Total effective surface area principle for enhancement of capacitive humidity sensor of thick-film nanoporous alumina

•Enhancing total effective surface area can enhance alumina humidity sensing.•Reducing anodizing potential for the increased surface area benefits capacitance sensing.•The alumina humidity sensor formed at 20 V enhances 2–3 times capacitance than those at 40–50 V.•An enhanced linear-like relationship between the capacitance and humidity occurs.•The theoretic surface area enhancement mechanism is presented. Total effective surface area is an important factor for environmentally sensing performance. The porous anodic aluminum oxide (AAO) film with a high density of nanopores leads to atremendous surface area for absorbing water molecules. But such an AAO humidity sensor formed in oxalic acid exhibits a low response of capacitance, especially under the low relative humidity (RH). Here, we demonstrate total effective surface area principle to greatly enhance the performance of AAO capacitive humidity sensor using small anodizing potential in oxalic acid. For pore-dependent surface area, the AAO pore wall would directly affect the absorbance of water molecules and the response of capacitive sensor. Decreasing the anodizing potential reduces both of the pore diameter and interpore distance proportionally but increases the surface area inversely. Therefore, the AAO sensor formed at small 20 V can greatly increase the amount of water molecules absorbed on the wall for enhancing 2–3 times response under low-to-high RH compared to those at 40 and 50 V. The good stability and reliable response/recovery time are also obtained for the AAO sensor synthesized at 20 V in oxalic acid.