Supercapacitors cannot fulfill their potential as energy storage devices without substantially improving their comparatively low energy density. This requires improving their capacitance. Unfortunately, predicting the capacitance of the carbon-based materials that typically make up supercapacitor electrodes is difficult. For example, remarkably we lack a theoretical understanding of the capacitance of even the most basic example of a carbon electrode: highly oriented pyrolytic graphite. (HOPG) This material has a capacitance that is an order of magnitude lower than both standard metals and theoretical expectations. Here, we use new quantum mechanical calculations in combination with a critical analysis of the literature to demonstrate that the standard explanations of this unusually low capacitance are inadequate. We then demonstrate that a layer of hydrocarbon impurities which has recently been shown to form on graphite is the most plausible explanation. We develop a model of this effect which accounts for the penetration of solvent into the hydrocarbon layer as the voltage increases. This model explains the characteristic V shape of the capacitance as a function of voltage. Finally, we present evidence that this layer also forms and limits the capacitance in real supercapacitor materials such as activated carbon. Methods of modifying or removing this layer could therefore potentially lead to significant improvements in the capacitance of typical supercapacitors.
Minor changes to the text and inclusion of an extra plot.