Humidity sensors are a common, but important type of sensors in our daily life and industrial processing. Graphene and graphene-based materials have shown great potential for detecting humidity due to their ultrahigh specific surface areas, extremely high electron mobility at room temperature, and low electrical noise due to the quality of its crystal lattice and its very high electrical conductivity. However, there are still no specific reviews on the progresses of graphene-based humidity sensors. This review focuses on the recent advances in graphene-based humidity sensors, starting from an introduction on the preparation and properties of graphene materials and the sensing mechanisms of seven types of commonly studied graphene-based humidity sensors, and mainly summarizes the recent advances in the preparation and performance of humidity sensors based on pristine graphene, graphene oxide, reduced graphene oxide, graphene quantum dots, and a wide variety of graphene based composite materials, including chemical modification, polymer, metal, metal oxide, and other 2D materials. The remaining challenges along with future trends in high-performance graphene-based humidity sensors are also discussed.

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Nickel phosphides have been widely explored for electrocatalytic hydrogen evolution reaction (HER) due to their high activity and durability. To date, however, it is still a big challenge to develop facile and scalable approaches to preparing nickel phosphide structures with high HER performance for practical applications. Here, a general strategy is demonstrated for facile preparation of porous nickel phosphide nanosheets on arbitrary substrates by electroless Ni plating on the substrate followed by a convenient and stable phosphidation reaction. Typical porous nickel phosphide nanosheets (Ni5P4/NiP2/Ni2P) supported on carbon cloth show excellent electrocatalytic HER activities in acidic electrolyte with very small overpotentials of 63 mV and 120 mV to attain current densities of 10 mA cm–2 and 100 mA cm–2, respectively, and a very low Tafel slope of 47.3 mV dec–1, which are among the best results compared to other non-noble HER electrocatalysts. Furthermore, the electrode exhibits superior flexibility and outstanding durability with negligible degradation under either an accelerated degradation test for 5000 cyclic voltammetry cycles or a durability test under a constant current density of 10 mA cm–2 for 168 h. The excellent HER performance is contributed by the high specific surface area of porous nanosheets and the synergistic effect among Ni5P4, NiP2, and Ni2P phases. Besides, the porous nickel phosphide nanosheets grown on a large-area carbon cloth film via the same method show nearly the same high HER activities, suggesting a high potential for practical application. In addition, this strategy is employed to prepare porous nickel phosphide nanostructures on arbitrary substrates, even elaborate leaf vein and silkworm cocoon, with remarkable HER activities. The preparation method reported here is practical and scalable and can be extended to produce transition-metal-based structures on appropriate substrates for various applications.

A high-efficient and low-cost catalyst on hydrogen isotope separation between hydrogen and water is an essential factor in industrial application for heavy water production and water detritiation. In past studies, Pt-based catalysts were developed but not practical for commercial use due to their high cost for vapor phase catalytic exchange (VPCE), while for impregnated nickel catalysts with a lower cost the problems of agglomeration and low Ni utilization existed. Therefore, to solve these problems, in-situ grown Ni-based catalysts (NiAl-LDO) derived from a layered double hydroxide (LDH) precursor were fabricated and first applied in VPCE in this work. Compared with traditional impregnated Ni-based catalysts, NiAl-LDO catalysts own a unique layered structure, homogeneous dispersed metallic phase, higher specific surface area as well as stronger metal-support interactions to prevent active metal from agglomerating. These advantages are beneficial for exposing more active sites to improve dynamic contacts between H2 and HDO in a catalyst surface and can bring excellent catalytic activity under a reaction temperature of lower than 400 °C. Additionally, we found that the dissociative chemisorption of HDO and H2 occurs not only in Ni (111) but also in NiO species where chemisorbed H(ads), D(ads), OH(ads) and OD(ads) are formed. The results highlight that both of the Ni2+ species and Ni0 species possess catalytic activities for VPCE process.

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