Surface Diffusion and Adsorption in Supercapacitors

The prospect of double layer capacitors relies on the high specific surface area provided by microporosity of carbon. Since there is not enough space within narrow micropores for forming a double layer, recent theoretical/computational studies have aimed at explaining the mechanism in micropores. The problem is that the available models suggest substantial differences in the mechanism of energy storage by microporous and other types of carbon, but the electrochemical behaviors are similar to a significant degree. Here, a conceptual model is proposed empirically, which is in full agreement with the experimental data reported in the literature, to reasonably explain a universal mechanism of all carbon-based capacitors including microporous, mesoporous, graphene, etc. It is described that none of the available models for double layer charging from Helmholtz to Graham is valid for carbon-based capacitors, as no double layer is formed within the micropores, as well as the partial contribution of double layer charging in larger pores or on graphene sheets. The mechanism of charge accumulation in supercapacitors is based on the adsorption of electroactive species over active sites of the carbon nanomaterial, and the surface diffusion of the adsorbed species collects the charge over the high surface area of carbon. The rate-determining step is usually controlled by the availability of active sites, which defines the rate capability of supercapacitors. This explains why the rate capability of double layer capacitors is much lower than the theoretical expectations, and why the alignment of graphene sheets results in fast performance in the so-called kilohertz supercapacitors. Any factors, such as narrow pores and irregularities, slowing down the subsequent surface diffusion cause the deviation from ideal capacitive behavior. The present paper attempts to highlight two points: surface diffusion might be a critical step in the mechanism of supercapacitors (probably a rate-determining step) and if narrow micropores exhibit capacitive behavior without forming a double layer, larger pores may do the same since no sudden change in the capacitive behavior with response to the pore size is observed (to represent a crucial change in the mechanism).