Abstract:

Although they show high power density, supercapacitors often suffer from low energy density. As a new kind of supercapacitors, colloidal ion supercapacitors that include various transition metal cations and rare earth cations, such as Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, Sn²⁺, Sn⁴⁺, La³⁺, Ce³⁺, Er³⁺, Yb³⁺, etc, show both high energy density and high power density. Proper evaluation of specific capacitance of colloidal ion supercapacitors can deepen our understanding of the electrochemical mechanism of electroactive cations in pseudocapacitive electrode materials. This review firstly outlines the basic concept of colloidal ion supercapacitors and in situ coprecipitation synthesis methods. Then the specific capacitance based on active cations(named cationic capacitance) is used to evaluate the performance of colloidal ion supercapacitors. We then calculate the specific capacitances on the basis of hydroxides and oxides(named hydroxide capacitance and oxide capacitance), which are traditional evaluation methods for supercapacitors. Compared with three kinds of specific capacitances, cationic capacitance can really reflect the intrinsic reaction mechanism of pseudocapacitive materials. It is expected that colloidal ion supercapacitors can overcome the technical bottleneck of the existing electrochemical energy storage devices and be the next-generation high-energy storage devices.

Key words: capacitance evaluation, colloid ionic supercapacitor, pseudocapacitance, Faradic reaction