Nickel and Cobalt Selenite Hydrates as Broad Solar Absorbers for Enhanced Solar Water Evaporation

Inorganic black materials possessing hydrophilicity are scarce but can be of great importance in areas such as solar water evaporation and solar steam generation. Herein, for the first time, transition-metal selenite hydrates (specifically, Earth-abundant metals Ni and Co) not only possess high solar absorbance (>96 %) in the solar spectral range (UV–vis–NIR) but also excellent hydrophilicity, which plays a key role in water transport in the solar steam generation. The hydrophilic behavior in selenite hydrates originates from trapped “water of hydration” inside its crystal lattice, which can easily form hydrogen bonds with other water molecules, facilitating water transport. Owing to the abovementioned properties, the studied selenite hydrates are tested for solar water evaporation, showing excellent water evaporation rates of 1.83 and 2.34 kg m−2 h−1 for nickel selenite hydrate and cobalt selenite hydrate, exceeding the theoretical limit of 1.47 kg m−2 h−1.