Moisture-induced reversible material transition behavior of Nickel (II) bromide for low humidity detection

Low humidity monitoring has been attached to adequate attention in a wide range of industrial fields, such as medical device hermetic packaging, semiconductor manufacturing and transformer manufacturing. However, it remains challenging to detect slight moisture change in low-humidity environment for conventional ionic-type humidity sensors. Herein, Nickel (II) bromide (NiBr 2 ) is explored to be of a reversible moisture-induced discoloration behavior and highly sensitive to moisture in low humidity environment. NiBr 2 humidity sensor delivers a tremendous impedance change nearly to 5 orders of magnitude in the low relative humidity (RH) ranging from 7 % to 24 % and the response time is only about 2 s when RH increases from 11 % to 33 %. The excellent performance for low-humidity detection can be mainly attributed to the reversible bulk absorption behavior to water molecules that lead to a reversible material transition between NiBr 2 and NiBr 2 ⋅6H 2 O. Meanwhile, the sensitivity of the sensor toward the range of 7 %–24 % RH exhibits nearly 5 orders of magnitude enhancement than that in medium-high humidity range of 26 %–95 % RH, which indicates that the sensitivity of the sensor to water molecules caused by the bulk absorption behavior is better than that dominated by the surface adsorption behavior. The results demonstrate that the bulk absorption behavior-mediated reversible material transition provide efficient guidance to realize low-humidity detection.