Impurities Limit the Capacitance of Carbon-Based Supercapacitors

Supercapacitors cannot fulfill their potential as energy storage devices without substantially improving their comparatively low energy density. Doing so requires improving their capacitance. Unfortunately, predicting the capacitance of the carbon-based materials that typically make up supercapacitor electrodes is very difficult. Carbon materials can have an areal capacitance that is an order of magnitude lower than both that of standard metals and theoretical expectations. Here, we provide new quantum mechanical calculations to demonstrate that the standard explanation of this unusually low capacitance in terms of the space charge capacitance is inadequate. We then demonstrate that a layer of hydrocarbon impurities, which has recently been shown to form on graphite, is likely the dominant cause of the low capacitance of graphite. 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. We present evidence that this layer may also play a role in limiting the capacitance in real supercapacitor materials such as activated carbon.