performance of carbonshell-coated FeP/C (Figure S9 in Supporting Information). Furthermore, in a preliminary experiment, polydopamine coating and phosphidation procedures were successfully applied

hydrogen lowers the catalytic activity. On the other hand, if ΔGH ≫ 0, hydrogen adsorption does not take place spontaneously. On this basis, we calculated ΔGH of pristine FeP and surface-oxidized FeP (FeP-O) to compare their HER activity. We used a slab model with a vacuum slab of ∼15 Å to describe the surface hydrogen adsorption (see Computational Details and Figure S8 in Supporting Information). Figure 4a shows the hydrogen adsorption energy of FeP and FeP-O as functions of hydrogen coverage. In general, the higher coverage makes the hydrogen bonding less favorable due to increase of adsorbate−adsorbate repulsion. Since HER occurs most actively occurs when ΔGH is closest to zero, we can expect that the activity of FeP is highest when the coverage is 50%. With that coverage, ΔGH is calculated as −0.04 eV. The negative value of ΔGH indicates that the overall reaction rate of HER is determined by the H2 evolution step, which is consistent with our experimental data from the Tafel slope of FeP (Figure 2). The values of ΔGH for FeP-O are always negative (Figure 4a) because the shorter bond length of O−H (0.97−0.98 Å) compared to that of P−H (1.42 Å) leads to the stronger hydrogen bonding and more sluggish H2 evolution step. Comparing the values of ΔGH closest to zero (−0.04 eV at 50% for FeP and −0.13 eV at 100% for FeP-O), the activity of FeP-O is lower than that of FeP, which supports that the surface oxidation of FeP causes a decrease in catalytic activity. This is further corroborated by a controlled oxidation experiment. We deliberately exposed FeP NPs with and without a carbon shell at high potential (1.5 V vs RHE for 6672

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to cobalt/cobalt oxide NPs to obtain carbon-shell-coated cobalt phosphide NPs (Figure S10 in Supporting Information), which demonstrates general applicability of our method.

ASSOCIATED CONTENT

S Supporting Information *

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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b01530. Additional TEM, XRD, and electrochemical data, EELS line scan data, performance of various HER catalysts in comparison, and DFT calculation details, data on preparation of 4 nm sized FeP NPs and cobalt phosphide NPs (PDF)

CONCLUSION In summary, we developed a simple and effective approach to prepare a non-Pt electrocatalyst for HER with high activity and high durability. A single-step thermal treatment of polydopamine-coated iron oxide NPs leads to the formation of carbonshell-coated FeP NPs. The carbon shell provides physical and chemical protections preventing both aggregation of the NPs during phosphidation and surface oxidation of FeP NPs under HER conditions. EXAFS and EELS analyses in combination with DFT calculation reveal that oxidation resistance of carbonshell-coated FeP NPs is the key for the long-term durability. Moreover, we demonstrated that our preparation method can be easily scaled up for mass production. Overall, we believe that carbon shell encapsulation of NPs can be a promising approach for development of various robust nanoscale electrocatalysts for large-scale water splitting and hydrogen production.

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AUTHOR INFORMATION

Corresponding Authors

*[email protected] *[email protected] *[email protected] *[email protected] ORCID

Samuel Woojoo Jun: 0000-0002-0879-2191 Kisuk Kang: 0000-0002-8696-1886 Yung-Eun Sung: 0000-0002-1563-8328

EXPERIMENTAL SECTION

Author Contributions

Synthesis of Carbon-Shell-Coated FeP Nanoparticles. In a typical synthesis, iron oxide NPs were prepared by heating a mixture of 4.24 g of Fe(acac)3 in 120 mL of oleylamine from room temperature up to 300 °C at 20 K min−1 and then kept at the same temperature for 1 h.33 After being rinsed with acetone and dried in a vacuum oven, the NPs were loaded on a carbon support (Vulcan XC 72-R, 20 wt %). For polydopamine coating, carbon-loaded NPs were dispersed in aqueous solution of dopamine hydrochloride (3 mg mL−1) buffered at pH 8.5 and stirred for 1 h.32 The thickness of coating is proportional to stirring time. After being coated, the sample was collected and dried by vacuum filtering at 70 °C. Phosphidation of iron oxide NPs and carbonization of dopamine were carried out by thermal annealing of the sample at 400 °C for 2 h in a tube furnace under Ar flow. A crucible boat containing NaH2PO2 powder was placed upstream of gas flow as a phosphorus source. For the large-scale synthesis, iron oxide NPs were prepared by the mass production method previously reported in ref 48. Briefly, a mixture of 18 g of iron oleate [Fe(oleate)3] and 5.7 g of oleic acid in 100 g of 1-octadecene was heated to 320 °C at 3.3 K min−1 and then kept at that temperature for 30 min. The rest of the procedures were the same way as described above. Characterization. High-resolution transmission electron microscopy images were measured using a Technai F20 electron microscope operating at 200 kV, X-ray diffraction patterns by a Rigaku D/MAX 2500 with Cu Kα source (40 kV, 200 mA), and X-ray photoelectron spectroscopy data by a ThermoFisher Scientific Sigma Probe surface analysis instrument with Al Kα source. X-ray absorption spectroscopy was measured at 8C beamline in the Pohang Accelerator Laboratory (Pohang, Republic of Korea). Electrochemical Measurements. FeP/C (with and without carbon shell) or Pt/C (20 wt %, Johnson Matthey) catalyst was dispersed in 2-propanol containing Nafion (15 wt %, Sigma-Aldrich). Seven microliters of the dispersion was deposited on the glassy carbon electrode (PINE, rotating ring disk electrode) with the disk area of 0.2475 cm2. Electrochemical measurements were conducted using an Autolab potentiostat (PGSTAT302N) in a standard three-electrode cell with glassy carbon counter electrode and Ag/AgCl reference electrode. All potentials are referenced as reversible hydrogen electrode (RHE) by H2 oxidation/reduction using a Pt wire. The HER activity was measured under H2-saturated 0.5 M H2SO4 at 298 K, and the data were ir-corrected based on impedance measurement, which was obtained from 100 kHz to 50 mHz at open circuit potential with 10 mV. Long-term durability test was conducted by potential cycling from −0.25 to +0.1 V. The scan rate for cycling was fixed at 50 mV s−1 in all experiments.

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D.Y.C., S.W.J., and G.Y. contributed equally to this work.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by IBS-R006-D1, IBS-R006-G1, and IBS-R006-Y1. REFERENCES

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DOI: 10.1021/jacs.7b01530 J. Am. Chem. Soc. 2017, 139, 6669−6674