Chemical reaction, Oxygen meets hydrogen, for the reaction to occur the reaction must occur above the critical reaction temperature....

In the wider world the same reaction is called combustion...

As the reaction is exothermic it becomes self-sustaining....

So really a fuel cell is burning the hydrogen, just that the reaction occurs without any "flame" (just a 700 degree ceramic fuel cell) and the potential across the reaction is harvested as electicity....

Combustion IS a chemical reaction.