Pragma, a French company, has developed an h-bike (for lack of a better term) which is powered by a hydrogen fuel cell (this may be too simplistic for some). Looks like more potential controversy, but IMO, a separate class of bike from MTB.
Picture of prototype and basic principles of technology on their site (pragma.com). TBH, I didn't read all about it since it's (probably) going to be too expensive for awhile.