Graphene Layer Encapsulation of Non-Noble Metal Nanoparticles as

Jun 6, 2018 - Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka ... Sa, Park, Jung, Shin, Jeong, Kwak, and Joo...
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Graphene Layer Encapsulation of Non-Noble Metal Nanoparticles as Acid-Stable Hydrogen Evolution Catalysts Kailong Hu,† Tatsuhiko Ohto,*,§ Linghan Chen,¶ Jiuhui Han,∥ Mitsuru Wakisaka,‡,⊥ Yuki Nagata,# Jun-ichi Fujita,† and Yoshikazu Ito*,†,⊥ Downloaded via FORDHAM UNIV on June 29, 2018 at 17:41:12 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

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Institute of Applied Physics, Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan Graduate School of Engineering Science, Osaka University, 1-3 Machikaneyama, Toyonaka 560-8531, Japan ¶ Department of Materials Science, Graduate School of Engineering, Tohoku University, Sendai 980-8579, Japan ∥ Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan ‡ Graduate School of Engineering, Toyama Prefectural University, 5180 Kurokawa, Imizu, Toyama 939-0398, Japan ⊥ PRESTO, Japan Science and Technology Agency, Saitama 332-0012, Japan # Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany §

S Supporting Information *

ABSTRACT: Acid-stable, non-noble catalysts are promising for hydrogen evolution reaction (HER); however, they get easily damaged when used in acidic electrolytes, thus reducing the HER lifetimes. Moreover, completely blocking catalysts from acidic electrolytes degrades HER performance. To achieve a balance between the HER lifetime and performance, we vary the number of N-doped graphene layers (1−2, 2−3, and 3−5 layers) encapsulating NiMo nanoparticles as efficient HER catalysts and obtain the optimal number of protective layers. Our data show that 3−5 graphene layers achieved the best balance, with a stable current density of 100 mA cm−2 for 25 h in 0.5 M H2SO4. Density functional theory calculations are performed to show the effect of encapsulating graphene layer number on the catalytic activity and protection of non-noble NiMo in acidic electrolytes.

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chemically stable graphene layers (GLs) is particularly promising. Encapsulation using thick GLs19−22 provides high chemical stability but often reduces the catalytic activity significantly.23,24 In contrast, single-layer encapsulation affords high-performance catalysts, but with short lifetimes.25 To achieve excellent balance between HER performance and lifetime, holey GLs as a partial graphene protection were used to encapsulate bulk porous NiMo alloys to minimize the surface area for desirable chemical reactions in the holes and to protect NiMo dissolution in acidic electrolytes by avoiding corrosion with the holey graphene covering;26 this combination achieves better long-term durability in acidic electrolytes than pristine NiMo alloys without graphene covering. However, undesired dissolution arising from acid penetration through the holes of the GLs persisted. Therefore, on the basis of the theory that

evelopment of inexpensive and chemically stable catalysts for hydrogen evolution reaction (HER) is important for large-scale hydrogen production,1,2 necessitating the search for Pt-free catalysts such as metal alloys,3,4 transition metal chalcogenides,5,6 selenides,7,8 phosphides,9,10 and metal-free carbides.11,12 Despite the abundance of HER-active non-noble metal catalysts, their commercialization is limited by their poor durability. For example, although NiMo alloys exhibit a near-zero onset potential and remarkable HER activity,13,14 NiMo-based catalysts are rarely utilized in acidic media,15,16 presumably due to serious dissolution under this condition. Long HER lifetimes are required in acidic electrolytes for proton exchange membrane-type water splitting.17 Considering that HER under acidic conditions is much faster than that under basic conditions,18 the availability of acid-stable, non-noble catalysts is a prerequisite for practical HER applications. Among the approaches employed to fabricate catalysts, encapsulation of non-noble metal nanoparticles (NPs) using © XXXX American Chemical Society

Received: May 7, 2018 Accepted: June 6, 2018 Published: June 6, 2018 1539

DOI: 10.1021/acsenergylett.8b00739 ACS Energy Lett. 2018, 3, 1539−1544

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Cite This: ACS Energy Lett. 2018, 3, 1539−1544

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ACS Energy Letters

Figure 1. Preparation and characterization of N-doped graphene-encapsulated NiMo NPs. (a) Cross-sectional SEM image of ENNiMoNPs/ 3DNG. (b) DF-STEM image of uniformly dispersed NPs on 3DNG and corresponding SAED pattern (inset). (c) Typical HRTEM image of the NiMo NP encapsulated by a monolayer graphene sheet on 3DNG. (d) DF-STEM image and corresponding EDS elemental mappings of ENNiMoNPs/3DNG.

thin GLs significantly enhance HER performance,25 a critical additional step for the practical application of this technique is optimizing the number of GLs to balance the HER activity and catalyst lifetime. This requires precise control of the GLs present on the non-noble metal catalyst surfaces. Thus, optimizing the graphene encapsulation method and understanding the effect of GL number on HER activity are required. Herein, we systematically synthesized three-dimensional Ndoped graphene (3DNG)-supported NiMo NPs encapsulated by N-doped graphene sheet layers (NGLs) as HER catalysts with long-term stability and high activity in acidic environments, achieving a performance similar to that of Pt due to the high electrical conductivity/electron mobility, bicontinuous open porous structure, and high chemical stability of the NGLs.27,28 The NGL catalytic activity mechanism was explored by density functional theory (DFT) calculations. Our results revealed the effect of NGL number on the nanocatalyst performance, enabling the rational design of acid-stable and inexpensive catalysts for efficient and green hydrogen production. The fabrication of 3DNG-supported NiMo NPs encapsulated by N-doped graphene (ENNiMoNPs/3DNG) is illustrated in Figure 1. Initially, NiMoO4 nanofibers were reduced at 950 °C in a H2/Ar atmosphere,29 followed by growth of NGLs on the as-obtained 3D porous NiMo at 700 °C for 15 s in a mixed H2, Ar, and pyridine atmosphere using standard chemical vapor deposition (CVD). Subsequently, the 3D porous NiMo was fully covered by NGLs and then etched using HNO3 to isolate 3DNG as the substrate for NiMo NP deposition; this step was performed using [Ni(NH3)6]MoO4 precursor solution.30 With the loss of ammonia under vacuum, Ni/Mo oxide compounds were generated on the 3DNG surface. Finally, ENNiMoNPs/3DNG was obtained via reduction of the as-deposited oxide at 950 °C and subsequent NGL growth at 700 °C for variable deposition times (1, 2, and 5 s for 1−2, 2−3, and 3−5 NGLs, respectively). Scanning/transmission electron microscopy (SEM/TEM) imaging showed that ENNiMoNPs/3DNG had a bicontinuous and open porous structure (pore radius = 300−500 nm; Figures 1a and S1) containing interconnected graphene sheets (Figure S2a), whereas dark-field scanning TEM (DF-STEM) imaging (Figure 1b) demonstrated uniformly distributed reduced NPs with an average diameter of 11.3 nm on the

3DNG surface (Figure S2b−d). Selected area electron diffraction (SAED) patterns showed sharp diffraction spots, confirming the high crystallinity of multilayer graphene (inset of Figure 1b). High-resolution TEM (HRTEM) imaging revealed lattice fringes with distances of 2.2, 2.1, and 3.4 Å, corresponding to the (041) facet of NiMo, the (121) facet of MoNi4, and the (002) facet of graphene (Figures 1c and S3a), respectively. NiMo NPs encapsulated by 2−3 and 3−5 NGLs were also observed (Figure S3). Energy-dispersive X-ray spectroscopy (EDS) confirmed that Ni and Mo atoms were homogeneously distributed on the NPs (Figures 1d and S4), with a mismatch between the C and Mo/Ni distributions. Structural characterization was performed using X-ray diffraction (XRD) and Raman spectroscopy. The XRD pattern of 3DNG-supported NiMo NPs encapsulated by 3−5 NGLs (NiMoNPs@3−5NGL/3DNG) revealed that the above composite mainly comprised different NiMo alloy phases such as NiMo (JCPDS No. 65-6903) and MoNi4 (JCPDS No. 65-5480) with small amounts of oxidized species (MoO2 (JCPDS No. 32-0671) and MoO3 (JCPDS No. 21-0569); Figures 2a and S5). Further, no obvious peaks of Mo2C (JCPDS No. 65-8766) were detected. The Raman spectrum of the 3DNG substrate was characteristic of high-quality N-doped graphene (2D/G band intensity ratio, I2D/IG = 1.44) (Figure 2b and Table S1). After the second CVD process, the NiMoNPs@ 1−2, 2−3, 3−5NGL/3DNG retained high-quality N-doped graphene (I2D/IG = 0.95−1.39), which could function as a conductive supporter. Moreover, the ID/IG ratio of 3DNG and NiMoNPs@1−2, 2−3, 3−5NGL/3DNG increased from 0.29 to 0.67, indicating the formation of NGLs with a certain topological defect density on the surface of the deposited NiMo NPs. The chemical states of NiMoNPs@3−5NGL/3DNG were probed by X-ray photoelectron spectroscopy (XPS). The obtained C 1s spectrum (Figure S6a) was characteristic of highquality graphene with a small amount of Mo2C15 (283.5 eV). The Mo 3d spectrum (Figure 2c) could be deconvoluted into dominant Mo0 (228.1 and 231.2 eV)31 and MoNi413 (229.2 and 231.6 eV) peaks with small contributions of Mo oxide13 (232.4 and 235.3 eV) and Mo2C32 (228.5 eV), whereas the Ni 2p spectrum showed signals of Ni0 (852.9 eV) and Ni2+ (856.0 eV) (Figure 2d).33 The N 1s spectrum (Figure S6b) showed pyridinic, graphitic, and oxidized N peaks.34,35 The XPS spectra 1540

DOI: 10.1021/acsenergylett.8b00739 ACS Energy Lett. 2018, 3, 1539−1544

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mV vs a reversible hydrogen electrode (RHE), respectively. The HER activity decreased with increasing NGL number, and the turnover frequencies for NiMoNPs@1−2, 2−3, 3−5NGL/ 3DNG at an electrode potential of −100 mV (vs RHE) were estimated to be 0.31, 0.26, and 0.25 H2 s−1, respectively. To investigate the effects of Mo2C, metal Ni, nitrogen dopant, and MoO2 on HER activity, we studied the catalytic performances of 3DNG-supported Mo2C and Ni NPs encapsulated by 1−2 NGLs (Mo2CNPs@1−2NGL/3DNG and NiNPs@1−2NGL/ 3DNG), 3D pure graphene-supported NiMo NPs encapsulated by 1−2 pure GLs (NiMoNPs@1−2GL/3DG), and MoO2 NPs coated on 3DNG (MoO2NPs/3DNG). As a result, the η10 values of Mo2CNPs@1−2NGL/3DNG, NiNPs@1−2NGL/ 3DNG, NiMoNPs@1−2GL/3DG, and MoO2NPs/3DNG were 40, 30, 27, and 560 mV higher than that of NiMoNPs@1−2NGL/3DNG, respectively (Figures S8−S10). Thus, Mo2C, Ni, and MoO2 did not significantly influence the HER performance of the NiMo-based composites, and the encapsulation by NGLs significantly enhanced the HER activity compared to that by pure GLs. Subsequently, we utilized electrochemical impedance spectroscopy to measure chargetransfer resistance (Rct) at an electrode potential of −200 mV vs RHE (Figure S12), showing that NiMoNPs@1−2, 2−3, 3− 5NGL/3DNG and NiMoNPs@3−5GL/3DG exhibited similar solvent resistances of around 3 Ω. NiMo NPs encapsulated by NGLs showed lower Rct values (6−10 Ω) than those encapsulated by 3−5 pure GLs (25 Ω), indicating that nitrogen doping enhanced the charge-transfer ability, thus accelerating the HER kinetics. For ENNiMoNPs/3DNG, the HER mechanism was determined from the Tafel plots of the polarization curves, with Tafel slope values of 49, 53, and 54 mV per decade observed for NiMoNPs@1−2, 2−3, 3−5NGL/3DNG, respectively (Figure 3b). This demonstrates the occurrence of the Volmer−Heyrovsky process.1,18 The electrochemical surface areas (ECSAs) of the investigated catalysts were determined by measuring their double-layer capacitance (Cdl) at different sweep rates in a faradic current-free region (Figures S13 and S14), which equaled 21.74, 9.99, and 3.32 mF for NiMoNPs@ 1−2, 2−3, 3−5NGL/3DNG, respectively. The ECSAs decreased because the hydrophobic characteristics of the samples, which are strongly related to Cdl, were enhanced with increasing NGL number due to similar geometric surface areas, implying that HER activity was largely determined by the NGL number. The results of the electrochemical tests are summarized in Table S2. We then examined the long-term retention of electrocatalytic activity. Although pristine porous NiMo (without graphene) exhibited outstanding HER performance (Figure 3c), the corresponding current density became very small after 1000 cyclic voltammetry (CV) cycles due to severe catalyst dissolution. In contrast, η10 of NiMoNPs@1−2, 2−3, 3− 5NGL/3DNG increased by 51.7, 23.3, and 5.0% after 1000 CV cycles, respectively. Thus, with increasing NGL number, catalyst acid resistance drastically improved, with the best balance observed for NiMoNPs@3−5NGL. Chronoamperometry measurements at an electrode potential of −200 mV vs RHE demonstrated that NiMoNPs@3−5NGL/3DNG retained 91.3% of its initial current density after 1 day of testing, while the NiMoNPs@1−2, 2−3NGL/3DNG retained only 58.6 and 75.1% (Figure S15), respectively, demonstrating the much higher chemical stability of reported holey graphene-encapsulated bulk porous NiMo alloys26 (supporting discussion,

Figure 2. (a) Ex situ XRD pattern of NiMoNPs@3−5NGL/3DNG. (b) Raman spectra of NiMoNPs@1−2, 2−3, 3−5NGL/3DNG and 3DNG. High-resolution Mo 3d (c) and Ni 2p (d) spectra of NiMoNPs@3−5NL/3DNG.

of NiMoNPs@1−2, 2−3NGL/3DNG were similar to those discussed above (Figure S7). Thus, the ENNiMoNPs/3DNG surface mainly comprised NiMo/MoNi4 alloy NPs with a small amount of Mo2C. The HER performances of the ENNiMoNPs/3DNG samples were investigated in 0.5 M H2SO4 electrolyte using a threeelectrode system, and the polarization curves of the samples with different NGL numbers were compared to those of commercial 10 wt % Pt/C and NP-free 3DNG (Figure 3a). Notably, 3DNG showed the worst HER performance, whereas NiMoNPs@1−2, 2−3, 3−5NGL/3DNG exhibited low onset potentials (10, 28, and 30 mV) and achieved a current density of 10 mA cm−2 at electrode potentials (η10) of 56, 77, and 80

Figure 3. (a) HER polarization curves and (b) Tafel plots of 10 wt % Pt/C, NiMoNPs@1−2, 2−3, 3−5NGL/3DNG, and 3DNG. (c) Polarization curves of pristine porous NiMo and NiMoNPs@1−2, 2−3, 3−5NGL/3DNG before (solid lines) and after (dashed lines) 1000 CV cycles. (d) DFT-calculated Gibbs free energies of H adsorption for NiMoNPs@1, 3, 5NGL, NiMoNPs@1GL, NiMo(100), a pure graphene sheet, and an N-doped graphene sheet. 1541

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ACS Energy Letters Figures S26 and S27). To understand the HER activity− catalyst structure relationship, we elucidated the origin of the protective effect of the NGLs. SEM/TEM images, XRD patterns, and XPS spectra obtained after CV cycling confirmed that the original porous morphology and deposited NPs were preserved without any obvious dissolution and that their structure and chemical states resembled those of NiMo and MoNi4; however, NGLs were slightly oxidized due to surface redox processes during the HER test (Figures S16−S19). Notably, a broken NGL was observed in NiMoNPs@1− 2NGL/3DNG after the durability test, while dissolution of NiMo NPs was not observed in the 3−5 NGL sample. Therefore, 1−2 NGLs were too thin to offer long-term protection. Importantly, graphene protection prevented Ni and Mo oxidation, and the graphene character of NiMoNPs@3− 5NGL/3DNG (ID/IG = 0.67−0.76) was well preserved, in line with the XPS results (Figure S20 and Table S1). Finally, the extent of NiMoNPs@3−5NGL/3DNG leached after 1000 CV cycles was determined to be less than 2 wt % by inductively coupled plasma optical emission spectrometry, confirming that NiMo NPs were well preserved (Table S3) due to the protective effect of NGLs. The influence of the NGL number on the NiMo NP activity/ stability was further investigated by DFT calculations using the PBE+D3(BJ) theory36−38 under periodic boundary conditions, with initial coordinates of NiMo(100) and NiMo(100) encapsulated by 1, 3, or 5 NGLs (Figures S21−S24). The overall HER mechanism in acidic media was thought to involve an initial stage (H+ + e− → H*), an intermediate stage (H* adsorption), and a final stage (generation of 0.5H2).2,39,40 Herein, HER activity was evaluated based on the Gibbs free energies for H* adsorption, |ΔGH*|,39,40 which are summarized in Figure 3d (further computational details are given in Supporting Information). Positive ΔGH* values were observed for a pure GL, pyridinic nitrogen (pN)-doped graphene, and NiMo(100) encapsulated by a single GL, whereas a large negative ΔGH* value was observed for graphitic nitrogen (gN)doped graphene.41 The |ΔGH*| value for NiMo(100) (0.11 eV) was similar to that of a highly efficient Pt catalyst (0.08 eV), indicating that NiMo(100) showed the best HER performance among the studied materials, in agreement with previous data.14 Moreover, models of N-doped graphene with 1, 3, and 5 layers distributed over the NiMo(100) surface exhibited |ΔGH*| values of 0.16, 0.37, and 0.38 eV, respectively, which were comparable to those of Mo-based materials such as MoS2 (−0.36 to +0.39 eV)42 and Mo2C (around −0.80 eV).43 Importantly, as the NGL number increased, |ΔGH*| and its deviation from the above-mentioned Pt value (0.08 eV) increased, in agreement with the experimentally determined order of HER activities (Figure 3a). In addition, the |ΔGH*| of the 3DNG substrate including pN- and gN-doped graphene was much higher (0.68 and 0.54 eV, respectively) than that of the NiMo NP samples, indicating poor HER activity. Establishing the agreement between DFT calculations and experimental data, we investigated the charge distribution of NGLs on the NiMo(100) surface (Figure 4). The data indicated that an NGL shows strong contrast with positively/ negatively charged parts (at most −1.2e and +0.9e on N and C atoms, respectively; Figures 4 and S25 and Table S4). The partial imparity in the positive and negative charge distribution on the NGLs boosted the HER process.44 The presence of N dopants on each NGL improved ΔGH* and simultaneously promoted charge density redistribution around the NGL to

Figure 4. (a) Charge density difference for NiMo(100) encapsulated by five NGLs, with total charges of the first, third, and fifth NGLs on NiMo(100). Yellow and cyan areas represent electron accumulation and depletion. (b) Partial charges of the third NGL in the 5NGL/NiMo(100) model. The box represents the unit cell boundary.

form electron accumulation areas, thus facilitating H* adsorption/desorption during electrochemical hydrogen production. The NiMo substrate influenced the total layer-by-layer charge balance on the NGLs: the first, third, and fifth NGLs had values of −1.48e, +0.033e, and +0.014e, respectively, and electron/hole doping on the NGLs could enhance electrical conductivity/mobility, contributing to HER performance.12 This DFT prediction was confirmed by XPS experimental results (Table S5). The Ni0 and Mo0 peaks of encapsulated NiMoNPs experienced a positive shift in comparison to pristine NiMo alloy (no graphene covering). These peak shifts indicated that the electron transfer from NiMo alloy substrates to encapsulating GLs occurred, resulting in electron accumulations (negatively charging) on NGLs. Indeed, the C 1s sp2 peak of the 1−2 NGL sample tends to show a slight negative shift in comparison to that of the 2−3 NGL and 3−5 NGL samples. These XPS experimental results are in line with our DFT calculations of the first GL being negatively charged. As such, NiMo substrates affected both ΔGH* and the charge balance of the outer NGLs. The superior HER performance and chemical stability of the prepared non-noble metal catalysts in acidic media arise from the relationship between the encapsulating NGLs and the NiMo substrate. DFT calculations suggested that the N-doped graphene/NiMo system showed an improved Gibbs free energy for H* adsorption compared to that for pure N-doped graphene (Figure 3d) due to the strong influence of the NiMo substrate on the encapsulating NGLs, in agreement with the experimental results. This influence became weaker with increasing NGL number and was not obviously observed after the third layer. Moreover, experimental results showed that the NGL number was important for determining the balance between HER activity and chemical stability, i.e., the 1−2 and 2−3 NGL samples demonstrated better HER performance but exhibited severe dissolution, whereas almost no metal leaching was observed for the 3−5 NGL sample with slightly lower HER performances than the others, thus achieving the best balance. The disruption of the total charge balance on the NGLs induced by the NiMo substrate and the bicontinuous porous conductive graphene networks with open pore channels further enhanced electron transport. 1542

DOI: 10.1021/acsenergylett.8b00739 ACS Energy Lett. 2018, 3, 1539−1544

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In summary, we optimized the number of NGLs encapsulating NiMo NPs on 3D porous graphene, achieving an optimal balance between HER activity and chemical stability in acidic electrolytes. Experimental results and DFT calculations suggested that the number of encapsulating NGLs was significant for HER performance, chemical durability of acidsoluble catalysts, and charge distribution. The results showed that 3−5 NGLs afforded excellent balance between HER performance and lifetimes for protection of NiMo NPs at a current density of 100 mA cm−2 for 1 day under acidic media, confirming the remarkable acid resistance ability. The described graphene encapsulation technique provides a new direction for exploiting low-cost, non-noble metal catalysts in acidic electrolytes for efficient hydrogen production.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsenergylett.8b00739. Detailed experimental methods and supplemental figures and data showing typical SEM, TEM, HRTEM, EDS elemental mapping, BF-STEM images, partical size distributions, XRD patterns, polarization curves, cycling durability tests, EIS spectra, CVs, Raman spectra, top and side views of the systems for calculations, stacking patterns, and material morphologies (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.I.). *E-mail: [email protected] (T.O.) ORCID

Kailong Hu: 0000-0003-0489-5836 Yuki Nagata: 0000-0001-9727-6641 Yoshikazu Ito: 0000-0001-8059-8396 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Ms. Kazuyo Omura of the Institute for Material Research (Tohoku University) for her help with the XPS measurements. This work was funded by JST-PRESTO “Creation of Innovative Core Technology for Manufacture and Use of Energy Carriers from Renewable Energy” (JPMJPR1541, JPMJPR1444); JSPS KAKENHI (Grant Nos. JP15H05473, JP23246063, JP15H02195, JP16K17855); JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Discrete Geometric Analysis for Materials Design” (Grant Number JP18H04477); World Premier International Research Center Initiative (WPI), MEXT, Japan; NIMS microstructural characterization platform as a program of the “Nanotechnology Platform” of MEXT, Japan; University of Tsukuba Basic Research Support Program Type S, the Open Facility, Research Facility Center for Science and Technology, University of Tsukuba; and the TEPCO Memorial Foundation. DFT calculations were performed through the use of OCTOPUS at the Cybermedia Center, Osaka University. K.L.H. acknowledges a Japanese government (MONBUKAGAKUSHO: MEXT) scholarship and the Kato Foundation for Promotion of Science (KS-3032). 1543

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ACS Energy Letters

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DOI: 10.1021/acsenergylett.8b00739 ACS Energy Lett. 2018, 3, 1539−1544