Imagine a device floating on our oceans, it is a square box, and it has a high yield solar panel on the top and some air inlets on the side. From the bottom there is a constant precipitate of pure carbon, C. The device is made on an industrial scale and put out to see to just capture carbon and evolve oxygen. As far as I know it can be done.
Picking CO2 from the air is not hard. So you can concentrate pure CO2 in a space. This requires energy of course but CO2 scrubbing is nothing new. But the step from CO2 to Carbon is hard. Chemically the CO2 bond is super strong, which is why we use it to burn and generate heat. Solar processes to split CO2 into CO and O2 are known, there are several cathalysts and new methods are being developed every day.
As you can see above however, splitting CO2 in CO and O2 leaves you with CO which has three bonds and is highly reactive. It is in smoke if you burn wood (because of too little oxygen), and we all know that smoke itself burns quite nicely and can even explode. So how are we going to get around this obstacle. The answer is to change the space the CO2 molecules exist in, so by applying an electric field.
When an electric field is applied in a space where CO2 is floating around the shape of the CO2 molecule changes, as do the orbital configuration of its electrons, meaning the distribution of electron density over the nuclei if you where able to measure or image them. An electric field exists between to electrodes which have opposite charge.
As the bonds between Carbon and Oxygen exist because the electrons want to get closer to the positively charged nucleus, so it is charge driven, an electric field can simply rip the bonds apart. This leaves you with two O’s and one C, and the O’s can quickly become O2 and the C can find other C’s to form a structure with (there are many options). As a result the Carbon is deposited, mixed with some oxygen. The oxygen can probably be removed using a membrane that only lets smaller molecules through.
The depositing can be done with a needle or with a mask of needles, so basically a bed of nails, so that larger amounts of carbon can be deposited at once. This is at room temperature and not even using ludicrous voltages (mainly because the voltage gets concentrated by the points.)
More voltage means more carbon, longer deposit times means more carbon
This seems to be a technology that can be scaled up and be made autonomous to operate in the way described above, depositing carbon below while catching sunlight above. There are challenges, like removing the carbon from the substrate, but this would in my opinion be a great device to make, and also make at scale. The trouble with CO2 in our atmosphere is that it is everywhere in it, so up to 16 km high, while we and carbon capturing plants only occupy the lower 100 meter or so. We will therefore need to use large surfaces to recapture CO2, and we can if we do it with automated systems at scale using low abundant materials. This is a typical Roboeconomic solution.
Source Nanopatterning of carbonaceous structures by field-induced carbon dioxide splitting with a force microscope