THE UNIVERSITY OF TENNESSEE SPACE INSTITUTE RESEARCHED THIS STUFF BACK IN THE 1980S: You have a big tank of hydrogen (or any other gas, really) a much smaller tank of anti-hydrogen, and a thrust chamber full of tungsten or iridium sponge. You shoot the hydrogen in, bleed in pulses of tiny amounts of anti-hydrogen to heat it to the desired temperature, and the sponge structure stabilizes the temperature in between pulses. As I recall, you got specific impulses in the high five figures. Storing the antimatter is the biggest challenge; I think there was some thought of freezing it. There was an OTA report, I think. I couldn’t find anything on the UTSI website, but then it’s pretty old stuff.

Reason for this discussion: Is SpaceX Secretly Working on an Antimatter Propulsion System?