Thin films of poly(vinylidene fluoride-co-hexafluoropropylene)-ionic liquid mixtures as amperometric gas sensing materials for oxygen and ammonia

Abstract

Gas sensors are important devices used to monitor the type and amount of gas present. Amperometric gas sensors – based on measuring the current upon an applied potential – have been progressing towards miniaturised designs that are smaller, lower cost, faster responding and more robust compared to commercially available sensors. In this work, a planar thin-film electrode device is employed for gas sensing with a thin layer of gel polymer electrolyte (GPE). The GPE consists of a room temperature ionic liquid (RTIL, with two different imidazolium cations and the tetrafluoroborate [BF₄]⁻ anion) mixed with poly(vinylidene fluoride-co-hexafluoropropylene) (PVDF-HFP). The polymer acts as a scaffold, with the RTIL ions able to flow within the porous percolated channels, resulting in a highly robust gel with high conductivity. The chemical nature of the polymer allows thin-films (ca. 6 μm) to be evenly dropcast onto planar electrode devices, using minimal amounts of material. Remarkably, no significant effect of resistance was observed in the voltammetric response with such thin films. Oxygen (O₂) and ammonia (NH₃) gases were detected in the concentration ranges 1–20% O₂ and 1–10 ppm NH₃ in the two GPEs using both linear sweep voltammetry (LSV) and long-term chronoamperometry (LTCA). LTCA was the preferred detection method for both gases due to the steady-state current response compared to the sloping current response from LSV. The thin nature of the film gave fast response times for both gases – less than 10 seconds for O₂ and ca. 40 seconds for NH₃ – easily rivaling the commercially available porous electrode designs and allowing for continuous monitoring of gas concentrations. These materials appear to be highly promising candidates as gas detection electrolytes in miniaturised devices, with accurate and fast responses in both the cathodic and anodic potential regions.