Capacitance of Nanoporous Carbon-Based Supercapacitors Is a Trade-Off between the Concentration and the Separability of the Ions

Abstract

Nanoporous carbon-based supercapacitors store electricity through adsorption of ions from the electrolyte at the surface of the electrodes. Room temperature ionic liquids, which show the largest ion concentrations among organic liquid electrolytes, should in principle yield larger capacitances. Here we show by using electrochemical measurements that the capacitance is not significantly affected when switching from a pure ionic liquid to a conventional organic electrolyte using the same ionic species. By performing additional molecular dynamics simulations, we interpret this result as an increasing difficulty of separating ions of opposite charges when they are more concentrated, i.e. in the absence of a solvent which screens the Coulombic interactions. The charging mechanism consistently changes with ion concentration, switching from counter-ion adsorption in the diluted organic electrolyte to ion exchange in the pure ionic liquid. Contrarily to the capacitance, in-pore diffusion coefficients largely depend on the composition, with a noticeable slowing of the dynamics in the pure ionic liquid.Supercapacitors are a promising electrochemical energy storage technology. They may be separated into two families of devices, depending on the electricity storage mechanism, i.e. the electric double-layer capacitors (EDLCs) and pseudocapacitors. 1 . In EDLCs, the charge is stored by a simple process of ion adsorption at the surface of the electrodes, even if at the microscopic scale complex mechanisms are at play. The most prominently employed electrode materials are porous carbons with high accessible surface area. Research and development of such EDLCs is being driven by demand from emerging applications which require efficient energy storage, notably renewable energy, 2 electric vehicles, 3 and smartgrid management. 4,5 Currently, commercial supercapacitors have energy densities on the order of 5 W h kg −1 . 5 There is a large ongoing interest in the design of electrolytes with improved performance over the traditional organic liquids, such as acetonitrile-based electrolytes. Room temperature ionic liquids (RTILs) have raised a lot of interest due to their high window of electrochemical stability, 6 high thermal stability, 7 and low toxicity. 8 RTILs are also highly tunable, and there are many possible combinations available which is beneficial as they can be selected to match specific electrodes. 9 However, when used as a pure electrolyte in EDLCs with highly microporous carbon electrodes, they suffer from high viscosity and poor conductivity. 10 Superconcentrated electrolytes, which are highly-concentrated mixtures of RTILs and organic solvents, may offer a good compromise for these target properties, while keeping most of the RTIL benefits. They are therefore considered as promising alternatives in EDLCs.A difficulty for choosing the optimal electrolyte is that the ions they contain adsorb inside electrified nanoporous carbon electrodes, in which the structure changes significantly due to strong electric field and co...

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