Investigating the Electrochemical Behaviour of Different Carbons in Non-Aqueous Solvents Using Step Potential Electrochemical Spectroscopy

Abstract

Supercapacitors are high-capacity electrochemical capacitors that bridge the gap between electrolytic capacitors and rechargeable batteries. They are a promising energy storage technology for addressing many of the problems associated with the transition to renewable energy technologies from fossil fuel based energy. Supercapacitors use electrostatic double-layer capacitance or electrochemical pseudocapacitance but in reality it’s often a combination of both. The common electrochemical methods used to characterize supercapacitor electrode materials are not able to separate the contribution from these different phenomena. However, using the step potential electrochemical spectroscopy (SPECS) method we will be able to develop a model of specific capacitance behaviour in link with the porosity and the morphology of the active material used for the synthesis of the electrode. The SPECS experiment involves applying a small (±25 mV) potential step to the working electrode followed by a long equilibration time (300 s). This process is repeated over and entire charge-discharge cycle. By scanning at such a slow rate, the electrode has time to equilibrate at each potential, and the maximum charge storage capabilities of the electrode can be accessed. Each of the different charge storage processes occurring at the electrode has a unique time- dependent current response, and hence each potential step profile can be fitted to a model describing each of these processes. From this, values for series resistance (RS), double layer capacitance (CDL), diffusion limited capacitance (CD) and residual capacitance (CR) can be extracted. When the potential is stepped over an entire capacitor cycling range, contributions from each process can be determined at each point in the cycle. In this work, nine different kinds of carbon structures were studied with the SPEC method and the results had shown how it was possible to link the carbon structure and the morphology with the storage behaviour. The morphology of these materials were very different and consequently the specific area and the diffusion of the electrolyte inside were impacted. The effects of different non-aqueous electrolytes were also studied under the same conditions to understand how the performances are affected by the ion size variation in order to have a better idea of the storage mechanism at the interface.