Hydrogen Fuel Cells

[HydrogenVapour]

I understand that hydrogen fuel cells essentially separate the hydrogen atom into proton and electron, where the proton passes through a membrane and the electron through a circuit which powers a generator/motor. But the part that confuses me is the end. Why is the oxygen needed to bond the proton and electron together? Is it some sort of catalyst?

 

Also:

 

Why can't the electron be used over and over again? Does it lose some of its energy powering the generator? 

 

Any help would be much appreciated. I apologize if my questions have obvious answers, I'm a 13 year old who hasn't learned about the science of fuel cells in school, and I find websites rather vague in this part of the process. 

[HydrogenVapour]

Edit: I realized that the oxygen is needed due to its strong electrical charge which attracts the ions,so its not a catalyst. 

 

But what about the second part?