Polyantimonic acid-based materials evaluated as moisture sensors at ambient temperature

Abstract

Humidity sensors are in high demand for many applications, such as environmental monitoring and air and food quality control. Despite many inorganic and organic materials exhibit moisture sensing properties, the electrical response of many existing sensors is not stable along the time. Polyantimonic acid (PAA) is characterized by elevated proton conductivity and by high thermal stability: consequently, it is seen as promising proton conductor for usage in humidity sensing devices. In this work, for the first time, PAA-based bulk solid membranes were produced and tested as potential materials for relative humidity (RH) detection and their moisture sensitivity was evaluated. Two different amounts of binder were used for moulding the solid sensors: the ones with 10% of binder were designated as 90PAA, while the ones with 20% were named 80PAA. The structures of the solid samples were investigated by X-ray diffraction (XRD) technique, adsorption–desorption curves via Brunauer–Emmett–Teller (BET), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS) techniques. The electrical behaviour was examined at ambient temperature by electrical impedance spectroscopy in the entire relative humidity (RH) interval (0–100%) and in the frequency range of 40 Hz up to 60 MHz. Electrical response of the materials was correlated with the structural features of the membranes. Both 90PAA and 80PAA sensors showed total resistance 3 × 105 and 3.5 × 105 Ω at 10% RH, respectively. A linear decrease of the resistance on RH was observed in the range 30–90% RH for both sensors. The electrical response of the evaluated PAA-based sensors displays good repeatability and reproducibility: the ones with lower binder content showed higher moisture sensitivity as well as very good time stability over 1 year.

Enhancing Polyantimonic-Based Materials’ Moisture Response with Binder Content Tuning

“…The elements were chosen considering the previously carried out discussion of the electrical response of the samples to moisture, during which the authors already advanced a hypothesis regarding the contributions to their overall electrical response, hoping it would suit the under-study sensor samples. In a previous literature report, the authors discussed the humidity-sensing mechanisms behind PAA ionic conductors prepared using a fluoroplastic-type binder [40]. The mechanisms behind the present PAA samples prepared using PVA are the same, which is in line with the fact that the same electrical circuit model supports the discussion of the observed contributions in both works.…”

“…From the plotted fittings, the adequation of the studied and developed equivalent model could be confirmed; without a doubt, both the Nyquist and Bode fits show a high agreement with the obtained experimental data; see Figures 13 and 14. For all samples, Additionally, and when comparing to previous published results also obtained for PAA-based samples [40], obtained using a fluoroplastic-type binder, the present work's results, where PVA was used, allowed for enhancing moisture sensitivity and diminishing pedance modulus varies mainly in the hundreds of the kΩ range). Indeed, a close comparison between the best results previously published (see Figure 11) and the finest ones from the present work (see Figure 10) allows for demonstrating the just-mentioned facts.…”

“…Additionally, and when comparing to previous published results also obtained for PAA-based samples [40], obtained using a fluoroplastic-type binder, the present work's results, where PVA was used, allowed for enhancing moisture sensitivity and diminishing ageing (the estimated standard deviation using the determined impedance modulus traces for both periods is 5.7 kΩ, which is relatively low considering that the sensor impedance modulus varies mainly in the hundreds of the kΩ range). Indeed, a close comparison between the best results previously published (see Figure 11) and the finest ones from the present work (see Figure 10) allows for demonstrating the just-mentioned facts.…”

“…Thus, it is always necessary to add a binder so that it is possible to obtain more stable membranes that do not become brittle when handled [36,37]. The study of PAA has already been carried out in conjunction with some ligands such as polyvinylidene fluoride (PVDF) [30,38], polytetrafluoroethylene (PTFE or Teflon ® ) [39], and fluoroplastic [40].…”

“…In a previous paper by the authors [40], a comparative analysis between some of the aforementioned characteristics was performed between several materials and fabrication routes reported in the literature, which allowed for highlighting the benefits and drawbacks of the material and fabrication route chosen by the authors. The main ones are concerned with hysteresis performance and low fabrication costs [41][42][43][44].…”