The abbreviation may have meant milli-Faraday / second = milliequivalent/s.
The Faraday is about 96485 coulomb/mole. 1mF/s= 96.5coul/s=96.5 A
Dear Robert Rodgers,
I think you're right - the authors probably meant milli-Faraday (not millifarad).
I expected to see milli-Faraday symbolized as mF (not mf as noted in the paper).
Thanks for clearing that up.