an original vision of Audi
Audi will then partner with a biogas manufacturer, whose production process, in the purification phase, releases CO2. This CO2 will be mixed with hydrogen to produce methane which Audi calls e-gas, and it is this e-gas, of quality equal to current town gas, which will motorize cars. The advantage is that today there is everywhere in Germany (and in Europe), a vast network of storage and distribution of natural gas. This avoids the difficulties of setting up an intelligent electricity distribution network, with bidirectional meters in all houses. And individuals could continue to fill up in a few minutes at any time, as they are used to with a petrol car
Source Motor Nature
Natural gas is produced via the reaction CO2 + 2 H2 -> CH4 + H2O
H2O being hydrolyzed elsewhere with renewable electricity.
The idea is
1) to benefit from existing gas distribution networks
2) a means of storing intermittent energy from wind turbines for useful purposes (transport)
3) to recycle the CO2 of a "lambda emitter"
4) to have a high autonomy on the vehicle (with hydrocarbons)
This NASA document explains in more detail the catalysts required. Known reactions since 1899!
On the principle it is very interesting, on the other hand the yield of primary energy used for 100 km does not have to be enormous.