Highly Active Catalyst of Two-Dimensional CoS2/Graphene Nanocomposites for Hydrogen Evolution Reaction
© Xing et al. 2015
Received: 15 November 2015
Accepted: 12 December 2015
Published: 21 December 2015
Hydrogen evolution reaction (HER) by electrochemical water splitting using new promising non-precious metal catalysts shows great potential for clean energy technology. The design and fabrication of a high-performance electrode material based on cobalt disulfide/reduced graphene oxide (CoS2/RGO) nanocomposites is reported by a one-step hydrothermal method. Benefiting from its structural advantages, namely, large amount of exposed surface, fast charge transfer, and synergistic effect between CoS2 and RGO, the as-prepared nanocomposites are exploited as a catalyst for the HER. The results indicate that CoS2/RGO-5 % exhibits the best performance of hydrogen evolution and the smallest overpotential of 159 mV to achieve a 15 mA cm−2 current density, possessing the easiest releasing of hydrogen gas and the highest charge transfer rate, as well as remarkable stability.
Hydrogen, as a new type of renewable energy source, has attracted extensive concern due to its potential application in powering vehicles or electric devices. A promising method to produce hydrogen is electrocatalytic reduction of water via the hydrogen evolution reaction (HER) [1–3]. The platinum catalysts exhibit extraordinary activity in catalyzing the hydrogen evolution, but the high cost and scarcity seriously impede their practical applications. These limitations prompt the intensive investigations on inexpensive and earth-abundant electrocatalysts, e.g., metal sulfide , carbide , boride , and phosphide . Many efforts have been taken to synthesize inorganic metal sulfur complexes to create analogs to these active materials. Particularly, MoS2 and WS2 have been researched as HER catalysts for their lower cost and higher stability than other metal derivatives [8–10]. On the other hand, carbon materials with excellent electrical conductivity, such as carbon nanotube and active carbons, were generally applied as supports of HER catalysts to enhance their stability and electrical conductivity . Till now, only few reports were devoted to the study of 2D CoS2/graphene catalysts, which can expand the category of catalytic materials fitting for efficient HER .
Herein, we report a novel strategy to synthesize 2D cobalt disulfide/reduced graphene oxide (CoS2/RGO) nanocomposites with high HER activity by a hydrothermal method. In these nanocomposites, RGO serves as a matrix for the uniform growth of CoS2 nanoclusters. The presence of RGO facilitates both electrical conductivity and ionic transportation during the HER. Besides, the homogeneous dispersion of CoS2 nanoclusters on the surface of RGO could also incredibly increases the catalytic active sites . Furthermore, the interaction between graphene and CoS2 can also inhibit the aggregation of CoS2 nanoclusters, resulting in improved cycling stability . Benefiting from their high electrical conductivity, opened pore structure, excellent dispersion of CoS2, and positive synergistic effect between CoS2 and RGO, the CoS2/RGO nanocomposites exhibit excellent performance for HER process.
Natural graphite was provided by Qingdao Ruisheng Graphite Company. Cobalt(II) acetate tetrahydrate (Co(Ac)2·4H2O), thiourea (H2NCSNH2), ethyl alcohol, hydrogen peroxide (H2O2), potassium permanganate (KMnO4), concentrated sulphuric acid (H2SO4), and sodium nitrate (NaNO3) were purchased from Sinopharm Chemical Reagent Company and used without further purification. Nafion (5 wt%) water solution was purchased from Shanghai Yibang Technology Co., Ltd. Deionized water (DI) was prepared by a Millipore pure water system.
GO Preparation and Purification
GO was synthesized by using modified Hummer’s method. In a typical synthesis, 1 g of graphite, 1 g of NaNO3, and 46 mL of H2SO4 were firstly mixed together in ice bath. Afterwards, 6 g of KMnO4 was added, and the suspension was heated to 35 °C. After vigorous stirring for 60 min, 80 mL of deionized water was added, and the temperature of solution was heated to 95 °C. Finally, 200 mL of water and 6 mL of H2O2 (30 wt%) was dropped into the solution, turning the color from dark brown to light yellow. Afterwards, the solution was filtered and washed thoroughly with deionized water. The filter cake was redispersed in water by strong mechanical agitation. After that, centrifugation was performed at 1500 rpm for four times until the solution is free of any visible particles. The supernatant was centrifuged at 8000 rpm for 20 min to remove tiny GO pieces. Finally, the precipitate was then redispersed in certain amount of water by sonication, resulting in an exfoliated GO solution (1 mg mL−1).
Synthesis of CoS2/RGO Nanosheets
Co(Ac)2·4H2O (0.01 mol) and H2NCSNH2 (0.02 mol) were added to 60 mL of GO solution (containing 12.3 mg GO that is weighed by drying the GO solution). The mixture was then treated by sonication for about 30 min, giving a clear solution. Then, the solution was poured to a Teflon-lined autoclave (100 mL) and heated in an oven at 180 °C for 12 h. During this hydrothermal process, the GO was reduced to RGO. After being cooled to an ambient temperature, the resultant was separated by centrifugation, then washed with water and ethanol several times, and finally dried in an oven at 60 °C. For comparison, different GO dosages (36.9, 61.5, and 184.5 mg) were adopted in the preparation. The CoS2/RGO composites were denoted as CoS2/GO-X (X = 1, 3, 5, 15 %), where X is the mass ratio of GO and CoS2. Pure CoS2 was also synthesized under the same conditions in the absence of GO.
The morphology of the CoS2/RGO nanosheets was observed by field emission scanning electron microscope (SEM, FEI Sirion 200) and transmission electron microscope (TEM, JEM 2010, JEOL, Japan). Crystal phase analysis was performed on a PANalytical X-ray diffractometer (XRD). Nitrogen sorption isotherms were determined at 77 K using a porosity analyzer (Tristar 3000, Micromeritics, USA). The Brunauer-Emmett-Teller (BET) area of the samples was obtained using the adsorption branch within the relative pressure (P/P 0) ranging from 0.06 to 0.3. Pore size distribution (PSD) curve was obtained from the desorption branch by the BJH (Barrett-Joyner-Halenda) method.
All electrochemical studies were conducted on a CHI660D electrochemical work station in a three-electrode setup using the as-prepared samples as working electrode, a calomel electrode as the reference electrode, and a platinum foil as a counter electrode in 0.5 M H2SO4 electrolyte. The oxygen dissolved in the electrolyte was removed by purging with nitrogen for 10 min prior to the experiments. Linear sweep voltammetry was used to determine the electrocatalytic activities of the as-prepared samples (with an active area of 0.5 cm2) towards the HER at room temperature. HER measurement was also performed using a carbon counter electrode to preclude any Pt contamination. The results showed that the polarization curves from either counter electrode are almost the same. All the potentials were referred to the reversible hydrogen electrode (RHE) without specification.
Results and Discussion
Surface area and pore volume of the as-prepared catalysts
S BET a
S Micro b
S Meso c
To evaluate the stability of the CoS2/RGO-5 % catalyst, the electrode is operated continuously for 500 cycles of potential scans (scan rate 5 mV s−1). The results show that there is no apparent recession of the activity, demonstrating robust catalytic durability of the CoS2/RGO-5 % catalyst in acidic electrolyte (Fig. 6d). This should be attributed to the interaction between CoS2 and RGO, which stabilize the CoS2 nanoparticles in HER process.
In conclusion, we have proposed a novel method to synthesize active CoS2/RGO nanocomposites by hydrothermal treatment. The obtained sample can be employed as a catalytic material for the hydrogen evolution reactions. The synergistic effect of CoS2 and RGO contributes to the good HER activity of these nanocomposites. Besides, this effective synthesis method could be extended to manufacture more promising electrode materials for fuel cell, Li-ion or Na-ion batteries, and supercapacitors.
This work was financially supported by the National Natural Science Foundation of China (21476264, 51107076), the Distinguished Young Scientist Foundation of Shandong Province (JQ201215), the Taishan Scholar Foundation (ts20130929), and the Fundamental Research Funds for the Central Universities (15CX05029A, 15CX08009A).
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