Can an Impressionist painting be done using a sponge instead of a brush?
I don’t see why not. Impressionist paintings are loose and have lots of movement. The paintings are the impression the artist gets from the subject matter, they are alive with feeling, light and color; they are a snap shot or moment in time. if your painting exhibits these traits then it is an impressionistic painting. You could also cut your sponge down to be able to have a bit more control if you like. Have a look at some impressionist artist such as Monet and Renoir.