[Svartalfiar]

To test the nanoparticles’ reactivity, Xu placed the electrode in a beaker of solution and bubbled carbon dioxide into it. He applied a small voltage to the electrode, and measured the resulting current in the solution.

I would assume the solution contains mostly water. Which would, of course, contain the necessary H for the process.

[thackney]

I would assume the solution contains mostly water.

Then you need to add the energy required to separate the molecular bonds of water.