Energy Extraction from Capacitive Mixing: Experimental and Computational Analysis of Chemical Aspects

The double-layer expansion at the interfaces of charged electrodes when a concentrated salt solution (molar concentration C salt ) is replaced by a diluted solution ( C fres h ), produces an increase of the cell voltage. Experiments and simulations were performed in order to study the effect of several variables on the obtained transient voltage, V(t), and the rising of voltage at equilibrium, Δ φ e q . Carbon electrodes were prepared and characterized by cyclic voltammetry, amperometry, and N 2 -adsorption isotherms. The experimental results and finite elements simulations show that the shape of V(t) is influenced by the diffusion coefficients of the ions. On the other hand, experiments and simulations indicate that Δ φ eq present a Nernstian-like dependence with the dilution coefficient, C dil = C salt / C fresh , while is independent of the initial charge and charging time. Quantitative features are interpreted by proposing a partial blocking of the electrodes for the dilution process, associated to irreversible incorporation of solution into the porous, presenting a model to estimate the blocking degree. Finally, cycles of charging-dilution-work-concentration were performed, extracting energies in the order of 100 μ J cm −2 , which are in agreement with the model equations as function of C salt and C fresh .