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.

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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.