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.

The production of hydrogen through electrochemical water splitting driven by clean energy becomes a sustainable route for utilization of hydrogen energy, while an efficient hydrogen evolution reaction (HER) electrocatalyst is required to achieve a high energy conversion efficiency. Nickel phosphides have been widely explored for electrocatalytic HER due to their unique electronic properties, efficient electrocatalytic performance, and a superior anti-corrosion feature. However, the HER activities of nickel phosphide electrocatalysts are still low for practical applications in electrolyzers, and further studies are necessary. Therefore, at the current stage, a specific comprehensive review is necessary to focus on the progresses of the nickel phosphide electrocatalysts. This review focuses on the developments of preparation approaches of nickel phosphides for HER, including a mechanism of HER, properties of nickel phosphides, and preparation and electrocatalytic HER performances of nickel phosphides. The progresses of the preparation and HER activities of the nickel phosphide electrocatalysts are mainly discussed by classification of the preparation method. The comparative surveys of their HER activities are made in terms of experimental metrics of overpotential at a certain current density and Tafel slope together with the preparation method. The remaining challenges and perspectives of the future development of nickel phosphide electrocatalysts for HER are also proposed.

Oxygen vacancy-rich amorphous porous NiFe(OH)_x on carbon paper is prepared through a two-step electrochemical depostion of Prussian blue nanocrystal layer and Ni(OH)_x layer followed by an alkaline etching process for efficient OER catalysis.