Letter Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
pubs.acs.org/journal/ascecg
Chrysanthemum-Like CoP Nanostructures on Vertical Graphene Nanohills as Versatile Electrocatalysts for Water Splitting Linh Truong,†,‡ Sahng-Kyoon Jerng,‡ Sanjib Baran Roy,†,‡ Jae Ho Jeon,†,‡ Kiwoong Kim,§ Kamran Akbar,∥ Yeonjin Yi,§ and Seung-Hyun Chun*,† †
Department of Physics, Sejong University, Neungdong-ro 209, Gwangjin-gu, Seoul 05006, Korea Graphene Research Institute, Sejong University, Neungdong-ro 209, Gwangjin-gu, Seoul 05006, Korea § Institute of Physics and Applied Physics, Yonsei University, Yonsei-ro 50, Seodaemun-gu, Seoul 03722, Korea ∥ Department of Energy Science, Sungkyunkwan University, Seobu-ro 2066, Jangan-gu, Suwon 16419, Korea ‡
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S Supporting Information *
ABSTRACT: CoP is a promising catalyst material to replace noble metals in water electrolysis. To further explore the potential of CoP in hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), we utilize vertical graphene nanohills (VGNHs) that are known to enhance catalytic performances through superaerophobicity. Unique CoP chrysanthemum-like structures are formed on VGNHs through a facile, one-step electrodeposition reaction. Because of the highly conductive VGNH support and the modified CoP nanostructures, the optimized CoP/VGNHs hybrid catalyst exhibits excellent electrocatalytic activities toward HER in 0.5 M H2SO4, such as a low overpotential at 10 mA cm−2 (η10) of 51 mV, a small Tafel slope of 36 mV dec−1, and a long-term stability. Specifically, the overpotential at 100 mA cm−2 (η100) is merely 125 mV, an outstanding performance for a noble metal-free catalyst. Furthermore, the HER performance in 1.0 M KOH (η10 of 93 mV) and the OER performance in the same alkaline medium (η10 of 300 mV) are highly competitive, making CoP/VGNHs also an excellent bifunctional electrocatalyst yielding a current density of 10 mA cm−2 at a low voltage of 1.63 V. This novel nanostructure offers an efficient strategy for the development of nonprecious metal catalysts for water electrolysis. KEYWORDS: CoP, Graphene, Vertical graphene nanohills, Hydrogen evolution reaction, Oxygen evolution reaction, Electrodeposition
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electrocatalytic performance.15,17−19 For instance, MoS2 nanosheets/carbon nanotubes,20 MoS2 nanoparticles/reduced graphene oxide (RGO),17 and MoS2 nanosheets on a graphene substrate21 required much lower overpotentials at high current densities as compared with pure MoS2. Furthermore, significant improvements were reported when vertical graphene nanohills (VGNHs) were used as ideal supports of nanoscale materials because VGNHs could provide highly active edges, robust anchor sites, and large amounts of gaps for electrolyte infiltration even though VGNHs by themselves had very poor catalytic performances.22−24 In fact, the vertical graphene, grown by plasma-enhanced chemical vapor deposition (PECVD), has been shown to possess a superaerophobic property which significantly enhances the electrocatalytic performance in various applications.25 TMPs are being considered as representative electrocatalysts in hydrogen evolution research, free from noble metals. However, the majority of the TMP synthesis approaches require complicated steps, long reaction times, expensive systems, low efficiency with toxic chemical reagents,1 and high
INTRODUCTION The energy and environmental crisis is driving the urgent demand for exploring efficient and clean energy sources. Hydrogen (H2) is an ideal solution due to its high energy density and ecofriendly combustion production.1−3 Water electrolysis is a simple approach to produce H2 via the hydrogen evolution reaction (HER), which requires an efficient catalyst for sufficient HER to occur at a low overpotential. Pt-based groups have been considered as important electrocatalysts for hydrogen production despite their scarcity and high cost.4 The development of earthabundant, economical electrocatalysts with efficient HER performance is essential to drive the water splitting reaction as an alternative energy source.1,5 To replace Pt, tremendous attempts have been made with transition metals6,7 and their phosphides (TMPs), 8−10 chalcogenides,11,12 or carbides.13,14 The developments of these HER catalysts, however, are usually limited due to low efficiency,15 and efforts have been made to improve HER activity by modifying their structural morphology and/or electrical conductivity.15,16 Because of the unique physical and chemical properties such as excellent conductivity, large surface area, and good chemical stability, carbon materials have received great attention as a potential support to enhance © XXXX American Chemical Society
Received: December 11, 2018 Revised: January 28, 2019 Published: February 11, 2019 A
DOI: 10.1021/acssuschemeng.8b06508 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 1. (a) Schematic illustration of the fabrication process for CoP/VGNHs/CC hybrids. (b) Low- and (c) high-magnification SEM images of CoP/VGNHs/CC. (d), (e), and (f) EDS elemental mapping images of Co, P, and C for CoP/VGNHs/CC, respectively.
temperature and/or pressure systems with flammable phosphorus sources.9 Electrodeposition is a straightforward, fast, low-cost method to fabricate nanostructures.5,26 While there have been several reports on electrodeposition methods of CoP5,27 as well as combining CoP and graphene,15,28−30 the catalytic activity toward HER remained insufficient, stimulating the development of a scalable strategy with vertical graphene. In this report, we developed CoP chrysanthemum-like structures supported by VGNHs on a flexible, conductive carbon cloth (CC). This novel CoP/VGNHs/CC hybrid catalyst, fabricated in optimized CoP electrodeposition conditions, exhibited excellent HER activities both in acidic and alkaline media. In fact, the overpotential at 100 mA cm−2 in 0.5 M H2SO4 was 125 mV, a performance superior to other noble metal-free catalysts. Furthermore, this CoP/VGNHs/ CC composite possessed an outstanding activity toward oxygen evolution reaction (OER) in 1.0 M KOH with an overpotential of merely 300 mV at 10 mA cm−2. The Tafel slope (61 mV dec−1) was smaller than that of IrO2. In turn, water splitting with the hybrid catalyst as both cathode and anode in alkaline medium required a low voltage of 1.63 V for a current density of 10 mA cm−2, a highly competitive performance for a binder-free, robust electrocatalyst fabricated by scalable methods and free from noble metals.
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cm) were used as counter, reference, and working electrode, respectively. The deposition solution consisted of CoSO4 (50 mM), NaH2PO2 (0.5 M), and CH3COONa (0.1 M) with deionized water. After the electrolyte was stirred at 500 rpm, the electrodeposition process with a current density of −50 mA cm−2 was followed for 10 min at room temperature. After that, the products were removed from the deposition bath and rinsed carefully with deionized water. Electrochemical Measurement. A Biologic SP-300 workstation, containing a three-electrode setup, was used to conduct all electrochemical measurements. The as-prepared samples (CoP/ VGNHs/CC, CoP/CC, CC, and standards) and a graphite rod were directly used as the working and the counter electrode, respectively. A 0.5 M H2SO4 solution with an Ag/AgCl (Sat. KCl) reference electrode or 1.0 M KOH with a Hg/HgO (Sat. NaOH) reference electrode was used for HER and OER measurements in acidic or alkaline medium. The 1.0 M KOH electrolyte was purged with oxygen for 20 min before each measurement of oxygen evolution. All reported potentials were referred to reversible hydrogen electrode (RHE) scale according to the equations: E (vs RHE) = E (vs Ag/AgCl) + E0(Ag/AgCl) + 0.0592 × pH for acidic electrolyte and E (vs RHE) = E (vs Hg/HgO) + E0(Hg/HgO) + 0.0592 × pH for alkaline electrolyte. All electrocatalytic behaviors were evaluated by linear sweep voltammetry (LSV) in 0.5 M H2SO4 and 1.0 M KOH solution at a scan rate of 10 mV s−1.31 The Tafel plots were converted from the polarization curves, replotted as the overpotential (η) vs log current density (log j). The linear portion of the Tafel plot was fitted to the Tafel equation η = a + b log(j) to obtain the Tafel slope value (b). Electrochemical impedance spectroscopy (EIS) was tested at frequency range of 1 Hz−1 MHz with an applied voltage of 400 mV overpotential.
EXPERIMENTAL SECTION
Material. Commercial CC (nominal thickness of 0.356 mm) was purchased from the Fuel Cell Store. Co(II) sulfate heptahydrate (CoSO4·7H2O, 99.999%), sodium hypophosphite monohydrate (NaH 2 PO 2 ·H 2 O, 97+%), and sodium acetate anhydrous (NaOOCCH3, 99.0%) were purchased from Alfar Aesar, Thermo Fisher Scientific, Inc. Preparation of VGNHs/CC. CC (6 cm × 6 cm) was ultrasonically cleaned with acetone, deionized water, and ethanol for 10 min, respectively. The cleaned CC substrate was placed into a PECVD system for the growth of VGNHs. After the temperature of the CC substrate reached 750 °C, hydrogen plasma (20 sccm) was then generated at a plasma power of 50 W for 2 min to eliminate the organic pollutants on the surface of CC. Then, the growth of VGNHs was continued for 45 min with a gas mixture of hydrogen (20 sccm) and methane (10 sccm). Details can be found in our previous reports.25 Preparation of CoP/VGNHs/CC. The electrodeposition of CoP nanostructures on VGNHs/CC was carried out by using a standard three-electrode system with a Biologic SP-300 workstation. A graphite rod, an Ag/AgCl (Sat. KCl), and a part of VGNHs/CC (1 cm × 1
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RESULTS AND DISCUSSION The schematic of the hybrid catalyst is illustrated in Figure 1a. The details of experimental procedures and analysis conditions are presented in the Supporting Information. We chose VGNHs because they could be grown on almost any substrate withstanding the growth temperature and could serve as binder-free supports for other catalysts, enhancing the catalytic performance because of the high conductivity and the superaerophobic property.25 Indeed, the CC substrate was fully covered with nanoscale VGNHs fabricated in typical growth conditions (Figure S1). In this study, based on the HER performance in acidic electrolyte, a deposition current density of −50 mA cm−2 for CoP/VGNHs/CC was selected for detailed study; in the following, it is referred to as CoP/ VGNHs/CC. For comparison, CoP/CC was prepared by a B
DOI: 10.1021/acssuschemeng.8b06508 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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ACS Sustainable Chemistry & Engineering
Figure 2. Electrochemical HER activity of CoP/VGNHs/CC: (a) LSV curves, (b) Tafel slopes in 0.5 M H2SO4, (c) LSV curves, and (d) Tafel slopes in 1.0 M KOH.
Figure 3. (a) LSV curves and (b) Tafel slopes for the electrochemical OER of CoP/VGNHs/CC in 1.0 M KOH. (c) Bifunctional water electrolysis tested in 1.0 M KOH. (d) Long-term stability at a constant cell voltage of 1.63 V for 24 h using two CoP/VGNHs/CC electrodes.
deposition on microscopically irregular VGNHs/CC resulted in completely different nanostructures. Figure 1b and c show the low- and high-magnification scanning electron microscopy (SEM) images of CoP flower-like structures grown on VGNHs/CC. The energy dispersive spectroscopy (EDS) elemental mapping analysis demonstrated that CoP flowerlike structures were densely distributed on the VG matrix
single-step electrodeposition reaction with conditions similar to those for CoP/VGNHs/CC. An important observation we noticed was the different growth mode of electrodeposited CoP when the template surface morphology was changed. While the electrodeposition of CoP on microscopically flat CC yielded nanoparticles of typical diameters around 100 nm (Figure S1b), the same C
DOI: 10.1021/acssuschemeng.8b06508 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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and recently discovered superaerophobicity.22−25 These advantages were usually combined to provide fast kinetics. We used electrochemical impedance spectroscopy (EIS) to investigate HER kinetics and interface reactions between the electrode and the electrolyte. The charge-transfer resistance (Rct), determined by the diameter of the semicircles at the high frequencies region, was related to the charge-transfer ability of the electrode at the interface.2 The Rct values of CoP/ VGNHs/CC, CoP/CC, and bare CC were recorded by the Nyquist plots at 400 mV (Figure S7). In acidic conditions, the smallest Rct of only ∼1.2 Ω was derived from CoP/VGNHs/ CC, implying a really fast charge-transfer kinetics. The dramatic reduction of Rct could be ascribed to the high electrical conductivity and the huge surface area of VGNHs, resulting in excellent HER activity. Indeed, the corresponding electrochemical active surface areas (ECSAs) were estimated to be 613 and 376 cm2 for CoP/VGNHs/CC and CoP/CC, respectively (Figure S8). A peculiar feature of electrodeposited CoP on VGNHs was the diverse nanostructures depending on the current density during the electrodeposition. Figure S9 shows the SEM images of CoP/VGNHs/CC synthesized at different electrodeposition conditions. At the lowest current density, CoP grew as nanoparticles with open edges. As we increased the current density, the nanoparticles seemed to collapse to form chrysanthemum-like structures. The density of the chrysanthemum-like structure increased on further increase in the current density. However, beyond −80 mA cm−2, CoP formed dense nanowalls, and the underlying VGNHs were invisible. To relate the CoP growth mode with the catalytic performance, the CoP/VGNHs/CC catalysts with different electrodeposition current densities were tested in acidic electrolyte for HER activity. As shown in Figure S10, the HER activity and the charge-transfer characteristics depended strongly on the electrodeposition condition. The best performance was obtained for the current density of −50 mA cm−2, which yielded dense chrysanthemum-like structures (the condition used for Figures 1−3 and Figures S1−S8). When the coverage of the CoP nanostructure was excessive (Figure S9d and e), the Rct increased significantly, and in turn, the HER activity was degraded. Thus, for the best performance, the VGNHs should be exposed to the electrolyte so that a fast bubble release and charge transfer could occur. So far, VGNHs have been combined with two-dimensional transition metal dichalcogenide (TMD) materials such as MoS2, MoSe2, or WS2 to fully exploit the merit of layered nanostructures.22−25 In general, enhanced catalytic performances were observed and ascribed to the high density of active sites, fast charge transfer, improved bubble release, and physical robustness. Since the TMDs investigated were semiconducting, the high conductivity of VGNHs seemed critical in the enhancement. In contrast, TMPs are metallic, and the use of VGNHs does not guarantee any improvement in this case. Our results showed that even the metallic CoP could be assisted by VGNHs for significantly enhanced catalytic activity. We believe that, in addition to the general advantages of VGNHs such as superaerophobicity and robust anchoring, the flower-like CoP structure enhanced the exposure of edges to hydrogen ions.
(Figure 1d−f). The X-ray diffraction (XRD) measurements (Figure S2) did not show crystalline peaks of CoP, indicating the formation of amorphous CoP with some Co inclusion in accord with previous reports.5 High-resolution transmission electron microscopy (HRTEM) supported the amorphousness of CoP (Figure S3). The X-ray photoelectron spectroscopy (XPS) results also confirmed the formation of CoP (Figure S4). The increase in oxygen-related components in the C 1s spectra, compared to as-grown VGNHs, was due to the solution-based electrodeposition to form CoP. The Co 2p and P 2p spectra were also affected by oxygen bonding (Co-O at 781.6 eV, orthophosphate at 132.4 and 133.9 eV), common features observed in electrodeposited CoP films.32 The shifts of binding energy positions due to the charge transfer from Co to P were also observed.4,29,33 The nanostructures were spheres with many edges, resembling chrysanthemum flowers. In fact, the chrysanthemum-like structures were closely related to the catalytic activities as we show later. To evaluate the influence of VGNHs on the catalyst activity of CoP, the linear sweep voltammetry (LSV) polarization measurements of bare CC, VGNHs/CC, CoP/CC, and CoP/ VGNHs/CC were compared in a typical three-electrode system with 0.5 M H2SO4 and 1.0 M KOH electrolytes (also included were the LSV of CC and Pt/C for comparison). Figure 2a−d show that CoP nanoparticles on CC were indeed good HER catalysts in both acidic and alkaline conditions, as had been reported already.29,33,34 Our new finding is the significant enhancement of catalytic activity when VGNHs were used as supports for CoP despite the noncatalytic behavior of VGNHs/CC. The overpotential at a current density of 10 mA cm−2(η10) was reduced from 176 mV (141 mV) for CoP/CC to 51 mV (93 mV) for CoP/VGNHs/CC in acidic (alkaline) media, and the Tafel slope was also reduced (36 mV dec−1) and close to that of Pt/C in acidic electrolyte (30 mV dec−1). In consequence, our hybrid catalyst was more competitive at higher current densities: the overpotential at 100 mA cm−2 (η100) was merely 125 mV, among the smallest values for noble metal-free catalysts (Table S1). The use of VGNHs was also very effective for OER, the rest of the half reaction of water splitting process. As shown in Figure 3a and b, CoP/VGNHs/CC showed an excellent OER activity in a 1.0 M KOH electrolyte with an overpotential of 300 mV at a current density of 10 mA cm−2. The Tafel slope was 61 mV dec−1, even lower than that of the standard OER electrode, IrO2. From the comparisons to the LSV results of CoP/CC, it was clear that VGNHs enhanced the catalytic activity of CoP in HER and OER. The robustness of CoP/ VGNHs/CC was another merit for stable operation. Indeed, the hybrid catalyst showed an excellent long-term stability, especially in alkaline media (Figure S5). The next step was to investigate the bifunctional performance for overall water splitting in a 1.0 M KOH electrolyte, using the hybrid catalyst for the cathode as well as for the anode. Because of the low HER and OER overpotentials in alkaline media, the required cell voltage to drive a current density of 10 mA cm−2 was just 1.63 V (Figure 3c). More importantly, the bifunctional reaction continued with a high stability (Figure 3d), as designed by using chemically and structurally stable VGNHs. We note that the ratio of Co to P reached 1:1 after catalytic reactions (Figure S6). The role of VGNHs in the enhancement of catalytic activity could be diverse, as VGNHs possess a variety of characteristics, such as high conductivity, robustness, multiple active edges,
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CONCLUSION In summary, CoP chrysanthemum-like structures were successfully deposited on VGNHs via a facile electrodeposition D
DOI: 10.1021/acssuschemeng.8b06508 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
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(4) Jiang, P.; Liu, Q.; Ge, C.; Cui, W.; Pu, Z.; Asiri, A. M.; Sun, X. CoP Nanostructures with Different Morphologies: Synthesis, Characterization and a Study of Their Electrocatalytic Performance toward the Hydrogen Evolution Reaction. J. Mater. Chem. A 2014, 2 (35), 14634−14640. (5) Bai, N.; Li, Q.; Mao, D.; Li, D.; Dong, H. One-Step Electrodeposition of Co/CoP Film on Ni Foam for Efficient Hydrogen Evolution in Alkaline Solution. ACS Appl. Mater. Interfaces 2016, 8 (43), 29400−29407. (6) McKone, J. R.; Sadtler, B. F.; Werlang, C. A.; Lewis, N. S.; Gray, H. B. Ni-Mo Nanopowders for Efficient Electrochemical Hydrogen Evolution. ACS Catal. 2013, 3 (2), 166−169. (7) Evans, D. J.; Pickett, C. J. Chemistry and the Hydrogenases. Chem. Soc. Rev. 2003, 32 (5), 268−275. (8) Kibsgaard, J.; Tsai, C.; Chan, K.; Benck, J. D.; Nørskov, J. K.; Abild-Pedersen, F.; Jaramillo, T. F. Designing an Improved Transition Metal Phosphide Catalyst for Hydrogen Evolution Using Experimental and Theoretical Trends. Energy Environ. Sci. 2015, 8 (10), 3022−3029. (9) Shi, Y.; Zhang, B. Recent Advances in Transition Metal Phosphide Nanomaterials: Synthesis and Applications in Hydrogen Evolution Reaction. Chem. Soc. Rev. 2016, 45 (6), 1529−1541. (10) Zhou, Z.; Mahmood, N.; Zhang, Y.; Pan, L.; Wang, L.; Zhang, X.; Zou, J.-J. CoP Nanoparticles Embedded in P and N Co-Doped Carbon as Efficient Bifunctional Electrocatalyst for Water Splitting. J. Energy Chem. 2017, 26 (6), 1223−1230. (11) Kong, D.; Cha, J. J.; Wang, H.; Lee, H. R.; Cui, Y. First-Row Transition Metal Dichalcogenide Catalysts for Hydrogen Evolution Reaction. Energy Environ. Sci. 2013, 6 (12), 3553−3558. (12) Zhang, J.; Liu, S.; Liang, H.; Dong, R.; Feng, X. Hierarchical Transition-Metal Dichalcogenide Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution. Adv. Mater. 2015, 27 (45), 7426−7431. (13) Vrubel, H.; Hu, X. Molybdenum Boride and Carbide Catalyze Hydrogen Evolution in Both Acidic and Basic Solutions. Angew. Chem., Int. Ed. 2012, 51 (51), 12703−12706. (14) Š ljukić, B.; Vujković, M.; Amaral, L.; Santos, D. M. F.; Rocha, R. P.; Sequeira, C. A. C.; Figueiredo, J. L. Carbon-Supported Mo2C Electrocatalysts for Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3 (30), 15505−15512. (15) Ma, L.; Shen, X.; Zhou, H.; Zhu, G.; Ji, Z.; Chen, K. CoP Nanoparticles Deposited on Reduced Graphene Oxide Sheets as an Active Electrocatalyst for the Hydrogen Evolution Reaction. J. Mater. Chem. A 2015, 3 (10), 5337−5343. (16) Zou, X.; Zhang, Y. Noble Metal-Free Hydrogen Evolution Catalysts for Water Splitting. Chem. Soc. Rev. 2015, 44 (15), 5148− 5180. (17) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133 (19), 7296−7299. (18) Pan, Y.; Lin, Y.; Liu, Y.; Liu, C. A Novel CoP/MoS2-CNTs Hybrid Catalyst with Pt-like Activity for Hydrogen Evolution. Catal. Sci. Technol. 2016, 6 (6), 1611−1615. (19) Tahir, M.; Pan, L.; Idrees, F.; Zhang, X.; Wang, L.; Zou, J.; Wang, Z. L. Electrocatalytic Oxygen Evolution Reaction for Energy Conversion and Storage: A Comprehensive Review. Nano Energy 2017, 37, 136−157. (20) Dai, X.; Du, K.; Li, Z.; Sun, H.; Yang, Y.; Zhang, W.; Zhang, X. Enhanced Hydrogen Evolution Reaction on Few−layer MoS2 Nanosheets−coated Functionalized Carbon Nanotubes. Int. J. Hydrogen Energy 2015, 40 (29), 8877−8888. (21) Zhang, X.; Zhang, Q.; Sun, Y.; Zhang, P.; Gao, X.; Zhang, W.; Guo, J. MoS2-Graphene Hybrid Nanosheets Constructed 3D Architectures with Improved Electrochemical Performance for Lithium-Ion Batteries and Hydrogen Evolution. Electrochim. Acta 2016, 189, 224−230. (22) Wang, Y.; Chen, B.; Seo, D. H.; Han, Z. J.; Wong, J. I.; Ostrikov, K.; Zhang, H.; Yang, H. Y. MoS2-Coated Vertical Graphene
method. The optimized morphology and density of CoP material warranted the excellent HER performances such as low overpotentials, small Tafel slopes, and long-term stability in both acidic and alkaline media. Moreover, possessing a good OER activity, the CoP/VGNHs/CC was employed as an outstanding bifunctional catalyst with a competitive value (10 mA cm−2 at a cell voltage of 1.63 V) over CoP families for overall water splitting in alkaline solution. The successful combination of CoP and VGNHs, both fabricated by scalable methods, could guide a practical solution in the search of water electrolysis catalysts free from noble metals.
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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/acssuschemeng.8b06508.
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Detailed information on experimental produces for synthesis processes of VGNHs/CC and CoP/VGNHs/ CC and details of electrochemical measurements. SEM images for surface-dependent morphology variations, XRD patterns, HRTEM of CoP regions, XPS spectra of as-grown CoP, stability of CoP/VGNHs/CC for HER/ OER, EDS after HER reactions, electrochemical impedance spectroscopy measurements, cyclic voltammograms of CoP/VGNHs/CC and CoP/CC, morphology and electrocatalytic activity of CoP formed at different electrodeposition current densities, and comparisons of our work with the reported results. (PDF)
AUTHOR INFORMATION
Corresponding Author
*Fax: +82 2 3408 4316. E-mail:
[email protected] (S.-H. Chun). ORCID
Sahng-Kyoon Jerng: 0000-0001-6965-0309 Yeonjin Yi: 0000-0003-4944-8319 Seung-Hyun Chun: 0000-0001-8397-4481 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Research Foundation of Korea (NRF) funded by the Ministry of Science and ICT and Ministry of Education (2010-0020207, 2016R1E1A1A01942649, 2018R1A5A6075964, 2018K1A4A3A01064272, and 2017R1A2B4002442).
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DOI: 10.1021/acssuschemeng.8b06508 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX
