Design of triple and quadruple phase boundaries and chemistries for environmental SO2 electrochemical sensing

Abstract

Air pollution is a major environmental risk to the health of humans and vegetation, leading to increasing demand for real-time monitoring of toxins and harmful pollutants. Recently, the first type III electrochemical sensors tracking lethal concentrations of SO₂ were developed based on a fast-conducting Li garnet Li₇La₃Zr₂O₁₂ (LLZO) solid electrolyte. Despite the successful proof-of-concept demonstration, the ideal sensing electrode issues concerning its preferred microstructure and chemistry remain unresolved as no catalysts are involved to ensure the low cost of such future devices. The challenging task to secure an efficient ionic, electronic, and gas pathways are needed for further sensor performance improvement. Here, the focus was placed on manipulating the electrochemical reaction zones from triple phase boundaries (TPBs) to quadrupole phase boundaries (QPBs) by merely changing the processing temperature of a Li₂SO₄–CaSO₄ composite sensing electrode. The intended manipulation enforced the desired shift from TPB to QPB reaction zones and unlocked a larger effective surface area for the electrochemical reaction. The sensor operated at 480 °C with up to one-order-of-magnitude-lowered response time and up to a 75% decrease in the recovery time down to ∼5 min for the QPB-based configuration compared to the TPB-based one. This study demonstrates novel tools and strategies to favorably engineer sensing reaction zones through electrode processing techniques and enrich the functionality of the Li-garnet Li₇La₃Zr₂O₁₂ fast-conducting electrolyte for sensing applications beyond batteries.