Article pubs.acs.org/cm
Vertically Aligned Interlayer Expanded MoS2 Nanosheets on a Carbon Support for Hydrogen Evolution Electrocatalysis Manjunath Chatti,† Thomas Gengenbach,§ Russell King,‡ Leone Spiccia,†,⊥ and Alexandr N. Simonov*,† †
School of Chemistry, ARC Centre of Excellence for Electromaterials Science and ‡Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia § Commonwealth Scientific and Industrial Research Organization Manufacturing Flagship, Clayton, Victoria 3168, Australia S Supporting Information *
ABSTRACT: This work describes the facile microwave synthesis of interlayer expanded, nanosized MoS2 sheets that are vertically aligned on a well-conducting reduced graphene (rGO) support, as confirmed by X-ray diffraction, Raman and X-ray photoelectron spectroscopy, scanning electron microscopy with energy dispersive X-ray analysis, and high-resolution transmission electron microscopy. Such structure has been predicted to be highly favorable for efficient electrocatalysis of hydrogen evolution by MoS2 but could not be achieved until now. Films deposited from the microwave-synthesized MoS2-rGO composites demonstrate outstanding and stable hydrogen evolution performance in acidic solution. These catalysts exhibit an exchange current density as high as 1.0 ± 0.2 A g−1MoS2‑rGO, sustain a current density of 10 mA cm−2 (36 A g−1MoS2‑rGO) at an overvoltage of 0.104 ± 0.002 V, and maintain steady performance for many hours. Importantly, our simple synthesis affords several advantages over more sophisticated methods used previously to prepare MoS2 catalysts.
■
electrocatalyst for the HER.7 This discovery emanated from seminal theoretical studies, which were confirmed experimentally, and stimulated research on similar catalytic systems.8−11 It led to other metal chalcogenides as well as phosphides, nitrides, and carbides with reasonable activity and stability during H2 evolution in acidic solutions.9−11 Nevertheless, MoS2-based materials remain among the most efficient non-noble cathode catalysts for water electrolysis.8 As is generally the case for heterogeneous catalysis, the structure and performance of H2 evolution electrocatalysts are closely linked, since different structural motifs often exhibit distinct adsorption and catalytic properties. For MoS2, a typical layered 2D material, catalysis is believed to mainly involve sulfur atoms on the edges of the basal planes, whereas the surface of these planes is essentially inactive.7,12 Consequently, efforts have been made to prepare ultrathin MoS2 sheets with the highest possible density of active edge sites. This requirement and the lower electrical conductivity through the basal planes, as compared to the edges, have led to the idea of fabricating very small, vertically aligned MoS2 sheets on conductive supports. Several groups have pursued this strategy and achieved significant enhancements in electrocatalytic performance compared to stacked, “thick” MoS2 catalysts.13−21 21 It is noted, however, that creation of vertically aligned catalysts where nm-sized chalcogenide sheets are homoge-
INTRODUCTION Sooner or later, dihydrogen is expected to supersede carbonbased fossil fuels as the world’s major energy carrier due to the nonrenewability (on the humanity’s time scale) and anticipated shortage of the latter.1 Although there are no widely available natural resources of H2, it can be generated in an environmentally friendly manner via electrolysis of water using renewable hydro-, wind-, or photovoltaic power.2 To satisfy global demands, such electrolyzers should function at very high current densities of up to several A cm−2, necessitating the use of highly conductive electrolytes to minimize ohmic losses. Strongly acidic and alkaline aqueous solutions meet this requirement better than others owing to very high mobility of the hydronium and hydroxyl ions, but both have advantages and disadvantages. Alkaline conditions enable use of non-noble, more abundant cathode and anode materials3,4 that are unstable at very low pH, while acidic medium allows higher conductivity and faster electrocatalytic H2 evolution rates.5 Therefore, significant efforts are being invested in the development of non-noble metal electrocatalysts for the water oxidation (anode process) and hydrogen evolution reaction (HER; cathode process), which are highly active and stable in strongly acidic solutions. This work focuses on cathode HER catalysts that meet these criteria. Until recently, there have been no materials that are comparable in the activity and stability to the Pt-based electrocatalysts under the acidic conditions used for H2 evolution.6 A significant breakthrough was achieved when MoS2 was introduced as a promising and highly stable © 2017 American Chemical Society
Received: January 10, 2017 Revised: February 21, 2017 Published: February 22, 2017 3092
DOI: 10.1021/acs.chemmater.7b00114 Chem. Mater. 2017, 29, 3092−3099
Article
Chemistry of Materials
7500 rpm for 15 min, washed several times with water and ethanol, and dried in an oven at 60 °C. Hydrothermal Synthesis of MoS2/rGO. “MoS2/N-rGO” was synthesized via a solvothermal method as reported in ref 30. Briefly, GO (10 wt %) and phosphomolybdic acid (H3PMo12O40·nH2O, 0.11 mmol) were dissolved in 40 mL of distilled water by sonication for 30 min. Then thiourea (5.26 mmol) was added, and the solution was transferred to a Teflon lined autoclave that was tightly sealed. Synthesis was undertaken at 180 °C for 24 h, and the autoclave was allowed to cool down to room temperature naturally. The final product was collected by centrifugation, washed with water, and dried at 60 °C overnight. Physical Characterization. X-ray diffraction (XRD) measurements were undertaken using a D8 Advance BRUKER diffractometer with Cu−Kα source operating at 40 kV and 25 mA at a scan rate of 1° min−1. Raman spectra were collected using a Renishaw Raman microscope equipped with a 514 nm excitation laser. X-ray photoelectron spectroscopy (XPS) analysis was performed using an AXIS Nova spectrometer (Kratos Analytical Inc., Manchester, UK) with a monochromated Al Kα source at a power of 180 W (15 kV × 12 mA) and a hemispherical analyzer operating in the fixed analyzer transmission mode. The total pressure in the main vacuum chamber during analysis was approximately 10−8 mbar. Samples were filled into shallow wells of a custom-made sample holder and analyzed at an emission angle of 0° as measured from the surface normal. As the actual emission angle is ill-defined in the case of powders (ranging from 0−90°), the sampling depth may range from 0−10 nm. Binding energies were referenced to the C 1s peak at 284.8 eV for aliphatic hydrocarbon. The accuracy associated with quantitative XPS is about 10−15%. Precision depends on the signal/noise ratio but is usually much better than 5%. The latter is relevant when comparing similar samples. Scanning electron micrographs (SEM) were recorded on a FEI Magellan-FEG instrument. Samples were prepared by drop-casting a diluted suspension of a material in ethanol on a Si wafer glued (super glue) and electrically connected (Cu tape) to an SEM stub. Transmission electron micrographs (TEM) were taken using a Tecnai T20 microscope with a 200 keV electron source. Samples were dropcasted from ethanol dispersions on a holey carbon coated 300 mesh Cu grids and dried in air. Electrochemical Experiments. Electrochemical measurements were carried out at ambient temperature (24 ± 1 °C) in a three electrode configuration using a Bio-Logic VSP potentiostat. As an electrolyte solution, 0.5 M H2SO4 (pH = 0.30) saturated with high purity Ar gas was used for all experiments. Throughout the measurements, the headspace of the cell was continuously purged with Ar at a slow flow rate. The solution was intensively agitated with a magnetic stirrer (1200 rmp) during all electrocatalytic tests. The auxiliary electrode compartment was separated from the working electrode chamber with two low porosity glass frits (P4, average pore size 10−16 μm). High-surface area Pt wire was used as an auxiliary electrode to facilitate measurements at high currents, but many control experiments including the long-term stability tests of the most active materials were additionally undertaken using a high-surface area Ti or Au wire to confirm that no contamination of the working electrode with platinum metal occurred during measurements. Current densities achieved with all three types of auxiliary electrodes were the same and were not enhanced when H2 evolution was measured at 20 mA cm−2 current density for at least 20 h. The reference electrode was double junction Ag|AgCl|KCl(sat.), but all potentials are reported either versus reversible hydrogen electrode (RHE) or are discussed in terms of the HER overpotential (η). Ag|AgCl reference was calibrated versus the custom-made RHE filled with 0.5 M H2SO4, and potentials were corrected according to the relationship: E versus RHE = −η = E versus Ag|AgCl|KCl(sat.) + 0.218, which is very close to the predictions of the Nernst equation (E vs RHE = E vs Ag|AgCl|KCl(sat.) + 0.197 + 0.059 pH). An uncompensated resistance (Ru) of 12−15 Ω was derived from the electrochemical impedance spectra measured at 0.2 V versus RHE and
neously distributed over the whole support surface with minimal degree of agglomeration and horizontal alignment has been achieved very rarely. Another way to improve the performance of MoS2 is to expand the interlayer spacing via the intercalation of cations,22−24 which is believed to improve energetics of the H adsorption on the catalyst and thus facilitate kinetics of the reaction.25 On the down side, the synthesis of vertically aligned, and even interlayer expanded, MoS2 sheets with high edge densities has required sophisticated and/or resource intensive techniques such as chemical vapor deposition (CVD) and solvothermal synthesis (Table S1 in Supporting Information). Such synthetic strategies are often time-consuming, highly sensitive to O2, and involve the use of toxic and corrosive gases. Recently, microwave-assisted synthesis was reported to produce MoS2 materials with acceptable catalytic properties for H2 evolution.22,26 This widely applied environmentally friendly and costeffective method accelerates reaction rates via uniform heating resulting from dipolar polarization and ionic conduction.26 Application of this method afforded MoS2 with expanded interlayers (up to 9.4 Å, cf., 6.3 Å for pristine MoS2) and high HER electrocatalytic activity.22 To our knowledge, approaches capable of synergistically enhancing the H2 evolution activity of MoS2 by creating vertically aligned thin sheets and simultaneously enhancing the interlayer spacing have not been reported so far. Herein, we demonstrate, for the first time, the advantages of this strategy. We employ facile microwave synthesis to fabricate thin interlayer expanded MoS2 sheets vertically aligned on reduced graphene oxide (rGO) supports. There is much interest in this type of composite materials for different applications27,28 including the HER catalysis.
■
EXPERIMENTAL SECTION
Materials. All reagents were of analytical grade and were used as received from suppliers (Sigma-Aldrich, Alfa Aesar, and Merck) without any further purification. 20 wt.% Pt catalyst supported on XC72 graphitized carbon was also purchased from Sigma-Aldrich and contained platinum particles with the mean linear diameter of
