Well carbon is #sp^2-"hybridized"# in #"bicarbonate ion...."#
Explanation:
There are #4+3xx6+2=24*"valence electrons"# to distribute....
And so we got....#O=C(-O^(-))_2# the which structure distributes #12*"electrons"#.... All #/_O-C-O=120^@# as we would anticipate. And thus a formal bond order of #4/3#...