When did the US use copper to mint pennies?

Cents were made of copper from 1793 to 1858;

cupro-nickel from 1859 to mid-1864

bronze (copper plus tin and/or zinc) from mid-1864 to 1942 and 1944 to mid-1982

zinc-coated steel in 1943

copper-coated zinc from mid-1982 to the present