Redox potential and “pe”

pe = −log{e}, where {e} is the “activity” of electrons. This variable is used because of its similarity with pH. But “e is not an aqueous species, in contrast to H+. The “activity of electrons” may be interpreted to represent the relative tendency for a given dissolved oxidant to accept electrons, or for electrons to “leave” the electrode in a galvanic cell.

pe is related to the redox potential, ESHE:
   pe = ESHE F / (R T ln(10))
where F and R are the Faraday and gas constants, and T is the temperature in degrees Kelvin (2.303 R T / F = 0.059 V at 25°C).

ESHE is the potential that a platinum electrode would ideally have in contact with the aqueous solution when measured against a standard hydrogen electrode (SHE). Sometimes it is referred as Eh (instead of ESHE).