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I'm trying to find a plausible way to reduce global warming in a world past the tipping point.

I recently read this article : https://www.sciencedaily.com/releases/2017/11/171127173225.htm. It suggests that we may be able to use carbon dioxide as a fuel.

Would that work to reduce the greenhouse effect?

You can’t use carbon dioxide as fuel, and that’s not what the article you cite is about. You can turn carbon dioxide (plus hydrogen or water) into fuel, but the process will need more energy than you will later release by burning the fuel, so you’ll need to get that energy from somewhere.

But yes; if you get the energy without burning fossil fuels and you use atmospheric carbon dioxide (or carbon dioxide that would otherwise enter the atmosphere), it will be pretty much carbon-neutral and thus help to reduce the amount of global warming.

According to your link:

The process can work with any level of carbon dioxide concentration, Wu says -- they have tested it all the way from 2 percent to 99 percent -- but the higher the concentration, the more efficient the process is.

The atmospheric concentration of carbon dioxide is .0391%. That's well under 2%. This would not work well at reducing the concentration of carbon dioxide in the atmosphere.

It is designed to reduce the carbon dioxide emitted from something like a coal power plant. It is a mitigation strategy for burning fossil fuels, not a way to reduce the concentration of carbon dioxide in the atmosphere.

This will not decrease the carbon dioxide in the air. It would (assuming it works as hoped) reduce the rate of increase.

There are proposals that more directly address temperature increases or attempt to reduce the carbon dioxide concentration in the air. But this isn't that. This is simply rate of growth reduction.

It is technically possible to burn carbon dioxide, but not in a practical way. The reason burning carbon produces energy is that the total potential energy of carbon and oxygen is minimized by the CO2 configuration. Splitting them up into carbon and oxygen again requires an addition of energy. Therefore, in order to burn carbon dioxide, you need to find something that will produce even lower total potential energy by displacing the oxygen. This depends on the electronegativity of the atoms in question. Oxygen is extremely electronegative, but fluorine is even moreso, and fluorine compounds are notorious for burning things that ordinarily won't burn, including combustion products such as water and silicon dioxide (common sand).

There are two problems with the idea of using carbon dioxide and fluorine as a fuel, though. The first is that fluorine compounds are, on their own, rare and dangerous and hard to deal with. More importantly, when you take carbon dioxide and displace the oxygen with fluorine, you end up with fluorocarbons, which are worse greenhouse gases than the carbon dioxide was to begin with!

Yes, but not from the link in the question.

According to the link in the question, Carbon Dioxide can be used to produce an energy storage medium in a rudimentary way, a bit like the chemical production of alcohol out of sugars by yeast or even sugars out of CO₂ sunlight and water. In all these cases the net energy output is less than the energy put into the system.

Carbon can (theoretically) be used to power a nuclear fusion reaction, as can oxygen. This is what happens in massive stars in their old age. It requires temperatures of upwards of 500 Mega Kelvin (about 3000 times hotter than the center of the sun as modeled by NASA. Oxygen requires greater than three times the temperature. The pressures are equally enormous and beyond our current capabilities to sustain. The CNO reaction cycle can be found detailed in a straightforward way in this wiki article, and is common in stars slightly larger than the sun.

Carbon and Oxygen fusion proper need more high pressures than this. We'd need to be able to mimic the conditions supposed to exist in the center of stars at least 8 times more massive than the sun. I can't help but feel that the development of force-field technology would facilitate this. We're not there yet.

Where do you think fossil fuels came from? CO2 that was removed from the atmosphere by plants (converted into the carbohydrates &c that the plants were composed of), then captured underground by various geological mechanisms.

The obvious problems with using this process to address global warming are

1) The process takes tens to hundreds of millions of years to have an effect; and

2) If you then burn the plant material for fuel, you're right back where you started.

One option is to separate out the carbon as described here: Scientists turn carbon dioxide into coal at room temperature.

The process involves using a gallium-based catalyst at room temperature that generates carbon flakes from the carbon dioxide. The article refers to it as coal (but coal is only one formation of carbon), but the properties indicated in the article suggest it is closer to graphite/graphene in structure.