Large-area (∼cm2) films of vertical heterostructures formed by alternating graphene and transition-metal dichalcogenide (TMD) alloys are obtained by wet chemical routes followed by a thermal treatment at low temperature. In particular, we synthesized stacked graphene and WxMo1–xS2 alloy phases that were used as hydrogen evolution catalysts. We observed a Tafel slope of 38.7 mV dec–1 and 96 mV onset potential (at current density of 10 mA cm–2) when the heterostructure alloy was annealed at 300 °C. These results indicate that heterostructures formed by graphene and W0.4Mo0.6S2 alloys are far more efficient than WS2 and MoS2 by at least a factor of 2, and they are superior compared to other reported TMD systems. This strategy offers a cheap and low temperature synthesis alternative able to replace Pt in the hydrogen evolution reaction (HER). Furthermore, the catalytic activity of the alloy is stable over time, i.e., the catalytic activity does not experience a significant change even after 1000 cycles. Using density functional theory calculations, we found that this enhanced hydrogen evolution in the WxMo1–xS2 alloys is mainly due to the lower energy barrier created by a favorable overlap of the d-orbitals from the transition metals and the s-orbitals of H2; with the lowest energy barrier occurring for the W0.4Mo0.6S2 alloy. Thus, it is now possible to further improve the performance of the “inert” TMD basal plane via metal alloying, in addition to the previously reported strategies such as creation of point defects, vacancies and edges. The synthesis of graphene/W0.4Mo0.6S2 produced at relatively low temperatures is scalable and could be used as an effective low cost Pt-free catalyst.Keywords: density functional theory (DFT) calculations; heterostructures; hydrogen evolution reaction (HER) mechanism; reduced graphene oxide (rGO); transition-metal dichalcogenides (TMDs) alloy;

Developing high-performance noble metal-free and free-standing catalytic electrodes are crucial for overall water splitting. Here, nickel sulfide (Ni3S2) and nickel selenide (NiSe) are synthesized on nickel foam (NF) with a one-pot solvothermal method and directly used as free-standing electrodes for efficiently catalyzing hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) in alkaline solution. In virtue of abundant active sites, the Ni3S2/NF and the NiSe/NF electrodes can deliver a current density of 10 mA cm−2 at only 123 mV, 137 mV for HER and 222 mV, 271 mV for OER. Both of the hierarchical Ni3S2/NF and NiSe/NF electrodes can serve as anodes and cathodes in electrocatalytic overall water-splitting and can achieve a current density of 10 mA cm−2 with an applied voltage of ∼1.59 V and 1.69 V, respectively. The performance of as-obtained Ni3S2/NF||Ni3S2/NF is even close to that of the noble metal-based Pt/C/NF||IrO2/NF system.A facile solvothermal method was proposed to synthesize free-standing nickel sulfide (Ni3S2) and nickel selenide (NiSe) electrodes, which contain high density of active sites and could serve as highly efficient catalysts for overall water splitting. Download high-res image (168KB)Download full-size image

The sodium-ion battery (SIB) has been considered as one of the most important and promising candidates for large-scale storage of electrical energy. Developing cost-effective but high-performance anode materials is crucial for SIBs. Herein a hard carbon-based anode material is synthesized from polyurethane foam. In order to boost its performance, NaCl, one of the most abundant and low-cost salts in the ocean, is used to intercalate hard carbon. To the best of our knowledge, reports on NaCl intercalation in sodium ion battery anode materials are very scarce so far. After Na+ intercalation, the as-obtained sample delivers a capacity of over 210 mAh g−1 at 20 mA g−1. Moreover, a 90 mAh g−1 specific capacity with over 78% retention can be achieved after 1000 cycles at 1 A g−1 current density, which is 100% higher than that of untreated samples. The much enhanced performance can be attributed to a synergetic effect of both heteroatom doping and Na+ intercalation which can improve the electronic/ionic conductivity and enlarge the lattice spacing of the hard carbon as well. This work demonstrates that NaCl-intercalated nitrogen-rich hard carbons are very promising in their ability to serve as a kind of low-cost but efficient anode material for SIBs.Download high-res image (257KB)Download full-size image

Efficient absorption in the S-band (2–4 GHz) has been a very challenging task for developing high-performance microwave absorption materials. Three-dimensional reduced graphene oxide (3D-rGO) powders were prepared by using a hydrothermal method and subsequent thermal treatment. It is found that as-prepared 3D-rGO could significantly enhance the electromagnetic wave attenuation in 2–4 GHz. When the content in the paraffin matrix is 4%, the 3D-rGO shows the strongest absorption in S-band, and the absorption will get stronger with the increase of coating thickness. When the thickness is 5 mm, the bandwidth of reflection loss less than −5 dB is in the range of 2.3 to 4.1 GHz, that is, it can almost cover the whole S-band. The excellent microwave absorption could be attributed to the honeycomb-like structures and the strong polarization of 3D-rGO powders. Considering the low density and good corrosion resistance, 3D-rGO powders may serve as an excellent component for the design of lightweight electromagnetic wave absorption coatings.

Due to the low density of nanostructured materials, it is still a big challenge to realize high volumetric performance instead of high specific gravimetric capacity with many state-of-the-art electrodes for compact electrochemical energy storage. Moreover, developing high-performance flexible and binder-free electrode materials is also crucial for their future applications in diverse fields, such as portable electronics and wearable devices. In this work, we designed and synthesized a Ni3V2O8/carbon cloth (CC) hierarchical structure as a flexible anode for sodium-ion batteries. Morphology-controllable growth of different Ni3V2O8/CC hierarchical structures is achieved by optimizing the synthesis parameters (e.g. the growth temperatures). The high mass loading (4 mg cm−2), ultra-high areal specific capacity (2.6 mA h cm−2 at a current density of 500 mA g−1), no addition of binders or other additives and good flexibility facilitate their application in sodium-ion batteries.

Air-cathodes are a critical component for microbial fuel cells (MFCs) and need to have high catalytic performance for the oxygen reduction reaction (ORR). As an important two-dimensional material, graphene has been explored in various applications including ORR catalysts for MFCs. However, the reported graphene for MFC cathodes was usually small flakes/powders, which cannot be directly coated onto metal meshes without binders. Here, we report a binder-free nitrogen-doped graphene (NG) sheet in situ grown on nickel mesh as an efficient catalyst layer for MFC air-cathodes. By optimizing the growth parameters of NG, the maximum power density of MFCs based on NG can be boosted up to 1470 ± 80 mW m−2, which is 32% higher than that of the conventional Pt/C air-cathode. The optimized NG air-cathode has a low internal resistance (21 ± 3 Ω), only 20% of that of the Pt/C air-cathode. These results provide a proof-of-concept for the binder-free NG air-cathode as an alternative to the costly Pt cathode for MFCs.

The traditional fabrication of graphene-based devices requires polymer-assisted transfer of graphene and a removal procedure of polymer coatings. Here, we propose to turn this process on its head and demonstrate a novel strategy of polymer-coated graphene as an optically antireflective and transparent electrode used in a graphene/silicon (G/Si) solar cell. No additional polymer removal and antireflection coatings (e.g. TiO2 colloids) are needed in our strategy. By engineering the thickness of polymer protective coatings, the light absorption and short-circuit current density of graphene solar cells can be greatly enhanced. We also showed that retaining the polymer coatings avoided the degradation of electrical conductivity of graphene films. With HNO3 doping applied on PMMA-coated G/Si solar cells, the PCEs can reach up to 13.34%. The long-term stabilities of HNO3 doped G/Si solar cells are also improved by using fluoropolymer (CYTOP) coatings on graphene. Our approach provides a novel fabrication method of transparent graphene electrodes for graphene-based optoelectronic devices with excellent light absorption.

Air-cathodes are a critical component for microbial fuel cells (MFCs) and need to have high catalytic performance for the oxygen reduction reaction (ORR). As an important two-dimensional material, graphene has been explored in various applications including ORR catalysts for MFCs. However, the reported graphene for MFC cathodes was usually small flakes/powders, which cannot be directly coated onto metal meshes without binders. Here, we report a binder-free nitrogen-doped graphene (NG) sheet in situ grown on nickel mesh as an efficient catalyst layer for MFC air-cathodes. By optimizing the growth parameters of NG, the maximum power density of MFCs based on NG can be boosted up to 1470 ± 80 mW m−2, which is 32% higher than that of the conventional Pt/C air-cathode. The optimized NG air-cathode has a low internal resistance (21 ± 3 Ω), only 20% of that of the Pt/C air-cathode. These results provide a proof-of-concept for the binder-free NG air-cathode as an alternative to the costly Pt cathode for MFCs.

Due to the low density of nanostructured materials, it is still a big challenge to realize high volumetric performance instead of high specific gravimetric capacity with many state-of-the-art electrodes for compact electrochemical energy storage. Moreover, developing high-performance flexible and binder-free electrode materials is also crucial for their future applications in diverse fields, such as portable electronics and wearable devices. In this work, we designed and synthesized a Ni3V2O8/carbon cloth (CC) hierarchical structure as a flexible anode for sodium-ion batteries. Morphology-controllable growth of different Ni3V2O8/CC hierarchical structures is achieved by optimizing the synthesis parameters (e.g. the growth temperatures). The high mass loading (4 mg cm−2), ultra-high areal specific capacity (2.6 mA h cm−2 at a current density of 500 mA g−1), no addition of binders or other additives and good flexibility facilitate their application in sodium-ion batteries.