That photo is silly. A bunch of people will scroll past thinking oxygen can be a black solid haha.
Dark, in this context, simply means it is being produced without light. This paper and finding should not be surprising to anyone with a chemistry background. In fact, I wonder why it has any traction at all.
If you put a standard AA battery into saltwater, you’ll observe bubbles and hear a fizz—that’s the generation of hydrogen and oxygen gases as the electricity splits water molecules, a process known as electrolysis