The world's smallest working fuel cell has been created by US chemical engineers, at just 3 millimetres across. Future versions of the tiny hydrogen-fuelled power pack could replace batteries in portable gadgets.
While batteries are used to do that today, fuel cells are able to store more energy in the same space. Even the most advanced batteries have an energy density an order of magnitude smaller than that of a hydrogen fuel tank.
Yet batteries are much easier to make at the small scale than the pumps and control electronics of a fuel cell. And small pumps can use more energy than they generate.