I am confused about terminology in a paper that reports a "mutual diffusion coefficient" of a binary electrolyte (i.e. no supporting electrolyte). Is this the same quantity as the ambipolar diffusion coefficient?
The term "Ambipolar" refers to diffusion induced by the gradient of coloumbic charge(s) of the components or an electric field induced by the components at the diffusing boundary. This is typical of ionic compounds due to having cations(+) and anions(-). Usually people use an "effective diffusion coefficient" for those materials.
Mutual diffusion is the diffusion of one component through another at say infinite dilution. IE: sodium chloride ions diffusing in water, OR conversely, water diffusing through sodium chloride ions. I'm sure you already knew this but, diffusion is a two way process. The mutual coefficient is just a means of handling either process. Where-as self-diffusion is the diffusion of a compound through a medium of itself.