Cars running on gasoline (and diesel) convert the fuel energy into motion and heat. In this process they generate CO2. Electric cars do not generate CO2, but of course one has to look into the ways the electrical energy is produced and most of the times one would discover that the source is again fossil fuel that results in CO2 generation when converted in electricity.
By using renewables one can hope to have cars that are carbon neutral. Having cars that are carbon negative, that is that "consume" CO2 from the atmosphere seems a long shot indeed.
And yet, this is what a team of researchers at the Vanderbilt and George Washington University are claiming.
They have found a way to create carbon nanotubes from atmospheric CO2 using a Solar Thermal Electrochemical Process (STEP). These carbon nanotubes can be used to substitute the graphite electrodes used in lithium-ion batteries (the ones used in electric cars). The winning point is the cost: the value created by the manufacturing of carbon tubes based anode is higher than the cost of producing them extracting CO2 from the atmosphere. The researchers estimate that for a battery producing 1kWh and costing around 325$ the anode part made of carbon nanotube can be valued at 18$ per Kg of CO2 (that roughly corresponds to the weight of the carbon nanotube). That is quite a bit considering the the anode represents in Lithium-ion batteries between 12-15% of its weight. For an in-depth, and interesting, discussion on various types of Lithium-ion batteries and to understand the technical/economic relationships among anode, cathode and electrolyte you may want to read this paper.
This is not the case in other processes, like the production of methanol from atmospheric CO2 that results in a cost higher than the one resulting from another industrial process. Besides, the methanol, once used by the car, will return the CO2 in the atmosphere...
Notice that the anode represents 40% of the battery, hence you use quite a bit of CO2 from the atmosphere to produce the required amount of carbon nanotubes.
With the trend towards more and more electric cars (we have just saw the interest generated by he Tesla Model 3) and the huge amount of batteries that will be needed the replacement of graphite anodes with carbon nanotubes manufactured extracting CO2 from the atmosphere can represent a factor in decreasing its level.
And again, we see that the tipping point for innovation is the economic one.