Highly Active Catalyst of Two-Dimensional CoS2/Graphene Nanocomposites for Hydrogen Evolution Reaction

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.


Background
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][2][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 earthabundant electrocatalysts, e.g., metal sulfide [4], carbide [5], boride [6], and phosphide [7]. Many efforts have been taken to synthesize inorganic metal sulfur complexes to create analogs to these active materials. Particularly, MoS 2 and WS 2 have been researched as HER catalysts for their lower cost and higher stability than other metal derivatives [8][9][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 [11]. Till now, only few reports were devoted to the study of 2D CoS 2 /graphene catalysts, which can expand the category of catalytic materials fitting for efficient HER [12].
Herein, we report a novel strategy to synthesize 2D cobalt disulfide/reduced graphene oxide (CoS 2 /RGO) nanocomposites with high HER activity by a hydrothermal method. In these nanocomposites, RGO serves as a matrix for the uniform growth of CoS 2 nanoclusters. The presence of RGO facilitates both electrical conductivity and ionic transportation during the HER. Besides, the homogeneous dispersion of CoS 2 nanoclusters on the surface of RGO could also incredibly increases the catalytic active sites [13]. Furthermore, the interaction between graphene and CoS 2 can also inhibit the aggregation of CoS 2 nanoclusters, resulting in improved cycling stability [14]. Benefiting from their high electrical conductivity, opened pore structure, excellent dispersion of CoS 2 , and positive synergistic effect between CoS 2 and RGO, the CoS 2 /RGO nanocomposites exhibit excellent performance for HER process.

GO Preparation and Purification
GO was synthesized by using modified Hummer's method. In a typical synthesis, 1 g of graphite, 1 g of NaNO 3 , and 46 mL of H 2 SO 4 were firstly mixed together in ice bath. Afterwards, 6 g of KMnO 4 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 H 2 O 2 (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 CoS 2 /RGO Nanosheets
Co(Ac) 2 ·4H 2 O (0.01 mol) and H 2 NCSNH 2 (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 CoS 2 /RGO composites were denoted as CoS 2 /GO-X (X = 1, 3, 5, 15 %), where X is the mass ratio of GO and CoS 2 . Pure CoS 2 was also synthesized under the same conditions in the absence of GO.

Material Characterizations
The morphology of the CoS 2 /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.

Electrochemical Measurements
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 H 2 SO 4 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 cm 2 ) 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
The strategy for the synthesis of the CoS 2 /RGO nanosheets is illustrated in Fig. 1. CoS 2 nanoclusters were in situ grown on the RGO surface through a facile hydrothermal process. In the structure of CoS 2 /RGO nanocomposites, the highly conductive graphene and the 2D structure of the composites are beneficial for electron transfer and proton transport, respectively. This is supposed to facilitate the proton reduction at the catalyst/electrolyte interface. Besides, the high dispersion of CoS 2 nanoclusters on the surface of graphene will increase drastically the catalytic active sites for the hydrogen evolution reactions [15][16][17].
As shown in Fig. 2a-d, CoS 2 nanoclusters grow uniformly on both sides of the graphene sheets with a size of several micrometers, and the size of nanoclusters is about 50-80 nm. It also can be found from Fig. 2e-h that the morphology of CoS 2 nanoclusters changes from vertically dispersed nanosheets to uniformly distributed nanoparticles with the increase of GO dosage. This is because the amount of CoS 2 is only enough to form crystal nucleuses at the large dosage of GO. Higher GO dosage would be beneficial for better dispersion of CoS 2 nanoclusters, contributing to more exposed CoS 2 surface for HER. The high-resolution TEM images demonstrate that most of the CoS 2 nanosheets stand vertically on graphene for CoS 2 /RGO-1 % sample (Fig. 3a, b). When the GO dosage was increased to 5 wt%, the shape of CoS 2 turns from nanosheets to nanoparticles. (Fig. 3c, d). The SEM and TEM observations agree with each other.
The crystal phase of the CoS 2 /RGO composites was detected by X-ray diffraction, as is presented in Fig. 4. It is shown that all the peaks are attributed to the cubic CoS 2 phase (JCPDS number 41-1471), which should be an active phase for HER. The peak intensity decreases with the rise of GO dosage because of better dispersion of CoS 2 on more RGO. No diffraction peaks for RGO can be hardly detected in the spectrum due to its low content.
The pore texture of the as-prepared samples was determined by nitrogen sorption measurement at 77 K. As shown in Fig. 5a, all the CoS 2 /RGO nanocomposites show typical type IV isotherm and obvious hysteresis loop, revealing their mesoporous nature. The BET specific surface area of CoS 2 /RGO-5 % is 26.2 m 2 g −1 (Table 1), which is the highest among these samples. The larger porosity of CoS 2 /RGO-5 % sample is also manifested by the PSD curves in Fig. 5b. The large surface area and pore volume are also supposed to facilitate its catalytic activity to HER.
The as-prepared CoS 2 /RGO catalyst coated on glassy carbon was applied as a working electrode for the HER. Pure CoS 2 was also evaluated for comparison. As shown in Fig. 6a, the polarization curves is measured in 0.5 M H 2 SO 4 electrolyte with a voltage sweep rate of 5 mV s −1 . The polarization curve of CoS 2 /RGO-5 % sample shows a small overpotential of 143, 173, and 346 mV at the current densities of 10, 20, and 100 mA cm −2 , respectively, indicating a good HER activity. In contrast, pure CoS 2 exhibits overpotential of 414 mV at a current density of 20 mA cm −2 , which is apparently larger than the overpotential detected for CoS 2 /RGO composites, suggesting its worse HER activity. On the other hand, at the potential of 300 mV, the CoS 2 /RGO-5 % shows a large cathodic current density of 78.8 mA cm −2 which is about nine times that of the CoS 2 /RGO-1 % electrode (9.0 mA cm −2 ) and far larger than the pure CoS 2 electrode (6.0 mA cm −2 ). The Tafel slope of CoS 2 /   Fig. 6b is 285 mV dec −1 , thus leading to a faster increment of the HER rate with increasing overpotential. The polarization curve recorded on CoS 2 /RGO-5 % displays the lowest overpotential of 159 mV to get a 15 mA cm −2 current density (Fig. 6c), which is smaller than those of the other samples. Therefore, the CoS 2 / RGO-5 % with the smallest slope shows the highest activity in the HER. The excellent catalytic performance of CoS 2 /RGO-5 % is due to its large surface area, which leads to high dispersion of CoS 2 nanoparticles and more active sites. Compared with pure CoS 2 , the better performance of CoS 2 /RGO composites (1, 3, 5 %) also indicates the positive synergistic effect between CoS 2 and RGO for HER.

RGO-5 % in
To evaluate the stability of the CoS 2 /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 CoS 2 / RGO-5 % catalyst in acidic electrolyte (Fig. 6d). This should be attributed to the interaction between CoS 2 and RGO, which stabilize the CoS 2 nanoparticles in HER process.

Conclusions
In conclusion, we have proposed a novel method to synthesize active CoS 2 /RGO nanocomposites by hydrothermal treatment. The obtained sample can be employed as a catalytic material for the hydrogen evolution reactions. The synergistic effect of CoS 2 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.