Statistical thermodynamics of supercapacitors and blue engines

We study the thermodynamics of electrode-electrolyte systems, for instance supercapacitors filled with an ionic liquid or blue-energy devices filled with river- or sea water. By a suitable mapping of thermodynamic variables, we identify a strong analogy with classical heat engines. We introduce several Legendre transformations and Maxwell relations. We argue that one should distinguish between the differential capacity at constant ion number and at constant ion chemical potential, and derive a relation between them that resembles the standard relation between heat capacity at constant volume and constant pressure. Finally, we consider the probability distribution of the electrode charge at a given electrode potential, the standard deviation of which is given by the differential capacity.