Solvent granularity in the differential electrical capacitance of supercapacitor and mechanism analysis

Abstract

In this work, we study how solvent granularity influences the differential electrical capacitance (Cd) of supercapacitor using aqueous electrolyte and molecular mechanism modulating the curve morphology of Cd versus surface charge density σ. Main new findings and conclusions are summarized into several points: (i) the solvent granularity helps to raise the Cd value, and the effect tends to weaken as the surface charge strength σ increases; particularly, the threshold σ, beyond which the solvent granularity effect begins to become graphically unobservable, rises obviously with the pore size. (ii) The bulk electrolyte concentration and pore size evidently influence the transition between camel-shaped and bell-shaped Cd−σ curves; a smaller pore and/or a higher bulk concentration help to facilitate the change from camel-shaped into bell-shaped curve, and the two factors are cooperative. (iii) Whether the Cd−σ curve is camel-shaped or bell-shaped, and the counter-ion is univalent or multivalent, the Cd value always tends to drop with the σ value as the latter one is sufficiently large; particularly, under conditions of very small pore the Cd value eventually approaches zero as the σ is approximately over 0.7C/m2. (iv) High counter-ion valence causes rising of the Cd value and the threshold σ corresponding to the Cd peak in the case of the camel-shaped curve. (iv) A new concept is introduced: change rate of the whole ion adsorption capacity with the surface charge strength, which, combined with the crowding effect, co-ion exclusion effect, discloses the mechanism of action by which all factors considered influence Cd by changing the electrostatic screening environment in the EDL.