There appears to be a promising library call_py_fort that allows calling Python routines from Fortran.  It might be tricky here too, though, because you really want to hold a Python object, in your case a Potentiostat object, in your Fortran program to pass back and forth.  The object would be opaque to Fortran, but Python would be able to understand it. 

Another library that might work better is ForPy that seems to be a bit less numerical-calculation-specific.  It appears to handle passing opaque objects back to Fortran.  However, it looks a bit less maintained. 

Neither solution is particularly pleasant because you're attempting to interact with Python's interpreter from a compiled program.  Some reading suggests (and I might lean this way too) simply running Fortran's EXECUTE_COMMAND_LINE to call Python scripts to do what you want.  It's another alternative that avoids the possible nightmare of trying to interact with the Python shared libraries directly, but it would require quite a bit of Python programming as well to provide a command-line interface to everything you want to do.

Sorry if this wasn't the solution you were hoping for, but interfacing disparate languages is rarely simple.