The Bosch BME280 is a solid choice and there is already a library for it: GitHub - lucaong/elixir_bme680: An Elixir library to interface with the BME680 (and BME280) environmental sensor · GitHub. The BME680 also measures gas concentration but the library is compatible with both

Small correction: I meant this library: GitHub - elixir-sensors/bmp280: Use Bosch barometric pressure sensors in Elixir · GitHub which is also used in the Nerves weather station book

The BME680 takes those sensors to the next step in that it contains a small MOX sensor. The heated metal oxide changes resistance based on the volatile organic compounds (VOC) in the air, so it can be used to detect gasses & alcohols such as Ethanol, Alcohol and Carbon Monoxide, and perform air quality measurements. Note it will give you one resistance value, with overall VOC content, but it cannot differentiate gasses or alcohols.

So no, no real CO2 sensor but an indicator of your general air quality