Assuming you have been reading through the paper, not just the abstract, the freezing potential is defined in the first paragraph.
For the benefit of readers without access to the paper:
"Inequality between the distributions for solute anions and cations in ice and in solution produces an electric potential between the solution and the growing ice, which is called a “freezing potential”. Anions or cations accumulate in the water contiguous to the interface due to the electrostatic force generated. The potential is then neutralized by highly mobile OH- or H3O+ ions, and the pH of the solution is varied. Most of the reactions accelerated in the freezing process which have been reported so far are explained in the terms of freeze concentration and hydrolysis accelerated by accumulated OH- or H3O+."
There is no reference to chemical potential (though presumably it is affected), or to any buffers (salt solutions are not buffers).