Ionic-Based Electrochemical Gas Sensor for Ultra-Low SO₂ Detection

Sulfur dioxide (SO₂) is a toxic gas that is emitted during industrial activities, fossil fuel combustion, wildfires and volcanic eruptions. Studies have shown that at even low concentrations (<10 ppm), SO₂ can be linked to deleterious health outcomes such as chronic respiratory issues, increased risk of birth defects, and cardiovascular issues with older individuals, infants and those with existing conditions such as asthma being at particular risk. As the frequency of wildfires increases and as industrial process occur in proximity to population dense areas, the ability for individuals to detect their exposure to SO₂ is ever more important. Traditionally, gas detection is based on methods such as UV-Vis spectroscopy which require large and expensive equipment. In this contribution, we discuss the rational design, development, and optimization of a compact, inexpensive electrochemical gas sensor for detecting SO₂ at low concentrations (<5 ppm) with fast detection times (<1 min). Using a variety of characterization techniques—such as Raman, XRD, SEM, EIS, EDS and S/TEM—we determined that the primary sensing reaction is the formation of Li₂SO₄ on Li₃BO₃ upon exposure to SO₂. This reaction follows a Nernstian behavior to concentrations as low as 0.25 ppm of SO₂, well below the human nose SO₂ detection limit of 0.65 ppm, and among the highest sensitivity for type III electrochemical sensors which indirectly measure a species, SO₂ (gas) through an auxiliary phase Li₂SO₄ (solid). The performance of the gas detector is strongly influenced by the Li-ion kinetics at two interfaces: (1) between the Li₃BO₃ solid-state electrolyte and the sensing electrode and (2) at the surface of the sensing electrode exposed to SO₂. To optimize the rate at which the sensing reaction occurs across these interfaces, we utilized principles of ionic glass-ceramic design to produce a sensing electrode composed of Li₂SO₄:CaSO₂:Li₃BO₃:SiO₂. We demonstrate that the increased ionic conductivity and stability offered by the Li₂SO₄:CaSO₄:Li₃BO₃ composite and the increased surface area by the SiO₂ enable the performance of this electrochemical sensor.