Despite being only 13, me and a friend were thinking of new ways to advance automotive travel. We came up with the idea of a hydrogen-powered engine powered by water, but after further research found that our idea was already a reality.

I then thought of a new option: why not use a human by-product like urine? I'd seen it done before with vegetable oil, so why not use a similar reaction involving electrolysis to extract hydrogen from the urine? Responses to my theory please. 

Good idea but (like all the best ideas) someone has beaten you to it!

https://www.forbes.com/sites/matthewdepaula/2012/11/08/teens-create-a-way-to-use-urine-as-fuel/#7f305690558c

And there are various other approaches to extracting energy from urine as well, for example: http://www.iflscience.com/technology/microbes-turn-pee-electricity/

 

But maybe you can find ways of improving on these methods.