Apparent violations of Onsager reciprocity continue to be reported in models of transport through charged membranes and concentrated electrolytes, with asymmetric cross-coefficients frequently interpreted as evidence of broken microscopic reversibility. We develop a thermodynamic framework that clarifies when such asymmetries represent genuine non-reciprocal behaviour and when they are artefacts of modelling and data reduction. Using a charged-membrane cell model as a case study, we formulate entropy production in terms of conjugate flux–force pairs, construct the full symmetric Onsager matrix, and then analyse how variable reduction, spatial averaging, and concentration-dependent parameterisation affect the resulting reduced kinetic coefficients. Numerical reconstructions of published cross-coefficients, together with controlled examples based on explicitly symmetric Onsager matrices, show that nonlinear, concentration-dependent reductions can generate pronounced and systematically ordered separations between effective coefficients, even when the underlying phenomenological matrix remains exactly symmetric. We further distinguish equilibrium linear-response coefficients from finite-amplitude, steady-state effective parameters and discuss the impact of temporal memory effects in concentrated electrolytes. Taken together, these results demonstrate that the reported asymmetries in charged-membrane models do not constitute a violation of Onsager reciprocity and provide practical criteria for testing reciprocity claims in complex coupled transport systems.

In a recent review, “Size and/or charge asymmetry effects in coulombic fluids in the presence of external fields: From simple electrolytes to molten salts”, Guerrero-García, Biophysical Chemistry 282, 106747 (2022), one of the present authors analyzed some consequences of breaking the symmetry in the valence and/or ionic size of charged fluids, such as aqueous electrolytes, macroion solutions, or molten salts, when ion correlations and ionic excluded volume effects were taken into account. In this review article, we would like to discuss some additional effects of breaking the symmetry in the valence and/or size of charged particles of coulombic fluids (i) next to a rigid cylindrical charged polymer, and (ii) in the electrostatic properties of an electrical double layer planar supercapacitor. These effects were studied in approaches beyond the classical non-linear Poisson-Boltzmann theory of point ions, by using classical integral equations theory, the Modified Poisson-Boltzmann theory, and Monte Carlo simulations in implicit solvent. As a result, the relevance of the asymmetry in the valence and/or size of charged particles, at microscopic level, is illustrated here either in simple electrolytes or in aqueous colloidal suspensions of macroions, when both are under the influence of an external electric field.