These have been installed in hydrogen-powered buses in Canada. But they generate power rather inefficiently. To be economically viable, fuel cells have to be cheap, efficient, lightweight and long-lived. PEM fuel cells are neither cheap nor very efficient. Also the polymer membranes that separate the hydrogen and oxygen compartments of the cell, and complete the electrical circuit by letting positively charged hydrogen ions pass through are leaky.
Some fuel can slip across the membrane, short-circuiting the cell and lowering the power output.
Solid acids, such as casium hydrogen sulfate, that contain mobile hydrogen ions, will act as an electrolyte in cells that burn hydrogen. Operating at around 160 degrees Celsius, these devices create higher voltages than PEM cells, and should, once design is optimized, have greater overall efficiencies. There is still a lot of room for improvement before these prototypes approach the kind of performance needed for commercial applications. Whether the automobile industry will take heed is another matter.