Hi, At half the speed of light, the kinetic energy of a Hydrogen molecule is in excess of 200 MeV. At that energy it isn't going to bounce off. It's going to embed itself in the hull material and deposit all its energy, thereby ablating the hull material, irrespective of its composition.
Note also that all this power needs to be supplied by the ship itself, since it's effectively required to overcome the "wind resistance". Even fusing the Hydrogen into Helium in a Bussard ramjet is only going to supply about 13 MeV / H2 molecule, which is far short of the 200 MeV needed. For a Bussard ramjet to break even it couldn't travel faster than c/7. Which implies a 28 year trip to the nearest star beyond the Sun. However complete conversion of matter to energy should do the trick.
