This product from Alphasense offers a lower cost solution against NDIR based CO2 sensors which are very expensive.

“The Alphasense patented carbon dioxide sensor is a world first in CO2 monitoring:

Requires no power
Operates at room temperature
One sensor measures from 500ppm to 95% CO2
Small size
Low cost
Simple interfacing

Using the potentiometric measuring principle with our patented silver halide solid state electrolyte, this sensor does not have the high power requirement or high price tag of NDIR technology. This sensor is ideal where low cost and low power are critical for CO2 measurement.

This sensor is the result of seven years of research and development, and this work continues. While not as accurate as NDIR, and not suitable for IAQ measurements, this sensor is ideal for safety alarms, combustion gas monitoring and high CO2 process monitoring.”