[Venyaceleste]

I've read journal about harvesting hydrogen from air pollutants (which is volatile organic compound)
with unbiased gas phase photoelectrochemical cell (https://onlinelibrary.wiley.com/doi/pdf/10.1002/cssc.201601806).
 I was wondering is unbiased gas phase photoelectrochemical cell
always produce only CO2 on anode and H2 on cathode? If its yes, then where is the other substances of VOC going
(especially, in this system)? Thank you.

[Arkcon]

This is a pretty advanced topic.  You should probably get the paper from the local university, read it, and read what it says, particularly the reactions you mention.  Then, read the references list, particularly the ones numbered to the points you're unclear on.  You should also note the authors, and read their other papers, especially their review articles, so you learn the basics of the topic, and can understand their papers better.

Its unlikely a complex environmental problem has been solved with one paper.  Its unlikely this is completely false.  Neither unlikely situation is completely impossible 'tho.

I also think its unlikely that Undergraduate reaction balancing skills or Undergraduate-level thermodynamics are going to solve your questions.  However, leaving

unbiased gas phase photoelectrochemical cell

why not pick a random VOC, and try to draw out and balance such a reaction in air?  Include nitrogen, if you feel like it.  And see what you can learn.