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 (NiBr₂) is explored to be of a reversible moisture-induced discoloration behavior and highly sensitive to moisture in low humidity environment. NiBr₂ 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 NiBr₂ and NiBr₂⋅6H₂O. 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.

低湿度监测在医疗器械密封封装、半导体制造和变压器制造等广泛的工业领域中得到了足够的重视。然而，传统离子型湿度传感器在低湿度环境中检测轻微的水分变化仍然具有挑战性。在此，溴化镍 (NiBr ₂ ) 被研究为具有可逆的湿气引起的变色行为，并且在低湿度环境中对湿气高度敏感。溴化镍₂湿度传感器在 7 % 至 24 % 的低相对湿度 (RH) 范围内提供近 5 个数量级的巨大阻抗变化，当 RH 从 11 % 增加至 33 % 时，响应时间仅为约 2 秒。低湿度检测的优异性能主要归功于水分子的可逆体积吸收行为，导致 NiBr ₂和 NiBr ₂ ⋅6H ₂之间的可逆材料转变O. 同时，传感器在 7 %–24 % RH 范围内的灵敏度比在 26 %–95 % RH 中高湿度范围内提高了近 5 个数量级，这表明传感器的灵敏度由体吸收行为引起的对水分子的吸附要好于由表面吸附行为主导的。结果表明，体吸收行为介导的可逆材料转变为实现低湿度检测提供了有效的指导。