Widening the Range of Trackable Environmental and Health Pollutants for Li-Garnet-Based Sensors

Abstract

Classic chemical sensors integrated in phones, vehicles, and industrial plants monitor the levels of humidity or carbonaceous/oxygen species to track environmental changes. According to current projections, the gas sensor market is estimated to grow at an annual rate of 7% between 2021-2026 (valued at USD 1.5 billion by 2026).[1] The projections indicate the strong need to increase the ability of sensors to sense a wider range of chemicals for future electronics not only to continue monitoring environmental changes but also to ensure the health and safety of humans. To achieve this goal, more chemical sensing principles and hardware must be developed. The critical factors that determine the sensing performance for rather corrosive toxins such as SO2 are to develop a suitable electrochemistry and sensor material selection stable in this environment, and operating at low temperature (ideally below 300 °C) to assure a low energy footprint per sensing device volume. One of the best investigated SO2 electrochemical (type III) sensors are those based on the solid-state Na+ conductor NASICON a known conductor vastly applied also as a battery solid state electrolyte. Despite the promise, the limited Na+ conductivity at ambient around 10− 7 S cm− 1 challenges intrinsically to establish fast sensor response time and lower operation temperatures (energy footprint); which is also typically accompanied by degradation of the sensor performance and poor reproducibility. We propose in this work as a promising alternative cubic Li-garnet Li7La3Zr2O12 (LLZO) as a solid-state electrolyte for new SO2 sensors due to their three orders of magnitude increased ionic conductivity (~mS cm− 1) and higher electrochemical stability window, which allows a wider definition and choice for sensing material electrodes. The material class of Li-garnets is known for about a decade[2,3] and has proven success for solid state batteries, however, it had only recently been introduced to serve as electrolyte for type III sensors tracking less corrosive gases such as CO2 with fast sensing and recovery times.[4,5] Here, we provide a proof-of-principle for the specific electrochemistry, material selection, and design of a Li-garnet LLZO-based electrochemical sensor, targeting the highly corrosive environmental pollutant sulfur dioxide (SO2).[6] [7] For that, we explore the following sensor electrochemistry and investigated the major aspects that affect the electromotive force response according to the Nernstian behavior and the response/recovery time of the sensor, explicitly the auxiliary sensing electrode composition and microstructure. Novel composite sensing electrode designs using LLZO based on porous scaffold, employed to define a high number of reaction sites, allowed to successfully track SO2 at the dangerous levels of 0–10 ppm with close-to-theoretical SO2 sensitivity. The introduction of the composite sensing electrode Li2SO4–CaSO4–LLZO with the LLZO electrolyte conductor achieved close-to-theoretical sensitivity of 47.7 mV/dec at remarkably low operating temperature of the sensor of 240 °C. We wish to highlight that this outperforms previously reported SO2 type III electrochemical sensors operating on Zr4+ (400 °C) or Na+ (600 °C) ion-conducting solid electrolytes in terms of their operation temperature and has as a consequence impact on the sensor power consumption. The insights on the sensing electrochemistry, reactions involved and control over the interface sensing electrode/Li+ electrolyte structures and phase stability provide first guidelines for future Li-garnet sensors to monitor with fast response a wider range of environmental pollutants and toxins.