Effects of Edge Atoms and Channel Width on Charge Storage in Nanoporous Carbon Supercapacitors

Abstract

The amorphous structure of nanoporous carbon electrodes in supercapacitors complicates the establishment of clear links between electrode geometry and capacitance. In this work, we examine how specific structural features govern charge storage and explain capacitance variations among carbide-derived carbon (CDC) electrodes. Two key methodological and mechanistic advances are introduced. First, we propose a physically motivated, electric-field-based definition of the ion sphere of influence, which avoids the ambiguity of radial distribution function-based cutoffs and enables a more reliable characterization of local ion--electrode interactions. Second, we introduce an ion-resolved channel-width descriptor that quantifies pore accessibility by identifying the narrowest passage a counter-ion must traverse to enter a pore, directly linking accessibility to ion decoordination. Using atomistic molecular dynamics simulations of supercapacitors comprising realistic CDC electrodes and a room-temperature ionic liquid electrolyte under applied potentials, we show that carbon atoms located at the edges of graphitic sheets consistently accumulate higher charge than basal-plane atoms across all electrode types. The fraction of edge atoms increases with structural disorder and correlates with enhanced capacitance, in agreement with recent experimental findings linking disorder to charge storage. Furthermore, analysis of pore size and channel width reveals that ion decoordination is governed primarily by channel width rather than pore size alone. Together, these results establish edge atom concentration and pore channel width as decisive structural descriptors controlling charge accumulation in nanoporous carbon supercapacitors, providing clear design guidelines for optimizing electrode architectures.