Improving electrochemical active area of MoS2 via attached on 3D-ordered structures for hydrogen evolution reaction
Graphical abstract
Introduction
In order for hydrogen (H2) to be a real alternative to fossil fuels, a renewable production method is needed. Electrochemical hydrogen evolution reaction (HER) is regarded as one of the most promising clean and renewable techniques for developing H2 as an energy source [[1], [2], [3], [4]]. Noble metal electrocatalysts based on platinum (Pt) are known to catalyze HER very efficiently due to the exceptionally low overpotential, which is close to the thermodynamic potential for hydrogen generation [5,6]. However, the high cost and scarcity of noble metal electrocatalysts have greatly hindered their potential utilization in commercial applications. There has been much research attention dedicated to finding HER electrocatalysts that are both effective and based on earth-abundant elements. In order to find efficient electrocatalysts for HER, the hydrogen adsorption free energy (ΔGH) is a good descriptor for the rate of HER, with an optimal binding ΔGH ≅ 0 eV [7], which means an efficient electrocatalyst for HER will thus be one that binds hydrogen neither too strongly nor too weakly.
Recently, remarkable advances have been made in the utilization of earth-abundant transition-metal-dichalcogenides (TMDCs), MoS2, MoSe2, MoTe2, WS2, etc., as noble-metal-free electrocatalysts for the HER [[8], [9], [10], [11], [12], [13]]. Typical among them is MoS2, which has relatively low adsorption free energy for protons (ΔGH = 0.08 eV) near its edge sites [14], making it comparable to Pt for HER. However, the HER activity of MoS2 is linearly proportional to the number of edge sites and electrical hopping between layers of MoS2 [15] In this regard, extensive efforts have been made to developing highly efficient MoS2 and other TMDC electrocatalysts by maximizing the active edge sites, including the use of nanoparticles [3], nanowires [16], thin films [17,18], nanosheets [19,20], and defect-rich nanosheets [21]. It has been also reported that vertically aligned MoS2 nanostructures with exposed, highly active edge sites exhibited higher electrocatalytic activities than bulk MoS2 [22,23]. However, the poor electrical conductivity of vertical MoS2 along the adjacent interlayers hinders electron transfer, which is essential for highly efficient HER. Motivated by this understanding, various works have sought to improve HER of MoS2 through growth on various conductive substrates, such as nanoporous carbon [24], carbon nanotube (CNT) [25], nanoporous metal [26] or graphene [27] to facilitate electron transfer at the interfaces. Even though significant improvements have been made from free standing as well as grown on different conductive substrate vertical nanostructures of MoS2, poor stability as well as sluggish kinetics due to edge defects and difference between surface tension of the conductive substrate and MoS2 [28] hinders the practical applications. MoS2 sheets decorated on different 3-dimensional (3D) conductive structures could solve the stability issues of MoS2 heterostructures with conductive materials. In this respect, MoS2 has been grown on 3D graphene [29], 3D nickel foams [[30], [31], [32], [33]] and 3D silica [19] structures for efficient water splitting. However, inhomogeneity in these structures leads aggregation of MoS2 sheets, which results in less exposure of active edge sites and reduced activity for HER.
Herein, we demonstrate a significant HER enhancement of MoS2 by making a homogeneously attached structure on well-ordered conductive 3D nickel (3D-Ni) nanostructures [[34], [35], [36], [37], [38], [39], [40]]. First, 3D-Ni nanostructures are prepared by a well-established templating method based on the combination of PnP and electrodeposition [36]. Attached MoS2 sheets on the 3D-Ni nanostructures are synthesized by solvent-assisted hydrothermal technique followed by subsequent thermal annealing. We observed that homogeneously attached MoS2 structures on the 3D-Ni showed significantly better HER performance (onset potential −237 mV vs. NHE at 10 mA/cm2) than the MoS2 on 2D-Ni (onset potential −283 mV vs. NHE at 10 mA/cm2). More importantly, we found that homogeneously attached MoS2 structures on 3D-Ni have an electrocatalytic active area (50 cm2 per active sites) that is 5 times higher than MoS2 on 2D-Ni (9 cm2 per active sites). The attached structure of MoS2 on 3D-Ni is stable at higher applied voltage (−0.60 V and −0.70 V vs. Ag/AgCl) for 48 h of continuous electrocatalytic reaction and can be scalable for the practical applications. Therefore, we conclude that the attached MoS2 structures on well-ordered conductive 3D-Ni enhances the electrocatalytic activity of MoS2 and this homogeneous structure solves the stability issues of heterostructures in acidic electrolytes.
Section snippets
Preparation of the well-ordered 3D-epoxy template
A layer of Au (50 nm) and Cr (5 nm) was deposited on a SiO2/Si substrate as a metal seed layer using electron-beam evaporator (SNTEK) (the Au layer is on the top of Cr layer). The Cr/Au-deposited substrate was cleaned using an air plasma (CUTEMP, Femtoscience) for 2 min (50 sccm, 40 mTorr, 60 W. An epoxy-based photoresist film (SU-8, Microchem) with thickness of 10 μm was spin coated on the substrate as a template. After that the photoresist-coated substrate was soft-baked at 95 °C for 10 min.
Results
Fig. 1 shows the fabrication process of attached MoS2 structures on the well-ordered 3D-Ni nanostructure. At first, Au/Cr are deposited on Si/SiO2, afterwards the epoxy template is prepared by PnP, which produces periodic 3D nanostructures in a single exposure by Talbot interference from a conformal phase mask [[41], [42], [43], [44]]. The well-ordered 3D-Ni is produced by electrodeposition of Ni onto the 3D-epoxy template by a conventional two-electrode method (for more details please see
Conclusions
In summary, we propose a new method to improve electrocatalytic activity of MoS2 by synthesizing an attached MoS2 structure on 3D-Ni for HER. Our results demonstrated that the enhancement in the electrocatalytic activity of attached MoS2 on 3D-Ni is attributed to the homogeneous and well-ordered structure, which leads to abundant active sites for excellent HER performance. Furthermore, the attached MoS2 on 3D-Ni shows a low onset potential of −237 mV at 10 mA/cm2 current density, which is lower
Acknowledgements
This work was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2017R1D1A1B03032791, No. 2016R1E1A1A01943131, No. 2017M3D1A1039558 and 2017M3A7B4049507).
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