During the Second World War, Frumkin led a large team of scientists and engineers involved in defense issues. This contribution did not save him from being dismissed in 1949 as the director of the Institute of Physical Chemistry, when he was accused of cosmopolitanism.
Frumkin's most fundamental achievement was the fundamental theory of electrode reactions, which describes the influence of the structure of the interface between electrode and solution on the rate of electron transfer, especially emphasizing the involvement of the solvated electron and its free energy of solvation. This theory has been confirmed and extended within the framework of contemporary physical electron transfer models.  Frumkin introduced the concept of the zero charge potential, the most important characteristic of a metal surface. The effect of adsorption on observed electrode kinetics is known as the Frumkin effect.
Alessandro Voltas question—a topic of discussion for over 120 years—about the nature of the EMF of electrochemical circuits was resolved using Frumkins approach.  Frumkin has developed the Frumkin isotherm, an extension of the Langmuir isotherm in describing certain adsorption phenomena.  Frumkins students developed novel experimental methods that would, in time, become standard.  Several applied electrochemical processes, including ones related to chemical sources of electrical power, industrial electrolysis, and anti-corrosion protection, were successfully developed under Frumkins supervision.