Sense and sustainability

The efficient detection and capturing of CO₂ is an important strategy in slowing down global warming. Now, writing in Advanced Materials, Jennifer Rupp and colleagues report a simple, fast and stable potentiometric electrochemical lithium garnet-based CO₂ sensor. This device can selectively detect a wide range of CO₂ concentrations at substantially lower operating temperatures than state-of-the-art sensors.

Credit: Indigo molecular images/Getty images

In potentiometric gas sensors, the electrodes and the solid electrolyte determine analyte selectivity, stability, operating temperatures and measurable concentrations. Sodium-ion electrolytes are commonly used for the detection of CO₂, but with operating temperatures above 400°C, sensors based on these electrolytes require high power consumption.

Such sensors also lack long-term chemical stability in humid environments and can exhibit cross-reactivity with other gases. “A CO₂ sensor should have fast reaction kinetics, be small in device form factor and operate at low temperatures to ensure efficient CO₂ tracking in households and commercial areas,” says Rupp.

To address these requirements, the researchers used lithium garnets, which are a commonly used electrolyte in solid-state batteries. Li₇La₃Zr₂O₁₂ has high ionic conductivity at room temperature, but lacks stability in contact with CO₂ and H₂O owing to the formation of Li₂CO₃. However, this reaction makes it an interesting candidate for detecting CO₂.

Rupp and colleagues formed an electrochemical cell using tantalum-doped lithium garnets as electrolytes, gold as a reference electrode and gold–Li₂CO₃ as a working electrode. This device can operate at temperatures below 400°C, but remains stable up to 500°C.

Exposing the working and reference electrodes to the same gas environment revealed that the sensor is selective for CO₂ and can detect CO₂ concentrations between 0 ppm and 4,000 ppm in synthetic air.

The electromotive force (EMF) of the cell, which is the maximum potential difference between the two electrodes, responds to step changes in CO₂ concentration between temperatures of 278°C and 366°C. In this temperature range, the sensor is stable for a few days and EMF stabilization, that is, the response time, occurs within 60 seconds, which is substantially faster than the times achieved with sodium-ion based sensors at these temperatures.

“Employing lithium garnets as electrolytes substantially decreases the operating temperature by up to 200°C, while maintaining the response time at less than 1 minute,” explains Michal Struzik, first author of the paper.

Employing lithium garnets as electrolytes substantially decreases the operating temperature by up to 200°C

In the future, Rupp and colleagues would like to tune the chemical composition and reaction kinetics of the electrodes to sense analytes other than CO₂. Thinking more widely, the researchers envisage a new concept called lithionics, arising from the combination of solid-state lithium electrolyte conductors with sets of different electrodes. “The idea is to design a chip that can sense chemicals, compute data and store energy, based on one solid-state lithium conductor,” comments Rupp.