  Recently the reversible heat production during the electric double layer
(EDL) buildup in a sodium chloride solution was measured experimentally
[Janssen et al., Phys. Rev. Lett. 119, 166002 (2017)] and matched with
theoretical predictions from density functional theory and molecular dynamics
simulations [Glatzel et al., J. Chem. Phys. 154, 064901 (2021)]. In the latter,
it was found that steric interactions of ions with the electrode's walls, which
result in the so-called Stern layer, are sufficient to explain the experimental
results. As only symmetric ion sizes in a restricted primitive model were
examined, it is instructive to investigate systems of unequal ion sizes that
lead to modified Stern layers. In this work, we explore the impact of ion
asymmetry on the reversible heat production for each electrode separately. In
this context, we further study an extension of the primitive model where
hydration shells of ions can evade in the vicinity of electrode's walls. We
find a strong dependence on system parameters such as the particle sizes and
the total volume taken by particles. Here, we even found situations where one
electrode was heated and the other electrode was cooled at the same time during
charging, while, in sum, both electrodes together behaved very similarly to the
already mentioned experimental results. Thus, heat production should also be
measured in experiments for each electrode separately. By this, the importance
of certain ingredients that we proposed to model electrolytes could be
confirmed or ruled out experimentally, finally leading to a deeper
understanding of the physics of EDLs.

Soft Condensed Matter