We know that hydrogen burns itself but does not helps in burning. Thus when hydrogen burns it does not produce any further polluting gas. So when liquid hydrogen burned it does not pollute.
Liquid hydrogen used as a rocket fuel.....when combine liquid hydrogen and liquid oxygen will produce high trust( kinetic energy) it's pollution free method where it's byproduct is pure water vapor.... But this has huge limitations like cost of production and storage....and need solid state rocket boosters for initial lift off from the ground level where liquid state doesn't have that much lift off.... While in space orbit a space craft use liquid proportion b'coz in space v d't have combustion to happen so liquid propulsion is usually used in final stages of rocket projections