Supporting Information

Transcription

1 Supporting Information 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, Δ 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. Δ Pohang Accelerator Laboratory (PAL), Pohang University of Science and Technology (POSTECH), Pohang 37673, South Korea. Corresponding Author s: S1

2 Figure S1. TEM image of iron oxide nanoparticles (NPs) deposited on to the carbon support. Figure S2. XRD data of iron oxide, intermediate state (heat treatment at 250 o C) and FeP (heat treatment at 400 o C). S2

3 Figure S3. HR-TEM images of FeP nanoparticles with thin (~0.7 nm, left) and thick (~2 nm, right) carbon shell. Figure S4. Nitrogen 1s XPS data for FeP/C with and without the carbon shell. The peak at around ev indicates the presence of N-doped carbon. S3

4 Table S1. Performance of various catalysts for hydrogen evolution reaction in 0.5 M H2SO4. Catalyst Loading (mg cm -2 ) Overpotential (@ 10 ma cm -2 ) Tafel slope (mv decade -1 ) Reference* FeP NA/Ti (13) Ni 2P nanoparticles (12) Cu 3P NW/CF S1 MoC x S2 Ni 5P 4-Ni 2P nanosheet S3 FeP/Ti (14) CoNi@C (31) Fe 0.9Co 0.15S 2/CNT S4 CoMoS S5 CoPS NPIs (38) CoP/Ti 0.9/2 95/85** 50 S6 CoP nanowires (20) Fe 0.5Co 0.5P nanowires (21) Carbon shell-coated FeP/C Current study * Numbers in parentheses refer the reference in the main text. ** Overpotential at current density of 20 ma cm -2. S4

5 Preparation of 4 nm-sized carbon shell-coated FeP NPs By applying the polydopamine coating and heat treatment procedures to 4 nm-sized iron oxide NPs, carbon shell-coated FeP NPs of the same size are successfully obtained (Figures S5a-d). XRD data show that the NPs contain a trace amount of metallic iron and iron phosphate (Figure S5e), being different from 7.6 nm sample which is almost pure FeP (Figure S2). Overpotential at 10 ma cm -2 is 87 mv for 4 nm NPs and 71 mv for 7.6 nm ones (Figure S5f). When iron oxide NPs as small as 2 nm are subjected to the same procedures, they simply dissolve away in the dopamine solution at ph 8.5 during the coating procedure. These observations from 4 nm and 2 nm samples can be explained as follow. Due to Gibbs-Thomson effect, the chemical potential of a spherical NP with radius r increases proportional to 1/r. As a result, too small NPs ( 2 nm) are thermodynamically unstable to withstand the alkaline condition. For the same reason, 4 nm-sized FeP NPs are unstable so that they tend to form FeP-O bonding at the surface in order to lower the surface free energy, even with the carbon shell protection. This tendency in turn increases their overpotential due to the negative effect of surface oxidation to the activity (see Figures 4a and 4b). Such low catalytic activity of small NPs due to intrinsic instability is a general phenomenon. For example, it is known that the activity of Pt NPs for oxygen reduction reaction rapidly decreases as they become smaller than ~ 2 nm, due to strong oxygen binding at the surface. S7,S8 Figure S5. (a-d) TEM images of polydopamine-coated 4 nm NPs before (a, b) and after the heat treatment (c, d). (e) XRD data of the 4 nm NPs before and after the heat treatment. The peaks indicated with arrows correspond to iron phosphate (1) and metallic iron (2), respectively. (f) Polarization curves for 4 nm and 7.6 nm FeP/C with carbon shell. S5

6 Figure S6. Chronopotentiometry results of FeP NPs with and without carbon shell. The measurement was done at current density of -10 ma cm -2. Figure S7. EELS line scan analysis after 5000 cycles for FeP/C with carbon shell (a) and without carbon shell (b). Blue, red, and green lines correspond to P, Fe, and O, respectively. S6

7 Computational Details All calculations were performed using Vienna ab initio simulation package (VASP) S9 in density functional theory (DFT) framework. Exchange-correlation energies were corrected with revised Perdew-Burke- Ernzerhof (RPBE) S10 spin-polarized generalized gradient approximation (GGA). S11 Van der Waals interaction was included in all calculations by DFT-D3 scheme. S12 We used the projected-augmented wave (PAW) pseudopotentials, S13,S14 with plane-wave basis sets as implemented in VASP. An energy cutoff for plane wave basis sets was set to 800 ev to ensure the convergence of calculated systems. All geometric relaxations were repeatedly performed in order that the residual force in the system converges below 0.05 ev/å. For investigating the H adsorption energy in FeP and partially oxidized FeP (FeP-O), a slab model composed of ten layers of (011) surface FeP S15 and a vacuum slab of ~15 Å was adopted as shown in Figure S6. FeP-O model was generated by substituting a single layer of phosphorus to oxygen. Since P- terminated surface was calculated to be more stable than Fe-terminated one, we modeled the slab system with P-termination. Top surface layer was allowed to relax, whereas other layers were fixed to mimic the bulk properties. Hydrogen adsorption energy was calculated as the Gibbs free energy difference between the reaction product and reactant, which is hydrogen-adsorbed FeP system and pristine FeP plus hydrogen gas, respectively. DFT energy of hydrogen adsorption EE HH is defined as EE HH = EE ssssssssssssssssss+hh EE ssssssssssssssssss 1 2 EE HH 2 where EE ssssssssssssssssss+hh is DFT energy of hydrogen-adsorbed substrate, EE ssssssssssssssssss is DFT energy of pristine substrate and EE HH2 is DFT energy of gas phase hydrogen. Since the hydrogen adsorption reaction includes the gas phase in its equation, the entropy and zero-point energy (ZPE) contributions could not be neglected in describing EE HH at room temperature. Therefore, Gibbs free energy of hydrogen adsorption GG HH is defined as GG HH = EE HH + ZZZZZZ TT SS where ZZZZZZ is the zero-point energy difference between the hydrogen-adsorbed substrate and gas phase hydrogen and SS is the entropy difference between the hydrogen-adsorbed substrate and gas phase. ZZZZZZ was obtained by the calculation of the Hessian matrix and the vibrational frequencies as implemented in VASP. Since the energy contribution from the configurational and vibrational entropy in the hydrogen-adsorbed substrate could be neglected, S16 SS was determined by considering the standard entropy of hydrogen gas. The inclusion of the entropy and ZPE contributions results in the following relation: S7

8 GG HH = EE HH ev Since four surface P sites are exposed in the unit cell, there are four different states of hydrogen-adsorbed FeP; hydrogen coverages of 25%, 50%, 75% and 100%. For instance, GG HH at 25% coverage indicates the energy required for covering 25% of the FeP surface to hydrogen from pristine FeP. Identical methods were used for FeP-O model system as well. Figure S8. Slab models of (a) FeP (b) FeP-O used in this work, consisting of (011) surface of FeP with a vacuum slab of ~15 Å S8

9 Figure S9. HER data of carbon shell-coated FeP/C prepared via large scale (Portion 1-5) and typical small scale synthesis (Standard). Portions 1-5 were sampled from the product and tested separately in order to confirm that the large scale product is homogeneous in terms of catalytic property. For the curves of Portions 1-5, overpotential at 10 ma cm -2 is distributed in the range from 70 mv to 73 mv, which is close to that of Standard curve at 71 mv. S9

10 Preparation of carbon shell-coated cobalt phosphide NPs In order to test the applicability of our method to the other transition metal NPs, we applied polydopamine coating and phosphidation procedures to cobalt NPs that were synthesized following the procedure in ref. S17. In the polydopamine coating procedure, the shape of the NPs is changed from solid sphere to hollow shell structure (Figures S10a, b). This can be attributed to the Kirkendall effect that occurs during oxidation of metallic cobalt to Co3O4 in alkaline aqueous solution (Figure S10e). S17 According to XRD data, during the phosphidation, Co3O4 NPs were transformed to mixed phase of Co2P and CoP at first, and then CoP became dominant in the final product. TEM data confirm that the NPs were well confined inside the carbon shell and hollow shell structure is turned back to solid sphere after the phosphidation (Figures S10c, d). This result demonstrates that our method can be extended to other metal and metal oxide NPs provided that they are stable under weak alkaline condition. Figure S10. (a-d) TEM images of carbon-loaded Co3O4 NPs after polydopamine coating (a, b) and cobalt phosphide NPs after phosphidation (c, d). (e) XRD data of the carbon shell-coated NPs before and after phosphidation. S10

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