i'm not saying it wouldn't work, I recently rewatched an episode of James May's Toy Stories with a fuel cell powered toy train. That was a few years back so it could well have been shrunk down by now.
As for safety, like the fuel cell cars, I imagine the cell will be pretty safe, but you just know when anything like that comes along the transport authorities will be all over it. probably be ok till they know about it, then banned until someone explains it to them. still banned for at least a year, then a quiet lifting of it with no apology.
just as long as an eventual phone version doesn't have water vapour coming from an exhaust like the cars do. everyone would walk around like they'd wet themselves.